Collective Effects Influencing Fluorescence Emission - Advanced Fluorescence Reporters in Chemistry and Biology - page 110

longer wavelengths is more common because in these cases the stronger electronic
charge distribution in the excited state results in its stronger interaction with the
environment.
The most efficient factor in stabilizing the electronic state is the dipole-dipole
interaction. This creates a local electric field (reactive field) around the excited dye
interacting with its dipole [ 14 ]. If the charges are present in its vicinity, they create
an electric field that interacts with the dye dipole and induces electrochromic shifts
of absorption and fluorescence spectra. The direction of these shifts depends on the
relative orientation of the electric field vector and the dye dipole. These effects of
electrochromism are overviewed in [ 15 ].
The act of light absorption is so fast that only the electronic subsystem in the dye
environment can respond to it. In contrast, the finite values of the fluorescence
lifetime, t F , allow for different reactions in the excited state that involve the
motions of atoms and molecules before the emission. They provide additional
stabilization for the excited state and the shift of fluorescence bands to longer
wavelengths. The most common is the dielectric relaxation, which is the rotation
of dipoles surrounding the excited fluorophore. This process is dynamic [ 16 ], so the
dielectric relaxation time,

t R , can vary in very broad ranges. In solid environments
(vitrified solvent glasses, and polymers), the relaxation is slower than the emission
( t R > t F ) and the fluorescence spectrum occupies the short wavelength position,

whereas in liquid solvents, the relaxation is much faster than the emission (

t R <

t F ). There can be cases when t R t F (viscous solvents,and flexible polymers),

where the motion of fluorescence spectra to longer wavelengths can be observed as
a function of time. In common steady-state observation, the spectra in this case are
sensitive to variations of temperature influencing t R and to the dynamic quenchers

influencing t F .

In the excited state, the redistribution of electrons can lead to localized states
with distinct fluorescence spectra that are known as intramolecular charge transfer
(ICT) states. This process is dynamic and coupled with dielectric relaxations in the
environment [ 16 ]. This and other solvent-controlled adiabatic excited-state reac-
tions are discussed in [ 17 ]. As shown in Fig. 1 , the locally excited (LE) state is
populated initially upon excitation, and the ICT state appears with time in a process
coupled with the reorientation of surrounding dipoles.

Fig. 1 Simplified energy
diagram showing the
influence of molecular
relaxations (with lifetime R )

on the energies of LE and ICT
states. The ICT states can be
strongly stabilized in polar
media by orientation of
surrounding dipoles resulting
in substantial shifts of
fluorescence spectra to lower
energies (longer wavelengths)

t

S 1

h n LE h n ICT

S 0

NO RELAXATION
LE emission

RELAXATION
ICT emission