What is a singlet state?

In chemistry, a singlet state is a molecular electronic state in which all electron spins are paired. This means that the spin of an excited electron is continually paired with a ground state electron.

Electrons may transition between singlet states and triplet states, or an electron pair with parallel spin, which is called intersystem crossing. In a two electron system, intersystem crossing is only possible between the singlet and triplet states. According to Hund's rule, when electron spins are parallel, it is at a lower energy than electrons with paired spins. When electron spins are paired, their individual movements cancel out and a singlet state is achieved.