Electron transfer reaction and demetalation of phthalocyanines

Abstract

An electron-transfer reaction takes place in the ground state between phthalocyanines (as electron-donors) and pyrylium cations (as electron-acceptors) in polar solvents, reaction which leads to the phthalocyanine radical-cations as evidenced by its absorption spectra identical to the one of the species formed by electrochemical oxidation.The fluorescence emission of the phthalocyanines is quenched by electron-acceptors (principally quinones). The free energy changeΔGCTof the electron-transfer reaction has been evaluated for each electron acceptor with a solvation energy of 0.14 eV in the case of dimethylformamide and 1.2 eV in that of dichloromethane. The rate constant of the quenching of the phthalocyanines singlet excited state by a series of electronacceptors is found to be of the order of1.2−1.5×1010L⋅mol-1s-1.Metalated phthalocyanines are demetalated in the dark by hydroxy-anthraquinones with a rate constant of the order of10−2L⋅mol-1s-1at 292K and which increases (up to7×10−2L⋅mol-1s-1at 349 K) with temperature. The activation energy of the demetalation reaction has been determined to be ca 30kJ⋅mol-1for 1,4-dihydroxyanthraquinone and 35kJ⋅mol-1for 1,2,4-trihydroxyanthraquinone.