Homo- or Hetero-Triplet Triplet Annihilation? A Case Study with Perylene-BODIPY Dyads/Triads

The photophysical processes of intramolecular "ping-pong" energy transfers in the iodinated reference dyad BDP-I-2-Py, as well as the uniodinated dyad BDP-Py and triad BDP-2Py, were studied. For BDP-I2-Py, a forward Forster resonance energy transfer (FRET) from the perylene (Py) unit to the diiodoBDP unit (tau = 7 ps) and a backward triplet energy transfer (TTET, tau = 3 ns) from the diiodoBDP unit to the Py unit were observed. For the BDP-Py and BDP-2Py systems, a FRET (tau = 5-8 ps) and a photoinduced electron transfer (PET) (tau = 1-1.5 ns) were observed in acetonitrile. The uniodinated dyad and triad were used as the triplet energy acceptor and emitter for a triplet-triplet annihilation (TTA) upconversion with palladium tetraphenyltetrabenzoporphyrin as the triplet photosensitizer. A maximum upconversion quantum yield of 12.6% was observed. Given that the dyad (BDP-Py) contains one BDP unit and one Py unit while the triad (BDP-2Py) contains two Py units and one BDP unit and based on the results from steady-state femtosecond and nanosecond transient optical spectroscopies, it is concluded that neither intramolecular homo-TTA nor intramolecular hetero-TTA is possible during a TTA upconversion for those systems.