Dual-Functionality of Phosphorescence and Photothermal Conversion through Light-Activated Open-Shell Singlet Diradicals in Silver Metal-Organic Frameworks

Abstract

Thermally excited triplet radical functional materials are among the leading research areas in spin-induced optical and thermal applications. Nevertheless, it is a challenging task to manipulate the spin state accurately and to regulate the synergistic response of multiple properties efficiently. Radical-induced photochromic materials offer a new approach to addressing this issue. In this work, we exploited the structural advantages of crystalline MOFs materials, assembled by the photoactive TEPE ligands and the silver ions, and further introduced the large steric ADCA- (HADCA = 1-adamantane carboxylic acid) guests to form [Ag(TEPE)](ADCA)·9H2O (1). MOF 1 rapidly forms the photochromic product 1* upon the perturbation of 365 nm UV light, and time-dependent EPR characterization verifies that the photoinduced radicals maintained good stability against water and oxygen. Interestingly, the variable-temperature EPR spectrum exhibits a rare thermal enhancement phenomenon, which is a typical characteristic of thermally excited triplet diradicals. Steady-state photoluminescence spectra monitor the transition from 1 with blue emission to 1* with red/NIR emission, while transient-state decay spectra account for the lifetimes ranging from nanoseconds to microseconds, confirming the emergence of the thermally excited triplet state. Under the 0.5 W 808 nm laser, the photothermal conversion efficiency of 1* is 49.05%. The remarkable solar-driven water evaporation efficiency is 80%, and the average water evaporation amount is 1.232 kg m-2 h-1. The photoinduced thermally excited triplet radicals not only activate the room-temperature phosphorescence behavior, but also trigger the outstanding photothermal conversion performance and further realize the triple responses of magnetic/optical/thermal performances.