Carbon Dot-like Molecular Nanoparticles, Their Photophysical Properties, and Implications for LEDs

Carbon dot-like molecular nanoparticles (CMPs) exhibit strong photoluminescence (PL) emissions that can be tuned throughout the visible spectrum. Recently, CMPs gained attention due to excitation-independent PL emission and higher PL quantum yield (QY) compared to conventional carbon dots (CDs). In this study, CMPs composed of molecular fluorophores are synthesized, under mild conditions, by a solvothermal method and their structure is investigated. The CMPs have an average size of 19.5 nm with a single-digit PL QY from 3.5 to 9.4% depending upon the solvents they are dispersed in. Two distinct excitation-wavelength-independent PL emission bands are observed from the CMPs: one is a deep-violet emission band centered at similar to 400 nm and another is a green emission band at similar to 520 nm. Structural and optical characterizations reveal that the CMPs are composed of two different types of molecular fluorophores. The molecular fluorophores are separated by dissolving the CMPs in low-polarity solvents and separating the molecules by solvent extraction. The PL QY of the green-emitting molecules after separation increases to 53.6%. Nuclear magnetic resonance characterization reveals that the violet-emitting molecules mainly have an aliphatic structure with an abundance of oxygen, while the green-emitting molecules have an aromatic structure and are identified as N-4,N-11-dimethyldibenzo[a,h]phenazine-4,11-diamine (BPD). Finally, time-dependent density functional theory (TD-DFT) calculations are carried out to further study the excited states in the BPD molecules and to confirm that the green emission arises from the same molecules. Furthermore, BPD shows strong PL QY when dispersed in polymers, which is relevant for applications in UV-pumped light-emitting devices.