Switching of the triplet-triplet-annihilation upconversion with photoresponsive triplet energy acceptor: photocontrollable singlet/triplet energy transfer and electron transfer.

A photoswitchable fluorescent triad based on two 9,10-diphenylanthracene (DPA) and one dithienylethene (DTE) moiety is prepared for photoswitching of triplet-triplet annihilation upconversion. The DPA and DTE moieties in the triad were connected via Click reaction. The DPA unit in the triad was used as the triplet energy acceptor and upconverted fluorescence emitter. The fluorescence of the triad is switched ON with the DTE moiety in open form [DTE-(o)] (upconversion quantum yield ΦUC = 1.2%). Upon UV irradiation, photocyclization of the DTE-(o) moiety produces the closed form [DTE-(c)], as a result the fluorescence of DPA moiety was switched off (ΦUC is negligible). Three different mechanisms are responsible for the upconverted fluorescence photoswitching effect (i.e., the photoactivated fluorescence resonance energy transfer, the intramolecular electron transfer, as well as the photoactivated intermolecular triplet energy transfer between the photosensitizer and DTE-(c) moiety). Previously, the photoswitching of TTA upconversion was accomplished with only one mechanism (i.e., the triplet state quenching of the photosensitizer by DTE-(c) via either the intermolecular or intramolecular energy transfer). The photophysical processes involved in the photochromism and photoswitching of TTA upconversion were studied with steady-state UV-vis absorption and fluorescence emission spectroscopies, nanosecond transient absorption spectroscopy, electrochemical characterization, and DFT/TDDFT calculations.