Photoluminescence emission and Raman response of monolayer MoS2, MoSe2, and WSe2.

We mechanically exfoliate mono- and few-layers of the transition metal dichalcogenides molybdenum disulfide, molybdenum diselenide, and tungsten diselenide. The exact number of layers is unambiguously determined by atomic force microscopy and high-resolution Raman spectroscopy. Strong photoluminescence emission is caused by the transition from an indirect band gap semiconductor of bulk material to a direct band gap semiconductor in atomically thin form.

Mechanism of the living lactide polymerization mediated by robust zinc guanidine complexes.

Zinc bis(chelate) guanidine complexes promote living lactide polymerization at elevated temperatures. By means of kinetic and spectroscopic analyses the mechanism has been elucidated for these special initiators that make use of neutral N-donor ligands. The neutral guanidine function initiates the polymerization by a nucleophilic ring-opening attack on the lactide molecule. DFT calculations on the first ring-opening step show that the guanidine is able to act as a nucleophile. Three transition states were located for ligand rearrangement, nucleophilic attack, and ring-opening. The second ring-opening step was modeled as a representation for the chain growth because here, the lactate alcoholate opens the second lactide molecule via two transition states (nucleophilic attack and ring-opening). Additionally, the resulting reaction profile proceeds overall exothermically, which is the driving force for the reaction. The experimental and calculated data are in good agreement and the presented mechanism explains why the polymerization proceeds without co-initiators.

Lactide polymerisation with air-stable and highly active zinc complexes with guanidine-pyridine hybrid ligands.

The synthesis of zinc complexes of guanidine-pyridine hybrid ligands [Zn(DMEGpy)Cl(2)] (C1), [Zn(TMGpy)Cl(2)] (C2), [Zn(DMEGqu)Cl(2)] (C3), [Zn(TMGqu)Cl(2)] (C4), [Zn(DMEGpy)(CH(3)COO)(2)] (C5), [Zn(TMGpy)(CH(3)COO)(2)] (C6), [Zn(DMEGqu)(CH(3)COO)(2)] (C7), [Zn(TMGqu)(CH(3)COO)(2)] (C8), [Zn(DMEGqu)(2)(CF(3)SO(3))][CF(3)SO(3)] (C9) and [Zn(TMGqu)(2)(CF(3)SO(3))][CF(3)SO(3)] (C10) is reported. These zinc complexes were completely characterised and screened regarding their activity in the ring-opening polymerisation of D,L-lactide. They proved to be active initiators in lactide bulk polymerisation, and polylactides with molecular weights (M(w)) up to 176,000 g mol(-1) could be obtained. They combine high activity with robustness towards moisture and air. The influence of reaction temperature and of the anionic component of the zinc salt on the activity of the catalyst, as well as the occurrence of undesired side reactions, was investigated. By correlating these findings with the structural study on the zinc complexes we could deduce a structure-reactivity relationship for the zinc catalysts. This study was accompanied by DFT calculations. The bis-chelate triflate complexes C9 and C10, supported by quinoline-guanidine ligands L3 and L4, exhibit by far the highest reactivity. Systematic comparison of these complexes with their mono-chelate counterparts and their bis-guanidine analogues allows the attributes that promote polymerisation by neutral guanidine ligand systems to be elucidated: accessibility to the zinc centre and Lewis acidity.