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

A novel trans-A(2)B(2)-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, C(60)Im, 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: ImC(60) 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 C-60 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 C-60. 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 C(60)Im 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 C-60 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.