Design Strategies for Luminescent Titanocenes: Improving the Photoluminescence and Photostability of Arylethynyltitanocenes

Abstract

Complexes that undergo ligand-to-metal charge transfer (LMCT) to d0 metals are of interest as possible photocatalysts. Cp2Ti­(C2Ph)2 (where C2Ph = phenylethynyl) was reported to be weakly emissive in room-temperature (RT) fluid solution from its phenylethynyl-to-Ti 3LMCT state but readily photodecomposes. Coordination of CuX between the alkyne ligands to give Cp2Ti­(C2Ph)2CuX (X = Cl, Br) has been shown to significantly increase the photostability, but such complexes are not emissive in RT solution. Herein, we investigate whether inhibition of alkyne-Ti-alkyne bond compression might be responsible for the increased photostability of the CuX complexes by investigating the decomposition of a structurally constrained analogue, Cp2Ti­(OBET) (OBET = o-bis­(ethynyl)­tolane). To investigate the mechanism of nonradiative decay from the 3LMCT states in Cp2Ti­(C2Ph)2CuX, the photophysical properties were investigated both upon deuteration and upon rigidifying in a poly­(methyl methacrylate) film. These investigations suggested that inhibition of structural rearrangement may play a dominant role in increasing emission lifetimes and quantum yields. The bulkier Cp*2Ti­(C2Ph)2CuBr was prepared and is emissive at 693 nm in RT THF solution with a photoluminescent quantum yield of 1.3 × 10–3 (τ = 0.18 μs). Time-dependent density functional theory (TDDFT) calculations suggest that emission occurs from a 3LMCT state dominated by Cp*-to-Ti charge transfer.

A new era of LMCT: leveraging ligand-to-metal charge transfer excited states for photochemical reactions

“…13,17,126 Very few complexes with emissive LMCT excited states are known and of those, most are short-lived and/or only emissive at low temperatures. 20,49,50,56,66,70,74,76,78,86,127–129 Computational studies propose their low energy emission stems from a mixed 3 LMCT/ 3 LLCT excited state where the ground state is entirely ligand-based and the excited state exhibits mixed metal and ligand character, consistent with the experimentally observed long-lived excited state. 22,47 Interestingly, spectroscopic studies further deciphered the origin of this photoluminescence and found Zr(PDP) 2 complexes exhibit thermally activated delayed fluorescence whereupon reversible intersystem crossing enables emission from both the singlet and triplet states across a range of temperatures.…”