Particle in a Box: Absorption Spectrum of Conjugated Dyes PDF

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The purpose of this experiment is to determine the value of the empirical parameter α and determine the length of a conjugated carbon chain for a set series of dyes by a UV/VIS spectrometer....

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Physical Chemistry (1.30204) Practical: Particle in Box

Name ID # Major Lecturer Practical Date Due Date

Max Yeki 20181070 Chemistry Mr Yuguru 20th /07/ 2021 06th /08/ 2021

Title: Particle in a Box: Absorption Spectrum of Conjugated Dyes Abstract: The absorption spectrum of a series of conjugated dyes is produced by a UV/VIS spectrometer to calculate the empirical parameter α and to find the length of the conjugated chain using the free electron model. Diluted solutions of each dye were analyzed using a UV/VIS spectrometer to determine the wavelength of maximum absorbance (λmax) for each dye. Using this wavelength, the best fit α is determined to be equal to 1 and the length of the box of , 1, 1‟ – Diethyl -2, 2‟ – dicarboncyanine is 1.727 x 10-10 m. Purpose: The purpose of this experiment is to determine the value of the empirical parameter α and determine the length of a conjugated carbon chain for a set series of dyes by a UV/VIS spectrometer. Introduction: The majority of colors that we see result from transitions between electronic states that occur as a result of selective photon absorption. For a molecule to absorb a photon, the energy of the impinging photon must match the energy difference between the initial state and some excited state of the molecule. We can describe this concept using the equation. Ephoton = hv = Emolecule = Eupper state – Elower State Molecules of a colored object absorb visible light photons when they are excited from their lowest-energy electronic state (called the “ground state”) to a higher-energy electronic state (called an “excited state”).

The transition between energy levels gives out light when detected UV-Vis Spectrometer.

(1)

In this experiment, light absorption properties of a carefully chosen set of organic molecules were measured and relate the absorption spectra to the particle in a box quantum mechanical model for the electrons. Procedure: (done by others) The experimental procedure that was followed can be found in the practical manual. Results calculation: Part A A: Pinacyanol chloride

MM = 388.9 g/mol j = 3 double bonds N = 2j + 2 = 2(3) + 2 = 8 pi electrons L = 1.39 x 10-10 m a = (2j + 2)L

B: 1,1’-Diethyl-2,2’ cyanine iodide

j = 2 double bonds N = 2j + 2 = 2(2) + 2 = 6 pi electrons L = 1.39 x 10-10 m a = (2j + 2)L

C: 1,1’-Diethyl-2,2’-dicarbocyanine Iodide

N = 2j + 2 = 2(4) + 2 = 10 pi electrons L = 1.39 x 10-10 m a = (2j + 2)L

Part B: 1. Calculate the value of ‘a’. Formula: Where L can be calculated from the equation:

, where λ is taken from

highest maximum wavelength. So in this case was, 1, 1‟ – Diethyl -2, 2‟ – dicarboncyanine iodide for it has ranged with 700nm.

Therefore;

√

=√

=√

=1.727 x

10-10 m Now find the „a‟ value: a = (2j + 2)L = (2(3) + 2)(1.727 x 10-10) = 1.3816 x 10-9 m Since the „s‟ value (700 nm) has been known from the spectrum of , 1, 1‟ – Diethyl -2, 2‟ – dicarboncyanine iodide, use this to find the „a‟ values of other two wavelengths. Dye A B C (used)

a = (2j + 2 + s)L (2(4)+2 + 700 x 10-9) 1.727 x 10-10 (2(2) + 2 + 700 x 10-9)(1.727 x 10-10) (2(3) + 2 + 700 x 10-9)( 1.727 x 10-10)

Length of the box (m) =1.727 X 10-9 = 1.036 X 10-9 = 1.3816 X 10-9

2. Now find Predicted box length (nm). Hence L = 1.39 x 10-10 m Dye a = (2j + 2 + s)L Predicted box length (m) (2(4) + 2 +8.947 x 10-07) 1.39 x 10-10 1.39 x 10-10 A 8.34 x 10-10 (2(2) + 2 +5.0594 x 10-07) 1.39 x 10-10 B (2(3) + 2 + 700 x 10-9) 1.39 x 10-10 C 1.112 x 10-9

3. Calculation of maximum wave length of the dyes. Since „a‟ values have been known, put into equation:

to find the maximum length of two other spectrums

(pinacynol chloride and 1,1-Diethyl-2,2‟-cynide lodide.

4. Dye A (pinacynol chloride) B (1,1-Diethyl2,2‟-cynide lodide)

maximum wave length (m) 8.947 x 10-07 5.0594 x 10-07

Absorption Spectra for the dyes

Figure 1: The absorption spectra of each conjugated dye. All three dyes have a different λmax. The spectrum of each dye was shown above. All of the conjugated dyes used have the largest absorbance at different wavelengths of light. Using the spectra shown, the λmax is determined by observing for each dye where the absorbance was the highest. The calculated length of 1, 1‟ – Diethyl -2, 2‟ – dicarboncyanine was 1.727 x 10-10 m. The accepted length of was 1.23 x 10-10. Thus the percentage difference was approximately 40%. In the free electron model, α represents a constant for a series of dyes of a given type.1 In this lab, α represents the penetration distance which lies between 0-1. Therefore, if α = 1, this signifies that light or any electromagnetic radiation can penetrate a significant distance into the conjugated dyes. Finally, the length of the box for each dye molecule from the absorption spectra was calculated by treating the system as a 1-D particle in a box. Conclusion: The experiment showed that finding the wavelength of maximum absorbance of a conjugated dye can be used to find the empirical parameter α and the length of a conjugated system within the dye. It was also seen that the lengths of the box calculated increase with the number of carbon atoms.

Reference: 1. Atkins, J.P,: Paula, J.D. Physical Chemistry 10th ed. Oxford Press: UK. 2010 2. Garland, C.W.; Nibler, J.W.; Shoemaker, D.P. Experiments in Physical Chemistry; McGraw-Hill: New York, 2009; 393-398...