Poly(l-lactide)s with tetraphenylethylene: role of polymer chain packing in aggregation-induced emission behavior of tetraphenylethylene

Abstract

Development of biocompatible and biodegradable fluorescent polymers and understanding their fluorescent characteristics are highly desirable. In this work, we synthesized one-armed, two-armed and four-armed poly(L-lactide)s by ring-opening polymerization of L-lactide using hydroxyl functionalized tetraphenylethylenes (TPE) as a macroinitiator. PLLA with TPE is non-emissive in good solvents and it emits differently in the aggregate and gel states. Star-shaped poly(L-lactide) (SSPLLA) emits strongly compared to the one-armed and two-armed PLLAs in the gel state. Gels of SSPLLA in N,N-dimethylformamide (DMF) crystallized into the ε form and emitted cyan light under UV illumination at room temperature. Temperature-dependent UV-vis spectroscopy, photoluminescence spectroscopy, X-ray diffraction and differential scanning calorimetry of SSPLLA/DMF gel in the heating process revealed that the emission behavior is sensitive to the structural changes of PLLA. Upon heating, the crystal structure of the SSPLLA gel transforms from the ε to α form over a broad temperature range. Such crystal-to-crystal transition of PLLA resulted in the rotation of phenyl rings of TPE and leads to a significant blue-shift in the fluorescence of SSPLLA due to the shorter intramolecular conjugation length. On further heating, even in the semicrystalline state (α form), enhanced mobility of the amorphous chains leads to the complete quenching of emission due to the free rotation of phenyl rings of TPE. On the other hand, the precipitate of SSPLLA prepared using N,N-dimethylacetamide (DMA) crystallized into the α form. Unlike gel, the fluorescence emission decreased gradually with temperature and the blue shift was observed relatively at higher temperatures due to the vigorous mobility of amorphous chains. We believe that bio-based polymers with aggregation-induced emission (AIE) characteristics can find applications in bio-imaging and medical/pharmaceutical fields.