Principles of Fluorescence Spectroscopy

PRINCIPLES OF FLUORESCENCE SPECTROSCOPY 249
Figure 7.20. Time-dependent emission maxima of anthroyloxy fatty
acids in egg PC vesicles. Dots: 2-AS; triangles: 6-AS: boxes: 9-AS;
circles: 12-AS; asterisks: 16-AP. Revised from [29].
Figure 7.21. Correlation functions for spectral relaxation of the
anthroyloxy fatty acids in egg PC vesicles. Revised from [29].
ter, if the 12-AS and 16-AP probes are located near the surface.
Irrespective of the interpretation, the data demonstrated
that membranes relax on the ns timescale, but the details
are not completely understood.
7.5. PICOSECOND RELAXATION IN SOLVENTS
Advanced Topic
In fluid solvents at room temperature spectral relaxation is
usually complete within about 10 ps, which is prior to the
emission of most fluorophores. This rapid process is too
rapid to be resolved with the usual instrumentation for timedomain
or frequency-domain fluorescence. However,
advances in laser technology and methods for ultrafast
spectroscopy have resulted in an increasing interest in ps
and fs solvent dynamics. 49–61 Because of the rapid timescale
the data on solvent dynamic are usually obtained using fluorescence
upconversion. This method is described in Chapter
4. Typical data are shown in Figure 7.22 for coumarin
152 (C152). The intensity decays very quickly on the blue
side of the emission, and displays a rise on the long-wavelength
side of the emission. The total intensity decay of
coumarin 152 with a lifetime of 0.9 ns is not visible on the
ps timescale of these measurements, as is seen as the nearly
horizontal line after 1 ps in the lower panel. The wavelength-dependent
upconversion data were used to reconstruct
the time-resolved emission spectra, which are shown
in Figure 7.23 at 0.1 and 5.0 ps. By 5 ps the relaxation is
essentially complete, which is why time-dependent effects
are usually not considered for fluorophores in fluid solution.
Figure 7.22. Fluorescence upconversion intensity decays of 7-
(dimethylamino)coumarin-4-acetate (C152) in water. The solid line
through the points is a multi-exponential fit to the intensity decay at
each wavelength. The peak near zero time in the upper panel is the
instrument response function (280 fs FWHM). Revised and reprinted
with permission from [61]. Copyright © 1988, American Chemical
Society.