Crystallization-promoted emission enhancement of poly(l-lactide) containing a fluorescent salicylideneazine center with aggregation-enhanced emission properties

Abstract

Fluorescent 1,2-bis(2,4-dihydroxybenzylidene)hydrazine (CN₄OH) with aggregation-enhanced emission (AEE) properties was used as the initiator to induce the ring-opening polymerization (ROP) of L-lactide, resulting in polymer CN-PLLA(n)s containing an AEE-active CN center. With both pairs of p- and o-hydroxyl (OH) groups, CN₄OH initiates an ROP of L-lactide solely with the p-OH groups and the resulting CN-PLLA(n)s are highly-emissive due to the rotational restriction imposed by the remaining p-OHs of the central CN unit. A study on the solid emission of CN-PLLA(n)s reveals that crystallization of the neighbouring PLLA chains, instead of the fluorescent CN center itself, determines the AEE activity, e.g. the emission of crystalline CN-PLLA(n)s is much higher in emission intensity than amorphous CN-PLLA(n)s. As restricted molecular rotation is the main mechanism leading to AEE activity, effective rotational restriction imposed by crystalline polylactide chains is responsible for the high emission of crystalline CN-PLLA(n)s, in contrast to the weak emission of amorphous CN-PLLAs. The emission promotion of the fluorescent CN center by the neighbouring polylactide chains is designated as crystallization-promoted emission enhancement (CPEE) and is the focus of the study.