A Time-Dependent DFT Study of the Absorption and Fluorescence Properties of Graphene Quantum Dots

Absorption and fluorescence spectra of graphene quantum dots (GQDs) have been computed by using time-dependent density functional theory (TDDFT). Different functionals, including PBE, TPSSh, B3LYP, PBE0, CAM-B3LYP, and LC-PBE, have been tested and B3LYP/6-31G(d) has been proven to be the most accurate method for our work. The bulk solvent effects of toluene and dichloromethane have been assessed by using the polarizable continuum model (PCM). The absorption wavelength of GQDs in solvents is red-shifted compared with that in the gas phase. Edge functionalization effects analysis shows that a small number of substituted groups on GQDs induce a small redshift whereas a large redshift occurs when the edges of GQDs are all decorated. Little difference in the fluorescent emission was observed in solvents and in the gas phase. Molecular orbital transition and transition density matrix analysis have been performed. The electronic transition mainly occurs in the middle part of the structure of C132. The strong absorption of C132 corresponds to a S0S3 transition and the fluorescence emission is ascribed to a S1S0 transition, which indicates that Kasha's rule is obeyed.