tetraphenylborate [14]. So, such hypothesis should be rejected because overlap
between donor and acceptor orbitals is impossible by use of HPA.
Changes occurring by formation of IAs of polymethine dyes with HPAs or other
bulky organic ions in absorption spectra as well as other effects are satisfactorily
interpreted via the molecular exciton coupling theory, i.e. coupling of transition
moments of the component dye molecules [15, 16]. The result of exciton splitting of
excited states in the composite molecule may be the appearance of strong spectral shifts
or splittings (which may be of the order of 2000 cm
-1
) of the absorption bands for the
constituent molecules. The dye molecule, according to exciton theory, is regarded as a
point dipole and the excitonic state of the dye aggregate splits into two levels through
the interaction of transition dipoles. The angle between the line of centers of a column
of dye molecules and the long axis of any one of the parallel molecules is called the
‘angle of slippage’, R. Large molecular slippage ( R < 32°) results in a bathochromic
shift (J-aggregate), and small slippage ( R > 32°) results in a hypsochromic shift (H-
aggregate).
Inorganic salts can have decisive influence on the formation of the aggregates. To
determine the mechanism of this influence, one should consider different processes
that could be initiated by inorganic salts in the solutions. One of them mentioned in
published paper [16] is increase of the effective dielectric constant of the solvent. It is
clear that formation of aggregates at such low concentrations of HPA (10
-8
mol L
-1
)
completely excludes this hypothesis because change in dielectric constant is negligible.
Other possible process is crystallization of polymethine dyes or salts caused by a
shift in the ionic balance in the solution. Indeed, it is well-known that the presence of
a common ion from another source decreases the solubility of a salt. HPAs can decrease
the solubility of polymethine dyes. The solubility of the IAs of HPAs with all the
studied polymethine dyes is so low that aggregation proceeds at any studied
concentration of the dye. At the same time aggregation of 3 10
-5
mol L
-1
of
pseudoisocyanine one of the most easily aggregated polymethine dye takes place if
only high concentration of such salts as NaJ (1.1 mol L
-1
), KBr (3.59 mol L
-1
), and
NaCl (4.95 mol L
-1
) is created in the solution [17]. Even the big concentration of such
large anion as ferrocyanide cannot induce the aggregation of Astra Phloxine while the
same dye forms aggregates in the presence of any studied concentration of different
HPAs.
Stability of the IAs formed between polymethine dyes and HPAs is significantly
higher than for triphenylmethane dyes. For the IA of Astra Phloxine with
12-MPC stability constant was evaluated as 10
20
. Hence, it is not necessary to use the
dye excess to secure complete formation of IA.
Other analytical effects occurring as a result of formation of IAs cationic dye
– heteropoly anion. The second found effect consists in quenching of the dye
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