The optical and electronic properties of polythiophenes and other poly(heterocycles) have received extensive study due to their applications in photovoltaic devices and organic transistors. Several short oligomeric thiophenes are highly emissive making them promising materials for organic lighting and displays. This study focuses on the effect of steric hindrance on the electronic spectroscopy of a series of bithiophenes with emphasis on identifying those structural properties that give rise to high emission yields. The simplest member of this series is 2,2’-bithiophene (2T), which computations have shown to have a highly twisted ground state.(1-4) Substitution in the inner ring positions (3 and 3’) with bromo (33’Br2T) and methyl (33’Me2T) substituents show distinctly different optical properties from 2T or 55’Br2T, a derivative brominated in the outer ring positions (5 and 5’). 33’Br2T and 33’Me2T exhibit very large Stokes shifts compared to 2T and 55’Br2T. Perhaps most interestingly, the excitation spectra of 33’Br2T and 33’Me2T do not overlap their respective absorption spectra except in a low-absorbing red tail not present in 2T or 55’Br2T. Excitation in the red tail shows a greater than ten-fold increase in the quantum yield as compared to excitation at the absorption maxima. This study explores this unusual behavior by use of ultrafast spectroscopy to probe excited state dynamics and theoretical calculations of the torsional energy surfaces and excited-state structure. We theorize the mechanism is related to the presence of small populations of highly-emissive conformers and that torsion about inter-ring angle greatly influences the electronic properties of these thiophene oligomers.
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