Structural optimization of organic fluorophores for highly efficient photothermal therapy

Developing highly efficient organic photothermal agents is desirable and imperative because of the great potential of photothermal therapy in tumor treatment. In this work, three boron dipyrromethene (BDP) molecules (Ph-BDP, Na-BDP and Py-BDP) with different conjugate areas were rationally designed and synthesized to study the effects of molecular structures on photoproperties. With the increase of conjugate areas, the absorption and emission wavelengths of BDP molecules were red-shifted, and the radiative transitions were weakened along with the improvement of non-radiative transitions and enhancement of intersystem crossing (ISC). After formation of BDP nanoparticles, the radiative transitions and ISC were weakened. Importantly, Py-BDP nanoparticles with the largest conjugate area had highest photothermal conversion efficiency (η = 42%) among these BDP nanoparticles, and indicated the best tumor treatment effect in vitro and in vivo. This work provides a systematic strategy for exploiting efficient photothermal agents.