Insights into Fluorophores of Dual-Emissive Carbon Dots Derived by Naphthalenediol Solvothermal Synthesis

Versatile species have been increasingly recognized as important components and possible fluorescence origins in carbon dots synthesized via a bottom-up route. Herein, we use 2,7-naphthalenediol and dimethylformamide as precursors to synthesize a novel type of carbon dots via an ethanothermal reaction. Two main luminescent components with entirely different structures, including molecular fluorophores and carbogenic nanodots, are independently acquired by column chromatography separation and dialysis, respectively. Through detailed nuclear magnetic resonance studies, the chemical structure of molecular fluorophores has been elucidated as 8-ethoxy-3H-cyclopenta[a]naphthalen-3-one (ECNO), while the carbon dot (C-dot) is characterized as a carbon core with bound fluorophores. A comparative study between ECNO and C-dots is systematically conducted in terms of steady- and transient-state emission spectra, environmental sensitivity, and stability. ECNO shows a single green and excitation-wavelength-independent emission, but it is highly sensitive to the solution pH and solvent polarity. As the carbogenic counterpart, C-dots show dual emission comprising a blue broad and a yellow narrow-band emission. Besides, C-dots exhibit higher photo- and thermal stability than ECNO. The state of fluorophores (unbound state vs bound state) causes these dramatic differences in the optical behaviors between ECNO and C-dots. By virtue of their distinctive properties, ECNO can be used as a fluorescent filler to fabricate smart polymer composites and anti-counterfeiting inks, while C-dots find applications as phosphors in white-light emitting diodes.