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

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 2Cl 2. The characteristic charge-resonance band due to the charge delocalization over the πsystem 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 233K). 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. Multiple π bonds: A three-electron oxidation of a trefoil-like porphyrin trimer generates a radical trication, which spontaneously dimerizes. The dimerization of the trimer is much more favorable relative to the corresponding monomer, which results from the intermolecular multiple π-bond formation between porphyrin radical cations of the trimer (see picture). Copyright © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.