Photochemistry and Photophysics of Charge-Transfer Excited States in Emissive d10/d0 Heterobimetallic Titanocene Tweezer Complexes

Abstract

Transition-metal complexes that undergo ligand-tometal charge transfer (LMCT) to d0 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 Cp2Ti(C2Ar)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 Cp2Ti(C2Ph)2. TDDFT investigations suggest that emission is dominated by arylalkynyl-to-titanium 3LMCT in all cases except Cp2Ti(C2Ph)2・CuBr, which is dominated by CuBr-to-Ti charge transfer. In roomtemperature fluid solution, only Cp2Ti(C2Ph)2 and Cp2Ti(C2Ph)2・AgCl are emissive, albeit with photoluminescent quantum yields ≤2 × 10−4. The parent complexes photodecompose in room-temperature solution with quantum yields, Φrxn, between 0.25 and 0.99. The coordination of MX decreases Φrxn by two to three orders of magnitude. There is a clear trend that Φrxn 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 3LMCT state and the singlet transition state for decomposition.