Synthesis and characterization of fluorinated heterofluorene-containing donor-acceptor systems.

A series of oligothiophene-perfluoro-9-heterofluorene donor-acceptor (DA) compounds was synthesized via a combination of nucleophilic aromatic substitution (S(N)Ar(F)) and palladium coupling reactions. These compounds are of interest as possible building blocks for materials with useful electron transport properties, since they possess relatively low LUMO energy levels of -3.3 to -3.6 eV (as determined by differential pulse voltammetry). The HOMO-LUMO energy gaps, as determined by UV-vis spectroscopy, range between 2.4 and 2.5 eV, and photoluminescence emission spectra reveal lambda(ems) values in the range of 480-600 nm (corresponding to yellow-orange emission). Dilute solution-state photoluminescence quantum yields were significantly lower than those of the pure acceptor heterofluorenes (0.02-0.38 for the DA compounds vs approximately 1 for the pure acceptors), and notable solvatochromism in the fluorescence suggests emission from a charge-separated state. Theoretical calculations show that HOMO-level electron density is more localized on the thiophene fragment, while the LUMO level electron density is mostly associated with the electron-deficient portion of the molecule. Photovoltaic (PV) devices based on DA/poly-3-hexylthiophene (P3HT) blends exhibit improved performance over P3HT-only devices, suggesting the ability of these DA compounds to transport electrons in the solid state.