Synthesis of Aza[n]helicenes up to n = 19: Hydrogen-Bond-Assisted Solubility and Benzannulation Strategy.

Synthetic challenges toward anomalous structures and electronic states often involve handling problems such as insolubility in common organic solvents and oxidative degradation under aerobic conditions. We designed benzo-annulated aza[n]helicenes, which benefit from both the suppressed elevation of highest occupied molecular orbital (HOMO) energies and high solubility due to hydrogen bonding with solvent molecules to overcome these challenges. This strategy enabled the synthesis of six new aza[n]helicenes ([n]AHs) of different lengths (n = 9-19) from acyclic precursors via one-pot intramolecular oxidative fusion reactions. The structures of all of the synthesized aza[n]helicenes were determined by X-ray diffraction (XRD) analysis, and their electrochemical potentials were measured by cyclic voltammetry. Among the synthesized aza[n]helicenes, [17]AH and [19]AH are the first heterohelicenes with a triple-layered helix. The noncovalent interaction (NCI) plots confirm the existence of an effective π-π interaction between the layers. The absorption and fluorescence spectra red-shifted as the helical lengths increased, without any distinct saturation points. The optical resolutions of N-butylated [9]AH, [11]AH, [13]AH, and [15]AH were accomplished, and their circular dichroism (CD) and circularly polarized luminescence (CPL) were measured. Thus, the structural, (chir)optical, and electrochemical properties of the aza[n]helicenes were comprehensively analyzed.

NIR-II Absorption/Fluorescence of D-A π-Conjugated Polymers Composed of Strong Electron Acceptors Based on Boron-Fused Azobenzene Complexes.

Luminescence in the second near-infrared (NIR-II, 1,000-1,700 nm) window is beneficial especially for deep tissue imaging and optical sensors because of intrinsic high permeability through various media. Strong electron-acceptors with low-lying lowest unoccupied molecular orbital (LUMO) energy levels are a crucial unit for donor-acceptor (D-A) π-conjugated polymers (CPs) with the NIR-II emission property, however, limited kinds of molecular skeletons are still available. Herein, D-A CPs involving fluorinated boron-fused azobenzene complexes (BAz) with enhanced electron-accepting properties are reported. Combination of fluorination at the azobenzene ligand and trifluoromethylation at the boron can effectively lower the LUMO energy level down to -4.42 eV, which is much lower than those of conventional strong electron-acceptors. The synthesized series of CPs showed excellent absorption/fluorescence property in solution over a wide NIR range including NIR-II. Furthermore, owing to the inherent solid-state emissive property of the BAz skeleton, obvious NIR-II fluorescence from the film (up to λFL=1213 nm) and the nanoparticle in water (λFL=1036 nm, brightness=up to 29 cm-1 M-1) were observed, proposing that our materials are applicable for developing next-generation of NIR-II luminescent materials.

Near-Infrared Emissive Hypervalent Compounds with Germanium(IV)-Fused Azobenzene π-Conjugated Systems.

A novel molecular design for showing near-infrared (NIR) emission is still required for satisfying growing demands for NIR-light technology. In this research, hypervalent compounds with germanium (Ge)-fused azobenzene (GAz) scaffolds were discovered that can exhibit NIR emission (λPL =690∼721 nm, ΦPL =0.03∼0.04) despite compact π-conjugated systems. The unique optical properties are derived from the trigonal bipyramidal geometry of the hypervalent compounds constructed by combination of Ge and azobenzene-based tridentate ligands. Experimental and theoretical calculation results disclosed that the germanium-nitrogen (Ge-N) coordination at the equatorial position strongly reduces the energy level of the LUMO (lowest unoccupied molecular orbital), and the three-center four-electron (3 c-4 e) bond in the apical position effectively rises the energy level of the HOMO (highest occupied molecular orbital). It is emphasized that large narrowing of the HOMO-LUMO energy gap is achieved just by forming the hypervalent bond. In addition, the narrow-energy-gap property can be enhanced by extension of π-conjugation. The obtained π-conjugated polymer shows efficient NIR emission both in solution (λPL =770 nm and ΦPL =0.10) and film (λPL =807 nm and ΦPL =0.04). These results suggest that collaboration of a hypervalent bond and a π-conjugated system is a novel and effective strategy for tuning electronic properties even in the NIR region.

Effects of Hypervalent Bismuth on Electronic Properties of the Azobenzene Tridentate Ligand and Roles of Lewis Acidity in Controlling Optical Properties.

Organobismuth compounds have been studied in various fields, including electronic states, pnictogen bonds, and catalysis. Among them, one of the unique electronic states of the element is the hypervalent state. So far, many issues regarding the electronic structures of bismuth in hypervalent states have been revealed; meanwhile, the influence of hypervalent bismuth on the electronic properties of π-conjugated scaffolds is still vailed. Here, we synthesized the hypervalent bismuth compound, BiAz, by introducing hypervalent bismuth into the azobenzene tridentate ligand as a π-conjugated scaffold. The influence of hypervalent bismuth on the electronic properties of the ligand was evaluated from optical measurements and quantum chemical calculations. The introduction of hypervalent bismuth revealed three significant electronic effects: first, hypervalent bismuth shows position-dependent electron-donating and electron-accepting effects. Second, BiAz can have a larger effective Lewis acidity than the hypervalent tin compound derivatives reported in our previous research. Finally, the coordination of dimethyl sulfoxide transformed the electronic properties of BiAz, similar to the hypervalent tin compounds. The data from quantum chemical calculations showed that the optical properties of the π-conjugated scaffold were able to be changed by introducing hypervalent bismuth. To the best of our knowledge, we first demonstrate that the introduction of hypervalent bismuth should be a new methodology for controlling the electronic properties of π-conjugated molecules and developing sensing materials.

Discrete Macrocycles with Fixed Chirality and Two Distinct Sides: Dipole-Dependent Chiroptical Response.

Control of symmetry is fundamental in molecular design with aimed properties. Herein we report a set of chiroptical C5 -symmetric molecules with variable dipolar structures based on a rim-differentiated cylindrical macrocycle, pillar[5]arene. Incorporation of electron-withdrawing ester groups formed an explicit two-sided structure, leading to increase in response wavelength and luminescence efficiency. On the other hand, chiroptical measurement of separated enantiomers revealed that such a dipolar character diminished dissymmetry of the electronic transitions. By suppressing the dipole, the dissymmetry factor for luminescence was enhanced from 0.4×10-3 to 5.1×10-3 in a less dipolar methoxy-substituted molecule, which was larger than reported pillar[5]arene derivatives without C5 -symmetry around one order of magnitude.

Vapochromic Luminescent π-Conjugated Systems with Reversible Coordination-Number Control of Hypervalent Tin(IV)-Fused Azobenzene Complexes.

The dynamic and reversible changes of coordination numbers between five and six in solution and solid states, based on hypervalent tin(IV)-fused azobenzene (TAz) complexes, are reported. It was found that the TAz complexes showed deep-red emission owing to the hypervalent bond composed of an electron-donating three-center four-electron (3c-4e) bond and an electron-accepting nitrogen-tin (N-Sn) coordination. Furthermore, hypsochromic shifts in optical spectra were observed in Lewis basic solvents because of alteration of the coordination number from five to six. In particular, vapochromic luminescence was induced by attachment of dimethyl sulfoxide (DMSO) vapor to the coordination point at the tin atom accompanied with a crystal-crystal phase transition. Additionally, the color-change mechanism and degree of binding constants were well explained by theoretical calculation. To the best of our knowledge, this is the first example of vapochromic luminescence by using stable and variable coordination numbers of hypervalent bonds.

Discovery of Functional Luminescence Properties Based on Flexible and Bendable Boron-Fused Azomethine/Azobenzene Complexes with O,N,O-Type Tridentate Ligands.

Azomethine (C=N) and azo (N=N) scaffolds are a part of structural units in poly(p-phenylene azomethine) (PAM) and poly(p-phenylene azo) (PAZ), respectively. Poly(p-phenylene vinylene) (PPV) is known to be one of luminescent π-conjugated polymers, meanwhile PAM and PAZ, which are the aza-substituted PPV analogues, are regarded as weak or no emissive materials. However, by the boron complexation, intense emission can be induced. Furthermore, environment-sensitivity and stimuli-responsivity were also observed. In this review, we demonstrate unique and versatile luminescent properties based on "flexible and bendable" π-conjugated systems composed of the boron-fused azomethine and azobenzene complexes (BAm and BAz) with the O,N,O-type tridentate ligands. The "flexible and bendable" luminophores showed intriguing optical behaviors, such as thermosalient effect, aggregation-induced emission (AIE) and crystallized-induced emission (CIE). Moreover, highly efficient emissions both in solution and film states were observed from the polymers. We illustrate the results and mechanisms on these luminescent properties from the series of our recent studies with BAm and BAz complexes and polymers.

Preparation of Near-Infrared Emissive π-Conjugated Polymer Films Based on Boron-Fused Azobenzene Complexes with Perpendicularly Protruded Aryl Substituents.

Most organic luminescent dyes usually show poor emission in solid due to aggregation-caused quenching due to nonspecific intermolecular interaction, such as π-π stacking. Furthermore, since commodity molecules having near-infrared (NIR) emission properties tend to have extended π-conjugated systems, development of luminescent organic materials with solid-state NIR emission has been still challenging. Herein, the series of the azobenzene complexes with the perpendicularly-protruded aryl derivative at the boron atom toward π-conjugated system is synthesized. From the optical measurements, it is shown that these complexes can show crystallization-induced emission enhancement behaviors. The donor-acceptor type π-conjugated polymers composed of the azobenzene complexes are also synthesized. Highly-efficient NIR emission from the phenyl-substituted polymers both in solution (λPL  = 742 nm, ΦPL  = 15%) and film states (λPL  = 793 nm, ΦPL  = 9%) is obtained. Furthermore, emission wavelengths can be tuned by changing the substituent at the boron atom to the modified aryl groups. From mechanistic studies including theoretical calculations, it is shown that electronic interaction is allowable between the aryl substituent to the π-conjugated system through the tetradentate boron.

Enantioselective Synthesis of Triple Helicenes by Cross-Cyclotrimerization of a Helicenyl Aryne and Alkynes via Dynamic Kinetic Resolution.

Optically active triple helicenes (TH-1) were prepared via a palladium-catalyzed enantioselective cross-cyclotrimerization of two helicenyl arynes 5, which are generated in situ from 3, with dialkyl acetylenedicarboxylate 4. Enantiomeric ratios of up to 98:2 were obtained when using 4a and (S)-QUINAP as the alkyne and chiral ligand, respectively. The absolute stereochemistry of TH-1a was revealed to be (M,P,M) by a single-crystal X-ray diffraction analysis. Kinetic studies of the racemization of enantiomerically pure TH-1a at elevated temperatures were conducted based on a high-performance liquid chromatography analysis. The activation energy for the racemization was found to be 29.1 kcal mol-1. Density functional theory calculations revealed that the palladium-catalyzed enantioselective cross-cyclotrimerization reactions proceed via the dynamic kinetic resolution of a five-membered palladacycle 6a with two [5]helicenes. Several initially formed stereoisomers of 6a eventually isomerize into the most thermodynamically stable palladacycle intermediate (M,P,M)-6a by inversion of the [5]helicenyl moiety. Then, the insertion of 4 into 6a to form (M,P,M)-12a, followed by a reductive elimination, leads to the formation of (M,P,M)-TH-1a in a stereoselective manner. The optical properties of TH-1a were studied by circular dichroism and circularly polarized luminescence.

Near-Infrared Circularly Polarized Luminescence through Intramolecular Excimer Formation of Oligo(p-phenyleneethynylene)-Based Double Helicates.

Carbon-based double helicates consisting of two anthracene-containing oligo(p-phenyleneethynylene) units and two flexible chiral 1,1'-binaphthyl units or two rigid chiral 9,9'-spirobifluorene units were developed. The curved oligo(p-phenyleneethynylene) fragments in the double helicates were successfully constructed by tin-mediated reductive aromatization. Helical oligo(p-phenyleneethynylene) double strands fixed by two rigid spirobifluorene units showed little structural change under photoirradiation, thereby emitting circularly polarized luminescence (CPL) in the visible region with a high quantum yield (ΦPL =0.93). In contrast, flexible binaphthyl units induced dynamic structural change of the oligo(p-phenyleneethynylene) luminophores under photoirradiation, leading to strong CPL (|glum |=1.1×10-2 ) in the near-infrared (NIR) region. UV/Vis, circular dichroism (CD), CPL and NMR spectroscopic analyses of the binaphthyl-hinged double helicate suggested excimer formation between two π-conjugated strands in the excited state. Theoretical calculations highlight the importance of the tightly interlocked excimer structure of the carbon-based double helicate in controlling the angle between the electric and magnetic transition dipole moments for strong NIR CPL generation.

Hash-Mark-Shaped Azaacene Tetramers with Axial Chirality.

The tetramers of azapentacene derivatives with unique hash mark (#)-shaped structures were prepared in a quite facile manner. The #-shaped tetramers are optically active due to possessing extended biaryl skeletons, and the structure of the tetramer composed of four dihydrodiazapentacene (DHDAP) units (1) was investigated as the first example of this kind of molecule. The tetramer 1 showed characteristic chiroptical properties reflecting its orthogonally arranged quadruple DHDAP moieties, as well as redox activity. The solution of enantiopure 1 exhibited intense circularly polarized luminescence (CPL) with a dissymmetry factor of 2.5 × 10-3. The absolute configuration of the enantiomers of 1 was experimentally determined by X-ray crystal analysis for the dication salt of the enantiomer of 1 with SbCl6- counterions. The solutions of enantiopure 12+·2[SbCl6-] also showed NIR circular dichroism (CD) spectra over the entire range from visible to 1100 nm, enabling the modulation of the chiroptical properties by redox stimuli.

Synthesis of enantiopure planar chiral bis-(para)-pseudo-meta-type [2.2]paracyclophanes.

A new type of planar chiral (Rp )- and (Sp )-4,7,12,15-tetrasubstituted [2.2]paracyclophanes was prepared from racemic 4,7,12,15-tetrabromo[2.2]paracyclophane as the starting substrate. Regioselective lithiation and transformations afforded racemic bis-(para)-pseudo-meta-type [2.2]paracyclophane (4,15-dibromo-7,12-dihydroxy[2.2]paracyclophane). Its optical resolution was performed by the diastereomer method using a chiral camphanoyl group as the chiral auxiliary. The diastereoisomers were readily isolated by simple silica gel column chromatography, and the successive hydrolysis afforded (Rp )- and (Sp )-bis-(para)-pseudo-meta-type [2.2]paracyclophanes ((Rp )- and (Sp )-4,15-dibromo-7,12-dihydroxy[2.2]paracyclophanes). They can be used as pseudo-meta-substituted chiral building blocks.

A Highly Efficient Near-Infrared-Emissive Copolymer with a N=N Double-Bond π-Conjugated System Based on a Fused Azobenzene-Boron Complex.

Fused azobenzene-boron complexes (BAzs) show highly efficient near-infrared (NIR) emission from the nitrogen-nitrogen double bond (N=N) containing π-conjugated copolymer. Optical measurements showed that BAz worked as a strong electron acceptor because of the intrinsic electron deficiency of the N=N double bond and the boron-nitrogen (B-N) coordination which dramatically lowered the energy of the lowest unoccupied molecular orbital (LUMO) of the azobenzene ligand. The simple donor-acceptor (D-A) type copolymer of bithiophene (BT) and BAz exhibited intense photoluminescence (PL) in the NIR region both in the dilute solution (λPL =751 nm, ΦPL =0.25) and in the film (λPL =821 nm, ΦPL =0.038). The BAz monomer showed slight PL in the dilute solution, and aggregation-induced emission (AIE) was detected. We proposed that N=N double bonds should be attractive and functional building blocks for designing π-conjugated materials.

Optically Active Phenylethene Dimers Based on Planar Chiral Tetrasubstituted [2.2]Paracyclophane.

Optically active phenylethene dimers based on a planar chiral 4,7,12,15-tetrasubstituted [2.2]paracyclophane were synthesized. We succeeded in controlling the molecular motion by binding luminophores in close proximity with the [2.2]paracyclophane scaffold. For example, aggregation-induced emission (AIE)-active luminophores were converted to show intense photoluminescence (PL) even in a diluted solution at room temperature and the resulting compound worked as a single-molecule thermoresponsible material around room temperature. Because of the AIE-active unit, the molecular motion could be easily activated by heating, leading to variable and reversible PL intensity. Furthermore, the π-conjugated systems with the planar chirality of 4,7,12,15-tetrasubstituted [2.2]paracyclophane provided excellent characteristics on circular dichroism (CD) and circularly polarized luminescence (CPL). The obtained dimers showed high CPL performances both in a diluted solution and in an aggregation state. We succeeded in proving that simple molecular designs composed of only carbon and hydrogen atoms could create versatile optical functionalities.

A Flexible, Fused, Azomethine-Boron Complex: Thermochromic Luminescence and Thermosalient Behavior in Structural Transitions between Crystalline Polymorphs.

This manuscript reports the multi-functional boron complex presenting aggregation-induced emission (AIE), crystallization-induced emission enhancement (CIEE), and thermosalient behavior accompanying thermochromic luminescence during crystal-crystal transitions by employing the fused azomethine ligand. In particular, we propose that these properties can be explained by molecular "flexibility" toward external stimuli, including temperature changes and photo-excitation, and two types of crystal polymorphs with different absorption and luminescent properties were obtained. Optical measurements indicated that both polymorphs showed individual AIE and strong CIEE properties. From the investigations, the boron complex showed large structural relaxation and formed the bent structure in the excited state, followed by emission annihilation in the absence of structural restriction. In addition, it was shown that interconversion between these polymorphs could be reversibly induced by heating and cooling. The data from the single-crystal X-ray analyses suggested that alteration of crystal packing and intermolecular interaction should influence the luminescent chromism. Moreover, we also found that the crystals showed unusual mechanical behavior, such as hopping and fragmentation by heating and cooling, respectively, which is called thermosalient behavior. It was suggested that the loosely-fused structure could be responsible for expressing unique optical and mechanical properties.

Diarylamino- and Diarylboryl-Substituted Donor-Acceptor Pyrene Derivatives: Influence of Substitution Pattern on Their Photophysical Properties.

Dianisylamino donor (D) and dimesitylboryl acceptor (A) substituents were introduced at the 1,6- and 2,7-positions of pyrene to demonstrate that the substitution patterns influence the photophysical properties. The different pictures in orbital interactions between the pyrene core and the D-A substituents led to the outcome that 1,6-substituted pyrene derivative 1 had stronger electron-donating and electron-accepting properties in conjunction with a small HOMO-LUMO gap, as compared to the 2,7-substituted derivative. For these pyrene derivatives, modest (ΦF = 0.2) to strong (ΦF = 1.0) fluorescence was detected in degassed organic solvents; 1 exhibited a typical intramolecular charge transfer (ICT) emission obeying energy-gap law, while 2 displayed a moderate inverse energy-gap law, originating from the different substitution patterns. Although theoretical calculations predicted that both 1 and 2 adopt highly twisted ICT excited states (TICT excited states) even in the gas phase, but practically, it was suggested that the observed photophysical properties could be determined by the extent of twist angle of the TICT-like excited state in accordance with the solvent polarity. Moreover, the bulky D-A substituents inhibit the intermolecular direct π-π interactions, thereby resulting in the bright and moderate solid-state emissions for 1 (ΦF = 0.76) and 2 (ΦF = 0.21), respectively.

Development of the optical sensor for discriminating isomers of fatty acids based on emissive network polymers composed of polyhedral oligomeric silsesquioxane.

It was shown that water-soluble network polymers composed of polyhedral oligomeric silsesquioxane (POSS) had hydrophobic spaces inside the network because of strong hydrophobicity of the cubic silica cage. In this study, the water-soluble POSS network polymers connected with triphenylamine derivatives (TPA-POSS) were synthesized, and their functions as a sensor for discriminating the geometric isomers of fatty acids were investigated. Accordingly, in the photoluminescence spectra, different time-courses of intensity and peak wavelengths of the emission bands were detected from the TPA-POSS-containing solution in the presence of cis- or trans-fatty acids during incubation. Furthermore, variable time-dependent changes were obtained by changing coexisting ratios between two geometric isomers. From the mechanistic investigation, it was implied that these changes could be originated from the difference in the degree of interaction between the POSS networks and each fatty acid. Our data could be applicable for constructing a sensing material for generation and proportion of trans-fatty acids in the oil.

Enhancement of Aggregation-Induced Emission by Introducing Multiple o-Carborane Substitutions into Triphenylamine.

The enhancement of aggregation-induced emission (AIE) is presented on the basis of the strategy for improving solid-state luminescence by employing multiple o-carborane substituents. We synthesized the modified triphenylamines with various numbers of o-carborane units and compared their optical properties. From the optical measurements, the emission bands from the twisted intramolecular charge transfer (TICT) state were obtained from the modified triphenylamines. It was notable that emission efficiencies of the multi-substituted triphenylamines including two or three o-carborane units were enhanced 6- to 8-fold compared to those of the mono-substituted triphenylamine. According to mechanistic studies, it was proposed that the single o-carborane substitution can load the AIE property via the TICT mechanism. It was revealed that the additional o-carborane units contribute to improving solid-state emission by suppressing aggregation-caused quenching (ACQ). Subsequently, intense AIEs were obtained. This paper presents a new role of the o-carborane substituent in the enhancement of AIEs.

Synthesis of Optically Active, X-Shaped, Conjugated Compounds and Dendrimers Based on Planar Chiral [2.2]Paracyclophane, Leading to Highly Emissive Circularly Polarized Luminescence.

Optically active, Fréchet-type dendrimers containing an emissive X-shaped π-electron system as the core unit were synthesized. Gram-scale optical resolution and transformations of 4,7,12,15-tetrasubstituted [2.2]paracyclophanes were also carried out. The high-generation dendrons effectively absorbed UV light and transferred energy to the core, resulting in high photoluminescence (PL) from the core. In addition, the dendrons sufficiently isolated the emissive X-shaped conjugated core and bright emission was observed from both thin films and solutions. Intense circularly polarized luminescence (CPL) was observed from the thin film. The dendrimer films exhibited excellent optical properties, such as large molar extinction coefficients, high fluorescence quantum efficiencies, intense PL and CPL, and large CPL dissymmetry factors.

Planar chiral tetrasubstituted [2.2]paracyclophane: optical resolution and functionalization.

We achieved optical resolution of 4,7,12,15-tetrasubstituted [2.2]paracyclophane and subsequent transformation to planar chiral building blocks. An optically active propeller-shaped macrocyclic compound containing a planar chiral cyclophane core was synthesized, showing excellent chiroptical properties such as high fluorescence quantum efficiency and a large circularly polarized luminescence dissymmetry factor.