Solvent effects on some optical and spectroscopic properties of aromatic compounds

Abstract

In this work we want of report some solvent effects observed during studies on benzenoid and heterocylic compounds by the Faraday effect and Depolarized Rayleigh Scattering (DRS), and their TCNE-complexes, by UV-visible spectrophotometry. The measured properties, respectively molecular magnetooptical rotation ([ϱ] M), molecular optical anisotropy (γ 2) and maximum absorption wavenumber ( − v max) undergo rather large changes when moving the gas phase to pure liquid or dissolved state, and vary weakly from a solvent to another one. As regards weak interactions, the Faraday effect allows essentially to observed internal field changes (Lorentz correction). It is likely that this bulk effect is not restricted to ‘aromatic’ solute. Moreover most solutesolvent interactions are masked by this non-specific medium effect, since even with polar solvents (CHCl 3, alcohols) no specific interaction given by π-donors and/or polar solute ( e.g. halobenzenes, anisole, …) is detected. So, for a given substance, the true value of [ϱ] M is obtained in the pure liquid state or in solvent as refractive as the solute. On the contrary, DRS, very sensitive to oriented interactions, can detect diatropism or paratropism in liquids. Therefore, to get comparable, ‘intrinsic’ γ 2 values one must now work with pure liquids but rather with dilute solutions in the same ‘inert’ solvent, that is a non-polar, weakly anisotropic one, containing bonds of the same kind as the solute. Accordingly, for organic compounds (H bonds), cyclohexane is preferred to CCl 4 (which may also give specific interactions). On the other hand the molar volume of the solvent need not be taken into account. The cases of benzene and some heterocyclics illustrate these principles. Finally, spectrophotometric studies of molecular complexes show that the influence of non-polar solvents on − v max generally reduces to a gross medium effect involving a linear function of (n 2 - 1)(2n 2 + 1). As regards polar solvents, more complex interactions with aromatic compounds and acceptor are likely to occur, and no correlation can be found with polarity parameters like Z or E T. TCNE-complexes of benzene, benzothiophene (after deconvolution of spectra) and dibenzothiophene are considered.