Photophysical Tuning of σ-SiH Copper-Carbazolide Complexes To Give Deep-Blue Emission

A series of σ-SiH copper complexes with different carbazole derivatives have been synthesized and characterized that adopt a neutral Si^HP₂ ligand (Si^HP₂ = (2-Pr₂PC₆H₄)₂Si^HMe) and present photophysical properties. A previously reported copper complex (Si^HP₂)Cu(carbazolide), and its derivatives showed that tuning of the emission properties is possible by incorporating various substituents on the carbazolide moiety. Newly synthesized copper complexes (2-6) having 3,6-dichlorocarbazolide, 3,6-dibromocarbazolide, 1-fluorocarbazolide, 3,6-dimethylcarbazolide, and 3,6-diphenylcarbazolide show a range of λ_max values of emission from 418 to 511 nm. Detailed analysis supports that their emission bands originate from excited states with Cu metal-ligand charge transfer (MLCT) and/or ligand-centered (LC) π-π* transitions. Substitution of a methyl or trifluoromethyl group at the 1-position of the carbazolide moiety was also investigated to regulate the structural tuning of the copper emitters. From the X-ray crystallographic data of (Si^HP₂)Cu(1-methylcarbazolide) (7) and (Si^HP₂)Cu(1,3-di(trifluoromethyl)carbazolide) (8), unusual structural features, arising from the interaction of a SiH moiety with CH₃ and CF₃, respectively, were recognized. Such interaction forces the carbazolide moiety to tilt, while the copper geometry remains consistent with the other complexes. In the case of 8, a SiH···F₃C interaction locks the carbazolide moiety in place, restricting its orbital overlapping with a copper-based orbital, according to the theoretical analysis by using density functional theory (DFT) computations. Thus, the unusual tilting results in deep-blue emission with a λ_max of 430 nm.