Synthesis and steady-state spectroscopic study of 5-aryl-2,2′-bipyridyls.: New fluorescent compounds in solid state

The absorption and fluorescent properties of newly synthesized 5-aryl-2,2'-bipyridyls were examined both in solution and in the solid state. The 6-methoxy derivatives 5a-5c have weak fluorescence, whereas 6-amino derivatives 7a-7f showed strong fluorescence. Their spectroscopic properties were computationally analyzed using semi-empirical molecular orbital calculations. The geometries of the ground (S-0) and the first sin.-let excitation states (S-1) were optimized and employed to estimate the vertical transition energies. The drastic differences in fluorescent intensities between 5a-5c and 7a-7f can be qualitatively explained by the relative energy levels S, and the neighboring triplet level; the energy gaps supposedly regulate the non-radiative intersystem crossing (ISC) probabilities. On the relaxed S-1 geometry, 5a-5c have quite small S-1-triplet energy gaps, whereas 7a-7f have no such quasi-degeneracies, leading to their quantitatively different quantum yields.