Intramolecular charge transfer in 1:1 Cu(II)/pyrenylcyclam dendrimer complexes

Two newly synthesized pyrenylcyclam dendrons (1 and 2) exhibit a new emission band centered at 450 nm when coordinated with copper triflate. Observed fluorescence shifts induced by coordinative metalation indicate an unusual intramolecular charge transfer from a pyrenyl excited state to the coordinated metal ion that competes with pyrene excimer formation. This interaction likely proceeds by photoexcitation of pi-complex of the appended arene, followed by intramolecular charge transfer within the dendritic 1:1 cyclam/metal complex, effecting reduction of the bound Cu(II) metal ion. The appended dendritic groups not only decrease the equilibrium binding constant with Cu(II) but also participate in a new excited-state pathway as an alternative to energy-dissipative excimer formation.