SPECTROSCOPIC PROPERTIES OF NEMATIC DISCOTIC PHENYLETHYNYLBENZENE DERIVATIVES - SYMMETRY EFFECTS

The spectroscopic properties of two phenylethynylbenzene derivatives c apable of forming nematic discotic (N-D) liquid crystals are studied i n solution and in their N-D phase. Their electronic absorption spectra are analysed with the help of fluorescence anisotropy measurements an d on the basis of a previous theoretical study. The results presented here illustrate the effects of a symmetry lowering of a radial-symmetr ic chromophore, arising from the substitution of an ethynylbenzene gro up by an alkoxy chain, upon the positions and the intensities of the e lectronic transitions. The absorption and fluorescence spectra of the nematic discotic phases are compared to those obtained in solution. Th e observed spectral shifts are discussed in terms of both exciton coup ling and environment effects on the excitation energy.