N-methyl-21-thiaporphyrins

Abstract

We synthesized five N-methyl-meso-tetraaryl-21-thiaporphyrins in 55–60 % yield by treating the appropriate meso-tetraaryl-21-thiaporphyrin with CH₃I. The N-methyl-21-thiaporphyrins were characterized by HRMS, 1D and 2D NMR spectroscopy, absorption spectroscopy, fluorescence spectroscopy and electrochemical techniques and the structure of one of the compounds was obtained by X-ray crystallography. 1D and 2D NMR spectroscopy were used to identify all of the resonances observed in the ¹H NMR spectra of the N-methyl-21-thiaporphyrins. NMR studies indicated that the methylation occurred at the pyrrole ring that is opposite to the thiophene ring. In the ¹H NMR spectra, the methylation resulted in significant upfield shifts of the pyrrole and thiophene protons; the maximum shifts were noted for the protons of the N-methylated pyrrole ring of the N-methyl-21-thiaporphyrins. The X-ray structure of one of the N-methyl-21-thiaporphyrins revealed significant deviation of the N-methylated pyrrole ring from the reference plane defined by the four meso carbon atoms, and the porphyrin ring is strongly distorted compared to the 21-thiaporphyrin ring. The absorption studies revealed that the N-methyl-21-thiaporphyrins showed four Q bands and one strong Soret band, which were bathochromically shifted compared to those of the 21-thiaporphyrins. The electrochemical studies on the N-methyl-21-thiaporphyrins indicated that the oxidation potentials became less positive and there were negligible shifts in the reduction potentials; the HOMO–LUMO energy gap decreased compared with that of the 21-thiaporphyrins. The N-methyl-21-thiaporphyrins are weakly fluorescent with low quantum yields and singlet-state lifetimes. The spectral and electrochemical studies indicated that N-methylation of the pyrrole ring of 21-thiaporphyrins significantly alters the electronic properties of the 21-thiaporphyrin macrocycle.