Using Spiropyran Fragment Superposition To Assign the EPR Spectrum of Localized Triplet Excitation on a Distorted Aromatic Nitro Group

Abstract

On the supposition that the zero-field splitting tensors of analogous triplet states in different crystal packings should be similarly oriented with respect to the group of atoms on which the spin is localized, one can identify the excited group in a series of complex molecules by a combination of X-ray and EPR measurements. This technique of fragment superposition was used to select the correct set of tensor assignments among 512 possibilities for the short-lived triplet states of nitrated benzoindolinospiropyrans. It shows conclusively that the spin-polarized triplet state is localized on the nitro group and that the nitro group loses local C 2v symmetry on excitation. This use of eigenvector orientation constitutes a new method for identifying an excited state of a simple chromophore with puzzling spectroscopy.