Homomolecular Triplet–Triplet\nAnnihilation\nin Metalloporphyrin Photosensitizers

Metalloporphyrins are ubiquitous in their applications\nas triplet\nphotosensitizers, particularly for promoting sensitized photochemical\nupconversion processes. In this study, bimolecular excited state triplet–triplet\nquenching kinetics, termed homomolecular triplet–triplet annihilation\n(HTTA), exhibited by the traditional triplet photosensitizers–zinc(II)\ntetraphenylporphyrin (ZnTPP), palladium(II) octaethylporphyrin (PdOEP),\nplatinum(II) octaethylporphyrin (PtOEP), and platinum(II) tetraphenyltetrabenzoporphyrin\n(PtTPBP)were revealed using conventional transient absorption\nspectroscopy. Nickel(II) tetraphenylporphyrin was used as a control\nsample as it is known to be rapidly quenched intramolecularly through\nligand-field state deactivation and, therefore, cannot result in triplet–triplet\nannihilation (TTA). The single wavelength transients associated with\nthe metalloporphyrin triplet excited state decaymeasured as\na function of incident laser pulse energy in toluenewere well\nmodeled using parallel first- and second-order kinetics, consistent\nwith HTTA being operable. The combined transient kinetic data enabled\nthe determination of the first-order rate constants (<i>k</i><sub>T</sub>) for excited triplet decay in ZnTPP (4.0 × 10<sup>3</sup> s<sup>–1</sup>), PdOEP (3.6 × 10<sup>3</sup> s<sup>–1</sup>), PtOEP (1.2 × 10<sup>4</sup> s<sup>–1</sup>), and PtTPBP (2.1 × 10<sup>4</sup> s<sup>–1</sup>) as\nwell as the second-order rate constant (<i>k</i><sub>TT</sub>) for HTTA in ZnTPP (5.5 × 10<sup>9</sup> M<sup>–1</sup> s<sup>–1</sup>), PdOEP (1.1 × 10<sup>10</sup> M<sup>–1</sup> s<sup>–1</sup>), PtOEP (7.1 × 10<sup>9</sup> M<sup>–1</sup> s<sup>–1</sup>), and PtTPBP\n(1.6 × 10<sup>10</sup> M<sup>–1</sup> s<sup>–1</sup>). In most instances, triplet excited state extinction coefficients\nare either reported for the first time or have been revised using\nultrafast transient absorption spectroscopy and singlet depletion:\nZnTPP (78,000 M<sup>–1</sup> cm<sup>–1</sup>) at 470\nnm, PdOEP (67,000 M<sup>–1</sup> cm<sup>–1</sup>) at\n430 nm, PtOEP (51,000 M<sup>–1</sup> cm<sup>–1</sup>) at 418 nm, and PtTPBP (100,000 M<sup>–1</sup> cm<sup>–1</sup>) at 460 nm. The combined experimental results establish competitive\ntime scales for homo- and heteromolecular TTA rate constants, implying\nthe significance of considering HTTA processes in future research\nendeavors harnessing TTA photochemistry using common metalloporphyrin\nphotosensitizers.