Phosphorescence Spectrum of Acetophenone; An Example of Pseudo-Jahn-Teller Distortion

Abstract

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.