4385 trans host. Since the transitions of the cis and trans conformations share no common state, the energy of the cis relative t o the trans form cannot be determined from the data. Long-lived phosphorescence of the 3A, state of trans-biacetyl has been observed and analyzed; emission previously assigned3 t o this state is due mainly t o impurities. Biacetyl is photosensitive, and the “strong green” impurity emission is easily restored by irradiation as the dominant luminescence system. Curves reported for biacetyl emission, in hydrocarbon glasses at low temperatures are remarkably similar t o the “strong green” emission of the impure crystal and have
the same profile as that illustrated in Figure 2c. This profile appears in, for example, the (nonexponential) emission of biacetyl in 3-methylpentane at 1.2’K, measured by Chan and Clarke, l7 and the interpretation of this microwave-optical double resonance experiment should possibly be reconsidered. Acknowledgment. We thank Professors W. D. Crow and R. W. Rickards for making available the equipment used for gas-liquid chromatography and fractional distillation and Mr. M. Puza for the low-temperature recrystallization of BA. (17) I. Y. ChanandR. H. Clarke, Chem. Phys. Lett., 19,53 (1973).
Excited Electronic States of the a-Dicarbonyls J. F. Amett, G. Newkome, W. L. Mattice, and S . P. McGlynn* Contribution from the Coates Chemical Laboratories, The Louisiana State University, Baton Rouge, Louisiana 70803. Received February 19, 1974 Abstract: Emission and absorption spectra are presented for two a-dicarbonyls: an indanedione and a propanedione derivative. Pes spectra are also reported for the indanedione and benzil, and a circular dichroism spectrum of an indanedione is given. These data are used to describe the dependence of excited state energies on the CO/CO dihedral angle, 8. This dependence, in conjunction with CNDO/s computations on glyoxal of variable 8, provides excitations, in order of increasing energy, as IrntT+* IFl and an assignment of the two low-energy IFn,* t IFl. These assignments are not in accord with those which are generally accepted. lrn_,-*/lrn+_*
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T
he low-energy absorption properties of a-diketones have generally been rationalized in terms of a simple composite molecule approximation. Thus the R, n, and R* orbitals of the dicarbonyl system are generated by the interaction of the appropriate orbitals of the carbonyl subunits t o produce bonding (+) and antibonding (-) combinations with the resultant ordering (in terms of decreasing binding energy) of x+, x - , n+, n-, R+*, and x-*. Moreover, the energy separation between the highest filled MO’s (Le., n+ and n-) has been supposed to be rather small, -1000 cm-l in a synperiplanar conformation and -100 cm-I in an antiperiplanar conformation. Coupling these considerations with the consequences of orbital overlap, the dependence of the orbital energies upon the intercarbonyl dihedral angle, 8, is seen t o be that of Figure 1. Since only the T+* + n+ and R-* + n- configuration excitations are electric-dipole allowed, it has been usual to associate the two observed low-energy IFnT* lFl absorption bands of a-dicarbonyls with these excitations. Recently, theoreticalZ and experimental3 results have invalidated both the assumption of near degeneracy of the nrt pair and the orbital ordering embodied in Figure 1. In particular, the n* orbital pair splitting is now known to be of the order of 2 eV (16,000 cm-l),
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(1) H. Suzuki, “Electronic Absorption Spectra and Geometry of Organic Molecules,” Academic Press, New York, N. Y., 1967. (2) (a) R. Hoffmann, Accounts Chem. Res., 4 , 1 (1971); (b) J. R. Swensen and R. Hoffmann, Helc. Chim. Acta, 53, 2331 (1970). (3) (a) D. 0. Cowan, R. Gleiter, J. A. Harkmall, E. Heilbronner, and V. Harnuny, Angew. Chem., Int. Ed. Engl., 10, 401 (1971); (b) see also Table 111 of this text.
with n- being of higher binding energy than n+. Consequently, all electronic spectroscopic interpretations based on Figure 1 require further consideration. The major point of the present work, then, is the provision of an alternative set of assignments. In specific, we believe that the R-* n- assignment for the second IFn,* + IFl absorption band is incorrect and that the proper assignment is RT* -+n,. Toward this end, we have investigated the electronic spectroscopy of a synperiplanar (8 = 0’) a-diketone, 3,3-dimethylindanedione (I), and a twisted (70 < 0 < 110’) a-diketone, l-phenyl-l,2-propanedione(11). We accept the available data4 for an antiperiplanar (0 = 180’) dialdehyde, glyoxal (111), as being representative of a transoidal dicarbonyl system. Since we have good experimental reasons t o believe that the two IFnm* + IFl transitions have their excitation density almost wholly localized on the a-dicarbonyl group^,^ we feel free to develop a computational model based on the CNDO/ s-CI approximational scheme for a glyoxal molecule of variable 8 and to compare the results of such computations with the experimental data which are available for the three molecules mentioned. Our experimental investigations also lead us to some conclusions concerning the assignment of higher energy electronic states; provide some information on photorotamerism in the S1 and T1 states of a-dicarbonyls; elaborate a novel blue-shift effect of cooling on the
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( 4 ) G. N. Currie and D. A. Ramsay, Can. J . Phys., 49, 317 (1971), and references therein. (5) J. F. Arnett and S. P. McGlynn, J . Amer. Chem. Soc., submitted for publication.
McGlynn, et al.
1 Excited Electronic States
of the a-Dicarbonyls
4386 I
550
’
500
$50
400 i ----
-x
(mp) 350
U (cli 10’1
0
90
300
,
250
’ 1
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Figure 3. Absorption spectra of 3,3-dimethylindanedione (I) in solution at 300°K: (-) 3-methylpentane (3-MP) solvent, (- - -) ethanol solvent, (-,-)acetonitrile solvent.
I80
6, d e g r e e s Figure 1. Orbital energies of an a-dicarbonyl system derived on the basis of a simple composite molecule description. The intercarbonyl dihedral angle, 0, is defined in Figure 2. The two allowed transitions for the synperiplanar case are also shown.
&
Y
I 3’
( c l i xld’)
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Figure 4. Absorption spectra of l-phenyl-1,2-propanedione(11) in solution at 300°K: (-) 3-methylpentane (3-MP) solvent, (- - -) ethanol solvent, (-,-)acetonitrile solvent. Figure 2. Atom numbering conventions, axes designations in the Cz, point group (i.e.. synperiplanar glyoxal, 0 = O ” ) , and the CO/CO dihedral angle for a twisted glyoxal.
phosphorescence, excitation, and absorption spectra of a-dicarbonyls and suggest that this blue-shift is dependent on effects which produce a variation of 0; indicate that twisting processes are coupled t o Sl So and T1 So energy-dissipative channels; and, to some T1 thermally activated nature degree, validate a Sl for much of the room-temperature fluorescence intensity. These considerations are touched on only in the Results section since they are not particularly pertinent t o our central thesis.
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Experimental Section Benzil (IV) (Baker) was purified immediately prior to use by sublimation, recrystallization, and/or thin layer chromatography. l-Phenyl-1,2-propanedione (11) (Eastman) was subjected to multiple vacuum distillations prior to use. 3,3-Dimethylindanedione (I) was prepared by standard procedures5and purified as above for IV. 3-Ethyl-3methylindanedione (V) was synthesized6 from a (-)phenylvaleric acid of 4 0 % optical purity. The purification of camphorquinone (VI) is described elsewhere.5 Circular dichroism was measured using a Durrum-Jasco 5-20 recording spectropolarimeter. The molar ellipticities quoted are based on a calibration of [B]zsu.j = 7260 deg cmQ/dmolfor d-10camphorsulfonic acid in water. All other experimental techniques used have been described previously.7 Molecular orbital, configuration, and state energies were computed using the Del Bene-Jaff6 CNDO/s-CI quantum-chemical scheme.8 The CNDO/s program used was obtained from the Quantum Chemistry Program Exchange (QCPE 174) and was modi(6) R. IC=O groups, are not only made spatially separate but where the different symmetries also may impose considerably different selection rules on the various lrnr* ’I” transitions. Acknowledgment. This work was supported by contract between the U. S. Atomic Energy Commission (Division of Biomedical and Environmental ResearchPhysics and Technological Program) and the Louisiana State University. We wish to express our sincere thanks to Dr. Donald Larson (Rohm & Haas),for access to his comprehensive knowledge of a-dicarbonyl spectroscopy, and J. L. Meeks (Louisiana State University), for his extensive help in obtaining and interpreting photoelectron spectra.
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Vapor Phase Excimer Formation in Saturated Amines Arthur M. Halpern Photochemistry and Spectroscopy Laboratory, Department of Chemistry, Northeastern University, Boston, Massachusetts 02115. Received January 26, 1974 Abstract: The spectral and temporal properties of the excited state of the saturated amine, ABCO (l-azabicyclo[2.2.2]octane),in the vapor phase are reported. On the basis of these properties, it is shown that ABCO forms an excimer in the vapor phase. This system is analyzed in terms of the usual monomeriexcimer kinetic scheme
to which one additional rate process had been added, the quenching of the excimer by ground state amine. The rate constants pertaining to the ABCO system are determined by measuring the monomer and excimer decay parameters as a function of the amine vapor pressure. The inference is drawn from solution phase studies that the ABCO excimer is strongly bound and is formed in the head-on approach of two ABCO molecules. The excimer formation efficiency in the vapor phase is determined to be ca. 0.2. The effects of added n-hexane vapor on the monomer and excimer emission efficiencies and kinetics are examined. Significant excimer enhancement is observed up to n-hexane overpressures of ca. 90 Torr. This is interpreted in terms of vibrational relaxation (and stabilization) within the monomer and excimer manifolds. The emission intensity and decay data imply that both radiative and nonradiative processes are induced by collisions between ABCO excited monomer and n-hexane molecules. The role of termolecular processes in excimer formation is suggested.
T
he association between ground and excited state molecules (excimer formation) is a well known phenomenon which occurs in the bimolecular electronic relaxation of many aromatic and heteroaromatic molecules.’ It has been reported recently that certain saturated amines also undergo excimerization, both in the vapor phase and in solution.’ It appears that vapor phase excimers have not been reported for the aromatic systems, presumably as a consequence of the low vapor pressures which characterize many excimerforming aromatic molecules at ambient temperatures. At high temperature, where the vapor pressure is high enough to provide a sufficient collision frequency, the (1) J . B. Birks, “Photophysics of Aromatic Molecules,” Wiley-Interscience, London, 1970, pp 301-371. ( 2 ) A. M. Halpern and E. Maratos, J . Amer. Chem. SOC.,84, 8273
concomitant increase in the excimer dissociation rate results in a lower net excimer emission yield. It is of interest to study the dynamic and energetic properties of the excited monomer and dimer (i.e., excimer) states in the absence of perturbing solvent molecules which affect both the formation and dissociation steps, as well as other nonradiative (and radiative) processes. This paper reports the results of a study of the ABCO, I (l-azabicyclo[2.2.2]octane), excimer in the vapor phase at ambient temperatures.
I _
(1972).
Journal of the American Chemical Society
1 96:14 1 July
10, 1974
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(3) B. Stevens and P. J. McCartin, Mol. Phys., 8, 597 (1964).
