J. Am. Chem. SOC.1994,116, 10477-10485
10477
Singlet State Cis,Trans Photoisomerization and Intersystem Crossing of 1-Arylpropenes Frederick D. Lewis,’ Dario M. Bassani, Richard A. Caldwell,’ and David J. Unett Contribution from the Departments of Chemistry, Northwestem University, Evanston, Illinois 60208-3113, and The University of Texas at Dallas, Richardson, Texas 75083 Received July 12, 1994@
Abstract: The temperature dependence of the singlet state lifetime and photoisomerization and fluorescence quantum yields for trans- and cis- 1-phenylpropene have been determined in hexane solution. Calculated barriers for twisting about the double bond on the singlet potential energy surface are 8.8 and 4.6 kcaYmol for the trans and cis isomer, respectively. The barrier for the trans isomer is sufficiently high to prevent isomerization on the singlet state surface at or below room temperature. However, isomerization occurs at low temperatures as a consequence of intersystem crossing to the triplet state, which undergoes barrierless isomerization. The quantum yield for intersystem crossing, as determined by time-resolved photoacoustic calorimetry, is 0.60 f0.03 and the rate constant for intersystem crossing is 4.7 x lo7 s-l. While intemal conversion is not significant at or below room temperature, thermally activated intemal conversion competes with singlet isomerization at high temperatures. The cis isomer undergoes isomerization predominantly via the singlet state at room temperature. Both electron-donating(p-methoxy) and electron-withdrawing (m- and p-cyano, p-carbomethoxy, and p-trifluormethyl) aromatic substituents are found to lower the barrier for singlet state isomerization. Increased solvent polarity (acetonitrile vs hexane) results in variable decreases in the barrier for singlet state isomerization. Photoisomerization of the p-cyano derivative at room temperature occurs predominantly via the triplet state in hexane solution and via the singlet state in acetonitrile solution. The effects of substituents and solvent are better correlated with the magnitude of the energy gap than the stability of either zwitterionic or biradical intermediates. Rate constants for intersystem crossing are, in most cases, not highly dependent upon aromatic substitution or solvent polarity.
Introduction The behavior of the lowest electronically excited states of conjugated olefins has attracted the attention of photochemists, spectroscopists, and theoreticians, interested in elucidating the mechanism of double-bond photoisomerization. cis- and transstilbene (c-S and t-S) and their derivatives have been the subject of numerous investigations, and the nature of the stilbene excited singlet state and triplet state potential energy surfaces is understood in considerable detail. l s 2 In comparison, relatively little is known about the styrene excited singlet state potential energy surface, in spite of numerous spectros~opic~-~ and theoretical1°-14 investigations of the styrene singlet state. Photoisomerization cannot be experimentally investigated in Abstract published in Advance ACS Abstracts, October 1, 1994. (1) Saltiel, J.; Sun, Y.-P. In Photochromism, Molecules and Systems; Diim,H., Bouais-Laurent, H., Eds.; Elsevier: Amsterdam, 1990; p 64. (2) Waldeck, D. H. Chem. Rev. 1991, 91, 415. (3) Hui, M. H.; Rice, S . A. J. Chem. Phys. 1974, 61, 833. (4) Ghiggino, K. P.; Hara, K.; Salisbury, K.; Phillips, D. J. Chem. Soc., Farraday Trans. 2 1978, 607. (5)Lyons, A. L., Jr.; Turro, N. J. J. Am. Chem. Soc. 1978, 100, 3177. (6) Condirston, D. A.: Lauosa, J. D. Chem. Phys. Len. 1979, 63, 313. (7) Leopold, D. G.; Hemliy, R. J.; Vaida, V.; Roebber, J. L. J. Chem. Phys. 1981, 75, 4758. (8) Syage, J. A.; Al Adel, F.; Zewail, A. H. Chem. Phys. Lett. 1983, 103, 15. (9) Grassian, V. H.; Bernstein, E. R.; Secor, H. V.; Seeman, J. I. J. Phys. Chem. 1989, 93, 3470. (10) Fueno, T.; Yamaguchi, K.; Naka,Y. Bull. Chem. Soc. Jpn. 1972, 45, 3294. (11) Bmni, M. C.; Momicchioli, F.; Baraldi, I. Chem. Phys. Lett. 1975, 36, 484. (12) (a) Orlandi, G.; Palmien, P.; Poggi, G. J. Chem. Soc., Farraday Trans. 2 1981, 77, 71. (b) Michl, J.; Bonacic-Koutecky, V. Electronic Aspects of Organic Photochemistry; Wiley-Interscience: New York, 1990; p 310. (13) Said, M.; Malrieu, J.-P. Chem. Phys. Len. 1983, 102, 312. @
styrene; however, both its ,!?-deuterio3 and B-methyl derivativeslSJ6 are reported to undergo photoisomerization. While there have been suggestions that styrene can undergo twisting about its ethylenic bond on the singlet surface, current evidence indicates that the barrier for such twisting is too large to permit isomerization at room temperature. Several calculations of the styrene potential energy surface have resulted in values ranging from 4 to 25 kcaYmol for this The substantially longer singlet lifetime for styrene vs t-S (14.8vs 0.1 ns) permits intersystem crossing to compete with other singlet decay pathways for styrene, whereas intersystem crossing in not a significant decay pathway for t-S. Low quantum yields for intersystem crossing were reported by Zimmerman et al.17 for several phenylalkenes (ast I 0.11). Some years later Bonneaul* reported a value of aSt = 0.40for styrene and lower values (ast I0.14)for several phenylalkenes and phenylcycloalkenes. In contrast to the styrene excited singlet potential energy surface, the styrene triplet potential energy surface has been thoroughly investigated by Caldwell and co-workerslg and by Tokumaru and co-workers.20 The triplet states of trans- 1-phenylpropene (t-1) and cis- 1 -phenyl(14) Hemley, R. J.; Dinur, U.; Vaida, V.; Karplus, M. J. Am. Chem. SOC. 1985, 107, 836. (15) (a) Rockley, M. G.; Salisbury, K. J. Chem. Soc., Perkins Trans. 2 1973, 69, 1582. (b) Crosby, P. M.; Salisbury, K. J. Chem. Soc., Chem. Commun. 1975,477. (16) Lewis, F. D.; Bassani, D. M. J. Am. Chem. SOC. 1993, 115, 7523. (17) Zimmerman, H. E.; Kamm, K. S.; Werthemann, D. P. J. Am. Chem. SOC. 1974, 96, 7821. (18) Bonneau, R. J. Am. Chem. SOC. 1982, 104, 2921. (19) Caldwell, R. A.; Sovocool, G. W.; Peresie, R. J. J. Am. Chem. SOC. 1973, 95, 1496. (b) Ni, T.; Caldwell, R. A.; Melton, L. A. J. Am. Chem. SOC.1989, 111,457. (20) Arai, T.; Sakuragi, H.; Tokumaru, K. Bull. Chem. SOC.Jpn. 1982, 55, 2204.
0002-7863/94/1516-10477$04.50/0 0 1994 American Chemical Society
10478 J. Am. Chem. Soc., Vol. 116, No. 23, 1994 propene (c-1) are reported to undergo essentially activationless isomerization with quantum yields of 0.5, as expected for a perpendicular triplet which decays with equal probability to ground state t-1 and c-1. Salisbury and co-workers15 have investigated the room temperature fluorescence and photoisomerization of trans-1phenylpropene (t-1) and cis-1 -phenylpropene (c-1) both in the vapor phase and in solution. They concluded that isomerization occurs via intersystem crossing and that the barrier to twisting on the singlet state surface is sufficiently large (> 10 kcdmol) to prevent isomerization. We recently reported the results of an investigation of the temperature dependence of the singlet lifetimes and isomerization quantum yields of t-1 and c-1 in hexane solution over an extended temperature range.16 Analysis of these data indicated that the barriers for isomerization on the singlet state surfaces of t-1 and c-1 are 8.8 and 4.6 kcaY mol, respectively. As a consequence of these different barriers, isomerization of t-1 at room temperature occurs predominantly via intersystem crossing, as earlier suggested by Salisbury and co-worker~,'~ whereas isomerization of c-1 occurs predominantly via the singlet state. In addition to uncertainty over the barrier height for styrene singlet state isomerization, there has been disagreement as to the nature of the lowest twisted singlet state. Both zwitterionic (hole-pair)12 and diradical13 lowest twisted singlet states have been proposed by different groups of theoreticians. If the transition state for twisting on the singlet surface resembles the twisted singlet, investigation of the effects of aromatic substitutents and solvent polarity on the height of the singlet barrier might provide information about the nature of the twisted singlet. We report here the results of our investigation of the effects of temperature upon the fluorescence and photoisomerization of the 1-arylpropenes t-1-t-6 and c-1 in hexane and acetonitrile solution. Both aryl substituents and solvent polarity are found to influence the barrier for singlet state isomerization and hence the competition between singlet and triplet isomerization pathways. However, analysis of the Arrhenius parameters does not support a polar transition state for singlet state isomerization. Rate constants for intersystem crossing and fluorescence are independent of temperature whereas intemal conversion is temperature dependent and appears to be related to the singlet isomerization process. We have also determined the quantum yield for intersystem crossing in t-1 by means of time-resolved photoacoustic calorimetry and find a significantly higher value than that previously reported for styrene and phenylalkenes.
i
Q
X
f-1: X = H t - 2 : X = p-CN t-3: X = m-CN t-4: X = p-CO2CH3 t-5: X = p-CF3 t-6: X = p-OCH3
Results Electronic Spectra. The absorption spectra of c-1 and t-1 through t-621resemble that of styrene,1° consisting of a weak long-wavelength absorption band which displays some vibrational structure and a stronger, structureless band at higher energy. The wavelengths of the first maxima in the long(21) Available as supplementary material.
Lewis et al. Table 1. Observed Absorption Maxima and Results of INDOlS CI Calculations for the Electronic Transitions of Substituted 1-Atylpropenes
energy obs calcd oscstr
transition (nm) (nm) C-1
Si
Sz t-1
Si
Sz t-2
Si Sz
t-3
Si
t-4
Si
t-6
Sz Si
Sz
SZ
(au)
288 256 284 ' 284 250 260 300 287 268 263 298 291
0.008 0.700 0.002 0.814 0.009
254 300 274 298 260
0.553 0.004 0.763 0.016 0.695
248 294 281 288 252
description
+ +
0.34 [22-241 0.56 [23-251 [23-241 0.42 [26-281 0.50 [27-291 [27-281 0.25 [26-281 0.40 [27-291 0.16 [27-281 [27-281 0.45 [33-351 0.40 [34-361 [34-351 0.29 [28-301 0.62 [29-311 [29-301
+
+
+ +
wavelength bands and the maximum of the higher energy bands in hexane solution are reported in Table 1. Substituents shift both maxima to lower energy compared to t-1. The maxima of c-1 are at higher energy compared to t-1. In the more polar solvent acetonitrile the higher energy band is broadened, in some cases virtually obscuring the lower energy band. However, the band maxima show only minor solvent shifts (0-4 nm). In order to obtain further information about the effects of aryl substituents on the electronic structure and absorption spectra of these molecules, we have performed INDOE-SCFCI (ZINDO)22 calculations for several of the trans-l-arylpropenes. ZINDO calculations were not performed for t-5 due to the absence of accurate parameters for trifluoromethylsubstituted aromatics. The calculated energies and oscillator strengths for the two lowest energy singlet transitions and major configurations contributing to these transitions are reported in Table 1. In all cases it was found that the lowest energy transition possesses a low oscillator strength and can be described as a combination of the HOMO (LUMO 1) and (HOMO - 1) LUMO. The strongly allowed transition at higher energies can be described as a pure HOMO to LUMO transition. The molecular orbitals involved for t-1 are depicted graphically in Figure 1. The relative magnitudes of the coefficients for three highest occupied and three lowest unoccupied molecular orbitals of t-1 are shown in Figure 1. The appearance of the frontier orbitals for the other 1-arylpropenes is similar to those for t-1. The presence of substituents does not result in a change in the nature of the MO's involved in the two lowest energy transitions. The fluorescence spectra of the l-arylpropenes*ldisplay either two maxima or a high-energy shoulder and a single maximum in hexane solution. The wavelengths of the high-energy maxima or shoulders in hexane and acetonitrile solution are summarized in Table 2. As is the case for the absorption maxima, aryl substituents shift the fluorescence maxima to lower energy whereas increased solvent polarity (acetonitrile vs hexane) leads to broadening of the fluorescence but has little influence on the fluorescence maxima (0-3 nm shifts). No phosphorescence was observed from any of the 1-arylpropenes either at room temperature or when cooled to 77 K in a methylcyclohexane glass. Fluorescence quantum yields were determined at room temperature in dilute (< M) deoxygenated hexane solution
-
-
+
(22) (a) Bacon, A. D.; Zemer, M. C. Theor. China. Acra 1979, 53, 21. (b) %mer, M. C.; Loew, G. H.; Kirchner, R. F.; Mueller-Westemhoff, U. T. J. Am. Chem. SOC. 1980, 102, 589.
Photoisomerization of I -Andpropenes
J. Am. Chem. Soc., Vol. II6, No. 23, I994
10479
Table 3. Temperature Dependence of the Singlet Lifetime and Quantum Yields for Fluorescence and Isomerization of rruns- 1-Phenylpropene" hexane
T(K) 473 453 433 413 393 373 353 340 320 300 280 260 240 230 220
y2s
'f1?4
LUMO
acetonitrile
t(ns) 0.98 1.42 1.97 2.76 3.59
0.12 0.13 0. I4 0.14 0.15
11.6 12.1 12.5 12.8
0.13 0.14 0.13 0.13
0.30 0.30 0.3 1 0.32
13.2
0.14
0.32
@I
@r
5
(ns)
ai
2.63 4.15 5.50 6.28 8.85 0.3 8.14 1.6 1.8
0.3 1 0.29 0.25 0.24 0.19 0.2 1 0.17 0.15 0.15
12.2
0.12
'' Deoxygenated hexane or acetonitrile solution, excitation at 281 nm. y22
Figure 1. Frontier orbitals involved in the first two electronic transitions of rmns- 1-phenylpropene. Table 2. Photophysica! Data for the I-Arylpropenes in Hexane and Acetonitrile Solution at Room Temperature
c-1 hexane
r-1 r-2 1-3 r-4
r-5 r-6
acetonitrile hexane acetonitrile hexane acetonitrile hexane acetonitrile hexane acetonitrile hexane acetonitrile hexane acetonitrile
306 306 308 307 315 314 328 326 315 317 306 306 322 325
2.6 2.2 1 I .8 ( 12.7) 10.9 5.5 0.4 9.5 11.3 0.8 0.5 3.5 2.6 8.5 7.2
0.03 (0.02)
1.2
0.35 (0.30)
3.0
0.32 (0.15)
5.8
0.21 (0.26)
2.2
0.05 (0.03)
6.3
0.15 (0.09)
4.2
0.42 (0.38)
4.9
0.14(025 0.14 0.12 (0.20) 0.17 0.08 (0.12) 0.34 0.20 (0.15) 0.27 0.33 (0.33) 0.30 0.25 (0.23) 0.26 0.12 (0.20) 0.13
Values for lo-? M I-arylpropene or extrapolated to zero concentration (value in parentheses). " Values for excitation of 1-arylpropenes (5IO-? M) into S I or S2 (data in parentheses). Values for excitation M I-arylpropene at 281 or 254 nm (data in parentheses). of
using styrene (@f = 0.246) as a secondary standard. Data for excitation into both the first and second absorption bands are summarized in Table 2. Values of @f are highly dependent upon substituent, varying from 0.42 for r-6 to 0.05 for r-4 and 0.03 for e-1. Values of @f are also excitation wavelength dependent, displaying lower values for S2 vs SI excitation, except in the case of r-3. Fluorescence decay times (ts)were measured at room temperature in both hexane and acetonitrile solution by time-correlated single photon counting and are reported in Table 2. Good fits to single exponential decays were obtained in all cases for excitation into either S1 or S2. The singlet lifetimes are highly dependent upon substituent, but they display only minor dependence upon solvent polarity, except in the case of r-2. Fluorescence quantum yields (SI excitation) and lifetimes were measured in hexane solution at several temperatures in the range 220-300 K. Data for r-1 are reported in Table 3 while data for the other 1-arylpropenes are available as supplementary material. The values of both @f and t~ increase with decreasing temperature, resulting in temperature-independent values for the fluorescence rate constant (kf= @ f l S - l ) . As previously noted for styrene6 and r-6,23
the singlet lifetime of r-1 is concentration dependent. A Stem-Valmer plot of the lifetime data determined at several concentrations (0.0010.02 M) provides values of ts0 = 12.7 ns and the rate constant for self-quenching of k , = 7.5 x IO9 M-' s-l.
Photoisomeriiation. Quantum yields for photoisomerijrntion were determined at low conversions of reactant isomer (
