Principles of Fluorescence Spectroscopy

252 DYNAMICS OF SOLVENT AND SPECTRAL RELAXATION
Figure 7.25. Time-resolved emission center of gravity of Yt-base in npropanol
at –20°C. From [78].
ation times are required to explain the time-dependent shifts
(Figure 7.25). One can use the correlation functions C(t) to
compare the measured and expected relaxation times (Figure
7.26). For Yt-base in propanol the decay of C(t) is faster
than predicted from the dielectric relaxation time τ D , and
slower than predicted from the longitudinal relaxation time
Figure 7.26. Extent of spectral relaxation for Yt-base [C(t)], compared
to the dielectric (τ D ) and longitudinal (τ L ) relaxation times of npropanol
at –20°C. Revised from [78].
τ L . This result is typical of that observed for many fluorophores
in polar solvents. 79
7.6. MEASUREMENT OF MULTI-EXPONENTIAL
SPECTRAL RELAXATION
Measurement of the complete process of spectral relaxation
is a technological challenge. Such measurements are difficult
because relaxation occurs over a wide range of
timescales. For instance, the dielectric relaxation times for
a simple solvent like n-octanol range from 3 to 1780 ps
(Table 7.3). Hence the apparatus needs to have resolution of
both ps and ns processes. As discussed in Chapter 4,
Figure 7.27. Chemical structures of an amphiphilic starlike macromolecule
(ASM) and coumarin 102 (C102). The lower panel shows
the emission spectra of C102 in cyclohexane (blue), ethyl acetate
(green), octanol (orange), water (red) and bound to the ASM in water
(black). Reprinted with permission from [80]. Copyright © 2002,
American Chemical Society. Courtesy of Dr. Edward W. Castner, Jr.
from the Rutgers University, NJ.