Femtosecond study of exciton dynamics in 9,9-di-n-hexylfluorene/anthracenerandom copolymers

Exciton dynamics of 9,9-di-n-hexylfluorene/anthracene (DHF/ANT) statistical copolymers have been measured using femtosecond transient absorption spect roscopy. An investigation of the excitation intensity dependence over the r ange of 0.1-1.0 mJ/(pulse cm(2)) for solutions and 1.0-17 mu J/(pulse cm(2) ) for thin films has been conducted to explore exciton relaxation mechanism s below excitation densities where exciton-exciton interaction is important . Intrachain relaxation of photoexcited singlet excitons is observed in dil ute solutions. In contrast, interchain relaxation mechanisms become predomi nant in thin films. Decay dynamics are independent of excitation intensity for dilute solutions and thin films of DHF/ANT when probed at 790 and 750 n m. In addition, time-resolved measurements for a DHF homopolymer and two co polymer thin films have been carried out as a function of probe wavelength. A stimulated emission (SE) feature and a photoinduced absorption (PA) feat ure are observed in the visible region. The SE and PA dynamics are similar far the copolymers, suggesting that the same excited state species, the sin glet exciton, is responsible for both the SE and PA. There is a significant difference between the SE and PA dynamics for DHF thin films on the 0-3-ps timescale. The SE dynamics show a pulse-width limited rise and a subsequen t decay. In contrast, both the 600 and 750 nm PA dynamics show a "double" r ise that represents contributions from two separate photophysical processes . These results, in combination with the steady-state photoluminescence spe ctrum, which indicates excimer emission, lead to the conclusion that interc hain species, such as excimers, are formed in [1 ps in DHF homopolymer film s following photoexcitation. That the copolymer dynamics show no evidence o f excited state species other than the singlet, emissive exciton, is consis tent with the interpretation that anthracene substituents in the polymer ba ckbone prevent interchain interactions in films.