Tuning the decay time of lanthanide-based near infrared luminescence from micro- to milliseconds through d→f energy transfer in discrete heterobimetallic complexes

Inert and optically active pseudo-octahedral (CrN6)-N-III and (RuN6)-N-II chromophores have been incorporated by self-assembly into heterobimetallic triple-stranded helicates HHH-[CrLnL(3)](6+) and HHH-[RuLnL(3)](5+). The crystal structures of [CrLnL(3)](CF3SO3)(6) (Ln = Nd, Eu, Yb, Lu) and [RuLn(3)]- (CF3SO3)(5) (Ln = Eu, Lu) demonstrate that the helical structure can accommodate metal ions of different sizes, without sizeable change in the intermetallic M(...)Ln distances. These systems are ideally suited for unravelling the molecular factors affecting the intermetallic nd -> 4f communication. Visible irradiation of the (CrN6)-N-III and (RuN6)-N-III chromophores in HHH-[MLnL(3)](5/6+) (Ln = Nd, Yb, Er; M = Cr, Ru) eventually produces lanthanide-based near infrared (NIR) emission, after directional energy migration within the complexes. Depending on the kinetic regime associated with each specific d-f pair, the NIR luminescence decay times can be tuned from micro- to milliseconds. The origin of this effect, together with its rational control for programming optical functions in discrete heterobimetallic entities, are discussed.