Xylylethynyl Titanocene with a Microsecond Emission Lifetime Photosensitizes Singlet-Oxygen Formation and Photon Upconversion.

Coordination complexes containing d metals with long-lived ligand-to-metal charge transfer (LMCT) excited states are promising candidates for use as photosensitizers. Previously, CpTi(CPh)CuBr (where Cp = pentamethylcyclopentadienyl and CPh = phenylethynyl) was reported to be emissive in THF solution at room temperature (RT) from an excited state with significant Cp*-to-Ti LMCT character (λ = 693 nm, τ = 0.18 μs). However, the corresponding cyclopentadienyl complex was not emissive. Structural constraint was hypothesized as a reason for the enhanced photophysics of the Cp* complex. To further test this hypothesis, the corresponding complex with -xylylethynyl ligands, CpTi(CPhMe)CuBr, has been prepared and characterized. X-ray crystallography demonstrates significant steric congestion caused by the additional methyl substituents. This xylylethynyl complex is emissive in THF solution at RT and the lifetime is approximately 10-fold greater (λ = 734 nm, τ = 1.6 μs) than the corresponding phenylethynyl complex. Spectroscopic and computational data are consistent with this emission being phosphorescence with significant Cp-to-Ti and xylylethynyl-to-Ti LMCT character. The evidence is also consistent with an excited state that is less distorted for the xylylethynyl complex than the phenylethynyl complex. The sterically induced, long lifetime of the xylylethynyl complex enabled its use in photon upconversion and O formation.