Light-emitting properties of donor-acceptor and donor-acceptor-donor dyes in solution, solid, and aggregated states: structure-property relationship of emission behavior

In this paper, we report a systematic study on the light-emitting behavior of a series of triphenylamine-based donor-acceptor-type dyes in the solution and solid states as well as in the aggregated state in polar aqueous media. The emission band shifted bathochromically along with the decrease in the fluorescence quantum yield as the solvent polarity was increased from nonpolar cyclohexane to polar DMF. In a THF/water medium, the emission was quenched in a low water volume, whereas the emission was recovered and increased in a high water volume. In a low water volume, the dye molecules exist in a monomeric form, and the fluorescence quenching increases with increasing water fraction, similar to that observed in the solvent-polarity-dependence study. In contrast, the dye molecules aggregated in a high water volume. This is probably because the inside of aggregates is less polar than the outside, thus preventing nonradiative deactivation and recovering the emission. This unusual emission was achieved by triphenylamine-based dyes containing a relatively strong acceptor moiety such as quinoxaline, benzothiadiazole, and thiadiazolopyridine, providing longer-wavelength red and near-IR emission. In the benzothiadiazole-based dyes, when the phenyl groups in the donor moieties were replaced with methyl groups, the fluorescence quantum yield decreased, indicating that the triphenylamine donor moiety is suitable for emission in the aggregated state. The nonplanar structure of triphenylamine disrupts an ordered packing and produces a less-ordered spherical aggregate, leading to an efficient light emission even in polar aqueous media.