Photophysical studies of supramolecular triads involving zinc naphthalocyanines and pyridylfullerenes with a second electron donor

Supramolecular triads composed of zinc naphthalocyanine (ZnNc) as an electron donor, pyridylfullerenes as primary electron acceptors, and either ferrocene (Fc) or N,N-dimethyl-aminophenyl (DMA) entities as second electron donors were constructed via metal-ligand axial coordination. The formation of the supramolecular triads was monitored using optical absorption methods and the measured binding constants revealed a moderately stable complex formation. The B3LYP/3-21G(*) optimized structures showed the disposition of the three entities of the triads to be in a linear fashion. The redox behavior of the different components was studied using cyclic voltammetry in o-dichlorobenzene containing (n-C₄H₉)₄NClO₄. The donor ability evaluated from the oxidation potentials of the electron donors followed the trend: ZnNc < Fc < DMA. Efficient electron transfer from the excited singlet state of zinc naphthalocyanine to the fullerene entity was observed in all of the studied triads in o-dichlorobenzene by fluorescence quenching measurements. Clear evidence, that the charge-separation process formed the radical ions in the triads, was obtained from a nanosecond transient absorption spectral technique, indicating the contribution of the second electron donor in prolonging the charge-separated states.