Morphology-Mediated Photoresponsive and Fluorescence Behaviors of Azobenzene-Containing Block Copolymers

We investigated the relationship between the self-assembled morphology of poly(tert-butyl acrylate)-block-poly(6-[4-(4'methoxyphenylazo)phenoxy]hexyl methacrylate) (PtBA-b-PAzoMA) block copolymers and their photoresponsive and fluorescence behaviors. The morphology of PtBA-b-PAzoMA copolymers was manipulated by dissolving them in mixed dimethylformamide (DMF)/hexanol solvents. When PtBA-b-PAzoMA was dissolved in DMF-rich (neutral) solvents, a favorable interaction-between the DMF molecules and both blocks resulted in a random-coiled conformation. The unconfined morphology facilitated the formation of both nonassociated and head-to-head organized azobenzene mesogens, which promoted fluorescence emission. When hexanol, a PtBA-selective solvent, was added to DMF, the solvency of PtBA-b-PAzoMA worsened, leading to its assembly into micelles, with PAzoMA in the micelle core. The confinement of azobenzene moieties in the micelle core hindered their trans-to-cis photoisomerization, thereby considerably decreasing the kinetics of photoisomerization and the population of cis isomers. Additionally, a nanoconfined geometry resulted in compactly packed chromophores, causing fluorescence loss. When PtBA-b-PAzoMA was exposed to UV light, the increased number of cis isomers hampered the closely packed mesogens, resulting in a substantial enhancement of fluorescence emission. When the mole fraction of the PAzoMA block was increased, PtBA-bPAzoMA formed clusters, causing the slow kinetics of photoisomerization and fluorescence quenching.