Influence of solvent and solid-state environment on excited-state dynamics in donor-acceptor block oligomers

Donor-acceptor π-conjugated block oligomers provide a platform for probing how molecular electronic structure and medium govern excited-state dynamics in π-conjugated systems. Here, we examine a series of diblock and triblock oligomers consisting of a tetrathiophene (T4) donor segment coupled to acceptor segments with systematically varied frontier orbital energies. Electrochemical and computational analyses using density functional theory (DFT) show highest occupied molecular orbital localization on the T4 segment and lowest unoccupied molecular orbital localization on the acceptor segments, establishing a driving force for intramolecular charge transfer. Steady-state and time-resolved spectroscopy reveal strongly solvent-dependent photophysics: in non-polar media, excitation yields fluorescent locally excited (LE) states with nanosecond lifetimes, whereas in polar solvents, ultrafast conversion to charge-transfer (CT) states occurs, leading to strongly quenched fluorescence and sub-100 ps lifetimes. Femtosecond transient absorption (TA) confirms picosecond LE → CT conversion in polar environments, with CT lifetimes that decrease across the series with decreasing CT energy. In contrast, solid-state films consisting of blends with the oligomers in poly(methyl methacrylate) (PMMA) exhibit photophysics resembling non-polar solution conditions, with predominantly LE emission despite early-time CT signatures in ultrafast measurements, indicating heterogeneous local environments. These results demonstrate that medium polarity, rather than frontier orbital offsets alone, controls the balance of LE and CT states in donor-acceptor oligomers.