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

Transition-metal complexes that undergo ligand-to-metal charge transfer (LMCT) to 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 CpTi(CAr)·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 CpTi(CPh). TDDFT investigations suggest that emission is dominated by arylalkynyl-to-titanium LMCT in all cases except CpTi(CPh)·CuBr, which is dominated by CuBr-to-Ti charge transfer. In room-temperature fluid solution, only CpTi(CPh) and CpTi(CPh)·AgCl are emissive, albeit with photoluminescent quantum yields ≤2 × 10. The parent complexes photodecompose in room-temperature solution with quantum yields, Φ, between 0.25 and 0.99. The coordination of MX decreases Φ by two to three orders of magnitude. There is a clear trend that Φ 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 LMCT state and the singlet transition state for decomposition.