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Solvent polarity: the SPP scale 8

A suitable probe for assessing solvent polarity should meet various requirements including the following: (a) the modulus of its dipole moment should increase markedly but its orientation remain unaltered in response to electronic excitation: (b) it should undergo no structural changes by effect of electronic excitation or the nature of the solvent; (c) its basicity or acidity should change as little as possible upon electronic excitation so that any changes will be negligible compared to those caused by polarity; (d) the spectral envelope of the electronic transition band used should not change with the nature of the solvent; and (e) its molecular structure should facilitate the construction of a homomorph allowing one to offset spurious contributions to the measurements arising from the causes cited in (b) to (d) above .

A comprehensive analysis of the literature on the choice of probes for constructing polarity scales led our group to consider the molecular structure of 2-amino-7-nitrofluorene (ANF) for this purpose. This compound had previously been used by Lippert16 to define his well-known equation, which relates the Stokes shift of the chromophore with the change in its dipole moment on passing from the ground state to the excited state. He concluded that, in the first excited state, the dipole moment increased by 18 D on the 5.8 D value in the ground state. These data, together with the dipole moment for the first excited electronic state (23 D), which was obtained by Czekalla et al.17 from electric dichroism measurements, allow one to conclude that the direction of the dipole moment changes very little upon electronic excitation of this chromophore. Baliah and Pillay18 analyzed the dipole moments of a series of fluorene derivatives at positions 2 and 7, and concluded that both positions were strongly resonant and hence than an electron-releasing substituent at one and an electron-withdrawing substituent at the other would adopt coplanar positions relative to the fluorene skeleton. In summary, a change by 18 D in dipole moment of a system of these structural features reflects a substantial charge transfer from the donor group at position 2 to the acceptor group at 7 upon electronic excitation.

Although ANF seemingly fulfills requirements (a) and (b) above, it appears not to meet requirement (c) (i.e. that its acidity and basicity should not change upon electronic excitation). In fact, electronic excitation will induce a charge transfer from the amino group, so the protons in it will increase in acidity and the transition will be contaminated with specific contributions arising from solvent basicity. In order to avoid this contribution, one may in principle replace the amino group with a dimethylamino group (DMANF), which will exhibit appropriate charge transfer with no significant change in its negligible acidity. The increase in basicity of the nitro group upon electronic excitation (a result of charge transfer from the N,N-dimethyl group), should result in little contamination as this group is scarcely basic and its basicity is bound to hardly change with the amount of charge transferred from the N,N-dimethylamino group at position 2 to the fluorene structure.

The analysis of the absorption spectra for DMANF in a broad range of solvents suggests that this probe possesses several interesting spectroscopic properties as regards its first absorption band, which is that used to assess the polar properties of solvents. Thus,8 (a) its first absorption band is well resolved from the other electronic bands (an increase in solvent polarity results in no overlap with the other electronic bands in the UV–Vis spectrum for this probe); (b) the position of this band is highly sensitive to solvent polarity and is bathochromically shifted with increase in it (the bathochromic shift in the absorption maximum between perfluorohexane and DMSO is 4130 cm–1); (c) in less polar solvents, where the band appears at lower wavelengths, it is observed at ca. 376 nm (i.e. shifted to the visible region to an extent ensuring that no problems derived from the cut-off of the solvent concerned will be encountered); and (d) its first band becomes structured in non-polar solvents (see Fig. 1, from ref. 8).

In summary, DMANF is a firm candidate for use as a solvent dipolarity/polarizability probe since its absorption is extremely sensitive to changes in the nature of the solvent, largely as a result of the marked increase in its dipole moment on passing from the electronic ground state to the first excited state. In addition, the change does not affect the dipole moment direction, which is of great interest if the compound is to be used as a probe. Because it possesses a large, rigid aromatic structure, DMANF is highly polarizable; consequently, its first electronic transition occurs at energies where no appreciable interferences with the cut-offs of ordinary solvents are to be expected.

However, the change in structure of the first absorption band for DMANF in passing from non-polar solvents to polar solvents and the potential contaminating effect of solvent acidity on the position of this band entails introducing a homomorph for the probe in order to offset the detrimental effects of these factors on the estimation of solvent polarities.

The homomorph to be used should essentially possess the same structure as the probe, viz. a nitro group at position 7 ensuring the occurrence of the same type of interaction with the solvents and an electron-releasing group at position 2 ensuring similar, through weaker, interactions with the nitro function at 7 in order to obtain a lower dipole moment relative to DMANF). The most suitable replacement for the –NMe2 function in this context is a fluorine atom, which poses no structural problems and is inert to solvents. The homomorph chosen was thus 2-fluoro-7-nitrofluorene (FNF). The analysis of the absorption spectra for FNF in a broad range of solvents clearly revealed that its first absorption band behaves identically with that for DMANF (its structure changes in passing from non-polar solvents to polar ones). However, the bathochromic shift in this band with increase in solvent polarity is much smaller than that in DMANF (see Fig.1 from ref. 8).

Obviously, the difference between the solvatochromism of DMANF and FNF will cancel many of the spurious effects involved in measurements of solvent polarity. Because the envelopes of the first absorption bands for FNF and DMANF are identical (see Fig. 1 from ref. 8), one of the most common sources of error in polarity scales is thus avoided. The polarity of a solvent on the SPP scale is given by the difference between the solvatochromism of the probe DMANF and its homomorph FNF [ (solvent) = FNFDMANF] and can be evaluated on a fixed scale from 0 for the gas phase (i.e. the absence of solvent) to 1 for DMSO, using the following equation:

SPP(solvent) = [(solvent) - (gas)] / [(DMSO) - (gas)]

( 1 )

A brief analysis of these SPP data allows one to draw several interesting conclusions from structural effects on solvent polarity, namely:

(a) Cyclohexane is used as the non-polar reference in many scales. In the SPP scale, the polarity gap between cyclohexane and the gas phase (0.557 SPP units) is as wide as that between cyclohexane and the highest polarity (0.443 SPP units).

(b) Alkanes span a wide range of SPP values (e.g. 0.214 for perfluoro-n-hexane, 0.479 for 2-methylbutane and 0.601 for decalin).

(c) Unsaturation increases polarity in alcohols. Thus, the SPP values for n-propanol, allyl alcohol and propargyl alcohol are 0.847, 0.875 and 0.915, respectively.



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This web site is part of the work carried out in our laboratory by the following group of scientists: C. Díaz, L. Barrio, P. Pérez, V. López, J. L. G. de Paz, F. García-Blanco y J. Catalán

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Última actualización lunes, 1 de julio de 2002