The phosphorescence spectra and lifetimes of acetophenone, aceto-d3-phenone and acetophenone-d5 were recorded in a variety of close packed and relaxed matrices. The variable vibronic lifetime changes, temperature dependence, and deuteration effects on the acetophenone phosphorescence from solvent to solvent are manifestations of perturbation between close lying 3ππ* and 3nπ* states. The acetophenone phosphorescence problem is marked by environmental perturbations superimposed on pseudo-Jahn-Teller interactions. The result is an environmental sensitive potential surface for the lowest triplet state marked by failure of the Condon approximation. Five cases are distinguished. (i) Only the dynamical pseudo-Jahn-Teller effect is operative as in acetophenone vapor. The emission here is interpreted to be from a distorted, i.e., largely torsionally twisted acetophenone about the COCH3 group. (ii) and (iii) There are environmental (static) interactions larger than dynamical interactions leading to strong characteristic out-of-plane vibrations and medium lifetime, e.g., COCH3 group modes in methylbenzoate matrix, where 3nπ* is lowest, and ring out-of-plane modes in acetophenone-d5 matrix, where 3ππ* is lowest. (iv) There is a dynamical Jahn-Teller interaction stronger than the environmental interaction. In some instances the resulting triple minimum potential curve results in dual phosphorescence emissions even at low temperatures (1.68°K) from the lowest 3nπ* state—one with a very short lifetime from a highly distorted region of the potential surface (the zero point region) and the other with a medium lifetime from an excited vibronic region, e.g., in pentane and other relaxed hydrocarbon matrices. The upper vibronic region is kept from rapidly cascading to the lower region of the triplet state by a potential barrier leading to temperature dependent phosphorescence. Breakdown of the Condon approximation for the lowest triplet state is observed, probably due to rapid change of the singlet—triplet transition moment with the distortion coordinate in the vicinity of near degeneracy of the zeroth order states. (v) Again the dynamical Jahn-Teller interaction is stronger than the environmental interaction except that the zeroth order 3ππ* state is now lowest, e.g., at 77°K in a benzene matrix the emission with its vibrational structure and lifetime characteristic of 3nπ is from the zeroth order region—whose electronic wavefunction is described largely by 3nπ. At 4.2°K, the vibrational structure and lifetime represent phosphorescence from an excited vibronic region of the lowest triplet state whose electronic wavefunction is largely 3ππ*. The rapid change in transition moment with vibrational coordinate also renders the Condon approximation inappropriate in this case.

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The phosphorescence spectra of benzaldehyde-h6, −1d1, −4d1, −3, 5d2 and −d6 have been studied optically in close packed matrices (acetophenone and methyl benzoate) and relaxed matrices (methylcyclohexane and perfluoro-n-hexane) at 4.2°K. All the spectra were sharp and could be analyzed vibrationally. In the close packed matrices, of the 11 possible out-of-plane (a″) fundamentals, nine are observed in Herzberg—Teller borrowing of intensity. The strongest modes are the aldehyde-H wag, the CHO torsion and CHO wag, and their intensities were found to be stronger than those of any totally symmetrical bands including the origin band. 2 and 3 quanta of aldehyde-H-wag and CHO-torsional modes are found with intensity alternation. In addition to this intensity alternation, the lack of long sequences of any nontotally symmetric mode and the analysis of the Franck—Condon envelope show that the triplet benzaldehyde does not deviate far from the ground state geometry. We conclude it is coplanar or nearly so. Deuteration effects on the vibronic intensities are remarkable. The long lifetime, ∼4×10−2sec at 4.2°K, supports a planar geometry for the triplet. The large change of lifetime and spectral intensity on substitution of the aldehyde hydrogen by deuterium shows that the aldehyde H plays an important role in the vibronic interaction. In the relaxed matrices most of the out-of-plane fundamentals are too inactive to be identified in the phosphorescence. The only out-of-plane mode clearly identified is the fundamental CHO-torsional mode and its overtone. Higher overtones are not found. Most of the intensity is in the 0–0 band and the ν (C=O) progression which is longer than in the close packed matrices. Also active are in-plane modes such as δ(Φ-CHO), ν(Φ-CHO), ν1, ν9a, and ν8a. The short lifetime in the relaxed matrices (fully 20 times shorter than in the close packed matrices), ∼2×10−3sec at 1.68°K, without appreciable deuteration effect establishes that the triplet benzaldehyde is highly nonplanar with the aldehyde group twisted out of plane. We conclude from the lifetime that the twist angle exceeds 60°. The vapor phosphorescence resembles the relaxed matrix phosphorescence, but with still reduced lifetime. We conclude that in the free molecule the aldehyde group in the benzaldehyde triplet is also strongly out of plane.