The Observation of Solvent Dependent Intraligand and Ligand-to-Ligand Charge Transfer Excited States in Au(III) Chromophores Containing Fluorene-Diphenyl Amine Ligands.

The arylgold(III) complex Au(III)-DPA and its alkynyl analog Au(III)-ADPA are synthesized and characterized using steady-state and time-resolved optical spectroscopy and density functional theory calculations. Both complexes embed the fluorenyl moiety with substitution at the 2,7-positions, and both carry diphenylamine (DPA) substituents. The photophysical properties of the molecules are examined in toluene and tetrahydrofuran (THF) solutions. Both complexes possess similar ground-state absorption spectra, which are a combination of ligand centered π-π* transitions and metal-perturbed π-π* transitions. The excited-state luminescent properties of the molecules are solvent dependent. In toluene, Au(III)-DPA displays featureless phosphorescence, and Au(III)-ADPA exhibits structured phosphorescence both ascribed to a metal-perturbed triplet intraligand (IL) π-π* state. In THF, however, both Au(III)-DPA and Au(III)-ADPA exhibit broad, red-shifted phosphorescence indicative of a solvent stabilized triplet ligand-to-ligand charge transfer (LLCT) state. The intersystem crossing efficiencies of both complexes are also investigated. Pulsed laser experiments reveal high intersystem crossing values (>0.79) for both complexes in both solvents, indicating that intersystem crossing is the dominant pathway for singlet excited-state decay. Density functional theory calculations indicate frontier orbitals centered on the substituted fluorenyl ligands highest occupied molecular orbital (HOMO) and the cyclometalating ligand lowest unoccupied molecular orbital (LUMO), supporting the assignment of LLCT excited states in more polar solvents.