Exciton band of a one-molecule-per-unit-cell crystal: hexamethylbenzene first singlet

Abstract

The exciton band structure has been determined experimentally, from quantitative band—band fluorescence and absorption measurements, for the first singlet excited state of the low temperature phase of the hexamethylbenzene crystal. At 2 K, the center for the band is at 35156 cm −1, its bottom (the κ = 0 state) at 35134 cm −1 and its total extent about 40 cm −1 (i.e. much larger than anticipated earlier). We derive a dispersion relation, based on the trigonal topology of the crystal, with six out-of-plane nearest neighbor pairwise interactions of −3.3 cm −1 and six in-plane ones of −0.4 cm −1. These parameters, based on the exciton band shape, are inconsistent with a transition-multiple—transition-multipole model, and in particular with an octopole—octopole one, based on the π electrons. π-σ interactions are suggested. The neat crystal data are fully consistent with a complete concentration study on isotopic mixed samples of hexamethylbenzene, at 2 K, by absorption and fluorescence spectra with a 1 cm −1 resolution. The mixed crystal results are also consistent with the energy and spectral moments given by the separated-band model and with the behaviour expected from cluster percolation and exciton percolation.