Handbook of Solvents - George Wypych - ChemTech - Ventech!

10.3 Solvent effects based on pure solvent scales 593
concerned will be encountered); and (d) its first band becomes structured in non-polar solvents
(see Figure 1 in ref. 15).
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 -NMe 2 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) (9). 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 Figure 1 in ref. 15).
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 Figure 1
in ref. 15), 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 [Δv(solvent) = v FNF -
v DMANF] 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) =[Δv(solvent)-Δv(gas)]/[Δv(DMSO)-Δv(gas)] [10.3.9]
Table 10.3.1 gives the SPP values for a broad range of solvents, ranked in increasing
order of polarity. Data were all obtained from measurements made at our laboratory and
have largely been reported elsewhere 15,66-70 -some, however, are published here for the first
time.