Hydroxybenzimidazole-functionalized carbon dots with morphology-correlated solid-state emission

Transforming surface fluorophores into rigid emissive architectures remains a significant challenge for achieving efficient solid-state fluorescence in carbon dots. Herein, we report a reaction-pathway-switching strategy that converts the amino-rich surface states of o-phenylenediamine-derived carbon dots ( o PD-CDs) into fused benzimidazole emitters, thereby enabling tunable and enhanced solid-state fluorescence. By reacting o PD-CDs with 3,5-dichlorosalicylaldehyde under different sulfite-mediated conditions, two distinct surface structures are selectively constructed: conventional ortho-hydroxyl Schiff-base states (HSB-CDs) formed under Na 2 S 2 O 5 /ethanol, and cyclized benzimidazole-fused emissive states (HBI-CDs) formed under NaHSO 3 /H 2 O/DMF. Structural and spectroscopic characterizations demonstrate that the different reaction pathways lead to distinct structural and optical properties. Compared with HSB-CDs (479 nm), HBI-CDs exhibit markedly red-shifted solid-state emission (563 nm). Morphology-dependent fluorescence studies reveal that HBI-CDs undergo concentration-governed self-assembly into unique fiber and disc-shaped architectures, generating tunable dual-emission characteristics that are attributed to monomer and aggregate states. Substituent engineering across twelve salicylaldehyde derivatives further demonstrates broad tunability of solid-state emission wavelength (456-575 nm), quantum yield (up to 7.54%), and excited-state lifetime (0.31-3.81 ns). Through functional modification of the surface groups of o PD-CDs, this work provides an entirely new strategy for regulating solid-state fluorescence in carbon dots.