Comprehensive Summary

Here, we combined the photon antibunching analysis, fluorescence correlation spectroscopy, and time-domain fluorescence lifetime imaging microscopy (TD-FLIM) to study the emission properties of a representative AIE-luminogen—4,4’-(benzo[c][1,2,5] thiadiazole-4,7-diyl)bis(N,N-diphenylaniline) (TPA-BT) at the single emitter level in a tetrahydrofuran (THF)/water solution where water is a non-solvent for TPA-BT. Our findings suggest that, at a constant water fraction in the solution, the size of TPA-BT aggregates increases with the TPA-BT concentration; TPA-BT aggregates are not a quantum emitter at room temperature in the solution. Moreover, utilizing TD-FLIM and a gel trapping technique allowed us to study the fluorescence lifetime of individual TPA-BT aggregates. Adding a polar solvent like water does not result in an overall decrease in fluorescence lifetime. Rather, it causes the fluorescence lifetime distribution to become wider, and only some molecules experience a decrease in their fluorescence lifetime. These results could represent a step forward in further understanding the photophysics of AIE-luminogens.