Bathochromic and Hypsochromic Shifts

Bathochromic Shift

• The bathochromic shift occurs when an absorption maximum shifts towards longer wavelengths or lower energy.
• This shift is caused by:
  • The presence of an auxochrome
  • Changes in solvent polarity
• Red shift is another name for the bathochromic shift because red has a longer wavelength than other colors in the visible spectrum.

Hypsochromic Shift

• The hypsochromic shift occurs when an absorption maximum shifts towards shorter wavelengths or higher energy.
• This shift is caused by:
  • The presence of an auxochrome
  • Changes in solvent polarity
• Blue shift is another name for the hypsochromic shift because blue has a shorter wavelength than other colors in the visible spectrum.

Hyperchromic Effect

• The hyperchromic effect causes increased absorption intensity.
• This is typically caused by the introduction of an auxochrome.
• For example, aniline, with the auxochrome NH2, shows a higher εmax (1430) than benzene (200) at 256nm.

Hypochromic Effect

• The hypochromic effect causes decreased absorption intensity.
• It's caused by a group that distorts the geometry of the molecule.
• For example, 2-methyl biphenyl has a lower εmax (10250) than biphenyl (19,000) due to the methyl group distorting the chromophore.

Chromophore

• A chromophore is a group of atoms that absorbs UV/Vis radiation.
• It determines the wavelength of radiation absorbed.
• Examples include:
  • C=C (double bond)
  • C=O (carbonyl group)
  • C=N (imine group)
  • N=N (azo group)
  • R-NO2 (nitro group)
• A chromophore will appear colored to our eyes only if it absorbs light in the visible region of the spectrum.
• The wavelength and intensity of absorption are influenced by:
  • The molecular environment of the chromophore
  • The solvent used
  • pH
  • Temperature

Auxochrome

• An auxochrome is a substituent attached to a chromophore.
• It does not absorb UV/Vis radiation on its own, but alters the intensity and wavelength of the chromophore.
• Examples include:
  • Methyl (CH3)
  • Hydroxyl (OH)
  • Alkoxy (OR)
  • Halogen (Cl, Br, I)
  • Amino (NH2)
• The effect of an auxochrome depends on its polarity.
  • Groups like CH3, CH3CH2, and Cl have a small effect, often a small red shift.
  • Groups like NH2 and NO2 have a strong effect and significantly alter the spectrum.
• Auxochromes often increase the absorbance by extending conjugation through resonance, bringing the LUMO closer to the HOMO.

Origin of UV/Vis Spectra

• The UV/Vis spectra is produced by the absorption of electromagnetic radiation in the UV/Vis region.
• This absorption causes transitions between electronic energy levels in the molecule.
• Electrons from the ground state of a molecule become excited to a higher energy state.
• The most common transition is from the HOMO to the LUMO.

Important Terms:

• λmax: The wavelength at which maximum absorption occurs.
• εmax: The maximum molar absorptivity, representing the absorption intensity at λmax.
• HOMO: Highest Occupied Molecular Orbital.
• LUMO: Lowest Unoccupied Molecular Orbital.
• UV/Vis spectrum: A plot of absorbance vs. wavelength.
• Band: Broad bands in the UV/Vis spectra arise due to the transitions taking place over a wide range of wavelengths.