Near-infrared organic chromophores with pH-sensitive, non-radiative emission for intelligent disease treatment

Because of the remarkable efficiency in phototherapeutic modality, organic chromophores with the ability to absorb near-infrared (NIR) light have been regarded as powerful bullets against many intractable diseases. However, the suboptimal disease-targeting effect of NIR chromophores is still an obstacle to clinical applications because they may cause undesirable NIR injuries to healthy tissues. In light of the acidic conditions stemming from severe microenvironments in some diseased tissues, developing “turn-on”-type NIR chromophores with acid-responsive photophysical properties would be a feasible approach for promoting disease selectivity. In this focus review, strategies for designing pH-sensitive NIR organic chromophores are summarized based on the analysis of their photophysical relaxation processes, including the enhancement of internal conversion and intersystem crossing of those chromophores in response to acidic conditions. The former is beneficial for promoting the photothermal conversion efficiency, and the latter facilitates the generation of singlet oxygen species. In addition, the applications of some representative pH-sensitive NIR chromophores are also exemplified and highlighted to provide a perspective for disease-selective photothermal therapy and photodynamic therapy.