Chiroptical switch based on azobenzene-substituted polydiacetylene LB films under thermal and photic stimuli

The azobenzene-substituted diacetylene (NADA) LB films were fabricated and their chiroptical properties were investigated in detail. It was found that the NADA LB films showed supramolecular chirality, although the NADA molecule itself was achiral. Overcrowded packing of azobenzene chromophores were believed to be responsible for the supramolecular chirality formation of NADA LB films. When irradiated by left- and right-handed circular polarized ultra-violet light (CPUL), obviously opposite CD signals for azobenzene chromophores and polydiacetylene (PDA) chains were observed from the resulting polymerized NADA (PNADA) LB films. Reversible changes of CD signals for PDA chains could be observed in response to thermal stimulus. When irradiated by left- and right-handed circular polarized lasers (CPL, 442 nm), a reversible chiroptical switch between two enantiometric structures of PDA chains could be observed by alternating the stereoregular packing of azobenzene chromophores in the side chains. Strong collective non-covalent interactions (π-π stacking) and rapid conformational rearrangement of azobenzene chromophores under CPUL or CPL treatment were believed to be responsible for the supramolecular chirality formation and reversible chiroptical switches upon thermal and photic stimuli. This research provides a novel model system for understanding the detailed mechanism of the chiroptical introduction and modulation in the PDA backbone.