Weak CT complexes

The formation of a detectable quantity of weak CT complexes usually requires a large excess of one component, or high concentrations of both components. Such a situation may lead to mechanistic complications. Thus, 1 2 complexes may form in the ground state [54], or the photogenerated ion pairs may be rapidly (subnanosecond or nanosecond time scale) intercepted to form triplexes [55] or ionic dimers [56]. It may be expected that the chemical behavior of these aggregates will crudely resemble the reactivity of normal ion pairs. 

In crystals which are composed of donors and acceptors, one can distinguish two limiting cases weak CT complexes and strong CT complexes or radical-ion salts. The boundary between the two groups is fuzzy. 

In this chapter, we will not discuss the weak CT complexes further. In these complexes, at least in the ground state, the charge transfer S is not large. They in general crystallise in mixed, alternating stacks. We have already mentioned the example systems anthracene-tetracyanobenzene or A-TCNB (cf Fig. 1.6) and anthracene-pyromellitic acid-dianydride or A-PMDA (cf Fig. 6.14). 

High-temperature polyimides are used as an excellent electrical insulator in microelectronic devices. This is due to the fact that the CTCs formed in PI films are classified into a category of weak CT complex, and therefore, have practically no contribution to the charge-separated structure at the ground state. Fainshtein et al. [116] showed that the increases in pressured and temperature cause a decrease in the electrical resistance in the dark for Pis derived from a fixed diamine (ODA) with PMDA, BTDA, and 3,3 4,4 -diphenylsulphonetetracarboxylic dianhydride. These results were explained in terms of an electron conductance mechanism based on interchain CTC formation. 

Evaluation of the Work Term from Charge Transfer Spectral Data. The intermolecular interaction leading to the precursor complex in Scheme IV is reminiscent of the electron donor-acceptor or EDA complexes formed between electron donors and acceptors (21). The latter is characterized by the presence of a new absorption band in the electronic spectrum. According to the Mulliken charge transfer (CT) theory for weak EDA complexes, the absorption maximum hv rp corresponds to the vertical (Franck-Condon) transition from the neutral ground state to the polar excited state (22). 

The term charge tranter refers to a succession of interactions between two molecules, ranging from very weak donor-acceptor dipolar interactions to interactions that result in the formation of an ion pair, depending on the extent of electron delocalization. Charge transfer (CT) complexes are formed between electron-rich donor molecules and electron-deficient acceptors. Typically, donor molecules are p-electron-rich heterocycles (e.g., furan, pyrrole, thiophene), aromatics with electron-donating substiments, or compounds... 

Nitrobenzene (NB) is such a weak acceptor that it is not capable of initiating polymerization of VCZ in the dark. Nitrobenzene has been used as a solvent for polymerization of VCZ in several instances. However, from a spectroscopic study of VCZ-NB mixture, the formation of a CT complex is confirmed (33). Although no new absorption peak is observed when VCZ and NB are mixed in benzene solution, a shift of absorption of NB is clearly detected. This CT complex is the contact charge transfer type as shown in Fig. 4. 

Interaction of 1-vinylindole with halogenated organic electron acceptors, even such weak acceptors as alkyl halides, results in the formation of polymeric CT complexes (28) of varying composition, all of which contain paramagnetic centers and display resistivities in the insulator range (74IZV1837). 

As is apparent from Eq. (7), the energy of the electronic transition in CT complexes is determined as the sum of the two terms, the difference in the energy of the diabatic states (Ada) and the electronic coupling element (HDa). Depending on their relative values, there can be two limiting cases represented by weak complexes with Hda Ada and strong complexes with Ada HDA. 

Consideration of the spectral and thermodynamic properties of arene CT complexes thus indicates that they are reasonably described within the framework of recent developments of the Mulliken formalism, in the case of both weak and strong complexes in the highly en-dergonic and nearly isergonic regions. Accordingly, let us now consider the structural consequences attendant upon charge transfer from the donor to the acceptor in such complexes. 

Charge transfer (CT) complexes are kept together by rather weak forces, and it is not to be expected that such forces should influence their electrochemical behaviour significantly. Thus, the CT complex between tetracyanoethylene and hexamethylbenzene has its halfwave potential for reduction shifted 0-039 V towards a more negative potential as compared to tetracyanoethylene itself (Peover, 1967) as is predictable from theoretical considerations of the formation of the CT complexes. 

The absorption IR spectra of the organic conductors, of both ion-radical salts and charge-transfer (CT) complexes, created by a given electron acceptor with various donors, share plenty of characteristic features. In addition to rather narrow and weak bands characteristic of the donor D and acceptor A molecules, a few novel absorption bands appear. They are polarized in the plane perpendicular to that of D and A molecules, broad and very intensive. The presence of such unusually polarized bands can be accounted for by the activation of totally symmetric donor or acceptor vibrations resulting from e-mv coupling. Typical polarized reflection spectra of the triethylammonium (TEA) (TCNQ)2 salt for three light polarizations [18] are shown in Fig. 1. It is fascinating that the reflectivity for... 

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. 

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