J. Phys. Chem. 1992, 96, 9328-9331
93%
(33) Diesen, R. W. J . Chem. Phys. 1964,4I,3256. (34)Cheah, C. T.;Clyne, M. A. A.; Whitefield, P. D. J . Chem. Soc., Faraday Trans. 2 1980,76,711-728. (35) Rozenshtcin, V. B.;Bedzhanyan, Yu. R.; Genhenzon, Yu. M. Kiner. Katal. 1988,29,30.
(36)Quinones, E.;Habdas, J.; Setser, D. W. J. Phys. Chem. 1987,91, 5155. (37) Benard, D. J.; Winker, B. K.; Seder,T. A.; Cohn, R. H. J . Phys. Chem. 1989,93,4790. (38) Herbelin, J. M.Chem. Phys. Lou. 1976,42,367.
TlmcResdved EPR and Phosphorescence Studies of the Lowest Excited Triplet State of Benzil MasaBiro M a ,Seigo Yamauchi: Noboru Hirota,* Department of Chemistry, Faculty of Science, Kyoto University, Kyoto 606, Japan and Jiro Hipchi* Department of Physical Chemistry, Faculty of Engineering, Yokohama National University, Yokohama 240, Japan (Received: April 23, 1992; In Final Form: July 20, 1992)
Time-resolved EPR (TREPR) and phosphorescence studies of the excited triplet states of benzil in various environments are made. On the basis of the TREPR spectral pattem and the phosphorescence peak position (A,,), the T1state is classified into three types. A clear correlation is found between the zfs and A,,, for the TIstates in various environments. This correlation indicates that both the zfs and A,,, are determined by the geometry of the T1 state, particularly the dihedral angle (8) between the two carbonyl groups. A model calculation of the zfs can account for the observed change in terms of the change in 8.
Introduction
The structure of benzil is considered to be flexible with respect to the dihedral angle (e) between the two carbonyl groups. Its photophysical properties drastically depend on the environment reflecting the structural change. Several spectroscopic studies have been made to understand the relationships between the excited-state structure and photophysical pr~perties.~-~ The peak maximum of the phosphorescence of benzil varies remarkably depending on the system ranging from 490 nm in an x trap4 to 570 nm in crystalline methylcyclohexane (MCH)) and sol~tion.~ From ENDOR studies, Chan and Heath6 and Itoh et al.' determined 8 of the lowest excited triplet (TI) state of benzil in a neat crystal (phosphorescence maximum at 520 nm) to be 157 and 150°, respectively, which differ remarkably from 8 = 111 in the ground state.* On the other hand, from an ODMR study, Asano et al. concluded that the structure of TI benzil in crystalline MCH is trans-planar (8 = 180)? A dipolemoment measurement also supported a trans-planar structure of the T1state in solution? Though it is now generally accepted that the T1 state of benzil assumes a trans or a near trans structure, there still remain some ambiguities about the exact structure of the T1state. For example, Roy et al.1° observed a time-dependent change in the phosphorescence spectrum (530-570 nm) in ethanol at a temperature just above the melting point and ascribed this change to structural relaxation from a skew to a planar conformation. However, the skew structure of the TI state with the 530-nm phosphorescence peak was not certain. The triplet properties such as zero-field splittings (zfs) and sublevel populations are also expected to reflect a structural change. However, such data are scarce compared with the phosphorescence data. It is now well-established that the timeresolved EPR (TREPR) technique is a convenient means to study the sublevel properties of short-lived triplet states." Zero-field splittings and the relative populating rates of triplet benzil under various conditions could be determined easily by using the TREPR method. The present work was undertaken to obtain further information about the relationship between the structure of the T I state and 'Present address: Institute for Chemical Reaction Science, Tohoku University, Katahira, Sendai 980, Japan.
its properties. Our observation'* that the polarization of the CIDEP signals of the radicals produced by the photolysis of bcnzil in 2-propanol is opposite to that expected from the usual TI benzil motivated us to examine the sublevel properties of triplet benzil under various conditions. Here we first determine the sublevel properties of the TI states under various conditions and examine the correlation between the zfs and the phosphor-ce spectrum. The zfs of benzil are also compared with those of the related molecules. Next, the dependence of the zfs on the structure of benzil is examined on the basii of a model calculation of zfs. We show that the zfs decrease with the deviation from a trans-planar structure.
ExperimenClrl Section Benzil (Nacalai Tesque) was recrystallized from ethanol several times and was zone-refined through 200 band paths. Solvents of spcctrograde purchased from Nacalai Tesque were used without furtherpurification. The concentrations of benzil in solutions were 10-1-104 M. The solutions were deaerated by blowing nitrogen gas. In phosphorescence measurements, the solutions were deaerated by repeated freeze-pumpthaw cycles and then sealed in quartz sample tubes. The samples were excited by a 900-W Xe arc lamp through a NiS04 solution filter and a UVD33S glass filter or an excimer laser (A = 308 nm). Emissions were focused on a SPEX 1704 1-m monochromator equipped with an EM1 9502B photinndtiplier. The TREPR signalswere detected without field modulation with a JEOGFE3X EPR spectrometer at 0.4-1.0 after the laser excitation (a Lumonics TE861M excimer laser with a XeCl fill, A = 308 nm, and a power of -60 mJ/pulse or a Lumonica HE440UBB excimer laser with -200 mJ/pulse). The spectra were taken by feeding an output of a modified preamplifier of the microwave unit to a PAR 160 boxcar integrator. The phosphorescence and TREPR signal decays were monitored by a transient memory (Kawasaki Electronika MRSOE) and accumulated on a signal averager (Kawasaki Electronika TMC700) or on a HP 9816 computer. Results .ad Discussion (1) Correlation between Zero-Field Splitting8 and Pbosphoresceace M~ximum. Figure 1 shows typical TREPR spectra of the lowest excited triplet (TI) state of benzil at 77 K and 0.4
OO22-3654/92/2096-9328S03 .OO/O 0 1992 American Chemical Society
Lowest Excited Triplet State of Benzil
,%)I
The Journal of Physical Chemistry, Vol. 96, No. 23, 1992 9329
2 rz
106
4 70
L I-
I-
Figure 1. Time-rcsolved EPR spectra of the lowest excited triplet state of benzil under different conditions and their simulated spectra: (a) in benzene under slow cooling, (b) in benzene under rapid cooling, and (c) in MCH under slow cooling. The upper spectra are observed ones, and the lower ones are the simulated ones. (d) Zero-field sublevel schemes for the triplets of types A, B, and C.
Flgm+ 2. Phoephortscenccspectra of b e d at 77 K (a) in b e m e under slow cooling, (b) in benzene under rapid cooling, and (c) in MCH under slow cooling conditions. E
4
A A
~~
Ph-
triplet matrix and conditions peak nm neat crystal (trap I)' 506 A powder 517 benzene slow cooling 519 B benzene rapid cooling 535 MCH rapid cooling 535 C MCH slow cooling 572 MCH slow cooling3 570
'D I -3/2Z,E
LP 3.57 3.56 3.41 3.83 3.80 5.13 5.14
Py-Px: Eo Pz-Px -0.76 0.05:0.95 -0.76 0.1:0.9 -0.16 0.1:0.9 -0.78 -0.1:0.9 -0.78 -0.1:0.9 -0.43 0 1 -0.48 0 1
(X- Y)/2.
p after the laser excitation under various ~ondition~ together with
the simulated ones. The simulation was made by following the procedure by Kottis and L,efebvreI3and taking into account the populations in the sublevels. The spin Hamiltonianthat determines the energies of the three sublevels is
8 5
A
P c
0
0
b
00 O 4
B The zfs (D and E) and the populating rates (Px:Py:Pz)are estimated from the simulation. The observed spectra are classified into roughly three types depending on their shapes and peak positions. The type A spectrum shown in Figure l a is found in a benzil powder and in a benzene matrix under slow cooling at a relatively high concentration. The type B spectrum shown in Figure 1b is found in ethanol, 2-propanol, and 3-methylpentaneglass matrices and in polycrystalline methylcyclohexane (MCH) and benzene under rapid cooling. The type C spectrum shown in Figure IC is observed in MCH under slow coolir~g.'~ Though the spectral details depend on the environment, all the spectra show the same polarization pattern of A,AAA/EEE (A and E denote an absorption and an emission of the microwave, respectively) from the low-field side to the high-field side. This spectral pattern indicates more pop ulations in the lower spin sublevels in all cases. The triplet states responsible for these three types of spectra are designated as triplets A, B, and C,respectively. The typical values of the zfs and populating rates determined from the simulation are given in Table I. The zfs vary considerably depending on the type of triplet state, with the largest zfs in triplet C and the smallest in A. The line width of the B-type spectrum is much wider than those of the A- and C-type spectra. The phosphorescence spectrum of benzil also varies depending on the environment. Figure 2 shows the typical phosphorescence spectra of triplets A, B, and C. Triplets A, B, and C show the phosphorescence maximum, k,,,,at about 520,540, and 570 nm, respectively. Thus, there is a correlation between the zfs and A,,,; a larger zemfield splitting is accompanied by a larger A,,. In order
o O 0
0 0 0
0 Q
3 480
500
520
540
560
580
h m (nm)
3. Relationship between the zfs parameters D* (=(@ + 3@)L/2) and the phosphorescmce peaks. &nzil(O),4,4'dimethylbcnzil (a),and l-phmyl-1,2-propanedione(A). 8 indicates the benzil data obtained by ODMR at 1.2 K (ref 4). The data are given in Table 11.
F@e
to examine this correlation more closely, we have studied the relationships between the zfs and the A,,, for b e n d and other related adicarbonyls as tabulated in Table 11. The result is shown in Figure 3 in which D* = (02 + 3E2)1/2determined from the values of H- are plotted against A,. It is seen that there is a clear m l a t i o n between the zfs and ,A,, in the adiwbonyls. This result suggests that the changes of the zfs and A,,, have the same cause. The most probable cause of these changes is the change in the 0 between the two carbonyl group. According to the ENDOR data: the x-trap triplet with A,,, = 517 nm is considered to have 0 = 157O. The triplet in MCH with A, = 570 nm is believed to have a trans-planar (0 = 180°) geometry from an ODMR studyS3 A dipolemoment measurement9(D = 0) of b e d in solution also
Mukai et al.
9330 The Journal of Physical Chemistry, Vo1. 96,No. 23, 1992 TABLE U Observed z+ro-Fpeld SpUttiog Parameter (0.)d 4,4'-Di10ethyl~d WorpbonranccW r i " ).A( Of PPDO & s c r d colmuw
neat crystal4 type A type B
type c
condition Benzil trap I trap I1 powder benzene (slow) methanol 2-propanol benzene (rapid) MCH (rapid) 3-methylpentane MCH (slow)' MCH (slow)
D*, GHz
A,
nm
3.81 3.38 3.80 3.71 4.20 4.21 4.27 4.34 4.50 5.21 5.23
506 495 517 519 527 530 535 535 537 570 572
4,4'-Dimethylbenzil ethanol 2-propanol MCH 3-methylpentane benzene
3.55 3.71 3.68 3.80 3.86
520 524 53 1 531 534
bond and the twisted angles (4) of the C-phenyl bond. The molecular structure is given in Figure 4. The bond lengths of C - C , (2-0, C-phenyl, and C-C (phenyl ring) are taken as 1.470, 1.200, 1S00, and 1.397 A, respeatively.8 The 042-phenyl and 04.242 angles are taken to be 120'. The matrix elements of the D tensors of the T I state with a wave function of are expressed as follows:
PPDO ethanol 2-propanol benzene MCH 3-methylpentane
5.53 5.60 5.64 5.68 5.70
530 535 542 541 542
(3)
supports the assignment of a trans-planar geometry for the triplet with A, = 570 nm. We take these triplet states as references. Since the zfs obtained from the simulations of triplets A and C are in good agreement with those of the above reference systems, these TI states are considered to have 8 = 157 and 180°, respectively. On the other hand, triplet benzil in rigid ethanol (triplet B) with A, = 530 nm was considered to have a skew geometry (e = 90°), and the change of A, from 530 to 570 nm near the melting point was interpreted in terms of the geometry change from a skew to a trans-planar structure." However, the fact that the zfs and A, of triplet B are in between those of triplets A and C seems to suggest that 0 of triplet B is somewhere between 157 and 180'. Wider line widths of the B-type spectra indicate that the geometries of the triplet states have more fluctuations than those of A and C. The phosphorescence change with regard to the change in 0 is explained as follows. The TI state of benzil is known to be 3nr* in character. The energy of the r* orbital is lowered in the trans-planar geometry because of the increased ?r conjugation around the C-C bond of the two carbonyl groups. As 0 deviates from 180', r conjugation is decreased and the energy of the r* orbital is increased, increasing the energy of the nr* state. The energies corraponding to A, are 19 230,18 520, and 17 540 cm-l for triplets A, B, and C, respectively. As to the related molecules, 4,4'-dimethylbenzil and 1phcnyl-1,2-propanedione (Ph(CO),Me; PPDO), we consider that they have geometries similar to that of benzil. Their TREPR spectra also show the similar spectral pattern of A,AAA/EEE. Only one spin sublevel (2)is predominantly populated. The zfs of 4,4'-dimethylbcmii arc only slightly d e r than those of bcnzil, but the zfs of PPDO arc much larger than those of benzil, though a simple Hackel MO calculation shows that delocalization of the *-electron density into the phenyl rings of benzil in the ?r* orbital is relatively small. Much larger zfs of PPDO are perhaps due to the combined effects of higher spin densities on the carbonyl groups and a near planar structure. (2) clllcul.tioaof the Change of Zero-Field Splitt€ngswith 8. In order to examine the dependence of the zfs on the geometrical change, we have calculated the dependence of the zfs on B based on a simple model. The zfs of the triplet states of carbonyls are usually due to the contributions by the spin-spin (SS)and spinorbit (SO)interactions. However, for the sake of simplicity, we neglect the contribution of the SO interaction. The zf parameters, D and E, are calculated for various rotation angles (e,) of the C-C
-
Figwe 4. Molecular structure and axes of benzil. The X axis is taken parallel to the C2axis.
and so on. Since the T I state of benzil is nr* in character, \kT, consists of the n and x* orbitals. The n electron is delocalized over the entire molecule, but for simplicity, we only consider the contribution by the oxygen atoms to the n orbital. We use the 2pCy) A O s perpendicular to each C - 0 bond in the 0-C-C plane for the n orbitals of the two oxygen atoms and the simple Htickel *-electron MO for the r* orbital. For the Htickel parameters, we use the values of (YO= (YC 8, &< = 0.98 cos e,, 8~~~~~= 0.98 cos 4, and Ipc=o = 8. Dij (ij = x, y , z) calculated by using these n and r* orbitals have two terms, the onecenter integrals on the oxygen atoms and the two-center integrals. The D matrices are diagonalized to give the zero.field sublevel energies, X,Y,and Z. Using the n- and *-electron spin densities on the oxygen atoms (pori and po*, respectively), the values of X,Y,and Z due to the one-center terms are given as 2 Y = jApo*pon (4)
+
x=
(:z : --
+ - ~ae, ).po=pon
from which we obtain D* = (02 + 3E2)'I2= ((3/2)($
22)]'/2as
n . u .
1
+
+P+
3(1 + 3 COS 8,)2)'/2 (7) APo"P0" where A is the zero-field splitting of the oxygen atom due to the electron spin-spin interaction. D* due to this term decreases as 8, increases. The onbccnter terms are calculated by using the Hartree-Fock SCF 2p AOs, and the two-center terms are calculated by using a half-point-charge appr~ximation.'~Chan and Heath estimated pori to be 0.34 on the basis of the ENDOR resuk6 We use this value for the calculation of D*. The D* values due to both the one- and two-center terms with various 8, are given in Table I11 and Figure 5. Though the spin densities of the n electron on the atoms other than oxygen are neglected in the above calculation, the error induced by this procedure would not ex& 10%. The calculation shows that D* decreases with the increases of 8,. The = ;(16
The Journal of Physical Chemistry, Vol. 96, No. 23, 1992 9331
Lowest Excited Triplet State of B e n d TABLE Ilk c.lflllrted ZerO-Fiekl SpUttiog pulmeter (W, CHz) rritb v m e,& 4 D*, GHz one-center sum of one- and only
4, deg
0
0 10 20 30
40 50 60
IO 80
00
O0
10.31 10.16 9.10 8.97 8.03 6.91 5.87 4.81 3.68
1.92 7.11
1.33 6.64 5.18 4.82 3.87 3.02 2.33
two-center So 1.94 1.78 1.34 6.65 5.19 4.83 3.88 3.02 2.33
100 1.91 7.82 1.38 6.69 5.81 4.85 3.89 3.03 2.34
12
10
8
2
This delocalization is considerably larger than estimated from the ENDOR result. If such delocalization takes place, contribution of the oxygen one-center terms to zfs is further reduced. Though the carbon onecenter terms contribute to zfs, this contribution is much smaller than that by the oxygen terms for the following reasons. Since the value of the one-center integral is proportional to the cube of the effective nuclear charge, the carbon onecenter integral is only about 0.36 of the oxygen one. Therefore, the contribution of the carbon one-center terms to zfs is estimated to be only 14%of the oxygen one. On the other hand, the twocenter tenns due to the n spin densities on the carbon atoms would produces a negative contribution to zfs, making a further decrease of the zfs. Another possible cause of the discrepancy is the neglect of the second-order effect of the spin-orbit (SO)interaction. In the cape of benzil, the z sublevel is the bottom one. Since the upper x and y sublevels go down by the SO interaction with the T2 (r#) state,17the values of the zfs may become smaller when the contribution of the SO interaction is included. Furthermore, the dependence of the D* value on the twisted angle q5 shows that the D* value also increases with the increase of 4. The twist between the C-phenyl bond decreases the conjugation between the r orbital of the carbonyl and those of the phenyl groups. Then the delocarization of the r electron into the phenyl groups decreases, increasing the zfs. However, this effect is found to be rather small as shown in Table 111. In conclusion, we believe that the observed change of D* is rationalized by the present model calculation.
conclusion
c
The zfs and phosphorescence spectrum of benzil vary remarkably depending on the environment. It is found that there is a good correlation between the zfs and the phosphorescence maximum. A model calculation of the zfs based on the spin-spin interaction can account for the decrease of D* with the decrease of the dihedral angle.
b 6
Registry No. PPDO, 519-01-1; bcnzil, 134-81-6; 4,4’-dimethylbenzil, 4351-48-6; glyoxal, 101-22-2.
References and Notes
4
2 I
120
140
160
180
degree
Figure 5. Calculated dependence of D* on the rotation of the dihedral angle. (1) Onaccnter term only (A) and (2) sum of one- and twc-center terms (A).
change of B from 180 to 150° reduces D* by 1.3 GHz. This reduction is close to the observed decrease of D* on going from triplet A to C. However, the magnitudes of the calculated D* values are somewhat larger than the experimentally obtained ones. We now consider possible causes for this discrepancy. In order to examine the delocalization of the n orbital further, we have calculated the n-electron densities in the planar TI state of glyoxal in the RHF approximation using the AM1 method.16 The result gives 0.286 and 0.109 electron densities on the oxygen and carbon atoms, respectively, indicating large delocalization.
(1) Morantz, D. J.; Wright, A. J. C. J . Chem. Phys. 1971, 54, 692. (2) Amett, J. F.; Mdjlynn, S. P. J . Phys. Chem. 1975, 79, 626. (3) Asano, K.; Aita, S.; Azumi, T. J. Phys. Chem. 1984,88, 5538. (4) Chan, I. Y.; Nelson, B. N. J . Chem. Phys. 1975,62,4080. (5) Fang, T. S.; Brown, R. E.; Kwan, C. L.; Singer, L. A. J. Phys. Chem. 1978,82,2489. (6) Chan, I. Y.; Heath, B. A. J . Chem. Phys. 1979, 71, 1070. (7) Teki, Y.; Takui, T.; Hirai, M.; Itoh, K.; Iwamura, H. Chem. Phys. k r r . 1982, 89, 263. (8) Brown, C. J.; Sadanaga, R. Acru Crystollogr. 1965, IS, 158. (9) Fessenden. R. W.: Carton. P. M.: Shimamori. H.: Scaiano. J. C. J . Phvs: Chrm. 1982. 86. 3803. 110) Roy, D. S.; Bdattacharyya, K.; Bera, S. C.; Chowdhury, M. Chem. Phys. krr. 1980, 69, 134. (1 1) Terazima, M.; Yamauchi, S.; Hirota, N. (a) J. Chem. Phys. 1986, 84,3619; (b) J. Phys. Chem. 1984,88,2682; (c) J . Phys. Chem. 1985,89, .**A
ILLU.
(12) Mukai, M.; Yamauchi, S.;Hirota, N. J. Phys. Chem. 1989,93,4411. (13) Kottis, P.; Lefebvre, R. J. Chem. Phys. 1963, 39, 393. (14) A preliminary report on the TREPR spectra of bend in MCH has been made. Okutsu, T.; Yano, K.; Kawai, K.; Obi, K. J. Phys. Chem. 1991, 95, 5401. (IS) Hipchi, J. J. Chem. Phys. 1963,38, 1231; 1963.39, 1339; 1963.39, 1841. (16) Dewar, M. J. S.;Zocbish, E. G.; Healy, E. F.; Stewart, J. J. P. J. Am. Chem. SIX. 1985, 107, 3903. (17) Hayashi, H.; Nagakura, S. Mol. Phys. 1972, 24, 801.
