Sulfur-Bridged Terthiophene Dimers: How Sulfur Oxidation State Controls Interchromophore Electronic Coupling

Symmetric dimers have the potential to optimize energy transfer and charge separation in optoelectronic devices. In this paper, a combination of optical spectroscopy (steady-state and time-resolved) and electronic structure theory is used to analyze the photophysics of sulfur-bridged terthiophene dimers. This class of dimers has the unique feature that the interchromophore (intradimer) electronic coupling can be modified by varying the oxidation state of the bridging sulfur from sulfide (S), to sulfoxide (SO), to sulfone (SO₂). Photoexcitation leads to the formation of a delocalized charge resonance state (S₁) that relaxes quickly (<10 ps) to a charge-transfer state (S₁*). The amount of charge-transfer character in S₁* can be enhanced by increasing the oxidation state of the bridging sulfur group as well as the solvent polarity. The S₁* state has a decreased intersystem crossing rate when compared to monomeric terthiophene, leading to an enhanced photoluminescence quantum yield. Computational results indicate that electrostatic screening by the bridging sulfur electrons is the key parameter that controls the amount of charge-transfer character. Control of the sulfur bridge oxidation state provides the ability to tune interchromophore interactions in covalent assemblies without altering the molecular geometry or solvent polarity. This capability provides a new strategy for the design of functional supermolecules with applications in organic electronics