Luminescent Cationic Group 4 Metallocene Complexes Stabilized by Pendant N-Donor Groups

Abstract

Cationic group 4 metallocene complexes with pendant imine and pyridine donor groups were prepared as stable crystalline [B­(C6F5)4]− salts either by protonation of the intramolecularly bound ketimide moiety in neutral complexes [(η5-C5Me5)­{η5-C5H4CMe2CMe2C­(R)N-κN}­MCl] (M = Ti, Zr, Hf; R = t-Bu, Ph) by PhNMe2H+[B­(C6F5)4]− to give [(η5-C5Me5)­{η5-C5H4CMe2CMe2C­(R)NH-κN}­MCl]+[B­(C6F5)4]− or by chloride ligand abstraction from the complexes [(η5-C5Me5)­(η5-C5H4CMe2CH2C5H4N)­MCl2] (M = Ti, Zr) by Li­[B­(C6F5)4]·2.5Et2O to give [(η5-C5Me5)­(η5-C5H4CMe2CH2C5H4N-κN)­MCl]+[B­(C6F5)4]−. Solid state structures of the new compounds were established by X-ray diffraction analysis, and their electrochemical behavior was studied by cyclic voltammetry. The cationic complexes of Zr and Hf, compared to the corresponding neutral species, exhibited significantly enhanced luminescence predominantly from triplet ligand-to-metal (3LMCT) excited states with lifetimes up to 62 μs and quantum yields up to 58% in the solid state. DFT calculations were performed to explain the structural features and optical and electrochemical properties of the complexes.

Luminescent Zirconocene Complexes with Pendant Phosphine Chalcogenide Donor Groups

“…As in our previous contributions, 48,50 we found that the experimentally observed emission of the studied compounds originates from S83 and S84), all with rather poor or even negligible quantum yields of emission, exhibit non-negligible contributions of the PPh 2 (�E) (E = O, S, Se) moiety to the 3 LMCT transition. Comparing these trends with the observed luminescence quantum yields leads us yet again 48 to the conclusion that the most efficient S n → T 1 intersystem crossing and subsequent T 1 → S 0 emission is found in cationic complexes with the most localized LMCT transitions (in complexes 3/4 containing HOMO-polarizing Cp* ligands). On a final note, the auxiliary P�E (E = O, S, Se) ligand is a suitable group for modifying spin−orbit coupling in our cationic complexes, as it does not significantly shift the transition energy, yet introduces mass into the proximity of the central atom where the 3 LMCT SOMO is localized.…”

“…Methods applied in our quantum chemical study were adapted from our previously established computational protocol. 48,50 Optimized molecular geometries (1, 2-O, -S, -Se, and 6) and geometries of lone cations (3/4-O, -S, -Se, and 7) were in good agreement with the XRD-determined solid-state structures, where available (see the Supporting Information, Chapter 5.3). Calculated transition energies (Table 3 and Supporting Information, Chapter 5.2) corresponded well to the experimentally observed absorption bands in solution and emission wavelengths in the solid state (cf.…”

“…40 nm for 3-S compared with solid state luminescence confirms the effects of solvent and is in line with previous observations for related complexes. 48 For completeness, luminescence of these complexes in solution was also quenched in the presence of molecular oxygen (see Figure S14 in the Supporting Information) due to energy transfer yielding O 2 ( 1 Δ g ). 50 All selenium-bearing derivatives (2-Se, 3-Se, 4-Se) exhibited lower luminescence intensities compared to oxygen and sulfur analogues in the solid state.…”

Reduction of Electron Repulsion in Highly Covalent Fe-Amido Complexes Counteracts the Impact of a Weak Ligand Field on Excited-State Ordering

“…The most effective molecular photosensitizers for photocatalytic and photovoltaic applications broadly absorb visible light , and exhibit suitably long-lived charge-transfer (CT) excited states for efficient charge injection into semiconductor surfaces or amassing sufficient concentrations of reactive excited states for synthesis . Coordination complexes of precious metals in low oxidation states such as Ru­(II) or Ir­(III) with chelating, π-acceptor ligands can fulfill these criteria effectively, but cost and scarcity considerations have driven the search for chromophores based on more abundant materials that do not compromise on performance. , While creative ligand designs have enabled the development of an increasing number of exciting candidates based on Cu, , Zr, − Co, , and Cr, − iron has long represented a key target in this area . The realization of useful Fe­(II)-based photosensitizers, however, has been stymied by the inherent tendency of first-row transition metal complexes to undergo rapid deactivation of CT excited states via lower-lying metal-centered (MC) states .…”

Spin–Orbit Natural Transition Orbitals and Spin-Forbidden Transitions

“…Recent applications of the NTO method have been primarily in photochemistry and -physics and studies of excited states. A small selection of available studies is presented by refs − . Visualization of NTOs has been particularly useful for systems for which the descriptions of an electronic transition of interest involve a considerable number of pairs of occupied and unoccupied MOs with relatively low weights, which renders the assignment of the excitation difficult.…”