Regulation of molecular conjugation to realize mult-color room temperature phosphorescence of carbon dots in urea and ammonium pentaborate precursor matrices

Carbon dots (CDs) room temperature phosphorescent (RTP) materials have great application prospects in optoelectronic devices, advanced anti-counterfeiting, information encryption and bio-imaging due to its excellent optical properties. However, achieving long-lifetime phosphorescence with color-tunable RTP CDs is a huge challenge. In this work, precursor molecules with different degrees of conjugation: 4-ethoxycarbonyl phenylboronic acid (4-EpBA), 9-phenanthracenylboronic acid (9-PhBA) and 1-pyrenylboronic acid (1-PyBA) were combined with urea and ammonium pentaborate (AP) matrix through high temperature pyrolysis method, long-lifetime and colorful phosphorescent CDs composites of 4-EpBA@Urea, 9-PhBA@Urea and 1-PyBA@AP were successfully synthesized. As the degree of conjugation of the precursor molecules increased, the phosphorescence colors of 4-EpBA@Urea, 9-PhBA@Urea and 1-PyBA@AP were blue, green and orange, respectively. In addition, since 1-PyBA@Urea could not produce orange phosphorescence emission, we chose 1-PyBA@AP that was able to generate bright orange phosphorescence after UV excitation. Furthermore, the rationality behind phosphorescence emission mechanism and wavelength modulation is further conformed by density functional theory (DFT) calculations. Finally, RTP CDs composites are successfully applied for advanced anti-counterfeiting and optical information storage.