Outstanding near-infrared absorption and photothermal conversion properties of phosphorus(V)-phthalocyanines containing diverse axial substituents

Abstract

We present phosphorus(V)-phthalocyanines (P-Pcs) with diverse axial substituents, which exhibit outstanding second near-infrared (NIR-II) absorption and photothermal conversion efficiency. The P-Pcs display high molar extinction coefficients in the NIR-II regions, and the solubility and optical properties are adjusted depending on the substituents and counter ions. Among the prepared P-Pcs, triethylene glycol substituted P-Pc (TEG-P-Pc) shows a considerable improvement in solubility and NIR-II absorption due to the efficient electron-donating character. Electrochemical bandgaps obtained by cyclic voltammetry measurements agree well with the observed absorption spectra. Thermogravimetric analysis confirms high thermal stability due to the sturdy and stable Pc skeletons. Furthermore, TEG-P-Pc demonstrates superb photothermal conversion, evidenced by rapid temperature rise upon laser irradiation. The current P-Pcs, coupled with long-wavelength absorption, high thermal stability, and photothermal properties, prove a successful example of organic NIR-II probes suitable for heat-controlling applications.