Acid‐Triggered Aggregation of Carbon Dots Shifted Their Emission to Give Unexpected Deep‐Red Lasing

Abstract It is challenging to realize ultra‐pH‐responsive fluorescent carbon dots (CDs), especially reversibly adjustable multichromatic emission covering a broad wavelength range including deep‐red lasing. This study reports a one‐step solvothermal treatment of pyrogallol to obtain brilliant CDs with a substantial bathochromic shift (from blue to dark red emission) induced by pH changes. The CDs show an excellent reversible pH response: simple acid treatment instantaneously shifts their initial green emission by 140 nm to a deep‐red emission with high‐color‐purity and enhances brightness, subsequent alkali addition returned the green emission, suggesting smooth and reversible regulation of fluorescence. Detailed components analyses, morphology tracking, and photodynamic studies demonstrate the discrete, reversible shift of emission derived from structural transformations, which include protonation of the CDs and their subsequent assembly and partial aggregation triggered by hydrogen bonding. The abundant H‐bonds crosslink the network of CDs, ultimately raising the quantum yield from 11.3% to 16.0%. Furthermore, the aggregated CDs crystallized into sheet‐like structures that acted as a resonant cavity for lasing show excellent gain ability and lead to strong deep‐red lasing. This study draws together pH regulation, the effects of CD's assembly, and their lasing emission to demonstrate significant potential for the development of versatile CDs.