J. Phys. Chem. 1994, 98, 12521-12525
12521
Study of Conformers of trans-2,2’-Dipyridylethylenein the Lowest Excited Triplet States by Time-Resolved Electron Paramagnetic Resonance Yasushi ShioyaJ Kenji Mikuni, Jiro Higuchi, and Mikio Yagi’ Department of Physical Chemistry, Faculty of Engineering, Yokohama National University, Tokiwadai, Hodogaya-ku, Yokohama 240, Japan Received: July 15, 1994; In Final Form: September IS, 1994@
Time-resolved EPR spectra have been observed for the very weakly phosphorescent triplet state of trans2,2’-dipyridylethylene in EPA and in methanol-ethanol (1:l by volume) at 77 K. The assignment of the transient EPR signals was canied out with the aid of the stretched poly(viny1 alcohol) film method. Two sets of time-resolved EPR signals are assigned to the quasi-planar conformers originated by a 180” rotation of the pyridyl ring around the single bond with the ethylenic carbon. The time-resolved EPR spectra of the two conformers were separated from the spectrum of their mixture using the technique of selective excitation. The zero-field splitting (ZFS) E parameter and the anisotropy of the triplet sublevel populating rates are sensitive to the conformational change, while the ZFS D parameters and lifetimes of the two conformers in the lowest excited triplet states are nearly identical.
1. Introduction
MA
The photophysics and photochemistry of nonrigid stilbenelike molecules and their aza analogs have been a subject of study for many years. trans-2,2’-Dipyridylethylene(trans-2,2’DPE) is a typical trans-diazastilbene. Three planar conformers originated by the rotation of the pyridyl rings around the single bonds with the ethylenic carbons are possible in trans-2,2’-DPE, as shown in Figure Evidence for the existence of an equilibrium of the conformers in solution has been provided through the dipole moment measurements.’ In the ground (G) state, the most stable conformer has been considered to be DPE-1 through the experiments of dipole moment,’ NMR? and phot~electron.~Semiempirical calculations have shown that DPE-3 is energetically unfavored because of interference between the o-hydrogen and the ethylenic h y d r ~ g e n . ~ . ~ Bartocci et al. have succeeded in separating the fluorescence spectrum of trans-2,2’-DPE into the ‘‘almost pure” spectra of the two conformers by using the technique of selective excitation in EPA at 77 K.6 They assigned the conformers with the 0-0 energies of the lowest excited singlet (SI) G transitions of 29 370 cm-l and 30 170 cm-’ to DPE-1 and DPE-2, respectively. Marconi et al. calculated the energies of the S1 states of the three conformers shown in Figure 1 by means of modified neglect of differential overlap (MNDO) and intermediate neglect of differential overlap/spectroscopic (INDO/S) calc~lations.~ From INDO/S calculations, the SI energy difference between DPE-1 and DPE-2 was estimated to be 55 cm-’. Since the S1 energy difference between the conformers is very small, further detailed investigations seem to be required for the assignment of the observed fluorescence spectra to specified conformers. Recently, we have shown that the zero-field splitting (ZFS) parameters and the triplet sublevel populating rates are sensitive to the conformational change in 2,2’-bipyridine and 2,2’b i q u i n ~ l i n e .Consequently, ~~~ the EPR method is useful for the study of the conformers in the lowest triplet (TI) states. However, the EPR signals of the TI state of trans-2,2’-DPE have not yet been observed through steady-state measurements. This is because of its short T1 lifetime.9 An optically detected
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t Resent address: Tokyo Research Laboratories, Kao Corp., Bunka, Sumida-ku, Tokyo 131, Japan. Abstract published in Advance ACS Abstracrs, October 15, 1994. @
0022-3654/94/2098-12521$04.50/0
I
DPE-1
/y
DPE-2
DPEJ
Figure 1. Molecular structures and coordinate system for conformational isomers of trans-2,2’-DPE.
magnetic resonance (ODMR) technique would be a powerful tool to detect short-lived T1 molecules.lOsll However, the very weakly phosphorescent character of trans-2,Y-DPE has prevented the detection of ODMR signals.’* The time-resolved EPR technique f i s t proposed by Kim and Weissman is a powerful tool for the study of short-lived and very weakly phosphorescent T1 m01ecules.l~ In the present work, we have observed the time-resolved and steady-state EPR spectra of the T1 state of trans-2,2’-DPE in rigid solutions and in stretched poly(viny1 alcohol) (PVA) films at 77 K. The conformers of trans-2,2’-DPE are discussed from the ZFS parameters and the relative populating rates of individual T1 sublevels. We have also observed the luminescence spectra of trans-2,2’-DPE in rigid solutions at 77 K. The main purpose of the study is to elucidate the conformation of the very weakly phosphorescent and short-lived TI state of trans2,2’-DPE.
2. Experimental Section 2.1. Materials. trans-2,Y-DPE (Tokyo Kasei E. P. Grade) was purified by recrystallization from ethanol and 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. Methanol (Dotite Luminasol), ethanol (Katayama Kagaku Luminasol), 2-propanol (Dotite Luminasol), and water (Dotite Luminasol) were used without further purification. All solvents were carefully checked for the absence of extraneous EPR signals prior to use. The sample solutions of trans-2,2’-DPE 0 1994 American Chemical Society
Shioya et al.
12522 J. Phys. Chem., Vol. 98, No. 48, 1994 were prepared at the concentration of 3 x lop3 mol dm-3 in diethyl ether-isopentane -ethanol (EPA, 5 :5 :2 by volume), in methanol-ethanol (ME, 1:l by volume), and in 2-propanolwater (i-PrOH-H2O) mixtures. A pure PVA film was obtained by the same method as described previo~sly.’~After heat treatment for about 1 h at 80 “C and swelling in distilled water at 40 “C, the swollen film was soaked in the methanol solution of a sample until an appropriate amount had penetrated into the film by diffusion. Then the film was stretched at 70 “C using a Shibayama SS-60 film stretcher. The films thus obtained have about 250% of stretch in the stretched direction s. 2.2. Apparatus. The transient EPR signals were measured using a JEOL-FElXG spectrometer without field modulation. A Lumonics HE-420 excimer laser (XeC1,308 nm) and an NDC Japan JH-1000L nitrogen laser (337 nm) were used as an exciting light source. The transient EPR signals were recorded with an Iwatsu DM-901 digital memory. With the aid of an NEC PC-8801 microcomputer system, the transient signals were integrated numerically to obtain the time-resolved EPR spectrum. The apparatus for the conventional steady-state EPR measurements was described previo~sly.’~The excitation was carried out using a Canard-Hanovia 1 kW Xe-Hg arc lamp equipped with a Toshiba UV-D33S glass filter and a Copal DC494 electromechanical shutter. For the luminescence measurements, samples were excited at 308 nm or at 337 nm using an Ushio 500 W Hg lamp equipped with a Toshiba UV-D33S glass filter through a Jobin Yvon H-20 UV monochromator. The emissions from a sample were detected by a Hamamatsu Photonics R453 photomultiplier tube. All measurements were carried out at 77 K.
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3. Results and Discussion
400 500 300 B 1 mT Figure 2. (a) Steady-state and (b-d) time-resolved EPR spectra for the TIstate of rruns-2,2’-DPE (a, b) in EPA, (c) in PVA (B 11 s), and (d) in PVA (B 11 n) at 77 K. The excitations were carried out at 308 nm and the sampling time was set 0.30-0.94 ps after the laser pulse for the time-resolved measurements.
3.1. EPR Spectra. The steady-state and time-resolved EPR spectra of the T1 state of truns-2,2’-DPE were measured in EPA at 77 K, as shown in Figure 2. Cis-trans geometrical isomerization of organic molecules containing an ethylenic double bond, such as stilbene, has been investigated by many workers.16 The fluorescence quantum yield of trans-stilbene is 1.O and the quantum yield of truns cis photoisomerization is zero in rigid solutions at 77 K.”,’* The intensity of EPR signals of truns-2,2’-DPE does not change through the continuous ultraviolet light irradiation within our observation time of 30 min in EPA and in ME at 77 K. Consequently, the trans cis photoisomerization is likely to be negligible in rigid matrices at 77 K and the observed EPR spectra can be assigned to the trans isomer. The A M s = i l transition signals of truns-2,2’-DPE are very weak throughout the steady-state EPR measurements, as shown in Figure 2a. On the other hand, they are strong throughout the time-resolved EPR measurements. Therefore, the assignment of the observed EPR signals was carried out for the timeresolved spectrum with the aid of the stretched PVA film method,14 as shown in Figure 2, c and d. From the X-ray crystal analysis data of trun~-2,2’-DPE,’~,*~ the dihedral angle between the ethylenic and pyridyl groups was estimated to be 2.8”. Assuming the molecular planarity in the T1 state, the principal axes (x, y , z ) of the ZFS tensor were taken to be as shown in Figure 1. According to the general relationships concerning the orientation of guest molecules in stretched PVA films,21the assignment of the resonance field is straightforward. The intensity of the X signals is relatively enhanced when the external magnetic field B is parallel to the stretched direction of a film s, as can be seen in Figure 2c. On
the other hand, the intensity of the Z signals is relatively enhanced when B is parallel to the normal of the film plane n, as shown in Figure 2d. As a result, all the observed signals can reasonably be assigned as in Figure 2. The time-resolved EPR spectrum of the TI state of truns2,2’-DPE in ME at 77 K was measured through the excitation at 308 nm, as shown in Figure 3a. As is clearly seen in Figure 3a, there are at least two T1 species in ME at 77 K. In this , is paper the species which gives the absorptive B ~ , signal denoted hereafter as the A conformer (A-DPE) and the species which gives the emissive Bfi,, signal is denoted hereafter as the B conformer (B-DPE). The time-resolved EPR signals assigned to A-DPE and B-DPE should be attributed neither to signals from impurities nor to the formation of aggregates of truns-2,2’-DPE in ME. The reasons for this assignment are as follows: (1) the steady-state EPR Bfin signal decay is exponential within experimental error for all of the resonance magnetic fields monitored and gives the T1 lifetime of 11 ms, as shown in Figure 4;(2) only one set of the time-resolved EPR signals was observed in i-PrOH-HzO (30 wt % of i-PrOH) as mentioned in section 3.6; (3) the time-resolved EPR spectrum observed in ME does not depend on the concentration of the sample solution in the range from 3 x to 1 x mol dm-3. 3.2. Excitation Energy Dependence of Time-Resolved EPR Spectrum. The time-resolved EPR spectrum of the TI state of truns-2,2’-DPE in ME at 77 K was observed through the excitation at 337 nm using the nitrogen laser, as shown in Figure 3b. Only one set of time-resolved EPR signals was observed. The resonance fields observed through the excitation at 337 nm coincide with those of A-DPE observed through the
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Triplet State Conformers of tran~-2,2’-Dipyridylethylene
J. Phys. Chem., Vol. 98, No. 48, 1994 12523
n
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time J ms Figure 4. Decay curves of the steady-state EPR &in signal at (a) 147.1 mT and (b) at 149.4 mT for the TI state of fruns-2,2’-DPE in ME at 77 K. Each trace is the result of 100 computer accumulations.
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100
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B /mT
Figure 3. Time-resolved EPR spectra for the T1 state of fruns-2,2‘DPE in ME at 77 K through the excitation (a) at 308 nm and (b) at 337 nm. The sampling time was set (a) 0.30-0.94 ps and (b) 0.301.58 ps after the laser pulse. Computer-simulated time-resolved EPR spectra of (c)A-DPE obtained by using D = 0.0912 cm-’, E = -0.0443 cm-l, and Px:Py:Pi= 0:l:O and of (d) E-DPE obtained by using D = 0.0912 cm-l, E = -0.0414 cm-I, and P,:Py:Pz = 0.36:0.64:0. (e) Computer-simulated time-resolved EPR spectrum by superposing the spectrum of A-DPE on that of E-DPE. The intensity ratio of the lowfield Y signals of A-DPE and E-DPE is 0.6:0.4 in the superposed spectrum. All spectra are normalized at the low-filed Y signals.
excitation at 308 nm, as shown in Figure 3, a and b. Since the time-resolved EPR spectrum observed through the excitation at 337 nm can be regarded as the “almost pure” spectrum of A-DPE, the spectrum of B-DPE can be separated from the spectrum of the mixture of A-DPE and B-DPE as described below. The D values obtained are about 0.1 cm-’ in both A-DPE and B-DPE and the T1 lifetimes are about 10 ms. These values suggest that the T1 states possess mainly a 3nn*character in both A-DPE and B-DPE. As is known comprehensively, T, sublevels are the lowest in energy for 3nn*states.1° Consequently, the order of the TI sublevels was determined to be T,, Ty, and T, from the top in A-DPE and B-DPE. The polarities of the time-resolved EPR signals of A-DPE at the stationary fields are A, AEMEAE from the low-field to the high-field, as shown in Figure 3b. Here, A and E denote absorption and emission, respectively, of the microwaves. The spectrum of the randomly oriented triplet state with spin polarization was simulated in the same manner as presented by
Kottis and Lefebvre with some modifications.22 In the present simulation a Gaussian line width of 2.0 mT was used. With the aid of the computer simulation, the relative populating rates were estimated to be P,:P,:P,= 0:1:0, as shown in Figure 3c. The sublevel preferentially populated by intersystem crossing (ISC) process was found to be Ty , the middle sublevel. The time-resolved EPR spectrum of truns-2,2’-DPE in ME observed through the excitation at 308 nm could be well reproduced by superposing the simulated spectrum of A-DPE on that of B-DPE, as shown in Figure 3e. Here, the ZFS parameters and relative populating rates of B-DPE were assumed to be D = 0.0912 cm-l, E = -0.0414 cm-’ and P,:P,:P,= 0.36:0.64:0, as shown in Figure 3d. The polarities of the timeresolved EPR signals of B-DPE at the stationary fields are E, EEA/EAA(weak) from the low-field to the high-field, as shown in Figure 3d. The sublevels preferentially populated by ISC were found to be T, and T,, the in-plane sublevels. As a result, we can reasonably obtain the ZFS parameters and relative populating rates of A-DPE and B-DPE as shown in Table 1. We can see from Table 1 that the E value and relative populating rates are sensitive to the conformational change. The D value of ethylene has been calculated: ID1 = 0.1847 ~ m - l . * The ~ D value of the phosphorescent 3nn*state of 2,6dimethylpyridinehas been measured: ID1 = 0.136 cm-1.24Since the observed D values of A-DPE and B-DPE are fairly smaller than those of ethylenic and pyridyl fragments, the two unpaired electrons localize neither on the ethylenic fragment nor on the pyridyl one. The delocalized character of the unpaired electrons in the T1 state has also been observed for tr~ns-stilbene.~~ 3.3. Excitation Energy Dependence of Fluorescence Spectrum. The fluorescence spectra of truns-2,2’-DPE were observed through the excitation at 308 nm and at 337 nm in ME at 77 K, as shown in Figure 5 , a and b. The red-shifted fluorescence spectrum was observed through the excitation at 337 nm, as shown in Figure 5a. The fluorescence spectra observed in the present work are similar to those observed in EPA at 77 K.6 The fluorescence spectrum observed in ME at 77 K does not depend on the concentration of the sample solution in the range from 3 x to 1 x mol dm-3. This shows that the fluorescence from the aggregates is negligible. The S1 energy in ME at 77 K is estimated from the first vibronic band to be 29 470 cm-I in A-DPE and 30 120 cm-I in B-DPE.
12524 J. Phys. Chem., Vol. 98, No. 48, I994
Shioya et al.
TABLE 1: Zero-Field Splitting Parameters (cm-’), Lifetimes (ms), t, and Relative Populating Rates, Pi, in the TIStates of trans-DPE’s Observed at 77 K molecule A-DPE B-DPE C-DPE trans-4,4’-DPtif
host MeOH-EtOHd MeOH-EtOHd i-PrOH-HaO‘ EPA
X
Y
Z
D”
Eb
77
0.0747 0.0718 0.0761 0.0777
-0.0139 -0.0110 -0.0215 -0.0150
-0.0608 -0.0608 -0.0564 -0.0631
0.0912 0.0912 0.0845 0.0947
-0.0443 -0.0414 -0.0488 -0.0464
11 11 10
D = (-3/2)Z. E = (1/2)(Y - X). Obtained from the decay of the steady-state B, (30 wt % of i-PrOH). fReference 27. a
21
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P,
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1.o
0.36 0.22 0.5
0.64 0.78 0.5
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signal. MeOH-EtOH (1:l by volume). e i-PrOH-HzO
TABLE 2: Calculated Zero-Field Splitting Parameters (cm-’) and 8’s (deg) for the TIStates of trans-DPE’s and trans-Stilbene molecule X Y Z D ~e DPE-la 0.1063 -0.0406 -0.0658 0.0987 -0.0734 48 DPE-2“ 0.1059 -0.0396 -0.0663 0.0994 -0.0727 48 DPE-3“ 0.1050 -0.0384 -0.0666 0.0999 -0.0717 48 trans-4,4‘-DPEb 0.1210 -0.0529 -0.0681 0.1021 -0.0869 48 trans-stilbenec 0.1044 -0.0348 -0.0696 0.1044 -0.0696 47 Calculated by using the geometry of truns-2,Y-DPE in its c r y ~ t a l . ’Calculated ~ ~ ~ ~ by using the geometry of truns-4,4’-DPE in its crystal.19 Calculated by using the geometry of trans-stilbene in its crystal.30
325
350
375
400
425
wavelength I nm Figure 5. Fluorescence spectra of truns-2,2‘-DPE in ME at 77 K through the excitation (a) at 308 nm and (b) at 337 nm.
3.4. Assignment of Time-Resolved EPR Spectra to the Conformers. According to the X-ray crystal analysis data, the ethylenic and pyridyl groups are nearly coplanar.2o In general, there is a geometry change in the TI state relative to the G state in a nonrigid molecule like stilbene.26 However, only Z signals are relatively enhanced when B is parallel to the normal of the stretched PVA film plane n and the populating rates of the inplane sublevels are much larger than that of the out-of-plane sublevel in both A-DPE and B-DPE. These facts show that the deviation from the planar structure in the T I state is small in A-DPE and B-DPE. truns-4,4’-DPE, for which no conformers can exist in a planar geometry, shows only a very small excitation energy dependence of fluorescence.6 Only one set of time-resolved EPR signals has been observed for trun~-4,4’-DPE.~’On the other hand, the fluorescence spectrum and the time-resolved EPR spectrum depend on the excitation energy in truns-2,2’-DPE as mentioned above. These facts also suggest the molecular planarity in the T1 state for the conformers of truns-2,2’-DPE. The problem we are faced with is the assignment of A-DPE and B-DPE to specified conformers shown in Figure 1. The ISC rates are not highly anisotropic for 3zx*states of aromatic hydrocarbons, such as benzene and naphthalene, while they are fairly anisotropic for azaaromatics.10~11,28 One possible explanation of the anisotropy of ISC in diazastilbenes arises from a consideration of the local symmetry of the aza nitrogens and the dominant role they play in spin-orbit For DPE- 1, the y axis is nearly perpendicular to the lone pair orbitals on the nitrogen atoms in the molecular plane. The directions
of the principal axes of the ZFS tensor are discussed in section 3.5. We would therefore expect that the Ty sublevel is predicted to be dominantly populated as is observed for A-DPE. On the other hand, for DPE-2, the y axis makes angles of about 80” and 16” to the lone pair orbitals on the nitrogen atoms in the molecular plane. We would therefore expect that both Ty and T, sublevels are predicted to be preferentially populated as is observed for B-DPE. As a result, A-DPE and B-DPE can reasonably be assigned to DPE-1 and DPE-2, respectively. It should be noted here that the SI state of DPE-1 was determined to be lower in energy than that of DPE-2 from the excitation energy dependence of the time-resolved EPR spectrum. This is in accord with the results of the fluorescence experiments by Bartocci et a1.6 3.5. Calculation of ZFS Parameters. The ZFS parameters were calculated for the three conformers of truns-2,2’-DPE and its related molecules assuming their planar structures as observed in the crystalline states. The results are given in Table 2. Since the geometries of DPE-2 and DPE-3 are not known, those were obtained by the 180” rotation of the pyridyl rings around the quasi-single bond between the ethylenic carbons. The wave functions used were constructed from Pariser-Parr-Pople-type linear combination of atomic orbital (LCAO) molecular orbitals by including confgurations arising from all the single excitations relative to the G state. Then ZFS parameters were calculated with only Coulomb-type spin-spin interaction integrals evaluated using double-9 self-consistent field AO’s. The details of the procedure are the same as those described in the previous paper (Treatment A) by H i g ~ c h i .As ~ ~shown in Figure 1, 8 denotes the angle between the x axis of the ZFS tensor and the L axis. The L axis is taken to be parallel to the quasi-single bonds between the ethylenic and pyridyl groups. The observed IEl value of DPE-1 is larger than that of DPE-2. It may be noted here that such a trend of E value could not be obtained if the geometrical structure of truns-2,2’-DPE is assumed to be the same as that of trans-stilbene. The calculated E value is sensitive to the geometrical structure of the aromatic rings. Although the calculated ID1 and [El values are slightly larger than the experimental values, the calculated values can explain the observed ones semiquantitatively. It may be noted here that the calculated values of 8 of the three conformers are nearly identical, although the value of 8 has not yet been determined
Triplet State Conformers of trun~-2,2‘-Dipyridylethylene
J. Phys. Chem., Vol. 98, No. 48, 1994 12525
4. Conclusions The time-resolved EPR spectra of the two conformers were separated from the spectrum of their mixture using the technique of selective excitation. The observed two conformers have been assigned to the specified conformers through the analysis of the anisotropic ISC processes. We have clarified that the ZFS E parameter and the anisotropy of the triplet sublevel populating rates are sensitive to the conformational change. The timeresolved EPR method is useful for studying the conformational isomers of short-lived and very weakly phosphorescent TI molecules, where the steady-state EPR and ODMR methods are not applicable.
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Figure 6. (a) Time-resolved EPR spectrum for the TI state of frans2.2’-DPE in i-PrOH-HzO (30 wt % of i-PrOH) at 77 K through the excitation at 308 nm. The sampling time was set 0.30-0.94 ps after the laser pulse. (b) Computer-simulated spectrum obtained by using D = 0.0845 cm-l, E = -0.0488 cm-’, and Px:P,:Pz= 0.22:0.78:0.
experimentally. The present calculation shows that the observed change in the anisotropy of the triplet sublevel populating rates is not due to the effect of the change in 8 but to the intrinsic effect of the conformational change. 8 3.6. Time-Resolved EPR Spectra in i-PrOH-HzO. The time-resolved EPR spectra of the TI state of truns-2,2’-DPE were measured in i-PrOH-HzO at 77 K in the range from 20 to 34 wt % of i-PrOH, as shown in Figure 6a. Only one set of the time-resolved EPR signals was observed. The ZFS parameters of truns-2,2’-DPE obtained in i-PrOH-Hz0 are different from those of DPE-1 and DPE-2, as shown in Table 1. Consequently, truns-2,2’-DPE in i-PrOH-HzO (from 20 to 34 wt % of i-PrOH) at 77 K is expected to have a different conformation from those of DPE-1 and DPE-2. In this paper the species observed in i-PrOH-Ha0 (30 wt % of i-PrOH) is denoted hereafter as the C conformer (C-DPE). Although the conformation of C-DPE seems to be unclear, the ZFS pattern for this conformer may be assumed to be the same as those for the A and B conformers. The ZFS parameters and the T1 lifetime are listed in Table 1. The similar solvent effect on the TI state of 2,T-bipyridine has previously been r e ~ 0 r t e d . l ~ The polarities of the time-resolved EPR signals of C-DPE at the stationary fields are (obscure), AEA/EAE from the lowfield to the high-field, as shown in Figure 6a. The spectrum of the randomly oriented TI state with spin polarization was simulated for C-DPE in the same manner as for A-DPE. In this simulation a Gaussian line width of 3.5 mT was used. With the aid of the computer simulation, the relative populating rates were estimated to be P,:P,:P, = 0.22:0.78:0, as shown in Figure 6b. The sublevel preferentially populated is Ty, the middle sublevel. As the sublevel preferentially populated by ISC is expected to be T, for DPE-3 from the spin-orbit selection rule mentioned in section 3.4, the conformer C-DPE should not be assigned to DPE-3.
Acknowledgment. The authors thank Mr. Keiichiroh Matsushima and Mr. Kazumasa Takemura of our laboratory for their help in measuring the luminescence spectra and calculating the ZFS parameters. We also thank Dr. Kanekazu Seki of our faculty for his kind advice. References and Notes (1) Perkampus, H.-H.; Muller, P. Z. Naturforsch. B 1970, 25, 917. (2) Coletta, F.; Gambaro, A.; Pasimeni, L. Gau. Chim. Ztal. 1973,103, 265. (3) Novak, I.; Klasinc, L.; Knop, J. V. Z. Naturforsch. A 1977, 32, 886. (4) Knop, J. V. Croat. Chim. Acta 1973, 45, 419. ( 5 ) Marconi, G.; Orlandi, G.; Poggi, G. J. Photochem. 1982, 19, 329. (6) Bartocci, G.; Masetti, F.; Mazzucato, U.; Dellonte, S.; Orlandi, G. Spectrochim. Acta 1982, 38A, 729. (7) Yagi, M.; Schlyer, B. D.; Maki, A. H. Chem. Phys. 1991, 157, 209. (8) Yaei. M.: Saitoh. A.: Takano. K.: Suzuki. K.: Himchi. J. Chem. Phys. Ltt.1985, 118, 275. (9) Elisei. F.: Mazzucato. U.:Gomer, H.; Schulte-Frohlinde, D. J. Photochem. 1987, 37, 87. (10) Kinoshita, M.; Iwasaki, N.; Nishi, N. Appl. Spectrosc. Rev. 1981, 17, 1. (11) El-Sayed, M. A. Annu. Rev. Phys. Chem. 1975, 26, 235. (12) Gomer, H. J. Phys. Chem. 1989, 93, 1826. (13) Kim, S. S.; Weissman, S. I. J. Magn. Reson. 1976, 24, 167. (14) Higuchi, J.; Yagi. M; Iwaki, T.; Bunden, M.; Tanigaki, K.; Ito, T. Bull. Chem. Soc. Jpn. 1980, 53, 890. (15) Yagi, M.; Makiguchi, K.; Ohnuki, A.; Suzuki, K.; Higuchi, J.; Nagase, S . Bull. Chem. SOC.Jpn. 1985, 58, 252. (16) Langkilde, F. W.; Wilbrandt, R.; Brouwer, A. M.; Negri, F.; Zerbetto, F.; Orlandi, G . J. Phys. Chem. 1994, 98, 2254. (17) Malkin, S.; Fischer, E. J. Phys. Chem. 1964, 68, 1153. (18) Sumitani, M.; Nakashima, N.; Yoshihara, K.; Nagakura, S. Chem. Phys. Len. 1977, 51, 183. (19) Vanant, J.; Smets, G.; Declercq, J. P.; Germain, G.; van Meerssche, M. J. Org. Chem. 1980, 45, 1557. (20) Kitagawa, S.; Matsuyama, S.; Munakata, M.; Emori, T. J. Chem. Soc., Dalton Trans. 1991, 2869. (21) Ito, T.; Higuchi, J.; Hoshi, T. Chem. Phys. Lett. 1975, 35, 141. (22) Kottis, Ph.; Lefebvre, R. J. Chem. Phys. 1963, 39, 393. (23) Boorstein, S. A.; Gouterman, M. J. Chem. Phys. 1963, 39, I
2443. (24) Motten, A. G.; Kwiram, A. L. J. Chem. Phys. 1981, 75, 2608. (25) Yagi, M. Chem. Phys. Lett. 1986, 124, 459. (26) Ikeyama, T.; Azumi, T. J. Phys. Chem. 1994, 98, 2832. (27) Yagi, M.; Mikuni, K.; Takano,K.; Shioya, Y.; Higuchi, J. J. Phys. Chem. 1989, 93, 4714. (28) El-Sayed, M. A.; Moomaw, W. R.; Chodak, J. B. Chem. Phys. Lett. 1973, 20, 11. (29) Higuchi, J. Bull. Chem. SOC.Jpn. 1981, 54, 2864. (30) Finder, C. J.; Newton, M. G.; Allinger, N. L. Acta Crystallogr. B 1974, 30, 411.
