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Photocatalysis using complexes of d0 metals with ligand-to-metal charge-transfer (LMCT) excited states is an active area of research. Because titanium is the second most abundant transition metal in the earth’s crust, d0 complexes of TiIV are an appropriate target for this research. Recently, our group has demonstrated that the arylethynyltitanocene Cp2Ti(C2Ph)2CuBr is not emissive in room-temperature fluid solution, whereas the corresponding Cp* complex, Cp*2Ti(C2Ph)2CuBr, is emissive. The Cp* ligand is hypothesized to provide steric constraint that inhibits excited-state structural rearrangement. However, modifying the structure also changes the orbital character of the excited state. To investigate the impact of the excited-state orbital character on the photophysics, herein we characterize complexes similar to Cp*2Ti(C2Ph)2CuBr—but one with a more electron-rich arylethynyl ligand, ethynyldimethylaniline (C2DMA), and one with a more electron-poor arylethynyl ligand, ethynyl-α,α,α-trifluorotoluene. We have also prepared complexes with the C2DMA ligand but with different Cp ligands that adjust the steric bulk and constraint around the Ti, by replacing the Cp* ligands with either indenyl ligands or an ansa-cyclopentadienyl ligand where the two Cp ligands are bridged by a dimethylsilylene. All four target complexes have been characterized crystallographically and structure activity relationships are highlighted. 

Complexes that undergo ligand-to-metal charge transfer (LMCT) to d0 metals are of interest as possible photocatalysts. Cp2Ti(C2Ph)2 (where C2Ph = phenylethynyl) was reported to be weakly emissive in room temperature (RT) fluid solution from its phenylethynyl-to-Ti 3LMCT state, but readily photodecomposes. Coordination of CuX between the alkyne ligands to give Cp2Ti(C2Ph)2CuX (X = Cl or Br) has been shown to significantly increase the photostability, but such complexes are not emissive in RT solution. Herein, we investigate whether inhibition of alkyne-Ti-alkyne bond compression might be responsible for the increased photostability of the CuX complexes by investigating the decomposition of a structurally constrained analogue, Cp2Ti(OBET) (OBET = o-bis(ethynyl)tolane). To investigate the mechanism of nonradiative decay from the 3LMCT states in Cp2Ti(C2Ph)2CuX, the photophysical properties were investigated both upon deuteration and upon rigidifying in poly(methyl methacrylate) film. These investigations suggested that inhibition of structural rearrangement may play a dominant role in increasing emission lifetimes and quantum yields. The bulkier Cp*2Ti(C2Ph)2CuBr was prepared and is emissive at 693 nm in RT THF solution with a photoluminescent quantum yield of 1.3 x 10–3 ( = 0.18 s). TDDFT calculations suggest emission occurs from a 3LMCT state dominated by Cp*-to-Ti charge transfer.