Ruthenium complexes that break the rules: Structural features controlling dual emission

A family of heteroleptic Ru^II coordination complexes containing substituted 1,10-phenanthroline (phen) ligands with extended conjugation was found to exhibit two simultaneously emissive excited states at room temperature in fluid solution. These systems demonstrate a breakdown of the standard nonradiative decay pathways that normally lead to a single, dominant, lowest energy emissive excited state in Ru^II complexes and most other chromophores. The structural requirements for dual emission were explored through the synthesis and characterization of isomeric systems. Two features were found to be primarily responsible for resolvable dual emission. Extended conjugation at the 4-position of the 1,10-phenanthroline ligand was identified as an essential feature, and asymmetry in the phenanthroline ligand substitutions appears to greatly facilitate the production of these two nonequilibrated emissive states. Additional complexes were studied which displayed "tunable" emissive characteristics for the two excited states as a function of covalent and noncovalent modification.