Switching Photon Upconversion by Using Photofluorochromic Annihilator with Low-Lying Triplet.

Photon upconversion based on triplet-triplet annihilation (TTA-UC) has attracted great attention due to its remarkable features including the high upconversion quantum yield, low threshold, and flexible combination of sensitizer and annihilator. Endowing TTA-UC with responsiveness will offer additional application dimensions; however, it is a challenge to develop annihilators with responsive features in the excited triplet state. Here we demonstrate the synthesis and photophysical behaviors of photofluorochromic annihilators derived from fluorescent diarylethenes. A series of turn-on mode fluorescent diarylethenes based on 1,2-bis(2-ethyl-1-benzothiophen-1,1-dioxide-3-yl)perfuorocyclopentene were synthesized, and their photochromism and photofluorochromism behaviors were thoroughly investigated. When sensitized by near-infrared ruthenium phthalocyanine, TTA-UC could be observed under excitation of 730 nm, accompanied by upconverted emission ranging from 500 to 700 nm. Because of the photoresponsive properties of the annihilators, TTA-UC can be switched between "on" and "off" by alternating irradiation of ultraviolet and visible light.