Charge separated state

Williams R M, Koeberg M, Lawson J M, An Y-Z, Rubin Y, Paddon-Row M N and Verhoeven J W 1996 Photoinduced electron transfer to Cgg across extended 3- and 11 a-bond hydrocarbon bridges creation of a long-lived charge-separated state J. Org. Chem. 61 5055-62... 

Fukuzumi, S., Ohkubo, K., Wenbo, E., Ou, Z., Shao, J., Kadish, K.M., Hutchison, J.A., Ghiggino, K.P., Sintic, P.J. and Crossley, M.J. (2003) Metal-centered photoinduced electron transfer reduction of a gold(III) porphyrin cation linked with a zinc porphyrin to produce a long-lived charge-separated state in nonpolar solvents. Journal of the American Chemical Society, 125, 14984-14985. 

The mere exposure of diphenyl-polyenes (DPP) to medium pore acidic ZSM-5 was found to induce spontaneous ionization with radical cation formation and subsequent charge transfer to stabilize electron-hole pair. Diffuse reflectance UV-visible absorption and EPR spectroscopies provide evidence of the sorption process and point out charge separation with ultra stable electron hole pair formation. The tight fit between DPP and zeolite pore size combined with efficient polarizing effect of proton and aluminium electron trapping sites appear to be the most important factors responsible for the stabilization of charge separated state that hinder efficiently the charge recombination. 

For very long time (> 1 year), electron-hole pair recombination occurs and spectral features of neutral occluded DPB molecule are found to reappear (figure 5). Then, electron trapping in zeolite framework shows that ionization does not proceed as a simple oxidation but stands for a real charge separated state. 

DPB as well as other DPP molecules (t-stilbene, diphenyl-hexatriene) with relatively low ionization potential (7.4-7.8 eV) and low vapor pressure was successfully incorporated in the straight channel of acidic ZSM-5 zeolite. DPP lies in the intersection of straight channel and zigzag channel in the vicinity of proton in close proximity of Al framework atom. The mere exposure of DPP powder to Bronsted acidic ZSM-5 crystallites under dry and inert atmosphere induced a sequence of reactions that takes place during more than 1 year to reach a stable system which is characterized by the molecule in its neutral form adsorbed in the channel zeolite. Spontaneous ionization that is first observed is followed by the radical cation recombination according to two paths. The characterization of this phenomenon shows that the ejected electron is localized near the Al framework atom. The reversibility of the spontaneous ionization is highlighted by the recombination of the radical cation or the electron-hole pair. The availability of the ejected electron shows that ionization does not proceed as a simple oxidation but stands for a real charge separated state. 

Figure 10. Vibronic levels of an electron transfer system. Here the initial electronic state is the D A state and the final electronic state is the D+A charge separation state. The vibrational levels within the D A manifold are labeled by v and those within D+A by v. ...

Fullerenes such as C60 are excellent electron acceptors. In a fullerene-porphyrin-based dyad, the photoexcited state of the Qo accepts an electron from the linked zinc porphyrin group to give a charge-separated state. 

We have seen that, in photosynthetic bacteria, visible light is harvested by the antenna complexes, from which the collected energy is funnelled into the special pair in the reaction centre. A series of electron-transfer steps occurs, producing a charge-separated state across the photosynthetic membrane with a quantum efficiency approaching 100%. The nano-sized structure of this solar energy-conversion system has led researchers over the past two decades to try to imitate the effects that occur in nature. 

Milanesio ME, Alvarez MG, Rivarola V, Silber JJ, Durantini EN (2005) Porphyrin-fullerene C60 dyads with high ability to form photoinduced charge-separated state as novel sensitizers for photodynamic therapy. Photochem Photobiol 81 891-897. 

Quite differently, Pleux et al. tested a series of three different organic dyads comprising a perylene monoimide (PMI) dye linked to a naphthalene diimide (NDI) or C60 for application in NiO-based DSSCs (Fig. 18.7) [117]. They corroborated a cascade electron flow from the valance band of NiO to PMI and, finally, to C60. Transient absorption measurements in the nanosecond time regime revealed that the presence of C60 extends the charge-separated state lifetime compared to just PMI. This fact enhanced the device efficiencies up to values of 0.04 and 0.06% when CoII/m and P/Ij electrolytes were utilized, respectively. More striking than the efficiencies is the remarkable incident photon-to-current efficiency spectrum, which features values of around 57% associated to photocurrent densities of 1.88 mA/cm2. 

Fig. 1. The Marcus parabolic free energy surfaces corresponding to the reactant electronic state of the system (DA) and to the product electronic state of the system (D A ) cross (become resonant) at the transition state. The curves which cross are computed with zero electronic tunneling interaction and are known as the diabatic curves, and include the Born-Oppenheimer potential energy of the molecular system plus the environmental polarization free energy as a function of the reaction coordinate. Due to the finite electronic coupling between the reactant and charge separated states, a fraction k l of the molecular systems passing through the transition state region will cross over onto the product surface this electronically controlled fraction k l thus enters directly as a factor into the electron transfer rate constant...

The temperature dependence of kET(CR3> revealed only a moderate change (2.6-3.0 sec ) upon varying the temperature between 163 and 203 K [47]. The longest lifetime of the resulting charge-separated state (i.e., ferricenium ion Ceo radical anion pair) in frozen benzonitrile (PhCN) is determined as 0.38 sec [47], which is more than one order of magnitude louger than any other intramolecular... 

Figure4.12 TTF and extended TTF was attached to C q via the Prato-reaction in order to get dyads suitable for long-lived charge-separated states.

De la Escosura A, Martinez-Diaz MV, Guldi DM et al (2006) Stabilization of charge-separated states in phthalocyanine-fullerene ensembles through supramolecular donor-acceptor interactions. J Am Chem Soc 128 4112-4118... 

The presence of the charge-separated state involving electron transfer from Mg(II)TPP to Re(bpy) was shown by time-resolved IR spectroscopy . ... 

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