Electronic states of diphenyl- and dipyridyl-s-tetrazines: linear and magnetic circular dichroism, and quantum chemical calculations

The electronic structures of 3,6-diphenyl-s-tetrazine (I), 3,6-bis(4-pyridy l)-s-tetrazine (II), 3,6-bis(3-pyridyl)-s-tetrazine (III), and 3,6-bis(2-py ridyl)-s-tetrazine (IV) were investigated by linear dichroism (LD) W-Vis ab sorption spectroscopy using stretched polyethylene matrices, by magnetic ci rcular dichroism (MCD) spectroscopy, and by quantum chemical calculations. The LD spectra contain information on the transition moment directions of t he observed electronic transitions, and here in combination with the MCD sp ectra, they lead to resolution of the otherwise hidden transitions. The ele ctronic transitions predicted by time dependent density functional theory ( TD-B3LYP/6-31G*), using molecular geometries determined by X-ray crystallog raphy, were in excellent agreement with the observed transitions. The combi ned experimental and theoretical evidence lead to a consistent assignment o f electronic states. In particular, the second observed transition of I aro und 27 000 cm(-1) can be assigned to an in-plane short-axis polarized pi-pi * transition, in contrast to a previous assignment. (C) 2000 Elsevier Scien ce B.V. All rights reserved.