Effect of solvent on the electronic absorption spectral properties of some mixed β-octasubstituted Zn(II)-tetraphenylporphyrins.

A series of mixed β-octasubstituted Zn(II)-porphyrins, 2,3,12,13-tetra(chloro/cyano/methyl)-5,7,8,10,15,17,18,20-octaphenylporphinato zinc(II), ZnTPP(Ph)4 X4 (X=CN, Cl and CH3 ) have been examined by electronic absorption spectroscopy in various solvents. These Zn(II)-porphyrins exhibited varying degree of red-shift of absorption bands as high as 20-30nm in 'B' band and 50-60nm in longest wavelength band, 'Q(0,0)' band in polar solvents relative to that found in nonpolar solvents. The red-shift of B and Q(0,0) bands showed an unusual trend, ZnTPP(Ph)4 (CN)4 >ZnTPP(Ph)4 (CH3 )4 >ZnTPP(Ph)4 Cl4 but fails to follow an anticipated anodic shift in first porphyrin ring oxidation (vs Ag/AgCl) potential: ZnTPP(Ph)4 (CN)4 (1.02V)>ZnTPP(Ph)4 Cl4 (0.74V)>ZnTPP(Ph)4 (CH3 )4 (0.38V). Such a trend suggests the combined effect of non-planarity of the macrocycle and electronic effect of the peripheral substituents. The equilibrium constants for the binding of nitrogenous bases with the Zn(II)-porphyrins showed as high as twenty fold increase for ZnTPP(Ph)4 X4 (X=Br and CN) relative to ZnTPP(Ph)4 (CH3 )4 and follow the order: ZnTPP(Ph)4 (CN)4 >ZnTPP(Ph)4 Br4 >ZnTPP(Ph)4 (CH3 )4 ≤ZnTPP which is approximately in line with an increase in anodic shift of their first ring redox potentials (ZnTPP(Ph)4 (CN)4 (1.02V)>ZnTPP(Ph)4 Br4 (0.72V)>ZnTPP (0.84V)>ZnTPP(Ph)4 (CH3 )4 ) (0.38V).