bathocromico

Bathocromico, or bathochromic shift, is a term used in UV-Visible spectroscopy to describe the movement of a molecule’s maximum absorption wavelength (lambda max) to longer wavelengths, i.e., a shift toward lower energy. The term comes from Greek bathos, meaning depth, and chroma, color. In contrast, a shift to shorter wavelengths is called a hypsochromic or blue shift.

Causes and mechanisms are varied. Increased conjugation or planarity of a molecule lowers the energy gap between the ground and excited states, favoring absorption at longer wavelengths. Intramolecular charge-transfer interactions, often enhanced by donor–acceptor substituents, can produce bathochromic shifts. Solvent effects also play a major role: polar or hydrogen-bonding solvents can stabilize the excited state more than the ground state, leading to solvatochromism and red-shifts in many dyes. Aggregation, complexation with metals, and changes in the molecular environment (such as protonation) can also induce bathochromic shifts.

Applications and relevance include the design of dyes and pigments with visible or near-infrared absorption, optimization of organic light-emitting diodes and solar cells, and the tuning of spectral properties for fluorescence probes. The term is frequently used alongside related concepts such as solvatochromism and hypsochromic shifts to describe how environmental or structural changes modify a molecule’s optical properties.

See also: hypsochromic shift, solvatochromism, spectral tuning.