MLCT excited states of cuprous bis-phenanthroline coordination compounds

Cuprous bis-phenanthroline compounds possess metal-to-ligand charge transfe r, MLCT, excited states. Phenanthroline ligands coordinated to Cu(I) that a re disubstituted in the 2- and 9-positions with alkyl or aryl groups, abbre viated Cu-1(phen')(2)(+), have long-lived excited states at room temperatur e. The parent Cu-1(phen)(+)(2) compound is non-emissive under the same cond itions with a short excited state lifetime, tau < 10 ns. Disubstitution in the 2.9-positions stabilizes the Cu(I) state and increases the energy gap b etween the MLCT and the ground state. The prototypical and most well studie d compound is Cu-1(dmp)(2)(+), where dmp is 2.9-(CH3)(2)-1,10-phenanthrolin e. In dichloromethane solution at room temperature, Cu-1(dmp)(2)(+) display s broad MLCT absorption with lambda(max) = 454 nm, a broad unstructured emi ssion with lambda(max) = 730 nm, and an excited state lifetime of 85 ns. Th e emission arises from two closely spaced MLCT excited states, separated in energy by 1800 cm(-1), that behave as one state at room temperature. Cu-1( dmp)(2)(+)* excited states are quenched in the presence of Lewis bases and coordinating solvents. A 5-coordinate excited state complex, or exciplex, i s proposed to account for temperature dependent quenching data. The substan tial inner-sphere reorganizational energy changes that follow light excitat ion are novel features of these MLCT excited states. This review attempts t o cover all the existing data reported on Cu-1(phen')(2)(+) excited states and contrast it with well-known MLCT behavior of (d pi)(6) transition metal compounds. (C) 2000 Elsevier Science S.A. All rights reserved.