The UV-vis spectra of conjugated dyes are interpreted using the multielectron particle-in-a-box model

Type :

Research papers

Pages :

8 pages

Format :

.doc

Published date :

04/25/2009

Category :

Science & technology

Sub category :

Physics & chemistry

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Summary :

Introduction
Results
Discussion
Conclusion
References

Abstract

The purpose of this lab is to observe how a real system may be approximated using the simple quantum mechanical model of a one-dimensional particle-in-a-box. Due to the arrangement of the electrons in the conjugated dyes, some electrons are free to move along the molecule's carbon-carbon chain like a particle-in-a-box. Therefore, the main concept of this lab is to relate the UV-vis absorption spectrum of a dye to the particle-in-a-box model, and to find which model fits the experimental values best. model A is to set the edge of the box as the nitrogen atoms, model B is to extend the box one bond beyond the nitrogen atoms while keeping the number of electrons the same, and model C is adjusting the length by an empirical correction factor.
Absorption spectra have particular characteristics wherein the wavelength given is representative of the energy absorbed and is related to the electronic structure of the molecules through a simple quantum mechanical model (for a one dimension schema) known as particle in a box. The color of each of the dyes is due inherently to the chromophore, which is a system of conjugated double bonds.1 The structure of the chromophore can be represented as two equivalent resonant hybrid orbitals (depending on the particular pairs of atoms chosen to be connected with pi-bonds/ and on the dye). The molecular orbitals, constructed from the p-orbitals on the C and N atoms, delocalize the pi-electrons along the entire length of the system. The difference in energy of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) is small.1 Therefore, the energy of the photon required to promote the electron between the levels is quite low- and falls within the visible range of the spectrum- giving rise to the colors of the dyes.