Highly resolved electronic spectra and electronic structure of benzo[f]quinoxaline

Abstract

Vibrational analysis of highly resolved phosphorescence (P), flourescence (F) and absorption spectra of benzo[f]quinoxaline (BQ) in Shpolskii matrices, at 77 K, was carried out. The spectra were dominated by phenanthrene-type fundamental vibrations. The appearance of several out-of-plane (op) modes in the P spectrum as well as the P lifetime, much shortened relative to that of the parent hydrocarbon, point to the proximity of the lowest n, π ∗ and π, π ∗ states of the BQ molecule. High activity of op modes in the F spectrum and complex structure of the absorption spectrum onset have been explained in terms of the pseudo-Jahn-Teller interaction between close-lying S 1 (π, π ∗)and S 2(n, π ∗) states. According to calculations of the BQ electronic structure, performed using a modified INDO CI method, the T 1(π, π ∗)-T 2(n, π ∗) and S 1(π, π ∗-S 2(n, π ∗) energy gaps are about 1200 and 700 cm -1, respectively.