C. PXRKANYI, E. J. BAUM,J. WYATT,AND J. N. PITTS,JR.
1132
interface, small surface effects are present in some cases, but they may be neglected in the glc experiment provided that a large enough film thickness (greater than is used. These encouraging findings about 1000 now allow one to proceed with a detailed thermodynamic glc study of solutions involving these nem-
A)
atogenic solvents with confidence that the results will provide information about bulk liquid crystal behavior.
Acknowledgment. This work was supported through a basic research grant from the U. 8. Army Research Office, Durham, North Carolina.
Physical Properties and Chemical Reactivity of Alternant Hydrocarbons and Related Compounds. XVI. 1 Electronic Absorption and Phosphorescence Spectra of Aryl Phenyl Ketones
by C. Phrkbnyi,Z Institute of Physical Chemistry, Czechoslovak Academy of Sciences, Prague, Czechoslovakia
E. J. Baum, Oregon Graduate Center for Study and Research,s Portland, Oregon
J. Wyatt, and J. N. Pitts, Jr. Department of Chemistry, University of California, Riverside, California
91502
(Received September 1 4 , 1 9 6 8 )
Spectral properties of eight aryl phenyl ketones were investigated by Pariser-Parr-Pople type LCI-SCF-MO calculations. Reasonable agreement is found between calculated (SO+ SI) excitation energies and wavelengths of absorption band maxima of the ketones, Phosphorescence emission spectra for six of the ketones were obtained, and reasonable agreement is found between calculated (So-+ T) excitation energy and wavelength of the 0-0 emission band for those compounds assigned to have a lowest-lying (n,n*) triplet state. The position of the 0-0 band was confirmed by phosphorescence excitation methods where possible. I n all cases, the predicted transition energy was found to be too low. calculated values were found to deviate equally from observed values for both singlet and triplet transitions resulting in accurate predictions of S-T splittings for the lowest (a,n*) excited states. Major contributions to this error would be made by choice of calculation parameters for the keto group, deviations from planarity of the compounds studied, and solvent dependence of the spectral band positions.
Introduction Iln earlier paper in this series discussed the correlation of HMO characteristics with physicochemical properties of aryl phenyl ketones.' Among other results, linear correlations were found between calculated transition energies and the positions of maxima in the long wavelength electronic absorption bands of these ketones. The present paper reports the results of LC1-SCF-MO calculations of singlet-singlet transition energies and oscillator strengths for ketones I-VIII. The values are compared with those derived from experimental absorption curves. Furthermore, phosphorescence emission The Journal of Physical Chemistry
and phosphorescence excitation spectra are presented, and calculated values of triplet-singlet transition energies are compared with experimentally determined values. Prominent features of the emission and excitation spectra along with phosphorescence lifetimes are ( 1 ) Presented in part at the 2nd IUPAC International Symposium on Photochemistry. Enschede, The Wetherlands, July 16-22, 1967. Part X V : C. Pbrkbnyi, Z. Dolejiek, and R . ZahradnIk, Collect. Czech. Chem. Commun., 3 3 , 1211 (1968). (2) Gates and Crellin Laboratories of Chemistry, California Institute of Technology, Pasadena, Calif. 91109. (3) 9340 5. W.Barnes Road, Portland, Ore. 97225. (4) C. Pbrkbnyi, V. Horbk, J. Pecka, and R . Zahradnik, Collect. Czech. Chem. Commun., 31, 835 (1966).
~HOSPHORESCENCE SPECTRA OF
1133
ARYLPHENYL KETONES
used to characterize the low-lying triplet states of ketones I-VIII.
Nishimoto formula6
14.399
ypv
ir
I
QpQ i
0
P
Pi
: w * 0 00 00
0 j 00
Experimental Section LCI-SCF-MO Treatment. The usual version of the Pariser-Parr-Pople LCI-SCF-MO method was used. The parametrization and method have been described elsewhere.6 Interactions between monoexcited configurations formed by promotion of one ?r electron from one of the four highest occupied MO’s to one of the four lowest unoccupied MO’s were considered. The systems studied were assumed to be planar and to have idealized geometry. All C-C bond lengths and all bond angles (including the C-C-0 bond angle) were assigned to be 1.40A and 120”, respectively. The C-0 bond length was taken as 1.20A. SCF MO’s were used as the basis for the CI calculations. Only resonance integrals between nearest neighbors were considered. The following parameters were used (values in eV) . Atom
I,
A,
.Y#P
2,
PC -p
C 0
11.22 13.60
0.69 2.30
10.53 11.30
1 1
-2.318 -2.318
I , and A , are the ionization potential and electron affinity of atom p in the atomic valence state, respectively. The monocentric electronic repulsion integrals and core resonance integrals between nearest neighbors are represented by y,,, and Pc-,, respectively, and Z, is the core charge at atom p. The bicentric electronic repulsion integrals have been calculated by the Mataga-
=
R,,
+ 1.328 eV
where R,,#(A)is the distance between atoms p and v. The calculations were performed on a National Elliott 503 computer. Spectroscopic Methods. The apparatus for measuring phosphorescence spectra has been described el~ewhere.~J Ketones were dissolved in 4: 1 ethanol-methanol solvent (saturated solutions a t room temperature), and the solutions were ‘degassed by the freeze-pump-thaw method in 7-mm Pyrex tubes. After sealing under vacuum, the samples were cooled to 77°K in the phosphoroscope Dewar. Phosphorescence spectra were obtained with excitation wavelengths X >300 mp and were not corrected for spectral bandpass or wavelength response of the analyzing monochromator-photomultiplier section. S o + T spectra of benzophenone in 4 : l ethanolmethanol glass and of 2-naphthyl phenyl ketone in iodide glass were 16 :4: 1 ethanol-methanol-ethyl obtained at 77°K by phosphorescence excitation techniquesSl0 (the phosphorescence excitation spectrum of 2-naphthyl phenyl ketone was measured on an Aminco-Bowman spectrophosphorimeter) . The phosphorescence emission was monitored a t the wavelength of maximum phosphorescence intensity, I p , while the excitation wavelength was scanned. The excitation spectra were not corrected for the spectral distribution of exciting light intensity. The intensity was reasonably constant over the wavelength region of interest. Phosphorescence decay curves were obtained with a Tektronix 5358 oscilloscope, equipped with a Polaroid land camera, connected to the phosphoroscope. Phosphorescence lifetimes, TP, were found graphically from plots of In Ip us. time (slope = - ~ / T P ) . The fluorescence spectra of 9-aiithryl phenyl ketone and 1-pyrenyl phenyl ketone in methanol (at room temperature) were obtained on an Optica Milano C F 4 N I spectrophotometer by the usual technique. Materials and methods of purification were the same as those employed in earlier s t ~ d i e s . ~The So-+ S absorption spectra of ketone-methanol solutions a t room temperature were obtained previ~usly.~Jl (5) J. Kouteck?, P. Hochman, and J. Michl, J . Chem. Phys., 40, 2439 (1964). (6) N . Mataga and K. Nishimoto, Z . Phys. Chem. (Frankfurt am Main), 13, 140 (1957). (7) J. G. Calvert and J. N . Pitts, Jr., “Photochemistry,” John Wiley and Sons, Inc., New York, N . Y . , 1966, pp 806-808. ( 8 ) D. R. Kearns and W. A. Case. J. Amer. Chern. SOC.,8 8 , 5087 (1966). (9) R . F. Borkman and D. R . Kearns, J. Chem. P h y s . , 46, 2333 (1967). (10) A. P. Marchetti and D. R. Kearns, J . Arner. Chem. SOC.,89, 768 (1967). and references therein. (11) C. PLrkBnyi, R. Zahradnlk. and M. NepraH in “Absorption Spectra in the Ultraviolet and Visible Region,” Vol. V I , L. L&ng, Ed., Akademiai Kiadb, Budapest, and Academic Press, Inc., New York, N. Y . , 1966.
Volume 79,Number 4 April 1060
C. PLRKLNYI, E. J. BAUM,J. WYATT,AND J. N. PITTS,JR.
1134
Table I: Results of LCI Calculations4 Predominant -configuration-cos 9 d i , j4 Wt
Ketone
AEI~
Benzophenone (I)
4.413 4.422 4.521 4.724 5.595 5.629 5.695 6.171
0.028 0.002 0.347 0.491 0.276 0.037 0.328 0.062
0.000 -1.000 1.ooo 0.000 0.000 0.000 -1.000 1.000
4, -1 3, -1 1, -1 2, -1 1, -3 1, -2 1, -4 2, -2
0.528 0.578 0.895 0.830 0.476 0.916 0.413 0.362
4.100 4.512 4.595 4 768 5.193 5.622 5.759 5.931
1.041 0.094 0.389 0.007 0.114 0.264 0.771 0.043
-0.611 0.164 0.569 -0.005 0.421 0.155 0.969 0.997
1, -1 4, -1 2, -1 3, -1 1, -2 1, -3 1, -4 2, -2
0.929 0.526 0.691 0.422 0.865 0.402 0.448 0.826
3.690 3.979 4.419 4.486 4.695 5.184 5.494 5.556
0.432 0.010 0.193 0.024 0.036 1.014 0.151 0.190
0.597 0.621 0.976 -0.266 0.516 -0.919 0.325 0.860
1, -1 3, -1 2, -1 4, -1 1, -2 1, -3 1, -4 2, -2
0.930 0.546 0.798 0.560 0.903 0.461 0.293 0.724
3.815 4.115 4.465 4.617 4.804 5.191 5.386 5.660
0.104 0.086 0.036 0.521 1.044 0.184 0.791 0.257
-0.315 -0.232 0.032 0.912 0.410 -0.940 0.042 -0.718
1, -1 1, -2 4, -1 2, -1 1, -2 1, -3 2, -2 2, -4
0.641 0.288 0.489 0.496 0.478 0.269 0.485 0.312
3.630 3.856 4.463 4.533 4.578 4.766 5 032 5.412
0.408 0.153 0.410 0.060 0.390 0.547 0.774 0.434
-0.109 0.144 -0.769 -1.000 0.994 -0.741 0.360 -0.400
1, -1 2, -1 3, -1 4, -1 1, -2 2, -2 2, -2 2, -3
0.847 0.418 0.606 0.503 0.576 0.342 0.477 0.812
2,977 3.669 3.992 4.132 4.336 4.733 4.999 5.190
0.535 0.020 0.008 0.058 0.016 1.617 0.025 0.760
0.518 -0.889 -0.922 -0.836 -0.880 -0.846 -0.974 -0.688
1, -1 3, -1 1, -2 2, -1 4, -1 1, -3 1, -4 2, -2
0.965 0.592 0.737 0.620 0.864 0.545 0.930 0.910
3.149 3.515 4.142 4.286 4.415 4.708 5.174 5.359
0.883 0.041 0.016 0.331 0.241 0.941 0.724 0.152
0.254 0.079 0.998 0.984 0.514 -0.929 0.339 -0.971
1, -1 2, -1 4, -1 1, -2 3, -1 1, -3 2, -2 1, -4
0.950 0.407 0.687 0.615 0.650 0.552 0.800 0.616
4-Phenylbenzophenone (11)
I
1-Naphthyl phenyl ketone (111)
2-Naphthyl phenyl ketone (IV)
I-Phenanthryl phenyl ketone (V)
I
9-Anthryl phenyl ketone (VI)
1-Pyrenyl phenyl ketone (VII)
The Journal of Ph~aicalChemistrg
fC
Second most important -configurationf-i, js
Wt
AEa 0
1, -3 1, -4
0.155 0.146
4, -1 4, -1
0.132 0.318
3, -1 3, -4
0.374 0 270
2.264 3.196 3.575 3.715 3.719 3.863 4.567 4.572
...
...
...
... I
...
...
2, -1 4, -1 1, -3 2, -1 4, -1 3, -1 4, -3
0 136 0.084 0.194 0.094 0.179 0.270 0.057 I
2.404 3.230 3.627 3.939 4.257 4.637 4.714 4.887
...
...
1, -3
0.375
2, -4 2, -1 3, -1 4, -1 1, -4
0.190 0.061 0.410 0.222 0.070
2.015 2.822 3.515 3.587 3.896 4.022 4.218 4.384
1, -3 1, -1 2, -4 3, -1 3, -1 2, -2 1, -3 4, -1
0.101 0.273 0.159 0.243 0.337 0.221 0.347 0.311
2.240 2.664 3.402 3.622 3.810 3.917 4.144 4 625
2, -1 1, -2 4, -1 3, -4 3, -1 1, -3 1, -3 1, -2
0.050 0.280 0.099 0.159 0.187 0.272 0.413 0.057
2.206 2.824 3.310 3.581 3.632 3.851 3.952 4.508
...
...
1, -3 2, -1
0.373 0.228 0.204 0.073 0.328
1.372 2.715 3.338 3.705 3.771 3.878 4 * 494 4.583
...
1, -2 2, -1 3, -1
... ...
...
... .,.
...
...
1, -3 1, -2 4, -1 4, -1 2, -1 3, -1 3, -4
0.267 0.150 0.145 0.133 0.391 0.062 0.170
I
1.738 2.638 3.261 3.474 3.664 4.140 4.173 4.619
1135
PHOSPHORESCENCE SPECTRA OF ARYLPHENYL KETONES Table I (Continued) Predominant Ketone
AE~*
la
6Chrysenyl phenyl ketone (VIII)
3.375 3.659 4.334 4.385 4.538 4.654 4.897 5.064
0.757 0.014 0.059 0.090 0.156 1.399 0.216 0.531
cos a
d
-0.137 -0 * 954 -0.866 0.754 -0.971 -0.886 0.878 -0.509
-configuration---4 j0 Wt
1, -1 2, -1 1, -2 3,-1 4,-1 1, -3 1, -4 2,-4
Second most important -configuration'--i, 10
0.959 0.583 0.697 0.846 0.493 0.390 0.849 0.262
Wt
...
...
1, -3
4, -1
0.376 0.209
1, -2 2, -1 4, -1 2, -3
0.139 0.260 0.072 0.221
...
...
AEIE
1.994 2.748 3.193 3.339 3.714 3.976 4.146 4.452
0 Additional theoretical quantities concerning the models of compounds given in Table I can be obtained at request from the authors (C. P.). is the angle formed by the positive direction of *Excitation energies (eV) for the eight lowest excited singlet states. Oscillator strength. the axis shown in the formula and the direction of the transition moment vector read clockwise. 0 A combination of two figures is used to label Only configurationa a configuration. A positive number refers to an orbital occupied in the ground state, a negative number to a virtual orbital. with more than 501,weight. 0 Excitation energies (eV) for the eight lowest excited triplet states.
30
40
kK
20
30
40
kK
20
x,
a.
kK
20
3)
LK
Figure 1. Ultraviolet absorption spectra of ketones I-VI11 (methanol solutions). Calculated LCI transition energies and intensities are shown a5 full straight lines. Scale for the calculated oscillator strength is shown on the right-hand side.
Results and Discussion Calculations. The results of the LCI-SCF-MO calculations of the spectral properties of the ketones I-VI11 are summarized in Table I. Electronic Absorption Spectra. Absorption curves of the aryl phenyl ketones, R-CO-CaHs, are similar to those of the corresponding parent hydrocarbons, R-H, with less fine structure and a more intense band in place of the weak CY hydrocarbon band. Thus, the long wavelength region of the absorption spectra of aryl ketones shows only one band with a slight indication of structure rather than the original two bands in benzenoid hydrocarbons.' The experimental absorption curves for ketones I-VI11 are compared with the results of LCI-SCF-MO calculations in Figure 1. The locations of the first
excited singlet states are summarized in Table 11. It is seen from Figure 1 that acceptable agreement is usually obtained between theoretical and experimental values of oscillator strengths and band maxima positions with the exception of some longest wavelength bands. For example, the longest wavelength band of 9-anthryl phenyl ketone VI is predicted to be more intense and to lie further to the red than is experimentally observed. Lack of agreement could be ascribed to noncoplanarity in aryl phenyl ketones. (However, cf. a similar paper dealing with aryl phenyl methyl cations.12) Also, there is no way of accounting for solvent shifts in the calculations. Improvement of the correlation might be obtained by altering the parameters used for the keto (12) V. Hordk, C. Pdrkdnyi, J. Pecka, and R. Zahradnlk, Collect. Czech. Chem. Cornmun., 32, 2272 (1967).
Volume 79, Number 4 April 1969
C. PARKANYI, E. J. BAUM,J. WYATT,AND J. N. PITTS,JR.
1136 Table 11: Location of the Lowest Excited (n, T*) and Observed
Singlet and Triplet States"
Observed
Observed
Observed singlet state singlet state (n, T * ) triplet energyb energyc state energyd (n, T*)
Compound
(7, T*)
Benzophenone (I) 4-Phenylbenzophenone (11) 1-Naphthyl phenyl ketone (111) 2-Naphthyl phenyl ketone (IV) 1-Phenanthryl phenyl ketone (V) 9-Anthryl phenyl ketone (VI) 1-Pyrenyl phenyl ketone (VII) 6Chrysenyl phenyl ketone (VIII)
27.0 26.8 26.7
... ... ... ... ...
(T,T*)
39.6 34.4 32.6 34.2 32.5 26.1 26.3 28.5
24,66
... ..,
23.8(?)
...
... ... ...
Phosphorescence bands---------
(7, T*)
triplet state, energyd
... 21.29 20.19 21.3 20.7
... ...
18.9
0-0
0-1
24.1 21.2 20.1 20.7 20.7
22.4 19.8 18.6 19.2 19.3
...
... 18.9
19.5sh 20.3sh
...
...
17.5
Phosphorescence lifetime. T P , sec 4.7 X 0.28h 0.51; 1.01 0.92
... ...
0.59
a Energies are given in kK. * From ref 14, in ethanol-diethyl ether (2: 1) at 77°K. 0 In methanol at room temperature, taken from ref 4. Observed (n, T*)and (T,T*)triplet state energies for the compounds I and IV determined from their phosphorescence excitation spectra. For other compounds, the wave numbers of the phosphorescence 0-0 bands are given. e Reference 8 gives 24.7 kK. f Taken from ref 14, ref 8 gives T P = 4 X 10-8 sec; cf. also ref 15, 22. 0 In agreement with ref 14. Reference 14 gives T P = 0.30 f 0.02 sec. i Reference 14 gives TP = 0.74 f 0.03 sec.
group. The values employed here are those found suitable for planar ~hena1enones.l~ Aryl phenyl ketones are not exactly planar. This complicates the comparison of theoretical and experimental transition energies. However, conjugation is reduced only slightly if deviations from planarity are small. It is found empirically that the energies of the maxima of the longest wavelength bands of the ketones studied here are roughly linearly correlated with calculated HMO E ( N +. VI) e n e r g i e ~ . ~ For certain of the ketones, the LCI calculations predict considerable mixing of configurations, even of
40
(k\,
30
I
I
20
30
I
$-SI
(kK)
40
Figure 2. Plot of experimental (5) against theoretical LCI (So .--) &,see text) excitation energies for aryl phenyl ketones. Regression line: 5 (kK) = 1.170 SO3 SI(kK) - 2.619; correlation coefficient T = 0.979, number of compounds n = 8. All values are significant on 1% probability level. The Journal of Physical Chemistry
those of low energy. The squares of the expansion coefficient for the x1,-1 configuration in the CI function for the first excited state are 0.929, 0.930, 0.641, 0.847, 0.965,0.950,and 0.959 for ketones 11-VIII, respectively (cj. Table I). Generally, there is a close correlation between experimental and theoretical values for the energies of the first excited singlet states (Figure 2 ) . Phosphorescence Emission and Phosphorescence Excitalion Spectra. Triplet states of aromatic ketones may be designated as either (n, r * ) or ( r , a*). The ketones can then be classified according to which of the two triplet types lies lowest in energy. The criteria used to distinguish between So --f T,,,* and So --+ T,,,* transitions are summarized in Table III.s Phosphorescence spectra of ketones I-V and VI11 are shown in Figure 3, along with phosphorescence excitation spectra of ketones I and IV. We were not able to obtain excitation spectra of the other ketones discussed. The location of the lowest triplet states and phosphorescence lifetimes, TP, are summarized in Table 11. Benzophenone ( I ) . The lowest-lying triplet state of benzophenone is an (n, r * ) state. (This is in accord with previous a s s i g n m e n t ~ . ~ J ~ The J ~ ) phosphorescence lifetime is of the order of sec, singlet-triplet splitting is -2400 cm-', and a prominent 1700 cm-l vibrational progression is observed in the spectrum; the positions of the 0-0,0-1, 0-2, and 0-3 bands are 24.125, 22.470, 20.885, and 19.325 kK,respectively. No heavy atom solvent effect was noted on the intensity of the So +. T,,,* transition. %Naphthyl Phenyl Ketone ( I V ). The lowest-lying triplet state of compound IV was assigned as a (a,r * ) state on the basis of its phosphorescence spectrum and lifetime (singlet-triplet splitting -12.9 kK, irregular (13) R. Zahradnlk, M. TichG, and D. H. Reid, Tetrahedron, 24, 3001 (1968). (14) V. Ermolaev and A. Terenin, J. Chim. P h y s . , 5 5 , 698 (1958). (15) G. Porter and P. Suppan, Trans. Faraday Soc., 61, 1664 (1965).
PHOSPHORESCENCE SPECTRAOF ARYL PHENYL KETONES
1137
~~~
and SO+ T,,,*Transitions Table 111: Criteria Used to Distinguish between SO+ Tn,,*
-
Transition type Spectroscopic feature
SO4 Tmr*
S-T splitting of 0-0 bands
SO -+ TI,.*
-2000 cm-l
Large (no definite position relative to S + S,, * band)
Heavy atom solvent influence
Little or no enhancement
Increases absorption intensity
Lifetime of phosphorescence, TP
10-8 to 10-2 sec
10-1 sec or greater
Fine structure
C=O vibrational progression
Generally complex; no C=O progression
structure, TP = 1.01 sec). We were able to measure the phosphorescence excitation spectrum only in the presence of ethyl iodide. Besides the 0-0 band of the SO+ T,,,* transition a t 21.3 kK, a second, poorly resolved band was observed at 23.8 kK in the excitation spectrum of IV. The large splitting between the two S-T bands (-2500 cm-I) and the high intensity of the second (23.8kK) band suggests that it is not due to the 0-1 band of the SO+ T,,,. transition. One would expect the SO--+ Sn,,* transition of 2-naphthyl phenyl ketone to occur a t about 26.7 kK (the So --+ S,,+ transitions of benzophenone, 4-phenylbenzophenone, and 1-naphthyl phenyl ketone are observed a t 27.0, 26.8, and 26.7 kK, respectively). If the 23.8-kK band is assumed to be due to the So + T,,,*transition, a reasonable value of 2900 cm-1 for the
I
P (kK1
1
I
2fO
205
20'0 I
16
1
I
18
ig
so-6 ( k K 1 Figure 4. Plot of experimental (C) against theoretical LCI (So + Tz)excitation energies for aryl phenyl ketones. The position of the 0-0 phosphorescence band is used in the correlation. The point for ketone VI11 does not fit the line.
Figure 3. Phosphorescence emission spectra of ketones I-V and VIII. For the ketones I and IV, phosphorescence excitation (PE) spectra are shown also. Relative phosphorescence intensity, Ih., is given.
(n, T*) S-T splitting is obtained. However, this assignment is not unequivocal as the 23.8-kK band also lies within 11.2 kK of the So--+ S2,,,* transition of 2-naphthyl phenyl ketone (cf. Figure 1). The expected S-T splitting for the second (T,T*) state is about 12.9 kK. Ketones II, 111, V , and V I I I . Phosphorescence lifetimes for these compounds are of the order of 0.1-1 sec. No characteristic vibrational structure resembling that for benzophenone was noted in the phosphorescence spectra. Singlet-triplet splittings are all larger than 9000 crn-l. The lowest-lying triplet states are assigned as ( T , T*) for ketones 11, 111, V, and VIII. This is in agreement with earlier assignments for 4-phenylbenzophenone (11) and 1-naphthyl phenyl ketone (111) .l6J6 The positions of the 0-0 phosphorescence bands for (16) V. Ermolaev and A. Terenin, Soolet Phys. U s p . (English Transl.), 3, 423 (1960);U s p . F i z . Nauk. 71, 137 (1960).
Volume 75,Number 4 April 1969
C. PARKANYI, E. J. BAUM,J. WYATT,AND J. N. PITTS,JR.
1138
ketones 11-V can be plctted against theoretical values of the energy of the lowest-lying triplet state of these compounds (Figure 4). A good regression line is obtained; however, the absolute values of the calculated energies are much lower than those experimentally observed. Nevertheless, the predictions are about equally good for the So -+ S1 and So -+ TI transitions of the same molecule (Table IV) . As a result of this, the S-T splittings for the lowest excited ( T , T * ) states are remarkably accurate, even when the predictions of the absolute positions of bands are not good.
%
I
I
-
- 10 -5 I
10 Table IV: Energy Difference, AE, between Calculated and Observed Transition Energies and the Singlet-Triplet Splittings for the Lowest Excited (7, T*) States of the Ketones 11-V and VI11 (Values in eV) ----AE
Compound
SO+ SI,,,*
I1 I11 IV V VI11
0.16 0.35 0.42 0.40 0.15
15
20
I
kK
Figure 5. Comparison of the absorption and fluorescence emission spectra of 1-pyrenyl phenyl ketone (methanol). I,..of fluorescence 19.4 kK (515 mp). Absorption spectrum is represented by the full, fluorescence spectrum by the dashed line.
7
SO
+
TI,,,*
0.23 0.47 0.40 0.36 0.35
AEs,T obsd
1.63 1.55 1.60 1.46 1.19
AEs,T oalcd
1.70 1.68 1.58 1.42 1.38
We did not observe any phosphorescence from ketones VI and VII. It follows from Figure 4 and theoretical values of triplet levels for ketones VI and VI1 (1.371 and 1.378 eV, respectively) that the expected values of the 0-0 phosphorescence bands of these compounds should be 18.19 and 19.25 kK, respectively ( L e . , 550 and 520 mp) . However, we observed a weak fluorescence of 9-anthryl phenyl ketone with Pmax a t 22.6 kK (442 mp). The theoretical So -+ S1 value for V I is 2.977 eV, ie., 24.1 kK. The agreement in this case is somewhat better than with the longest wavelength absorption band. A good fluorescence spectrum was obtained for 1-pyrenyl phenyl ketone (VII) which is a mirror image of its absorption spectrum (Figure 5 ) . Photochemical reactivity of aryl ketones can be correlated with relative ordering of their (n, T * ) and ( T , T * ) triplet states. It has been demonstrated that (n, r*) states are more reactive than ( T , T * ) states toward hydrogen atom abstraction reactions. Quantum yields of photoreduction are of the order of 10 to 100 times larger for compounds having lowest-lying (n, T * ) triplet states than for those having lowest-lying ( T , T * ) triplet states."
The Journal o/ Physical Chemistry
Thus, 4-phenylbenzophenone (11) is less reactive than benzophenone (I) toward intermolecular hydrogen atom abstraction (quantum yields of ketone consumption in isopropyl alcohol solution are 0.2 and 2.0, respectively, for these compounds) .18 It follows from our results that aryl phenyl ketones 111-VI11 should be less reactive than benzophenone as well, their reactivity approaching that of 4-phenylbenzophenone toward photoreduction. The results of a recent photochemical study of the reduction of aryl phenyl ketones with sodium borohydride in dimethylformamide show that, under these conditions, the reactivity of 4-phenylbenzophenone resembles that of benzophenone rather than that of other aryl phenyl ketones.lg However, these results may be complicated by the increased donating ability of sodium borohydride compared to that of alcohols. It has been shown that good hydrogen donors, such as tributyltin hydride, efficiently reduce photoexcited ketones having lowest-lying (r,T * ) triplet states.20-22 (17) See N. J. Turro, "Molecular Photochemistry," W. A. Benjamin, Inc., New York, N. Y., 1965, and ref 7 for detailed discussions. (18) J. N. Pitts, Jr., H. W. Johnson, Jr., and T. Kuwana, J . P h y s .
Chem., 6 6 , 2456 (1962). (19) V. Hor&kand J. Pecka, private communication. (20) G. S. Hammond and P. A. Leermakers, J . Amer. Chem. SOC., 84, 207 (1962).
(21) G. S. Hammond and P. A. Leermakers, J . Phys. Chern., 6 6 , 1148 (1962). (22) 0. Parker and C. G. Hatchard, A n a l y s t , 8 7 , 664 (1962).
