Spontaneous charge shift

While such a device has yet to be constructed, Debreczeny and co-workers have synthesized and studied a linear D-A, -A2 triad suitable for implementation in such a device.11641 In this system, compound 6, a 4-aminonaphthalene monoimide (AN I) electron donor is excited selectively with 400 nm laser pulses. Electron transfer from the excited state of ANI to Ai, naphthalene-1,8 4,5-diimide (NI), occurs across a 2,5-dimethylphenyl bridge with x = 420 ps and a quantum yield of 0.95. The dynamics of charge separation and recombination in these systems have been well characterized.11651 Spontaneous charge shift to A2, pyromellitimide (PI), is thermodynamically uphill and does not occur. The mechanism for switching makes use of the large absorption cross-section of the NI- anion radical at 480 nm, (e = 28,300). A second laser pulse at 480 nm can selectively excite this chromophore and provide the necessary energy to move the electron from NI- to PI. These systems do not rely on electrochemical oxidation-reduction reactions at an electrode. Thus, switching occurs on a subpicosecond time scale. 

For oxidation of G in duplex DNA, Steenken concluded that the proton on N-1 of G shifts spontaneously to N-3 of the cytosine in the normal Watson-Crick base pair to generate [C+(H)/G ]. Consistent with this proposal, calculations indicate that charge transfer in oxidized DNA is coupled with proton transfer from G to Experiments carried out in D2O also reveal a kinetic isotope effect for G oxidation, implicating a concerted proton-coupled electron transfer mechanism. However, density functional theory (DFT) calculations in the gas phase predict that the structure with a proton on G N-1 [C/HG ] is more stable than [C (H)/G ] by 1.4kcal/mol. " ... 

The charge yield (shown on the plots 3 and 4) is related to the mean field that detunes the dot level from the resonance. The detuning also includes the frequency shifts due to the bonding interaction in the harmonic approximation near the potential minimum. The use of the Lennard-Jones potential could be made at longer distances, where the attraction to the surface is created by the van der Waals or Casimir-Polder potentials. This "spontaneous" interaction is relevant for the shuttle with a large amplitude of vibrations close to electrodes surfaces. 

Deviations from OGM were recognized early on spectroscopic properties of molecular crystals Davydov shifts and splittings of absorption bands in molecular crystals are clear deviations from OGM and were rationalized based on the excitonic model (EM) [10, 14, 15, 16, 17]. This same model proved extremely successful to describe the complex and technologically relevant spectroscopy of molecular aggregates, i.e. of clusters of molecules that spontaneously self-assemble in solution or in condensed phases [IS]. Much as it occurs in molecular crystals, due to intermolecular electrostatic interactions the local bound electron-hole pair created upon photoexcitation travels in the lattice and the corresponding wave function describes an extended delocalized object called an exciton. We explicitly remark that the Frenkel picture of the exciton, as a bound electron-hole pair, both residing on the same molecule, survives, or better is the basis for the excitonic picture. The delocalization of the exciton refers to the fact that the relevant wave function describes a Frenkel exciton (a bound e-h pair) that travels in the lattice, and this is of course possible even when electrons and/or holes are, separately, totally localized. In other terms, the EM describes localized charges, but delocalized excitations. 

Figure 10-1. Energy scheme of the relevant electronic levels that are affecting stimulated emission in an isolated dye molecule in solution (a) and in a conjugated polymer (b). In both systems absorption of a photon induces a transition from the singlet ground state Sq to a vibronically excited state within the Si-manifold. After vibronic relaxation, a red-shifted emission can be observed. This transition can occur in a spontaneous as well as in a stimulated manner. A photoinduced absorption may occur as competing processes in both systems (PIA) due to the population of the triplet state T. A pair of oppositely charged carriers (GP) can be generated in the conjugated polymer due to dissociation of the 5i-state. This can lead to additional photoinduced absorption bands that compete with the stimulated emission.

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