Homomolecular Triplet–Triplet Annihilation in Metalloporphyrin Photosensitizers

Metalloporphyrins are ubiquitous in their applications as triplet photosensitizers, particularly for promoting sensitized photochemical upconversion processes. In this study, bimolecular excited state triplet–triplet quenching kinetics, termed homomolecular triplet–triplet annihilation (HTTA), exhibited by the traditional triplet photosensitizers–zinc(II) tetraphenylporphyrin (ZnTPP), palladium(II) octaethylporphyrin (PdOEP), platinum(II) octaethylporphyrin (PtOEP), and platinum(II) tetraphenyltetrabenzoporphyrin (PtTPBP)–were revealed using conventional transient absorption spectroscopy. Nickel(II) tetraphenylporphyrin was used as a control sample as it is known to be rapidly quenched intramolecularly through ligand-field state deactivation and, therefore, cannot result in triplet–triplet annihilation (TTA). The single wavelength transients associated with the metalloporphyrin triplet excited state decay–measured as a function of incident laser pulse energy in toluene–were well modeled using parallel first- and second-order kinetics, consistent with HTTA being operable. The combined transient kinetic data enabled the determination of the first-order rate constants (k_T) for excited triplet decay in ZnTPP (4.0 × 10³ s^–1), PdOEP (3.6 × 10³ s^–1), PtOEP (1.2 × 10⁴ s^–1), and PtTPBP (2.1 × 10⁴ s^–1) as well as the second-order rate constant (k_TT) for HTTA in ZnTPP (5.5 × 10⁹ M^–1 s^–1), PdOEP (1.1 × 10¹⁰ M^–1 s^–1), PtOEP (7.1 × 10⁹ M^–1 s^–1), and PtTPBP (1.6 × 10¹⁰ M^–1 s^–1). In most instances, triplet excited state extinction coefficients are either reported for the first time or have been revised using ultrafast transient absorption spectroscopy and singlet depletion: ZnTPP (78,000 M^–1 cm^–1) at 470 nm, PdOEP (67,000 . Authors: Arshad, Azka [North Carolina State University, Raleigh, NC (United States); North Carolina State University] (ORCID:0000000269172869); Castellano, Felix N. [North Carolina State University, Raleigh, NC (United States)] (ORCID:0000000175468618). DOE Contract: SC0019370. Subjects: 14 SOLAR ENERGY; 37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY; Excited states; photosensitization; pyrroles; triplet-triplet annhilation