We synthesized five N-methyl-meso-tetraaryl-21-thiaporphyrins in 55-60% yield by treating the appropriate meso-tetraaryl-21-thiaporphyrin with CH 3I. 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.
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