The low-lying electronic states and ultrafast relaxation dynamics of the monomers and J-aggregates of meso-tetrakis (4-sulfonatophenyl)-porphyrins.

This article focuses on the computational and experimental investigation of the electronic and vibrational spectra and dynamics of meso-tetrakis(4-sulfonatophenyl)-porphyrins (TSPP) in monomeric and J-aggregate forms. Using the PFD-3B density functional combined with time-dependent density functional theory (TD-DFT), the study accurately reproduces UV-Vis absorption and fluorescence spectra under varying pH conditions and predicts an infrared absorption near 1900 cm⁻¹ unique to the excited singlet and triplet states. Transient mid-infrared absorption spectroscopy at this frequency was employed to probe excited-state vibrational dynamics, revealing that internal conversion from the S₂ to S₁ state, vibrational relaxation within S₁, and S₁ state lifetimes occur on timescales consistent with prior electronic studies, with relaxation processes in J-aggregates occurring roughly three times faster than in monomers. These findings enhance understanding of porphyrin excited-state behavior and may inform the design of more efficient photosensitizers for applications such as dye-sensitized solar cells.