bathochromic

Bathochromic shift, also called a red shift, is a phenomenon in spectroscopy where the absorption maximum (lambda max) moves to longer wavelengths. This corresponds to a decrease in the energy difference between the ground state and the first excited state. The term is commonly used for absorption spectra but can also describe shifts in emission spectra. The opposite phenomenon is a hypsochromic, or blue, shift.

Causes and mechanisms include several factors that lower the energy of electronic transitions. Extending the conjugation length or increasing molecular planarity reduces the HOMO–LUMO gap, moving lambda max toward longer wavelengths. Substituents that donate electrons or promote intramolecular charge transfer can stabilize the excited state and produce a bathochromic shift. Coordination to metals and formation of certain aggregates can likewise lower transition energies. Solvent effects also contribute; solvatochromism occurs when a polar solvent stabilizes the excited state more than the ground state, leading to a bathochromic shift.

Applications and examples: In dyes and pigments, bathochromic shifts help explain color changes in different environments. In organic electronics and photochemistry, controlling the shift allows tuning of light absorption. In spectroscopy and sensing, bathochromic shifts indicate changes in conjugation, solvent polarity, protonation state, or binding events. Cyanine dyes show pronounced bathochromic shifts with longer polymethine chains, while metal complexation or aggregation can shift characteristic absorption bands to longer wavelengths.