Handbook of Solvents - George Wypych - ChemTech - Ventech!

592 Javier Catalán
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)
(6) for this purpose. This compound
had previously been used by Lippert 63 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. This data, together
with the dipole moment for the first excited electronic state (23 D), which was obtained by
Czekalla et al. 64 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 Pillay 65 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 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, 7), 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 used to assess the polar properties of solvents. Thus, 15 (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