Complexes with ligand-to-metal charge-transfer (LMCT) excited states involving d0 metals represent a new design for photocatalysts. Herein, the photochemistry and photophysics of d0 titanocenes of the type Cp2Ti­(C2R)2, where C2R = ethynylphenyl (C2Ph), 4-ethynyldimethylaniline (C2DMA), or 4-ethynyltriphenylamine (C2TPA), have been investigated. Cp2Ti­(C2Ph)2 and Cp2Ti­(C2DMA)2 have also been characterized by single-crystal X-ray diffraction. The two aryl rings in Cp2Ti­(C2DMA)2 are nearly face-to-face in the solid state, whereas they are mutually perpendicular for Cp2Ti­(C2Ph)2. All three complexes are brightly emissive at 77 K but photodecompose at room temperature when irradiated into their lowest-energy absorption band. The emission wavelengths and photodecomposition quantum yields are as follows: Cp2Ti­(C2Ph)2, 575 nm and 0.65; Cp2Ti­(C2TPA)2, 642 nm and 0.42; Cp2Ti­(C2DMA)2, 672 nm and 0.25. Extensive benchmarking of the density functional theory (DFT) model against the structural data and of the time-dependent DFT (TDDFT) model against the absorption and emission data was performed using combinations of 13 different functionals and 4 basis sets. The model that predicted the absorption and emission data with the greatest fidelity utilized MN15/LANL2DZ for both the DFT optimization and the TDDFT. Computational analysis shows that absorption involves a transition to a 1LMCT state. Whereas the spectroscopic data for Cp2Ti­(C2TPA)2 and Cp2Ti­(C2DMA)2 are well modeled using the optimized structure of these complexes, Cp2Ti­(C2Ph)2 required averaging of the spectra from multiple rotamers involving rotation of the Ph rings. Consistent with this finding, an energy scan of all rotamers showed a very flat energetic surface, with less than 1.3 kcal/mol separating the minimum and maximum. The computational data suggest that emission occurs from a 3LMCT state. Optimization of the 3LMCT state demonstrates compression of the C–Ti–C bond angle, consistent with the known products of photodecomposition.

Transition-metal complexes that undergo ligand-tometal charge transfer (LMCT) to d 0 metals are of interest as possible photocatalysts due to the lack of deactivating d−d states. Herein, the synthesis and characterization of nine titanocene complexes of the formula Cp 2 Ti(C 2 Ar) 2 •MX (where Ar = phenyl, dimethylaniline, or triphenylamine; and MX = CuCl, CuBr, or AgCl) are presented. Solid-state structural characterization demonstrates that MX coordinates to the alkyne tweezers and CuX coordination has a greater structural impact than AgCl. All complexes, including the parent complexes without coordinated MX, are brightly emissive at 77 K (emission max between 575 and 767 nm), with the coordination of MX redshifting the emission in all cases except for the coordination of AgCl into Cp 2 Ti(C 2 Ph) 2 . TDDFT investigations suggest that emission is dominated by arylalkynyl-to-titanium 3 LMCT in all cases except Cp 2 Ti(C 2 Ph) 2 •CuBr, which is dominated by CuBr-to-Ti charge transfer. In roomtemperature fluid solution, only Cp 2 Ti(C 2 Ph) 2 and Cp 2 Ti(C 2 Ph) 2 •AgCl are emissive, albeit with photoluminescent quantum yields ≤2 × 10 −4 . The parent complexes photodecompose in room-temperature solution with quantum yields, Φ rxn , between 0.25 and 0.99. The coordination of MX decreases Φ rxn by two to three orders of magnitude. There is a clear trend that Φ rxn increases as the emission energy increases. This trend is consistent with a competition between energy-gap-law controlled nonradiative decay and thermally activated intersystem crossing between the 3 LMCT state and the singlet transition state for decomposition.

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