Photoswitching of the triplet excited state of diiodobodipy-dithienylethene triads and application in photo-controllable triplet-triplet annihilation upconversion.

Dithienylethene (DTE)-2,6-diiodoBodipy triads were prepared with the aim to photoswitch the triplet excited state of the 2,6-diiodoBodipy moiety. Bodipy was selected due to its low T1 state energy level to avoid sensitized photocyclization of DTE, which is very often encountered in DTE photoswitches, so that the photochemistry of DTE and the organic chromophore can be addressed independently. This is the first time that DTE was covalently connected with an organic triplet photosensitizer. For the triad with DTE-o structure, selective photoexcitation into the diiodoBodipy part did not initiate photocyclization of DTE-o. Upon photoirradiation at 254 nm, thus the DTE-o → DTE-c transformation, the intersystem crossing (ISC) of 2,6-diiodoBodipy moiety was competed by the photoactivated resonance energy transfer (RET), with Bodipy as the intramolecular energy donor and DTE-c as energy acceptor. The fluorescence of Bodipy was quenched and the triplet state lifetime of Bodipy was reduced from 105.1 to 40.9 μs. The photoreversion is O2-independent, but can be greatly accelerated upon selective photoexcitation into the diiodoBodipy absorption band (at 535 nm). We concluded that ISC is not outcompeted by RET. The photoswitching of the triplet state was transduced to the singlet oxygen photosensitizing, as well as triplet-triplet annihilation upconversion.