Photoisomerization of azobenzene chromophores in organic/inorganic zirconium phosphonate thin films prepared using a combined Langmuir-Blodgett and self-assembled monolayer deposition

The photoinduced trans to cis isomerization of two new azobenzene-derivatized phosphonic acids, 4-(4'-(3"-pentyloxy)phenyl)azophenylbutylphosphonic acid (2a) and 4-(4'-propoxyphenyl)azophenylbutylphosphonic acid (2b), has been studied in zirconium phosphonate thin films. Multilayer films were assembled using a three-step combined Langmuir-Blodgett (LB) and monolayer self-assembly procedure that forms a layered metal phosphonate lattice in the polar region of asymmetric bilayers which are composed of octadecylphosphonate and the azobenzene chromophore. Multilayer films containing 2a and 2b undergo rapid and reversible photoisomerization. For both molecules, thermal relaxation of the cis form shows both fast- and slow-relaxing components that arise from cis isomers experiencing different degrees of strain in the films. The relaxation rates and the ratio of fast/slow relaxing components can be changed through synthetic modification of the chromophore and by altering the thin-film architecture. When the cis form of 2a is deposited directly, virtually all of the fast-relaxing component is eliminated, and the thermal half-life for cis to trans isomerization is greater than 11 h.