Intermolecular charge transfer

The examples we will use are TICT (twisted intermolecular charge transfer) amino-benzonitrile (ABN) compounds (Figs. 9.15 and 9.16). We have recently completed theoretical work on this class of compound and the reader is referred to reference 14 for a complete bibliography. 

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... 

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. 

As described most elegantly elsewhere in this volume, the halogen bond is an intermolecular, charge-transfer interaction between a Lewis base and an electron-deficient halogen. Other chapters that accompany this chart its use in, for example, supramolecular chemistry, molecular conductors and coordination chemistry. In this chapter, a much more recent application of halogen bonding is described, namely in the realisation of liquid-crystalline materials. 

These equations of motion can be integrated by many standard ensembles constant energy, constant volume, constant temperature and constant pressure. More complex forms of the extended Lagrangian are possible and readers are referred to Ref. [17] for a Lagrangian that allows intermolecular charge transfer. 

Table I lists a variety of organic nonlinear materials which have appeared in the literature their relative powder efficiencies, absorption cutoffs and /3 values (if available) are also provided. These materials are "typical" only in that they represent results from the few classes of organic compounds investigated to date, yet they are instructive in that one learns which molecular properties may be important. A few caveats are in order to avoid misinterpretation of the data in Table I. Except for compound 10 (19) all the powder efficiency and cutoff data are from our own measurements. Powder measurements were performed on ungraded samples using the Nd YAG output at 1.06/t as fundamental since powder efficiency is a function of particle size distribution and a variety of other factors (3) these values are only semiquantitative. The cutoff values are the wavelengths for which 10-4M solutions in ethanol (unless otherwise indicated) have no absorbance. The cutoff values will be similar to those found in crystal state except where intermolecular charge transfer is important in the crystal or the molecule is solvatochromic, this latter effect being quite common for cyanine dyes such as...

The monomer AH becomes slightly anionic and B becomes slightly cationic as a result of the nB - charge transfer Qci = Qb - Qah from Lewis base to Lewis acid ... 

Table 5.3. The NBO descriptors of binary B HA H-bonded complexes (see Fig. 5.1), showing net intermolecular charge transfer A0b a, ctah bond ionicity / ah. and PNBO overlap integrals for attractive ri j ctah ( Vncr.) and repulsive iib-ctah (5no ) interactions...

Consistently with (5.30b)-(5.30e), the intermolecular charge transfer A0b a (Table 5.3, first column) clearly correlates with the strength of nB— oah interactions (Fig. 5.1), the A b h binding energy (Table 5.1, first column), and the (inverse) Rb...h distance (Table 5.1, second column). Furthermore, the transferred... 

Figure 5.14 Correlation of net H-bond energy (A2shb, squares) and principal n-a stabilization energy (AEn a,(2), circles) with intermolecular charge transfer (Qcr) cf. Table 5.15. (Approximate trend-lines are shown for each quantity to aid visualization.)...

In summary, the covalent/ionic-resonance picture can be used to describe the entire range of neutral and charged H-bonding phenomena. The NRT resonance weights (wcov and / , ) and bond orders (6a—h and 6b...h) are correlated in the expected manner with bond lengths, IR frequencies, intermolecular charge transfer, and other properties. 

The examples of Section 5.2 can be summarized by saying that H-bonds are an unusually strong form of n—a donor-acceptor interaction exhibiting the expected strong cooperative effects associated with intermolecular charge transfer. 

Oxyhalide Complexes. (Ph4A)[Cr03X] (A = P or As and X = F or Cl) have been precipitated from a solution of the corresponding potassium salt in dilute HF or HCl and their unit cells, i.r., and electronic spectra reported.The interactions of Cr02Cl2 with aromatic hydrocarbons and fluorocarbons have been examined and the intermolecular charge-transfer transitions recorded. 

The electronic absorption spectrum of nickelocene has been recorded and analyzed in considerable detail by Scott and Becker.11 They found band maxima at 1920, 3075, and 6920 A and shoulders at 2700, 3450, 4400, and 5700 A. The band at 1920 A is believed to be an allowed transition which is designated as a W-F transition in ferrocene, but the intensity of this band is less than the N-V band in ferrocene. The shoulder at 2700 A is denoted as a N-Vor N-Q transition similar to the 2300 and 2600 A shoulders in ferrocene. The band at 3075 A is relatively intense and occurs at approximately the same wavelength as the intermolecular charge transfer band of ferrocene in carbon tetrachloride described by Brand and Snedden. The absorption spectrum of nickelocene in carbon tetrachloride shows no new bands other than those found in cyclohexane or ethanol. 

The degree of intermolecular charge transfer can be expected to be smaller in compound oc.5 than in compounds oc.2 and oc.4, because oc.5 does not have the three cyano substituents. The 2PA spectrum of oc.5 shows a broad band around 770 nm, with 3max = 3.8 x 10 GM [121]. Even if a direct comparison with the oc.2 series [119] is not possible because of the different nature of the conjugated linker and of the solvent used in the experiment, the cross section of oc.5 is significantly smaller than for oc.2 with the R2 branch (which is shorter than the branch in oc.5) and a dialkylamino substituent (W= 1.4x103 gm[119]). 

In addition to this influence of the molecular structure, which correlates with the number and the delocalization of -electrons, steric effects resulting from the internal molecular geometry and the nature of the substituents may play an important role in intermolecular charge transfer because of the packing of adjacent molecules in the solid 80>. For instance, the observation that the introduction... 

In some compounds the doping effect may be the result of charge-carrier generation brought about by an intermolecular charge-transfer transition 72,83)... 

The bonding of ions to metals is dominated by Coulomb attraction since there is a significant difference in electron affinity between the metals and ions. The bonding also involves a redistribution of charge through intermolecular charge transfer (between adsorbed ions and the surface) and intramolecular polarization (in ions and on the surface), which reduces the Pauli repulsion. 

As is shown in Fig. 8, the polymers containing excess neutral TCNQ are much more highly conductive. These compounds, which are soluble in amide solvents, are slightly yellow in color and have conductivities in the range 10-10 to 10-8 ohms-1 cm-1. Since the conductivity is so markedly dependent on the presence of excess TCNQ it is possible that intermolecular charge transfer is facilitated. However this will require studies of mobility rather than conductivity alone. 

Photoinduced electron transfer occurs through excitation of the 400-nm absorption bands of the donor chromophores based on the aminonapthalene-dicarboximide derivatives. The tails of the dopants absorption bands extend to at least 500 nm, which allows for the use of an Ar+ laser. Figure 8 illustrates the ground-state absorption spectra of the donor and acceptor for both the intramolecular and intermolecular charge transfer dopants in toluene. The spectra are similar for all of the dopants, with the exception of 2, which has a 50-nm red-shifted absorption band. The inset illustrates the broadened spectra in the liquid crystalline environment. The extinction coefficient at 457 nm varies from approximately 1000 M-1 cm-1 for 4, 2000 M-1 cm-1 for 1, 5000 M-1 cm-1 for 3, and 10,000 M-1 cm-1 for 2. 

Figure 9 Diffraction efficiency of photorefractive grating in the composite systems. Note that high diffraction efficiency for the composites containing the intramolecular charge transfer dopants 3 and 4 occurs at lower applied voltages than those for the intermolecular charge transfer dopants.

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