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

摘要

Herein we report three new gold(i) complexes with a benzothiazole-2,7-fluorenyl moiety bound through a gold-carbon sigma-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 S-1 and T-2 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. S