Controlling Triplet–Triplet Annihilation Upconversion by Tuning the PET in Aminomethyleneanthracene Derivatives

Activatable triplet–triplet annihilation upconversion was achieved using aminomethyleneanthracene derivatives. The molecular structures of the anthracene derivatives were varied by changing the number of phenyl substituents on the anthracene core (A-1, A-2, and A-3 containing no phenyl and one and two phenyl substituents, respectively). The structural modifications tune the intersystem crossing (ISC), the fluorescence, as well as the distance between the electron donor (amino group) and the fluorophore by using methylene (A-1 and A-2) or a benzyl moiety (A-3) as a linker. Triplet–triplet annihilation upconversion is mainly tuned by photoinduced electron transfer (PET). Hence, the fluorescence of A-1 and A-2 can be switched on by protonation or acetylation of the amino group, whereas A-3 gives persistent strong fluorescence. Determination of the Gibbs free energy changes indicated significantly different PET driving forces for the three compounds. The mechanism of the fluorescence switching was studied wit...