Structure−Optical Property Relationship of Carbon Dots with Molecular-like Blue-Emitting Centers

The determination of physical mechanisms governing photoluminescence (PL) of carbon dots (CDots) is required for fabrication of CDots with desired optical properties. Using the density functional theory, we comprehensively analyze how the changes in the structure of molecular centers during the synthesis of blue-emissive CDots affect their optical properties. Our calculations predict a pronounced red shift of the PL peak and a decrease in the PL intensity of CDots by up to 2 orders of magnitude when the monomer-like molecular centers are replaced by the covalent or noncovalent dimers of the molecular fluorophore IPCA (5-oxo-1,2,3,5-tetrahydroimidazo[1,2-α]pyridine-7-carboxylic acid). The predicted red shift of the PL peak can be significantly reduced by shortening the length of the conjugated π-system in the IPCA-like molecular centers caused by presence of oxygen atoms, which can be achieved through substitution of nitrogen with oxygen atoms in the conjugated π-system. To verify our theoretical predictions, we varied the synthesis time of the blue-emissive CDots with the IPCA-like molecular centers made from citric acid and ethylenediamine. The PL intensity of these CDots is found to initially rise for the synthesis times between 1 and 2 h and then to dramatically decrease by a factor of 2 and remain almost constant for synthesis times of up to 6 h, with a weak PL peak appearing at longer wavelengths. These trends could be well reproduced by numerical simulations assuming they are caused by the structural changes in the blue-emissive IPCA-like molecular centers of the CDots. The revealed structure-function relationship of CDots with molecular-like centers can explain similar trends in the PL spectra of CDots obtained from other precursors.