Preparation of seven-coordinated hypervalent tin(IV)-fused azobenzene and applications for stimuli-responsive π-conjugated polymer films.

Heavy atoms can form highly coordinated states, and their optical properties have attracted much attention. Recently, we have demonstrated that a reversible coordination-number shift of hypervalent tin(IV) from five to six can provide predictable hypsochromic shifts in light absorption and emission properties in small molecules and a π-conjugated polymer film. Herein, we show the preparation of seven-coordinated tin and reveal that the binding constant of the seven coordination with ethylenediamine (EDA, K = 2900 M-1) is 200 times higher than that of six coordination with propylamine (PA, K = 14 M-1) owing to the chelate effect. Moreover, reversible vapochromism of the π-conjugated polymer film was observed upon exposure (λabs = 598 nm and λPL = 697 nm) and desorption (λabs = 641 nm and λPL = 702 nm) of EDA vapor. Furthermore, as a unique demonstration, the thermochromic film was prepared by fixing the seven coordination as the initial state using 1,10-phenanthroline. These optical variations are predictable by quantum chemical calculations. Our findings are valuable for the development of designable and controllable stimuli-responsive materials focusing on the inherent properties of the elements.

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.

Solvent-free synthesis and chiroptical properties of a C-N axially chiral cruciform dimer of benzo[b]phenoxazine.

A novel C-N axially chiral molecule composed of two tert-butyl-substituted benzo[b]phenoxazine (BPO) was synthesized via solvent-free reactions. The absolute configurations of the enantiomers were determined by X-ray single-crystal analysis. The enantiomers had a sufficiently high racemization barrier to ignore racemization at room temperature (149 ± 20 kJ mol-1), and the solutions exhibited dual circularly polarized emissions stemming from fluorescence and phosphorescence of |gCPL| = ca. 1 × 10-3.

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.

Synthesis of Regioregular and Random Boron-Fused Azomethine Conjugated Polymers for Film Morphology Control.

Regioregular and random conjugated polymers based on a boron-fused azomethine unit were synthesized by Sonogashira-Hagihara cross coupling reaction. Although these polymers exhibited similar optical properties in the solution states, a distinct difference was observed in the aggregation forming ability in the film states; scanning electron microscope (SEM) observation indicated the existence of fiber-like aggregates in the spin-coated film of the regioregular polymer, while regiorandom polymer showed no aggregate in the film state. Accordingly, the UV-vis absorption spectrum of the regioregular polymer showed an increased shoulder peak due to the aggregate formation, whereas the random one showed no change. Furthermore, an absolute fluorescence quantum efficiency of the regioregular polymer was enhanced in response to the aggregate disassembly via thermal annealing treatment. In this study, we demonstrate that controlling regioregularity of the conjugated polymers can induce the different morphological structures and thermal-responsive behaviors. These findings could be beneficial for the design strategy and potential applications of thin-film optoelectronic devices with stimuli-responsive properties.

Benzannulated Double Aza[9]helicenes: Synthesis, Structures, and (Chir)optical Properties.

A benzannulated double aza[9]helicene 1 was successfully synthesized via a one-pot oxidative fusion reaction. 1 was derivatized to N-alkylated double aza[9]helicene 1-Et and 1-Bu, whose structures were determined by X-ray diffraction analysis. 1-Et and 1-Bu exhibited red-shifted absorption and fluorescence spectra compared to single aza[9]helicene. The double aza[9]helicenes were expected to have two different conformers. Consistent with solid-state structure, the chiral-isomer was estimated to be more stable by 16 kcal/mol relative to meso-isomer. Indeed, enantiomers of 1-Et and 1-Bu were optically resolved by HPLC and showed mirror-imaged CD and CPL spectra with the CPL brightness up to 19.2 M-1cm-1 for 1-Bu.

Stimuli-Responsive π-Conjugated Polymers Showing Solid-State Emission Based on Boron-Fused Azomethine Complexes with NNO-Tridentate Ligands.

We report stimuli-responsive luminescent π-conjugated polymers involving the boron-fused azomethine (BAm) structure with the NNO-tridentate ligands and demonstrate their application to film-type sensors. The acid-responsive properties of the polymers are provided by the free lone-pair electrons of the nitrogen atom on the BAm unit. The polymer films showed red shifts in emission wavelengths under acid vapor, and the responses were reversible and corresponded to deprotonation. Furthermore, owing to environmental sensitivity in the film state, unique luminescent color changes were observed. The bilayer films of the BAm polymer coated by poly(n-butyl methacrylate) exhibited vapochromic luminescence with gradual blue shifts (orange → yellow → green) by vapor annealing with chloroform depending on the exposure time. We demonstrated that these changes were applicable for evaluating solvent-vapor permeability of target membranes. This study shows that nitrogen substitution into existing organic compounds is effective for creating stimuli-responsive materials.

Efficient synthesis and unit-selective π-extension of π-fused [4.3.3]propellane as a chiral building block.

A three-dimensional skeleton, π-fused [4.3.3]propellane, was constructed and derivatized by selective π-extension at the two naphthalene units. The obtained propellanes existed as stereoisomers different in spatial arrangement, one of which displayed a chiroptical response originating from through-space interactions between 5-azachrysenes in a skew position.

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.

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.

Cyclic arrays of five pyrenes on one rim of a planar chiral pillar[5]arene.

Spatial arrangement of multiple planar chromophores is an emerging strategy for molecule-based chiroptical materials via easy and systematic synthesis. We attached five pyrene planes to a chiral macrocycle, pillar[5]arene, producing a set of chiroptical molecules in which pyrene-derived absorption and emission were endowed with dissymmetry by effective transfer of chiral information. The chiroptical response was dependent on linker structures and substituted patterns because of variable interactions between pyrene units. One of these hybrids showed larger dissymmetry factor and response wavelength (g lum = 7.0 × 10-3 at ca. 547 nm) than reported pillar[5]arene-based molecules using the pillar[5]arene cores as parts of photo-responsive π-conjugated units.

Development of a fluoride-anion sensor based on aggregation of a dye-modified polyhedral oligomeric silsesquioxane.

We report a new concept for the turn-on fluoride sensor based on the aggregation of dye-modified polyhedral oligomeric silsesquioxane (POSS). The dye-modified POSS aggregation initially shows weak fluorescence, while intense fluorescence can be obtained when fluoride breaks POSS cores following dye release.

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.

CPL on/off control of an assembled system by water soluble macrocyclic chiral sources with planar chirality.

Herein, we report the synthesis and planar chiral properties of a pair of water-soluble cationic pillar[5]arenes with stereogenic carbons. Interestingly, although units of the molecules were rotatable, only one planar chiral diastereomer existed in water in both cases. As a new type of chiral source, these molecules transmitted chiral information from the planar chiral cavities to the assembly of a water-soluble extended π-conjugated compound, affording circularly polarized luminescence (CPL). The chirality transfer process and resulting CPL were extremely sensitive to the feed ratio of the chiral pillar[5]arenes owing to the combined action of their planar chirality, bulkiness, and strong binding properties. When a limited amount of chiral source was added, further assembly of the extended π-conjugated compound into helical fibers with CPL was triggered. Unexpectedly, larger amounts of chiral source destroyed the helical fiber assemblies, resulting in elimination of the chirality and CPL properties from the assembled structures.

Asymmetric Lumino-Transformer: Circularly Polarized Luminescence of Chiral Eu(III) Coordination Polymer with Phase-Transition Behavior.

A chiral Eu(III) coordination polymer with phase-transition behavior, [Eu(+tfc)3(m-dpeb)]n, (+tfc: (+)-3-trifluoroacetylcamphorato, m-dpeb: 1,3-bis(diphenylphosphorylethynyl)benzene) was reported for understanding the effect of polymer chain arrangement (orientation effect) on the circularly polarized luminescence (CPL) in a solid system. The phase-transition behavior of the transformable Eu(III) coordination polymer was characterized using differential scanning calorimetry and powder X-ray diffraction. The Eu(III) coordination polymer exhibited phase transition at approximately 180 °C. The magnitude of the CPL intensity was drastically changed because of the phase transition, without coordination geometrical change around the Eu(III) ion. In this study, the orientation effect of a chiral Eu(III) coordination polymer on the CPL properties in crystalline solid is demonstrated.

Recent Progress on Designable Hybrids with Stimuli-Responsive Optical Properties Originating from Molecular Assembly Concerning Polyhedral Oligomeric Silsesquioxane.

In this review, we describe recent progress on stimuli-responsive hybrid materials based on polyhedral oligomeric silsesquioxane (POSS) and their applications as a chemical sensor. In particular, we explain the unique functions originating from molecular assembly concerning POSS-containing soft materials mainly from our studies. POSS has an inorganic cubic core composed of silicon-oxygen (Si-O) bonds and organic substituents at each vertex. Owing to intrinsic properties of POSS, such as high thermal stability, rigidity, and low chemical reactivity, various robust hybrid materials have been developed. From the numerous numbers of POSS hybrids, we herein focus on the environment-sensitive optical materials in which molecular assembly of POSS itself and functional units connected to POSS should be a key factor for expressing material properties. We also explain the mechanisms of chemical sensors originating from these stimuli-responsive optical properties. Stimuli-responsive excimer emission and pollutant detectors, nanoplastic sensors with the water-dispersive POSS networks, trans fatty acid sensors, turn-on luminescent sensors for aerobic condition and fluoride anion sensors are described. We also mention the mechanochromic polyurethane hybrids and the thermally-durable mechanochromic luminescent materials. The roles of the unique optical properties from soft materials composed of rigid POSS, which doesn't have significant light-absorption and emission properties in the visible region, are surveyed.

Development of NIR emissive fully-fused bisboron complexes with π-conjugated systems including multiple azo groups.

Development of novel near-infrared (NIR) emitters is essential for satisfying the growing demands of advancing optical telecommunication and medical technology. We synthesized elemental skeletons composed of robust π-conjugated systems including two boron-fused azo groups, which showed an intense emission in the red or near-infrared (NIR) region both in solution and solid states. Two types of bisboron complexes with different aromatic linkers showed emission properties with larger bathochromic shifts and emission efficiencies in solution than the corresponding monoboron complex. Transient absorption spectroscopy disclosed that the inferior optical properties of the monoboron complex can be attributed to fast nonradiative deactivation accompanied by a large structural relaxation after photoexcitation. The expanded π-conjugated system through multiple boron-fused azo groups can contribute to rigid molecular skeletons followed by improved emission properties. Moreover, the anti-form of the bisboron complex with fluorine groups in the opposite directions to the π-plane exhibited crystallization-induced emission enhancement in the NIR region. The molecular design by using multiple boron-fused azo groups is expected to be a critical strategy for creating novel NIR emitters.

Molecular design and application of luminescent materials composed of group 13 elements with an aggregation-induced emission property.

Complexation of π-conjugated ligands by metal or semimetal ions leads to the enhancement of the planarity and rigidity of π-conjugated systems. Boron, especially, has played a central role in the design of luminescent main-group complexes. However, these complexes still suffer the disadvantage of aggregation-caused quenching as well as typical organic fluorophores. It has recently been reported that some types of boron complexes exhibit the aggregation-induced emission (AIE) property. Moreover, AIE behavior from complexes and organometallic compounds composed of the other group 13 elements, such as aluminum and gallium, has emerged in this decade. These observations greatly encourage us to develop advanced functional materials based on the group 13 elements. Indeed, recent research has demonstrated that these classes of materials are potentially versatile scaffolds for constructing chromic luminophores, efficiently emissive π-conjugated polymers and so on. This review mainly describes AIE-active group 13 complexes with four-coordinate structures and their application as photo-functional materials. Proposed mechanisms of the origins of AIE behavior are briefly discussed.

Stimuli-Responsive Self-Assembly of π-Conjugated Liquids Triggers Circularly Polarized Luminescence.

We developed novel room-temperature stimuli-responsive N-heteroacene-based liquid materials bearing a chiral alkyl chain. When these liquid materials were exposed to HCl vapor as an external stimulus, a disordered-ordered state change occurred immediately to yield self-assembled solid states from fluidic liquids. The self-assembly mechanism during this state change was evaluated by experimental in situ observations and molecular dynamics simulations over various spatiotemporal scales. These self-assembled structures led to supramolecular chirality through the influence of the chiral alkyl chain. As a result, circularly polarized luminescence (CPL) was triggered in the solid state, which was absent in the precursor liquid, thereby rendering this the first report on a stimuli-responsive CPL on/off liquid material. In addition, the initial state was recovered by exposure to air or upon heating. Moreover, the synergy between the experimental and the theoretical studies opens a new avenue to develop a novel class of stimuli-responsive materials and to discover novel phenomena in such materials.

Double Heterohelicenes Composed of Benzo[b]- and Dibenzo[b,i]phenoxazine: A Comprehensive Comparison of Their Electronic and Chiroptical Properties.

Heterohelicenes are potential materials in molecular electronics and optics because of their inherent chirality and various electronic properties originating from the introduced heteroatoms. In this work, we comprehensively investigated two kinds of double NO-hetero[5]helicenes composed of 12H-benzo[b]phenoxazine (BPO) and 13H-dibenzo[b,i]phenoxazine (DBPO). These helicenes exhibit good electron-donor properties reflecting the electron-rich character of their monomers and were demonstrated to work as p-type semiconductors. The enantiomers of these helicenes show the largest class of dissymmetry factors for circularly polarized luminescence (CPL) (|gCPL| > 10-2) among helicenes reported to date. Interestingly, the signs of CPL are opposite for BPO and DBPO double helicenes of the same helicity. The origin of the large gCPL values and the inversion of the CPL sign was addressed by analysis of the transition electronic dipole moments and transition magnetic dipole moments based on TD-DFT calculations.