Origin of Anisotropic Photoluminescence in Heteroatom-Doped Carbon Nanodots

Because of the origin of anisotropic photoluminescence (PL) in carbon nanodots (CDots) is unclear, which greatly hampers precisely controlling their luminescence properties and relevant applications. Here, the anisotropic PL in heteroatom-doped CDots using polarization-dependent femtosecond transient absorption spectroscopy and computational simulation is studied. Anisotropic absorption and stimulated emission of CDots are observed in different solvents. Solvent effect on solvation relaxation is observed for CDots in different solvents, reflecting the dipole-dipole interaction between CDots and polar solvent molecules. Depolarization process is observed not only dependent on solvent viscosity but also related to the proton donation capability of solvent. The experimental and computational results reveal the anisotropic PL of CDots originate from neither intrinsic nor extrinsic states, but originates from n-pi* transition of a hybrid state, in which the highest occupied molecular orbital is localized at dopant atoms, such as pyridinic N and carbonyl O, and the lowest unoccupied molecular orbital is delocalized over graphene-like domain.