CL-200031 Received: January 15, 2020 | Accepted: February 15, 2020 | Web Released: February 20, 2020 Highlight Review Supramolecular Polymerization and Functions of Isoxazole Ring Monomers Takeharu Haino* and Takehiro Hirao Graduate School of Science, Hiroshima University, 1-3-1 Kagamiyama, Higashi-Hiroshima, Hiroshima 739-8526, Japan E-mail: haino@hiroshima-u.ac.jp Takeharu Haino earned his PhD in 1992 from Hiroshima University under the supervision of Prof. Yoshimasa Fukazawa. Thereafter, he moved to Sagami Chemical Research Center to work with Prof. Daisuke Uemura. In 1993, he was appointed as an Assistant Professor in the Department of Chemistry at Hiroshima University. He then joined the group of Prof. Julius Rebek Jr. at The Scripps Research Institute (1999­2000). In 2000, he was promoted to an Associate Professor of Chemistry at Hiroshima University. He has been a Full Professor since 2007. His research interests embrace the development of photoactive supramolecular polymer materials and graphene-based functional materials. Takehiro Hirao received a BSc degree in chemistry in 2011, an MSc degree in organic chemistry in 2013, and a Ph.D. degree in 2016 in organic chemistry from Hiroshima University, Japan, under the supervision of Professor Takeharu Haino. After working as a postdoctoral researcher with Professor Jonathan L. Sessler at the University of Texas at Austin for two years, he accepted a position as an Assistant Professor of Chemistry at Hiroshima University in April 2018. His research currently focuses on the development of supramolecular polymeric ensembles. Abstract Head-to-tail dipole-dipole arrays of isoxazole rings lead to supramolecular helical assemblies where the assembly and disassembly are regulable based on temperature and solvent properties. The cooperative supramolecular polymerization characterized by a two-step polymerization consisting of nucleation and elongation is driven by the multiple dipole array as well as the induced dipoles in the supramolecular organization. The helical supramolecular assemblies are fabricated with the aid of multiple dipole­dipole interactions. The chiroptical properties, such as CD and CPL, are determined by the right-handed and left-handed helicities of the supramolecular organizations, which are directed by the stereogenic side chains. The AIE and AIEE are established in the supramolecular assemblies. The AIE feature of the platinum complex is inherent in the supramolecular assemblies, which results in luminogenic micelles. Emissive supramolecular micelles are fabricated. In this review, these aspects are briefly described, emphasizing the importance of the intermolecular dipole-dipole interactions in supramolecular chemistry. Keywords: Isoxazole | Supramolecular polymerization | Helical assembly Introduction Supramolecular chemistry deals with a dynamic molecular 574 | Chem. Lett. 2020, 49, 574–584 | doi:10.1246/cl.200031 system that is made up of molecular components bound together by noncovalent or dynamic covalent bonds.1 In a supramolecular assembly mechanism, molecular recognition of a molecular component to its counterpart is driven by structural and electronic complementarities, which generate elaborate supramolecular structures. Various noncovalent interactions govern molecular recognition that determines novel supramolecular structures.2 The association and dissociation of multiple molecular components are completely reversible; thus, a monomeric state dynamically equilibrates with its self-assembled state on the relevant timescale. A supramolecular compound with complex structure and properties thus becomes visible as a thermodynamic minimum in an ordered molecular organization. Such a supramolecular system is often found in living entities such as DNA and proteins, which inspires us to generate novel supramolecular systems with a precise set of desired functions.3 The fusion concept, “supramolecular polymer chemistry,” consists of a combination of polymer science and supramolecular chemistry.4 Supramolecular polymers constitute an innovative class of infinite and dimensionally controlled molecular assemblies, leading to unique functions only in the assembled state.5 Their polymer-like chain structures are completely reversible; thus, their soft properties, such as gelation, liquid crystallinity, and elasticity, rely on their structural and dynamic features, such as size, dimension, lifetime, and conformational © 2020 The Chemical Society of Japan Complementary intermolecular interactions a) Isodesmic mehcanism Self-assembly Ki Supramolecular monomer Ki Ki Ke Ke ••••• ••••• Self-assembled oligomer b) Cooperative mehcanism Kn Supramolecular polymer Figure 1. Supramolecular polymerization of a supramolecular monomer. flexibility of the supramolecular chains.6 Thus, functional supramolecular materials are expected to be stimulus-responsive, which attracts a considerable amount of interest in material science as well as supramolecular chemistry. Chain-like supramolecular polymeric structures are constructed via the self-assembly of molecular monomers, where intermolecular interactions are complementarily preprogrammed (Figure 1). Host-guest, multiple hydrogen-bonding, donor-acceptor, hydrophobic, and other structures are precisely implemented into small supramolecular monomers for supramolecular polymerization. Among supramolecular monomers, synthetically accessible small aromatic molecules are potential molecular platforms for the development of supramolecular polymers and nanostructures.7 Although a planar structure generally promotes intermolecular π­π stacking interactions, directional and controlled supramolecular polymerization requires the effective aid of other directional noncovalent interactions.8 Hydrogen bonding is a common noncovalent interaction that is often employed in the construction of supramolecular polymeric assemblies.9 An amide functionality primarily offers intermolecular head-to-tail hydrogen bonding that directs highly directional and ordered supramolecular stacked polymeric assemblies. Seminal work demonstrating the use of head-to-tail hydrogen bonding for the creation of supramolecular polymeric assemblies was reported by Matsunaga et al.10 and Shirota et al.11 Helically stacked supramolecular structures with hydrogen bonds were demonstrated by Meijer et al.12 Aida et al. reported that an amphiphilic structure inserted onto a hexabenzocolonene core facilitates supramolecular polymerization where hydrophobic units and hydrophilic units are phaseseparated, leading to tubular nanostructures with unique photoconductivity.13 Metal-metal bonds such as Pt-Pt,14 Au-Au,15 etc.16 also stabilize supramolecular assemblies. Charge transfer interactions show good affinity with π­π stacking interactions because both donor and acceptor units are generally π­ conjugated aromatic molecules.17 A dipole-dipole interaction drives the molecular organization in supramolecular polymeric nanostructures. Würther et al. reported that merocyanine forms a dimeric complex through an antiparallel dipole-dipole interaction, which is strong enough to produce supramolecular polymeric assemblies.18 Our group has been continuously contributing to supramolecular polymer chemistry using a variety of intermolecular interactions.19 During the course of study, an isoxazole ring was discovered to drive the supramolecular polymerization of flat aromatic monomers. In this review article, we describe our latest enterprises to develop Chem. Lett. 2020, 49, 574–584 | doi:10.1246/cl.200031 •••• •••• Figure 2. Schematic representation of isodesmic and cooperative supramolecular polymerization of rigid and flat molecules. In this case, a nucleus is the dimer. supramolecular polymerizations directed by intermolecular dipole-dipole interactions of isoxazole rings and supramolecular functions emerging in polymeric states. Supramolecular Polymerization Supramolecular Polymerization Mechanism. In the supramolecular polymerization of small monomers, there are two major growth mechanisms, the so-called isodesmic and cooperative processes (Figure 2).20 In an isodesmic process, the association behavior of a monomer generally obeys a “step-bystep” reversible association model where a single noncovalent bond is reversibly formed, being equivalent at all the steps in the supramolecular polymerization process. An equal association constant (Ki) is given at each association step in an isodesmic supramolecular polymerization.21 Similar to step-growth polymerization, supramolecular polymer chains are characterized by high polydispersity, with values of up to 1.5­2.0. The association constant of each monomer unit can determine the degree of polymerization. There is no clear transition with a critical concentration or a critical temperature in an isodesmic process; therefore, a monomeric state is broadly transferred into a polymeric state. An isodesmic process is also known as a multistage open association (MSOA). A cooperative process in supramolecular polymerization is composed of initial nucleation and subsequent elongation.22 In an initial nucleation process, the supramolecular polymerization is governed by an association constant Kn for the binding of a monomer at each step. When the association process is transferred into an elongation regime, a monomer binds to a nucleus with an association constant Ke. The cooperative factor Ke/Kn is larger than unity in positive, cooperative supramolecular polymerization. Therefore, a sharp transition from nucleation to elongation occurs at a critical concentration or critical temperature in cooperative supramolecular polymerization. After the transition, all monomers are completely assembled into supramolecular polymers with high molecular weights. This cooperative supramolecular polymerization has recently been debated in terms of its potential use for supramolecular controlled chaingrowth polymerization that offers a way to achieve a high degree of polymerization and low dispersity, in sharp contrast to isodesmic supramolecular polymerization.23 Recently, Meijer et al. reported a convenient method to distinguish an isodesmic process from a cooperative process using T- and C-dependent plots of absorption changes upon a self-assembly process.24 In particular, a T-dependent plot is very useful. When a T-dependent plot is sigmoidal, supramolecular polymerization is isodesmic. A nonsigmoidal plot gives rise © 2020 The Chemical Society of Japan | 575 1 a) Te N O Top view OR b) αagg Side view 0 O N N O Ki NO N O ON RO N N O N O 1a: R = n-C10H23 O N O N O O 300 N O N OR (S)-1b: R = (R)-1b: R = c) Figure 3. (a) Structure of 3,5-diphenylisoxazole, (b) Schematic representation of helical supramolecular polymerization of 1,3,5-tris(phenylisoxazolyl)benzenes 1a, (R)-1b and (S)-1b, and (c) calculated structure of the stacked dimer. The red and green arrows denote the dipole moments of the isoxazole ring. Adapted from Ref. 26 with permission from The Royal Society of Chemistry. to an obvious transition at a critical temperature (Te), which indicates that supramolecular polymerization is cooperative. An isodesmic process and a cooperative process are quantitatively discussed using a van der Schoot model.25 Isodesmic Supramolecular Polymerization Directed by Dipole-Dipole Intermolecular Interactions. 3,5Diphenylisoxazoles were originally discovered as mesogenic compounds (Figure 3a).27 An isoxazole ring possesses a fairly large dipole moment of 1.7 Debye that is located on the unsaturated C=N bond. A lack of peri-steric repulsion directs a perfectly planar structural feature in the molecular geometry of 3,5-diphenylisoxazoles, which facilitates intermolecular π-π stacking interactions.26 Structurally extended 1,3,5-tris(phenylisoxazolyl)benzene 1 offers an entirely planar π-conjugated aromatic surface that facilitates intermolecular π-π stacking interactions (Figure 3b, c).28 The three local dipoles located on the C=N double bonds of 1 aid the intermolecular π-π stacking interaction.29 A head-to-tail dipole-dipole arrangement generally results in an attractive noncovalent interaction with a fairly large stabilization energy. When the three isoxazole rings adopt a circular arrangement, 1 is stacked along its C3 axis in a slightly slipped manner, where the iterative head-to-tail dipole-dipole interactions become more stabilized. Thus, the helical supramolecular assembly of 1 grows in piles along the C3 axis to gain attractive energy from the head-to-tail multiple dipole interactions. 576 | Chem. Lett. 2020, 49, 574–584 | doi:10.1246/cl.200031 T/K 320 Figure 4. Melting curve in the supramolecular polymerization of 1a in MCH. Table 1. Self-association constants (Ki) and enthalpic and entropic contributions of the self-assembly of 1a in MCH, chloroform-d1, and DMSO.28 Solv. MCH CDCl3 DMSO Ki/mol¹1 1.0 © 105 3.7 4.4 © 105 ¦H/kJ mol¹1 ¹63 ¹11 ¹42 ¦S/J mol¹1 K¹1 ¹121 ¹24 ¹38 During supramolecular polymerization, the melting curve gives rise to a sigmoidal curvature with a transition temperature of 313 K (Figure 4). During isodesmic supramolecular polymerization, the broad transition occurs from the monomer to the supramolecular polymers with a lack of a critical concentration and a critical temperature. The supramolecular polymerizations of 1a are nicely described by applying an isodesmic model in methylcyclohexane (MCH), chloroform-d1, and dimethylsulfoxide (DMSO). The solvent effect is quite evident in the self-assembly. Chloroform-d1 is a destructive solvent for self-assembly, where self-assembly occurs with a small self-association constant. The self-assembly is facilitated in methylcyclohexane and dimethylsulfoxide. Noncovalently directed intermolecular associations are generally influenced by competitive solvation. The supramolecular polymerizations of 1a always occur by a balance of enthalpy-driven and entropy-opposed associations (Table 1). Although the enthalpic gain mostly comes from the dipole-dipole interaction and π-π stacking interaction upon polymerization, a difference in enthalpy is clearly described by the extent of the competitive solvation that interferes with supramolecular polymerization. Thus, a well-solvated molecule receives relatively small enthalpic gain because of the significant enthalpic penalty that is required in desolvation. Desolvation has a significant influence on isodesmic supramolecular polymerization. The number of isoxazole rings influence the supramolecular polymerization of π-conjugated supramolecular monomers.29 The enthalpic components of supramolecular polymerization are fairly correlated with the number of isoxazole rings. However, the association constants are not proportional most likely due to a difference in conformational freedom being lost when assembling, which might be determined by the symmetries of supramolecular monomers. Cooperative Supramolecular Polymerization. Cooperative supramolecular polymerization is often observed in hydrogen-bonded supramolecular polymers, e.g., an amide group on a π-conjugated aromatic molecule forms head-to-tail © 2020 The Chemical Society of Japan hydrogen bonding in its stacked piles.30 The amide groups are polarized in their head-to-tail linear array, which facilitates intermolecular dipole-dipole interactions to gain additional stabilization in linear polymer growth. Non-hydrogen bonding noncovalent interactions are rarely employed for cooperative supramolecular polymerization. The non-hydrogen bonding dipole-dipole interactions of the isoxazole ring in carbazole 2 direct cooperative supramolecular polymerization in decalin (Figure 5).31 A T-dependent plot of the absorption changes resulted in the nonsigmoidal melting curve in the cooling process. A sharp transition was observed at approximately 290 K, which corresponds to cooperative supramolecular polymerization (Figure 6a).24 The T-dependent plot was evaluated by using the Van der Schoot model,25b providing quantitative parameters in cooperative polymerization. Cooling the solution gave rise to the transition from a nucleation process to an elongation process at a transition temperature (Te) of 290.28 K. Above the transition temperature (Te), the nuclei are smaller than the size of the nuclei (Nn(Te): 20-mer). Below the transition temperature (Te), the supramolecular polymerization is greatly accelerated in the elongation regime, where the degree of polymerization (DP) becomes larger than Nn(Te) through the molecular association of the nuclei with the monomer. A dimensionless equilibrium constant (Ka) between nucleation and N RO O N O RO N nucleation N O N N N O RO N O RO N O RO N N O RO N N N O O O 2 RO O N N O RO N O RO N RO RO RO RO RO RO RO RO RO RO N N N O N O N N N O RO N N N O N O RO N N N O N O N N N O N O N N N O N O RO RO N N N O N O N N O N O RO RO elongation N N O RO N O O O O O O O O O N O Figure 5. Schematic representation of the cooperative twostage association of carbazole 2. The red arrows denote the dipole located on the C=N double bond. a) 1 b) Te elongation N O N O N O N O C10HC 21OH O 10 21 N O N ON O C10 C10H21 OH21O N O N O CO C10H21 10H21O N O O N C10HC OH21O 2110 C10 C10H21 OH21O N ON O C10 C10H21 OH21O N ON O C10H21O N O C10H21O N C10H21O C10H21O N O N O N N N N O N O N O OR N O N O N O N O N O N O N O N S N O N RO S N O N OR N O N N N N N O N N N O N O N O N O N O N N N O N O N O O N N O O N CO C10H21 10H21O OR N O C10H21O C10H21O N O N N N O O N N O N O C10HC OH O 2110 21 C10HC OH21O 2110 C10HC OH21O 2110 N O N N ON O C10HC OH21O 2110 C10HC2110OH21O C10HC21OH O 10 21 nucleation N O C10H21O C10H21O CO C10H21 10H21O elongation describes the extent of the cooperativity. The fairly small dimensionless equilibrium constant (Ka: 1.2 © 10¹4) determined by the analysis indicates that the nucleation process makes a transition to the elongation process in a steady cooperative fashion. The association constant (Ke: 3.3 © 104 L mol¹1) at the transition temperature is large enough to accelerate the supramolecular polymerization in the elongation. The elongation process releases an enthalpy (¦He) of ¹88 kJ mol¹1. Supramolecular polymerization is commonly enthalpy-favored in the elongation process. The intermolecular dipole-dipole interaction participates in supramolecular polymerization (Figure 6b). The crystal structure corroborates a linear dipole arrangement and a π-π stacking interaction among the monomers. The planar conformation of the carbazole ring with two phenyisoxazole side wings enhances a π-π stacking interaction. The peri-steric interaction causes the rest of the isoxazole rings to vertically adjust to the carbazole ring. The linearly oriented C=N double bonds evidently establish the existence of head-to-tail dipole-dipole interactions. Thus, the direction of the polymer growth is directed in the piled assembly of 2. An energetic perspective of the cooperative process is proposed by the theoretical calculation of the stacked polymeric structure. The molecular dipole moment of 2 (4.3 Debye) is significantly enhanced to 5.0 Debye in the crystal structure. Accordingly, piling the monomer in the directional growth most likely facilitates the polarization of the molecular dipole, which facilitates cooperative supramolecular polymerization. Solvent Participation in the Determination of an Isodesmic Process and a Cooperative Process. A sulfur atom is highly polarizable; therefore, an induced-dipole interaction is highly enhanced. The supramolecular polymerization of sulfur-containing flat aromatic molecule 3 is facilitated (Figure 7). Supramolecular polymerization occurs both in an isodesmic process and in a cooperative process.32 The supramolecular polymerization is cooperative in MCH and decalin, while isodesmic polymerization occurs in chloroform. A Goldstein­Stryer model was used to evaluate the cooperative supramolecular polymerization. The association constants and thermodynamic parameters (¦H and T¦S) were determined both in a nucleation regime and in an elongation regime (Table 2). The cooperative factor (· = Kn/Ke) was determined to be precisely 0.03, and the size of the nuclei was 5-mer in decalin solution. The cooperative factor is far less than unity, which indicates that the supramolecular polymerization is strongly cooperative, with enthalpic gains and entropic penalties in the solvent systems. The enthalpy changes during elongation are more advantageous than those during nucleation, implying N O N O S N O N O RO T/K N O S O N O N RO N O 3: R = n-C10H21 S RO S N O N O S OR SS O SS O S O N N OR N OR OR OR Figure 6. (a) Melting curve in the assembly of 2 and (b) its crystal structure. Reprinted with permission from Ref. 31. Copyright (2016) American Chemical Society. Chem. Lett. 2020, 49, 574–584 | doi:10.1246/cl.200031 Figure 7. Schematic representation of the cooperative selfassembly of 3. © 2020 The Chemical Society of Japan | 577 Table 2. Summary of the thermodynamic parameters of the self-assembly of 3.32 ¦Se/ ¦He/ ¦Sn/ ¦Hn/ kcal mol¹1 cal mol¹1 K¹1 kcal mol¹1 cal mol¹1 K¹1 CDCl3 ¹4.2 ¹3.7 ® ® Decalin ¹7.3 ¹7.3 ¹12.6 ¹18 MCH ¹9.9 ¹14 ¹14.7 ¹22 nucleation Solv. a) b) Free Energy ΔGn in chloroform ΔGi ΔGe small assembly (nuclei) =3 –10 = solvent molecule large assembly –10 –15 –20 –15 –10 –5 ΔH / kcal mol–1 0 elongation ΔΔG monomer ΔH / kcal mol–1 TΔS / kcal mol–1 in decalin, MCH –5 0 –5 isodesmic assembly cooperative assembly 2 3 4 dielectric constant 5 Figure 8. (a) An enthalpy­entropy compensation plot of the self-assembly of 3 in chloroform (circle), decalin (triangle), and MCH (square). A line of best fit is displayed. (b) A plot of the enthalpy gain from the self-assembly of 3 versus dielectric constant of the solvents. Reprinted with permission from Ref. 32. Copyright (2017) American Chemical Society. that the supramolecular polymerization during elongation is greatly enhanced by receiving the extra enthalpic gains, most likely arising from the induced dipoles formed in the linear array of the polarizable sulfur atoms. The enthalpy­entropy compensation relationship was first empirically defined by Inoue et al. and often applies to various supramolecular systems.33 This relationship provides a systematic discussion in view of conformational change and desolvation upon supramolecular association. In this supramolecular polymerization process, the thermodynamic parameters in each of the isodesmic and cooperative self-polymerization processes in chloroform-d1, decalin, and MCH resulted in a good linear relationship (Figure 8a); thus, the enthalpy­entropy compensation relationship is operative, which implies that the isodesmic process and the elongation process are considered in the same mechanistic aspect. The slope (¡) is related to the flexibility in conformation, and the intercept (T¦S0) describes the contribution of desolvation; a penalty in freedom is lost by the association of a flexible molecule, resulting in a large ¡. In general, a small ¡ value is given in the supramolecular association of a preorganized host molecule and a rigid complimentary guest molecule due to the lack of a certain conformational change upon complexation. Based on the slope (¡) of 0.58 obtained from the plot, 42% of the enthalpic gains grant the stabilization of the assembly, while the entropic penalty in losing freedom is paid by the remaining 58%. The fairly small ¡ for the self-assembly of 3 suggests that the monomer is conformationally quite rigid, resulting in a modest entropic penalty in conformation upon the supramolecular polymerization process. The remarkably large intercept (T¦S0) of 1.6 kcal mol¹1 describes the favorable contribution to entropy arising from desolvation upon supramolecular polymerization. The linear relationship between the enthalpy and entropy com- 578 | Chem. Lett. 2020, 49, 574–584 | doi:10.1246/cl.200031 Figure 9. Energy landscape of the isodesmic and cooperative self-assemblies of 3. Reprinted with permission from Ref. 32. Copyright (2017) American Chemical Society. ponents implies that the isodesmic process and the cooperative process are firmly associated with each other in a mechanism. The solvent effect in the supramolecular polymerization is described by plotting the enthalpic gains and the dielectric constants of the solvents (Figure 8b). The monomer is solvated more than the polymers, which becomes more obvious in the polar solvent chloroform; therefore, the intermolecular association interferes with competitive solvation, losing the enthalpic benefit upon supramolecular polymerization. Subsequently, the assemblies cannot grow to be sufficiently sizable to transition from the nucleation process into the elongation process in chloroform but can transition in the other solvents. The overall energy landscape in the supramolecular polymerization process is illustrated (Figure 9). Chloroform is a good solvent for solvating the monomeric form, which leads to an isodesmic process where the size of the nuclei becomes too small to be transferred into a cooperative regime. The less polar solvents enhance the supramolecular polymerization process due to less solvation. The supramolecular polymerization results in more sizable nuclei that are transferred to the elongation regime. Given that all the supramolecular polymerization processes are on the same mechanistic landscape, the isodesmic self-assembly observed in chloroform is consistent with a nucleation regime in the cooperative process. There are two free energies (¦Gn, ¦Ge) in the cooperative process. The free energy change in the nucleation is smaller than that in the elongation. The free energy difference (¦¦G = ¦Ge ¹ ¦Gn) corresponds to the cooperative process upon self-assembly of 3. Supramolecular Functions Supramolecular Polymers in Helical Helical Supramolecular Polymer. When 1a stacks as a pile, there are two energetically close conformations in the dipole-dipole interactions: One possesses a dissymmetric helical conformation where the local dipoles align in a head-to-tail fashion, and the other possesses an achiral eclipsed conformation where the local dipoles adopt an anti-parallel conformation (Figure 10a). In a helical conformation, the right-handed (P) and © 2020 The Chemical Society of Japan a) Helical conformation Anti parallel conformation (P)-helix R1O O N N OR1 O N O N O (S)-4: R1 = (R)-4: R1 = R2 = O b) N R2 left-CPL O (M)-helix C6H13 C6H13 right-CPL O Figure 11. Schematic representation of supramolecular helical assemblies of PBI-implemented tri(phenylisoxazolyl)benzene (R)- and (S)-4. Adapted from Ref. 44 with permission from The Royal Society of Chemistry. Figure 10. (a) Stereoplots of (a) helical assembly and eclipsed assembly of 1,3,5-tris(phenylisoxazolyl)benzene. (b) Helical supramolecular polymers of 1a, b, and (P)­(M ) interconversion. Reprinted with permission from Ref. 28. Copyright (2011) American Chemical Society. left-handed (M ) helical forms equilibrate at ambient temperature (Figure 10b). Therefore, helical supramolecular polymers exist in racemic forms without a chiral auxiliary. Upon locating a chiral substituent to achiral monomer 1a, the (P)- and (M )helical forms become diastereomeric; therefore, the (P)­(M ) interconversion is biased. The stereogenic centers of (S)- and (R)-1b direct the right-handed and left-handed helical senses of the supramolecular helical polymer, respectively. Unique chiral amplification behavior, the “majority rules effect,” 34 is often observed in helical supramolecular polymers.35 The circular dichroism (CD) intensities against the enantiomeric excess of the chiral monomer 1b result in a deviation from linearity. A disproportionate impact on the helicity is caused by the enantiomers in excess. Accordingly, helical supramolecular polymers are formed through the coassembly of (S)- and (R)-1b. The majority of enantiomers determine the helical sense of supramolecular polymers. The “sergeants-and-soldiers principle” 36 is in effect for the supramolecular polymer. The (P)- and (M )-helical supramolecular polymers of achiral 1a are dynamically interconvertible, which is shifted upon the addition of a tiny amount of 1b. A small amount of chiral 1b is brought together into the helical polymer of achiral 1a, which greatly intensifies the CD signals; thus, chiral information from the stereogenic center is enhanced in the supramolecular helical polymers. CPL Emissive Helical Supramolecular Polymer. A differential emission in right- and left-handed circularly polarized light is attributed to circularly polarized luminescence (CPL), an intrinsic chiroptical property of chiral luminogenic molecules and molecular assemblies.37 CD is observed at the absorption of an electronic ground state, which provides structural information on their chiralities. In contrast, chirality in the excited state of a luminogenic molecule is directly associated with CPL. Many applications are expected, such as information storage, 3D displays, CPL lasers, biological probes, security technology, and sensing devices.38 Lanthanide complexes were initially employed to study their CPL behaviors, emitting from Chem. Lett. 2020, 49, 574–584 | doi:10.1246/cl.200031 the excited state through f-f transitions.39 Currently, small chiral hydrocarbons, such as helicenes and cyclophanes, have been discovered to show intense emission with high dissymmetric factors (glum).40 Conjugated polymers with chiral side chains exhibit intense CPL.41 Supramolecular polymers often produce high CPL dissymmetry and are good platforms that generate intense CPL emissions.42 The structures of supramolecular polymers are generally sensitive to environmental conditions such as temperature and solvent properties. When CPL emission properties originate from the assembled states of a luminogenic monomer, the CPL emitting properties (i.e., inversion of handedness, on-off regulation, modulation of emission wavelength, and intensity) are triggered by its supramolecular polymerization process through the facile structural modification of a monomer structure. Thus, supramolecular polymerization possesses a great advantage in developing CPL-emitting devices through the polymerization and depolymerization processes induced by environmental conditions. A helical supramolecular assembly is a promising scaffold in which luminogenic molecular components are organized in a chiral fashion, which is responsible for generating CPL emission (Figure 11). Luminogenic perylene bisimide (PBI) emits intensely in the visible region with a high quantum yield. Thus, functionalized PBIs have often been employed in supramolecular photochemistry.43 Placing a PBI unit onto the helical supramolecular polymers of tris(phenylisooxazolyl)benzene results in helically ordered PBI units that can emit CPL.44,45 The polymerization and depolymerization processes can be dominated by stimulation, bringing about the on-off regulation of CPL. The supramolecular polymers of PBI-implemented tri(phenylisoxazolyl)benzenes (S)- and (R)-4 result in supramolecular polymers where the PBI unit located at the periphery is in adjacent contact with its neighbors (Figure 10). The J-aggregation of the PBI unit leads to a redshift in its absorption band in the decalin solution. The stereogenic center of the monomer determines the helical sense of the supramolecular polymers, displaying strong Cotton effects in the characteristic PBI absorption band. The (R)- and (S)-stereogenic centers determine a left-handed helical sense and a right-handed helical sense of the helical supramolecular polymers of 4, respectively. A decalin solution of 4 generates a mixture of the monomeric form and the polymeric forms that display a welldefined fluorescence band assigned to the PBI unit and a broad emission band assigned to the stacked assemblies of the PBI units, respectively. CPL was clearly observed in the broad © 2020 The Chemical Society of Japan | 579 R1O R2 N O S Ge right-handed CPL R2 O N S O N O N R1O left-handed CPL OR1 nucleation elongation c) 1 R1O Te (R)-5: R1 = R2 = a) b) agg (S)-5: R1 = elongation nucleation 0 270 280 290 300 310 T/K Figure 12. Self-assembly of bis(phenylisoxazolyl)benzene-connected dithienogermole (R)- and (S)-5 and cooperative associationdirected CPL inversion. AFM images (300 © 300 nm) of the fibers prepared from the (a) nucleation regime and (b) elongation regime. (c) A T-dependent plot of the solution of 5. Adapted from Ref. 47 with permission from The Royal Society of Chemistry. emission of the chiral assemblies in decalin, while there was no CPL signal in the monomeric fluorescence band of the PBI unit. A mirror image relationship of the CPL signals between (R)- and (S)-4 is established in the assembled states. Therefore, the helical supramolecular polymers of the tris(phenylisoxazolyl)benzene generate the helical platform in which the PBI units are arranged to be narrowly stacked in a helical fashion. The right-handed and left-handed helical organizations of the PBI units direct the positive and negative signs of CPL, respectively. The polymerization and depolymerization processes rely on the solvent nature and the temperatures, which results in the on-off regulation of the CPL. Switchable CPL Emission during Cooperative Supramolecular Polymerization. Dithienogermole (DTG) is a class of group 14 metalloles condensed with aryl groups.46 This luminogenic aromatic core DTG is functionalized with phenylisoxazolyl groups to construct supramolecular luminogenic helical assemblies. When functionalized DTG 5 assembles to form helical supramolecular polymers, the chiral supramolecular DTG organization results in CPL properties (Figure 12).47 Monomers (R)- and (S)-5 exhibit cooperative supramolecular polymerization (Figure 12c). Cooling an MCH solution of 5 results in a clear transformation from a nucleation regime to elongation at approximately 282 K. The monomeric form equilibrates with its aggregate during the nucleation regime. At the transition, the aggregate reaches a size of 7-mer that is transferred into the elongation regime, where the elongation process is commonly considered an enthalpy-favorable and entropy-opposed association. Strong cooperativity is represented by the small dimensionless equilibrium constant between nucleation and elongation (Ka: 0.002). The helical sense of the supramolecular polymer is directed by the stereogenic center of the side chain. The helically organized DTG units in the supramolecular polymer are sensitive to the supramolecular polymerization processes. The helical sense of the DTG units is inverted through the transition from the nucleation regime to the elongation regime, although 580 | Chem. Lett. 2020, 49, 574–584 | doi:10.1246/cl.200031 the helical supramolecular polymers are formed both in the nucleation regime and in the elongation regime. The supramolecular polymeric fibers are detected in both the nucleation and elongation regimes using atomic force microscopy (Figure 11a, b). Right-handed helical fibers with a pitch of 11 nm and a diameter of 10 nm are clearly formed. The transition to the elongation process results in doubly thickened fibers. The morphological difference between the supramolecular polymeric fibers leads to a dramatic difference in emission. The initial nuclei of (S)-5 generate the right-handed CPL. The CPL sign is completely inverted through the transition. The elongation of the supramolecular polymer results in the left-handed CPL. Metal-metal-interaction-directed Emissive Helical Supramolecular Polymer. Emissive metal complexes can be inserted onto the supramolecular helical organization formed through self-assembly. d8- and d10-metal-to-metal interactions are attractive supramolecular interactions that direct polymeric metal organization, bringing about unique optoelectronic features. The square-planar structural feature of Pt(II) complexes drives stacked molecular assemblies through a Pt-Pt interaction that shows fine phosphorescence in an assembled state. The helical supramolecular polymer of a tris(phenylisoxazolyl)benzene is fused with a Pt(II) complex to control the strong emissive feature through supramolecular polymerization. The blended molecular structure of (S)- and (R)-6 facilitates cooperative supramolecular polymerization through dipoledipole, Pt­Pt, and π­π stacking interactions (Figure 13).48 Monomer 6 is associated with the aid of Pt-Pt, dipoledipole, and π-π stacking interactions. The supramolecular polymerization is isodesmic in chloroform. The typical 3MMLCT emission upon supramolecular polymerization comes from the MMLCT band. Although the supramolecular polymer of chiral monomer 6 is evidently stabilized by Pt-Pt interactions, the supramolecular polymers are nonhelical on the basis of a lack of CD. A toluene solvent system results in a difference in the selfassembly process and structure. The supramolecular polymers of (R)- and (S)-6 have a mirror image relationship in the helical © 2020 The Chemical Society of Japan hydrophilic shell N N N non-helical assembly Pt O RO AIE N Pt N O OR (S)-6: R = O TEGO AIEE AICPL N N O hydrophobic core N THF/water (v/v) 10/0 7: TEG = -(CH2CH2O)3CH3 9/1 8/2 7/3 6/4 5/5 4/6 3/7 2/8 1/9 OTEG helical assembly (R)-6: R = Figure 13. Nonhelical self-assembly and helical self-assembly of bis(phenylisoxazolyl)benzene-connected platinum complexes (R)- and (S)-6 and the nonemissive and emissive solids from the chloroform solution and the toluene solution, respectively. Adapted from Ref. 48 with permission from The Royal Society of Chemistry. θ / mdeg 60 0 0 100 t / min 200 Figure 14. Time-dependent CD evolution in the supramolecular polymerization of (S)-6. Adapted from Ref. 48 with permission from The Royal Society of Chemistry. structure. Time-dependent supramolecular polymerization was observed at ambient temperature (Figure 14). After a certain lag time, helical supramolecular polymerization slowly occurred; thus, a fairly sizable kinetic barrier exists between the polymerization and depolymerization processes. Based on sigmoidal growth, the supramolecular polymerization process is autocatalytic and cooperative, where nuclei, formed in the lag time, catalyze the propagation process that accelerates supramolecular polymerization in the elongation regime. In cooperative supramolecular polymerization, depolymerization is suppressed, most likely due to the strong Pt-Pt interaction being enhanced in toluene. This type of cooperative supramolecular polymerization is often found in biological systems, such as the supramolecular polymerization of amyloid proteins. A toluene solvent system directs the self-assembly of 6, resulting in unusual aggregation-induced enhancement of emission (AIEE). The emission in the toluene solution of 6 is strongly intensified with increasing quantum yields when supramolecular assemblies are formed. The emission lifetimes of 0.25 and 1.1 ¯s suggest that 6 exists in photochemically nonequivalent states, which are most likely the monomeric form and the polymers, respectively. The AIEE behavior is attributed to the supramolecular polymers with a longer lifetime. Therefore, the dynamic motion of the molecular unit is partly constrained in the supramolecular polymers, where the nonradiative decay channels are most likely reduced, thus gaining a fraction of the radiative decay. At the emission bands corresponding to Chem. Lett. 2020, 49, 574–584 | doi:10.1246/cl.200031 Figure 15. Supramolecular micelles formed through the selfassembly of bis(phenylisoxazolyl)benzene-connected platinum complex 7 with hydrophilic triethylene glycol units, and optical image of the emissive micelles formed in THF/water mixed solution. Adapted from Ref. 53 with permission from The Royal Society of Chemistry. the supramolecular polymers, intense mirror image CPLs are produced with a high dissymmetric factor of 0.01, whereas the monomeric form is not CPL-active. The assembly and disassembly behaviors result in control over the strong aggregationinduced circularly polarized luminescence (AICPL). This AICPL behavior is enhanced in the gel phase.49 AIE-active Supramolecular Micelles. Micelles are in the class of supramolecular assemblies with three-dimensionally extended architectures such as spheres, cylinders, disks, and tubes.50 An amphiphilic molecular component is directed to selfassemble micellar aggregates. Luminescent micelles are quite useful for tissue imaging in biological systems.51 Charged platinum amphiphiles are often employed to fabricate luminescent micelles due to their aggregation-induced emission (AIE).52 An neutral AIE-active platinum complex 7 also self-assembles to form emissive supramolecular micelles (Figure 15).53 Hydrophilic triethylene glycol (TEG) groups are equipped to result in amphiphilic features that facilitate micelle formation through self-assembly. In the assembled state, the intermolecular π-π stacking and dipole-dipole interactions are enhanced due to the planer structural feature of the diphenylisoxazole ring. The intermolecular metal-metal interactions cooperatively participate in the stabilization of the supramolecular micelles.54 Complex 7 exists as a monomer in THF. In the presence of water, large spherical aggregates with a diameter of 143 nm are formed at a specific water content of 70%. Water contents above 70% generate the formation of micron-sized aggregates. The AIE behavior is clearly observed via the self-assembly of 7 in a water/THF mixed solvent. The emission of the platinum core is greatly induced upon the formation of micelles, although the monomer, small aggregates, and large aggregates are not emissive. The AIE with a quantum yield of 0.16 is turned on when the micelles are formed at the specific water content in THF. Summary and Outlook Hierarchical supramolecular structures such as DNA duplex, α-helix, β-sheet, etc. are stabilized by supramolecular interactions. Hydrogen bonding is one of the key supramolecular interactions, which directs the DNA duplex formation through the self-assembly of individual single-stranded conjugates. The © 2020 The Chemical Society of Japan | 581 directionality of hydrogen bonding plays a key role in the specificity of biomolecular associations in biological systems. In contrast, a dipole-dipole interaction might be considered less directional because there are head-to-tail and anti-parallel dipole-dipole arrangements. Twelve years ago, the head-to-tail dipole-dipole interactions between isoxazole rings were first reported to direct the helical supramolecular stacked assemblies of 1,3,5-tris(phenylisoxazolyl)benzene with the aid of an aromatic π-π stacking interaction. This example demonstrates that multiple dipole-dipole interactions can be a potential alternative to organize hierarchical supramolecular structures. Supramolecular polymerization, directed by the dipole-dipole interactions, controls the size and dimension of the supramolecular polymers in a designated manner. The cooperative supramolecular polymerization is also directed by the headto-tail dipole-dipole arrangement. In addition, A variety of functional groups can be placed on desired supramolecular polymers, resulting in supramolecular functional materials. In this article, circular dichroism and circularly polarized light absorption, emerging only in the assembled state, are illustrated to be typical supramolecular functions. The supramolecular polymerization also directs the AIEE and AIE. The three-dimensionally extended supramolecular polymer results in AIE-active luminogenic micelles fabricated through the amphiphilic features designed in the molecular core. Weak supramolecular interactions might restrict the amenable use of supramolecular materials in real applications. This defect can be compensated by strengthening supramolecular polymer structures using multiple supramolecular interactions. The reversible nature of robust supramolecular polymers establishes advanced supramolecular materials with adaptivity that results in a simple process, facile recycling, stimuli responsivity, environmental adaptivity, and self-healability.55 A bright future is coming with supramolecular polymer materials for the next generation of polymer chemistry. 3 4 5 6 7 8 9 10 11 12 This work was supported by Grants-in-Aid for Scientific Research (JSPS KAKENHI Grant Number JP15H03817 to T. Haino, JP18H05980 and 19K21130 to T. Hirao) and by the Grants-in-Aid for Scientific Research on Innovative Areas (JSPS KAKENHI Grant Numbers JP17H05375 (Coordination Asymmetry), JP19H04585 (Coordination Asymmetry), and JP17H05159 (π-figuration) to T. Haino). Funding from the Nippon Sheet Glass Foundation, The Ogasawara Foundation for the Promotion of Science & Engineering, Takahashi Industrial and Economic Research Foundation, Shorai Foundation for Science and Technology, The Futaba Electronics Memorial Foundation, Iketani Science and Technology Foundation, Tokuyama Science Foundation, and Fukuoka Naohiko Memorial Foundation is gratefully acknowledged. References and Notes 1 a) D. J. Cram, Angew. Chem., Int. Ed. Engl. 1988, 27, 1009. b) J.-M. Lehn, Angew. Chem., Int. Ed. Engl. 1988, 27, 89. c) C. J. Pedersen, Angew. Chem., Int. Ed. Engl. 1988, 27, 1021. 2 a) H.-J. Schneider, Angew. Chem., Int. Ed. 2009, 48, 3924. b) F. Biedermann, H.-J. Schneider, Chem. Rev. 2016, 116, 5216. 582 | Chem. Lett. 2020, 49, 574–584 | doi:10.1246/cl.200031 13 14 S. Zhang, Nat. Biotechnol. 2003, 21, 1171. a) J. M. Lehn, Angew. Chem., Int. Ed. Engl. 1990, 29, 1304. b) L. Brunsveld, B. J. B. Folmer, E. W. Meijer, R. P. Sijbesma, Chem. Rev. 2001, 101, 4071. c) T. 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