On-Surface Synthesis of Helicene Oligomers.

We report on-surface synthesis of heterochiral 1D heptahelicene oligomers after deposition of a racemic heptahelicene monomer on an Au(111) surface followed by Ullmann coupling under ultrahigh vacuum conditions. Structure, chirality and mode of adsorption of the resulting dimers to octamers are inferred from the scanning probe microscopy and theoretical calculations.

Helical Bilayer Nanographenes: Impact of the Helicene Length on the Structural, Electrochemical, Photophysical, and Chiroptical Properties.

Helical bilayer nanographenes (HBNGs) are chiral π-extended aromatic compounds consisting of two π-π stacked hexabenzocoronenes (HBCs) joined by a helicene, thus resembling van der Waals layered 2D materials. Herein, we compare [9]HBNG, [10]HBNG, and [11]HBNG helical bilayers endowed with [9], [10], and [11]helicenes embedded in their structure, respectively. Interestingly, the helicene length defines the overlapping degree between the two HBCs (number of benzene rings involved in π-π interactions between the two layers), being 26, 14, and 10 benzene rings, respectively, according to the X-ray analysis. Unexpectedly, the electrochemical study shows that the lesser π-extended system [9]HBNG shows the strongest electron donor character, in part by interlayer exchange resonance, and more red-shifted values of emission. Furthermore, [9]HBNG also shows exceptional chiroptical properties with the biggest values of gabs and glum (3.6 × 10-2) when compared to [10]HBNG and [11]HBNG owing to the fine alignment in the configuration of [9]HBNG between its electric and magnetic dipole transition moments. Furthermore, spectroelectrochemical studies as well as the fluorescence spectroscopy support the aforementioned experimental findings, thus confirming the strong impact of the helicene length on the properties of this new family of bilayer nanographenes.

Nonaqueous capillary electrophoresis and quantum chemical calculations applied to investigation of acid-base and electromigration properties of azahelicenes.

Nonaqueous capillary electrophoresis (NACE) using methanol (MeOH) as a solvent of the BGEs and quantum mechanical density functional theory (DFT) have been applied to determine the thermodynamic acidity (ionization) constants (pKa ) of mono- and diaza[5]helicenes, mono- and diaza[6]helicenes, and their dibenzo derivatives in MeOH and water. First, the mixed acidity constants, pKa,MeOHmix${\rm{p}}K_{{\rm{a,MeOH}}}^{{\rm{mix}}}$ , of ionogenic pyridinium groups of azahelicenes and their derivatives in MeOH were obtained by nonlinear regression analysis of pH dependence of their effective electrophoretic mobilities. The effective mobilities were measured by NACE in a large series of methanolic BGEs within a wide conventional pH range (pHMeOH 1.6-12.0) and at ambient temperature (21-26°C) in a home-made CE device. Prior to mixed acidity constant calculation, the effective mobilities were corrected to reference temperature (25°C) and constant ionic strength (25 mM). Then, the mixed acidity constants were recalculated to the thermodynamic acidity constants pKa,MeOH by the Debye-Hückel theory of nonideality of electrolyte solutions. Finally, from the methanolic thermodynamic pKa,MeOH values, the aqueous thermodynamic pKa,H2O${\rm{p}}{K_{{\rm{a,}}{{\rm{H}}_{\rm{2}}}{\rm{O}}}}$ constants were estimated using the empirical relations between methanolic and aqueous acidity constants derived for structurally related pyridine derivatives. Depending on the number and position of the nitrogen atoms in their molecules, the analyzed azahelicenes were found to be weak to moderate bases with methanolic pKa,MeOH in the range 2.01-8.75 and with aqueous pKa,H2O${\rm{p}}{K_{{\rm{a,}}{{\rm{H}}_{\rm{2}}}{\rm{O}}}}$ in the range 1.67-8.28. The thermodynamic pKa,MeOH obtained by the DFT calculations were in a good agreement with those determined experimentally by NACE.

Electrochemistry of Tetrathiafulvalene Ligands Assembled on the Surface of Gold Nanoparticles.

The synthesis of a tetrathiafulvalene (TTF) derivative, S-[4-({4-[(2,2'-bi-1,3-dithiol-4-ylmethoxy)methyl] phenyl}ethynyl)phenyl] ethanethioate, suitable for the modification of gold nanoparticles (AuNPs), is described in this article. The TTF ligand was self-assembled on the AuNP surface through ligand exchange, starting from dodecanethiol-stabilized AuNPs. The resulting modified AuNPs were characterized by TEM, UV-Vis spectroscopy, and electrochemistry. The most suitable electrochemical method was the phase-sensitive AC voltammetry at very low frequencies of the sine-wave perturbation. The results indicate a diminishing electronic communication between the two equivalent redox centers of TTF and also intermolecular donor-acceptor interactions manifested by an additional oxidation wave upon attachment of the ligand to AuNPs.

Helically Chiral Aromatics: The Synthesis of Helicenes by [2 + 2 + 2] Cycloisomerization of π-Electron Systems.

Advanced molecular nanocarbons are now in the spotlight reflecting the basic discoveries of fullerenes, carbon nanotubes, and graphene. This research area includes also the chemistry, physics, and nanoscience of nonplanar polycyclic hydrocarbons, many of which exhibit helical chirality, such as iconic helicenes and their congeners. The combination of unique π-electron systems with the chirality phenomenon makes them highly attractive in various fields of science. Helicenes are polyaromatic compounds that are composed of all-angularly annulated benzene units, but other (hetero)cycles can also be embedded into their backbone. Even though they do not contain any stereogenic center, they are inherently chiral owing to the helical shape they adopt. Hexahelicene and higher homologues are conformationally stable within a reasonable range of temperatures and, therefore, can be obtained in an enantiopure form through a racemate resolution or asymmetric synthesis. An amazing array of synthetic methods for their preparation has been developed, but only a few of them have passed the tough scrutiny to be general, robust and practical methods such as traditional photocyclodehydrogenation of diaryl olefins and recently developed transition-metal-catalyzed [2 + 2 + 2] cycloisomerization of π-electron systems, which is discussed in this Account. Alkyne [2 + 2 + 2] cycloisomerization is a highly exergonic process and is therefore suitable for forming the strained helicene backbone, three (or more) cycles of which are closed in a single operation. The typical starting materials are aromatic triynes (optionally cyanodiynes or ynedinitriles) or tetraynes with diynes that undergo intramolecular or intermolecular cyclization, respectively, catalyzed by various complexes mainly of Ni0, CoI, or RhI. Utilizing this synthetic methodology, various [5]-, [6]-, [7]-, [9]-, [11]-, [13]-, [16]-, [17]-, and [19]helicenes or their congeners, including functionalized derivatives, can be effectively prepared. Moreover, asymmetric synthesis (both catalytic and stoichiometric) of nonracemic helicenes has already been demonstrated. It relies on [2 + 2 + 2] cycloisomerization of centrally chiral triynes followed by an asymmetric transformation of the first order (controlled by the 1,3-allylic-type strain) or on enantioselective [2 + 2 + 2] cycloisomerization of alkynes catalyzed by chiral complexes mainly of Ni0 or RhI. Intriguingly, advanced helical architectures were formed such as the longest helicenes (up to oxa[19]helicene by closing 12 rings in a single synthetic operation) or laterally extended helicenes (e.g., pyreno[7]helicenes). Utilizing the aforementioned synthetic methodology, the tailor-made helical molecular nanocarbons are now better accessible to be applied in enantioselective catalysis, chirality sensing, spintronics (based on chirality induced spin selectivity), chiroptics (to produce circularly polarized light emission), organic/molecular electronics, or chiral single molecule devices.

Synthesis of Racemic, Diastereopure, and Enantiopure Carba- or Oxa[5]-, [6]-, [7]-, and -[19]helicene (Di)thiol Derivatives.

A series of carba- or oxa[5]-, [6]-, [7]-, and -[19]helicene (di)thiols was prepared. The Miyazaki-Newman-Kwart rearrangement of (dimethylcarbamothioyl)oxy (oxa)helicenes in a flow reactor or nucleophilic substitution of dichloro (oxa)helicenes with alkanethiolates were used in the sulfanylation step. Despite the high temperatures employed in this key step, no conformational scrambling was observed during the asymmetric synthesis of the diastereo- and enantiopure oxahelicenes. Single-molecule conductivity of the longest oxa[19]helicene dithiol derivative was studied by the scanning tunneling microscopy break-junction method.

Chirality-Controlled Self-Assembly of Amphiphilic Dibenzo[6]helicenes into Langmuir-Blodgett Thin Films.

Racemic and highly enantioenriched 3-methoxycarbonyl, 3-carboxy, and 3-hydroxymethyl derivatives of dibenzo[6]helicene were prepared. The Langmuir layers of these helicenes were formed at the air-water interface and transferred onto solid substrates to afford Langmuir-Blodgett films, which were then studied by ambient atomic force microscopy and (chir)optical spectroscopy. Significant differences were found in the behaviour of the Langmuir layers as well as in the morphology, UV/Vis, electronic circular dichroism (ECD), and fluorescence spectra of the Langmuir-Blodgett thin films depending on the molecular chirality and nature of the polar group. The experimental results were supported by molecular dynamics simulations.

Aromatic Azide Transformation on the Ag(111) Surface Studied by Scanning Probe Microscopy.

Chemical transformation of 9-azidophenanthrene on the Ag(111) surface was studied by nc-AFM in UHV. High-resolution imaging supported by first-principle calculations revealed the structure of the final products that originated from a common and elusive 9-phenanthryl nitrenoid intermediate chemisorbed on the Ag(111) surface. A formal nitrene insertion into the C-H bond along with its dimerisation and hydrogenation were identified as main reaction channels. Thus, the ability of aryl azides to form covalent σ- and π-bonds between their transformation products on a solid surface was demonstrated at a single-molecule level.

Large Converse Piezoelectric Effect Measured on a Single Molecule on a Metallic Surface.

The converse piezoelectric effect is a phenomenon in which mechanical strain is generated in a material due to an applied electrical field. In this work, we demonstrate the converse piezoelectric effect in single heptahelicene-derived molecules on the Ag(111) surface using atomic force microscopy (AFM) and total energy density functional theory (DFT) calculations. The force-distance spectroscopy acquired over a wide range of bias voltages reveals a linear shift of the tip-sample distance at which the contact between the molecule and tip apex is established. We demonstrate that this effect is caused by the bias-induced deformation of the spring-like scaffold of the helical polyaromatic molecules. We attribute this effect to coupling of a soft vibrational mode of the molecular helix with a vertical electric dipole induced by molecule-substrate charge transfer. In addition, we also performed the same spectroscopic measurements on a more rigid o-carborane dithiol molecule on the Ag(111) surface. In this case, we identify a weaker linear electromechanical response, which underpins the importance of the helical scaffold on the observed piezoelectric response.

Helicenes as Chirality-Inducing Groups in Transition-Metal Catalysis: The First Helically Chiral Olefin Metathesis Catalyst.

Helical chirality is a novel enantioselectivity-inducing property in transition-metal-catalyzed transformations. The principle is illustrated herein for the example of asymmetric olefin metathesis. This work reports the synthesis of the first helically chiral Ru-NHC alkylidene complex from an aminohelicene-derived imidazolium salt, which was ligated to the first generation Hoveyda-Grubbs catalyst. Kinetic data were acquired for benchmark test reactions and compared to an achiral catalyst. The helically chiral Ru-catalyst was evaluated in asymmetric ring-closing metathesis (RCM) and ring-opening metathesis-cross-metathesis (ROM/CM) reactions, which proceeded with promising levels of enantioselectivity. Extensive NMR-spectroscopic investigations and a DFT geometry optimization were performed. These results led to a topographic steric map and calculation of percent-buried-volume values for each quadrant around the metal center.

Asymmetric Synthesis of Nonracemic 2-Amino[6]helicenes and Their Self-Assembly into Langmuir Films.

Alternative ways of preparing nonracemic 2-amino[6]helicene derivatives were explored. The enantioselective [2 + 2 + 2] cycloisomerization of a nonchiral triyne under Ni(cod)2/( R)-QUINAP catalysis delivered the enantioenriched (+)-( P)-2-aminodibenzo[6]helicene derivative in 67% ee. An ultimate "point-to-helical" chirality transfer was observed in the cyclization of enantiopure triynes mediated by Ni(CO)2(PPh3)2 affording (-)-( M)- or (+)-( P)-7,8-bis( p-tolyl)hexahelicen-2-amine in >99% ee as well as its benzoderivative in >99% ee. The latter mode of stereocontrol was inefficient for a 2-aminobenzo[6]helicene congener with an embedded five-membered ring. The rac-, (-)-( M)-, and (+)-( P)-7,8-bis( p-tolyl)hexahelicen-2-amines formed Langmuir monolayers at the air-water interface featuring practically identical surface pressure vs mean molecular area isotherms. The corresponding Langmuir-Blodgett films on quartz or silicon substrates were characterized by UV-vis/ECD spectroscopy and AFM microscopy, respectively.

Reversal of the sense of enantioselectivity between 1- and 2-aza[6]helicenes used as chiral inducers of asymmetric autocatalysis.

Reversal of the sense of enantioselectivity was observed between 1-aza[6]helicene 2 and 2-aza[6]helicene 3 employed as chiral inducers of asymmetric autocatalysis of pyrimidyl alkanol. In the presence of (P)-(+)-1-aza[6]helicene 2, the reaction of pyrimidine-5-carbaldehyde 1 with diisopropylzinc afforded, in conjunction with asymmetric autocatalysis, (S)-pyrimidyl alkanol 4 with high ee. Surprisingly, the reaction in the presence of (P)-(+)-2-aza[6]helicene 3 gave the opposite enantiomer of (R)-alkanol 4 with high ee. In the same manner, (M)-(-)-2 and (M)-(-)-3 afforded (R)- and (S)-alkanol 4, respectively. The sense of enantioselectivity is controlled not only by the helicity of the azahelicene derivatives but also by the position of the nitrogen atom.

Synthesis of Long Oxahelicenes by Polycyclization in a Flow Reactor.

A series of oxahelicenes composed of ortho/meta-annulated benzene/pyridine and 2H-pyran rings were synthesized on the basis of the cobalt(I)-mediated (or rhodium(I)- or nickel(0)-mediated) double, triple, or quadruple [2+2+2] cycloisomerization of branched aromatic hexa-, nona-, or dodecaynes, thus allowing the construction of 6, 9, or 12 rings in a single operation. The use of a flow reactor was found to be beneficial for the multicyclization reactions. The stereogenic centers present in some of the oligoynes steered the helical folding in such a way that the final oxa[9]-, [13]-, [17]- and [19]helicenes were obtained in both enantiomerically and diastereomerically pure form. Specifically, the oxa[19]helicenes beat the current record in the length of a helicene backbone. Single-molecule conductivity was studied by the mechanically controllable break-junction method with a pyridooxa[9]helicene.

[2+2+2] Cycloisomerisation of Aromatic Cyanodiynes in the Synthesis of Pyridohelicenes and Their Analogues.

We have developed a methodology for the synthesis of pyridohelicenes and their analogues based on the Ni(0) -, Co(I) - or Rh(I) -mediated intramolecular [2+2+2] cycloisomerisation of cyanodiynes. It allows for folding the linear precursors into the corresponding helical backbones comprising the newly formed pyridine unit in their central part. Along with racemic pyrido[n]helicenes (n=5,6,7) and their derivatives, both enantio- and diastereomerically pure pyrido[n]helicene-like molecules (n=5,6) were prepared by employing the chiral substrate-controlled cyclisation of the corresponding enantiopure cyanodiynes.

Growth kinetics of racemic heptahelicene-2-carboxylic acid nanowires on calcite (104).

Molecular self-assembly of racemic heptahelicene-2-carboxylic acid on a dielectric substrate at room temperature can be used to generate wire-like organic nanostructures consisting of single and double molecular rows. By means of non-contact atomic force microscopy, we investigate the growth of the wire-like pattern after deposition by experimental and theoretical means. From analyzing the time dependence of the mean row length, two distinct regimes were found. At the early post-deposition stage, the mean length grows in time. Subsequently, a crossover to a second regime is observed, where the mean row length remains nearly constant. We explain these findings by a mean-field rate equation approach providing a comprehensive picture of the growth kinetics. As a result, we demonstrate that the crossover between the two distinct regimes is accomplished by vanishing of the homochiral single rows. At later stages only heterochiral double row structures remain.

An Ultimate Stereocontrol in Asymmetric Synthesis of Optically Pure Fully Aromatic Helicenes.

The role of the helicity of small molecules in enantioselective catalysis, molecular recognition, self-assembly, material science, biology, and nanoscience is much less understood than that of point-, axial-, or planar-chiral molecules. To uncover the envisaged potential of helically chiral polyaromatics represented by iconic helicenes, their availability in an optically pure form through asymmetric synthesis is urgently needed. We provide a solution to this problem present since the birth of helicene chemistry in 1956 by developing a general synthetic methodology for the preparation of uniformly enantiopure fully aromatic [5]-, [6]-, and [7]helicenes and their functionalized derivatives. [2 + 2 + 2] Cycloisomerization of chiral triynes combined with asymmetric transformation of the first kind (ultimately controlled by the 1,3-allylic-type strain) is central to this endeavor. The point-to-helical chirality transfer utilizing a traceless chiral auxiliary features a remarkable resistance to diverse structural perturbations.

Chimerical pyrene-based [7]helicenes as twisted polycondensed aromatics.

Chimerical pyrene-based dibenzo[7]helicene rac-1 and 2H-pyran[7]helicene (M,R,R)-(-)-2, in which two pyrene subunits are fused to the [7]helicene/[7]heterohelicene scaffold, were synthesised by means of Ni(0) - or Co(I) -mediated [2+2+2] cycloisomerisation of dipyrenyl-acetylene-derived triynes. Pyrene-based dibenzo[7]helicene 1 was obtained in enantioenriched form by enantioselective cycloisomerisation under Ni(0) /QUINAP catalysis (57 % ee) or in enantiopure form by racemate resolution by liquid chromatography on a chiral column. 1,3-Allylic-type strain-controlled diastereoselective cycloisomerisation was employed in the synthesis of enantiopure (M,R,R)-(-)-2. Physicochemical properties of 1 and 2 encompassing the helicity assignment, stability to racemisation, X-ray crystal structure, UV/Vis, experimental/calculated electronic circular dichroism and fluorescence spectra were studied. Accordingly, comparison of the X-ray crystal structure of (M,R,R)-(-)-2 with calculated structures (DFT: B3LYP/cc-pVDZ, B97D/cc-pVDZ) indicated that its helical backbone is slightly over-flattened owing to intramolecular dispersion forces between tert-butylated pyrene subunits. Both 1 and 2 are fluorescent (with quantum yields in dichloromethane of ΦF =0.10 and 0.17, respectively) and are suggested to form intramolecular excimer states upon excitation, which are remarkably stabilised and exhibit large Stokes shifts (296 and 203 nm, respectively).

On the physicochemical properties of pyridohelicenes.

A comprehensive study on the physicochemical properties of a series of mono- and diaza[5]helicenes as well as mono- and diaza[6]helicenes is reported. Through the use of both computational and experimental methods, these helically chiral pyridohelicenes with the nitrogen atom(s) in various positions are characterised according to their inversion barriers, protonation constants and redox potentials. By using DFT calculations, kinetic measurements, UV/Vis titrations, cyclic voltammetry and EPR spectroscopy, a self-contained picture of their behaviour under conventional treatment by heat, acids and oxidising/reducing agents is provided.

The use of cobalt-mediated cycloisomerisation of ynedinitriles in the synthesis of pyridazinohelicenes.

A cobalt-mediated [2+2+2] cycloisomerisation of ynedinitriles to helical pyridazines in good to high yields was developed. The construction of the pyridazine nucleus from one alkyne and two nitrile units is proposed to follow either a conventional organometallic mechanism or to be triggered by a single-electron transfer from a Co(II) species. Various [5]-, [6]- and [7]helicene pyridazines were prepared.

Tetrathiafulvalene-oligo(para-phenyleneethynylene) conjugates: formation of multiple mixed-valence complexes upon electrochemical oxidation.

Short monodisperse oligo- (para-phenyleneethynylene) (pOPE) units bearing laterally attached tetrathio-substituted tetrathiofulvalene (TTF) units have been synthesised from functionalised aromatic building blocks by using the Sonogashira cross-coupling methodology. The unusual redox properties of these TTF-pOPE conjugates were observed by employing electrochemical methods, such as cyclic voltammetry and exhaustive electrolysis. We found that formally one half of the TTF units in the pOPE monomer 1, dimer 2, and trimer 3 (with 2, 4, and 6 TTF units, respectively) are electrochemically silent during the first-step oxidation at 0.49 V. We propose the formation of persistent mixed-valence complexes from the TTF and TTF(+·) units present in an equal ratio. Such mixed-valence dyads (single or multiple in the partially oxidised 1-3) exhibit an unusual stability towards oxidation until the potential of the second oxidation at 0.84 V is achieved. This finding suggests that below this potential the oxidation of the respective mix-valence complexes is extremely slow.