Bhyrappa P, Sankar M. Effect of solvent on the electronic absorption spectral properties of some mixed β-octasubstituted Zn(II)-tetraphenylporphyrins.
SPECTROCHIMICA ACTA. PART A, MOLECULAR AND BIOMOLECULAR SPECTROSCOPY 2018;
189:80-85. [PMID:
28802858 DOI:
10.1016/j.saa.2017.07.059]
[Citation(s) in RCA: 2] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Received: 03/21/2017] [Revised: 07/21/2017] [Accepted: 07/30/2017] [Indexed: 06/07/2023]
Abstract
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)4X4 (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)4Cl4 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)4Cl4 (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)4X4 (X=Br and CN) relative to ZnTPP(Ph)4(CH3)4 and follow the order: ZnTPP(Ph)4(CN)4>ZnTPP(Ph)4Br4>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)4Br4 (0.72V)>ZnTPP (0.84V)>ZnTPP(Ph)4(CH3)4) (0.38V).
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