Fluorescent J-Aggregates and Their Biological Applications - Advanced Fluorescence Reporters in Chemistry and Biology - page 140

dependence of the Coulombic interaction energy on the solvent dielectric constant.
In highly polar solvents when the charged dye molecule and its counter ion are
surrounded by the solvate shells and separated one from another, the counter ions
do not participate in an aggregation process. Consequently the Coulombic repulsion
between the dye molecules hinders the aggregate formation. Thus, the higher the
dielectric constant, the weaker this repulsion, and hence, the stronger are the
aggregation processes. As the value of the dielectric constant of water is extremely
high (

81), the dye aggregation in water is the most intensive. Besides, the
hydrophobic interactions also occur in this solvent enhancing the aggregation
process. Addition of other organic solvents to the dye aqueous solution destroys
aggregates, this destruction being stronger for the solvents with lower dielectric
constant. On the other hand, increasing of the solvent ionic strength (i.e., addition of
salts which would dissociate into ions) results in the screening of the Coulombic
repulsion forces and thus promotes aggregation. At the same time, in a nonpolar
solvent (e.g., hexane), the charged dye molecules as well as their counter ions
cannot be solvated and separated from one another, and thus can be located closely.
As a result, the total charge of the dye molecule is zero, but an additional dipole
moment appears to be formed by the charged dye and its counter ion. As the
dielectric constant of the nonpolar solvent is low and thus electrostatic forces are
not weakened, the aggregates are formed by the dye - counter ion pairs because of
dipole-dipole and van der Waals interactions [ 19 ].
One of the most interesting points to be elucidated upon the study of an
aggregate is its geometry, i.e., the structure of the dye molecule packing within
an aggregate. First of all, if the absorption spectrum of an aggregate contains only
one band, then its either short-wavelength or long-wavelength shift as compared to
that of the monomer dye permits to decide about “card-pack” or “head-to-tail”
structure, respectively. In the case where several bands are present in the aggregate
absorption spectrum, the elementary cell of an aggregate contains several mole-
cules. Here, polarization measurements are useful. Particularly, linear dichroism
(LD) and fluorescence polarization study could reveal the angles between the dif-
ferent permitted absorption transitions in an aggregate [ 20 ], while circular dichro-
ism (CD) measurements point to the chirality of the aggregate structure [ 21 ]. At the
same time, nonzero LD and birefringence of the sample could also cause contribu-
tion to the CD spectrum measured with the polarization-modulation method
[ 22 ]. As both LD and birefringence values could be nonzero even in solution in
the case of large aggregates (the ones observed in [ 22 ] were 1-2 m m wide and
several tens of micrometer long), this possibility should be taken into account [ 22 ].
To study the size of the formed aggregate, several approaches could be used.
First of all, large aggregates could be observed directly in a microscope. For
example, an electron-microscope investigation of 12.5

e ¼

10 3

(that is rather
high concentration) aqueous solution of pseudoisocyanine (Fig. 3 ) allowed
observing the cylindrical aggregate structure with the length near 350 nm and
width about 2.3 nm, that corresponds to about 3,000 molecules [ 23 ]. Besides,
aggregate size could be estimated by the indirect methods, such as light scattering
or centrifugation [ 17 ].

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