CT excitation

The charge transfer Scheme V is akin to the adiabatic electron transfer cycle in Scheme IV. In this case the work term Wp required to bring the products D+ and A together to the mean separation rp in the CT excited state is given by ... 

The photophysical properties of [Ru(TBP)(CO)(EtOH)], [Ru(TBP)(pyz)2], [Ru(TBP)(pyz)] (Fl2TBP = 5,10,15,20-tetra(3,5-tert-butyl-4-hydroxyphenyl)porphyrin) have been investigated by steady-state and time-resolved absorption and emission spectroscopies. The complexes are weakly luminescent, and the origins of this behavior is discussed.Transient Raman spectroscopic data have been reported for [Ru(TPP)(py)2], [Ru(TPP)(CO)(py), and [Ru(TPP)(pip)2] (pip = piperidine),and nanosecond time-resolved resonance Raman spectroscopy has been used to examine the CT excited states of [Ru(0EP)(py)2] and [Ru(TPP)(py)2]. " ... 

Figure 6 Typical pulse sequence used in experiments with a signal-enhancement scheme (which can involve DFS, RAPT or application of adiabatic pulses) placed before the CT excitation and detection, with formation of a spin-echo.

The CT excitation and the ensuing CR are described within the framework of the time-dependent perturbation theory in the electronic coupling V. The time dependent CR rate constant is [3] ... 

Just after CT excitation of the DACs, the TG spectrum consists of a band centred at 670 nm (see Figure 3) and due to the radical anion of PMDA. Its decay, which can be reasonably well reproduced with a monoexponential function, is due to CR to the neutral ground state. 

If the charge transfer (CT) complex is sufficiently strong, the ion-radical pair would dissociate to induce ionic and/or radical reactions. The mechanism of this photoexcitation is different from the n — n or n — n excitations. The later process is the excitation of isolated molecules whereas the CT excitation requires two molecules in contact. Surprisingly, rather limited attention has been directed to this field of photosensitized CT process from the viewpoint of organic reactions. 

CT excitation is more common in the photochemistry of coordination compounds. Besides the d — d transition (ligand field band), the CT band is often observed and the redox reaction between central metal ion and ligand is frequently induced by photoirradiation. This process has been used to initiate vinyl polymerization. 

The transition from CT excited state to triplet state has been shown for the TMPD-a-methyl naphthalene system. The triplet level of naphthalene is considerably lower than the CT level of this pair (20). 

The insight of photoinitiation is complicated. Even when CT absorption is observed, the initiation process may not start from a charge transferred state or form ion-radicals. An alternative mechanism is triplet excitation via charge transfer absorption. Namely, when the CT excited level is higher than the triplet level, a considerable amount of the CT excitation would be converted to the triplet state. The TMPD+-naphthalene pair fits in this case (20). Conversely, the contribution of CT might be predominant even when the CT interaction in the ground state is not observed. As shown in Eqs. (14) and (16), charge transfer interaction will not take part in photoexcitation but occurs in the excited state. Possible reaction mechanismus may be explained as follows. 

Figure 4.57 Reversal of n—n and CT excited states in 4-aminobenzophenone with change of solvent polarity...

In order to explain the stabilization of the CT excited state even in a nonpolar solvent, the orthogonal intramolecular charge-transfer (OICT) mechanism, which is a conceptually similar term to the TICT mechanism but is used to stress the orbital orthogonality rather than the twisting of the molecular frame, has been proposed. The OICT mechanism for the CT fluorescence of aryldisilanes in nonpolar solvents was supported by the remarkable dependence of the fluorescence spectra on the dihedral angle between a benzene pjr... 

In relation to the reactions of CT excited states of aryldisilanes, photochemistry of LE and ICT states of the rigid, p-cyano-substituted styryldisilane 7 was investigated by Steinmetz and coworkers154. Although no 1,3-silyl shift is observed in this sytem, 7 affords a major product attributable to addition of alcohol across the Si—Si bond in the CT state. The roles of LE and CT states in the formation of additional minor products of silylene extrusion and homolytic Si—Si cleavage have also been elucidated. 

Charge transfer (CT) excited states are characterized by a radial displacement of charge from one group or atom to another. In arylboranes (Figure 6.18), CT excited states are responsible for the compound s luminescence.50 The reverse solvatochro-mism of the fluorescence and the CT absorption bands appear to be related to an inversion of the dipole moment in the electronic transition, S(l S1 that is, from the... 

The energy of a vertical transition to a CT excited state, /(, (7 = LMCT, MLCT, or LLCT), is expressed by using the ionization potentials and electroafbnities of the... 

A further property associated with the radial displacement of charge associated with CT electronic transitions is a change in the dipolar moment of the molecule. If the electronic transition causes, for example, an increase in the dipolar moment, the energy of the CT excited state will decrease (other factors aside) with the polarity of the solvent. Therefore, the CT absorption bands will experience solvatochromic shifts of tens of nanometers. Related solvatochromic effects will be detected in the emission spectrum of CT excited states. While the solvatochromism of absorption bands is a tool for the assignment of CT transitions in the absorption spectrum of complexes, the rationalization of such effects in terms of the solvent properties, for example, the dielectric constant, is not always possible. 

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