Substituent

About: Substituent is a research topic. Over the lifetime, 42877 publications have been published within this topic receiving 516716 citations. The topic is also known as: side chain & side group.

Substituent effects on the electronic structure of metalloporphyrins: a quantitative analysis in terms of four-orbital-model parameters

Abstract: The effects of substituents at the periphery of the porphyrin ring on its a - a* absorption spectrum are determined in a quantitative fashion by using a new approach based on the four-orbital model. To accomplish this we systematically investigated with UV-visible absorption spectroscopy the Mg (Sn), Zn, Cu, Ni, and Pt series of metals incorporated into porphyrins with different substituents at the periphery of the macrocycle. The metal dependence of the UV-visible spectrum is used to determine relative energy splittings of the frontier orbitals, intensity parameters for a - a* transitions, and configuration interaction energies for metalloporphyrins with a variety of patterns of peripheral substitution (P, OEP, UroP, CoproP, ProtoP, and TPP). Differences in these MO parameters for distinct porphyrins are related to inductive and conjugative effects of the substituents. In particular, a good correlation between the splitting of the Lu-band and the Soret band for a series of copper porphyrins and the Hammett constants for the substituents is explained by a relationship between the electron-withdrawing ability of substituents and the strength of the configuration interaction that mixes one-electron excited states of the porphyrin. Variations among the porphyrins in the configuration interaction energy result from delocalization of ring charge caused by increased mixing of p-orbitals of the substituent a-carbons with ring orbitals as the "electronegativity" of the substituents increases. Delocalization of ring charge onto substituent carbons lowers the electron-electron repulsion, and therefore the configuration interaction. Anomalous features of the spectra of the metalloporphines and metalloprotoporphyrins are also explained. The spectra of the metalloporphines are unusual because suitable substituent p-orbitals are not available on the hydrogens for mixing with ring a-orbitals. The spectra of the metalloprotoporphyrins are of biological interest and are found to be only mildly anomalous. The unusual features can be traced to enhanced mixing with and delocalization of ring charge onto the a-system of the two vinyl substituents. For a complete understanding of substituent effects on the absorption spectra, changes in the configuration interaction, orbital energies, and transition dipoles must all be considered.

The “Diindene” ArInInAr (Ar = C6H3-2,6-Dipp2, Dipp = C6H3-2,6-Pri2). Dimeric versus Monomeric In(I) Aryls: para-Substituent Effects in Terphenyl Ligands

Abstract: The synthesis and structure of a “diindene” with significant metal−metal bonding are described. It has an In−In distance of 2.9786(5) A, an In−In−C angle of 121.23(6)°, and an In−In bond order that is probably less than unity.

Conjugated macrocycles related to the porphyrins. part 16.1 synthesis of hexa- and heptaalkyl-substituted inverted or n-confused porphyrins by the 3 + 1 methodology

Abstract: Pyrrole-2,4-dicarboxaldehydes 13a−c were condensed with a tripyrrane dicarboxylic acid 12 in 1% TFA−CH2Cl2 to give, following oxidation with 0.1% FeCl3, a series of “etio-type” N-confused porphyrins 14a−c. Excellent yields were obtained when the diformylpyrrole possessed a 5-alkyl substituent, although the products were most easily isolated in the form of their hydrochloride salts. Addition of TFA to the free base inverted porphyrins led to initial protonation onto the external nitrogen atom to produce monocations 19 and further acidification afforded a dications 21. UV−vis spectroscopy showed strong Soret bands for 14, 19, and 21, features that are associated with fully aromatic porphyrinoid systems, and proton NMR spectroscopy confirmed the presence of powerful diamagnetic ring current where the meso-protons resonated near 10 ppm while the interior CH was strongly shielded to −6 ppm for the free base structures and −4 ppm for the dications 21. Taking into account the effect of the introduction of deloca...