Synthesis and Characterization of meso-Triarylsubporphyrins

Abstract

The synthesis of several meso-triaryl-subporphyrins, based on utilizing tripyrrolylborane as a precursor in a reaction with arylaldehydes (where aryl = phenyl, 4- and 3-pyridyl, tolyl, 4-methoxyphenyl, and 4-(trifluoromethyl)phenyl) under reflux in propionic acid, is reported. All of the compounds have been successfully characterized by 1H-, 13C-, and 13C-1H 2D NMR, electronic absorption, magnetic circular dichroism (MCD), IR, and fluorescence spectroscopy, together with cyclic (CV) and differential pulse (DPV) voltammetry. The X-ray structure of tris{(trifluoromethyl)phenyl}subporphyrin was found to be slightly domed and similar to that of the recently reported hexaethylsubtriazaporphyrin (Inorg. Chem. 2006, 45, 6148). The electronic absorption spectra of all of the subporphyrins contain intense Soret bands in the 370-380 nm region and weaker Q bands in the 420-550 nm region, which are at shorter wavelengths than those observed (at ca. 400-420 nm and ca. 450-650 nm, respectively) for tetrapyrrole porphyrins. The intensity of the Q00 band decreases as the meso-aryl groups become more electron withdrawing. These characteristics can be rationalized by using Gouterman's four-orbital model as a conceptual framework. The MCD bands observed in the Q band region of the subporphyrins (subPs) spectra consistently show a -ve/+ve intensity pattern in ascending energy, while, in contrast, the sign sequence of the bands observed in the Soret band region change dramatically depending on the nature of the aryl groups: from +ve/-ve in the case of the electron withdrawing 4-pyridyl, 4-(trifluoromethyl)phenyl and 3-pyridyl groups to -ve only for phenyl and -ve/+ve for the electron donating 4-tolyl and 4-methoxyphenyl groups. S1 fluorescence emission was observed in the 490-620 nm region. The quantum yields (phiF) in benzene (phiF = 0.10-0.12) are similar to that of metal-free tetraphenylporphyrin (H2TPP) (phiF = 0.11) but are somewhat lower in the case of ethanol (phiF = 0.06-0.07) due to the higher polarity. The redox potential differences observed between the first oxidation and reduction steps are in the 2.52-2.64 V range, which is larger than that of normal porphyrins (ca. 2.25 V). Molecular orbital (MO) calculations of these compounds help to provide an enhanced understanding of the spectroscopic and electrochemical properties. A byproduct of the synthesis was characterized using X-ray crystallography and a range of spectroscopic techniques. A subporphyrin mu-oxo dimer was prepared and characterized.