TTET

Triplet–triplet energy transfer, abbreviated TTET, is a nonradiative energy transfer process in photochemistry and photophysics. In TTET, a donor molecule that is in its triplet excited state transfers energy to an acceptor molecule that is also in a triplet state. The transfer is typically described as a Dexter-type exchange mechanism, which requires direct orbital overlap and close physical contact between the donor and acceptor, rather than long-range dipole–dipole interactions.

A key condition for TTET is the relative triplet energies of the two species. The donor’s triplet energy must be higher than the acceptor’s triplet energy to allow energetically favorable transfer. Because TTET relies on exchange interactions, it is highly distance-dependent and occurs over short ranges, often via diffusion-controlled encounters in solution or within rigid matrices.

TTET has several important applications. It is a fundamental step in triplet–triplet annihilation upconversion (TTA-UC), where TTET populates the triplet state of an acceptor that can subsequently undergo annihilation to generate higher-energy singlet emission. TTET is also involved in various photoredox and photocatalytic schemes, where energy is transferred from a photosensitizer to a substrate or catalyst to drive reactions without net electron transfer.

Oxygen can quench triplet states, so TTET processes often require inert or controlled environments to minimize quenching. Experimental study of TTET typically employs transient absorption spectroscopy, phosphorescence quenching measurements, and related kinetic analyses to determine energy-transfer rates and efficiencies. TTET is distinct from Förster resonance energy transfer (FRET), which operates via long-range dipole–dipole coupling and is not appropriate for triplet–triplet pathways.