Star-Shaped Poly(l -lactide) with a Dipyridamole Core: Role of Polymer Chain Packing on Induced Circular Dichroism and Photophysical Properties of Dipyridamole

Abstract

The dipyridamole is emissive in the solution state; however, the emission is weakened in the solid state due to the aggregation-caused quenching. The aggregation of dipyridamole can be prevented by synthesizing star-shaped poly(L-lactide) (SSPLLA) using dipyridamole core. During crystallization of the polymer, chromophore molecules were expelled out of crystalline lamellae and reside at the interface of crystalline and amorphous phase. The helical chains of poly(L-lactide) (PLLA) can induce the helicity to chromophore moieties during the crystallization of PLLA. The photoluminescence (PL) quantum yield measured for the semicrystalline polymer is 46%. When the polymer is melt-quenched to amorphous, PLLA chains remain in the random conformation, and no induced circular dichroism of dipyridamole was found. In the amorphous polymer, the dipyridamole molecules are scattered and show higher PL quantum yield ∼55%. Transparency of the polymer and the disruption of π-stacking of chromophore resulted in the higher quantum yield in the amorphous star-shaped PLLA. These results suggest that the morphology and the polymer chain packing could play an important role in determining the solid-state optical properties of chromophores. In addition, a detailed investigation is carried out to understand the relationship between chain conformation, the growth of crystal lamellae, and superstructure of star-shaped PLLA.