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

Abstract

The electronic structures of 3,6-diphenyl- s-tetrazine ( I), 3,6-bis(4-pyridyl)- s-tetrazine ( II), 3,6-bis(3-pyridyl)- s-tetrazine ( III), and 3,6-bis(2-pyridyl)- s-tetrazine ( IV) were investigated by linear dichroism (LD) UV–Vis absorption spectroscopy using stretched polyethylene matrices, by magnetic circular dichroism (MCD) spectroscopy, and by quantum chemical calculations. The LD spectra contain information on the transition moment directions of the observed electronic transitions, and here in combination with the MCD spectra, they lead to resolution of the otherwise hidden transitions. The electronic transitions predicted by time dependent density functional theory (TD-B3LYP/6-31G*), using molecular geometries determined by X-ray crystallography, were in excellent agreement with the observed transitions. The combined experimental and theoretical evidence lead to a consistent assignment of electronic states. In particular, the second observed transition of I around 27 000 cm −1 can be assigned to an in-plane short-axis polarized π–π * transition, in contrast to a previous assignment.