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 SiHP2 ligand (SiHP2 = (2-iPr2PC6H4)2SiHMe) and present photophysical properties. A previously reported copper complex (SiHP2)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 λmax 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 (SiHP2)Cu(1-methylcarbazolide) (7) and (SiHP2)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 λmax of 430 nm.