Homomolecular Triplet-Triplet Annihilation in Metalloporphyrin Photosensitizers

• Published in: The journal of physical chemistry. A, Molecules, spectroscopy, kinetics, environment, & general theory Molecules, spectroscopy, kinetics, environment, & general theory, 2024-09, Vol.128 (36), p.7648-7656
• Main Authors: Arshad, Azka, Castellano, Felix N
• Format: Article
• Language: English
• Subjects: Excited states; INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY; photosensitization; pyrroles; SOLAR ENERGY; triplet-triplet annhilation
• ISSN: 1089-5639; 1520-5215; 1520-5215
• DOI: 10.1021/acs.jpca.4c05052

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 ( ) for excited triplet decay in ZnTPP (4.0 × 10 s ), PdOEP (3.6 × 10 s ), PtOEP (1.2 × 10 s ), and PtTPBP (2.1 × 10 s ) as well as the second-order rate constant ( ) for HTTA in ZnTPP (5.5 × 10 M s ), PdOEP (1.1 × 10 M s ), PtOEP (7.1 × 10 M s ), and PtTPBP (1.6 × 10 M s ). 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 cm ) at 470 nm, PdOEP (67,000 M cm ) at 430 nm, PtOEP (51,000 M cm ) at 418 nm, and PtTPBP (100,000 M cm ) at 460 nm. The combined experimental results establish competitive time scales for homo- and heteromolecular TTA rate constants, implying the significance of considering HTTA processes in future research endeavors harnessing TTA photochemistry using common metalloporphyrin photosensitizers.