[90–92]. In specific cases, the changes in the orientation led to the formation of
photoactive molecular aggregates, resulting in a phenomenon called aggregationinduced emission [93]. Systems based on organic dyes and layered nanoparticles are
mostly composed of a mixture of various types of molecular assemblies coexisting
together with non-aggregated dye molecules [5]. Although a parallel alignment
between interacting transition dipole moments is the most suitable for FRET to
occur, this phenomenon proceeds at a lower extent with variable configurations
[23]. A typical example is the energy transfer from monomers and H-dimers to
J-aggregates; the result of such an interaction can be monitored by the red-shifted
fluorescence [23]. The energy transfer produces a partial depolarization of the
polarized light used for the excitation, but a complete depolarization is rarely
achieved [35, 44].
It has been concluded that the orientation is individually related to dye structure
and in particular to the interactions that occur between molecules and particles
[92]. The topography of a particle surface can also influence the molecular orientation that results from the structure of the particle surface. Different orientations can
occur in colloidal systems and in solids, even if the components of the materials are
the same. It is not simple to generalize the relatively different behaviors of different
types of organic dyes depending on the properties of both of the components. In rare
cases, a perpendicular orientation is preferred. If the dye has more ionic groups, these
cause electrostatic interactions with the particle surface supporting a parallel orientation [43, 94]. Significant effects of various solvents have also been observed
which, due to the interactions between the individual components, could significantly influence or possibly alter the orientation of dye molecules by changing the
vapor pressure of the solvent [43, 56]. The phenomenon was investigated for
porphyrin dyes whose molecules were adsorbed onto the basal surfaces of
phyllosilicate particles. When changing the composition of the vapors surrounding
this hybrid material, the angle of the orientation of the adsorbed dye molecules
changed in some cases. It has been assumed that the change in orientation resulted in
significant changes in the efficiency of FRET between different porphyrin molecules. In this way, the energy transfer can be changed by an external stimulus, which
is caused by the change of molecular orientation. This technique could be useful in
constructing photofunctional switches or sensors [43].
4.2.8 Cascade Energy Transfer
Cascade FRET or multi-step FRET can be defined as a non-radiative process of the
transfer of excitation energy that occurs between more than one pair of ED and EA
on the scale of intermolecular distances <10 nm. By its nature, multi-step FRET is
more efficient than the one-step process, which is limited to only a pair of the first
and end member of the cascade. It is claimed that a multi-step process can be realized
beyond the distance conventionally achieved for single-step processes [95]. If there
are several molecules of different dyes with different photophysical properties in the
vicinity of an excited ED molecule, then a selective transfer of energy takes place not
232
J. Bujdák
photoactive molecular aggregates, resulting in a phenomenon called aggregationinduced emission [93]. Systems based on organic dyes and layered nanoparticles are
mostly composed of a mixture of various types of molecular assemblies coexisting
together with non-aggregated dye molecules [5]. Although a parallel alignment
between interacting transition dipole moments is the most suitable for FRET to
occur, this phenomenon proceeds at a lower extent with variable configurations
[23]. A typical example is the energy transfer from monomers and H-dimers to
J-aggregates; the result of such an interaction can be monitored by the red-shifted
fluorescence [23]. The energy transfer produces a partial depolarization of the
polarized light used for the excitation, but a complete depolarization is rarely
achieved [35, 44].
It has been concluded that the orientation is individually related to dye structure
and in particular to the interactions that occur between molecules and particles
[92]. The topography of a particle surface can also influence the molecular orientation that results from the structure of the particle surface. Different orientations can
occur in colloidal systems and in solids, even if the components of the materials are
the same. It is not simple to generalize the relatively different behaviors of different
types of organic dyes depending on the properties of both of the components. In rare
cases, a perpendicular orientation is preferred. If the dye has more ionic groups, these
cause electrostatic interactions with the particle surface supporting a parallel orientation [43, 94]. Significant effects of various solvents have also been observed
which, due to the interactions between the individual components, could significantly influence or possibly alter the orientation of dye molecules by changing the
vapor pressure of the solvent [43, 56]. The phenomenon was investigated for
porphyrin dyes whose molecules were adsorbed onto the basal surfaces of
phyllosilicate particles. When changing the composition of the vapors surrounding
this hybrid material, the angle of the orientation of the adsorbed dye molecules
changed in some cases. It has been assumed that the change in orientation resulted in
significant changes in the efficiency of FRET between different porphyrin molecules. In this way, the energy transfer can be changed by an external stimulus, which
is caused by the change of molecular orientation. This technique could be useful in
constructing photofunctional switches or sensors [43].
4.2.8 Cascade Energy Transfer
Cascade FRET or multi-step FRET can be defined as a non-radiative process of the
transfer of excitation energy that occurs between more than one pair of ED and EA
on the scale of intermolecular distances <10 nm. By its nature, multi-step FRET is
more efficient than the one-step process, which is limited to only a pair of the first
and end member of the cascade. It is claimed that a multi-step process can be realized
beyond the distance conventionally achieved for single-step processes [95]. If there
are several molecules of different dyes with different photophysical properties in the
vicinity of an excited ED molecule, then a selective transfer of energy takes place not
232
J. Bujdák