Charge-transfer donor-acceptor complexes

The solvent dependence of the conformational equilibria of trans-1,4-dimethoxycyclohexane and tra s,cA-l-Me-3,5-dimethoxycyclohexane was studied by low temperature ll NMR spectroscopy (90JCR(S)152) (cf. Table VI). The conformational equilibria of trans-1 -acceptor-2-donor-substituted cyclohexanes 10ee=10aa were studied with respect to intramolecular charge-transfer (donor-acceptor) complexes (CT) in the di-equatorial conformation (02JOC6938) the CT absorptions and the... 

C C CT tr,V tr,2 proportionality constant Coulomb charge charge-transfer (donor—acceptor) complex ktr/kp, apparent transfer constant apparent transfer constant of monomer 1 or monomer 2, respectively... 

The group of Meijer and Schenning has constructed ambipolar field-efifect transistors from imides-diaminotriazines H-bonded p-n dyad complexes 29 based on OPV4T in combination with PBI-2 [89]. The transistors show two independent pathways for charge transport. In contrast, processing of OPV and PBl that are not connected by H-bonds formed charge transfer donor-acceptor complexes. They showed no mobility in field-effect transistors, presumably due to an unfavorable supramolecular organization. 

Also, an alternating free-radical addition copolymerization of cyclohexene and formic acid, perhaps via charge transfer, donor-acceptor complexes, yields polyesters ... 

Wynne K, Galli C and Hochstrasser R M 1994 Ultrafast charge transfer in an electron donor-acceptor complex J. Cham. Phys. 100 4796-810... 

The donor-acceptor formation can be considered by transfer of electrons from the donor to the acceptor. In principle one can assume donor-acceptor interaction from A (donor) to B (acceptor) or alternatively, since B (A) has also occupied (unoccupied) orbitals, the opposite charge transfer, from B to A. Such a view refers to mutual electron transfer and has been commonly estabUshed for the analysis of charge transfer spectra of n-complexes [12]. A classical example for a donor-acceptor complex, 2, involving a cationic phosphorus species has been reported by Parry et al. [13]. It is considered that the triaminophosphines act as donor as well as an acceptor towards the phosphenium cation. While 2 refers to a P-donor, M-donors are in general more common, as for example amines, 3a, pyridines, 3b, or the very nucleophilic dimethylaminopyridine (DMAP) [ 14], 3c. It is even a strong donor towards phosphorus trichloride [15]. 

As we have seen from reaction 4.49 donor-acceptor complexes (Lewis- or 7r-type) occur in a fairly inert medium (such as cyclohexane) via charge transfer between a base (electron donor) and an acid (electron acceptor by its electron deficiency). In a few instances, e.g., in the Bonitz titration29 of the precatalyst diethylalaminium chloride with isoquinoline, the complex constists of an ion-pair ionizate. 

Mulliken [3] presented a classification of electron donor-acceptor complexes based on the extent of intermolecular charge transfer that accompanies complex formation. An outer complex is one in which the intermolecular interaction B- XY is weak and there is little intra- or intermolecular electric charge redistribution, while an inner complex is one in which there is extensive electric charge (electrons or nuclei) redistribution to give [BX] + - -Y . Inner complexes are presumably more strongly bound in general than outer complexes. 

The donor-acceptor complexes [Ir(/r-dmpz)(CO)(PPh2 0(CH2)2R )]2 exhibit photo-induced electron-transfer rate constants of 1012s—1 and charge recombination rates slower than 2 x 10los-1 when R = pyridine and 4-phenylpyridine.534 Further studies on these complexes revealed that recombination reactions were temperature dependent and slower for the deuterated acceptors.535... 

Molecules of this type are often called donor-acceptor complexes or sometimes charge transfer complexes (because charge is transferred from the donor to the acceptor as the nonbonding electron pair of the donor atom is shared with the acceptor atom). In other words, there is a formal transfer of one electron, which is evident in the formal charges on the atoms in the complex. Once formed, however, the bond is simply a covalent bond consisting of a pair of shared electrons, whose origin is irrelevant to the nature of the... 

Blackstock, S. C., J. P. Lorand, and J. K. Kochi. 1987. Charge Transfer Interactions of Amines with Tetrahalomethanes. X-ray Crystal Structures of the Donor-Acceptor Complexes of Quinuclidine and Diazabicyclo-[2.2.2]octane with Carbon Tetrabromide. J. Qrg. Chem. 52,1451. 

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). 

A theoretical formalism is available for understanding optical charge transfer processes in a variety of chemical systems (mixed-valence ions, donor-acceptor complexes, metal-ligand charge transfer chromophores, etc) where the extent of charge transfer is large and where electronic coupling between the electron donor and acceptor sites is relatively small. 

The Patterno-Buchi coupling of various stilbenes (S) with chloroanil (Q) to yield fran -oxetanes is achieved by the specific charge-transfer photo-activation of the electron donor-acceptor complexes (SQ). Time-resolved spectroscopy revealed the (singlet) ion-radical pair[S+% Q" ] to be the primary reaction intermediate and established the electron-transfer pathway for this Patterno-Buchi transformation. Carbonyl quinone activation leads to the same oxetane products with identical isomer ratios. Thus, an analogous mechanism is applied which includes an initial transfer quenching of the photo-activated (triplet) quinone acceptor by the stilbene donors resulting in triplet ion-radical pairs. ... 

Attack on the aromatic ring and formation a n-complex or electron donor-acceptor complex N02 + ArH —> ArH N02. This complex involves high electrostatic and charge-transfer interactions between the n-aromatics and nitroninm ion. 

For instance, Kochi and co-workers [89,90] reported the photochemical coupling of various stilbenes and chloranil by specific charge-transfer activation of the precursor donor-acceptor complex (EDA) to form rrans-oxetanes selectively. The primary reaction intermediate is the singlet radical ion pair as revealed by time-resolved spectroscopy and thus establishing the electron-transfer pathway for this typical Paterno-Biichi reaction. This radical ion pair either collapses to a 1,4-biradical species or yields the original EDA complex after back-electron transfer. Because the alternative cycloaddition via specific activation of the carbonyl compound yields the same oxetane regioisomers in identical molar ratios, it can be concluded that a common electron-transfer mechanism is applicable (Scheme 53) [89,90]. 

Photoinduced intramolecular electron transfer in the donor-acceptor complex 87 (R = H) generates transient charge-separated open-shell species with the remarkably long lifetime of about 75 ps [89]. Dyads that contain Tt-extended tetrathiafulvalene units also form stable cationic species upon oxidation [90]. The dumbbell shaped triad 91 [91-93] (Scheme 4.13) was obtained by carrying out the reaction with the in situ generated bis-diene at room temperature, in the dark and in o-dichlorobenzene as a solvent in 50% yield. The product is thermally unstable and easily undergoes a retro-Diels-Alder reaction [91]. 

This exciton diffuses to the donor/acceptor interface via an energy-transfer mechanism (i.e., no net transport of mass or charge occurs). (3) Charge-transfer quenching of the exciton at the D/A interface produces a charge- transfer (CT) state, in the form of a coulombically interacting donor/acceptor complex (D A ). The nomenclature used to describe this species has been relatively imprecise, and has... 

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