Collective Effects Influencing Fluorescence Emission - Advanced Fluorescence Reporters in Chemistry and Biology - page 121

Fig. 6 Illustration of antenna
effect. Great number of
donors (D) absorb light and
transfer the excitation energy
to a small number of
acceptors (A). Excitation of
acceptors via FRET increases
their emission intensity and,
often, lifetime in comparison
with direct excitation.
Homo-FRET between the
donors is not shown

D

h n F

D D

D A D

D D

D

acceptor, one should not disregard the fact that its photodegradation also occurs at a
much faster rate, in accordance with the frequency of the acts of excitation [ 66 ].
The theory describing the light-harvesting effects suggests going beyond the
dipole-dipole approximation of F¨rster theory [ 67 , 68 ]. Its development for mod-
eling the natural systems of photosynthesis is beneficial for the design of fluores-
cence reporters.
The light-harvesting and cascade-transfer wavelength shifting are quite compat-
ible. Moreover, the generation of “superquenching” effects can be easily produced
in cascades and under the action of light harvesters. So, if the transfer is efficient,
the quenching of the final acceptor quenches the whole ensemble. This opens up
enormous possibilities in sensor design.

5.4 Red-Edge Effects in Energy Transfer

The easiest way to observe homo-FRET is to detect its influence on emission
anisotropy in rigid environments, where the anisotropy in diluted systems is high
[ 69 ]. If one dye molecule absorbs light and some others being in different orienta-
tions emit, the emission in the case of efficient FRET becomes totally depolarized.
Surprisingly, this effect decreases or even disappears when fluorescence is excited
at the long wavelength edge of the emission band (the Red-Edge effect in FRET).
Its present interpretation is based on the inhomogeneous broadening of spectra [ 13 ]
and on the idea that the species selected at the Red-Edge are unable to serve as
FRET donors [ 40 , 45 ].
To understand this effect, one can imagine an overlap integral (see Fig. 2 )as
a composition of overlap integrals formed by individual donor-acceptor pairs
in an inhomogeneous ensemble of dye molecules. As explained in Sect. 5.1, at