A new class of porphyrins in which intervening ethynyl moieties connect aryl groups to the porphyrin 5- and 15-positions is described. The synthesis and optical spectroscopy of five new compounds are reported:  [5,15-bis[(4‘-dimethylaminophenyl)ethynyl]-10,20-diphenylporphinato]zinc(II), [5,15-bis[(4‘-methoxyphenyl)ethynyl]-10,20-diphenylporphinato]zinc(II), [5,15-bis[(phenyl)ethynyl]-10,20-diphenylporphinato]zinc(II), [5,15-bis[(4‘-fluorophenyl)ethynyl]-10,20-diphenylporphinato]zinc(II), and [5,15-bis[(4‘-nitrophenyl)ethynyl]-10,20-diphenylporphinato]zinc(II). The X-ray structure of one of these complexes, [5,15-bis[(4‘-fluorophenyl)ethynyl]-10,20-diphenylporphinato]zinc(II), is unusual in that it shows the arylethynyl-phenyl rings to be coplanar with the porphyrin macrocycle. This arrangement of aryl rings with respect to the porphyrin macrocycle underscores the fact that the steric barrier to rotation between the porphyrin and arylethynyl-phenyl aromatic systems has been removed and highlights the enha...

Exceptional electronic modulation of porphyrins through meso-arylethynyl groups. Electronic spectroscopy, electronic structure, and electrochemistry of [5,15-bis](aryl)ethynyl]-10,20-diphenylporphinato]zinc(II) complexes

X-ray structures solved for many tetrapyrrole-containing biomolecules have indicated that the tetrapyrrole unit adopts a nonplanar conformation and the degree of nonplanarity varies from one system to other. One of the reasons for choosing a family of tetrapyrrole macrocycles at the active sites of many biomolecules is due to the fact that they are highly flexible and can adopt a range of conformations. Recent realisation that the nonplanar conformations are essential for the biological functions has led to a surge of research activity on studies of nonplanar porphyrins. A large number of synthetic nonplanar porphyrins with varying degree of nonplanarity have been studied as models. The present article reviews the latest developments in such studies. Specifically, the structural deformation effect on various spectral properties such as optical, redox behaviour in the ground and excited singlet and triplet states are described for a series of nonplanar porphyrin systems. They include, hydroporphyrins, N-substituted porphyrins, heterosubstituted porphyrins and sterically crowded porphyrins. Furthermore, the dependence of nature of magnetic coupling on structure of porphyrin core in paramagnetic metalloporphyrin cation radicals is highlighted.

Nonplanar porphyrins and their biological relevance: ground and excited state dynamics

First synthesis of perfluorinated corrole and its mn=o complex.

A potent superoxide dismutase mimic: manganese beta-octabromo-meso-tetrakis-(N-methylpyridinium-4-yl) porphyrin.

Advances in Modern Synthetic Porphyrin Chemistry

The free-base octabromotetraphenylporphyrin (H2OBP) has been prepared by a novel bromination reaction of (meso-tetraphenylporphyrinato)copper(II). The metal [V(IV)O, Co(II), Ni(II), Cu(II), Zn(II), Pd(II), Ag(II), Pt(II)] derivatives exhibit interesting electronic spectral features and electrochemical redox properties. The electron-withdrawing bromine substituents at the pyrrole carbons in H2OBP and M(OBP) derivatives produce remarkable red shifts in the Soret (50 nm) and visible bands (100 nm) of the porphyrin. The low magnitude of protonation constants (pK3 = 2.6 and pK4 = 1.75) and the large red-shifted Soret and visible absorption bands make the octabromoporphyrin unique. The effect of electronegative bromine substituents at the peripheral positions of the porphyrin has been quantitatively analyzed by using the four-orbital approach of Gouterman. A comparison of MO parameters of MOBP derivatives with those of the meso-substituted tetraphenylporphyrin (M(TPP)) and unsubstituted porphine (M(P)) derivatives provides an explanation for the unusual spectral features. The configuration interaction matrix element of the M(OBP) derivatives is found to be the lowest among the known substituted porphyrins, indicating delocalization of ring charge caused by the increase in conjugation of p orbitals of the bromine onto the ring orbitals. The electron-transfer reactivities of the porphyrins have been dramatically altered by the peripheral bromine substituents, producing large anodic shifts in the ring and metal-centered redox potentials. The increase in anodic shift in the reduction potential of M(OBP)s relative to M(TPP)s is found to be large (550 mV) compared to the shift in the oxidation potential (300 mV). These shifts are interpreted in terms of the resonance and inductive interactions of the bromine substituents.

V. Krishnan

Papers

Octabromotetraphenylporphyrin and its metal derivatives: Electronic structure and electrochemical properties