electronic structure

Absorption Spectra of Conjugated Dyes

Introduction

In this experiment Ultraviolet-Visible spectroscopy is used to explore the electronic structure of several conjugated polyene dyes, and the Particle-in-a-Box model is used to extract structural information. The bands for polyene dyes arise from electronic transitions involving the π- electrons along the chain of the molecule and the associated wavelength of these bands depends on the spacing of the electronic energy levels.

Conjugated polyenes, such as β-carotene, are ubiquitous pigments in nature and generally absorb light in the visible portion of the electromagnetic spectrum. These polyenes have a structure with alternating π-electron character (i.e., double bonds) in which the electrons are delocalized over the entire conjugated system. By changing the size of the conjugated system, the effective “box” length over which the electrons can move is changed. This crude model of electrons moving along a chain of carbon atoms can be successfully modeled with the Particle-in-a-Box quantum mechanical model.

*Note on wavelengths: If only changes in electronic energy accompany absorption of light, a very sharp maximum in absorption should be observed at the characteristic wavelength. Although sharp lines are observed for isolated atoms, broad absorption bands are observed for substances in liquid phases (due to accompanying vibrational and rotational transitions). In this experiment, we shall assume that the wavelength maximum (the wavelength at which the dyes absorb most strongly) is the wavelength to use in the calculations.

EXPERIMENT

  1. Prepare stock solutions of the dyes with methanol as the solvent (approx. 10^-5 M)

  1. Prepare dilute solutions of select dyes such that the final concentration will give an

absorbance between 0.5 and 1.0 for a 1cm path length

  1. Recall Beer’s Law; A = εcb & reference the extinction coefficient

  1. Please check your calculations in order to minimize the amount of solvent

  1. Record spectra against a solvent reference, scanning from 400-800 nm

  1. Save and print your spectra

  1. Determine maximum wavelength for each dye

  1. Dispose of used methanol/dye solutions in the appropriate waste container

*Safety note: The dyes being used in this experiment are toxic. Be advised and avoid getting the solutions on your skin. In addition, these dyes slowly degrade in the presence of light, so attempt to keep the solutions stored in the dark when not in use.

ADDITIONAL ANALYSIS

  1. Complete the analysis questions found in the text book under experiment #34 a. Do not do the “theoretical calculations” section

  1. Plot the UV-Vis absorption spectra yourself (Do NOT just include a screen shot) and label the maximum wavelength for each dye

  1. Calculate the effective box length, , for the dyes

a. Include a sample calculation

  1. Include a table showing the name of the dye, total number of π-electrons, maximum wavelength, and box length

  1. Compare experimental length with geometric box length xb, where is the number of bonds in the conjugated system and b=0.139 nm (assuming uniform spacing)

a. Evaluate and tabulate the difference 6. Is the Particle-in-a-Box an appropriate description of these conjugated systems?

Strawberries

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