Long Excited-State Lifetimes in Three-Coordinate Copper(I) Complexes via Triplet–Triplet Energy Transfer to Pyrene-Decorated Isocyanides

There has been much effort to improve
excited-state lifetimes
in
photosensitizers based on earth-abundant first-row transition metals.
Copper(I) complexes have gained significant attention in this field,
and in most cases, sterically driven approaches are used to optimize
their lifetimes. This study presents a series of three-coordinate
copper(I) complexes (Cu1–Cu3) where
the excited-state lifetime is extended by triplet–triplet energy
transfer. The heteroleptic compounds feature a cyclohexyl-substituted
β-diketiminate (CyNacNacMe) paired with aryl isocyanide
ligands, giving the general formula Cu(CyNacNacMe)(CN-Ar)
(CN-dmp = 2,6-dimethylphenyl isocyanide for Cu1; CN-pyr
= 1-pyrenyl isocyanide for Cu2; CN-dmp-pyr = 2,6-dimethyl-4-(1-pyrenyl)phenyl
isocyanide for Cu3). The nature, energies, and dynamics
of the low-energy triplet excited states are assessed with a combination
of photoluminescence measurements at room temperature and 77 K, ultrafast
transient absorption (UFTA) spectroscopy, and DFT calculations. The
complexes with the pyrene-decorated isocyanides (Cu2 and Cu3) exhibit extended excited-state lifetimes resulting from
triplet–triplet energy transfer (TTET) between the short-lived
charge-transfer excited state (3CT) and the long-lived
pyrene-centered triplet state (3pyr). This TTET process
is irreversible in Cu3, producing exclusively the 3pyr state, and in Cu2, the 3CT and 3pyr states are nearly isoenergetic, enabling reversible TTET
and long-lived 3CT luminescence. The improved photophysical
properties in Cu2 and Cu3 result in improvements
in activity for both photocatalytic stilbene E/Z isomerization via triplet energy transfer and photoredox
transformations involving hydrodebromination and C–O bond activation.
These results illustrate that the extended excited-state lifetimes
achieved through TTET result in newly conceived photosynthetically
relevant earth-abundant transition metal complexes.