Local Excitation

As the result of localized excitation, an electron from the highest occupied orbital (HOMO) is promoted to the lowest unoccupied orbital (LUMO) in the same molecule [1,2,6], or on much rarer occasions, if the photon energy is sufficient, an electron can be ejected into the medium [23], Either process generates both a very strong reductant and a strong oxidant. The promoted (or ejected) electron can be easily transferred to a lower-laying LUMO of the other component (acceptor) or an electron from the other component (donor) can fill the vacancy created by the electron promotion (or ejection). Thus, in principle, the excited component can serve both as the donor and acceptor, and its ultimate role is determined by the electronic properties of the other component. 

The feasibility of electron transfer between the excited component and the ground state reagent is determined largely by thermodynamic considerations. As pointed out by Weller [24], the free energy for electron transfer can be estimated from Eq. 5  

It has to be noted that bimolecular forward electron transfer is the only situation where the presence of the inverted region has not been confirmed experimentally [26]. Several plausible explanations have been proposed to account for the 

The presence of the inverted region, however, has not yet been demonstrated [29], With the appropriate design (see below) some of these ions can be considered as a new type of an ion pair [42], a penetrated ion pair (PIP) in which one of the ions is buried inside the other. Research into PET in organized media is very active [40-41] and encompasses a variety of topics from semiconductors [43] and zeolites [44], through various photoconductive polymers [45] to PET-initiated polymerizations and depolymerizations [46] that are generally outside of the scope of this review. 

The non-diffusional methods of bringing D and A together may also result in payment of an ultimate price of diminished overall efficiency since BET within contact ion pairs is usually more rapid than in solvent separated ion pairs (see below). Indeed, the most important aspect of the forward electron transfer is that it presets the conditions for the competition between BET and the fragmentation reaction. The reactive intermediates (ion pairs) are generated in specific solvation and spin states. That state can be controlled or at least influenced by a selection of the excited state component, the ground state component and solvent [20], as well as by magnetic and electric fields [36]. 

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Kovalenko S A, Ernsting N P and Ruthmann J 1996 Femtosecond hole-burning spectroscopy of the dye DCM in solution the transition from the locally excited to a charge-transfer state Chem. Phys. Lett. 258 445-54... 

Reilly P D and Skinner J L 1993 Spectral diffusion of single molecule fluorescence a probe of low-frequency localized excitations in disordered crystals Phys. Rev. Lett. 71 4257-60... 

Route (1) is referred to as local excitation and route (2) as CTC excitation. It has been observed that the different routes bring about the polymerization of AN with different kinetic behaviors. A 365-nm light will irradiate the CTC only, and in this case the rate of polymerization for different aromatic tertiary amines descends in the following order ... 

It is in the same order as the equilibrium constants of CTC of amine-FN. That is, the stronger the ability of an amine to form CTC with electron acceptors, the faster the rate of pholopolymerization. However, under 313-nm irradiation, local excitation plays a principal role and the rate of polymerization is observed to descend in a different order [80] ... 

Local excitation was also studied for primary and secondary amines under irradiation at 313 nm. The results are summarized in Table 11. In order to estimate the photoinitiating efficiency of the amines, the measurement was performed at a chosen constant absorbance (0.40) of the reaction mixture. The rates of polymerization were found to be in the following order ... 

The LUMOs are it of alkenes and of ketenes. The LUMO-LUMO interaction occurs between the C=C bond of alkenes and the C=0 bond of ketenes, promoting the reaction across the C=0 bond of kentenes. The important pseudoex-cited configuration D A is the locally-excited nn configuration of alkenes. 

The delocalization of excessive a- (or P-) spins and the bond polarization can take place among radical orbitals, p and q, and the central n (or o) and n (or o ) orbitals, resulting in the electron transferred configurations (T) and locally excited configurations (E), respectively (Fig. 5a). The delocalization-polarization mechanisms are different between singlet and triplet states, as addressed in the following subsections. 

Imura, K. and Okamoto, H. (2008) Development of novel near-field microspectroscopy and imaging of local excitations and wave fimctions of nanomaterials. BuU. Chem. Soc.Jpn., 81, 659-675. 

When the electron coupling between locally excited-state (LE) and charge transfer state (CT) is weak, the electron transfer rate kcl can be expressed as (7)... 

From the excited-state conversion rates (ka and kd), it is possible to calculate the ratio of the quantum yields between emission from the twisted and LE states. The ratio of the steady-state quantum yields from the locally excited state (f>LE and the TICT state r/>TI, T for DMABN and other aminobenzonitrile derivatives is described in (1) [12],... 

Bosch LI, Mahon MF, James TD (2004) The B-N bond controls the balance between locally excited (LE) and twisted internal charge transfer (TICT) states observed for aniline based fluorescent saccharide sensors. Tetrahedron Lett 45(13) 2859-2862... 

Before reviewing existing examples, a very brief explanation on the mechanisms of decoherence for molecular spin qubits is necessary more details are available elsewhere [67]. Broadly speaking, the three decoherence sources for these systems are spin bath decoherence, oscillator bath decoherence and pairwise dipolar decoherence, and can be regulated by a combination of temperature, magnetic field and chemical design of the system [70]. The spin bath mainly consists of nuclear spins, but in general it also includes any localized excitations that can couple to the... 

Thermal or photochemical activation of the [D, A] pair leads to the contact-ion pair D+, A-, the fate of which is critical to the overall efficiency of donor/acceptor reactivity as described by the electron-transfer paradigm in Scheme 1 (equation 8). In photochemical reactions, the contact ion pair D+, A- is generated either via direct excitation of the ground-state [D, A] complex (i.e., CT path via irradiation of the charge-transfer (CT) absorption band in Scheme 13) or by diffusional collision of either the locally excited acceptor with the donor (A path) or the locally excited donor with the acceptor (D path). 

Coherent optical phonons can couple with localized excitations such as excitons and defect centers. For example, strong exciton-phonon coupling was demonstrated for lead phtalocyanine (PbPc) [79] and Cul [80] as an intense enhancement of the coherent phonon amplitude at the excitonic resonances. In alkali halides [81-83], nuclear wave-packets localized near F centers were observed as periodic modulations of the luminescence spectra. 

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