Photochemistry of porphyrins and their metal complexes in ...wxjs.chinayyhg.com/.../1978-Volume-2/2/107-138.pdf · Photochemistry of porphyrins and their metal complexes 109 tioning

Photochemistry of porphyrins and their metal complexes in solution and organized media

David G. Whitten Department of Chemistry, Univeraity of North Carolina, Chapel Hill, North Carolina 27514

C o n ~ n ~

I. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107 H. Porphyrins as Photosensitizers in Solution: Electronic Energy Transfer . . . . 108

A. Metalloporphyrin excited states . . . . . . . . . . . . . . . . . . . . 108 B. Triplet energy transfer with porphyrins and metalloporphyrins . . . . . . 109 C. Intramolecular triplet energy transfer in metalloporphyrin complexes . . . 1 I0 D. Luminescent triplet metalloporphyrins as scnsitizers in quantum chain

processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I 16 IlL Photoinduced redox reactions of porphyrins . . . . . . . . . . . . . . . . 121

A. General redox patterns . . . . . . . . . . . . . . . . . . . . . . . . 121 B. Electron transfer quenching and exciplex formation . . . . . . . . . . . 122 C. Photoreduction and oxidation of the porphyrin ring . . . . . . . . . . . 123

IV. Photoreactions of porphyrins in organized media . . . . . . . . . . . . . . 127 A. General behavior ofporphyrins in monolayers and micelles . . . . . . . . 127 B. Ligand photoejection and exchange in ruthenium and iron complexes in mono-

layer assemblies and micelles . . . . . . . . . . . . . . . . . . . . . 131 C. Photooxidation of surfactant protoporphyrins in monolayer films, organized

monolayer assemblies and micelles . . . . . . . . . . . . . . . . . . . 133 V. References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 136

L Introduction

The photochemical reactions of porphyrins and their metal complexes and related compounds (chlorophylls, chlorins, etc.) have received much attention f rom workers in a wide variety of areas approaching the subject with various interests and points of view. I t is beyond the scope of this article to a t tempt to review the broad topic ofporphyrin photochemistry. In fact, a resurgence of interest in porphyrin chemistry has led to two excellent review series 1.2 both of which include reviews on the general area of porphyrin photochemistry 3 as well as some specific topics such as the photochemistry of chlorophyll and its analogs in membranes as related to photosynthesis. 4 In the present review, we will restrict our discussion to a limited number of photoproccsses that have been studied extensively in our laboratories. Much of this work focuses on the special relationship between the metal and. porphyrin macrocycle in metaIloporphyrin complexes. In particular, this relationship is manifested in processes involving light induced energy transfer and redox phenomena. The material covered in this article will therefore be limited compared to other recent reviews and will deal largely with work done by our group and investigations related to these studies.

© 1978, by Verlag Chemie International, Inc.

107

108 D. G. Whitten

This review is divided into three main areas. The first two deal with light- induced reactions of porphyrins and their metal complexes in solution, in particular energy transfer and electron transfer reactions of the porphyrin. The third section reviews the reactions of porphyrins in organized media including monolayer films, monolayer assemblies and micelles and will deal with non- photochemical processes as well as photoreactions.

II. Porphyrins as photosensitizers in solution: electronic energy transfer

The commonly studied porphyrins fall into two general groups distinguished by the substitution pattern on the porphyrin ring. The naturally occurring and biologically important porphyrins such as protoporphyrin IX have substituents on the eight t-positions of the four pyrrole rings and hydrogen at the four "bridge" positions and are referred to as octaalkylporphyrins. The synthetically easy-to-prepare but non-naturally occurring tetraphenylporphyrins and tetra- arylporphyrin have hydrogens on the fl-pyrrole positions and aromatic groups at the bridges. Although there are characteristic red shifts in absorption and luminescence spectra on going from the octaalkyl to the tetraphenylporphyrins, the shifts are small even for the free-base porphyrins since the phenyl groups in most complexes are largely twisted out of the porphyrin plane thus minimizing both steric and conjugative interactions, a Free-base porphyrins exhibit characteristic fluorescence and phosphorescence from the lowest singlet and triplet states respectively. The fluorescence has a lifetime in the nanosecond region and can be observed in both solution and rigid glasses. 3 The triplets of most free-base porphyrins do not phosphoresce in room temperature solutions but they can be easily observed by conventional (micro-second) flash spectroscopic techniques; typical lifetimes are in the range 100-500 tzsec. Interestingly, a significant deuterium isotope effect has been observed for triplet lifetime for only the N - - H protons of free-base porphyrins. 6'~ It has been suggested that relaxation of the triplet to ground state is coupled with interconversion of different tautomeric forms in the free base. 7 Typical singlet energies for octaalkylporphyrins are in the range 47-48 kcal tool- x while that for tetraphenylporphyrin is 45 kcal mol- z. Triplet energies are generally 6--7 keal mol-X lower than the singlet energies. "~

A. Metalloporphyrin excited states

A great number of porphyrin metal complexes have been prepared, particularly during the last 15 years. In many eases, there has been at least a cursory investigation of excited state properties for these complexes such as the deter- mination of luminescence spectra and lifetimes. In general, for most metal complexes, it is clear that the excited states are best described as discrete singlet and triplet states associated basically with the porphyrin ligand but whose lifetimes and reactivities are strongly affected by the contained metal. While it is beyond the scope of this review to cover in detail the luminescence behavior of the numerous complexes prepared to date, a few generalizations are worth men-

Photochemistry of porphyrins and their metal complexes 109

tioning. The complexes of closed-shell diamagnetic metal ions such as Cd(II), Mg(II), Zn(II), and Sn(IV) are strongly fluorescent but do not phosphoresce in solution; triplet states are easily detected by flash photolysis and have lifetimes in the 50-500/~sec range, a'e Complexes of most open-shell metal ions of the first transition period are either non-fluorescent or very weakly fluorescent and their triplet lifetimes are very short in solution. In contrast, complexes of open-shell but diamagnetic metal ions of higher transition periods (e.g. Pd(II), Pt(II), Ru(II)) show both intense fluorescence and phosphorescence in solution at room temperature) ,8-z: Triplet lifetimes for some of these complexes range upwards into the msec range. In several cases, the singlet-triplet splittings are small enough to allow appreciable repopulation of the singlet such that delayed fluorescence is a significant channel for excited state deactivation. 9,~z A major part of this review will deal with photoreactions in which the strongly luminescent triplet of Pd(II), Pt(II), or Ru(II) is an intermediate.

B. Triplet energy transfer with porphyrins and metalloporphyrins Since several free-base and metaUoporphyrins have prominent uv and visible

absorption spectra, small singlet-triplet splittings, high intersystem crossing efficiencies and long excited triplet lifetimes, they should be excellent sensitizers for a variety of photoprocesses. In fact, their use has been relatively limited until recently. It has been recognized that porphydns play an important role in a number of light induced biological disorders, many of which also involve molecular oxygen. In several cases, it has been suggested that the key step involves energy transfer from porphyrin or metaUoporphyrin triplets to ground state (triplet) oxygen with production of excited singlet oxygen, z2-1~ Hematoporphyrin IX, protoporphyrin IX, and some metalloporphyrins have been widely used as sensitizers of singlet oxygen in more conventional photo-oxygenation reactions. That metaUoporphyrins can function well as classical photosensitizers has been demonstrated in studies of sensitization of azobenzene and thioindigo dyes. zs.z6

Detailed studies of triplet energy transfer by several groups over the last 20 years have had a major impact in elucidating mechanistic details of photochemical reactions and determining properties of excited states. Although several different types of molecules have been studied as donors and acceptors, including ketones, lanthanide complexes and other spectroscopically or chemically reactive systems, one of the most widely used probes has been the cis-trans isomerization of olefins such as the stilbenes and related compounds. 17-2° These compounds have been used both as donors and, more widely, as acceptors of triplet excitation and indeed their use has led to the observation in several cases of unexpected new phenomena. Probably, the most interesting phenomena are those occurring where the stilbenes are used as triplet energy acceptors from potential donors having lower triplet energies than the indicated or "spectroscopic" triplets of the stilbenes, zT-za The "' spectroscopic" or vertical triplets of the stilbenes have been estimated from singlet-triplet absorption spectra to be 48 and 57 kcal tool-z for trans and cis isomers respectivelyZV'~°; those of the 1,2-diphenylpropenes (~-methylstilbenes) are indicated as 50 and 56 kcal tool -z for trans and cis

110 D. G. Whitten

isomers. 17 Since, as indicated earlier, the porphyrins and their metal complexes have excited triplets in the 40-45 kcal tool- x range, a their convenient excitation in the visible region where the stilbenes do not absorb and their relatively long triplet lifetimes make them particularly suitable low energy sensitizers for investigation of triplet energy transfer phenomena occurring with stilbenes and related systems. In investigations conducted in these laboratories, we have investigated energy transfer phenomena in the metalloporphyrin-stilbene system in both intra- and intermolecular situations.

C. Intramolecular triplet energy transfer in metalloporphyrin complexes

The zinc, magnesium and cobalt (III) porphyrins are well suited for use in a study of intramolecular energy transfer since the metal in the simple porphyrin complex is not fully coordinated. Thus, zinc porphyrins readily accept one additional nitrogen, phosphorous or oxygen ligand while the corresponding magnesium and cobalt complexes can accept up to two apiece. 21-2a Although ligand exchange is indicated to be very rapid for zinc and magnesium complexes in both the ground and excited state, 2a the cobalt (III) porphyrins do not readily exchange ligands and it is easy to isolate the diligated complex, MPL2. The stilbene-like ligands that were chosen for the study of intramolecular energy transfer were 1-(~-naphthyl)-2-(4-pyridyl) ethene (NPE) and 4-stilbazole. In a preliminary study, it was found that both of these olefins undergo cis-trans isomerization under direct and sensitized photolysis. 24 Results with different sensitizers and the observation of weak singlet-triplet spectra indicate that triplet levels for the uncomplexed 4-stilbazole isomers are similar to those of the stilbenes while slightly lower energies are indicated for the isomers of NPE. 25 Both NPE and 4-stilbazole isomers form complexes with zinc and magnesium porphyrin complexes in which the long-wavelength transitions (porphyrin ~r--,~r* bands) are essentially identical to those of pyddine or piperidine complexes. Since excess 4-stilbazole or NPE must be added to solutions containing the zinc and magnesium porphyrins, it is not possible to determine the effect complex formation produces on the ligand transitions; however, for the cobalt (III) complexes, which do not undergo exchange, a subtraction spectrum indicates that the ligand ~r ~ ~r* transitions are shifted to longer wavelengths (for NPE the ~,~x shifts from 331 to 353 nm with a corresponding but smaller shift in the band onset) and that excitation energies may be lowered by several kcal tool- 1. Similar behavior has been observed for ligand olefins in other metal complexes.

Studies ofintermolecular energy transfer with the stilbenes and 1,2-diphenylpropenes indicate that these olefins quench sensitizers having energies in the range 40-45 kcal tool -1 with rate constants in the range k = 104-107 1 tool -1 s-1. la Therefore, at the very high effective acceptor concentrations assumed for the intramolecular stilbazole-porphyrin or NPE-porphyrin complexes, one would anticipate energy transfer to dominate radiationless decay of the metalloporphyrin triplet with efficient cis-trans isomerization of the bound ligand as a consequence. Since photostationary states reported for the stilbenes with

Photochemistry o f porphyrins and their metal complexes 111

trans-NPE trans-4-stilbazole

sensitizer in the 40-45 kcal mol - 1 energy range are 50--70% cis, one might expect that intramolecular sensitization should involve efficient conversion in both directions. In fact, irradiation o f NPE and 4-stilbazole complexes o f zinc and magnesium porphyrins (etioporphyrin I and mesoporphyrin IX dimethylester complexes) does lead to efficient isomerization of the olefinic ligand but the results are quite different from those obtained in intermolecular studies. 2s First of all, although the trans to cis efficiencies are moderate in several cases (Table 1)

Table 1. I$omerization of ligand-olefats in metalloporphyrin complexe~ .~

.Photo- Stationary

Porphyrit~ Ligand.Olef~ O,-.t O~-,c State

Zn etio I Stilbene 0.01 0.001 Zn etio I 4-Stilbazole 0.4 0.001 99% trans Zn eao I NPE 6.6 + 1 0.2 96°7, trans Zn meso IX NPE 6.6 _+ 1 96% trans Mg etio I 4-Stilbazole 0.17 99% trans Mg etio I NPE 3 + 1 95% trans

• Reprinted with permission from D. G. Whitten, P. D. Wildes, and C. A. DeRosier, I. Amer. Chem. Soc., 94, 7811 (1972). Copyright by the American Chemical Society.

b Dcgassed benzene solutions irradiated at 25-28 ° with 405 and 436 nm light.

"5 x 10-s M. a5 x 10-3 M.

for the complexed ligand, the cis to trans efficiencies are much greater and in several cases substantially exceed unity, the anticipated limit for an isomerization occurring via radiationless decay of the olefin triplet. The photostationary states wcrc trans-rich in all cases and far from the values obtained in intermolecular sensitization. Perhaps the most striking feature of the intramolecular photo- reaction is the finding by flash photolysis that the triplet lifetime of the metalloporphyrin is unaffected by complex formation or the occurrence of the isomerization process. In fact the triplet-triplet absorption spectrum obtained for the complex is unchanged from that obtained for complexes with ligands

112 D. G. Whitten

possessing no low excited states. Several experiments indicate that the metalloporphyrin is the precursor for the isomerization and that the reaction does require complexation between the metaUoporphyrin and the olefin. For example, it is found that addition of substancas quenching the porphyrin triplet strongly quench the isomerization. ~s Addition of bases such as pyridine which compete for the metal coordination site reduces the extent but does not completely eliminate the isomerization. The observation that trans-rich photostationary states are obtained might tend to suggest the intermediacy of radicals or radical ions which might initiate a radical chain-induced isomerization in the energetically favored c/a to trans direction. However, such a mechanism, though difficult to absolutely rule out, appears to be unlikely for several reasons. The quantum yield for such a process would likely increase rapidly with c/s olefin concentration yet the limiting quantum yields are reached at concentrations in the 1 × 10 -3 M range for NPE and are essentially invariant at higher concentration. I f a radical ion were involved in a chain process, an increase in the efficiency with solvent polarity might be anticipated; however, a change to solvents more polar than benzene decreases the efficiency in the cases studied to date. Further considera- tions of the various possibilities for metalloporphyrin ligand electron transfer indicate that such processes should be highly energetically unfavorable from the lowest singlet and triplet.

The fact that isomerization occurs readily with the metalloporphyrin localized triplet as a necessary precursor but without causing observable quenching of the triplet suggest that the metaUoporphyrin is a photocatalyst and that a mechanism involving use of the triplet in promoting the isomerization should also include its regeneration. Although, as indicated earlier, porphyrin to olefin energy transfer should be likely with the complexes investigated, a simple energy transfer followed by isomerization concurrent with non-radiative decay can be ruled out as the probable mechanism. However, a mechanism involving reversible energy transfer is possible and in fact appears to be the most likely explanation for the observed phenomena. The basis for the reversible energy transfer mechanism lies in the particular triplet energy surface for the stilbenes and other 1,2-diarylethyl- enes and the relatively long triplet lifetimes for these olefins. A potential energy surface such as indicated in Figure 1 is believed to occur for several olefins similar to stilbene; the key feature is the broad minimum or double minimum with a small-barrier occurring between transoid and twisted geometries. For stilbene triplets, several paths for deactivation are evidently possible. Unassisted decay evidently occurs only from the twisted state resulting in formation of c/s and trans ground states in an approximately 3:2 ratio. However, several molecules can quench the excited state in competition with non-radiative decay in processes which yield different c/s: trans ratios. Quenching by azulene 2o and by /~-carotene, 26 which is believed to occur by triplet energy transfer, results in selective production of only trans-stilbene. This has been taken as evidence that quenching by energy transfer involves only the transoid form of the triplet, which has a large potential energy gap to the ground state, as a potential donor. 2° In contrast, quenching by oxygen, which probably does not involve energy


E

0 i RANS 90" C15

Figure 1. Proposed triplet (upper) and ground state (lower) potential energy curves for stilbene as a function of angle of twist about the carbon-carbon central bond. Reproduced with permission from J. A. Mercer-Smith and D. G. Whitten, J. Amer. Chem. Soc., in press. Copyright by the American Chemical Society.

transfer, shortens the triplet lifetime without affecting the decay ratio; here presumably only the twisted form is quenched) ° Quenching by nitroxide radicals, also a process not involving energy transfer, leads to a slightly c/s-rich stationary s ta teY The key feature in the intramolecular ligand-olefin-porphyrin complexes is that the metalloporphyrin triplet should lie below the transoid triplets of NPE and 4-stilbazole; thus once energy is transferred to produce the olefin-localized triplet state, the bound metalloporphyrin should be able to quench the olefin triplet by energy transfer with selective production of trans ground state and metalloporphyrin triplet in a process analogous to the intermolecular eases occurring with azulene and B-carotene as quenchers. The overall mechanism for the process is described by eqs. (1)-(I0) where L * = c/s-ligand- olefin and L t = trans-ligand-olefin and MP = metalloporphyrin.

M P - L c h. , M p 1 , _ L c (1) M p 1 , _ L c k , , M P - L c (2)

M p 1 , _ L C k, ~ M p a . _ L t (3)

MP a* - L ~ ~ MP - L '3. (4) MP a * - L t *~, M P - L to* (5)

MP - U a* k , , Mpa, - Lt (6)

MP a * - U + L ~ k, ~ M p a , _ L ~ + L t (7)

MP a * - L ~ ~ M P - L ~ (8)

M p a , _ L t ko, M P - L t (9)

M P - L ta* ..k~,~ ~ ( M P - L t) + (1 - ~ ) ( M P - L ~) (lO)

114 D. G. Whitten

The key steps are the "uphill" energy transfer, eqs. (4) and (5), the reverse energy transfer eq. (6), and ligand exchange eq. (7). An expression for the cis --> trans quantum yield using this mechanism may be easily derived (eq. (11)).

k4[k, + kT(L~)][ke + ak~o]4,~o (11) ~c-.t ffi [ks + kT(LC)][ks + k~o][k, + ks] - kT(LC)kek4

Since ligand exchange is rapid compared to metalloporphyrin triplet decay (k~(L ~) >> ks) and the observation of no quenching of the porphyrin triplet by the olefin-ligand indicates ks >> k~o, the relationship reduces to the much simpler eq. (12):

~..,, = k,~,,Jk8 (12)

Although a value for k, cannot be measured for NPE or 4-stilbazole, estimated values (vide supra) based on intermolvcular examples suggest that k~ > k8; thus with the large indicated value of ~,-o for both magnesium and zinc porphyrins a predicted value of ~c-.t > 1 can be estimated in accord with the observed results. 25

A more quantitative picture of the process as well as confirmation of eqs. (I)-(10) as the mechanism for the photocatalysis can be gained by examination of intramolecular photosensitization with some modified derivatives of NPE and 4-stilbazole. These ligand olefins, 1-(a-naphthyl)-2-(4-pyridyl) propene (PPP) are analogs of the methyl-substituted stilbene, 1,2-diphenylpropene. This olefin undergoes photochemical cis-trans isomerization under direct and sensitized irradiation in processes similar to those occurring with the stilbenes. 28 Some major differences occur, however, and these can be attributed to slightly different excited state potential energy surfaces for the diphenylpropvnes. Figure 2 indicates an approximate potential surface for the triplet as a function of the angle of rotation about the olefinic bond. The major difference is the presence of only a single energy minimum corresponding to the near 90 ° twisted geometry.

u5

0 TRANS 90" C IS

Figure 2. Proposed triplet (upper) and ground state (lower) potential energy curves for 1,2-diphenylpropene as a function of angle of twist about the central carbon-carbon bond.


The consequence of the single minimum near the ground state maximum is a very short lifetime for the triplet; unlike the stilbenes, quenching of planar excited states is not possible for 1,2-diphenylpropene and no "azulene" effect occurs. 2'.8° Thus, for metalloporphyrin complexes of NPP and PPP, the mechanism described by eqs. (1)-(10) would have to be modified to delete the reverse energy transfer step (eq. (6)). As a consequence, if the isomerization described above for NPE and 4-stilbazole occurs via energy transfer, the use of NPP and PPP as ligands should result in some pronounced changes including ff,*t values lower than unity and the observation of quenching of the porphyrin triplet for these complexes.

H3

trans-NPP

~ H CH3

trans-PPP

The predicted changes are in fact readily observable with zinc porphyrin complexes of NPP and PPP (Table 2)) 5,28 The triplet lifetime of the zinc

Table 2. Photoisomerization of 1,2-diphenylpropene analogs as ligands in zinc etioporphyrin complexes a'~

Photo- Stationary

Ligand-Olefin ~ ~Pc-.t ~t~c State

NPP 0.4 0,2 31 70 cis PPP 0.1 0.05 30% c/s

Reprinted with permission from D. G. Whitten, P. D. Wildes, and C. A. DeRosier, J. Amer. Chem. Soc., 94, 7811 (1972). Copyright by the American Chemical Society.

Degassed benzene solutions irradiated at 25-28 ° with 504 and 436nm light; [zinc etio], 5 × 10 -5 M.

c Concentration, 5 x 10 -3 M.

etioporphyrin I-cis-NPP complex is reduced to ca. { of the value obtained with other zinc etioporphyrin I-nitrogen base complexes indicating the introduction of a new channel for non-radiative decay. Quantum yields for the isomerization of both NPP and PPP are substantial but below unity in all cases suggesting that no chain process is involved for these olefins. The $o*t for the zinc etio-cis NPP complex is 0.4; assuming a 1:1 decay ratio from the NPP excited state this

116 D.G. Whiten

indicates that ca. 807o of the excitation energy delivered to the complex is degraded by nom'adiative decay of the ligand-olefin triplet. This agrees well with the calculated value of 84% estimated from a comparison of relative triplet lifetimes of the c/s-NPP and pyridine complexes. A value for the uphill energy transfer step (eq. (4)) can be estimated from the same data to be 1.1 × 10' s -1. Since the potential surface from the c/s sides of NPP and NPE should be very similar, the values for k, for both complexes should be quite similar. Substitution of the value for NPP in eq. (12) leads to a "predicted" value for ~c-.t for NPE of 5.S in good agreement with the measured value of 6.6. An interesting aspect of the study of intramolecular triplet energy transfer in the zinc porphyrin-NPP complexes was the finding through a study of temperature effects on the quenching process, eq. (4), that the activation energy for the process is only 2.5 kcal tool- 1.25 Since the difference in "spectroscopic" triplets of zinc porphyrin and olefin is estimated to be 10-12 kcal tool -~, it is clear that the triplet populated must not be the cisoid triplet but more likely one of twisted geometry. Not surprisingly the value of AS* for the process is - 34 eu.

D. Luminescent triplet metalloporphyrins as sensitizers in quantum chain processes

Having observed a quantum chain process mediated by reversible intramolecular transfer of triplet excitation in the zinc and magnesium porphyrin- ligand olefin complexes, we thought it would be of interest to determine whether an intermolecular counterpart of such a process could be observed. Inter- molecular quantum chain reactions have been reported for conjugated diene isomerization processes, ax-~s These reactions occur at high diene concentrations and are indicated to involve energy transfer between diene triplets and ground states. Although such a process could conceivably occur at very high stilbene concentrations, we have seen no evidence for it with conventional sensitizers. The mechanism proposed for the intramolecular quantum chain process is insensitive to stilbene concentration above the point where energy transfer to the stilbene is dominant and the limiting value for the quantum chain is reached at relatively low stilbene concentrations. Although a number of compounds having energies in the 40-45 kcal tool-x range could have potentially mediated such a process, the palladium (II) and platinum (II) porphyrins appeared to be particularly attractive candidates for such a study for a number of reasons. These complexes, in which the porphyrin is bound to a d 8 metal ion, are very stable as square planar complexes and there is little tendency to bind extra ligands, x°.xl Thus the use of simple stilbenes and 1,2-diphenylpropenes with these porphyrins should involve no ground state interactions and any sensitization observed should be purely due to dynamic interaction in the excited state. The strong phosphorescence of these complexes coupled with long excited state lifetimes in solution indicate that even relatively slow bimolecular energy transfer processes should occur with moderate efficiency and be readily detectable, x°'xx

Preliminary experiments indicated that palladium (II) octaethylporphyrin (PdOEP), palladium (If) mesotetraphcnylporphyrin (PdTPP), and platinum (II)


mesotet raphenylporphyr in (PtTPP) do not form complexes with the stilbenes or 1,2-diphenylpropenes in the g round state in benzene or pentane solution, a4 I r radia t ion of benzene or pen tane solut ions o f the pal ladium or p la t inum porphy- t ins with the stilbenes or d iphenylpropenes leads to cis-trans isomerizat ion of the ol¢fin as the only detectable reaction. For sensit ization of the 1,2-diphenylpropenes the observed results are completely in accord with those to be expected for a conventional sensitization process as indicated in eqs. (13)-(17). 3~ Dynamic quenching of the metalloporphyrin triplets can be followed by quenching of

MP ~ ' MP 1. ' M P a* (13) MP a* , MP" (+he) (14)

MP a* + c/s-DPP ) DPP a* + MP (15) M P a* + trar~-DPP ) DPP a* + MP (16)

DPP a* ) ~(trans-DPP) + (l-a)(cis-DPP) (17)

phosphorescence and the quenching constants obtained (Table 3) are in good agreement with those obtained for the diphenylpropenes with aromatic hydro- carbons and ketones having comparable triplet energies. The photostafionary

Table 3. Quenching of metalloporphyrin triplets by stilbenes and 1,2.diphenylpropene: .~

Sensitizer

])dOEP PdTPP PtTPP (pentane) PdOEP (benzene) (benzene) (benzene)

~,° ~sec 555 500 i o c i o ~ vo a

Method ~- T

eis-stilbcne k~ ° 3.28 x 10 e 1.37 x 108 correlation coefficient 1.00 0.993

trans-stilbcne k~ 3.34 x 108 1.52 x 10 e 5.32 x l0 s correlation coefficient 0.999 0.982 0.946

cis-l,2-diphenylpropen¢ ke 1.98 x 10 s 9.69 x 108 correlation coefficient 1.00 0.809

trans- l,2-diphenylpropene kq 6.81 x l0 s correlation coefficient 0.995

358 54 i o c i o c

I I

1.75 x l0 s 6.68 x 10 s

0.995

1.03 x lO s 5.02 x IO s

0.986 0.998

Reprinted with permission from J. A. Mercer-Smith and D. G. Whitten, J. Amer. Chem. Sot., in press. Copyright by the American Chemical Society.

b Degasscd solutions, rate constants determined by linear regression analysis. c Measured by phosphorescence intensity quenching. d Measured by reduction of triplet lifetime as determined by photolysis. • Units of k~ are I tool- 1 s- 1.

118 D. G. Whitten

states (Table 4) are cis-rich and the measured values for PdOEP sensitization correlate well with those calculated using the mechanism outlined in eqs. (13)--(17). The measured quantum yield, ~o_., = 0.37, is reasonable for a normal process and it can be used to estimate a value of 0.84 for the PdOEP intersystem crossing etficiency.

Table 4. Photostationary atates for metalloporphyrin sensitized isomerization o f stilbenea and 1,2-diphenylpropenes ~

Photostationary State, ~ cis

Sensitizer Stilbene 1,2-Diphenylpropene

PdOEP 17 PdTPP 16 PtTPP 37

68

Degassed benzene solutions, averages of several concentrations.

Sensitization of the stilbenes by PdOEP, PdTPP, and P tTPP is more complicated than that for the diphenylpropenes and cannot be adequately explained by the simple mechanism outlined above. Although the results suggest a quantum chain process is occurring (Table 5), the behavior in the intermolccular case is not so simple as that observed intramolecularly with the zinc and magnesium porphyrins. 34 First of all relatively rapid quenching of metalloporphyrin triplets is observed by both isomers (Table 3). This contrasts with the intramolecular situation where no quenching of metalloporphyrin triplets is observed (vide supra). The quenching is viscosity-dependent as indicated by an approximately

Table 5. Quantum yields for metalloporphyrin sensitization of stilbene isomerization ~

Sensitizer

Process b PdO EP PdTPP PtTPP

~c~t 1.58 _+ 0.03 c 1.36 + 0.3" 1.34 _+ 0.01 f ~J~c 0.30 + 0.01 c 0.24 :i: 0.01 ~

Reprinted with permission from J. A. Mercer-Smith and D. G. Whitten, J. Amer. Chem. Soe., in press. Copyright by the American Chemical Society.

b Degassed benzene solutions, porphyrin concentrations 5 - I00 x 10-5 M; irradiation wavelengths 435 nm for PtTPP and 546 nm for PdOEP and PdTPP.

c Concentration 0.186 M. a Concentration 1.44 M. * Concentration 0.646 M. I Concentration 2.84 M.


threefold increase when the solvent is changed from benzene to the less viscous pentane. Also, in contrast with the intramolecular eases described above, the observed photostationary states (Table 4) are less rich in trans isomer than the 95-9970 obtained for the NPE and 4-stilbazole complexes. However, the photostationary states for the stilbenes are trans-rich compared to those obtained for the 1,2-diphenylpropenes and they are dearly much more trans-rich than predicted using known decay ratio and the mechanism described by eqs. (13)--(17).

The fact that quantum yields for the stilbene isomerization sensitized by triplets of Pd(II) and Pt(II) porphyrin triplets exceed unity in all three cases examined (Table 5) indicates a chain process is occurring. However, the observation of net quenching of the porphyrin triplet suggests that some non- radiative decay by stilbene triplets is occurring. The fact that both quenching constants and photostationary state composition are influenced by viscosity suggests that either a discrete excited state complex or at least a solvent-caged encounter complex is playing a role as indicated in eqs. (18)-(20) where st = stilbene. If simple porphyrin to stilbene energy transfer were to occur with

MP a* + st ~ (MPa*---st) (MP---st) 7 - - -* (MP---st a*)

(MP---st a*) ~ st a* + MP

(18) (19) (20)

production of a kinetically "free" stilbene triplet, there would be no possibility of reverse energy transfer or a quantum chain mechanism since the porphyrin concentrations used (max 5 x 10 -s M) are too Iow to permit "'dynamic'" quenching to compete with non-radiative decay (k ~ 107 s-l). 85 If an excited porphyrin-stilbene complex or cage encounter assembly exists in which equilibra- tion between essentially non-perturbed stilbene and porphyrin triplets occurs, an equilibrium constant can be calculated for the PdOEP-stilbene system:

[stilbene]8* = 8 x 10 -~ K~q = [porphyrin]a,

Considering relative decay rates for metalloporphyrin and stilbene triplets of 2 x 10 a and 107 s -1 a5 respectively, it can be predicted that decay from the stilbene triplet should be an important process and in fact dominate porphyrin decay by ca. 10 to 1. Therefore, it is easy to explain qualitatively why quenching should occur; however, the fact that no quantitative agreement is observed suggests that the interaction between porphyrin and stilbene in the excited state may not be a simple one. If the differences between results obtained in benzene and pentane can be ascribed purely to viscosity effects, a6 the increase in quenching constant and the fraction cis in the photostationary state can be attributed to enhanced formation of"f ree" stilbene triplets in the latter solvent. This suggests that the stilbene-porphyrin excited state complex or cage encounter complex must have either a strong preference for decay to trans-stilbene, an equilibrium

120 D. G. Whitten

constant other than that calculated above or a much longer lifetime for the stilbene triplet in the complex.

Two mechanisms which can account for the observed results are outlined below:

Mechanism L

Mechanism IL

MP h" , MP 1. ' MP a*

MP a* + trans k't , [MP + st a*]

MP a* + c i s kg , [ M P + s t a*] I M P + s t a*] t'*t, M P + s t 3.

[MP + st a*] k, Mpa, , + trans

st a* , ~ t a(trans) + [l-e] (cis)

MP a* *[ , MP

MP ~ MP 1. , MP a*

MP a* + trans kl , [MP, st] a*

MP a* + cis kg , [MP, st] a*

IMP, st] a* k~t, MP + st a* ~ (l-~) cis + a(trans)

[MP, st] a* k, Mpa, + trans

[MP, st] a* *°~ ~ MP + (1-fl) cis + [3 trans

For mechanism I in which [MP + sP*] represents a loose or cage-encounter complex, the relationships given by eqs. (21)-(23) can be readily derived.

4~.t = 4~,~ kp + ¢k~t (21) kst

~,.c = ¢ , ~ (1 - a) (22)

[trans] k~ a + k f l k , t = (23)

The mechanism allows for the quantum chain process, the observed viscosity effect, and the trans-rich photostationary state. Calculated and measured photostationary states are in reasonable agreement. However, values of ,~t.o calculated using the well-established value of 0.41 f o r , and measured ~ o are substantially higher than the measured values with PdOEP and PdTPP. Thus, it appears that an additional decay channel beyond that outlined in mechanism I must be occurring. Mechanism II, in which IMP, st] °* is used to represent a " t ighter" excited state complex than the simple encounter complex of mechanism I, adds an additional decay channel, kox, which likely involves selective production


of trans-stilbene ground states. This allows for a lower ~t,~ than mechanism I predicts and also can account partially for the enhanced quenching in the intermolecular case as compared to the intramolecular complexes described in Section II C. Otherwise, mechanism II predicts qualitatively similar results to those of eqs. (21)-(23) obtained for mechanism I but with expressions that are somewhat more complex.

In conclusion, the intermolecular sensitization ofstilbene isomerization by the platinum and palladium porphyrin triplets appears to involve several complica- tions which are described most simply by mechanism II in which a discrete complex intervenes. The complex evidently can dissociate to give free stilbene and free metalloporphyrin triplets but it also has competitive non-radiative process of its own. Apparently, the complex formation induces a decrease in the non- radiative rate of the stilbene-localized triplet. Consideration of redox potentials for stilbcne and the metalloporphyrins indicate that the complex is almost certainly not strongly charge-transfer in nature. Although there is clearly not enough evidence available to permit a description of the complex, a reasonable possibility appears that complex formation involves a configuration in which the ~.-electron systems of trans-stilbene and the metalloporphyrin overlap in some- thing close to a "sandwich" arrangement. This would be in agreement with the known tendency of metalloporphyrins to associate in ground and excited states 87 and also in accord with lack of evidence for such a complex in the intramolecular complexes (where direct bonding of the olefin-ligand to the metal through nitrogen precludes such an interaction). 25 A complex of this sort would be anticipated to have a longer lifetime, particularly if complexation stabilizes the trans form of the stilbene relative to the twisted configuration. Decay of the complex might be expected to give almost exclusively trans-stilbene.

The results of the inter- and intramolecular sensitization studies with metalloporphyrin triplets as potential energy donors suggest that triplet energy transfer can be considerably more complicated than previous experiments have suggested. It appears reasonable that complex formation of other than a charge transfer type can play a general role, particularly where relatively long-lived excited states are involved in low energy sensitization processes. If formation of such a complex involves an alteration of excited state potential surfaces in the donor or acceptor as is observed here for stilbene, appreciable modification of photoreactivity may occur.

Ill. Photoinduced redox reaction of porphyrins

A. General redox patterns

It is now fairly well established through a number of different investigations that porphyrins and their metal complexes undergo a series of (generally) reversible one-electron oxidations and reductions, as For metal complexes these reactions can involve either the metal or the ring as the site of the redox reactions and it has been found in several cases that interesting phenomena involving "redox isomerism" and cooperativity can result via these reactions. *~.45 In

122 D. G. Whitten

recent years, it has been found that excited state redox reactions can occur as a general process for both organic molecules and metal complexes. In several cases, these reactions have been found to involve very low activation energies and in examples where the excited state energy exceeds the energy required to drive the ground state redox process, excited state quenching via redox can proceed at rates very close to diffusion-controUed. .7.48 Given the rich array of redox levels available for porphyrins and metailoporphyrins and their long-lived excited states, it is not surprising that quenching of porphyrin and metalloporphyrin excited states by electron donors and acceptors is a prominent process.

B. Electron transfer quenching and exciplex formation

Although quenching of porphyrin excited states by both electron donors and acceptors has been observed, the more thoroughly studied process has been quenching by electron acceptors. It has been found that both excited singlets and triplets can be quenched readily at rates which correlate with ease of quencher reduction. 4a'5° In polar solvents such as N-methylformamide, acetonitrile, and ethanol, it is clear that both singlet and triplet quenching by electron acceptors such as nitroaromatics, pyridinium ions, and quinones produces free ions which generally recombine with back electron transfer to yield ground states of the starting material as outlined in eqs. (24)-(27). Rates of quenching are generally

p h , , p1. , pa , (24) px. + A" ' P f + A .~-1 (25) pa . + A" , P f + A .n-x (26)

P* + A . ~-x , P + A " (27)

near diffusion controlled provided the energy of the excited state equals or exceeds the energy of the redox state as determined by eq. (28) developed by

En+,A,,-I = Elt2(P/P +) - EltS(An-1/A) + 0.10 + 0.10eV (28)

Weller and co-workers for aromatic systems. For example, quenching of the luminescent triplet of PdOEPby the electron acceptor paraquat 2 + (N,N'-dimethyl- 4,4'-bipyridine) occurs to give the products PdOEP + and paraquat + with a rate constant k = 1.5 x 109 1 mol -x s-1. 5x The bimolecular back reaction, which can be observed by flash photolysis, occurs with a rate constant k = 1.4 x 10 ~ 1 mo1-1 s -I. Excited state electron transfer reactions with free base and metalloporphyrins can be followed by fluorescence or phosphorescence quenching in the case of luminescent complexes. For non-luminescent, but long lived triplets such as free base porphyrins or the zinc (II), tin (IV), or magnesium (II) complexes, direct quenching can be easily observed by flash photolysis, s2 Interestingly, however, we have recently found that even short-lived non- luminescent excited states not detectable by flash spectroscopic techniques can be quenched by electron transfer to yield unstable ionic products. 63 For example, we have found that Ru(TPP) (pyridine)2, which does not emit measurably at room temperature in N,N-dimethylformamide, can be quenched by 0.05 M


Ru(NHa)] + to yield transient spectral changes consistent with formation of Ru(TPP) (pyddine)~" and Ru(NHa)~ +. The transients formed decay with second- order kinetics yielding a value k = 1.2 x 10 a 1 tool -1 s -1 for the back electron transfer process. 53 The possibility of quenching very short lived non-luminescent excited states demonstrated by this reaction offers the prospect of examining numerous electron transfer processes not previously observable.

In non-polar solvents, and especially for neutral acceptors, quenching of porphyrin excited states readily occurs, but in many cases no production of ions can be observed. 4',52 In several cases, electron transfer quenching doubtless occurs but the initial product is an ion-pair whose separation to free ions is inhibited in low dielectric constant media. 52 With singlets of zinc, magnesium, and free-base porphyrins quenching by electron acceptors in non-polar solvents produces no transient detectable by conventional (microsecond) flash photolysis. Evidently, the ion-pair has a lifetime too short to permit its detection on this time scale. For quenching of triplet states of zinc and magnesium porphyrins, the situation appears more complicated both for the quenching process and with the behavior and type of transient formed.

As mentioned above, quenching of free base, zinc, and magnesium porphy- tins by electron acceptors occurs at near diffusion-controlled rates provided the "redox state" lies at or lower than the energy of the porphyrin triplet. From the behavior observed with several other types of systems, it would be anticipated that rates of quenching should fall off when the energy of the redox state exceeds the triplet energy of the substrate porphyrin and that the fall off should give a linear relationship between log kq and AE with a slope = - 1/2.3RT. Although a monotonic decrease in k~ with increasing AE is observed, the fall-off is not as rapid as expected; and in several cases a near diffusion controlled quenching rate is observed where AE is several kcalmol-L 49'5° In several cases, triplet quenching produces relatively long lived transients having spectra similar to but slightly different from the porphyrin triplet. 49'5° These transients appear to be themselves excited states since they can be quenched by substances such as azulene or tetracene having very low-lying triplets (in contrast the ions produced in polar solvents cannot be quenched). The most reasonable explanation for the above results is that quenching of the porphyrin triplets in non-polar solvents can occur by either complex formation or electron transfer. The product of the quenching can be properly called an "exciplex" in both cases but it is clear that formation of the exciplex in the cases where the "redox state" lies well above the triplet in energy must involve other than exclusively charge transfer interactions.49 Although we have observed such behavior with metalloporphyrins and a variety of acceptors, the results do not seem a property of porphyrins alone since quenching of the anthracene triplet by the same acceptors leads to similar results.~,, so

C. Photoreduction and oxidation of the porphyrin ring

Although, in many cases, the photoreduction and oxidation reactions of porphyrins and their metal complexes consist simply of reversible electron

124 D. G. Whitten

transfer reaction, there are several examples of reactions where the ring can be photooxidized or reduced. This section will deal with two specific examples: the photoreduction of tin (IV) porphyrins which appears to be a fairly general reaction for a number of different tin porphyrins and certain other metal complexes and the photooxidation of protoporphyrin free base which appears to be a fairly specific process but one of moderate biological significance.

The tin (IV) porphyrin photoreduction can be mediated by irradiating solutions of the metalloporphyrin in the presence of a variety of reducing agents including SnCIa.2H20, 54.55 N,N-dimethylaniline, 56 triethylamine 56 and other aliphatic amines) 7 The reaction produces in consecutive steps first chlorins (ring-reduced dihydroporphyrins) and subsequently the corresponding vic- tetrahydroporphyrin or/sobacteriochlorin, eq. (29). We have studied the reaction

H

H H

(29)

most thoroughly with SnCI2 as the reducing agent. B5 Under these conditions, we find that the porphyrin triplet is the reaction precursor since it is efficiently quenched by SnCla while the fluorescent singlet is unaffected under the reaction conditions. When octaalkyl porphyrins such as tin (IV) complexes ofetioporphy- rin I, mesoporphyrin IX or octaethylporphyrin are reacted, an intermediate in which the bridge positions have been reduced can be detected. With these compounds, reduction apparently proceeds via electron transfer quenching (whether one or two electrons are transferred in the initial step is uncIear) followed by proton transfer to yield a bridge-reduced species which subsequently rearranges to give the chlorin. The second reduction (evidently) proceeds by an analogous mechanism. The proton source in the SnC12.2H20/pyridine reactions is the water of hydration; deuterium incorporation, predominantly into the bridge positions, results when SnCI2.2D20 is used. 55

The reaction proceeds with very low quantum efficiency when the octaalkyl-


porphyrin complexes are used. ss When MS-tctraphenylporphyrin tin (IV) is the substrate the quantum yield is approximately I00 times greater, s8 Although tetraphenylporphyrin complexes are generally easier to reduce than the corresponding octaalkylporphyrins, g8 the greater case of reduction does not appear itself to simply account for the greater efficiency since under the reaction conditions used even the octaethylporphyrin tin (IV) triplet is more than 903 quenched by SnCl2. Preferential reactivity of the intermediates to chlorin products and/or greater stability of the intermediates very likely does play a role. Interestingly it is found that simple mctaIlation of MS-tctraphcnylporphydn by SnCI2.2H20 in dcgassed pyridine solution leads to quantitative formation of the chlorin in the dark. se Here, presumably as with octaalkylporphyrins, the first product produced by treatment of the free-base with SnCI2 is the porphyrin tin (II) complex which is a "redox isomer" of the porphyrin tin (IV) dianion. Although the porphyrin tin (II) complex can be isolated from octaethylporphydn in the absence of water, metallation of OEP with SnCI2.2H20 leads directly to the tin (IV) porphyrin and not to the chlorin suggesting a different partitioning of intermediates for the different porphyrins.

As mentioned above, we have observed similar reactivity for tetraphenylporphyrin with a variety of reducing agents including SnCI2, tricthylamine and N,N-dimethylaniline. Although we have not performed extensive experiments to determine intermediates involved in the reaction with amines, a similar mechanism involving the amine as a quencher of the mctalloporphyrin triplet by electron transfer appears reasonable. In related experiments, Harel and Manassen s7 have observed intermediates detectable by esr produced by photolysis of Sn (IV) tetraphenylporphyrin in benzene-N-methylpyrrolidine. They suggest the role of porphyrin free radicals in the reduction which are formed by decay ofa porphyrin- amine exciplex. Interestingly, they have found that intermediates occurring in the tin tetraphenylporphyrin photoreduction can be trapped chemically by added substrates such as nitrobenzene. 57

The photooxidation of protoporphyrin in the presence of molecular oxygen has been known for some time although details of the reaction and structure of the products have only recently been determined.Sa'sa Inhoffen and co-workers 60 were able to show that the major products formed in solution were the isomeric hydroxy aldehydes as shown in eq. (30). The mechanism for the reaction is likely as shown in eqs. (31)-(34) where PP = protoporphyrin IV dimethyl ester and PPO2 is the photooxidation product "photoprotoporphyrin" consisting of the two isomers shown in eq. (30). The initial adduct is believed to be a cyclic product of oxygen 1,4-addition to the diene unit of rings 1 or 2. 60 The reaction proceeds with relatively low efficiency (~ ranges from 0.006--0.03 depending upon solvent) ~a and evidently is retarded under conditions where more reactive acceptors of singlet oxygen are present, la,1~

Several features of the photooxidation of protoporphyrin are unique and interesting. Whereas most porphyrin photooxidations result in a net bleaching in which ultimately colorless products are formed, the protoporphyrin reaction proceeds in good yield to form the indicated products which are relatively stable

126 D. G. Whitten

CH=CH2 CHa

H 3 C ~ CH=CH2

H s C ~ - - C H a

CH2 CH2 I 1

CH2 CH2 I I

COOCHa COOCHa

CH=CH2 CHa ~ OH/cHO

HaC ~ ~ ~ ' - C H a

CH2 CH2 I I

CH2 CH2 I I

COOCH.~ COOCH3

CHa CHa I / ~ , ~ O H / C H O

( Z . )

2 CH3 CH i COOCHa U CH2

+

(3o)

pp '~ , pp1, , ppa, (31) ppa, + O2 , PP + O~* (32) PP + O12" , "Adduct" (33)

"Adduct" ~ ' PPO2 (34)

to further irradiation. Interestingly the oxidation essentially stops with incorporation of one molecule of 02 even though a similar reaction to yield a doubly oxidized product would seem reasonable. The failure of the photoproduct to react with oxygen photochemically could be due to an inertness of the photoproduct to reaction with singlet oxygen or to a failure of the photoproduct to sensitize singlet oxygen. While there is presently no evidence regarding the former possibility, it appears that the latter may play a role. Although the photoproduct is strongly fluorescent, we have been unable to detect a triplet for it by flash photolysis. 61 Since singlet oxygen is produced only via triplet semi- tizers, 62'6a the presence of a very short-lived triplet for photoprotoporphyrin could account at least in part for the product stability. The ir spectrum of the photoproduct indicates strong intramolecular hydrogen bonding in the hydroxyaldehyde 6° which could play a role in rapid deactivation of the triplet.

In our reinvestigation of the protoporphyfin photooxidation, we have


observed formation of a second product having a "porphyrin-like" electronic sp~trum but one whose absorption maxima are shifted ca. 22 nm to longer wavelengths. Although this product, which appears to be a formyl derivative formed via 1,2 photoaddition of O2, is a very minor product in solution photooxidation, it is produced in relatively higher proportions in the reaction of surfactant protoporphyrins in films, micelles, and monolayers which will be discussed in the next section. This product is not formed from the normal photoproduct in the reaction and we have not been able to effect its interconversion with the normal product (in either direct) by treatment with acid, alumina, silica gel or other potential catalysts.

IV. Photoreactions of lm~hyrins in organized media

,4. General behavior of porphyrins in monolayers and micelles

One of the chief goals of much of the research involving porphyrins, their metal complexes, and related molecules has been the construction of relatively simple model systems which simulate the behavior of the more complex biological systems in which porphyrins or related molecules such as the chlorophylls serve as prosthetic groups. The development of simple models has been hampered to a great extent by the strong dependency of the behavior of the porphyrins upon environment. For example, the reversible binding of molecular oxygen to ferrous porphyrins in the hemoproteins such as hemoglobin and myoglobin is not readily simulated by simple iron (II) porphyrins in solution. The recent preparation of successful models for hemoprotein behavior has only been accomplished by the construction of porphyrins containing special substituents such that (a) a hydrophobic "pocket" not containing a tightly-bound ligand is present and (b) bimolecular contact of oxygenated iron (II) porphyrins culminating in the formation of oxygen bridged (/~-oxo) dimers is inhibited, e4-66 For some time there has been evidence that in concentrated solutions as well as in solid or colloidal systems chlorophylls and related pigments can form aggregates having properties quite different from isolated monomers.67 Recently, some of the reaction centers from photosynthetic organisms have been investigated and it has been suggested that the active " t rap" probably involves a pair (or perhaps even a trimer) s6-7° of chlorophyll molecules which donate an electron to an acceptor molecule following excitation of the dimcric site by energy transfer from the light absorbing or "bulk" pigments. The luminescence of chlorophyll and related pigments is particularly solvent dependent; for example, in non-polar solvents almost no fluorescence is observed while a change to polar solvents or addition of coordinating molecules is accompanied by a dramatic increase in the fluorescence. 71 Although some information concerning the state of aggregation and the immedi- ate environment of the complex has been obtained in certain cases, it is often difficult to obtain definitive evidence as to the origin of the luminescence changes and the structure of the complexes in solution. Studies of chlorophyll and some metalloporphyrins in membranes and colloidal systems clearly suggest that aggregation plays a major role and that the structure and size of these aggregates

128 D. G. Whitten

are effected strongly by polar ligands coordinating with the metal. 4 Chlorophyll itself is well known to exhibit surfactant properties such that monolayer films are formed by spreading on a water surface. Langrnuir TM found that these films can be transferred to various rigid supports and that the usual solution fluorescence disappeared in films and assemblies. Subsequent work with chlorophyll 73-~s has shown that rapid degradation of the chlorophyll occurs in the films unless precautions are taken. 78 In the films formed from pure chlorophyll the molecule is folded such that both the bulk of the porphyrin ring and the phytol chain are in the "hydrophobic" region and "liquid-like" properties are reported. 7°-76 When the phytol chains of chlorophyll are replaced by methyl groups as in methyl chlorophyllide, monolayers are also formed but these are less stable and absorption spectra indicate more interaction between the porphyrin rings. 74'76 It has also been found that mixed monolayers and membranes containing chlorophyll can be formed. For example bacteriochlorophyU can diffuse into membranes with a dramatic shift in the fluorescence from the solution value (785-790 nm) to the in vivo maximum at 885 nm. 77 In contrast to the earlier reports by Langmuir, TM Krasnovsky TM reports that solid films of chlorophyll fluoresce with a A,.~x near the in vivo value.

Techniques for the preparation, manipulation and study of monolayer films and assemblies have been greatly developed and refined during recent years. 7~'al We have recently begun an investigation of porphyrin chemical and photochemical reactivity in films, micelles and monolayer assemblies using the techniques to isolate porphyrins and their metal complexes in a controlled environment. We have found these techniques particularly useful in the preparation of reactive intermediates by ligand photoejection such that these intermediates are "trapped" and can be diverted from their usual solution reactions. We have also examined interfacial reactions occurring between porphyrins immo- bilized in a micelle, film or assembly and reagents and substrates present in a solution or vapor phase.

In our investigations, we have prepared and utilized the surfactant and porphyrins shown below, 1-5. Some investigations have already been carried out with monolayer assemblies generated from these compounds. Results of these investigations give some idea of what sort of information can be obtained regarding environmental effects on luminescence and reactivity. For example, it has been found that both films and assemblies can be readily formed from the free base of 1. However, both the absorption and fluorescence spectra obtained for the assemblies of I are quite different from those obtained in dilute solutions of 1 or other free base porphyrins at room temperature. 82 The spectra are however very similar to those obtained for concentrated solutions at low temperatures (~ -80°C) where thermodynamic data suggest a dimerization is occurring. A study of the surface area/molecule gives a value of ,-, 56 A 2 for closely packed (23 dyne/era) spread films on water which is about ½ the anticipated area of 120 A 2 obtained from crystallographic data. Thus the results suggest that the films contain dimeric units where 2 porphyrin molecules are stacked face to face. 82 This evidently results from a packing phenomenon and


should probably occur with most metal complexes as well. Thus with complexes of 1, formation of monolayer assemblies allows construction of dimedc units having known and controlled geometry. Dimedc structures are also formed when 1 is incorporated into micelles with octyl tdmethylammonium bromide (CTAB) as a host. The absorption spectrum in the CTAB micelles is nearly identical to that obtained in the assemblies. A dimeric structure, based on surface pressure- area measurements and absorption spectra, is also indicated for the free bases of 2and 3.

Surface-pressure area isotherms have also been measured for some of the metal complexes. Although by no means all of the possible luminescent complexes of I have been prepared or studied, it appears that packing into dimeric units is a general phenomenon more or less independent of the central metal ion.

CH2CHa CHa CH2CHa CHa

CH2 CH2 CH2 CH2 I [ [ I

i ll2 CH2 CH2 CH2 I I I

COO--C18Ha7 COODHC C02OHC COOC~Ha~

1 2 COOC18Ha7

Ha7C~802C COOC~aHa7

COOClaHa7 3

130 D. G. Whitten

(~xsHsl CO

Cx~Hax [ ( - ~ [ I Cx,Hal I t , ,~ '~ - -NH I

CO " Y CO I I

~ NH--CO--ClsHal

t # a , a ) a , a m = I s o m e r

4

C02--DHC CO2~DHC

C02DHC


Preliminary experiments suggest that 2 and 3 and their metal complexes also exist in dimeric units although more study will be necessary to determine the precise spatial relationship between adjacent chromophores. Apparently in 1-3 the area of the hydrophilic porphyrin chromophore is too large to be effectively balanced by the hydrophobic hydrocarbon side chains. The stacking of two porphyrin rings face-to-face, perhaps in a staggered arrangement, results in a better balanced and more stable assembly.

That such geometry can control reactivity is indicated by a recent study of the iron (III) complexes of I and 3. 88 Both iron complexes are clearly monomeric in solution and show "normal" solution spectra. However, when monolayer assemblies are constructed, the assemblies show spectra nearly identical to the "p-oxo-dimer," PFe-O-FeP, ofthe parent porphyrins. Extraction of the porphy- tin from the assemblies confirms that reaction of the monomer to give the p-oxo-dimer has occurred. A careful study of the process indicates that the dimer is formed rapidly in the spread films on water, sa Evidently both the rate of reaction is accelerated and the equilibrium is dramatically shifted to favor the dimer in the film.

In an effort to avoid dimerization, the free base and iron complexes of 4 have recently been prepared, s' With the free base of 4 excellent films and monolayer assemblies can be obtained for mixtures with arachidic acid. Here the absorption spectrum is nearly identical to that obtained for dilute solutions and most evidence indicates assemblies in which the porphyrin is monomerically dispersed are obtained. 8' The iron (III) complex of 4 also exhibits spectra in assemblies that are nearly identical to those obtained in dilute solution. More importantly, the iron (III) complex of 4 does not form p-oxo-dimer either in thin films or in the assemblies. The fact that 4 is monomerically dispersed in the assemblies has enabled us to prepare the corresponding iron (I]) complexes which, in the assemblies, exhibit properties which may make them reasonable models for hemoprotein systems, s4

B. Ligand photoejection and exchange in ruthenium and iron complexes in monolayer assemblies

The photoejection of ligands such as carbon monoxide has been investigated for a large number of metal complexes, including iron porphyrins. In many cases, ligand photoejection generates a very reactive species which may react further with a variety of reagents, especially potential new ligands or electron donors) In solution studies, we found that the highly thermally stable CO complexes of ruthenium (II) porphyrins undergo photolysis in the presence of reagents such as pyridine, aliphatic amines, ethers or water to yield the corresponding complex in which the oxygen or nitrogen ligand has replaced CO. 9 For several of the oxygen ligands such as tetrahydrofuran, the complexes are unstable and in the dark in the presence of even small amounts of CO they revert to the starting CO complex. Although the reaction proceeds with very low overall quantum efficiency, primary ligand ejection appears to be a moderately efficient process since an intermediate subsequent to the phosphorescent state can be readily detected by flash photolysis.

132 D. G. Whitten

Attempts to isolate the CO-free ruthenium porphyrin in the absence of potential ligands have been only marginally successful since even under reduced pressure recapture of CO or scavenging of traces of potential ligands occurs, g,85

The possibility of isolating porphyrins and their metal complexes in monolayer films or assemblies suggested that ligand photoejection processes might proceed under these conditions to afford different results and products not obtainable in solution. Thus in the absence of a solvent, cage recombination processes should be minimized and the retardation of diffusion might together be expected to facilitate isolation of the reactive intermediates not isolable in solution. Toward this goal, we prepared surfactant derivatives of Ru (II) CO mesoporphyrin IX as the bis(dihydrocholesteryl) and bis(octadecyl) esters. These complexes afforded good monolayer assemblies when spread as mixtures with dodecanoic (arachidic) acid. Assemblies of the ruthenium porphyrin CO complex gave spectra nearly identical to those obtained in solution, aa On irradiation of these assemblies in vacuum with visible light, there is a spectral change similar to that occurring transiently in solution under flash photolysis; the product formed is stable indefinitely under vacuum but reacts instantane- ously with CO to regenerate the initial spectrum. Admission of nitrogen to the species generated from photolysis of the CO complex in the assemblies produces still a further change in the visible spectrum. The product from nitrogen addition can itself be photolyzed in vacuum to regenerate the species formed by photolysis of the CO complex. A product having the same spectrum as that produced by photolysis of the CO complex in vacuum and subsequent addition of nitrogen can be produced by photolysis of the CO complex in a nitrogen stream, as The results are most consistent with photolysis of the CO complex to yield an isolable ruthenium porphyrin in the assemblies in which a reactive site at the metal is generated. The overall chemistry is summarized in the Scheme below. Similar

hv N2 Stream 1

PRuCO h, , ~' -~ PRuN2 v g g u u m

, co PRu ~, [ ( VagUUlll ] hv

CO atmosphere

reactivity has been observed when oxygen replaces nitrogen as the substituting agent, ae The PRu also reacts rapidly with ligands such as pyridine in the assemblies to generate a product having a spectrum nearly identical to that of the pyridinate complex generated by photolysis of the CO complex in solution. 9 The PRuN2 complex prepared in the assemblies also reacts rapidly with pyridine vapour or pyridine from an aqueous pyridine solution contacted with the assemblies to yield what is evidently the pyridine complex.

The most interesting aspect of these experiments is the isolation and selective reaction of the reactive PRu species which is afforded by the monolayer technique in this case. Thus what is only an intermediate in solution can be isolated in the


assemblies and made to react selectively with nitrogen in oxygen in reactions we have not yet observed in solution. An obvious choice for extending the study is an investigation of the corresponding iron (II) porphyrins. As mentioned above, the strong tendency to dimerize has been a perplexing problem in the case of the iron porphydns since the iron (III) porphyrins rapidly and irreversibly form reoxodlmers in the assemblies. 8a Fortunately, we find that assemblies can be formed from the iron (III)"picket fence" porphyrin tetrahexadecanoic acid amide 4 in which the porphyrin is monomerically dispersed and ~oxodimers are not formed. Preliminary investigations show that in the assemblies the iron (III) bromide is readily reduced by pipiridine vapor to yield a relatively stable iron (II) complex. Our experiments in this area are still incomplete but preliminary evidence suggests that labile CO and O2 adducts of the iron (II) complex are readily and reversibly generated by exposure of the complex to moderate pressures of these gases, a~

C Photooxidation ofsurfactant protoporphyrins in monolayer films, organized monolayer assemblies and micelles

As outlined in Section III-C, the oxidation of protoporphyrin IX free base is a prominent and relatively unusual photooxidation reaction whose mechanism involves several steps and at least two electronically excited species, the porphyrin triplet and ringlet excited oxygen. We felt that this reaction might be especially attractive to study in membrane-like organized media for a number of reasons. For example, it might be anticipated that restriction of diffusion or molecular motion might either inhibit the reaction entirely or lead to different products. Secondly, if the reaction were to occur, the rate and efficiency might be quite medium-sensitive and it should be especially interesting to determine the role and effectiveness of potential quenchers and scavengers in the different environments.

The surfactant photoporphyrin IX used in this study was the bis(dihydrocholesteryl) ester, Proto IX(DHC)=. Both the Proto IX(DHC)= and the corresponding dimethylester, Proto IX DME, were found to undergo the "normal" photooxidation in air-saturated methylene chloride solution to yield predominantly the green hydroxyaldehyde6°; a small amount of the " red" product in which a formyl group has replaced the vinyl group was also obtained but it was estimated to be no more than a few percent (~ 1.5) of the total obtained. The Proto IX(DHC)= was found to form stable films and monolayer assemblies in mixtures with dodecanoic acid; however, the spectra obtained indicated that the porphyrin was dimeric in the assemblies.a2 Irradiation of both the assemblies and films led to the formation of green photooxidation products. The product formed in the assemblies was separated by medium-pressure liquid chromo- tography; although the product consisted chiefly of the two products formed in solution photolysis there was substantially more (ca. 40°7o of the total) of the " red" product. The major product was the mixture of hydroxyaldehyde isomers. 61 Photolysis of Proto IX(DHC)2 in monolayer assemblies was carried out under a variety of conditions. Rapid formation of the photooxidation product occurred for a single outer layer ofporphyrin or for assemblies containing

134 D. (7. Whitten

the porphyrin "submerged" under several layers of dodecanoic acid. Photo- oxidation of assemblies immersed in water proceeds at comparable rates to those obtained for the same assemblies surrounded by air. That the reaction does require oxygen was verified by the finding that assemblies irradiated under vacuum are photostable, sl The finding that what are evidently formyl products are formed in enhanced proportions in the monolayer films and assemblies is especially striking and illustrative of the influence the monolayer environment can produce. The formyl product likely arises from 1,2-addition of singlet oxygen as shown in eqs. (35) and (36). 1,4-Addition to the diene unit to form the

0~0

Os)

O~O

+ CH2=O (36)

hydroxyaldehyde is reasonably the preferred path in unhindered non-viscous solutions. However, in the monolaycr films and assemblies, it is likely that the tight packing into dimeric units inhibits the 1,4-addition while still allowing the less favorable 1,2-addition to occur and thus giving enhancement of the formyl/hydroxyaldehyde product ratio.

Both protoporphyrin esters were investigated in micellar solution. It was found that both Proto IX DME and Proto IX(DHC)2 can bc incorporated into the micelles formed from cctyltrimethylammonium bromide (CTAB). Both porphyrins exhibited spectra very much like those obtained for the monolaycr assemblies suggesting that the porphyrins are also dimeric in the micellcs. Irradiation of CTAB-Proto 1X(DME) micellcs in water leads to slow oxidation of the porphyrin to form what appears spectrally to be predominantly the hydroxyaldehyde mixture. In contrast, irradiation of the CTAB-Proto IX(DHC)2 solutions under similar conditions led to even slower product formation. The pronounced retardation of the reaction in micelles is probably due to unfavorable partitioning of oxygen between the micelles and bulk solvent. The micelles are small and probably contain much less than one oxygen molccule/micelle. Oxygen can move freely across the micclle-solution interface as evidenced by the lack of observable triplet-triplet absorption from the porphyrins in undegasscd micelles following flash excitation. However, excited oxygen likely escapes from the miccllcs rapidly and has little probability of re-entering a micelle before deactivation. The differences in reactivity between the Proto IX DME and Proto


IX(DHC)= could be due to different positioning of the two porphyrins in the micelles. It would appear reasonable that the surfactant Proto IX(DHC)= might have the porphyrin more anchored near the edge of the micelle while the nonsurfactant Proto IX DME might be more "dissolved" in the hydrophobic region.

The effect of quenchers on the photolysis in monolayer assemblies has been investigated using two different types of quenchers. 1-(p-Methoxyphenyl)-6- phenyl-l,3,5-hexatriene has been used as a potential quencher of the porphyrin triplet while cholesterol has been used as a potential singlet oxygen scavenger. The triene quenches protoporphyrin triplets in solution with a rate constant, k~ = 5 × 10 81 tool -z s -1 as measured by flash photolysis for both esters. Cholesterol is a reasonable singlet oxygen scavenger as indicated by its role as the primary acceptor in the disorder porphyria. 18':* Interestingly, we find that the triene is ineffective as a quencher even in very high concentrations. For example, an assembly prepared with Proto IX (DHC)=, triene and dodecanoic acid in a 1 : 2: 3 ratio undergoes photooxidation at the same rate (in air) as those of assemblies with no tdene, el In contrast, assemblies prepared from Proto IX(DHC)2 cholesterol and dodecanoic acid in a 1: 2:3 ratio react at about half the rate of assemblies without cholesterol. Cholesterol is also affective at retarding the rate of photooxidation of Proto IX D ME in solution and Proto IX (DHC)= in spread films on water surface. A reasonable explanation of the "nonquenching" effect of the triene could be that even though energy transfer quenching of the porphy- fin triplet occurs in the assemblies, the triene triplet may have a sufficient lifetime and energy to sensitize singlet oxygen. In contrast, cholesterol scavenging of singlet oxygen should definitely retard photooxidation of the porphyrin. In an effort to investigate the effective "range" of singlet oxygen in the assemblies, we have varied the distance between porphyrin and cholesterol by constructing assemblies of different configuration. As mentioned above, with the assemblies containing porphyrin and cholesterol in the same layer, both in high "concentra= tion" there is ca. 50% quenching of the photooxidation. When assemblies are constructed with comparable "concentrations" of cholesterol and Proto IX(DHC)= in adjacent layers in hydrophilic contact there is a reduction of the rate of photooxidation of ca. 35%. However, when two or more layers of dodecanoic acid separate the layers containing porphyrin from those containing cholesterol, there is no measurable retardation of the photooxidation. We are currently investigating the use of other quenchers including species with triplets lower than singlet oxygen, quenchers such as nitrogen, which do not quench by energy transfer, and more powerful scavengers of zO=.

Acknowledgment

Much of the research described in this review is the work of students and postdoctoral associates who have worked in these labs during the last eleven years. Especially important contributions are due to the following: J. C. Yau,

136 D. G. Whitten

I. G. Lopp, P. D. Wildes, F. R. Hopf , F. A. Carroll , J. Mercer-Smith, R. H. Schmehl and B. E. Horsey. We are grateful to the Nat iona l Institutes o f Heal th (Gran t G M 15,238) and the U.S. A r m y Research Office (Gran t D A A G 2 9 - 7 6 - G - 0079) for financial support o f various por t ions o f the w o r k described.

V. References

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Wagner and I. Kochevar, Jr. Amer. Chem. See., 90, 2232 (1968). (37) J. B. Callis, J. M. Knowles, and M. Gouterman, Jr. Phys. Chem., 77, 154 (1973). (38) J. H. Fuhrhop, in ref. 2, p. 593. (39) G. L. Closs and L. E. Closs, Jr. Amer. Chem. See., 85, 818 (1963).


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Ware, Eds., Academic Press, New York, 1975, p. 247. (51) R. C. Young, unpublished results. (52) J. K. Roy and D. G. Whitten, J. Amer. Chem. Soe., 94, 7162 (1972). (53) R. C. Young, J. Nagle, T. J. Meyer, and D. G. Whitten, submitted for publication. (54) D. G. Whitten and J. C. N. Yau, Tetrahedron Lett., 3077 (1969). (55) D. G. Whitten, J. C. Yau, and F. A. Carroll, J. Amer. Chem. Soc., 93, 2291 (1971). (56) S. Peskin, unpublished results. (57) Y. Harel and J. Manassen, unpublished manuscript. We thank Prof. Manasseh for a

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p. 43. For reviews of chlorophyll and photosynthesis see: "The Chlorophylls," L. P. Vernon and G. R. Seely, Eds., Academic Press, New York, 1966; E. Rabinovitch and Govindjee, "Photosynthesis," John Wiley and Sons, Inc., New York, 1969; R. K. Clayton, "Molecular Physics of Photosynthesis," Blaisdell, New York, 1965.

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Part 3b, A. Weissberger and B. Rossiter, Eds., John Wiley and Sons, Inc., New York, N.Y., 1972, p. 577.

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138 D. O. Whitten

(81) H. Kuhn, Pure dppL Chem., 27, 421 (1971). (82) K. A. Zachariasse and D. G. Whitten, Chem. Phys. Lett., 22, 527 (1973). (83) F. R. Hopf, D. Mobius, and D. G. Whitten, J. Amer. Chem. Soc., 98, 1584 (1976). (84) R. H. Schmehl and F. R. Hopf, unpublished results. (8:5) F. R. Hopf, unpublished results. (86) F. R. Hopf and D. G. Whitten, J. Amer. Chem. Soc., 98, 7422 (1976).

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