Charge transfer transitions, donor-acceptor

The experimental challenge is determining the I2 and I2—Lewis base adduct concentrations these are often obtained via UV-Visible spectroscopy by examining charge transfer and donor-acceptor transitions (Section 6.4.2). 

Much of chemistry occurs in the condensed phase solution phase ET reactions have been a major focus for theory and experiment for the last 50 years. Experiments, and quantitative theories, have probed how reaction-free energy, solvent polarity, donor-acceptor distance, bridging stmctures, solvent relaxation, and vibronic coupling influence ET kinetics. Important connections have also been drawn between optical charge transfer transitions and thennal ET. 

As it concerns the band in the UV region (at 315 nm in the present case), Benesi and Hildebrand [5] assigned this absorption to a charge-transfer transition, where the phenyl ring acts as an electron donor (D) and the iodine as an electron acceptor. The interaction can be described in resonance terms as D-I2 <-> D+I2", the band being assigned to the transition from the ground non polar state to the excited polar state. 

During the last decade one paper concerning the use of ESCA on heterocyclic compounds has appeared. The observed effects are interpreted in terms of intramolecular charge-transfer transitions between a donor and an acceptor group of the molecule.41... 

Electronic absorption spectroscopy charge transfer transitions, 19 71 d-d transitions, 19 70, 71 flavocytochrome b, 36 269-271 intraligand transitions, 19 71-80 of organometallics, 19 69-80 Electronic coupling, between donor and acceptor wave functions, 41 278 Electronic nuclear double resonance spectroscopy, molybdenum center probes, 40 13... 

Charge transfer transitions are best discussed in the language of MO theory (Section 8.2). An L—> M transition involves the transfer of an electron from a nonbonding or weakly-bonding MO localised mainly on the ligand moieties L , to a vacancy in the partly-filled nd subsheli of M. Such a transition is likely to appear in the visible or near UV spectrum if the energy separation between the donor and acceptor orbitals is relatively small. This will be the case if ... 

Figure 7.17 Modes of Ti02 photosensitization (a) photosensitization with organic or inorganic chromophores chemisorbed onto titanium dioxide surface (b) formation of surface complexes exhibiting metal-to-band charge transfer transitions (MBCT) (c) bulk doping resulting in formation of acceptor or donor levels and (d) formation of composite semiconductors. A denotes the electron acceptor, D the electron donor...

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 simplest model consists of two centres, one donor (D) and one acceptor (A), separated by a distance I and contains two electrons. Here we consider this simple system to illustrate some general relations between charge transfer, transition intensities and linear as well as non-linear optical polarizabilities. We will show below that the electro-optic parameters and the molecular polarizabilities may be described in terms of a single parameter, c, that is a measure of the extent of coupling between donor and acceptor. Conceptually, this approach is related to early computations on the behaviour of inorganic intervalence complexes (Robin and Day, 1967 Denning, 1995), Mulliken s model for molecular CT complexes (Mulliken and Pearson, 1969) and a two-form/two-state analysis of push-pull molecules (Blanchard-Desce and Barzoukas, 1998). 

Its electronic transition is associated with an intramolecular charge-transfer between donor and acceptor group, producing an excited state with a dipole moment (1, ) appreciably different from that in the ground state p ). 

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