Bis-4-aza[6]helicene: A Bis-helicenic 2,2′-Bipyridine with Chemically Triggered Chiroptical Switching Activity

Switchable molecular tweezers: design and applications

Switchable molecular tweezers are a unique class of molecular switches that, like their macroscopic analogs, exhibit mechanical motion between an open and closed conformation in response to stimuli. Such systems constitute an essential component of artificial molecular machines. This review will present selected examples of switchable molecular tweezers and their potential applications. The first part will be devoted to chemically responsive tweezers, including stimuli such as pH, metal coordination, and anion binding. Then, redox-active and photochemical tweezers will be presented.

Helicoselective Synthesis of Indolohelicenoids through Organocatalytic Central-to-Helical Chirality Conversion

We report the helicoselective and convergent construction of indolohelicenoids with excellent efficiency and stereocontrol. This reaction proceeds through a chiral-phosphoric-acid-catalyzed enantioselective cycloaddition and eliminative aromatization sequence, which can be finely controlled by adjusting the reaction temperature. Mechanistic studies reveal that the chiral phosphoric acid cooperatively serves as both a bifunctional and Brønsted acid catalyst, enabling one-pot central-to-helical chirality conversion. Additionally, the optical properties of the synthesized indolohelicenoids were characterized to explore their potential applications in organic photoelectric materials.

Acid/Base-Triggered Photophysical and Chiroptical Switching in a Series of Helicenoid Compounds

A series of molecules that possess two quinolines, benzoquinolines, or phenanthrolines connected in a chiral fashion by a biaryl junction along with their water-soluble derivatives was developed and characterized. The influence of the structure on the basicity of the nitrogen atoms in two heterocycles was examined and the photophysical and chiroptical switching activity of the compounds upon protonation was studied both experimentally and computationally. The results demonstrated that changes in the electronic structure of the protonated vs. neutral species, promoting a bathochromic shift of dominant electronic transitions and alternation of their character from π-to-π* to charge-transfer-type, when additionally accompanied by the high structural flexibility of a system, leading to changes in conformational preferences upon proton binding, produce particularly pronounced modifications of the spectral properties in acidic medium. The latter combined with reversibility of the read-out make some of the molecules in this series very promising multifunctional pH probes.

Circularly polarized luminescence in the one-dimensional assembly of binaphtyl-based Yb(iii) single-molecule magnets

Lanthanide ions have attracted great interest owing to their optical and magnetic properties. Single-molecule magnet (SMM) behavior has been a fascinating science for thirty years. Moreover, chiral lanthanide complexes allow the observation of remarkable circularly polarized luminescence (CPL). However, the combination of both SMM and CPL behaviors in a single molecular system is very rare and deserves attention in the design of multifunctional materials. Four chiral one-dimensional coordination compounds involving 1,1'-Bi-2-naphtol (BINOL)-derived bisphosphate ligands and the Yb(iii) centre were synthesized and characterized by powder and single-crystal X-ray diffraction. All the Yb(iii)-based polymers displayed field-induced SMM behavior with magnetic relaxation occurring by applying Raman processes and near infrared CPL in the solid state.

Two-ring chirality generated by the alignment of two achiral phenylacetylene macrocycles

When two achiral rings are bound mechanically, a chiral source is generated in the assembly. The chiroptical properties could be modulated according to the relative occupation of each ring in the assembly. In fact, we have found that two isomeric assemblies (1 and 2) show unique properties in each assembly with two achiral rings of phenylacetylene macrocycle (PAM). When considering the difference in the chiroptical properties of these two isomeric assemblies (6PAM × 2), no comparison was available based on no activity of the achiral component element itself (6PAM). In this work, we synthesized a two-ring chiral analog (4) by the ring-fusion of two 6PAMs to an 11PAM, and examined the chiroptical properties of 4, since the single helix was imparted as a chiral source. By comparison of the chiroptical properties (molar circular dichroism and molar optical rotation) of 1 and 2 to those of 4, we demonstrated that the disparity was related to the alignment of the two achiral rings.

Circularly polarized luminescence of helicenes: A data-informed insight

Helicenes are an interesting scaffold for chiroptical properties and in particular circularly polarized luminescence (CPL). In this short review, we collect the luminescence (g_lum ) and absorption (g_abs ) dissymmetry factors associated to the first Cotton effect of the electronic circular dichroism (ECD) spectrum. Considering the data for 170 [n]-helicenes (n = 4-11), overall we found reasonable correlations between g_lum and g_abs . Despite a few notable exceptions, this would confirm a similarity in the stereochemistry of the ground and emitting excited states for most helicenes. These results may be useful in rationalizing chiroptical data and help chemists in designing new helicene structures with the desired CPL properties.

A Dualistic Arrangement of a Chiral [1]Rotaxane Based on the Assembly of Two Rings and Two Rods

We demonstrate the synthesis and chiroptical properties of doubled molecules of a chiral [1]rotaxane, based on the assembly of an achiral ring of a phenylacetylene macrocycle (6PAM) and a p-phenylene ethynylene rod. Two molecules of [1]rotaxane constituted the doubled molecule through the ring fusion of 6PAMs to a 10PAM, which assured stationary occupation relative to each optically active unit. The absorption properties of the 10PAM-based doubled molecule and 6PAM-based original unit were consistently characterized by the independent existence of m-phenylene ethynylene ring(s) and p-phenylene ethynylene rod(s). Thus, molar circular dichroism (CD) was directly compared between the doubled molecule (n = 2) and the original unit (n = 1) to show that molar CD was increased more than expected by an increase in the number of units, or by an increase in absorbance. Due to the invariance of the configuration and the relative occupation of two units arranged adjacent to each other in 10PAM, one more comparison was available with an isomeric molecule of two rings and two rods in a threaded-and-unthreaded form. The additional arrangement of an optically inactive unit in an unthreaded form also led to an increase in molar CD, compared to that of the original chiral unit in a threaded form.

Synthesis and Coordination Chemistry of a Benzannulated Bipyridine: 6,6'-Biphenanthridine

The synthesis, characterization, and coordination chemistry of a doubly π-extended bipyridine analogue, 6,6'-biphenanthridine (biphe), is presented. The structure of the molecule has been determined in the solid state by X-ray diffraction, showing an angle of 72.6° between the phenanthridine planes. The free, uncoordinated organic molecule displays blue fluorescence in solution. It can be singly protonated with strong acids, and the protonated form displays more intense yellow emission. The effect of acid on the excited states is interpreted with the aid of TDDFT calculations. Two Ru(II) coordination complexes, tris(6,6'-biphenanthridine)ruthenium(II) dichloride, [Ru(biphe)₃]Cl₂, and bis(2,2'-bipyridine)(6,6'-biphenanthridine)ruthenium(II) tetraphenylborate, [Ru(bpy)₂(biphe)](BPh₄)₂, are also reported and their structures determined in the solid state by X-ray diffraction. Both complexes display emission at 77 K that is strongly bathochromically shifted by almost 200 nm compared to that of the archetypal ³MLCT emitter [Ru(bpy)₃]²⁺. Such a red shift is consistent with the more extended conjugation and lower-energy π* orbitals associated with the biphe ligand, lowering the energy of the ³MLCT excited state, as revealed by TDDFT calculations. The efficient non-radiative decay that is typical of such low-energy emitters renders the phosphorescence extremely weak and short-lived at ambient temperature, and rapid ligand photodissociation also competes with radiative decay, especially in the heteroleptic complex. Electrochemical analysis illustrates the effect of biphe's stabilized vacant π* manifold, with multiple reversible reductions evident at much less negative potentials than those observed for [Ru(bpy)₃]²⁺.

Synthesis of Nitrogen-Doped Aza-Helicenes with Chiral Optical Properties

Aza-helicenes are one of the most important series of heterohelicenes; herein, a series of novel aza-helicenes (5H, 6H, 6S, and 8S) were prepared via Bischler-Napieralski cyclization, and the interconversion dynamic process of these aza-helicenes was revealed using density functional theory calculations. The novel nitrogen-doped [6]helicene (6H) possesses a very high interconversion energy barrier of 36.0 kcal/mol. Two enantiomers of 6H were successfully resolved by high-performance liquid chromatography and showed desired chiral optical properties. 6H with chiral optical activity and lone electrons can be a potential candidate for chiral switches, which was demonstrated using the UV and circular dichroism spectra obtained upon titration with an acid and a base.

Synthesis of Aza[n]helicenes (n = 4-7) via Photocyclodehydrochlorination of 1-Chloro-N-aryl-2-naphthamides

A series of aza[n]helicenes (n = 4-7) was synthesized using a photocyclodehydrochlorination of 1-chloro-N-aryl-2-naphthamides as a general synthetic procedure introducing the nitrogen atom to the third ring of the helicene framework. The effect of the nitrogen presence in the helicene skeleton on the physicochemical properties of the prepared aza[n]helicenes was studied and compared to those of the parent carbo-analogues. The insertion of a nitrogen atom into the outer edge of the helicene molecule has a severe impact on certain physicochemical properties such as optical rotation, electrostatic potentials, and intermolecular interactions. On the other hand, some other properties such as UV/vis, fluorescence, and phosphorescence spectra remained almost unaffected when compared to the parent carbohelicenes. A nitrogen atom can be also used for further derivatization, which can lead to further modification of helicene properties, as manifested here in the fluorescence changes induced by protonation.

Synthesis of Functionalized Six-Membered-Ring Azahelicenes

Functionalization, namely the introduction of side groups onto the molecular scaffold of a helicene, may have either the purpose of modifying the electronic properties of the parent helicene, e.g., by adding electron-withdrawing or electron-donating groups, or the scope of providing the helicene with a “handle”, which can be reacted to bind the molecule to another molecule or to a solid structure, such as a carbon or metal surface, or again to allow for complexation of the helicene with metal ions. The possible approaches are two-fold: the synthesis of the helicene can be performed using starting materials that already contain a side group, or the side group can be introduced after the synthesis of the parent helicene. As azahelicenes are helicenes bearing one or more nitrogen atom(s) in the molecular framework, parent azahelicenes can be functionalized on carbon atoms by exploiting the presence of the electron-withdrawing nitrogen atom. Moreover, they can be transformed into quaternary salts, whose properties are quite different from those of the parent azahelicenes in terms of the solubility and electronic properties. This review aims to provide a survey of the different synthetic methods available to attain this fascinating class of compounds.

The synthesis and crystal structure of pH-sensitive fluorescent pyrene-based double aza- and diaza[4]helicenes

Novel pyrene-based double aza- and diaza[4]helicenes have been prepared through a five-step synthetic sequence in overall good yields. Commercially available 2,3-dihaloazines (2,3-dibromopyridine, 2,3-dichloropyrazine and 2,3-dichloroquinoxaline) were used as starting materials. The synthesis employs electrophile-induced cyclizations of ortho-alkynyl bihetaryls as the key steps, leading to the formation of a helical skeleton. To discern the effect of merging azine and pyrene moieties within a helical skeleton, the X-ray structures, UV-vis absorption and fluorescence spectra of the helicenes were investigated and compared with those of the parent [4]helicene, aza- and diaza[4]helicenes. It was found that the emission properties of the synthesized helicenes can be modulated as a function of pH. The basicity of pyrene-based double aza[4]helicenes was estimated by the direct fluorimetric titration method; the pK_a value was found to be equal to 1.4.

Circularly polarized luminescence of Eu(III) complexes with chiral 1,1'-bi-2-naphtol-derived bisphosphate ligands

The interest for lanthanide circularly polarized luminescence (CPL) has been quickly growing for 10 years. However, very few of these studies have involved correlation between the dissymmetry factor (g_lum ) and the chemical modifications in a series of chiral ligands. Four polymeric compounds of Eu(III) were prepared by using a series of binaphtyl derivatives for which the size of the π system as well as the number of stereogenic elements (i.e., the binaphtyl moiety) are modulated. The resulting {[Eu(hfac)₃ ((S)/(R)-L^x )]}_n (x = 1 and 3) and {[Eu(hfac)₃ ((S,S,S)/(R,R,R)-L^x )]}_n (x = 2 and 4) have been characterized by powder X-ray diffraction by comparison with the X-ray structures on single crystal of the Dy(III) analogs. In solution, the structure of the complexes is deeply modified and becomes monomeric. The nature of the ligand induces change in the shape of the CPL spectra in CH₂ Cl₂ solution. Furthermore, a large |g_lum | = 0.12 of the magnetic-dipole transition for the [Eu(hfac)₃ ((S,S,S)/(R,R,R)-L² )] complex involving the ligand with three stereogenic elements and an extended 𝜋 system has been measured. This report also shows CPL measurements in solid state for the series of {[Eu(hfac)₃ ((S)/(R)-L^x )]}_n (x = 1 and 3) and {[Eu(hfac)₃ ((S,S,S)/(R,R,R)-L^x )]}_n (x = 2 and 4) polymers.

Recent Advances in the Synthesis of 2,2′-Bipyridines and Their Derivatives

The sustained interest in the synthesis of new analogues of 2,2′-bipyridines is supported by the importance of compounds featuring bipyridine core in diverse areas of chemical, biomedical and materials research, which is relayed into the development of new approaches and the expansion of existing synthetic methods. This short review covers advances in the synthesis of 2,2′-bipyridines, including both the synthesis of compounds with a given substitution pattern and the development of new methods for assembling the bipyridine core. Special attention is directed toward the use of pyridine N-oxides and metal-free protocols to facilitate the formation of bipyridines. This short review focuses primarily on reports published in the last 5–6 years.

1 Introduction

2 Ullmann-Type Homocoupling Reactions

3 Cross-Coupling Reactions in the Synthesis of Bipyridines

4 Coupling Reactions Employing Pyridine N-Oxides

5 Other Methods for the Synthesis of 2,2′-Bipyridines

6 Conclusions and Outlook

Enantioenriched Ruthenium-Tris-Bipyridine Complexes Bearing One Helical Bipyridine Ligand: Access to Fused Multihelicenic Systems and Chiroptical Redox Switches

The synthesis and photophysical and chiroptical properties of novel aza[n]helicenes (6a-d, 10a,b, n = 4-7) substituted with one or two 2-pyridyl groups are described. The preparation was performed via an adapted Mallory reaction using aromatic imines as precursors. The obtained novel class of helical 2,2'-bipyridine ligands was then coordinated to Ru(bipy)₂²⁺ units, thus affording the first diastereomerically and enantiomerically pure [RuL(bipy)₂]²⁺ (11a,c, L = 6a,c) or [Ru₂L'(bipy)₄]⁴⁺ (12, L' = 10b) complexes. The topology and stereochemistry of these novel metal-based helical architectures were studied in detail, notably using X-ray crystallography. Interestingly, the coordination to ruthenium(II) enabled the preparation of fused multihelical systems incorporating aza- and ruthena-helicenes within the same scaffold. The photophysical, chiroptical, and redox properties of these complexes were examined in detail, and efficient redox-triggered chiroptical switching activity was evidenced.

Heterohelicenes through 1,3-Dipolar Cycloaddition of Sydnones with Arynes: Synthesis, Origins of Selectivity, and Application to pH-Triggered Chiroptical Switch with CPL Sign Reversal

Regioselective access to heterohelicenes through the 1,3-dipolar cycloaddition of sydnones with arynes is described. Novel access to sydnones and poly(hetero)aromatic aryne precursors allowed the introduction of chemical diversity over multiple positions of the helical scaffolds. The origins of the unconventional regioselectivity during the cycloaddition steps was systematically investigated using density functional theory (DFT) calculations, unveiling the key features that control this reactivity, namely, face-to-face (π···π) or edge-to-face (C-H···π) interactions, primary orbital interactions and distortion from coplanarity in the transition structures (TSs) of the transformation. From the library of 24 derivatives synthesized, a pyridyl containing derivative displayed reversible, red-shifted, pH-triggered chiroptical switching properties, with CPL-sign reversal. It is found that protonation of the helicene causes a change of the angle between the electric and magnetic dipole moments related to the S₁ → S₀ transition, resulting in this rare case of reversible CPL sign inversion upon application of an external stimulus.

Rhodium-Catalyzed Enantioselective Synthesis, Structures, and Properties of Single and Double Azahelicene-Like Molecules

The enantioselective synthesis of aza[6] and [7]helicene-like molecules have been achieved by the cationic rhodium(I)/axially chiral biaryl bisphosphine complex-catalyzed intramolecular [2+2+2] cycloaddition of cyanodiynes. This protocol was successfully applied to the diastereo- and enantioselective synthesis of an S-shaped double aza[6]helicene-like molecule with a high ee value of 89 %. Although no epimerization and racemization were observed in the double carbo[6]helicene-like molecule at 80 °C, epimerization and racemization of the double aza[6]helicene-like molecule proceeded at 80 °C. This double aza[6]helicene-like molecule showed good fluorescent quantum yields and chiroptical responses under both neutral and acidic conditions.

Synthesis and Properties of Partially Saturated Fluorenyl-Derived [n]Helicenes Featuring an Overcrowded Alkene

The straightforward, multigram-scale synthesis of the partially saturated H₆ -fluoreno[n]helicenes (n=5 or 7) featuring a central, overcrowded alkene is described. The key cyclization step was based on an intramolecular McMurry reaction from the corresponding 1,5-diketones. Chiral stationary phase HPLC analysis and isomer separation indicate that each helicenic compound is constituted of three diastereoisomers at room temperature, i. e. the configurationally stable (R,R,P)/(S,S,M) pair of enantiomers and an apparently achiral compound resulting from the rapid interconversion between the (R,S,P) and (S,R,M) enantiomers. The partially saturated H₆ -fluoreno[n]helicenes are oxidatively aromatized to give an efficient access to the corresponding fluoreno[n]helicenes. The chiroptical properties (vibrational and electronic circular dichroism) of the chiral, enantiopure compounds have been measured and analyzed by quantum-chemical calculations, confirming their helicoidal nature.

Solid-State Near-Infrared Circularly Polarized Luminescence from Chiral YbIII -Single-Molecule Magnet

A field-induced chiral Yb^III Single-Molecule Magnet (SMM) displayed an unprecedented near-infrared circularly polarized luminescence (NIR-CPL) in the solid-state. The bridging bis(1,10-phenantro[5,6b])tetrathiafulvalene triad (L) allowed an efficient sensitization of the NIR ² F_5/2 →² F_7/2 emission while the NIR-CPL is associated to the f-f transitions of the Yb^III ion bearing chiral β-diketonate derived-camphorate ancillary ligands.

Origins of Optical Activity in an Oxo-Helicene: Experimental and Computational Studies

Helicenes are known to provide extremely strong optical activity. Prediction of the properties of helicenes may facilitate their design and synthesis for analytical or materials sciences. On a model 7,12,17-trioxa[11]helicene molecule, experimental results from multiple spectroscopic techniques are analyzed on the basis of density functional theory (DFT) simulations to test computational methodology and analyze the origins of chirality. Infrared (IR), vibrational circular dichroism (VCD), electronic circular dichroism (ECD), magnetic circular dichroism (MCD), and Raman optical activity (ROA, computations only) spectra are compared. Large dissymmetry factors are predicted both for vibrational (ROA/Raman ∼ VCD/IR ∼ 10^-3) and electronic (ECD/Abs ∼10^-2) optical activity, which could be verified experimentally except for ROA. Largest VCD signals come from a strong vibrational coupling of the C-H in-plane and out-of-plane bending modes in stacked helicene rings. The sum-over-states (SOS) approach appeared convenient for simulation of MCD spectra. Our results demonstrated that selected computational methods can be successfully used for reliable modeling of spectral and chiroptical properties of large helicenes. In particular, they can be used for guiding rational design of strongly chiral chromophores.

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.

Irreverent Nature of Dissymmetry Factor and Quantum Yield in Circularly Polarized Luminescence of Small Organic Molecules

Recently, a rational modification of small organic molecules has attracted considerable attention for designing advanced materials with enhanced circularly polarized luminescence (CPL) activity. A particular emphasis has been placed on fully allowed π-π^* transition of rigid aromatic systems, due to their relatively superior emission properties or quantum yields of luminescence (Φ_lum). However, their dissymmetry factors (g_lum), differential left and right CPL intensities, are typically disappointingly low at least in one to two orders of magnitude. Truly useful organic CPL materials, rated by a circular polarization luminosity index (Λ_CPL) per single molecule, possess both |g_lum| and Φ_lum values high. However, how to improve these two factors simultaneously with a proper molecular design is an open question. Here, we addressed this issue by theoretical and statistical inspection on a possible relation of the g_lum and Φ_lum values. According to the analysis, we propose simple, unpretentious, yet pertinent guidelines for designing superior organic CPL materials for the future with large Λ_CPL values.

Enantioselective Control of Both Helical and Axial Stereogenic Elements though an Organocatalytic Approach

A highly diastereo- and enantio-selective method for the asymmetric synthesis of molecules containing helicenes and stereogenic axes was developed based on organocatalysis. Various compounds bearing both helical and axial stereogenic elements were obtained in excellent enantioselectivities. The mechanism study revealed that the reaction proceeded through two stages: 1) The first cyclization produces a reaction intermediate containing a stereogenic axis. 2) The dynamic kinetic resolution of helix reaction intermediate following with cyclization generates a helix and another stereogenic axis.

Reaction-based chiroptical sensing of ClO− using circularly polarized luminescence via self-assembly organogel

A chiral amino acid functionalized probe, PTZ-D, could self-assemble into a chiral organogel displaying unprecedented chiroptical monitoring of ClO⁻ with switchable CPL signals.