Triplet-Triplet Annihilation in a Single Dendritic Macromolecule with Pt Porphyrin As a Sensitizer-Core

Abstract

Triplet-Triplet Annihilation (TTA) is a process in which two molecules in their triplet spin states interact to produce one excited and one ground state singlets. In traditional diffusive TTA systems, Pt or Pd porphyrins are typically used as 'sensitizers'. Upon excitation, followed by ultrafast intersystem crossing, they transfer the excitation energy to molecules of an 'annihilator' by way of collisional triplet-triplet energy transfer (TTET), thus sensitizing the annihilator triplet states. Annihilator triplets undergo TTA, followed by emission of delayed fluorescence, which usually occurs at wavelengths shorter than the wavelength required for the excitation of sensitizer. The resulting energy upconversion may present interest for bioimaging applications. However, TTA in diffusive systems requires very high local concentration of the sensitizer and annihilator, making realization of this process in a biological system practically non-feasible. Here we present a rare example of a system where the TTA process occurs in a single dendritic macromolecule, charting a path to a new class of upconverting imaging probes. The sensitizer (donor) is a metalloporphyrin, which constitutes the core of the dendrimer, while multiple annihilator (acceptor) perylenes are positioned at the dendrimer periphery. Two consecutive cycles of photon absorption by the sensitizer, intersystem crossing and sensitization of a peripheral annihilator moiety by intramolecular triplet-triplet energy transfer (TTET) create a single dendrimer bearing two triplet excitons, which subsequently undergo TTA followed by the annihilator fluorescence. The overall efficiency of the process greatly exceeds that of conventional multiphoton-induced emission at low excitation energies.