Ion Pairing of Cationic and Anionic Ir(III) Photosensitizers for Photocatalytic CO2 Reduction at Lipid-Membrane Surfaces.

We synthesized an ion pair comprising cationic and anionic Ir(III) photosensitizers ([Ir1+][Ir2-]) for photocatalytic CO2 reduction and showed that the cationic component imparts stability, while the cyclometalating ligands in the anionic component ensure effective visible-light absorption. The triplet excited state of [Ir1+] is the key photoredox species in this system and is mainly generated through the transfer of triplet excitation energy from the anionic moiety due to Coulombic interactions and appropriate triplet energy alignment between the two ionic components. The positive photosensitization effect of ion pairing was demonstrated by photocatalytic CO2 reduction in cooperation with a Re(I) molecular catalyst incorporated into a vesicle membrane.

Spontaneous preparation of a fluorescent ratiometric chemosensor for metal ions using off-the-shelf materials.

A self-assembled chemosensor prepared using off-the-shelf materials has shown various fluorescence responses including ratiometric and simple ON-OFF switching profiles by adding different toxic metal ions. The unique fingerprint-like responses have been applied to pattern recognition of metal ions in river water for environmental analysis.

Zn(II)-Dipicolylamine-Attached Amphiphilic Polythiophene for Quantitative Pattern Recognition of Oxyanions in Mixtures.

Herein, we propose a novel amphiphilic polythiophene-based chemosensor functionalized with a Zn(II)-dipicolylamine side chain (1poly ⋅ Zn) for the pattern recognition of oxyanions. Optical changes in amphiphilic 1poly ⋅ Zn can be induced by the formation of a random coil from a backbone-planarized structure upon the addition of target oxyanions, which results in blueshifts in the UV-vis absorption spectra and turn-on-type fluorescence responses. Dynamic behavior in a polythiophene wire and/or among wires could be a driving force for obtaining visible color changes, while the molecular wire effect is dominant in obtaining fluorescence sensor responses. Notably, the magnitude of optical changes in 1poly ⋅ Zn has depended on differences in properties of oxyanions, such as their binding affinity, hydrophilicity, and molecular geometry. Thus, various colorimetric and fluorescence response patterns of 1poly ⋅ Zn to oxyanions were obtained, albeit using a single chemosensor. A constructed information-rich dataset was applied to pattern recognition for the simultaneous group categorization of phosphate and carboxylate groups and the prediction of similar structural oxyanions at a different order of concentrations in their mixture solutions.

Triarylamine-Substituted Benzimidazoliums as Electron Donor-Acceptor Dyad-Type Photocatalysts for Reductive Organic Transformations.

Triarylamine-substituted benzimidazoliums (BI+-PhNAr2), new electron donor-acceptor dyad molecules, were synthesized. Their photocatalytic properties for reductive organic transformations were explored using absorption and fluorescence spectroscopy, redox potential determinations, density functional theory calculations, transient absorption spectroscopy, and reduction reactions of selected substrates. The results show that irradiation of BI+-PhNAr2 promotes photoinduced intramolecular electron transfer to form a long-lived (∼300 µs) charge shifted state (BI•-PhN•+Ar2). In the pathway for photocatalysis of reduction reactions of substrates, BI•-PhN•+Ar2 is subsequently transformed to the neutral benzimidazolyl radical (BI•-PhNAr2) by single-electron transfer from the donor 1,3-dimethyl-2-phenylbenzimidazoline (BIH-Ph) serving as a cooperative agent. Among the benzimidazoliums explored, the bromo-substituted analogue BI+-PhN(C6H4Br-p)2 in conjunction with BIH-Ph demonstrates the most consistent catalytic performance.

Hydrogen Production from Hydrophobic Ruthenium Dye-Sensitized TiO2 Photocatalyst Assisted by Vesicle Formation.

Dye-sensitized H2 evolution photocatalysts have attracted considerable attention as promising systems for the photochemical generation of H2 from water. In this study, to mimic the reaction field of natural photosynthesis artificially, we synthesized a hydrophobic Ru(II) dye-sensitized Pt-TiO2 nanoparticle photocatalyst, RuC9@Pt-TiO2 (RuC9 = [Ru(dC9bpy)2(H4dmpbpy)]2+; dC9bpy = 4,4'-dinonyl-2,2'-bipyridine, H4dmpbpy = 4,4'-dimethyl phosphonic acid-2,2'-bipyridine), and integrated it into 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) lipid bilayer vesicle membranes. The photocatalytic H2 production activity in 0.5 M l-ascorbic acid aqueous solution enhanced by more than three times in the presence of DPPC vesicles (apparent quantum yield = 2.11%), whereas such a significant enhancement was hardly observed when the vesicle formation was omitted. These results indicate that the highly dispersed state of the hydrophobic RuC9@Pt-TiO2 nanoparticles in the DPPC bilayer vesicles is a key factor in achieving enhanced photocatalytic H2 production activity in aqueous solution.

Light-induced electron transfer/phase migration of a redox mediator for photocatalytic C-C coupling in a biphasic solution.

Photocatalytic molecular conversions that lead to value-added chemicals are of considerable interest. To achieve highly efficient photocatalytic reactions, it is equally important as it is challenging to construct systems that enable effective charge separation. Here, we demonstrate that the rational construction of a biphasic solution system with a ferrocenium/ferrocene (Fc+/Fc) redox couple enables efficient photocatalysis by spatial charge separation using the liquid-liquid interface. In a single-phase system, exposure of a 1,2-dichloroethane (DCE) solution containing a Ru(II)- or Ir(III)-based photosensitizer, Fc, and benzyl bromide (Bn-Br) to visible-light irradiation failed to generate any product. However, the photolysis in a H2O/DCE biphasic solution, where the compounds are initially distributed in the DCE phase, facilitated the reductive coupling of Bn-Br to dibenzyl (Bn2) using Fc as an electron donor. The key result of this study is that Fc+, generated by photooxidation of Fc in the DCE phase, migrates to the aqueous phase due to the drastic change in its partition coefficient compared to that of Fc. This liquid-liquid phase migration of the mediator is essential for facilitating the reduction of Bn-Br in the DCE phase as it suppresses backward charge recombination. The co-existence of anions can further modify the driving force of phase migration of Fc+ depending on their hydrophilicity; the best photocatalytic activity was obtained with a turnover frequency of 79.5 h-1 and a quantum efficiency of 0.2% for the formation of Bn2 by adding NBu4+Br- to the biphasic solution. This study showcases a potential approach for rectifying electron transfer with suppressed charge recombination to achieve efficient photocatalysis.

A Photocatalytic System Composed of Benzimidazolium Aryloxide and Tetramethylpiperidine 1-Oxyl to Promote Desulfonylative α-Oxyamination Reactions of α-Sulfonylketones.

A new photocatalytic system was developed for carrying out desulfonylative α-oxyamination reactions of α-sulfonylketones in which α-ketoalkyl radicals are generated. The catalytic system is composed of benzimidazolium aryloxide betaines (BI+-ArO-), serving as visible light-absorbing electron donor photocatalysts, and 2,2,6,6-tetramethylpiperidine 1-oxyl (TEMPO), playing dual roles as an electron donor for catalyst recycling and a reagent to capture the generated radical intermediates. Information about the detailed nature of BI+-ArO- and the photocatalytic processes with TEMPO was gained using absorption spectroscopy, electrochemical measurements, and density functional theory calculations.

Triplet Excited States Modulated by Push-Pull Substituents in Monocyclometalated Iridium(III) Photosensitizers.

A series of novel monocyclometalated [Ir(tpy)(btp)Cl]+ complexes (Ir2-Ir5) were synthesized using 2,2':6',2″-terpyridine (tpy) and 2-(2-pyridyl)benzo[b]thiophene (btp) ligands, as well as their derivatives bearing electron-donating tert-butyl (t-Bu) and electron-withdrawing trifluoromethyl (CF3) groups. Ir2-Ir5 exhibited visible-light absorption stronger than that of the known complex [Ir(tpy)(ppy)Cl]+ (Ir1; ppy = 2-phenylpyridine). Spectroscopic and computational studies revealed that two triplet states were involved in the excited-state dynamics. One is a weakly emissive and short-lived ligand to ligand charge-transfer (LLCT) state originating from the charge transfer from the btp to the tpy ligand. The other is a highly emissive and long-lived ligand-centered (LC) state localized on the btp ligand. Interestingly, the excited state dominant with 3LLCT was completely changed to the 3LC state upon the introduction of substituents on both the tpy and btp ligands. For instance, the excited state of the parent complex Ir2 was weakly emissive (Φ = 2%) and short-lived (τ = 110 ns) in CH2Cl2; conversely, Ir5, fully furnished with t-Bu and CF3 groups, displayed intense phosphorescence with a prolonged lifetime (τ = 14 µs). This difference became increasingly prominent when the solvent was changed to aqueous CH3CN, most probably due to the 3LLCT stabilization. The predominant excited-state nature was switchable between the 3LLCT and 3LC states depending on the substituents employed; this was demonstrated through investigations of Ir3 and Ir4, bearing either the t-Bu or the CF3 group, where the complexes exhibited properties intermediate between those of Ir2 and Ir5. All of the Ir(III) complexes were tested as photosensitizers in photocatalytic H2 evolution over a Co molecular catalyst, and Ir5 outperformed the others, including Ir1, due to improvement in the following key properties: visible-light-absorption ability, excited-state lifetime, and reductive power of the one-electron-reduced species against the catalyst.

Easy-to-Prepare Mini-Chemosensor Array for Simultaneous Detection of Cysteine and Glutathione Derivatives.

We herein propose an easy-to-prepare mini-chemosensor array constructed by two-types of off-the-shelf coumarin dyes for simultaneous classification and quantification of sulfur-containing amino acids (SCAAs) (i.e., l-cysteine, l-cystine, l-homocysteine, glutathione, and glutathione disulfide). The detection mechanism of SCAAs relied on a coordination-based sensor array (CBSA) utilizing the competitive binding among the coumarin dye, a Zn2+ ion and SCAAs. The reversible feature of the coordination bond of the coumarin-Zn2+ resulted in UV/vis spectral shifts and fluorescence enhancement in response to the analytes, offering the multianalyte detection in high accuracy. Furthermore, a mixture of glutathione and l-cysteine was successfully quantified in a sample containing human blood serum. This study would be an important example of the optical chemosensor array toward the rapid and accurate detection of biomarkers.

A Water-Gated Organic Thin-Film Transistor for Glyphosate Detection: A Comparative Study with Fluorescence Sensing.

Invited for the cover of this issue is the group of Tsuyoshi Minami at the University Tokyo. The image illustrates that despite being fabricated with the same polythiophene material, a water-gated organic thin-film transistor is a more sensitive device than a fluorescence sensor chip. Read the full text of the article at 10.1002/chem.202003529.

A Water-Gated Organic Thin-Film Transistor for Glyphosate Detection: A Comparative Study with Fluorescence Sensing.

This work reports the design of a highly sensitive solid-state sensor device based on a water-gated organic thin-film transistor (WG-OTFT) for the selective detection of herbicide glyphosate (GlyP) in water. A competitive assay among carboxylate-functionalized polythiophene, Cu2+ , and GlyP was employed as a sensing mechanism. Molecular recognition phenomena and electrical double layer (EDL) (at the polymer/water interface) originated from the field-effect worked cooperatively to amplify the sensitivity for GlyP. The limit of detection of WG-OTFT (0.26 ppm) was lower than that of a fluorescence sensor chip (0.95 ppm) which is the conventional sensing method. In contrast to the previously reported insulated molecular wires to block interchain interactions, molecular aggregates under the field-effect has shown to be effective for amplification of sensitivity through "intra"- and "inter"-molecular wire effects. The opposite strategy in this study could pave the way for fully utilizing the sensing properties of polymer-based solid-state sensor devices.

Supramolecular Sensor for Astringent Procyanidin C1: Fluorescent Artificial Tongue for Wine Components.

An artificial tongue that detects astringent components for a comprehensive evaluation of taste has not been established to date. Herein, we first propose fluorescent polythiophene (PT) derivatives (S1-S3) modified with 3-pyridinium boronic acid as supramolecular chemosensors for wine components including astringent procyanidin C1. After numerous attempts for the synthetic conditions, more than 95 mol % of the PT unit was modified with the pyridinium boronic acid moiety. To evaluate the PT derivatives as chemosensors of the artificial tongue, qualitative and quantitative analyses were performed with four types of wine components (i.e., sweet, sour, bitter, and astringent tastes) in combination with pattern recognition models. Notably, procyanidin C1 in the actual wine sample was successfully detected in a quantitative manner. In other words, we have established an authentic artificial tongue using PT based supramolecular chemosensors.

Protocol for Visible-Light-Promoted Desulfonylation Reactions Utilizing Catalytic Benzimidazolium Aryloxide Betaines and Stoichiometric Hydride Donor Reagents.

An unprecedented photocatalytic system consisting of benzimidazolium aryloxide betaines (BI+-ArO-) and stoichiometric hydride reducing reagents was developed for carrying out desulfonylation reactions of N-sulfonyl-indoles, -amides, and -amines, and α-sulfonyl ketones. Measurements of absorption spectra and cyclic voltammograms as well as density functional theory (DFT) calculations were carried out to gain mechanistic information. In the catalytic system, visible-light-activated benzimidazoline aryloxides (BIH-ArO-), generated in situ by hydride reduction of the corresponding betaines BI+-ArO-, donate both an electron and a hydrogen atom to the substrates. A modified protocol was also developed so that a catalytic quantity of more easily prepared hydroxyaryl benzimidazolines (BIH-ArOH) is used along with a stoichiometric hydride donor to promote the photochemical desulfonylation reactions.

Simplest Chemosensor Array for Phosphorylated Saccharides.

We believe that "the simpler we are, the more complete we become" is a key concept of chemical sensing systems. In this work, a "turn-on" fluorescence chemosensor array relying on only two self-assembled molecular chemosensors with ability of both qualitative and quantitative detection of phosphorylated saccharides has been developed. The easy-to-prepare chemosensor array was fabricated by in situ mixing of off-the-shelf reagents (esculetin, 4-methylesculetin, and 3-nitrophenylboronic acid). The fluorescence-based saccharide sensing system was carried out using indicator displacement assay accompanied by photoinduced electron transfer (PeT) under various pH conditions. The simultaneous recognition of 14 types of saccharides including glucose-6-phosphate (G6P) and fructose-6-phosphate (F6P) was achieved with a successful classification rate of 100%. We also succeeded in the quantitative analysis of a mixture of glucose (Glc), as an original substrate, G6P and F6P, as enzymatic products in pseudoglycolysis pathway. Finally, levels of Glc and F6P in human induced pluripotent stem (hiPS) cells were indirectly monitored by using our proposed chemosensor array. Glc and F6P in supernatants of hiPS cells were classified by linear discriminant analysis as a pattern recognition model and the observed clusters represent the activity of hiPS cells. The results show the high accuracy of the proposed chemosensor array in detection of phosphorylated and similarly modified saccharides.

Photofunctions of iridium(iii) complexes in vesicles: long-lived excited states and visible-light sensitization for hydrogen evolution in aqueous solution.

Recently, the photochemistry of cationic bis-cyclometalated Ir(iii) complexes has gained increasing attention in interdisciplinary studies owing to the characteristic photofunctions of the complexes, including long-lived and strong phosphorescence, visible-light absorption, and photosensitizing properties. Herein, two Ir(iii) complexes, Ir2 and Ir3, composed of coumarin 6 and different ancillary ligands (i.e., 4,4'-dimethyl-2,2'-bipyridine and 2-(2-pyrazolyl)pyridine) were prepared and characterized to investigate their photophysical properties in several solvents. These compounds were incorporated into DPPC vesicles to investigate their photosensitizing properties in an aqueous solution. Ir2 and Ir3 absorbed light strongly in the visible region and exhibited similar phosphorescence spectral profiles dominated by the coumarin ligands in all tested environments. In contrast, the ancillary ligands and the surrounding environment influenced the excited-state deactivation pathways. For instance, the phosphorescence of Ir2 was weak in highly polar solvents and the vesicle membrane, and in contrast, Ir3 exhibited intense and surprisingly long-lived phosphorescence in these media (Φ = 25-38% and τ = 32-41 µs). Visible-light illumination of Ir2 and Ir3 bound to the membrane surface resulted in H2 evolution when an electron donor and a water-soluble Ni(ii) catalyst were present in the outer aqueous phase. Under these conditions, the reaction system incorporating Ir2 performed much better than the Ir3 system in terms of photosensitizer stability and turnover frequency.

An anionic iridium(iii) complex as a visible-light absorbing photosensitizer.

In this work, a new anionic Ir(iii) complex (Ir2) was synthesized using coumarin 6 and orotate ligands, and its use as a photosensitizer in photochemical hydrogen (H2) generation was demonstrated. For comparison purposes, we also prepared a known anionic Ir(iii) complex (Ir1) bearing 2-phenylpyridine and orotate ligands, which was previously reported as an emitting material by another research group. The photophysical properties of complexes Ir1 and Ir2 were examined in several solvents including aqueous acetonitrile (CH3CN-H2O), which is frequently used in photocatalytic H2-generating experiments. Ir1 was found to be weakly emissive in CH3CN-H2O, which is probably due to the effect of hydrogen bonding between the orotate ligands and H2O molecules. In sharp contrast, Ir2 exhibited remarkable properties that include strong visible-light absorption (ε = 78 800 M-1 cm-1 at 462 nm), a high phosphorescence quantum yield (50%), and a long-lived excited state (15 µs), even in the same solvent. Actually, Ir2 functioned as a photosensitizer during visible-light-driven H2 generation using sodium ascorbate and a cobalt(iii) diglyoxime complex as the electron donor and water-reduction catalyst, respectively. To the best of our knowledge, this is the first example of the use of an anionic Ir(iii) sensitizer in a photoinduced electron-transfer reaction. The results of this study suggest that Ir2 can be applied to future photochemical H2-generating systems that are based on ion-paired photocatalysts.

Benzimidazolium Naphthoxide Betaine Is a Visible Light Promoted Organic Photoredox Catalyst.

Benzimidazolium naphthoxide (-ONap-BI+) was first synthesized and utilized as an unprecedented betaine photoredox catalyst. Photoexcited state of -ONap-BI+ generated by visible light irradiation catalyzes the reductive deiodination as well as desulfonylation reactions in which 1,3-dimethyl-2-phenylbenzimidazoline (Ph-BIH) cooperates with as an electron and hydrogen atom donor. Significant solvent effects on the reaction progress were discovered, and specific solvation toward imidazolium and naphthoxide moieties of -ONap-BI+ was proposed.

Impact of Substituents on Excited-State and Photosensitizing Properties in Cationic Iridium(III) Complexes with Ligands of Coumarin 6.

A series of bis-cyclometalated cationic iridium (Ir) complexes were synthesized employing two coumarin 6 ligands and a 2,2'-bipyridine (bpy) with various substituents as new sensitizers, realizing both features of strong visible-light absorption and long-lived excited state. Complexes 2-4, with electron-donating methyl and methoxy groups, absorbed visible light strongly (ε: 126 000-132 000 M(-1) cm(-1)) and exhibited room-temperature phosphorescence with remarkably long lifetimes (21-23 µs) in dichloromethane. In contrast, the excited state of prototype complex 1 without any substituents was short-lived, particularly in highly polar acetonitrile. Phosphorescence of complex 5 with the strong electron-withdrawing CF3 groups was too weak to be detected at room temperature even in less polar dichloromethane. The triplet energies of their coumarin ligand-centered ((3)LC) phosphorescent states were almost invariable, demonstrating that selective tuning of the excited-state lifetime is possible through this "simple chemical modification of the bpy ligand" (we name it the "SCMB" method). The spectroscopic and computational investigations in this study suggest that a potential source of the nonradiative deactivation is a triplet ligand-to-ligand charge-transfer state ((3)LLCT state, coumarin 6 → bpy) and lead us to conclude that the energy level of this dark (3)LLCT state, as well as its thermal population, is largely dependent on the substituents and solvent polarity. In addition, the significant difference in excited-state lifetime was reflected in the photosensitizing ability of complexes 1-5 in visible-light-driven hydrogen generation using sodium ascorbate and a cobalt(III) diglyoxime complex as an electron donor and a water-reduction catalyst, respectively. This study suggests that the SCMB method should be generally effective in controlling the excited state of other bis-cyclometalated cationic Ir(III) complexes.

Solvent dependent photosensitized singlet oxygen production from an Ir(III) complex: pointing to problems in studies of singlet-oxygen-mediated cell death.

A cationic cyclometallated Ir(III) complex with 1,10-phenanthroline and 2-phenylpyridine ligands photosensitizes the production of singlet oxygen, O2(a(1)Δ(g)), with yields that depend appreciably on the solvent. In water, the quantum yield of photosensitized O2(a(1)Δ(g)) production is small (ϕ(Δ) = 0.036 ± 0.008), whereas in less polar solvents, the quantum yield is much larger (ϕ(Δ) = 0.54 ± 0.05 in octan-1-ol). A solvent effect on ϕ(Δ) of this magnitude is rarely observed and, in this case, is attributed to charge-transfer-mediated processes of non-radiative excited state deactivation that are more pronounced in polar solvents and that kinetically compete with energy transfer to produce O2(a(1)Δ(g)). A key component of this non-radiative deactivation process, electronic-to-vibrational energy transfer, is also manifested in pronounced H2O/D2O isotope effects that indicate appreciable coupling between the Ir(III) complex and water. This Ir(III) complex is readily incorporated into HeLa cells and, upon irradiation, is cytotoxic as a consequence of the O2(a(1)Δ(g)) thus produced. The data reported herein point to a pervasive problem in mechanistic studies of photosensitized O2(a(1)Δ(g))-mediated cell death: care must be exercised when interpreting the effective cytotoxicity of O2(a(1)Δ(g)) photosensitizers whose photophysical properties depend strongly on the local environment. Specifically, the photophysics of the sensitizer in bulk solutions may not accurately reflect its intracellular behavior, and the control and quantification of the O2(a(1)Δ(g)) "dose" can be difficult in vivo.