Interaction of para-substituted meso-tetraphenylporphyrins and meso-tetra(n-propyl)porphyrin with weak and strong carboxylic acids: A UV–Vis spectroscopic study

Abstract

N-protonation of H2t(n-Pr)p and a series of para-substituted meso-tetraphenylporphyrins (arylpor), using carboxylic acids of different strength and size, CF3COOH, HCOOH, H2C2O4, CH2ClCOOH, CH3COOH and CH3CH2COOH, has led to a shift of the Soret and Q(0,0) bands with respect to those of the corresponding free base porphyrins. The size of central acceptor as well as the strength of N–H⋯carboxylate hydrogen bonding are the main factors influencing the red shift of the Soret band; the Soret band of H2t(n-Pr)p(CH3CH2COOH)2 is red shifted in comparison to those of the other dications of H2t(n-Pr)p. It may be shown that the out-of-plane deformation of the porphyrin core, which decreases the antibonding interactions in the frontier orbitals of the porphyrin core, leads to greater stabilization of the eg orbitals relative to the a1u one and consequently causes the red shift of the Soret band. Although the Q(0,0) band is blue shifted with respect to the free base porphyrin, the blue shift may be shown to be due to the protonation of the pyrrolenine nitrogen atoms rather than the saddling of the macrocycle. A comparison of the spectral data of the dications of arylpor, bearing strong electron donating para-substituents, with those of the related H2t(n-Pr)p adducts clearly indicates the importance of the π-resonance interactions in the observed spectral shifts of the Q(0,0) bands and, to less an extent, the Soret bands. The unusually large red shift observed for the Soret band of the H2t(4-NO2)pp dications seems to be due to the stabilization of eg orbitals through overlap with the proper unoccupied π∗ orbital of the electron withdrawing (4-NO2)phenyl in the excited state, a1u1eg1.