The Journal of
Physical Chemistry
0 Copyright, 1991. by the American Chemical Society
VOLUME 95, NUMBER 10 MAY 16, 1991
LETTERS Solvent Dependence of the Twisted Excited State Energy of Tetraphenyiethyiene: Evidence for a Zwitterionic State from Picosecond Optical Calorimetry Joseph Morais, Jangseok Ma, and Matthew B. Zimmt* Department of Chemistry. Brown University, Providence, Rhode Island 0291 2 (Received: July 30, 1990: In Final Form: February I I , 1991)
The energy and decay rate constant of the twisted excited singlet state of tetraphenylethylene are determined as a function of solvent by using picosecond optical calorimetry. The energy of the twisted excited state, relative to the planar ground state, in alkane solvents, diethyl ether, and tetrahydrofuran are 67.0 1.3,66.2 1.4, and 65.3 1.7 kcal/mol, respectively. A dipole moment of 6 D for the twisted excited state is obtained from a dielectric continuum analysis of the solvent dependence of the twisted state energy. Using the activation energy for thermal cis-trans isomerization to obtain an estimate of the twisted ground state energy, a linear relationship is found between the logarithm of the decay rate constant and the energy gap at the twisted geometry. These results confirm a previous explanation for the solvent-dependent lifetimes of the twisted state and provide evidence for zwitterionic character in the lowest energy excited singlet state of tetraphenylethylene.
*
*
Introduction The electronic nature of the lowest energy, twisted, excited, singlet state of alkenes, the *phantom" state 'p*, has been the subject of considerable interest.l The phantom state lies within the potential energy surface funnel responsible for the cis-trans photoisomerization of alkenes2 Extensive theoretical calculations3 for ethylene, the parent alkene, predict that the 'p* state is zwitterionic. The lowest energy geometry exhibits C, symmetry, with one carbon pyramidalized, and a dipole moment exceeding 2 D. Conjugated substituents introduce (r,r*)excited diradical
electronic configurations which, potentially, are lower in energy than a zwitterionic state at the twisted geometry. On the basis of a b initio gas-phase calculations, Malrieu' has suggested that the Ip* states of polyenes and arylalkenes with three or more double bonds have excited diradical configurations. Experimental evidence for zwitterionic Ip* states exists in a number of alkene system^.^ Recently, Schilling and Hilinski6 reported that the Ip* lifetime of tetraphenylethylene (TPE) decreases with increasing solvent polarity. Solvent polarity was quantified by using Dimroth's ET(30) parameter.' The authors
(1) (a) Saltiel. J.; Charlton, J. L. Rearrangement in Ground and Excited States: deMayo, P., Ed.:Academic Press: New York, 1980 Vol. 3, p 25. (b) Mulliken, R. S.Phys. Reu. 1932,4I,751. (c) Saltiel, J. J . Am. Chem. Soc. 1968, 90,6394. (d) Salem, L. Science 1976. 191, 822. (e) Doany, F. E.; Heilweil, E. J.; Moore, R.: Hochstrasser, R. M. J . Chem. Phys. 1984,80,201. (2) Michl, J. Mol. Photochem. 1972,4, 243. (3) (a) Brooks, B. R.; Schaefer, H . F. 111 J . Am. Chem. Soc. 1979,101, 307. (b) Buenker, R. J.; Peyerimhoff, S.D. Chem. Phys. 1976,9, 7 5 .
(4) (a) Nebot-Gil, 1.: Malrieu, J.-P. J . Am. Chem. Soc. 1982. 104, 3320. (b) Nebot-Gil, 1.; Malrieu. J.-P. Chem. Phys. Lett. 1981, 84,571. (5) (a) Maeda, Y.; Okada, T.; Mataga, N. J . Phys. Chem. 1984,88,2714. (b) Dauben, W. G.: Ritscher. J. S. J . Am. Chem. Soc. 1970, 92, 2925. (c) See ref 2 in ref 6 below. (d) Klett, M. W.: Johnson, R. P. Tetrahedron Lett. 1983.24, 1107. (6) Schilling, C. L.; Hilinski, E. F. J . Am. Chem. Soc. 1988, 110, 2296. (7) Reichardt, C.; Dimroth, K. Fortschr. Chem. Forsch. 1968, 11. 1.
0022-365419 1 12095-3885$02.50/0
0 1991 American Chemical Society
3886 The Journal of Physical Chemistry, Vol. 95, No. 10, 1991
Letters
TABLE I: Solvent d TetrapbcnykthykwParameters ET(W? solvent pentane
hexane
4:'
E('p*),b
k,,,b*
a,bd
i3,bJ
U,,bJ
109 s - ~
IO' s-'
0.595 f 0.026
4.1 f 0.3
0.830 f 0.013
67.4 f 1.2
0.619 f 0.019
4.6 f 0.3 (5.1) 5.8 f 0.3
107 s-1 2.1 f 0.5 (2.1) 2.2 0.4 (2.5) 1.9 0.3 (2.3) 2.1 f 0.4 (2.3) 3.0 f 0.2
m/s
0.808 f 0.012
kcal/mol 66.8 f 1.1
kcal/mol
€10
rf
mL/kJ
fb
31.0
1.84
1.3575
1.11
31.0
1.89
1.3751
0.912
nonane
31.0
1.97
1.4054
0.678
0.854 f 0.008
66.9 f 0.7
0.687 f 0.015
diethyl ether
34.6
4.34
1.3526
0.970
0.813 f 0.015
66.2 f 1.4
3.66 f 0.08
tetrahydrofuran
37.4
7.32
1.4050
0.657
0.843 f 0.019
65.3 f 1.7
11.0 f 0.2 (9.1 f 1.0)
2.9 f 0.1 (3.0) 11.0 f 0.3
*
995 (1006)
1070
(1077) 1190 ( 1206) 981 (975) 1320
oValuesfrom refs 23 and 27e. bError limits represent 1 obtained from multiple fits of at least six separate data sets at an excitation angle of 50'. Sample temperature was 23 f 1 O C for these experiments. CValuesin parentheses are from ref 6. dValues in parentheses are from ref 25a. cValues in parentheses are from ref 26. /Values in parentheses are from refs 22 and 25b. (I
proposed that their observations could be explained using radiationless relaxation theory: provided the Ip* state was zwitterionic. Dipolar interaction with the solvent lowers the Ips state energy more than the energy of the twisted, diradical, ground state, Ip, to which relaxation is presumed to occur.1*2The reduced energy gap results in more rapid radiationless decay.* The authors6 noted that the picosecond absorption spectra of Ip* were consistent with the zwitterionic assignment, but no information on Ip* energetics was available. This Letter describes the results of picosecond optical calorimetry studies on TPE that quantitate the solvent dependence of the Ip* state energy and establish a linear correlation between the logarithm of the Ip* radiationless decay rate constant and the Ip*-'p energy gap. These experiments demonstrate the zwitterionic nature of 'p* in TPE.
Experimental Section The picosecond optical calorimetry waveforms are proportional to the time-dependent diffraction efficiency of the optically generated phase grating9 The grating is generated from two excitation beams (317 nm, 200 nJ, 400 pm, 37 ps) which are crossed a t an angle 8 = 50' in a 1-mm path length Suprasil flow cell. The diffracted signal from the nonresonant probe beam (634 nm, 200-450 nJ, 220 pm, 52 ps) was spatially isolated, filtered (Hoya R-62,2 ND-13), and detected by a Hammamatsu R928 phototube (550 V) terminated in 3 Mohm and a lock-in amplifier. The probe beam delay (-0.8 to 8.0 ns) was obtained with an optical delay stage (Gaertner Scientific). Tetraphenylethylene (Aldrich) was recrystallized twice from ethanol/benzene and sublimed twice. Solvents were distilled from CaH, and had optical densities less than 0.001 (317 nm) in a 1-mm cell. TPE solution optical densities ranged from 0.8 to 1.2 (317 nm) in a I-mm cell. The basic equations for the calculation of picosecond optical calorimetry diffraction waveforms have been described elsewhere.'& The method employed to obtain calorimetric quantities from the experimental data is described below. The experimental waveforms were fit by minimizing x2 IIpusing Powell's method.llb The convergence criterion was a fractional tolerance of IO4. Parameters resulting from each fitting procedure were reintroduced as initial guesses to check for local stability. Each data set was refit starting a t five or more locations in parameter space. The results reported for each parameter represent averages of the best fits from at least six independent experiments. Eight parameters are extracted from the fitting procedure: A, a waveform amplitude (8) (a) Englman, R.; Jortner, J. Mol. Phys. 1970.18, 145. (b) Freed, K. F.; Jortner, J. J . Chem. Phys. 1970, 52, 6272. (c) Siebrand, W . J . Chem.
Phys. 1967. 46, 440. (9) (a) Fayer, M. D. IEEE J . Quanrum Elecrron. 1986, QE-22. 1437. (b) Pohl, D. W.;Schwarz, S. E.; Irniger, V. Phys. Reo. Lett. 1973, 31, 32. (IO) (a) Miller, R. J. D. Time Rcsolved Spectroscopy. In Advances in Spcrrarcopy; Clark, R. J. H., Hater, R. E.,Eds.;John Wiley and Sons: New York, 1989 Vol. 18, p I . (b) Nelson, K. A.; Fayer, M.D. J. Chem. Phys. 1980, 72, 5202. (c) Zimmt, M. B. Chem. Phys. Lerr. 1989, 160, 564. (11) (a) Bevington, P. R. Data Reduction and Error Analysis in the Physical Sciences; McGraw-Hill: New York, 1969; Chapters 5,6. (b) Press, W. H.; Flannery, B. P.;Teukolsky, S. A.; Vetterling, W . T. Numerical Recipes; Cambridge University Prcss: Cambridge, 1988; Chapter 10.5. p 294.
scaling factor;f, the ratio of (1) the volume change produced by the conversion of the twisted excited state to the planar ground state to (2) the volume change produced by the conversion of the photon energy to heat; k,,, the twisted state decay rate constant; o,the acoustic wave frequency; CY,the acoustic wave attenuation time constant; 8, the thermal diffusion time constant; At, the delay line position o f t = 0; B, the real index contribution from the Ip* state at 634 nm. Once w has been determined, the sound velocity, CY,and /3 can, in principle, be obtained from the literature. Where available, literature and extracted values agreed to within 20%. At varied by less than 8 ps between data sets. (The digital resolution of the delay line is 3 ps/point.)
Results and Discussion The theory of picosecond optical calorimetry,l& a form of transient grating spectroscopy,Iqvb has been described previously. The experimental observable, the diffracted intensity from the time delayed, nonresonant probe pulse, is proportional to the square of the grating peak-null difference in the real component of the refractive i n d e ~ . ~The J ~ principal sources of real refractive index changes in these experiments are (1) thermal expansion resulting from photoinduced, exothermic radiationless relaxationlo and (2) differences in the molar volume of TPE ground and excited states. Both contributions weakly modulate the solvent volume (density) and, thereby, the refractive index. Solvent volume changes accompanying the formation and the decay of Ip* are readily distinguished by the large difference in the rate constants of these two radiationless transitions (vide infra). A third contribution to the grating peak-null refractive index difference has the following characteristics. It is positive in the grating excitation peaks, present at t = 0, and decays with the same rate as Ip*. We assign this to a weak dispersive contribution from the TPE Ip* absorption spectrum? Since the magnitude of this contribution changes less than 10% between -50 and 23 O C in diethyl ether, it cannot arise from an equilibrium between 430 nm) and the planar excited state (A,,,= 600 the Ip* state (A, nm). Radiationless relaxation from the vertically excited state generates the Ip* state of TPE within 6 ps.I2 The heat released per unit volume by this relaxation, Hf, is the product of the quantum yield of Ip* formation, cp, the number density of excited states, N, and the energy difference, (hu - E('p*)); E('p*) is the energy of the Ip* state relative to the planar ground state, and hu is the excitation photon energy. The photoinduced conversion of So into Ip* is also accompanied by a change in the molecular volume, A V = (V(lp*) - V ( S , ) ) . The volume change resulting from the production of Ip* is Vf = N q [ ( h u - E('p*))X, + Av]. X,is the thermal expansivity of the solvent. Radiationless relaxation from Ip* to 'p is the rate-limiting step in the re-formation of the planar ground state."I2 The volume change attending the relaxation from Ip* to the planar ground state is V, = Ncp[(E(lp*))X, - Av]. The combined quantum yield of fluorescence, intersystem crossing, and dihydrophenanthrene formation is less than 0.05.6-'3 Thus, ( 1 2 ) Greene, B. 1. Chem. Phys. Le??. 1981, 79, 51.
The Journal of Physical Chemistry, Vol. 95, No. IO, 1991 3887 3000
2000 t OW
0 0
2
6
4
3000 2000
1
DEE 0
2
3000
0
I " " I "
'-j
68
2
0
4
0.6 X, (ml/kJ)
0.8
1
1.2
IT
h
6
Delay (nrec)
66
Figure 1. Experimental (points) and calculated (solid line) picosecond optical calorimetry waveforms for TPE in pentane (PN), diethyl ether (DEE),and tetrahydrofuran (THF). X-axis units are in nanoseconds. The full time base for the THF data is 1.5 ns less than for PN and DEE. The Y-axis units are arbitrary.
. Y
v
65
h
E,
n
I
w
(1)
The experimental and best fit calculated diffraction waveforms for TPE in pentane (PN), diethyl ether (DEE) and tetrahydrofuran (THF) are presented in Figure 1. The grating wavevector, K , in the 6 = 50' experiments was 1.676 X lo' m-I. Table I lists solvent properties and the averaged best fit values of E('p*), k,,, a,0, and u, for each solvent. The sound velocities, v,, were obtained from the simulations as W / K . The statistical uncertainties infi the Ips decay rate constants, and sound velocities are less than 5% and less than 20% in the acoustic attenuation and thermal diffusion time constants. Both E(lp*) and AV in eq t are unknown. Following the precedent of Peters, Choy, and Goodman,*I these quantities may (13) (a) Goerner, H. J . fhys. Chem. 1982, 86, 2028. (b) Olsen, R. J.; Buckles. R. E. J . Photochem. 1979, IO, 215. (c) Sharafy, S.;Muszkat, K. A. J . Am. Chem. Soc. 1971, 93,4119. (14) Dihydrophenanthrene formation is an activated The shorter lifetime and decreased energy in the more polar solvents should decrease DHP quantum yields. Thus, to the extent that DHP is formed, the actual value offin the least polar solvents could k higher than reported here. A large quantum yield of relaxed fluorescence would appear as an artifically lowfvalue, also. Neither effect is significant in this system. ( I 5) The variation of E('p*) is monotonic in solvent dielectric constant and nonmonotonic in solvent refractive index. Thus, dipoltdipole interactions are primarily responsible for the solvent-induced energ changes. Currently, we cannot distinguish whether the dipole moment of Yp* is intrinsic (see ref 3) or induced as a consequence of its very large polarizability (see Wulfman, C. E.: Kumei, S.Science 1971, 172, 1061). (16) (a) &ens, H.; Knibk, H.; Weller, A. J . Chem. fhys. 1%7,47, 1183. (b) For these experiments, the relevant quantity is the solvation enthalpy. The enthalpy changes are normally I0-20% larger than the free energy changes.Iw (c) Marcus, Y.J. Chem. Soc., Faraday Trans. I 1987,83,339. (d) Marcus, Y.J. Chem. Soc., Faraday Trans. I 1987,83, 2985.
= 68.4 kcal/mole
Slope = -7.3 kcal/mole
v
formation of Ip* and its radiationless decay to the ground state are the only significant sources of volume changes. The ratio VJ( V, + V f ) is therefore equated to the fitting parameter f. Equation 1 provides a relationship betweenf, E('p*), and AV. f X h v = XsE('p*) - AV
0.4
0.2
Figure 2. Plot of the molar expansion (mL/mol) accompanying the decay of the twisted excited state, huXd as a function of thermal expansivity, X, (mL/J), in the alkane solvents. Error bars represent one standard deviation based on the experimentally determined f.
6
4
0
63
0.1
0.2
0.4
0.3 ( EB- 1 )/(zss+
0.5
I)
+
Figure 3. Plot of E('p*) vs (e, - I)/( 1 2%) and the best fit line through the data. Error bars represent one standard deviation based on the experimentally determinedfin these solvents. The values of the abscissa and the ordinate for the alkane solvents have been averaged.
be determined through variation of X, and analysis according to eq 1. To minimize changes in 'p*solvent interactions, only the results from alkane solvents are included in this correlation (Figure 2). From the slope, E(lp*) in alkane solvents is 67.0 f 1.2 kcal/mol, and from the intercept, AV = -29 f 7 mL/mol. The significantly smaller molar volume of Ip* suggests an increase of alkane-TPE interactions upon formation of the twisted excited state. Independent evaluation of AVin a single ether solvent is not easily achieved. By use of eq 1, the experimentallydetermined ffor each ether, and AVmeasured for TPE in the alkanes, E('p*) in diethyl ether and T H F are estimated to be 66.2 f 1.1 and 65.3 f 1.8 kcal/mol, re~pectively.~~ The energy of the TPE twisted excited singlet state, E('p*), decreases with increasing solvent polarity. Although the energy differences between solvents are comparable to the uncertainties, ~
~
~~~~
(17) The molecular volume of TPE is 326 A'. The equivalent volume sphere has a 4.27-A radius. (18) (a) Caspar, J. V.; Meyer, T. J. J . Am. Chem. Soc. 1983. 105, 5583. (b) Caspar, J. V.;Meyer, T. J. J . fhys. Chem. 1983, 87. 952. (c) Caspar. J. V.; Kober, E. M.; Sullivan, B. P.; Meyer, T. J. J . Am. Chem. Soc. 1982, 104,630. (d) Siebrand, W. J . Chem. fhys. 1967, 47, 241 I . (19) Leigh, W. J.; Arnold, D. R. Can. J . Chem. 1981, 59, 609. The activation energy differs by less than 0.25 kcal/mol for benzene (ET(30) = 34.5) or benzonitrile ( E ~ ( 3 0 = ) 42.0) as solvent. (20) The electronic coupling between Ip* and 'p could vary with solvent. We do not have sufficient information to comment on this possibility. (21) (a) Westrick, J. A.; Goodman, J. L.; Peters, K. S.Biochemistry 1987, 26, 8313. (b) Leung, W. P.; Cho, K. C.; Chau, S. K.; Choy, C. L. Chem. fhys. Le??.1987, 141,220. (c) Herman, M. S.;Goodman, J. L. J . Am. Chem. SOC.1989, 111, 1849.
1; 1 \
4
3888 The Journal of Physical Chemistry, Vol. 95, No. 10, 1991 25
the charge distribution in the solvated Ip* state is described by 5 the transfer of an electron between the rotated diphenylmethyl SlopeWork = -0.89 - 0(kcal/mole)-'
h
r"
Y
E
22 21
I
Dielectric Continuum Model Minimum Energy Gap: 27.2 kcal/mole Minimum Lifetime: 40 ps ( a )
19
Letters
27
28
E('p')-E(
29
'p)-Es
30
31
32
(kcal/mole)
Figure 4. Plot of In (knr)vs Ip*-lp energy gap for TPE in nonpolar and
weakly polar solvents.
the trend is apparent.24 The variation of E(lp*) with solvent is approximately -0.3 times the change in ET(30), indicating a significant difference in the dipole moments of Ip* and the ground ~ t a t e . ~Since , ' ~ the latter has no dipole moment, the Ip* state of TPE must be zwitterionic in the weakly polar solvents. If one uses a dielectric continuum model, the solvation free energy16 of a dipole, p, centered in a spherical cavity of radius a within a dielectric with static constant c, is -IpZrxn/21. The reaction field generated within the cavity by the induced polarization of the dielectric is The intercept and slope of the best fit h e l l a (Figure 3) of a plot of E(lp*) vs (c, - 1)/(1 + 2 4 provide estimates of the Ip* gas-phase energy (68.4 f 3.4 kcal/mol, cs = 1) and of -p2/a3 (-7.3 f 2.8 kcal/mol), respectively. Using a = 4.27 & I 7 we obtain a dipole moment of 6.3 D for Ip*. This is comparable to the dipole moment generated from unit positive and negative charges separated by the distance of a C-C single bond and indicates that (22) (a) Handbook of Chemisrry and Physics; Weast, R. C., Ed.; CRC Press: Boca Raton, FL. 1984; p E-43. (b) Braslavsky, S.; Heihoff, K. Handbook oforganic Phorochemisrry;Scaiano,J. C., Ed.; CRC Press: Boca Raton, FL, 1989; Vol. I , p 327. (23) (a) The Chemist's Companion; Gordon, A. J., Ford, R. A,, Eds.; John Wiley & Sons: New York, 1972; pp 1-13. (b) Oevering, H.; Paddon-Row, M. N.; Heppener, M.;Oliver, A. M.;Cotsaris, E.;Verhoeven, J. W.; Hush, N. S . J. Am. Chem. Soc. 1987, 109, 3258; The Aldrich Catalog, Aldrich Chemical Company: Milwaukee, 1990. (24) The error limits represent one standard deviation for the distribution of individual experimental determinations of/. The uncertainty in the mean value is the standard deviation divided by the square root of the number of independent determinations.11'Thus, the uncertainty in the auerage values is smaller by a factor of from 2.4 to 3.0. The contribution to the error limits originating from uncertainty in AV(alkanes) amounts to 1.5, 1.8, 2.5, 1.7, and 2.5 kcal/mol in pentane, hexane, nonane, diethyl ether, and THF, respectively, and is not included in the stated error limits since any deviation from the actual value shifts the calculated 'p' energy in each solvent in the same direction and by nearly q u a l amounts. (25) (a) Schaafs, W. Landoli-Boernstein N.S.,Group f l Atomic and Molecular Physics: Hellwege, K. H., Hellwege, A. M., Eds.;Springer-Verlag:
Berlin. 1967: Volume 5, Chapter 3. (b) Reference 25a, Chapter 2. (26) (a) Palavra, A. M. F.; Wakeham, W. A.; Zalaf, M. fnl. J . Thermophys. 1987.8. 305. (b) Kashiwagi, H.; Oishi, M.; Tanaka, Y.;Kubota, H.; Makita, T. Int. J . Thermophys. 1982,3, 101. (c) Calado, J. C. G.; Fareleira, J. M. N.A.; Nieto de Castro. C. A.; Wakeham, W. A. fnr. J . Thermophys. 1983.4, 193. (d) D'Ans. J.; Bartels, J.; Bruggencate, P. Ten; Eucken, A,; J m , G.; Roth, W. A. Landoh-Boermtein: Zahlenwerre und Funklionen, 6th ed.; Borchers, H., Hausen, H., Hellwege. K. H.,Schaefer, KI., Schmidt, E., Eds.; Springer-Verlag: Berlin, 1968; Volume 2, Chapter 5. (27) (a) The thermal expansion coefficient, X,,is equal to the product of the cubic expansion coefficient and the molecular weight divided by the product of the density and the constant-prtssure heat capacity. Solvent density and molccular weight from ref 23. Cubic expansion coefficients from ref 27b-d. Heat capacities from ref 27d-c. (b) Brostow, W.Phys. Chem. Liquids 1972,3,91. (c) Obama, M.;Oodera, Y.; Kohama, N.; Yanase. T.; Saito, Y.; Kusano. K. J . Chem. Eng. Dara 1985,30, 1. (d) Marcus, Y . fon Soluarion; John Wiley and Sons: Chichester, 1985; pp 133-135. (e) Shaw, R. J . Chem. Eng. Data 1979, 14, 461.
units. It remains to discuss the quantitative relationship between E(Ip*) and the radiationless relaxation rate constant, knr. Englman, Jortner, and Freed* have derived expressions for the radiationless relaxation rate constant between two states separated by an energy gap, AE. Meyer and co-workerslgS* have demonstrated that this expression is consistent with the linear correlations observed between the logarithm of the nonradiative decay rate constant and emission maximum in numerous series of inorganic and organic molecules. The slopes from plots of In (/cnr) vs AE vary from -0.241gd to -0.83188-c (kcal/mol)-'. For TPE, the appropriate energy gap'.2 can be approximated as AE = E('p*) - (E(lp) + Es). E('p), the energy of the twisted ground state, is approximated as the activation barrier for thermal cis-trans isomerization of TPE, 35.5 kcal/mol.lg E,, the free energy cost1& of the solvent orientation polarization generated by the excited state dipole moment, is given by E, = ( p 2 / a 3 ) [ ( c -s 1)/(1
+ 2 4 - (n2 - 1)/(1 + 2nZ)]
(3)
where n is the solvent optical refractive index. The slope of -0.89 (kcal/mol)-', obtained from the plot of In (knr)vs AE for TPE in the solvents listed in Table I (Figure 4), is large but comparable to previously reported values. Thus, the solvent dependence of the Ip* decay rate constant is consistent with a change in the Franck-Condon factorsm caused by variation of the Ip*-'p energy gap. This correlation seems to apply even in the alkane solvents. The 'p* decay rate constant increases with increasing dielectric constant (pentane to nonane) despite the concomitant increase in the solvent viscosity. Schilling and Hilinski6 reported Ip* decay rate constants in acetonitrile and methanol of (6.7 f 3.3) X 1O'O and >5 X 1O'O s-I, respectively. The dielectric continuum model described above predicts an energy gap of 27.2 kcal/mol in acetonitrile or methanol and a maximum Ip* decay rate constant of 2.6 X 1O'O s-I. Our model underestimates the decay rate constants (by a factor of 2-3) in these strongly polar, organic solvents. As we are unable to resolve the formation and decay of 'p* in these solvents, we can not determine which part of the model is responsible for the discrepancy. It must be emphasized that the quantitative aspects of the dipole moment and energy gap analyses are based on the assumption of constant AVin solvents for which c varies from 1.7 to 7.3. The development of charge separation in Ip* should cause an apparent decrease of the TPE molecular volume due to electrastriction. This effect may be responsible, in part, for the large negative AV observed in Figure 2 and could vary with solvent polarity. We are not aware of an experimental method that enables separation of enthalpy and volume contributions in a single, nonaqueous solvent. In summary, the picosecond optical calorimetry technique has been used to determine the energies and decay rate constants of excited states with lifetimes as short as 100 ps. Analysis of the solvent dependence of the TPE twisted excited singlet state energy using a dielectric continuum model demonstrates that the Ip* state posesses a dipole moment of approximately 6 D, comparable to the moment obtained by the transfer of an electron between the twisted alkene carbons. The variation of the Ip* decay rate constant in nonpolar and weakly polar solvents is consistent with the predicted variation of the Ip*-'p energy gap and previously observed rate-energy gap correlations. These results provide evidence for zwitterionic character in the twisted singlet excited state of tetraphenylethylene. Acknowledgment is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for partial support of this research. Financial support from the Camille and Henry Dreyfus Foundation, Brown University, and from an NSF Presidential Young Investigator Award to M.B.Z. are gratefully acknowledged. We also thank Professors Hilinski, Stratt, Lawler, and Goodman for numerous, informative discussions and the reviewers for important suggestions.
