Photobasicity of excitation dependent full color light emitting carbon Dots: role of surface functional groups

Photobases are materials that exhibit enhanced basicity in their excited states and play a significant role in applications such as photocatalytic chemical transformations, water splitting, and polymerization. In this study, we investigated the photobasicity of excitation dependent full color light emitting carbon Dots (C-Dots) and examined the role of nitrogen sites, specifically pyridinic, pyrrolic, and graphitic nitrogen in the core, along with various electron-donating (-NH₂) and electron-withdrawing (-COOH) surface functional groups in regulating their photobase characteristics. Exploring the photobasic properties and proton transfer pathways of differently emitting C-Dots is challenging due to their complex internal structure. To address this, the ground- and excited-state pK_a values, along with the protonation sites of different emitting C-Dots, were first determined using pH-metric titration. Furthermore, optical studies were carried out to investigate the photobase properties of the C-Dots. The combined results suggest the protonation pathways in various emitting C-Dots. In B-C-Dots, proton abstraction occurs directly at the core from water, whereas in G-C-Dots and R-C-Dots, protonation occurs with the assistance of functional groups present on the surface of the C-Dots upon the photoexcitation of C-Dots. Ultrafast spectroscopic measurements reveal that proton transfer occurs in different emitting C-Dots within 180-480 fs after excitation, with the fastest process observed in G-C-Dots at approximately 185 fs. Therefore, this study provides a strategy for controlling the photobasicity of C-Dots, which may open up promising applications in related fields.