Excimer emission from self-assembly of fluorescent diblock copolymer prepared by atom transfer radical polymerization

Well-defined fluorescent copolymers of methyl methacrylate with 1-pyreneylmethyl methacrylate were synthesized by atom transfer radical polymerization (ATRP). The random and block copolymer could be clearly distinguished by their glass-transition temperature (T_g) values, with a single T_g value (124 °C) for the random polymer, and two T_g values (115 and 158 °C) for the block copolymer. The emission spectra of the copolymers were different in excimer emission, allowing analysis of the ordering of the two polymers, by determining the ratio between excimer emission (I_E) and monomer emission (I_M). The fluorescence spectra of the random copolymer exhibited both monomer and excimer emission of pyrene with a I_E/I_M ratio of 1.20-1.39 at a concentration of 0.001-0.05 mg/mL. The block copolymer exhibited strong excimer emission with an emission quantum yield for the excimer (φ_E) of 42%. The I _E/I_M ratio from the block copolymer was >25, even in a very dilute solution. The φ_E value increased to 68% when the block copolymer solution was processed to a thin film, indicating increased interactions among the pyrene block by self-assembly. In addition, nanopores were formed from the block copolymer, while no specific morphology was found from the random copolymer. The average diameter of the nanopores from block copolymer was ∼300 nm. Upon thermal annealing of the block copolymer film, a dramatic increase in excimer emission was observed to give a high φ_E value of 89%. A face-to-face pyrene assembly in the block copolymer was observed on the high-resolution transmission electron microscopy (HR-TEM) images, from which the average packing period of the well-defined pyrene block was estimated to range from 4.5 Å for pyrene block width to 5.6 Å for the width of PMMA mainchain.