Synthesis and Structural Properties of Aza[n]helicene Platinum Complexes: Control of Cis and Trans Stereochemistry

HPLC Enantioseparation of Rigid Chiral Probes with Central, Axial, Helical, and Planar Stereogenicity on an Amylose (3,5-Dimethylphenylcarbamate) Chiral Stationary Phase

The chiral resolving ability of the commercially available amylose (3,5-dimethylphenylcarbamate)-based chiral stationary phase (CSP) toward four chiral probes representative of four kinds of stereogenicity (central, axial, helical, and planar) was investigated. Besides chirality, the evident structural feature of selectands is an extremely limited conformational freedom. The chiral rigid analytes were analyzed by using pure short alcohols as mobile phases at different column temperatures. The enantioselectivity was found to be suitable for all compounds investigated. This evidence confirms that the use of the amylose-based CSP in HPLC is an effective strategy for obtaining the resolution of chiral compounds containing any kind of stereogenic element. In addition, the experimental retention and enantioselectivity behavior, as well as the established enantiomer elution order of the investigated chiral analytes, may be used as key information to track essential details on the enantiorecognition mechanism of the amylose-based chiral stationary phase.

Helical donor-acceptor platinum complexes displaying dual luminescence and near-infrared circularly polarized luminescence

A series of chiral platina[5]helicenes displaying dual luminescence, i.e., fluorescence between 450 and 600 nm and red/NIR phosphorescence between 700 and 900 nm, has been synthesised, characterised and studied by first-principle calculations. This unusual behavior has been attributed to limited electronic interactions between the d orbitals of the metal and the π-orbitals of the organic ligand on the excited-state. Accordingly, the electron richness of the donor group on the helical ligand does not affect the energy of the phosphorescence process but does play a role on its efficiency. Interestingly, near-infrared circularly polarized luminescence can be obtained for the three complexes with dissymmetry factors up to 3 × 10^-3 at 750 nm.

Ligand isomerism in Pt(II) complexes – structural aspects

This review has focused on ligand isomers in Pt(II) complexes. There are a variety of inner coordination spheres about the platinum(II) atom (PtN4, PtN2Cl2, PtP2Cl2, PtPNC2, PtPNCl2, PtP2CBr, PtP2CS), build up by mono- and bidentate ligands. The bidentate ligands create a variety of metallocyclic rings. The L–Pt–L bite angle (mean values) open in the sequence: 73.1° (PNP) < 78.7° (NC2C) < 80.4° (NC2N) < 86.4° (PC2P) < 86.7° (PNNP) < 93.0° (CC3S). There are three types of isomers: ligand, mixed – (ligand + distortion), and mixed – (ligand + cis-trans), isomers, which are rarity.

Application of Innovative Analytical Modeling for the Physicochemical Analysis of Adsorption Isotherms of Silver Nitrate on Helicenes: Phenomenological Study of the Complexation Process

The interaction between the silver ion and the cyclic aromatic molecules, namely, the helicenes, is the subject of this paper. In fact, a silver complexation system based on quartz crystal microbalance (QCM) sensor with a functional film of helicenes has been designed and developed at four temperatures. The developed system, in which the sensor response reflects the adsorption of the hexahelicene and the heptahelicene, was able to control the complexed mass of silver for each concentration. Experimental outcomes indicated that the quartz crystal coated with heptahelicene is the adequate material for silver adsorption. Then, a theoretical study has been performed through two statistical physics models (SMPG and SMRG) in order to analyze the experimental adsorption isotherms of the two helicenes at the ionic scale. The SMRG model was developed using the real gas law and was satisfactorily applied for the microscopic investigation of the hexahelicene isotherms indicating that the lateral interactions between the adsorbates are responsible of the decrease of the adsorbed quantity at saturation. The interpretation of the two models’ parameters indicated that the adsorption of the two helicenes is an endothermic phenomenon. Interestingly, the heptahelicene is recommended for silver complexation because it shows the highest adsorption energies involving chemical bonds during the complexation process. The SMPG model and the SMRG model also allow prediction of three thermodynamic functions (configurational entropy, Gibbs free enthalpy, and internal energy) which govern the adsorption mechanism of silver on the two helicenes.

Dinuclear Rhenium Complexes with a Bridging Helicene-bis-bipyridine Ligand: Synthesis, Structure, and Photophysical and Chiroptical Properties

By attaching pyridine groups to a diaza[6]helicene, a helical, bis-ditopic, bis-N N-coordinating ligand can be accessed. Dinuclear rhenium complexes featuring this bridging ligand, of the form [{Re(CO)3 Cl}2 (N N-N N)], have been prepared and resolved to give enantiopure complexes. These complexes are phosphorescent in solution at room temperature under one- and two-photon excitation. Their experimental chiroptical properties (optical rotation, electronic circular dichroism and circularly polarized emission) have been measured. They show, for instance, emission dissymmetry factors of c.a. ±3x10-3 . Quantum-chemical calculations indicate the importance of stereochemistry on the optical activity, pointing towards further design improvements in such types of complexes.

Triggering Emission with the Helical Turn in Thiadiazole-Helicenes

Introduction of heterocycles into the helical skeleton of helicenes allows modulation of their redox, chiroptical, and photophysical properties. This paper describes the straightforward preparation and structural characterization by single-crystal X-ray diffraction of thiadiazole-[7]helicene, which was resolved into M and P enantiomers by chiral HPLC, together with its S-shaped double [4]helicene isomer, as well as the smaller congeners thiadiazole-[5]helicene and benzothiadiazole-anthracene. A copper(II) complex with two thiadiazole-[5]helicene ligands was structurally characterized, and it shows the presence of both M and P isomers coordinated to the metal center. The emission properties of the heterohelicenes are highly dependent on the helical turn, as the [7]- and [5]helicene are poorly emissive, whereas their isomers, that is, the S-shaped double [4]helicene and thiadiazole-benzanthracene, are luminescent, with quantum efficiencies of 5.4 and 6.5 %, respectively. DFT calculations suggest quenching of the luminescence of enantiopure [7]helicenes through an intersystem-crossing mechanism arising from the relaxed excited S1 state.