Sequential, Ultrafast Energy Transfer and Electron Transfer in a Fused Zinc Phthalocyanine-free-base Porphyrin-C Supramolecular Triad.

A supramolecular triad composed of a fused zinc phthalocyanine-free-base porphyrin dyad (ZnPc-H P) coordinated to phenylimidazole functionalized C via metal-ligand axial coordination was assembled, as a photosynthetic antenna-reaction centre mimic. The process of self-assembly resulting into the formation of C Im:ZnPc-H P supramolecular triad was probed by proton NMR, UV-Visible and fluorescence experiments at ambient temperature. The geometry and electronic structures were deduced from DFT calculations performed at the B3LYP/6-31G(dp) level. Electrochemical studies revealed ZnPc to be a better electron donor compared to H P, and C to be the terminal electron acceptor. Fluorescence studies of the ZnPc-H P dyad revealed excitation energy transfer from H P* to ZnPc within the fused dyad and was confirmed by femtosecond transient absorption studies. Similar to that reported earlier for the fused ZnPc-ZnP dyad, the energy transfer rate constant, k was in the order of 10  s in the ZnPc-H P dyad indicating an efficient process as a consequence of direct fusion of the two π-systems. In the presence of C Im bound to ZnPc, photoinduced electron transfer leading to H P-ZnPc :ImC charge separated state was observed either by selective excitation of ZnPc or H P. The latter excitation involved an energy transfer followed by electron transfer mechanism. Nanosecond transient absorption studies revealed that the lifetime of charge separated state persists for about 120 ns indicating charge stabilization in the triad.