Lab TF:__________________________________ Student Name:_______________________________
Physical Sciences 1
Experiment 2
Spectroscopy of Conjugated Molecules
Please read this experiment and complete the
attached prelab before your lab next week.
This lab will meet Feb. 10th and Feb. 11th
1
Scientific Background
In molecules containing conjugated double bonds, electrons can delocalize along the π-electron system due to
the overlap of the p orbitals (Figure 1.1b). The electrons of these molecules absorb ultraviolet and/or visible
radiation, and can be crudely modeled as particles in a box , where the length of the box is approximately the
distance between the two carbons at each end of the π-electron system.
π -e le c tro n s y s te m
(a) double bonds not conjugated
(b) conjugated double bonds and the π-electron system
Figure 1.1
(a) double bonds that are separated by two or more single bonds are not conjugated (b) conjugated
double bonds are present when there are two or more continuous sets of double bonds within a molecule.
Let’s consider what happens when butadiene absorbs UV radiation at 217 nm. The π-electron structure of
butadiene is shown in Figure 1.2. In the normal state of the butadiene molecule, called the ground state, the
four π electrons occupy the two lowest-energy molecular orbitals. When butadiene absorbs energy from
light, one π electron in the highest occupied molecular orbital (HOMO) is promoted to the next vacant
molecular orbital, the lowest-unoccupied molecular orbital (LUMO). The resulting state of the butadiene
molecule is called an excited state. The energy required for this absorption must match ∆E, the difference in
the energies of the HOMO and LUMO, which also corresponds to the lowest energy transition that can occur
for this molecule. The wavelength of the photons with energy ∆E corresponds to a maximum in the
absorption spectrum of the molecule and is denoted λmax.
n = 4
LU M O
n = 3
DE
b u ta d ie n e
H O M O
U V lig h t
e n e rg y = DE
n = 2
n = 1
g r o u n d sta te
e x c ite d s ta te
Figure 1.2 A diagram showing the placement of four π electrons in the molecular orbitals of butadiene, both in the
ground state and the excited state. When an electron in the highest occupied molecular orbital (HOMO) absorbs light of
energy ∆E, the electron is elevated to the lowest-unoccupied molecular orbital (LUMO).
The structural feature of a compound that is most important in determining the λmax is the number of
consecutive conjugated double bonds. The longer the conjugated π-electron system (that is, the more
consecutive conjugated multiple bonds), the higher the wavelength of the absorption (why?). If a molecule
has enough double bonds in conjugation, one or more of its λmax values will be large enough to fall within the
visible region of the electromagnetic spectrum, and the compound will appear colored.
2
Recall that a molecule’s absorption spectrum determines its color. The
wavelengths of light that are not absorbed reach your eye and are interpreted by
your brain as a specific color. For molecules with one dominant absorption
peak, the observed color is the opposite of the color of light that is most
strongly absorbed. You recall the color wheel, which relates colors with their
opposites (Figure 1.3). For instance, if the absorption peak of the above
molecule were in the green region of the visible spectrum, the molecule would
absorb green light and would appear red.
Figure 1.3 Color wheel
In this experiment, you will use a UV-visible spectrometer to measure the absorbance of various conjugated
molecules including β-carotene (from carrots), lycopene (from tomatoes), and some thiacyanine dyes (Figure
1.4). Once you have determined the λmax for each molecule, you will estimate the length of the π-electron
system through the particle-in-a-box model calculation.
Figure 1.4
Chemical structures of the conjugated molecules that are used in this experiment.
*Remember, the above structures do NOT show carbon or hydrogen atoms as well as lone pairs of electrons. Lone
pairs can participate in a conjugated π-electron system*
Measuring absorbance using a UV-visible spectrometer
Spectrometers can be used to measure emission/absorption quantitatively, and to detect emission/absorption
at wavelengths which the human eye cannot. Spectrometers measure the intensity of light ( I) as a function of
the wavelength of that light. They have three basic components:
•A light source, which generates a broad range of wavelengths.
•A sample, which absorbs the light passing through it.
•A detector, which measures the intensity of light transmitted by the sample at various
wavelengths.
3
I(λ)
light source, I0
lens
sample
diffraction
grating
detector
array
A simplified diagram of a modern visible-light spectrometer is shown above. Light from the broad-spectrum
source passes through the sample, is shaped by a lens into a parallel beam, and passes through a diffraction
grating. Diffraction gratings are very fine periodic structures etched in glass or drawn on a reflective surface
and work due to the interference (summation or subtraction) of light waves interacting with the neighboring
strips of the grating. These devices differentially deflect incident light of different wavelengths: the angle at
which light exits from the device depends on its wavelength. The diffraction grating in the spectrometer is a
higher-quality version of the ones in the glasses that were used in class.
A position-sensitive light detector is located beyond the diffraction grating. Typically, the detector is a linear
array of very small photodetector cells, which are very similar to the individual pixels of a digital camera’s
image sensor. Each of the photodetectors is illuminated only by light of a specific wavelength (see the
diagram). To obtain a spectrum, one has to measure the intensity of light collected by each of the
photodetector cells and know the relationship between wavelength and cell position. Both of these tasks are
performed by the spectrometer’s electronics. The Vernier Spectrometer that you will use in this experiment is
very similar to the diagram above, although it uses a reflective, rather than transmissive, diffraction grating,
and its optical system is more elaborate than the single lens pictured in the diagram.
Although spectrometers can be designed to observe wavelengths from 10 –11 m to 10 m, particular
instruments usually cover only a small range of wavelengths. For example, the instrument we will use for
this experiment, the Vernier Spectrometer, is sensitive to wavelengths between 380 nm and 950 nm. The
human eye is sensitive to light at wavelengths between approximately 380 nm and 700 nm.
Calculating the length of the molecules
Once the value of λmax has been determined, we can calculate the energy of the light absorbed and estimate
the length of the π-electron system. Remember that these electrons can be crudely modeled as particles in a
box, where the length of the box is approximately the distance between the two carbons at each end of the πelectron system. The energy of the absorbed photon is related to the electronic transition and the length of
the π-electron system (“box”) via the following equations:
where h
= Planck’s constant
L = length of the π-electron system (“box”)
nf = the final energy level
ni = the initial energy level
c
m
∆E
λmax
4
=
=
=
=
speed of light
mass of an electron
energy of absorbed photon
wavelength of maximum absorbance
Lab TF:__________________________________ Student Name:_______________________________
Experiment 2: Procedure, Lab Report and Prelab
Before You Come to Lab:
• Read the entire lab report, including the previous introduction and discussion, and the entire procedure.
• Complete the Prelab, which is the last two pages of the lab report, and turn in the prelab to your TF as you
enter the lab.
Safety in the Laboratory
• Safety glasses or safety goggles and lab coats must be worn at all times in the laboratory.
• Gloves must be worn while performing experiments or working with chemicals.
• The chemicals used in this experiment are toxic. Wash your hands and change gloves if you suspect that
you have spilled any chemical on your gloves.
Waste Disposal and Cleanup
• Empty all cuvettes into the “Rinse Beaker” at your lab bench.
• Use a squirt bottle to rinse the cuvettes with water and pour the rinse into the “Rinse Beaker” .
• Dispose of empty plastic cuvettes in the trash.
• When you are done with the lab, empty the “Rinse Beaker” into the waste collection bucket in
the back of the lab.
• Leave everything else at your lab bench.
• When you leave the lab, your lab bench should look exactly as it did when you arrived.
Before You Leave the Lab
•
Have your TF check your lab bench for cleanup.
• Submit your data and lab report to your TF. This page and all subsequent pages must be stapled and turned
in.
• Wash your hands before leaving the lab.
Grading:
Prelab:
_____ / 10
Lab Report:
_____ / 20
Safety:
_____ / 3
Cleanup:
_____ / 2
Total:
_____ / 35
5
Using the Vernier Spectrometer and LoggerPro
• For this lab we will be using the Vernier spectrometers. They are the small black boxes connected to the
computers at your lab bench. Using these spectrometers with the computers will streamline data
collection.
• The Vernier Spectrometer has a small square opening where you can insert the sample cuvette. Only use
the plastic cuvettes in the Vernier Spectrometers. Never attempt to insert a glass test tube into the
Vernier Spectrometer. In addition, please be careful not to spill any solutions onto the computers or the
spectrometers. All “wet work” including preparation and mixing of solutions should be done on the lab
benches across the sink from the computers.
Starting the LoggerPro Software
• If your computer is not already logged in, login to the computer by clicking on “Physical Sciences” and
entering the password “ps1”.
• On the menu bar at the top of the screen, click on “Go → Applications → Logger Pro 3” and open
“LoggerPro”. You should see a plot of absorbance versus wavelength appear with a spectrum of colors.
Important Notes on the Plastic Cuvettes
• The plastic cuvettes have a clear side where the light will pass through, and a shaded partially opaque
side. Only handle the cuvettes on the opaque sides. Fingerprints on the clear sides will interfere with
your measurements.
• Do not overfill the cuvettes. In this lab you will be filling the cuvettes with exactly 1.0 mL of solution.
This will fill the cuvettes approximately as full as they should be filled without overflowing. If you
overfill a cuvette or spill any solution on the side of a cuvette, pour a little out into the “Rinse Beaker”
and wipe off the outside of the cuvette with a paper towel before proceeding. Be careful not to get any
solution inside the spectrometer equipment.
• When inserting the plastic cuvettes into the spectrometer, make sure that the light beam is passing
through the clear sides of the plastic cuvette, not through the cloudy opaque sides. You can do this by
looking for the light beam and observing the cuvette closely, or by lining up the arrow on the upper edge
of the cuvette with the blue dot on the spectrometer.
• The cuvette should be inserted firmly into the spectrometer. Less than 1 cm of the cuvette should stick
out of the top of the spectrometer, and the cuvette should not be able to move back and forth much
within the spectrometer. If you are having trouble inserting the cuvette all the way into the spectrometer,
ask your TF for assistance.
6
Collecting Absorbance Spectra
Data, Observations, and Notes
Take the plastic cuvette holder at your lab
station to the center bench in the laboratory room
and fill it with 5 clean plastic cuvettes. Note that
each cuvette has clear sides and opaque sides.
Never touch the clear sides; fingerprints will
interfere with your measurements. Handle the
cuvettes only by the opaque sides.
You will see a set of bottle-top dispensers
that are calibrated to dispense exactly 1.0 mL of
solution. Each cuvette will hold only one solution,
so remember the order in which you dispense
them. Carefully “squirt” 1.0 mL of solutions 1-4
(the thiacyanine dyes) into the cuvettes by
smoothly pulling up the top of the dispenser and
pushing it down slowly. Your cuvette should be
filled almost to the top with 1.0 mL of solution.
Note the color each solution appears.
When you get to your bench, fill up the
fifth cuvette with roughly 1 mL of distilled water.
This will serve as your reference solution.
Color Solution 1 Appears:
Color Solution 2 Appears:
Color Solution 3 Appears:
Color Solution 4 Appears:
Calibrating the Spectrometer
Gently insert the reference cuvette into the
spectrometer. Note the orientation of the cuvette
when it is inserted; the path of the beam of light
should pass through the clear sides of the cuvette.
Make sure the cuvette is inserted all the way into
the spectrometer.
In Logger Pro, go to the “Experiment”
menu. Select “Calibrate → Spectrometer.” Once
the dialog box appears, click “Finish Calibration.”
You will have to wait a few seconds, and then you
can click “OK.” Now click on the spectrometer
icon at the top of the window (this icon has a
“rainbow” appearance”). A new window will
appear. Select the button labeled “Abs vs.
Wavelength” and click “OK.” Remove the
reference cuvette from the spectrometer.
7
Data Collection
Data, Observations, and Notes
Thiacyanine Dyes
Gently insert sample #1 into the
spectrometer. Click on the green button at the top
of the screen to begin recording data. The screen
will appear blank for a few seconds and then you
should see an absorbance spectrum appear. Once
you see the absorbance spectrum, click on the red
stop button at the top of the screen. Remove
sample #1 from the spectrometer and insert sample
#2. Click on the green button to collect the next
spectrum. A window will pop up asking if you
want to save your current data. Click the button
that says “Store latest run.” (This will allow you
to record all of your spectra on the same graph.)
After a few seconds, you should see the spectrum
for sample #2. Click the stop button. Repeat this
procedure with samples #3 and #4.
Solution 1 λmax :
nm
Color Absorbed:
When you are done collecting data, notice
that as move your cursor over each of the peaks
you can see the wavelength in the lower left
corner. Some solutions will produce multiple
peaks in the visible spectra, but we are only
interested in the peak at the maximum wavelength.
Record the wavelength of maximum absorbance
of each sample (λmax) and the color region the peak
appears in.
Solution 2 λmax :
nm
Color Absorbed:
Solution 3 λmax :
nm
Color Absorbed:
β-Carotene and Lycopene
Unfortunately these two substances are not
very stable, so the necessary data has been
included on the lab report.
Solution 4 λmax :
Color Absorbed:
8
nm
Lab TF:__________________________________
Student Name:__________________________________
Lab Report
Molecule
Literature
λmax (nm)
Experimental
λmax (nm)
1. 3,3’- Diethylthiacyanine Iodide
423*
2. 3,3’- Diethylthiacarbocyanine Iodide
558*
3. 3,3’- Diethylthiadicarbocyanine Iodide
650*
4. 3,3’- Diethylthiatricarbocyanine Iodide
758*
5. β-Carotene
460†
460
6. Lycopene
480†
480
ΔEabsorb
(J)
Electronic
transition
(ni ! nf)
L (nm)
*Journal of the American Chemical Society, 62, 1116 (1940).
†
Biochem Journal, 38(3), 279-282 (1944).
1.
For each molecule, fill in the table above with: (Do the calculations on a separate sheet of paper)
a)
The maximum absorbance wavelength (λmax). How do your experimental values compare to the literature
values? Explain any differences you observe.
b.
The energy of light absorbed that corresponds to the λmax.
c.
The initial and final energy levels of the electronic transition that corresponds to the λmax. Consult with
your TF before proceeding to d). Hint: It would help to draw the resonance structure of a cyanine dye.
d.
The length of the π-electron system. You should do your calculations using the particle in a box model and
your experimental values for λmax.
9
2.
Compare the color the solution appeared and the color it absorbed. Do they correlate as you would
expect? Are there any discrepancies between the two? Why?
3.
For the thiacyanine dyes, compare the calculated lengths of the π-electron system to the λmax and explain
any trend you observe.
4.
Using the particle in a box model, demonstrate that ΔEabsorb is greater for 3,3’- diethylthiacyanine than for
3,3’- diethylthiacarbocyanine (which has a wider “box”).
10
Lab TF:__________________________________
Student Name:________________________________
Prelab
To be completed and handed in as you enter the lab.
1.
2.
3.
In this experiment, you will measure the absorbance of conjugated molecules including β-carotene,
lycopene, and some thiacyanine dyes. Given the color of each molecule, predict the approximate
maximum absorption wavelength (λmax) in the visible region (380 nm – 750 nm). Remember, the
eye perceives the unabsorbed light. Hint: A color wheel may prove useful.
Molecules
Observed Color
Approx. λmax (nm)
3,3’- Diethylthiacyanine Iodide
yellow
420-430 nm
3,3’- Diethylthiacarbocyanine Iodide
bright pink
550-560 nm
3,3’- Diethylthiadicarbocyanine Iodide
blue
650-660 nm
3,3’- Diethylthiatricarbocyanine Iodide
dark blue
755-765 nm
β-Carotene
orange
460-470 nm
Lycopene
red
A conjugated molecule 1,4-diphenyl-1,3-butadiene (shown at right)
has the λmax of 330 nm. In what region of the electromagnetic
radiation does this wavelength falls? Which electronic transition
does this wavelength correspond to (i.e. n = 1 to n = 2, etc.)? You
may ignore the electrons in the rings.
1 ,4 - d ip h e n y l- 1 ,3 - b u ta d ie n e
Do you expect the λmax of 1,6-diphenyl-1,3,5-hexatriene to be
lower or higher than that of 1,4-diphenyl-1,3-butadiene above?
Briefly explain why.
1 ,6 - d ip h e n y l- 1 ,3 ,5 - h e x a tr ie n e
11
Lab TF:__________________________________ Student Name:_______________________________
Experiment 2: Procedure, Lab Report and Prelab
ANSWER KEY
Before You Come to Lab:
• Read the entire lab report, including the previous introduction and discussion, and the entire procedure.
• Complete the Prelab, which is the last two pages of the lab report, and turn in the prelab to your TF as you
enter the lab.
Safety in the Laboratory
• Safety glasses or safety goggles and lab coats must be worn at all times in the laboratory.
• Gloves must be worn while performing experiments or working with chemicals.
• The chemicals used in this experiment are toxic. Wash your hands and change gloves if you suspect that
you have spilled any chemical on your gloves.
Waste Disposal and Cleanup
• Empty all cuvettes into the “Rinse Beaker” at your lab bench.
• Use a squirt bottle to rinse the cuvettes with water and pour the rinse into the “Rinse Beaker” .
• Dispose of empty plastic cuvettes in the trash.
• When you are done with the lab, empty the “Rinse Beaker” into the waste collection bucket in
the back of the lab.
• Leave everything else at your lab bench.
• When you leave the lab, your lab bench should look exactly as it did when you arrived.
Before You Leave the Lab
•
Have your TF check your lab bench for cleanup.
• Submit your data and lab report to your TF. This page and all subsequent pages must be stapled and turned
in.
• Wash your hands before leaving the lab.
Grading:
Prelab:
_____ / 10
Lab Report:
_____ / 20
Safety:
_____ / 3
Cleanup:
_____ / 2
Total:
_____ / 35
1
Using the Vernier Spectrometer and LoggerPro
• For this lab we will be using the Vernier spectrometers. They are the small black boxes connected to the
computers at your lab bench. Using these spectrometers with the computers will streamline data
collection.
• The Vernier Spectrometer has a small square opening where you can insert the sample cuvette. Only use
the plastic cuvettes in the Vernier Spectrometers. Never attempt to insert a glass test tube into the
Vernier Spectrometer. In addition, please be careful not to spill any solutions onto the computers or the
spectrometers. All “wet work” including preparation and mixing of solutions should be done on the lab
benches across the sink from the computers.
Starting the LoggerPro Software
• If your computer is not already logged in, login to the computer by clicking on “Physical Sciences” and
entering the password “ps1”.
• On the menu bar at the top of the screen, click on “Go → Applications → Logger Pro 3” and open
“LoggerPro”. You should see a plot of absorbance versus wavelength appear with a spectrum of colors.
Important Notes on the Plastic Cuvettes
• The plastic cuvettes have a clear side where the light will pass through, and a shaded partially opaque
side. Only handle the cuvettes on the opaque sides. Fingerprints on the clear sides will interfere with
your measurements.
• Do not overfill the cuvettes. In this lab you will be filling the cuvettes with exactly 1.0 mL of solution.
This will fill the cuvettes approximately as full as they should be filled without overflowing. If you
overfill a cuvette or spill any solution on the side of a cuvette, pour a little out into the “Rinse Beaker”
and wipe off the outside of the cuvette with a paper towel before proceeding. Be careful not to get any
solution inside the spectrometer equipment.
• When inserting the plastic cuvettes into the spectrometer, make sure that the light beam is passing
through the clear sides of the plastic cuvette, not through the cloudy opaque sides. You can do this by
looking for the light beam and observing the cuvette closely, or by lining up the arrow on the upper edge
of the cuvette with the blue dot on the spectrometer.
• The cuvette should be inserted firmly into the spectrometer. Less than 1 cm of the cuvette should stick
out of the top of the spectrometer, and the cuvette should not be able to move back and forth much
within the spectrometer. If you are having trouble inserting the cuvette all the way into the spectrometer,
ask your TF for assistance.
2
Collecting Absorbance Spectra
Data, Observations, and Notes
Take the plastic cuvette holder at your lab
station to the center bench in the laboratory room
and fill it with 5 clean plastic cuvettes. Note that
each cuvette has clear sides and opaque sides.
Never touch the clear sides; fingerprints will
interfere with your measurements. Handle the
cuvettes only by the opaque sides.
You will see a set of bottle-top dispensers
that are calibrated to dispense exactly 1.0 mL of
solution. Each cuvette will hold only one solution,
so remember the order in which you dispense
them. Carefully “squirt” 1.0 mL of solutions 1-4
(the thiacyanine dyes) into the cuvettes by
smoothly pulling up the top of the dispenser and
pushing it down slowly. Your cuvette should be
filled almost to the top with 1.0 mL of solution.
Note the color each solution appears.
When you get to your bench, fill up the
fifth cuvette with roughly 1 mL of distilled water.
This will serve as your reference solution.
Color Solution 1 Appears: YELLOW
Color Solution 2 Appears: RED/PINK
Color Solution 3 Appears: BLUE
Color Solution 4 Appears: BLUE/VIOLET
Calibrating the Spectrometer
Gently insert the reference cuvette into the
spectrometer. Note the orientation of the cuvette
when it is inserted; the path of the beam of light
should pass through the clear sides of the cuvette.
Make sure the cuvette is inserted all the way into
the spectrometer.
In Logger Pro, go to the “Experiment”
menu. Select “Calibrate → Spectrometer.” Once
the dialog box appears, click “Finish Calibration.”
You will have to wait a few seconds, and then you
can click “OK.” Now click on the spectrometer
icon at the top of the window (this icon has a
“rainbow” appearance”). A new window will
appear. Select the button labeled “Abs vs.
Wavelength” and click “OK.” Remove the
reference cuvette from the spectrometer.
3
Data Collection
Data, Observations, and Notes
Thiacyanine Dyes
Gently insert sample #1 into the
spectrometer. Click on the green button at the top
of the screen to begin recording data. The screen
will appear blank for a few seconds and then you
should see an absorbance spectrum appear. Once
you see the absorbance spectrum, click on the red
stop button at the top of the screen. Remove
sample #1 from the spectrometer and insert sample
#2. Click on the green button to collect the next
spectrum. A window will pop up asking if you
want to save your current data. Click the button
that says “Store latest run.” (This will allow you
to record all of your spectra on the same graph.)
After a few seconds, you should see the spectrum
for sample #2. Click the stop button. Repeat this
procedure with samples #3 and #4.
Solution 1 λmax :
Color Absorbed:
When you are done collecting data, notice
that as move your cursor over each of the peaks
you can see the wavelength in the lower left
corner. Some solutions will produce multiple
peaks in the visible spectra, but we are only
interested in the peak at the maximum wavelength.
Record the wavelength of maximum absorbance
of each sample (λmax) and the color region the peak
appears in.
Solution 2 λmax :
Color Absorbed:
Solution 3 λmax :
Color Absorbed:
β-Carotene and Lycopene
Unfortunately these two substances are not
very stable, so the necessary data has been
included on the lab report.
Solution 4 λmax :
Color Absorbed:
4
410 - 430
nm
VIOLET
540 - 560
nm
GREEN
640 – 660
nm
ORANGE
740 - 760
RED
nm
Lab Report
(8pts – 2pts for each part of #1)
Molecule
Literature
λmax (nm)
Experimental
λmax
(nm)
ΔEabsorb
(J)
Electronic
transition
(ni $ nf)
L (nm)
1. 3,3’- Diethylthiacyanine Iodide
423*
422
4.71 x 10-19
3$4
0.95
2. 3,3’- Diethylthiacarbocyanine Iodide
558*
556
3.58 x 10-19
4$5
1.23
3. 3,3’- Diethylthiadicarbocyanine Iodide
650*
648
3.07 x 10-19
5$6
1.47
4. 3,3’- Diethylthiatricarbocyanine Iodide
758*
758
2.62 x 10-19
6$7
1.73
5. β-Carotene
460†
(460)
4.32 x 10-19
11 $ 12
1.79
6. Lycopene
480†
(480)
4.14 x 10-19
11 $ 12
1.83
*Journal of the American Chemical Society, 62, 1116 (1940).
†
Biochem Journal, 38(3), 279-282 (1944).
1.
For each molecule, fill in the table above with: (Do the calculations on a separate sheet of paper)
a.
The maximum absorbance wavelength (λmax). How do your experimental values compare to the literature
values? Explain any differences you observe.
See table above.
The experimental values are very close to the literature values. (There shouldn’t be a significant
deviation between the literature values and the experimental values.)
b.
The energy of light absorbed that corresponds to the λmax.
See table above.
c.
The initial and final energy levels of the electronic transition that corresponds to the λmax. Consult with
your TF before proceeding to d). Hint: It would help to draw the resonance structure of a cyanine dye.
See table above.
d.
The length of the π-electron system. You should do your calculations using the particle in a box model
and your experimental values for λmax.
See table above.
2.
Compare the color the solution appeared and the color it absorbed. Do they correlate as you would
expect? Are there any discrepancies between the two? Why? (3pts)
This answer will vary depending on their results. The color the solution appears should
appear opposite of what it absorbs. Solution 4 will not be as consistent because we are observing the
second excited state.
3.
For the thiacyanine dyes, compare the calculated lengths of the π-electron system to the λmax and explain
any trend you observe. (3pts)
As the calculated length of the π-electron system increases the λmax increases.
This is because as the conjugated pi system becomes longer, it absorbs a lower energy
of light because the energy spacing of adjacent orbitals becomes smaller. Since it is
absorbing a lower energy of light, it is absorbing a longer wavelength of light, so λmax
increases.
2
4.
Using the particle in a box model, demonstrate that ΔEabsorb is greater for 3,3’- diethylthiacyanine than for
3,3’- diethylthiacarbocyanine (which has a wider “box”). (6pts)
This question could be interpreted in several different ways. Give credit for any reasonable
discussion and interpretation. Here is one possible interpretation of the question:
h2
2
2
2 n f - ni
8mL
(
DEabsorb =
)
For 3,3’- diethylthiacyanine (“a”) the n initial to nfinal transition is 3 ! 4.
For 3,3’- diethylthiacarbocyanine (“b”) the n initial to nfinal transition is 4 ! 5.
Plugging these values in, we have, for 3,3’- diethylthiacyanine (we’ll call this “a”):
DEabsorb
h2
h2
h2
2
2
2
2
=
n - ni =
4 -3 =
( 7)
8mL2 f
8mL2
8mL2
(
)
(
)
For 3,3’- diethylthiacarbocyanine (let’s call this “b”) we have:
h2
h2
h2
2
2
2
2
n
n
=
5
4
=
( 9)
i
8mL2 f
8mL2
8mL2
(
DEabsorb =
)
(
)
Taking a ratio of the energies here:
2
∆Ea
∆Eb
=
7h
8 mL a 2
9h 2
8 mL b
" # " #
7 Lb
=
9 La
2
2
7 Lb
=
9 La
2
2
As long as (Lb/La)2 > 9/7, then ∆Ea > ∆Eb.
We can approximate the length of the conjugated π-system by the number of bonds between
nitrogen and nitrogen in the structures. Using this ballpark approximation, “a” has a length of 4
bonds, and “b” has a length of 6 bonds. Using this approximation, we can assume that the ratio
of the lengths of the π-system in “b” to the π-system in “a” is:
" # "#
2
Lb
6
≈
La
4
2
= 2 . 25 >
9
7
Note that any approximation of relative “box” length from the number of bonds (i.e. 4 and 6, 5
and 7, or 6 and 8 are all potentially reasonable) would lead to a similar result.
Since(Lb/La)2 > 9/7, then we can conclude that ∆Ea must be greater than ∆Eb.
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Answer Key
Prelab
1.
In this experiment, you will measure the absorbance of conjugated molecules including β-carotene,
lycopene, and some thiacyanine dyes. Given the color of each molecule, predict the approximate
maximum absorbtion wavelength (λmax) in the visible region (380 nm – 750 nm). Remember, the
eye perceives the unabsorbed light. Hint: A color wheel may prove useful. (4pts)
Molecules
Observed Color
Approx. λmax (nm)
3,3’- Diethylthiacyanine Iodide
yellow
420 around 40030 nm
3,3’- Diethylthiacarbocyanine Iodide
bright pink
550- around 57060 nm
3,3’- Diethylthiadicarbocyanine Iodide
blue
650-6around 62060 nm
3,3’- Diethylthiatricarbocyanine Iodide
dark blue
77around 600 (??)65
β-Carotene
orange
460-4around 45070 nm
Lycopene
red
around 520
NOTE: There is a considerable possible range for these values—but they should not be the exact values
listed in the lab report. Also, solution #4 would be almost impossible to predict anywhere near correctly
from the color that is given.
2.
A conjugated molecule 1,4-diphenyl-1,3-butadiene (shown at right)
has the λmax of 330 nm. In what region of the electromagnetic
radiation does this wavelength falls? Which electronic transition
does this wavelength correspond to (i.e. n = 1 to n = 2, etc.)? You
may ignore the electrons in the rings. (3pts)
1 ,4 - d ip h e n y l- 1 ,3 - b u ta d ie n e
•330 nm is in the ultraviolet range of the electromagnetic spectrum.
•This corresponds to the transition n = 2 to n = 3.
3.
Do you expect the λmax of 1,6-diphenyl-1,3,5-hexatriene to
lower or higher than that of 1,4-diphenyl-1,3-butadiene
above? Briefly explain why. (3pts)
be
1 ,6 - d ip h e n y l- 1 ,3 ,5 - h e x a tr ie n e
The λmax of 1,6-diphenyl-1,3,5-hexatriene should be higher (larger) because this
molecule has a longer conjugated pi system, which means it will absorb light with
a smaller energy, and thus it will absorb light with larger wavelength.
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