Charge-transfer absorption band acceptor

Various enol silyl ethers and quinones lead to the vividly colored [D, A] complexes described above and the electron-transfer activation within such a donor/acceptor pair can be achieved either via photoexcitation of charge-transfer absorption band (as described in the nitration of ESE with TNM) or via selective photoirradiation of either the separate donor or acceptor.41 (The difference arising in the ion-pair dynamics from varied modes of photoactivation of donor/acceptor pairs will be discussed in detail in a later section.) Thus, actinic irradiation with /.exc > 380 nm of a solution of chloranil and the prototypical cyclohexanone ESE leads to a mixture of cyclohexenone and/or an adduct depending on the reaction conditions summarized in Scheme 5. 

The scope of the Patemo-Buchi cycloaddition has been widely expanded for the oxetane synthesis from enone and quinone acceptors with a variety of olefins, stilbenes, acetylenes, etc. For example, an intense dark-red solution is obtained from an equimolar solution of tetrachlorobenzoquinone (CA) and stilbene owing to the spontaneous formation of 1 1 electron donor/acceptor complexes.55 A selective photoirradiation of either the charge-transfer absorption band of the [D, A] complex or the specific irradiation of the carbonyl acceptor (i.e., CA) leads to the formation of the same oxetane regioisomers in identical molar ratios56 (equation 27). 

The UV-vis spectral analysis confirms the appearance of a new charge-transfer absorption band of the complexes of colorless a-donors (R3MH) and the n-acceptor (TCNE). In accord with Mulliken theory, the absorption maxima (Act) of the [R3MH, TCNE] complexes shift toward blue with increasing ionization potential of the metal hydrides (i.e., tin > germanium > silicon) as listed in Table 8. 

Fig. 2 Direct relationship of the charge-transfer absorption bands of various arene-iodine complexes (ordinate) with those of the corresponding aromatic complexes with different acceptors (abscissa) as indicated, T,... 

Photoactivation of the bis(arene)iron(II) complexes with ferrocene and arene donors by the selective irradiation of the charge-transfer absorption bands as in (6) uniformly results in the de-ligation of the acceptor moiety... 

The nitrosonium cation bears a formal relationship to the well-studied halogens (i.e. X2 = I2, Br2, and Cl2), with both classes of structurally simple diatomic electron acceptors forming an extensive series of intermolecular electron donor-acceptor (EDA) complexes that show well-defined charge-transfer absorption bands in the UV-visible spectral region. Mulliken (1952a,b 1964 Mulliken and Person, 1969) originally identified the three possible nonbonded structures of the halogen complexes as in Chart 7, and the subsequent X-ray studies established the axial form II to be extant in the crystals of the benzene complexes with Cl2 and Br2 (Hassel and Stromme, 1958, 1959). In these 1 1 molecular complexes, the closest approach of the... 

Tetranitromethane produces strongly coloured electron donor-acceptor (EDA) complexes with derivatives of the anthracene213, in dichloromethane. Specific irradiation of the charge transfer absorption band at X > 500 nm produces a rapid fading of the colour of the solutions. From these solutions, adduct 91 is obtained (reaction 24) its structure is ascertained by X-ray crystallographic diffraction. 91 is derived from an anti-addition of fragments of tetranitromethane by a multistep pathway214. 

Bulk crystalline radical ion salts and electron donor-electron acceptor charge transfer complexes have been shown to have room temperature d.c. conductivities up to 500 Scm-1 [457, 720, 721]. Tetrathiafiilvalene (TTF), tetraselenoful-valene (TST), and bis-ethyldithiotetrathiafulvalene (BEDT-TTF) have been the most commonly used electron donors, while tetracyano p-quinodimethane (TCNQ) and nickel 4,5-dimercapto-l,3-dithiol-2-thione Ni(dmit)2 have been the most commonly utilized electron acceptors (see Table 8). Metallic behavior in charge transfer complexes is believed to originate in the facile electron movements in the partially filled bands and in the interaction of the electrons with the vibrations of the atomic lattice (phonons). Lowering the temperature causes fewer lattice vibrations and increases the intermolecular orbital overlap and, hence, the conductivity. The good correlation obtained between the position of the maximum of the charge transfer absorption band (proportional to... 

Fluorescence from the excited state complexes of t-1 and electron poor alkenes has been observed only with dimethylfuma-rate and fumaronitrile, both of which form weak ground state complexes with t-1 (76). Fluorescence of the same wavelength and lifetime is observed upon quenching of t or excitation in the charge-transfer absorption band of the complexes of t-1 with these acceptors. Some properties of these excited complexes and other exciplexes of t-1 are summarized in Table 7. Fluorescence maxima, like the absorption maxima, of related charge-transfer complexes, can be correlated with the donor ionization potentials (eq. 16). As shown in Fig. 3, the point for t-1 falls well below the line obtained by Shirota and co-workers (87) for the com-... 

Photochemical electron-transfer can be effected by irradiation of the charge-transfer absorption band of the electron donor-acceptor complex.15 Alternatively, photochemical electron-transfer may proceed by actinic activation of RH followed by quenching with A, or by the reverse sequence involving activation of A and quenching with RH. 

The existence of the "charge-transfer absorption band" at 300 nm in the UV region, bright color of the solutions and a number of other factors suggest formation of donor-acceptor complexes in the toluene solutions of C60. 

The existence of the charge-transfer absorption bands is characteristic of important electron donor-acceptor contributions to the contact ion pair that is the direct precursor in the formation of metal-metal dimers by the mutual annihilation of carbonylcobalt(I) cations and carbonylcobal-tate(—I) anions (79). The diverse results cannot be explained by any single process in which the metal-metal bond for the dimer is formed by the... 

Recently, a further example of photopolymerization of a donor-acceptor substituted monomer (methyl-2-octadecanamidopropenate) without any additive has been described [16]. The co (polyacrylates) containing electron-acceptor moieties as pendant groups are also photosensitive [17,18]. Their photosensitivity coincides with the charge-transfer absorption band peak and, therefore, such compounds possess a photoinduced memory effect. 

Charge-transfer (CT) transition An electronic transition in which a large fraction of an electronic charge is transferred from one region of a molecular entity, called the electron donor, to another, called the electron acceptor (intramolecular CT) or from one molecular entity to another (intermolecular CT). Typical for donor-acceptor complexes or multichromophoric molecular entities. In some cases the charge transfer absorption band may be obscured by the absorption of the partners. 

Synonyms for EPDjEPA complex are electron donor acceptor (EDA) complex [50], molecular complex [57, 58], and charge-transfer (CT) complex [51]. Since normally the term molecular complex is only used for weak complexes between neutral molecules, and the appearance of a charge-transfer absorption band does not necessarily prove the existence of a stable complex, the more general expression EPDjEPA complex, proposed by Gutmann [53], will be used here. This will comprise all complexes whose formation is due to an interaction between electron-pair donors (Lewis bases) and electron-pair acceptors (Lewis acids), irrespective of the stabilities of the complexes or the charges of the components. 

Whether the deligation process occurs spontaneously in a thermal reaction or requires photoactivation depends on the donors employed. For example, most EDA complexes of 6/s(arene)iron(II) acceptors with aromatic donors such as anthracene or durene or with ferrocene are thermally stable and can be isolated in crystalline form (see Figure 3) [124]. Photoactivation of these complexes in acetonitrile by the selective irradiation of their charge-transfer absorption bands uniformly results in the deligation of the acceptor moiety (Eq. 36). 

Similar photoinduced dimerizations and ligand substitutions in the presence of additives such as triphenylphosphine are observed with ion-pairs salts of Mn(CO)s and V(CO)6" with cobaltocenium or other cationic acceptors such as Ph2Cr", pyr-idinium, quinolinium, etc [118]. Most importantly, all photochemical transformations of the various carbonyl metallate salts are initiated by actinic light that solely excites the charge-transfer absorption bands of the contact ion pairs whereas local excitation of the separate ions is deliberately excluded. 

Photoinitiated SET has been used to drive a molecular machine and absorption and fluorescence spectroscopy have been used to monitor it. A 1 1 pseudoro-taxane forms spontaneously in solution as a consequence of the donor-acceptor interactions between the electron-rich naphthalene moiety of the thread (380) and the electron-deficient bipyridinium units of the cyclophane (381). The threading process is monitored by the appearance of a charge transfer absorption band and disappearance of the naphthalene fluorescence. Excited state SET from 9-anthracenecarboxylic acid (9-ACA) reduces a bipyridinium moiety of the cyclophane, lessening the extent of interaction between the thread and the cyclophane and dethreading occurs. On addition of oxygen the reduced cyclophane is reoxidised and threading reoccurs. ... 

One shortcoming of the donor-bridge-acceptor design illustrated in Figures 1 and 2 is that increasing the nonlinearity is correlated with a red shift of the charge-transfer absorption band (the so-called nonlinearity-transparency trade-off). 

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