DIPHENYLPOLYENE DYE SPECTRA UV-VISIBLE SPECTROSCOPY PARTICLE IN THE BOX

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…

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)

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.

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.