Dual functionality of phosphorescence and photothermal conversion through light-activated open-shell singlet diradicals in silver metal–organic frameworks

Abstract

Synergistically regulating the photoluminescence properties and photothermal conversion performance is a highly concerning and challenging task, and constructing radical-induced photochromic materials is one of the effective strategies. In this work, we exploited the structural advantages of crystalline MOF materials, assembled using the photoactive TEPE ligands and the silver ions, and further introduced the large steric ADC⁻ (ADCA = 1-adamantane carboxylic acid) guests to form [Ag(TEPE)](ADC)·9H₂O (1). MOF 1 rapidly forms photochromic product 1* under 365 nm light  irradiation, 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 showcase 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 a thermally excited triplet state. Under a 0.5 W 808 nm laser, the photothermal conversion efficiency of 1* is 49.05%. The remarkable solar-driven water evaporation efficiency is 80% under Xenon lamp irradiation, 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 triple responses of magnetic/optical/thermal effects can be further realized.