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 [kappa] = 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 [pi] electrons. [pi]-[sigma] 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.