Excitation-Dependent Two-State Photoluminescence of Microwave-Synthesized Carbon Dots: Solvent and pH Sensitive Behaviour.

In this study, carbon dots (C-dots) were synthesized from resorcinol and trisodium phosphate in ethylene glycol using a microwave-assisted method. The C-dots exhibited predominantly green photoluminescence when excited at 460 nm. Their absorption and emission spectra showed strong solvent-dependent variations, and the quantum yield varied between 45% and 62% depending on the solvent. Both absorption and emission maxima exhibited a noticeable red shift with increasing solvent polarity. Upon varying the excitation wavelength from 300 to 480 nm, two distinct emission bands blue and green emerged. However, no continuous spectral shift was observed in any solvent. Excitation spectra recorded at different emission wavelengths also revealed two well-defined excitation peaks. The blue emission originates from π→π* transitions within the CC sp domains at the core of the C-dots, whereas the green emission arises from surface states associated with phenolic -OH and phosphate functional groups. The absence of continuous spectral shift in emission suggests the absence of heterogeneity in CC sp domains in the structure. The pH-dependent study revealed two emissive states: a dark state at acidic pH due to protonation of surface groups, and a highly emissive state at higher pH due to their deprotonation. Time-resolved photoluminescence showed specific solvent effects on the emission originating from core and surface of the C-dots. These findings establish that the green emission is strongly governed by the surface functional groups, while the core-related blue emission remains distinct.