Investigating the excited state dynamics and two-photon absorption of β-functionalized neutral and cationic meso-tetraphenylporphyrins

Abstract

Photophysical characterization of recently developed meso-tetraphenylporphyrin-pyrrolidine/pyrroline conjugates with potential in photodynamic inactivation was performed, including both neutral and cationic derivatives. Cationic compounds exhibited red-shifts in both absorption and emission spectra, alongside reduced absorption intensities, likely reflecting conformational distortions. Fluorescence quantum yields ranged from 0.6 % to 7.1 %, with cationic porphyrins consistently less emissive than their neutral analogues, and their fluorescence lifetimes were likewise decreased. Triplet quantum yields were determined using the Consecutive Laser Pulse Excitation (CLPE) method, reaching up to 34 % for neutral and 26 % for cationic derivatives. Internal conversion was the dominant decay pathway across all the derivatives. According to the calculated rates, the decrease in fluorescence and triplet formation observed for cationic derivatives was mostly due to the increase in the non-radiative decay path. Heavy-atom effect promoted by the iodide counterion may be one of the main causes for this effect. Two-photon absorption cross-section spectra were determined for all the samples using a fluorescence-based technique. Measurements between 820 nm and 1400 nm revealed that two-photon absorption enables the same transitions as one-photon absorption. This behavior indicates a symmetry-breaking effect in the molecule, induced by the solvent and peripheral substituents. A region of enhanced two-photon absorption was observed near 800 nm, aligning with the first biological window, making it particularly promising for biomedical applications. While the added charge reduced one-photon absorption values, it had minimal impact on the two-photon absorption cross-section or two-photon brightness of these samples.