Design Strategies for Luminescent Titanocenes: Improving the Photoluminescence and Photostability of Arylethynyltitanocenes

Complexes that undergo ligand-to-metal charge transfer (LMCT) to d(0) 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 (LMCT)-L-3 state but readily photodecomposes. Coordination of CuX between the alkyne ligands to give Cp2Ti(C2Ph)(2)CuX (X = Cl, 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 (LMCT)-L-3 states in Cp2Ti(C2Ph)(2)CuX, the photophysical properties were investigated both upon deuteration and upon rigidifying in a 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*Ti-2(C2Ph)(2)CuBr was prepared and is emissive at 693 nm in RT THF solution with a photoluminescent quantum yield of 1.3 x 10(-3) (tau = 0.18 mu s). Time-dependent density functional theory (TDDFT) calculations suggest that emission occurs from a (LMCT)-L-3 state dominated by Cp*-to-Ti charge transfer.