Optical Property Control by the Interligand Charge Transfer Excited State in Brominated Homoleptic and Heteroleptic Aluminum Dinuclear Triple-Stranded Helicates.

The utilization of aluminum, an abundant and inexpensive element, for the synthesis of novel functional complexes is extremely important, but the design and control of photofunctionality are still unexplored. In this study, we focused on our previously developed dinuclear triple-stranded helicates incorporating two aluminum ions (ALPHY) to synthesize both homoleptic and heteroleptic complexes with bromine atoms at the 3-position of the pyrrole moiety in the Schiff base ligands. The brominated Schiff base ligands were reacted with AlCl3 to synthesize homoleptic complexes, while different ligands were mixed to prepare heteroleptic complexes. Single-crystal X-ray structural analysis revealed the structures of these novel complexes. We found that increasing the degree of bromination resulted in a tunable emission color, shifting progressively from 550 (yellow) to 566 nm (orange). Optical resolution of the complexes facilitated the observation of mirror-image circular dichroism and circularly polarized luminescence. Furthermore, employing ultrafast spectroscopy techniques, we have elucidated that the optical properties are governed by the interligand charge transfer (ILCT) among the three ligands. The formation of heteroleptic complexes induces the ILCT state even in nonpolar environments, thereby accelerating nonradiative decay and intersystem crossing. These findings mark significant advancements in photofunctional materials based on multinuclear complexes.

Crystallographic and spectroscopic studies on persistent triarylpropargyl cations.

By acid treatment of precursor alcohols, mesitylethynyl-substituted diarylmethyl cations were isolated as stable solids, X-ray structural analyses of which revealed a planar geometry. Furthermore, the ion pairs including these triarylpropargyl cations form charge-segregated assemblies in the crystal, and effective intermolecular interaction induces a red-shift of absorption in the crystal.

Cation-Stacking Approach Enabling Interconversion between Bis(xanthylium) and its Reduced Species.

Cyclophane-type dications with two units of xanthylium were designed, with the expectation that intramolecular interaction between cation units could induce changes in absorption and redox behavior. The desired dications were synthesized via the macrocyclic diketone as a key intermediate, which was efficiently obtained by a stepwise etherification. X-ray and UV/Vis measurements revealed that the cyclophane-type dications adopt a stacking structure in both the crystal and solution. Due to the intramolecular interaction caused by π-π stacking of the xanthylium units, a considerable blue shift compared to the corresponding monocations and a two-stage one-electron reduction process were observed in the dications. Furthermore, upon electrochemical reduction of dications, the formation of biradicals via radical cation species was demonstrated by UV/Vis spectroscopy with several isosbestic points at both stages. Therefore, the cation-stacking approach is a promising way to provide novel properties due to perturbation of their molecular orbitals and to stabilize the reduced species even though they have open-shell characters.

Structural-Change-Induced Two-Stage Redox Behavior of Pentacenebisquinodimethane with Tricolor Chromism.

Tricolor electrochromism was realized through the interconversion among the neutral (yellow), dicationic (green), and tetracationic (blue) states, even though only one kind of chromophore is generated upon oxidation. Both dicationic and tetracationic states were isolated as stable salts, and their different colors come from the effective interchromophore interaction only in the tetracationic state but not in the dicationic state. Despite the negligible Coulombic repulsion in the tetracationic state with four cyanine-type chromophores, pentacenebisquinodimethane undergoes stepwise two-stage two-electron oxidation when radical-stabilizing 5-(4-octyloxyphenyl)-2-thienyl groups are attached on the exomethylene bonds. A contribution from the biradical form only in the neutral state but not in the dicationic state is the reason for the observed negative cooperativity during the electrochemical oxidation.

Thermal Equilibrium Between Quinoid/Biradical Forms Enhancing Electrochemical Amphotericity.

Upon dibenzo annulation on Thiele's hydrocarbon (tetraphenyl-p-quinodimethane), the quinoid form and the biradical form adopt quite different geometries, and thus are no longer resonance structures. When these two forms can interconvert rapidly due to the small energy barrier (ΔG≠), the equilibrated mixture contains both forms in a ratio that is determined by the energy difference (ΔGo) between the two forms. For a series of tetrakis[5-(4-methoxyphenyl)-2-thienyl]-substituted derivatives, the more stable quinoid form and the metastable biradical form coexist in solution as an equilibrated mixture due to small ΔG≠ (<15 kcal mol-1) and ΔGo (1-4 kcal mol-1), in which the proportion of the two forms can be regulated by temperature. Since the biradical form can undergo easy two-electron (2e) oxidation to the corresponding dications as well as easy 2e-reduction to the dianions, it exhibits very high electrochemical amphotericity. This character with a record-small span for not only the first oxidation and reduction potentials but also the second those, [E1 sum≈E2 sum=E2 ox-E2 red=ca. 1.4 V], is attained through thermally enhanced conversion to the biradical form from the corresponding quinoid form, the latter of which is less amphoteric due to higher Eox and lower Ered values.

Domino-Redox Reaction Induced by An Electrochemically Triggered Conformational Change.

The concept of a domino-type reaction has been applied in a wide range of fields such as synthetic organic chemistry, material engineering, and life science. To extend the domino concept to redox chemistry, we designed and synthesized a dimeric quinodimethane (QD) with a nonplanar dithiin spacer. The domino-redox properties can be activated by raising the temperature, based on a thermally equilibrated twisted conformation of QD, which has a higher HOMO level that is more readily oxidized. After one QD unit is oxidized (trigger), steric repulsion and electronic interaction between electrophores make the neighboring QD unit adopt a twisted conformation (domino process), which facilitates the following oxidation. Thus, a domino-redox reaction was achieved for the first time by a change in the HOMO level due to a drastic change in the molecular conformation.

On-Surface Synthesis of Multiple Cu Atom-Bridged Organometallic Oligomers.

A metal-metal bond between coordination complexes has the nature of a covalent bond in hydrocarbons. While bimetallic and trimetallic compounds usually have three-dimensional structures in solution, the high directionality and robustness of the bond can be applied for on-surface syntheses. Here, we present a systematic formation of complex organometallic oligomers on Cu(111) through sequential ring opening of 11,11,12,12-tetraphenyl-1,4,5,8-tetraazaanthraquinodimethane and bonding of phenanthroline derivatives by multiple Cu atoms. A detailed characterization with a combination of scanning tunneling microscopy and density functional theory calculations revealed the role of the Cu adatoms in both enantiomers of the chiral oligomers. Furthermore, we found sufficient strength of the bonds against sliding friction by manipulating the oligomers up to a hexamer. This finding may help to increase the variety of organometallic nanostructures on surfaces.

Smart Lipid Nanoparticle that Remodels Tumor Microenvironment for Activatable H2S Gas and Photodynamic Immunotherapy.

Hydrogen sulfide (H2S) has shown promise for gas therapy. However, it is still controversial whether H2S can remodel the tumor microenvironment (TME) and induce robust antitumor immunity. Here, a tumor-targeting and TME-responsive "smart" lipid nanoparticle (1-JK-PS-FA) is presented, which is capable of delivering and releasing H2S specifically in tumor tissues for on-demand H2S gas and photodynamic immunotherapy. 1-JK-PS-FA enables a burst release of H2S in the acidic TME, which promptly reduces the embedded organic electrochromic materials and consequently switches on near-infrared fluorescence and photodynamic activity. Furthermore, we found that high levels of H2S can reprogram the TME by reducing tumor interstitial fluid pressure, promoting angiogenesis, increasing vascular permeability, ameliorating hypoxia, and reducing immunosuppressive conditions. This leads to increased tumor uptake of 1-JK-PS-FA, thereby enhancing PDT efficacy and eliciting strong immunogenic cell death during 808 nm laser irradiation. Therefore, 1-JK-PS-FA permits synergistic H2S gas and photodynamic immunotherapy, effectively eradicating orthotopic breast tumors and preventing tumor metastasis and recurrence. This work showcases the capacity of H2S to reprogram the TME to enhance H2S gas and immunotherapy.

Carbon-based Biradicals: Structural and Magnetic Switching.

Sterically hindered C═C double bonds often deform into a bent or twisted geometry. Thus, many overcrowded ethylenes or anthraquinodimethanes can adopt multiple conformations, such as a folded form or a twisted form, which are interconvertible under the application of external stimuli. A perpendicular form with biradical character can also be adopted when designed to incorporate a stable carbon-based radical unit, which is involved in stimuli-responsive magnetic switching accompanied by a structural change. This review focuses on recent advances in the development of such strained π-electron systems and reveals the factors that affect the mutual interconversion and switching behavior. The energy barrier for the interconversion of conformational isomers is affected by the tricyclic skeleton or bulky substituents on the C═C double bonds, whereas the relative stability of the perpendicular biradical form increases with the additional insertion of 9,10-anthrylene units into the C═C double bonds.

One-Pot Synthesis of Helical Azaheptalene and Chiroptical Switching of an Isolable Radical Cation.

A nitrogen-centered heptalene, azaheptalene, was designed as a representative of a new class of redox-responsive molecules with a large steric strain that originates from the adjacent seven-membered rings. The pentabenzo derivative of azaheptalene was efficiently synthesized by a palladium-catalyzed one-pot reaction of commercially available reagents. Bromination led to mono- and dibrominated derivatives, the latter of which is interconvertible with isolable radical cation species exhibiting near-infrared absorption. Since the azaheptalene skeleton shows configurationally stable helicity with a large torsion angle, enantiomers could be successfully separated. Thus, optically pure azaheptalenes with P- or M-helicity showed strong chiroptical properties (|gabs |≥0.01), which could be changed by an electric potential.

Double Dynamic Structural Change Enabling Tricolor Chromism by the Realization of Apparent Two-Electron Transfer to Skip the Open-Shell State.

Most redox systems generally cannot avoid the involvement of open-shell species upon generating multiply charged species, which often reduces reversibility in multi-color electrochromic systems. In this study, we newly synthesized octakis(aminophenyl)-substituted pentacenebisquinodimethane (BQD) derivatives and their hybrids with alkoxyphenyl analogues. Thanks to apparent two-electron transfer accompanied by double dramatic changes in the structure of the arylated quinodimethane skeleton, the dicationic and tetracationic states were generated and isolated quantitatively because of the negligible steady-state concentration of intermediary open-shell species such as monocation or trication radicals. When two electrophores with different donating abilities are attached to the BQD skeleton, a dicationic state with a different color can be isolated in addition to the neutral and tetracationic states. For these tetracations, an interchromophore interaction induces a red-shift of the NIR absorptions, thus realizing tricolor UV/Vis/NIR electrochromic behavior involving only closed-shell states.

Bis(triarylmethylium)-type Macrocyclic Dications: Mechanochromic Emission Extending to the Red Region.

Macrocyclic dications 22+ composed of two triarylmethylium units were designed and synthesized. In contrast to the reference monocations 1+ , macrocyclic dications 22+ exhibited mechanochromic emission extending to the red region (-900 nm), since the luminescence color in a solid state can reversibly change due to their constrained structures granted by alkylene linkers and the choice of a proper counterion. X-ray diffraction and spectroscopic analyses revealed that such mechanochromic behavior was induced by the crystal-to-amorphous transition. A change in the intermolecular interaction of macrocyclic dications 22+ would be the key to realizing a change in the emission pattern, since the color of the molecules did not change by applying mechanical stimuli. These findings may suggest a design strategy for creating a variety of stimuli-responsive materials, especially for carbocation-based fluorescent materials.

Dibenzotropylium-Capped Orthogonal Geometry Enabling Isolation and Examination of a Series of Hydrocarbons with Multiple 14π-Aromatic Units.

A series of six dications composed of pure hydrocarbons with one to six non-substituted 9,10-anthrylene units end-capped with two dibenzotropyliums were designed and synthesized to elucidate the electronic properties of huge oligo(9,10-anthrylene) backbones. Their structures were successfully determined by X-ray analyses even in the case of eight planar 14π-electron units, revealing that all dications adopt almost orthogonally twisted structures between neighboring units. Spectroscopic and voltammetric analyses show that neither the significant overlap of orbitals nor the delocalization of electrons between 14π-electron units occurs due to the orthogonally twisted geometry even in solution. As a result, sequential oxidation processes were observed with the reversible formation of multivalent cations with the release of the same number of electrons as the number of anthrylene units. Upon two-electron reduction, a closed-shell butterfly-shaped form was obtained from the dication containing one anthrylene unit, whereas open-shell twisted biradicals were isolated as stable entities in the cases of derivatives containing three to six anthrylene units. Notably, from the derivative with two anthrylene units, a metastable open-shell isomer was obtained quantitatively and underwent slow thermal conversion to the most stable closed-shell isomer (Ea = 23.1 kcal mol-1). There is a drastic change in oxidation potentials between two neutral species (ΔE = 1.32 V in CH2Cl2). Since the present dications were regenerated upon oxidation of the isolated reduction products, these systems may contribute to the development of advanced response systems capable of switching color, magnetic properties, and oxidative properties by using a "cation-capped orthogonal geometry".

Enhancement of NIR-Absorbing Ability of Bis(diarylmethylium)-Type Dicationic Dyes Based on an Ortho-Substitution Strategy.

Electrochromic systems capable of switching near-infrared (NIR) absorption are fascinating from the viewpoint of applications in the materials and life sciences. Although 11,11,12,12-tetraaryl-9,10-anthraquinodimethanes (AQDs) with a folded form undergo one-stage two-electron oxidation to produce twisted dicationic dyes exhibiting NIR absorption, there is a need to establish a design strategy that can enhance the NIR-absorbing abilities of the corresponding dicationic dyes. In this study, we designed and synthesized a series of AQD derivatives with various substituents introduced at the ortho-position(s) of the 4-methoxyphenyl group. X-ray and spectroscopic analyses revealed that NIR-absorbing properties can be changed by introduction of the ortho-substituents. Thus, control of the steric and electronic effects of the ortho-substituents on the 4-methoxyphenyl groups was demonstrated to be an effective strategy for fine-tuning of the HOMO and LUMO levels for neutral AQDs and twisted dications, respectively, resulting in the modification of electrochemical and spectroscopic properties under an "ortho-substitution strategy".

Strain-Sensitive On-Surface Ladderization by Non-Dehydrogenative Heterocyclization.

On-surface cyclodehydrogenation recently became an important reaction to planarize π-conjugated molecules and oligomers. However, the high-activation barrier to cleave the C-H bond often requires high-temperature annealing, consequently restricting structures of precursor molecules and/or leading to random fusion at their edges. Here, we present a synthesis of pyrrolopyrrole-bridged ladder oligomers from 11,11,12,12-tetrabromo-1,4,5,8-tetraaza-9,10-anthraquinodimethane molecules on Ag(111) with bond-resolved scanning tunnelling microscopy. This non-dehydrogenative cyclization between pyrazine and ethynylene/cumulene groups has a low-activation barrier for forming intermediary dimeric oligomer containing dipyrazinopyrrolopyrrolopyrazine units, thus giving new insight into the strain-sensitive in ladder-oligomer formation.

Near-Infrared Electrochromic Behavior of Dibenzothiepin Derivatives Attached with Two Michler's Hydrol Blue Units.

10,11-Bis[bis(4-dimethylaminophenyl)methylene]dibenzo[bf]thiepin (1) and -oxepin (2) were prepared as stable yellow crystalline compounds, which are the cyclic analogues of electron-donating hexaarylbutadienes. Upon two-electron oxidation, they are reversibly transformed into the title dications (12+ and 22+ ) exhibiting near-infrared (NIR) absorptions, which were also isolated as stable salts. These redox pairs can serve as new entries into less well-explored organic NIR-electrochromic systems, and the separation of redox peaks (electrochemical bistability) was attained for 1/12+ and 2/22+ , thanks to drastic geometrical changes between neutral and dicationic states, as revealed by a series of X-ray analyses. Thiepin-S,S-dioxide analogue (3/32+ ) exhibits quite similar dynamic redox behavior due to nonaromatic nature of the dibenzothiepin and -oxepin unit in 12+ and 22+ , whereas the thiepin-S-oxide derivative (4/42+ ) does not exhibit bistability due to the smaller change in geometry upon electron transfer, showing that a subtle change of a bridging atom in the central seven-membered ring can modify the redox properties.

Multiple molecular interactions between alkyl groups and dissociated bromine atoms on Ag(111).

Multiple intermolecular interactions offer a high degree of controllability of on-surface molecular assemblies. Here, two kinds of molecular networks were formed by depositing 11,11,12,12-tetrabromo-1,4,5,8-tetraaza-9,10-anthraquinodimethane derivatives with two different alkyl groups in length (C4 and C8) on clean Ag(111) surfaces under ultrahigh vacuum. The detailed structures of each network before and after the cleavage of the C-Br bonds were investigated with high-resolution scanning tunneling microscopy at low temperature. We found that the diffusion of the Br atoms by high-temperature annealing plays a role in the formation of Br-mediated self-assembly. While dissociated Br atoms interacted with alkyl groups by hydrogen bonding through C-H⋯Br contacts in both systems, the different strengths of the van der Waals interactions between the alkyl groups resulted in the formation of different structures.

Geometrical and Electronic Structure of Cation Radical Species of Tetraarylanthraquinodimethane: An Intermediate for Unique Electrochromic Behavior.

Two tetraarylanthraquinodimethane (Ar4 AQD) derivatives having two different aryl groups (aminophenyl and methoxyphenyl) were prepared by sequential dibromomethylation and Suzuki-coupling reactions. X-ray analyses showed that they adopt a folded structure in the neutral state whereas the corresponding dications have a planar anthracene ring, to which diarylmethylium units are perpendicularly attached. Different from Ar4 AQD having the same substituents that undergoes facile two-electron transfer during interconversion with the dicationic state, the intermediary cation radical becomes long-lived in the newly prepared unsymmetric derivatives. The geometric and electronic structures of the open-shell intermediates were elucidated through electrochemical and theoretical investigation, with revealing that the cation radicals adopt the twisted geometry like dications. Upon electrolyses of the dications, the twisted cation radicals were involved in the electrochromism whereas their steady-state concentration is negligible in the oxidation process, thus realizing unique tricolor electrochromic behavior with a hysteretic pattern of color change (colorless -> purple -> blue -> colorless).

A Ratiometric Photoacoustic Probe with a Reversible Response to Hydrogen Sulfide and Hydroxyl Radicals for Dynamic Imaging of Liver Inflammation.

Reversible imaging probes that allow for the dynamic visualization of the redox cycle between hydroxyl radical (⋅OH) and hydrogen sulfide (H2 S) are vital to probe the redox imbalance-involved pathological process in vivo. Herein, we report a reversible ratiometric photoacoustic (PA) imaging nanoprobe (1-PAIN) for the real-time imaging of ⋅OH/H2 S redox cycle in vivo. 1-PAIN displays a low PA ratio between 690 and 825 nm (PA690 /PA825 ), which significantly increases by ≈5-fold upon oxidation by ⋅OH, and is switched back to the initially low PA690 /PA825 value upon reduction by H2 S. 1-PAIN could dynamically report on the hepatic ⋅OH production in mice during the lipopolysaccharide (LPS)-induced liver inflammation process, and visualize hepatic H2 S generation during the N-acetyl cysteine (NAC)-induced anti-inflammation process. 1-PAIN can act as a useful tool to probe the redox state in living biology, beneficial for the study of redox imbalance-related diseases.

Redox-Active Hydrocarbons: Isolation and Structural Determination of Cationic States toward Advanced Response Systems.

Invited for this month's cover is the group of Prof. Yusuke Ishigaki at Hokkaido University, Japan. The cover picture shows a cyclic voltammogram drawn in the style of an ink painting, along with the structures of cationic species composed of pure hydrocarbons. The sunrise, a metaphor for X-rays irradiated on cations, is reminiscent of the dawn of redox chemistry of hydrocarbons. Elucidation of their redox properties should provide important insights into the development of organic electronic devices. The cover was designed by YAP Co., Ltd. More information can be found in the Review by T. Harimoto and Y. Ishigaki.