Electronic spectrum and structure of 11,11,12,12-tetracyano-1,4-naphthaquinodimethane

Abstract

The lower π-electronic states of 11,11,12,12-tetracyano-1,4-naphthaquinodimethane (TCNNQ) were calculated by the usual SCF-CI and the VESCF-CI methods within a Pariser-Parr-Pople scheme. The absorption spectrum of TCNNQ was measured in cyclohexane and methanol. The observed lower excitation energies were 3·14, 4·32, 4·5, 5·6 and 5·96 eV. The lower three excitation energies calculated by both methods are in good agreement with the observed values. It was shown that the effect of doubly excited configurations is far from being negligible. As for charge distribution, the VESCF method was found to give more reasonable results than the usual SCF method. Theoretical bond lengths obtained from bond orders suggest partial breakdown of the quinoid structure, which may be associated with the fact that the electron affinity of TCNNQ is relatively small compared with those of the 2,6-analogue and TCNQ.