7087
J. Phys. Chem. 1989, 93, 7087-7091 mental conditions (low-temperature solid-state environment), but we can expect that the nonradiative decay mechanisms, which are active under these conditions, are not slowed down at higher temperatures in the liquid state. It therefore appears that the isomerization reaction cannot compete with the decay processes of SI (already active at low temperatures) and will not take place on the excited-state singlet surface. It is also very unlikely that the isomerization can take place on the ground-state surface; this would require that the reaction speed for the hot molecule created by the internal conversion from SI increase by about 15 orders of magnitude from its value measured for a thermalized molecule at room t e m p e r a t ~ r e(5.4 ~ ~ x IO4 s-l at 120 "C). While our results rule out a singlet mechanism for the cis to trans isomerization, they are fully compatible with a triplet route. The occurrence of this isomerization on the triplet surface was demonstrated by the observation of this reaction after triplet sensitizing.l2 A triplet mechanism is also consistent with a number of time-resolved experiments: the observation, after excitation of the cis isomer, of a long-lived transient absorption attributed to a T, TI t r a n s i t i ~ n , ' ~and , ~ ~the * ~recovery ~ of the trans ground state on the time scale of the decay of this TI state.16 In nanosecond experiments, the same transient absorption, attributed to a planar trans triplet species, was observed after excitation of either isomer,16*18 while recent picosecond experiments on thioindigo derivatives show the appearance of different transient absorptions after exciting the cis or the trans isomer. In the case of a perinaphthothioindigoid dye, the buildup of the absorption is much more rapid when the cis isomer is excited, (3.7 f 1.2) X 1Olos-l, as compared to (1.4 f 0.3) X lo9 s-I for the trans isomer,21
-
(32) Erler, M.; Haucke, G.; Paetzold, R. Z . Phys. Chem. ( k i p z i g ) 1977, 258, 315. (33) Krysanov, S.A.; Alfimov, M. V. Dokl. Phys. Chem. 1981,258,460.
consistent with our observation of an increased intersystem crossing in the cis isomer of thioindigo. Conclusion
In this work we have shown the following: The comparison of the resonance Raman spectra (and, in a more restricted way, of the absorption spectra) of the two isomers of thioindigo confirms predictions, based on molecular orbital calculations, of a very similar electronic structure of the first excited singlet state. The lifetime of this state in the cis configuration is in the order of 100 fs, in contrast to a value of 15 ns for the trans isomer. We suggest that the rapid decay of S1of cis-thioindigo reflects an increase of the intersystem crossing rate, due to a nearby nx* triplet state and possibly also related to the proximity of the two oxygen atoms. Even then this observation is difficult to rationalize and presents a challenge for a theoretical treatment of the question. Preliminary results of molecular orbital calculations support the hypothesis put forward here of the presence of a n i * triplet state close to or below the a x * singlet state. The rapid decay of the excited singlet state of cis-thioindigo should not be slowed down in liquid solutions as compared to the solid-state environment; this observation, therefore, excludes a singlet pathway for the cis trans isomerization, while all evidence is consistent with a triplet route.
-
Acknowledgment. We are grateful to W. Liittke for the gift of the dye samples and many fruitful exchanges concerning indigo dyes. Stimulating discussions with P. Barbara during the completion of this work have been very helpful. We thank H. Cardy for performing the MO calculations and communicating the unpublished results. Registry No. Thioindigo, 522-75-8; nonane, 11 1-84-2.
Time-Resolved ESR Studies on the Excited Triplet States and Photoenolization of 2-Methylacetophenone and Related Molecules Tadaaki Ikoma, Kimio Akiyama, Shozo Tero-Kubota, and Yusaku Ikegami* Chemical Research Institute of Non-Aqueous Solutions, Tohoku University, Katahira 2-1 -1, Sendai 980, Japan (Received: January 23, 1989)
Nonphosphorescent excited triplet (%ri*) states of photoenols (3E)generated from the intramolecular hydrogen transfer of o-methylacetophenone (OMAP) and related compounds were measured, together with their phosphorescent excited triplet by time-resolved ESR techniques at low temperatures. The triplet ESR spectrum with zero-field splitting parameters, state (3K), 1 0 1 = 0.060 and IEI = 0.0025 cm-', clearly different from those of the parent molecules (3K), was assigned to 'E. The observed small ID1 value suggests the biradical character of 'E. It was pointed out from the examination of substituent and matrix effects that the ease of photoenol generation clearly depends on the character of the T1state of the parent molecules and also on the coplanarity of the carbonyl group with the aromatic ring. The presence of water induces a striking change in the character of the TI state of OMAP; i.e., n?r* of Tl in nonpolar or absolute ethanol glassy matrices is altered by T X * in 0.5% HzO/ethanol. CIDEP spectra observed from the quenching reaction with methylviologen proved that 'E behaves as an excellent electron donor.
Introduction During recent decade, photoenolization1-3 as well as the Nomish particular, a Type 113-5 has received considerable attention.
number of studies on the reaction mechanism and kinetics have been reported for the transient species in the photoenolization Of o-alkyl-substituted aromatic ketones6-lZ and o-alkylbenzo-
( 1 ) Haag, R.; Wirz, J.; Wagner, P. J. Helu. Chim. Acta, 1977, 60, 2595. (2) Sammes, P. G.Tetrahedron 1976, 32, 405. (3) Scaiano, J. C. Acc. Chem. Res. 1982, 15, 252. (4) Wagner, P. J. Acc. Chm. Res. 1971, 4, 168. (5) Scaiano, J. C.; Lissi, E. A.; Encina, M. V. Reu. Chem. Inrermed. 1978,
(6) Findlay, D. M.; Tchir, M. F. J . Chem. SOC.,Faraday Trans. 1 1976, 72, 1096. (7) Lutz, H.; Breheret, E.; Lindqvist, L. J . Chem. SOC.,Faraday Trans. 1 1973, 69, 2096. (8) Wagner, P. J.; Chen, C.-P. J . Am. Chem. SOC.1976, 98, 239. (9) Small, R. D.; Scaiano, J. C. J . Am. Chem. SOC.1977, 99, 7713.
2, 139.
0022-3654/89/2093-7087$01.50/0
0 1989 American Chemical Society
7088
The Journal of Physical Chemistry, Vol. 93, No. 20, 1989
Ikoma et al. a
phenones.l3-I8 The enols produced from the excited triplet state (jK)of these ketones are usually called biradical because of the reactivity appearing in the addition reaction to double bond^,^,'^ intramolecular c y c l i ~ a t i o n , ~electron * ~ - ~ ~t r a n ~ f e r , ~and ~ ~ *hy~J~ drogen a b ~ t r a c t i o n . ~ ’ -Scaiano ~~ and his co-workersIo,lldetermined the lifetime (7B = 580 ns) of 3E generated from omethylacetophenone (OMAP) in water/acetonitrile (1:4, V/V)at room temperature and led to a conclusion that the 7Bis controlled by an isc (intersystem crossing) process since singlet enol (E) has a very short lifetime to produce original ketone. A tunneling mechanism is proposed for the back-reaction to the ketone from E on the basis of the large isotope effect.24 Then, the photoenolization reaction of OMAP is expressed in eq 1.
I,
C
tA d OMAP (K)
V
OMAP (3E )
The triplet states of aromatic ketones have long been the subject of many spectroscopic studies because of their unique reactivity and consequently a number of zero-field splitting (zfs) parameters have been accumulated by ESR and ODMR (optical detection of magnetic resonance) measurement^.^^-^^ It is known that the properties of TI states of aromatic carbonyls vary with substituents and also with environment, since the energy separation between 3 n ~ and * 3 7 r ~ *states is usually sma11.27*28However, there has been little study of the electronic structure of the T1 states of aromatic carbonyl compounds taking part in photoenolization. Recent developement of the time-resolved ESR (TRESR) technique with laser excitation provides useful information about short-lived triplet and nonphosphorescent triplet states.2g We have applied this technique to examine the paramagnetic species formed in the photoenolization of OMAP and related molecules?0 since zfs parameters give valuable information on the electronic structure of the triplet states. 5,8-Dimethyltetralone (5,8-DMT), 2,4,6-trimethylacetophenone(TMAP), and 2,4,6-trimethylbenz-
(10) Das, P. K.; Encinas, M. V.; Small, R. D., Jr.; Scaiano, J. C. J. Am. Chem. SOC.1979, 101, 6965. ( 1 1) Scaiano, J. C. Chem. Phys. Lett. 1980, 73, 319. (12) Scaiano, J. C. Tetrahedron 1982, 819. (13) Porter, G.; Tchir, M. F. J. Chem. SOC.D 1970, 1372; J . Chem. SOC. A 1971, 3772. (14) Uji-ie, K.; Kikuchi, K.; Kokubun, H. Chem. Left. 1977, 499; J . Photochem. 1979, 10, 145. (IS) (a) Matsuura, T.; Kitaura, Y. Tetrahedron 1969, 25, 4487. (b) Hayashi, H.; Nagakura, S.; Ito, Y.;Umemura, Y.; Matsuura, T. Chem. Lett. 1980, 939. (16) Nakayama, T.; Hamanoue, K.; Hidaka, T.; Okamoto, M.; Teranishi, H. J. Photochem. 1984, 24, 71. (17) Wilson, R. M.; Hannemann, K.; Peters, K.; Peters, E.-M. J. Am. Chem. SOC.1987, 109, 4741. (18) Wilson, R. M.; Hannemann, K.; Heinman, W. R.; Kirchhoff, J. R. J. Am. Chem. SOC.1987, 109, 4743. (19) Heindel, N. D.; Sarver, E. W.; Pfau, M. A. Tetrahedron Lett. 1968, 3579. (20) Yang, N. C.; Yang, D.-D. H. J. Am. Chem. SOC.1958, 80, 2913. (21) Wagner, P. J.; Zepp, R. G. J. Am. Chem. SOC.1972, 94, 287. (22) Wagner, P. J.; Kelso, P. A.; Zepp, R. G. J. Am. Chem. SOC.1972, 94, 7480. (23) Encinas, M. V.; Wagner, P. J.; Scaiano, J. C. J. Am. Chem. SOC. 1980, 102, 1357. (24) Grellmann, K.-H.; Weller, H.; Tauer, E. Chem. Phys. Lett. 1983, 95, 195. (25) Kinoshita, M.; Iwasaki, N.; Nishi, N. Appl. Specfrosc.Rev. 1981, 17, 1.
( 2 6 ) Chan, I. Y. Rev. Chem. Intermed. 1987, 8, 339. (27) Cheng, T. H.; Hirota, N. Chem. Phys. Lett. 1972, 14, 415. (28) Cheng, T. H.; Hirota, N. Mol. Phys. 1974, 27, 281. (29) Hirota, N.; Yamauchi, S.;Terazima, M. Rev. Chem. Intermed. 1987, 8, 189. (30) Briefly communicated: Akiyama, K.; Ikegami, Y.; Tero-Kubota, S. J . Am. Chem. SOC.1987, 109, 2538.
I
0.0
I
I
0.1
I
I
0.2
I
I
I
0.3
I
0.4
I
1
.
0.5
I
.
0.6
B/T Figure 1. Transient ESR spectra observed a t 1 ws after laser pulse excitation of OMAP in an MCH glassy matrix a t 77 K (a) and at 15 K (c). Computer-simulated spectra of a for ’E and of c for 3K are shown as b and d, respectively.
aldehyde (TMBA) were chosen for the examination of substituent effects on the photoenolization. Transient ESR spectra of 3E
5,EDMT
TMAP
TMBA
and 3K of OMAP, substituent effects on the photoenol generation, matrix effects on the triplet states, and quenching of 3E with methylviologen are described. Experimental Section
All samples were purified by distillation under reduced pressure before use. 5,8-DMT was synthesized according to the previously reported p r o c e d ~ r e . ~Spectrograde ~ solvents, methylcyclohexane (MCH), acetonitrile (CH3CN),dimethylformamide (DMF), and ethanol (EtOH), were used. Absolute EtOH was obtained by reflux with Mg metal for 48 h after drying with molecular sieves M was degassed (4A). The sample solution prepared at 5 X by using five freeze-thaw cycles. For CIDEP measurements in the quenching of 3E, sample solutions were deoxygenated by bubbling with argon gas. The solutions were flowed in a quartz tube within the ESR cavity. The transient ESR signal was directly detected with an X-band ESR spectrometer (Varian Model E109E or JEOL Model FE2XG) modified for a wide-band preamplifier and taken into a boxcar integrator ( N F BX-531 or PAR Model 162) at arbitrary times after the laser pulse. A nitrogen laser (homemade or Molectron Model UV-24, 337 nm) and also an excimer laser (Lumonics Model HE-420; XeCl, 308 nm) were used as the pulse light source. For continuous irradiation, a high-pressure 500-W Hg lamp equipped with an UV-25 glass filter was used. The measurements at very low temperatures were performed by using a helium flow cryostat (Air Product Model LTD-3-110). Phosphorescence spectra and triplet lifetimes were measured with a Hitachi Model 850 spectrophotometer. (31) Mosby, W. L. J. Am. Chem. SOC.1952, 74, 2564
Excited Triplet States and Photoenolization of OMAP TABLE I. Zfs Parameters and Spin Polarization of the Triplet Species Observed by the Laser Irradiation of OMAP in Glassy Matrices
The Journal of Physical Chemistry, Vol. 93, No. 20, 1989 7089 a
spin polarization zfs/cm-'
.
- 2
-0
ID1 = 0.25
(El = 0.005
axis
PP
Z
1.oo 0.00 0.00
Y X
Y
K' a
x,
