Organic triplet excited states of gold(I) complexes with oligo(o-or m-phenyleneethynylene) ligands: Conjunction of steady-state and time-resolved spectroscopic studies on exciton delocalization and emission pathways

A series of mononuclear and binuclear gold(I) complexes containing oligo(o- or m-phenyleneethynylene) (PE) ligands, namely [PhC=C(C6H 4-1,2-C=C)n-1Au(PCy3)] (n = 2-4, 4a-c), [μ-{C=C-(1,2-C6H4C=C)n}{Au(PCy 3)}2] (n = 1-6, 8, 5a-g), [PhC=C(C6H 4-1,3-C=C)n-1Au(PCy3)] (n = 2-4, 6a-c), and [μ-{C=C-(1,3-C6H4C=C)n}{Au(PCy 3)}2] (n = 1, 2, 7a,b), were synthesized and structurally characterized. Extensive spectroscopic measurements have been performed by applying combined methods of femtosecond transient absorption (fs-TA), fs time-resolved fluorescence (fs-TRF), and nanosecond time-resolved emission (ns-TRE) coupled with steady-state absorption and emission spectroscopy at both ambient and low (77 K) temperatures to directly probe the temporal evolution of the excited states and to determine the dynamics and spectral signatures for the involved singlet (S1) and triplet (T1) excited states. The results reveal that S1 and T1 both feature ligand-centered electronic transitions with ππ* character associated with the phenyl and acetylene moieties. The 3ππ* emission of the PE ligands is switched on by the attachment of [Au(PCy 3)]+ fragment(s) due to the heavy-atom effect. T 1(3ππ) was found to form with nearly unity efficiency through intersystem crossing (ISC) from S1( 1ππ). The ISC time constants were determined to be ∼50, 35, and 40 ps for 4b and 6a,b, respectively. Dual emission composed of fluorescence from S1 and phosphorescence from T1 were observed for most of the complexes except 5a and 7a, where only phosphorescence was found. The fluorescence at ambient temperature is accounted for by both the short-lived prompt fluorescence (PF) and long-lived delayed fluorescence (DF, lifetime on microsecond time scale). Explicit evidence was presented for a triplet-triplet annihilation mechanism for the generation of DF. Ligand length and substitution-dependent dynamics of T1 are the key factors governing the dual emission character of the complexes. By extrapolation from the plot of emission energy against the PE chain length of the [Au(PCy3)] + complexes with oligo(o-PE) or oligo(m-PE) ligands, the triplet emission energies were estimated to be ∼530 and ∼470 nm for poly(o-PE) and poly(m-PE), respectively. Additionally, we assign the unusual red shifts of 983 cm-1 from [PhC=CAu(PCy3)] (1) to [μ-{1,3-(C=C) 2C6H4}{Au(PCy3)}2] (7a) and 462 cm-1 from 7a to [μ3-{1,3,5-(C=C) 3C6H3}{Au(PCy3)}3] (8) in the phosphorescence energies to excitonic coupling interactions between the C=CAu(PCy3) arms in the triplet excited states. These complexes, together with those previously reported [Au(PCy3)]+ complexes containing oligo(p-PE) ligands (J. Am. Chem. Soc. 2002, 124, 14696-14706), form a collection of oligo(phenyleneethynylene) complexes exhibiting organic triplet emission in solution under ambient conditions. The remarkable feature of these complexes in exhibiting TTA prompted DF in conjunction with high formation efficiency of T1(3ππ ) affords an opportunity for emission spectra to cover a wide range of wavelengths. This may have implication in the development of PE-based molecular materials for future optical applications.