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.

学界已开展大量研究，旨在提升基于地壳丰产第一过渡系金属的光敏剂的激发态寿命。一价铜配合物在此领域受到广泛关注，多数情况下研究者采用空间位阻调控策略优化其激发态寿命。本研究报道了一系列三配位一价铜配合物（Cu1~Cu3），其激发态寿命通过三重态-三重态能量转移（triplet–triplet energy transfer, TTET）得以延长。该类杂配合物采用环己基取代的β-二亚胺配体（CyNacNacMe）与芳基异氰配体配位，通式为Cu(CyNacNacMe)(CN-Ar)，其中各配合物的具体配体如下：Cu1对应CN-dmp为2,6-二甲基苯基异氰；Cu2对应CN-pyr为1-芘基异氰；Cu3对应CN-dmp-pyr为2,6-二甲基-4-(1-芘基)苯基异氰。研究者结合室温与77 K下的光致发光测试、超快瞬态吸收（ultrafast transient absorption, UFTA）光谱以及密度泛函理论（DFT）计算，对低能三重激发态的本质、能级与动力学行为进行了表征。带有芘修饰异氰配体的配合物（Cu2与Cu3），其激发态寿命得以延长，这源于短寿命电荷转移三重激发态（³CT）与长寿命芘中心三重激发态（³pyr）之间的三重态-三重态能量转移（TTET）过程。在Cu3中，该TTET过程为不可逆过程，仅生成³pyr态；而在Cu2中，³CT与³pyr态能级近乎简并，可实现可逆TTET过程并维持长寿命的³CT态发光。Cu2与Cu3优异的光物理性能，使其在两类反应中均展现出更优的催化活性：一是通过三重态能量转移实现的芪类化合物E/Z异构化光催化反应，二是涉及氢脱溴反应与C-O键活化的光氧化还原转化反应。本研究结果表明，通过TTET策略实现的激发态寿命延长，可开发出具有光催化应用潜力的新型地壳丰产过渡金属配合物。