Studies of the electronic absorption and emission spectra of 2,1,3-benzothiadiazole

Abstract

A study has been made of the spectral properties of 2,1,3-benzothiadiazole (BTD) and of BTD-d4. Absorption spectra have been obtained in polar and nonpolar solvents at room temperature and in an EPA rigid glass at 77 K. The complete loss of resolution of the first singlet absorption in ethanol at room temperature is explained in terms of the formation of weak solute solvent complexes. This solvent effect is taken as evidence that the heterocyclic ring portion of the molecule is involved in the electronic transition. Fluorescence and phosphorescence emission spectra have been obtained in polar, nonpolar, and heavy atom rigid glasses at 77 K. Triplet decay lifetimes have been obtained both by directly measuring the phosphorescence decay and by monitoring the decay of triplet-triplet (T-T) absorption. The T-T absorption spectrum of BTD-d4 has been obtained in EPA rigid glass at 77 K in the regions from 20 000 cm−1 to 28 000 cm−1. The deuterium effect on the triplet state lifetime and the external heavy atom effect on phosphorescence intensity are interpreted as providing evidence for ππ∗ character in the lowest triplet state. On the other hand, the phosphorescence energy and the triplet lifetime are interpreted as providing evidence for nπ∗ character in the same state. Similary, the T-T spectrum is suggestive of nπ∗-ππ∗ interaction in the triplet manifold. A difference in the triplet state lifetime obtained by the two measurement techniques is discussed.