PMDR studies of the magnetic and dynamic properties of the lowest excited triplet states of aromatic carbonyl molecules : acetophenone, benzaldehyde and their derivatives

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Abstract
The results of the phosphorescence microwave double resonance studies of the lowest excited triplet (T 1) states of acetophenone, benzaldehyde and their derivatives in various mixed crystals are presented and analysed. The zero field splittings (ZFS), the total decay rates and the relative radiative decay rates and populating rates were determined for a series of compounds in different host crystals. The correlations between ZFS, the decay rates and the energy separation between the 3nπ* and 3ππ* states were carefully examined and the results were explained in terms of the mixing of the 3nπ* and 3ππ* states. The mixing coefficients of the 3nπ* states in the T 1 states were estimated from the decay rates. Changes of the ZFS, decay rates, vibrational structures of the phosphorescence spectra of the 3ππ* aromatic carbonyl molecules were also analysed. The mechanisms responsible for the S 1 → T 1 and T 1 → S 0 processes of the 3ππ* states of these aromatic carbonyl molecules are discussed in view of the obtained results. Effects of substitution and host on the properties of the T 1 states of the aromatic carbonyl molecules were investigated and the causes of these effects were considered. External heavy atom effects on the 3nπ* and 3ππ* carbonyl molecules are also investigated and the results are discussed.