Strategic Lifetime Tuning of Visible-Light Absorbing Two-Coordinate Metal Complexes

This paper highlights how the singlet and triplet amide ligand centered (1,3LC) and interligand charge transfer (1,3ICT) states in (carbene)M(amide), M = Cu, Au (cMa) complexes influence the excited state properties when the triplet states are close in energy. To that end we prepared a set of five cMa complexes, in which the amide (i.e., 5H-benzo[b]carbazole, H-BnCz) was kept constant giving a 3LC energy of ca. 2.15 eV. Four different carbene ligands were selected to develop MBnCzCarbene complexes which have energies for the ICT state that vary from being either markedly higher, lower, or close to the energy of the 3LC state on the amide ligand. Steady-state and time-resolved spectroscopic studies show that the emission spectrum of the cMa complex mirrors the phosphorescence of the parent H-BnCz amide and has a luminescence decay lifetime in the millisecond regime when the lowest energy excited state is 3LC. When the lowest energy states are 1,3ICT in nature, the emission band is broad and featureless, giving a lifetime in the nanosecond regime. As the 3LC and 1,3ICT states have comparable energies, dynamic equilibrium between the states is observed with the luminescence consisting of a mixture of 3LC and 1,3ICT transitions. The measured lifetime of this equilibrating system is between 45 and 350 μs depending on the solvent, for both copper- and gold-based cMa complexes. These complexes demonstrate the ability to “park” the excited state population in the 3LC state, allowing it to act as a reservoir for thermally activated emission, while still maintaining a very rapid equilibrium between LC and ICT states.