Tetraphenylethylene-derived organic cages with persistent photogenic radicaloids and efficient photothermal conversion

Abstract

Transformation of the electronic spin states has received significant interest in recent years because of its applications in the magnetic information storage materials and optical response switches. However, it can be formidably challenging to use ultraviolet (UV) light as a contactless stimulus to alter the electronic spin states of the supramolecular cages. Inspired by the approach on the radical cation mechanism of the Scholl reaction and the photocyclization reactivity of tetraphenylethylene (TPE) derivatives, we report two photochromic cages (cage-TPE-1 and cage-TPE-2) that can be photochemically transformed from electron paramagnetic resonance (EPR)-silent to EPR-active form via UV irradiation due to the photo-induced electron transfer process at the TPE moiety. EPR provided a strong single-electron signal with a g value of 2.003 in cage-TPE-1 and cage-TPE-2 but no signal was detected for the pristine samples without the UVexposure. The transformation can be monitored by ultraviolet–visible–near-infrared (UV–vis–NIR) and photoluminescence (PL) spectroscopy. The photogenic radical cage intermediate was unstable in solution resulting in the degradation and intramolecular cyclization of the reactive species, while the radicals were found to be stable and persistent in the solid state due to the spin delocalization, the steric protection of confined cavities, and the oxygen isolation. Both theoretical calculations and spectral measurements suggest that the photogenic radical cage-TPE-2^(•+) experiences photocyclization on the TPE core. Compared with the close-shell cage, the radical cage-TPE-1^(•+) and cage-TPE-2^(•+) exhibit better photothermal conversion performance due to the incubated infrared absorption from open-shell electron feature and reduced band gap.