Triplet state quantum yield formation and two-photon absorption in Tetra- Phenyl Porphyrins derivatives

Author(s)

Júlia Akiyama da Silva

• Document type: Master's Dissertation
• Press: São Carlos.
• Institution: Universidade de São Paulo (USP). Instituto de Física de São Carlos (IFSC/BT)
• Defense date: 2024-11-12

Examining board members

Leonardo De Boni; Leandro Henrique Zucolotto Cocca; Bernardo Almeida Iglesias

• Advisor: Leonardo De Boni

Abstract

In this work, an unprecedented spectroscopic study was conducted on a class of neutral and cationic porphyrins, encompassing absorption and fluorescence measurements, time-resolved fluorescence, determination of quantum efficiencies for fluorescence and triplet states, and the two-photon absorption cross-sections. The aim was to explore their application within the therapeutic window, assessing the potential of these compounds for phototherapeutic treatments, where selective and efficient activation is essential. These organic molecules, with nonlinear optical responses, offer significant advantages such as increased light penetration into tissues, a critical feature for photosensitizers used in photodynamic therapy (PDT). Porphyrins are particularly attractive for this nonlinear study due to their efficiency in triplet state formation, which results in the production of reactive singlet oxygen, a key factor in PDT. Notably, only a few studies in literature have reported measurements of two-photon absorption (TPA) cross-sections in porphyrin molecules, which is known to be challenging due to the tendency of these molecules to aggregate at low molar concentrations. Initially, the quantum efficiency of triplet formation was reported using the double-pulse excitation technique, which monitors the fluorescence signal decrease due to the formation of non-emissive triplet states, a characteristic of these studies. To further explore the nonlinear optical properties of these molecules, a study of two-photon absorption (TPA) spectra was performed using the fluorescence technique via two-photon absorption, where the two-photon absorption cross-section is determined by monitoring the fluorescence signal as a function of excitation power. This approach allows measurements in very low concentration solutions, avoiding aggregation. It was expected that the electronic nature of porphyrins would lead to significant changes in the TPA cross-sections due to different electron-accepting groups and symmetry changes, influencing the TPA magnitude in various chalcone derivatives, which is crucial for applications such as fluorescent bioprobes. As a result of the measurements conducted on chalcone porphyrins dissolved in DMSO, the studied porphyrins showed quantum yields around 30%, and approximately 20% for a neutral and a cationic porphyrin. The main decay rates fluorescence, internal conversion, and intersystem crossing were analyzed. An increase in the intersystem crossing rate was observed with the addition of the iodine counterion, enhancing non-radiative decay while maintaining a high triplet formation rate. Regarding two-photon absorption, an increase in the TPA crosssection was observed in the Soret band, possibly due to the resonance enhancement region of the Q-band one-photon absorption or a gerade parity state near the Soret band. Furthermore, in the Q-bands, a maximum absorption cross-section of approximately 10 GM at 570 nm was measured, explained by TPA-allowed transitions (gerade-type state) into vibronic states observed in non-centrosymmetric molecules. This supports the hypothesis that chalcone groups attached to TPP can increase distortions in molecular geometry and symmetry. (AU)

• FAPESP's process: 22/07652-2 - Triplet state quantum yield formation a two-photon absorption in Tetra-Phenyl Porfirins derivatives
• Grantee: Júlia Akiyama da Silva
• Support Opportunities: Scholarships in Brazil - Master