Supramolecular electron transfer by anion binding.
ABSTRACT Anion binding has emerged as an attractive strategy to construct supramolecular electron donor-acceptor complexes. In recent years, the level of sophistication in the design of these systems has advanced to the point where it is possible to create ensembles that mimic key aspects of the photoinduced electron-transfer events operative in the photosynthetic reaction centre. Although anion binding is a reversible process, kinetic studies on anion binding and dissociation processes, as well as photoinduced electron-transfer and back electron-transfer reactions in supramolecular electron donor-acceptor complexes formed by anion binding, have revealed that photoinduced electron transfer and back electron transfer occur at time scales much faster than those associated with anion binding and dissociation. This difference in rates ensures that the linkage between electron donor and acceptor moieties is maintained over the course of most forward and back electron-transfer processes. A particular example of this principle is illustrated by electron-transfer ensembles based on tetrathiafulvalene calixpyrroles (TTF-C4Ps). In these ensembles, the TTF-C4Ps act as donors, transferring electrons to various electron acceptors after anion binding. Competition with non-redox active substrates is also observed. Anion binding to the pyrrole amine groups of an oxoporphyrinogen unit within various supramolecular complexes formed with fullerenes also results in acceleration of the photoinduced electron-transfer process but deceleration of the back electron transfer; again, this is ascribed to favourable structural and electronic changes. Anion binding also plays a role in stabilizing supramolecular complexes between sulphonated tetraphenylporphyrin anions ([MTPPS](4-): M = H(2) and Zn) and a lithium ion encapsulated C(60) (Li(+)@C(60)); the resulting ensemble produces long-lived charge-separated states upon photoexcitation of the porphyrins.
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ABSTRACT: Multi-modular supramolecular systems capable of undergoing photoinduced energy and electron transfer are of paramount importance to design light-to-energy and light-to-fuel converting devices. Often, this has been achieved by linking two or more photo-active or redox-active entities with complementary spectral and photochemical properties. In the present study, we report a new triad made out of two entities of subphthalocyanine covalently linked to BF2-chelated azadipyrromethene ((SubPc)2-azaBODIPY). The triad was fully characterized by spectral, computational, electrochemical and photochemical techniques. The B3LYP/6-31G* calculations revealed a structure wherein the donor, SubPc, and the acceptor, azaBODIPY, were well separated with no steric crowding. The different redox states were established from the differential pulse voltammetry studies and the data were used to estimate free-energy change associated with charge separation. Such calculations revealed the charge separation from either the (1)SubPc* or (1)azaBODIPY* to be thermodynamically feasible for yielding the (SubPc)SubPc˙(+)-azaBODIPY˙(-) radical ion-pair. Steady-state fluorescence studies revealed quantitative quenching of (1)SubPc* in the triad and solvent dependent quenching of (1)azaBODIPY* indicating participation of both fluorophores in promoting photochemical events. In nonpolar toluene, singlet-singlet energy transfer from the (1)SubPc* to azaBODIPY was observed, while in polar benzonitrile, evidence of energy transfer was feeble. Femtosecond laser flash photolysis studies provided concrete evidence for the occurrence of ultrafast photoinduced electron transfer by providing spectral proof for the formation of the (SubPc)SubPc˙(+)-azaBODIPY˙(-) charge separated state. The charge recombination followed populating the (3)azaBODIPY* prior to returning to the ground state.Physical Chemistry Chemical Physics 07/2014; · 4.20 Impact Factor
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ABSTRACT: High oxidation potential perfluorinated zinc phthalocyanines (ZnFnPcs) are synthesised and their spectroscopic, redox, and light-induced electron-transfer properties investigated systematically by forming donor–acceptor dyads through metal–ligand axial coordination of fullerene (C60) derivatives. Absorption and fluorescence spectral studies reveal efficient binding of the pyridine- (Py) and phenylimidazole-functionalised fullerene (C60Im) derivatives to the zinc centre of the FnPcs. The determined binding constants, K, in o-dichlorobenzene for the 1:1 complexes are in the order of 104 to 105 M−1; nearly an order of magnitude higher than that observed for the dyad formed from zinc phthalocyanine (ZnPc) lacking fluorine substituents. The geometry and electronic structure of the dyads are determined by using the B3LYP/6-31G* method. The HOMO and LUMO levels are located on the Pc and C60 entities, respectively; this suggests the formation of ZnFnPc.+–C60Im.− and ZnFnPc.+–C60Py.− (n=0, 8 or 16) intra-supramolecular charge-separated states during electron transfer. Electrochemical studies on the ZnPc–C60 dyads enable accurate determination of their oxidation and reduction potentials and the energy of the charge-separated states. The energy of the charge-separated state for dyads composed of ZnFnPc is higher than that of normal ZnPc–C60 dyads and reveals their significance in harvesting higher amounts of light energy. Evidence for charge separation in the dyads is secured from femtosecond transient absorption studies in nonpolar toluene. Kinetic evaluation of the cation and anion radical ion peaks reveals ultrafast charge separation and charge recombination in dyads composed of perfluorinated phthalocyanine and fullerene; this implies their significance in solar-energy harvesting and optoelectronic device building applications.ChemPhysChem 05/2014; · 3.35 Impact Factor
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ABSTRACT: The oxidizing ability of organic dyes is enhanced significantly by photoexcitation. Radical cations of weak electron donors can be produced by electron transfer from the donors to the excited states of organic dyes. The radical cations thus produced undergo bond formation reactions with various nucleophiles. For example, the direct oxygenation of benzene to phenol was made possible under visible-light irradiation of 2,3-dichloro-5,6-dicyano-p-benzoquinone (DDQ) in an oxygen-saturated acetonitrile solution of benzene and water via electron transfer from benzene to the triplet excited state of DDQ. 3-Cyano-1-methylquinolinium ion (QuCN(+)) can also act as an efficient photocatalyst for the selective oxygenation of benzene to phenol using oxygen and water under homogeneous and ambient conditions. Alkoxybenzenes were also obtained when water was replaced by alcohol under otherwise identical experimental conditions. QuCN(+) can also be an effective photocatalyst for the fluorination of benzene with O2 and fluoride anion. Photocatalytic selective oxygenation of aromatic compounds was achieved using an electron donor-acceptor-linked dyad, 9-mesityl-10-methylacridinium ion (Acr(+)-Mes), as a photocatalyst and O2 as the oxidant under visible-light irradiation. The electron-transfer state of Acr(+)-Mes produced upon photoexcitation can oxidize and reduce substrates and dioxygen, respectively, leading to the selective oxygenation and halogenation of substrates. Acr(+)-Mes has been utilized as an efficient organic photoredox catalyst for many other synthetic transformations.Organic & Biomolecular Chemistry 07/2014; · 3.57 Impact Factor