Collective Effects Influencing Fluorescence Emission - Advanced Fluorescence Reporters in Chemistry and Biology - page 115

photophysical process, such as FRET. Similarly, photoselection can be provided by
variation of excitation energy. The dye molecule can absorb only the light quanta
that correspond to its electronic transition energy.
These basic considerations permit the explanation of a group of phenomena that
refer to fluorescence spectroscopy and have a common name “Red-Edge effects”
[ 40 - 45 ]. It has been found that in solid or viscous polar environments, the fluores-
cence spectroscopic properties do not conform to classical rules. Thus, when
fluorescence is excited at the red (long wavelength) edge of the absorption spec-
trum, the emission spectra start to depend on the excitation wavelength [ 40 ]
(Fig. 3 ), and FRET, if present, fails at the “red” excitation edge. The Red-Edge
excitation can profoundly influence not only FRET but also other excited-state
reactions, such as electron and proton transfers if they occur in rigid or highly
viscous environments [ 41 ].
The best tool for observation of these phenomena is time-resolved spectroscopy
[ 42 , 43 ] that makes it possible to observe the excitation-wavelength-dependent
evolution of the spectra in time. The steady-state observations can be complicated
by the existence of ground-state heterogeneity [ 44 ] that originates not only from
the presence of different dyes but also from the same dyes participating in different
(e.g., H-bonding) interactions.
The role of the conditions in which these phenomena are observed is now well
understood [ 40 , 45 ]. The chromophore should be solvatofluorochromic, that is,
its fluorescence spectra should respond to changes in interaction energy with its
environment by significant shifts. This environment should be relatively polar, but
rigid or highly viscous, so that the relaxation times of its dipoles, t R , are comparable

or longer than the fluorescence lifetime t F (in the case of recording the steady-state

spectra) or on the time scale of observation (in time-resolved spectroscopy). Thus,
these effects are coupled with molecular dynamics in condensed media.

λ ∗ ex

τ R << τ F

τ R ≈τ F

τ R >> τ F

λ ex

max
F

Fig. 3 Dependencies of positions of fluorescence band maxima, l , on excitation wavelength,

l ex , for different correlations between the dipole relaxation time, t R , and fluorescence lifetime,

t F . When relaxations are slow, the fluorescence band occupies extreme short wavelength position

and the Red-Edge effect is the most significant, and when they are faster than the emissions,
the spectrum is at long wavelengths and the Red-Edge effect is absent. The excitation spectrum,
F ( Dl em and em are the magnitudes of Red-Edge effect, and

Dl ex is the isorelaxation point (the excitation wavelength at which the position of fluorescence
band does not depend on relaxations) [ 32 ]

l ex ), is also presented schematically. Dl