Energy transfer in zinc porphyrin-phthalocyanine heterotrimer and heterononamer studied by fluorescence resonance energy transfer (FRET)

Sequential, Ultrafast Energy Transfer and Electron Transfer in a Fused Zinc Phthalocyanine-free-base Porphyrin-C60 Supramolecular Triad

A supramolecular triad composed of a fused zinc phthalocyanine-free-base porphyrin dyad (ZnPc-H₂ P) coordinated to phenylimidazole functionalized C₆₀ via metal-ligand axial coordination was assembled, as a photosynthetic antenna-reaction centre mimic. The process of self-assembly resulting into the formation of C₆₀ Im:ZnPc-H₂ 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 H₂ P, and C₆₀ to be the terminal electron acceptor. Fluorescence studies of the ZnPc-H₂ P dyad revealed excitation energy transfer from ¹ H₂ 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_ENT was in the order of 10¹²  s^-1 in the ZnPc-H₂ P dyad indicating an efficient process as a consequence of direct fusion of the two π-systems. In the presence of C₆₀ Im bound to ZnPc, photoinduced electron transfer leading to H₂ P-ZnPc^.+ :ImC₆₀ ^.- charge separated state was observed either by selective excitation of ZnPc or H₂ 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.

Sn(IV) porphyrin based axial-bonding type porphyrin triads containing heteroporphyrins as axial ligands

The thiaporphyrin building blocks with N(3)S and N(2)S(2) cores containing one hydroxyphenyl functional group at the meso position were synthesized by adopting the unsymmetrical thiophene diol method. These monohydroxy thiaporphyrins were used to construct the first examples of axial bonding type Sn(IV) porphyrin triads in which Sn(IV) porphyrin acts as basal unit and the two thiaporphyrin units as axial ligands by treating with SnTTP(OH)(2) in benzene at refluxing temperature. The axial bonding type triads were confirmed by mass, 1D and 2D NMR studies. The absorption and electrochemical studies support weak ground state interaction among the porphyrin subunits within the porphyrin triads. The fluorescence studies indicate there is a possibility of energy transfer at the singlet state from basal Sn(IV) porphyrin unit to axial thiaporphyrin units.