Aggregation-Driven Dual-State Emission for Nanomolar-Level Photodynamic Therapy and Acid-Base/pH Responses in Water

The coumarin-based fluorophore, 7-(diethylamino-3-(5-(trifluoromethyl)benzo[d]thiazol-2-yl)-2H-chromen-2-one (DTBTC), as synthesized in this work, exhibits excellent potential as a multifunctional photosensitizer for bioimaging and photodynamic therapy. DTBTC enables confocal cellular imaging at an exceptionally low concentration of 500 nM and efficiently generates reactive oxygen species under light irradiation, producing potent photocytotoxicity with IC50 values of 332 and 646 nM against HeLa and MCF-7 monolayer cells, respectively. It also demonstrates significant therapeutic efficacy in the corresponding tumors pheroid models (IC50 < 20 μM). Compared with reference compounds R1 [7-(diethylamino)-2-oxo-2H-chromene-3-carboxylic acid] and R2 [coumarin-3-carboxylic acid], DTBTC displays distinctive aggregation-dependent photophysical behavior. In solution, it exhibits bright green emission with a high photoluminescence quantum yield (81.14%), a narrow emission bandwidth (FWHM = 41 nm), and a small Stokes shift (39 nm), characteristic of monomeric emission. Upon aggregation, the appearance of a dominant blueshifted absorption band (449 nm) and a weaker redshifted band (491 nm) indicates the coexistence of H- and T-type aggregates. While H-aggregates promote emission quenching, T-aggregates give rise to orange-red emission. This aggregation-sensitive luminescence further enables fluorescence turn-off detection of acids (acid detection threshold corresponds to an estimated pH of ~6.01) in water, with high sensitivity even toward weak acids such as acetic acid.