First-principles study on the modulation mechanism of solvent effect on the fluorescence emission of carbon dots

Abstract

Solvent effect is one of the effective pathways to regulate the fluorescence emission spectra of carbon dots (CDs). In this work, the solvent effect on fluorescence emission of CDs with functional groups has been investigated using density functional theory (DFT) and time-dependent density functional theory (TD-DFT) methods. Solvation model based on density (SMD) and microsolvent-SMD models have been employed to simulate the solvent effects. The electron donating or withdrawing ability of the surface functional groups of CDs, the polarity of the solvent, and the interaction between solvent molecules and CDs were analyzed. The results indicate that microsolvent-SMD model comprehensively considers the solvent polarity and the interaction between solvent molecules and functional groups, which is more advantageous to revealing the physical nature of solvent molecules participating in fluorescence emission, resulting in the calculated fluorescence spectra closer to experimental results. The hydrogen bonds can modulate the fluorescence spectra of CDs more significantly than the solvent polarity, when the CDs are primarily characterized by n-π* transitions, hydrogen bonding induces a blue shift in the fluorescence spectrum, whereas when the CDs mainly undergo π-π* transitions, hydrogen bonding causes a red shift in the spectrum. This work provides theoretical guidance to exploit the solvent effect to achieve effective modulation of the fluorescence properties of CDs.