Exciplex

An exciplex is a transient excited-state complex formed between two chemically distinct species, typically a donor molecule in an excited electronic state (D*) and an acceptor molecule in the ground state (A). The complex [D*A] is held together by noncovalent interactions and forms upon photoexcitation or energy transfer. If the excited complex relaxes, it emits light in a process known as exciplex emission.

Exciplex emission is usually broad, structureless, and red-shifted compared with the monomer emissions of the individual components, reflecting its charge-transfer character. The formation often occurs in solution or condensed phases where the donor and acceptor can diffuse into contact; solvent polarity can influence stability and spectral properties. Exciplexes are commonly observed with electron-rich donors and electron-deficient acceptors, and they are distinct from excimers, which involve two identical molecules in an excited state.

Applications and significance: Exciplexes are used to study intermolecular interactions and can contribute to luminescence in organic light-emitting devices, photochemical systems, and sensors. They also provide insight into energy transfer processes and the factors that govern excited-state stability. Lifetimes of exciplex emission typically range from several nanoseconds to microseconds, depending on the specific molecular pair and environment.