Collective Effects Influencing Fluorescence Emission - Advanced Fluorescence Reporters in Chemistry and Biology - page 109

a matrix that can protect them against chemical destruction, photodestruction, and
quenching. The second category comprises reporters serving as probes or those
involved in molecular sensors . As probes, they should respond to changes in their
molecular environment and as essential parts of the sensors, they should be coupled to
recognition units and respond to the target binding through change in their fluores-
cence parameters. Here, improvement of response can be provided in several dimen-
sions, particularly, at achieving the broadest range of the change of intensity and the
strongest shifts on the wavelength scale.
These aims can be achieved by incorporating dyes into nanostructures and
nanocomposites, but the routes to follow have to be different. If we wish to obtain
the highest degree of brightness, the dyes should not interact, but their local
environment (e.g., polymer matrix) should provide optimal protection against
various quenching and bleaching effects. Alternatively, if we wish to attain the
highest dynamic range of response in terms of variations of intensity or wavelength
of emission together with high brightness, we need to induce dyes incorporated into
the nanostructures to interact. These interactions exhibit new phenomena, the
proper knowledge of which could lead to new dimensions for optimization of
fluorescence response. This chapter describes these possibilities.

2 Spectroscopy of Intermolecular Interactions

A short excursion into the physics and spectroscopy of intermolecular interactions
is intended to illustrate the effects of fluorescence spectra change on the transition
of dye molecules from liquid solvents to solid environments, on the change of
polarity and hydration in these environments, and on the formation of excited-state
complexes (excimers and exciplexes).

2.1 Universal Intermolecular Interactions

Interactions that exist between molecules in any media are universal. Their physical
modeling can be made on a mesoscopic level of description on which the solute can
be represented by a point dipole with dipole moment, , and its environment, by its

averaged macroscopic parameters, such as refractive index, n , and dielectric con-
stant,

m

. On this level, the theory explaining the shifts of light absorption and
fluorescence bands is well developed [ 10 - 12 ], see also [ 13 ]. The concept of
“solvation” or “stabilization” energy, which determines the position of an elec-
tronic state with the variation of intermolecular interactions is important. Since the
position of the absorption or emission spectrum depends on the difference in the
energies of the corresponding states, the higher salvation energy of the ground state
(compared to that of the excited state) increases this energy gap and the spectra
move to higher energies (shorter wavelengths). Meanwhile, the shift of spectra to

e