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J . Phys. Chem. 1989, 93, 47 14-47 15
Time-Resolved Electron Spin Resonance Study of the Nonphosphorescent Triplet State of trans-Diazastilbene Mikio Yagi,* Kenji Mikuni, Keiichi Takano, Yasushi Shioya, and Jiro Higuchi Department of Physical Chemistry, Faculty of Engineering, Yokohama National University, Tokiwadai, Hodogaya-ku, Yokohama 240, Japan (Received: September 30, 1988; In Final Form: December 21, 1988)
Time-resolved ESR spectra have been observed for the lowest triplet (TI)state of tram-diazastilbene (tran~-4,4’-dipyridylethylene, trans-DPE) in EPA rigid glass and in stretched poly(viny1 alcohol) films at 77 K. Zero-field splitting parameters and relative populating rate constants of triplet spin sublevels are determined. These values indicate that the distortion from the planar structure by the rotation around the central C=C bond is small for the T I state of trans-DPE in rigid matrices at 77 K.
1. Introduction
The nature of the spin sublevels of the lowest excited triplet (TI) states of organic molecules containing an ethylenic C=C double bond, such as stilbene, is poorly known because of its nonphosphorescent or very weakly phosphorescent character.’ Only two reports on the direct observation of the T1 state of trans-stilbene are available. The first one is concerned with the zero-field splitting (ZFS) parameter D* observed in glassy matrices by the steady-state ESR measurement.2 The second is concerned with the ID1 and IEI values of the photosensitized TI state of stilbene observed in a micellar aggregate by the timeresolved ESR m e a ~ u r e m e n t . ~However, the time-resolved ESR spectrum of the T, state of stilbene has not yet been observed through the direct excitation. Recently, Ikeyama and Azumi have succeeded in observing the extremely weak phosphorescence of the trans-stilbene single c r y ~ t a l . They ~ determined the total and relative radiative decay rate constants of the individual sublevels by the optical detection of magnetic resonance (ODMR) technique.5 However, the individual sublevels have not yet been assigned to the molecular axes because the time-resolved ESR experiment was carried out in a random rigid solution and the ODMR experiment was carried out in the absence of a magnetic field.3v5 In the present work, we have chosen trans-diazastilbene, tran~-4,4’-dipyridylethylene (trans-DPE), as a typical example of organic molecules containing an ethylenic C=C double bond, because the quantum yield6,’ and the anisotropy* of the S, T, intersystem crossing (ISC) in N-heterocyclics are larger than those in the parent hydrocarbons. We have observed the time-resolved ESR spectra of the nonphosphorescent T, state of trans-DPE in EPA at 77 K. The assignment of the transient ESR signals was carried out with the aid of the stretched poly(viny1 alcohol) (PVA) film method. The ZFS parameters, lifetime, and relative populating rate constants are reported. The main purpose of the study is to elucidate the geometry of the TI state of the molecule containing an ethylenic C=C double bond.
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2. Experimental Section
trans-DPE (Tokyo Kasei G.R. Grade) was purified by sublimation in vacuo. Isopentane (Tokyo Kasei G.R. Grade) was purified by distillation. Diethyl ether (Merck Uvasol) was purified by passing through activated alumina and thereafter by distillation. (1) Saltiel, J.; Khalil, G . E.; Schanze, K. Chem. Phys. Lett. 1980, 70, 233. (2) Yagi, M. Chem. Phys. Lett. 1986, 124, 459. (3) Murai, H.; Yamamoto, Y.; I’Haya, Y. J. Chem. Phys. Lett. 1986, 129, 201. -. ~
(4) Ikeyama, T.; Azumi, T. J . Phys. Chem. 1985, 89, 5332. (5) Ikeyama, T.; Azumi, T. J . Phys. Chem. 1988, 92, 1383. (6) Malkin, S.; Fischer, E. J . Phys. Chem. 1964, 68, 1153. (7) Bartocci, G.; Masetti, F.; Mazzucato, U.; Dellonte, S.; Orlandi, G. Spectrochim. Acta, Part A 1982, 38, 729. (8) El-Sayed, M. A. Annu. Rev. Phys. Chem. 1915, 26, 235.
Ethanol (Wako S.S. Grade) was used without further purification. The sample solution of trans-DPE had concentrations of 1 X mol dm-3 in EPA (diethyl ether + isopentane + ethanol, 5 5 2 by volume). The method of preparation of the stretched PVA film was identical with that described in the previous paper.9 The experimental setup for the time-resolved ESR measurements is the same as that reported previously.IOJ1 The transient ESR signals were integrated numerically between 0.30 and 0.94 ~s after the laser excitation to obtain the time-resolved ESR spectra.
3. Results and Discussion Assuming the molecular planarity in the T, state, the principal axes (x,y,z) of the ZFS tensor were taken to be as shown in Figure 1. The steady-state and time-resolved ESR spectra of the TI state of trans-DPE were measured at 77 K, as shown in Figure 2. The AMs = f l transition signals are very weak throughout the steady-state ESR measurement, while these are strong enough throughout the time-resolved ESR measurement. This is because of a short lifetime in the TI state and a highly anisotropic ISC process of trans-DPE. To determine the direction of the magnetic principal axes, we measured the time-resolved ESR spectra in stretched PVA films, as shown in Figure 2c,d. According to the general relations concerning the orientation of guest molecules: the assignment of the resonance field is straightforward. The intensity of the Z signals is strong when the external magnetic field B is parallel to the normal of the film plane n, as can be seen in Figure 2c. On the other hand, the intensity of the X signals is relatively enhanced when B is parallel to the stretched direction of a film s, as shown in Figure 2d. As a result, we can reasonably assign all of the observed signals as in Figure 2. The ZFS parameters and lifetimes obtained are listed in Table I. We can see from this table that the ZFS parameters and lifetime of trans-DPE are similar to those of trans-stilbene. Therefore, the TI state of trans-DPE has a 3mr*character mainly. As is known comprehensively,T, sublevels are the lowest in energy for the 3mr* states.12 Consequently, the order of the triplet sublevels was determined to be T,, T,,, and T, from the top. The spectrum of the randomly oriented triplet state with spin polarization was simulated in the same manner as presented by Kottis and Lefebvre13 with some modifications. In the present simulation, a Gaussian line width of 3 mT was used. The polarities of the signals at the stationary fields are E, EEA/EAA from the (9) Higuchi, J.; Ito, T.;Yagi, M.; Minagawa, M.; Bunden, M.; Hoshi, T. Chem. Phys. Lett. 1911, 46, 477. (IO) Kim, S. S.;Weissman, S. I. J . Magn. Reson. 1916, 24, 167. ( 1 1 ) Yagi, M.; Deguchi, Y.; Shioya, Y.; Higuchi, J. Chem. Phys. L e f t . 1988, 144, 412. (12) Kinoshita, M.; Iwasaki, N.; Nishi, N. Appl. Spectrosc. Reu. 1981, 17, I. (13) Kottis, Ph.; Lefebvre, R. J . Chem. Phys. 1963, 39, 393.
0022-365418912093-4714$01.50/0 , 0 1989 American Chemical Society I
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Time-Resolved EPR Study of Diazastilbene
The Journal of Physical Chemistry, Vol. 93, No. 12, 1989 4715
TABLE I: ZFS Parameters (in cm-l) and Lifetimes (in s), 7, in the TI States of baas-DPE a d trans-StUbene Observed in EPA at 77 K molecule X Y 2 Do Eb D" P trans-DPE 0.0777 -0.0150 -0.0631 0.0947 -0.0464 0.1235 0.021 trans-stilbene 0.0673' -0.0087' -0.0587' 0.088oC -0.038CP 0.1116 0.02 11 OD = (-3/2)2. b E = (1/2)(Y- X). CObtainedfrom the observed resonance field of the B,, signal. dObtained from the decay of the B ~ signal. " 'Reference 5 (determined in a single crystal at 1.3 K). 'Reference 2. U
.---+--
(a)
I1 I I
xz
Y
I/
z x
,Y
X'
Bll"
Figure 1. Molecular structure and coordinate system for trans-DPE.
low-field to the high-field, where E and A denote emission and absorption of the microwaves, respectively. With the aid of the computer simulation, the relative populating rates were estimated as shown in Figure 2e. The sublevels to be Px:P+Pz= 0.5:0.5:0, initially populated by ISC process were found to be T, and Ty, in-plane sublevels. We conclude that the distortion from the planar structure by the rotation around the central C=C bond is small for the T1state of trans-DPE because of the following three reasons. (1) The intensity of the 2 signals of trans-DPE is very strong in the stretched PVA film with Blln. (2) The timeresolved ESR signal of tram-DPE is strong enough to be easily observed. This fact shows that there is an obvious difference between the populating rate constants of the in-plane sublevels and that of the out-of-plane sublevel in trans-DPE, although the computer simulation of the time-resolved ESR spectrum only gives the value of (P, - P,)/(P, - P,). In aromatic molecules with planar T, states, the relative populating rate constant of out-of-plane sublevel is expected to be much smaller than that of the in-plane sublevel.12 (3) The ZFS parameter E should be zero, if the TI state of trans-DPE has a perpendicular configuration that is caused by twisting the central C-C bond by -90' from trans-DPE. However, the observed E value is -0.0464cm-'.
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B/ T figure 2. (a) Steady-state and (b-d) time-resolved ESR spectra for the TIstate of trans-DPE (a, b) in EPA and (c, d) in stretched PVA films at 77 K. (e) Computer-simulated spectrum obtained by using D = 0.0947 cm-', E = -0.0464 cm-', and P,:P,:P, = 0.5:0.5:0. Acknowledgment. The present work was supported by a Grant-in-Aid for Scientific Research from the Ministry of Education, Science and Culture (No. 63540330).
Registry No. trans-DPE, 13362-78-2.
