Semi-random conjugated polymers based on 3-hexylthiophene have been extensively investigated within the past several years as an effective route to broadening absorption without losing desirable properties of poly(3-hexylthiophene) (P3HT). Here, we closely investigate the structural, optical, and electronic differences found in randomized analogues of poly(3-hexylthiophene–thiophene–2,3-dimethylthieno[3,4-b]pyrazine) (P3HTT-TP) semi-random polymers. Proton nuclear Overhauser effect spectroscopy is used to identify linkages within the conjugated backbone, and UV–vis, cyclic voltammetry, and grazing incidence X-ray diffraction are used to determine the influence of the subtle differences in monomer connectivities. Semi-random P3HTT-TP was found to have a stronger intramolecular charge transfer absorption band and higher thermal transitions compared to its randomized analogue. Additionally, the function of the thiophene unit in the semi-random polymers is observed and demonstrated to have key electronic effects as well as maintaining a higher HOMO level, aside from providing needed stoichiometric balance to the acceptor monomer. This work demonstrates the utility of the semi-random polymer approach as a robust tool for incorporating small amount of electron-poor monomers for the purpose of improving the absorption breadth of P3HT-based polymers.
Low-coordinate titanium(III) amido complexes were readily prepared via treatment of β-TiCl 3 with 1 or 2 equiv of Li[N(Si i Pr 3 )DIPP] (DIPP = 2,6-di-iso-propylphenyl) to form dinuclear [{Ti(N(Si i Pr 3 )DIPP)Cl(μ-Cl)} 2 ] (1) or mononuclear Ti[N(Si i Pr 3 )DIPP] 2 Cl (2), respectively. Both complexes were characterized by a variety of methods, including NMR spectroscopy, Evans method magnetic susceptibility, and single-crystal X-ray diffraction studies. Complex 1 was shown to be a versatile precursor for salt metathesis reactions to form heteroleptic complexes bearing alkoxide or silyl ligands. Alternatively, 1 formed adducts with Lewis bases such as 4-dimethylaminopyridine (DMAP) that are paramagnetic, mononuclear species. The base-stabilized complex 5 was alkylated with 1 or 2 equiv of LiCH 2 SiMe 3 to form mono-or dialkyl products, respectively. In contrast, treatment of complex 1 in the absence of DMAP with 2 equiv of LiCH 2 SiMe 3 results in the ultimate formation of a dinuclear cyclometalated complex 9 bearing a [Ti 2 (μ-Cl) 2 (μ:η 1 -CH 2 SiMe 2 CH 2 -)] core. This study demonstrates the convenient synthesis of a family of low-coordinate titanium(III) amido complexes possessing a variety of neutral and monoanionic ligands.
The pentadentate tetrapyrazolylpyridyl diborate ligand B 2 Pz 4 Py has been complexed to Ti III to form the chloro complex (B 2 Pz 4 Py)Ti III Cl, which is a convenient starting material for preparing alkyl and hydride derivatives of this ligand. The former (R = CH 3 and CH 2 SiMe 3 ) are highly thermally stable and do not react with dihydrogen to form (B 2 Pz 4 Py)Ti III H. Rather, treatment of the chloro starting material with NaHBEt 3 affords the desired hydrido complex in 85% yield. This Ti III hydride was fully characterized and exists in both solution and the solid state as a dimeric species; dissociation into monomers faces a high barrier of over 60 kcal/mol, according to density functional theory computations. This is due to stabilization of the dimer by dispersion forces. The computations show that the dimer has an open-shell singlet (S = 1) ground state, which is consistent with the magnetic susceptibility of 1.73 μ B per titanium center measured by the Evans method. However, the triplet state is very close in energy. The large barrier to dissociation into reactive monomers is reflected in the observed lack of reactivity with carbon dioxide to form the expected formato derivative, which can be prepared separately from (B 2 Pz 4 Py)Ti III Cl and sodium formate. Both the hydrido and alkyl Ti III compounds react with water to form a μ-oxo dinuclear species, which reacts further with dioxygen to form oxidized peroxo and oxo Ti IV complexes. All three of these compounds were fully characterized.
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