Phosphorescence microwave double resonance and spectroscopic studies of the radiative mechanisms of benzaldehyde type3ππ* aromatic carbonyls

We have made a detailed investigation of the vibronic band dependence of the radiative decay rates from the spin sublevels of the 3ππ* aromatic carbonyls of the benzaldehyde type. The sublevel phosphorescence spectra were obtained for representative systems. It is shown that the emissions from the z and x sublevels are active at the 0-0 and the vibronic bands involving both a′ and a″ vibrations, while the y sublevel emission is strong at the bands involving a′ vibrations. Correlations between the relative intensities of the various vibronic bands and the energies among 3ππ*, 3nπ* and 1nπ* states were investigated. It is shown that the direct interaction between 3ππ* and 3nπ* states is important in causing radiative decay. The radiative decay from the z sublevel is primarily due to the three mechanisms : (i) direct electrostatic mixing between 3ππ* and 3nπ* states, (ii) vibronic mixing between 3ππ* and 3nπ* states, (iii) direct spin-orbit mixing between 3ππ* and 1nπ* states. The factors which determine the relative importance of each mechanism is considered in detail. The emission from the y sublevel is likely due to the direct spin-orbit mixing with the 1σπ* (1πσ*) state. The mechanism for the x sublevel emission is the same as that for the z sublevel emission and possible explanations for this observation are given. Critical examinations of the criteria for distinguishing 3ππ* and 3nπ* aromatic carbonyls are given based on the results of the present work.