Photostability investigation of a near-infrared-II heptamethine cyanine dye.

The development of small-molecule organic fluorophores in the second near-infrared region (NIR-II, 1000-1700 nm) has recently received immense attention due to deep tissue light penetration and reduced autofluorescence. Polymethine cyanine dyes as important building blocks are frequently used to design NIR-II light-activated fluorophores. However, applications of these fluorophores were restricted by their poor photostability. Herein, we synthesized a NIR-II heptamethine cyanine dye 4 as the model compound to investigate its photostability and identify its photoproducts. Compound 4 shows improved photostability compared to ICG, and remains relatively stable even under sunlight irradiation. The cyanine scaffold's good photostability promoted its stable photothermal properties. The effective photothermal activity of cyanine dye 4 and the low toxicity of its one major photoproduct demonstrated its potential for further photo-therapeutic applications. This work would pave the way for the development of NIR-II cyanines with photostability through delicate structure design.