Surface-state triggered solvatochromism of carbonized polymer dot and its two-photon luminescence

This study reports a new nitrogen-doped carbonized polymer dot (CPD)-based solvatochromic probe. Its color-changing for different solvents was explored in detail by the measurements of photophysical parameters (involving Stokes shifts, fluorescence quantum yield, fluorescence lifetime, radiative decay rate constant, and non-radiative decay rate constant) and the following analyses according to Lippert-Mataga equation and Kamlet-Taft model. The hydrogen bonding effect of the CPD in protonic solvents was semi-quantitatively assessed. It takes charge of the solvatochromic phenomenon, especially in proton solvents. Interestingly, this CPD exhibits two-photon solvatochromism. Moreover, the relations between the photophysical parameters and the surface states of CPD in aprotic and proton solvents were depicted. The results reveal the nitrogen doping in the CPD impacts the up- and down-conversion solvatochromic features from the comparison between N-free and N-doped CPDs in many characterizations. In addition, this N-doped CPD was dispersed into polymer matrices to fabricate tunable solid-state luminescent films, which is another model for evidencing the interactions of the CPD with surroundings. This study is of significance in understanding the surface-state controlled luminescence in the CPDs, and will be beneficial for developing new smart, responsive carbon-based nanoprobes.