Ultrafast spectroscopic study of donor-acceptor benzodithiophene light harvesting organic conjugated polymers

New light harvesting organic conjugated polymers containing 4,8-bis(2-ethylhexyloxy)benzo[1,2-b;3,4-b′]dithiophene(BDT) donor groups and thiophene with various electron-withdrawing acceptor groups were investigated. Also investigated was poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl]] (PTB7), which is one of the most efficient photovoltaic conjugated polymers. In this study, the steady state absorption, steady state emission, ultrafast fluorescent decay dynamics, and nonlinear optical properties of these light harvesting conjugated polymers were probed in solution. All of the conjugated polymers investigated have significant absorption over much of the visible spectrum due to small band gaps due to low lying LUMO energies created by the electron-withdrawing groups. Fluorescence upconversion studies on the conjugated polymers showed short decay dynamics for conjugated polymers with strong electron-withdrawing groups. Two-photon absorption spectroscopy showed large two-photon absorption cross sections for the conjugated polymers with strong electron-withdrawing acceptors. Fluorescence anisotropy decay studies showed contributions from both hopping and a coherent energy migration process for some of the polymers. The polymers were investigated for their photovoltaic efficiency and correlated with both the steady-state and time-resolved dynamics of the investigated donor-acceptor polymers.