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| Intensity and Absorption Shifts in UV-Visible Spectroscopy |
There are different terms related to UV-visible spectroscopy that describe the types of spectral shifts observed in the sample, including hyperchromic, bathochromic, hypsochromic, and hypochromic shifts. These shifts are defined below:
Hyperchromic Shifts
A hyperchromic shift is an increase in the absorbance intensity or the magnitude of the electronic transition after changes in the environment, concentration, or other factors. A hyperchromic effect occurs when the absorbance intensity of the electronic transition is increased due to a change in the environment.
One example of a hyperchromic shift is the denaturation of proteins, such as albumin, caused by increase temperature or other factors. It has been found that the absorbance of albumin increases with temperature, leading to a hyperchromic shift. Hyperchromic shifts have also been observed in particular organic dyes that exhibit a positive solvatochromic effect.
Hypochromic Shifts
A hypochromic shift is a decrease in the absorbance intensity or magnitude of the electronic transition for a given concentration of an analyte. A hypochromic effect occurs when light absorption intensity decreases due to the interactions between the molecules in the solution.
The hypochromic shift, also known as a blue shift, refers to a decrease in the absorption intensity or molar absorptivity (ε) of a molecule. This shift is observed as a reduction in the peak height or the absorbance value in the UV-Vis spectrum. The hypochromic effect can be caused by various factors, including changes in the molecular environment, intermolecular interactions, or alterations in the electronic structure of the molecule.
An example of a hypochromic shift is the non-additive behavior of spectral shifts observed in binary mixtures of dyes. The interaction between the two dyes reduces the absorbance intensity due to the lowering of the dye’s transition dipole moment.
Bathochromic Shifts
A bathochromic shift is a spectral shift to lower energy or longer wavelength. A bathochromic shift takes place when a conjugated system inspires the energy levels to lower energy than those of an isolated molecule.
The term "bathochromic" is derived from the Greek words "bathos" meaning depth and "chroma" meaning color. A bathochromic shift, also known as a red shift, refers to a shift in the absorption wavelength of a molecule to longer wavelengths, resulting in a change towards the red end of the visible spectrum. In UV-Vis spectroscopy, the absorption of light by a molecule causes the promotion of electrons from lower energy levels (ground state) to higher energy levels (excited states). The energy difference between these levels corresponds to a specific wavelength of light.
A classic example of a bathochromic effect is the shifting of an azo dye's maximum absorbance towards a longer wavelength upon the addition of an electron-donating substituent to the molecule. This long-wavelength shift is due to the donation of electrons extending the π-electron system, which lowers the energy levels of the molecular orbitals involved in the electronic transition.
Hypsochromic Shifts
A hypsochromic shift is a spectral shift towards higher energy or shorter wavelength. Hypsochromic shifts happen whenever a transition is motivated into higher energy levels of an organic chromophore.
An example of a hypsochromic effect is the shift of the maximum absorbance of a -CN group in phenyl-CN towards shorter wavelengths compared to phenyl-H.
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| Shifts in UV-Visible Spectroscopy |
In conclusion, the shifts observed in UV-visible spectroscopy are due to changes in the molecular electronic structure, concentration, environment, and intermolecular interactions in the sample. Spectral shifts, such as hyperchromic, bathochromic, hypsochromic, and hypochromic, provide valuable information on the molecular characteristics of the sample, making UV-visible spectroscopy a powerful analytical technique.
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