Molecular Orbital Studies on Novel Fluorescent Arylbipyridyls in Solution and Solid State

Abstract

The absorption and fluorescent properties of newly synthesized 5-aryl[2,2']bipyridyls were examined both in solution and in the solid state. 6-methoxy derivatives 1a-c have weak fluorescence whereas 6-amino derivatives 2a-f showed strong fluorescence. There seems no notable differences in their structural characteristics indicated by X-ray crystallographic and MO-optimized geometries. In aiming to find rational explanations for the fluorescent differences from the theoretical viewpoints, their spectroscopic properties were computationally analyzed using semi-empirical molecular orbital calculations. The geometries of the ground (S0) and the first singlet excited state (S1) were optimized and employed to estimate the vertical transition energies. The drastic differences in the fluorescent intensities between 1a-c and 2a-f can be qualitatively explained due to the relative energy levels S1 and the neighboring triplet level; the energy gaps supposedly regulate the non-radiative intersystem crossing (ISC) probabilities. On the relaxed S1 geometry, 1a-c have quite small S1-triplet energy gaps whereas 2a-f have no such quasi-degeneracies, leading to their quantitatively different quantum yields.