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

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journal contribution
posted on 11.05.2016, 00:00 by Zhou Yu, Xiangru Guo, Qi Zhang, Lang Chi, Ting Chen, Ruidong Xia, Longfei Wu, Larry Lüer, Juan Cabanillas-Gonzalez
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.