Structural and photoisomerization cross studies of polar photochromic monomeric glasses forming surface relief gratings

A novel class of photochromic materials based on polar azo derivatives containing functionalized biphenyl substituents has been synthesized. Once melt and cooled, these compounds formed amorphous transparent monomeric materials whose glass transition temperatures depend mainly on the bulkiness of the biphenyl substituent; by contrast, a change in polarity, evidenced by absorption spectroscopy and DFT calculations, exerts insignificant influence. Photoisomerization of the azo groups occurred both in solution and thin films; the rate constants of the thermal back-relaxation in both media increase with compound polarity. Photoconversion yields display smaller values in solid state than in solution, due to more packed surroundings; surprisingly, they increase substantially with the monomer bulkiness, despite raising T(g)s. Comparative dynamic surface relief grating writings and AFM surface modulation measurements of irradiated thin films showed that the efficiency of the writing process improves dramatically for glasses consisting of more hindered and less polar compounds with a 4- and 2-fold increase in the diffraction efficiency and peak-to-trough amplitude, respectively. Such results highly contrast with those obtained in polymer matrices, where competitive polymer chain reorganization occurs. Free volume enhancement upon back-and-forth photoisomerization of bulky photochromes, facilitating the azo photoisomerization, has been put forward to explain these results.