Analysis of Cyanine Dye Spectra (Worksheet)
- Page ID
- 67399
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Different members of your team should analyze the data in the following table (Table of Data for Cyanine Dyes) for different series of cyanine dyes. Each person should do at least 2 series. You should combine and use all of the results in answering the following questions. Also, more than one person should analyze each series to validate the result.
- Construct an expanded version of the table. The following additional information should be included in your table for each molecule: the number of pi electrons, the number of pi-bonds in the chain, the length L, and for each series the average bond length β and overshoot δ. L, β, and δ will be determined in parts (b) and (c) below.
- For each molecule in the table, use the particle-in-a-box model to determine the length L of the pi electron box.
- For each series of dyes, determine a value for the average bond length β and the overshoot δ from a plot of L vs b, where b is the number of bonds. Plot all series are the same graph. Explain why the plots are essentially parallel but displaced from each other.
- Do the bond length values β differ significantly from that for benzene (0.139 nm) or vary significantly within a series or between series? Can variations in the overshoot, δ, be explained by the nature of the end groups?
- For series A, predict the peak absorption wavelength for the next molecule in the series.
- Prepare an energy-level chart for the cyanine dyes. Plot En on the left vertical axis, n on the right vertical axis, and the number of carbon atoms in the pi system on the horizontal axis. Include at least eight energy levels in your diagram. How does the spread of energy levels vary with increasing number of atoms in the conjugated chain? For each of the molecules indicate in your diagram (by drawing vertical arrows from a lower state to a higher state), the transition that gives rise to the intense absorption band. How does the energy of these intense transitions vary with increasing length of the conjugated section of the molecules? Are the relationships between energy and the quantum number n, and energy and the length L, discrete or continuous? Write a paragraph describing the diagram for one series of cyanine dyes.
- Write a paragraph discussing how and why the absorption coefficient for a cyanine dye changes with the size of the molecule and the number of pi electrons.
- Comment on how well you think this model describes the electronic energy level structure of these molecules. What features of these molecules would you take into account if you were to develop a better theoretical description of the electronic structure?
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Absorption maximum (nm) |
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| Simpson I |
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| Simpson II |
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| Simpson III |
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| Kuhn I |
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| Kuhn II |
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| Kuhn III |
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| Kuhn IV |
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| Kuhn V |
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| Kuhn VI |
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| Kuhn VII |
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| Kuhn VIII |
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| Kuhn IX |
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| Kuhn X |
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| Kuhn XI |
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| Kuhn XII |
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| Kuhn XIII |
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- Series A: W.T. Simpson, J. Chem. Phys., 1948, 16, p 1124. Series B-D: H. Kuhn, Helv. Chim. Acta, 1948, 31, p 1441, J. Chem. Phys., 1948 16, p 840. See also: Richard S. Moog, J. Chem. Educ., 1991, 68 (6), p 506