Excitation localization

This regioselective example of a di-w-methane photorearrangement is of particular importance because in this case both n moieties have very similar singlet energy levels. This suggests that excitation localization does not control the reaction. 

LAL LLE LN lateral accessory lobe long-lasting excitation local interneuron... 

Comparing the reactants and the products, the reactions are apparently nonredox processes. Using a spin-trapping EPR technique it was shown [114] that irradiation of the complexes leads to an alkyl radical formation (CH3 or C2Hj). The efficiency of the homolytic metal-carbon bond splitting depends on the electronic properties of the other axial ligand. The ostensibly non-redox photoinsertions are thus a product of two redox reactions. As far as the photoreactive excited state is concerned, the metal-carbon bond is either indirectly activated by a ir-nt excitation localized on the tetrapyrrole ring [112] or there is an... 

The localized excitation wave functions are the analogues of expressions (2), namely (5) for excitation localized on a host... 

To explain the idea of the method [13,14], let us consider a two-phonon decay of a highly excited local mode caused by the interaction /7irl( = QY.rf V3 A,, where V3 lI/ are the cubic anharmonicity interaction parameters, Q is the coordinate... 

Let us consider first the decay of a strongly excited local mode due to simultaneous emission of k > 2 phonons. The Hamiltonian of the system under consideration is... 

In general, it has been found that under the correct combination of electronic coupling, thermodynamic driving force, solvent and temperature, P-Q systems readily undergo photoinitiated electron transfer as shown in Fig. 2. Singlet excitation localizes on the porphyrin, which has the energetically lowest-lying... 

For arbitrary initial conditions, the solution of (4.27) may be derived from the knowledge of the response, at an arbitrary site m, to an excitation localized at t = 0 on an arbitrary site n (this is the Green s function of the system see Appendix A). As we are interested in spectral data, it is natural to use the Fourier transform of (4.27) corresponding to an initial condition of one excitation localized on site n> ... 

In the case of a metal substrate, the experimental evidence shows that metal excitation is dominated by surface photon absorption. Optical radiation excites surface charge carriers, usually free or sub-vacuum-level electrons that can efficiently couple to the adsorbate. This often leads to enhanced photolysis cross sections or altered product distributions. Excitation localized on the adsorbed molecule in close proximity to a metallic solid may efficiently couple to the electronic states of the surface, leading to excitation quenching. When light-absorbing molecules are separated from the surface by spacer molecules, the influence of the surface on molecular excitation and relaxation decreases [4,21],... 

From a theoretical point of view, a balanced description of the initially excited local tttt on one side of the barrier and the tttt CT state on the other side is a big challenge, even for the CASSCF and CASPT2 methods [29, 85], The potential curve including the barrier height predicted by the ROKS method [56] is remarkably close to the CC2 result of Ref. [87],... 

Scheme 3 After irradiation into the lowest Q bands calculated at 2.40 (lA ) and 2.42 eV (lA") (vs. experimental values of 2.8 eV (Qj band) and 2.30 eV (Qj, band)) and corresponding to tt —tt excitations localized on the porphyn cycle, the system wiU evolve on the bound part of the associated potentials (Figure 3).

Two separate experimental approaches, diffuse reflectance and photoluminescence spectroscopy, were then taken both led to similar results. The latter technique is the more sensitive, and well-resolved spectra can often be observed, but only when a radiative decay of the excited state occurs. The diffuse reflectance spectra are broader in scope but the absorption bands appear as shoulders. The reflectance spectra of alkaline earth oxides were examined by Zecchina et al. (77, 78), Garrone et al. (79), and Zecchina and Scarano (80), but an overpressure of a quenching gas (usually oxygen) had to be used to suppress the fluorescence and to allow observ ation of the reflectance absorption bands (Fig. 10). In addition to usual bands in the U V region due to bulk excitations (bulk cxcitons), new absorption bands which correspond to excitations localized on the surface ions are present. 

In the expansion 22 the expressions in parentheses represent zeroth-order excitations localized at the carbonyl or cyclopropane moieties, respectively. The term CT includes charge-transfer components and depends upon the extent of mesomeric interactions between the subunits. The sum in equation 22 is over all the HOMO - LUMO (3e, 4e ) transitions of cyclopropane (cf. Subsection III.A). Actually, of course, more excitations have to be included into expansion 22. In particular, expansion 22 does not include Rydberg excitations. 

Expression (2.30) follows from the initial approximation regarding excitation localization. According to this approximation in the limit when A - 0, the inelastic cross section coincides with the thermodynamic limit of... 

The importance of complex formation in singlet energy transfer was pointed out earlier by Hammond and Cooke (78) in the race-mlzatlon of the sulfoxide group in compound [23]. They proposed a mechanism Involving two excited states a singlet excited state with excitation localized on the naphtalene ring and an excited state complex with delocalized excitation. Only the former could be quenched and no emission from the second could be observed. 

These studies show that the effect of hydrogen bonding between a chromophore and its environment on the electronic excitations localized in the chromophore be accurately represented by means of the orbital-free embedding potential of the Eq. 53... 

Danilova, Y.E., Lepcshkin, N.N., Rautian, S.G., Safonov, V.P. Excitation localization and nonlinear optical processes in colloidal silver aggregates. Physica A 241, 231—235 (1997)... 

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