Charge Transfer Photophysics of Tetra(α-amino) Zinc Phthalocyanine

The absorption, fluorescence, and transient absorption spectra of Tetra(alpha-amino) zinc phthalocyanine, ZnPc(alpha-NH2)(4), have been measured in polar solvents and compared with that of ZnPc(alpha-R)(4) (R = H, NO2, OCH(CH3)(2)). While the latter three showed the typical photophysics of phthalocyanines, ZnPc(alpha-NH2)(4) exhibits distinct spectral properties, a very low fluorescence quantum yield and a relatively long fluorescence lifetime. These observations are explained by the substantial charge transfer characters in the absorption and fluorescence spectra of ZnPc(alpha-NH2)(4). NMR indicates that intramolecular H-bonding makes atoms in NH2 actually coplanar with other elements of ZnPc(alpha-NH2)(4). The local excited state is non emissive and the weak emission is assigned to its charge transfer state. The transient absorption bands from laser flash photolysis located at 630 nm, 645 nm is assigned to the mono-charge transfer state, while that at 545 nm is assigned to the di-charge transfer state.