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 with steady state UV–vis absorption, fluorescence emission spectroscopy, nanosecond transient absorption spectroscopy, and ab initio computations. We found that the PET exerts different quenching effects on the singlet and triplet excited states of the anthracene derivatives. The triplet–triplet annihilation upconversion using these compounds as triplet acceptors/emitter was studied as well, and it was found that upconversion can be switched on by inhibition of the PET through acetylation and protonation.