20π-Electron Antiaromatic Benziphthalocyanines with Absorption Reaching the Near-Infrared-II Region.

Although second near-infrared (NIR-II, 1000-1500 nm) light has attracted considerable attention, especially for life sciences applications, the development of organic dyes with NIR-II absorption remains a formidable challenge. Herein we report the design, synthesis, and electronic properties of 20π-electron antiaromatic benziphthalocyanines (BPcs) that exhibit intense absorption bands in the NIR region. The strong, low-energy absorption of the antiaromatic BPcs is attributed to electric-dipole-allowed HOMO-LUMO transitions with narrow band gaps, enabled by the reduced structural symmetry of BPc compared with regular porphyrins and phthalocyanines. The combination of peripheral substituents and a central metal decreases the HOMO-LUMO energy gaps, leading to the extension of the absorption bands into the NIR-II region (reaching 1100 nm) under reductive conditions.

Dearomative Construction of 2D/3D Frameworks from Quinolines via Nucleophilic Addition/Borate-Mediated Photocycloaddition.

Dearomative construction of multiply-fused 2D/3D frameworks, composed of aromatic two-dimensional (2D) rings and saturated three-dimensional (3D) rings, from readily available quinolines has greatly contributed to drug discovery. However, dearomative cycloadditions of quinolines in the presence of photocatalysts usually afford 5,6,7,8-tetrahydroquinoline (THQ)-based polycycles, and dearomative access to 1,2,3,4-THQ-based structures remains limited. Herein, we present a chemo-, regio-, diastereo-, and enantioselective dearomative transformation of quinolines into 1,2,3,4-THQ-based 6-6-4-membered rings without any catalyst, through a combination of nucleophilic addition and borate-mediated [2+2] photocycloaddition. Detailed mechanistic studies revealed that the photoexcited borate complex, generated from quinoline, organolithium, and HB(pin), accelerates the cycloaddition and suppresses the rearomatization that usually occurs in conventional photocycloaddition. Based on our mechanistic analysis, we also developed further photoinduced cycloadditions affording other types of 2D/3D frameworks from isoquinoline and phenanthrene.

Near-Infrared Fluorescence Enhancement by Deuteration of Phthalocyanine.

Organic compounds with near-IR (NIR) fluorescence have many potential applications in materials and life sciences, but the much weaker intensity of fluorescence in the NIR region than in the UV-visible region is a major obstacle. Herein we show that deuteration of phthalocyanines, a representative class of organic NIR dyes, increases both the fluorescence quantum yield and the fluorescence lifetime compared with non-deuterated phthalocyanines.

Origin of Large Near-Infrared Solvatochromism of 18π-Electron Aromatic Monohydroxybenziphthalocyanine.

Near-IR (NIR) organic dyes have been widely utilized in life sciences and materials science. Herein we report an unusually large NIR solvatochromism of monohydroxybenziphthalocyanine, an analogue of 18π-electron aromatic phthalocyanine in which a single isoindoline unit is replaced with a phenol ring. The solvatochromism is attributed to deprotonation of the phenol moiety in highly polar solvents, leading to the generation of a strongly NIR-absorptive 18π-electron aromatic quinoidal monoanion.

A Switchable Near-Infrared-Absorbing Dye Based on Redox-Bistable Benzitetraazaporphyrin.

Activatable near-infrared (NIR) dyes responsive to external stimuli are used in medical and other applications. Here, we describe the design and synthesis of bench-stable 18π- and 20π-electron benzitetraazaporphyrins (BzTAPs) possessing redox-switchable NIR properties. X-Ray, NMR, and UV/Visible-NIR analyses revealed that 20π-electron BzTAP 1 exhibits NIR absorption and antiaromaticity with a paratropic ring-current, while 18π-electron BzTAP 2 shows weakly aromatic character with NIR inertness. Notably, the NIR-silent BzTAP 2 was readily converted to the NIR-active BzTAP 1 in the presence of mild reducing agents such as amine. The intense NIR absorption band of BzTAP 1 is in sharp contrast to the very weak absorption bands of previously reported antiaromatic porphyrinoids. Molecular orbital analysis revealed that symmetry-lowering perturbation of the 20π-electron porphyrinoid skeleton enables the HOMO-LUMO transition of 1 to be electric-dipole-allowed. BzTAPs are expected to be useful for constructing activatable NIR probes working in reductive environments.

Shining Visible Light on Reductive Elimination: Acridine-Pd-Catalyzed Cross-Coupling of Aryl Halides with Carboxylic Acids.

Despite the recent tremendous progress on transition-metal/photoredox dual catalysis in organic synthesis, single transition-metal catalysis under visible-light irradiation, which can utilize light energy more efficiently, is still underdeveloped. Herein, we report the design of photosensitizing phosphinoacridine bidentate ligands for visible-light-induced transition-metal catalysis, expecting that the electron-accepting acridine moiety would create a highly reactive electron-deficient metal center toward reductive elimination via metal-to-ligand charge transfer (MLCT). Using these ligands, we have achieved a palladium-catalyzed cross-coupling reaction of aryl halides with carboxylic acids under visible-light irradiation. Electronic tuning of the phosphinoacridine ligands not only enabled the use of a variety of aryl halides as the coupling partner, including less reactive aryl chlorides, under blue light irradiation, but also realized the employment of lower-energy green and red light for the cross-coupling. Experimental mechanistic studies have proved that the reductive elimination of aryl esters is induced by photoirradiation of phosphinoacridine-ligated arylpalladium(II) carboxylate complexes. The theoretical calculation suggests that the reductive elimination in the excited state is promoted by decreasing the electron density of the Pd center through photoinduced intramolecular electron transfer, i.e., MLCT, in the transition state owing to the electron-deficient acridine scaffold. This is a very rare example of photoinduced reductive elimination on palladium(II) complexes.

Metal-Free Photoredox-Catalyzed Hydrodefluorination of Fluoroarenes Utilizing Amide Solvent as Reductant.

A metal-free photoredox-catalyzed hydrodefluorination of fluoroarenes was achieved by using N,N,N',N'-tetramethyl-para-phenylenediamine (1) as a strong photoreduction catalyst. This reaction was applicable not only to electron-rich monofluoroarenes but also to polyfluoroarenes to afford non-fluorinated arenes. The experimental mechanistic studies indicated that the amide solvent NMP plays an important role for regeneration of the photocatalyst, enabling additive-free photoreduction catalysis.

Rhodium-Catalyzed meta-Selective C-H Carboxylation Reaction of 1,1-Diarylethylenes via Hydrorhodation-Rhodium Migration.

A meta-selective C-H carboxylation reaction of 1,1-diarylethylene derivatives with CO2 by using a rhodium catalyst with NaOi Pr as a stoichiometric reductant has been achieved. Together with hydrogenation of the ethylene moiety, a carboxyl group was introduced to the meta-position of the aryl ring with high selectivity over the ortho-positions. Experimental and computational mechanistic studies indicate that this carboxylation reaction proceeds via hydrorhodation on the ethylene moiety, followed by 1,4-rhodium migration and successive 1,2-rhodium migration on the aryl ring. The use of a bulky phosphine ligand seems to be the key to this unusual aryl-to-aryl 1,2-rhodium shift.

Design of Photostable, Activatable Near-Infrared Photoacoustic Probes Using Tautomeric Benziphthalocyanine as a Platform.

Near-infrared (NIR) imaging techniques have attracted significant attention for biological and medicinal applications due to the ability of NIR to penetrate deeply into tissues. However, there are very few stable, activatable molecular probes that can utilize NIR light in the wavelength range beyond 800 nm. Herein, we report a new activatable NIR system for photoacoustic imaging based on tautomeric benziphthalocyanines (BPcs). We found that the existence of a free hydroxyl group is crucial for NIR absorption of BPcs. Synthesized water-soluble hydroxy BPcs exhibited high photostability and no fluorescence, which are desirable features for photoacoustic imaging. We synthesized BPcs in which the free hydroxyl group was masked by an esterase-labile or an H2 O2 -labile group. The photoacoustic signals of these hydroxy-masked BPcs were increased upon NIR excitation at 880 nm in the presence of esterase or H2 O2 , respectively. These are rare examples of activatable probes utilizing NIR light at around 900 nm.

N-Alkynylpyridinium Salts: Highly Electrophilic Alkyne-Pyridine Conjugates as Precursors of Cationic Nitrogen-Embedded Polycyclic Aromatic Hydrocarbons.

We achieved the first synthesis of N-alkynylpyridinium salts, by reacting pyridines with alkynyl-λ3-iodanes. The N-alkynylpyridiniums exhibit highly electron-accepting character with extended π-conjugation. The electrophilic alkynyl groups were readily susceptible to Michael addition and 1,3-dipolar cycloaddition to afford various N-alkenylpyridiniums. Ring-fused pyridiniums were synthesized through intramolecular cyclization, demonstrating the utility of N-alkynylpyridiniums for the design of various electron-deficient cationic nitrogen-embedded polycyclic aromatic hydrocarbons with unique optical and electrochemical properties.

Near-Infrared Fluorescence from In-Plane-Aromatic Cycloparaphenylene Dications.

Cycloparaphenylenes (CPPs) are hoop-shaped conjugated hydrocarbons corresponding to partial structures of fullerenes or armchair carbon nanotubes. Here, we examined the fluorescence properties of a series of [ n]cycloparaphenylene dications ([ n]CPP2+, n = 5-9), which have unique in-plane aromaticity. The fluorescence peak positions of the [ n]CPP2+s shifted to the longer-wavelength region with increasing ring size, reaching the near-infrared region for those with n > 5. The fluorescence quantum yield of [6]CPP2+ was the highest among the [ n]CPP2+s examined in this study, and the value was on the same order as that of carbon nanotubes. The Stokes shifts of [ n]CPP2+s were smaller than those of neutral [ n]CPPs, which do not have in-plane aromaticity. Theoretical calculations indicate that [ n]CPP2+s undergo smaller structural changes upon S0-S1 transition than [ n]CPPs do, and this is responsible for the difference of the Stokes shift. Furthermore, molecular orbital analysis reveals that the S0-S1 transition of smaller [ n]CPP2+s has an electric-dipole-forbidden character due to HOMO → LUMO/HOMO → LUMO+1 mixing. The relatively high fluorescence quantum yield of [6]CPP2+ is considered to arise from the balance between relatively allowed character and the dominant effect of energy gap.

Effects of N-Oxidation on Heteroaromatic Macrocycles: Synthesis, Electronic Structures, Spectral Properties, and Reactivities of Tetraazaporphyrin meso-N-Oxides.

Heteroaromatic N-oxides such as pyridine and quinoline N-oxides are well studied in organic chemistry, and N-oxide formation has long been utilized for tuning the reactivities of heteroaromatics. However, the scope of aromatic N-oxidation is still restricted to relatively small azine or azole skeletons, and there has been little investigation of the photophysical/chemical effects of N-oxidation on larger heteroaromatic systems. Here, the synthesis and unique properties of new macrocyclic heteroaromatic N-oxides, tetraazaporphyrin (TAP) meso-N-oxides, are reported. N-Oxidation of TAP reduced the 18π-aromaticity of the TAP ring compared with that of the parent TAP owing to the cross-conjugated resonance structure. The optical properties of TAPs were significantly changed by N-oxidation: the N-oxides did not exhibit azaporphyrin-like but instead porphyrin-like optical properties, that is, weak Q absorption bands, strong Soret absorption bands, and weak fluorescence. These features can be explained by the near-degenerate frontier molecular orbitals resulting from N-oxide formation. Singlet oxygen quantum yields were greatly increased to almost quantitative levels by N-oxidation. The N-oxides showed near-IR-responsive photoredox properties and were suitable as both oxidants and sensitizers for oxidation reactions. Protonation of the N-oxides restored TAP-like intense Q bands and red fluorescence, offering a potential design strategy for fluorescence switches.

In-plane aromaticity in cycloparaphenylene dications: a magnetic circular dichroism and theoretical study.

The electronic structures of [8]cycloparaphenylene dication ([8]CPP(2+)) and radical cation ([8]CPP(•+)) have been investigated by magnetic circular dichroism (MCD) spectroscopy, which enabled unambiguous discrimination between previously conflicting assignments of the UV-vis-NIR absorption spectral bands. Molecular orbital and nucleus-independent chemical shift (NICS) analysis revealed that [8]CPP(2+) shows in-plane aromaticity with a (4n + 2) π-electron system (n = 7). This aromaticity appears to be the origin of the unusual stability of the dication. Theoretical calculations further suggested that not only [8]CPP(2+) but also all [n]CPP (n = 5-10) dications and dianions exhibit in-plane aromaticity.

Dialkylzinc-mediated cross-coupling reactions of perfluoroalkyl and perfluoroaryl halides with aryl halides.

A highly chemoselective perfluoroalkylation reaction of aromatic halides is reported. Thermally stable perfluoroalkylzinc reagents, generated by a rapid halogen-zinc exchange reaction between diorganozinc and perfluoroalkyl halide species, couple with a wide range of aryl halides in the presence of a copper catalyst, in moderate to high yields. Good stability of the perfluoroalkylzinc species was indicated by DFT calculation and the reagents were storable for at least three months under argon without loss of activity. This method is applicable to gram-scale synthesis, and its functional group tolerance compares favorably with reported protocols.

Perfluoroalkyl and -aryl zinc ate complexes: generation, reactivity, and synthetic application.

A combination of dimethylzinc, perfluoroalkyl iodide, and LiCl afforded a new type of perfluoroalkyl (RF ) zinc ate complex. These complexes show much greater thermal stability than conventional perfluorinated metal species, such as RF -lithium species and Grignard reagents, and they can be used at room temperature or higher. The results of DFT calculations on the origin of the enhanced stability are reported and the synthetic utility of RF -zincate complexes is demonstrated.

Unraveling the electronic structure of azolehemiporphyrazines: direct spectroscopic observation of magnetic dipole allowed nature of the lowest π-π* transition of 20π-electron porphyrinoids.

Hemiporphyrazines are a large family of phthalocyanine analogues in which two isoindoline units are replaced by other rings. Here we report unambiguous identification of 20π-electron structure of triazolehemiporphyrazines (1, 2) and thiazolehemiporphyrazine (3) by means of X-ray analysis, various spectroscopic methods, and density functional theory (DFT) calculations. The hemiporphyrazines were compared in detail with dibenzotetraazaporphyrin (4), a structurally related 18π-electron molecule. X-ray analysis revealed that tetrakis(2,6-dimethylphenyloxy)triazolehemiporphyrazine (1b) adopted planar geometry in the solid state. A weak absorption band with a pronounced vibronic progression, observed for all the hemiporphyrazines, was attributed to the lowest π-π* transition with the electric-dipole-forbidden nature. In the case of intrinsically chiral vanadyl triazolehemiporphyrazine (2), a large dissymmetry (g) factor was detected for the CD signal corresponding to the lowest π-π* transition with the magnetic-dipole-allowed nature. Molecular orbital analysis and NICS calculations showed that the azolehemiporphyrazines have a 20π-electron system with a weak paratropic ring current.

18π-Electron tautomeric benziphthalocyanine: a functional near-infrared dye with tunable aromaticity.

Dihydroxybenziphthalocyanine 1, with bulky aryloxy groups, has been synthesized and characterized by X-ray crystallography, NMR and UV/Vis-NIR spectroscopy, and theoretical calculations. Macrocycle 1 is the first example of an aromatic benziphthalocyanine with an 18π-electron structure, and was found to exist as an equilibrium mixture of weakly aromatic and strongly aromatic tautomers. The aromaticity and near-IR absorption can be controlled by chemical modification at the reactive resorcinol moiety and by variation of the solvent.

Visible-Light-Enabled Carboxylation of Benzyl Alcohol Derivatives with CO2 Using a Palladium/Iridium Dual Catalyst.

A highly efficient carboxylation of benzyl alcohol derivatives with CO2 using a palladium/iridium dual catalyst under visible-light irradiation was developed. A wide range of benzyl alcohol derivatives could be employed to provide benzylic carboxylic acids in moderate to high yields. Mechanistic studies indicated that the oxidative addition of benzyl alcohol derivatives was possibly the rate-determining-step. It was also found that a switchable site-selective carboxylation between benzylic C-O and aryl C-Cl moieties could be achieved simply by changing the palladium catalyst.

Visible-Light-Enabled Carboxylation of Benzyl Alcohol Derivatives with CO2 Using a Palladium/Iridium Dual Catalyst.

Invited for this month's cover is the group of Nobuharu Iwasawa at Tokyo Institute of Technology. The image shows a switchable site-selective carboxylation between benzylic C-O and aryl C-Cl moieties enabled by a palladium/iridium dual catalyst under visible light irradiation. The Communication itself is available at 10.1002/cssc.202102095.

Catalytic direct hydrocarboxylation of styrenes with CO2 and H2.

A three-component hydrocarboxylation of an olefin with CO2 and H2 could be regarded as a dream reaction, since it would provide a straightforward approach for the synthesis of aliphatic carboxylic acids in perfect atom economy. However, this transformation has not been realized in a direct manner under mild conditions, because boosting the carboxylation with thermodynamically stable CO2 while suppressing the rapid hydrogenation of olefin remains a challenging task. Here, we report a rhodium-catalysed reductive hydrocarboxylation of styrene derivatives with CO2 and H2 under mild conditions, in which H2 served as the terminal reductant. In this approach, the carboxylation process was largely accelerated by visible light irradiation, which was proved both experimentally and by computational studies. Hydrocarboxylation of various kinds of styrene derivatives was achieved in good yields without additional base under ambient pressure of CO2/H2 at room temperature. Mechanistic investigations revealed that use of a cationic rhodium complex was critical to achieve high hydrocarboxylation selectivity.