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The main bands in the fluorescence excitation spectrum of perylene occur at 367, 388, 409 and 436 nm. In the fluorescence emission spectrum, the...

The main bands in the fluorescence excitation spectrum of perylene occur at 367,
388, 409 and 436 nm. In the fluorescence emission spectrum, the bands occur at 435.5 463,
496 and 530 nm. Construct an approximate energy level diagram for the fluorescence
transitions between the ground and first excited electronic states of perylene.
2. (6 pts) A fluorescence quantum yield of unity (1.00) is rarely, if ever observed.
a. List the processes which are responsible for quantum yields below 1?
b. The fluorescence decay rate, kF, and nonradiative decay rate, kNR, for perylene in an
unspecified solvent are 1.00x109s-1 and 4.00x109s-1, respectively. Calculate the
fluorescence quantum efficiency and lifetime of the molecule.
c. How much would the rate of the non-radiative processes have to be reduced to improve
the quantum efficiency by 30%?
Chemistry 437
Homework #2
2
3. (5 pts) Design and draw a
diagram of a portable,
battery-operated
fluorometer designed to
measure benzo[a]pyrene
(Bap), one of the more
carcinogenic PAHs, in
natural waters like rivers,
lakes and streams. (The
excitation and emission
maxima are indicated in
the figure on the right.)
Show all components that
come in contact with the
light path or form the
measured signal, show as
much of the detection
circuitry as you can.
4. (5 pts) Most commercial IR absorbance spectrometers and many commercial Raman
spectrometers are Fourier transform instruments, but a few are not. What is the alternative to
Fourier transform measurements for IR spectra? Draw an instrument diagram illustrating the
construction of an alternative instrument.
5. (5 pts) Single walled carbon nanotubes
(SWNT) also can be formed in
processes that produce soot. Since they
are so inert, they are not very soluble
which complicates working with them.
Investigators add functional groups to
the ends of the tubes to make them
soluble. The infrared (IR) absorption
spectrum of carbon nanotubes change
markedly with functionalization as the
figure at right shows. (Use reflectance
as you would transmittance for
calculations.) Top spectrum: SWNT; Bottom spectrum: soluble (functionalized) SWNT This
change can be used to measure the extent of functionalization of the tubes.
The IR absorbance of a SWNT sample prepared by diluting a 0.50 mL aliquot of a 100.0 mL
reaction mixture to 10.0 mL was 0.480 at 1465 cm-1 and 0.032 at 1200 cm-1 in a 1 cm
cuvette. The molar absorptivities of the soluble SWNT at 1465 cm-1 and 1200 cm-1 are 554
cm-1M-1 and 16 cm-1M-1, respectively. The molar absorptivities of unreacted SWNT at 1465
cm-1 and 1200 cm are 64 cm-1M-1 and 124 cm-1M-1, respectively. Calculate the %unreacted
SWNT in the sample assuming the stoichiometry of the reaction is 1:1. (If you are
completely stumped by this one, I can give you a hint for the bargain price of 1 point.
Chen et al., Science, 282(5386), 95 - 98 (1998)
Beltrán, et al., Analytica Chimica Acta 373(2-3), 311-319 (1998).
Chemistry 437
Homework #2
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6. (4 pts) Figure 13-7 in Skoog depicts the contribution of the various kinds of instrument noise
to the relative uncertainty in the concentration calculated from a UV-VIS absorbance
measurement. Use your understanding of the differences in the components of UV-VIS and
IR absorbance spectrometers to estimate the contribution of the various kinds of instrument
noise to the relative uncertainty in the concentration calculated from an IR absorbance
measurement. This is not a mathematical question; it is qualitative. So the shape of the
curves matter more than accurate locations for maxima or minima, but do label t

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