Fluorophore-Dependent Optical Properties of Multicolor Carbon Dots for Bioimaging and Optoelectronic Devices

To achieve the high tunability and quantum yield of carbon dots (CDs) suitable for applications to bioimaging and optoelectronic devices, optical features are systematically investigated in three types of CDs. Phenylenediamine isomers are employed to develop the CDs of blue, green, and red. Despite the comparable chemical structures and functional groups, the predominant optical transitions are distinct in the three CDs, which is mainly attributed to the configurations of fluorophores in aggregated structures. Another type of optical transition is also observed, whose energies are similar in the three CDs, implying the involvement of surface states. The first-principles calculations indicate that the functional groups change the electronic structures and transition energies, suggesting that oxidation-related states are responsible for the surface states. In a wide range of pH, the optical transitions remain stable, although the protonated chromophores result in shifted transitions in highly acidic conditions. The protonation of the nitrogen moiety influences the electronic structures, while the protonated position controls the relaxation processes. These findings offer strategies to improve the multicolor CDs with a green chemistry approach, which are suitable for applications to bioimaging and optoelectronic devices as alternatives to metal-based semiconductor quantum dots.