Greatly enhanced intermolecular π-dimer formation of a porphyrin trimer radical trications through multiple π bonds

Abstract

A trefoil-like porphyrin trimer linked by triphenylamine (TPA-TPZn(3)) was synthesized. A three-electron oxidation of TPA-TPZn(3) forms a radical trication (TPA-TPZn(3)(3+)), in which each porphyrin ring undergoes a one-electron oxidation. The radical trication TPA-TPZn(3)(3+) spontaneously dimerizes to afford (TPA-TPZn(3))(2)(6+) in CH(2)Cl(2) . The characteristic charge-resonance band due to the charge delocalization over the π system of (TPA-TPZn(3))(2)(6+) was observed in the NIR region. The initial oxidation potential of TPA-TPZn(3) is negatively shifted relative to that of the corresponding monomer porphyrin, which results from the stabilization of the oxidized state of TPA-TPZn(3) associated with the dimerization. The thermodynamic parameters (i.e., ΔH, ΔS, and ΔG) for the formation of (TPA-TPZn(3))(2)(6+) were determined by measuring Vis/NIR spectra at various temperatures. The formation constant of (TPA-TPZn(3))(2)(6+) is significantly larger than that of the radical cation dimer of the corresponding monomer porphyrin (e.g., over 2000-fold at 233 K). The electronic states were investigated using EPR spectroscopic analysis. The greatly enhanced dimerization of TPA-TPZn(3)(3+) results from multiple π-bond formation between the porphyrin radical cations.