Photophysical properties of organogold(i) complexes bearing a benzothiazole-2,7-fluorenyl moiety: selection of ancillary ligand influences white light emission

Abstract

Herein we report three new gold(I) complexes with a benzothiazole-2,7-fluorenyl moiety bound through a gold–carbon σ-bond and either an N-heterocyclic carbene or organophosphine as ancillary ligands. The complexes have been characterized by NMR spectroscopy, X-ray crystallography, high resolution mass spectrometry, elemental analysis, and static and time-resolved optical spectroscopy. These compounds absorb almost strictly in the ultraviolet region and exhibit dual-luminescence following three freeze–pump–thaw cycles in toluene. The selection of the ancillary ligand significantly influences the excited-state dynamics of the complexes. The two phosphine containing complexes have similar fluorescence and phosphorescence quantum yields leading to generation of white light emission. The carbene containing complex exhibits a higher fluorescence quantum yield compared to its phosphorescence quantum yield resulting in a violet emission. Extensive photophysical characterization of these compounds suggests that the phosphine complexes undergo intersystem crossing more efficiently than the carbene complex. This is supported by a three-fold increase in luminescence lifetime, a halving in fluorescence quantum yield, and an increase in intersystem crossing efficiency by 25 percent for the phosphine complexes. Density-functional theory calculations support these observations where the energy gap between the S₁ and T₂ states for the carbene is roughly twice that of the phosphine complexes. To our knowledge this is the first example of single-component mononuclear gold(I) complexes exhibiting non-excimeric state white light emission, although a similar phenomenon has been realized for gold(III) aryl compounds. Further, the triplet lifetimes of all three complexes are on the order of one ms in freeze–pump–thaw degassed toluene. These molecules also exhibit delayed fluorescence; all of the complexes display diffusion-controlled rate constants for triplet–triplet annihilation. Strong excited-state absorption is observed from the singlet and triplet excited-states in these molecules as well. The singlet states have excited-state extinction coefficients on the order of 1.5 × 10⁵ M⁻¹ cm⁻¹ and the triplet states have excited-state extinction coefficients on the order of 1.0 × 10⁵ M⁻¹ cm⁻¹.