Conjugate molecules, effect

The wavelength necessary to effect the tt — tt transition in a conjugated molecule depends on the energy gap between HOMO and LUMO, which in turn depends on the nature of the conjugated system. Thus, by measuring the UV spectrum of an unknown, we can derive structural information about the nature of any conjugated tt electron system present in a molecule. 

Molecules with donor and acceptor in trans- and a s-configurations give much higher nonlinearities, due to favorable linear donor-acceptor conjugation, than those with substituents at the geminal position, where only the weaker cross-conjugation is effective (see 89 vs 86 and 92, Fig. 8). 

The effects of metal-to-metal electronic coupling are observed for several complexes where a ferrocene moiety is bound to transition-metal complexes with a -conjugated chain, 68 (167), 69 (168), and 70 (169). Tertiary amine-ferrocene conjugated molecules, 71, show two-step le oxidation, and their monocationic forms exhibit strong LMCT bands at 600-700 nm (170). 

Based on the fundamental dipole moment concepts of mesomeric moment and interaction moment, models to explain the enhanced optical nonlinearities of polarized conjugated molecules have been devised. The equivalent internal field (EIF) model of Oudar and Chemla relates the j8 of a molecule to an equivalent electric field ER due to substituent R which biases the hyperpolarizabilities (28). In the case of donor-acceptor systems anomalously large nonlinearities result as a consequence of contributions from intramolecular charge-transfer interaction (related to /xjnt) and expressions to quantify this contribution have been obtained (29). Related treatments dealing with this problem have appeared one due to Levine and Bethea bearing directly on the EIF model (30), another due to Levine using spectroscopically derived substituent perturbations rather than dipole moment based data (31.) and yet another more empirical treatment by Dulcic and Sauteret involving reinforcement of substituent effects (32). 

Generally, the transparency of a molecular x electron system narrows with increasing conjugation (bathochromic effect) while its nonlinear efficiency increases. The urea molecule is a small conjugated molecule transparent up to 2000 A with a low (3 value 1.3 10-3 e.s.u. (10). For 4-nitro 4 -dimethylaminostilbene the... 

Crosstalk has been discussed fairly extensively, as one of a series of interference phenomena that can lead to a different kind of control of molecular transport than has been discussed in Sect. 7.4. It is also possible to observe intramolecular interference effects. For example, with cross-conjugated molecules [163] or benzene dithiol linked in the 1,3 (or meta) configurations [164-171], both are expected to show substantially reduced transport. 

In conjugated molecules one or other of the possible protonation sites may be more or less favoured by solvation effects and for this reason sites of protonation are often solvent dependent. In some instances, similar stability of two possible cations results in tautomeric equilibria and these too may be solvent dependent. Just as solute-solvent interactions have an effect on the relative stability of two possible cations formed from a conjugated molecule, so in solid salts stability relationships depend on the mode of packing of ions, which determines interactions with the nearest neighbours. Therefore the types of cation observed in solid salts are not necessarily the most stable ones in solution. 

It is important to emphasize that the presence of two or three basic centres of comparable basicity in a conjugated molecule may lead to protonation processes at all of them. These processes may occur at various rates, but if all the rates are fast, nmr spectra do not enable us to say what are the relative amounts of variously protonated species present at equilibrium. Increasing the acidity of the medium and lowering the temperature in order to observe the resonances of the captured proton may have the effect of shifting the tautomeric equilibria in favour of one or other of the protonation sites. Information on the position of tautomeric equihbria in protonation processes is thus not obt iinable from nmr spectra under the conditions of rapid proton exchange. 

The length of an alkyl chain in a conjugated molecule has also a significant effect on the solubility of the compound. For example, methyl acrylate is fully miscible with [BMIMJPF, but butyl acrylate is only partially miscible, simply because of the increased alkyl chain length of the ester (87). 

Another property of these equivalent orbitals is that they include, in themselves effects of delocalisation. Such effects are most important in conjugated molecules, although they arc present in all molecules to a greater or lesser extent. In a highly conjugated system such as benzene only limited amount of localisation can be achieved by transforming the orbitals. 

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