Stereomutation and chiroptical bias in the kinetically controlled supramolecular polymerization of cyano-luminogens

The synthesis of two pairs of enantiomeric cyano-luminogens 1 and 2, in which the central chromophore is a p-phenylene or a 2,5-dithienylbenzene moiety, respectively, is described and their supramolecular polymerization under kinetic and thermodynamic control investigated. Compounds 1 and 2 form supramolecular polymers by quadruple H-bonding arrays between the amide groups and the π-stacking of the central aromatic moieties. In addition, the peripheral benzamide units are able to form intramolecularly H-bonded pseudocycles that behave as metastable monomer M* thus affording kinetically and thermodynamically controlled aggregated species AggI and AggII. The chiroptical and emissive features of compounds 1 and 2 strongly depend on the aggregation state and the nature of the central aromatic unit. Compounds 1 exhibit a bisignated dichroic response of different intensity but with similar sign for both AggI₁ and AggII₁ species, which suggests the formation of helical aggregates. In fact, these helical supramolecular polymers can be visualized by AFM imaging. Furthermore, both AggI and AggII species formed by the self-assembly of compounds 1 show CPL (circularly polarized light) activity of opposite sign depending on the aggregation state. Thienyl-derivatives 2 display dissimilar chiroptical, morphological and emissive characteristics for the corresponding kinetically and thermodynamically controlled aggregated species AggI and AggII in comparison to those registered for compounds 1. Thus, a stereomutation phenomenon is observed in the AggI₂ → AggII₂ conversion. In addition, AggI₂ is arranged into nanoparticles that evolve to helical aggregates to afford AggII₂. The dissimilar chiroptical and morphological features of AggI₂ and AggII₂ are also appreciated in the emissive properties. Thus, whilst AggI₂ experiences a clear AIE (aggregation induced emission) process and CPL activity, the thermodynamically controlled AggII₂ undergoes an ACQ (aggregation caused quenching) process in which the CPL activity is cancelled.

The emissive features of supramolecular polymers formed by cyano-luminogens depends on the nature of the central aromatic unit. Intramolecular H-bonded species influence the chiroptical properties and the resulting aggregated species can be kinetically controlled.     

Ruthenium‐Grafted Vinylhelicenes: Chiroptical Properties and Redox Switching

The properties of mono‐ and bis‐Ru–vinyl[6]helicene complexes ( 2 a and 2 b , respectively), recently synthesized by using molecular engineering of helicenes based on the grafting of lateral organometallic substituents on the π‐helical backbone through a vinyl bridge, are presented. These helicene derivatives are thoroughly characterized, with special attention given to their chiroptical properties and redox switching activity. The UV/Vis and electronic circular dichroism (ECD) spectra of P and M enantiopure species, both in the neutral and oxidized states ([ 2 a ] ^{. +}, [ 2 b ] ^{. +}, and [ 2 b ]²⁺), are analyzed with the aid of quantum‐chemical calculations. The extended π‐conjugation facilitated by the vinyl moiety, clearly visible in the electronic structures of 2 a , b , introduces new active bands in the ECD spectra that consequently lead to a significant increase in optical rotation of Ru–vinylhelicenes compared with the organic precursors. The vibrational circular dichroism (VCD) spectra were measured and calculated for both the organic and organometallic species and constitute the first examples of VCD for metal‐based helicene derivatives. Finally, the redox‐triggered chiroptical switching activity of 2 a , b is examined in detail by using ECD spectroscopy. The modifications of the ECD spectra in the UV/Vis and NIR region are well reproduced and rationalized by calculations.     

Polyphenol content and antioxidant activity of fourteen wild edible fruits from Burkina Faso

A total of fourteen (14) species of wild edible fruits from Burkina Faso were analyzed for their phenolic and flavonoid contents, and their antioxidant activities using the DPPH, FRAP and ABTS methods. The data obtained show that the total phenolic and total flavonoid levels were significantly higher in the acetone than in the methanol extracts.Detarium microcarpum fruit had the highest phenolic and the highest flavonoid content,followed by that of Adansonia digitata, Ziziphus mauritiana, Ximenia americana and Lannea microcarpa. Significant amounts of total phenolics were also detected in the other fruit species in the following order of decreasing levels: Tamarindus indica > Sclerocaryabirrea > Dialium guineense > Gardenia erubescens > Diospyros mespiliformis > Parkiabiglobosa > Ficus sycomorus > Vitellaria paradoxa. Detarium microcarpum fruit also showed the highest antioxidant activity using the three antioxidant assays. Fruits with high antioxidant activities were also found to possess high phenolic and flavonoid contents. There was a strong correlation between total phenolic and flavonoid levels and antioxidant activities.     

An Enantiopure Cyclometallated Iridium Complex Displaying Long‐Lived Phosphorescence both in Solution and in the Solid State

A new enantiopure cyclometallated iridium complex bearing a [4]helicenic ‐coordinating and two ‐coordinating dfppy (2‐(2,4‐difluorophenyl)‐pyridyl) ligands was prepared. This complex displayed long‐lived phosphorescence both in solution and in the solid state. Its chiroptical properties, namely electronic circular dichroism and circularly polarized luminescence, were also examined. Comparison with former chiral complexes enabled assignment of the Δ_Ir‐(?) and Λ_Ir‐(+) absolute configurations.     

Phosphahelicenes: From Chiroptical and Photophysical Properties to OLED Applications

Herein, the experimental physicochemical and chiroptical properties of a series of phosphahelicenes are reported, focusing on their UV/Vis absorption, luminescence, electronic circular dichroism, optical rotations, and circularly polarized luminescence. Furthermore, detailed analysis of absorption and ECD spectra performed with the help of quantum-chemical calculations allowed us to highlight general features of these helicenic phosphines. Finally, due to well-suited electrochemical properties and thermal stability, the systems were successfully used as emitters in organic light-emitting diodes.     

Optical Activity of Spin-Forbidden Electronic Transitions in Metal Complexes from Time-Dependent Density Functional Theory with Spin-Orbit Coupling

The calculation of magnetic transition dipole moments and rotatory strengths was implemented at the zeroth-order regular approximation (ZORA) two-component relativistic time-dependent density functional theory (TDDFT) level. The circular dichroism of the spin-forbidden ligand-field transitions of tris(ethylenediamine)cobalt(III) computed in this way agrees very well with available measurements. Phosphorescence dissymmetry factors and the corresponding lifetimes are evaluated for three N-heterocyclic-carbene-based iridium complexes, two of which contain helicene moieties, and for two platinahelicenes. The agreement with experimental data is satisfactory. The calculations reproduce the signs and order of magnitude of , and the large variations of phosphorescence lifetimes among the systems. The electron spin contribution to the magnetic transition dipole moment is shown to be important in all of the computations.     

Absolute configuration of C76 from optical rotatory dispersion.

The absolute configuration of C76 has been determined as (+)589-(fC)-C76 , for the first time, by comparing the experimental and predicted optical rotatory dispersion (ORD) patterns. The experimental ORD pattern was derived from the experimental electronic circular dichroism (ECD) spectrum using the Kramers-Kronig (KK) transform. The theoretical ORD spectra were calculated in the resonant region using linear response theory, and also using the KK transform of the theoretical ECD spectrum, at different theoretical levels, namely BHLYP/6-31G*, B3LYP/6-31G*, BLYP/6-31G*, and HF/6-31G*. Good agreement noted between experimental and predicted spectra allows for an unambiguous determination of the absolute configuration.