 Hypochromic effect

The hypochromic effect is a decrease in the intensity of an absorption band, the opposite of the hyperchromic effect. It describes absorption strength rather than a shift of the band's wavelength.^[ref]

Under the Beer-Lambert law, $A=\epsilon c\ell$, so comparing molecular absorption strength requires accounting for concentration $c$ and optical path length $\ell$. Diluting a sample decreases its absorbance without establishing a decrease in its molar absorption coefficient.

An original example keeps concentration, path length and band shape fixed, but reduces the peak absorbance from0.80 to0.60. The relative decrease in the peak molar absorption coefficient isa25% hypochromic effect. Yet the transmitted fraction increases from $10^{-0.80}\simeq 0.158$ to $10^{-0.60}\simeq 0.251$. Weaker absorption means more transmitted light, not less.

A useful measurement compares complete spectra in matched cuvettes with a spectrophotometer, using corresponding solvent blanks and correcting any dilution caused by mixing. Full spectra help distinguish a decrease in band strength from a band moving away from one monitored wavelength. If the corrected peak strength changes while the peak position stays fixed, the measurement directly separates those two effects. A reduced band alone does not uniquely establish the molecular arrangement responsible for it.