Aggregation of Naphthobisthiadiazole-Based Donor-Acceptor-Donor Dyes That Restrict Quenching in Solution and Emit Red Light in Polar Aqueous Media

Triphenylamine-naphthobisthiadiazole-based donor-acceptor-donor dyes were prepared and their aggregation and light-emitting behaviors were investigated. Deep-red light was emitted between 650-680 nm in a solution of tetrahydrofuran and dichloromethane with moderate fluorescent quantum yield of 0.36-0.48. However, when the solvent system was changed to THF/water, emission behavior became dependent on the solvent ratio. For lower water concentrations, emission intensity decreased significantly when the ratio of water to THF was increased. The donor-acceptor-donor molecules exist as monomers, and an increase in solvent polarity favors the fluorescence quenching that results from solvation. However, emission intensity recovered and intensified progressively at higher water concentrations. The molecules form aggregates, which provide a less polar hydrophobic space within to restrict solvation and quenching, ultimately allowing emission.