Singlet excited state

A **singlet excited state** is a quantum mechanical state of a molecule or atom in which one of the electrons has been promoted to a higher energy orbital, but all electron spins remain paired (opposite). This is in contrast to a triplet excited state, where two unpaired electrons have parallel spins. The term "singlet" refers to the overall spin multiplicity (2S+1 = 1, where S=0); thus, all spins are paired and there is no net magnetic moment[5][7]. When a photon is absorbed by a molecule in its (typically singlet) ground state, it is usually promoted to a singlet excited state of higher energy (often denoted S1 for the first excited singlet), from which it can undergo various processes such as fluorescence, internal conversion (non-radiative loss of energy), or intersystem crossing to a triplet state[1][3][5]. Singlet excited states typically have very short lifetimes (on the order of nanoseconds)[3]. The concept of "singlet excited state" is a fundamental quantum state, not a discrete molecular entity or druggable target. It is widely referenced in photochemistry, photophysics, and spectroscopy[1][3][7]. Caveats/Limitations: - **"Singlet excited state" is not a molecule, receptor, or structured biological target**, but rather a general electronic state that any molecule with paired electrons can transiently occupy after absorbing energy[5][7]. - It is not used as a therapeutic target, and no drugs are known or designed to act directly on "the singlet excited state." - The term does not map to a unique protein, nucleic acid, or other biomolecule, and is best treated as a physical/quantum chemistry concept, not a targetable entity.

Other names

Excited singlet stateS1 (when referring specifically to the first singlet excited state)electronically excited singlet state