Novel Fluorene-Based Copolymers Containing Branched 2-Methyl-butyl-Substituted Fluorene-co-benzothiadiazole Units for Remarkable Optical Gain Enhancement in Green-Yellow Emission Range

We develop novel fluorene-based green-yellow emission copolymers (namely, nF1/4F8BT) composed of poly(9,9-dioctylfluorene- co -benzothiadiazole) (F8BT) and branched 2-methyl-butyl substituted fluorene- co -benzothiadiazole units (F1/4BT) in different ratios. Upon blending nF1/4F8BT copolymers with poly(9,9-dioctylfluorene) (PFO), a systematic increase in photoluminescence quantum efficiency is observed as the ratio of the branched monomers increase. Likewise, nF1/4F8BT and PFO:nF1/4F8BT blends exhibit superior optical gain properties with respect to F8BT, manifested as a 30% reduction of amplified spontaneous emission threshold and 50% increase in optical gain with respect to F8BT blends at the same emission wavelength. The optical-gain-related properties were studied to understand the influence of the branched side chain unit on stimulated emission properties. Femtosecond transient absorption studies confirm exciton-annihilation hindrance in the new copolymers likely caused by interference of branched substituents with molecular packing in films. Branch side-chain substitution of a limited number of monomers is an efficient strategy to boost the optical gain properties of conjugated polymers.