Photophysics of the lowest triplet state in 2-benzoylpyridine crystals. I. Optical spectra

The optical spectra of the lowest triplet state in 2-benzoylpyridine single crystals at 4.2 K have two features which are unique among the benzophenones. The first is the crystal blue shift of the 0-0 origin compared with its position in host crystals. This is ascribed to the loss of conjugation of the pyridyl ring in the excited state, due to its rotation out of the plane of the carbonyl group in the pure crystal. The second feature is the broad, almost structureless nature of phosphorescence, singlet absorp­tion and triplet absorption. This is accounted for in terms of strong elec­tron-phonon coupling consequent upon a relative translation of the molecules in response to carbonyl group charge redistribution on excita­tion. This is also a consequence of the crystal structure which places the carbonyl group and nitrogen atom of one molecule over the reversed arrangement of these atoms in the neighbouring molecule of the c-axis stack.