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Ruthenium-Grafted Vinylhelicenes: Chiroptical Properties and Redox Switching
Chemistry - A European Journal
Abstract
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.
... Moreover, chiral molecules unable to undergo 3D enantiospecific crystallisation may be able to enantio-segregate, by forming small homochiral clusters and even extended homochiral domains upon surface SA. Such assemblies have several applications in energy materials: as blueprints for the growth of helical organic nanotubes with elevated non-linear optical properties [28][29][30], as sites or modifiers for heterogeneous chiral catalysis [31][32][33][34], or as building blocks for molecular motors [35][36][37]. Restriction of the assembly process to two dimensions selects a part of the potential energy surface on which the recognition process occurs, thus chiral specificity is more easily attained. ...
The development of materials with specific properties and controllable response to external stimuli is of central importance in the energy industry and manufacturing. A large part of the current research effort in energy materials is dedicated to understanding how to control matter at the molecular level. If highly complex materials are to be manufactured at a large scale, in a sustainable and efficient manner, the advantages of control science will have to be materialised, and bottom-up methods of development, such as molecular self-assembly will have to be employed. Here, we review our group’s efforts in developing a fundamental understanding of self-assembly processes, in particular self-assembly at the vacuum-solid interface, using computational and theoretical materials science. A combination of simulation approaches, from quantum-based methods, to classical atomistic calculations, to mean-field approximations of bulk mixed monolayers is used in an attempt to approach the various length scales characteristic to self-assembled pattern formation for functional materials.
Oxidative fusion reaction of cyclic heteroaromatic pentads consisting of pyrrole and thiophene gave closed‐heterohelicene monomers and dimers depending on the oxidation conditions. Specifically, oxidation with [bis(trifluoroacetoxy)iodo]benzene (PIFA) gave closed‐[7]helicene dimers connected at the β‐position of one of the pyrrole units with the remarkably elongated C–C bonds about 1.60 Å. Although this bond was intact against thermal and physical activations, homolytic bond dissociation took place upon UV irradiation in DMSO to give the corresponding monomers. Thus, interconversion between the closed‐helicene monomer and dimer was achieved. The optically pure dimer was photo‐dissociated into the monomers associated with circularly polarized luminescence (CPL) turn‐ON.
Oxidative fusion reaction of cyclic heteroaromatic pentads consisting of pyrrole and thiophene gave closed‐heterohelicene monomers and dimers depending on the oxidation conditions. Specifically, oxidation with [bis(trifluoroacetoxy)iodo]benzene (PIFA) gave closed‐[7]helicene dimers connected at the β‐position of one of the pyrrole units with remarkably elongated C−C bonds of about 1.60 Å. Although this bond was intact against thermal and physical activations, homolytic bond dissociation took place in DMSO upon irradiation with UV light to give the corresponding monomers. Thus, interconversion between the closed‐helicene monomer and dimer was achieved. The optically pure dimer was photo‐dissociated into the monomers associated with a turn‐on of circularly polarized luminescence (CPL).
Helicenes are fascinating molecules that combine helical chirality and π‐conjugated systems. Recently, the chemistry of helicenes has experienced a boom, owing to the exploitation of their unique properties in many fields such as asymmetric synthesis, catalysis, supramolecular recognition, materials science, and medicinal chemistry. The use of the general toolbox of coordination and organometallic chemistry applied to helicenic (pro)ligands constitutes an appealing way to create structural diversity and to introduce new properties in the area of helicenes. To this aim, many different types of monodentate or polydentate ligands have been exploited to prepare different families of metal‐based helical architectures and supramolecular assemblies with controlled topology and stereochemistry. The metal endows the helicene with either unusual or improved properties such as strong chiroptical properties, among which are high optical rotation values or intense electronic circular dichroism, improved emission properties, and most particularly circularly polarized luminescence, redox switching activity, magnetic properties, and chiral sensing, to name a few. The obtained metallohelicenes are thus very appealing for the development of novel chiral functional materials.
Metal–organic systems incorporating helicene moieties have been attracting growing attention, because they combine large‐magnitude chiroptical properties with additional appealing features such as redox activity and phosphorescence. This chapter provides an overview of experimental and theoretical advances regarding the photophysical and optical activity (OA) of metal‐helicene derivatives. In particular, representative joint experimental‐computational case studies are presented, including multihelicenic assemblies, redox‐ and acid‐/base‐triggered chiroptical switches, and transition‐metal‐based helicenic phosphors. The focus is on calculated results with the aim to demonstrate the importance of quantum‐chemical calculations in affording the basis for interpreting measured data and understanding the factors responsible for any observed trends, and, ultimately, in providing new insights into corresponding structure–property relationships in such systems.
Bimetallic complexes of hexapyrrole-[Formula: see text],[Formula: see text]-dicarbaldimines consisting of a pair of four-coordinate metal sites adopt a helical closed [Formula: see text]-symmetric form or sigmoidal open forms depending on whether the 2,2[Formula: see text]-bipyrrole subunit at the center of the hexapyrrole chain takes cis- or trans-conformation. X-ray crystallography of a bisNi complex having N-[([Formula: see text]-1-cyclohexylethyl]carbaldimine units at both ends of the hexapyrrole chain revealed a non-symmetric heterohelical open form where the metal coordination sites of opposite helical sense sit on opposite sides of the central 2,2[Formula: see text]-bipyrrole subunit. BisPd complexes preferred a closed [Formula: see text] form and a steric bulk at the 3,3[Formula: see text]-position of the 2,2[Formula: see text]-bipyrrole subunit improved the helical sense bias. A bisPd complex with N-[([Formula: see text]-1-cyclohexylethyl]carbaldimine units adopts a helical closed [Formula: see text] form exclusively with full bias for a [Formula: see text]-helical sense. These bimetallic single stranded helicates were reversibly oxidized to [Formula: see text]-cation radicals at 0.1[Formula: see text]0.3 V vs. a ferrocene/ferrocenium couple and spectroelectrochemistry revealed remarkable absorption and CD spectral changes in the Vis-NIR region.
We report a computational study on vibronic effects in the spectroscopy, photoinduced processes and decay back to the ground state of aza[7]helicene, a helicene with an unusually high fluorescence quantum yield (QY = 0.39). In a first step, we compute and assign the absorption and electronic circular dichroism (ECD) spectra in its full frequency range from 2.7 to 5.0 eV, accounting for nonadiabatic effects. Then we compute the quantum dynamics of the cascade of ultrafast internal conversions of the highly-excited singlet states to the lowest-energy one S1. Finally we adopt Fermi golden rule rates to compute the QY of the dye, taking into account the competition between the radiative decay and the nonradiative decays to the ground state and to the energy-accessible triplet states. We use time-dependent density functional theory (TD-DFT), including solvent (dichloromethane) effects within the polarizable continuum model, to parameterize a linear vibronic coupling (LVC) model involving the first lowest 12 singlet states and all the normal coordinates. Nonadiabatic spectra and internal conversions dynamics are then computed through wavepacket propagations with the Multilayer (ML) extension of the Multiconfigurational Time Dependent Hartree method (ML-MCTDH). We highlight the molecular vibrations playing a major role in determining the shape of the spectra and analyse the effect of inter-state couplings. At the same time we report a breakdown of perturbative Herzberg-Teller approach. The computed QY is in perfect agreement with experiment and allows us to ascertain that intersystem crossings are the processes limiting the fluorescence from S1. They involve the three lowest triplet states and are made effective by spin-orbit coupling and vibronic effects.
While the development of chiral molecules displaying circularly polarized luminescence (CPL) has received considerable attention, the corresponding CPL intensity, g lum, hardly exceeds 10-2 at the molecular level owing to the difficulty in optimizing the key parameters governing such a luminescence process. To address this challenge, we report here the synthesis and chiroptical properties of a new family of π-helical push-pull systems based on carbo[6]helicene, where the latter acts as either a chiral electron acceptor or a donor unit. This comprehensive experimental and theoretical investigation shows that the magnitude and relative orientation of the electric (μe ) and magnetic (μ m ) dipole transition moments can be tuned efficiently with regard to the molecular chiroptical properties, which results in high g lum values, i.e. up to 3-4 × 10-2. Our investigations revealed that the optimized mutual orientation of the electric and magnetic dipoles in the excited state is a crucial parameter to achieve intense helicene-mediated exciton coupling, which is a major contributor to the obtained strong CPL. Finally, top-emission CP-OLEDs were fabricated through vapor deposition, which afforded a promising g El of around 8 × 10-3. These results bring about further molecular design guidelines to reach high CPL intensity and offer new insights into the development of innovative CP-OLED architectures.
Chiroptical switches, whose chiral optical signals such as optical rotatory dispersion (ORD), circular dichroism (CD) and circularly polarized luminescence (CPL) are reversibly interchangeable between two states, offer many promising applications in the fields of chiral sensing, optical displays, information storage, asymmetric catalysis and so on. Through various non-covalent interactions, supramolecular chiroptical switches have been constructed by combining the chiral and responsive functional components. This review summarizes the recent progress in the construction of supramolecular chiroptical switchable systems that reversibly respond to various stimuli, such as light, electricity, magnetic fields, mechanical force, solvents, pH, temperature, and chemical additives. The switching of supramolecular chirality in the forms of on/off, amplification/weakening and chirality inversion is shown. Additionally, the design of chiroptical switchable systems for chiral logic gates, data communication, chiral separation and asymmetric catalysis has been demonstrated. Future challenges in developing supramolecular chiroptical switches are also discussed.
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)3Cl}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.
We present a systematic study of vibrationally resolved absorption (ABS), electronic circular dichroism (ECD), emission (EMI), and circularly polarized luminescence (CPL) of aza[7]helicene. Because of the rare experience of theoretical CPL calculation, a variety of harmonic models have been employed to compute the vibronic structures. To fully understand the vibronic effects on the spectral shapes, Franck-Condon (FC) and Herzberg-Teller (HT) contributions, Duschinsky mixings and temperature effect have all been taken into consideration. The performance of different alternative approximate methods has been carefully compared and discussed in detail. The results show that Vertical Hessian (VH) model has a slight better performance on the spectral shapes than Adiabatic Hessian (AH), especially for CPL spectrum. The thermal excitation effect has led to a reduced resolution and a broader spectral width. The moderate HT effects on the different spectral shapes have been addressed. The dissymmetry factors have been correctly reproduced and the main vibronic features of the four different spectral shapes have been successfully captured. A good estimation of the overall spectral width, relative position and relative height of different spectral bands has been presented. The nice agreement with the experiment allows us to present a detailed interpretation of the spectral shapes.
We describe herein the synthesis of 2,6‐bis(pyrazol‐1‐yl)pyridine (1‐bpp) ligands substituted by helicene units in the 4‐position of the pyridine ring. The two‐step procedure afforded the molecules bpp[n]helicene (n = 4, 5 and 6) and methyl‐bpp[4]helicene, abbreviated as 4, 5, 6 and 4m respectively, with overall good yields. This series of ligands shows fluorescence in the violet region, with a red shift of the emission when increasing the number of aromatic rings in the helicene unit. The tridentate fluorophores have been reacted with Re(CO)5Cl and [Ru(terpy)Cl3] to prepare a series of transition metal complexes formulated as [ReI(n)(CO)3Cl] (n = 4 and 5) and [RuII(n)(terpy)](PF6)2 (n = 4, 4m, 5 and 6). The compounds [ReI(4)(CO)3Cl]·(CH2Cl2)0.5(H2O) and [RuII(4m)(terpy)](PF6)2 crystallized in the centrosymmetric space groups P21/n and P1 respectively, and thus present both M and P helicene units in their crystal structures. The absorption and emission properties of the complexes have been studied. All complexes show luminescence at 77 K in frozen DCM solution. The tricarbonyl complex [ReI(5)(CO)3Cl] shows a strong emission band with two peaks at 540 nm and 580 nm, which could be attributed to ligand‐localized π–π* phosphorescence due to the extended π system of the ligand. The heteroleptic RuII complexes show emission bands above 600 nm attributed to the terpyridine‐based ³MLCT state.
Vibrational circular dichroism (VCD) spectroscopy is one of the most powerful techniques for the determination of absolute configurations (AC), as it does not require any specific UV/Vis chromophores, no chemical derivatization and no growth of suitable crystals. In the last decade it has become increasingly recognized by chemists from various fields of synthetic chemistry such as total synthesis and drug discovery as well as from developers of asymmetric catalysts. This review gives an overview about the most important experimental aspects of a VCD-based AC determination and explains the theoretical analysis. The comparison of experimental and computational spectra that leads to the final conclusion about the AC of the target molecules is described. In addition, the review summarizes unique VCD studies carried out in the period 2008-2018 that focus on the determination of unknown ACs of new compounds, which were obtained in its enantiopure form either through direct asymmetric synthesis or chiral chromatography.
A novel enantiopure bis-helicenic 2,2'-bipyridine system was prepared using a Negishi coupling. Thanks to the bipyridine unit, the coordination with ZnII and protonation processes were studied revealing efficient tuning of photophysical (UV/Visible and emission) and chiroptical properties (electronic circular dichroism and circularly polarized emission) of the system. The coordination/decoordination and protonation/deprotonation processes appeared reversible thus constituting novel chiroptical switches.
7-Cyanoexahelicene has been prepared in racemic form through a short photochemical synthetic approach and its enantiomeric separation was studied and allowed to obtain (P)- and (M)-enantiomers in 100% ee and >98.5% ee, respectively. The optical rotatory dispersion and electronic circular dichroism (ECD) spectra of the resolved ones were measured. Other optical properties such as UV and photoluminescence as well as the electrochemical behavior were also examined in solutions.
The coordination of a helicene functionalized bipyridine to various lanthanide complexes (ln = Y, Eu, Yb, Gd) led to the formation of an isostructural series. The photophysical study indicated that lanthanides act as heavy atoms strongly enhancing the singlet oxygen generation. We also demonstrated that the sensitization of the f‐f luminescence is in competition with the singlet oxygen generation. image John Wiley & Sons, Ltd.
Straightforward synthesis and resolution of a series of chiral fluorescent macrocycles are presented, together with their electronic circular dichroism (ECD), strong excimer fluorescence (EF, λ 300 to 650 nm) and allied highly circularly polarized luminescence (CPL, glum up to 1.7×10⁻²). The ECD, EF and CPL responses are strongly affected by the presence of metal ions (Na⁺, Ba²⁺) thanks to deep conformational changes. While ECD signals can be almost completely reversibly inverted upon the complexation/decomplexation of metal ions in a typical binary response, CPL signals are reversibly quenched concomitantly. The designed macrocycles display thus a remarkable combination of both +/− ECD and on/off CPL reversible switching.
Introducing one or two alkynyl‐iron moieties onto a carbo[6]helicene results in organometallic helicenes (2a,b) displaying strong chiroptical activity combined with efficient redox‐triggered switching. The neutral and oxidized forms have been studied in detail by electronic and vibrational circular dichroism (ECD, VCD) as well as by Raman optical activity (ROA) spectroscopies. The experimental results were analyzed and spectra were assigned with the help of first‐principles calculations. In particular, a recently developed method for ROA calculations under resonance conditions has been used to study the intricate resonance effects on the ROA spectra of mono‐iron‐ethynyl‐helicene 2a.
For the first time, chirality in (3Z,9Z)-1,2,5,8-dithiadiazecine-6,7(5H,8H)-dione series was recognized. Enantiomers of the 4,9-dimethyl-5,8-diphenyl analogue were isolated at room temperature and their thermal stability determined. X-ray confirmed the occurrence of a pair of enantiomers in the crystal. Absolute configurations were assigned by comparing experimental and calculated vibrational/electronic circular dichroism spectra of isolated enantiomers. A distorted tesseract (four-dimensional hypercube) was used to visualize the calculated enantiomerization process which requires the rotation around four chirality axes. Conformers of higher energy as well as several concurrent pathways of similar energies were revealed.
A short and efficient synthetic pathway leading to a new chiral π-conjugated system is reported. The X-ray structure of the target compound was determined and showed a helical conformation. Its resolution was successfully accomplished, leading to two enantiomers in high optical purity, and their chiroptical properties were examined experimentally. The photophysical properties of the organic material were also evaluated, showing an emission in the visible region, and HOMO and LUMO levels have been estimated experimentally, demonstrating an electrochemical band gap of 2.37 eV.
A simple and efficient approach leading to a well functionalized helically chiral π-conjugated system is reported. Optical resolution of racemic helicene has been well accomplished affording both enantiomers (+)- and (-)-4 in high optical purity and their chiroptical properties were experimentally evaluated. Based on electronic circular dichroism (ECD) absolute configurations of dextrorotary and laevorotatory antipodes were assigned as P and M, respectively. Photophysical properties of the helical compound have been also examined in solutions and show a remarkable behaviour.
The charged, electroactive bipyridine-helicene-ruthenium(III) complex [4].+,PF6⁻ has been prepared from 3-(2-pyridyl)-4-aza[6]helicene and a Ru-bis-(β-diketonato)-bis-acetonitrile precursor (β-diketonato: 2,2,6,6-tetramethyl-3,5-heptanedionato). Its chiroptical properties (electronic circular dichroism and optical rotation) were studied both experimentally and theoretically and suggest the presence of 2 diastereoisomers, namely (P,Δ)- and (P,Λ)-[4].+,PF6⁻ (denoted jointly as (P,Δ*)-[4].+,PF6⁻) and their mirror-images (M,Λ)- and (M,Δ)-[4].+,PF6⁻ ((M,Δ*)-[4].+,PF6⁻). The electrochemical reduction of (P,Δ*)-[4].+,PF6⁻ to neutral complex (P,Δ*)-4 was performed and revealed strong changes in the UV-vis and electronic circular dichroism spectra. A reversible redox-triggered chiroptical switching process was then achieved.
We describe the synthesis of mono- and bis([6]helicene-≡-Ru(dppe)2-≡)-DTE complexes 1o,c and 2o,c. The photochromic and electrochromic properties were studied, and attractive isomerization processes were observed, i.e., spontaneous reopening of [1c]•+ to [1o]•+ and ring closure of [2o]²⁺ to [2c]²⁺. Due to strong chiroptical responses, all of these chiral complexes correspond to new types of chiroptical switches (triggered by light and/or redox stimuli) and could be described as either “NOR” or “OR” logic gates.
Molecular switches are of interest for the construction of molecular-scale memory devices. Switches based on redox-triggered helicenes can achieve intense chiroptical read-out values but most systems only display a small potential difference between the “ON” and the “OFF” switch states (redox hysteresis). Although a larger hysteresis could be achieved by coupling the electron transfer to a fast follow-on reaction, this approach has been limited to the intramolecular ring-opening of helicene-like systems to axially chiral biaryls. Here, we present the first intermolecular follow-on reaction: an azoniahelicene system that dimerises upon reduction, leading to intense, reversible chiroptical switching with a >500 mV hysteresis and a >300 mV “read-only” potential range. The reported helicene dimerization is supported by detailed electrochemical investigations and the comparison of experimental and calculated electronic circular dichroism (ECD) spectra.
Enantiomeric glutamate gelators containing a spiropyran moiety are designed and found to self-assemble into a nanohelix through gelation. Upon alternating UV and visible light irradiation, the spiropyran experiences a reversible change between a blue zwitterionic merocyanine state and a colorless closed ring state spiropyran in supramolecular gels. This photochromic switch causes a series of subsequent changes in the optical, chiroptical, morphological properties from supramolecular to macroscopic levels. While the solution of the gelator molecules does not show any circular dichroism (CD) signal in the region of 250–700 nm due to the fact that the chromophore is far from the chiral center, the gel shows chiroptical signals such as CD and circularly polarized luminescence (CPL) because of the chirality transfer by the self-assembly. These signals are reversible upon alternating UV/vis irradiation. Therefore, a quadruple optical and chiroptical switch is developed successfully. During such process, the self-assembled nanostructures from the enantiomeric supramolecular gels also undergo a reversible change between helices and fibers under the alternating UV and visible light trigger. Furthermore, a rewritable material fabricated from their xerogels on a glass is developed. Such rewritable material can be efficiently printed over 30 cycles without significant loss in contrast and resolution using UV and visible light.
Five tris(β-diketonato) complexes of ruthenium(III), chromium(III), and cobalt(III) [Ru(Buacac)3 (1), Ru(Oacac)3 (2), Cr(Buacac)3 (3), Cr(Oacac)3 (4), and Co(Buacac)3 (5), where Buacac = 3-butylpentane-2,4-dionato and Oacac = 3-octylpentane-2,4-dionato] with a chiral propeller-like structure have been prepared. Ligands and complexes syntheses are presented together with characterization of the compounds by 1H and 13C NMR spectroscopy, mass spectrometry, IR, UV–vis, electronic circular dichroism (ECD) spectroscopy, electrochemistry studies, and first-principles calculations. The crystal structures of 1 and 5 have also been obtained and analyzed. A comparison of the 1H NMR spectra of diamagnetic (ligands and 5) and paramagnetic (1 and 2) species is presented. Optical resolution of the five complexes has been achieved for the first time by supercritical fluid chromatography using a chiral column, giving rise to very high purity grades in all cases. ECD measurements and calculations have led to the assignment of the absolute configuration, Δ or Λ, of each enantiomer for 1−5. Spectroelectrochemical UV–vis and ECD studies have been performed on ruthenium Λ-2 and chromium Λ-4 complexes, revealing their redox-triggered chiroptical switching confirming the noninnocence character of the β-diketonate ligands.
This chapter describes the reactions and transformations of helicenes, which are classified into four sections. First, transformation of the helical skeletons is discussed. By intramolecular Diels–Alder reaction, unusual [2+2] cycloaddition of C=C double bonds on the terminal rings, and dioxygen-triggered transannular dearomatization of helicene, some unprecedented reaction products can be obtained. The second section describes the direct C–H functionalization, which provides the most useful method for modification of the helicene structures. Functionalization of helicenes can be easily achieved by electrophilic aromatic substitution including nitration, halogenation, and acylation. For heterohelicenes, some C–H bonds can be activated by themselves, while the functionalizations of helicenium cation and azahelicenes are also shown. In third section, the transformations of bromohelicenes and hydroxyl helicenes are introduced, which can conveniently produce various functionalized helicene derivatives. Finally, recent advances in the preparation of helicene-embedded organometallics are discussed. When two Cp rings are embedded into the termini of the helicene skeleton, helical metallocene or polymeric metallocene can be obtained. With helicenes as monodentate ligands, bidentate ligands, or multitopic ligands, various metal complexes of helicenes and metallahelicenes are produced, and they exhibit the enhanced chiroptical properties of helicene core, redox switching, and acid–base switching phenomena, and even the acid/base-triggered switch of circularly polarized luminescence. Moreover, they are also potential candidates for the application of asymmetric synthesis.
Both the helicity and the π–conjugated structure of helicenes endow them with a wide range of potential applications in molecular recognition and fluorescent sensing, and the environmentally responsive switches, especially in the chiral recognition and chiroptical switches. Thus, chiral recognition of helicene crown ethers toward racemic amine salts, recognition of diacids by helicopodand, chiral recognition between the BINOL-modified Au nanoparticles and Helquat via donor–acceptor interactions, and chiral recognition between the helicene quinone radical anion and BINAPO were sequentially found. It was also found that stereoregular helical acetylenes with [6]helicene units as the pendants generated by Rh-catalyzed polymerization display the ability to adsorb one enantiomer preferentially in a racemic mixture solution. Moreover, 2,15-dihydroxyl[6]helicene as fluorescence sensor for chiral amines and aminoalcohols, diaza[4]helicenes with pyridine and amino moieties for pH-sensitive sensing, tetrahydro[5]helicene thioimide-based chemodosimeter for Hg2+, and a humidity sensor based on [6]helicene-derived imidazolium salt were also developed. Recently, a series of fast-responsive explosive sensors based on the polymerization of [5]helicenes were reported as well. Furthermore, helicenes and their derivatives can be applied in the design and development of various acid-base responsive switches, redox-responsive switches, photo-responsive switches, and chiroptical switches.
We report two new helicenes derived from the double fusion of an acene with two perylene diimide (PDI) subunits. These PDI-helicene homologs exhibit very different structural and electronic properties, despite differing by only a single ring in the link between the PDI units. The shorter inter-PDI link brings the two PDI subunits closer together, and this results in the collision of their respective ?-electron clouds. This collision facilitates intramolecular through-space electronic delocalization when the PDI-helicene is reduced.
We report two new helicenes derived from the double fusion of an acene with two perylene diimide (PDI) subunits. These PDI-helicene homologs exhibit very different structural and electronic properties, despite differing by only a single ring in the link between the PDI units. The shorter inter-PDI link brings the two PDI subunits closer together, and this results in the collision of their respective ?-electron clouds. This collision facilitates intramolecular through-space electronic delocalization when the PDI-helicene is reduced.
Four new mononuclear alkynyl complexes each featuring a terminal 2-fluorenyl group, namely Fe(η5-C5Me5)(CO)2[C≡C(2-C21H25)] (2), Ru(κ2-dppe)2Cl[C≡C(2-C21H25)] (3), Ru(κ2-dppe)2[C≡C(4-C6H4NO2)][C≡C(2-C21H25)] (4), and [Fe(η5-C5Me5)(κ2-dppe){C≡C(C5H4N)-CH2(2-C13H9)}][PF6] (5[PF6]), have been synthesized and characterized, and their redox, absorption, and emission properties have been studied. For the two ruthenium derivatives 3 and 4, these studies are complemented by spectroelectrochemical investigations, Z-scan measurements, and DFT calculations. Fluorimetric studies reveal that these compounds are poorly or not luminescent, and, when luminescent, that the detected weak luminescence most likely originates from a higher lying ligand-centred (LC) excited state presumably located on fluorene. Finally, the third-order nonlinear optical (NLO) properties of 3 and 4 are reported. It is shown that the bis-alkynyl complex 4 is significantly more active than 3 and that both compounds exhibit two-photon absorption (TPA) around 860–1050 nm, with TPA cross-sections above 350 GM. In addition, it is shown that both species should give rise to a marked switching of their cubic NLO properties in this spectral range upon oxidation.
The combination of a bis-alkynyl-helicene moiety with two iron centers leads to novel electroactive species displaying unprecedented redox-triggered chiroptical switching. Upon oxidation, strong changes of vibrational modes (either local or extended coupled modes) are detected by vibrational circular dichroism and Raman optical activity. Remarkably, the sign of the optical rotation at 1.54 µm (that is, at wavelengths typically used for telecommunications) changes upon oxidation while the topology and stereochemistry of the helicene remain unchanged.
The combination of a bis-alkynyl-helicene moiety with two iron centers leads to novel electroactive species displaying unprecedented redox-triggered chiroptical switching. Upon oxidation, strong changes of vibrational modes (either local or extended coupled modes) are detected by vibrational circular dichroism and Raman optical activity. Remarkably, the sign of the optical rotation at 1.54 µm (that is, at wavelengths typically used for telecommunications) changes upon oxidation while the topology and stereochemistry of the helicene remain unchanged.
The molecular conformation of a bis-helicenic terpyridine system is strongly modified upon binding to Zn(ii) ion, a process that is accompanied by large changes in the optical and chiroptical properties. This system affords a new type of helicene-based chiroptical switching.
Monometallic gold(I)-alkynyl-helicene complexes (1 a,b) and bimetallic gold(I)-alkynyl-helicene architectures featuring the presence (2 a,b) or absence (3 a,b) of aurophilic intramolecular interactions were prepared by using different types of phosphole ligands (mono-phosphole L1 or bis-phospholes L2,3). The influence of the Au(I) d(10) metal center(s) on the electronic, photophysical, and chiroptical properties of these unprecedented phosphole-gold(I)-alkynyl-helicene complexes was examined. Experimental and theoretical results highlight the importance of ligand-to-ligand-type charge transfers and the strong effect of the presence or absence of Au(I) -Au(I) interactions in 2 a,b.
Helicenes are mols. with ortho-fused arom. rings that adopt an inherently chiral helical shape. This helical topol. combined with the extended π-conjugated system provides them with excellent chiroptical and photophys. properties. These properties and peculiarities make helicenes important candidates for the conception of new chiroptical switches. Several examples of light-, redox- and pH-triggered helicene-based switches have recently appeared in the literature including examples from our group. Some of them are multi-input and/or multi-output systems in which the changes can be triggered by different stimuli and/or read-out by different techniques, an attractive advantage for functional materials.
Four stereoisomers of sulfinyl ferrocenyl-substituted helicenequinones having central, planar, and helical elements of chirality were stereoselectively formed, in one step, from reaction between enantiopure sulfinyl ferrocenyl dienes and a sulfinyl quinone. Asymmetric synthesis, kinetic resolution, or chemical resolution processes occurred in sequential cycloaddition, sulfoxide elimination, and partial aromatization steps.
Helquat dyes are the first helicene-like cationic styryl dyes obtained as separate enantiomers. Their remarkable chiroptical properties are due to unique combination of cationic hemicyanine chromophore with helicene-like motif. Magnitude of the ECD response and the pH switching along with their positioning in the visible region are unprecedented among helicenoids.
The coordination and/or organometallic chemistry of π-helicenic ligands is a powerful tool to generate multifunctional molecules displaying optimized chiroptical properties combined with new properties furnished by the metallic center. In this review, we relate the different examples that have been described in this field of research to date.
This literature overview demonstrates that helically chiral ligands and organocatalysts have been largely neglected so far. However, a few recent studies on helical pyridine, the corresponding ammonium salts and N-oxides have highlighted the significant potential of these compounds as organocatalysts for Michael type additions, aldehyde propargylations, epoxide openings, and others. In addition, helicenes displaying a fused phosphole ring at the end of their polyaromatic structures, have been used as ligands in enantioselective gold promoted cycloisomerization reactions, giving both excellent catalytic activity and high enantiomeric excesses. These recent results are expected to stimulate further research on the catalytic applications of helically chiral auxiliaries, in the next few years.
Circularly polarized light is central to many photonic technologies, including circularly polarized ellipsometry-based tomography1, 2, optical communication of spin information3 and quantum-based optical computing and information processing4, 5. To develop these technologies to their full potential requires the realization of miniature, integrated devices that are capable of detecting the chirality or ‘handedness’ of circularly polarized light. Organic field-effect transistors, in which the active semiconducting layer is an organic material, allow the simple fabrication of ultrathin, compact devices6, 7, 8. Here we demonstrate a circularly polarized light-detecting organic field-effect transistor based on an asymmetrically pure, helically shaped chiral semiconducting molecule known as a helicene9. Importantly, we find a highly specific photoresponse to circularly polarized light, which is directly related to the handedness of the helicene molecule. We believe that this opens up the possibility for the detection of the chirality of circularly polarized light in a highly integrated photonic platform.
Cationic triangulenes and helicenes are highly stable carbocations with planar and helical conformations respectively. These moieties are effective dyes with original absorption and emission properties. Over the last decade, they have received greater attention and are considered as valuable tools for the development of innovative applications. In this review, the synthesis of these unique compounds is presented together with their core chemical and physical properties. Representative applications spanning from surface sciences to biology and chemistry are presented.
Turbomole is a highly optimized software package for large‐scale quantum chemical simulations of molecules, clusters, and periodic solids. Turbomole uses G aussian basis sets and specializes on predictive electronic structure methods with excellent cost to performance characteristics, such as (time‐dependent) density functional theory ( TDDFT ), second‐order M øller– P lesset theory, and explicitly correlated coupled cluster ( CC ) methods. These methods are combined with ultraefficient and numerically stable algorithms such as integral‐direct and Laplace transform methods, resolution‐of‐the‐identity, pair natural orbitals, fast multipole, and low‐order scaling techniques. Apart from energies and structures, a variety of optical, electric, and magnetic properties are accessible from analytical energy derivatives for electronic ground and excited states. Recent additions include post‐ K ohn– S ham calculations within the random phase approximation, periodic calculations, spin–orbit couplings, explicitly correlated CC singles doubles and perturbative triples methods, CC singles doubles excitation energies, and nonadiabatic molecular dynamics simulations using TDDFT . A dedicated graphical user interface and a user support network are also available.
This article is categorized under: Software > Quantum Chemistry
The first helicene-based carbene-osmium complex has been prepared from a vinyl-osmium derivative and this system has been shown to behave as a potential acid-base triggered chiroptical switch.
We report the implementation of the computation of rotatory strengths, based on time-dependent density functional theory, within the Amsterdam Density Functional program. The code is applied to the simulation of circular dichroism spectra of small and moderately sized organic molecules, such as oxiranes, aziridines, cyclohexanone derivatives, and helicenes. Results agree favorably with experimental data, and with theoretical results for molecules that have been previously investigated by other authors. The efficient algorithms allow for the simulation of CD spectra of rather large molecules at a reasonable accuracy based on first-principles theory. The choice of the Kohn–Sham potential is a critical issue. It is found that standard gradient corrected functionals often yield the correct shape of the spectrum, but the computed excitation energies are systematically underestimated for the samples being studied. The recently developed exchange-correlation potentials “GRAC” and “SAOP” often yield much better agreement here with experiments for the excitation energies. The rotatory strengths of individual transitions are usually improved by these potentials as well. © 2002 American Institute of Physics.
Helicity and axial chirality are reversibly interconverted in the novel redox pair 1 and 22+ ; the compounds exhibit drastic UV/Vis and circular dichroism spectral changes as well as dynamic structural changes upon electron transfer, thus furnishing an unprecedented multi-output response system with high bistability. X=S or O.
This book provides an introduction to the important methods of chiroptical spectroscopy in general, and circular dichroism (CD) in particular, which are increasingly important in all areas of chemistry, biochemistry, and structural biology. The book can be used as a text for undergraduate and graduate students and as a reference for researchers in academia and industry, with or without the companion volume in this set. Experimental methods and instrumentation are described with topics ranging from the most widely used methods (electronic and vibrational CD) to frontier areas such as nonlinear spectroscopy and photoelectron CD, as well as the theory of chiroptical methods and techniques for simulating chiroptical properties. Each chapter is written by one or more leading authorities with extensive experience in the field.
This book provides an introduction to the important methods of chiroptical spectroscopy in general, and circular dichroism (CD) in particular, which are increasingly important in all areas of chemistry, biochemistry, and structural biology. The book can be used as a text for undergraduate and graduate students and as a reference for researchers in academia and industry. Experimental methods and instrumentation are described with topics ranging from the most widely used methods (electronic and vibrational CD) to frontier areas such as nonlinear spectroscopy and photoelectron CD, as well as the theory of chiroptical methods and techniques for simulating chiroptical properties. Applications of chiroptical spectroscopy to problems in organic stereochemistry, inorganic stereochemistry, and biochemistry and structural biology are also discussed, and each chapter is written by one or more leading authorities with extensive experience in the field.
Introduction to Vibrational Optical ActivityOrigin and Discovery of Vibrational Optical ActivityVCD Instrumentation DevelopmentROA Instrumentation DevelopmentDevelopment of VCD Theory and CalculationsDevelopment of ROA Theory and CalculationsApplications of Vibrational Optical ActivityComparison of Infrared and Raman Vibrational Optical ActivityConclusions
References
Herein we report on the theoretical–experimental analysis of the one-and two-photon absorption and circular dichroism spectra of two intrinsically chiral aromatic molecules – hexahelicene derivatives – with helical chirality and intramolecular charge transfer (ICT). The primary outcomes of our investigation demonstrate that the TPA cross-section and the amplitude of the TPCD signal of this type of helicenes are strongly affected by the strength of the ICT and the nature of the extension of the electronic delocalization, i.e. beyond (EXO-ICT) or within (ENDO-ICT) the helicene core. These results were corroborated through the comparative theoretical analysis of the corresponding contributions of the magnetic dipole transition moment and the electric quadrupole transition moment to the TPA rotatory strength on a series of five similar helicene derivatives with different molecular electron delocalization disposition. Two-photon absorption (TPA) and two-photon circular dichroism (TPCD) spectra were obtained using the double L-scan technique over a broad spectral range (400–900 nm) using 90 fs pulses at a low repetition rate (2–50 Hz) produced by an amplified femtosecond system. The theoretical simulations were performed using modern analytical response theory within the Time-Dependent Density Functional Theory (TD-DFT) approach using B3LYP and CAM-B3LYP, and the aug-cc-pVDZ and 6-311++G(d,p) basis sets.
The unpolarized absorption and circular dichroism spectra of the
fundamental vibrational transitions of the chiral molecule,
4-methyl-2-oxetanone, are calculated ab initio. Harmonic force fields
are obtained using Density Functional Theory (DFT), MP2, and SCF
methodologies and a 5S4P2D/3S2P (TZ2P) basis set. DFT calculations use
the Local Spin Density Approximation (LSDA), BLYP, and Becke3LYP (B3LYP)
density functionals. Mid-IR spectra predicted using LSDA, BLYP, and
B3LYP force fields are of significantly different quality, the B3LYP
force field yielding spectra in clearly superior, and overall excellent,
agreement with experiment. The MP2 force field yields spectra in
slightly worse agreement with experiment than the B3LYP force field. The
SCF force field yields spectra in poor agreement with experiment.The
basis set dependence of B3LYP force fields is also explored: the 6-31G
and TZ2P basis sets give very similar results while the 3-21G basis set
yields spectra in substantially worse agreements with experiment.
Electronic circular dichroism and circularly polarized luminescence acid/base switching activity has been demonstrated in helicene-bipyridine proligand 1 a and in its “rollover” cycloplatinated derivative 2 a. Whereas proligand 1 a displays a strong bathochromic shift (>160 nm) of the nonpolarized and circularly polarized luminescence upon protonation, complex 2 a displays slightly stronger emission. This strikingly different behavior between singlet emission in the organic helicene and triplet emission in the organometallic derivative has been rationalized by using quantum-chemical calculations. The very large bathochromic shift of the emission observed upon protonation of azahelicene-bipyridine 1 a has been attributed to the decrease in aromaticity (promoting a charge-transfer-type transition rather than a π–π* transition) as well as an increase in the HOMO–LUMO character of the transition and stabilization of the LUMO level upon protonation.
We present the synthesis and characterization of enantiomerically pure [6]helicene o-quinones (P)-(+)-1 and (M)-(-)-1 and their application to chiroptical switching and chiral recognition. (P)-(+)-1 and (M)-(-)-1 each show a reversible one-electron reduction process in their cyclic voltammogram, which leads to the formation of the semiquinone radical anions (P)-(+)-1(•-) and (M)-(-)-1(•-), respectively. Spectroelectrochemical ECD measurements give evidence of the reversible switching between the two redox states, which is associated with large differences of the Cotton effects [Δ(Δε)] in the UV and visible regions. The reduction of (±)-1 by lithium metal provides [Li(+){(±)-1(•-)}], which was studied by EPR and ENDOR spectroscopy to reveal substantial delocalization of the spin density over the helicene backbone. DFT calculations demonstrate that the lithium hyperfine coupling A((7)Li) in [Li(+){(±)-1(•-)}] is very sensitive to the position of the lithium cation. On the basis of this observation, chiral recognition by ENDOR spectroscopy was achieved by complexation of [Li(+){(P)-(+)-1(•-)}] and [Li(+){(M)-(-)-1(•-)}] with an enantiomerically pure phosphine oxide ligand.
Tetrathia[7]helicene ([7]TH)-based complexes substituted at the thienyl ring ends by a ferrocenyl group (Fc) or by a (η5-cyclohexadienyl)Mn(CO)3 derivative have been prepared by Sonogashira coupling reactions starting from the mono- or diiodo [7]TH compounds. The molecular structure of one of the diferrocenyl [7]TH complexes was established by X-ray analysis. Electrochemical investigation on the Fc-[7]TH systems show that the Fc groups are significantly electron poorer with respect to Fc (ΔE° ≈ 0.15 V), due to the effective conjugation of the Fc redox moiety with the triple bond + helicene system, as also confirmed by spectroscopic data. Potential cycling around the second oxidation peak, assigned to the thiahelicene moiety, affords fast, regular growth of electrodeposited conducting films, provided that one terminal α-thiophene position be available for coupling; on the other hand, long alkyl chains hamper film formation. The conducting films feature a broad oxidation wave resulting from the merging of several redox peaks, having its onset at the Fc oxidation. Since conducting films obtained by electrooligomerization of parent tetrathiahelicene have their onset potentials 0.45 V more positive than the Fc redox sites in this studied Fc-[7]TH conjugates, the above continuity could point to some coupling between Fc redox centers and conjugated π systems, favored by solid-state stacking.
Four different hexahelicenes, 5-aza-hexahelicene (1), hexahelicene (2), 2-methyl-hexahelicene (3), and 2-bromo-hexahelicene (4), were prepared and their enantiomers, which are stable at r.t., were separated. Vibrational circular dichroism (VCD) spectra were measured for compound 1; for all the compounds, electronic circular dichroism (ECD) and circularly polarized luminescence (CPL) spectra were recorded. Each type of experimental spectrum was compared with the corresponding theoretical spectrum, determined via Density Functional Theory (DFT). Following the recent papers by Nakai et al., this comparison allowed to identify some features related to the helicity and some other features typical of the substituent groups on the helical backbone. The Raman spectrum of compound 1 is also examined from this point of view.
Conspectus Kohn-Sham theory (KST) is the "workhorse" of numerical quantum chemistry. This is particularly true for first-principles calculations of ground- and excited-state properties for larger systems, including electronic spectra, electronic dynamic and static linear and higher order response properties (including nonlinear optical (NLO) properties), conformational or dynamic averaging of spectra and response properties, or properties that are affected by the coupling of electron and nuclear motion. This Account explores the sometimes dramatic impact of the delocalization error (DE) and possible benefits from the use of long-range corrections (LC) and "tuning" of functionals in KST calculations of molecular ground-state and response properties. Tuning refers to a nonempirical molecule-specific determination of adjustable parameters in functionals to satisfy known exact conditions, for instance, that the energy of the highest occupied molecular orbital (HOMO) should be equal to the negative vertical ionization potential (IP) or that the energy as a function of fractional electron numbers should afford straight-line segments. The presentation is given from the viewpoint of a chemist interested in computations of a variety of molecular optical and spectroscopic properties and of a theoretician developing methods for computing such properties with KST. In recent years, the use of LC functionals, functional tuning, and quantifying the DE explicitly have provided valuable insight regarding the performance of KST for molecular properties. We discuss a number of different molecular properties, with examples from recent studies from our laboratory and related literature. The selected properties probe different aspects of molecular electronic structure. Electric field gradients and hyperfine coupling constants can be exquisitely sensitive to the DE because it affects the ground-state electron density and spin density distributions. For π-conjugated molecules, it is shown how the DE manifests itself either in too strong or too weak delocalization of localized molecular orbitals (LMOs). Optical rotation is an electric-magnetic linear response property that is calculated in a similar fashion as the electric polarizability, but it is more sensitive to approximations and can benefit greatly from tuning and small DE. Hyperpolarizabilities of π-conjugated "push-pull" systems are examples of NLO properties that can be greatly improved by tuning of range-separated exchange (RSE) functionals, in part due to improved charge-transfer excitation energies. On-going work on band gap predictions is also mentioned. The findings may provide clues for future improvements of KST because different molecular properties exhibit varying sensitivity to approximations in the electronic structure model. The utility of analyzing molecular properties and the impact of the DE in terms of LMOs, representing "chemist's orbitals" such as individual lone pairs and bonds, is highlighted.
A combination of IR spectroelectrochemistry and DFT calculations has been used to demonstrate that the vinyl ligands in the complexes [Ru(CH═CHC6H4Me-4)Cl(CO)(PMe3)3] and [{RuCl(CO)(PMe3)3}2(μ-CH═CHC6H4CH═CH)] are redox noninnocent, and one-electron oxidation results in radical cations that are best described in terms of metal-stabilized organic radicals.
Enantiopure mono-cycloplatinated-[8]helicene and bis-cycloplatinated-[6]helicene derivatives were prepared through column chromatography combined with crystallization of diastereomeric complexes using a chiral ancillary sulfoxide ligand. The UV-visible spectra, circular dichroism, molar rotations, and (circularly polarized) luminescence activity of these new helical complexes have been examined in detail and analysed with the help of first-principles quantum-chemical calculations.
The enantioselective synthesis of azahelicenes and S-shaped double azahelicenes has been achieved via the gold-catalyzed sequential intramolecular hydroarylation of alkynes. The use of excess AgOTf toward a gold(I) complex is crucial for this transformation. Interestingly, the circularly polarized luminescence activity of the S-shaped double azahelicenes was significantly higher than that of the azahelicenes.
Two-step redox switching in enantiopure helquat system [P-1]2+ → [P-1]●+ → [P-1]0 is demonstrated. The viologen-type electroactive unit embedded directly in the helical scaffold of 1 is responsible for the prominent chiroptical switching at 264 nm. This process is associated with a marked Cotton effect sign-reversal ramping between Δε = +35 M-1 cm-1 for [P-1]2+ and Δε = -100 M-1 cm-1 for [P-1]0. This helically chiral system features the most intense chiroptical switch response documented in the field of helicenoids.
In this work, we describe the preparation and the properties of the novel bis(vinylphenylene)-bridged diruthenium complexes {Ru(CO)(eta(2)-O2C-p-C6H4SAc)((PPr3)-Pr-i)(2)}(2)(mu-CH=CH-C6H4-CH=CH-1,3 and -1,4) (6 and 7), the bis(ethynylphenylene)-bridged complex trans-[AcS-p-C6H4-C C-Ru(dppe)(2)-C C-p-C6H4-C C-Ru(dppe)(2)-C C-p-C6H4-SAc] (11), the bis(1-ethynyl-4-vinylphenylene)-bridged triruthenium complex trans-[{Ru(dppe)(2)}{-C C-p-C6H4-CH=CH-Ru-(CO)(eta(2)-O2C-p-C6H4SAc, (PPr3)-Pr-i)(2)}(2)] (8), and the monometallic congeners Ru(CH=CH-p-C6H4SAc)(CO)(eta(2)-O2C-p-C6H4SAc)((PPr3)-Pr-i)(2) (4) and trans-{Ru(dppe)(2)(-C C-p-C6H4-SAc)(2)] (10). These mono-, bi-, and trimetallic complexes feature terminal acetyl-protected thiol functions for covalent binding to gold surfaces or for bridging the gaps of gold nanoelectrodes. All complexes display low oxidation potentials, and IR studies of the neutral complex 8 and of its various oxidized forms 8(n+) indicate the high vinyl/ethynyl bridging ligand contribution to the oxidation processes and complete charge delocalization in all available oxidation states (n = 1-3). Strong delocalization of the relevant occupied frontier MOs over the entire pi-conjugated {Ru}-bridge-{Ru'}-bridge-{Ru} backbone is also supported by DFT calculations on the parent complexes V8 and V8(OMe). The benzoate ligand bearing the functional group for gold binding is outside the conjugation path and insulates the wirelike central portion of these molecules from their periphery. Upon insertion into molecular junctions, these molecules are expected to enhance sequential tunneling and to facilitate Coulomb blockade behavior. They will thus contribute to our understanding of structure property relationships for metal-containing molecular wires.
We show that the VCD signal intensities of amino acids and oligopeptides can be enhanced by up to two orders of magnitude by coupling them to a paramagnetic metal ion. If the redox state of the metal ion is changed from paramagnetic to diamagnetic the VCD amplification vanishes completely. From this observation and from complementary quantum-chemical calculations we conclude that the observed VCD amplification finds its origin in vibronic coupling. with low-lying electronic states. We find that the enhancement factor is strongly mode dependent, and that it is determined by the distance between the oscillator and the paramagnetic metal ion. This localized character of the VCD amplification provides a unique tool to specifically probe the local structure surrounding a paramagnetic ion, and to zoom in on such local structure within larger biomolecular systems.
We investigate the performance of the approximate coupled cluster singles- and doubles model CC2 in the prediction of optical rotations of organic molecules. For this purpose we employ a combination of two test sets from the literature which include small and medium-sized rigid organic molecules and a series of helicenes. CC2 calculations on molecules as large as 11-helicene became possible through a recent implementation of frequency-dependent second-order properties for CC2 which makes use of the resolution-of-the-identity approximation for the electron repulsion integrals. The results are assessed with respect to the accuracy of the absolute values of the optical rotation and the prediction of the correct sign, which is crucial for the determination of absolute configurations. The performance of CC2 is compared with that of density functional theory at the B3LYP and CAM-B3LYP levels. Furthermore we investigated the influence of the molecular geometry and the one-electron basis set and tested to which extent spin-component scaling changes the results.
Homochiral and heterochiral cis-bis-cycloplatinated-[6]helicene derivatives 1 b(1, 2) , as representative examples of platina[6]helicenes that share a common platinum center, have been prepared. A diastereo- and enantioselective synthesis, which combines CH activation and dynamic isomerization from heterochiral structure 1 b(2) into homochiral structure 1 b(1) , is also described. Overall, this isomerization process results in the transfer of chiral information from one helicene moiety to the other one. The chiroptical properties of homochiral (P)- and (M)-1 b(1) were greatly modified upon oxidation into their corresponding (P)- and (M)-diiodo-Pt(IV) complexes (5). The changes were also analyzed by performing theoretical calculations. CH activation in the synthesis of organometallic helicenes is further demonstrated by the preparation of cis-bis-cycloplatinated-[8]helicene 1 c.
Electroactive fused ethylenedithiotetrathiafulvalene[4]helicene and -[6]helicenes have been synthesized through a strategy that involved the preparation of 2,3-dibromo-helicene derivatives as intermediates. The dihedral angles between the terminal helicenes, as determined by single-crystal X-ray analysis, are 22.7° and 50.7° for the [4]helicene and [6]helicene, respectively. Their solid-state architectures show interplay between S⋅⋅⋅S and π⋅⋅⋅π intermolecular interactions. The chiroptical properties of the enantiopure EDTTTF[6]helicene derivatives have been investigated and supported by TDDFT calculations. Remarkable redox switching of the circular dichroism (CD) signal between the neutral and radical-cation species has been achieved.
The comparison between experimental and calculated VCD spectra allowed the unequivocal assignment of the absolute configuration of heptahelicene C30H18 as P(+).
New carbo[6]helicene derivatives grafted with π-conjugated cyano-phenyl arms were synthesized in enantiopure forms and their π-conjugation examined by UV-vis spectroscopy. The influence of the π-conjugation on the circular dichroism spectra and molar rotations is discussed based on comparing experimental data with results from quantum-chemical calculations. The results highlight the fact that increasing the spatial extension of the π-system in a helicene molecule is an efficient way of increasing its molar rotation. Chirality 25:455-465, 2013. © 2013 Wiley Periodicals, Inc.
Summary Nonrelativistic and quasirelativisticab initio pseudopotentials substituting the M(Z-28)+-core orbitals of the second row transition elements and the M(Z-60)+-core orbitals of the third row transition elements, respectively, and optimized (8s7p6d)/[6s5p3d]-GTO valence basis sets for use in molecular calculations have been generated. Additionally, corresponding spin-orbit operators have also been derived. Atomic excitation and ionization energies from numerical HF as well as from SCF pseudopotential calculations using the derived basis sets differ in most cases by less than 0.1 eV from corresponding numerical all-electron results. Spin-orbit splittings for lowlying states are in reasonable agreement with corresponding all-electron Dirac-Fock (DF) results.
The divinylphenylene-bridged diruthenium complexes (E,E)-[{(P i Pr 3) 2 (CO)ClRu} 2 (µ-HCdCHC 6 H 4 CHd CH-1,3)] (m-2) and (E,E)-[{(P i Pr 3) 2 (CO)ClRu} 2 (µ-HCdCHC 6 H 4 CHdCH-1,4)] (p-2) have been prepared and compared to their PPh 3 -containing analogues m-1 and p-1. The higher electron density at the metal atoms increases the contribution of the metal end groups to the bridge-dominated occupied frontier orbitals and stabilizes the various oxidized forms with respect to those of m-1 and p-1. This has been confirmed and quantified electrochemically, because the two reversible oxidation waves were observed at considerably lower potentials than for the PPh 3 complexes. Owing to their greater stability, the one-and two-electron-oxidized forms m-2 n+ and p-2 n+ of both complexes could be generated and spectroscopically characterized inside an optically transparent thin layer electrolysis cell. UV/vis/near-IR and ESR spectroelectrochemistry indicates that the oxidation processes are centered at the organic bridging ligand. σ-Bonded divinyl-phenylenes thus constitute an unusual class of "noninnocent" ligands for organometallic compounds. Electronic transitions observed for the mono-and dioxidized forms closely resemble those of donor-substituted phenylenevinylene compounds, including oligo(phenylenevinylenes) (OPVs) and poly-(phenylenevinylene) (PPV) in the respective oxidation states. Strong ESR signals and nearly isotropic g tensors are observed for the monocations in fluid and frozen solutions. The metal contribution to the redox orbitals is illustrated by a shift of the CO stretching bands to notably higher energies upon stepwise oxidation. The shifts strongly exceed those observed for the PPh 3 containing, six-coordinated species (E,E)-[{(PPh 3) 2 (CO)Cl(L)Ru} 2 (µ-HCdCHC 6 H 4 CHdCH)] n+ (L) substituted pyridine). IR spectroelec-trochemistry reveals the presence of two electronically different transition-metal moieties in m-2 + , while they resemble each other more closely in p-2 + . Differences in electronic coupling are illustrated by the charge distribution parameters calculated from the spectra. Bulk electrolysis experiments confirm the results from the in situ spectroelectrochemistry and the overall stoichiometry of the redox processes. Quantum-chemical calculations were performed in order to provide insight into the nature and composition of the frontier orbitals. The electronic transitions observed for the neutral forms were assigned by TD DFT. IR frequencies calculated for m-2 and p-2 in their various oxidation states retrace the experimental observations. They fail, however, in the case of m-2 + , where a symmetrical structure is calculated, as opposed to the distinctly asymmetric electron distribution observed by IR spectroscopy. Geometry-optimized structures were calculated for all accessible oxidation states. The structural changes following stepwise oxidation agree well with the experimental findings: e.g., a successive low-energy shift of the CdC stretching vibration of the bridge. The radical cation m-2 + displays a broad composite electronic absorption band at low energy that extends into the mid-IR region.
The absolute configurations of two large molecules that possess chiral axes, but no chiral centers, have been determined by vibrational circular dichroism (VCD): an annelated heptathiophene (a helical molecule with C2-symmetry) and a π-conjugated chiral derivative of o-tetraphenylene (a D2-symmetric dimer of 1,1‘-binaphthyl). In both cases, the size of these molecules exceeds the current limit of published structures for which VCD has been used to determine the absolute configuration. In the case of the annelated heptathiophene, 3 different elements, and 11 total (7 S atoms, 2 Si atoms, and 2 Cl or Br atoms), beyond the second row in the periodic table are included in the calculated structure. In the case of the tetraphenylene molecule, a total of 40 C atoms constitutes the hydrocarbon structure, which is a new upper limit for the number of atoms beyond H for which VCD has been calculated for the determination of absolute configuration. The excellent agreement between observed IR and VCD spectra and spectra calculated at the density functional theory (B3LYP/6-31G*) level for these molecules provides definitive determination of their absolute configurations and establishes a new regime of molecular size and elemental variety for which accurate comparisons of VCD calculations to experiment can be conducted.
“Helicene” is the name introduced by Newman in 1955, to describe the benzologues of phenanthrene in which the extra ortho-condensed rings give rise to a (regular) cylindrical helix. The pioneer work of Newman in this field cannot be overemphasized; his brillant synthesis and resolution of [6]helicene, achieved eighteen years ago, will remain as a landmark, for it opened the way to the study of a fascinating class of synthetic molecules. In the following review, an attempt is made to summarize the present state of our knowledge in this rapidly expanding field.
Dinuclear acetylide-type complexes bridged by a photochromic dithienylethene unit (DTE), M-C≡C-DTE-C≡C-M 1 (M= MCp*(dppe); M= Fe (1Fe), Ru (1Ru)), are prepared, and their wire-like and switching behavior as well as oxidation chemistry has been investigated. The DTE complexes 1 exhibit photochromic behavior in a manner similar to organic DTE derivatives; UV-irradiation causes ring closure of the open isomer 1O to form the closed isomer 1C and visible light-irradiation of the resultant 1C causes reverse ring opening to regenerate 1O. But the performance is dependent on the metals. With respect to the interconversion rates and the 1C content at the photostationary state under UV irradiation, the ruthenium complex 1Ru is superior to the iron analogue 1Fe. The wire-like performance turns out to be associated with the photochromic processes, and the efficient switching performance has been verified for 1Fe as characterized by the Vab values (derived from IVCT bands): Vab(1FeC; ON)= 0.047 eV vs. Vab(1FeO; OFF)= 0 eV) and supported by the large switching factor (SF= KC(C; ON)/KC(O; OFF)= 39) as well. SF for 1Ru is determined to be 4.2. The remarkable switching behavior arises from the different π-conjugated systems in the two isomeric forms, i.e. cross-conjugated (1O) and fully conjugated π-systems (1C). It is also found that, in contrast to the reversible redox behavior of the iron complex 1Fe, the ruthenium complex 1RuO undergoes oxidative ring closure to form the dicationic species of the closed isomer 1RuC2+ and, thus, the ruthenium system 1Ru shows dual photo- and electro-chromism. The distinct oxidation behavior of 1Fe and 1Ru can be ascribed to the spin distribution on the diradical intermediates 1FeO2+ and 1RuO2+, as supported by DFT calculations of them.
Pick and choose: Novel cationic diaza-, azaoxo-, and dioxo[6]helicenes are readily prepared and functionalized selectively by orthogonal aromatic electrophilic and vicarious nucleophilic substitutions. Reductions, cross-coupling, or condensation reactions introduce additional diversity and allow tuning of the absorption properties up to the near-infrared region. The diaza salts can be resolved into single enantiomers.
Ab initio calculations of conformational stabilities, IR absorption and vibrational circular dichroism spectra of four bridged triarylamine helicenes are reported, using DFT/B3LYP/6-31G(d, p) method. We find that the bridged triarylamine helicenes in M configuration are more stable than the helicenes in P-configuration (Scheme 1). The two atropisomers, M and P of the helicene 1 and 2 as well as the two diastereomers of the camphanate derivatives, 3 and 4, show no significant dissimilarities in the IR absorptions, but the VCD spectra of these molecules are the characteristic feature of the particular isomer and can be used for identification of absolute configuration of these chiral molecular systems, along with experimentally obtained VCD spectra.
Starting from the screening in conductors, an algorithm for the accurate calculation of dielectric screening effects in solvents is presented, which leads to rather simple explicit expressions for the screening energy and its analytic gradient with respect to the solute coordinates. Thus geometry optimization of a solute within a realistic dielectric continuum model becomes practicable for the first time. The algorithm is suited for molecular mechanics as well as for any molecular orbital algorithm. The implementation into MOPAC and some example applications are reported.
Carbohelicenes are a class of fascinating chiral helical molecules which have a rich history in chemistry. Over a period of almost 100 years, chemists have developed many methods to prepare them in a racemic or in a non-racemic form. They also possess a series of interesting chiral, physical, electronic and optical properties. However, their utilization in chemistry or chemistry-related fields has rarely appeared in a detailed and comprehensive review. It is the purpose of this review to collect fundamental applications and functions involving carbohelicenes in various disciplines such as in materials science, in nanoscience, in biological chemistry and in supramolecular chemistry. From the numerous synthetic methodologies reported up to now, carbohelicenes and their derivatives can be tailor-made for a better involvement in several subfields. Among those domains are: nanosciences, chemosensing, liquid crystals, molecular switches, polymers, foldamers, supramolecular materials, molecular recognition, conductive and opto-electronic materials, nonlinear optics, chirality studies and asymmetric synthesis. Helicene chemistry is now at a developmental stage, where sufficient application data are now collected and are extremely useful. They provide many more ideas for setting up the basis for future innovative applications.
Diruthenium complexes (X)(dppe)2Ru−C≡C−1,4-C6H4−CH═CH−RuCl(CO)(PiPr3)2 (X = Cl, 1a; X = C≡CPh, 1b) containing an unsymmetrical (ethynyl)(vinyl)phenylene bridging ligand are compared to their symmetrical 1,4-bis(ethynyl)phenylene- and 1,4-divinylphenylene-bridged congeners and their mononuclear alkynyl precursors. Electrochemical and UV/vis/NIR, IR, and EPR spectroscopic studies on the neutral complexes and their various oxidized forms indicate bridging ligand-centered oxidation processes and uniform charge and spin delocalization over both dislike organoruthenium moieties despite differences in their intrinsic redox potentials. Comparison between the chloro and the phenylacetylide-terminated derivatives 1a,b suggests further that the conjugated organometallic π-system extends over the entire unsaturated backbone including the terminal ligand at the alkynyl ruthenium site. This paves the way to even more extended π-conjugated organoruthenium arrays for long-range electronic interactions.
The carbonyl hydrido complexes MHCl(CO)(P-i-Pr3)2 (M = Ru, Os) react with alkynes HC≡CR (R = Ph, H) by insertion to give the five-coordinate vinylmetal compounds M(CH=CHR)Cl(CO)(P-i-Pr3)2 (3-6) in nearly quantitative yields. Addition of CO or displacement of the chloride ligand in 3-6 by acetate or acetylacetonate produces the six-coordinate complexes M(CH=CHR)Cl(CO)2(P-i-Pr3)2 (R = Ph; 7, 8), M(CH=CHR)(η2-O2CMe)(CO)(P-i-Pr3) 2 (R = Ph; 9, 10) and M(CH=CHR)(η2-acac)(CO)(P-i-Pr3)2 (R = H, Ph; 11-14), respectively. Treatment of RuHCl(CO)(P-i-Pr3)2 with HC≡CPh and KOH in methanol leads to a mixture of Ru(C≡CPh)2(CO)(P-i-Pr3)2 (15) and Ru(C≡CPh)(CH≡CHPh)(CO)(P-i-Pr3)2 (16), the latter of which reacts with CO to form the octahedral compound Ru(C≡Ph)(CH=CHPh)(CO)2(P-i-Pr3)2 (17). The molecular structure of Os(CH=CHPh)Cl(CO)(P-i-Pr3)2 (4) has been determined by X-ray investigation. 4 crystallizes in the space group P21/n with a = 16.437 (5) Å, b = 10.980 (4) Å, c = 17.086 (4) Å, and β = 105.56 (2)°. The osmium atom has a nearly perfect square-pyramidal coordination with the vinyl ligand in the apical position and the two phosphines trans to each other in the base of the pyramid. With regard to the stereochemistry at the vinyl C=C bond, only the E isomer is formed. A stereo ORTEP diagram illustrates that the metal atom, although coordinatively unsaturated, nevertheless is well shielded at the sixth position of the (uncompleted) octahedron by four of the 12 methyl groups of the phosphine ligands. IR as well as 1H, 13C, and 31P NMR data of the complexes 3-17 are reported.
The adequate treatment of solvation effects in the calculation of UV/vis spectra is a complicated task and a general solution of this problem is still outstanding. Continuum solvation models like COSMO at least allow one to take into account a large part of the different solvent effects on electronic excitations, if the dielectric screening is properly implemented within the CI part of the underlying MO program. A comprehensive development of the respective theory is given. This ends up in a rather simple result, and it turns out that most of the former approaches to the problem have been in severe error. Geometry changes of the solute in different solvents have been taken into account for the first time and appear to be of considerable importance. A few example applications of a COSMO-MOPAC implementation are presented.
In this paper, we report the first systematic experimental and theoretical investigation of the electric field induced second harmonic response of some tetrathia[7]helicene-based NLOphores. We studied six model compounds carrying the NO2, CHCHCN, and COCF3 units as accepting groups on both the terminal thiophene positions as well as on the central benzene ring of the helicene backbone. These groups, known to be of medium and medium-strong accepting strength, allow tuning of both the electronic and structural properties of the helicenes studied. This experimental/theoretical study should set a milestone in addressing new structure-properties relationship of this class of nonconventional chiral chromophores able to show second order as well as third order NLO phenomena.
Pd(0)-catalyzed reactions of 2,15-diethynyl[6]helicene with derivatives of p- and o-diiodobenzene give, respectively, polymers 4 and cyclophanes 5, in which helicenes are linked by diethynylbenzenes. Both are very soluble in many common solvents. The molar rotation of polymer 4 is greater than that of monomeric analogue 12, and peaks in its UV and CD spectra at wavelengths greater than 350 nm are shifted to the red of those in 12. Cyclophane 5a, which contains two helicene rings, could be isolated from the reaction mixture in pure form. Its maxima in the UV and CD spectra are not shifted to the red of those of 12, most likely because the two helicene rings in 5a are twisted with respect to one another.