Collective Effects Influencing Fluorescence Emission - Advanced Fluorescence Reporters in Chemistry and Biology - page 116

The major factor that produces broadening of the spectra is the so-called
inhomogeneous broadening [ 13 , 40 ]. It originates from the nonequivalence of
chromophore environments in an ensemble of otherwise identical molecules result-
ing in the distribution of solute-solvent interaction energies. Therefore, the elec-
tronic transition energies for every species become distributed on the scale of
energy, and their superposition forms an inhomogeneously broadened contour.
Excitation at the band edge selects a part of this distribution, the spectroscopic
properties of which are different from that of the mean. At the long wavelength
edge of the emission band, the species for which the excitation energy with the
environment is the strongest are excited, and for them the emission spectrum
becomes shifted correspondingly.
Due to molecular motions, the excitation energies fluctuate in time causing
redistribution within this ensemble, “mixing” different environments. When the dye
is incorporated into a solid (polymers, low-temperature solvent glasses) under the
condition t R > t F , the distribution persists during the time of emission, and the

broadening is static. The broadening is dynamic if the motions in the chromophore
environment occur simultaneously (

t R < t F ).
Thus, the inhomogeneous broadening effects contain information about the dynamic
properties of condensed systems, and the rate of fluorescence emission provides the
necessary time scale for these observations [ 45 ].
The Red-Edge effects are popular tools to study the dynamics and interactions in
nano-scale objects, such as proteins and biomembranes [ 45 ] or reverse micelles
[ 46 ]. In this case, organic dyes are introduced into the system or, like tryptophan in
proteins [ 47 ] or intrinsic fluorophore in green fluorescent protein [ 48 ], they are a
part of it. Studies on polymer films with incorporated dyes [ 49 ] are quite frequent,
suggesting the broader use of this tool in the studies of dye-doped nanoparticles.
These effects were detected in conjugated polymers, particularly in polyfluorene
copolymers [ 50 ], and more detailed studies on them are expected to be made in
future. Regarding metal nanoclusters and Quantum Dots, the selective excitation
may reveal a different property - their size distribution, though inhomogeneous
broadening effects can also be detected [ 51 ].

t R t F ) or faster than the emission (

5 Collective Effects Observed with Organic Dyes

Incorporation of organic dyes into nanoparticles in concentrations that allow
efficient FRET to proceed between them due to short interchromophore distances
can dramatically increase the range of variation of their spectroscopic properties.
Due to the long-range but steep distance dependence of the effect, one can either
activate or eliminate the exchange of their excited-state energies by a variation in
dye concentration. Thus, in addition to variations in the spectroscopic properties of
donor and acceptor, the collective effects in energy transfer can be efficiently
explored. Based on effects in FRET, such as directed transfer and antenna effect,
powerful tools can be developed for the design of advanced sensors.