Lifetime Of Charge-separated State Research Articles

A dramatic elongation of the lifetime of charge-separated state by complexation with yttrium triflate in ferrocene-anthraquinone linked dyad.

Seven million times elongation of the lifetime of charge-separated state is attained in the presence of yttrium triflate [Y(OTf)3] in the photoinduced electron-transfer reaction of a ferrocene-anthraquinone dyad (Fc-AQ) with a rigid amide spacer in benzonitrile at 298 K as compared with the lifetime in its absence. Such remarkable elongation of the CS lifetime in the presence of Y(OTf)3 results from the strong binding of Y(OTf)3 with the AQ*- moiety of Fc+-AQ*-.

Superexchange‐Mediated Charge Separation and Charge Recombination in Covalently Linked Donor—Bridge—Acceptor Systems

AbstractFor Abstract see ChemInform Abstract in Full Text.

Nanosecond time-resolved studies of long-lived photoinduced charge separation in the dyad fluorescein–anthraquinone–methyl ester adsorbed on TiO 2 colloids

A dyad composed of fluorescein and 2-methyl-anthraquinone (FL–AQ) was synthesized and its photophysical properties were examined by absorption, fluorescence spectroscopy, and fluorescence lifetime. The charge-separated state formed by photoinduced intramolecular electron transfer was detected by nanosecond transient absorption spectroscopy for the first time. When FL is excited in solution, the photoinduced electron transfer from FL to AQ proceeds efficiently. The rate constant and the efficiency of intramolecular electron transfer are 3.95×10 9 s −1 and 95%, respectively. Its charge-separated state lifetime is too short to detect by transient absorption spectroscopy. Adding nanometer colloidal TiO 2 to an FL–AQ ethanol solution prolongs the lifetime of the charge-separated state, so its transient absorption signal is recorded significantly. The lifetimes of FL +· at 480 nm and AQ −· at 560 nm in the FL–AQ/TiO 2 colloidal system are 11.1 and 8.93 μs, respectively.

Superexchange-Mediated Charge Separation and Charge Recombination in Covalently Linked Donor - Bridge - Acceptor Systems

Evidence is presented in support of the concept that electron transfer (ET) between a pair of chromophores may take place efficiently over large distances (>10 Å) by the mediation of an intervening saturated hydrocarbon medium. For example, ET is found to take place on a sub-nanosecond timescale through saturated norbornylogous bridges greater than 13 Å in length, by a superexchange (through-bond coupling) mechanism. The dependence of the ET dynamics on the bridge length and configuration are consistent with the operation of a superexchange mechanism. The distinction between molecular wire behaviour and superexchange-mediated ET is made. The distance dependence of ET dynamics through different types of bridges—saturated and unsaturated hydrocarbon bridges, proteins, and duplex DNA—is discussed and explained. Strategies for prolonging the lifetimes of charge-separated states are explored and discussed. In general, long-lived charge-separated species have been generated using giant multichromophoric systems in which the charges are separated by large distances, often exceeding 20 Å. In contrast, it is shown that very long-lived charge-separated states, possessing the triplet multiplicity, may be generated using short ‘dwarf’ dyads, in which the charges are less than 6 Å apart. Charge recombination in these species is slowed by the difference in electron spin multiplicity between the charge-separated state and the ground state.

Vectorial photoinduced electron transfer in alternating Langmuir–Blodgett films of phytochlorin–[60]fullerene dyad and regioregular poly(3-hexylthiophene)

Alternating Langmuir–Blodgett (LB) bilayer structures, consisting of a donor–acceptor (DA) layer of a phytochlorin–fullerene (PF) dyad and a layer of a regioregular poly(3-hexylthiophene) (PHT) polymer were used to study interlayer vectorial photoinduced electron transfer (VPET). As the dyad PF undergoes, under light illumination, an intramolecular ET from the phytochlorin to the fullerene moiety an intralayer VPET takes place in the LB monolayer. When PF was deposited on the PHT layer and excited the second ET took place from the PHT layer to the phytochlorin cation. Thus the PHT layer can act as a secondary electron donor and accompany the primary photoinduced electron transfer in the PF layer by a spontaneous interlayer electron transfer. Important characteristic properties of the VPET bilayer are the longer distance of charge separation (CS) and the longer lifetime of the charge separated state (lifetime from microsecond to second) as compared to VPET of the PF monolayer alone (where the lifetime of CS state was ≈30 ns). The CT measurements were carried out for different molecular orientations and film structures. Models for the multistep photochemical reactions are discussed.

Synthesis and photophysical studies of a porphyrin–viologen dyad covalently linked by a flexible seven-atom chain

A dyad with terminal porphyrin (P) and viologen (MV 2+) chromophores covalently linked by a flexible seven-atom hexanoate chain has been synthesized. Ab initio and semi-empirical solvent continuum calculations suggest that the preference for ‘extended’ or ‘folded’ conformations of the dyad depend strongly on solvent. Fluorescence lifetime and transient absorption studies in acetonitrile indicate that approximately 80% of the dyad population are in a suitable conformation to undergo photoinduced electron transfer (PET) to produce P +–MV + with a rate of 5×10 8 s −1. A long-lived charge-separated state lifetime of 170 ns is observed. It is proposed that electrostatic repulsion between the positively charged end groups resulting from PET increases the average donor–acceptor separation thus retarding the charge recombination process.

Polymer effects in photoinduced charge separation

Reverse electron transfer between photogenerated redox pair in bulk aqueous solutions can be efficiently suppressed by collecting the photoliberated electron to the positively charged surface of molecular assemblies such as micelles, bilayers and polymers. Zwitterionic viologen molecules serve as efficient electron carriers, which shuttle between the bulk phase and the surface of molecular assemblies. The molecular assemblies with aligned viologen units at the surface are particularly useful as molecular device for the electron enrichment and storage. Quantitative charge separation of the cage-escaped redox pair can be achieved by the use of the amphiphilic viologen molecular assemblies in combination with the zwitterionic viologen. The lifetime of charge-separated state can be extremely extended in these systems : ca 30 ms for the viologen bilayer and above 1 s for the polymer with pendant viologen. Amphiphilic viologen molecular assemblies also help to increase charge-separation of the photoinduced, intramolecular electron transfer in sensitizeracceptor linked system incorporated into the molecular assemblies. All of these phenomena are ascribed to cooperativity effects of aligned viologen units, which induce electron hopping a long the surface via successive electron exchange between viologen molecules.