Microwave-induced delayed phosphorescence studies of the total and radiationless decay processes of3ππ* aromatic carbonyls

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Abstract
We have investigated the details of the radiationless decay processes of 3ππ* aromatic carbonyls of benzaldehyde type using microwave-induced delayed phosphorescence (MIDP) technique. Correlations between the decay rates from the spin sublevels and the energy separations between the 3ππ* and 3nπ* state (ΔE TT) and those between the 3ππ* and 1ππ* states (ΔE ST) were examined in detail for a series of systems. It is shown that the decay rates from the z and x sublevels depend on both ΔE TT and ΔE ST strongly, but the y sublevel decay is independent of them. An empirical model to explain the correlations among the decay rates, ZFS, ΔE TT and ΔE ST is presented. It is shown that the decays from the z and x sublevels of the 3ππ* aromatic carbonyls are dominantly radiationless and are primarily determined by the mixing with the 1nπ* states unless ΔE TT is very small. On the other hand, the radiationless decay from the y sublevel is likely due to the mixing with the 1σπ* (1πσ*) states. The mechanism for the radiationless decay from each spin sublevel is discussed in view of the results obtained.