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Spectral shifts upon reversible modifications of formyl peripheral substituents in porphyrin chlorine and bacteriochlorin a phenomenological explanation for the red shift of protonated schiff base

, : Spectral shifts upon reversible modifications of formyl peripheral substituents in porphyrin chlorine and bacteriochlorin a phenomenological explanation for the red shift of protonated schiff base. Journal of the American Chemical Society 106(14): 3943-3950

It was previously demonstrated that the formation of a protonated Schiff base metalloporphyrin resulted in a 50-nm red shift of the long wavelength absorption band relative to the unprotonated imine. A systematic study is presented on the spectral properties of 5 formylporphyrin, chlorin and bacteriochlorin systems upon formation of their protonated Schiff bases. The compounds used were Ni complexes of 4-vinyl-8-formyl, 1,4-diformyl and 4,8-diformyl derivatives of an alkyl phorphyrin; Cu 2,6-di-n-pentyl-4-vinyl-7-hydroxy-8-acroleinyl-1,3,5,7-tetramethylchlorin; and Cu 2,6-di-n-pentyl-3,7-dihydroxy-4,8-diacroleinyl-1,3,5,7-tetramethylbacteriochlorin. Pyrrolidine adducts and the neutral, but electron deficient condensates of malononitrile and ethyl cyanoacetate were also examined. The unusual red shift of absorption maxima in the visible region and the splitting/broadening of the Soret peak observed in protonated Schiff base porphyrin, chlorin and bacteriochlorin are attributable to the resonance effect of a strong electron-withdrawing group. Studies of solvent and counterion effects further indicated that the conversion of a carbonyl to a Schiff base peripheral group would subject the spectral and redox properties of chlorin and porphyrin to a greater extent of environmental control. A Schiff base chlorophyll could be an attractive model for the photosynthetic reaction centers.

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