A Photosensitive Liquid Crystal Studied by 14N NMR, 2H NMR, and DFT Calculations

An azobenzene derivative, namely diheptylazobenzene, showing the nematic and smectic A liquid crystalline phases, was investigated by means of a combined approach based on NMR and DFT calculations. 14N NMR quadrupole- and chemical-shift-perturbed spectra were acquired in the whole mesophasic range, providing both experimental quadrupolar splittings and chemical shift anisotropy values. On the same mesogen, deuterium labelled at the α-position of the hydrocarbon chain, 2H NMR quadrupole-perturbed spectra were recorded. The analysis of these NMR data was performed with the help of ab initio calculations, in vacuo and by taking into account the effect of the anisotropic environment typical of liquid crystals, by using the IEF-PCM model. The geometry optimizations of the azomesogen in the trans and cis configurations were performed by DFT calculations employing the combination of B3LYP functional with the 6-311G(d) basis set. The analysis of experimental NMR data was performed by considering the trans configuration as the most populated one and the corresponding quadrupolar tensors and chemical shielding tensors were determined at the DFT level of theory. The main result of this work is the determination of a relatively high and temperature-dependent molecular biaxiality of the trans state of this azomesogen.