Charge-transfer absorption band

A nearly universal feature of EDA complexation is the presence of new absorption bands in the electronic spectrum of the complex that are not found in the spectrum of uncomplexed donor or acceptor [137-140]. These spectral bands are observed even in cases where no other evidence of complexation exists, i.e., where Keda is too small to measure. The charge-transfer resonance theory of Mulliken [141] was originally formulated to account for these striking spectral features. According to Mulliken, the ground-state wave function for the complex can be formulated as 

The charge-transfer absorption band results from the promotion of an electron from the EDA ground state to the excited state. For weakly interacting donors and acceptors (in which a b), populating this excited state by irradiation of the CT band essentially promotes an electron from the donor orbital to one located on the acceptor — usually represented by the HOMO and LUMO, respectively. This transition effectively corresponds to a direct electron transfer without the intermediacy of a (local) excited state of either D or A. The energy of this transition, and thus the frequency (wavelength) of the absorption is given by 

Acceptor CP2M0H2 T KEDA (nm) (M-l) CP2WH2 (nm) keda (M-1) Cp2ReH T KedA (nm) M h  

The linearized relationship in equation 11 and 1 la is expected not to hold for strong CT complexes (owing to the enhanced magnitude of the bracketed term in equation 10).This conclusion has been verified for the [NO+,arene] series of strong complexes (vide supra), in which the principal charge-transfer band at 340 nm does not shift over a potential range of AEda = 1.0 V [130]. Further studies of the strong complexes are particularly desirable. 

The energy of the charge-transfer transition varies with solvent. For molecular complexes, the shift is minor and attributable to rather weak EDA 

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Solvatochromic pareuaeters, so called because they were Initially derived from solvent effects on UV/visible spectra, have been applied subsequently with success to a wide variety of solvent-dependent phenomena and have demonstrated good predictive ability. The B jo) scale of solvent polarity is based on the position of the intermolecular charge transfer absorption band of Reichardt s betaine dye [506]. Et(io> values are available for over 200 common solvents and have been used by Dorsey and co-%rarkers to study solvent interactions in reversed-phase liquid chromatography (section 4.5.4) [305,306]. For hydrogen-bonding solvents the... 

As examples. Table 8 records some observations on d—d and charge transfer absorption bands in metal/protein systems. The examination of the spectrum of cobalt carbonic anhydrase (d—d) and of iron conalbumin (charge-transfer) permitted a prediction of the ligands from the protein to the metal. The predictions have now been substantiated by other methods. 

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. 13 Charge-transfer absorption bands from dichloromethane solutions containing Os04 and various (a) benzene, (b) naphthalene, and (c) anthracene donors (as indicated) showing the progressive bathochromic shift with aromatic donor strength. Reproduced with permission from Ref. 96b.

The electron-transfer paradigm in Scheme 1 (equation 8) is subject to direct experimental verification. Thus, the deliberate photoactivation of the preequilibrium EDA complex via irradiation of the charge-transfer absorption band (/ vCT) generates the ion-radical pair, in accord with Mulliken theory (equation 98). 

Mulliken,6 the energetics of the latter (generated in the course of the photoirradiation of the charge-transfer absorption band) is given by... 

Of the various anthracenedione isomers, only the 9,10-compound is used for the synthesis of dyes it is usually referred to simply as anthraquinone (6.1). The parent compound is pale yellow in colour, having a weak absorption band in the visible region (n—>tt transition). The presence of one or more electron-donating substituents leads to significant bathochromic effects so that relatively simple derivatives are of commercial importance as dyes. The colour of such compounds, which usually contain amino or hydroxy groups, can be attributed to the existence of a charge-transfer absorption band [1]. 

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,... 

Fig. 4 Charge-transfer absorption bands of the EDA complexes of tropylium cation with various donors (A) benzenes, (B) naphthalenes and (C) anthracenes.

Fig. 5 (A) Typical time-resolved picosecond absorption spectrum following the charge-transfer excitation of tropylium EDA complexes with arenes (anthracene-9-carbaldehyde) showing the bleaching (negative absorbance) of the charge-transfer absorption band and the growth of the aromatic cation radical. (B) Temporal evolution of ArH+- monitored at Amax. The inset shows the first-order plot of the ion...

Fig. 6 (A) The charge-transfer absorption band (- --) obtained as a difference...

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. 

Laser flash photolysis at wavelengths within the charge-transfer absorption bands of 2,2,6,6-tetramethylpiperidine-./V-oxyl (TEMPO) and carbon tetrachloride yields theoxoam-monium chloride of TEMPO 291 (Xmax = 460 nm) and the trichloromethyl radical in an essentially instantaneous 18 ps) process152. The primary photochemical reaction is an electron transfer from TEMPO to carbon tetrachloride followed by immediate decomposition of the carbon tetrachloride anion radical to chloride and trichloromethyl radical (equation 140). The laser flash photolysis of TEMPO and of other nitroxides in a variety of halogenated solvents have confirmed the generality of these photoreactions152. 

In most complexes, the charge transfer band is well separated from the MC band, so that it is easy to obtain excited states of different nature by exciting with radiation of suitable wavelengths. Co(phen)] and Co(bpy) [146] do not present charge transfer absorption bands (LMCT or MLCT) in the visible but only a weak MC band and n-n transitions in the UV. 

As shown In Figure 5, azide bound to a single copper gives rise to one relatively Intense charge transfer absorption band. In analogy to peroxide, this transition originates from the level the... 

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... 

LB films prepared from trimethylocta-decylphosphonium TCNQ, transferred to substrates, and exposed to iodine vapor FTIR Lateral d.c. conductivity was estimated (from energy of the maximum of the charge transfer absorption band) to be about 20-50 Scm-1 731... 

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