Fluorescence Spectrometry
463
! Attention
Before ascribing the enhancement in fluorescence intensity for any soft-nano composite; it is recommended to plot normalized fluorescence curves and then to compare
the emission characteristics. The Stokes shift in the LCM and their composites with
nanomaterial significantly depends upon the molecular ordering.
5.3 Energy Transfer Mechanism in Mesogenic-Nano
Composites
The energy transfer at a particular distance occurs between two molecules, a donor
(the excited fluorophore) and an acceptor (a chromophore or a fluorophore). The
energy-transfer mechanism can be described as: D
∗
+ A → D + A
∗ . The efficiency
of energy transfer depends on four parameters; the distance between the donor and
acceptor, the spectral overlap between the donor fluorescence spectrum and the acceptor absorption spectrum, quantum yield, and the orientation factor, which renders the
relative alignment of the dipoles of the acceptor in the ground state and of the donor
in the excited state. Energy transfer mechanisms are described below.
5.3.1 Donor-Acceptor Energy Transfer (DAET)
When the emission band of the donor molecule partially overlaps with the absorption
band of the acceptor molecule, the photon re-absorption, and nonradiative energy
transfer can take place leading to the quenching of donor fluorescence attributing
to the donor-acceptor energy transfer. The rate of energy transfer and its efficiency
are already given in Eqs. (17) and (18), respectively . If the transfer rate is much
faster than the decay rate, then energy transfer will be efficient; otherwise, it will be
inefficient. Another factor, known as the orientation factor, (κ
2 ), in the analysis of the
energy transfer efficiency can be written as: κ
2 = (sinθ D sinθ A cosφ - 2 cosθ D cosθ A )
2 ;
where, θ D and θ A are the angles between these dipoles and the vector joining the donor
and the acceptor, and φ is the angle between the planes. Depending upon the relative
orientation of donor and acceptor, the value of κ
2 can range from 0 to 4. For headto-tail parallel transition dipoles: κ
2 = 4, and for parallel dipoles: κ
2 = 1. Most of the
LCM-nanoparticle composites and proteins bound to semiconductor nanoparticles
render the quenching of fluorescence via donor-acceptor energy transfer [21, 40, 41].
Since energy transfer decreases the intensity of the donor, an increased contribution
of impurity fluorescence to the data for the D-A pair could result in different values
of τ D for each mode of analysis. The extent of energy transfer also depends on the
concentration of the D-A pairs.
463
! Attention
Before ascribing the enhancement in fluorescence intensity for any soft-nano composite; it is recommended to plot normalized fluorescence curves and then to compare
the emission characteristics. The Stokes shift in the LCM and their composites with
nanomaterial significantly depends upon the molecular ordering.
5.3 Energy Transfer Mechanism in Mesogenic-Nano
Composites
The energy transfer at a particular distance occurs between two molecules, a donor
(the excited fluorophore) and an acceptor (a chromophore or a fluorophore). The
energy-transfer mechanism can be described as: D
∗
+ A → D + A
∗ . The efficiency
of energy transfer depends on four parameters; the distance between the donor and
acceptor, the spectral overlap between the donor fluorescence spectrum and the acceptor absorption spectrum, quantum yield, and the orientation factor, which renders the
relative alignment of the dipoles of the acceptor in the ground state and of the donor
in the excited state. Energy transfer mechanisms are described below.
5.3.1 Donor-Acceptor Energy Transfer (DAET)
When the emission band of the donor molecule partially overlaps with the absorption
band of the acceptor molecule, the photon re-absorption, and nonradiative energy
transfer can take place leading to the quenching of donor fluorescence attributing
to the donor-acceptor energy transfer. The rate of energy transfer and its efficiency
are already given in Eqs. (17) and (18), respectively . If the transfer rate is much
faster than the decay rate, then energy transfer will be efficient; otherwise, it will be
inefficient. Another factor, known as the orientation factor, (κ
2 ), in the analysis of the
energy transfer efficiency can be written as: κ
2 = (sinθ D sinθ A cosφ - 2 cosθ D cosθ A )
2 ;
where, θ D and θ A are the angles between these dipoles and the vector joining the donor
and the acceptor, and φ is the angle between the planes. Depending upon the relative
orientation of donor and acceptor, the value of κ
2 can range from 0 to 4. For headto-tail parallel transition dipoles: κ
2 = 4, and for parallel dipoles: κ
2 = 1. Most of the
LCM-nanoparticle composites and proteins bound to semiconductor nanoparticles
render the quenching of fluorescence via donor-acceptor energy transfer [21, 40, 41].
Since energy transfer decreases the intensity of the donor, an increased contribution
of impurity fluorescence to the data for the D-A pair could result in different values
of τ D for each mode of analysis. The extent of energy transfer also depends on the
concentration of the D-A pairs.