Synthesis, characterization and photophysical properties of mixed ligand tris(polypyridyl)chromium(III) complexes, [Cr(phen)2L]3+

A series of new heteroleptic, tris(polypyridyl)chromium(III) complexes, [Cr(phen)2L]3+ (L = substituted phenanthrolines or bipyridines), has been prepared and characterized, and their photophyical properties in a number of solvents have been investigated. X-ray crystallography measurements confirmed that the cationic (3+) units contain only one ligand L plus two phenanthroline ligands. Electrochemical and photophysical data showed that both ground state potentials and lifetime decays are sensitive to ligand structure and the nature of the solvent with the exception of compounds containing L = 5-amino-1,10-phenanthroline (aphen) and 2,2′-bipyrimidine (bpm). Addition of electron-donating groups in the ligand structure shifts redox potentials to more negative values than those observed for the parent compound, [Cr(phen)3]3+. Emission decays show a complex dependence with the solvent. The longest lifetime was observed for [Cr(phen)2(dip)]3+ (dip = 4,7-diphenylphenanthroline) in air-free aqueous solutions, τ = 273 μs. Solvent effects are explained in terms of the affinity of hydrophobic complexes for non-polar solvent molecules and the solvent microstructure surrounding chromium units.