No abstract

Photoinduced electron transfer (PET) in newly assembled dyads formed via metal‐ligand axial coordination of phenylimidazole‐functionalized bis(styryl)BODIPY (BODIPY(Im)2) and zinc tetrapyrroles, that is, zinc tetratolylporphyrin (ZnP), zinc tetra‐t‐butyl phthalocyanine (ZnPc) and zinc tetra‐t‐butyl naphthalocyanine (ZnNc), in non‐coordinating o‐dichlorobenzene (DCB) is investigated using both steady‐state and time‐resolved transient absorption techniques. The structure of the BODIPY(Im)2 was identified by using single crystal X‐ray structural analysis. The newly formed supramolecular dyads were fully characterized by spectroscopic, computational and electrochemical methods. The binding constants measured from optical absorption spectral studies were in the range of ∼104 M−1 for the first zinc tetrapyrrole binding and suggested that the two imidazole entities of bis(styryl)BODIPY behave independently in the binding process. The energy level diagram established using spectral and electrochemical studies suggested PET to be thermodynamically unfavorable in the ZnP‐bearing complex while for ZnPc‐ and ZnNc‐bearing complexes such a process is possible when zinc tetrapyrrole is selectively excited. Consequently, occurrence of efficient PET in the latter two dyads was possible to establish from femtosecond transient absorption studies wherein the electron transfer products, that is, the radical cation of zinc tetrapyrrole and the radical anion of BODIPY(Im)2, was possible to spectrally identify. From target analysis of the transient data, time constants of circa 3 ns for ZnPc⋅+:BODIPY⋅− and circa 0.5 ns for ZnNc⋅+:BODIPY⋅− were obtained indicating persistence of the radical ion‐pair to some extent. The electron acceptor property of bis(styryl)BODIPY in donor‐acceptor conjugates is borne out from the present study.

A 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.

The Front Cover picture shows the optimized structure of a trans‐A2B2‐functionalized multimodular zinc porphyrin/fullerene supramolecular donor–acceptor conjugate and the geometry‐governed photochemical events occurring upon photoexcitation of the porphyrin. More information can be found in the Full Paper by F. D′Souza and co‐workers on page 17 in Issue 1, 2017 (DOI: 10.1002/cptc.201600017).

The front cover artwork is provided by the D′Souza group at the Department of Chemistry, University of North Texas. The image shows the optimized structure of a trans‐A2B2‐functionalized multimodular zinc porphyrin/fullerene supramolecular donor–acceptor conjugate and the geometry‐governed photochemical events which occur upon photoexcitation of the porphyrin. Read the full text of the article at 10.1002/cptc.201600017.