Publications by authors named "Youngmin You"

54 Publications

Boosting Photoredox Catalysis Using a Two-Dimensional Electride as a Persistent Electron Donor.

ACS Appl Mater Interfaces 2021 Sep 31;13(36):42880-42888. Epub 2021 Aug 31.

Division of Chemical Engineering and Materials Science, Ewha Womans University, Seoul 03760, Republic of Korea.

Electrides, which have excess anionic electrons, are solid-state sources of solvated electrons that can be used as powerful reducing agents for organic syntheses. However, the abrupt decomposition of electrides in organic solvents makes controlling the transfer inefficient, thereby limiting the utilization of their superior electron-donating ability. Here, we demonstrate the efficient reductive transformation strategy which combines the stable two-dimensional [GdC]·2e electride electron donor and cyclometalated Pt(II) complex photocatalysts. Strongly localized anionic electrons at the interlayer space in the [GdC]·2e electride are released via moderate alcoholysis in 2,2,2-trifluoroethanol, enabling persistent electron donation. The Pt(II) complexes are adsorbed onto the surface of the [GdC]·2e electride and rapidly capture the released electrons at a rate of 10 s upon photoexcitation. The one-electron-reduced Pt complex is electrochemically stable enough to deliver the electron to substrates in the bulk, which completes the photoredox cycle. The key benefit of this system is the suppression of undesirable charge recombination because back electron transfer is prohibited due to the irreversible disruption of the electride after the electron transfer. These desirable properties collectively serve as the photoredox catalysis principle for the reductive generation of the benzyl radical from benzyl halide, which is the key intermediate for dehalogenated or homocoupled products.
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http://dx.doi.org/10.1021/acsami.1c12363DOI Listing
September 2021

Electronic structures and optical characteristics of fluorescent pyrazinoquinoxaline assemblies and Au interfaces.

Sci Rep 2021 Aug 20;11(1):16978. Epub 2021 Aug 20.

Department of Physics, Ewha Womans University, Seoul, 03760, South Korea.

Understanding the excitonic processes at the interfaces of fluorescent π-conjugated molecules and metal electrodes is important for both fundamental studies and emerging applications. Adsorption configurations of molecules on metal surfaces significantly affect the physical characteristics of junctions as well as molecules. Here, the electronic structures and optical properties of molecular assemblies/Au interfaces were investigated using scanning probe and photoluminescence microscopy techniques. Scanning tunneling microscopy images and tunneling conductance spectra suggested that the self-assembled molecules were physisorbed on the Au surface. Visible-range photoluminescence studies showed that Au thin films modified the emission spectra and reduced the lifetime of excitons. Surface potential maps, obtained by Kelvin probe force microscopy, could visualize electron transfer from the molecules to Au under illumination, which could explain the decreased lifetime of excitons at the molecule/Au interface.
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http://dx.doi.org/10.1038/s41598-021-96437-xDOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC8379188PMC
August 2021

Chiral polymer hosts for circularly polarized electroluminescence devices.

Chem Sci 2021 Jul 21;12(25):8668-8681. Epub 2021 May 21.

Division of Chemical Engineering and Materials Science, Graduate Program in System Health Science and Engineering, Ewha Womans University Seoul 03760 Republic of Korea

Polymer electroluminescence devices producing circularly polarized luminescence (CP PLEDs) have valuable photonic applications. The fabrication of a CP PLED requires a polymer host that provides the appropriate chiral environment around the emitting dopant. However, chemical strategies for the design of chiral polymer hosts remain underdeveloped. We have developed new polymer hosts for CP PLED applications. These polymers were prepared through a free-radical polymerization of 3-vinylcarbazole with a chiral -alkyl unit. This chiral unit forces the carbazole repeat units to form mutually helical half-sandwich conformers with preferred ()-helical sense along the polymer main chain. Electronic circular dichroism measurements demonstrate the occurrence of chirality transfer from chiral monomers to achiral monomers during chain growth. The ()-helical-sense-enriched polymer interacts diastereoselectively with an enantiomeric pair of new phosphorescent ()- and ()-dopants. The magnitude of the Kuhn dissymmetry factor ( ) for the ()-helically-enriched polymer film doped with the ()-dopant was found to be one order of magnitude higher than that of the film doped with the ()-dopant. Photoluminescence dissymmetry factors ( ) of the order of 10 were recorded for the doped films, but the magnitude of diastereomeric enhancement decreased to that of . The chiral polymer host permits faster energy transfer to the phosphorescent dopants than the achiral polymer host. Our photophysical and morphological investigations indicate that the acceleration in the chiral polymer host is due to its longer Förster radius and improved compatibility with the dopants. Finally, multilayer CP PLEDs were fabricated and evaluated. Devices based on the chiral polymer host with the ()- and ()-dopants exhibit electroluminescence dissymmetry factors ( ) of 1.09 × 10 and -1.02 × 10 at a wavelength of 540 nm, respectively. Although challenges remain in the development of polymer hosts for CP PLEDs, our research demonstrates that chiroptical performances can be amplified by using chiral polymer hosts.
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http://dx.doi.org/10.1039/d1sc02095aDOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC8246120PMC
July 2021

Twist to Boost: Circumventing Quantum Yield and Dissymmetry Factor Trade-Off in Circularly Polarized Luminescence.

Inorg Chem 2021 Jun 24;60(11):7738-7752. Epub 2021 Mar 24.

Division of Chemical Engineering and Materials Science, and Graduate Program in System Health Science and Engineering, Ewha Womans University, Seoul 03760, Republic of Korea.

Circularly polarized luminescence (CPL) enables promising applications in asymmetric photonics. However, the performances of CPL molecules do not yet meet the requirements of these applications. The shortcoming originates from the trade-off in CPL between the photoluminescence quantum yield (PLQY) and the photoluminescence dissymmetry factor (). In this study, we developed a molecular strategy to circumvent this trade-off. Our approach takes advantage of the strong propensity of [Pt(N^C^N)Cl], where the N^C^N ligand is 1-(2-oxazoline)-3-(2-pyridyl)phenylate, to form face-to-face stacks. We introduced chiral substituents, including ()-methyl, ()- and ()-isopropyl, and ()-indanyl groups, into the ligand framework. This asymmetric control induces torsional displacements that give homohelical stacks of the Pt(II) complexes. X-ray single-crystal structure analyses for the ()-isopropyl Pt(II) complex reveal the formation of a homohelical dimer with a Pt···Pt distance of 3.48 Å, which is less than the sum of the van der Waals radii of Pt. This helical stack elicits the metal-metal-to-ligand charge-transfer (MMLCT) transition that exhibits strong chiroptical activity due to the electric transition moment making an acute angle to the magnetic transition moment. The PLQY and values of the MMLCT phosphorescence emission of the ()-isopropyl Pt(II) complex are 0.49 and 8.4 × 10, which are improved by factors of ca. 6 and 4, respectively, relative to the values of the unimolecular emission (PLQY, 0.078; , 2.4 × 10). Our photophysical measurements for the systematically controlled Pt(II) complexes reveal that the CPL amplifications depend on the chiral substituent. Our investigations also indicate that excimers are not responsible for the enhanced chiroptical activity. To demonstrate the effectiveness of our approach, organic electroluminescence devices were fabricated. The MMLCT emission devices were found to exhibit simultaneous enhancements in the external quantum efficiency (EQE, 9.7%) and the electroluminescence dissymmetry factor (, 1.2 × 10) over the unimolecular emission devices (EQE, 5.8%; , 0.3 × 10). These results demonstrate the usefulness of using the chiroptically active MMLCT emission for achieving an amplified CPL.
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http://dx.doi.org/10.1021/acs.inorgchem.1c00070DOI Listing
June 2021

Modeling Electron-Transfer Degradation of Organic Light-Emitting Devices.

Adv Mater 2021 Mar 15;33(12):e2003832. Epub 2021 Feb 15.

Division of Chemical Engineering and Materials Science, and System, Health and Engineering Convergence Major, Ewha Womans University, Seoul, 03760, Republic of Korea.

The operational lifetime of organic light-emitting devices (OLEDs) is governed primarily by the intrinsic degradation of the materials. Therefore, a chemical model capable of predicting the operational stability is highly important. Here, a degradation model for OLEDs that exhibit thermally activated delayed fluorescence (TADF) is constructed and validated. The degradation model involves Langevin recombination of charge carriers on hosts, followed by the generation of a polaron pair through reductive electron transfer from a dopant to a host exciton as the initiation steps. The polarons undergo spontaneous decomposition, which competes with ultrafast recovery of the intact materials through charge recombination. Electrical and spectroscopic investigations provide information about the kinetics of each step in the operation and degradation of the devices, thereby enabling the building of mass balances for the key species in the emitting layers. Numerical solutions enable predictions of temporal decreases of the dopant concentration in various TADF emitting layers. The simulation results are in good agreement with experimental operational stabilities. This research disentangles the chemical processes in intrinsic electron-transfer degradation, and provides a useful foundation for improving the longevity of OLEDs.
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http://dx.doi.org/10.1002/adma.202003832DOI Listing
March 2021

Photodelivery of β-phenylethylamines.

Org Biomol Chem 2020 Oct;18(39):7842-7847

Division of Chemical Engineering and Materials Science, Ewha Womans University, Seoul 03760, Korea.

We have developed the first photodonors for the trace amino neurotransmitters, β-phenylethylamine (DPSY1) and β-methylphenylethylamine (DPSY2). Our photodonors react rapidly with photosensitized singlet dioxygen (1O2) to yield the amines. The photodelivery of β-phenylethylamine into aqueous solutions by employing liposome scaffolds is successfully demonstrated.
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http://dx.doi.org/10.1039/d0ob01792bDOI Listing
October 2020

Benzothiazole Synthesis: Mechanistic Investigation of an In Situ-Generated Photosensitizing Disulfide.

J Org Chem 2020 09 3;85(18):11835-11843. Epub 2020 Sep 3.

Department of Chemistry, Chung-Ang University, 84 Heukseok-ro, Dongjak-gu, Seoul 06974, Republic of Korea.

The use of a visible light absorbing intermediate as a photosensitizer makes a chemical process simple and sustainable, obviating the need for the use of chemical additives. Herein, the formation of a photosensitizing disulfide in benzothiazole synthesis from 2-aminothiophenol and aldehydes was proposed and confirmed through in-depth mechanistic studies. A series of photophysical and electrochemical investigations revealed that an in situ-generated disulfide photosensitizes molecular oxygen to generate the key oxidants, singlet oxygen and superoxide anion, for the dehydrogenation step.
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http://dx.doi.org/10.1021/acs.joc.0c01598DOI Listing
September 2020

Deep-Red-Fluorescent Zinc Probe with a Membrane-Targeting Cholesterol Unit.

Inorg Chem 2020 Aug 27;59(16):11562-11576. Epub 2020 Jul 27.

Division of Chemical Engineering and Materials Science, Ewha Womans University, Seoul 03760, Republic of Korea.

Organelle-targeting fluorescence probes are valuable because they can provide spatiotemporal information about the trafficking of analytes of interest. The spatiotemporal resolution can be improved by using low-energy emission signals because they are barely contaminated by autofluorescence noises. In this study, we designed and synthesized a deep-red-fluorescent zinc probe (JJ) with a membrane-targeting cholesterol unit. This zinc probe consists of a boron-azadipyrromethene (aza-BODIPY) fluorophore and a zinc receptor that is tethered to a tri(ethylene glycol)-cholesterol chain. In aqueous solutions buffered to pH 7.4, JJ exhibits weak fluorescence with a peak wavelength of 663 nm upon excitation at 622 nm. The addition of ZnCl elicits an approximately 5-fold enhancement of the fluorescence emission with a fluorescence dynamic range of 141000. Our electrochemical and picosecond transient photoluminescence investigations indicate that the fluorescence turn-on response is due to the zinc-induced abrogation of the formation of a nonemissive intramolecularly charge-separated species, which occurs with a driving force of 0.98 eV. The fluorescence zinc response was found to be fully reversible and to be unaffected by pH changes or the presence of biological metal ions. These properties are due to tight zinc binding with a dissociation constant of 4 pM. JJ was found to be nontoxic to HeLa cells up to submicromolar concentrations, which enables cellular imaging. Colocalization experiments were performed with organelle-specific stains and revealed that JJ is rapidly internalized into intracellular organelles, including lysosomes and endoplasmic reticula. Unexpectedly, probe internalization was found to permeabilize the cell membrane, which facilitates the influx of exogens such as zinc ions. Such permeabilization does not arise for a control probe without the tri(ethylene glycol)-cholesterol chain (JJC). Our results show that the membrane-targeting cholesterol unit can disrupt membrane integrity.
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http://dx.doi.org/10.1021/acs.inorgchem.0c01376DOI Listing
August 2020

Visible-Light-Induced Trifluoromethylation of Unactivated Alkenes with Tri(9-anthryl)borane as an Organophotocatalyst.

J Org Chem 2019 Oct 22;84(20):12925-12932. Epub 2019 Aug 22.

Department of Chemistry , Chung-Ang University , 84 Heukseok-ro , Dongjak-gu, Seoul 06974 , Republic of Korea.

Tri(9-anthryl)borane was successfully applied as an organophotocatalyst for the visible-light-induced trifluoromethylation of unactivated alkenes with CFI. The mild reaction conditions tolerated a variety of functional groups, and the reaction could be extended to perfluoroalkylations with CFI and CFI. Mechanistic studies revealed that the photoredox catalysis involves an oxidative quenching pathway.
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http://dx.doi.org/10.1021/acs.joc.9b01624DOI Listing
October 2019

Generation of N-Centered Radicals via a Photocatalytic Energy Transfer: Remote Double Functionalization of Arenes Facilitated by Singlet Oxygen.

J Am Chem Soc 2019 07 19;141(26):10538-10545. Epub 2019 Jun 19.

Department of Chemistry , Chung-Ang University , 84 Heukseok-ro, Dongjak-gu , Seoul 06974 , Republic of Korea.

An unprecedented approach to the generation of an N-centered radical via a photocatalytic energy-transfer process from readily available heterocyclic precursors is reported, which is distinctive of the previous electron transfer approaches. In combination with singlet oxygen, the in-situ-generated nitrogen radical from the oxadiazoline substrate in the presence of fac-Ir(ppy) undergoes a selective ipso addition to arenes to furnish remotely double-functionalized spiro-azalactam products. The mechanistic studies provide compelling evidence that the catalytic cycle selects the energy-transfer pathway. A concurrent activation of molecular oxygen to generate singlet oxygen by energy transfer is also rationalized. Furthermore, the occurrence of the electron transfer phenomenon is excluded on the basis of the negative driving forces for one-electron transfer between oxadiazoline and the excited state of fac-Ir(ppy) with a consideration of their redox potentials. The necessity of singlet oxygen as well as the photoactivated oxadiazoline substrate is clearly supported by a series of controlled experiments. Density functional studies have also been carried out to support these observations. The scope of substrates is explored by synthesizing diversely functionalized cyclohexadienone moieties in view of their utility in complex organic syntheses and as potential targets in pharmacology.
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http://dx.doi.org/10.1021/jacs.9b05572DOI Listing
July 2019

Artificial Control of Cell Signaling Using a Photocleavable Cobalt(III)-Nitrosyl Complex.

Angew Chem Int Ed Engl 2019 07 13;58(30):10126-10131. Epub 2019 Jun 13.

Department of Emerging Materials Science, DGIST, Daegu, 42988, Republic of Korea.

Cells use gaseous molecules such as nitric oxide (NO) to transmit both intracellular and intercellular signals. In principle, the endogenous small molecules regulate physiological changes, but it is unclear how randomly diffusive molecules trigger and discriminate signaling programs. Herein, it is shown that gasotransmitters use time-dependent dynamics to discriminate the endogenous and exogenous inputs. For a real-time stimulation of cell signaling, we synthesized a photo-cleavable metal-nitrosyl complex, [Co (MDAP)(NO)(CH CN)] (MDAP=N,N'-dimethyl-2,11-diaza[3,3](2,6)pyridinophane), which can stably deliver and selectively release NO with fine temporal resolution in the cytosol, and used this to study the extracellular signal-regulated kinases (ERKs), revealing how cells use both exogenous and endogenous NO to disentangle cellular responses. This technique can be to understand how diverse cellular signaling networks are dynamically interconnected and also to control drug delivery systems.
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http://dx.doi.org/10.1002/anie.201903106DOI Listing
July 2019

Amplified circularly polarized phosphorescence from co-assemblies of platinum(ii) complexes.

Chem Sci 2019 Feb 29;10(5):1294-1301. Epub 2018 Nov 29.

Division of Chemical Engineering and Materials Science , Ewha Womans University , Seoul 03760 , The Republic of Korea . Email:

Molecules capable of producing zero-field circularly polarized phosphorescence (CPP) are highly valuable for chiroptoelectronic applications that rely on triplet exciton. However, the paucity of tractable molecular design rules for obtaining CPP emission has inhibited full utilization. We report amplification of CPP by the formation of helical co-assemblies consisting of achiral square planar cycloplatinated complexes and small fractions of homochiral cycloplatinated complexes. The latter has a unique Pfeiffer effect during the formation of superhelical co-assemblies, enabling versatile chiroptical control. Large dissymmetry factors in electronic absorption ( , 0.020) and phosphorescence emission ( , 0.064) are observed from the co-assemblies. These values are two orders of magnitude improved relative to those of individual molecules. In addition, photoluminescence quantum yields (PLQY) also increase by a factor of ten. Our structural, photophysical, and quantum chemical investigations reveal that the chiroptical amplification is attributable to utilization of both the magnetically allowed electronic transition and asymmetric coupling of excitons. The strategy overcomes the trade-off between and PLQY which has frequently been found for previous molecular emitters of circularly polarized luminescence. It is anticipated that our study will provide new insight into the future research for the exploitation of the full potential of CPP.
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http://dx.doi.org/10.1039/c8sc04509gDOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6357861PMC
February 2019

Birch Reduction of Aromatic Compounds by Inorganic Electride [CaN]e in an Alcoholic Solvent: An Analogue of Solvated Electrons.

J Org Chem 2018 Nov 30;83(22):13847-13853. Epub 2018 Oct 30.

Department of Energy Science , Sungkyunkwan University , Suwon 440-746 , Republic of Korea.

Birch reduction of aromatic systems by solvated electrons in alkali metal-ammonia solutions is widely recognized as a key reaction that functionalizes highly stable π-conjugated organic systems. In spite of recent advances in Birch reduction with regard to reducing agent and reaction conditions, there remains an ongoing challenge to develop a simple and efficient Birch reaction under mild conditions. Here, we demonstrate that the inorganic electride [CaN]e promotes the Birch reduction of polycyclic aromatic hydrocarbons (PAHs) and naphthalene under alcoholic solvent in the vicinity of room temperature as a solid-type analogy to solvated electrons in alkali metal ammonia solutions. The anionic electrons from electride [CaN]e are transferred to PAHs and naphthalene via alcoholysis in a polar cosolvent medium. It is noteworthy that a high conversion yield to the hydrogenated products is ascribed to the extremely high electron transfer efficiency of 98%. This simple protocol utilizing an inorganic electride offers a direct and practical strategy for the reduction of aromatic compounds and provides an outstanding reducing agent for synthetic chemistry.
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http://dx.doi.org/10.1021/acs.joc.8b02094DOI Listing
November 2018

Monocycloplatinated Solvento Complex Displays Turn-on Ratiometric Phosphorescence Responses to Histamine.

Inorg Chem 2018 Nov 22;57(21):13985-13997. Epub 2018 Oct 22.

The study of biological histamine (HA) requires probes capable of ratiometric photoluminescence detection of HA. We discovered that a monocycloplatinated complex having two solvento ligands ([Pt(2-(2-naphthyl)quinolinate)(NCCH)]ClO) could produce ratiometric phosphorescence responses to HA in aerated aqueous solutions buffered to pH 7.4. The HA response was characterized with a hypsochromic shift of an emission peak wavelength from 635 to 567 nm. The corresponding phosphorescence intensity ratio (i.e., I/ I) increased from 0.26 to 1.90. Spectroscopic and spectrometric investigations indicated an occurrence of spontaneous displacement of the labile CHCN ligands with HA. An independently prepared HA adduct supported this notion. The ratiometric phosphorescence responses to HA were highly tolerant to other biological stimuli, including changes in pH and the presence of biometals and biological Lewis bases such as amino acids, nucleosides, biothiols, neurotransmitters, and small molecular metabolites. Of note was the high selectivity toward HA over common biological ligands, including histidine, cysteine, and homocysteine, which was ascribed to tighter HA binding. Our phosphorescence measurements employing Boc-protected derivatives of HA suggested that the bis-chelate motif involving imidazolyl and terminal amino groups was crucial for eliciting the ratiometric phosphorescence signaling. Finally, the bioimaging utility of the HA probe was validated using RAW 264.7 macrophages that were exogenously supplemented with HA or stimulated with thapsigargin to enrich intracellular HA. Ratiometric phosphorescence imaging microscopy experiments demonstrated the ability of the probe for monitoring intracellular HA uptake. In addition, photoluminescence lifetime imaging microscopy techniques could be applied for visualization of HA within the RAW 264.7 cells, because the HA binding elongated the photoluminescence lifetime. Our study demonstrated the promising utility of inner-sphere interactions of phosphorescent Pt(II) complexes for detection of biological HA.
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http://dx.doi.org/10.1021/acs.inorgchem.8b02612DOI Listing
November 2018

Development of quantum dot aptasensor and its portable analyzer for the detection of di-2-ethylhexyl phthalate.

Biosens Bioelectron 2018 Dec 30;121:1-9. Epub 2018 Aug 30.

Department of Environmental Science and Engineering, Ewha Womans University, 52 Ewhayeodae-gil, Seodaemun-gu, Seoul 03760, Republic of Korea. Electronic address:

We have developed a quantum dot aptasensor (QD-aptasensor) and its accompanying portable analyzer for the detection of di-2-ethylhexyl phthalate (DEHP). This sensor is based on a newly screened aptamer (60-mer) via SELEX and shows a binding affinity of 213 nmol/L with DEHP. The 60-mer aptamer together with its three shorter truncated aptamers (45, 28, and 22-mer) as well as three different DNA probes (12, 9, and 13-mer) were further investigated to form the best combination for the QD-aptasensor. Using a 22-mer-truncated aptamer and a 12-mer DNA probe combination, the QD-aptasensor demonstrated excellent DEHP sensitivity with an LOQ = 0.5 pg/mL as well as good selectivity in the presence of other phthalate analogs. The binding between the truncated aptamers and DEHP was also characterized. Finally, a QD-aptasensor-based portable analyzer was also developed, and its equivalence to the laboratory protocol was established with a correlation coefficient r = 0.86 for DEHP concentrations ranging from 0.0005 to 100 ng/mL.
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http://dx.doi.org/10.1016/j.bios.2018.08.065DOI Listing
December 2018

Tailoring Hydrophobic Interactions between Probes and Amyloid-β Peptides for Fluorescent Monitoring of Amyloid-β Aggregation.

ACS Omega 2018 May 11;3(5):5141-5154. Epub 2018 May 11.

Division of Chemical Engineering and Materials Science, Ewha Womans University, Seoul 03760, Republic of Korea.

Despite their unique advantages, the full potential of molecular probes for fluorescent monitoring of amyloid-β (Aβ) aggregates has not been fully exploited. This limited utility stems from the lack of knowledge about the hydrophobic interactions between the molecules of Aβ probes, as well as those between the probe and the Aβ aggregate. Herein, we report the first mechanistic study, which firmly establishes a structure-signaling relationship of fluorescent Aβ probes. We synthesized a series of five fluorescent Aβ probes based on an archetypal donor-acceptor-donor scaffold (denoted as -). The arylamino donor moieties were systematically varied to identify molecular factors that could influence the interactions between molecules of each probe and that could influence their fluorescence outcomes in conditions mimicking the biological milieu. Our probes displayed different responses to aggregates of Aβ, Aβ and Aβ, two major isoforms found in Alzheimer's disease: , having pyrrolidine donors, showed noticeable ratiometric fluorescence responses (Δν = 797 cm) to the Aβ and Aβ samples that contained oligomeric species, whereas , having -methylpiperazine donors, produced significant fluorescence turn-on signaling in response to Aβ aggregates, including oligomers, protofibrils, and fibrils (with turn-on ratios of 14 and 10 for Aβ and Aβ, respectively). Mechanistic investigations were carried out by performing field-emission scanning electron microscopy, X-ray crystallography, UV-vis absorption spectroscopy, and steady-state and transient photoluminescence spectroscopy experiments. The studies revealed that the probes underwent preassembly prior to interacting with the Aβ species and that the preassembled structures depended profoundly on the subtle differences between the amino moieties of the different probes. Importantly, the studies demonstrated that the mode of fluorescence signaling (i.e., ratiometric response versus turn-on response) was primarily governed by stacking geometries within the probe preassemblies. Specifically, ratiometric fluorescence responses were observed for probes capable of forming J-assembly, whereas fluorescence turn-on responses were obtained for probes incapable of forming J-aggregates. This finding provides an important guideline to follow in future efforts at developing fluorescent probes for Aβ aggregation. We also conclude, on the basis of our study, that the rational design of such fluorescent probes should consider interactions between the probe molecules, as well as those between Aβ peptides and the probe molecule.
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http://dx.doi.org/10.1021/acsomega.8b00286DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6641720PMC
May 2018

Chemical tools for the generation and detection of singlet oxygen.

Authors:
Youngmin You

Org Biomol Chem 2018 06;16(22):4044-4060

Division of Chemical Engineering and Materials Science, Ewha Womans University, Seoul 03760, Korea.

Growing evidence indicates intermediacy of singlet dioxygen (1O2) in a variety of pathophysiological processes. 1O2 has also found great utility of destructive actions for clinical and environmental applications. However, many details of the molecular mechanisms mediated by 1O2 remain insufficiently understood. Efforts to elucidate the 1O2 chemistry have been hampered by the lack of chemical tools capable of generation and detection of 1O2. In this review, I summarize the recent advances in the development of the chemical tools of 1O2. This article focuses on two topics. The first part introduces chemical methods for ground-state generation of 1O2. Designs of the molecular carriers of 1O2 are also explained. The second part discloses molecular probes of 1O2. The probes are categorized into three groups, depending on signaling modalities: absorption-based probes, photoluminescent probes, and chemiluminescent probes. Focus is on the molecular design to maximize the signaling actions. Disadvantages of using the probes are also discussed to motivate the future research. I hope that this review will serve as helpful guidance to the exploitation and development of the chemical tools of 1O2.
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http://dx.doi.org/10.1039/c8ob00504dDOI Listing
June 2018

Degradation of blue-phosphorescent organic light-emitting devices involves exciton-induced generation of polaron pair within emitting layers.

Nat Commun 2018 03 23;9(1):1211. Epub 2018 Mar 23.

Division of Chemical Engineering and Materials Science, Ewha Womans University, Seoul, 03760, Republic of Korea.

Degradation of organic materials is responsible for the short operation lifetimes of organic light-emitting devices, but the mechanism by which such degradation is initiated has yet to be fully established. Here we report a new mechanism for degradation of emitting layers in blue-phosphorescent devices. We investigate binary mixtures of a wide bandgap host and a series of novel Ir(III) complex dopants having N-heterocyclocarbenic ligands. Our mechanistic study reveals the charge-neutral generation of polaron pairs (radical ion pairs) by electron transfer from the dopant to host excitons. Annihilation of the radical ion pair occurs by charge recombination, with such annihilation competing with bond scission. Device lifetime correlates linearly with the rate constant for the annihilation of the radical ion pair. Our findings demonstrate the importance of controlling exciton-induced electron transfer, and provide novel strategies to design materials for long-lifetime blue electrophosphorescence devices.
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http://dx.doi.org/10.1038/s41467-018-03602-4DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5865184PMC
March 2018

Photocatalytic function of the B complex with the cyclometalated iridium(iii) complex as a photosensitizer under visible light irradiation.

Dalton Trans 2018 Jan;47(3):675-683

Department of Chemistry and Biochemistry, Graduate School of Engineering, Kyushu University, Motooka, Fukuoka 819-0395, Japan.

A visible light induced three-component catalytic system with the cobalamin derivative (B) as a catalyst, the cyclometalated iridium(iii) complex (Irdfppy, Irppy, Irpbt and [Ir{dF(CF)ppy}(dtbpy)]PF) as a photosensitizer and triethanolamine as an electron source under N was developed. This catalytic system showed a much higher catalytic efficiency than the previous catalytic system using [Ru(ii)(bpy)]Cl as the photosensitizer for the dechlorination reaction of 1,1-bis(4-chlorophenyl)-2,2,2-trichloroethane (DDT). Noteworthy is the fact that the remarkable high turnover number (over ten thousand) based on B, which ranks at the top among the reported studies, was obtained when Irdfppy was used as a photosensitizer. This photocatalytic system was also successfully applied to the B enzyme-mimic reaction, i.e., the 1,2-migration of the phenyl group of 2-bromomethyl-2-phenylmalonate. The plausible reaction mechanism was proposed, which involved two quenching pathways, an oxidative quenching pathway and a reductive quenching pathway, to be responsible for the initial electron transfer of the excited-state photosensitizers during the DDT dechlorination reaction. Transient photoluminescence experiments revealed that the oxidative quenching of the photosensitizer dominated over the reductive quenching pathway.
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http://dx.doi.org/10.1039/c7dt03742bDOI Listing
January 2018

Visible light-driven photogeneration of hydrogen sulfide.

Chem Commun (Camb) 2017 Aug;53(86):11830-11833

Division of Chemical Engineering and Materials Science, Ewha Womans University, Seoul 03760, Korea.

The combined use of a singlet oxygen photosensitizer and 1,3-diarylisobenzothiophene enables efficient generation of hydrogen sulfide under visible light illumination.
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http://dx.doi.org/10.1039/c7cc06990aDOI Listing
August 2017

Bistable Solid-State Fluorescence Switching in Photoluminescent, Infinite Coordination Polymers.

Chemistry 2017 Jul 10;23(42):10017-10022. Epub 2017 Jul 10.

Center for Supramolecular Optoelectronic Materials and Department of Materials Science and Engineering, Seoul National University, Daehak-dong, Gwanak-gu, Seoul, Korea.

Photo-functional infinite coordinated polymers (ICPs) were synthesized that consist of the photochromic dithienylethene (DTE) and a luminescent bridging unit to give enhanced fluorescence in the solid state. We could fabricate well-ordered micropatterns of these ICPs by a soft-lithographic method, which repeatedly showed high contrast on-off fluorescence switching.
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http://dx.doi.org/10.1002/chem.201701656DOI Listing
July 2017

Low-Affinity Zinc Sensor Showing Fluorescence Responses with Minimal Artifacts.

Inorg Chem 2017 Apr 5;56(8):4332-4346. Epub 2017 Apr 5.

Department of Applied Chemistry, Kyung Hee University , Yongin, Gyeonggi-do 17104, Korea.

The study of the zinc biology requires molecular probes with proper zinc affinity. We developed a low-affinity zinc probe (HBO-ACR) based on an azacrown ether (ACR) and an 2-(2-hydroxyphenyl)benzoxazole (HBO) fluorophore. This probe design imposed positive charge in the vicinity of a zinc coordination center, which enabled fluorescence turn-on responses to high levels of zinc without being affected by the pH and the presence of other transition-metal ions. Steady-state and transient photophysical investigations suggested that such a high tolerance benefits from orchestrated actions of proton-induced nonradiative and zinc-induced radiative control. The zinc bioimaging utility of HBO-ACR has been fully demonstrated with the use of human pancreas epidermoid carcinoma, PANC-1 cells, and rodent hippocampal neurons from cultures and acute brain slices. The results obtained through our studies established the validity of incorporating positively charged ionophores for the creation of low-affinity probes for the visualization of biometals.
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http://dx.doi.org/10.1021/acs.inorgchem.6b02786DOI Listing
April 2017

Mechanism and Applications of the Photoredox Catalytic Coupling of Benzyl Bromides.

Chemistry 2016 Dec 28;22(49):17790-17799. Epub 2016 Oct 28.

Division of Chemical Engineering and Materials Science, Ewha Womans University, Seoul, 03760, Republic of Korea.

The photoredox catalytic coupling of halomethyl arenes to bibenzyl derivatives has been demonstrated. The catalytic protocol employed the Hantzsch ester, potassium phosphate, and a photoactive cyclometalated Ir complex catalyst. A photochemical quantum yield as high as 20 % was obtained. The catalytic mechanism was investigated in detail by performing photophysical and electrochemical measurements, as well as by quantum chemical calculations. The results suggest that two-electron mediation might be responsible for the improved photon economy. The reaction protocol was compatible with halomethyl arenes that contain a variety of functional groups. Finally, the synthetic utility of our protocol was demonstrated by the preparation of a natural dihydrostilbenoid, brittonin A.
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http://dx.doi.org/10.1002/chem.201603517DOI Listing
December 2016

Controlled Fluoroalkylation Reactions by Visible-Light Photoredox Catalysis.

Acc Chem Res 2016 10 14;49(10):2284-2294. Epub 2016 Sep 14.

Department of Chemistry, Chung-Ang University , 84 Heukseok-ro, Dongjak-gu, Seoul 06974, Republic of Korea.

Owing to their unique biological, physical, and chemical properties, fluoroalkylated organic substances have attracted significant attention from researchers in a variety of disciplines. Fluoroalkylated compounds are considered particularly important in pharmaceutical chemistry because of their superior lipophilicity, binding selectivity, metabolic stability, and bioavailability to those of their nonfluoroalkylated analogues. We have developed various methods for the synthesis of fluoroalkylated substances that rely on the use of visible-light photoredox catalysis, a powerful preparative tool owing to its environmental benignity and mechanistic versatility in promoting a large number of synthetically important reactions with high levels of selectivity. In this Account, we describe the results of our efforts, which have led to the development of visible-light photocatalytic methods for the introduction of a variety of fluoroalkyl groups (such as, -CF, -CFR, -CHCF, -CF, and -CF) and arylthiofluoroalkyl groups (such as, -CFSPh, -CFSAr, and -CFSAr) to organic substances. In these studies, electron-deficient carbon-centered fluoroalkyl radicals were successfully generated by the appropriate choice of fluoroalkyl source, photocatalyst, additives, and solvent. The redox potentials of the photocatalysts and the fluoroalkyl sources and the choice of sacrificial electron donor or acceptor as the additive affected the photocatalytic pathway, determining whether an oxidative or reductive quenching pathway was operative for the generation of key fluoroalkyl radicals. Notably, we have observed that additives significantly affect the efficiencies and selectivities of these reactions and can even change the outcome of the reaction by playing additional roles during its course. For instance, a tertiary amine as an additive in the reaction medium can act not only as a sacrificial electron donor in photoredox catalysis but also as a hydrogen atom source, an elimination base for dehydrohalogenation of the intermediate, and also a Brønsted base for deprotonation. In the same context, the selection of solvent is also critical since it affects the rate and selectivity of reactions depending upon its polarity and reagent solubilizing ability and plays additional roles in the process, for example, as a hydrogen atom source. By clearly understanding the roles of additives and solvent, we designed several controlled fluoroalkylation reactions where different products were formed selectively from the same starting substrates. In addition, we could exploit one of the most important advantages of radical reactions, that is, the use of unactivated π-systems such as alkenes, alkynes, arenes, and heteroarenes as radical acceptors without prefunctionalization. Furthermore, fluoroalkylation processes under mild room-temperature reaction conditions tolerate various functional groups and are therefore easily applicable to late-stage modifications of highly functionalized advanced intermediates.
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http://dx.doi.org/10.1021/acs.accounts.6b00248DOI Listing
October 2016

Single-Electron-Transfer Strategy for Reductive Radical Cyclization: Fe(CO) and Phenanthroline System.

Org Lett 2016 10 27;18(19):4900-4903. Epub 2016 Sep 27.

Department of Applied Chemistry, Kyung Hee University , Yongin 17104, Korea.

An electron-transfer strategy using low-valent iron pentacarbonyl [Fe(CO)] to generate radical species from alkyl iodides was achieved. A range of pyrrolidines, tetrahydrofurans, and carbocycles were synthesized via 5-exo cyclization reactions of alkyl radical intermediates generated by electron transfer from a system involving Fe(CO), 1,10-phenanthroline, and diisopropylamine. Moreover, tandem addition reactions with Michael acceptors were also explored. Photophysical and electrochemical studies support a mechanism that involves electron transfer from the low-valent Fe reductant to alkyl iodide.
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http://dx.doi.org/10.1021/acs.orglett.6b02375DOI Listing
October 2016

Molecular dyad approaches to the detection and photosensitization of singlet oxygen for biological applications.

Authors:
Youngmin You

Org Biomol Chem 2016 Jul;14(30):7131-5

Division of Chemical Engineering and Materials Science, Ewha Womans University, Seoul 03760, Republic of Korea.

The principles and prospects of a molecular dyad strategy for photocontrolling biological singlet oxygen are highlighted.
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http://dx.doi.org/10.1039/c6ob01186aDOI Listing
July 2016

Coreactant Strategy for the Photoredox Catalytic Generation of Trifluoromethyl Radicals under Low-Energy Photoirradiation.

J Org Chem 2016 08 16;81(16):7072-9. Epub 2016 Jun 16.

Department of Chemistry, Chung-Ang University , Seoul 06974, Republic of Korea.

Photoredox catalysis has emerged as a valuable alternative to dark-state catalysis. For the full potential of photoredox catalysis to be utilized, it is imperative to make use of low-energy photons in photoinduced radical processes. We have demonstrated that the use of oxalate as a coreactant provides a useful principle for the photocatalytic production of trifluoromethyl radicals (•CF3) from CF3I upon green or red LED photoirradiation of narrow-bandgap photocatalysts. The photocatalytic cycle involves a radical anion of carbon dioxide (CO2(•-)) as a reductant for CF3I, which is generated through photoinduced oxidative decarboxylation of oxalate. Electrochemical characterizations and steady-state and transient photophysical investigations were performed to reveal that there are two photoinduced electron-transfer pathways for oxalate-mediated •CF3 generation.
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http://dx.doi.org/10.1021/acs.joc.6b00966DOI Listing
August 2016

A singlet oxygen photosensitizer enables photoluminescent monitoring of singlet oxygen doses.

Chem Commun (Camb) 2016 Jan;52(4):780-3

Division of Chemical Engineering and Materials Science, Ewha Womans University, Seoul 120-750, Korea.

A molecular dyad that can photosensitize and visualize singlet oxygen ((1)O2) was developed. The dual photofunction enables ratiometric photoluminescence monitoring of the progress of (1)O2-induced cell death.
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http://dx.doi.org/10.1039/c5cc08411cDOI Listing
January 2016

Phosphorescent Zinc Probe for Reversible Turn-On Detection with Bathochromically Shifted Emission.

Inorg Chem 2015 Oct 5;54(20):9704-14. Epub 2015 Oct 5.

Department of Chemistry and Nano Science and ‡Division of Chemical Engineering and Materials Science, Ewha Womans University , Seoul 120-750, Korea.

Phosphorescent molecules are attractive complements to fluorescent compounds for bioimaging. Time-gated acquisition of the long-lived phosphorescence signals provides an effective means to eliminate unwanted background noises due to short-lived autofluorescence. We have previously investigated the molecular principles governing modulation of photoinduced electron transfer in phosphorescence zinc probes that were based on biscyclometalated Ir(III) complexes (Woo, H. et al. J. Am. Chem. Soc. 2013, 135, 4771-4787). The studies established that phosphorescence turn-on responses would be attainable for Ir(III) complexes with high triplet-state energies. This sets an upper limit to an emission wavelength, restricting the development of red- or near-IR-phosphorescence turn-on probes. To address this challenge, we designed and synthesized a new phosphorescent probe having an electron-deficient 2-(2-pyridyl)pyrazine diimine ligand tethering a di(2-picolyl)amine (DPA) zinc receptor. This ligand control led to red phosphorescence emission (λ(ems) = 596 nm), with an excited-state reduction potential (E*(red)) retained as high as 1.44 V versus standard calomel electrode (SCE). The E*(red) value was more positive than the ground-state oxidation potential of DPA (1.05 V vs SCE), permitting an occurrence of photoinduced electron transfer at a rate of 2 × 10(7) s(-1). Zinc binding at DPA abolished the electron transfer to produce phosphorescence turn-on signaling. The probe was capable of detecting zinc ions selectively over other competing biological metal ions in aqueous buffer solutions (pH 7.4, 20 mM piperazine-N,N'-bis(2-ethanesulfonic aid)) with the zinc dissociation constant of 109 pM. Finally, bioimaging utility of the probe has been successfully demonstrated by visualizing exogenously supplied zinc ions in live HeLa cells. The research described in this paper demonstrates that judicious ligand control enables retention of turn-on responses in the low-energy phosphorescence region.
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http://dx.doi.org/10.1021/acs.inorgchem.5b00967DOI Listing
October 2015

Mechanisms and applications of cyclometalated Pt(ii) complexes in photoredox catalytic trifluoromethylation.

Chem Sci 2015 Feb 24;6(2):1454-1464. Epub 2014 Nov 24.

Department of Advanced Materials Engineering for Information and Electronics , Kyung Hee University , Yongin , Gyeonggi-do 446-701 , Korea . Email:

The incorporation of a trifluoromethyl group into an existing scaffold can provide an effective strategy for designing new drugs and agrochemicals. Among the numerous approaches to trifluoromethylation, radical trifluoromethylation mediated by visible light-driven photoredox catalysis has gathered significant interest as it offers unique opportunities for circumventing the drawbacks encountered in conventional methods. A limited understanding of the mechanism and molecular parameters that control the catalytic actions has hampered the full utilization of photoredox catalysis reactions. To address this challenge, we evaluated and investigated the photoredox catalytic trifluoromethylation reaction using a series of cyclometalated Pt(ii) complexes with systematically varied ligand structures. The Pt(ii) complexes were capable of catalyzing the trifluoromethylation of non-prefunctionalized alkenes and heteroarenes in the presence of CFI under visible light irradiation. The high excited-state redox potentials of the complexes permitted oxidative quenching during the cycle, whereas reductive quenching was forbidden. Spectroscopic measurements, including time-resolved photoluminescence and laser flash photolysis, were performed to identify the catalytic intermediates and directly monitor their conversions. The mechanistic studies provide compelling evidence that the catalytic cycle selects the oxidative quenching pathway. We also found that electron transfer during each step of the cycle strictly adhered to the Marcus normal region behaviors. The results are fully supported by additional experiments, including photoinduced ESR spectroscopy, spectroelectrochemical measurements, and quantum chemical calculations based on time-dependent density functional theory. Finally, quantum yields exceeding 100% strongly suggest that radical propagation significantly contributes to the catalytic trifluoromethylation reaction. These findings establish molecular strategies for designing trifluoromethyl sources and catalysts in an effort to enhance catalysis performance.
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http://dx.doi.org/10.1039/c4sc02537gDOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5811153PMC
February 2015
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