Photoinduced radicals modulated NIR photothermal conversion in a photochromic inorganic–organic complex

Abstract

Realizing precise temperature regulation under low constant near-infrared (NIR) laser density is of great significance for the clinical application of photothermal therapy. However, photo-modulated photothermal conversion materials are still in their infancy, and it is challenging and vital to develop photochromic materials with extraordinary radical stability to realize tunable photothermal conversion under an acceptable NIR laser density. In this work, we present a UV-induced photochromic inorganic–organic complex (NEU20) bearing dense packing mode of 4,4′-bipyridine (bpy)/deprotonated 4,4′-bipyridine (H₂bpy²⁺) dications and the unique sandwich π-aggregates consisting of infinite H₂bpy²⁺–bpy–H₂bpy²⁺ interaction, which can generate tunable and stable H₂bpy˙⁺ radicals through the UV-induced photoinduced electron transfer (PIET) process and cause absorption bands in the red and IR regions, thus modulating the NIR photothermal effect under a low constant NIR laser density. NEU20 enables precise control of photothermal temperatures through UV exposure duration besides changing the NIR laser power, and its photothermal conversion exhibits good recyclability, a high conversion efficiency and high temperature increase rate. The clear and adjusted imaging performance based on the photothermal effect is also presented.