Charge-transfer complexes from tetracyanoethylene

Most of the information presently available has been obtained with a view to comparing selenophene and tellurophene with furan and thiophene, and has already been discussed in Chapter 3.01. The resulting ionization potentials are in good agreement with those deduced from the spectra of charge transfer complexes with tetracyanoethylene (82CS(20)214, 75JCS(F1)2045). 

Cyclisation of 1-hydroxy-2-napt ho ic acid in dry toluene in the presence of phosphorous pentoxide as a dehydrating agent gives a small yield of a cyclic tetramer, termed tetra-l-napthoid (7.34). Host 7.34 forms a number of 1 2 clathrates with small molecules such as CHC13 and benzene, which are very unstable with respect to desolvation. Complexes of 2-bromobutyric acid and napthalene are much more stable. A charge transfer complex with tetracyanoethylene (TCNE) is also known in which the electron-poor TCNE accepts electron density from the electron-rich aryl rings of the macrocycle. 

All of the conformers of the tetra-n-propyl ether of 4 form 1 1 charge-transfer complexes with tetracyanoethylene (TCNE), but in varying degree the strength of the complex diminishes from = 280 to / assoc = 30 in the order ... 

The viscosity dependence of emission from the charge-transfer complexes of tetracyanoethylene with benzene and a number of alkylbenzenes has been studied. At high viscosities the fluorescence spectra of alkylbenzenes have a double-band character and the two sub-bands change in different ways with changing viscosity. These observations and the dependence of the excitation spectrum on observation wavelength are discussed in terms of different orientational isomers of the ground-state charge-transfer complexes.1 7... 

M.p. 296 C. Accepts an electron from suitable donors forming a radical anion. Used for colorimetric determination of free radical precursors, replacement of Mn02 in aluminium solid electrolytic capacitors, construction of heat-sensitive resistors and ion-specific electrodes and for inducing radical polymerizations. The charge transfer complexes it forms with certain donors behave electrically like metals with anisotropic conductivity. Like tetracyanoethylene it belongs to a class of compounds called rr-acids. tetracyclines An important group of antibiotics isolated from Streptomyces spp., having structures based on a naphthacene skeleton. Tetracycline, the parent compound, has the structure ... 

The dipole moment varies according to the solvent it is ca 5.14 x 10 ° Cm (ca 1.55 D) when pure and ca 6.0 x 10 ° Cm (ca 1.8 D) in a nonpolar solvent, such as benzene or cyclohexane (14,15). In solvents to which it can hydrogen bond, the dipole moment may be much higher. The dipole is directed toward the ring from a positive nitrogen atom, whereas the saturated nonaromatic analogue pyrroHdine [123-75-1] has a dipole moment of 5.24 X 10 ° C-m (1.57 D) and is oppositely directed. Pyrrole and its alkyl derivatives are TT-electron rich and form colored charge-transfer complexes with acceptor molecules, eg, iodine and tetracyanoethylene (16). 

Figure 5.50 The optimized structure (a) and leading 7i

Adams and Cherry (78) have investigated the effects of stilbene substitution on the behavior of their excited complexes with fumaronitrile and find that the rate constants for fluorescence and nonradiative decay are insensitive to substitution, but that the rate constant for intersystem crossing is increased by electron-donating substituents (lower stilbene oxidation potential). This trend is attributed to a decrease in the energy gap between the excited complex and locally excited 3t (Fig. 4). The observed energy gap dependence of the exciplex lifetime could also account for the absence of fluorescence (or cycloadduct formation, see Section IV-B) from the excited charge-transfer complexes of t-1 with stronger electron acceptors such as maleic anhydride (76) or tetracyanoethylene (85). 

Although the two benzene rings in 56 are linked by almost single bonds, there is ample evidence to demonstrate that they are not independent and that substantial delocalization is present over the entire jr-system. Thus, the electronic spectrum has two sets of hands at 235-260 and 330—370 nm, significantly different from biphenyl which has only one absorption band at 250 nm. A charge-transfer complex of 56 with tetracyanoethylene, presumably of structure 57, was shown to be more stable than the corresponding fluorene complex73. The infra-red absorptions for... 

Figure 4.14 Energy for the maximum absorbance for charge transfer complexes of s-trinitrobenzene, tetracyanoethylene, and chloranil with various donors plotted against the adiabatic ionization potential of the donor. Recent ionization potentials from the NIST tables were used. The vertical displacement results from the differences in the Ea of the molecules. The calculated curves were obtained by using a two-parameter nonlinear least squares. The values of the constants are given in Table 4.5, where they are compared with published values. Data from [8, 30, 32].

Figure 4.15 Electron affinities of charge transfer complex acceptors calculated from C2 = 2.9 versus the current best adiabatic electron affinities. This is a precision and accuracy plot. The zero intercept slope indicates that the same quantities are measured. The compounds are maleic anhydride, tetrachlorophthalic anhydride, benzoquinone, trinitro-flourenone, s-trinitrobenzene, chloranil, tetracyanoquinodimethane, and tetracyanoethylene in order of their electron affinities.

Explain why the oscillator strengths of charge-transfer transitions in EDA complexes are low and why the absorption maximum of the charge-transfer band of tetracyanoethylene complexes with methylated benzenes is red shifted upon increasing methyl substitution (from vmax = 2.6 pm 1 for benzene to = 1.9 pm 1 for hexamethylbenzene), 343... 

Tetracyanoquinodimethane (TCNQ) and tetracyanoethylene (TCNE) are known to be strong electron acceptors, and the investigation of their properties is of interest since a number of stable charge transfer complex orystals with high electrical conductivities can be formed from them using a variety of electron donors. 

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