Lab 5.
Procedure.
Parts of this lab are adapted from GNS1 (pp.380-385), Potts2 and analogous experiment
at Dartmouth College3.
You will collect data with a lab partner (who
should be noted in your write-up). However, all the data analysis
and write-up should be done without collaboration.
Section A
This part of the lab is partially described in GNS (pp.380-385) You will study the visible absorbance
spectra of six cyanine dyes, labeled A through F; structures for
these dyes appear in the introduction section. Note
that each structure shown is a positive ion; as supplied by the
manufacturer these are accompanied by halide ions (Cl-, Br- or I-) as the
counter ion.
Before the experiment
- You should have stock solutions ~ 10-4 M in
methanol for each dye
During the experiment
Prepare ~ 10 -5
M solutions of each dye in methanol (keep track of
the concentration in each case). Be patient, they
dissolve slow. Maximum absorbance should not exceed 1. If
they do, add more methanol. Keep track of the
resulting concentration, so that you can calculate the
extinction coefficients for each dye.
Measure the spectra of all 6 solutions in the range from
300 nm to 900 nm. Use Ocean
Optics spectrophotometer in room CB109. Plastic
cuvettes tend to stain by these dyes. So, either wash
cuvettes thoroughly or use a new cuvette for each dye;
another option - use a quartz (or glass) 1 cm cuvette.
Make sure that you rinse the glass cuvette with methanol
thoroughly as well. Use the same cuvette with methanol
for the background. Save all 6 files on a disk.
Determine lmax for
the given cyanine dye solutions A through F.
After the experiment
- Dispose used methanol/dye solutions in the appropriately
marked container. Clean the glass cuvettes thoroughly or,
if you used plastic cuvettes, throw them away.
Section B
In this section you will investigate
aggregation of two dyes, one of them forms J- aggregates and
another - H-aggregates. The aggregation will be induced by change
in concentration and ionic strength. Both dyes are positively
charge at neutral pH, thus you will see the effect of NaCl on
absorption, identify the type of aggregates and suggest their
structures.
Before the experiment
You should
have stock solutions of ~10-4 M oxazine 4
(another name LD 690), structure of which is shown on the
right (e615=1.09×105M-1cm-1),
and ~10-4 M 1,1'-Diethyl-2,2'-cyanine iodide
(Dye A from the previous section) in water (its
extinction coefficient at the maximum is e524=5.4×104M-1cm-1).
- You should also have solid NaCl
During the experiment.
Prepare aqueous
solutions of these dyes at concentrations
3*10-6 M (2 mL) , 10-5 M (2
mL) and 3*10-5 M (1 mL)
by dissolving with DI water
Proceed as in section A to record
the absorption spectra for each solutions (in 1cm and 3
mm plastic cuvettes, respectively) in the wavelength
range from 300 nm to 900 nm. Remember to use water filled
cuvette for background measurements. Save files on the
disk for future analysis
Determine lmax of
peaks for each solution and their intensities.
Add appropriate amount of solid NaCl to each cuvette to
make 3.0 M NaCl. Repeat the same absorption measurements
after salt fully dissolves. NB. Gently shake
before measurements; avoid formation of bubbles. Use aqueous 1.0 M NaCl filled
cuvette for background measurements. Pay attention to new
bands appearing and whether they change with time. Wait
till they stabilize!!
Determine lmax of
old and peaks for each solution and their
intensities. NB. If you observe no
changes then you have done something wrong; one dye
should form H-aggregates (a high energy shifted new band)
and another - J-aggregates (a lower energy shifted new
band)
After the experiment
- Solutions must be disposed off properly
when you finish. You will find a waste solvent bottle in
the hood for this purpose.
References
- Garland, Nibler, and Shoemaker; Experiments in Physical
Chemistry, 7th edition. McGraw-Hill, 2003, pp.380-385
- R.A. Potts, J. Chem. Ed. 51, 539
(1974).
- Chem 10 Lab manual, Department of Chemistry, Dartmouth
College
- I. Struganova, I. J.
Phys. Chem. A 104, 9670-9674 (2000)
- T.D. Slavnova, A.K. Chibisov, H. Gorner, J.
Phys. Chem. A 109, 4758-4765 (2005)
Last updated 03/14/07.