Bathochromic shift and hypsochromic shift are terms used in the context of UV-Vis (ultraviolet-visible) spectroscopy to describe changes in the absorption wavelength of a molecule. These shifts are often caused by changes in the molecular environment, such as solvent polarity, or by structural changes in the molecule itself (e.g., conjugation, protonation/deprotonation).


1. Bathochromic Shift ("Red Shift")

  • Definition: A bathochromic shift refers to a shift in the absorption maximum to a longer wavelength (lower energy).
  • Explanation: This shift occurs when the energy difference between the ground state and the excited state decreases, meaning it requires less energy (longer wavelength) for the electron to be excited.
  • Causes:

    • Increased conjugation: A molecule with extended conjugation (like a longer Ï€-system in conjugated organic compounds) often shows a bathochromic shift.

    • Solvent effects: Polar solvents can stabilize the excited state more than the ground state, causing a bathochromic shift.

    • Example: If a molecule’s absorption shifts from 300 nm to 350 nm, this is a bathochromic shift.

2. Hypsochromic Shift ("Blue Shift")

  • Definition: A hypsochromic shift refers to a shift in the absorption maximum to a shorter wavelength (higher energy).
  • Explanation: This shift occurs when the energy difference between the ground and excited states increases, requiring more energy (shorter wavelength) to excite an electron.
  • Causes:

    • Decreased conjugation: If a molecule's conjugation is disrupted (e.g., by protonation or changing the structure), it often leads to a hypsochromic shift.

    • Solvent effects: Non-polar solvents or other environmental factors that stabilize the ground state more than the excited state can result in a hypsochromic shift.

    • Example: If a molecule’s absorption shifts from 400 nm to 350 nm, this is a hypsochromic shift.

Summary

  • Bathochromic Shift (Red Shift): Shift to longer wavelengths, lower energy.
  • Hypsochromic Shift (Blue Shift): Shift to shorter wavelengths, higher energy.

Both shifts provide valuable information about the molecular structure and the environment of a substance when analyzed using UV-Vis spectroscopy.