UV-Visible Spectroscopy

Introduction

Ultraviolet-visible spectroscopy corresponds to the region of the electromagnetic spectrum between 200 - 400 nm for UV and 400 - 800 nm for visible light. The process involves the excitation of the highest energy electrons in the molecule. These electrons would be located in the HOMO (the Highest Occupied Molecular Orbital), which are excited to the LUMO (the Lowest Unoccupied Molecular Orbital). In general these transitions would involve either π or lone pair electrons (n) of the molecule. Extended conjugated systems tend to have the stronger absorption.
The lowest energy transition possible is between the HOMO and LUMO as shown in the figure right. The absorption of electromagnetic energy excites one of the HOMO electrons moving it to the LUMO, creating an "electronically excited state". The HOMO - LUMO gap, ΔE, depends on the extent of the π system. The longer or larger the π system, the lower the ΔE, and therefore the longer the wavelength of light absorbed. The colors seen in dyes, inks, flowers etc. are due to highly conjugated organic molecules (examples are given below). The region of the molecule responsible for absorbing the EM energy is called the chromophore, and most commonly involve C=C (π → π*) and C=O (n → π*) transitions. electronic excitation in UV-Vis spectroscopy

The table below lists the wavelength of strongest absorption and the maximum absorbency (εmax) for three π systems of increasing length.

H(CH=CH)nH
     
λmax (nm)
     
εmax
     
Compound
1
   
170
   
15,000
   
ethene
2
   
217
   
21,000
   
1,3-butadiene
3
   
258
   
35,000
   
1,3,5-hexatriene

Color

So how does the wavelength of absorption relate to the color of a compound? First we need to know the color of the different λ of visible light.

The Visible Spectrum
Color
Wavelength (nm)
ΔE HOMO - LUMO gap (eV)
color wheel
UV
100 - 400
12.4 - 3.10
Violet
400 - 425
3.10 - 2.92
Blue
425 - 492
2.92 - 2.52
Green
492 - 575
2.52 - 2.15
Yellow
575 - 585
2.15 - 2.12
Orange
585 - 647
2.12 - 1.92
Red
647 - 700
1.92 - 1.77
Near IR
700 - 10,000
1.77 - 0.12
If the compound absorbs in one region of the spectra, it appears with the opposite (complimentary) color, since all of the absorbed color has been removed. For example:
  • the material absorbs violet light ⇒ color is yellow
  • the material absorbs blue light ⇒ color is orange
  • the material absorbs yellow-green light ⇒ color is red-violet

Example Spectra
 
UV-Vis spectra of:                
UV-Vis Spectra of some common dyes and indicators
             
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© 2007 R. Spinney