Collective Effects Influencing Fluorescence Emission - Advanced Fluorescence Reporters in Chemistry and Biology - page 112

form the H-bonds. This may lead to differences in their spectra and influence the
resonance energy transfer between them.

2.3 Excimers and Exciplexes

When excited, the molecules of organic dyes tend to form complexes with unex-
cited molecules like themselves. These excited dimeric complexes are called the
excimers. The excimer emission spectrum is easy to observe because it is very
different from that of a monomer. It is usually broad and strongly shifted to longer
wavelengths, and it does not contain vibrational structure. If the excimer is not
formed, we observe emission of the monomer in the fluorescence spectra, and upon
its formation there appears a characteristic emission of the excimer.
Selection of excimer formers is limited to aromatic hydrocarbons. Pyrene deri-
vatives are more frequently used because of the unique property of these fluoro-
phores to form stable excimers with long fluorescence lifetimes. The structured
band of a pyrene monomer is observed at about 400 nm, whereas that of the excimer
is located at 485 nm. Long lifetimes ( 40 ns for

excimer) allow for easy rejection of background emission and application of
lifetime sensing [ 27 ]. Different fluorescence sensor technologies are based on the
target-induced shifts between monomer and excimer emissions. They use confor-
mational changes in the sensor unit that brings the pyrene groups together [ 28 ]. It is
not easy to provide this response at the level of nanoparticles.
The formation of excimers requires close location and proper orientation
between the partners - specifically, the formation of a cofacial sandwich between
two heterocycles rich in

300 ns for monomer and

-electrons. Weak ground-state complexes can be formed
between proximate monomers that become excimers upon excitation. It has been
observed that when pyrene groups are appended to a flexible polymer in solution,
fluorescence shows the presence of a significant number of excimers [ 29 ]. When
pyrene is incorporated into nanoparticles formed by miniemulsion polymerization,
it shows noneven distribution indicated by an increased number of excimers, which
means that some complexes between monomers are formed prior to excitation [ 30 ].
Depending on the polymer matrix, for some derivatives the excimers are not formed
[ 31 ]. At the same time, it has been reported that highly emissive conjugated
polymer excimers can be formed in the condensed states of these polymers [ 32 ].
These examples suggest many possibilities to explore monomer-excimer transi-
tions in the sensor design. The excimers are also attractive because they suc-
cessfully combine such properties as high degrees of brightness, long lifetimes,
and dramatic Stokes shifts.
Exciplexes are the excited-state complexes that can be formed by partners of
different origin [ 33 ]. Their formation on intermolecular interaction can provide a
fluorescence reporting signal [ 28 , 34 ]. The advantage of their formation in high-
concentration matrices is the large Stokes shift that, as we will see below, can
prohibit the homo-FRET.

p