Search for Photochemical Processes Peculiar to Excited Triplet Molecules and Selective Properties

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In this project we focused on aromatic carbonyl molecules and investigated the excited state dynamics, particularly the intersystem crossing by the analysis of the vibronic structure in the excited triplet state. The triplet molecule emits phosphorescence with the very long Lifetime. We have developed laser spectroscopy system by which we can detect phosphorescence sensitively in a supersonic jet, and observed high resolution phosphorescence excitation spectra for a number of molecules. The typical molecule is benzaldehyde and we have observed the accurate spectra in the wide wavelength region. We also performed the quantum chemical molecular orbital calculations and assigned the vibrational levels in the excited states. The S1 state is nπ^*, and S_2 is ππ^*. Both are strongly coupled with the triplet states and show fast iutersystem crossing. As a result, fluorescence from the singlet state is weak, but phosphorescence from the triplet is very strong. We analyzed the deuterated compound and the rate of the iutersystem crossing was not changed by the deuterations. In pyridinecarboxyaldehyde, which is the nitrogen substituted compound, we found two nπ^* states. The intersystem crossing is suppressed in the lower energy, but enhanced in the higher energy region. As the molecular size becomes larger, the photochemical processes are expected to be faster because the level number is increased. However, it was not so fast in 1indanone and 1tetralone in which the aliphatic ring is attached. In contrast with this, the intersystem crossing is very fast in xanthone. It is concluded that the properties of the triplet molecules are various and peculiar for each compound. It is necessary to apply this to chemical reaction by probing the origin of the variety.