Titanocene as a New Acceptor (A) for Arylamine Donors (D) in D−π–A Chromophores

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

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.

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

Transition-metal complexes that undergo ligand-to-metal 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 room-temperature 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, Φ_rxn, between 0.25 and 0.99. The coordination of MX decreases Φ_rxn by two to three orders of magnitude. There is a clear trend that Φ_rxn 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.

Ligand-to-Metal Charge-Transfer Photophysics and Photochemistry of Emissive d0 Titanocenes: A Spectroscopic and Computational Investigation

Complexes with ligand-to-metal charge-transfer (LMCT) excited states involving d⁰ metals represent a new design for photocatalysts. Herein, the photochemistry and photophysics of d⁰ titanocenes of the type Cp₂Ti(C₂R)₂, where C₂R = ethynylphenyl (C₂Ph), 4-ethynyldimethylaniline (C₂DMA), or 4-ethynyltriphenylamine (C₂TPA), have been investigated. Cp₂Ti(C₂Ph)₂ and Cp₂Ti(C₂DMA)₂ have also been characterized by single-crystal X-ray diffraction. The two aryl rings in Cp₂Ti(C₂DMA)₂ are nearly face-to-face in the solid state, whereas they are mutually perpendicular for Cp₂Ti(C₂Ph)₂. All three complexes are brightly emissive at 77 K but photodecompose at room temperature when irradiated into their lowest-energy absorption band. The emission wavelengths and photodecomposition quantum yields are as follows: Cp₂Ti(C₂Ph)₂, 575 nm and 0.65; Cp₂Ti(C₂TPA)₂, 642 nm and 0.42; Cp₂Ti(C₂DMA)₂, 672 nm and 0.25. Extensive benchmarking of the density functional theory (DFT) model against the structural data and of the time-dependent DFT (TDDFT) model against the absorption and emission data was performed using combinations of 13 different functionals and 4 basis sets. The model that predicted the absorption and emission data with the greatest fidelity utilized MN15/LANL2DZ for both the DFT optimization and the TDDFT. Computational analysis shows that absorption involves a transition to a ¹LMCT state. Whereas the spectroscopic data for Cp₂Ti(C₂TPA)₂ and Cp₂Ti(C₂DMA)₂ are well modeled using the optimized structure of these complexes, Cp₂Ti(C₂Ph)₂ required averaging of the spectra from multiple rotamers involving rotation of the Ph rings. Consistent with this finding, an energy scan of all rotamers showed a very flat energetic surface, with less than 1.3 kcal/mol separating the minimum and maximum. The computational data suggest that emission occurs from a ³LMCT state. Optimization of the ³LMCT state demonstrates compression of the C-Ti-C bond angle, consistent with the known products of photodecomposition.

Insights into the charge-transfer character of electronic transitions in RCp2Ti(C2Fc)2 complexes using solvatochromism, resonance Raman spectroscopy, and TDDFT

A series of complexes with low-energy FeII to TiIV metal-to-metal charge-transfer (MMCT) transitions, Cp2Ti(C2Fc)2, Cp*2Ti(C2Fc)2, and MeOOCCp2Ti(C2Fc)2, was investigated using solvatochromism and resonance Raman spectroscopy (RRS) augmented with time-dependent density functional theory (TDDFT) calculations in order to interrogate the nature of the CT transitions. Computational models were benchmarked against the experimental UV-Vis spectra and B3LYP/6-31G(d) was found to most faithfully represent the spectra. The energy of the MMCT transition was measured in 15 different solvents and a multivariate fit to the Catalán solvent parameters - solvent polarizability (SP), solvent dipolarity (SdP), solvent basicity (SB), and solvent acidity (SA) - was performed. The effect of SP indicates a greater degree of electron delocalization in the excited state (ES) than the ground state (GS). The small negative solvatochromism with respect to SdP indicates a smaller dipole moment in the ES than the GS. The effect of SB is consistent with charge-transfer to Ti. Upon excitation into the MMCT absorption band, the RRS data show enhancement of the alkyne stretching modes and of the out-of-plane bending modes of the cyclopentadienyl ring connected to Fe and the alkyne bridge. This is consistent with changes in the oxidation states of Ti and Fe, respectively. The higher-energy transitions (350-450 nm) show enhancement of vibrational modes consistent with ethnylcyclopentadienyl to Ti ligand-to-metal charge transfer (LMCT). The RRS data is consistent with the TDDFT predicted character of these transitions. TDDFT suggests that the lowest-energy transition in Cp2Ti(C2Fc)2CuI, where CuI is coordinated between the alkynes, retains its FeII to TiIV MMCT character, in agreement with the RRS data, but that the lowest-energy transitions have significant CuI to Ti character. For Cp2Ti(C2Fc)2CuI, excitation into the low-energy MMCT absorption band results in selective enhancement of the symmetric alkynyl stretching mode.

Synthesis and characterization of a tert-butyl ester-substituted titanocene dichloride: t-BuOOCCp2TiCl2

Bis[η5-(tert-butoxyca-rbonyl)-cyclo-penta-dien-yl]di-chlorido-titanium(IV), [Ti(C10H13O2)2Cl2], was synthesized from LiCpCOOt-Bu using TiCl4, and was characterized by single-crystal X-ray diffraction and 1H NMR spectroscopy. The distorted tetra-hedral geometry about the central titanium atom is relatively unchanged compared to Cp2TiCl2. The complex exhibits elongation of the titanium-cyclo-penta-dienyl centroid distances [2.074 (3) and 2.070 (3) Å] and a contraction of the titanium-chlorine bond lengths [2.3222 (10) Å and 2.3423 (10) Å] relative to Cp2TiCl2. The dihedral angle formed by the planes of the Cp rings [52.56 (13)°] is smaller than seen in Cp2TiCl2. Both ester groups extend from the same side of the Cp rings, and occur on the same side of the complex as the chlorido ligands. The complex may serve as a convenient synthon for titanocene complexes with carboxyl-ate anchoring groups for binding to metal oxide substrates.

Rise of Conjugated Poly-ynes and Poly(Metalla-ynes): From Design Through Synthesis to Structure-Property Relationships and Applications

Conjugated poly-ynes and poly(metalla-ynes) constitute an important class of new materials with potential application in various domains of science. The key factors responsible for the diverse usage of these materials is their intriguing and tunable chemical and photophysical properties. This review highlights fascinating advances made in the field of conjugated organic poly-ynes and poly(metalla-ynes) incorporating group 4-11 metals. This includes several important aspects of conjugated poly-ynes viz. synthetic protocols, bonding, electronic structure, nature of luminescence, structure-property relationships, diverse applications, and concluding remarks. Furthermore, we delineated the future directions and challenges in this particular area of research.

Oxidatively stable ferrocenyl-π-bridge-titanocene D–π-A complexes: an electrochemical and spectroscopic investigation of the mixed-valent states

Coordination of Cu^I into ^RCp₂Ti(C₂Fc)₂ compounds results in well-separated Fe^III/II couples and mixed-valent states with distinct spectroscopic signatures.

A trans-bidentate bis-pyridinyl ligand with a transition metal hinge

A titanocene based metalloligand, Cp*₂Ti(C₂2-py)₂, was synthesized and ligated to either Cu(i) or Pd(ii), binding Cu(i) between its alkynes and Pd(ii) between its pyridinyl rings.