Photochemistry and Photophysics of Charge-Transfer Excited States in Emissive d 10 / d 0 Heterobimetallic Titanocene Tweezer Complexes.

Transition-metal complexes that undergo ligand-to-metal 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 room-temperature 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.