Essay UV-Vis Spectroscopy and Photochemistry – Cyanine Dyes, Carrot Colour and Iodine – Science Assignment Help

Assignment Task:

Task:

Colour and chemistry are closely related because the spectroscopic properties that determine colour are linked directly to molecular structure and energetics. In this experiment, you will explore this relationship.
 

In Part I of this experiment, you will study electronic spectra of some linear conjugated molecules. You will look at some dye molecules that will act as models particle-in-a-box theory. The structure of the dye is varied systematically in a way that illustrates the relationship between the theory, molecular structure and absorption of photons. You will use cyanine dyes as real-world examples of the “box”. The dyes and other related chemical species are used in writeable CD and DVD media, and as fluorescent probes for biological applications, such as proteomics studies. You will also look at a biologically relevant example of a linear conjugated molecule, that being β- carotene, the molecule that gives carrots and many other biological systems their colour.
 

We will be exploring the interaction of ultraviolet and visible light with molecules. The energy of the photon is sufficiently large that electrons in the molecules will be excited to higher energy states; hence we are employing electronic spectroscopy in these studies. Electronic spectra may also contain vibrational, and even rotational, fine structure. In Part II of this experiment, you will record the absorption due to a single electronic transition of molecular iodine, I2. The spectrum comprises many peaks, arising from different amounts of vibrational excitation accompanying the change in the energy of the excited electron. The ability to relate peaks observed in a spectrum to transitions on an energy level diagram is an important skill for all types of spectroscopy. The measured transition energies will also be used to estimate dissociation energies for I2 in both its ground and electronically excited states.

This experiment is comprised of two parts.

Part I: Electronic Spectra of Cyanine Dyes and β-Carotene – Particle- in-the-Box

• To record the uv-vis (electronic) spectra of three cyanine dyes.
• To record the uv-vis spectrum of β-carotene
• To analyse cyanine dye spectra in terms of the particle-in-a-box model and to compare the electronic transition energy with predictions of the model.
• To explain the colour of carrots in terms of the particle-in-a-box model.
• To relate absorption energies to colour.
Part II: Electronic Spectrum of Iodine Vapour – Vibronic Spectra
• To record the electronic absorption spectrum of gaseous iodine and interpret it in terms of the Franck-Condon principle.
• To use the vibrational energies to calculate vibrational parameters including the anharmonicity constant, for the excited electronic state.
• To use the wavelength of the continuum limit to calculate the dissociation energy of the electronically excited state.
• To estimate the dissociation energy of the ground electronic state from the continuum limit and the excitation energy of atomic iodine.