4.2: Proceedure (Part I) – Collect Spectra, Apply PIB

Materials

• Equipment:
  • UV/Vis spectrometer (you will use either an Agilent Cary 60 Spectrometer or an Agilent Cary 100 Spectrometer)
  • Cuvettes (you will use glass cuvettes, which are optically transparent to visible (but not to UV) light)
• Reagents:
  • Reagent grade methyl alcohol (methanol)
  • Stock solutions of four polymethine dyes dissolved in methanol (solutions are approximately 10^–4 M):
    • 1,1'-diethyl-2,2'-cyanine iodide
    • 1,1'-diethyl-2,2'-carbocyanine chloride (also known as pinacyanol chloride)
    • 1,1'-diethyl-2,2'- dicarbocyanine iodide
    • 1,1'-diethyl-4,4'-carbocyanine iodide (also known as cryptocyanine)

Note

The cyanine dyes used in this experiment are toxic irritants. Solutions of these dyes in methanol must be handled wearing gloves at all times. Perform all dilutions in a fume hood. Read the safety data sheets for these dyes (http://www.sigmaaldrich.com) prior to the lab.

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Procedure

The Group Notebook (ELN) Template for this experiment is here: Dyes Group Notebook Template.  

This template is provided as a guide so that you know what data and information should be kept in a notebook. You may keep your personal notebook in any way you'd like, but you should translate your notes into this template for the group submission.

A. Prepare the Instrument

To ensure that the instrument is calibrated and functioning well, vialidate the wavelength accuracy.  To do this, perform instrument diagnostics tests using the Validate application. (See Operation Instructions for Agilent Cary Spectrophotometers)

B. Determine Experimental  \(\lambda_{max}\) from Absorption Spectra:

1. Fill four standard glass cuvettes approximately 4/5 full with methanol (about 2.5 - 3 mL). Then add 3-15 drops of one dye to each cuvette. Add enough dye that you can see a light color for each solution. 
2. Use the UV-vis to record the spectrum of each dye. If needed, adjust the solution concentration of one or more of the dye samples until the peak maximum absorbance reading is approximately 0.2 to 1.0 absorbance units.
3. Overlay the spectra of the four dyes.  Annotate the resulting overlaid spectra: label each dye spectrum and and label its associated \( \lambda_{max} \) value.
4. Save the samples for steps C and D.

C. Determine the effect of Concentration on the spectrum, and specifically on \(\lambda_{max}\)

1. Choose one of the four dyes to use as a representative sample.
2. Create a sample in which the concentration of that dye is different by a factor of 2 (divide in half or double it).
3. Use the UV-vis to record the spectrum.
4. Create an overlay that shows the effect of concentration on the spectrum. Annotate the spectra to include the name of the dye and the \(\lambda_{max}\) determined at each concentration.

D. Determine the effect of Mixing on the spectrum, and specifically on \(\lambda_{max}\).  

1. Choose two dyes that have \(\lambda_{max}\) values that differ by at least 100 nm. 
2. Using the solutions from part B, create a new sample by mixing two of the dyes together in equal volumes.  
   *Note that this does not necessarily mean that the concentrations are equal.
3. Use the UV-vis to record the spectrum.
4. Create an overlay that experimentally demonstrated the effect of mixing. Annotate the spectra to include the names of each sample the \(\lambda_{max}\)'s determined for each sample.

E. Use Particle in the Box model to calculate theoretical \( \lambda_{max} \)

1. Determine \(\gamma\) for the series of homologous dyes (using data from part B).

   Use the Matlab script called dye_v3 to determine the "best" value of \( \gamma \) This script will prompt you for information about the four dyes, and the experimentally measured values of \( \lambda_{max} \). It will solve the particle in a box equation for \( \lambda \), separately for each dye, over the range of \( \gamma \) from 0.00 to 2.00 in steps of 0.01. It will sum the absolute deviation between the calculated \( \lambda \) and your experimentally measured \( \lambda_{max} \) for each value of \( \gamma \), for each dye. It will output the one value of \( \gamma \) that produces the smallest summed deviation over the series of the four dyes. That is, the script will calculate the value of gamma which gives the best fit between peak wavelengths calculated using the free electron model and peak wavelengths measured experimentally. To use this script, start up a Matlab session and type the dye_v3 command:

   dye_v3

   Then simply enter the information as prompted by the script.

2. Use the output value of \( \gamma \) to calculate a value for \( \lambda_{max} \) for each of the four dyes using the equation \[ \lambda=\frac{8mca^2}{h} \frac{\left ( p+3+\gamma \right )^2}{p+4} \]

(Remember that the value of the average bond length of a carbon-carbon bond, a, is 1.39 x 10^–8 cm the mass of the electron, m, is 9.1 x 10^–31 kg.)

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Thinking about the Data (Post-Experiment Assignment)

This part of the assignment is where you will solidify and demonstrate your understanding of the theories applied to experimental results.  Each question below should be answered thoroughly and clearly in typed text.  Your writing should be accurate and grammatically correct. While you may discuss these ideas with your peers, the work submitted for grading must be your own.  Please refer to the Individual ELN Template for the latest version of this assignment.

A. Tabulate your data.

First, Tabulate your data.  An example table is provided:

 B. Describe Experimental Data (Results)

• Compare λ_max values to literature: Reference the table above and assess how your experimental results compare to literature λmax values for each dye. Report all values with appropriate uncertainty and explain how you determined the uncertainty. 

• Experimental trends: Explain how λ_max changes across the dye series in terms of conjugation length and π-electron count.

C. Discuss Beer’s Law

• Interpret the effect of concentration and mixing: Explain how dilution and mixing affect λ_max of each dye, drawing on the experimental evidence and the theory of Beer’s Law to justify your reasoning.

• Compare the experimental results to your initial predictions and explain any discrepancies.

D. PIB Model

• Evaluate the PIB model: Assess how well PIB predictions agree with experiment. Use quantitative comparisons to support your evaluation. Report PIB values with appropriate uncertainty and explain how you determined the uncertainty.

• Justify deviations: If values differ from predictions or trends, provide evidence-based explanations (structural features, model assumptions, or experimental error).

• Reflect on models: Consider the usefulness and limitations of the PIB model. How might more sophisticated models (e.g., Hückel MO) improve predictions?

• Connect to practice: Explain how Part I of this experiment illustrates the role of simple models in physical chemistry—what do we gain, and what do we lose, when we rely on approximations.

E. Extension Exercise:

Calculate the wavelength (in nm) and minimum energy (eV) of the molecule 1,3,5,7-octatetrene ( CH2=CHCH=CHCH=CHCH=CH2 ) using the particle-in-a-box model and the HMO models. What color does the molecule absorb from white light? What color does it then appear to the eye? 

PIB Calculations: 

Hint: The PIB equation given for the conjugated dyes should be re-worked before doing this calculation. The skeletal structure is drawn for you:

[calculate \(\Delta\)E in nm and answer the question about the color of light absorbed and thus the color the molecule would appear.]