J. Phys. Chem. 1989, 93, 5410-5414
5410
the BSSE has confirmed the results obtained in our previous studies on neutral hydrogen-bonded dimers. The picture of the interaction resulting after C P corrections is less basis set dependent for the interaction along the approaching path as well as for the quantities having the status of observables, Le., R , and hE(R,). The modifications introduced by C P corrections give more emphasis to the electrostatic character of the interaction, at the expense of other contributions. The approximation of using the electrostatic component to get an estimate of AE in noncovalent dimers was proposed many years and successively tested on many systems. In more recent times, this approximation has been proposed again66 and cor-
roborated by a number of examples. This approximation is not invalidated by C P corrections; on the contrary there is numerical evidence that artifacts due to the basis set dependence of Em may be reduced to a noticeable extent, making it easier to model H-bond interactions for large molecular systems.
Acknowledgment. We are grateful to the CNUCE Institute for a generous grant of computer time that allowed us to carry out the computations with the 3-21G+ and 6-31G** basis sets on the carboxylate- and phosphate-water complexes. Registry No. HCOO-,71-47-6; CH,COO-, 71-50-1 ; H2P0Lr 14066-20-7; H20, 7732-18-5.
~
(65) Bonaccorsi, R.; Petrongolo, C.; Scrocco, E.; Tomasi, J. Theor. Chim. Acra 1971, 20, 331.
(66) Buckingham, A. D.; Fowler, P. W. J. Chem. Phys. 1983, 79, 6426.
Influence of Solvent Polarity on the Excited Triplet States of Nonphosphorescent 1,P-Naphthoquinone and Phosphorescent 8,lO-Phenanthrenequinone: Time-Resolved Triplet ESR and CIDEP Studies Hirami Shimoishi? Shozo Tero-Kubota,t**Kimio Akiyama,t and Yusaku Ikegami*.t Chemical Research Institute of Non-Aqueous Solutions, Tohoku University, Katahira 2-1 -1. Sendai 980, Japan, and Coordination Chemistry Laboratories, Institute for Molecular Science, Okazaki 444, Japan (Received: November 17, 1988; In Final Form: February 28, 1989)
The excited triplet states of 1,2-naphthoquinone and 9,lO-phenanthrenequinonewere observed in several glassy matrices at low temperatures by time-resolved ESR method. Notable solvent effects on the zfs parameters were observed, indicating the small energy separation between 3nr* and 3rr*states in these o-quinones. It was found that the TI state has dominantly rr* character in ethanol while it is an n r * in nonpolar solvents. CIDEP spectra of the semiquinone radicals produced by photoinduced hydrogen abstraction from the solvent reflect the electronic character of the T1states.
Introduction Photochemistry of quinones has been extensively studied, and in particular, recent CIDEP (chemically induced dynamic electron polarization) studies in this field made a fundamental contribution to our understanding of the reaction mechanism of pquinones.14 It has been well established that the excited state taking part in the hydrogen abstraction and electron-transfer reactions of pquinones is the triplet (TI) state, and the ODMRS and time-resolved ESR (TRESR)6*7studies assigned the TI state to be n r * in character. In contrast to the case of p-quinones, there have been only a few studies on the electronic structure and photochemistry of o-quinones. It is known that several o-quinones are photoreactive in the presence of hydrogen donors.*-1° From the phosphorescence measurements, the TI state of 9,lO-phenanthrenequinone (9,lO-PQ) is identified as nr*." However, no information has been obtained for 1,2-naphthoquinone (1,2-NQ) because of its nonphosphorescent character. TRESR technique is a powerful tool for investigating the short-lived and nonphosphorescent TI states.l2-I4 In our recent work,15 anion radicals of several o-quinones were detected for the electron-transfer reactions of amines in acetonitrile. The CIDEP spectra were emissive for 1,2-NQ and 9,lO-FQ and absorptive for acenaphthenequinone in polar solvent. This result induced us to do a detailed study on the o-quinone triplet states. This paper deals with the influence of solvent polarity on the TI states of 1,2-NQ and 9,lO-PQ (Figure 1). The observed Present address: RI Laboratory, Fukushima Medical College, Hikarigaoka 1, Fukushima 960-12. Japan. 'Tohoku University. f Institute for Molecular Science.
0022-3654/89/2093-5410$01.50/0
zero-field splitting (zfs) parameters in some glassy matrices are discussed in terms of na*-ar* interaction. Photoinduced hydrogen abstraction of these o-quinones in several solvents afforded the CIDEP spectra, which also provided information for confirming the nature of the TI states.
Experimental Section Commercial 1,2-NQ, 9,1O-PQ, and 1,4-NQ were carefully purified by sublimation after recrystallization from ethanol. Solvents were purified by distillation after dehydration by pre(1) Wan, J. K.; Wong, S.-K.; Hutchinson, D. A. Acc. Chem. Res. 1974, 7, 58. (2) Wan, J. K. S.; Elliot, A. J. Acc. Chem. Res. 1977, 10, 161. (3) Wong, S. K. J. Am. Chem. SOC.1978, 100, 5488. (4) Primer, T.; Dobbert, 0.;Dinse, K. P.; van Willigen, H. J. Am. Chem. SOC.1988, 110, 1622. (5) Kinoshita, M.; Iwasaki, N.; Nishi, N. Appl. Spectrosc. Rev. 1981, 17, 1. (6) Murai, H.; Hayashi, T.; I'Haya, Y. J. Chem. Phys. Lett. 1984, 106, 139. (7) Murai, H.; Minami, M.; Hayashi, T.; I'Haya, Y. J. Chem. Phys. 1985, 93, 333. (8) Rubin, M. B.; Zwitkowits, P. J . Org. Chem. 1964, 29, 2362. (9) Maruyama, K.; Otsuki, T.; Naruta, Y. Bull. Chem. SOC.Jpn. 1976, 49, 791. (10) Takuwa, A,; Soga, 0.;Maruyama, K. J. Chem. SOC.,Perkin Trans. 2 1985, 409. (11) Kuboyama, A,; Yabe, S. Bull. Chem. SOC.Jpn. 1967, 40, 2475. (12) Akiyama, K.; Ikegami, Y.; Tero-Kubota, S.J. Am. Chem. Soc. 1987, 109, 2538. (13) Tero-Kubota, S.;Migita, K.; Akiyama, K.; Ikegami, Y. J. Chem. Soc., Chem. Commun. 1988, 1067. (14) Hirota, N.; Yamauchi, S.;Terazima, M. Rev. Chem. Intermed. 1987, 8, 189. (15) Shimoishi, H.; Akiyama, K.; Tero-Kubota, S.; Ikegami, Y. Chem. Lerr. 1988, 251.
0 1989 American Chemical Society
1,2-Naphthoquinone and 9,lO-Phenanthrenequinone n
n
L
X
9,lO-PQ
1,Z-NQ
Figure 1. Molecular structure and principal axes of o-quinones. In-plane molecular axes for 1,2-NQ are ambiguous because of its low symmetrical structure.
1”he Journal of Physical Chemistry, Vol. 93, No. 14, 1989 5411
TABLE I: Zero-Field Splitting Parameters (cm-I) in the TI States of 1,2-Naphthoquinone and 9,10-Phenanthrenequinonein Several Matrices 1,2-NQ
matrix EtOH DMF MTHF n-BuzO
9,lO-PQ
D*’
D
E
D*’
D
E
0.1275 0.1352 0.1711 0.25
0.114 0.12 0.16
0.033 0.036 0.035
0.1016 0.1308 0.1911
0.0935 0.12 0.19
0.023
.D* = (Dz+ 3E2)’lZ.
Y I
0.1
I
0.3 B / T
ixi , 0.1
0.03 0.035
I ‘
0.5
0.1
.
I
0.3
0.5
BIT
Figure 3. Observed (a-c) and simulated (a’+’) TRESR spectra for the 9,lO-PQ observed in (a) EtOH, (b) DMF, and (c) MTHF matrices at 15 K. The spectra were taken 1 ps after the laser irradiation.
TIstate of
, 0.3 BIT
,
,
0.5
0.l
0.3
Q5
BIT
Figure 2. Observed (a-d) and simulated (a’+’) TRESR spectra for the TI state of 1,2-NQ observed in (a) EtOH, (b) DMF, (c) MTHF, and (d) n-butyl ether matrices. The spectra were taken 1 1 s after the laser pulse irradiation at 77 K for a-c and at 15 K for d.
viously reported methods.16 Measurements of transient ESR spectra were carried out by using an X-band ESR spectrometer (Varian E-109E or JEOL FE2XG) without field modulation. The ESR signal was taken into a boxcar integrator ( N F BX531 or PAR 162) at arbitrary times after the laser pulse. A nitrogen laser (NDC JH-1000L or Molectron UV-24) was used as the source of the light pulse. The sample solutions for the measurements of the excited triplet state were degassed by three freeze-thaw cycles. A helium cryostat (Air Product, Ltd., Model 3-1 10) was used for the measurements at very low temperatures. For the CIDEP experiments, the solutions were deoxygenated by bubbling with pure argon gas for 2 h before use and were allowed to flow into a quartz tube within the ESR cavity (flow rate N 100 mL/h). All sample solutions were prepared at a concentration mol dm-3. of 1 X
Results Triplet ESR Spectra. TRESR spectra of the T I state of 1,2N Q were clearly observed in various glassy matrices with N2 laser excitation at low temperatures as shown in Figure 2. A conventional ESR spectrum due to the triplet state has never been detected upon continuous photolysis of these systems. The spectra of Figure 2 suggest that the TIstate of 1,2-NQ received significant matrix effects, implying a small energy separation between h r * and %7r* states. In an EtOH matrix, a very sharp lAMsl = 2 transition (emissive) was observed at 0.15 T. The lAMsI. = 1 transitions show the spin polarizations of EEA at the low-field half and of EAA at the high-field half, where E is emissive and A is enhanced (16) Riddick, J. A.; Bunger, W. E.;Sakano,T. K. Organic Soluenrs; Wiley: New York, 1986.
absorption. This polarization pattern suggests that preferential intersystem crossing (isc) occurred to the highest sublevel of the TI state. From the zfs parameters of 1 0 1 = 0.1 14 cm-’ and IEl = 0.033 cm-’ determined by computer simulation and the sharp line width of the TRESR spectrum, the T I state is considered to be m* in character in an EtOH matrix. With decreasing polarity of solvent matrices, spectral width increases and the lAMs( = 2 line shifts to the lower field, indicating the increase of the 1 0 1 value. The canonical points in the IAMsl = 1 transitions were unclear in nonalcoholic matrices because of the marked broadening of the spectral components. Therefore, on the basis of the value of D* = (D2 + 3E2)1/2derived from the IAMsl = 2 signal, the zfs parameters were estimated by iteration of the simulation to get the simulated spectra of Figure 2. Most appropriate values obtained from the procedure are listed in Table I. In an M T H F 0 1 = 0.16 cm-I (2-methyltetrahydrofuran) matrix, zfs parameters, 1 and IEl = 0.035 cm-’, and the spin polarization pattern of EEEE/AAA were obtained. However, the relative peak height of the lAMsI = 2 transition to the lAMsI = 1 signal is larger in the simulated spectrum than in the observed one. Inhomogeneous distributions of the ID1 and IEl values may be responsible for the spectral feature as pointed out for the p-quinone triplet This is probably because the environment of the quinones has some distributions and/or the molecular structure is deformed in the matrices, since the TI state clearly has na* character. In a Bu20 matrix, the triplet ESR spectrum observed only at very low temperature shows notable broadening and no canonical points due to the IAMsl = 1 transitions. Therefore, the simulation was not carried out, and the D* value estimated from the lAMs( = 2 transition is shown in Table I. The absorptive signal at g = 2.00 is due to the paramagnetic center in the quartz tube generated by laser irradiation. At 77 K, no TRESR signal was detected in this matrix, implying very fast electron spin relaxation. In the hydrocarbon matrices, such as M C H (methylcyclohexane) and toluene, no signal due to the triplet state was observed, even at 10 K. This may be ascribed to the large ID1 value and also to an inhomogeneous distribution of the zfs parameters induced by the interaction with the matrices. The above results strongly suggest that the TI state of 1,2-NQ is mr* in character in EtOH, while it is strongly n r * in nonpolar solvents. Although
5412
The Journal of Physical Chemistry, Vol. 93, No. 14, 1989 a
Shimoishi et al. TABLE 11: Observed Hyperfine Splitting Constants (eH,) and Experimental (pcxptl)and Calculated Spin Densities (paw) of the 1,2-Naphthoquinone Radical
posn I 2 3 4
NQH' produced by the photochemical hydrogen abstraction from (a) EtOH, (b) 2-PrOH, and (c) m-xylene by 1,2-NQ at -40 OC. The observed ones were taken 1 ps after the laser pulse. Ratios of TM and RPM contributions for 1,2-NQH': a', 6:4; b', 5 : 5 ; c'. 3:7. the almost pure a a * was observed for the T, state of 1,2-NQ in an EtOH matrix, no phosphorescence was detected for this system at 77 K. As shown in Figure 3, remarkable solvent effects on the triplet ESR spectrum were similarly observed for 9,lO-PQ. The TRESR measurements were carried out at 15 K, since N2 laser irradiation of the solution at 77 K easily produced a different signal overlapped with the triplet spectrum. The simulated spectra are shown in Figure 3, and the zfs parameters determined are shown in Table I. The ID1 value obtained in an EtOH matrix was smaller than that of 1,2-NQ. From the spectral pattern in an MTHF matrix, wide distribution of the zfs values was deduced. In BuzO and hydrocarbon matrices, no TRESR spectrum was due to the TI state of 9,lO-PQ was observed even at 10 K because of the large ID1 value and also the distribution of the zfs parameters. The observed zfs parameters and spectral shape suggest that the TI state of 9,lO-PQ has dominantly a a * character in an EtOH matrix and it is an na* state in nonpolar matrices. On the other hand, small solvent effects on the lifetime of the ~ 8.7 ms in EtOH and 4.1 ms in M C H at phosphorescence ( T = 77 K) were observed for 9,1O-PQ, though the spectrum showed broadening in the alcohol. These results agree well with those reported previously." The T~ observed in an EtOH matrix is unusually short for the phosphorescence from the 3 a a * state. However, if the mixing with the h a * is large, a broader TRESR spectrum and wide distribution of zfs parameters should be observed in the alcoholic matrix as a result of the environmental effects. Thus, the T, state of 9,lO-PQ is considered to be m * in an EtOH matrix. The small mixing with 3na* would induce the fast relaxation. The short lifetime is probably also due to a relatively large mixing with the nearby S,(na*) state by direct spin-orbit coupling, as will be discussed in later text, since the a * molecular orbital is mainly localized on the carbonyl groups loading large spin densities on the two oxygen atoms. CIDEP Spectra. Figure 4 shows the transient ESR spectra observed 1 ps after the laser irradiation of 1,2-NQ in EtOH, 2-PrOH, and m-xylene at -40 OC together with the computersimulated results. The simulations were based on the sets of ESR parameters of neutral semiquinone (1,2-NQH'; g = 2.0040, aH(1 H ) = 0.80 mT, aH(2 H ) = 0.18 mT, and aH(2 H ) = 0.14 mT) and of the radicals produced from each solvent.'' Participation (17) ESR parameters of the solvent radicals determined by computer simulation are as follows: g = 2.0030, a H ( C H 3 )= 2.22 mT, aH = 1.54 mT, and a H ( O H )= 0.10 mT for C H 3 6 H O H ;g = 2.0030 and a H ( C H 3 )= 1.96 mT for ( C H 3 ) , 6 0 H ;and g = 2.0026, a H ( C H 2 )= 1.622 mT, and a H ( C H 3 ) = 0.338 mT for the 3-methylbenzyl radical.
PCXPt?
PCalCdb
0.210 0.221
5
0.14 0.80 0.18
6
C
7 8 9 10 11 12
Figure 4. Observed (a-c) and simulated (a'-c') CIDEP spectra of 1,2-
mT
0.03 0.18
0.059
-0.062
0.338 0.076
0.265
0.013 0.076
0.091 0.028
0.038 0.080 -0.007 -0.022
0.125 0.034
0.14
OAssuming the value of Q = -2.37 mT.I9 *Parameters of the calculationZ0of l ,2-NQH': Coulomb integral, a l l = a + l .60 and a 1 2= 01 + 2.00; resonance integral, = 1.00 and p2,12 = 0.80; McLachlan parameter, X = 1.2. 'Not resolved. a
a'
Figure 5. Temperature dependence of the CIDEP spectra observed 1 ps after the laser pulse irradiation of the m-xylene solution of 1,2-NQ at (a) +20, (b) 0, and (c) -20 OC and the simulated spectra (a'-?), respectively.
of both triplet mechanism (TM) and radical-pair mechanism (RPM) was taken into account for the present CIDEP observations. The contribution of RPM is larger than that of TM in these solutions, though the phase of T M polarization depends on the solvent polarity, as will be discussed in later text. The small contribution of T M to these spectra implies that the rate of hydrogen abstraction by the triplet 1,2-NQ is relatively slow. The total width and the g value of 1,ZNQH' are both different from those of the anion radical reported previously.l* The hfs constants were tentatively assigned to each proton by comparing them with the spin densities calculated by McLachlan's procedure, as shown in Table 11. The observed spin densities on the carbon atoms in 1,2-NQH' were evaluated from the McConnell's relation pi = aHi/Q, where the value of Q = -2.37 mT was used." Parameters for the McLachlan calculation were chosen from the values by Streitwieser.*O The result indicates that about 80% of unpaired electron spin is delocalized on the naphthalene ring. Calculations for another possible structure of 1-hydroxy-2-oxysemiquinone could not explain the experimental spin densities. It is interesting that the spin distribution of 1,2-NQH' observed in the present work is significantly different from those of ion-pair (18) Pedersen, J. A. Handbook of EPR Spectra from Quinones and Quinol; C R C Press: Boca Raton, FL, 1985. (19) Karplus, M.; Fraenkel, G. K. J . Cftem. Phys. 1961, 35, 1312. (20) Streitwiesser, A,, Jr. Molecular Orbital Theory f o r Organic Chemists; Wiley: New York, 1967.
1,2-Naphthoquinone and 9,lO-Phenanthrenequinone
The Journal of Physical Chemistry, Vol. 93, No. 14, 1989 5413
-.
a nn*
,,*F T 2 XY /.
- T2Z _ _ __ __ -.-_.?!Q TIZ
ISC
1 nn*
? = 2TIZ TIY
Figure 6. CIDEP spectra observed 1 ps after the laser irradiation of 1,4-NQ in (a) EtOH, (b) 2-PrOH, and (c) m-xylene at -40 OC.
and metal complexes of 1,2-NQ.I8 The largest spin density at the 4-position of 1,2-NQH' well explains the result that the photochemical reaction of this quinone in the presence of a hydrogen donor mainly produced the adduct attached at this position.1° In alcoholic solvents, emissive T M polarization was observed. This phase is consistent with the result of the low-temperature experiment in an alcoholic matrix: Preferential isc to the highest sublevel of the TI state (aa*)of 1,2-NQ induces E polarization in the CIDEP spectrum. On the other hand, absorptive polarization of T M was observed in m-xylene. The same phase was observed in the cases of toluene and cumene solutions. The contribution of T M increases with increasing temperature, as shown in Figure 5. The spectral simulation indicates that the ratio TM/RPM is altered from 0.25 at -40 OC to 1.5 at +20 "C. This is ascribed to an increase of the hydrogen abstraction rate of the triplet 1,2-NQ from the solvent with the rise in temperature. Similar results were obtained for 9,lO-PQ. ESR parameters of the neutral semiquinone radical (9,lO-PQH') determined by simulation are g = 2.0038, aH(l H ) = 0.31 mT, aH(2 H ) = 0.29 mT, and aH= 0.10 mT. Solvent radicals with the same parameters as those obtained in the 1,2-NQ system were detected. The T M polarization was emissive in alcoholic solvents and absorptive in aromatic hydrocarbons. In contrast with the present o-quinones, no solvent effects on the CIDEP spectra were observed for p-quinone systems. In Figure 6 are shown the CIDEP spectra obtained from the laser photolysis of 1,4-NQ in EtOH, 2-PrOH, and m-xylene at -40 OC. Observations of 1,4-NQH' and solvent radicals with all E polarization indicate that these spectra can be interpreted by pure T M . It is clear that there are few solvent effects on the TI state (na*) of 1,4-NQ, indicating very little mixing of the 37ra*state, which may be ascribed to a large separation between the TI and the T2 states.
Discussion In EtOH, the TI states of the present o-quinones were found to be dominantly aa* in character on the basis of relatively small ID1 values (0.114 cm-I for 1,2-NQ and 0.0935 cm-I for 9,lO-PQ) and the sharp line width of the triplet ESR components. The spin polarization pattern of EEA/EAA observed in these quinones suggested that preferential population occurred to the highest triplet sublevel in the isc process. As has been reported for oquinones,21*22 the SI state is na* ( B , symmetry) and the lowest level of the ar* states has Bz symmetry. Thus, a dominant isc, 'nn*(B,) %a*(B2), is expected to occur to the T, sublevel ( Z / / C 2 ,axis, where the principal axes are taken as shown in Figure l), since direct spin-orbit coupling mixes S,(na*) only with the TZ sublevel in the pa* state. It can be easily deduced from the present results that the order of spin sublevels in 3aa* of
-
(21) Kuboyama, A. Bull. Chem. SOC.Jpn. 1960, 33, 1027. (22) Kuboyama, A.; Kozima, Y.; Maeda, J. Bull. Chem. SOC.Jpn. 1982, 55, 3635.
'-
SOC
TIX
EtOH
MTHF
(Dl = 0 . 1 1 4
= 0.160
[El = 0.033
= 0.035
TIXY
i
IC
TIXY
T1 z
nn*
MCH 2 0.3
Figure 7. Proposed energy diagram of the lowest h*and 1,2-NQ in several matrices.
%1r*
of
9,lO-PQ is T,, TY,and out-of-plane Tx from the top. In 1,2-NQ, in-plane principal axes are ambiguous because of its low symmetry. In aprotic polar solvents, such as DMF, triplet ESR spectra of the present o-quinones showed a broadening of line width and an increase of the ID1 value. Decreasing of matrix polarity tends to increase the zfs parameters. In nonpolar hydrocarbon matrices, no triplet ESR spectrum was detected probably by reasons of the large 101value and of wide distribution of the zfs parameters. It has been clarified by ODMR2*z6 and ESR2' studies, semiempirical calculation,28and phosphorescence analysis29that spin-orbit interaction between nearby 3na* and 3 ~ 7 r *states contributes to zfs parameters in aromatic carbonyl compounds. Spin-orbit mixing, which is sublevel selective, causes energy shifts in the triplet sublevels, leading to large 101values in the TI state. Therefore, increasing of the ID1 value observed in the present system may be due to the spin-orbit coupling between T2(na*) and T,(aa*) states. Although no triplet ESR spectrum was observed in nonpolar hydrocarbon matrices at very low temperature, CIDEP spectra observed in the hydrogen abstraction reaction showed absorptive T M in several hydrocarbon solvents in contrast with emissive T M detected in ethanol. These facts clearly indicate the change of the TI character. Then, the TI state of the present o-quinones in nonpolar solvents can be assigned to na* in character. The interpretation is summarized in Figure 7 . A similar diagram on the excited triplet states for xanthone has been reported.30 However, in the present system, solvent effects reverse the order of the energy levels of 3na* and 3aa*states. In hydrocarbon solvents, isc induced by spin-orbit coupling populates the T2,(aa*) state, and then, the internal conversion with spin conservation occurs to the lowest TIz(nr*). Free radicals generated from the reaction of the spin-polarized T I state gave the CIDEP spectra with absorptive TM. In our previous work,Is photochemical electron transfer for acenaphthenequinone yielded the anion radical with all absorptive CIDEP, even in polar solvent. This suggests that the energy separation between the TI and T2 (23) Cheng, T. H.; Hirota, N. J. Chem. Phys. 1972, 56, 5019. (24) Cheng, T. H.; Hirota, N. Mol. Phys. 1974, 27, 281. (25) Jones, C. R.; Maki, A. H.; Kearns, D. R. J . Chem. Phys. 1973, 59, 873. (26) Despres, A,; Migirdicyan, E. Chem. Phys. 1980, 50, 381. (27) Batley, M.; Bramley, R. Chem. Phys. Lett. 1972, 15, 337. (28) Hayashi, H.; Nagakura, S. Mol. Phys. 1972, 24, 801. (29) Laposa, J. D.; Bramley, R. J . Phys. Chem. 1984, 88, 4641. (30) Murai, H.; Minami, M.; I'Haya, Y. J. J . Phys. Chem. 1988,92,2120.
J . Phys. Chem. 1989, 93, 5414-5418
5414
states is relatively large and solvent effects on the TI state are small in this molecule. The present results clearly show that the T I states of both 1,2-NQ and 9,lO-PQ are influenced by solvent polarity, though the luminescent properties are significantly different from each other. Lim3' proposed that vibrational coupling between closelying 3 n ~ and * 3 ~ via~the*out-of-plane bending mode causes potential energy distortion in the excited states (proximity effect), leading to a remarkable increase in the nonradiative relaxation rate of the TI state. However, no phosphorescence is observed for 1,2-NQ in EtOH, while the TI state is assigned to the almost pure AT* state. This fact seemed to suggest that the proximity effect is not the main factor for the nonphosphorescenceof 1,2-NQ. This suggestion is supported by the result of 9,lO-PQ emits phosphorescence in every matrix while 3 n ~ * - 3 interaction ~~* is observed. It has been that in aromatic a-dicarbonyl (31) Lim, E. C. J . Phys. Chem. 1986,90, 6770. (32) Morantz, D. J.; Wright, A. J. C. J . Chem. Phys. 1971,54, 692. (33) Arnett, J. F.; McGlynn, S. P. J . Phys. Chem. 1975,79, 626.
molecules photorotamerization occurs in the excited state; that is, for benzil, although stable conformation in the ground state is a skew structure (dihedral angle between two carbonyls, 0 N goo), it is a trans-planar form (0 N 180O) in the S, and T, states. Therefore, the vanishing of luminescence in 1,2-NQ considered to be ascribed to the shallow potential surface in the excited state rather than to a proximity effect. Two carbonyl groups in 1,2-NQ can easily move toward the opposite out-of-plane sites because of the loose molecular structure, while they are fixed by two benzene rings in 9,lO-PQ.
Acknowledgment. We are grateful to Professors J. Higuchi and M. Yagi for their help in the spectral simulation. The present work was partially supported by Grant-in-Aids for Scientific Research No. 62470001 and 63540324 from the Japanese Ministry of Education, Science and Culture. Registry No. 1,2-NQ, 524-42-5; 9,10-PQ, 84-1 1-7. (34) Roy, D. S.;Bhattacharyya, K.; Bera, S. C.; Chowdhury, M. Chem. Phys. Lett. 1980,69, 134.
Penning Ionization Electron Spectroscopy of Group IVB Trimethylphenyls: (CH,),MCBH, (M = C, Si, Ge, Sn, Pb) Masahide Aoyama, Shigeru Masuda, Koichi Ohno, Yoshiya Harada,* Department of Chemistry, College of Arts and Sciences, The University of Tokyo, Komaba, Meguro- ku, Tokyo 153, Japan
Mok Chup Yew, Huang Hsing Hua, and Lee Swee Yong Department of Chemistry, National University of Singapore, Kent Ridge, Singapore 051 1 (Received: December 1.5, 1988)
He*(2%) Penning ionization electron spectra (PIES) and He I photoelectron spectra (UPS) of group IVB trimethylphenyls (CH3),CC6H5,(CH3)$iC6H5, (CH3)sG&6H5,(CH3)$nC6H5, and (CH3),PbC6H5were measured to study the spatial electron distributions of individual molecular orbitals. The relative band intensities in PIES were analyzed on the basis of ab initio MO calculations, and all the UPS bands were assigned to molecular orbitals. Except for (CH3),CC6H5,most bands in the PIES and UPS can be interpreted in terms of the superpositionsof the correspondingspectra of metal tetramethyls and benzene, showing that interaction between the metal trimethyl and phenyl moieties is weak. The phenyl A orbitals give strong bands in PIES, because they are widely distributed outside the molecule and easily interact with metastables. With increasing size of the central atom, the relative intensity of the M ns band decreases in PIES, although the spatial expanse of the M ns atomic orbital increases. This indicates that the electron densities on the surrounding moieties, which diminish on going from (CH3)$C6H5 to (CH3)3PbC6H,,contribute most to the band intensity as in the case of (CH3)4M.
Introduction From a chemical point of view, the spatial distribution of molecular orbitals is of great interest since it is one of the most important factors determining the reactivity of molecules. Recent studies of Penning ionization electron spectroscopy have revealed that the relative band intensity of the spectrum reflects the distribution of individual molecular orbitals. In Penning ionization' of molecules M (M A * M+ + A + e-), metastable rare gas atoms A* attack occupied orbitals of M, from which an electron is transferred to the inner-shell orbital of A* in association with an electron emission from the outer shell of A* into a continuum state.2 Kinetic energy analyses of ejected electrons provide Penning ionization electron spectra (PIES), which are similar in many respects to UV photoelectron spectra (UPS).' Recent studies, however, have revealed that occupied molecular orbitals whose electron distributions are extending
+
-
( I ) Penning, F. M. Narurwissenshafren 1927,1 5 , 818. (2) Hotop, H.; Niehaus, A. Z . Phys. 1969,228, 68-88 (3) CermPk, V. J . Chem. Phys. 1966,44, 3781-3786.
0022-3654/89/2093-5414$01.50/0
outside the molecule give strong bands in PIES.4,5 This characteristic of PIES has been understood in terms of the electronexchange mechanism in which exterior electron distributions of molecular orbitals are selectively probed by incoming metastable atoms. Such a nature of PIES provides a valuable means for the assignment of UV photoelectron spectra, the studies of the spatial (4) Ohno, K.; Mutoh, H.; Harada, Y. J . Am. Chem. SOC.1983, 105, 4555-4561. (5) Ohno, K.; Matsumoto, S.; Harada, Y. J . Chem. Phys. 1984, 81, 4447-4454. (6) Kajiwara, T.;Masuda, S.;Ohno, K.; Harada, Y. J . Chem. Soc., Perkin Trans. 2 i988,507-5 1 1 . (7) Ohno. K.; Fuiisawa, S.; Mutoh, H.; Harada, Y. J . Phys. Chem. 1982, 86, 440-441. ( 8 ) Fujisawa, S.; Ohno, K.; Masuda, S.; Harada, Y. J . Am. Chem. SOC. 1986,108, 6505-651 1. (9) Munakata, T.; Kuchitsu, K.; Harada, Y. Chem. Phys. Lert. 1979,64, 409-412. Munakata, T.; Ohno, K.; Harada, Y.; Kuchitsu, K. Chem. Phys. Letr. 1981,83, 243-245. (IO) VeszprBmi, T.;Bihatsi, L.; Harada, Y.; Ohno, K.; Mutoh, H. J. Organomet. Chem. 1985,280, 39-43. (11) Harada, Y.; Ohno, K.; Mutoh, H. J Chem. Phys. 1983, 79, 3251-3255.
0 1989 American Chemical Society
