66
M.C. Rezende et al. / Spectrochimica Acta Part A 87 (2012) 61–66
dyes, cause hypsochromic shifts of their solvatochromic band. As
suggested in the introduction, inorganic salts, like hydroxylic
solvents, may be classified by a more general term, as electrophile-
bond donors, or EBD species.
On the contrary, tetraalkylammonium, and other organic
cations, affect the medium-dependent S0 → S1 transition energy
of these dyes in a different way. By forming an ion-pair with
the phenolate dye, they shield its charged oxygen atom from
the hydroxylic solvent, replacing its solvation shell by a more
hydrophobic microenvironment, provided by the alkyl chains of
the ammonium cation.
In agreement with the conclusions of the present paper, the
authors invoked nonspecific and specific effects (described in gen-
eral terms as a “specific solvation” of the dyes by Bu4NBr) to
rationalize the positive halochromism of the ET(30) and ET(8) dyes
present work, a more detailed view of this specific solvation was
developed, by simulating the dye–cation interactions. The forma-
tion of these complexes, postulated as a consequence of the formal
equilibria of the solvent-exchange model [25], acquired a more tan-
gible reality, being corroborated by their tendency to elicit the same
positive halochromism observed experimentally.
5. Conclusions
Acknowledgements
The positive halochromism of phenolate dyes 1, 2 and 4
in hydroxylic solvents, in the presence of tetraalkylammonium
cations was examined in a systematic way, by varying the sol-
vent, the organic cation and the dye structure. Positive halochromic
shifts increased with the HBD strength of the solvent and with
the hydrophobic nature of the tetraalkylammonium salt. Sterically
hindered phenolate dyes exhibited smaller shifts than their unhin-
dered analogues.
These trends were interpreted by means of nonspecific and spe-
cific effects. In the first case, the known reduction of the solvent
permittivity of aqueous solutions of tetraalkylammonium salts was
invoked to explain the positive halochromism of these dyes in the
presence of organic salts. A more detailed explanation for these
observations in water was based on the structure-forming ability
of these cations, and the formation of ice-like cages around the
large tetraalkylammonium ion, which had the effect of depleting
the solvating shell of the dye of water molecules.
Specific dye–cation interactions were also mimicked by means
of a simple model of a dye–cation pair separated by a variable dis-
tance dO–N. The calculated energy of the S0 → S1 transition of the
dye decreased with the increased value of dO–N−3, a parameter
assumed proportional to the cation concentration [5]. These the-
oretical results were thus in agreement with the observed positive
halochromism by these organic cations.
The interpretations based on nonspecific effects do not depart
from the conclusions based on specific dye–cation interactions.
Both approaches arrive at the same conclusion: addition of
hydrophobic tetraalkylammonium cations to solutions of pheno-
late dyes in hydroxylic solvents tends to deplete the solvating shell
of the dye of its solvent molecules. This depletion may be the result
of an increased order of the solvating hydroxylic molecules around
the large, hydrophobic cation or its ion-pairs; or it may be the result
of replacing the hydrophilic solvating shell of the phenolate dye by
alkyl chains.
This work was financed by Fondecyt project 1100022 and Coni-
cyt project 24090025. We are grateful to Conicyt for grants to M.D.
and A.A.
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