Donor Franck-Condon excited state

Fig. 10. Excitation of a donor (or acceptor) takes place as a vertical transition generating a Franck-Condon (FC) state with a different electron configuration but same nuclear geometry as the ground state. The Franck-Condon state undergoes equilibration to a thermalized excited state. In PET, the nuclei of the thermalized excited state (in this case, an electron donor), acceptor, and surrounding molecules undergo a reorganization to the geometry of the transition state. Electron transfer takes place rapidly within the transition state without any appreciable nuclear motion, generating a Franck-Condon-like radical ion pair. The excited radical ion pair subsequently undergoes equilibration to a thermalized ion pair...

These data can be explained in terms of the high stabilization energy resulting from solvation of the excited t state. High p values in these cases indicate that the polar solvent-solute intermolecular stabilization ofthe zwitterionic excited t state is very sensitive to intramolecular substituent effects. In contrast, there is no dependence of Stokes shifts on a-constants in cyclohexane, which is nonpolar aprotic solvent, where the vibrational relaxation ofthe Franck-Condon state plays a primary role in stabilizing the excited state. This implies that the vibrational relaxation is not sensitive to intramolecular donor-acceptor interactions. The observations showed that the ultrafast intra- and intermolecular electronic polarization plays a major role in determining the position of the Franck-Condon zwitterionic state and its sensitivity to the relaxation of polar-substituted stilbenes. 

Evaluation of the Work Term from Charge Transfer Spectral Data. The intermolecular interaction leading to the precursor complex in Scheme IV is reminiscent of the electron donor-acceptor or EDA complexes formed between electron donors and acceptors (21). The latter is characterized by the presence of a new absorption band in the electronic spectrum. According to the Mulliken charge transfer (CT) theory for weak EDA complexes, the absorption maximum hv rp corresponds to the vertical (Franck-Condon) transition from the neutral ground state to the polar excited state (22). 

According to the Franck-Condon principle, the photoexcitation triggers a vertical transition to the excited state, which is followed by a rapid nuclear equilibration. Without donor excitation, the electron transfer process would be highly endothermic. However, after exciting the donor, electron transfer occurs at the crossing of the equilibrated excited state surface and the product state. 

Significant electronic coupling V between donor and bridge as well as between bridge and acceptor, represented by the matrix element (x u y), must exist. In other words, the transition between the initial (neutral), intermediate (locally excited) and final (charge separated) state must be Franck-Condon allowed... 

Electron transfer reactions and spectroscopic charge-transfer transitions have been extensively studied, and it has been shown that both processes can be described with a similar theoretical formalism. The activation energy of the thermal process and the transition energy of the optical process are each determined by two factors one due to the difference in electron affinity of the donor and acceptor sites, and the other arising from the fact that the electronically excited state is a nonequilibrium state with respect to atomic motion (P ranck Condon principle). Theories of electron transfer have been concerned with predicting the magnitude of the Franck-Condon barrier but, in the field of thermal electron transfer kinetics, direct comparisons between theory and experimental data have been possible only to a limited extent. One difficulty is that in kinetic studies it is generally difficult to separate the electron transfer process from the complex formation... 

The CT complexes are characterized by a new absorption band which is usually red-shifted as compared to local excitation bands [47-49], According to the Mulliken formulation the CT-exdtation corresponds to an electronic transition from the HOMO of the donor to the LUMO of the acceptor, i.e. it accomplishes full electron transfer [47], The transition is instantaneous, producing two intermediates (ions) in a direct contact but in a non-equilibrium, Franck-Condon state. The relaxation of the pair competes with BET, diminishing the quantum yield for ion generation [49], This process is believed to take... 

The photophysical processes of semiconductor nanoclusters are discussed in this section. The absorption of a photon by a semiconductor cluster creates an electron-hole pair bounded by Coulomb interaction, generally referred to as an exciton. The peak of the exciton emission band should overlap with the peak of the absorption band, that is, the Franck-Condon shift should be small or absent. The exciton can decay either nonradiatively or radiative-ly. The excitation can also be trapped by various impurities states (Figure 10). If the impurity atom replaces one of the constituent atoms of the crystal and provides the crystal with additional electrons, then the impurity is a donor. If the impurity atom provides less electrons than the atom it replaces, it is an acceptor. When the impurity is lodged in an interstitial position, it acts as a donor. A missing atom in the crystal results in a vacancy which deprives the crystal of electrons and makes the vacancy an acceptor. In a nanocluster, there may be intrinsic surface states which can act as either donors or acceptors. Radiative transitions can occur from these impurity states, as shown in Figure 10. The spectral position of the defect-related emission band usually shows significant red-shift from the exciton absorption band. 

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