Main-Group-Based Electro- and Photoactive Chiral Materials

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Cite This: Chem. Rev. XXXX, XXX, XXX−XXX

Main-Group-Based Electro- and Photoactive Chiral Materials Flavia Pop, Nicolas Zigon, and Narcis Avarvari*

Chem. Rev. Downloaded from pubs.acs.org by UNIV OF LOUISIANA AT LAFAYETTE on 04/03/19. For personal use only.

Laboratoire MOLTECH-Anjou, UMR 6200 CNRS-Université d’Angers, UFR Sciences, Bât. K, 2 Bd. Lavoisier, 49045 Angers Cedex, France ABSTRACT: This Review discusses the structure−property relationships in chiral molecules, macromolecules (polymers), and supramolecules (crystals, liquid crystals, or thin films) containing main-group elements. Chirality is a major property in our world, having a prominent influence on processes in biology, chemistry, and physics. Its impact in optics due to its interaction with electromagnetic waves gave rise to a multitude of effects, such as the Cotton effect and circularly polarized luminescence, making possible applications such as 3D displays and polarized sunglasses. Herein, a particular emphasis will be given to the influence of chirality on the conducting and optical properties of molecules or materials containing frontier heteroelements, particularly boron, silicon, phosphorus, and sulfur. These synergic materials are expected to become game-changers in the field of materials science by bringing new properties into the realm of reality, such as chirality-induced spin-selectivity, circularly polarized luminescence, and electrical magnetochiral anisotropy. This Review should be of interest for chemists and also physicists working in the fields of molecular and supramolecular chemistry, and molecular materials in the broadest sense.

CONTENTS 1. General Introduction 2. Chiral Electroactive Precursors and Conducting Materials 2.1. Introduction on Chirality and Conductivity 2.2. Tetrathiafulvalenes 2.2.1. TTF-Containing Stereogenic Centers 2.2.2. TTF with Axial Chirality 2.2.3. TTF with Planar Chirality 2.3. Metal Dithiolenes 2.4. Oligothiophenes and Polythiophenes 2.5. Heterohelicenes 3. Chiral Photoactive Precursors and Materials 3.1. Introduction on Circularly Polarized Luminescence (CPL) 3.2. CPL-Active Heterohelicenes 3.3. Boron Derivatives 3.4. Siloles for AIE-CPL 3.5. Chiral Phosphines 3.6. Macromolecular and Supramolecular Chiral Materials 3.6.1. Polymers and Oligomers 3.6.2. Other Supramolecular Assemblies 4. Conclusions and Outlook Author Information Corresponding Author ORCID Notes Biographies Acknowledgments References

1. GENERAL INTRODUCTION Chirality generally refers to objects, materials, and aggregates and ultimately to molecules which can coexist in two nonsuperimposable forms, called enantiomers, showing a mirror image relationship; chirality is present in many scientific areas related to chemistry, biology, astronomy, and physics,1 but also in art and architecture.2 The most distinctive manifestation of chirality in chemistry is in the optical activity of chiral compounds, expressed as optical rotation or circular dichroism.3 The controlled preparation of chiral functional structures is a contemporary challenge for materials science because of the interesting properties that can arise from the resulting assemblies. Chiral materials in the broadest sense have been known for a while, yet the direct influence of chirality on physical properties such as magnetism, luminescence, or conductivity constitutes a relatively recent challenge. The importance of chirality in main-group chemistry has been recognized and extensively discussed over the years in many review articles dedicated to the huge impact of chiral compounds in catalysis and asymmetric synthesis, illustrated for example by the important role of chiral phosphines4−6 and chiral organosulfur ligands and catalysts.7 However, no comprehensive reports but one have specifically focused on chirality-related physical properties, such as conductivity and luminescence, of materials and precursors based on main-group elements. Indeed, one review article described, back in 2009, chiral tetrathiafulvalene (TTF) precursors and derived conducting materials,8 whereas chiral conducting polythiophenes have been discussed within

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Special Issue: Frontiers in Main Group Chemistry Received: December 14, 2018

© XXXX American Chemical Society

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DOI: 10.1021/acs.chemrev.8b00770 Chem. Rev. XXXX, XXX, XXX−XXX

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of the structural disorder present in the former was reported in 2005.25 The precursors involved in these salts were racemic and enantiopure EDT-TTF-oxazolines (EDT = ethylenedithio)26−28 (Figure 1), which provided, by electrocrystallization,

more general studies for interests related to chiral electrochemical sensors,9 electrodes for asymmetric synthesis,10 and spin-dependent electrochemistry11 as a manifestation of the chirality-induced spin selectivity (CISS) effect.12 In the field of chiral luminescent compounds and materials, some of those containing heteroelements have been included, for example, in reviews dealing with the aggregation-induced emission (AIE) phenomena13 or with luminescent small organic molecules.14 As far as the magnetic properties are targeted,15 interesting synergic phenomena involving chirality such as magnetochiral dichroism,16−18 Faraday effect,19 or the switching of magnetization by chiral self-assembled monolayers have been observed.20 However, in all these chiral magnetic materials, the heteroelements, if any, play only the role of ligands and do not themselves provide the magnetic properties. Moreover, the ligands involved in the very large majority of the magnetically active transition metal and rare earth complexes are based on nitrogen and oxygen, elements which are largely represented in traditional organic chemistry and in molecular materials chemistry, and will not be discussed herein. The heteroelements we will refer are the nonmetals and metalloids of groups III−VI, namely boron, silicon, germanium, phosphorus, arsenic, antimony, sulfur, selenium, and tellurium, commonly considered by the heterochemistry community as belonging to this classification. We will comprehensively and specifically discuss in this Review chiral electroactive and photoactive precursors and materials derived therefrom, in which the heteroelements play a central role in the targeted physical property. The first part of the Review (section 2) is dedicated to the electroactive/ conducting chiral materials classified according to the molecular building unit, while in the second part (section 3) chiral luminescent derivatives are inventoried according to the type of the heteroelement and the nature of the resulting material, i.e., molecular, supramolecular, polymer, liquid crystal, etc.

Figure 1. EDT-TTF-oxazoline precursors (left) and detail of the singlecrystal X-ray structure of [(rac)-EDT-TTF-Ox]2(AsF6), highlighting the structural disorder, with a 1:1 occupational site disorder of the (R) (dark gray) and (S) (light gray) enantiomers.

conducting mixed-valence salts formulated as (EDT-TTFOx)2(AsF6), where the single-crystal conductivity (∼100 S cm−1 at room temperature) of the enantiopure (R) and (S) salts was 1 order of magnitude higher than that of the racemic (rac) salt (∼10 S cm−1 at room temperature).25 The three salts showed the same crystal cell parameters and similar packings and band structures; the difference was attributed to the structural disorder observed in the latter, where the two enantiomers were statistically found on the same crystallographic site in a 1:1 ratio (Figure 1). This conclusion was confirmed later through the preparation of the isostructural series of (EDT-TTF-Ox)2(PF6) salts showing similar metal-like behavior in the high-temperature regime and differences in the conductivity values between the racemate and enantiopure compounds, while for the mixedvalence salts of the series (EDT-TTF-Ox)2[Au(CN)2], where the structural disorder was absent, the same conductivity values of ∼120−130 S cm−1 were measured for racemic and enantiopure forms.29 These series of radical cation salts (EDT-TTF-Ox)2X (X = AsF6, PF6, Au(CN)2) provided the first strong evidence of the influence of chirality on the conducting properties of TTF conductors and motivated further studies in the community. Other families of chiral TTF-based conducting salts will be discussed in the following section. It should be noted that the modulation of the conducting properties of racemic versus corresponding enantiopure compounds through the structural disorder or different packing constitutes a somehow indirect influence of chirality and should result, in any case, in the same conducting behavior and conductivity values for both enantiomers. However, the conducting properties of the two enantiomers of a conducting material could be, in principle, discriminated upon applying a magnetic field parallel to the direction of the current. Accordingly, on the basis of symmetry arguments, Rikken et al.30 deduced that the two-terminal electrical resistance of a chiral conductor measured in a magnetic field B is expressed as

2. CHIRAL ELECTROACTIVE PRECURSORS AND CONDUCTING MATERIALS 2.1. Introduction on Chirality and Conductivity

The influence of chirality on the conducting properties of a material in the absence of an applied magnetic field can manifest, in principle, either as a consequence of the different solid-state packing of the enantiopure conductors compared to the corresponding racemate or as a modulation of the structural disorder of the enantiopure versus racemic forms, resulting in a difference in conductivity. Indeed, concerning the latter, since the two enantiomers of a racemate can possibly co-crystallize on the same crystallographic site, it constitutes an inherent source of structural disorder compared to the enantiopure forms. Knowing that the structural disorder strongly influences the electron transport properties,21 different conducting behaviors can be thus expected for racemic compounds showing partial crystallographic occupancy for the two enantiomers with respect to the corresponding enantiopure phases. As the very large majority of the heteroatom-based molecular conductors have been provided by tetrathiafulvalene (TTF)-type precursors,22−24 it is not surprising that most of the chiral conductors were prepared using chiral TTFs8 and that, consequently, several chirality-triggered effects on conductivity were observed within these series of molecular conductors. Accordingly, the first complete family of chiral TTF radical cation salts showing metal-like behavior and huge differences in single-crystal resistivity between the racemic and enantiopure forms because

RD/L(B , I) = R 0(1 + γ D/L B ·I + βB2 )

(1)

where I is the electrical current. Parity symmetry requires γ = −γL for the magnetochiral anisotropy parameter, where D and L correspond respectively to the right- and left-handedness of the conductor. The parameter β characterizes the normal magnetoresistance that is allowed in all conductors, be they chiral or not. The effect corresponding to the linear dependence in B (eq 1) is called electrical magnetochiral anisotropy (eMChA), in analogy with the magnetochiral anisotropy or dichroism previously described in optics,16 which corresponds to an enantioselective D

B


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in 2014, the first observation by Avarvari, Rikken, et al. of the electrical magnetochiral anisotropy in single crystals of the conducting salts (DM-EDT-TTF)2(ClO4) for the (R,R) and (S,S) enantiomers (vide infra).36 This represents the only report to date of the effect in the case of a bulk crystalline molecular conductor. Very recently, Ideue, Iwasa, et al. reported on the non-reciprocity of superconductivity in individual chiral tungsten sulfide (WS2) nanotubes measured via ionic gating technique and resistance measurement on both the first and second harmonic signals in alternating current (AC) mode (Figure 3).37 Ambipolar transfer in the electrostatic doping region and the emergence of superconductivity by electrochemical doping have been observed. The nonreciprocal signal is largely enhanced in the superconducting state, being associated with unprecedented quantum Little−Parks oscillations38 originating from the interference of supercurrent along the circumference of the nanotube. These particularly milestone results, involving an inorganic chiral material, open the way for studying the interplay between superconductivity and chirality and surely will motivate chemists’ endeavors toward the preparation of chiral molecular superconductors, possibly based on TTF or related electroactive precursors. In close analogy with the eMChA effect, Wagnière and Rikken suggested that the flow of electric charge through a chiral conductor should produce a magnetic field collinear with the direction of the current, a phenomenon referred to as the inverse magnetochiral anisotropy effect.39 Only very recently, this effect was experimentally demonstrated in the case of single crystals of elemental tellurium, which is a nonmagnetic semiconductor crystallizing in the enantiomorphic trigonal space groups P3121 (right-handed) and P3221 (left-handed). Current-induced magnetization was observed by measuring the 125Te NMR spectra of a right-handed single crystal at 100 K under an applied pulsed electric current (Figure 4).40 Once again this effect was observed within an inorganic conducting material. Therefore, in order to establish structure− property relationships, it is important to investigate it in various series of chiral molecular conductors, which could open the way toward future developments in the area of magnetic field generation. We have outlined so far pioneering results concerning chiral conductors in the broadest sense, highlighting the synergy between chirality and electron transport properties and pointing out magnetochiral anisotropy phenomena which have been only recently experimentally observed. In the next subsections we will specifically describe the most representative classes of heteroatom-based chiral electroactive molecular precursors, with a special focus on the conducting materials derived therefrom and examples of chiroptical redox switches.

contribution to the optical properties of chiral media that depends on the relative orientation of the direction of light and magnetic field and is independent of the polarization of light. The existence of these synergetic effects is the direct consequence of the simultaneous breaking of parity symmetry by chirality and of time-reversal symmetry by a magnetic field. When considering eq 1, the eMChA effect can be quantified by ΔR(I,B) ≡ R(I,B) − R(−I,B), from which it can be deduced that ΔR = 2γ D/L B ·I R

(2)

This cross-effect, allowing the modulation of the electrical resistance of a chiral conductor with the absolute configuration of the material and by reversing the direction of the field or current, was observed for the first time in a model system consisting of mechanically produced bismuth helices or twists, with an anisotropy factor γ = 10−3T−1A−1.30 The two microscopic mechanisms responsible for eMChA in this model system were the current-induced magnetic field in combination with the magnetoresistance (the β term in eq 1), predicting a relatively strong effect in materials with large magnetoresistance, and scattering by chiral screw dislocations. The latter has been elegantly evidenced in the case of the twisted Bi wires showing opposite linear dependence of the relative resistance with the field for the two helicities, the effect disappearing upon annealing of the wire as a consequence of the melting of the screw dislocations (Figure 2).

Figure 2. Two-terminal magnetochiral resistance anisotropy difference ΔR(I,Bext) − ΔR(I,−Bext) of D and L twisted bismuth wires for I = 0.2 A at 77 K. Also shown is the behavior of the L wire after annealing. The inset shows the geometry of the experiment. Reproduced with permission from ref 30. Copyright 2001 American Physical Society.

2.2. Tetrathiafulvalenes

As outlined in the Introduction on Chirality and Conductivity, tetrathiafulvalenes (TTF) represent the most popular family of electroactive precursors for crystalline molecular conductors.22,23,41 Chiral TTF precursors and conducting materials were reviewed in 2009;8 therefore, herein we will briefly describe the main families of chiral TTFs classified according to the type of chirality, i.e., point chirality, axial chirality, and planar chirality, and we will detail the most representative examples of conducting materials within the past 10 years. 2.2.1. TTF-Containing Stereogenic Centers. The first enantiopure TTF derivative was the (S,S,S,S) enantiomer of tetramethyl-BEDT-TTF (TM-BEDT-TTF, 1, Figure 5),

Further, the effect was also observed in individual singlewalled carbon nanotubes at low temperatures, by the same group, with anisotropy factors γD/L = 102T−1A−1,31 in agreement with model calculations of a free electron on a helix,32 and later on confirmed by other groups.33−35 Until recently, the eMChA effect was observed only in the two chiral materials mentioned above. However, the intensive work in the past 15 years dedicated to chiral TTF precursors allowed, C


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Figure 3. WS2 chiral nanotubes (NTs) and asymmetric superconducting transport in chiral NTs. (a, b) Illustrations of the unidirectional electric transport in superconducting chiral NTs. Because of the broken inversion symmetry, asymmetric magnetoresistance is expected under magnetic field parallel to the tube axis. (c) Transmission electron microscopy image of a single WS2 NT. (d) Electron diffraction pattern of a single WS2 NT. The red arrow represents the direction of the tube axis. The tilted hexagonal diffraction pattern (white lines) confirms the chiral structure in addition to the zigzag-type NT (yellow line). (e) Scanning electron microscopy image of a WS2 NT device. (f) Illustration of the electric double-layer transistor device. The electrolyte KClO4/polyethylene glycol is used as gate medium. Reproduced with permission from ref 37. Copyright 2017 Nature Publishing Group.

Figure 4. Pulse-current dependence of line H in the 125Te NMR spectrum. The NMR spectra are shown as a function of deviation of magnetic field felt by the 125Te nuclei from 7.38034 T. The black, red, and blue lines correspond to data obtained under pulsed electric current densities of 0, +82, and −82 A cm−2, respectively. Reproduced with permission from ref 40. Copyright 2017 Nature Publishing Group.

Figure 5. Achiral BEDT-TTF and EDT-TTF and their enantiopure methylated derivatives TM-BEDT-TTF (1), DM-BEDT-TTF (2), and DM-EDT-TTF (3).

provide (S,S)-DMEDT-EDO-TTF,51 or oxidizing one external sulfur atom into sulfone,52 in some cases with charge-transfer complexes or radical cation salts being obtained,53 complete families of conducting salts containing at least both enantiomers, and sometimes also the racemic form, have been exclusively obtained with the donors 1−3. This last feature is particularly important in view of comparing the conducting properties of the enantiopure and racemic forms. Although it was synthesized only with the (S,S,S,S)-1 enantiomer, we shall mention, however, the chiral metallic radical cation salt (1)x[MnCr(ox)3]·DCM showing ferromagnetic behavior thanks to the presence of a [MnCr(ox)3]− honeycomb network, with a magnetic ordering temperature Tc = 5.5 K, reported by Galán-Mascarós, Coronado, et al.54 The temperature-dependent single-crystal resistance measurements show a rather peculiar behavior, with metal-like behavior in the

42

reported by Dunitz and Wallis in 1986, while the (R,R) enantiomer of the ortho-dimethylated analogue DM-BEDTTTF (2) was described several years later by Sugawara et al.43 Quite surprisingly, the enantiopure dimethylated version of EDT-TTF, namely DM-EDT-TTF (3), was reported only in 2013 by Avarvari, Pop, et al.,44 together with its chiroptical properties and a theoretical conformational study on the relative stability of the axial or equatorial orientation of the methyl substituents.45 Although many other enantiopure BEDT-TTF- and EDTTTF-like derivatives have been described,8 formed by varying the number of stereogenic centers,46,47 replacing both or one dimethylethylene bridge with dimethylpropylene,48,49 using hydrogen-bonding donor−acceptor groups,50 replacing one ethylenedithio unit in DM-BEDT-TTF with ethylenedioxo to D


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Figure 6. Temperature dependence of the electrical resistivity ρ for single crystals of [1]2AsF6 (left) and [1]2SbF6 (right). Adapted with permission from ref 56. Published 2014 Open Access by the Royal Society of Chemistry.

racemic trisphat anion, the donor molecules forming strong dimers inserted into the channels of the honeycomb-like anionic network. Although no conducting properties were reported for these salts, they are very likely insulating because of the strong dimerization of the donors. Honeycomb anionic networks have been observed as well in the structures of the complete series of radical cation salts of donor 1 with the heterometallic dianionic complex [KIFeIII(Cl2An)3]2− (Cl2An = dichloroanilate) containing the ditopic bis(chelating) dichloroanilate ligand. In the crystalline salts formulated as (1)3PPh4[KIFeIII(Cl2An)3]·3H2O, the donors are arranged in parallel columns in the organic layers, which alternate with 2D heterometallic double layers also containing PPh4 cations (Figure 7).59 In the enantiopure salts

high-temperature regime and a room-temperature conductivity value of 65 S cm−1, the occurrence of a first smooth charge localization around 190 K inducing semiconducting behavior, re-entrance in a metallic regime below 10 K and, finally, appearance of a second metal-to-insulator (MI) transition at around 5 K. Although no evidence for the eMChA effect was provided, it would be highly interesting to go further into this series by preparing the opposite enantiomer and also the racemate. Semiconducting radical cation salts of (S)-1 with the series of octahedral anions XF6− (X = P, As, Sb), formulated as [(S)1]2(XF6), shortly followed the first report on the synthesis of (S)-1,42 yet only the structure of the PF6 salt was detailed.55 Recently, Avarvari, Wallis, et al. described the full series of the AsF6 and SbF6 salts with their single-crystal X-ray structures and electron transport measurements.56 The compounds are isostructural except for the difference in space groups between the enantiopure and racemic forms that were, respectively, triclinic non-centrosymmetric P1 and centrosymmetric P1̅, with a slight increase of ∼1.2% of the cell volume when going from AsF6 to SbF6. Despite the disorder of the ethylene bridge in the salts with the racemic donor, all the salts show similar semiconducting behavior, with room-temperature conductivity of 0.5−1 S cm−1 and activation energies ranging between 920 and 1340 K (Figure 6). Donor 1 provided 1:1 radical cation salts by electrocrystallization with [NBu4](I3), the three compounds being isostructural with the exception of the space group, which was monoclinic C2 for the enantiopure forms and monoclinic C2/m for the racemate.57 As in the previous series, the donor was disordered in the latter, with both enantiomers equally occupying the same crystallographic site. Because of the complete oxidation of the donors, formation of dimeric units was observed, leading to semiconducting behavior of the crystalline materials, and as a consequence of the structural disorder, the racemate shows a room-temperature conductivity 3 orders of magnitude lower than that of the enantiopure counterpart, that is, 4 × 10−7 S cm−1 compared to 2 × 10−4 S cm−1. Both enantiomers of 1 have also been engaged in electrocrystallization experiments with the Δ enantiomer of the trisphat anion in an attempt to combine chiral donors with chiral anions.58 It turned out, however, that during the experiments the anion racemized. Therefore, in the crystalline salts thus obtained, one enantiopure donor was associated with one

Figure 7. Crystal packing of [(R)-1]3PPh4[KIFeIII(Cl2An)3]·3H2O in the ac plane, showing the organic−inorganic layer segregation. Crystallization water molecules are omitted for clarity. Adapted with permission from ref 59. Copyright 2015 American Chemical Society.

there are, in fact, six independent donors and two heterometallic [KIFeIII(Cl2An)3]2− units of inverted Δ−Λ configurations, indicating that the chirality of the donor was not transferred to the anionic layer. Interestingly, two of the six independent donors show an allequatorial conformation of the methyl substituents, while the four others show a very rare mixed (ax,ax,eq,eq) conformation (Figure 8). The three compounds show similar semiconducting behavior, with room-temperature conductivity of 3 × 10−4 S cm−1 and a E


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compounds crystallized in the monoclinic P21 space group with four donors for two anions in the asymmetric unit, while the racemic form showed a 2:1 stoichiometry in the triclinic centrosymmetric space group P1̅ with only one independent donor and half of the anion in the asymmetric unit.44 The donors arrange in columns in the organic layer, a twist of 32° being observed from one column to the other in the enantiopure compounds, while parallel columns are observed in the racemate with short S···S intermolecular contacts (Figure 10).

Figure 8. Molecular structure of donor 1 in the all-equatorial conformation (top left (S) enantiomer, top right (R) enantiomer) and in the mixed (ax,ax,eq,eq) conformation (bottom left (S) enantiomer, bottom right (R) enantiomer) in the structure of [1]3PPh4[KIFeIII(Cl2An)3]·3H2O. Reproduced with permission from ref 59. Copyright 2015 American Chemical Society.

magnetic response due to the presence of high-spin S = 5/2 FeIII centers. To date, no complete series of conducting salts based on donor 2 has been reported. Indeed, Sugawara et al. reported an isostructural series of semiconducting salts [(R)-2]X (X = PF6, ClO4, ReO4), crystallizing in the orthorhombic space group P2221 and containing only the (R) enantiomer,60 while Zambounis et al. described a monoclinic P21 κ-phase [(S)2]ClO4, showing a superconducting transition at 2 K for applied pressures higher than 5 kbar.61 This result, however, was not confirmed later on, especially through magnetic field-dependence measurements, thus leaving uncertainty about the real nature of the phase which was measured and the corresponding experimental observations. In a more recent study, both enantiomers of 2 were engaged in electrocrystallization to provide the mixed-valence enantiomeric salts (2)4ReCl6, containing four independent donor molecules in different oxidation states, that were activated semiconductors with room-temperature conductivity of 0.17 S cm−1 and activation energy of 133 meV.62 The magnetic properties are dominated by the presence of structurally isolated high-spin S = 3/2 Re(IV) ions. The anions play not only a magnetic role but also a templating one, since CH···Cl intermolecular short contacts are observed (Figure 9). An even more important effect of the anion through its templating role has been observed in the three complete series (3)2XF6 (X = P, As, Sb), containing the enantiopure and racemic donor DM-EDT-TTF (3), which was only recently described in an enantiopure form.44 In a first report dealing with the mixedvalence salts of 3 with the PF6− anion, the enantiopure

Figure 10. Crystal packing of [(S)-3]4(PF6)2 (top) and [(rac)-3]2PF6 (bottom) with an emphasis on the short S···S intermolecular contacts. Adapted with permission from ref 44. Copyright 2013 American Chemical Society.

The different packing between the racemate and enantiomers together with the charge localization observed in the latter lead to completely different conducting properties. Accordingly, the enantiopure salts show semiconducting properties with σRT ≈ 10−5 S cm−1 and an activation energy Ea ≈ 3900 K, whereas the racemic compound is metallic down to 110 K, the temperature at which a Mott-type localization occurs and the compound becomes a semiconductor. Applying hydrostatic pressure makes it possible to increase the value of the room-temperature conductivity σRT = 250 S·cm−1 by 17% kbar−1, to gradually shift

Figure 9. Packing in the structure of [(S)-2]4[ReCl6] with an emphasis on CH···Cl short contacts of 2.69−2.84 Å. Reproduced with permission from ref 62. Copyright 2014 John Wiley & Sons. F


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the localization toward lower temperatures, and, eventually, to completely suppress it at 11.5 kbar, where the material remains metallic down to 50 mK (Figure 11).

Figure 11. Temperature dependence of the electrical resistivity ρ for a single crystal of [(rac)-3]2PF6 for different pressures. The inset shows the same data with a linear scale for resistivity. Adapted with permission from ref 44. Copyright 2013 American Chemical Society. Figure 12. Packing in the structures of [(S,S)-3]2SbF6 (top) and [(S,S)3]2PF6 (bottom). The short CH···F contacts are highlighted. Adapted with permission from ref 63. Published 2016 Open Access by the Royal Society of Chemistry.

In a follow-up development, the use of the larger isostructural anions AsF6− and SbF6− in combination with 3 provided complete series of salts in which the type of packing was clearly influenced by the size of the anions. Indeed, with SbF6−, the largest of the series, the three saltsracemic and enantiopure (R,R) and (S,S)are now isostructural and adopt the triclinic cell found in [(rac)-3]2PF6, P1̅ for the former and P1 for the enantiopure forms.63 The situation is more complex for AsF6−, as the racemate is isostructural with the PF6− and SbF6− congeners, while for the enantiopure forms both the monoclinic P21 and triclinic P1 phases crystallized during the same electrocrystallization experiments. In agreement with the previous results of electron transport measurements on [(rac)3]2PF6, the triclinic phase, occurring now for racemic and enantiopure salts with AsF6− and SbF6−, is metallic. The templating role of the anion modulating the packing type can be disclosed when analyzing the intermolecular hydrogen bond interactions established between the fluorine atoms and vinyl CCH, methyne CHMe, or methyl protons. In the triclinic racemic phases PF6, AsF6, and SbF6, all the fluorine atoms engage in such hydrogen bond interactions, while small differences are observed when compared to the triclinic enantiopure phases with AsF6 and SbF6. This may be because the disposition of the stereogenic centers is slightly different, and therefore the donors shift in the stack in order to optimize the CH···F interactions. Nevertheless, the CH···F distances are larger, on average, for the AsF6 salt than for the SbF6 one in spite of shorter AsF than SbF bond lengths, indicating that the interactions are less effective in the former and thus suggesting that, for PF6, such a triclinic enantiopure phase is disfavored. Interestingly, in the monoclinic phase of 4:2 stoichiometry, the donors adopt such a packing that all the fluorine atoms of the two independent anions are efficiently engaged in hydrogen bonding for both AsF6 and PF6 (Figure 12). As outlined in the Introduction, the combination of chirality, conductivity, and magnetic field gives rise to the synergetic effect referred to as electrical magnetochiral anisotropy (eMChA). While the effect was described by Rikken et al. in the case of metallic Bi wires30 and single-walled carbon nanotubes,31 its observation in bulk molecular conducting materials remained

elusive until 2014 when Avarvari, Rikken, et al. reported the first chiral TTF-based conductors showing this phenomenon.36 Electrocrystallization of enantiopure 3 in the presence of (TBA)ClO4 provided mixed-valence radical cation salts formulated as (3)2ClO4 which crystallized in the enantiomorphic space groups P6222 and P6422 for the (S) and (R) enantiomers, respectively. At the same time, the racemate crystallized as a 1:1 monoclinic P21/c phase. Once again, the anion establishes intermolecular hydrogen bond interactions of CH···O type (Figure 13).

Figure 13. Detail of the packing in the structure of [(S,S)-3]2ClO4. The short CH···O contacts are highlighted. Adapted with permission from ref 36. Copyright 2014 Nature Publishing Group.

Both enantiopure salts show metal-like conductivity down to 40 K under ambient pressure, then further cooling triggered an abrupt metal-insulator (MI) transition. Band structure calculations are in agreement with the metallic character of the salts and suggest a possible Peierls transition at low temperature. The most striking result on this material was the evidence for the eMChA effect when crystals of the two enantiomers were measured under a parallel magnetic field. Linear and opposite dependences of the relative resistance anisotropy ΔR/R on current and field were observed, with the anisotropy factor g amounting to −0.6 × 10−2T−1A−1 and +0.6 × 10−2T−1A−1 for the (R) and (S) enantiomers, respectively (Figure 14). G


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Figure 14. eMChA results obtained on [(R)-3]2ClO4 (left) and [(S)-3]2ClO4 (right). γ = VeMChA /2GRBI2, where G is a preamplifier gain, equal to 185. Top panels show the odd dependence of the effect on the magnetic field, bottom panels show the quadratic dependence of the heterodyne voltage on the current, which corresponds to a linear current dependence of the eMChA. Reproduced with permission from ref 36. Copyright 2014 Nature Publishing Group.

have been also described,68,69 yet no conducting salts based on these families of donors have been reported so far. TTF-containing chiral chains and units able to promote selfassembly through hydrogen bonding and π−π stacking represent valuable building blocks toward electroactive helical aggregates provided with supramolecular chirality. Accordingly, Tatewaki et al. described chiral TTF-(tetra)amides forming semiconducting helical doped fibers in the presence of the strong electron acceptor TCNQF4,70,71 while Avarvari and Amabilino reported a series of chiral C3 tris(TTF) compounds based on a benzene-1,3,5-tricarboxamide (BTA) platform, 3,3′bis(acylamino)-2,2′-bipyridine linkers, and TTF terminal groups with chiral chains such as 2-methyl-butyl72,73 and citronellyl.74 Homochiral helical fibers have been obtained in the solid state and characterized by optical and electronic microscopy. 2.2.2. TTF with Axial Chirality. Axial chirality has been first introduced in TTF precursors through binaphthyl units as in the flexible compounds 7 and 8, described by Martin75 and Zhang,76 respectively, or in the more rigid compounds 9−11 reported by Fourmigué et al. (Figure 16).77−79 Redox modulation of the circular dichroism (CD) signal has been observed in compound 8 as a consequence of the variation of the binaphthyl dihedral angle with the oxidation state of TTF,76 while a very interesting homochiral electrochemical recognition was shown by compound 9.77 Hasegawa et al. introduced the allene platform to attach TTF units to obtain electroactive axially chiral TTF compounds such

This milestone result should certainly motivate further activity in this direction and raises the question on the microscopic mechanism of the phenomenon in bulk crystalline materials and the correlation between the magnitude of the effect with the composition of the material and the level of expression of the chiral information. As discussed in the Introduction to this section, EDT-TTFMe-oxazolines 5 (Figure 15) provided the first complete series

Figure 15. EDT-TTF-oxazoline derivatives.

of chiral conducting salts in which a difference of conductivity between the racemic and enantiopure salts was observed because of the structural disorder in the former.25 Substitution of the second carbon atom provided EDT-TTF-SMe-Meoxazolines 564 and EDT-TTF-bis(Me-oxazolines) 6.65 Conducting radical cation salts were reported only with rac-5. Note also that bis(pyrrolo)-TTFs containing chiral Nappended groups,66 together with charge-transfer complexes with I3−,67and TTF-sulfoxides with stereogenic sulfur atoms H


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Figure 16. TTF compounds with axial chirality.

as 12 (Figure 16), showing strong redox modulation of the CD signal (Figure 17).80,81

Figure 18. (A) Molecular structure of dimeric TTF linked to chiral [2.2]paracyclophane (13). (B) Possible redox scheme of (Rp)-13. Adapted with permission from ref 82. Copyright 2014 John Wiley & Sons.

Figure 17. ECD spectra of (S)-122+ (dashed red line), (S)-124+ (solid red line), (R)-122+ (dashed blue line), and (R)-124+ (solid blue line) in DCM−MeCN (v/v = 4:1) solution. Adapted with permission from ref 80. Copyright 2011 American Chemical Society.

complexes that show conducting properties. The first examples of chiral metal dithiolene complexes investigated as conducting materials were reported in 2001 by Kisch et al.85 Ni(II)-based complexes with fused dioxolane tetrahydro-1,4-dithiocine-2,3dithiolate ligand (diotte2− 14:dioxolane-tetrathiaethylene, Figure 19) were obtained as both racemic and enantiopure (R,R)/(S,S) materials. The authors combined in their study the anionic [Ni(diotte)2]− complexes with viologen (V) dications, such as the chiral bis(2-methyl-3-hydroxypropyl)-4,4′-bipyr-

However, no conducting materials have been reported based on these axially chiral TTFs. 2.2.3. TTF with Planar Chirality. Planar chirality has been reported for TTF-paracyclophane derivatives, first described by Hasegawa et al. in bis(TTF)-paracyclophanes 13 (Figure 18), showing redox modulation of the CD signal.82 More recently, Avarvari, Pop, et al. reported mono-TTFparacyclophanes including single-crystal X-ray structure for an enantiopure derivative and chiroptical properties correlated with TD-DFT calculations.83 2.3. Metal Dithiolenes

The various strategies to control the packing through chirality in chiral metal dithiolenes, which involve the use of either chiral cations or chiral ligands or associate both within the same material, have been recently addressed elsewhere.84 Within this section we will mainly focus on the chiral metal dithiolene

Figure 19. Chiral dithiolene ligands used in neutral Au and anionic Ni complexes. I


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Figure 20. Crystal packing in [Au{(R,R)-dm-dddt}2] (left). Temperature dependence of the electrical resistivity versus the inverse temperature for single crystals of [Au{(S,S)-dm-dddt}2] at two different applied pressures and [Au{(R,R)-dm-dddt}2] at ambient pressure (right). The lines are the fit giving the activation energy. Reproduced with permission from ref 86. Copyright 2016 American Chemical Society.

idinium dication (HiBV2+), in materials formed by like and unlike diastereomeric pairs: [(R,R)-HiBV][Ni{(R,R)-diotte}2]2/[(S,S)-HiBV][Ni{(S,S)-diotte}2]2 and [(S,S)-HiBV][Ni{(R,R)-diotte}2]2/[(R,R)-HiBV][Ni{(S,S)-diotte}2]2. Resistivity measurements of these materials on compressed pellets showed semiconducting behavior with noticeable differences between the diastereomeric like and unlike pairs (S,S)(S,S)/ (R,R)(R,R) and (S,S)(R,R)/(R,R)(S,S) most probably due to the nature of the packing within the materials. Similarly, some of us have studied anionic Ni(II) complexes with both enantiomers of the 5,6-dimethyl-5,6-dihydro-1,4-dithiin-2,3-dithiolate ligand (dm-dddt2− 15, Figure 19).86 Analysis of the single-crystal X-ray structures did not evidence any short intermolecular S···S contacts between the complexes separated by bulky TBA cations, but the preference to establish CH···S hydrogenbonding interactions was notable. Thus, the two enantiomeric salts are poor semiconductors according to temperaturedependent single-crystal resistivity measurements, which give a value on the order of ∼10−6 S cm−1 at room temperature. Aiming at improving the charge transport of the chiral Nidithiolene anionic complexes by using electron-rich planar dithiolene ligands and chiral cations, Fourmigué et al.87 explored enantiopure trimethylammonium cations, such as (R)-Ph(Me)HC*-NMe3+, (S)-(tBu)-(Me)HC*-NMe3+, and (S)-(1-Napht)MeHC*-NMe3+, in combination with anionic [Ni(dmit)2]− complexes (dmit2− = 1,3-dithiole-2-thione-4,5-dithiolate). In all the systems the Ni complexes arrange in dimers or trimers with radical anions alternating or being sandwiched between neutral species. The units forming dimers and trimers have strong interactions between them and so with the complexes of the neighboring dimers and trimers. As a consequence, charge localization occurs within these materials, giving semiconductors with room temperature conductivities between 0.02 and 0.03 S cm−1 and activation energies of 0.12−0.17 eV. Another class of metal dithiolene complexes with high potential for use in conducting materials comprises neutral Au(III) complexes. The interest in neutral Au bis(dithiolene) complexes resides in their radical character, which constitutes one of the main conditions to access single-component conductor (SCC) materials, possibly endowed with quasi-2D or 3D electron transport properties. Indeed, the absence of cations in the solid structure could increase the dimensionality of the material, yet the bulkiness of the chosen ligands can play an important role in the packing and thus the charge transport of

the materials. One such example is based on the bornylenedithiolato (bordt2− 16, Figure 19) ligand, neutral gold complexes of which obtained as both enantiopure (D) and rac/meso mixtures being reported by Fourmigué et al.88 Despite the lack of disorder in comparison with the rac/meso counterpart, the enantiopure complexes are quite isolated within the structure due to the bulkiness of the bordt ligands and interact poorly through the sulfur atoms, thus providing insulating compounds. Similarly, within the neutral Au(III)[(R,R)-dmdo-pdt]2 (dmdo-pdt = dimethyl-dioxolane-propylenedithio dithiolene 17, Figure 19) complex, reported recently by the same group,89 formation of dimers separated by the bulky side of the ligand has been observed, thus precluding any efficient electron delocalization between the complexes. Recently, some of us reported the first example of a chiral SCC based on Au(III) and the dm-dddt 15 ligand (Figure 19) as both (R,R) and (S,S) enantiomers. Despite the favorable stacking of the neutral molecules due to the preferential equatorial orientation of the methyl units of the ligand, the two materials are semiconductors (Figure 20). The low conductivity within these materials very likely comes from the dissymmetry of the two dithiolene ligands, a phenomenon that has been previously observed in other neutral Au(III) dithiolene complexes and which has been recently rationalized through DFT calculations.86 In the crystal structures of several neutral Au(III) dithiolene complexes, the two dithiolene units have different degrees of oxidation, as suggested by the lengths of the CC bonds. DFT calculations run on a series of neutral Au and anionic Ni bis(dithiolene) complexes clearly indicate higher stability for the symmetric structure for the anionic Ni complexes, while the asymmetric structures were found to be more stable for the neutral Au complexes, with the exception of the complexes based on dithiolene ligands with extended conjugation or aromatic backbones. This study demonstrates that the experimentally observed structural asymmetry is inherent to neutral [Au(dithiolene)2] species. As a step forward, the authors evaluated the contribution of the intermolecular interactions and solidstate architecture on the spin and charge localization versus delocalization in these complexes with the aim of predicting the transport properties. Overall, the symmetric structure would be more favorable than the asymmetric one in two cases: (1) when dithiolene ligands with extended delocalization are used and (2) when the formation of chains is induced in the bulk structure. J


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Figure 21. Chemical structures of chiral thiophenes, phenylquaterthiophenes, and benzo-dithiophenes used in semiconducting materials.

the electroactive species as a consequence of hydrogen bond formation; however, they did not achieve enantiomer discrimination. Nevertheless, chiral discrimination toward chiral anions has been observed with both enantiomers of a similar polymer, poly(22), synthesized by Roncali et al.97 In this case the enantioselectivity was probed as the difference in the current density during the oxidation/reduction processes of the two polymeric enantiomers in the presence of (+)- and (−)-camphorsulfonic anions. Chiral discrimination using conducting chiral poly(thiophenes) detected by cyclic voltammetry works by changing either the oxidation rates or the current density and can be a consequence of both regioregularity of the conjugate system and interchain interaction due to the bulkiness of the chiral substituents. Taking advantage of the benefits brought by chiral alkyl units in polymers such as increased solubility, regioregularity, reduced structural defects, and self-organization, Ochiai and Rikukawa98−100 used chiral poly(25) and stearic acid in Langmuir− Blodgett (LB) films with the aim to correlate chirality and electrical properties. Stable monolayers of poly(25) and stearic acid were transferred on solid substrates, forming uniform LB films that were conducting in neutral form and when doped with SbCl5 (10−5 and 10−1 S cm−1, respectively). When used in organic light-emitting diode (OLED) devices, the LB films had a turn-on voltage higher than that of spin-coated films due to the insulating stearic acid,100 whereas the third-order nonlinear susceptibility χ3 was almost 10-fold higher than for the spincoated counterpart.99 Similarly, Reynolds et al. tried to gain insight into the role of interchain interactions in such chiral conducting poly(thiophenes) by comparing the spectroscopic and electrical properties of chiral helical aggregates of poly(26a) and poly(26b).101 Although poly(26a) proved to form more heterogeneous aggregates than poly(26b), the conductivity of the spray-coated films doped with iodine was not significantly different between the two chiral polymers and was very similar to that of the racemic counterpart of poly(26b) (4 S cm−1 for poly(26a), 0.3 S cm−1 for poly(26b), and 0.4 S cm−1 for racpoly(26b)). Similar results on electron mobilities have been reported by Ikai and Maeda102 with co-polymers of 27 and noctyl-thieno[3,4-b]thiophene-2-carboxylate. When used in devices for solar cells, the chiral co-polymers (with (S,S) or (R,R) enantiomers, 27a or 27b) did not show any difference in

Both situations should favor the formation of Au(III) bis(dithiolene)-based chiral neutral SCCs with metal-like conductivities. One aspect which should not be neglected, however, is the influence of pressure. As can be seen in Figure 20 (right), the activation energy of the electron transport process is strongly decreased upon application of moderate pressures. 2.4. Oligothiophenes and Polythiophenes

Oligothiophenes are among the most investigated classes of electroactive materials because of their high chemical stability, ease of functionalization and thus versatility, and tunable optical and electrical properties.90−92 Oligo- and polythiophenes embedded with chirality have recently been developed with the aim of improving regioregularity and microscopic organization in the search for specific properties (enantiomer recognition, chemical sensing, asymmetric reactions, circularly polarized luminescence, organic photovoltaics, or simply superior conductivity).10 The existent strategies employed in their synthesis make use of punctual chirality onto monomers (that are electro-oligomerized and -polymerized) or quaterthiophenes (Figure 21), which are used in conducting chiral surfaces, bulk heterojunctions, liquid crystals, and aggregates. The first chiral conducting polymer film of optically active thiophene (18) developed in the context of “chiral metals” showed conductivity significantly lower than that of the classical achiral poly(alkyl-thiophenes) despite its mechanical robustness.93 Later, the increase in stereoselectivity of heterogeneous reactions was clearly shown to benefit from the chiral auxiliaries present in the conducting polymers. Thus, Pellon et al.94 showed the heterogeneous catalysis of modified electrodes to be drastically influenced by the amount of chiral units in the polymers (no enantiomeric excess (ee) with chiral poly(19) and 57% ee with a chiral co-polymer of 19 with thiophene). Similarly, stable conducting films on various surfaces (platinum and ITO) were observed when chiral alkyl-thiophene (20) was co-polymerized with 3,4-ethylenedioxythiophene (EDOT) instead of self-polymerized.95 In a similar study, Chahma et al.96 showed the formation of stable chiral conducting surfaces (with polymers of 21) on electrode surfaces (platinum, gold, and glassy carbon) as well as the formation of hydrogen bonds on the surfaces that create a barrier and decrease the electron transfer constant through the chiral conducting layer. These chiral surfaces have been able to recognize amino acids in solution due to changes in conductivity accompanied by a potential shift of K


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power conversion energy compared to the achiral counterpart (with a mixture of (S,S), (R,R), and (S,R) enantiomers, 27c). In semiconducting liquid crystals, the dependence of the carrier mobility on the temperature and the pre-transitional organization in chiral nematic phases were investigated by Funahashi and Tamaoki.103−105 The self-organization of the chiral phenylquaterthiophene derivatives in the chiral nematic phases was modified by using either alkyl chains of different lengths or different methyl substitution (23 and 24, Figure 21). Although the three derivatives formed chiral nematic phases, compound 23 crystallized above 87 °C, derivative 24 (R = C3H7) formed a chiral smectic phase below the chiral nematic one, and 24 (R = C12H25) exhibited a different smectic phase. The changes in supramolecular organization of the three oligothiophenes drastically impact the positive charge carrier mobility, which is of electronic type, as well as its dependence on temperature, while no noticeable differences were seen in the negative carrier mobility (Figure 22). While in the liquid crystals of 23 the positive carrier mobility increases with increasing temperature, in 24 (R = C3H7) it is constant over the whole temperature range of the chiral nematic phase and increases with increasing temperature in the chiral smectic phase, whereas in the case of 24 (R = C12H25) a decrease in the mobility was observed with increasing temperature. In another approach, Sannicolo et al. made use of inherently chiral oligomers as driving units to form chiral electroactive films on various electrode surfaces (Figure 23).106 Besides the asymmetry due to axial chirality, the advantages of using blocks with a central twist translate in high intrinsic regioregularity and 3D character. Oligothiophenes 28 have been electrochemically polymerized106 to give stable conducting electrode materials and copolymerized with various co-monomers to improve the conductivity of the resulting co-polymer or to be used as recognition materials due to induced 3D structure.107 Electrooligomerization of 28 (giving mixtures of oligomers from dimer to hexamer) afforded materials with good enough conductivity and selectivity toward enantiomer recognition. Thus, enantiopure films of 28 showed very good enantio-discrimination ability toward electroactive chiral probes ((S)- and (R)-N,N-dimethyl1-ferrocenylethylamine or -3,4-dihydroxyphenylalanine), seen as a shift in the oxidation potentials of the probes due to electron transfer through the chiral films (Figure 24).108 Chemical oxidation of the twisted (S) or (R) monomer of 28 with iron(III) chloride afforded mixtures of cyclic chiral oligothiophenes which had the particularity to be formed by a single stereoisomer.109 The difference in oxidation potential between the two enantiomeric probes formed using electrodes coated with enantiomeric cyclic dimers and trimers was similar to that of the electrochemically deposed oligomers, whereas in all cases the potential separation was superior when using racemic probes. In contrast, when electrodes were modified with poly(29), the opposite effect was observed, with larger potential peak separation on the enantiopure probes than the racemic probe.110 The open-chain stereoisomers of 28 dimers have been compared with those of the cyclic oligomers (dimers and trimers) as donors in bulk heterojunction solar cells for optoelectronic applications.111 Their performances in solar cells, associated with either C60 or PCBM, were correlated to the geometry of the moleculemore favorable in the case of the cyclic trimer and open-chain dimer than the cyclic dimerand were in total agreement with theoretical calculations.

Figure 22. Carrier mobility in the chiral nematic and isotropic phases of derivatives 23 (a) and 24 (b, R = C3H7; c, R = C12H25). Reproduced with permission from ref 105. Copyright 2007 American Chemical Society.

Changes in the substitution of the twisted dibenzothiophene core with ethylenedioxythiophene (30)112 and dithieno[3,3b:2′,3′-d]pyrrole (32) gave stable polymeric chiral materials on the electrodes with higher conductivity and rigidity due to increased planarity, whereas the electrochemical formation of poly(31) proved to be very slow and the material showed very low stability.113 However, both oligothiophenes 31 and 32 and the corresponding polymers obtained by chemical oxidation were stable enough and were CPL active (Figure 25). 2.5. Heterohelicenes

Among derivatives that possess inherent chirality, helicenes are one of the most investigated class of chiral materials due to their helical shape.114 Heterohelicenes have the advantage of associating unique structural features and good electronic properties supplied by the heteroatom.115 In the field of conducting materials, thia-heterohelicenes (formed either only by β-annealed oligothiophenes or by alternating thiophene and phenyl rings within the helix, Figure 26) have superior L


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Figure 23. Chemical structures of inherently chiral thiophene monomers.

conjugation and electron-donating properties compared to the carbohelicenes and can be seen as electronic analogues to αoligothiophenes. For example, [7]-TH (37), decorated with bromine and TMS units, forms the radical cation and dication at potential values similar to those used with α-terthiophene but higher than the ones used with α-sexithiophene or polythiophene.116 This observation suggests a smaller electron delocalization compared to that observed in equivalent oligothiophenes because of helical distortion, despite the larger number of electron-rich sulfur atoms in the conjugation. Nevertheless, the first configurationally stable radical cation, obtained by chemical oxidation of (M)-[7]-TH with [NO][PF6], was reported by Rajca et al.117 The same authors described the electrochemical oxidation of [9]-TH (35) and [11]-TH (36) that gave access to a larger number of oxidation states (1+, 2+, 3+, and 4+).118 Superior terms of thiahelicenes show a trend similar to that observed with classical αoligothiophenes, i.e., shorter band gaps due to increased conjugation. The ease of tuning their electronic properties was recently proved on aza-thia-helicene [7]-TAH (38), which has oxidation potential inferior to that of its [7]-TH (37) analogue

Figure 24. Potential separation of chiral redox probes (S)-, (R)-, and rac-N,N-dimethyl-1-ferrocenylethylamine (compound 3 in ref 108) using enantiopure films of oligo(28) (28 corresponds to compound 1a in ref 108). Reproduced with permission from ref 108. Copyright 2014 John Wiley & Sons.

and shows inversion of the HOMO−SOMO levels due to the presence of an electron-deficient pyrrole unit.119 Mussini et al.120 reported that tetrathia-phenyl-helicene materials (TPH 39−41, Figure 26) had oxidation potentials similar to those of all-thiophene helicenes (TH 33−37) but shifted to higher values compared to those of α-oligothiophene analogues with the same number of thiophene rings. Whereas the torsion of the conjugated system in the helix makes crossconjugation less efficient, in TH it can be counter-balanced by the large electron-donating character due to the higher number of thiophene rings compared to TPH analogues. Oligomerization or dimerization of [7]-TPH (39) of types A and B resulted in a decrease in the band gap, whereas the bulkiness of the substitution in positions 7 and 8 was shown to affect the M


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instead of thiophene gave semiconductors and insulators as racemic 2:1 radical cation salts with PF6− anions and racemic 1:1 charge transfer complex with TCNQ.124 The first circularpolarization-selective organic semiconductors were made using enantiomerically pure aza-helicenes in field effect transistors, in which the changes in the off-current with the circularly polarized illumination were related to the handedness of the helicene used.125 Some of the strategies employed by Hasobe et al. to tune the chiroptical properties of thia-helicenes include sulfonation126 and the use of push−pull molecules.127 Thus, the fluorescence quantum yield of tetrasulfone[9]helicene ([9]-PTSH (43), Figure 26) has been enhanced to 0.27, compared to 0.03 for the parent tetrathia[9]helicene [9]-PTH (42). In addition, the first oxidation and reduction potential were anodically shifted by the electron-accepting sulfone units. When the helical skeleton was modified by quinoxaline electron-acceptor units within a push− pull molecule ([9]-QPTH (44) and [9]-H-QPTH (45), Figure 26), the fluorescence quantum yield and the first oxidation potentials remained unchanged, whereas the reduction potential was anodically shifted. Both the oxidation potential and the quantum yields could be changed by introducing substituents on the phenyl rings. Increasing the push−pull character by the addition of substituents, the fluorescence quantum yield for [9]Me2N-QPTH (47, 0.43 in benzene) was shown to have one of the highest values for conjugated helicenes.127 Similarly, the amine groups on the phenyls seem to have the greatest influence on the shift of the oxidation potential (Table 1). In another work, Chan et al.128 saw for sultam-based thia-aza-sulfonehelicenes [5]-PTASH (49) a significant increase accompanied by a blue shift of the photoluminescence efficiency in the solid state compared to solution (0.28 in the solid compared to 0.13 in dichloromethane (DCM) solutions). Nakamura et al.129 studied azaboradibenzo[6]helicenes [6]AB-H (52, Figure 27) in both racemic and enantiopure forms.

Figure 25. CD and CPL spectra of (S)- or (R)-31 and the corresponding materials obtained by chemical oxidation. Reproduced with permission from ref 113. Copyright 2018 John Wiley & Sons.

oligomerization ability through combined steric and solubility effects.121 Despite the lack of data on the contribution of chirality to their charge transport properties, these materials have the advantage of combining extended electron delocalization and strong chiroptical properties due to the twist angle of the annealed thiophene rings, which makes them very appealing for use in chiral conductors and chiroptical switches.122 For instance, with the racemic form of naked 39 (which exhibits a helical nanowire structure formed by alternating stacks of the two enantiomers), very poor thin-film transistor activity and low mobility were observed.123 Similarly, 39 with edge phenyl units

Figure 26. Thia-heterohelicene (TPH, TH, TAH, and PTH) derivatives. N


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Table 1. Redox Potentials and Fluorescence Quantum Yields of [9]-PTH (42), [9]-PTSH (43), and [9]-QPTH (44) heterohelicene [9]-PTH (42) [9]-PTH (42), R = nPr [9]-PTSH (43) [9]-QPTH (44) [9]-QPTH (45) [9]-MeO-QPTH (46) [9]-Me2N-QPTH (47) [9]-CN-QPTH (48)

Ered3

−2.04

Ered2

Ered1

Eox1

Eox2

ΦFL

ref

− −2.39 −1.34 −1.63 −1.69 −1.66 −1.68 −1.58

−1.65 −1.86 −0.91 −1.26 −1.25 −1.28 −1.36 −1.06

1.25 1.27 − 1.21 1.17 1.16 0.90 1.25

1.63 1.57

0.02 0.03 0.27 0.04 0.05 0.04 0.39 (0.43) 0.26 (0.3)

127 126

1.26 1.63

127

Figure 28. Solid-state packing in single crystals of racemic and (P) forms of 52 together with the electronic couplings (in meV) between the HOMO orbitals of the neighboring molecules; the electronic couplings between LUMOs are shown in parentheses. Reproduced with permission from ref 129. Copyright 2012 American Chemical Society.

Figure 27. Saddle-like double heterohelicenes.

thia-helicenes (double-[6]-dithia-H (53), Figure 27). Within these thia-helicenes, the radical cation has stability superior to that of all phenyl analogues due to the presence of the sulfur atoms.133 In addition, time-resolved microwave conductivity measurements on crushed crystals showed superior charge mobility of rac-53 (with respect to the all phenyl analogues). The field-effect transistor of a film of 53 worked as a p-type semiconductor with a hole mobility of 3.3 × 10 −2 cm2 V−1 s−1. An interesting saddle-like, enantiomer-separable [4]helicene is the double-[4]-phenothiazine-H (50, Figure 27).134 This [4]helicene was resolved by chiral chromatography, and the enantiomers showed a racemization barrier of 112.8 kJ/mol. The large difference between the first and second oxidation potentials of the racemic form indicated good electron communication between the two phenothiazine units in contrast to the open ring analogue. In addition, the stability of the radical cation forms under ambient conditions (in both solution and solid state) leads to saddle-like [4]helicene materials with high potential for chiral conductors. Heterohelicenes have been modified with photoswitchable or redox units for applications in chiroptical switches. For instance, thia-helical derivatives have been reversibly formed using a dithienylcyclopentene photochrome,135−138 whereas helicenes modified with pyrazine-dithienylethene photoswitchable units were shown to stabilize the cyclic form.139 A very interesting example of multifunctionality has been given with fused TTFhelicenes (Figure 29), especially with the enantiomers of [6]-HTTF (55) that had redox chiroptical responses (Figure 30).140 These derivatives represent promising candidates for chiral

Interestingly, different charge carrier transport properties were observed between the racemic and the (P) enantiomer films of this azabora-helicene by time-of-flight (TOF) measurements. While the racemic form shows high hole mobility (μh = 4.6 × 10−4 cm2 V−1 s−1) and no transient photocurrent in electron mobility, the (P) form shows both hole and electron mobilities, the latter being higher than the former (μe = 4.5 × 10−3 cm2 V−1 s−1, μh = 7.9 × 10−4 cm2 V−1 s−1). The carrier inversion between the two forms is due to the different structure packing (Figure 28) and has been confirmed by theoretical calculation of electronic couplings. In addition, the estimated reorganization energy was in agreement with the superior electron mobility in the (P) enantiomer.130 This was further confirmed by the larger changes in bond length of the reduction process than those of the oxidation process. Interestingly, these bond length changes were mainly localized on the heteroatoms, suggesting an important contribution of the heteroatoms to the electronic properties of these helical structures. Ambipolar conductivity was measured by TOF on a vacuumdeposited amorphous film of the racemic unsubstituted oxaborahelicene double-[5]-OB-H (51, μh = 5.7 × 10−3; μe = 7.9 × 10−3 cm2 V−1 s−1).131 This behavior was not surprising, given its peculiar 3D arrangement in the solid state, which seems to have been preserved in the amorphous film. Moreover, the fluorescence quantum yields of these double-[5]-OB were reported to be the highest among double heterohelicene derivatives (Figure 27 and Table 2). The semiconductivity of racemic heterohelicenes was observed by Segawa, Itami, et al.132 on saddle-like π-extended O


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polarized light in a chiral medium. Usually, CD is quantified by the absorptive dissymmetry factor gabs:14

Table 2. Values of Emission Maxima, Quantum Yields, and CPL Dissymmetry Factors for Heterohelicene CPL Emitters heterohelicene

λlum

Φem

|gem| × 10−3

ref

[7]-Si-H (60)

450

0.23

3.5

157

Triphenylene [7]-Si-H (61)

482

0.15

16

158

[7]-P-H(=O) (64) Triphenylene [7]-P-H (65) Benzopicene [9]-P-H (66)

470 487 502

0.078 0.22 0.085

1.5 0.81 0.48

159

[9]-PTH (42) [9]-PTSH (43)

450 430

0.02 0.27

− 0.83

126

gabs =

600

0.26

3

127

[5]-AB-H (H) (67) [5]-AB-H (OMe) (67) [5]-AB-H (NMe2) (67)

495 502 586

0.29 0.30 0.13

0.25 0.95 3.5

162

[6]-AB-H (68) [8]-AB-H (69) [6]-bis-AB-H (70) [10]-bis-AB-H (71)

430 442 430 473

0.21 0.069 0.49 0.074

0.9 0.7 2.3 1

163

[4]-TA-H (62) [6]-TA-H (63)

470 500

− 0.0157

− (60)a 9 (10)a

164

Double [5]-OB-H,(t-Bu) (51)

436

0.65

1.7

131

with

ε(λ) = 1 2 {εL(λ) + εR (λ)}

where εL and εR are the molar extinction coefficients for left- and right-handed polarized light at a given wavelength. Similarly, for chiral fluorescent molecules or materials, the difference in magnitude for the emission of left- and righthanded polarized light is called circularly polarized luminescence (CPL). CPL is usually expressed by the luminescence dissymmetry factor, written gem or glum:142,143 gem =

[9]-CN-QPTH (48)

Δε(λ) ε( λ )

1

IL − IR ΔI =2 I 2 (IL + IR )

Both CD and CPL arise from the interaction of light and chirality. The fundamental difference between them is that CD is related to the ground state of the molecule, whereas CPL is informative about the structure of the excited state of the molecule. Thus, a careful analysis of both CD and CPL spectra gives valuable information on the electronic states of the molecule or material. As Faraday said, “polarized light is a most subtle and delicate investigator of molecular condition”. These chiroptical properties are of high interest for the development of smart materials, such as spintronics devices, where light could be used in spin-based electric circuits,144 security-enhanced information storage and transportation,145−147 photoswitches for responsive materials,148 3D displays with chiral LEDs,149−151 sensing of chirality within biological substrates,152 or ellipsometry-based tomography with improved sensitivity compared to classical ellipsometry.153 Theoretically, the values for gabs and gem are between −2 and +2. Lanthanide complexes have been extensively studied in this regard but are beyond the scope of this Review; detailed discussion about this topic can be found in ref 154. The highest |gem| valuestypically between 0.05 and 0.5have mainly been obtained with these complexes. Organic molecules generally have |gem| values in the 10−5−10−2 range for single molecules (these low values make the spectra usually noisy) and 10−3− 10−1 for polymers or aggregates, but can rise to high levels of CPL by self-organization, for example in liquid crystals or rigid nanoscale cylinders.155 Plus, they usually are less sensitive to their environment than the lanthanide complexes.

a

Values in parentheses are from calculated CPL spectra.

conductors due to the stability of the radical cation forms. The same authors studied helicenes modified with thiadiazole units, which proved to have emission properties highly influenced by the helical turn.141 Accordingly, Bis-[4]-H-Td (58) and [4]-HTd (59) helicenes were luminescent, with quantum yields of 5.4 and 6.5%, respectively, whereas the emission of [5]-H-Td (56) and [7]-H-Td (57) was strongly diminished by intersystem crossing. The fluorescence quenching was supported by theoretical calculations from the favorable energy alignment of the relaxed S1 states with the nearby triplet states (Figure 31).

3. CHIRAL PHOTOACTIVE PRECURSORS AND MATERIALS

3.2. CPL-Active Heterohelicenes

Helicene materials, with their unique rigid π-system, represent a promising class of active CPL materials, as shown by Mori et al. in their study on small organic molecules.14 Those authors found a quantitative correlation between |gabs| and |gem| (unique to the class of CPL-active small molecules) which demonstrated that the CPL and CD dissymmetry factors have proportional values, with the former being smaller. Existing heterohelicenes

3.1. Introduction on Circularly Polarized Luminescence (CPL)

The difference in UV−visible absorption between a left- and right-handed light beam by a chiral molecule (or material) is called electronic circular dichroism (ECD).3 This effect is due to the different propagation speeds of the left- and right-handed

Figure 29. TTF and thiadiazole (Td) heterohelicenes together with the anthracene-benzothiadiazole derivative. P


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Figure 30. CD spectra of (P)-(+)/(M)-(−) enantiomers of [6]-H-TTF (55) as neutral and oxidized forms, together with the redox chiroptical response of (M)-(−)-[6]-H-TTF observed by CD at 306 nm. Reproduced with permission from ref 140. Copyright 2013 John Wiley & Sons.

same order of magnitude only for triphenylene [7]-P-H (65),159 whereas the other two reported phospha-[7] and [9]helicenes had very low luminescence (Table 2). When CPL activity is compared within the two families of heterohelicenes, significant differences are observed between the sila- and the phospha-helicenes. Surprisingly, the triphenylene [7]-Si-H (61) had the highest |gem| value of all the existing CPL-active heterohelicenes.158 Another work160 reported a similar high value of |gem| = 3.2 × 10−2 for fluorene-based carbo-helicenes equivalent to triphenylene sila- and phospha-[7]helicene, which is the highest reported to date for helicene derivatives. Interestingly, the solid structure of rac-[7]-P-H(=S) (64) is reported to be chirally anisotropic, comprising alternating M and P forms which have opposite dipole moment orientation with respect to each other.161 Very good fluorescence quantum yields and CPL activity were observed by Hasobe et al.126,127 on modified phenyl-thiahelicenes. [9]-PTH (42, Figure 26), which had very low emission and no CPL activity, was subjected to chemical transformations in order to enhance its emission. Oxidation to 43 proved to be efficient enough to increase the luminescence quantum yield by an order of magnitude and to activate the CPL luminescence. When changes were made to the conjugated system by forming push−pull molecules, the Φem was preserved and the CPL enhanced by 1 order of magnitude ([9]-CNQPTH, Table 2). In this case the substituents were shown to play a crucial role, together with the extended conjugation. Boron atoms have rarely been employed in the structure of helical derivatives. The few existing reports on boron-helicenes involve azabora and oxabora single and double helicenes. In the series of [5]-AB-H (67, X = H, OMe, and NMe2,)162 the nature of the substituent seems to have little impact on the fluorescence quantum yields, whereas the electron-donating groups seem to red shift the emission maxima. Interestingly, 67 (X = NMe2) showed CPL emission maxima close to 600 nm and had the highest |gem| as a consequence of an increase of the chargetransfer character of the emissive state due to the strong electron-donating character of the −NMe2 substituent. Double [5]-OB-H (51, Figure 27) had record values of the quantum yield of emission among the discussed helicenes and known double helicenes, while the CPL activity was comparable to

Figure 31. Calculated energy levels (eV) of ground and excited singlet and triplet states for (P)-[7]-H-Td (57). Reproduced with permission from ref 141. Copyright 2017 John Wiley & Sons.

have, in general, CPL dissymmetry factors in the range of typical values for small organic molecules in the non-aggregated state. From the existing literature on heterohelicenes, we could not find any obvious correlation between the nature of the heteroatom within the structure of the helicene and the intensity of the CPL response. The intensity of the CPL response in the heterohelicene family can be greatly influenced by the presence and the position of the heteroatom(s) and the substituents and so by the nature of the π-conjugation within the helix.156 In the following we will discuss families of CPL-active sila-, phospha-, thia-, azabora-, and thiaza-helicenes presented in Figures 26, 27, and 32. The presence of silole in the structure of organic molecules is known to favor good luminescence and electron transport properties. Indeed, sila-helicenes [7]-Si-H (60)157 and triphenylene [7]-Si-H (61)158 show fluorescence quantum yields of 23 and 15%, respectively. When silole was replaced with phosphorus in similar [7]helicenes, the Φem remained of the Q


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Figure 32. CPL-active sila-, phospha-, azabora-, oxabora-, and thiaza-helicenes.

those of other heterohelicenes.131 Similar high Φem and |gem| were observed for 68 and [6]-bis-AB-H (70).163 Increasing the size of the helicene has a negative impact on the Φem (7% for both [8]-AB-H and [10]-bis-AB-H); nevertheless, the CPL dissymmetry factor was similar to those of other azaborahelicenes. In thiaza systems, such as [6]-TA-H (63), the experimental and calculated CPL response was almost of 1 order of magnitude larger than those of other small organic molecules, despite its very low quantum yield of emission (1.6%).164 CPL activity has been also observed in the first helical Pt(II)(diimine)(helicene-dithiolene) 73b complexes, recently reported by Avarvari et al. (Figure 33).165

Figure 33. Synthesis of Pt(II)-helicene luminescent complexes. Adapted with permission from ref 165. Copyright 2017 Royal Society of Chemistry.

Figure 34. (Top) Absorption spectra of (P)-73b and (M)-73b and emission spectrum of (M)-73b in acetonitrile (2.5 × 10−4 M) at T = 298 K, λex = 532 nm. (Bottom) CD and CPL spectra of (P)-73b and (M)73b. Adapted with permission from ref 165. Copyright 2017 Royal Society of Chemistry.

The [4]helicene and [6]helicene complexes show low-energy emission bands at 720 and 715 nm in CH3CN solutions at room temperature, originating from a MMLL′CT transition, typical for Pt(diimine)(dithiolene) complexes, with emission quantum yields of 0.15% and 0.19%, respectively. The [6]helicene complexes, prepared in enantiopure forms, are CPL active in spite of the low quantum efficiency, with a gem dissymmetry factor of ±3 × 10−4 (Figure 34).

3.3. Boron Derivatives

Boron derivatives, and particularly boron-dipyrromethene (also abbreviated BDP or BODIPY), have been in the spotlight in the past decades because of their excellent optical properties (high R


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Figure 35. Boron derivatives 74−88.

investigated by the Gossauer group to gather more information on the relation between chirality and the signs of CD signals in the BODIPY family.169 It displayed a strong CD band at 518 nm and Φem = 0.39. Ziessel and co-workers also used the propensity of −BF to accept H-bonds in a BODIPY with an asymmetric boron atom (see 76, Figure 35).170 Lateral differentiation by a polar formyl group engendered H-bonds that stabilized the boron configuration and prevented its racemization even in daylight at room temperature. Induction of helicity or twisting in single molecules (vide supra), or even in the aggregates (vide infra), is an excellent way to achieve CPL with appreciable intensity. In the case of helicenes, it is commonly observed that the distortion of the aromatic core decreases the emission efficiency. An original strategy to induce a twist in the BODIPY skeleton was pursued by Gobo et al.171 77 was obtained as a racemate and separated via chiral HPLC before measuring its high quantum yield of 0.73, its Cotton effect Δε = 60 M−1 cm−1, and |gem| = 6 × 10−4, which is in the range of CPL for single organic molecules (SOMs).

molar absorptivity and almost quantitative quantum yields), ease of functionalization (for a fine-tuning of the optical properties), and stability and solubility in common organic solvents.14,166−168 Introduction of chirality on the side chains, by atropoisomer formation or by chiral induction of the media, will be discussed herein, along with the resulting chiroptical properties of the molecules thus obtained. Difluoroboron urobilinoids were among the first reported examples of BODIPY as CPL emitters. Urobilinoids are products of heme catabolism in mammals, which present a chelating N,N-dipyrrin moiety. In order to study their folded conformation by abstracting intramolecular H-bonds arising from a protonated pyrrolo part, difluoroboron urobilinoid derivatives such as 74 (Figure 35) were synthesized.169 Changes of conformationin particular formation of P or M helices depending of the solvent polarity were observed by CD and NMR. Additionally, NH−F bonds stabilize the folded structure. The quantum yield was 0.48, and |gem| reached 9.4 × 10−4. Using the trifluoromethyl-methoxy-methyl group as an established chiral moiety for desymmetrization, chiral BODIPY 75 was S


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59° relative to the BODIPY plane. Indeed, at room temperature no CPL is observed, and the main CD signals are centered on the benzene ring transitions below 300 nm. When the temperature is decreased below 10 °C, the propeller chirality is in equilibrium between the clockwise and counterclockwise diastereoisomers, with the clockwise enantiomer being slightly more stable by 0.8 kcal/mol. Below −70 °C, strong CD effects appear in the BODIPY region of the UV spectrum, attributed to the formation of an H-type head-to-tail dimer with a good |gem| for SOMs of 2.0 × 10−3 and Φem = 0.45. Binaphthyl moieties have been widely used to create asymmetry in chemical systems, and several examples of BINAP-substituted BODIPY have been reported in the literature. Beer et al. reported the synthesis and spectroscopic properties of BODIPY mono- and disubstituted 1,1′-binaphthyl derivatives 81 and 82.176 Interestingly, spectroelectrochemical investigation showed that a reversible reduction takes place and promotes the appearance and disappearance of the CD signal at 501 nm (Figure 37). This on−off switching is combined with good quantum yields (79% for 81 and 69% for 82). These materials proved their possible utility as sensors, as 82 also showed chiral recognition toward the enantiomers of 1phenylethylamine, observed by fluorescence quenching.177 De la Moya and co-workers synthesized the R and S enantiomers of 83 (Figure 35), where a BINOL is attached directly to the boron atom through B−O bonds.178 These compounds exhibited Φem ≈ 0.45, |gem| = 1.0 × 10−3, and a light polarization of opposite sign for the two enantiomers in CPL. On the other hand, the analogue 84, where a nitrogen replaced the methylene bridge between the pyrroles, sees its fluorescence totally quenched due to a strong charge delocalization in the excited state and the lack of overlapping between HOMO and LUMO, according to DFT calculations.179 When the BINAP is linked on a pyrrole instead of the boron atom as in 85, a very low fluorescence is observed (Φem = 0.001−0.08 depending on the solvent), thus making CPL non-observable in this system. Some BINAP-substituted BODIPY showed AIE properties. The zwitterion 86 displayed AIE with red fluorescence when dispersed into a polystyrene matrix polymer.180 Its B−N bond is rather labile and is sensitive to hydrolysis, competitive coordination, or Lewis bases. Its Φem reaches 4% in the solid state. It was used for Lewis acids/bases enantiomer pairs recognition, such as menthol or methylbenzylamine, but more interestingly it was used to study the microscopic morphology of a blend of a coordinating polyethylene glycol (PEG) polymer and a non-coordinating polystyrene (PS) polymer. The oxygen atoms of the PEG chains coordinated to the nitrogen and quenched the luminescence in the PEG domains, while the PS

The Ortiz and de la Moya groups examined the properties of the helical bis-BODIPYs 78 and 79 (Figure 35).172−174 These helical molecules do not aggregate in solution up to mM concentrations in chlorinated solvents. The two BODIPY moieties experience an efficient exciton coupling, computation showing electron transfer from one BODIPY unit to the other (Figure 36). The helical conformation of the molecule induces a

Figure 36. (Left) Calculated preferential conformation for R,R-78. (Right) CPL (top, solid line = R,R enantiomer; dotted line = S,S enantiomer) and fluorescence spectra (bottom, degassed 1 mM CHCl3 solution). Adapted with permission from ref 173. Copyright 2016 John Wiley & Sons.

mutual chiral orientation of the two moieties and therefore a strong rotational strength. |gem| peaks at 0.001, but interestingly its sign changes between R,R-78 and R,R-79. According to calculations, both molecules adopt the same helical conformation in solution. Such a small changegoing from an amino to an oxo bridgeproduced a complete change of the CPL sign. Despite calculations and studies on the influence of solvent polarity on the excited state, the exact reason for this change remains unclear. The influence of the organization of the aromatic surrounding the BODIPY core is also well illustrated in the work of Mori and co-workers.175 The perphenylated BODIPY 80 (Figure 35) is surrounded by small chiral aliphatic chains. By decreasing the temperature, a propeller-like chirality is induced in the benzene rings, which, due to the steric hindrance, should be tilted by ca.

Figure 37. (a) UV−visible spectra and (b) CD spectra upon reduction of 82 in CH3CN. (c) Cyclability of the process followed by CD (n = number of cycles). Adapted with permission from ref 176. Copyright 2000 John Wiley & Sons. T


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Figure 38. (a) CD spectra of R-88 in THF/H2O mixtures at 1.0 × 10−5 M. (b) CPL spectra of R/S-88 in THF/H2O mixtures at 1.0 × 10−5 M. SEM images for S-88 in THF/water 40/60 (c) and 5/95 (d). Adapted with permission from ref 182. Published 2016 Open Access by the Royal Society of Chemistry.

domains remained red under the fluorescence microscope. Similarly, Cheng’s group developed BINAP-substituted BOPHY (bis(difluoroboron)-1,2-bishydrazine) dyes bearing the AIE inducer tetraphenylethylene (TPE), 87 and 88 (Figure 35).181,182 Fluorescence from the π−π* transition of the BODIPY or BOPHY moiety was triggered by excitation in the TPE π−π* transition around 330 nm, to avoid the selfabsorption caused by the small Stokes’s shifts of these boron dyes, thanks to an intramolecular energy transfer. In DCM/ hexane mixture an AIE is clearly observed, the fluorescence quantum yield going from 0.014 in DCM to 0.582 in DCM/ hexane 10/90 for 87, and from 0.11 in pure DCM to 0.31 in DCM/hexane 5/95 for 88. In THF/water mixture, the compounds precipitated, and the aggregation caused quenching of the fluorescence. From scanning electron microscopy (SEM), the formation of nanowires and nanorods was observed for 88 in THF/water mixtures (Figure 38). Interestingly, following this aggregation by CD upon increasing the water content showed the reversion of CD signals, which was attributed to the axial chirality transfer to the supramolecular helical nanowires. The helical nanowires formed at high water content were not fluorescent, but for a water content of 40%, the nanorods of 88 were emissive, with a CPL of 5.5 × 10−4, similar to the value in DCM/hexane mixtures. For 88 in DCM/hexane, the |gem| reached 2 × 10−3. Axial chirality can also be generated from the arrangement in a nonplanar fashion of chemical moieties located on both sides of a chemical bond, arising from the restricted rotation around this

bond. Among atropisomers, binaphthyl derivatives are surely some of the most well known and widely used (vide supra), but chirality based on atropisomery of BODIPY dyes was also investigated.183−186 Compound 89 shows very weak luminescence (Φem < 1%),184 while compound 90 is described as highly luminescent and showing proper Cotton effect;183 unfortunately, neither the luminescence nor the CPL spectra have been recorded. On the other hand, compounds 91 and 92 (Figure 39) have been investigated in depth, the enantiomers being separated on a chiral HPLC phase.185,186 91 in particular showed a strong Cotton effect, due to a strong coupling of the transition dipole moments of the BODIPYs. The respective enantiomers of 91 and 92 exhibited mirror images in CPL and ECD (Figure 40), with a respectable |gem| for 91 of 3.8 × 10−3 and a weaker one of 4 × 10−4 for 92. Extensive DFT calculations on the ground and excited states of the molecules revealed that 92 is more flexible than 91, leading to the co-existence of several conformations and weaker chiroptical properties for 92. Hall’s group produced the family of N,N,O,O compounds 93 and the compound N,N,O,C 94 boron dipyrromethenes, aiming to develop highly efficient CPL emitters in the red region of the visible spectrum.187 For 93-H in CH3CN, the Φem reaches 0.65 and |gem| 4.7 × 10−3, while |gem| remains at 3.7 × 10−3 for 94. Calculations performed on the excited states outlined the importance of the exciton chirality rule as well as the fine interplay between the electric and transition dipole moments and the angle between them to ensure a high dissymmetry factor for the emission.188 Following a similar design principle with U


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Figure 39. Boron derivatives 89−99.

rather simple method allows one to change the CPLE sign by changing the solvent chirality. Modulation of the chiroptical properties using a chemical effector, more specifically anion or cation binding and ion pair formation, was also achieved in attempts to obtain chiral responsive materials. The chiral azacrown 97 was designed to bear a luminescent BODIPY moiety.192 The fluorescence quantum yields were low (∼0.5%) in the case of the free ligand, but upon addition of cations in MeCN, significant luminescence enhancement (up to a factor of 10) was obtained in most cases. Unfortunately, 97 showed no preferential recognition for chiral ammonium cations due to its relative high flexibility, and one can assume that, for a similar reason, the CPL induction would have been low in this system. Maeda and co-workers synthesized the flexible anion receptor 99 (Figure 39), where chirality is induced by BINOL moieties linked to the boron atom.193 Although the Φem reaches 0.51 in DCM, almost no CPL for the free molecule was observed. Upon complexation of MeCOO− or Cl−, |gem| rose to 2 × 10−3 (Figure 41). By using chiral bis-binaphthylammonium as counterion (Figure 39), Haketa et al. showed the influence of the counterion on the chirality of the helix formed by the complexation of Cl− (or Br−) by 98.194 Low-temperature CD spectra and NMR suggested the formation of diastereomeric

N,N,O,O boron dipyrromethene, Saikawa et al. synthesized a figure-eight compound, 95 (Figure 39).189 The twisted structure caused by the tetrahedral geometry of the boron promoted the chiroptical properties of 95 (Φem = 0.58, and |gem| = 9 × 10−3), with an emission maximum in the red region (λ = 655 nm). Notably, its non-borylated precursor has its emission quenched (Φem < 0.01). Moraleja et al. also used dimers of N,N,O,O BODIPY, where the boron atoms were connected by tartaric acid, in 96 (Figure 39).190 The various isomers (R,R-, S,S-, and meso-R,S-) commercially available for tartaric acid allowed the exploration of the influence of the relative spatial orientations between the two moieties on the optical and chiroptical properties, while inducing the chirality at the same time. The dyes showed quantum yields in the 80−90% range, except in the case of R,RH-96, where the quantum yield decreased to 33% due to intramolecular excitonic interaction upon photoexcitation. On the other hand, the less flexible R,R-Me-96 showed a bright emission from a J-type intramolecular aggregate. Interestingly, circularly polarized laser emission (CPLE) was observed in ethyl acetate with a high degree of light polarization (DOP) close to 1. Notably, CPLE was also observed with achiral BODIPY dyes dissolved in optically active limonene by the group of Cerdán.191 They observed a moderate DOP between 0.04 and 0.11. This V


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Figure 40. Absorbance and emission spectra for 91 (a) and 92 (c) and ECD and CPL spectra for 91 (b) and 92 (d) in 5 × 10−5 M DCM solutions. Reprinted with permission from ref 186. Copyright 2016 John Wiley & Sons.

pairs through ion-pair formation: R,R+ and S,S+ bis-BINAP combined with a P or M helix (Figure 42). Cation exchange occurs faster than the racemization between M and P helices. Despite modest diastereomeric excess (24% for 98-Cl−/R,R+ in DCM), the CPL dissymmetry factor for these highly luminescent complex is 8.4 × 10−3 for the chloride complex and 1.3 × 10−2 for the bromide complex in DCM (Figure 42), and increased to 1.8 × 10−2 and 2.1 × 10−2 in DCM/octane, respectively.

vibrations. For example, Dai and co-workers studied several tetraphenylene (TPE) chiral molecules displaying AIE and having thiourea moieties for chiral guest sensing and recognition.195,196 Along with TPE, fluorescent silole derivatives represent one of the most common motifs for AIE materials. It was proposed that the intramolecular rotation of the phenyl rings surrounding the silole core plays a prevalent part in the non-radiative energy transfer process, and that these rotations are prevented in the packed state.197 These molecules, highly fluorescent in the solid state, exhibit CPL upon induction of chirality on their side chains. The first example in this regard is provided by the groups of Wong and Tang, who synthesized silole 100 substituted by chiral mannose side groups (Figure 43).198 Aggregation under controlled conditions is directed toward the formation of right-handed helical nanoribbons that further assemble in micrometer-sized right-handed ropes, as observed by SEM and TEM. Upon aggregation, the quantum yield rises from 0.6% in pure DCM to 81.3% in thin films. The CPL dissymmetry factor depends highly on the external conditions: In heterogeneous suspension or cast films, gem is between −0.12 and −0.08, while for dispersions in a poly(methyl methacrylate) (PMMA) matrix, it reaches values between −0.17 and −0.13.

3.4. Siloles for AIE-CPL

Most of the luminescent single molecules have their quantum yields reduced or even obliterated upon aggregation through the formation of excimers by π−π interactions. This phenomenon is called aggregation-caused quenching (ACQ). Classically, planar molecules with a π-conjugated core would pack closely and present ACQ due to non-radiative energy transfer pathways. The antagonistic effect, namely the appearance or the enhancement of luminescence upon aggregation, already introduced above, is aggregation-induced emission (AIE).13,167 Typically, non-planar molecules with a π-conjugated core would see their intramolecular motions slowed or stopped upon aggregation, thus preventing a non-radiative energy transfer from the excited state to the ground state to occur via intramolecular rotations or W


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Upon evaporation of a DCM/toluene solution in microfluidic channels, gem climbs to −0.32 (Figure 44). Induction of chirality in the supramolecular aggregate is primordial to enhance the CD and CPL dissymmetry factors. Thiourea-functionalized silole 101 shows neither CD nor fluorescence in solution in a good solvent.199 In poor solvents or thin films, it reaches an aggregated luminescent state (Φem up to 95% in thin films) but still shows no CD nor CPL, despite its pendent chiral phenylethanamine groups. Enantiopure mandelic acid or phenylacetic acid can complex with the compound, thus inducing a chirality transfer and engendering a CD signal around 370 nm, corresponding to the silole absorption in the UV region. In thin films of 101 and R-(−)- or S-(+)-mandelic acid (in a molar ratio of 1/40), CPL is detected, and gem reaches values of about −0.01 and +0.01 for the R-(−) and S-(+)forms, respectively (Figure 45). The large amount of acid used for the thin films’ formation implies that the acid might form crystalline domains, thus generating a chiral environment inducing a supramolecular helical arrangement of the silole moieties. This kind of chiral induction was also exploited in liquid crystals (vide infra). By using chiral amino acids as side groups for chiral induction, silole derivatives showing both AIE and CPL were obtained, as for compounds 102 and 103 (Figure 43).200,201 102 is functionalized with valine−menthol moieties, while 103 bears methylated leucine groups. In solution, the CD and fluorescence quantum yield are negligible (Φem = 0.33% for 102 in THF and almost non-emissive for 103 in DMSO or DMSO/H2O mixtures with the water fraction f w below 30%). With increasing fractions of water, Φem is enhanced in both cases to 18.9% at f w = 90% in THF; it even reaches 80.3% in thin films for 102 (Figure 46a) and shows a 57 times enhancement for 103 at f w = 90% in DMSO compared to a pure DMSO solution (Figure 46b). 102 showed CPL with gem reaching −0.016, the sample preparation having proven once again primordial: the best CPL signals were obtained by evaporation of DMF in microfluidic channels. 103 displayed an averaged dissymmetry factor of −0.05 in thin films obtained by evaporation of 1,2-dichloroethane (DCE). In both cases, the obtained materials were studied by imaging techniques such as SEM or atomic force microscopy (AFM). Both compounds showed helical assemblies, and in the case of 102 the pitch showed a dependency on the poor solvent polarity (right-handed in THF/hexane and left-handed in THF/water mixtures). The interplay between hydrophobic and hydrophilic parts of the molecule and the surrounding solvent is subtle: the exact packing manner of the molecules should be determined with theoretical simulation. For 103, helical arrangements were also observed by fluorescence microscopy (Figure 46d). Functionalization of a chiral BINOL core with two silole moieties (Figure 47) was investigated by Yang et al.202 The compound displayed CD and fluorescence enhancement upon aggregation induced by increment of the water content in a THF solution of 104. Notably, similar binaphthol derivatives functionalized with TPE displayed aggregation-annihilated CD.203,204 This opposite effect is caused by the twisting of the naphthalene rings. Furthermore, the luminescence for 104 was quenched upon addition of picric acid, whose detection was possible from 1 ppm concentration. It was hypothesized that the acid intercalates between the phenyl substituents of the silole and induces the de-aggregation of the helices. Surprisingly, the achiral hexaphenylsilole 105 (Figure 47) formed chiral helical aggregates and showed the appearance of CD and CPL upon aggregation, without the addition of chiral

Figure 41. Spectral changes for 99 upon TBACl addition in (a) UV− visible (bottom) and CD (top) (1.0 × 10−5 M in DCM) and (b) fluorescence (bottom) and CPL (top). For UV−visible, 50 equiv of TBACl was used, while in CD, fluorescence, and CPL measurements, 200 equiv was used. Insets shown pictures of the corresponding solutions without and with irradiation. Adapted with permission from ref 193. Copyright 2011 American Chemical Society.

Figure 42. (a) Interconversion between the M- and P-helices and the ion-pair cation exchanges. (b) CPL (top) and fluorescence (bottom) spectra in a 1 mM DCM solution (λexc = 483 nm) of 98-Cl−/R,R+. Adapted with permission from ref 194. Copyright 2012 John Wiley & Sons. X


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Figure 43. Chiral silole derivatives 100−103 used for AIE-induced CPL.

additives in the media or the use of chiral external stimuli.205 While 105 does not show CD nor CPL in solution, it does so in the thin film state after evaporation from a THF solution. The CPL dissymmetry factor gem is in the 0.001 to −0.0125 range. AFM displayed nanotubes and left-handed self-assembled helical fibers, whose intertwining formed left-handed superhelices up to several micrometers in length. Similar spontaneous appearance of chirality was observed by Dong et al. when 105 was adsorbed on Cu(111) and Ag(111) surfaces.206 The molecules form extended 2-D domains, whose basic structures are U-shaped clusters made by six molecules of 105. Here, the left-oriented and right-oriented U-clusters are both present on the surface and are arranged in parallel zigzag chains.

of copper(I) halide bridged complexes, 108, bearing chiral chelating bis(phosphine) ligands.208 These Cu2X2 (X = F, Cl, Br, or I) complexes exhibited yellow-green light emission upon UV irradiation in the solid state, with quantum yield below 1% for the fluorine and chlorine complexes, 1.7% for the bromine, and 3.9% for the iodine complexes. The emission originates from a (Cu + X)LCT excited state according to DFT calculations. Unfortunately, for these two examples, no CPL data were recorded. A cluster constituted by three Au atoms and surrounded by chiral 2,2′-bis(di-p-tolylphosphino)-1,1′-binaphthyl ligands 109 showed no emission but strong CD in DCM solution. Upon addition of hexane, the discrete cluster aggregated into nanocubes that displayed luminescence centered around 583 nm with a Φem peaking at 3.6% for a hexane fraction of 70%.209 In the aggregated state, the restricted intramolecular rotation of the tolyl moieties blocks the non-radiative decay, thus allowing the generation of a CPL signal with |gem| = 7 × 10−3, with an emission from a 3MMLCT state. By employing Pt(phenyl-2,2′bipyridine) complexes bearing chiral pinene moieties, and linking two of them with a bis-phosphine, Zhang et al. obtained chiral Pt−Pt dimers 110.210 The complex displayed luminescence around 600 nm in solution at room temperature with gem ≈ 5 × 10−3. Notably, the length of the alkyl chain (either a −CH2− or a −CH2CH2−) separating the two Pt centers has a strong influence on the CPL spectra: the CPL emissive state was

3.5. Chiral Phosphines

Chiral phosphines with point or axial chirality have been widely used as ligands, in particular for asymmetric catalysis. Nevertheless, examples exploiting the luminescence of the complexes to generate chiroptical properties remain scarce (with the exception of lanthanide complexes, which are beyond the scope of this Review). Using a phosphine-substituted carbohydrate (altropyranose) and phenylpyridine, Che and co-workers synthesized cyclometalated Pt(II) complexes 106 and 107 (see Figure 48) that have a broad emission band centered around 510 nm with a Φem reaching 5%.207 The formation of a 5membered rigid chelate is primordial in the rigidity increase observed and the appearance of a stable excited 3MLCT state in solution at room temperature. Gibbons et al. synthesized a series Y


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Figure 44. Mannose-functionalized silole 100. (a) Quantum yield in DCM/hexane solutions as a function of the percentage of hexane (c = 10−5 M, λexc = 356 nm). The inset picture shows a DCM solution, a 90% hexane solution, and the solid sample under irradiation (λexc = 364 nm). (b, c) Fluorescence microscopy images under UV irradiation of a sample prepared by evaporation of DCE (b) and by evaporation of DCM/toluene (c) in microfluidic channels. (d) (IL− IR) and (e) gem as a function of wavelength and under various conditions (a DCM 2 × 10−4 M solution, a neat cast film from DCE 2 mg/mL evaporation, a suspension in 2 × 10−4 M DCM/hexane 1/9, a 10 wt% dispersion in PMMA, and a fabricated pattern from evaporation of DCM/toluene in microfluidic channels). Adapted with permission from ref 198. Copyright 2012 Royal Society of Chemistry.

Figure 45. CD spectra of thiourea-functionalized silole 101 (compound 1 in ref 199) in (a) THF solution and (b) thin films in the presence of chiral acids ([101] = 1 mM and [acid] = 40 mM before evaporation). Reproduced with permission from ref 199. Published 2014 Open Access by the Royal Society of Chemistry.

of polymers has advantages over small molecules: good film formation, better control over the 3-D arrangement, and scalability.213,214 Several strategies exist for induction of chirality in polymers or in material: the introduction of chiral moieties in the polymer backbone or side chains, the placement of a luminescent polymer in a chiral solvent or a chiral nematic liquid

either 3MMLCT or 3MLCT, and the sign of the CPL activity was opposite. 3.6. Macromolecular and Supramolecular Chiral Materials

In general, polymerization or oligomerization of an optically active monomer strengthens its chiroptical properties (1 order of magnitude higher than SOM on average).211,212 Plus, the use Z


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Figure 46. (a) Fluorescence quantum yield vs water content in THF for 102 (c = 10−5 M). (b) Relative emission peak intensity at λ = 490 nm (I/I0) vs water content in DMSO for 103. (c) AFM for 102 upon evaporation of a 10−3 mg/mL THF solution. (d) Fluorescence image for 103 after evaporation of a 10−4 M DCE/hexane 1/1 solution, with a SEM image of the same sample inset. Adapted with permission from ref 200 (copyright 2014 Royal Society of Chemistry) and ref 201 (copyright 2016 John Wiley & Sons).

properties. A dependence between the g values and the material thickness is therefore observed. In this case, the g value no longer depends only on the difference in the probability for the absorption or emission of left- and right-handed polarized light at the molecular site.215 3.6.1. Polymers and Oligomers. 3.6.1.1. With Boron. Boron dipyrromethene derivatives have been widely investigated for their chromophoric properties, as strong absorbers and good emitters. As stated above, single-molecule chiral BODIPY derivatives have been investigated for their circularly polarized light emission properties. Several chiral polymers including boron have thus been designed. One of the most common axial chirality inducers is the well-known binaphthyl, or BINAP. Wu et al. synthesized BODIPY/BINAP polymers 111 by Sonogashira cross-coupling, tuning the emission wavelength by attaching styryl groups to the BODIPY fragment (Figure 49).216 They show moderate Mw (in the 7800−10 900 g·mol−1 range) and a polydispersity index (PDI) around 2.0. Despite a low quantum yield between 7 and 15% for the four polymers, the CPL dissymmetry factor gem is in the 0.06−0.08 range. Cheng and co-workers investigated the influence of the BINAP dihedral angle on the light polarization, on compounds 112-c (109° dihedral angle), 112-d (79° dihedral angle), and 112-e (56°

Figure 47. Binol derivative 104 and perphenylated silole 105.

crystal to induce chirality, or the addition of chiral molecules as chirality inducers in an achiral polymer matrix. It is worth noting that, for aggregated materials, the longrange order can have a strong influence on the chiroptical AA


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Figure 48. Chiral phosphine derivatives 106−110.

dihedral angle).217 Mw values were in the 11 700−15 250 g· mol−1 range, and the PDI was around 1.45 for the three polymers. Both the ECD and CPL signals showed a strong influence on the dihedral angles, the absorption dissymmetry factor going from 0.0012 for 112-c to 0.0027 for 112-d, and |gem| going from 0.001 for 112-c to 0.002 for 112-d and -e. Notably, a similar structure was obtained using 1,2-diaminocyclohexane as chiral inducer, either in its imine form 112-a or in its reduced amine form 112-b.218 Again, the flexibility and angles were shown to be primordial factors for obtaining chiroptical properties, as 112-a had a proper |gem| = 0.0015, while the more flexible 112-b showed neither ECD nor CPL. Another factor of importance for tuning the luminescence of the material is its self-aggregation behavior. Nagai et al. showed that, for polymer 113, mixing fully R and fully S polymers had a strong impact on the aggregation properties.219 While S-113 had a quantum yield of 0.81, a 30/70 S/R polymer mixture sees its Φem rising to 0.98. SEM revealed that the pure polymers formed fiber-like aggregates, while in a 30/70 ratio, spherical particles in the 0.5−1.2 μm range were observed. Similarly, in 114-a−c, the CPL dissymmetry factor for the monomer is below 0.01, while in the polymers it increases up to ca. 0.30.220 These high values can be explained by a π−π stacking between the polymer chains and a determined chiral environment around the polymer backbone. Aggregation was observed upon heating and cooling the solution or changing the solvent polarity, causing variations in the CPL intensities. High |gem| values of 0.349 and 0.105 were also measured for salen-BF2 derivatives 115-a and -b, respectively.221 These values are much higher than that of the precursor monomer (|gem| = 0.042). The derivatives have moderate Φem values of 0.37 and 0.32. Despite being measured in solution at 10−5 M, in this case

the organizing role of the polymer backbone seems to be primordial for an efficient chiral induction. The chiral conjugated polymer 116 (Figure 49) has been shown to aggregate in nanoparticles (NPs) upon rapid mixing of water in a THF solution of 116 under ultrasonication.222 The chiroptical properties depend upon the sizes of the NPs: while the CD decreased upon particle size increase, the fluorescence dissymmetry increased. The NPs were used for cell fluorescence imaging, an application also met by chiral BODIPY SOMs.223 Triptycene was also used recently by Swager, Ikai, and coworkers as chirality inducer in polymers such as 117.224 Triptycene-based polymers do not form higher order aggregated structures, and therefore it was assumed that the chiral information on the triptycene was only transmitted to the adjacent units. The polymers also showed no dependency of their chiroptical properties on external factors (solvent, temperature, or concentration), implying that the angles between a pair of transition dipole moments were fixed around the robust chiral unit. The luminescence dissymmetry factor reaches around 0.001 in absolute value. 3.6.1.2. With Sulfur. Thiophenes and polythiophenes have been thoroughly investigated for their fluorescence properties (in addition to their conduction properties, vide supra), in particular in the visible red and NIR region of the electromagnetic spectrum.225 A straightforward synthesis and the ease of functionalization allow researchers to tune their emission properties, as well as to introduce chirality on their backbone. One way to introduce chirality on thiophene-based polymers is to add chiral alkyl side chains (Figure 50). Meijer and coworkers synthesized the chiral polythiophene 118, which is ordered in n-decanol at room temperature by the formation of small aggregates, while keeping a |gem| on the order of 0.005.226 Notably, in DCM, where the polymer does not aggregate, no AB


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Figure 49. Polymeric boron derivatives 111−117.

Cotton effect was observed. The relationship between CD and CPL properties and the thickness of spin-coated thin films of polythiophene 119 was studied by Lakhwani et al.215 One should indeed take particular note of the fact that the circular polarization originates in some cases in the long-range order of the material rather than the different probabilities for the emission of a left- or a right-handed photon. 119 showed a weak dependency of its gem and gabs values on the film thickness, meaning that the circular polarization comes directly from the differential probability in left and right light absorption and emission. In contrast, fluorene/thiophene polymers 120 showed a strong dependency of their gem and gabs values on the film thickness, implying a long-range order correlation in these cases, with values up to 0.2 for |gem| and 0.3 for gabs in 120-a (Figure 51).227 Notably, a similar effect was observed in NPs in 116 (vide supra, part 3.6.1.1 on boron polymers) where the CPL increased along with the increase in the NPs’ size.222 The direct influence of CD absorption on CPL spectra when absorption and emission spectra overlap was investigated by Abbate and coworkers with the polymer 121.228

Chiral polypropylenedioxythiophene 122 displayed a strong dependency of its chiroptical properties on its aggregation, caused by a poor solvent, by the solution temperature, or by the annealing temperature of thin films of 122.229 After annealing, thin films of 122 had gabs as high as 0.97, but no CPL was recorded. Remarkably, by sandwiching a thin film of 122 in a three-electrode system and applying a voltage, the yellow fluorescence is switched on and off by varying the potential, due to the reversible oxidation of the polymer. Akagi and co-workers synthesized the photoresponsive chiral polymers 123 in order to obtain a reversible CPL emissive material.230 In solution, no chiroptical effect was observed, but in thin films, gem on the order of 10−2 was observed, with either blue or green luminescence (insets Figure 52). Interestingly, upon irradiation with UV light for ca. 1 min, the fluorescence was quenched (from 0.65 to 0.005 for 123-a,b and from 0.23 to 0.01 for 123-c,d), and it was regenerated upon visible light irradiation because of the reversible photoswitching of the dithienylethene moiety. Ergo, the CPL signal can be switched on and off upon photoirradiation of the thin films. AC


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Figure 50. Sulfur-containing polymers 118−127.

thiophene oligomers.233 Unfortunately, no CPL data were recorded for these three polymers. Another way to introduce chirality in thiophene-based polymers is to introduce it directly in the polymer backbone, mainly achieved using axially or planar chiral molecules. A cyclophane unit alternately polymerized with quaterthiophene yielded the chiral and optically active polymer 127.234 The polymer apparently did not form higher order structures in 10−6 M CHCl3 solution in the excited state, and the |gem| remained between 4 × 10−4 and 5 × 10−4. The Ikai group developed a class of chiral polymer based on the natural motif of elligitannins: a biphenyl linked to a sugar, thus having both chiral centers on the sugar moiety and axial chirality on the biphenyl part.235−237 Polymerization of these moieties with mono-, di-, or terthiophene or with a thieno[3,4b]thiophene yielded the compounds 129a−d, respectively (Figure 53). The formation of helicoidal structures is triggered by the solvent and dependent on the length of the polymer. Typically, for 129a−c, ca. 15−25 repeating units are necessary to stabilize the helical structure, as the chain length required to ensure the folding decreases with a decreasing number of thiophene rings in the chain. In CHCl3, no strong chiroptical effect could be observed, but the addition of CH3CN promoted the folding and an enhancement of CD and CPL signals. In thin films, the helical chirality is also retained, as indicated by the luminescence dissymmetry factor between 0.9 × 10−2 and 1.9 ×

Figure 51. (a) Emission (bottom) and CPL (λexc = 380 nm) for an 80nm-thick film of 120-a and (b) gem and gabs values as a function of the film thickness. Reproduced with permission from ref 227. Copyright 2012 The Chemical Society of Japan.

Fronk et al. studied the relationship between the phenyl/ thiophene rotational barrier and the strength of the Cotton effect in polymers 124, 125, and 126.231,232 Aggregation of the polymers was caused by changing the temperature or adding a poor solvent. 125 has the highest rotational barrier (9 kcal/mol) and did not retain its chirality in thin films, because a helical conformation of the polymers could not be reached. On the other hand, 124 and 126 have rotational barriers around 4 kcal/ mol, giving a more flexible backbone to the polymers, able to accommodate a helical conformation. Panda and co-workers performed calculations to study similar behaviors with pyridineAD


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Figure 52. (a) Fluorescence spectra and (b) CPL spectra of 123-a,b. (c) Fluorescence spectra and (d) CPL spectra of 123-c,d (thin films). Insets: Photographs of thin films of 123 under irradiation with λ = 370 nm wavelength. Reproduced with permission from ref 230. Copyright 2010 John Wiley & Sons.

Figure 53. Sulfur-containing polymers 128−133.

10−2, while in CHCl3/CH3CN mixture, the reduced order in the polymer chain left a |gem| between 3.2 × 10−3 and 4.5 × 10−3. For 129-d the helix is further stabilized by hydrogen bonds between

the pendant amide groups, and the presence of only ca. six repeating units (less than two helical turns according to molecular modelizations) is sufficient for helix stabilization. As AE


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Figure 54. Silicon-containing polymers 134−139.

addition of R- or S-limonene in a good solvent/poor solvent mixture, with modest |gem| = 3 × 10−4.242 Further measurements underlined the importance of the sample homogeneity, as CPL signal is dependent on the size and morphology of the aggregates.243 Another chiral polysaccharide polymer known to adopt a helical conformation was used for inducing chirality in polythiophene, reaching a CPL dissymmetry factor of 0.0045 in solution and on the order of 10−3 in the solid state.244 Fujiki and co-workers showed that an achiral thiophene/benzothiazole polymer was endowed with chirality from a chiral polysilane polymer template.245 After removal of the chiral template, the thiophene/benzothiazole polymer retained its chirality and chiroptical properties with |gem| ≈ ±0.02. 3.6.1.3. With Silicon. Silicon-based molecules and polymers have been known to display a high activity in optics. In particular, polysilafluorenes have received a great deal of interest, along with alkylpolysilanes. Asymmetric catalysis to access inherently chiral small silole or silafluorenes has been devised.246,247 Polysilafluorene oligomers were studied by Chen et al. using DFT computations, which showed that polysilafluorenes with ortho and para linkage would be the most effective for chiral luminescent polymers.248 Dai et al. used the silole/Eu(III) mixed polymer 134 (Figure 54) for emission in the red region of the visible spectrum.249 Although the polymer is achiral, chiral induction by coordination of N-Boc proline (L or D) on the Eu(III) center induces a chiroptical response due to the direct chirality induction on the metal-containing emissive part of the polymer. Based on the CD sign, the enantiomers of the amino acids could be discriminated. Poly(diphenylacetylene)s constitute a class of polymers known to be highly emissive and to adopt a non-planar, twisted geometry in solution and in the solid state. Chiral side groups could be introduced by functionalization of the phenyl rings, among which silicon tetrahedral chirality has been used, such as in 135 and 136.250,251 136 shows a good quantum yield of 56% in solution and 6.2% in films, and, interestingly, showed an increase in its CD signal upon annealing at 80 °C up to gabs =

a result, 129d retains a well-defined conformation in solution as well as in thin films, with |gem| reaching 1.6 × 10−2 in CHCl3 and 1.0 × 10−2 in thin films. Binaphthyl derivatives are also popular chiral inducers. These moieties have been introduced in polymer backbones, with either phenothiazine or benzothiadiazole as the luminophores in 128 and 130 (Figure 53), respectively.238,239 For 128, ACQ of the luminescence is observed upon addition of hexane above 40% v/v as the poor solvent in a DCM solution of the polymer. Below 40% hexane, the material kept its luminescence and exhibited |gem| ≈ 0.001. In contrast, 130 possesses a TPE moiety that is known to induce AIE upon addition of a poor solvent, typically water. In 130, two Förster resonance energy transfer (FRET) pairs co-exist: one fluorine/TPE and the other TPE/ benzothiazine, allowing a red emission from the benzothiazine with quantum yields reaching 0.20 in THF/water 20/80. The CPL dissymmetry factor in the red region of the visible spectrum reaches 0.002 in the aggregated state in solution. By using triptycene as the chiral inducer, Ikai et al. showed that the chirality was transmitted to the luminophores’ thiophene derivatives in 131a,b.224 In the same study the BODIPY compound 117 (Figure 49) was also developed and investigated. Variations of the luminophores promoted the emission of circularly polarized light of various wavelengths, from blue to red, with |gem| ≈ 0.001. Thiophene oligomers such as 132 or 133 (Figure 53) have also shown chiroptical properties.240 While 132 displays strong luminescence and Cotton effect, the CD signal in 133 is about 1 order of magnitude smaller than in 132. As a result, only 132 displays a measurable |gem| between 0.001 and 0.002. Oligomers of 28 (Figure 23, vide supra) exhibit axial chirality and therefore produce CD and CPL signals.241 Calculations matched the experimental data and evidenced the promising properties of this family of compounds not only for their electrical but also for their chiroptical properties. Supramolecular induction of chirality by the environment was achieved for achiral polyfluorene/thiophene polymers by the AF


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Figure 55. UV−visible spectra (bottom) and CD spectra (top) upon annealing at 80 °C for various times of a 5 × 10−4 M toluene solution of 136. Reprinted with permission from ref 250. Copyright 2016 American Chemical Society.

Figure 56. LC molecules 140−149.

annealing on the chiroptical properties of m- and p-135 has been observed as well. While the UV−visible and luminescence spectra were not affected, the CD response was multiplied by a

0.0071 (Figure 55). The annealing process should allow the system to overcome the energetic barrier toward a more defined axial chirality of the polymer core. A similar positive effect of AG


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Figure 57. (a) XRD pattern for S-147 with (b) the proposed structure with the assigned reflections and (d) the helical π-stack of 147. (c) Fluorescence, circularly polarized fluorescence, and gem for 141, 143, and 147 as-cast films. (d) Schematic representation of the interchain helically π-stacked structure of 147 assembly. Reprinted with permission from ref 262. Copyright 2012 American Chemical Society.

3.6.2. Other Supramolecular Assemblies. 3.6.2.1. Liquid Crystals. Liquid crystals (LCs) are characterized by their unique ability to provide an ordered structure while keeping a dynamic and flexible nature. More specifically, chiral phases such as the chiral nematic, chiral smectic C, twisted grain boundary, or blue phase LC can be obtained upon self-assembly of chiral LC molecules. One should distinguish between active liquid crystalline devices, that actually emit directly circularly polarized light, and passive liquid crystalline devices, that reflect circularly polarized light at a wavelength close to their helical pitch, as demonstrated by Meskers and co-workers in chiral nematic LCs based on benzothiadiazole,256 or Nabiev and co-workers, who obtained high |gem| = 1.3 by embedding CdSe/ZnS quantum dots within a cholesteric liquid crystalline phase.257 In these cases, the propagation of one handedness of the circularly polarized light is inhibited through the material. For instance, the R- or S-140 molecule (Figure 56) showed a |gem| value up to 1.5 at its maximum.258 The position of the filtering stop band is modulated by the helical twist, and therefore the R/S ratio, thus allowing the tuning of the stop band between UV and infrared. Since the CPL in these systems is related to light transmission, the thickness of the film is impacting the |gem| = 1.2 and 1.5 for 5 and 9 μm films, respectively. The oligothiophene moiety confers it charge carrier transport property. By applying a

factor of ca. 4 upon annealing in toluene. The quantum yields were equal to 27.8 and 2.6% for p- and m-135, respectively. Polysilane polymers also display efficient photoluminescence properties, and chirality induction has been performed on these systems simply by the introduction of a pendent chiral group, as in the different 137 polymers (Figure 53).252,253 Fujiki and coworkers were the first to measure |gem| = 5 × 10−3 for a quantum yield Φem ≈ 1% for a chiral silane polymer.252 Later on, they studied the dependency of chiral polysilanes’ aggregation of the optical and chiroptical properties on the solvents and molecular weights of the polymers 137.253 More specifically, the aggregation of the polymers (with optimized good-to-bad solvent ratio and molecular weight of the polymer) induced the formation of ca. 5 μm particles with gem = −0.7 and Φem = 53% at room temperature. Supramolecular induction of chirality in the polymer backbone of polysilanes and silafluorenes was achieved with achiral 138 and 139.254,255 Achiral 138 gave CD- and CPLactive aggregates in a trisolvent mixture of CHCl3/R- or Slimonene/CH3OH. The sign and amplitude of the CD signal were dependent on the volume fraction of limonene, and |gem| increased to 0.23 in 138. Without the presence of the poor solvent CH3OH, no aggregates and therefore no chiroptical properties were observed in these systems. AH


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Figure 58. Structures of the compounds 150−152.

BINOL derivative, they move to a chiral structure which displays CD. 3.6.2.2. Supramolecular Polymers and Gels. Supramolecular ordering in supramolecular polymers built through noncovalent interactions is known to induce a chiral arrangement of the constituting parts. This strategy of synthesis by self-assembly avoids a sometimes complicated synthetic route. As stated above, BODIPYs are strong fluorophores. The small peptide diphenylalanine is a commonly employed chiral binding motif for the construction of supramolecular assemblies. Karikis et al. combined these two moieties to obtain spherical (for a Fmoc-protected molecule) or fibrillar (for Boc-protected molecules) assemblies of micrometric sizes in the proper solvent mixture.265 After drop-casting of the fibers, a strong Cotton effect was recorded, while the spherical assemblies remained silent. By using the proper solvent system, a gel can be formed. Yang et al. demonstrated that by mixing a well-known chiral nonluminescent gelatora Boc-protected glutamic acid derivativewith a luminescent achiral polymer such as a polysilafluorene, the obtained gel incorporated the polymer.266 The chiral co-gel forms chiral helical assemblies with right or left handedness which retain the blue fluorescence of the polymers upon UV irradiation. The chirality is transmitted to the luminescent polymer, with |gem| values in the range of 1.27 × 10−3−1.41 × 10−3. Interestingly, even after removal of the gelator by washing, the achiral polymer retained the chiral information, with |gem| in the 1.16 × 10−3−1.45 × 10−3 range. Following a similar procedure with a C3-symmetric chiral glutamic acid, several dyes covering the whole visible spectra (including a hexaphenylsilole derivative) were embedded into the chiral nanotubes formed upon gelation, giving inner aggregates with AIE.267 All of the dyes displayed CPL in the 10−3−10−2 range, and the silole derivative displayed |gem| = 0.2 × 10−3, with a quantum yield around 90%. The AIE in chiral gels was also observed with silole/cholesterol,268 with Ag+-based metallosupramolecular polymers,269 or in the recognition of chiral amines with boron esters.270 3.6.2.3. Macrocycles and Cages. Macrocycles and cages constitute an important part of supramolecular entities. Natural compounds such as cyclodextrins are known chiral macrocycles, able to encapsulate various hosts in their hydrophobic cavity. In this confined environment, the encapsulated molecules are

voltage, the helicity of the material is lost, and therefore the fluorescence is no longer polarized. LCs were also used to provide a chiral environment for the polymerization of polythiophene derivatives by Goto and coworkers. A cholesteric molecule induces chirality in a nematic LC phase, and achiral fluorophores are electropolymerized within this templating environment, thus forming a chiral thin film of polymer on the electrode.259−261 |gem| values up to 0.005 have been observed in these systems. Notably, heating of the obtained polymers with induced chirality has been shown in some cases to decrease the chiroptical properties by driving back the polymers into a non-chiral form.260 Watanabe et al. studied a wide variety of alternating phenyl/ thiophene-based polymers 141−147 bearing chiral side chains.262 In solutions and films, circularly polarized fluorescence was in the order of |gem| = 10−3−10−2, but upon annealing of the films at temperatures in the liquid crystalline region, the polymers further self-assembled following the higherorder structure provided by liquid crystallinity, resulting in |gem| values increased by ca. 1 order of magnitude up to −0.15 for 147. Annealing is believed to promote here the helical π-stacking of the polymer backbone into the chiral nematic phase. 141, 143, and 147 emit in the blue, green, and red regions of the visible spectrum, respectively (Figure 57). A mixture of the three polymers prepared as a cast film emits red fluorescence, due to the polymer aggregation and subsequent energy transfer between the polymers. 141, 143, and 147 were dispersed in a polystyrene matrix, and thanks to the different wavelengths for emission of the different polymers, circularly polarized white light was emitted with gem ≈ 10−3. The lower value was explained by the fact that the polymers are no longer aggregated in the matrix. 148 demonstrated a similar ability as chiral LC to produce a stop band to one handedness of the light and to form thin films of chiral nematic phase with |gem| of up to 1.77.263 By doping the chiral LC phase with a Coumarin 5 dye (2.5% weight ratio), the wavelength is shifted to the emission wavelength of the dye by non-radiative energy transfer originating from the LC luminescent host, while keeping a high |gem| up to 1.8. The sergeant-and-soldier effect is known to be an efficient method to achieve chirality in supramolecular assemblies, including LCs.264 Achiral polymers 149a−f are electroactive and display electrochromism as thin films. Upon mixing with a AI


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fundamental and applications points of view. Then, in section 3, chiroptical properties such as circular dichroism and circularly polarized luminescence (CPL) of heteroelement-based materials were targeted. Compared to the electroactive/conducting materials, a larger variety of heteroelements are involved in photoactive materials, which were discussed according to the type of material, i.e., molecular, consisting of heterohelicenes, boron-containing siloles, phosphines, and supramolecules including polymers and oligomers classified following the nature of the heteroelement, supramolecular polymers and gels, liquid crystals, macrocycles, and cages. The access to molecular and supramolecular materials with switchable chiroptical properties and high CPL activity depending on the nature of the heteroelement, its chemical environment and reactivity, and the self-assembly process represents an important field of active research, the development of which will certainly continue in the near future. The possibility to reach CPL dissymmetry factors comparable to those of lanthanide complexes by considering the large variety offered by the heteroelements and the richness of supramolecular chemistry in terms of self-assembly and aggregation phenomena impacting the photophysical properties, such as aggregation-induced emission, makes this field particularly interesting. Across this interdisciplinary Review, it was our ambition to demonstrate that combining chirality and heteroelements provides new avenues in the field of molecular materials and to address future challenges which we and other research groups consider to be of great importance for the field.

protected from undesirable interactions with the external media, and their shapes and conformations might be influenced by the cage shape. Tang and co-workers used the amphiphilic cage 150 to encapsulate thiophene diketopyrrolopyrrole (TDPP), the assembly keeping the luminescence properties of both the cage and the TDPP, which emitted white light in the aggregated state (Figure 58).271 The cage preserved TDPP from ACQ by inclusion in its hydrophobic cavity. Interestingly, the phenyl rings in 150 adopt a propeller rotational pattern, thus inducing chirality in the cage, and CD spectra in solution revealed an enantiomeric excess, so far neither explained nor transmitted to the guest. Kato et al. used chiral tetrahedral phosphorus for the construction of chiral macrocycle 151, which exhibited a quantum yield of 0.76 and |gem| = 0.002,272 while Shimada et al. used silicium-based cyclophane 152 as a green emitter in solution and in the solid state, with Φem = 0.05 and 0.09, respectively, and |gem| ≈ 0.0016.273

4. CONCLUSIONS AND OUTLOOK The field of chiral molecular materials is in continuous development, especially from the perspective of finding experimental evidence for theoretically predicted synergic phenomena between chirality and physical properties such as charge transport, magnetism, light absorption, and emission, and also to uncover new effects of fundamental scientific interest situated at the interface of chirality with diverse research fields. The pivotal role of heteroelements, commonly belonging to the main groups III−VI in the periodic table, in chiral materials endowed with conducting and optical properties has been thoroughly addressed in this Review. Chiral electroactive precursors and derived conducting materials classified in four families, i.e., tetrathiafulvalenes (TTF), metal dithiolene complexes, oligo-/polythiophenes, and heterohelicenes, have been discussed from the perspectives of the chirality input and the differences between the racemic and enantiopure forms and also between the enantiomers. As the reader could easily figure out, the first three families were exclusively based on sulfurcontaining compounds. Knowing the importance and the added value of selenium in conductors, especially those based on tetraselenafulvalene (TSF),274 the preparation of chiral TSF precursors and conductors appears to be an interesting future direction. In the family of metal dithiolene complexes, a realistic challenge would be the discovery of chiral metallic compounds, possibly as single components, while for both TTFs and metal dithiolenes access to superconducting materials remains elusive and requires a continuous motivation and endeavor from research groups in this field. Closely related to metal dithiolene single-component conductors are the stable neutral radicals which can show conducting properties275,276 and could possibly be decorated with chiral motifs. Electroactive heterohelicenes are appearing more and more frequently in the literature; therefore, one future challenge to address in the preparation of conductors is to show the helical packings. As mentioned in the Introduction on Chirality and Conductivity, in the emerging field of magnetochiral phenomena, there are plenty of opportunities for molecular electroactive and conducting materials to be investigated in these areas. Synergic effects such as electrical magnetochiral anisotropy (eMChA), inverse eMChA, photoconductivity with polarized light irradiation or with natural light under a magnetic field, and electrocrystallization under a magnetic field applied parallel to the direction of the current are of paramount importance from both

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. ORCID

Flavia Pop: 0000-0003-3524-9781 Nicolas Zigon: 0000-0002-7539-2921 Narcis Avarvari: 0000-0001-9970-4494 Notes

The authors declare no competing financial interest. Biographies Flavia Pop studied chemistry at the Babeş-Bolyai University, ClujNapoca (Romania), and obtained her Ph.D. in 2009 from the BabeşBolyai University and the University of Angers (France) under the joint supervision of Prof. I. Grosu and Dr. J. Roncali. She continued her research with a postdoctoral position in France with Dr. N. Avarvari in the field of molecular materials based on tetrathiafulvalene. She carried on as a postdoctoral researcher at The University of Nottingham (UK), working on chiral chromophores and their aggregation for active materials in solar cells with Prof. D. B. Amabilino. In 2017, she obtained a CNRS research position in Angers (France), where she is currently developing chiral molecules for conducting materials. Nicolas Zigon received his Ph.D. in 2013 from the University of Strasbourg, under the supervision of Prof. Mir Wais Hosseini, studying molecular turnstiles and metallo-organic networks. In 2014, he started a postdoctoral internship as a JSPS fellow in the group of Prof. Makoto Fujita, working on the development of the crystalline sponge method, and completed his training in supramolecular chemistry and supramolecular polymerization in the laboratory of Prof. Frank Würthner as a Humboldt Postdoctoral fellow. In 2017, he was recruited as an assistant professor at the University of Angers, where investigating the interactions between magnetism, conductivity, and chiralitymore specifically in helicene derivativesis his major research theme. AJ


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Narcis Avarvari is CNRS Director of Research at the Laboratory Moltech-Anjou, University of Angers (France). He was born in Iasi, Romania, where he studied at the University “Al. I. Cuza” before moving to the Ecole Polytechnique in Paris, where he obtained a master’s degree in organic and organometallic chemistry. He received his Ph.D. in chemistry in 1998 at the Ecole Polytechnique (France) under the supervision of Prof. F. Mathey and Dr. P. Le Floch. After a one-year postdoctoral stay at the ETH Zürich (Switzerland) with Prof. H. Grützmacher, he obtained in 1999 a permanent research position with the CNRS in Nantes (France), and then he moved in 2001 to the University of Angers. Since 2009, he also holds an associate professor position at the Ecole Polytechnique in Paris. He was awarded with the 2007 Prize of the French Chemical Society, Coordination Chemistry Division, and he was promoted to CNRS Director of Research in 2010. He currently heads a research team dealing with molecular materials, crystal engineering, and coordination chemistry, as well as self-assembly and chirality. For several years, he has been developing various projects related to the synthesis, coordination chemistry, and properties of chiral tetrathiafulvalenes, metal dithiolene complexes, and functional helicenes.

ACKNOWLEDGMENTS This work was supported by the CNRS, the University of Angers, and the National Agency for Research (ANR Blanc, ANR-15-CE29-0006, ChiraMolCo Project). The authors warmly thank all their co-workers on the papers cited in the references. REFERENCES (1) Wagnière, G. H. On Chirality and the Universal Asymmetry; WileyVCH: Weinheim, 2007. (2) Avnir, D.; Huylebrouck, D. On Left and Right: Chirality in Architecture. Nexus Network J. 2013, 15, 171−182. (3) Berova, N.; Nakanishi, K.; Woody, R. W. Circular Dichroism. Principles and Applications; Wiley-VCH: New York, 2000. (4) Pietrusiewicz, K. M.; Zablocka, M. Preparation of Scalemic PChiral Phosphines and Their Derivatives. Chem. Rev. 1994, 94, 1375− 1411. (5) Li, Y.-M.; Kwong, F.-Y.; Yu, W.-Y.; Chan, A. S. C. Recent Advances in Developing New Axially Chiral Phosphine Ligands for Asymmetric Catalysis. Coord. Chem. Rev. 2007, 251, 2119−2144. (6) Wei, Y.; Shi, M. Applications of Chiral Phosphine-Based Organocatalysts in Catalytic Asymmetric Reactions. Chem. - Asian J. 2014, 9, 2720−2734. (7) Otocka, S.; Kwiatkowska, M.; Madalińska, L.; Kiełbasiński, P. Chiral Organosulfur Ligands/Catalysts with a Stereogenic Sulfur Atom: Applications in Asymmetric Synthesis. Chem. Rev. 2017, 117, 4147− 4181. (8) Avarvari, N.; Wallis, J. D. Strategies Towards Chiral Molecular Conductors. J. Mater. Chem. 2009, 19, 4061−4076. (9) Ibanez, J. G.; Rincón, M. E.; Gutierrez-Granados, S.; Chahma, M.; Jaramillo-Quintero, O. A.; Frontana-Uribe, B. A. Conducting Polymers in the Fields of Energy, Environmental Remediation, and Chemical− Chiral Sensors. Chem. Rev. 2018, 118, 4731−4816. (10) Kane-Maguire, L. A. P.; Wallace, G. G. Chiral Conducting Polymers. Chem. Soc. Rev. 2010, 39, 2545−2576. (11) Mondal, P. C.; Fontanesi, C.; Waldeck, D. H.; Naaman, R. SpinDependent Transport through Chiral Molecules Studied by SpinDependent Electrochemistry. Acc. Chem. Res. 2016, 49, 2560−2568. (12) Gohler, B.; Hamelbeck, V.; Markus, T. Z.; Kettner, M.; Hanne, G. F.; Vager, Z.; Naaman, R.; Zacharias, H. Spin Selectivity in Electron Transmission Through Self-Assembled Monolayers of DoubleStranded DNA. Science 2011, 331, 894−897. (13) Mei, J.; Leung, N. L. C.; Kwok, R. T. K.; Lam, J. W. Y.; Tang, B. Z. Aggregation-Induced Emission: Together We Shine, United We Soar! Chem. Rev. 2015, 115, 11718−11940. AK


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