4522
The Lowest Triplet State of Stilbene" William G . Herkstroeterlband Donald S. McClure Contribution from the James Franck Institute and Department of Chemistry, University of Chicago, Chicago, Illinois 60637. Received February 27, 1968 Abstract: The lowest triplet state of stilbene has been observed by means of its absorption spectrum following flash excitation of stilbene in glassy solvents at liquid nitrogen and liquid argon temperatures. The triplet lifetime is a strong function of solvent viscosity and decreases rapidly as the solvent becomes more fluid. Evidence that the transient absorption is truly the triplet-triplet absorption is: (1) similar transient spectra are found in several stilbene derivatives and nonisomerizable model compounds including perdeuteriostilbene, 1,2-dihydro-3-phenylnaphthalene, 4,4'-dichlorostilbene, and 5H-dibenzo[a,~cycloheptene;(2) deuteration markedly increases the lifetime; (3) the observed spectrum has approximately the correct energy and f number according to Pariser-Parrtype calculations.
T
he lowest triplet state of stilbene is of considerable the trans triplet to a geometry intermediate between interest because of its role in the cis-trans isomerizacis and trans is predicted experimentally by the work of tion of this molecule. Numerous attempts have been Hammond and Saltiel*2 as well as Stegemeyer'o and made to study this electronically excited state by either also theoretically from the calculations of Ting,4 alphosphorescence or triplet-triplet absorption, but though this barrier is probably small. The intersection without success. 2-5 of the potential surfaces for triplet and 'ground states The origin of the first singlet-triplet transition in upon twisting 90" may well be responsible for highly trans-stilbene lies at 2.2 eV (17,300 cm-l), according to facile, radiationless return to the isomeric ground absorption spectra taken in ethyl iodideea and in oxystates, a likely path for the is~rnerization.~~~~~'~~'~ gen under high pressure.6b The corresponding transiAside from the inability to detect phosphorescence or tion in the cis isomer of stilbene appears without structriplet-triplet absorption, there is, in fact, experimental ture even in heavy-atom solvents, but the origin has evidence that the lifetime of the stilbene triplet in solubeen estimated to occur at 2.5 eV (20,000 ~ m - - ~ ) . ~ ,tion * at room temperature is on the order of l F 7 sec or The lowest triplet state is believed to be a reactive intereven less. 12,19,20 mediate in the photochemical isomerization of stilbene The quantum yield for fluorescence of trans-stilbene upon both direct irradiation and triplet energy trans~,~~,~~,~~ varies inversely with t e m p e r a t ~ r e . ~ , ~Fischer Although fer from appropriate ~ensitizers.~~'~,~~,',~-~' and coworkers11n13*14 have further demonstrated the spectroscopic transitions involve no change in geomeinverse interdependence of the quantum yields of fluotry, isomerization leads to a 180" twist about the cenrescence and isomerization from trans- to cis-stilbene. tral bond. This raises the question of the shape of the They have evidence that the process of intersystem potential surface of the lowest stilbene triplet state in crossing in trans-stilbene from the first excited singlet the twisting coordinate. A barrier to twisting 90" from to the triplet via intermediate singlet and triplet states is temperature dependent with an activation energy of (1) (a) Supported by Public Health Service Research Grant No. RO/GM 13519, NIGMS, RGB. (b) National Science Foundation about 1.2 kcal/mol. Such a barrier is also predicted by Postdoctoral Fellow, 1965-1966. Address correspondence to this the calculations of Ting.4 author: Research Laboratories, Eastman Kodak Co., Rochester, Fischer 1 3 , 1 4 also has observed that the quantum yield N. Y. 14650. (2) ,(a) G. Porter, unpublished results as reported in (b) D. Schulteof isomerization from trans- to cis-stilbene is reduced at Frohlinde, H. Blume, and H. Giisten, J. Phys. Chem., 66, 2486 (1962). higher viscosities, but that the reverse process is largely (3) W. G. Herkstroeter and G. S. Hammond, J . Am. Chem. Soc., 88, independent of viscosity. He proposed that this is 4769 (1966). (4) C. H. Ting, Ph.D. Thesis, University of Chicago, 1965. consistent with the fact that cis-stilbene has a larger (5) Just prior to submission of this paper, we read the prelimivan der Waals volume than the lrans isomer. He nary results of G. Heinrich, H. Blume, and D. Schulte-Frohlinde, Tefrahedron Lerters, 4693 (1967), who tentatively conclude that the transient further hypothesizes that the viscosity dependent step absorption observed by them is the triplet-triplet absorption of transis either the conversion of the lowest energy trans stilbene. In the light of our work, w e feel that their interpretation is triplet to the twisted triplet or the deactivation of the correct. (6) (a) R. H. Dyck and D. S. McClure, J. Chem. Phys., 36, 2326 twisted triplet to ground-state cis-stilbene. (1962); (b) D. F. Evans,J. Chem. Soc., 1351 (1957). Curiosity about the various processes involved in the (7) E. Lippert, 2.Physik. Chem. (Frankfurt), 42, 125 (1964). triplet-state population and depopulation and their (8) H. Stegemeyer, ibid., 51, 95 (1966). (9) Th. Forster, 2.Elektrochem., 56, 1716 (1952). relationship to photoisomerization in stilbene helped (10) H. Stegemeyer, J. Phys. Chem., 66,2555 (1962). to prompt this investigation. It would appear that the (11) S. Malkin and E. Fischer, ibid., 68, 1153 (1964). (12) G. S. Hammond, J. Saltiel, A. A. Lamola, N. J. Turro, J. S . lifetime of the lowest triplet of trans-stilbene depends Bradshaw, D. 0. Cowan, R. C. Counsell, V. Vogt, and C. Dalton, upon its resistance to rotation about the central double J . A m . Chem. Soc., 86,3197 (1964). bond and the resistance to radiationless transitions to (13) K. A. Muszkat, D. Gegiou, and E. Fischer, ibid., 89, 4814 (1967). the ground state. For this reason, we decided to look (14) D. Gegiou, K. A. Muszkat, and E. Fischer, ibid., 90, 12 (1968). (15) Saltiel'e, 17 recently presented evidence that the direct photoisomerization of stilbene proceeds through the first excited singlet state rather than the triplet. (16) J. Saltiel, E. D. Megarity, and K. G. Kneipp, ibid., 88, 2336 (1966). (17) J. Saltiel, ibid., 89, 1037 (1967).
(18) P. Borrell and H. H. Greenwood, Proc. Roy. SOC.(London),
A298, 453 (1967). (19) E. F. Ullman, J . Am. Chem. Soc., 86, 5357 (1964). (20) E. F. Ullman and W. A. Henderson, Jr., ibid., 89, 4930 (1967).
Journal of the American Chemical Society J 90:17 1 August 14, 1968
4523 -
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Figure 1. The triplet-triplet absorption spectrum of trans-stilbene. The spectrum was recorded in units of absorbance following flash excitation of 4.3 X l(r6 M rrans-stilbene in glass 1 at 77°K in a cell of 8-cm optical length.
I
for metastability of the stilbene triplet under conditions where we believed such rotation would be restricted.
Results and Discussion If the stilbene triplet lifetime is indeed short, the radiative lifetime would well be expected to be orders of magnitude longer than the nonradiative lifetime, which could readily explain the inability to detect phosphorescence from stilbene. Detection of the triplet by viewing its absorption, however, requires only that the triplet lifetime be increased to the point where it can be resolved by flash-excitation methods. Since previous attempts at viewing the triplet-triplet absorption of stilbene had all been with fluid solvents, we attempted the same objective in rigid host media. In the first experiment, carried out in conjunction with Ottolenghi,zl a transient species with an absorption maximum near 26,300 cm-' (3800 A) was detected both in EPA22 and 3-methylpentane solutions of transstilbene at 77°K following flash excitation. The calculations of Ting4 predict a strong triplet-triplet absorption band in trans-stilbene at 29,500 cm-' (3400 A) with no other strong band in the visible. It seemed likely that this band was, indeed, being observed. In order to make possible a more positive identification of the transient observed, similar experiments were repeated in other rigid host media and the absorption spectrum itself was resolved as shown in Figure 1. This transient could not be detected at higher temperatures where the solvents became fluid. Nothing appeared in the absence of stilbene. Photochemical reaction between stilbene and solvent also seemed unlikely. In order to test for possible hydrogen abstraction from the alkane solvents, a glass made up of a mixture of n-pentane and 2,2-dimethylbutane containing no tertiary carbon atoms23 was employed as solvent. N o (21) M. Ottolenghi, W. G. Herkstroeter, and D. S. McClure, unpublished results. (22) EPA is a 5 : 5 :2 mixture of diethyl ether, isopentane, and ethanol. (23) K. Rosengren and S. Sunner, Acra Chem. Scand., 16, 521 (1962).
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Figure 2. The triplet-triplet absorption spectrum of trans-stilbene&. The spectrum was recorded in units of absorbance following flash excitation of 7.2 X 10-6 M trans-stilbene-dlz in glass 1 at 77°K in a cell of 8-cm optical length.
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cm-1 x 10-3 Figure 3. The triplet-triplet absorption spectrum of 1,2-dihydro-3phenylnaphthalene. The spectrum was recorded in units of M 1,Zdihydro-3absorbance following flash excitation of 5 X phenylnaphthalene in glass 1 at 77'K in a cell of 8-cm optical length.
differences in the stilbene transient spectra were observed relative to those obtained with two other alkane solvent mixtures containing tertiary carbon atoms. Similar transient spectra were obtained for molecules structurally closely related to trans-stilbene. The transient spectra with trans-stilbene and trans-stilbenedI2,shown in Figures 1 and 2, respectively, have absorption peaks near 26,500,27,950, and 29,400 cm-l (3780, 3580, and 3410 A), which appears to represent a progression of a 1450-cm-' vibration. This compares with 1635- and 1599-cm-I vibrational progressions of the central double bond in the ground state and first excited singlet state, respectively.6a The transient spectrum of 1,2-dihydro-3-phenylnaphthalene(1) (FigHerkstroeter, McClure
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Lawest Triplet State of Stilbene
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Figure 4. The triplet-triplet absorption spectrum of 4,4'-dichlorostilbene. The spectrum was recorded in units of absorbance following flash excitation of 6.2 X 1 0 - 6 M 4,4'-dichlorostilbene in glass 1 a t 77°K in a cell of 8-cm optical length.
ure 3) is similar to the first two, but red-shifted by approximately 400 cm- l, while that of 4,4'-dichlorostilbene (Figure 4) is shifted by 150 cm-1 and has a vibrational progression of 1550 cm-l.
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Table I. First-Order Rate Constants for Triplet-State Depopulation and Their Variation with Viscosity, Temperature, and Isotopic Substitution
rrans-Stilbene trans-Stilbene-d12 kdkD trans-Stilbene rruns-Stilbene-dl2 kdkn 1,2-Dihydro-3-phenylnaphthalene 1,2-Dihydro-3-phenylnaphthalene 1,2,5,6-Dibenzanthracene 1,2,5,6-Dibenzanthracene 4,4'-Dichlorostilbene SH-Dibenzo[a,dlcycloheptene
-ki, la
190 88 2.2
sc-1
___ 7
Glass 2a
Glass 3a
170 50 3.4
77 77 77 87 87 87 77
>104b
>io4
56 11 5.1 200
>io*
>io4
40
290
...
87
>io4
...
77
...
0.76
87
...
0.85
77 77
210 140
...
Glass no. 1
2 3
The decay of transient absorption was first order in all cases, and the rate constants measured for the various species are listed in Table I. Strong evidence for
Glass
Table 11. Glass-Forming Hydrocarbon Solvents0 and Their Estimated Viscosities
a
1
Temp, "K
stilbene transient, The triplet lifetime enhancement upon substitution of deuterium for hydrogen is a confirmed fact in many hydrocarbons and is controlled by the influence of C-H and C-D vibrational modes on the magnitude of the overlap of the zero-point vibrational wave function of the triplet state and the isoenergetic, vibrational wave functions of the ground state. 2 4 The rate constants listed in Table I for the decay of the triplets of trans-stilbene, trans-stilbene-d12,and 1,2dihydro-3-phenylnaphthalene are quite sensitive to the viscosity of the host medium. These rate constants were measured employing as solvents three different glass-forming hydrocarbon mixtures at both liquid nitrogen and liquid argon temperatures. In Table 11 are listed the compositions of the three glasses together with their viscosity values at 77 and 87"K, which were determined as follows. Rosengren27found that, over a range of low temperatures, the relative viscosities of the solvents are in the numerical order assigned in Table I1
...
5.0 75 190
... ... ...
... ...
...
a See text and Table 11. b Since no transient was observed, the rate constant must be greater than IO4sec-*.
the identification of these transients as triplets is provided by the deuterium effect on the lifetime of the transJournal of the American Chemical Suciet2 ; 90.17
'
Estimated log viscosity, poise 77°K 87°K
Composition 6 parts isopentane and 1 part 3-methylpentane 3 parts n-pentane and 8 parts 2,2-dimethylbutane 3-Methvl~entane
6.8
5.0
7.3
5.5
12.0
10.2
See ref 27.
and that, for each solvent, the temperature dependence of viscosity is nearly identical, with the viscosity decreasing by a factor of 3 for a 2.5"K rise in temperature. The viscosities of glasses 1 and 3 at 77°K can be assigned as 6.9 X lo6 and 9.4 X 10" poise, respectively.28 B/T (where 17 is the visThe relationship log 17 = A cosity, A and B are constants, and B is the same for all glasses) can be applied in order to calculate the viscosities of glasses 1 and 3 at 87°K. From Rosengren's work, 27 glass 2 has the same viscosity at 86 "K as glass 1 at 83"K,and this relationship can be extrapolated in order to estimate the viscosity of glass 2 at 77 and 87 OK. The triplet lifetimes of trans-stilbene, trans-stilbene-d12, and 1,2-dihydro-3-phenylnaphthalene are, with one exception, in the same order as the viscosities of the host media. This correlation confirms the dependence of triplet lifetime upon viscosity of the solvent. Also included in Table I are rate constants for the triplet-state depopulation of 1,2,5,6-dibenzanthracene measured in glass 2 at both 77 and 87°K. Since these rate constants are, first of all, smaller than those measured for stilbene and its derivatives and also independent within experimental error of the change in viscosity and temperature, one can discount the possibility that the lifetime differences in stilbene are controlled by the diffusion of adventitious quenchers
+
(24) See ref 25 and 26 and other references therein. (25) J. D. Laposa, E. C. Lim, and R. E. Kellogg, J . Chern. P h y s . , 42, 3025 (1965). (26) S. I
