Geometry‐Controlled Photoinduced Charge Separation and Recombination in a Trans‐A2B2‐Functionalized Donor–Acceptor Conjugate Composed of a Multimodular Zinc Porphyrin and Fullerene

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AbstractA novel trans‐A2B2‐functionalized multimodular zinc porphyrin (TPA‐BT)2ZnP has been synthesized through a multistep sitedirected synthesis as an electron donor for light‐energy‐harvesting applications. The Zn porphyrin was functionalized by appending two triphenylamine–bithiophene moieties (TPA‐BT) at the trans‐located meso‐positions of the porphyrin ring to improve the electron‐donor capability. Facile oxidation of the Zn porphyrin due to electron‐rich substituents was evidenced by cyclic voltammetry, and the spectrum of oxidized (TPA‐BT)2ZnP was obtained using spectroelectrochemical methods. The donor–acceptor conjugate was subsequently built by coordinating an electron acceptor, C60Im, through metal–ligand axial coordination. The conjugate was characterized by spectral, electrochemical, and computational techniques. The geometry and electronic structure of the (TPA‐BT)2ZnP:ImC60 conjugate, deduced from the B3LYP/6‐31G* method, revealed the frontier HOMO to be delocalized over the (TPA‐BT)2ZnP macrocycle whereas the LUMO was localized on the C60 entity. Free‐energy calculations performed according to Rehm–Weller's approach suggested the possibility of the occurrence of photoinduced electron transfer processes from the singlet excited Zn porphyrin to C60. Efficient photoinduced charge separation in the conjugate was demonstrated from studies involving femtosecond transient absorption techniques. The measured rate of charge separation was slightly higher than previously reported donor–acceptor systems formed from simple Zn porphyrin and C60Im due to facile oxidation of (TPA‐BT)2ZnP. The charge recombination rate was also found to be faster and this has been attributed to the closely located C60 and triphenylamine entities of (TPA‐BT)2ZnP, facilitating through‐space charge recombination process. The present study highlights the importance of the 3D organization of the self‐assembled donor–acceptor conjugate in governing the speed of electron transfer.