Regioselective 2,3-disubstituted porphyrins: synthesis, spectral, structural and electrochemical properties

Abstract

A series of regioselective β-disubstituted meso-tetraphenylporphyrins, H₂TPP(X)₂ (where X = methyl (CH₃), phenyl (Ph), bromo (Br) and phenylethynyl (PE)), as well their metal complexes (Cu(II), Co(II), Ni(II) and Zn(II)) were synthesized and characterized using various spectroscopic techniques. The absorption band of the phenylethynyl-substituted porphyrin, H₂TPP(PE)₂, exhibited a considerable red shift of 6–18 nm in the Soret band and 10–25 nm in the Q_x(0,0) band compared with the methyl- and phenyl-disubstituted porphyrins, viz. H₂TPP(CH₃)₂ and H₂TPP(Ph)₂. This red shift is assigned to the inductive and resonance effect of the phenyl and phenylethynyl groups present on the porphyrin π-skeleton and follows the trend of H₂TPP(CH₃)₂ < H₂TPP(Ph)₂ < H₂TPP(PE)₂. Two successive one-electron ring reduction and oxidation reactions occur in these regioselective β-disubstituted free base porphyrins, and additional metal-centred redox reactions [Co(II)/Co(I) and Co(II)/Co(III)] were observed in the Co(II) complexes. The first ring reduction potential of free base β-disubstituted-meso-tetraphenylporphyrins, H₂TPP(X)₂ (where X = CH₃, Ph, Br and PE), lies in the range of −1.22 V to −0.99 V, and shows an anodic shift in redox behaviour, which can be directly attributed to the increment in the electron withdrawing nature and follows the sequence of H₂TPP(PE)₂ > H₂TPP(Br)₂ > H₂TPP(Ph)₂ > H₂TPP(CH₃)₂. A reduction in the HOMO–LUMO gap was observed in these porphyrins compared with the unsubstituted metal complexes (MTPPs), and thus redox tunability was achieved upon β-disubstitution. Single-crystal X-ray structure analysis of H₂TPP(CH₃)₂ and CuTPP(PE)₂ revealed quasi-planar and ruffled conformations of the porphyrin core, respectively.