1. DIPHENYLPOLYENE DYE SPECTRA
UV-VISIBLE SPECTROSCOPY
PARTICLE IN THE BOX

2. QUANTUM MECHANICAL MODEL
Delocalized pi electrons in a conjugated dye molecule are constrained to move along the C-C backbone.
What energy states are the pi electrons in? The particle in the box model provides the answer.
En = n2h2/8mL2 where L is the length of the dye molecule, m is the electron mass, h is Planck’s constant and n is 1,2 ,3…

3. QM Model - 2
Light absorbed by the dye molecule will cause the electron to be promoted from ni to nf.
The energy of this transition is
ΔE = (nf2 -ni2)h2/8mL2 = hc/
Measure the absorption spectra for three dye molecules (with increasing L values)

4. BEER’S LAW
Beer’s Law describes the dependence of absorption intensity on dye molecule concentration, [J].
A = ε [J]ℓ where ε is the molar absorption coefficient and ℓ is the cell path length.
A linear least analysis of A vs [J] yields ε.

5. Analysis
Calculate the Particle in the Box predictions for absorption wavelength and compare them to the experimental values.
Modify the Box model using a factor α.
Use the absorbance vs dye concentration data to determine the extinction coefficient for one dye.