Spectroscopic properties of nematic discotic phenylethynylbenzene derivatives: Symmetry effects

Abstract

The spectroscopic properties of two phenylethynylbenzene derivatives capable of forming nematic discotic (N _D ) liquid crystals are studied in solution and in their N _D phase. Their electronic absorption spectra are analysed with the help of fluorescence anisotropy measurements and on the basis of a previous theoretical study. The results presented here illustrate the effects of a symmetry lowering of a radial-symmetric chromophore, arising from the substitution of an ethynylbenzene group by an alkoxy chain, upon the positions and the intensities of the electronic transitions. The absorption and fluorescence spectra of the nematic discotic phases are compared to those obtained in solution. The observed spectral shifts are discussed in terms of both exciton coupling and environment effects on the excitation energy.