Photo-induced and thermal isomerization dynamics of p,p′-diheptylazobenzene dispersed in a nematic liquid crystal

We report on a systematic study of the photo-induced trans-cis isomeric conversion of p,p’-diheptylazobenzene molecules dispersed in 4-cyano-4′- n -pentylbiphenyl host liquid crystal carried out by time-resolved absorption measurements as a function of the pump beam intensity, wavelength and sample temperature. From the experimental data the trans-cis isomerization rate could be established. In all cases a very good agreement was found of the dependence from the pump beam intensity of the isomerization rate and of the photostationary isomer concentration fractions with the predictions of a theoretical model. Measurements of the cis to trans thermal relaxation rates carried out at different sample temperatures revealed an Arrhenius-like dependence with an activation energy value very similar to that obtained in bulk samples by Nuclear Magnetic Resonance characterizations. The light induced cis-trans relaxation rates were also found to scale favorably with the predictions of the considered theoretical model.