Photoluminescence study on exciton migration and trapping in a copolymer based on poly(fluorene)

Steady-state and time-resolved photoluminescence spectra of poly[9,9-bis(2'-ethylhexyl)fluorene-2,7-diyl] and a copolymer of poly[9,9-bis(2'-ethylhexyl)fluorene-2,7-diyl-co-alpha-cyanostilbene-4,4'-diyl] were investigated. In THF, the copolymer exhibits the characteristic blue emission band of the homopolymer together with another emission band arising from chromophoric segments containing comonomer units. A total suppression of the homopolymer emission band in copolymer thin film demonstrates the efficient exciton migration and trapping into the segments containing comonomer units. Exciton migration and trapping in copolymer thin film gives rise to a red-shift in photoluminescence as compared with the homopolymer. Time-resolved photoluminescence results are also consistent with the facile exciton migration from fluorene polymer units to trapping sites composed of cyanostilbene comonomer units. The exciton trapping processes result in a red-shift in the emission spectrum with enhanced color stability in the copolymer relative to the homopolymer.