Photochemical reaction in azobenzene-containing rigid poly(amide acid) networks

Photochemical reaction of azobenzenes embedded into rigid poly(amide acid) (PAA) networks, which undergo reversible photomechanical response upon alternating irradiation of 405 nm and 532 nm lights, was explored. Azobenzene functionalities are incorporated into the PAA backbone that is end-linked with tri- and tetra-armed crosslinkers, 1,3,5-tris(4-aminophenyl)benzene (TAPB) and tetra(4-aminophenyl)methane (TAPM), respectively, to give photoresponsive polymer networks. It was discovered that the trans-to-cis photoisomerization rate of azobenzene moiety is highly dependent on the network architecture. Before crosslinking, the photoisomerization rate is considerably low and totally independent on molecular weights of the backbone whereas, after crosslinking, the rate is dramatically enhanced and highly dependent on the molecular weights. A crosslinker effect on the photomechanical response is also disclosed. These results are attributed to an inter-chain aggregation of PAA backbones. The bulky structure and superior connectivity of TAPM make the network structure spatially extended with less defects, which not only prevents the aggregation but also contributes to the nano-to-macroscopic propagation of molecular deformation of azobenzene.