J. Phys. Chem. 1993,97, 8713-8717
8713
Photoisomerization of Arylethylenes: Exploring the Singlet Potential Energy Surface of a Partially Planar, Specially Stabilized Compound B. S. Udayakumar, C. Devadoss, and Gary B. Schuster' Department of Chemistry, Roger Adams Laboratory, University of Illinois, Urbana, Illinois 61801 Received: March 25, 1993
A uniquely substituted tetraarylethylene was prepared and studied spectroscopically and photochemically. 9-Fluorenylidenetribenzocycloheptatriene (1) has a singlet lifetime of 4.1 ns at room temperature and fluorescence and transient absorption spectra that are independent of temperature and solvent. The fluorescence emission is assigned to the vertical (Franck-Condon) state. The relatively long lifetime of this state is a consequence of the barrier to planarization of the tribenzosuberenyl group. There are no photochemical reactions of 1 in cyclohexane, acetonitrile, or methanol solution. From the results it is clear that planarization of the groups on both ethyleniccarbon atoms is required to facilitate rotation about this double bond. Further, the perpendicular excited state even in this highly stabilized example appears to be zwitterionic.
Introduction Experimental investigationof geometric isomerizationreactions of aryl-substituted ethylenic systems has provided an important framework for the development of modern theories of photochemistry and photophysics.1 Foremost among these systems are landmark investigations of cis- and trans-stilbene and tetraphenylethylene. These compounds have been probed theoretically and experimentally by steady-state and time-resolved spectroscopicmethods culminating in a recent report of the direct measurement of transition-state dynamics for the excited singlet state of cis-stilbene by femtosecond time scale analysis of vibrational motion along its reaction coordinate.2 The seemingly simple geometric isomerization reaction of these aryl-substituted ethylenic system has been shown to involve complex motions on the excited state potential energy surface proceeding through a series of structurally distinct states. The number and specific nature of these excited states is not certainly defined in all cases, but normally it appears that at least three states are involved. Excitation leads to the vertical (Franck-Condon) singlet state, torsional relaxation about the aryl-ethylenic carbon bond gives a relaxed, sometimes emissive, excited state, and rotation around the ethylenic carbon-carbon bond leads to formation of the nonemissive perpendicular ("phantom") excited state. In this paper we report steady-state and time-resolved spectroscopic investigation of the tetrasubstituted aryl ethylene 1. This compound was designed specially to stabilize the zwitterionic configuration of the perpendicular excited state. Fischer and co-workers first reported the solvent viscosity dependenceof the fluorescenceand trans to cis photoisomerization of stilbenes and a number of their simple derivatives3 This work led later to the conclusion4 that radiationless deactivation of the fluorescing Franck-Condon state requires a change in shape of the molecule. On the basis of single configuration,semiempirical molecular orbital calculations, the required change in shape was attributed to variation in the equilibrium torsional angles about the phenyl-ethylene and ethylenic carbon-carbon bonds in the ground and excited states.$ Orlandi and Siebrand6have presented theoretical and experimental evidencesuggestingthat the barrier to isomerization results from a nonadiabatic crossing to a doubly excited singlet state (lAg, S,) of stilbene having an energy minimum at the perpendicular configuration. The potential energy surface they present emphasizes relaxation to the perpendicular state by torsional rotation about the ethyleniccarboncarbon bond. In contrast, Troe and Weitzel7 report MNDO calculations which suggest that the isomerization of stilbene in the singlet state must overcome an adiabatic barrier on the SI 0022-365419312097-8713$04.00/0
state (lBJ imposed by a complicated combination of ethylene and phenyl group rotations and stretching of the ethylenic and phenyl-ethylene bonds. In a pioneering study, Greene*reported detection of the perpendicular state formed from tetraphenylethylene by means of picosecond absorption spectroscopy. At room temperature the vertical state of this molecule relaxes in ca. 5 ps (Amx = 640 nm) to a long-lived (3.0 ns, A, = 417 nm) state formed by twisting about its ethylenic carbon-carbon bond. The analogous vertical state was detected in a "stiff stilbene" analog (1,l'-biindanylidene) by Hochstrasser and co-workers9 and found to have a 10-ps lifetime. It has been known for a long time that the fluorescence of tetraphenylethylene shows two components4 which may be resolved by varying the temperature. Rentzepis and co-workers showed that the motion responsible for this temperature-dependent spectral shift is independent of the rearrangement required for inducing the radiationless deactivation.10 The identity of the motion responsible for the spectral shift was clearly delineated by Shultz and Fox11 who studied a series of tetraarylethylenes tethered with alkyl chains that restrict motion about the arylethylene bond. For these model compounds, the singlet lifetime and fluorescence efficiency of the vertical singlet state increases. This observation was interpreted to show that an important contribution to deactivation of the Franck-Condon singlet state is torsional motion about the aryl-ethylene bond. The electronic properties and relative energies of the three excited singlet states of aryl-substituted ethylenes have recently been studied by time-resolved spectroscopic and calorimetric techniques. Schilling and HilinskiIz and later Sun and Fox13 reported that the lifetime of the perpendicular state of tetraphenylethylene depends dramatically on solvent polarity. In cyclohexane solution, the perpendicular state has a lifetime of 1.3 ns, but in acetonitrile solution its lifetime is reduced to 14 ps. The change in lifetime is attributed to a reduction in the energy of the perpendicular state in the polar solvent and is taken as evidence that this state is zwitterionic in character and not a biradical. This conclusion was confirmed very recently by time-resolved microwave conductivity mea~urements.1~ Zimmt and co-workersI5 measured the energy of the perpendicular state of tetraphenylethylene directly in several solventsand found that it decreases from 67 kcallmol above the ground state in hexane solution to 65 kcal/mol in tetrahydrofuran solution. This is to be compared with an energy of 77 kcallmol for the vertical excited state. In more recent work, Zimmt and Mal6 showed for tetraphenylethylene and for some related substituted tetraarylethylenes that the excited perpendicular state is in equilibrium with the relaxed 0 1993 American Chemical Society
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The Journal of Physical Chemistry, Vo1. 97, No. 34, 1993
Udayakumar et al.
CHART I
7 30000
1
"Pemendicular"
stateresponsible for the spectral shift observedin the fluorescence spectrum. A kinetic analysis of these data indicates that the energy of the perpendicular state is no more than ca. 3 kcal/mol below that of the relaxed, emitting singlet state." This finding is consistent with the report by Saltiel and co-workersl*of the adiabatic conversion of singlet excited cis-stilbene to the trans isomer with significant efficiency. The excited singlet potential energy surface for the simply substituted aryl ethylenes must be relatively flat. A thorough examination of the photophysics for a series of donor-acceptor substituted stilbenes was recently reported by Lapouyade, Rettig, and their co-workers.lg They prepared and investigated cyclic, bridged stilbene analogs substituted at the 4 and 4' positionswith dialkylaminoand nitrile groups, respectively. The cyclic bridging groups were positioned so that torsional motion about the ethyleniccarbon-carbon bond, the aryl-ethylene bond, and the aryl amino group bond were independently restricted. This study, too, revealed that there are at least three states on the excited singletpotential energy surface. Excitation generates the emissive vertical state which can relax irreversibly either to a nonemissive perpendicularstate, thought to be a diradical rather than a zwitterion, or to a twisted (about the aryl amino group bond) emissive state. Wereport herein thesynthesis and investigationof arylethylene 1which permits examination of key aspects of the excited state potential energy surface of these compounds. In some ways arylethylene 1 resembles the tethered compounds of Shultz and Fox in that torsional motions about the aryl-ethylenic carbon bonds are restricted. In other aspects 1resembles the compounds of Lapouyade and Rettig since a zwitterionic perpendicular state can be specially stabilized. The well-known aromaticity of the fluorenylanion and the tropyium cation (i.e., the tribenzosuberenyl group) will lower the energy of the perpendicular state of 1 significantlycompared with that of tetraphenylethylene (seeChart I). This stabilization might be sufficient to switch electronic configurationsofthe perpendicular stateandmakethezwitterionic form the ground state. Unlike previous examples of related tetraarylethylenes, the ethylenic carbon atom of 1 incorporated in the fluorenyl group is coplanar with its attached substituents and the ethylenic carbon atom contained within the tribenzosuberenylgroupis not coplanar withits substituents. Asignificant energetic bamer must be overcome to achieve planarization of this center, and this fact strongly affects the photophysical properties of this compound.
Experimental Section General. NMR spectra were recorded on a Varian XL 200 (200 MHz) spectrometer. Low-resolution mass spectra were recorded on a Varian-MAT CH-5 mass spectrometer (70 eV). Steady-state emission and excitation spectra were measured on Spex Fluorolog spectrometer. The fluorescence lifetimes were measured by single photon counting with a 0.5 ns pulse width flash lamp. The picosecond and nanosecond transient absorption spectrometers have been previously described.20 Tribenzo[a.c.e]cyclobeptaMene-l-thioae. A 1-mL toluene solution containing 100 mg (0.39 mmol) of tribenzocyclohep tatriene-l-one21 and 94.5 mg (0.23 mmol) of Lawesson's reagent
200
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Wavelength, nm
Figorel. Absorptionspectrumof1(1.8X 10-5M)incyclohexanemlutim at room temperature (solid line). Fluorescence spectra of 1 (broken lines) in cyclohexane solution at room temperature (lower curve) and in
glassy methylcyclohexane at 77 K (upper curve). (Aldrich) was heated at reflux overnight under an Ar atmosphere. After cooling, the toluene was removed under vacuum and the residue was subjected to column chromatography (silica gel, 1:4 CH2Clz/hexane as eluent) to give 22.7 mg (21%) of pure product as a blue solid GC-MS, m / z (relative intensity) 272 (M+, 74), 271 (23), 269 (lo), 239 (14), 228 (loo), 227 (lo), 226 (21), 134 (ll), 113 (15). 9--1tribenzocyclobepbibienetMhrae A2-mLdrybenzene solution containing 23 mg (0.08 "01) of tribenzo[a.c.e]cycloheptatrien-1-thioneand 26 mg (0.13 "01) of 9-diazofluorene was heated at reflux under N2 for 2 h. After cooling, the benzene was removed under reduced pressure and the residue was purified by column chromatography (silica gel, CHzC12 in hexanes as eluent) to give 16 mg (44%)of pure product: MS-EI, m / z (relative intensity) 436 (M+, 100). 435 (1 I), 404 (24), 403 (9), 329 (13), 328 (33), 240 (9), 196 (16). 9 - F l m u e n y l i a e a e M b y c l o ~ p t (1). a ~ ~A 2-mL xylene solution containing 16 mg (0.04 "01) of 9-fluorenyltribenzocycloheptatrienethiirane and 500 mg (7.9 "01) of Cu powder was heated at reflux for 14 h.22 After cooling, the Cu powder was removed by filtration and the residue was washed with 5 mL of xylene. The combined filtrate was concentrated and subjectcd to column chromatography (silica gel, 30% CHzCl2 in hexanes as eluent) to give 4.9 mg (33%) of pure product: mp 310-312 OC; l H NMR (CDC4) 6 6.80-6.96 (m, 4 H, aromatic), 7.167.71 (m, 16 H, aromatic); MS-EI, m / z (relative intensity) 404 (M+, loo), 403 (23), 240 (3), 239 (12), 202 (9), 187 (lo), 165 (7), 163 (lo), 77 (18); uv (C&I12), A 254 (log c 4.60), 261 (4.62), 300 (4.13), 350 (3.15),HR-EIcalcdforC30H2~404.1565, found 404.1576.
RWlkS 1. Steady-StateSpectra. The absorption spectrum of arylethylene 1in cyclohexane solution at room temperature is shown in Figure 1. This spectrum is nearly identical with that of tetratolylethylene reported by Shultz and Fox.ll The lowest energy absorption band of 1 has a maximum at 323 nm with an excinction coefficient of 14 0oO M-' cm-l. For comparison, the lowest energy absorption maximum of tetratolylethylene occurs at ca. 3 14 nm with an extinction coefficient of 11 200 M-l cm-l. In contrast, the lowest energy absorption for the benzylidene analog of 1 has a maximum absorption at ca. 270 nm and for fluorene itself this band is at ca. 300 nm. On the basis of the similar absorption spectra, we consider the lowest energy vertical singlet state (lBJ of 1 to be analogous to that of the stilbenes and to tetraphenylethylene. The fluorescence spectra of 1 in methylcyclohexane solution at room temperature and at 77 K are also shown in Figure 1.
The Journal of Physical Chemistry, Vol. 97, No. 34, 1993 8715
Photoisomerization of Arylethylenes
1.o
0
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"
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Wavelength, nm Figure 2. Transient absorption spectrum of 1in cyclohexane solution at rwm temperature recorded 100 ps after excitation with a 18-ps laser pulse at 355 nm. The inset shows the decay of the absorption band when
Figure 3. Transient absorption spectra recorded 200 and 10 000 ps after excitation of 1 in cyclohexane solution at room temperature. These two spectra are identical within experimental error.
excitation of 1 with a 20-11s pulse shows no evidence for a longlived transient that would indicate formation of its triplet state.
monitored at 520 nm.
Discussion
Neither spectrum reveals much vibronic structure, though there are at least three apparent bands in the low-temperature spectrum. The fluorescence quantum efficiency for 1at room temperature is 0.025. Significantly, unlike tetraphenylethylene, apart from the increase in fine structure and an apparent increase in fluorescence quantum yield, there is virtually no difference between the spectra of 1 recorded at room temperature and at 77 K. The Stokes loss for 1 in cyclohexane solution at room temperature is 3500 cm-1 and appears to be the same at 77 K. In contrast, the Stokes loss for tetraphenylethylene decreases dramatically when the temperature is changed from 300 to 77 K and the solution becomes rigid.10 Essentially identical spectra for 1 are observed in acetonitrile solution at room temperature as in cyclohexane solution except that the fluorescence quantum yielddecreasmto0.018. Thefluorescenceefficiencyof 1decreases with increasing temperature in p-xylene solution between room temperature and 69 "C. Analysis according to the Arrhenius formalism gives an activation energy for the nonradiative processes competing with fluorescence of 5.2 kcal/mol. No emission attributable to phosphorescence was observed under any conditions. There is no evidence for any irreversible photochemistry of 1in cyclohexane, acetonitrile, or methanol solution even when it is subjected to extensive irradiation. These findings indicate that there are no measurably emissive nonvertical states on the singlet potential energy surface of 1, that intersystem crossing to the triplet of 1 is not an efficient process, and that no reactive, long-lived zwitterionic states are formed. 2. Time-ResolvedSpectroscopy. The fluorescence lifetime of 1recorded at room temperature in cyclohexane solution is 4.4 ns (4.3 ns in acetonitrile solution). Its fluorescence decay under these conditions is monoexponential. Similarly, at 77 K in a frozen methylcyclohexane solution, 1 shows a single exponential decay with a lifetime of 35 ns. Excitation of a cyclohexane solution of 1 with a 20-ps laser pulse at 355 nm gives the transient spectra shown in Figure 2. The three apparent absorption bands in this spectrum at 420, 520, and 620 nm appear instantaneously on this time scale and are attributed to absorption of the SIstate of 1. These three bands decay simultaneously as may be judged by comparing the spectra in Figure 3 recorded 200 and 10 000 ps after the laser pulse. The kinetic decay of this transient absorption was fit to a first-order function which gives a lifetime of 4.7 f 0.3 ns. Essentially identical results are obtained when the excited state of 1is probed in acetonitrile solution. Spectral analysis following
1has revealed some interesting features that add insight into the
The spectroscopic analysis of specially substituted arylethylene nature of singlet excited state potential energy surfaces for these compounds. Two features in particular merit discussion. First, the special structure of 1 restricts torsional motion about the dihedral angle 4, defined as that between the plane of the phenyl rings and the plane containing the two ethylenic atoms and their adjacent carbon atoms. However, further analysisof the structure for 1 indicates that there is no additional restriction beyond that of tetraphenylethylene on the rotation about the ethylenic double bond. Second,the special aromatic stabilizationof the zwitterionic form of perpendicular 1 may invert the electronic configuration of this state and cause the ground state to be zwitterionic. 1. Ground-State Structure of 1. The structures of arylsubstituted ethylenes have been examined by both experimental and computational meth0ds.2~The geometry of these compounds are characterized by a series of bond lengths, bond angles, and dihedral angles. For example, trans-stilbene in its ground state has essentially normal bond lengths and angles with a dihedral angle 4 of 33". Similarly for cis-stilbene,bond lengths and angles are found to be nearly identical with the trans isomer but the value for $I found experimentally is 43". Molecular orbital calculations reproduce these structural parameters with reasonable accuracy. We carried out a series of molecular mechanics calculations (MMX) on 1 to ascertain the details of its groundstate conformation. The accuracy of this approach was tested by comparison of similarcalculationswith the experimental values for the stilbenes and tetraphenylethylene. Thevalue of 4 calculated for cis-stilbeneby the MMX method is 37O. The heat of formation predicted by this calculation for cis-stilbeneis 56.9 kcal/mol. Estimation of the heat of formation of cis-stilbene by the Benson group equivalent method24yields a value of 58.5 kcal/mol. These findings give us some confidence that the MMXcalculationgivesgoodestimates for bothenergetic and structural features in these systems. MMX calculations of the ground-state structure for tetraphenylethylene gives 4 = 49O. When this method is applied to 1, the structure that is predicted to have the lowest energy in the ground state is show in Figure 4. The fluorenyl group is essentially planar, but the tribenzosuberenyl group is, as expected, nonplanar. The value of 4 calculated for the minimum energy conformation of 1 is 1.So for the fluorenyl group and 67" for the tribenzosuberenyl group. As can be appreciated from inspection of Figure 4, the tribenzosuberenyl group of 1 is "butterfly" shaped with the opposing benzo groups forming the "wings", the remaining benzo
8716 The Journal of Physical Chemistry, Vol. 97, No. 34, 1993
Udayakumar et al.
B
I
Multidimensional Reaction Coordinate
Barrierto Planarization Figure 5. (A) Schematic singlet potential energy surface for tetraphenylethylene(energiesare not to scale) showing minima correspondingto the ground state (SV),thevertical excited state (Slv), the relaxed excited and the perpendicular excited state ('P*). (B) The proposed state (SIR), singlet potential energy surface for 1, the symbols have the same meaning as those in A. (C) Detail of the surface crossing region for the singlet potential energy surfaces of tetraphenylethylene; the thermal barrier to reach the crossing region is low because the excited singlet surface is easily planarized. (D) Detail of the surface crossing region for the singlet potential energy surfaces of 1, the bamer is higher than that in tetraphenylethylenebecause it is more difficult to planarize this system.
Figme4. Midmumenergystructurefor 1 predicted from thePCMODEL force field calculation.
group forms the "tail", and the ethylene group is in the "head" position of the butterfly. This structure leads to the prediction that appropriately substituted tribenzosuberenyl groups will be chiral. In fact, optically active compounds based upon such structures have been prepared25 and a barrier to thermal racemization of ca. 31 kcal/mol has been measured experimentally. This finding is relevant to the analysis of the photophysical behavior of 1 since racemization of these compounds probably proceeds through a transition structure where the tribenzosuberenyl group has become planar. 2. The ExcitedS i e t Stateof 1. The most remarkable feature of the behavior of the excited singlet state of 1 is that it is unremarkable. The absorption spectrum of 1 is essentially identical with that of alkyl-substituted tetraarylethylenes. However, while the fluorescence of tetraphenylethylene shows an extraordinary temperature-dependent Stokes shift, the fluorescence of 1 is independent of temperature and the Stokes shift is modest. Similarly, studies of the decay of the tetraphenylethylene excited singlet state clearly show two components-an initial fast relaxation attributed to thevertical excited state and a slower, solvent-polaritydependent component attributed to the perpendicular excited state. In contrast, 1 shows only a single "slow", solvent-polarityindependent component in its excited singlet state decay whether probed by fluorescence or by time-resolved absorption. The spectroscopic behavior of 1 is simply interpreted. The excited state detected by fluorescence and by time-resolved absorption is assigned to the vertical, Franck-Condon, singlet state. Its lifetime, 4.5 ns, is nearly loo0 times longer than that of the comparable state of tetraphenylethylene. This increase in lifetime of 1 must be attributed either to the imposition of a barrier for formation of its relaxed singlet state, which must be nonemissive, or to a barrier for formation of the perpendicular state. The spectroscopicfeatures of 1are therefore more typical
of a "rigid" chromophoric system than they are of a conformationally mobile compound such as stilbeneor tetraphenylethylene. 3. The Potential Energy Surface for the Singlet State of 1. It is useful to consider the excited singlet state potential energy surface proposed for tetraphenylethylene when considering that for 1. Figure SA is a representation of the singlet surface for tetraphenylethylene based on the recent report of Ma and Zimmt.15 The reaction coordinate in this figure is not specified exactly but consists, at least, of ethylenic torsional motion, torsion about 4, and changes in some bond lengths and angles. The key point we emphasize here is that the excited-state surface is relatively flat-the barriers for conversion between the vertical (Slv), relaxed (SIR),and perpendicular (lP*) states are small, and the potential well for the perpendicular state must be shallow since Ma and Zimmt report that it is in equilibrium with SIR. Moreover, the energy gap between the excited perpendicular state and the ground perpendicular state (1P)for tetraphenylethylene is solvent-polarity dependent and is relatively large since the lifetime of the excited state can be rather long. A possible singlet potential energy surface for 1 based on our spectroscopic and chemical observations is shown in Figure 5B. It differs from the surface depicted for tetraphenylethylene in three significant ways. First, the Franck-Condon state (Slv) sits in a comparatively deep potential energy well. The study of the temperature dependence of the fluorescence of 1 shows that the activation barrier of its decay is 5.2 kcal/mol. Second, we do not include a minimum in the surface of 1 for the relaxed state S I R since there is no evidence for its existence. In fact, as will be discussed later, the equivalent to S I R in this case may represent a maximum on the excited surface. Third, the excited perpendicular state lP*of 1 exists in a deep potential energy well close to the energy of the ground perpendicular state 1P. Unlike tetraphenylethylene, there is no evidence for the 1P*state in the time-resolved absorption spectra we record for 1. The differencesbetween the proposed potential energy surfaces for tetraphenylethylene and for 1 originate from two unique structural features of 1. First, there is a significant energetic barrier to planarization of the tribenzosuberenyl group. Fox and Schulzl have shown that imposition of a barrier to rotation about
Photoisomerization of Arylethylenes the torsional angle I#J increases the lifetime of S1v for tethered ethylenes. This fiiding was interpreted to indicate that this phenyl ring motion is the largest contributor to the deactivation of the Slvstate. It is reasonable to assume that this is the motion leading to formation of S I R and that the additional conjugation caused by increasing the planarity of the ethylenic carbon atoms and the phenyl groups will stabilize the transition structure for rotation about the ethylenic bond that leads to the zwitterionic or biradial IPSstate. The structure for 1calculated by the MMX force field indicates that the ethylenic carbon atom and the phenyl groups incorporated in the fluorenyl unit are essentially planar in the ground state. If planarization about only one of the ethylenic carbon atoms was sufficient to stabilize 1P* and promote rotation about the ethylenic double bond then the lifetime of Slv for 1 should be shorter than it is for tetraphenylethylene. Since the lifetime of 1 is ca. 1000 times longer than that for tetraphenylethylene, it is clear that planarization of both ethylenic carbon atoms is required to facilitate formation of lP*. Consequently, we attribute the long lifetime for the vertical singlet state of 1 to the barrier to planarization of the tribenzosuberenyl group. Complete planarization of this group in the ground state must overcome a 31 kcal/mol barrier. The barrier height may be somewhat less in the excited state. A possible singlet potential energy surface for 1 is presented graphically in Figure 5C and D. Figure 5C is a schematic representation of the crossing between the B, (SI)and A, (S,) singlet states of tetraphenylethylene. The key point is that the SIsurface is shallow indicating a low barrier to planarization. In contrast, the potential surface for planarization of the tribenzosuberenyl group of 1 in its S1 state, shown in Figure 5D,is steeper. Simply put, the motion controlling the lifetime of the vertical singlet state of 1is not torsion about its ethylenic double bond but planarization of the tribenzosuberenyl group. This proposal extends the conclusion of Fox and Shultzll to require the planarization of both ethylenic carbon atoms to facilitate formation of the perpendicular state. Also, unlike tetraphenylethylene, the greater barrier to planarization of 1 appears to remove the minimum energy conformation assigned to S I R . Lastly we address the issue of the electronic configurations for IP* and 'P. In particular, we sought evidence for an inversion from tetraphenylethylene with the ground state taking on zwitterionic character. Some evidence in support of this view comes from the insensitivityof the singlet lifetime of 1 to solvent dielectric constant. If the transition structure leading to lP* reflects the polar character of a zwitterionic state, then there might be a measurable solvent effect. Since there is not, either the lP* state is not zwitterionic or the far greater part of the barrier leading to this transition structure is due to planarization of the tribenzosuberenyl group. We favor the latter explanation since we can detect no net photochemistry when 1 is irradiated in methanol. If 1P were zwitterionic it should be trapped by the solvent under these conditions.
Conclusions Tetraarylethylene 1 has unique spectroscopic properties that reveal features about the potential energy surface for its singlet
The Journal of Physical Chemistry, Vol. 97, No. 34, 1993 8717 state. Thevertical, Franck-Condon, state is relatively long lived. This observation requires that there be a barrier to molecular motions leading to formation of the perpendicular state. This barrier is not due to torsional rotation about the ethylenic double bond or planarization of the fluorenyl group but is attributed to planarization of the tribenzosuberenyl group. This finding requires that both ends of the ethylenic system become planar before rotation about the double bond occurs. Further, even though the fluorenyl and tribenzosuberenyl groups strongly stabilize the zwitterionic perpendicular state, this configuration is still the excited rather than the ground state.
Acknowledgment. This work was supported by grants from the National Science Foundation and from the Army Research Office for which we are grateful. References and Notes (1) Saltiel, J.; Sun, Y.-P. In Photochromism: Molecules and Systems; Durr, H., Bouas-Laurent, H., Eds.; Elsevier: Amsterdam, 1990, p 64. (2) Pederson, S.; Banarcs, L.; Zewail, A. H. J. Chem. Phys. 1992, 97, 8801. (3) Gegiou, D.; Muszkat, K. A,; Fischer, E. J . Am. Chem.Soc. 1968,90, 13. Sun, Y.-P.; Saltiel, J. J . Phys. Chem. 1989, 93, 8310. (4) Fischer, G.; Seger, G.; Muszkat, K. A,; Fischer, E. J. Chem. Soc., Perkin Trans. 2 1975, 1569. (5) Saltiel, J. J. Am. Chem. SOC.1967, 89, 1036. (6) Orlandi, G.; Siebrand, W. Chem.Phys. Lett. 1975,30,352. Orlandi, G.; Palmeri, P.; Poggi, G. J. Am. Chem. Soc. 1979, 101, 3492. Negri, F.; Orlandi, G. J. Phys. Chem. 1991, 95, 748. (7) Troe, J.; Weitzel, K.-M. J . Chem. Phys. 1988, 88, 7030. (8) Greene, B. I. Chem. Phys. Lett. 1981, 79, 51. (9) Doany, F. E.; Heilweil, E. J.; Moore, R.; Hochstrasser, R. M. J. Chem. Phys. 1984,80,201. (10) Barbara, P. F.; Rand, S. D.; Rentzepis, P. M. J. Am. Chem. Soc. 1981, 103,2156. (11) Shultz, D. A.; Fox, M. A. J. Am. Chem. SOC.1989,111,6311. (12) Schilling, C. L.; Hilinski, E. F. J . Am. Chem. Soc. 1988,110,2296, (13) Sun, Y.-P.; Fox, M. A. J. Am. Chem. 1993, Z15, 747. (14) Schuddeboom, W.; Jonker, S.A.; Warman, J. M.; De Haas, M. P.;
Vermeulen, W. J. M.; Jager, V. F.; de Lange, B.; Feringa, B. L.; Fasenden, R. W. J. Am. Chem. Soc. 1993, 115, 3286. (15) Zimmt, M. B. Chem. Phys. Lett. 1989, 160, 564. Morais, J.; Ma, J.; Zimmt, M. B. J . Phys. Chem. 1991,95, 3885. (16) Ma, J.; Zimmt, M. B. J. Am. Chem. Soc. 1992, 114, 9723. (17) Personal communicationwith Professor Matthew Zimmt, Department of Chemistry, Brown University. (18) Saltiel, J.; Waller,A.; Sun, Y.-P.;Sears, D. F., Jr. J . Am. Chem.Soc. 1990,112, 4580. (19) Lapouyade, R.; Czescha, K.; Majenz, W.; Rettig, W.; Gilabert, E.; Rulliere, C. J. Phys. Chem. 1992, 96, 9643. (20) Zhu, Y.; Koefed, R. S.;Devadosss, C.; Shapley, J. R.; Schuster, G. B. Inorg. Chem. 1992,31, 3505. (21) Tochtermann, W.; Oppenlander, K.;Walter, U. Chem. Ber. 1964, 97, 1318. Treibs, W.; Klinkhammer, H.-J. Chem. Ber. 1951, 84, 671. (22) Feringa, B. L.; Jager, W. F.; de Lange, B. Tetrahedron Lett. 1992, 33, 2887. (23) Traetterberg, M.; Frantsen, E. B.; Mijhoff, F. C.; Hoekstra. J. J . Mol. Struct. 1975,26,56. Traetterberg, M.; Frantsen, E. B. J. Mol. Struct. 1975, 26, 69. (24) Benson, S.W.; Cruickshank, D. M.; Golden, D. M.; Haugen, G. R.; ONeal, H. E.; Rodgers, A. S.; Shaw, R.; Walsh, R. Chem. Rev. 1969, 69, 279. (25) Bergman,E. D.;Klein, J.J. Org. Chem. 1958,23,512. Tochtermann, W.; Kuppers, H. Angew. Chem. 1965, 77, 173.
