Exciplexes

Exciplexes are excited-state complexes formed between two molecular species that do not interact in the ground state. They typically arise when a molecule acting as a donor is photoexcited and then interacts with a second molecule, the acceptor, to form a charge-transfer complex in the excited state. In the exciplex, partial electron transfer creates a D+–A− character, and the emission arises from this excited-state complex rather than from either component alone.

Exciplex emission is usually red-shifted and broader than the fluorescence of the individual molecules, often appearing as a broad, structureless band with a large Stokes shift. The formation and emission depend on the relative energies of the donor's excited state and the charge-transfer state, as well as molecular proximity and orientation. In solution, formation can be diffusion-controlled; in rigid films or aggregates, preassociated donor–acceptor pairs or specific packing can favor exciplex formation. The spectra and lifetimes are sensitive to solvent polarity, temperature, and the donor–acceptor distance.

Exciplexes are of particular interest in organic electronics and photophysics. In organic light-emitting diodes, donor–acceptor exciplexes can contribute to efficiency through thermally activated delayed fluorescence when the charge-transfer state is near-degenerate with singlet and triplet states, enabling reverse intersystem crossing. They also play roles in fluorescence quenching, photoredox chemistry, and sensing, where the appearance or growth of exciplex emission reports on donor–acceptor interactions.

Exciplexes differ from excimers, which are excited-state dimers of identical species; exciplexes involve two distinct molecules and an electron-transfer–driven CT character.