I. Photochem. Photobiol. A: Chem., 65 (1992) 2940 29

Dynamics of cis-stilbene photoisomerization: the adiabatic pathway to excited trans-stilbene

J. Saltiel, A. S. Waller and D. F. Sears, Jr. Depariment of Chemise, The Florida State University, Tallahassee, FL 32306-3006 (USA)

Abstract

Fluorescence excitation spectra of cis-stilbene in n-hexane monitored at 350 nm and 404 nm, are resolved into &-stilbene and trans-stilbene contributions using principal component analysis with self-modeling. The results confirm that both trans and cis fluorescence originating from excitation of pure ckstilbene solutions are due to single photon excitation of ‘c to ‘c*. The quantum yield for adiabatic ‘c* * It* conversion is &* -0.0020, independent of excitation wavelength. The corrected fluorescence excitation spectrum of cis-stilbene faithfully tracks its UV absorption spectrum. The fluorescence spectrum of c&-stilbene is nearly temperature independent in n-hexane, O-58 “C. Assuming a lc* ---, ‘p*+ It* pathway for adiabatic ‘t* formation, where ‘p* is the perpendicular phantom singlet state, leads to the conclusion that ‘p* and It* are nearly isoenergetic.

1. Introduction

Adiabatic cis-trans photoisomerization was first envisioned by Olson [l] who formulated the reaction as

molecule + h Y - molecule’ + h Y’ (1)

Olson proposed that the barrier to rotation is greatly diminished in the excited state and was the first to discuss the reaction in terms of potential energy curves [l-3]. The quantum mechanical basis for expecting a perpendicular geometry for the olefin’s vibrationally relaxed lowest excited singlet state is due to Mulliken’s expansion [4] of Hiickel’s description of the double bond [5]. Mulliken’s potential energy curves for rotation about the CC double bond of the lowest state of ethylene served as early valuable guides to discussions of cis-trans photoisomerization (for reviews see [6--&X]).

The questions posed by Olson and by Mulliken were addressed in the pioneering study of Lewis, Magel and Lipkin [9]. The problems of photoisomerism and rotamerism that have occupied photochemists up to the present time were clearly outlined in the first paragraph of that paper. Regarding isomerism it was stated that “If a substance under ordinary conditions exists in two stereoisomeric forms it is possl%le, when light is absorbed, that they both produce the same electronically excited molecule” and “in the state of electronic excitation, the two isomers may lose their individual existence and become merely phases of rotation or vibration belonging to a single state”. A less explicit forerunner of Havinga’s NEER (non-equill%ration of excited rotamers) principle [IO] was the statement “it is conceivable that even when no isomers exist in the normal state, they may be found in a state of electronic excitation.”

lOlO-6030/92/$5.00 0 1992 - Elsevier Sequoia. All rights reserved

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The experimental task undertaken by Lewis et al., in their study of cis-trans photoisomerization of the stilbenes was to attempt to resolve the questions: (i) “If the electronically excited state is essentially the same for the cis and trans forms, why are there no fluorescence bands corresponding to transitions to the cis form?“; (ii) “Would the same fluorescence as is found for h-ans-stilbene be observed when cir- stilbene is illuminated?” In their work they failed to detect cis-stilbene fluorescence under various conditions and concluded “that if cis-stilbene has fluorescence, it must be less than 1% of that from trans-stilbene under simiIar conditions.”

Structureless fluorescence, easily distinguishable from trans-stilbene fluorescence, was first observed from cis-stilbene in high viscosity media [11-E]. Since under these conditions the medium largely inhibits photoisomerization, these observations fall short of addressing the questions raised by Lewis et al. An early report of cis-stilbene fluorescence in methanol, resembling the fluorescence of trans-stilbene [16] was, no doubt correctly, attributed to buns-stilbene contamination [9]. Until our recent report [17], all other attempts to record the fluorescence spectrum of cis-stilbene in fluid solution had failed.

The currently accepted potential energy curve for twisting about the central bond of the lowest singlet excited state of stilbene was first proposed by Saltiel [IS] based on empirical considerations. It involves a common perpendicular intermediate ‘p*, the phantom singlet state, that is attained from the trans side, It*, by crossing a small torsional barrier (approximately 3 kcal mol-I) and by nearly barrierless torsional displacement from the cis side, k* Numerous experimenta and theoretical studies . in accord with this proposal have been reviewed recently [8]. Since quantum mechanical calculations are not trustworthy in this respect, the depth of the energy well at rp* is unknown and has been open to speculation. Our laboratory’s interest in returning to the questions raised by Lewis et al. concerning the fluorescence of cis-stilbene dates back to some erroneous fluorescence lifetimes that led Birch and Birks to postulate that twisting from It* to ‘p* is reversible [19]. Birks further proposed that in flexible molecules such as stilbene one may distinguish between vertical transitions to lower electronic states and “horizontal transitions to states of the same multiplicity and similar energy, but differing in configuration and symmetry” [20]. In a careful kinetics analysis of the fluorescence lifetimes and quantum yields of Birch and Birks the inconsistencies between their observations, the observations of others and their inter- pretation were explained [21]. Charlton and Saltiel concluded that to the extent that the%*+l * p reversibility exists ‘Yuorescence should be observable following excitation of cis-stilbene, provided that ‘c* + ‘p* is important as is generally thought” and that a planned study of cLr-stilbene fluorescence quantum yields (footnote 24 in ref. 21) might provide an accurate direct experimental measure of trans-stilbene fluorescence arising from the ‘p* + ‘t * process [213. However, the failure of numerous subsequent studies [7, 8, 22-301 to reveal the long-lived fluorescence component in h-uns-stilbene fluorescence that was claimed by Birch and Birks dampened our interest in pursuing the fluorescence of cis-stilbene in solution. Furthermore, our early results suggested that it would be very difficult to obtain sufficiently pure cis-stilbene to allow detection of its Auorescence in the absence of interference by trans-stilbene fluorescence.

We returned to the problem in earnest following an intriguing report by Petek et al. of Auorescence with an unexpectedly long lifetime, 20 ns, from supersonic beams of cis-stilbene vapor seeded in inert gas expansions [31]. A broad structureless emission similar to that observed in rigid media was attributed to vibrationally relaxed ‘c* trapped in an inherent minimum on the S1 potential energy surface [31]. A rough estimation of the radiative constant of cis-stilbene, k,= 1.6 X ~O’S-~ in glassy hydrocarbon

31

media at 77 K was available from its lifetime [32] and its fluorescence quantum yield under these conditions [13, 141. Together with known lifetimes of 0.9-1.35 ps and 1.0 ps obtained by monitoring the decays of ‘c* absorption [27, 33-341 and ‘c* fluorescence

[351, respectively, in n-hexane solution at room temperature, &= 10m4 could be estimated. This value was well within the range of detection of our spectrophotometer since we had shown earlier that we could easily detect the prompt fluorescence of benzophenone whose quantum yield is about an order of magnitude smaller [36]. We were also encouraged because in our determination of the enthalpy difference between the two stilbene isomers in So, we had succeeded in obtaining samples of cis-stilbene that were free of both bibenzyl and trans-stilbene contaminants [37]. In addition we were confident that even if entirely trans free samples of cis-stilbene could not be obtained, we could apply principal component analysis with self-modeling (PCA-SM), a powerful method of curve resolution that we had successfuily employed on other problems, e.g. ref. 36, to determine the cis-stilbene fluorescence spectrum.

As described in a recent preliminary report, our expectations were rewarded [38]. Fluorescence measurements were carried out at 30.0 “C in Fisher HPLC grade n- hexane. Spectra were measured with a modified Perkin-Elmer MPF-2A spectropho- tometer with operation and data acquisition controlled by a Dell Corporation 80286/ 87 (12 MHz) microcomputer. A two component fluorescence input matrix was obtained consisting of the emission spectrum of our purest cti-stilbene (2.79X lo-’ M) solution (99.979% c, 0.021% t, by GLC) and a set of spectra containing known small amounts of trans-stilbene (up to 1.16 x lo- 7 M) added to the initial cis-stilbene solution. Photoisomerization (cis + trans) during the course of spectral acquisition was minimized by circulating 500 mL solutions from a reservoir through the 4 mL cell compartment.

350 nm

280 360 440 520 600

IL - (nm)

Fig. 1. Absorption and emission (corrected) spectra of tmns- and cir-stilbene in n-hexane at 30 “C.

32

9.0

I I I I I I 1 I I I

-0.1 0.30 0.70 1.10 1.50 [transj x IO’, M

Fig. 2. Relative truns-stilbene fluorescence area as a function of added trans-stilbene concentration; the initial solution was 2.79~ lo-’ M cis-stilbene and 5.9X lo-’ M trans-stilbene.

Even with this precaution, it was necessary to correct the spectra for small systematic photochemically induced increases in iruns-stilbene fluorescence. PCA-SM applied to a matrix of 76 fluorescence spectra obtained by excitation at 268.0, 269.6 and 272.0 nm yielded the pure component ‘c* fluorescence spectrum shown in Fig. 1 [38], in good agreement with ‘c* emission spectra obtained in rigid media at low temperature [II-H]. The quantum yield of this emission, (8.9 Ifr 0.7) X 10e5, is approximately 0.2% of the quantum yield for trans-stilbene fluorescence. Especially gratifying was the observation that fluorescence from pure cis-stilbene consisted of nearly equal contri- butions of ‘c* and ‘t* fluorescence. This can be seen in Fig. 2, where the relative area of trans-stilbene fluorescence is plotted ws. the concentration of trans-stilbene in the cis-stilbene solutions. The intercept at the abscissa of Fig. 2, 3.67~ lo-’ M, exceeds the known level of trans impurity in cis-stilbene by more than a factor of 6. The linear dependence of trans fluorescence intensity on excitation intensity and preliminary results from fluorescence excitation spectra led to the conclusion that tmns-stilbene fluorescence from pure cis-stilbene solutions represented the adiabatic process of Olson’s eqn. (l), albeit with much lower efficiency than he envisioned. Our best current estimate for the ‘c*+-It* quantum yield is & * =0.0020. If in fact, the ‘p* state corresponds to a planar 4 state as proposed by Orlandi and Siebrand [39], the adiabatic conversion of ‘c* + It* on the lowest excited singlet state surface of stilbene would represent two successive horizontal transitions of the type defined by Birks [20]. Alternatively, the rotation may occur entirely within the ‘B, state 1401.

2. Results

We now present a preliminary report of the resolution of fluorescence excitation spectra from solutions of cis-stilbene without and with added small known quantities

33

of trans-stilbene (as in the above fluorescence study) into pure cis and trans fluorescence excitation spectra. Two fluorescence excitation spectral input matrices were subjected to PCA-SM, each utilizing the fluorescence wavelength corresponding to the maximum intensity of the trans (350 nm) and cis (404 nm) fluorescence spectra respectively (Fig. 1). The details of this study will be published elsewhere. We give here a summary of our results. Both sets of fluorescence excitation spectra from our purest cis-stilbene solution were shown to be composed primarily (about 80%) of cis-stilbene excitation, consistent with the known 0.021% trans impurity. Figure 3 shows a plot of the relative area of the trans-stilbene fluorescence excitation spectrum as a function of added trans- stilbene concentration. The intercept at the abscissa of this plot corresponds to 4.9 X IO-’ M bans-stilbene, or 0.020% of the total stilbene concentration, in excellent agreement with the trans contamination level determined independently by gas-liquid chroma- tography (GLC) analysis. This confirms that both the trans and cis fluorescence originating from excitation of pure cis-stilbene solutions originate from single photon excitation of ‘c to k*. As expected, excitation at the red edge of cis-stilbene absorption (338 nm), where trans-stilbene is essentially transparent, does not alter the It* fluorescence contribution in the emission spectrum of cti-stilbene.

The analysis of the fluorescence excitation spectra provides an independent value for the quantum yield of ‘t * formed adiabatically from k* giving & = 0.0020, independent of excitation wavelength. Furthermore, the quantum yield of the ‘c* + ‘c portion of the fluorescence is also independent of excitation wavelength. The same Auorescence excitation spectrum for ck-stilbene is obtained, within experimental uncertainty, from both the 350 nm and 404 nm spectral input matrices. Figure 4 shows that the fluorescence excitation spectrum from the 350 nm matrix, corrected for self-absorption and non-

1.5 I

0.0 0.50 1.00

[trans] x 10’. M

Fig. 3. Relative tins-stilbene fluorescence excitation area (with 350 nm and 404 nm monitored wavelengths) as a function of added Zrans-stilhene concentration; the initial solution was 2.47X lo-’ M cir-stilbene and 5.0 X lo-’ M tins-stilbene.

0.4 -

h .Z B 0.3 - 3 d

zf ‘5 2 0.2 - ti

0.1 -

oo* . 250 280 310 340

h - (nm)

Fig. 4. The corrected fluorescence excitation spectrum of cFF-stilbene (-) superposed on its absorption spectrum (-o-).

linearity in instrumental response, faithfully tracks the UV absorption spectrum of c&- stilbene.

The temperature dependence of the emission spectrum of pure cis-stilbene in n- hexane has been studied in the 0 to 58 “C range. The spectrum is nearly temperature independent so that to a first approximation & of ‘c* is constant, while &* exhibits the same temperature effect as does (I-&) of It*.

3. Discussion

The lowest electronically excited singlet state of &s-stilbene undergoes cyclization to 4a,4b_dihydrophenanthrene (DHP) in addition to double bond isomerization.

In degassed solutions DHP reverts to cis-stilbene both photochemically and thermally, while in the presence of air or other oxidation agents it gives phenanthrene (for a review see ref. 41). The formation of DHP as an unstable yellow product [9] was first reported by Lewis et OZ., although its structure [42] and spectrum 1433 were assigned much later by Moore et al. and Muszkat and Fischer respectively. In fluid solutions quantum yields for c+ DHP and c 4 t of 0.10 [43, 441 and 0.35 [4345] respectively have been reported [43, 441 in good agreement with initial estimates by Lewis et al.

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[9]. The reverse process DHP +c has been reported to have a quantum yield of 0.70 even at very low temperatures where medium rigidity significantly attenuates the photochemical reactions of cis-stilbene [44].

Following Saltiel’s proposal [7], the cis-trans photoisomerization of the stilbenes has been discussed with minor modifications in terms of the potential energy curves for twisting about the central bond shown in Fig. 5. The ‘c* reaction channel for DHP formation proceeds along a different vibrational mode probably involving torsional motions about the vinyl-phenyl bonds. DHP formation has been discussed as an allowed electrocyclic adiabatic process, ‘c* ---, ‘DHP*, by Mallory and Mallory [46]. The possibility that ‘DHP* forms reversibly and may serve as a reservoir for ‘c* has also been considered [45]. At ambient temperatures the photoreaction quantum yields, provided in major part by Fischer and co-workers, are consistent to a good approximation with the fractional distribution of decay paths shown in Scheme 1 (see also Fig. 5). The postulations of vibrationally relaxed electronically excited common intermediates, ‘p* for ‘t and ‘c, and ‘DHP* for ‘c and ‘DHP, are in agreement with predicted sums of unity for the decay fractions of each intermediate. It appears that ‘p* and ‘DHP* once formed retain no memory of their source.

100

75

E (kcalhol)

50

25

-‘C

0 0 7v2 n

8

Fig. 5. Potential energy curves for twisting about the central bond of stilbene in So and S1.

;;;;X;[;;ir+g_ *

A DHP

Scheme 1. The fate of excited states in stilbene photochemistry in fluid hydrocarbon solution.

36

The mechanism in Scheme 1 readily accounts for our observations and is also consistent with a recently published study by Fleming and co-workers on the decay kinetics of ‘c* fluorescence in several solvents including n-hexane (T= 1.03f0.15 ps, 293 K) [47]. The excitation wavelength independence of the fluorescence spectrum and the fluorescence quantum yield of ‘c* suggest rapid (relative to 1 ps) essentially vertical relaxation of the initially formed vibrationally hot ‘c* followed by fluorescence from vibrationally relaxed ‘c* in competition with very fast (10” s-l) “horizontal” radiationless decays to ‘p* and ‘DHP *. Since product quantum yields (It, ‘DHP) are also temperature independent in the temperature range employed in our fluorescence studies [44], the 70~30% partitioning shown in Scheme 1 is insensitive to temperature changes, suggesting that torsional relaxation towards ‘p* dominates the decay of ‘c*. The possibility that torsional motions leading to ‘c*

(mainly along 6) and to ‘DI-IP* (mainly among & and A) are coupled has long been considered [6] and has been advocated most recently to account for vapor phase spectroscopic observations on cis-stilbene and relatively rigid cyclic analogues [48].

As pointed out in the results section, our observations leave no doubt concerning the origin of t ’ * fluorescence by an adiabatic pathway from ‘c*. Since the quantum yield of this process, &* = 0.0020, is independent of the excitation wavelength and thus the excess vibrational energy of nascent ‘c*, it must also occur following vibrational relaxation of this state. Clearly, if torsional displacements along the 8 and/or 4 coordinates leading to either ‘p* or ‘DHP* were more favorable from the initially formed vibrationally hot ‘c* molecules, neither &* nor the +r of ‘c* would be independent of excitation wavelength. It seems reasonable to assume that the pathway from relaxed ‘c* to It* follows the potential energy curve in Fig. 1, thus passing through the relaxed phantom singlet intermediate, ‘p*. Since &,* =0.70 and +ct* = &p*&t*. it follows that r#+* = 0.0030. By analogy with the known temperature independence of the decay rate constant of the phantom triplet state, 3p* [49], we assume that the lifetime of ‘p* is similarly temperature independent. According to Scheme 1, the fluorescence quantum yield of It* formed adiabatically from ‘c*, c& is given by:

ht.= &D*+,C*&t (2)

where &, is the fluorescence quantum yield of t r * formed directly by excitation of ground state pans-stilbene. Since &, is strongly temperature dependent due to the small (approximately 3.5 kcal mol - ’ in n-hexane) torsional energy barrier along the It* + ‘p* path, our observation that +rta is, in contrast, essentially temperature in- dependent requires that the decrease in & with increasing temperature be, nearly exactly, compensated for by the increase in c&*. If our assumptions are valid, It* and lp* must represent nearly isoenergetic regions on the S1 surface. The failure, therefore, to observe doubly exponential fluorescence decay following excitation of trans-stilbene, as had been erroneously proposed by Birch and Birks [19], can be attributed to the short lifetime of ‘p*.

37

Direct information concerning the involvement and lifetime of ‘p* has been sought in recent femtosecond-pulse laser studies. In a very elegant study, Hochstrasser and co-workers took advantage of the short It* lifetime of fruns-l,l’-biindanylidene, a stiff stilbene analogue [S, SO], to demonstrate the presence of a bottleneck on the path from *t* to ‘c [51). The kinetics analysis of transient absorption in n-hexane solvents was consistent with a medium independent lifetime for ‘p*, 7,, = 10 f 3 ps [51]. A 3 f 2 ps bottleneck on the way from ‘c* to ground state stilbenes has been assigned to 7p in stilbene based on the time evolution of transient absorption of cis-stilbene probed and pumped at 312.5 nm [52]. However, in an extension of this study new observations were interpreted to show that any intermediate in the ‘c* *product (It) path must have a lifetime of less than 150 fs [53]. It was further concluded that hot pans-stilbene molecules are produced in the isomerization [53]. Thus, transient absorptions in the 310-350 nm regions observed to decay to products with lifetimes of approximately 15 ps following femtosecond-pulse excitation of ‘c in n-hexane, as well as a bottleneck (7~ 14 ps) in the appearance of It in n-hexadecane monitored by fluorescence intensity induced by a second pulse, were attributed to rate determining transfer of internal energy from hot ground state products to the solvent [54]. This conclusion was based on the premise that the relaxation of hot stilbene products must be as slow as in the rigid aromatic molecules anthracene 1551 and azulene [56]. On the other hand, the biexponential ‘t * fluorescence rise times observed following femtosecond-pulse excitation of lc in Ar and Kr clusters (approximately 60% fast component, approximately 1.7 ps in Ar, approximately 3.0 ps in Kr; approximately 40% slow component, approximately 16f 1 ps in Ar or Kr) were attributed tentatively by Petek et al. to different decay rates from vibrationally hot (short T) and relaxed (long 7) ‘p* [57]. It seems that the question of the lifetime of ‘p* is not yet settled.

The suggestion of Hochstrasser et al. that stilbene photoproducts are formed hot and undergo relatively’slow vibrational relaxation [53, 543 is reminiscent of the Lewis mechanism for stilbene photoisomerization [9]. Many of the thoughts expressed in that highly seminal paper remain just as fresh more than 50 years later, having found new proponents who have couched them in modern terminology. Lewis et al. attributed the very rapid decay of electronically excited cis-stilbene to a “Loose bolt” effect involving a coupling of the electronic oscillation with low frequency torsional vibrations: “The phenyl groups are constrained between the tendency of resonance to make them lie in a plane and their mutual repulsions. At the time of electronic excitation the resonance forces change, and therefore the phenyl groups start toward an altered equilibrium position. This motion will in itself set up the transverse vibrations which we regard as responsible for the early dissipation of the electronic energy” [9]. According to Lewis et al. radiationless decay produces highly vibrationally excited ground state molecules having energies well in excess of the energy of the torsional barrier for ‘c+ ‘t rotation in the ground state [9]. These molecules find themselves in the “no- man’s land” where “the two isomeric configurations are merely phases in the rotation of a single type of molecule.” Following this logic, Lewis et al. concluded that since &ens-stilbene molecules fluoresce even when they are initially formed in S1 with more than 20 kcal mol-’ excess vibrational energy, the barrier to rotation about the central bond of It* must be high. This prediction may reveal a weakness in both the original Lewis proposal and the new version by Hochstrasser and co-workers. The absence of an excitation wavelength effect on both trans- and cir-stilbene fluorescence quantum yields shows that excitation into the “no-man’s land” of the S1 state is followed by rapid transfer of excess vibrational energy to the medium without loss of geometrical identity even in the case of ‘c* for which torsional relaxation is essentially barrierless.

38

In the latter case this must occur with a significantly smaller relaxation time than the approximately 1 ps fluorescence Iifetime of rc*. Actually, a “loose bolt” effect of sorts may operate in torsionally flexible molecules if we redefine the term as involving very rapid transfer of excess vibrational energy to the medium as a result of highly effective damping of low frequency torsional motions, at least in the excited state. There appears to be no reason to assume that vibrational relaxation must be slower by orders of magnitude in the ground state of such molecules. The failure of highly vibrationally excited ‘t * and ‘c* molecules to pass into a “no-man’s land” in the excited state may provide evidence favoring the Orlandi and Siebrand mechanism for the photoisom- erization [39]. According to this mechanism the small torsional barrier in the It* + ‘p* direction and the much smaller barrier in the k* ---, ‘p* direction are due to crossings between the lowest singly excited B state and a higher doubly excited A state. Molecules excited above these weakly avoided crossings [58] may still experience significant torsional barriers in the B state.

The advantage of the Hochstrasser mechanism is that a lifetime close to 150 fs for ‘p* is consistent with &,t * = 0.003 inferred from our fluorescence measurements. Setting +Pt * = kpt 7p and assuming krp = k,,, in view of the insensitivity of &, to temperature leads to rP= 170 fs, in agreement with Hochstrasser’s limiting value. We are left with the dilemma of explaining why transfer of excess vibrational energy to the medium is so much slower in the ground state than in the lowest excited singlet state. Furthermore, if the lifetime of ‘p* is close to 170 fs, the assumption that thermal equilibrium has been achieved at that geometry may be questioned [59].

Acknowledgment

This research was supported by National Science Foundation Grant CHE 90- 14060.

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