7664
J. Phys. Chem. 1991, 95, 7664-1667
is only 17 cm-l for pyrrole. These values compare very well to the local mode frequency differences of 35 and 31 cm-l for furan and thiophene, respectively, and agree with the experimental observation that the two aryl oscillators in the pyrrole spectrum are poorly resolved. Substituent-induced shifts in aryl C H stretching frequencies are also well reproduced, as is evident from the results of Tables V and VI. The frequencies derived from potential energy surfaces calculated with the split valence basis sets reproduce both the direction and magnitude of the frequency shift in 2,5-dichlorothiophene, the methylthiophenes, and the methylfurans. Let us return to the C H bond length-frequency correlation equation (eq 3). In this equation, the C H bond length of benzene (1.084 A) is used as a reference, and shifts in overtone peak positions (A?) are measured relative to the position of the corresponding transition in benzene. In order for this equation to hold, clearly frequency and anharmonicity cannot vary independently. Examination of the a b initio isolated oscillator parameters, which are based on the quartic potentials in eqs 4 and 5 , offers some insight. Both the frequency and anharmonicity parameters contain a contribution from the diagonal quadratic force constant. For the frequency, the quadratic contribution is in the numerator, while for the anharmonicity it is in the denominator. Thus, if the quadratic term is the dominant one, a higher value of the frequency will always be associated with a lower value of the anharmonicity. As a result, the anharmonicity difference will contribute to an increasing separation between overtone peaks associated with distinct XH oscillators at increasing levels of X H excitation. This is far and away the most usual situation, and the assumption that it pertains is implicit in the correlation equation. The relative values of the local mode parameters for the two t y p of oscillators in furan are opposite to the usual trend. Thus, in furan, one expects the higher order cubic and quartic diagonal force constants to be relatively more important and eq 3 to fail a t a quantitative level. However, as we have noted, the bond length-frequency correlation is still qualitatively useful in assigning the overtone spectra, even in this case. Conclusion Comparison of the overtone spectra of furan and thiophene with those of symmetrically substituted species unequivocally identifies
the CH stretching progressions in the spectra of the parent molecules. The assignments are substantiated further by the linearity of the Birge-Sponer plots and by C H bond lengths obtained from geometry-optimized a b initio calculations at the SCF level. The relative agreement between the experimental local mode frequencies and computed a b initio isolated oscillator frequencies provides even further support. Substitution on the aromatic ring was found to have a similar effect on the aryl overtone spectra of aromatic heterocycles as it did on the spectra of substituted benzenes. The experimental parameterization of the substituent effect was largely absorbed by the local mode aryl C H stretching frequency parameters. Again ab initio isolated oscillator frequencies reproduce the trends observed in the experimental parameters. The ability of small split valence basis sets to be sensitive to such subtle changes in the potential energy surface is perhaps surprising. Despite the small basis sets employed, the parameters obtained from, for example, 3-21 G split valence computations for the methyl- and chloro-substituted species support the local mode assignment of the substituent-induced shift in the aryl C H stretching maxima. Force fields derived from small split valence basis sets do appear to be useful in helping with the assignment of trends in vibrational overtone spectra. The experimental local mode parameters derived from the assignment of the overtone spectra of the aromatic heterocycles are somewhat unusual as compared to those encountered in benzene ring systems. This is particularly true for furan and substituted furans. A trend between frequency and anharmonicity, which is observed in the overtone spectra of benzene systems, is reversed in the furans. This reversal has important consequences when established C H stretching-CH bond length correlation equations are used.
Acknowledgment. We are grateful to the Natural Sciences and Engineering Research Council of Canada for financial support. M.G.S.is grateful to NSERC and the University of Manitoba for postgraduate fellowships. B.R.H. is grateful to the Chemistry Department of the Australian National University for a visiting fellowship and to H. G. Kjaergaard for helpful discussions. Registry No. Furan, 110-00-9; thiophene, 110-02-1; 2,5-dichlorothiophene, 3 1 72-52-9; 2,5-dimethylthiophene.638-02-8; 2,5-dimethylfuran, 625-86-5; 2-methylfuran, 534-22-5; 1 H-pyrrole, 109-97-7.
Dipole Moments of Excited Triplet States of Substituted Benzophenones Hiroshi Shimamori,* Hisakazu Uegaito, and Kazushi Houdo Fukui institute of Technology, 3-6-1 Gakuen, Fukui 910, Japan (Received: December 4 , 1990; In Final Form: April 17, 1991) Dipole moments of the lowest excited triplet states of substituted benzophenones in benzene solutions are determined by the time-resolved microwave dielectric absorption. The values for compounds with electron-attracting groups (CIand F) located at 2-, 4-, and 4,4'-positions of the aromatic rings decrease with respect to the ground state. Even compounds with electron-donating sroups (CH3, NH2,and OCH3) show similar decreases in the dipole moments. These results are consistent with the nn* character of the lowest triplet states of those compounds. When two amino groups are substituted into one phenyl ring, the dipole moment in the excited state, measured in dioxane solvent, is slightly lower than the ground state, and the nr* nature is reduced considerably in this case. A marked increase in the dipole moment is observed for 44dimethy1amino)benzophenone and implies that the lowest triplet is mostly nn* or charge transfer in nature. Introduction In a polar molecule, the magnitude of the dipole moment reflects well the distribution of the electron density and is closely related to the electronic state. When the molecule is an electronically excited state, the knowledge of the dipole moment is important not only in characterizing the nature of the excited state but also in understanding its reactivity. Although a volume of data on the dipole moment has been accumulated for molecules in the 0022-3654/9 1/2095-1664$02.50/0
ground state, there have been very few for those in the excited states.'V2 The determination of dipole moments for excited electronic states of molecules has been carried out primarily by observing the effects of electric fields on the shift and splitting (1) McClellan, A. L. Tables of Experimental Dipole Moments; Vol. 11; Rahara Enterprises: El Cerrito, CA, 1974. (2) Liptay, W . In Excited States; Lim, E. C . , Ed.; Academic Press: New York, 1974: p 129.
0 1991 American Chemical Society
Dipole Moments of Substituted Benzophenones of rotational lines of vibronic bands (Stark effects) and on the intensity of these spectra (electrochromic effects) and also by observing their changes with solvent (solvatochromic effect^).^" These data provide mainly the dipole moment for the excited singlet state, while very few data have been obtained for the triplet state. These spectroscopic methods require rather complex procedures to derive a dipole moment value from the data. Recently, a new method (time-resolved microwave dielectric absorption (TRMDA)) has been developed that measures the temporal variation of a microwave dielectric loss caused by a laser-induced change in the dipole moment of a compound.e13 Owing to its limited time resolution, the technique is suitable for observing the lowest triplet state. The absolute values of dipole moments can be determined by comparison with data for a reference compound. So far the method has been applied to some aromatic aldehydes and ketones4J2or and has further been extended to charge-transfer complexes8-10 and contact ion p a i r ~ . ~ J ~ Here we have applied this technique to the determination of the dipole moments of excited triplet states of substituted benzophenones in nonpolar solvents. It is well-known that in aryl ketones substituted functional groups strongly affect the nature and the reactivity of electronically excited states.le16 Studies of hydrogen-transfer reactions as well as measurements of phosphorescence and ESR spectra have been used to characterize the lowest excited triplet state (n**, TU*, and CT), but the exact assignment has not been easy and is sometimes confusing. Such difficulties have been demonstrated in the assignments of the lowest triplet states of 4-aminoben~ophenone’~J~ and Michler’s keIn the present study we have examined the effects of substituted functional group(s) on the dipole moments of the excited triplet state, and the results are discussed in light of the nature of the excited state deduced from other studies.
Experimental Section The measurements were made as previously described.’Ll3 The sample was irradiated with the third harmonic (355 nm) of a Nd:YAG laser. An X-band microwave circuit was used. A silica cell containing a sample solution was placed within the resonant cavity (TElol mode, the resonant frequency -8.8 MHz). The signal was averaged, mostly over 256 shots, by using a Tektronix 2430 digital oscilloscope to improve the signal-to-noise ratio. Upon determination of the dipole moment of the excited species, the absolute intensity of the signal was compared with that of a reference compound (diphenylcyclopropenone) for which the data (3) Bakhshiev, N. G.; Knyazhanashii, M. I.; Minkin, V. I.; Osipov, 0. A.; Saidov. G. V. Russ. Chem. Rev. (End. Trawl.) 1969. 38. 740. (4) Fessenden, R. W.; Carton,‘P.h.; Shimamori, H,;Scaiano, J. C. J. Phys. Chem. 1982,86, 3803. ( 5 ) Warman, J. M.; Visser, R.-J. Chem. Phys. Len. 1983, 98, 49. (6) Fessenden, R. W.; Hitachi, A.; Nagarajan, V. J . Phys. Chem. 1984, 88. 107. (7) Fessenden, R. W.; Scaiano, J. C. Chem. Phys. Lett. 1985, 117, 103. (8) Warman, J. M.; de Haas, M. P.;Oevering, H.; Verhoeven, J. W.; Paddon-Row, M. N.; Oliver, A. M. Chem. Phys. Lett. 1986, 128.95. (9) Warman, J. M.; de Haas, M. P.; Oevering, H.; Paddon-Row, M. N.; Contsans, E.; Hush, N. S.;Oliver, A. M.; Verhoeven, J. W. Narure 1986,230,
615.
(IO) Weisenborn, P. C. M.; Vanna, C. A. G. 0.;de Haas, M. P.; Warman, J. M. Chem. Phys. Letr. 1986, 129, 562. (11) Fessenden, R. W.; Hitachi, A. J. Phys. Chem. 1987, 91, 3456. (12) Shimamori, H.; Houdo, K.; Uegaito, H.; Nakatani, Y . ;Uchida, K. Nippon Kagaku Kaishi 1989 (8). 1379. (13) Shimamori, H.; Uegaito, H. J. Phys. Chem., in press. (14) Koizumi, M.; Kato, S.;Mataga, N.; Matuura, T.; Usui, Y. Phorosensitized Reactions; Kagakudojin: Kyoto, 1978; Chapter 4 and references therein. ( I 5 ) Turro, N. J. Modern Molecular Photochemisrry; Benjamin/Cummings: Menlo Park, CA, 1978; Chapter IO and references therein. (16) Horspool, W. H. In Photochemistry in OrganicSynrhesis;Coyle, J. D., Ed.; The Royal Society of Chemistry: London, 1986; Chapter 4. ( 1 7) Porter, G.; Suppan, P. Trans. Faraday Soc. 1965.61, 1664. (18) Porter, 0.;Suppan. P. Trans. Faraday Soc. 1966, 62, 3375. (19) Schuster, D. 1.; Goldstein, M . D. J . Am. Chem. Soc. 1973, 95, 986. (20) Suppan, P. J. Chem. Soc., Faraday Trans. I 1975, 71. 539. (21) Schuster, D. I.; Goldstein, M.D.; Bane, P. J. Am. Chem. Soc. 1977, 99, 187. ( 2 2 ) Brown, R. G.; Porter, G. J . Chem. Soc., Faraday Trans. I 1976, 72, 1569.
The Journal of Physical Chemistry, Vol. 95,No. 20, 1991 7665 1
-I
a 2
a .
T I M E Figure 1. Time dependence of the output (amplitude) of the dielectric absorption apparatus for solutions of benzophenone and its derivatives in benzene: (a) benzophenone, (b) 4-chlorobenzophenone, (c) 3,4-diaminobenzophenone, (d) 4-(dimethylamino)benzophenone, and (e) diphenylcyclopropenone in benzene. Time scale: (a) 5 , (b) 5 , (c) 2, (d) 1, and (e) 1 rsldivision. Amplitude scale: (a) 1, (b) I , (c) 1, (d) 10, and (e) 5 mV/division.
were taken under the same irradiation conditions as in the sample compound. Measurements of reflected microwave powers as a function of concentration of each solute (the “static meas~rement”~) were made for the purpose of cancellation of parameters common for both the sample and the reference compounds. The principle and the method of this measurement were described previo~sly.~J 2~1 Benzophenone and its substituted derivatives (2-chloro-, 4chloro-, 4fluoro-, 2-methyl-, 4-methyl-, 4-methoxy-, 3,4diamino-) and diphenylcyclopropenone(all purchased from Aldrich Chemical Co.) were purified by recrystallization from ethanol. Only 4amino- and 4-(dimethy1amino)benzophenone were subject to further purification by vacuum sublimation. Benzene and dioxane (Wako Chemicals, Spectrograde) were dehydrated by contact with molecular sieve 3A. All the samples were deoxygenated by bubbling with Ar gas. The measurements were made at room temperature (-295 K).
Results Samples containing benzophenone or its various substituted derivatives were measured in benzene. Only 3,4-diaminobenzophenone was measured in dioxane. The concentration was chosen so that the optical density of the solution at 355 nm was unity. All the sample compounds except 4-(dimethy1amino)benzophenone gave negative signals initially which returned then to the base line. Some examples of the observed signals are shown in Figure 1, where the resutls for benzophenone, 4-~hlorobenzophenone,3,4diaminobenzophenone, and 4-(dimethy1amino)benzophenoneare represented along with that for diphenylcyclopropenone as the reference compound. Only 4-(dimethylamino)benzophenone showed a rapid growth of the initial amplitude. The initial part in all signals corresponds to the formation of the lowest excited triplet state, and the following decays should reflect the destruction of the triplet state, caused by the natural decay, quenching by trace amounts of impurities, and triplet-triplet annihilation at higher concentrations. The negative and positive signals correspond respectively to the decrease and increase in the dipole moment ( F ) of the photoabsorbing species. Thus, these signals
7666 The Journal of Physical Chemistry, Vol. 95, No. 20, 1991
Shimamori et al.
TABLE I: Dimle Moments ( h )of Triplet States of Substituted Benzophenones'
molecule benzophenone
k
2-chlorobenzophenone 4-chlorobenzophenone 4-fluorobenzophenone 4,4'-dichlorobenzophenone
@c
2.98
1.o
[1.7,8 l.8,* 2.1'1 2.3 f 0.3 1.2 f 0.4 1.3 f 0.5 0.6 i 0.3'
3.46 2.75 2.66
e I .o
1.75
e
2.69
e
3.14 AJ 4.73
e
CJ 5.3
e e 1.01
[-Ik]
2-meth ylbenzophenone
4-methylbenzophenone 4-methoxybenzophenone 4-aminobenzophenone 4-(dimethy1amino)benzophenone
3,4-diaminobenzophenone Michler's ketone
Cc,d (8.9 - 4.4/@)'/'
1:
2.1 f 0.1
1.5 f 0.2 2.4 f 0.2 (0.80 f 0.04) A 3.4 f 0.2 (1.35 i: 0.05) B (0.97 f 0.02) C 7.8 f 0.y [8.4']
(1 1.97 - 6.82/@)'/'
(7.56 - 6.04/@)'/' (7.1 - 5.4/@)'/' (3.06 - 2.64/@)'/'
e
(7.2 - 5.0/@)'/' (9.86 - 4.25/1$)'/~ (1 - 0.36/-$)'/' A
1.o
1 .o
Br
(22.4 - 10.8/@)'/' (1 + 0.82/@)'/' B (1 - 0.05/@)'/' C
(28.1 + 32.8/@)'/'
'Values in debye; measured in benzene solution except for 3,4-diaminobenzophenonewhich was measured in 1,4-dioxane solution. bValuesfor the ground states (in debye); taken from ref I . CQuantumyields for triplet formation; taken from CRC Handbook ojOrganic Photochemistry; Vol. I; Scaiano, J. C., Ed.; CRC Press: Boca Raton, FL, 1989. dExpressionof p, including 6. cAssumed to be 1.0. /Since the values are not reported, they are simply denoted as A , B, and C. EReference 23. *Reference 24. 'Reference 4. /Reference 12. kReference 25. reflect the sign of the variation of the dipole moment when molecules are excited to the triplet state. The downward signal for the reference compound corresponds to the decomposition of diphenylcyclopropenone ( p = 5.1 D) into diphenylacetylene ( p = 0 D) and carbon monoxide ( p = 0.1 D). In determining the dipole moment of the excited triplet state, we only need the initial amplitude of the dielectric loss signal. The ratio of the initial amplitude V for a sample compound to that for the reference compound can be expressed as4 -v* =
BS-I
[SI,A(c1,2)g(7),
Vr
8;'
[sIrA(~,2)g(~)r
+ /3-')/2 + /T'C[S]
(2)
where P,is the reflected microwave power, Po is the incident power, and C = Fg(r)p2( F is a factor associated with the geometry of the cavity used). Examples of plots, based on eq 2, for the result of the static measurement for some compounds are shown in Figure 2. Each slope gives the corresponding value of C. So the ratio g(T)s/g.(r)r in eq 1 can be replaced by (Cs/Pg>)/(Cr/I,,Z), and we obtain -=
,
I
A
-
h
0
e > e
1.5
v
I 7
Y
(1)
where 0 is the coupling factor of the cavity, [SI is the concentration of the transient, A(p2) = p t - p [p, is the dipole moment of the transient (in the present case t i e triplet state); pg is the dipole moment in the ground state], g(7) = ~ / { +l ( w T ) ~ ][w is the microwave radian frequency: T is the dielectric relaxation time for the transient], and the subscripts s and r refer to the sample and the reference compounds. Since it is difficult to know the absolute value of g ( ~ )we , replace the ratio g ( ~ ) ~ / gwith ( ~ another )~ factor determined from the "static measurement" in which the microwave power reflected from the cavity is measured as a function of the concentration of the solute (in the ground state). The results of static measurements are analyzed by using the following equation4 [ l - (Pr/P,)1/2]-I = (1
2.0
(3)
From eq 3 we can determine the value of A(p2) and therefore the value of p, by knowing p . If the value of p8 is not known, only the ratio p l / p g can be dktermined. The values of p, thus determined are summarized in Table I, where the values for 4,4'-dichlorobenzophenone and Michler's ketone determined previously'* are also included. The error bars come from the variation of signal heights observed for several samples of the same compound. Another uncertainty may be associated with the quantum yield 4 of the triplet formation. Most known values of $J equal unity, and the error in assuming that 4 = 1 for the other compounds may not be large. However, expressions for p, including $J are also listed in Table I.
1 .o
0
5
10
CONCENTRATION
15
20
25
(mM)
Figure 2. Plot of appropriate function of reflected power as a function of concentration of solutes in benzene. Solute: 0,diphenylcyclopropenone; 0,4-aminobenzophenone; V, benzophenone.
Discussion The electronic character of the lowest excited triplet states of aryl ketones has been discussed in relation to its capability of the hydrogen atom abstraction from various hydrogen-donating solvents such as 2-propanol, ethanol, and t ~ l u e n e . l ~ -These '~ ketones have an oxygen atom with lone-pair electrons that give rise to the na* transition. The nn* transitions also exist as in usual aromatic compounds. In the nn* transition, one of the lone-pair electrons on the oxygen atom is promoted to an antibonding n* orbital which extends over the whole conjugated system, so that the oxygen atom acquires some electrophilic reactivity in comparison with that in the ground state. This nature is mainly responsible for the hydrogen abstraction. In the nu* transition, on the other hand, one of the n electrons is promoted to the n* orbital, and the electron density on the oxygen atom does not decrease but rather increases, thus showing less reactivity in the hydrogen abstraction. These characteristics are closely related to the change of the molecular dipole moment in going to the lowest excited triplet state. It is obvious that the dipole moment of the molecule decreases in the nn* state, whereas it increases in the nn* state. There are, however, some complications in relating the measured dipole moment value to the nature of the lowest triplet state, since there may be mixing of the nn* and ns* ~ t a t e s . ' ~In . ' ~principle, the nn* state of any aryl ketone should possess some nn* character. The mixing may be enhanced when the two states lie very close in energy. Alternatively, the two states can both be populated by thermal activation. Therefore, the dipole moment data obtained here may not lead to a unique assignment of the nature of the lowest triplet state. We can only say which state configuration, nn* or nn*,is contributing more strongly in the actual state which may be a mixed one or an averaged one over thermally populated states. It should also be noted that as
Dipole Moments of Substituted Benzophenones only the absolute value of the change of the dipole moment is measured the direction of the change is unknown in asymmetric substituted compounds. In such molecules the shift of the electron density upon excitation could be larger than that inferred from the measured dipole moment value. In benzophenone and all the derivatives except those with dimethylamino groups pl is smaller than pg. The lowest excited triplet state of benzophenone is known to be an na* state,”+j and the dipole does not reverse direction in going from the ground to the lowest triplet state.23 The present result is consistent with these. The absolute value of p, for benzophenone determined here is in good agreement with values determined previously4 by using the same technique as the present one and is slightly higher than the values 1.8 and 1.7 D corresponding to that in crystal fieldsz4 and that in the absence of the crystal e n ~ i r o n m e n trespectively. ,~~ As shown in Table I, similar reduction of the dipole moment occurs in most of the substituted benzophenones, for which values of p, have not been reported previously. When benzophenone is substituted with an electron-attracting CI atom, the reduction of the dipole moment appears to be somewhat larger for substitution in the 4-position (Ap = -1.6 D) than that in the 2-position (Ap = -1.2 D). There is a slight difference between 4-C1 (Ap = -1.6 D) and 4-F (Ap = -1.3 D) substitutions. In the case of 4,4’dichlorobenzophenone a very small value (-0.6 D) for pl is obtained,12 reflecting a strong electron-attracting effect of two Cl atoms, though a large error is involved in the determined value because of a small value of Ap2 (due to a rather small value of pg (=1.75 D)). The above value of p, may be compared to the value of p, = 1 D (or Ap = -0.7 D with respect to the ground state) determined from the analysis of the Stark effects on the absorption and emission lines associated with the 3na* state of this compound in pure crystals.2s A reduction of the dipole moment is observed for molecules substituted with an electron-donating group such as CH3, W H 3 , and NH2. In the case of the CH3group the reduction of the dipole moment for 4-CH3 (Ap = -0.7 D) is smaller than that for 2-CH3 ( A p = -1.2 D). It appears that the electron-donating nature of CH3 is contributing more strongly in the former case. The lowest excited triplet states of 4-amino-, 4-hydroxy, and 4-(dimethylamino)benzophenone exhibit photochemical properties that depend on the polarity of the solvent: in 2-propanol these compounds scarcely undergo the hydrogen abstraction reaction, while the reaction proceeds in cyclohexane.”J8 This difference has been interpreted as being due to a change of the lowest excited triplet state from a,* or intramolecular CT character in polar solvents to ,a* in nonpolar s01vents.l~ The values of p, for 4-methoxyand 4-aminobenzophenone in benzene are lower than those in the ground state, consistent with an na* transition. On the other hand,
-
(23) Barker, J. W.; Noe, L . J. J. Chem. Phys. 1972,57, 3035. (24) Hochstrasser, R. M.;Noe, L. J. J. Mol. Spectrosc. 1971, 38, 175. (25) Hochstrasser, R. M.;Michaluk, J. W. J . Mol. Specrrosc. 1972,42, 197.
The Journal of Physical Chemistry, Vol. 95, No. 20, 1991 7667 in 4-(dimethy1amino)benzophenone the value of p, definitely increases, though the absolute value is uncertain because p is unknown. This result implies that the excited state has aa0 or CT character, in disagreement with results of photoreduction measurements in nonpolar solvents which suggest that the lowest excited triplet is the na* state.”] Let us check whether or not this discrepancy is explained by the presence of a significant mixing between the na* and TU* states. The dielectric absorption signal is proportional to A(p2) (= pit2 - p:). We expect that the value of pg for this compound is about 5 D [a value between 4aminobenzophenone (4.73 Dl) and Michler’s ketone (5.3 DI)], and the experimentally determined value of pl is therefore about 6.7 D. We may assume that pl for the “pure” a a * (or CT) state is about 8 D in analogy with the value for Michler’s ketone (= 7.8 or 8.4 D (see Table I)), for which the lowest triplet is the C T state in nonpolar solvents.u Assuming also p, = 3 D for the “pure” na* state, we can calculate A(& for the “pure” a a * and na* states to be 39 and -16, respectively. Thus, about 65% mixing of TU* with na* can account for the actual dipole moment of 6.7 D. Although the percentage of mixing may change slightly depending on the values of pl for the “pure” states, this estimation suggests that even if a significant state mixing is present the character of the lowest triplet is more like aa* than na*. On the other hand, if the na* state is still the lowest but thermal energy populates two states, one needs to assume a small difference (about 0.02 eV) between energy levels for the two states to explain the observed increase of p. In contrast with 4-aminobenzophenone the decrease of p is very small in 3.4-diaminobenzophenone. Apparently, the further addition of an amino group in the 3-position considerably reduces the na* character of the excited state. The state mixing or the population of the two states is again possible, but the na* character appears to dominate slightly over the a a * character in this case. In summary, for most of benzophenone derivatives substituted with various functional groups, both electron-attracting and electron-donating, the dipole moment in nonpolar solvent decreases in the lowest triplet state with respect to the ground state. This decrease is due to the na* character of the excited state in nonpolar solvents. It seems that only a dimethylamino group, because of its strong electron-donating nature, can make the a a * or CT character dominant even in nonpolar solvents.
Acknowledgment. We are very grateful to Prof. H.Mikawa for helpful discussion and suggestions. This work was supported in part by the Grant-in-Aid for Scientific Research (C) from The Ministry of Education, Science and Culture. Registry No. Michler’s ketone, 90-94-8; benzophenone, 1 19-61-9; 2-chlorobenzophenone, 5 162-03-8; 4-chlorobenzophenone, 134-85-0; 4fluorobenzophenone, 345-83-5; 4,4’-dichlorobenzophenone,90-98-2; 2methylbenzophenone, I3 1-58-8; 4-methylbenzophenone, 134-84-9; 4methoxybenzophenone, 61 1-94-9; 4-aminobenzophenone, 1137-41-3; 4-(dimethylamino)benzophenone,530-44-9; 3,4-diaminobenzophenone, 39070-63-8.
