only on the basis of the smallest intermolecular distance but also on the basis of
spectral overlap criterion. Assuming that the distances in a macrosystem are statistically the same for all the dye components involved, then the spectral properties are
the most important criterion.
There are several examples of systems with layered nanoparticles as hosts of at
least three different types of dyes exhibiting multi-step FRET. Blue-emitting neutral
poly(vinylcarbazole),
tris-(8-hydroxy-quinoline)
aluminum,
tris
[2-(4,6-difluorophenyl) pyridinato-C 2,N] iridium (III), and 4-(dicyano-methylene)-2-methyl-6-(4-dimethylamino-styryl)-4H-pyran were chosen as fluorophores
with blue, green, and orange emission, respectively [84]. The fluorophores were
assembled in the films of LDH applying an LbL assembly method and efficiently
interacted as ED and EA in FRET. Two-dimensional FRET was achieved with
enhanced orange fluorescence from the final member of the cascade. The efficiency
of FRET was significantly affected by the presence of different organic solvents
[84]. The phenomenon of controlled multi-step energy transfer can lead to novel
materials with efficient light absorption over a broad spectral range. One such
material is Sap nanolayers with three adsorbed cationic dyes, one fluorine derivative,
and two porphyrins. The material exhibited efficient absorption over a broad range of
wavelengths [96]. FRET in more than one step was also observed for the dyes
pyrene, acriflavine, and RB [95]. Pyrene and RB played the role of ED and EA,
respectively, and acriflavine molecules mediated and increased the efficiency of the
transfer. The presence of Lap significantly increased the FRET efficiency [95]. The
two-step FRET process was observed for films of Sap and three laser dyes [35]. A
FRET process was confirmed by the results of steady-state and time-resolved
fluorescence spectroscopies. The depolarization of light determined by fluorescence
anisotropy helped to identify the most efficient steps of FRET [35]. Complex
systems based on the films of Sap with embedded six dyes were also constructed,
and multi-step FRET proceeding at a very low concentration of the dyes was
characterized [44].
4.2.9 Energy Transfer in Solids and Assembled Films
In addition to the variability of the individual components that make up the hybrid
systems, there is a relatively broad spectrum of material types. Besides conventional
colloidal systems and solid powders or films that can be prepared from colloidal
precursors, there are some special types of solid hybrid materials [7, 12, 24, 97]. The
most frequent examples of solid materials are transparent thin films that can be
prepared by various procedures (Fig. 11). Very thin films can be made by depositing
LbL assemblies using the charge of individual components in the deposition process
to prepare alternating assemblies of the layers. Several films of this type exhibited
efficient FRET [29, 39, 40, 64, 78, 83, 98, 99]. The films of LB type which represent
monomolecular assemblies can be prepared by combining nanoparticles and surfactant molecules. Luminescent LB films can be prepared by applying either the
surfactants with luminescent groups or incorporating common dye molecules into
Resonance Energy Transfer in Hybrid Systems of Photoactive Dye Molecules and. . .
233
spectral overlap criterion. Assuming that the distances in a macrosystem are statistically the same for all the dye components involved, then the spectral properties are
the most important criterion.
There are several examples of systems with layered nanoparticles as hosts of at
least three different types of dyes exhibiting multi-step FRET. Blue-emitting neutral
poly(vinylcarbazole),
tris-(8-hydroxy-quinoline)
aluminum,
tris
[2-(4,6-difluorophenyl) pyridinato-C 2,N] iridium (III), and 4-(dicyano-methylene)-2-methyl-6-(4-dimethylamino-styryl)-4H-pyran were chosen as fluorophores
with blue, green, and orange emission, respectively [84]. The fluorophores were
assembled in the films of LDH applying an LbL assembly method and efficiently
interacted as ED and EA in FRET. Two-dimensional FRET was achieved with
enhanced orange fluorescence from the final member of the cascade. The efficiency
of FRET was significantly affected by the presence of different organic solvents
[84]. The phenomenon of controlled multi-step energy transfer can lead to novel
materials with efficient light absorption over a broad spectral range. One such
material is Sap nanolayers with three adsorbed cationic dyes, one fluorine derivative,
and two porphyrins. The material exhibited efficient absorption over a broad range of
wavelengths [96]. FRET in more than one step was also observed for the dyes
pyrene, acriflavine, and RB [95]. Pyrene and RB played the role of ED and EA,
respectively, and acriflavine molecules mediated and increased the efficiency of the
transfer. The presence of Lap significantly increased the FRET efficiency [95]. The
two-step FRET process was observed for films of Sap and three laser dyes [35]. A
FRET process was confirmed by the results of steady-state and time-resolved
fluorescence spectroscopies. The depolarization of light determined by fluorescence
anisotropy helped to identify the most efficient steps of FRET [35]. Complex
systems based on the films of Sap with embedded six dyes were also constructed,
and multi-step FRET proceeding at a very low concentration of the dyes was
characterized [44].
4.2.9 Energy Transfer in Solids and Assembled Films
In addition to the variability of the individual components that make up the hybrid
systems, there is a relatively broad spectrum of material types. Besides conventional
colloidal systems and solid powders or films that can be prepared from colloidal
precursors, there are some special types of solid hybrid materials [7, 12, 24, 97]. The
most frequent examples of solid materials are transparent thin films that can be
prepared by various procedures (Fig. 11). Very thin films can be made by depositing
LbL assemblies using the charge of individual components in the deposition process
to prepare alternating assemblies of the layers. Several films of this type exhibited
efficient FRET [29, 39, 40, 64, 78, 83, 98, 99]. The films of LB type which represent
monomolecular assemblies can be prepared by combining nanoparticles and surfactant molecules. Luminescent LB films can be prepared by applying either the
surfactants with luminescent groups or incorporating common dye molecules into
Resonance Energy Transfer in Hybrid Systems of Photoactive Dye Molecules and. . .
233