Ethyne Functionalized <i>Meso</i>-Phenothiazinyl-Phenyl-Porphyrins: Synthesis and Optical Properties of Free Base Versus Protonated Species

• Main Authors: Eva Molnar, Emese Gál, Luiza Găină, Castelia Cristea, Luminița Silaghi-Dumitrescu
• Format: Article
• Language: English
• Published: MDPI AG 2020-10-01
• Series: Molecules
• Subjects: phenothiazine; porphyrin; photophysical properties
• Online Access: https://www.mdpi.com/1420-3049/25/19/4546

Synthesis, structural characterization and photophysical properties for a series of new <i>trans</i>-A₂B₂- and A₃B-type ethynyl functionalized <i>meso</i>-phenothiazinyl-phenyl porphyrin derivatives are described. The new compounds displayed the characteristic porphyrin absorption spectra slightly modified by weak auxochromic effects of the substituents and fluorescence emission in the range of 651–659 nm with 11–25% quantum yields. The changes recorded in the UV-vis absorption spectra in the presence of trifluoroacetic acid (TFA) are consistent with the protonation of the two internal nitrogen atoms of the free-base porphyrin (19 nm bathochromic shift of the strong Soret band and one long wave absorption maxima situated in the range of 665–695 nm). Protonation of the phenothiazine substituents required increased amounts of TFA and produced a distinct hypsochromic shift of the long wave absorption maxima. The density functional theory (DFT) calculations of a porphyrin dication pointed out a saddle-distorted porphyrin ring as the ground-state geometry.