Cis,trans-isomerization of the dibenzylideneacetone ligand: Intramolecular quenching effected by ferrocenyl group substitutions and coordination with Pd(0) and Pt(0)

Abstract

The structural properties, UV—visible absorption and emission spectra and emission lifetimes of the dibenzylideneacetone ligand (dba), 4-ferrocenyldibenzylideneacetone (dba-Fc) and diferrocenylideneacetone (dFca) in solution and in the solid state are reported. The primary non-sensitized cis,trans-isomerization reactions of dba ▪ at 298 K in solution were investigated by 1H nuclear magnetic resonance (NMR), UV—visible and Fourier transform IR spectroscopy. The photoinduced cis,trans-isomerization reaction exhibits a large photochemical quantum yield (Φ cis,trans = 0.85 ± 0.05) and is not affected by the presence of oxygen in solution. In contrast, dba-Fc, dFca and the M 2(dba) 3 complexes (M  Pd, Pt) are photochemically inert, even under prolonged broad band irradiation (more than 48 h). An efficient intramolecular energy transfer quenching mechanism is proposed involving energy transfer from the upper intraligand excited states to the lower triplet ligand field states in the dba-Fc and dFca compounds and to the lower triplet metal-to-ligand charge transfer state in the M 2(dba) 3 complex. Finally, the reported results on the delayed fluorescence of dba (τ e = 28.5 ± 2.0 μs) provide the lower energy limit for the non-luminescent triplet state of dba ( E T ≥ 13 880 cm −1).