photochromic dyes in FRET can be applied for special systems that enable switching
between two forms, thus modulating FRET by the two optical forms of either ED or
EA molecules. Another example is the ring-open and ring-closed form of rhodamine
dyes controlled by environmental conditions such as the polarity of the solvent. The
open-ring forms have high molar absorption coefficients in the vis range, whereas
those with the closed ring do not exhibit significant absorption in this region. In
addition, these two forms are significantly different in terms of fluorescence and
photoactivity. There are several examples of systems including layered silicates
exhibiting the phenomena of modulating FRET by changing the structure of
interacting dye molecules. For example, using light-sensitive merocyanine as part
of the FRET system, UV light can be used to switch off the energy transfer in the
system [78]. The process was fully reversible, and the switching on could be
performed using vis light irradiation. The switching on/off was not a fast process,
taking seconds to minutes [78].
In many cases, the pH of the system can significantly affect the properties of dye
molecules participating in FRET. Acidic forms significantly change their properties
with respect to the energies of absorbed and emitted light, but also in terms of
fluorescence efficiency and the value of molar absorption coefficients. One such
example is xanthene dyes [101]. Another example of FRET modulated by pH is the
sensitive response of fluorescein to the pH of the medium [47]. The spectral changes
of fluorescein sensitively influenced the overlap between its emission spectrum and
the absorption spectrum of R6G, as the EA. The electrostatic interaction between the
anionic and the cationic dye also led to an efficient FRET in mixed solutions, most
likely due to the formation of ionic pairs. However, the presence of Lap contributed
to the increase in FRET efficiency. These systems could be used as a pH sensor over
a relatively broad range [47].
In addition to pH, other conditions can also affect FRET. For example,
solvatochromic properties can be used to influence not only the spectral properties
of the dyes but also the efficiency of FRET [56]. Fluorophores can change their
properties due to their interaction with layered hosts, which can also be applied in
FRET. For example, poly(styrene) chains carrying fluorescent terfluorene side
groups significantly changed their spectral properties when intercalated between
silicate layers [102]. The hybrid material exhibited deep-blue electroluminescence
mainly thanks to FRET taking place between terfluorene and fluorophore groups
occurring in different conformations (Fig. 12). The excitons of terfluorene groups in
the bulk phase transferred energy to the intercalated polymer chains. The emission
occurred mainly from the planar fluorenyl moieties intercalated between the silicate
layers, which exhibited enhanced quantum efficiency [102].
4.2.11 Influence of Electrical Properties of Hosts on FRET
The optical properties of the system and the efficiency of FRET can be influenced by
the electronic properties of the layered particles. This is essential whether the layered
particles has insulating, semiconductive, or electrically conductive properties, or if
Resonance Energy Transfer in Hybrid Systems of Photoactive Dye Molecules and. . .
235
between two forms, thus modulating FRET by the two optical forms of either ED or
EA molecules. Another example is the ring-open and ring-closed form of rhodamine
dyes controlled by environmental conditions such as the polarity of the solvent. The
open-ring forms have high molar absorption coefficients in the vis range, whereas
those with the closed ring do not exhibit significant absorption in this region. In
addition, these two forms are significantly different in terms of fluorescence and
photoactivity. There are several examples of systems including layered silicates
exhibiting the phenomena of modulating FRET by changing the structure of
interacting dye molecules. For example, using light-sensitive merocyanine as part
of the FRET system, UV light can be used to switch off the energy transfer in the
system [78]. The process was fully reversible, and the switching on could be
performed using vis light irradiation. The switching on/off was not a fast process,
taking seconds to minutes [78].
In many cases, the pH of the system can significantly affect the properties of dye
molecules participating in FRET. Acidic forms significantly change their properties
with respect to the energies of absorbed and emitted light, but also in terms of
fluorescence efficiency and the value of molar absorption coefficients. One such
example is xanthene dyes [101]. Another example of FRET modulated by pH is the
sensitive response of fluorescein to the pH of the medium [47]. The spectral changes
of fluorescein sensitively influenced the overlap between its emission spectrum and
the absorption spectrum of R6G, as the EA. The electrostatic interaction between the
anionic and the cationic dye also led to an efficient FRET in mixed solutions, most
likely due to the formation of ionic pairs. However, the presence of Lap contributed
to the increase in FRET efficiency. These systems could be used as a pH sensor over
a relatively broad range [47].
In addition to pH, other conditions can also affect FRET. For example,
solvatochromic properties can be used to influence not only the spectral properties
of the dyes but also the efficiency of FRET [56]. Fluorophores can change their
properties due to their interaction with layered hosts, which can also be applied in
FRET. For example, poly(styrene) chains carrying fluorescent terfluorene side
groups significantly changed their spectral properties when intercalated between
silicate layers [102]. The hybrid material exhibited deep-blue electroluminescence
mainly thanks to FRET taking place between terfluorene and fluorophore groups
occurring in different conformations (Fig. 12). The excitons of terfluorene groups in
the bulk phase transferred energy to the intercalated polymer chains. The emission
occurred mainly from the planar fluorenyl moieties intercalated between the silicate
layers, which exhibited enhanced quantum efficiency [102].
4.2.11 Influence of Electrical Properties of Hosts on FRET
The optical properties of the system and the efficiency of FRET can be influenced by
the electronic properties of the layered particles. This is essential whether the layered
particles has insulating, semiconductive, or electrically conductive properties, or if
Resonance Energy Transfer in Hybrid Systems of Photoactive Dye Molecules and. . .
235