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J. Phys. Chem. 1983, 87,380-382
Evidence That the Excited-State Geometry of Diphenylbutadiene Is Nearly Planar W. A. Yee, J. S. Horwltz,
R. A. Goldbeck, C. M. Elnterz, and D. S. Kllger’
Dlvlslon of Natural Sciences, University of California at Santa Cru.?, Santa Cru.?, Californla 95064 (Received: October 12, 1982: I n Flnal Form: December 1, 1982)
A structural analogue of diphenylbutadiene (DPB), 1,5-diphenyl-2,3,4,6,7,8-hexahydronaphthalene (HHN), was synthesized and its spectral properties were studied. Absorption and fluorescence spectra indicate that HHN has a ground-state geometry in which the phenyl rings are twisted relative to the polyene unit and a nearly planar lowest excited ‘B, state geometry. The S, SI spectra of DPB and HHN are similar as are the T, T1spectra. DPB and HHN show temperature dependences of fluorescence lifetimes with similar Arrhenius activation energies and similar radiative decay constants. It is concluded that relaxation of excited DPB occurs from an excited state in which the polyene moiety is nearly planar.
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There has been intense interest in recent years in unphysics has thus held special interest. derstanding polyene spectroscopy and mechanisms of To explain the fluorescence properties of a,w-diisomerization. This interest has been spurred in part by phenylpolyenes, Birks and Birchlo proposed a model in the spectral characteristics of visual pigments, which which the excited molecule undergoes a conformational contain a polyene chromophore, and by the role of polyene change in the polyene unit to a more stable twisted form. photoisomerization in visual transduction. The a,o-diVelsko and Fleming” examined the solvent viscosity and phenylpolyenes have been the subject of particular interest temperature dependences of the nonradiative decay rate of DPB. Their results are consistent with a mechanism as their fluorescent properties have made them easier to of rotational relaxation that competes with fluoresence. study than the nonemitting unsubstituted polyenes. We recently reported a study of the excited-state absorpAn interesting issue for spectroscopists and photochemtion spectroscopy of diphenyl polyene^.^ Experimental ists has been the nature of the lowest excited state of polyenes. Two-photon spectroscopic studies have indicated spectra were compared with PPP-SCF MO calculations that the lowest excited singlet states of the diphenylwhich included both single and double excitation configpolyenes are of lA, Recent fluorescence uration interaction. For DPB, we found that only if the spectroscopic studies4 and excited-state absorption speclowest excited state were the ’B, state would our calculated troscopic studies5 confirm these assignments in diand measured spectra agree. To achieve that agreement phenylhexatriene and diphenyloctatetraene. In diwe needed to introduce a correlation correction to our phenylbutadiene (DPB) one and two-photon ~ t u d i e s ~ ? ~molecular orbital calculation. Agreement could also be found, however, if no correlation correction was made but indicate that the and the ‘B, excited states are nearly one instead assumed that the lB, state geometry is highly degenerate so the state ordering may be solvent dependent. twisted. In hydrocarbon solvents the lowest singlet state has ‘B, As can be seen, twisted excited-state geometries in DPB ~haracter.~ have been suggested from a number of studies. To test In 1964 Hammond and co-workerse performed a classic the possibility of a highly twisted excited state of DPB we study of triplet-sensitized cis-trans isomerization. They have synthesized and studied the properties of 1,5-dishowed that the lowest triplet state of stilbene was a phenyl-2,3,4,6,7,8-hexahydronaphthalene (HHN), a DPB “phantom” triplet state which lay below the spectroscopic triplet. Saltiel and co-workers’ also found a similar “phantom” state to be the lowest excited singlet-state of stilbene. By analogy to calculations of excited-state potential energy surfaces of ethylene8 and stilbene? these “phantom” states have been described as states with highly twisted geometries. The studies of stilbene have led to the HHN common belief that excited states of polyenes, in general, analogue constrained to a near planar diene geometry. A have highly twisted geometries. The role of highly discomparison of the properties of DPB and HHN leads us torted or perpendicular excited states in polyene phototo conclude that the geometry of the lowest excited lB, state of DPB is nearly planar. Synthesis of HHN was accomplished by a straightfor(1)H.L. Fang, R. J. Thrash, and G. E. Leroi, J.Chem. Phys,, 67,3389 ward modification of a procedure given by Bailey.I2 (1977). (2)H. L. Fang, R. J. Thrash, and G. E. Leroi, Chem. Phys. Lett., 57, Chromate oxidation of 1,5-decalindiol gave the corre59 (1978). sponding diketone which was subsequently reacted with (3)J. A. Bennett and R. R. Birge, J . Chem. Phys., 73, 4234 (1980). 2 equiv of phenyllithium. The resulting diphenyldecalin(4)D. J. S.Birch and R. E. Imhoff, Chem. Phys. Lett., 88,243(1982). (5)R. A. Goldbeck, A. J. Twarowski, E. L. Russell, J. K. Rice, R. R. diol was dehydrated with formic acid in dimethylformBirge, E. Switkes, and D. S. Kliger, J. Chem.Phys., 77, 3319 (1982). amide (DMF) to yield product. Recrystallization from (6)G. Hammond, J. Saltiel, A. Lamola, N. Turro, J. Bradshaw, D. DMF/water or from ethanol gave colorless, long, thin Cowan, R. Counsell, V. Vogt,and C. Dalton, J . Am. Chem. SOC.,86,3197
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(1964). (7)J. Saltiel, J. D’Agostino, E. Megarity, L. Metts, K. Neuberger, M. Wrighton, and 0. Zafriou, Org. Photochem., 3, 1 (1973). (8)A. J. Merer and R. S.Mulliken, Chem. Reo., 69, 639 (1969). (9)G.Orlandi and W. Siebrand, Chem. Phys. Lett., 30,352 (1975). 0022-365418312087-0380~01.5010
(10)J. B. Birks and D. J. S.Birch, Chem. Phys. Lett., 31,608(1975). (11)S.P.Velsko and G. R. Fleming, J.Chem. Phys., 76,3553(1982). (12)A. S.Bailey, Can. J. Chem., 37,541 (1959).
0 1983 American Chemical Society
The Journal of Physical Chemistry, Vol. 87, No. 3, 1983
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(nm)
Figure 1. Absorption and emission spectra of DPB (- - -) and HHN in cyclohexane at room temperature. The emission intensities are not corrected for detector response.
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needles, mp 143 "C.Proof of structure was obtained from mass spectra and 'H NMR.13 The ground-state absorption and fluorescence spectra for DPB and HHN in cyclohexane are shown in Figure l.14 The fluorescence quantum yields in cyclohexane at room temperature were measured to be 0.42 for DPB and 0.06 for HHN. The excited-state lifetimes were found to be 600 ps for DPB and less than 100 ps for HHN under these ~0nditions.l~ In Figure 1, it can be seen that both DPB and HHN exhibit absorption features near 235 nm. Our calculations indicate that this absorption is due to phenyl-localized excitation so it is reasonable to expect similar DPB and HHN transitions in this region. This conclusion agrees with the assignment of Bennett and Birge in their analysis of the DPB absorption ~ p e c t r u m . ~While the 235-nm transitions in DPB and HHN are similar, the lowest energy absorption band and fluorescence spectra of the two molecules differ significantly. In DPB, the absorption and emission bands show good mirror image symmetry and overlap. In HHN, the corresponding bands are structureless, with an absorption maximum in HHN at 280 nm instead of at 330 nm as found in DPB. More significantly, there is a very large fluorescence Stokes shift of 10700 cm-'. A Stokes shift as large as that in HHN must result from either different absorbing and emitting electronic states or from a large change in excited-state geometry. If there were an emitting state 10700 cm-' below the 'B, and should be easily detected However, no such low-lying l?i? state was seen in a two-photon fluorescence experiment. Let us now consider that HHN undergoes a large geometry change upon excitation. Because the u framework in HHN does not allow significant twisting of the polyene unit, a large geometrical change must involve phenyl ring twists. Our calculation of the ground-state absorption spectrum indicates that a 280-nm transition is expected if the phenyl rings are twisted 60' out of the diene plane. In an MM2 calculation of the ground state, a phenyl twist
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(13) We note here that the melting point and the ground-state absorption spectrum of HHN differ from those given in ref 12. No shcture proof was provided there as HHN was an intermediate in the synthesis of 1,S-diphenylnaphthalene. (14) Though the fluorescence spectrum is uncorrected, the detector response is relatively flat in this spectral region. (15) DPB fluorescence w a ~excited by a 300-ps N2 laser and HHN fluorescence was excited by a frequency doubled N2 laser-pumped dye laser. Fluorescence was detected by a photomultiplier with a 600-ps risetime and a sampling oscilloscope. (16) R. R. Birge, private communication.
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500
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Wavelength ( n m )
Flgwe 2. Excited singlet-state spectra of HHN and DPB in cyclohexane at room temperature.
angle of 74O is obtained." Such a twisted phenyl conformation is consistent with the lower molar absorptivity observed for HHN as compared to DPB. The large fluorescence Stokes shift is expected if the phenyl rings become nearly planar with the diene unit in the excited state. This behavior is directly analogous to the case of 1,4-diphenylnaphthalene which has been shown to have its phenyl rings 55O out of plane in the ground state and nearly planar in its excited state.18 Thus, analysis of the absorption and the fluoresence spectra of HHN indicates that ita ?r-electronicsystem is nearly planar in the excited state. The S, S1 excited-state absorption spectra of DPB and HHN in room temperature cyclohexane are shown in Figure 2. The methods used to obtain these spectra are described el~ewhere.~J~ As can be seen, the excited-state spectra of the two molecules are quite similar. Only one absorption band appears in the region 4 5 " O nm.I9 The HHN spectral maximum is shifted from that of DPB by about 800 cm-' (0.1 eV). This small difference is taken to mean that the geometries of the absorbing excited states of HHN and DPB are very similar. MM2 calculations indicate that the polyene portion of HHN is planar in the ground state. However, upon excitation the bond orders of the double bonds will decrease. This could result in small twists in the polyene moiety due to constraints imposed by the structure of the cyclohexene rings. According to our PPP calculations, the 5, S1transition of a planar DPB would red shift by -0.14 eV if a twist of 20° were introduced into the diene unit.5 A slight red shift is also predicted for small phenyl rotations about the diene unit. Since our calculations indicate that any deviation from planarity resulta in a red-shifted excited-state absorption and since the excited-state absorption of HHN lies to the red of the DPB excited-state absorption, we conclude that the excited-state geometry of HHN deviates from planarity more than that of DPB. This agrees with the theoretical
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(17) Peter Kollman, private communication. (18) E. L. Russell, A. J. Twarowski, D. S. Kliger, and E. Switkes, Chem. Phys., 22, 167 (1977). (19) Recently Chattopadhyay and Das (Chem. Phys. Lett., 87, 145, (1982))reported an S, SImaximum for DPB at 430 nm. In our study of DPB (ref 5) we carefully examined this wavelength region and found no excited-state absorption band.
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382 The Journal of Physical Chemistry, Vol. 87,No. 3, 1983
predictions of Pierce and Birgea20 Two other results deserve mention. First, we obtained the T, T1 spectrum for HHN by direct excitation at room temperature in cyclohexane. The triplet-triplet absorption maximum appears at 385 nm, which is near the value of 390 nm reported for DPB previously.21s22 The ratio the triplet absorption intensity to the singlet absorption intensity is approximately six times larger in HHN than DPB. Thus, if we assume the same extinction coefficients of excited singlet and triplet states for the two molecules, the triplet yield in HHN is about six times larger than that in DPB. Second, we studied the temperature dependence of the fluorescence decay times for DPB and HHN in 3-methylpentane (3MP). While the decay rates of HHN are significantly larger than those of DPB, the Arrhenius plots have virtually the same slope. The singlet decay processes in both molecules have activation energies of about 850 cm-'. Furthermore, we estimated the room temperature lifetime of HHN in 3MP to be between 75 and 100 ps from the Arrhenius plot. This leads to a radiative decay constant of 5 X 108-7 X lo8 s-l, which is near the mean value of 7.7 X lo8 s-l obtained for DPB in a variety of alkane solvents." Hence, the similar magnitudes of the fluorescence transition moments is consistent with a near planar geometry of the 'B, states in HHN and DPB. The larger nonradiative rate constant
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(20)B. M.Pierce and R. R. Birge, J. Phys. Chem., 86,2651 (1982). (21)S.K.Chattopadhyay, P. K. Das, and G. L. Hug, J.Am. Chem. SOC.,104,4507 (1982). (22)H.Gorner, J.Photochem., 19, 342 (1982).
Letters
in HHN compared to DPB is probably due to larger phenyl motions resulting from steric interactions with the ring system. These motions apparently enhance the rates of both intersystem crossing and internal conversion. It is possible that, subsequent to excitation into the 'B, state, the molecule is transformed to a twisted 'A, state. This must involve an activation barrier analogous to that suggested for stilbene.B If the twisted '$ state had a very short lifetime or a small absorption oscillator strength, we would not have observed its formation in this experiment. We thus cannot completely rule out the involvement of twisting motions in the decay of DPB excited states. If, however, the lowest excited state of DPB is the lBu, the unimolecular decay of the lowest excited state might occur through small polyene twisting motions or through twisting motions of the phenyl rings. Large polyene twisting motions, however, cannot be involved. We recognize that HHN is not an entirely satisfactory model for studying DPB photophysics since phenyl motions are more severe in HHN than DPB. Further work with completely rigid DPB analogues is in progress.
Acknowledgment. We gratefully acknowledge Mr. Jim Loo for obtaining mass spectra and NMR spectra; Professor David J. Morgans, Jr., for discussion on synthesis; Professor Robert Birge for the two-photon work as well as helpful discussions; and Professor Peter Kollman for the MM2 calculations. This work was supported by NIH Grant EY00983. (23)R. M.Hochstrasser, Pure Appl. Chem., 52, 2683 (1980).
