Handbook of Solvents - George Wypych - ChemTech - Ventech!

602 Javier Catalán
to be able to characterize the acidity of its environment. In addition, its basicity should increase
upon electronic excitation such that its electronic transitions will be sensitive to the
acidity of the medium. This behavior will result in a bathochromic shift in the absorption
band the magnitude of which will increase with increasing acidity of the environment. The
probe should also be free of potential conformational changes that might influence the electronic
transition to be evaluated. Finally, its molecular structure should be easily converted
into a homomorph lacking the basic site without any side effects potentially affecting the resulting
solvent acidity.
Our group has shown 24,75 that the extremely strong negative solvatochromism of the
chromophore stilbazolium betaine dye (11), about 6500 cm -1 , is not a result of a change in
the non-specific effect of the solvent but rather of change in acidity. This was confirmed 75
by a study of the solvatochromic effect of a derivative of (11), o,o�-di-tert-butylstilbazolium
betaine dye (DTBSB) (12) in the same series of solvents. In this compound, the basic site in
the betaine dye (its oxygen atom) is protected on both sides by bulky tert-butyl groups; the
specific effect of the solvent is hindered and the compound exhibits only a small
solvatochromic effect that can be ascribed to non-specific solvent effects. 75
Since the carbonyl group of stilbazolium betaine possesses two lone pairs in the plane
of the quinoid ring, one can have two (11), one (TBSB, 13) or no channels (DTBSB) to approach
hydrogen bond-donor solvents, depending on the number of o-tert-butyl groups
present in the molecular structure of the (11) derivative concerned. Our group 24 has also
shown that, for at least 20 alkanols, the wavenumber difference between the maximum of
the first absorption bands for (11) and TBSB, and for TBSB and DTBSB, is virtually identical.
These results show that an oxygen lone pair in (11) is basic enough, as a result of hydrogen
bonding, for (11) to be used as an acidity probe in UV-Vis spectroscopy; thus, the
absorption maximum for TBSB shifts by about 1000 cm -1 from 1-decanol to ethanol, 24
whereas that for DTBSB shifts by only 300 cm -1 . This sensitivity to acidity, and the structural
likeness of the probe and its homomorph -which must endow them with a similar sensitivity
to solvent basicity and dipolarity/polarizability-, suggest that the two compounds
make an appropriate probe-homomorph couple for developing a pure solvent acidity scale.
For this purpose, the first visible absorption band for these compounds exhibits quite an in-