Analytica Chimica Acta 496 (2003) 155–163

Raman excitation profile of a sterically protecteddiphosphene [ArP= PAr]

Tiffany Copeland, Michael P. Shea, Matt C. Milliken, Rhett C. Smith,John D. Protasiewicz, M. Cather Simpson∗

Department of Chemistry, Case Western Reserve University, Clapp Hall, Cleveland, OH 44106, USA

Received 29 January 2003; accepted 24 July 2003

Abstract

The resonance Raman excitation profile (RREP) of the bis[2,6-di(m-xylyl)phenyl]P2 diphosphene (DxpP=PDxp) in CDCl3was measured in the region of overlap between the�–�∗ (368 nm) and n+–�∗ (459 nm) UV/visible electronic absorptionbands. The P=P stretch (622 cm−1) was enhanced much more strongly in resonance with the allowed�–�∗ than with thesymmetry-forbidden n+–�∗. The observed differences are attributed to differences in the geometries of the excited states andto the different mechanisms of enhancement for resonance with allowed and forbidden electronic absorptions.© 2003 Elsevier B.V. All rights reserved.

Keywords:Resonance; Raman spectroscopy; Diphosphene; Photochemistry

1. Introduction

Diphosphenes are an exciting new class of main-group compounds that violate the empirical “doublebond rule,” that predicts that stable multiple bonds donot occur beyond the second row of the periodic table.The P=P double bond in these molecules is very reac-tive and resisted successful synthesis for many years.However, in 1981 Yoshifuji et al.[1] broke this bar-rier by reporting the first synthesis and isolation of astable diphosphene compound.

Since then, these remarkable compounds have gar-nered much interest. More than 20 stable (i.e. isolatedand stored at room temperature under anhydrous andanaerobic conditions) P=P compounds have been re-ported and research into heavier atom multiply bondedsystems, heteroatomic doubly bonded systems, transi-

∗ Corresponding author. Fax:+1-216-368-3006.E-mail address:mcs9@po.cwru.edu (M.C. Simpson).

tion metal diphosphene complexes, and other relatedareas has blossomed[2–4]. The synthesis, structuralanalysis and reactivity of diphosphenes and relatedcompounds have been reviewed[5–10]. In addition,the parent compound HP=PH has been studied fairlyextensively by theoretical means, [see6 and referencestherein].

Unlike similar systems with second-row doublebonds, diphosphenes generally have absorption bandsin the visible region of the spectrum[6,8]. Thesehave been assigned to an allowed�–�∗ transitionand a formally symmetry-forbidden n+–�∗ at lowerenergy. Crystals are usually red, orange or yellow,depending upon the substituents. The relatively smallHOMO–LUMO gaps giving rise to their visible ab-sorption make diphosphenes excellent candidates forapplications in electronic, optical and magnetic de-vices. Such devices based upon stilbene and azoben-zene architectures have been well-studied. However,the potential for analogous molecular and polymeric

0003-2670/$ – see front matter © 2003 Elsevier B.V. All rights reserved.doi:10.1016/S0003-2670(03)00996-6

156 T. Copeland et al. / Analytica Chimica Acta 496 (2003) 155–163

P

P

Fig. 1. DxpP=PDxp.

systems based upon diphosphenes remains largelyuntapped.

Because of the weaker P=P �-bond, diphosphenesare significantly more reactive than their C=C andN=N counterparts. They participate in electrophilic,nucleophilic, cycloaddition and other types of reac-tions. They also are photoactive, and can undergophotoisomerization, photoinduced dimerization, andcleavage [for reviews on diphosphene reactivity see[5–7]. However, these compounds may be renderedkinetically stable through steric protection of the re-active P=P bond by bulky substituents like 2,4,6-tri-tert-butylphenyl (Mes∗) [1], 2,6-dimesitylphenyl(Dmp) [11], and 2,6-di(m-xylyl)phenyl (Dxp) [12](Fig. 1).

The photochemistry of diphosphenes is particularlyinteresting, and holds much promise for future appli-cations of these molecules. However, spectroscopiccharacterization of these systems is in its early stages,and excited state characterization has not really be-gun. Several questions remain to be answered areWhat are the photoreactive coordinates in this sys-tem? Are the photoaccessible excited states consistentwith the proposed mechanism of free phosphinidineformation? What is the origin of the wavelength de-pendence of photoreactivity? Resonance Raman ex-citation profiles (RREPs) can begin to address thesequestions.

The RREP measures the dependence of the Ra-man scattering intensities (relative or absolute) ofindividual vibrational modes upon the wavelength ofthe incident laser beam. RREPs provide informationabout the structures and dynamics of excited states,[for reviews see[13,14]. Though the positions andlinewidths of Raman spectral lines reflect electronic

and vibrational characteristics of the ground state,the scattering intensities of normal modes in the res-onance Raman spectrum is related to excited stateproperties. For allowed electronic transitions, such asthe �–�∗ in diphosphenes, totally symmetric modesthat exhibit large displacements in the excited staterelative to the ground state are most intense. Reso-nance with forbidden electronic transitions, like thediphosphene n+–�∗, leads to enhancement of othertypes of modes and analysis is a bit more complex.By identifying which modes are strongly enhanced byresonance and which are not, the dynamics on the ex-cited state surface immediately after excitation can beelucidated and related to reactivity. For example, thisapproach been applied with great success in evaluatingreorganization energies in electron transfer reactions[15].

The RREP approach is relatively underutilized as ananalytical tool, particularly for molecules that do notabsorb strongly in the visible region of the spectrum.This is largely due to the challenges associated withweak signal intensities. Here we have overcome theselimitations and clearly demonstrated that this methodcan be used to characterize the geometry of the ex-cited state and the reaction coordinate associated witha photoinduced reaction of a somewhat weakly ab-sorbing (ε ∼1000–10,000) molecule.

This paper reports the first RREP study of adiphosphene. We have focused this initial study uponthe RREP of the P=P double bond stretch of theDxpP=PDxp system. The reported vibrational spec-tra and a limited normal coordinate analysis haveassigned the P=P stretch assigned to∼600 cm−1

[16–20]. DxpP=PDxp was chosen because it doesnot undergo any detectable, irreversible photoinducedchanges under our experimental conditions. Nonethe-less, its study is directly relevant to photoactivediphosphenes with, for example, Mes∗ ligands[5,21].The molecular structures and electronic states givingrise to the UV/visible transitions are very similar. Therange of excitation wavelengths spans the importantregion of overlap between the�–�∗ and the n+–�∗electronic transitions. Due to limitations of the ex-perimental apparatus, the range over which we havebeen able to measure is relatively small. Nonetheless,the data are consistent with a well-behaved molecularsystem and yield important insight into diphospheneexcited state dynamics.

T. Copeland et al. / Analytica Chimica Acta 496 (2003) 155–163 157

2. Experimental methods

DxpP=PDxp (Fig. 1) was synthesized as previouslyreported[12]. Crystalline material was dissolved inCDCl3 to a concentration of 5–10 mM and placed in aquartz NMR tube under N2 gas.31P NMR was used tomonitor the quality of the sample during the resonanceRaman experiments. UV/visible absorption spectrumwas also measured at the end of the resonance Ramanexperiments to ensure no photodegradation occurred.The UV/visible spectrum inFigs. 3 and 6was mea-sured at a concentration of 1.4 × 10−4 M in CDCl3.

RREPs were measured using the experimental ap-paratus inFig. 2. The incident laser was operated at20 Hz with 10 ns pulses. The laser power was attenu-ated between 5 and 15 mW using neutral density (ND)filters, as necessary, to prevent sample photodegrada-tion. The beam was focused on the sample using a longfocal length quartz cylindrical lens, and data were col-lected in a 135◦ backscattered configuration. The en-trance slit on the half-meter spectrometer (Chromex,500i; 2400/300 grating) was set to 100�m for all spec-tra. No experimental line broadening was observed atthis slit-width. Final spectra were averaged from ei-

L1

ND

L2 L3

dP

F

CCD

S

PM

Chr

omex

0.5

m I

mag

ing

Spec

trom

eter

(50

0i)

Continuum ND6000 Tunable Dye Laser

Continuum Surelite I-20

Nd:YAG Laser

DM

Fig. 2. Apparatus for measuring Raman excitation profiles. The tripled (355 nm) output of a Surelite I-20 Nd:YAG laser (Continuum, Inc.)is separated from residual 532 and 1064 nm by a set of dichroic mirrors (DM) and used to pump a tunable ND6000 dye laser (Continuum,Inc.). For the experiments outlined here, Stilbene 420 (Exciton, Inc.) or Coumarin 440 (Exciton, Inc.) were used as appropriate. The incidentlaser power (measured at PM) was attenuated as necessary to 5–15 mW using the Q-switch delay of the Nd:YAG laser and ND filters.The beam was focused on the sample (S) in∼135◦ backscattering geometry using a cylindrical lens (L1). Alignment irises, aluminummirrors, and a quartz prism used for steering are not shown. Scattered light was collected in a quartz complex triple lens (L2; Triptar; focallength 70 mm) and collimated before passing through a quartz f-matching lens (L3; Oriel, Inc.; focal length 350 mm). The collected lightpassed through a polarization scrambler (dP; Oriel, Inc.) and a notch filter (F; Kaiser Optical Systems, Inc.) centered at 424 nm or 436 nm,as appropriate. The signal then entered the half-meter, single grating (2400/300), imaging spectrometer (500is; Chromex, Inc.) through a100�m entrance slit. Signal was collected with a liquid N2-cooled CCD camera (CCD; SDS9000, Photometrics, Inc.) and evaluated usinga PC computer.

ther 2 (436, 432, 428, 424, and 420 nm) or 3 (416 nm)scans. Each scan was accumulated for 60 s. Spectrawere calibrated using Ar, Kr, and Xe lamps, as appro-priate. The calibration was fine-tuned in data process-ing using the CDCl3 internal standard solvent line at650 cm−1.

The Raman spectra were fit using a non-linearleast-squares curve fitting routine based upon theLevenberg–Marquardt approach[22]. Only the rangeof interest (590–690 cm−1) was fit, and all fits con-verged. The curve fits used Lorentzian lineshapesand linear baselines. The relative intensity of the P=Pstretch (622 cm−1) was evaluated using the CDCl3solvent line at 650 cm−1 as an internal standard.

3. Results and discussion

3.1. Geometry and UV/visible absorption

As with most of the diphosphenes synthesizedthus far, the stable geometry of DxpP=PDxp has atrans-configuration (Fig. 1). Initial examination ofthe X-ray crystal structure indicates that the geometry

158 T. Copeland et al. / Analytica Chimica Acta 496 (2003) 155–163

of DxpP=PDxp is very similar to that of its bet-ter characterized cousin DmpP=PDmp [11,23]. InDmpP=PDmp, the dihedral angle about the –P=P–bond is 177◦ and the P=P double bond length is1.9849 Å. Unlike in the carbon analog stilbene, thesymmetry of the backbone phenyl–P=P–phenyl is notD2h. Rather, one of the phenyls is nearly coplanarwith the –P=P– architecture (168◦ C–C–P=P dihe-dral) and the other is nearly perpendicular (105◦C–C–P=P dihedral). This arrangement is presumablyadopted to accommodate the steric bulk of the sub-stituents. These bulky units thus form a protectivepocket around the reactive P=P double bond. BothDmpP=PDmp and DxpP=PDxp are stable to pho-toexcitation at the wavelengths and incident powersused in this study; no irreversible photoreactivity orphotodegradation was observed.

The UV/visible absorption spectra of diphospheneshas been fairly well-characterized[6,7]. The absorp-tion spectrum of DxpP=PDxp is shown inFig. 3.The two observed transitions arise from�–�∗ andn+–�∗ electronic excitations. The assignments arebased upon their relative intensities, and have beenrecently strengthened by electron deformation densitydetermination[24]. Computational studies of HP=PH[6 and references therein], phenyl–P=P–phenyl[25,26], and Mes∗P=PMes∗ [24] also confirm that thetwo highest occupied molecular orbitals (HOMOs)are the n+ and � and the lowest unoccupied molec-ular orbital (LUMO) is the�∗. Though the orderof the HOMOs depends upon the method and basisset employed, the results are entirely consistent withobserved absorption spectra.

0

2000

4000

6000

8000

10000

12000

300 325 350 375 400 425 450 475 500 525 550 575 600

Wavelength / nm

Ext

inct

ion

Coe

ffic

ient

/ M

-1 c

m-1

π-π*

n+-π*

Fig. 3. Absorption spectrum of DxpP=PDxp in CDCl3.

The �–�∗ transition is symmetry-allowed, and isthe more intense of the two. It appears at higherenergy than the weaker n+–�∗ band. In CDCl3,the solvent used in the studies reported here, the�–�∗ and n+–�∗ are observed at 368 nm (ε =11,500 M−1 cm−1) and 459 nm (ε = 668 M−1 cm−1),respectively. The solvent dependence of these tran-sitions is limited. In hexanes, a much less polarsolvent, the�–�∗ has a maximum value at 368 nm(ε = 10,200 M−1 cm−1) and the n+–�∗ is observedat 458 nm (ε = 543 M−1 cm−1). This solvent insensi-tivity is perhaps not surprising, given the localizationof these electronic transitions upon the –P=P– andthe protection of this central unit by the bulky ligandarchitecture.

3.2. Resonance Raman excitation profile

The photochemistry of the diphosphenes is very in-teresting, from both fundamental science and practicalapplication perspectives. Spectroscopic characteriza-tion is yet in its early stages, and many issues needto be addressed before diphosphene photochemistry isunderstood as well as that of its second-row counter-parts. We have begun to explore the low-lying excitedstates of this system using RREPs. This powerful ap-proach has the potential to unlock details of excitedstate structures and dynamics that will lend invaluableinsight into the mechanism(s) involved in diphosphenephotoinduced reactivity.

The intensity of the Raman signal of a particularvibration is directly proportional to the square of theRaman polarizability |α|2. In the resonance limit, the

T. Copeland et al. / Analytica Chimica Acta 496 (2003) 155–163 159

Kramers–Heisenberg–Dirac description of the polar-izability [27,28] can be expressed as

(α)i→f =∑

|e〉

〈f |µ|e〉〈e|µ|i〉Eie − EL + iΓ

(1)

where |i〉, |e〉, and |f〉 are initial, intermediate, andfinal vibronic states, respectively. |i〉 and |f〉 reflectdifferent vibrational levels associated with the groundstate. The states are coupled via the transition dipoleµ. Eie is the energy difference between the initialand intermediate vibronic states andEL the energyof the incident laser. The polarizability thus becomeslarge when the laser is in resonance with an absorp-tion transition of the molecule(Eie − EL ∼ 0). Thelinewidth is denoted byΓ .

By using the Born–Oppenheimer approximationand treating the nuclear coordinate dependence of thedipole moment operator properly, the polarizabilitybecomes

(α)i→f = |µ0|2∑

|ve〉

〈vf |ve〉〈ve|vi〉(εe − εi + E0) − EL + iΓ

+ higher order terms (2)

Expression (2) was originally derived by Albrecht[29,30] and the dominant, leading term is called theAlbrecht A-term. The Franck–Condon overlaps in thenumerator of the A-term are large for totally symmet-ric modes with significant displacement in the excitedstate relative to the ground state. Hence these modesappear intense in the resonance Raman spectrum.

An equivalent, more convenient analysis approachhas been developed that employs a time-dependentpicture with wave-packets traveling on the excitedstate surface[31–33]. Unfortunately, our data are notyet complete enough to allow implementation of thismethod. We will therefore focus on qualitative con-clusions that can be drawn from the sum-over-statespicture discussed above.

In this initial study, we have examined the RREPof DxpP=PDxp in the region of overlap between the�–�∗ and n+–�∗. We were able to access a 20 nmrange of the absorption spectrum (416–436 nm), andfocused upon the P=P stretching mode at 623 cm−1.The CDCl3 solvent was chosen because of its vi-brational spectrum. There is a strong, Raman-activevibration at 650 cm−1 (near the P=P stretch but notobscuring it) that was used as an internal standard.

Though the accessible range encompasses a rathernarrow subsection of the∼200 nm broad absorptionspectrum, the data nonetheless provide important andpreviously untapped insight into the excited states ofthe diphosphene system.

The results of the excitation wavelength-dependentintensities of the P=P stretching vibration (622 cm−1)are shown inFig. 4. The solvent peak (650 cm−1)heavily dominates the spectrum when the excitationwavelength is 436 nm. As the incident wavelength isdecreased, the relative intensity of the P=P stretch in-creases significantly. The signal-to-noise ratio of thespectra noticeably decreases at the higher energy ex-citations, as the fluorescence from the sample beginsto become significant.

In order to quantify the incident wavelength depen-dence, the spectra were fit with Lorentzian lines and alinear background. The results are shown inFig. 5andare summarized inTable 1. The integrated intensity ofthe P=P stretch increases dramatically as the incidentexcitation wavelength decreases, by a factor of 6 fromexcitation at 436 nm (0.134) to excitation of 416 nm(0.823). Over the range of wavelengths studied here,the increase is monotonic.

The position of the P=P stretch is independent ofexcitation wavelength over the range employed; itsvalue does not change relative to that of the solventpeak. In addition, the linewidths of both the solventand diphosphene lines remain constant. No evidenceis seen for photoreactivity in the Raman spectrum.31PNMR and absorption spectra confirm this assessment.

The RREP is depicted inFig. 6, along with therelevant portion of the absorption spectrum. Thecross-section of the resonance Raman scattering in-creases concomitantly with the absorption from the�–�∗ transition. The n–�∗ transition does not appearto significantly resonantly enhance the P=P stretch.Further experiments extending to the red wouldstrengthen this conclusion.

While quantitative analysis is precluded by the rel-atively narrow region covered by the RREP, the re-sults yield some interesting qualitative conclusions.The resonance enhancement of the P=P stretch is sig-nificantly greater for the�–�∗ transition than for then+–�∗. Two factors likely contribute to these findings.

First, the�–�∗ is an allowed electronic transitionand the P=P stretch is a symmetric vibration. The P=Pstretch should be resonantly enhanced via the A-term

160 T. Copeland et al. / Analytica Chimica Acta 496 (2003) 155–163

500 520 540 560 580 600 620 640 660 680 700 720 740 760 780 800

Raman Shift / cm-1

Nor

mal

ized

Ram

an In

tens

ity

416 nm

420 nm

424 nm

428 nm

432 nm

436 nm νP=P

*

Fig. 4. Raman spectra of DxpP=PDxp in CDCl3 at various excitation wavelengths. The sample concentration sample was 5–10 mM. Theincident laser power was maintained between 5 and 15 mW to prevent photodamage. Spectra are the average of either 2 (420–436 nm) or3 (416 nm) 60 s accumulations, and are displayed normalized to the solvent peak at 650 cm-1 (marked with∗).

(2) mechanism above, as well as by the much smallerhigher order terms in the polarizability expression,when the excitation is in or near this electronic ab-sorption. In contrast, the n+–�∗ is formally forbid-den. Resonance enhancement of Raman scattering viathis much weaker electronic absorption occurs mainlythrough the higher order terms in the polarizability,and thus it is expected to be significantly smaller, asobserved. In addition, any resonance enhancement as-sociated with the n+–�∗ via the A-term will also be

Table 1Curve fit results for resonance Raman spectra of DxpP=PDxp measured at different incident wavelengthsa

Incident wavelength (nm) P=P stretch Internal standard solvent line Relative integrated areab

Position (cm−1) Width (cm−1) Position (cm−1) Width (cm−1)

436 622 11 649 7 0.13432 622 8 650 7 0.18428 622 9 649 7 0.23424 622 8 650 7 0.34420 623 9 650 7 0.41416 622 9 650 7 0.82

a Non-linear, least-squares curve fits employed Lorentzian line shapes and linear baselines. The fits are shown inFig. 5.b Relative integrated area is the integrated area of the P=P stretch divided by the integrated area of the internal standard solvent peak

at 650 cm−1.

significantly weaker because of the smaller extinctioncoefficientε ∼ |µ0|2 for this band.

Second, the nature of the excited state also impactsthe resonance Raman cross-section. A simple, bondorder argument points out how differences in the na-tures of the�–�∗ and n–�∗ excited states are likely tocontribute to the observations. Excitation from the�bonding to the�∗ antibonding P=P orbital should, tofirst order, generate a whole unit bond order change.The bond order change associated with promoting an

T. Copeland et al. / Analytica Chimica Acta 496 (2003) 155–163 161

436 nm

428 nm

432 nm

416 nm

420 nm

424 nm

590 610 630 650 670 690 590 610 630 650 670 690

Nor

mal

ized

Ram

an I

nten

sity

Raman Shift / cm-1

Fig. 5. Curve fits of the 590–690 cm−1 region of resonance Raman spectra of DxpP=PDxp at various excitation wavelengths. The fitsemployed Lorentzian line shapes and linear baselines.

400 425 450 475 500 525 Wavelength / nm

Ext

inct

ion

Coe

ffic

ient

/ M

-1 c

m-1

R

elative Intensity of νP=

P

0.0

0.5

1.0

1.5

2.0

0

1200

2400

Fig. 6. Comparison of the absorption spectrum and the RREP of the P=P stretch in DxpP=PDxp. The closed circles are the integratedintensities of the P=P stretch (623 cm−1) from the curve fits depicted inFig. 5 and listed inTable 1, normalized for the solvent line at650 cm−1.

162 T. Copeland et al. / Analytica Chimica Acta 496 (2003) 155–163

electron from the nonbonding, positive combinationof the lone-pair orbitals on the phosphorus (n+) to the�∗ antibonding P=P molecular orbital should producea smaller change in the P=P bond strength. The P=Pbond length associated with the�–�∗ excited stateshould therefore be greater than that associated withthe n+–�∗ excited state. As discussed above, the mag-nitude of the Franck–Condon coupling terms in thenumerator of expression (2) is related to extent of thechange along the vibrational coordinate that occurswith the electronic excitation. Resonance enhance-ment associated with the�–�∗ excitation should begreater, because the distortion from the ground stategeometry along the P=P coordinate should be largerfor this excited state.

4. Summary

This study reports the first RREP information fora diphosphene. This system is challenging for theRREP analytical tool because of diphosphene’s rela-tively weak absorption and the location of the�–�∗UV/visible transition at the blue/UV end of the vis-ible spectrum. The results indicate that the RREPmethod can be used to determine the geometry of themolecular excited states and explore their dynamics.Clearly, there is superior enhancement of the P=Pstretch through resonance with the�–�∗ transitionrelative to the n+–�∗. This is due to differences inthe geometries of the excited states and to the dif-ferent mechanisms of enhancement for resonancewith allowed and forbidden electronic absorptions.These preliminary studies suggest that an early stepin the photoreaction of diphosphenes may be rapid,significant elongation of the P=P bond. However,further studies need to be performed to extend the re-gion covered by the RREP throughout the absorptionband. Despite the instrumental challenges associatedwith extending the excitation to higher energy (intothe near-UV where excitation is more difficult) andto lower energy (where the absorption is weaker),doing so would allow quantitative rather than qual-itative analysis of the excited state geometries anddynamics. Once this vital information is known, otherintriguing issues, including wavelength selectivity inphotoreactivity and functional interactions betweenthe two electronic transitions can be put into context.

The experiments reported here are a crucial first stepin probing and controlling the photochemistry of thispromising class of molecules.

Acknowledgements

We thank the National Institutes of Health(GM056816) (MCS) and the National Science Foun-dation (CAREER CHE-9733412) (JDP) for supportof this work.

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M. Cather Simpson is currently an as-sistant professor in the Chemistry De-partment at Case Western Reserve Uni-versity in Cleveland, OH, where she isa co-director of the Center for ChemicalDynamics. She joined the faculty therein January 1997. She received her un-dergraduate degree in Echols-Interdisci-plinary Studies at the University of Vir-ginia and a Ph.D. in medical sciences

from the University of New Mexico with a Howard Hughes Pre-doctoral Research Fellowship. Her dissertation research with Dr.M. Ondrias focused upon the vibrational behavior of heme pro-teins and its relationship to function. She continued her researchinto porphyrin behavior as a Department of Energy DistinguishedPostdoctoral Researcher at Sandia National Laboratories under theguidance of Dr. J. Shelnutt. Since joining the faculty at CWRU,she has focused upon the study of vibrational energy dynamics inlarge molecules, including porphyrins, in solution. She receivedan NIH FIRST award to support this work. In addition, she wasawarded a CWRU Glennan Fellowship for her innovations in gen-eral chemistry.