AIE-ESIPT Photoluminescent Probe Based on 3-(3-Hydroxypyridin-2-yl)isoquinolin-4-ol for the Detection of Intracellular Hydrogen Peroxide.

Excited-state intramolecular proton transfer (ESIPT) molecules, which feature large Stokes shifts to avoid self-absorption, play an essential role in photoluminescent bioimaging probes. Herein, we report the development of an ESIPT molecule 3-(3-hydroxypyridin-2-yl)isoquinolin-4-ol (PiQ). PiQ not only undergoes a distinct ESIPT process unlike the symmetrical 2,2'-bipyridyl-3,3'-diol but also exhibits aggregation-induced emission (AIE) characteristics. PiQ self-assembles into aggregates with an average size of 241.0±51.9 nm in aqueous solutions, leading to significantly enhanced photoluminescence. On the basis of the ESIPT and AIE characteristics of PiQ, the latter is functionalized with a hydrogen peroxide-responsive 4-pinacoratoborylbenzyl group (B) and a carboxylesterase-responsive acetyl group (A) to produce a photoluminescent probe B-PiQ-A. The potential of PiQ for applications in bioimaging and chemical sensing is underscored by its efficient detection of both endogenous and exogenous hydrogen peroxide in living cells.

MMP-2-Activatable Photoacoustic Tumor Imaging Probes Based on Al- and Si-Naphthalocyanines.

Enzyme-activatable photoacoustic probes are powerful contrast agents to visualize diseases in which a specific enzyme is overexpressed. In this study, aluminum and silicon naphthalocyanines (AlNc and SiNc, respectively) conjugated with matrix metalloprotease-2 (MMP-2)-responsive PLGLAG peptide sequence and poly(ethylene glycol) (PEG) as an axial ligand were designed and synthesized. AlNc-peptide-PEG conjugates AlNc-pep-PEG formed dimeric species interacting with each other through face-to-face H-aggregation in water, while SiNc-based conjugates SiNc-pep-PEG hardly interacted with each other because of the two bulky hydrophilic axial ligands. Both conjugates formed spherical nanometer-sized self-assemblies in water, generating photoacoustic waves under near-infrared photoirradiation. The treatment of MNc-peptide-PEG conjugates (M = Al, Si) with MMP-2 smoothly induced the cleavage of the PLGLAG sequence to release the hydrophilic PEG moiety, resulting in the aggregation of MNcs. By comparing the PA signal intensity changes at 680 and 760 nm, the photoacoustic signal intensity ratios were shown to be enhanced by 3-5 times after incubation with MMP-2. We demonstrated that MNc-peptide-PEG conjugates (M = Al, Si) could work as activatable photoacoustic probes in the in vitro experiment of MMP-2-overexpressed cell line HT-1080 as well as the in vivo photoacoustic imaging of HT-1080-bearing mice.

Bis-Metal Complexes of Doubly N-Confused Dioxohexaphyrins as Potential Near-Infrared-II Photoacoustic Dyes.

We investigated the detailed photophysical properties of a series of bis-metal (Zn and Cu) dioxohexaphyrin complexes as potential second near-infrared (NIR-II)-light responsive dyes. A cisoid-configured 28π-electron-conjugated dioxohexaphyrin analogue (c-3a) containing two peculiar "confused pyrrole" moieties in the framework is identified as a reduced isomer derivative of a transoid 26π-dioxohexaphyrin (t-2a). The symmetry-altered structure of c-3a affords a heteroleptic inner environment within the NNNN/NNOO donor core, which imparts its highly flexible electronic features and nonplanar geometry. The macrocycle c-3a can be transformed into the corresponding 26π-electron congener (c-2a) having a coplanar rectangular structure by unique solvent-mediated redox reactivity. Furthermore, upon metal complexation, saddle-distorted bis-metal complexes (c-M2-2a) were formed as the 26π-conjugated structural isomer of the trans-dioxohexaphyrin species (i.e., t-M2-2a). These isoelectronic dioxohexaphyrins demonstrate precise geometry-dependent photophysical properties. Broad tailing NIR-II absorption, weak emissive character, and rapid-decay of the S1 state are observed for c-Zn2-2a. In contrast, the coplanar t-M2-2a exhibits efficient photoacoustic response upon laser excitation with NIR-II light (λ > 1000 nm). To the best of our knowledge, this is the first example of an expanded porphyrin-based photoacoustic contrast agent responsive to NIR-II light.

π-Conjugated Macrocycles Bearing Angle-Strained Alkynes.

Angle-strained alkyne-containing π-conjugated macrocycles are attractive compounds both in functional materials chemistry and biochemistry. Their interesting reactivity as well as photophysical and supramolecular properties have been revealed in the past three decades. This review highlights the recent advances in angle-strained alkyne-containing π-conjugated macrocycles, especially their synthetic methods, the bond angles of alkynes (∠sp at C≡C-C), and their functions. The theoretical and experimental research on cyclo[n]carbons and para-cyclophynes consisting of ethynylenes and para-phenylenes are mainly summarized. Related macrocycles bearing other linkers, such as ortho-phenylenes, meta-phenylenes, heteroaromatics, biphenyls, extended aromatics, are also overviewed. Bond angles of strained alkynes in π-conjugated macrocycles, which are generable, detectable, and isolable, are summarized at the end of this review.

An enzyme-triggered turn-on fluorescent probe based on carboxylate-induced detachment of a fluorescence quencher.

We developed a new class of turn-on fluorescent probes for an esterase. After the esterase-mediated hydrolysis produced carboxylate (as a fluorescence activator), the fluorescence intensity was markedly increased through the detachment of a quencher moiety from the quenched Cy5 fluorophore. Because the probes based on this new activator-induced quencher-detachment (AiQd) adopt a non-immolative linker between the cleavable site and the fluorophore, the rate of the enzymatic reaction is greatly improved, without the generation of any by-products.

Recent Progress on Cyclic Nitrenoid Precursors in Transition-Metal-Catalyzed Nitrene-Transfer Reactions.

Nitrene-transfer reactions are powerful synthetic tools for the direct incorporation of nitrogen atoms into organic molecules. The discovery of novel nitrene-transfer reactions has been dominantly supported not only by improvements in transition-metal catalysts but also by the employment of novel precursors of nitrenoids. Since pioneering work involving the use of organic azides and iminoiodinanes as practical synthetic tools for nitrogen-containing compounds was reported, a new approach using various N-heterocycles containing strain energy or a weak bond has emerged. In this review, we briefly summarize the history of nitrene-transfer chemistry from the viewpoint of its precursors. In particular, the use of N-heterocycles such as 2H-azirines, 1,4,2-dioxazol-5-ones, 1,2,4-oxadiazol-5-ones, isoxazol-5(4H)-ones, and isoxazoles is comprehensively described, showing the recent remarkable progress in this chemistry.

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.

Unique Tube-Ring Interactions: Complexation of Single-Walled Carbon Nanotubes with Cycloparaphenyleneacetylenes.

Carbon nanotubes (CNTs) interlocked by cyclic compounds through supramolecular interaction are promising rotaxane-like materials applicable as 2D and 3D networks of nanowires and disease-specific theranostic agents having multifunctionalities. Supramolecular complexation of CNTs with cyclic compounds in a "ring toss'' manner is a straightforward method to prepare interlocked CNTs; however, to date, this has not been reported on. Here, the "ring toss" method to prepare interlocked CNTs by using π-conjugated carbon nanorings: [8]-, [9]-, and [10]cycloparaphenyleneacetylene (CPPA) is reported. CPPAs efficiently interact with CNTs to form CNT@CPPA complexes, while uncomplexed CPPAs can be recovered without decomposition. CNTs, which tightly fit in the cavities of CPPAs through convex-concave interaction, efficiently afford "tube-in-ring"-type CNT@CPPA complexes. "Tube-in-ring"-type and "ring-on-tube"-type complexation modes are successfully distinguished by spectroscopic, thermogravimetric, and microscopic analyses.

Asymmetric Synthesis of 2H-Azirines with a Tetrasubstituted Stereocenter by Enantioselective Ring Contraction of Isoxazoles.

Highly strained 2H-azirines with a tetrasubstituted stereocenter were synthesized by the enantioselective isomerization of isoxazoles with a chiral diene-rhodium catalyst system. The effect of ligands and the coordination behavior support the proposed catalytic cycle in which the coordination site is fixed in favor of efficient enantiodiscrimination by a bulky substituent of the ligand. In silico studies also support the existence of a rhodium-imido complex as a key intermediate for enantiodiscrimination.

Generation of Stable Ruthenium(IV) Ketimido Complexes by Oxidative Addition of Oxime Esters to Ruthenium(II): Reactivity Studies Based on Electronic Properties of the Ru-N Bond.

The reaction of an oxime ester with [Ru(PPh3 )3 X2 ] proceeded smoothly at room temperature to afford a stable RuIV ketimido complex as oxidative adduct. The structure of the complex was unambiguously determined by X-ray crystallographic analysis, which showed an almost linear Ru-N-C array. The electronic properties of the nitrogen atom were estimated by DFT calculations, and the results suggested double-bond character of the Ru-N bond. Kinetic studies and consideration of the substituent effect on the oxime ester led to the proposal of a reaction mechanism involving oxidative addition, which could proceed by N,O-chelating coordination to the Ru center prior to N-O bond cleavage. The obtained Ru ketimido complex could be transformed into a ruthenacycle by C-H activation by a concerted metalation-deprotonation mechanism in dichloromethane/methanol. Ru ketimido complexes with a tethered alkyne or alkene moiety underwent chloroamination of unsaturated C-C bonds followed by C-H activation, which resulted in the formation of a ruthenacycle. Considering the LUMO of an isolated Ru ketimido complex, the chloroamination should proceed by a synchronous 1,3-dipolar cycloaddition-type mechanism. Insight into the character and reactivity of Ru ketimido complexes will be helpful for developments in the catalytic transformation of oxime esters.

Polymeric Self-Assemblies with Boron-Containing Near-Infrared Dye Dimers for Photoacoustic Imaging Probes.

Polymers containing pyrrolopyrrole aza-BODIPY (PPAB) and thiophene-bridged BODIPY dimers (TBD) having poly(ethylene glycol) (PEG) or PEGylated hyaluronic acid (HA) were prepared by facile conjugation approaches. Self-assemblies consisting of TBD-conjugated polymers more efficiently generated photoacoustic (PA) signals than PPAB-PEG conjugate upon irradiation with near-infrared pulsed laser light. Among dye-conjugated polymers examined, TBD-HA-PEG conjugates efficiently generated photoacoustic signals, 1.49-1.83 times stronger than that of commercially available indocyanine green (ICG). We found that the following two factors are essential to enhance PA signals from self-assemblies: (1) the formation of strongly interacting TBD aggregates and (2) enhancement of the elastic modulus of self-assemblies by conjugating TBDs with HA. TBD-conjugated HA derivatives circulated in blood vessels for a longer time (15.6 ± 4.9% injected dose (ID) in blood 24 h after injection) and more specifically accumulated in tumor tissues (17.8 ± 3.5% ID/g in tumor 24 h after injection) than ICG-conjugated HA derivatives, visualizing a tumor site more clearly. The cell uptake experiment of dye-HA conjugates indicates that ICG-conjugated polymers internalized into cells or merged with cell walls to emit strong fluorescence, while TBD-conjugated polymers were not internalized into cells. Because the disassembly of the TBD-conjugated HA derivatives is suppressed, aggregated TBDs emit weak fluorescence but efficiently generate strong PA signals in tumor tissues.

Synthesis of 2H-Azirines by Iridium-Catalyzed Decarboxylative Ring Contraction of Isoxazol-5(4H)-ones.

A phosphine-free iridium-catalyzed reaction of isoxazol-5(4H)-ones (isoxazolones) has been developed, and affords 2H-azirines through decarboxylation and ring contraction. This method provides an efficient and environmentally benign protocol which could replace the conventional approaches used to synthesize 2H-azirines.

Near-infrared dye-conjugated amphiphilic hyaluronic acid derivatives as a dual contrast agent for in vivo optical and photoacoustic tumor imaging.

Amphiphilic hyaluronic acid (HA) derivatives bearing hydrophobic indocyanine green dye derivatives and hydrophilic poly(ethylene glycol) were synthesized through the use of condensation and copper-catalyzed click cyclization reactions. The amphiphilic HA derivatives dissolved in water and formed self-assemblies in which the near-infrared dyes were tightly packed and arranged to form dimers or H-aggregates. By irradiating an aqueous solution of HA derivatives with near-infrared light, photoacoustic signals were detected along with fluorescence emission. Self-assemblies consisting of HA derivatives could smoothly accumulate in tumor tissues by passive tumor targeting. By utilizing HA derivatives as a contrast agent, tumor sites were clearly visualized by optical imaging as well as by photoacoustic tomography.

Sonication-induced formation of size-controlled self-assemblies of amphiphilic Janus-type polymers as optical tumor-imaging agents.

In this study, amphiphilic Janus-type polymers were synthesized via ring-opening metathesis polymerization (ROMP), multiple vicinal diol formation, and grafting of poly(ethylene glycol) monomethyl ether (mPEG). These amphiphilic polymers formed self-assemblies, which were a mixture of micelles and multimicellar aggregates, in water. By choosing suitable Janus-type polymers and irradiating an aqueous solution of polymers using a sonicator, either small micelles or large multimicellar aggregates were obtained selectively. Hydrophobic substituents controlled the aggregation-disaggregation behavior, leading to the formation of metastable self-assemblies by sonication. The formation of self-assemblies with a uniform size was affected by ultrasonic frequency, rather than power. In vivo optical tumor imaging revealed that the large-size multimicellar aggregates persisting for a long time in blood circulation slowly accumulated in tumor tissues. In contrast, the tumor site was rapidly, clearly visualized using the small-size micelles.

Palladium-catalyzed aza-Wittig-type condensation of isoxazol-5(4H)-ones with aldehydes.

This paper describes the development of a palladium-catalyzed decarboxylative inter- and intramolecular condensation reaction of isoxazol-5(4 H)-ones with carbonyl compounds in the presence of PPh3 , giving various 2-azabuta-1,3-dienes or pyrroles in moderate to high yields.

Stereoselective construction of 1,3-disilylcyclopentane derivatives by scandium-catalyzed [3+2] cycloaddition of allylsilanes to ß-silylenones.

The Sc(OTf)3-catalyzed [3+2] cycloaddition of allylsilanes to ß-silyl-α,ß-unsaturated ketones (ß-silylenones) has been developed to form five-membered syn-1,3-disilylketones diastereoselectively through the rearrangement of the silicon substituents on the allylsilane. Stabilization of the carbocation intermediates by a double silicon effect plays a key role in directing the course of the reaction to favor the [3+2] cycloaddition pathway over simple allylation.

A reductively convertible nickel phthalocyanine precursor as a biological thiol-responsive turn-on photoacoustic contrast agent.

A nickel phthalocyanine precursor bearing poly(ethylene glycol) as a turn-on contrast agent for photoacoustic imaging was prepared. The water-soluble polymeric chains were smoothly eliminated through thiol-mediated reductive aromatization in cancer cells, enabling the detection of endogenous biological thiols in vitro and in vivo.

Dual-responsive near-infrared turn-on fluorescent probe for cancer stem cell-specific visualization.

Aldehyde dehydrogenase 1A1 (ALDH1A1) stands out as one of the most reliable intracellular biomarkers for stem cells because it is expressed in both cancer stem cells (CSCs) and normal somatic stem cells (NSCs). Although several turn-on fluorescent probes for ALDH1A1 have been developed to visualize CSCs in cancer cells, the discrimination of CSCs from NSCs is difficult. We here report an AND-type dual-responsive fluorescent probe, CHO_ßgal, the near-infrared fluorescence of which can be turned on after responding to both ALDH1A1 and ß-galactosidase. The AND-type dual responsiveness enables CSCs to be clearly visualized, whereas NSCs are non-emissive in microscopy. CSC-positive metastasis model lungs were successfully discriminated from normal lungs in ex vivo staining experiments using CHO_ßgal, whereas the single-input ALDH1A1-responsive probe failed to achieve this discrimination owing to pronounced false-positive fluorescence output from lung NSCs. In tissue slice staining experiments, even in the presence of adjacent normal tissues, the peripheral region-specific localization of CSCs was clear. The versatility of CHO_ßgal holds promise not only as a fundamental in vitro research tool for visualizing CSCs but also as a valuable asset in practical tissue staining diagnosis, significantly contributing to the assessment of cancer malignancy.

Light-controllable cell-membrane disturbance for intracellular delivery.

Highly polar and charged molecules, such as oligonucleotides, face significant barriers in crossing the cell membrane to access the cytoplasm. To address this problem, we developed a light-triggered twistable tetraphenylethene (TPE) derivative, TPE-C-N, to facilitate the intracellular delivery of charged molecules through an endocytosis-independent pathway. The central double bond of TPE in TPE-C-N is planar in the ground state but becomes twisted in the excited state. Under light irradiation, this planar-to-twisted structural change induces continuous cell membrane disturbances. Such disturbance does not lead to permanent damage to the cell membrane. TPE-C-N significantly enhanced the intracellular delivery of negatively charged molecules under light irradiation when endocytosis was inhibited through low-temperature treatment, confirming the endocytosis-independent nature of this delivery method. We have successfully demonstrated that the TPE-C-N-mediated light-controllable method can efficiently promote the intracellular delivery of charged molecules, such as peptides and oligonucleotides, with molecular weights ranging from 1000 to 5000 Da.

Acid-catalyzed direct conjugate alkenylation of α,ß-unsaturated ketones.

New tricks, old reaction: The title reaction proceeds under mild and transition-metal-free conditions (see scheme; Tf=trifluoromethanesulfonyl). Various combinations of substrates are applicable to the synthesis of γ,δ-unsaturated ketones by employing silicon substituents at the ß-position of the enones. The silicon substituents play a key role in stabilization of the cationic intermediate by hyperconjugation.