7119
J . Phys. Chem. 1991,95, 71 19-7121
Time-Resolved EPR Study on the Excited Triplet State of Nonphosphorescent Tropone Tadaaki Ikoma, Kimio Akiyama, Shozo Tero-Kubota, and Yusaku Ikegami* Institute for Chemical Reaction Science, Tohoku University, Katahira 2- I - I , Aobaku, Sendai 980, Japan (Received: June 4, 1991; I n Final Form: August 5, 1991)
The magnetic property and decay kinetics of the nonphosphorescent triplet (T1) state of tropone in glassy matrices have been studied by employing the time-resolved EPR (TREPR) method with excimer and dye laser excitations. The TI state 1 0 1= 0.077 cm-I and was assigned to be nearly pure 3rr*in character from the small zero-field splitting (zfs) parameters ( IEI = 0.01 1 cm-I in a toluene glassy matrix) and their small dependency on the matrix polarity. The directions of the principal axes of the zfs tensor were determined by magnetophotoselection experiments with two colors of lasers. It was found that the T1(rr*) state of tropone has very short lifetime; the average rate constant from the zero-field spin sublevels is 2 X lo5 S-'.
Introduction
TABLE I: Zero-Field Splitting Parameters a d Relative Population
Time-resolved EPR (TREPR) technique has made a significant contribution to our understanding of the short-lived and nonphosphorescent triplet (TI) states of various molecules. Since tropone is a typical nonbenzenoid aromatic molecule, the electronic structure in the ground state and assignment of the absorption spectrum have been the subject of much interest.'+ In the ground state, tropone has a planar configuration with alternating bond lengths.'" Although photoinduced isomerization and dimerization reactions of tropone and its derivatives have been reported?+' there has been little study on the excited states. Especially, the nonphosphorescent character has prevented the investigation of electronic structure in the TI state of tropone. In the present study, we detected the TI state of tropone in rigid glassy matrices. Using magnetophotoselection (MPS) method with two colors of lasers, we have succeeded in assigning the directions of the zero-field splitting (zfs) principal axes of the TI(***) state. The very short lifetime of the TI state was obtained from the analyses of decay kinetics of the transient EPR signals. Experimental Section
Tropone was purified by distillation under reduced pressure before use. All solvents used were dehydrated over molecular sieves (4A). The sample solution was degassed by freezepump-thaw cycles and sealed in a quartz tube. The concentration of tropone was 0.01 mol dm-'. EPR measurements at very low temperature were performed using a helium cryostat (Oxford Model ESR 900). An excimer laser (Lumonics Model HE-420, XeCI, 308 nm) and a dye laser (Lumonics Model HD-300, PBD 357 nm) pumped by the excimer laser were used as the light pulse source. The TREPR system used, which has a response time of 0.35 ps, was described in a previous paper.'O The time profiles of the transient EPR signals were taken into a combined system of a digitizer (Tektronix Model RTD 710) and a personal computer (NEC Model PC-9801XL) for accumulation and data analysis. (1) Barrow, M. J.; Mills, 0.S.;Filippini, G. J . Chem. Soc., Chem. Com-
mun. 1973, 66.
(2) Creswell, R. A. J . Mol. Sprctrosc. 1974, 51, 111. (3) Ogasawara. M.: Iijima, T.; Kimura, M. Buff.Chem. Soc. Jpn. 1972, 45* 3271. (4) Weltin, E.; Heilbronner, E.; Labhart, H. H e l a Chim. Acta 1963,46, 2041. (5) Yamaguchi, H.; Amako, Y.; Azumi, H. Tetrahedron 1967, 24, 267. (6) Hastie, S. B.;Rava, R. F. J . Am. Chem. SOC.1989, 111, 6993. (7) Paitto, D. J. Organic Photochemistry; Chapman, 0. L.,Ed.;Marcel Dekker: New York, 1967; Vol. I , p 155. (8) Mukai, T.; Kimura, M. Tetrahedron Lett. 1970, 717. (9) Mukai, 7.; Tezuka, T.; Akasaki, Y. J. Am. Chem. Soc. 1966,88,5025. (IO) Ikoma, T.; Akiyama, K.; Two-Kubota, S.;Ikegami, Y. J . Phys. Chem. 1989, 93, 7087.
Difference between the Zero-Field Sublevels of Tropone in Various Rigid Solvents at 77 K solvent pl/cm-' )El/cm-l (p, - pz):(py- p,) to1uene 0.077 0.011 1 .oo:o.oo
MTHF EtOH TFE
0.077 0.077 0.077
0.01 1 0.0 15 0.0 17
Results and Discussion Zero-Field Splitting. The TREPR spectrum of the TI state of tropone was clearly observed in a toluene glassy matrix by the excimer laser irradiation at 77 K as shown in Figure la. The spectrum was obtained 0.5 ps after the laser pulse. A weak = 2 transition. emissive signal a t 0.15 T corresponds to the IMSJ The lAMsI = 1 transitions show the spin polarization of EAE at the low-field half and of AEA a t the high-field half, where E is emission and A is enhanced absorption of microwave. The spectrum was reproduced by computer simulation (Figure lb) resulting in the zfs parameters of 1 4 = 0.077 cm-I and 14 = 0.01 1 cm-I. In various glassy matrices of 2-methyltetrahydrofuran, ethanol, and 2,2,2-trifluoroethanol, similar TREPR spectra were observed, whereas a conventional EPR spectrum and luminescence were never detected even at 4.2 K in any matrices. The zfs parameters estimated from the computer simulation are summarized in Table I. As is distinct from simple )nr* carbonyl molecules," no matrix dependence of the 1 0 1value was observed though the IEI value increased by the hydrogen bonding in protic solvents. The small matrix effects on the zfs levels suggest that the TI state of tropone has nearly pure m* character. The 0 1 value and the narrow width (2 mT) of the relatively small 1 spectral component support the assignment. Thus, zfs parameters can be denoted mainly by electron spin dipolar interaction. MPS experiments are useful in determining the principal axis directions of zfs tensor. Tropone has a broad absorption in the wavelength region 250-370 nm. From the theoretical calculationsS and the dichromism of the spectrum induced by the external electric field: it has been clarified that the absorption band consists of two types of TU* transitions with a different transition dipole moment. The directions of transition moments are perpendicular and parallel to the C, symmetry axis of tropone for the S1(7r44*s*) and S 2 ( r s r s * excitations, ) respectively. On the basis of the extinction coefficient corresponding to each transition, we examined the selective excitation to the SIand S2states. It can be considered from the analysis of the absorption spectrum that the wavelength of 357 nm excites only the SIstate, whereas the excimer laser irradiation (308 nm) induces mainly the S2state transition. Figure 2 shows time-resolved MPS spectra obtained from the irradiation of tropone with these wavelengths. When the electric vector (E) ( 1 1 ) Cheng, T. H.; Hirota, N. Mol. Phys. 1974, 27, 281.
0022-3654/91/2095-7 119302.50lO , 0 1991 American Chemical Society ,
I
~
1.oo:o.oo 0.9k0.05 0.95:0.05
7120 The Journal of Physical Chemistry, Vol. 95, No. 19, 1991
Letters
a
I I'
A -
\
-z
b
0.1
0.2
0.3
Figure 3. Molecular axes and zero-field sublevel schemes of tropone. P, and k, are relative population rates during the isc process and decay rate constants from the sublevels, respectively.
05
0.4
B/T Figure 1. Observed (a) and simulated (b) TREPR spectra for the TI state of tropone in a toluene glassy matrix. The observed spectrum was taken at 0.5 ps after the laser pulse (308 nm) at 77 K. Y X
Z
X Y
Z
f /v 0 01
02
0.3 B/T
04
02
03
04
0.5
B/T
Figure 2. TREPR spectra of the T, state of tropone in a toluene glassy matrix excited with linearly polarized laser lights of 357 nm with EllB (a) and E I B (b) and of 308 nm with EllB (c) and E I B (d). The spectra were observed at 0.5 ps after the laser pulse at 77 K. of the 357-nm light was parallel to the static magnetic field (B), the signals of the middle canonical fields (u)gained in intensity relative to the other signals (Figure 2a). With E I B, the intensities of the middle pairs (Y) significantly were decreased (Figure 2b). On the other hand, in the 308-nm excitation polarized by a Glan-Thompson prism, the intensities of the innermost pairs ( X ) increased with EllB (Figure 2c). In the present paper, the molecular axis system is taken as shown in Figure 3. Directions of the zfs tensor of tropone can be definitely determined from the above MPS measurements. The signals of middle pairs (Y)increased in the excitation of S,state with the light of EllB correspond to those from the molecules aligned as YllB. The signals of innermost pair ( X ) are due to those from the molecules aligned as XllB, since they are intensified in the S2excitation with EIIB. It can be assumed that the energy level of the out-of-plane TZsublevel locates at the lowest position in analogy with many mr* states. Therefore, we can determine the order of the triplet sublevels of tropone as shown in Figure 3, if the deviation of the structure is relatively small in the TI state. The definitive MPS spectra observed implies that the deviation from the C, symmetry group in the TI state is relatively small in tropone. The polarization pattern of the TREPR spectrum indicates that preferential population occurs at the middle Tx sublevel during the intersystem crossing (isc) process. Table I shows very small solvent effects on the spin polarization. Supposing C, symmetry
1
10
1
1
20
1
1
1
30
1
1
40
1
50
1
1
60
1
1
70
1
80
Time/ps Figure 4. Time evolutions of the transient EPR signals in toluene at 4.2 K observed with B(IXon the low-field side (a) and the high-field side (b). Insets: triplet TREPR spectra of tropone observed at the indicated times.
group, both SIand TI states have the same electronic configuration B2(r4rs*).Thus, direct spin-orbit coupling (soc) between them is unimportant and some vibronic-soc schemes are effective. The dominant processes are described as follows:
The anisotropic isc observed is well explained by these schemes. Sublevel Decay Rates. The polarization inversion of the triplet
EPR signals was clearly observed. The EAE/AEA polarization pattern in the IAMsl = 1 transitions changed to AEA/EAE at about 7 ps after the laser pulse (Figure 4, inset). The typical time evolutions of the transient EPR signals are shown in Figure 4. The time profiles of the decay curves were independent of the microwave power below 1 mW indicating the ineffectiveness of the Torrey osciIlation.l2 Any shifts of the canonical points were not observed, suggesting no participation of photochemical reactions in the present measurements. It can be, therefore, considered that the observed polarization inversion is caused by the (12) Torrey, H. C. Phys. Reu. 1949, 76, 1059.
7121
J. Phys. Chem. 1991,95, 7121-7124 difference of decay rates in the triplet sublevels. We analyzed the time developmentsof the transient EPR signals to determine the decay rate constants. The time evolutions were measured at the magnetic fields of the innermost X and the outermost Z canonical orientations with low microwave power (0.5 mW) at 4.2 K. Thus, it may be assumed that the nutation and the electron spin relaxation are neglected. The signals observed at the Y canonical fields with a low microwave power were too weak for the analysis. The observed time profiles were reproduced by a sum of two exponential functions taking account of apparatus response time ( 7 ) as expressed with eq 1. where PI
It has been reported that flexible monocyclic enones have twisted form in the T,(mr*) We could not observe any evidence for 3mr*-3n** mixing in tropone. Therefore, it can be considered that the short lifetime is due to a large Frank-Condon factor induced by the difference of the potential between the Tland So states. In tropone, the atomic coefficients of HOMO(r4) and LUMO(r5*) are very different each other. The values obtained from a semiempirical PPP type SCF MO calculation are summarized as follows: /7
I(r) = l l d t ' r - l exp(-(t
- t?/7)(P1exp(-klt?
y 0.000
Q)o.530
- P2 exp(-k2t?} (1)
0.417
0.492
and P2 are the relative populating rates of the Zeeman sublevels. The decay rate constants k, ( i = 1 , 2 ) of Zeeman sublevels are expressed as follows:
k, = c,&
+ C&y + C,&
(2)
where C, and k, (j= X, Y,Z)are mixing coefficients and the decay rate constants of the zero-field sublevels, respectively. The response time of 0.35ps was used for the deamvolution calculation. We obtained kx = 5 X IOs s-' and &,, N kz N 5 X 10, s-I, which leads to an average decay rate constant of k., = (kx + ky + kz)/3 Y 2 X IO5 s-l. from the analyses of the transient signals at Z canonical points. Similar results were obtained from the decay signals at the X canonical fields. The rate constants and the populating ratios of the zero-field sublevels determined are summarized in Figure 3. The results indicate that the %**state of tropone has very large radiationless rates from the triplet sublevels. The rates are smaller than those of flexible monocyclic enones.') (13)Yamauchi, S.;Hirota, N.; Higuchi, J. J. Phys. Chem. 1988,92,2129.
Since the C2-C3and C4-C, bonds have an antibonding character in the rs*orbital, the excitation into the TI state would accompany the increase of bond lengths in these bonds and the twisting around these bonds. The difference between the molecular structures of TI and So states may be responsible for the fast radiationless transition rate. Further detailed investigations are in progress by comparing the present results with the characters of several benzotropone derivatives.
Acknowledgment. The present work was partially supported by Grants-in-Aid of Scientific Research No. 02453002 and of Scientific Research on Priority Area No. 02245204 from the Japanese Ministry of Education, Science and Culture. (14) Bonncau, R. J. Am. Chem. Soc. 1980,102, 3816.
Complementary Grating Effect in Forced Rayieigh Scattering Sangwook Park, Jungmoon Sung, Hongdoo Kim? and Taihyun Chang* Department of Chemistry, POSTECH, P.O. Box 125, Pohang, 790-600. Korea, and Division of Organic Materials. RIST, P.O. Box 135, Pohang, 790-600, Korea (Received: June 12, 1991: In Final Form: July 26, 1991)
The problem of non-single-exponential decay profiles of the forced Rayleigh scattering (FRS) is addressed by considering the contributions from complementary phase gratings. Since the technique relies on creating transient optical gratings by means of photolabels and reading the total diffraction signal intensity arising from the complementary gratings, a small difference in the decay time constants of optical fields derived from a complementary pair of such gratings could produce extremely nonexponential FRS decay profiles. By detailed simulation studies, we show how different profiles can be obtained with only minor differences in the decay constants, and how single-exponential analyses of such profiles can mislead in deducing the correct diffusion coefficients.
Introduction Since its inception as a tool for the study of mass diffusion, the forced Rayleigh scattering (FRS)technique has been applied to a variety of translational diffusion problems in complex media and self-diffusion such as anisotropic diffusion in liquid and probe diffusion in polymer solutionsc7 and in the bulk state.&I1 The technique requires a photolabel undergoing changes in its optical properties upon irradiation by the optical excitation source, Le., writing beams. If a fringe pattern is imprinted onto a sample To whom correspondence should be addrwaed. 'Polymer Materials Lab.. KIST, P.O.Box 131, Cheongryang, Seoul, 130-650.Korea.
OO22-3654/9 1/2095-7 121$02.50/0
by either crossing two writing beams or relying on a ruled mask, a periodic concentration profile of photochromically shifted or ( 1 ) Hervet, H.; Urbach, W.; Rondelez, F. J. Chem. fhys. 1978.68.2725. ( 2 ) Takezoe, H.; Ichikawa, S.; Fukuda, A,; Kuzc, E. Jpn. J . Appl. fhys.
1983,23,L78. (3)Urbach, W.;Hervet, H.; Rondelez, F. J . Chem. fhys. 1985,83, 1877. (4) Hervet, H.; LCger, L.; Rondelez. F. fhys. Rev. Let?. 1979,42,1681. (5) W w n , J. A.; Noh, 1.; Kitano, T.; Yu, H.Macromolecules 1984,17,
782.
(6)Kim, H.;Chang, T.; Yohanan, J. M.; Wang, L.; Yu, H. Macromolecules 1986,19,2737. (7) Landry, M.R.; Gu, Q.;Yu. H. Macromolecules 1988,21, 1 1 58. 29,(8) 2261. Tran-Cong, Q.; Chang, T.; Han, C. C.; Nishijima. Y. Polymer 1988, Q 1991 American Chemical Society
