Walking the Emission Tightrope: Spectral and Computational Analysis of Some Dual-Emitting Benzothiadiazole Donor-Acceptor Dyes.

The synthesis, spectroscopic characterization, and computational modeling of seven benzo[ c][2,1,3]thiadiazole-based donor-acceptor dyes is reported. Using a range of linker units, it is possible to alter the lowest energy transition in terms of intensity (from 8000 to 25000 L mol-1 cm-1) and wavelength (from 350 to 430 nm). Resonance Raman spectroscopy was used in concert with DFT calculations to indicate that the linker unit participates in charge transfer processes. In each compound the excited state behavior appears to be primarily described by a BTD●--Linker-TPA●+ state. Stokes shift versus solvent parameter gradients are on the order of 15000 cm-1, indicating Δμ values are large. Dual emission is observed in six of the seven compounds and it can be modulated as a function of solvent. TD-DFT calculations, including excited state optimizations (linear response and state specific), indicate that the lowest energy emission is charge transfer in character. The high energy emissive state is assigned as n-π*. In nonpolar solvents, only the low energy charge transfer emission band is observed and this band generally has a high quantum yield (Φ ≈ 0.9). For compounds with phenyl and triazolyl linkers, in polar solvents only the high energy n-π* emission is observed. The high energy n-π* emission has a low quantum yield regardless of solvent.