Effect of aggregation configuration of molecular fluorophore CZA on photoluminescence properties of carbon dots

The effect of aggregation configuration of molecular fluorophore citrazinic acid (CZA) on the photoluminescence (PL) properties of carbon dots (CDs) has been investigated using first-principles method. The structural stability of all aggregates has been analyzed, and the results show that the most stable structures are J-type CZA aggregates with head-to-tail configurations and the CZA/CD aggregates are bonded by replacing H atoms on the CD edges with de-OH from the pyridine ring of CZA. The luminescent properties of CZA/CD aggregates are mainly affected by the binding modes and binding sites. When the sites belong to electron-donating groups, electron-withdrawing groups or sp2 domain, the PL spectra of CDs are shifted and the luminescent intensities are significantly enhanced. The results suggest that covalently bonded CZA/CD aggregates are responsible for the high fluorescence quantum yield of CD. Moreover, the distance between the centers of the two pyridine rings in H-type CZA dimers less than 3.5 Å is prone to π-π stacking, leading to fluorescence quenching of aggregates. The present work is helpful in understanding the effect of molecular fluorophores on the PL properties of CDs and provides theoretical guidance for the controllable synthesis of CDs.