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

• Published in: Chemphyschem 2019-01, Vol.20 (1), p.163-172
• Main Authors: Seetharaman, Sairaman, Follana‐Berná, Jorge, Martín‐Gomis, Luis, Charalambidis, Georgios, Trapali, Adelais, Karr, Paul A., Coutsolelos, Athanassios G., Fernández‐Lázaro, Fernando, Sastre‐Santos, Ángela, D'Souza, Francis
• Format: Article
• Language: English
• Subjects: Absorption; Ambient temperature; Buckminsterfullerene; charge separation; Electron transfer; Electrons; Energy transfer; Excitation; Fluorescence; Fullerenes; NMR; Nuclear magnetic resonance; Photosynthesis; phthalocyanines; porphyrins; supramolecular triads; ultrafast energy transfer; Zinc
• ISSN: 1439-4235; 1439-7641
• DOI: 10.1002/cphc.201800847

A supramolecular triad composed of a fused zinc phthalocyanine‐free‐base porphyrin dyad (ZnPc‐H2P) 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 C60Im:ZnPc‐H2P 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 H2P, and C60 to be the terminal electron acceptor. Fluorescence studies of the ZnPc‐H2P dyad revealed excitation energy transfer from 1H2P* 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, kENT was in the order of 1012 s−1 in the ZnPc‐H2P dyad indicating an efficient process as a consequence of direct fusion of the two π‐systems. In the presence of C60Im bound to ZnPc, photoinduced electron transfer leading to H2P‐ZnPc.+:ImC60.− charge separated state was observed either by selective excitation of ZnPc or H2P. 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. One step at a time: Ultrafast excitation transfer from singlet excited free‐base porphyrin to zinc phthalocyanine and electron transfer from the singlet excited zinc phthalocyanine to fullerene, leading to a long‐lived charge‐separated state in the subsequent step, are demonstrated in a supramolecular triad as photosynthetic antenna reaction‐center mimicry.