Highly efficient NIR-II photothermal conversion from a 2,2′-biquinoline-4,4′-dicarboxylate-based photochromic complex

Abstract

The development of second near-infrared (NIR-II, 1000–1700 nm) absorbing photothermal materials has garnered significant attention in various fields, such as photothermal therapy, imaging, and photothermal catalysis, owing to the advantages of a large tissue penetration depth, high maximum allowable energy, and good sensitivity and signal-to-noise ratio. However, exploring novel NIR-II absorbing photothermal complexes with high photothermal conversion efficiency (PCE) still remains a significant challenge. Herein, we present a novel photochromic complex, [Sr(BCA)₂(H₂O)₂]_n (1, BCA = 2,2′-biquinoline-4,4′-dicarboxylate), for NIR-II photothermal conversion. Benefitting from the synergy of the coordination effect, planar conjugation, and π–π stacking arrangements of ligands, complex 1 exhibits heat-stable charge-separated states after photoinduced electron transfer (PET), accompanied by a broad optical absorption ranging from 250 to 1500 nm. Remarkably, the colored state 1P displays excellent performance of photothermal conversion under the irradiation of NIR lasers (808 and 1064 nm). Specifically, under 1064 nm laser irradiation (1.25 W cm⁻²), 1P achieves an exceptional PCE value of 84.5%, with the sample temperature rapidly increasing from 14 °C to 86.8 °C. The experimental data and time-dependent density functional theory (TD-DFT) calculations suggest that the highly efficient NIR-II photothermal conversion of 1P can be attributed to the non-irradiative decay of excited states facilitated by the formation of intra- and inter-molecular charge transfer states, as well as intermolecular π–π interactions among BCA ligands.