Effects of pH on the ultrafast transient absorption of iron (III) meso-tetrakis(4-N-methyl-pyridiniumyl) porphyrin (Fe3+TMPyP) molecular complexes

Author(s)

Goncalves, Pablo J. ^{[1, 2]} ; Sciuti, Lucas F. ^[3] ; Barbosa Neto, Newton M. ^[4] ; Silva, Rodrigo Costa e ^{[1, 5]} ; Silveira-Alves Jr, Eli ; Mendonca, Cleber R. ^[3] ; Zilio, Sergio C. ^{[2, 3]} ; Borissevitch, Iouri E. ^{[2, 6]} ; de Boni, Leonardo ^[3]

Affiliation

^[1] Univ Fed Goias, Inst Quim, BR-74001970 Goiania, Go - Brazil

^[2] Univ Fed Goias, Inst Fis, BR-74001970 Goiania, Go - Brazil

^[3] Univ Sao Paulo, Inst Fis Sao Carlos, Caixa Postal 369, BR-13560970 Sao Carlos, SP - Brazil

^[4] Fed Univ Para, Inst Ciencias Exatas & Nat, Programa Posgrad Fis, Rua Augusto Correa, 01 Guama, BR-66075110 Belem, Para - Brazil

^[5] Univ Fed Sao Carlos, Dept Quim, CP 676, BR-13565905 Sao Carlos, SP - Brazil

^[6] Univ Sao Paulo, Fac Filosofia Ciencias & Letras Ribeirao Preto, Dept Fis, Ribeirao Preto - Brazil

• Document type: Journal article
• Source: JOURNAL OF PHOTOCHEMISTRY AND PHOTOBIOLOGY A-CHEMISTRY; v. 408, MAR 1 2021.
• Web of Science Citations: 0

Abstract

The meso-tetrakis (methylpyridiniumyl) porphyrin complexed with Fe3+ (Fe3+TMPyP) is an irregular porphyrin, possessing unfilled d external shells with d(6) electronic configuration. In aqueous solution, Fe3+TMPyP exists in a complex equilibrium of different forms. Changing the solution pH, these complexes can bind to other species (as OH- and H2O) or form physical aggregates (mu-oxo-dimers), that alter their excited states properties and affect their potential applications. In the present work, we report excited state absorption dynamics of Fe3+TMPyP molecular complexes in different pH employing femtosecond Z-Scan and ultrafast transient absorption techniques. Complementary data were obtained by UV-vis absorption and fluorescence emission spectroscopies. For 532 nm excitation, the Z-Scan technique shows a decrease in the absorption coefficient for Fe3+TMPyP at pH 4.0 and an increase at pHs 7.0 and 9.0. Ultrafast transient absorption reveals the same behavior in the 525-570 nm region for samples at pHs 7.0 and 9.0. Fast relaxations, with lifetimes around 2.0 picoseconds, were obtained for all samples studied, and were attributed to internal conversion relaxation processes. Although the molecular aggregation accelerates the relaxation process of regular porphyrins, mu-oxo-dimers slightly affect the relaxation of Fe3+TMPyP. The prompt conversion of absorbed light energy in heat observed to Fe3+TMPyP porphyrins can be useful for biomedical photothermal applications, such as Photoacoustic Imaging (PAI) and Photothermal Therapy (PTT). (AU)

• FAPESP's process: 18/11283-7 - Nonlinear photonics: spectroscopy and advanced processing of materials
• Grantee: Cleber Renato Mendonça
• Support type: Research Projects - Thematic Grants
• FAPESP's process: 16/20886-1 - Ultrafast nonlinear optical spectroscopy: Transient Absorption and optical Kerr Gate with polarization control
• Grantee: Leonardo De Boni
• Support type: Regular Research Grants