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.