Excitation transfer in metal-ligand coordinated free-base porphyrin-magnesium phthalocyanine and free-base porphyrin-magnesium naphthalocyanine dyads

Singlet excitation transfer in self-assembled dyads formed via axial coordination of imidazole appended free-base tetraphenylporphyrin, H2PIm , to either a magnesium phthalocyanine, MgPc , or a magnesium naphthalocyanine, MgNc is investigated in non-coordinating solvents using spectroscopic (optical and mass), electrochemical, and time-resolved transient absorption techniques. The newly assembled dyads are fully characterized by spectroscopic, computational and electrochemical methods. The binding constants measured from optical absorption spectral data are found to be in the range of 103–104 M-1 for the 1:1 dyads suggesting fairly stable complex formation. However, the anticipated 2:1 complex for Mg coordination is not observed under the present solution conditions. Electrochemical and computational studies suggested that photoinduced electron transfer to be a thermodynamically unfavorable process when free-base porphyrin is excited in these dyads. However, selective excitation of H2PIm entity in these dyads resulted in rapid excitation transfer and the position of the imidazole linkage on the H2P entity seem to direct the overall efficiency of excited energy transfer. Kinetics of energy transfer is monitored by transient absorption measurements using pump-probe technique and is compared with the earlier reported H2PIm:ZnPc and H2PIm:ZnNc dyads. The time constants are in the order of 0.2–18 ps depending upon the type and relative orientation of the donor and acceptor entities of the dyad indicating ultrafast excitation transfer, and agree fairly well with theoretically estimated values assuming Förster energy transfer mechanism.