Synthesis and photoisomerization characteristics of a 2,4,4′-substituted azobenzene tethered to the side chains of polymethacrylamide

A side-chain methacrylamide copolymer, poly{(N-[3-(4-(4'-methoxyphenylazo)-2-nitrophenoxy)propyl]-N-[3-(dimethylamino)propyl]methacrylamide)-co-(N-[3-(dimethylamino)propyl]methacrylamide)} (PNA) functionalized with a 2,4,4'-substituted chromophore, 2-nitro-4'-methoxyazobenzene was prepared and characterized. The branched structure of the azo chromophore was specifically designed to prevent the aggregate formation by minimizing the ground-state aromatic stacking interactions between the azo molecules. This cationic polyelectrolyte, together with the oppositely charged iota-carrageenan (CAG) polysaccharide, was alternately dip-coated onto a fused silica substrate to produce a multilayered film by layer-by-layer electrostatic self-assembly. A UV/vis spectroscopic study of PNA, performed during both a heating-cooling (25-85 degrees C) and an E-Z-E photoisomerization cycle, revealed that the maximum absorbance wavelength of the chromophore did not shift in either DMSO/H2O (1:20) solution or the multilayer assembly comprising 10 PNA/CAG bilayers. This result indicates that the PNA chromophores do not undergo aggregation. Monitoring of the E/Z photoisomerization of the solution- and film-based PNA chromophores revealed that the intensity of the pi-pi* transition band of its E-isomer decreased during irradiation with UV light. Also, the Z/E reverse isomerization induced by visible light (lambda > 460 nm) was observed with almost full recovery of the E isomer. The kinetics of the photoisomerization step was determined to be monoexponential, indicating that photoisomerization proceeds via a single pathway. Unlike in general cases, one distinguishable property of the film-based PNA chromophore is the E/Z switching speed under experimental conditions (360 nm, P = 2 mW/cm(2)), which is remarkably similar to that observed in solution.