molecules over time. Within a few seconds, mixed systems with a random distribution of both types of dyes emerged [11]. On the other hand, the segregation of dye
molecules has been experimentally proven for some specific systems. Such segregation could be promoted when the dye molecules are structurally very different
species. Examples are an ED and EA pair based on a polymer and a compound of
much smaller molecular size [59, 60]. In this case, the segregation has been proven
and led to an increase in molecular aggregation. Segregation can also relate to the
strong adsorption of dye molecules, which prevents their migration and redistribution. For example, some porphyrins can migrate onto the surface of individual clay
mineral particles, but cannot move from one particle to another [36]. The segregation
of the molecules can be greatly influenced by an appropriate interaction between the
dye molecules and the particle surface. If the adsorption of the dye molecules is
dominated by electrostatic forces, the distribution of charged groups in the dye
molecules should match sites with an opposite charge on the particles. Appropriate
parameters would lead to the optimal adsorption and distribution of the molecules
(random distribution). A model that describes the rules for achieving these optimal
conditions has been elaborated in detail [34, 36, 38, 56, 61–63]. However, in some
cases, nanoparticles may exhibit a heterogeneous charge distribution, which may
lead to the distribution of qualitatively different properties of a portion of the
particles with respect to dye adsorption. If ED and EA are dyes with significantly
different properties, some molecules can be selectively adsorbed onto part of the
particles and another dye on another part, which is, in fact, the segregation of the
molecules. One example is a system based on porphyrin and viologen molecules on
the surface of a synthetic Sap [53]. Although porphyrin was effectively quenched at
high concentrations of viologen molecules, some of the porphyrin fluorescence
remained to indicate the presence of the segregated phases of the molecules. To
reduce molecular segregation, the proper selection of components and preliminary
tests by FRET seem to be the best strategies. However, an encapsulation of dye
molecules in cavitands can also play a significant role and may improve the
properties of the system in this respect [52].
The distances between the molecules can be controlled not only by the number of
dye molecules with respect to the surface of particles but also by incorporating
another component to dilute the concentration of the adsorbed dye molecules. In
some cases, organic surfactants, which are inactive in terms of photophysics, were
used as a part of the mixture with dye molecules. This helped to reduce the
concentration of the dye molecules and prevented their aggregation. An example
is multilayer hybrid films containing amphiphilic cationic complexes of Ir
III
adsorbed onto synthetic Sap [42]. The distance between the two Ir
III complexes,
representing the emitters of blue and red light, was controlled by the presence of
stearylammonium cations. Another strategy was adopted for LbL films [29]. The
efficiency of FRET was strongly influenced by the change in the distance between
the interacting dye molecules, which was altered by the insertion of one or multiple
photo-inactive layers of poly(styrene sulfonic acid) between the layers containing
the photoactive dyes [29]. A similar strategy was applied for the LbL assemblies
based on laser dyes and Sap particles [64].
224
J. Bujdák
molecules has been experimentally proven for some specific systems. Such segregation could be promoted when the dye molecules are structurally very different
species. Examples are an ED and EA pair based on a polymer and a compound of
much smaller molecular size [59, 60]. In this case, the segregation has been proven
and led to an increase in molecular aggregation. Segregation can also relate to the
strong adsorption of dye molecules, which prevents their migration and redistribution. For example, some porphyrins can migrate onto the surface of individual clay
mineral particles, but cannot move from one particle to another [36]. The segregation
of the molecules can be greatly influenced by an appropriate interaction between the
dye molecules and the particle surface. If the adsorption of the dye molecules is
dominated by electrostatic forces, the distribution of charged groups in the dye
molecules should match sites with an opposite charge on the particles. Appropriate
parameters would lead to the optimal adsorption and distribution of the molecules
(random distribution). A model that describes the rules for achieving these optimal
conditions has been elaborated in detail [34, 36, 38, 56, 61–63]. However, in some
cases, nanoparticles may exhibit a heterogeneous charge distribution, which may
lead to the distribution of qualitatively different properties of a portion of the
particles with respect to dye adsorption. If ED and EA are dyes with significantly
different properties, some molecules can be selectively adsorbed onto part of the
particles and another dye on another part, which is, in fact, the segregation of the
molecules. One example is a system based on porphyrin and viologen molecules on
the surface of a synthetic Sap [53]. Although porphyrin was effectively quenched at
high concentrations of viologen molecules, some of the porphyrin fluorescence
remained to indicate the presence of the segregated phases of the molecules. To
reduce molecular segregation, the proper selection of components and preliminary
tests by FRET seem to be the best strategies. However, an encapsulation of dye
molecules in cavitands can also play a significant role and may improve the
properties of the system in this respect [52].
The distances between the molecules can be controlled not only by the number of
dye molecules with respect to the surface of particles but also by incorporating
another component to dilute the concentration of the adsorbed dye molecules. In
some cases, organic surfactants, which are inactive in terms of photophysics, were
used as a part of the mixture with dye molecules. This helped to reduce the
concentration of the dye molecules and prevented their aggregation. An example
is multilayer hybrid films containing amphiphilic cationic complexes of Ir
III
adsorbed onto synthetic Sap [42]. The distance between the two Ir
III complexes,
representing the emitters of blue and red light, was controlled by the presence of
stearylammonium cations. Another strategy was adopted for LbL films [29]. The
efficiency of FRET was strongly influenced by the change in the distance between
the interacting dye molecules, which was altered by the insertion of one or multiple
photo-inactive layers of poly(styrene sulfonic acid) between the layers containing
the photoactive dyes [29]. A similar strategy was applied for the LbL assemblies
based on laser dyes and Sap particles [64].
224
J. Bujdák