Ultrafast Excited-State Dynamics of the Organic Photoredox Catalyst DDQ.

The electron-deficient oxidant 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) has recently emerged as a promising visible-light photoredox catalyst. However, its excited-state behavior remains poorly understood. Here, we investigate the ultrafast dynamics of photoexcited DDQ in acetonitrile using transient electronic and infrared absorption spectroscopy, supported by quantum chemical calculations. Upon 395 nm excitation, we identify rapid intersystem crossing (ISC) from the singlet to triplet manifold within 1.5 ps, followed by internal conversion and vibrational relaxation on a 10.9 ps time scale. Our findings demonstrate that DDQ exhibits near-unity ISC quantum yield and long triplet lifetime. Together with the high reduction potential of the triplet state, these properties make it a viable metal-free alternative to conventional iridium- and ruthenium-based photocatalysts.