In spectroscopy, bathochromic shift (from Greek βαθύς (bathys) 'deep' and χρῶμα (chrōma) 'color'; hence less common alternate spelling "bathychromic") is a change of spectral band position in the absorption, reflectance, transmittance, or emission spectrum of a molecule to a longer wavelength (lower frequency).^[1] Because the red color in the visible spectrum has a longer wavelength than most other colors, the effect is also commonly called a red shift.

Hypsochromic shift is a change to shorter wavelength (higher frequency).

It can occur because of a change in environmental conditions: for example, a change in solvent polarity will result in solvatochromism.^[2]

A series of structurally-related molecules in a substitution series can also show a bathochromic shift. Bathochromic shift is a phenomenon seen in molecular spectra, not atomic spectra; it is thus more common to speak of the movement of the peaks in the spectrum rather than lines.

${\displaystyle \Delta \lambda =\lambda \!_{{\text{state 2}} \atop {\text{observed}}}-\,\lambda \!_{{\text{state 1}} \atop {\text{observed}}}}$

where ${\displaystyle \lambda }$ is the wavelength of the spectral peak of interest and ${\displaystyle \lambda \!_{{\text{state 2}} \atop {\text{observed}}}>\,\lambda \!_{{\text{state 1}} \atop {\text{observed}}}.}$

Bathochromic shift is typically demonstrated using a spectrophotometer, colorimeter, or spectroradiometer.

• Chromism
• Solvatochromism
• Spectroscopy