Synthesis and Characterization of <i>meso</i>-Triarylsubporphyrins

Abstract

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