Influence of inter- and intramolecular hydrogen-bonding interactions of azo dyes on the dynamics of photo-induced anisotropy, The

Abstract

In a previous study the orientation dynamics of an o-derivitized methyl red monolayer (o-dMR ML) was found to exhibit unusual sub-diffusive behavior caused by molecular interactions. From our prior study of the cis to trans thermal relaxation rate of o-dMR in solution-based experiments, we discovered an unexpected behavior that appeared to be associated with the hydrogen bonding properties of the solvents. The objective was to further investigate the influence of hydrogen bonding on cis to trans thermal relaxation and to possibly connect it to the orientational dynamic of the monolayer. The o-methylated_dMR, which has the hydrogen on the amide group of o-dMR methylated, was synthesized. The influence of the hydrogen bonding on o-dMR and o-methylated_dMR was studied through solution experiment: UV-Vis absorption spectroscopy, the measurement of the cis-trans thermal relaxation rate, and the quantum mechanical modeling. The surface characterization of the conformity, thickness and the volume density for o-dMR and o-methylated_dMR ML were done through atomic force microscopy and angle-resolved x-ray photoelectron spectroscopy. The effect of intermolecular interactions within the MLs was studied through the UV-Vis absorption spectroscopy, the measurement of the cis-trans thermal relaxation rate and the measurement of the birefringence of the MLs. The solution study indicated that the competition between intermolecular and intramolecular hydrogen bond leads to the decrease of the cis-trans thermal relaxation rate. By contrast, the intermolecular hydrogen bonding between the electron-withdrawing group of o-dMR and the hydrogen-bond donor solvent molecules increases the cis-trans thermal relaxation rate of o-dMR. The monolayer study of UV-Vis absorption spectroscopy and thermal relaxation measurement indicates that the transition and excited state of the molecules are affected by intermolecular hydrogen interactions and [pi]-[pi] stacking interactions. However, the result of the birefringence study implied that the unusual sub-diffusion kinetics of the o-dMR ML is mainly caused by the [pi]-[pi] stacking interactions rather than the hydrogen bonding interactions. The o-dMR ML develops a better photo-induced anisotropic layer than the o-methylated_dMR ML, which is mainly contributed by a faster thermal relaxation rate and a higher volume density of the o-dMR ML.