Biradical

A biradical, also called a diradical, is a molecule that contains two unpaired electrons occupying two singly occupied molecular orbitals. The two unpaired electrons may reside on the same atom or on different atoms, and their spins can couple to form either a singlet state (paired spins, total spin S = 0) or a triplet state (parallel spins, total spin S = 1). The energy difference between these spin states, known as the singlet–triplet gap, determines the ground-state spin characterization of the molecule.

Biradicals are typically highly reactive and short-lived under normal conditions because each unpaired electron can participate in chemical bonding or radical reactions. Stability can be enhanced by steric protection, extended conjugation, or by rigid frameworks that strongly couple the two radical centers; such species are often referred to as biradicaloids. Their behavior is a topic of interest in physical organic chemistry and materials science, where subtle changes in spin coupling influence reactivity and electronic properties.

Generation and detection of biradicals rely on photochemical or thermal activation, redox methods, or radical-generating precursors. Their presence is commonly confirmed by electron spin resonance (ESR or EPR) spectroscopy, sometimes complemented by UV–visible spectroscopy and computational studies to characterize the electronic structure and the singlet–triplet character.

Examples of relevance include biradical intermediates formed in Bergman cyclization of enediynes and other conjugated systems where two radical centers interact. The concept also applies to biradical character in certain polycyclic hydrocarbons and in processes such as singlet fission in organic electronics.