Term: π → σ* transition
https://doi.org/10.1351/goldbook.S04387

Definition:
An electronic transition described approximately as a promotion of an electron from a 'bonding' orbital π to an 'antibonding' orbital σ designated as σ*. Such transitions generally involve high transition energies and appear close to or mixed with Rydberg transitions.

Related Terms:
1) promotion (http://doi.org/10.1351/goldbook.P04877).
2) rydberg transitions (http://doi.org/10.1351/goldbook.R05432).

Source: PAC, 1996, 68, 2223. 'Glossary of terms used in photochemistry (IUPAC Recommendations 1996)' on page 2266 (https://doi.org/10.1351/pac199668122223)

Citation: 'π → σ* transition' in IUPAC Compendium of Chemical Terminology, 5th ed. International Union of Pure and Applied Chemistry; 2025. Online version 5.0.0, 2025. https://doi.org/10.1351/goldbook.S04387

License: The IUPAC Gold Book is licensed under Creative Commons Attribution-ShareAlike CC BY-SA 4.0 International (https://creativecommons.org/licenses/by-sa/4.0/) for individual terms.

Disclaimer: The International Union of Pure and Applied Chemistry (IUPAC) is continuously reviewing and, where needed, updating terms in the Compendium of Chemical Terminology (the IUPAC Gold Book). Users of these terms are encouraged to include the version of a term with its use and to check regularly for updates to term definitions that you are using.