Generalizing Reversible Triplet–Triplet Energy Transfer as a Strategy to Increase Excited-State Lifetime in Copper(I) Charge-Transfer Chromophores

In this work, we generalize reversible triplet-triplet energy transfer (TTET) as a strategy to achieve long (μs range) excited-state lifetimes in copper(I) photosensitizers of the general formula Cu(RNacNac)(CN-Ar) (RNacNac is a substituted β-diketiminate and CN-Ar is an aryl isocyanide). Six new complexes are presented where the isocyanide is substituted with a triplet-acceptor moiety─anthracene, naphthalene, or phenanthrene. In-depth photophysical analysis reveals that energetic matching of the charge-transfer (3CT) and acceptor-localized 3(π→π*) states is key to achieving long triplet excited-state lifetimes via reversible TTET. With 2-isocyanoanthracene as the acceptor ligand, 3CT and 3(π→π*) are not energetically proximate, and irreversible quenching of 3CT occurs. With a small 0.1-0.2 eV gap between the two triplet states, luminescence originates from the 3CT state with a moderate excited-state lifetime of 250 or 283 ns observed. As key to the success of this approach, we demonstrate that altering the N-substituent on the RNacNac ligand modulates the energy of the 3CT state, giving three complexes where the 3CT and 3(π→π*) energies are perfectly matched and the excited-state lifetimes are much longer, spanning 1.9-2.7 μs. As such, the key lesson of this work is that long triplet-state lifetimes in these compounds can be achieved by proper "mixing and matching" of the RNacNac ligand, which influences the 3CT energy, and the triplet acceptor group which dictates the energy of the 3(π→π*) state.