Issue 5, 2015

The origin of the absorption spectra of porphyrin N- and dithiaporphyrin S-oxides in their neutral and protonated states

Abstract

meso-Tetraphenylporphyrin N-oxide (1) and meso-tetraphenyl-21,23-dithiaporphyrin S-oxide (3) possess optical spectra that are distinctly different from their parent porphyrins, meso-tetraphenylporphyrin (2) and meso-tetraphenyl-21,23-dithiaporphyrin (4), respectively. The hyperporphyrin spectra were reproduced and classified using TD CAM-B3LYP and SCS-CC2 computational methods. Calculations revealed the electronic and conformational influences of the N- and S-oxide functionalities. While the N-oxide under acidic conditions forms a dication with a UV-vis spectrum that is nearly indistinguishable from that of the diprotonated parent porphyrin, the diprotonated S-oxide possesses a much different UV-vis spectrum from diprotonated parent dithiaporphyrin. A computational study of the protonation events revealed the site and degree of protonation and rationalized the regular and hyperporphyrin UV-vis spectra of the neutral and protonated species, respectively. The study illuminates the electronic effects of the relatively rare modification of the inner porphyrin heteroatoms. It also illustrates a case in which TD CAM-B3LYP reaches its limits to make reliable predictions about the optical properties of a porphyrinoid, making the use of higher methods essential.

Graphical abstract: The origin of the absorption spectra of porphyrin N- and dithiaporphyrin S-oxides in their neutral and protonated states

Supplementary files

Article information

Article type
Paper
Submitted
15 Oct 2014
Accepted
11 Dec 2014
First published
11 Dec 2014

Phys. Chem. Chem. Phys., 2015,17, 3560-3569

The origin of the absorption spectra of porphyrin N- and dithiaporphyrin S-oxides in their neutral and protonated states

T. Bruhn and C. Brückner, Phys. Chem. Chem. Phys., 2015, 17, 3560 DOI: 10.1039/C4CP04675G

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