Photoinduced charge-transfer dynamics in fluorescent electron donor-acceptor polymers

Electron donor-acceptor (D-A) polymers are emerging as promising candidates for the development of solid materials with tunable emission. Herein, we investigate the excited-state dynamics of polymers consisting of a central naphthalenediimide (NDI) acceptor with two polystyrene donor chains and copolymers with various secondary donors incorporated. We find strong differences in the dynamics when going from diluted polymer solutions to pure polymer films. In liquids, ultrafast intrachain electron transfer from a styrenic donor to the excited NDI, followed by sub-nanosecond charge recombination to the ground state is observed. Because of the tight packing in the film, ultrafast electron transfer occurs between donors and acceptors of different polymer chains. Emission is found to originate from the most strongly coupled D-A pairs, for which electron transfer is so fast that it leads to a lifetime broadening of the NDI absorption band. Because of this, these highly coupled pairs can be photoselected upon red-edge excitation. The charge-transfer state decays on the tens of ns timescale via radiative and non-radiative charge recombination to the ground state as well as via charge recombination to the triplet state of NDI. This latter pathway, which is detrimental to the fluorescence quantum yield, is almost suppressed with the strongest secondary donor. Finally, we show that excitation of the secondary donor instead of the NDI acceptor does not lead to the population of the charge-transfer state and thus does not contribute to the luminescence of the films.