Ligand-to-metal charge-transfer states in emissive d⁰ metallocenes: Design strategies for titanocene based photocatalysts

Authors

Paul S. Wagenknecht, Furman UniversityFollow

ACS Citation

Wagenknecht, P. S. Ligand-to-metal charge-transfer states in emissive d⁰ metallocenes: Design strategies for titanocene based photocatalysts. In Photochemistry and Photophysics of Earth Abundant Metal Compounds; Ford, P. C., van Eldik, R., Eds.; Advances in Inorganic Chemistry 83; Elsevier; Amsterdam, 2024.

Version of Record

https://doi.org/10.1016/bs.adioch.2023.12.001

Abstract

Complexes of d⁰ transition metals with photoactive ligand-to-metal charge-transfer (LMCT) excited states have recently shown significant promise as photocatalysts involving earth-abundant metals. Over the past four decades, a handful of examples of metallocenes with d⁰ metals (Sc^III, Zr^IV, Hf^IV, Nb^V, and Ta^V) have demonstrated emission from LMCT states in room-temperature (RT) fluid solution. Recently, Cp₂TiCl₂ has also been shown to act as a photocatalyst, despite a lack of emission in RT solution. In fact, emission from LMCT states for Ti^IV complexes in RT fluid solution has been elusive. This review summarizes the findings of earlier investigations into emissive Sc^III, Zr^IV, Hf^IV, Nb^V, and Ta^V metallocenes to extract clues regarding design principles for emissive metallocenes. Complexes of the type Cp₂TiX₂ (where X = halide or pseudohalide) are emissive only at 77 K, and are discussed as the first examples of emissive titanocenes. Complexes of the type Cp₂Ti(C₂R)₂ (where R = ferrocenyl or an aryl substituent) have recently been investigated for their C₂R-to-Ti^IV LMCT states. The first example of a Ti^IV complex with an emissive ligand-to-Ti^IV LMCT state in RT fluid solution is Cp₂Ti(C₂Ph)₂. However, this complex undergoes efficient photodecomposition. Coordination of CuX (X = Cl, Br) between the alkyne ligands results in complexes that are significantly more photostable but not emissive in RT fluid solution. Mechanistic investigations suggested that the main path for nonradiative decay for such complexes involves excited-state structural reorganization. A titanocene with greater steric bulk, Cp^⁎₂Ti(C₂Ph)₂CuBr, is emissive in RT THF solution (λ_max = 693 nm, Φ_em = 1.3 ×10^–3, τ = 0.18 μs). The possible role of the Cp^⁎ ligand in facilitating emission in titanocenes is discussed.

Source Name

Advances in Inorganic Chemistry

Publication Date

2024

Volume

83

Document Type

Citation

Citation Type

Book Chapter