Kyoung Chul Ko

KCN sensor: unique chromogenic and ‘turn-on’ fluorescent chemodosimeter: rapid response and high selectivity

An indole conjugated coumarin 1 for KCN chemodosimeter has been prepared and displayed considerable dual changes in both absorption (blue-shift) and emission (turn-on) bands exclusively for KCN. DFT/TDDFT calculations support that the fluorescence enhancement of 1-KCN is mainly due to blocking of the ICT process.

Coumarin-based thiol chemosensor: synthesis, turn-on mechanism, and its biological application.

A new chemodosimetric probe (1) is reported that selectively detects thiols over other relevant biological species by the turning on of its fluorescence through a Michael type reaction. The fluorogenic process upon its reaction was revealed to be mediated by intramolecular charge transfer, as confirmed by time-dependent density functional theory calculations. The application of probe 1 to cells is also examined by confocal microscopy, and its cysteine preference was observed by an ex vivo LC-MS analysis of the cellular metabolite.

Fluorescence turn-on sensors for HSO4-.

A coumarin-based derivative (1), a highly selective and sensitive turn-on fluorogenic probe for the detection of HSO4- ions in aqueous solution, has been designed and synthesized. Various spectroscopic and DFT calculations revealed that H-bonding between the phenolic -OH and imine nitrogen of 1 played a crucial role in its high selectivity for HSO4-.

Tandem Synthesis of Photoactive Benzodifuran Moieties in the Formation of Microporous Organic Networks

Over the last decade, microporous organic materials have been extensively prepared through various coupling reactions. In the early stages, relatively simple aromatic building blocks were used to prepare microporous organic networks (MONs) and relevant studies have focused on their physisorption behavior toward gas guests. Recently, more specific functionalities were achieved by the introduction of designed active sites into MONs. Usually, the active sites could be introduced by using the predesigned building blocks or by postmodification of the porous materials. If the active sites could be concomitantly formed in the network formation process for porous materials, this synthetic process would be very efficient and ideal for functional materials. For example, we have demonstrated the successful incorporation of active N-heterocyclic carbene metal species into metal– organic frameworks (MOFs) during self-assembly processes. However, this kind of synthetic approach is relatively rare, especially in the synthesis of MONs. Benzodifurans (BDFs) are very interesting materials owing to their unique optical and electrical properties. Their electron-rich nature has enabled them to be applied as redox-active hole transfer materials in organic lightemitting devices. Moreover, very recently, anti-benzodifuran-based organic materials have attracted significant attention as photoand redox-active materials in solar cells and organic field-effect transistors. anti-Benzodifurans can be prepared in the intramolecular cyclization reaction of 1,4hydroquinone with two alkyne groups at the 2,5-positions. Generally, tandem reactions in organic synthesis can be defined as a consecutive series of intramolecular reactions. In well-designed tandem processes, the functional groups for the following successive reactions can be generated in situ as a result of the previous reaction. Through the introduction of tandem processes to organic synthesis, the synthetic strategies become more atom-economical, because the work-up and isolation processes for intermediates can be reduced. Thus, much effort has been made for the development of smart tandem processes for complicated target organic materials. The Cooper research group and others have shown that MONs can be prepared by Sonogashira coupling between multialkyne connectors and multihalo arene building blocks. 7, 8] We have continued to develop functional MONs. We speculated that the generation of benzodifuran species can be induced in a tandem manner during the formation of the MON through a Sonogashira coupling. As far as we are aware, tandem synthetic strategies for the preparation of functional MONs have not been reported. Herein, we report the preparation of photoactive MONs with benzodifuran moieties through tandem synthetic processes, and their applications to photocatalytic coupling of primary amines. Figure 1 shows the synthetic strategy for the synthesis of a MON containing benzodifuran moieties (BDF-MON).

Rationally Designed Fluorescence Turn-On Sensors: A New Design Strategy Based on Orbital Control

Herein, we explore a new strategy in the chemo-sensor field for fluorescence amplification upon binding with metal ions based on controlled participation of the nitrogen lone pair orbital. The basic architecture of the sensor entails a fluorophore, the sp(2) hybridized nitrogen lone pair (-C═N-), and a chelator site referred to as the control part. Though nonplanar and nonfluorescent, compound IC1 achieved pseudo planarity from binding with Zn(2+) as indicated by the increased fluorescence signal. Its other analogue (IC2) is also planar, and unlike IC1-Zn(2+) was fluorescent with a lack of binding affinity to metal ions. The time-dependent density functional theory (TDDFT) calculations revealed that the fluorescence amplification was due to the blocking of the nitrogen lone pair orbital; unlikely geometrical rearrangements were insignificant. This could indicate a breakthrough concept in the future design of fluorescent turn-on sensors.

Photocatalysis by phenothiazine dyes: visible-light-driven oxidative coupling of primary amines at ambient temperature.

New phenothiazine based organic dyes were prepared for visible-light-driven organic transformations. The 3,7-disubstituted phenothiazine derivatives showed visible light absorption and reversible one-electron oxidation behavior. In the presence of 0.5 mol % of 3,7-disubstituted phenothiazines, primary benzylamines showed oxidative coupling under visible light irradiation from a blue LED. The electronic effect of substituents in phenothiazine dyes was observed in catalytic activities. The mechanistic pathway of oxidative coupling was discussed based on the detection of H(2)O(2) after the reaction.

Enhancement of Electrogenerated Chemiluminescence and Radical Stability by Peripheral Multidonors on Alkynylpyrene Derivatives

A very generous donor: The electrochemiluminescence (ECL) efficiency and radical stability of pyrene, a poor ECL luminophore, are markedly improved as the number of peripheral multidonor units increased in a series of compounds (see picture). Photophysical and electrochemical studies and theoretical calculations have contributed to the understanding of the ECL enhancement, which is a step forward in the development of new light-emitting materials.

Systematic Approach To Design Organic Magnetic Molecules: Strongly Coupled Diradicals with Ethylene Coupler

The intramolecular magnetic coupling constant (J) values of diradical systems linked with two monoradicals through a coupler (para-substituted phenyl acetylene (Model I), meta-substituted phenyl acetylene (Model II), ethylene (Model III)) were investigated by unrestricted density functional theory calculations. We divided eight monoradicals into α-group and β-group according to Mulliken spin density values of the connected atoms. The overall trends in the strength of magnetic interactions of diradicals were found to be identical in three different model systems. The diradicals with para-substituted phenyl acetylene coupler resulted in almost twice stronger intramolecular magnetic coupling interactions of the corresponding diradicals as compared to the meta-substituted one with opposite magnetism. NN-Ethylene-PO (nitronyl nitroxide radical coupled to phenoxyl radical via ethylene coupler) was calculated to have the strongest magnetic coupling constant with ferromagnetism, and to be even stronger (more than twice) than NN-ethylene-NN (nitronyl nitroxide diradical with ethylene coupler), which was reported to have strong antiferromagnetic interactions in a previous experiment. It was found that the spin density values of the connected atoms are closely related to the determination of magnetic interactions and J values. The spin states of the ground state in diradical systems were explained by means of the spin alternation rule.

Organic magnetic diradicals (radical-coupler-radical): standardization of couplers for strong ferromagnetism.

The intramolecular magnetic coupling constant (J) values of sets of diradicals linked to bis-DTDA, OVER, and NN radicals (DTDA, OVER, and NN groups) through an aromatic coupler were studied by unrestricted density functional theory calculations (UB3LYP/6-311++G(d,p)). Among 15 aromatic couplers, 9 compounds with an odd number of carbon atoms along its spin coupling path were found to interact ferromagnetically upon coupling with bisradicals while the other 6 couplers with an even number of carbon atoms along its spin coupling path give rise to antiferromagnetic coupling. The overall trends in the strength of magnetic interactions of aromatic couplers were preserved for DTDA, OVER, and NN groups so that the trend can be utilized as an index for the magnetic strength of a given coupler. It was found that the differences in the nucleus-independent chemical shift (NICS), bond order of connecting bonds, and Mulliken atomic spin density at connected atoms between triplet and BS states are closely related to the intramolecular magnetic behavior. 2,4- and 2,5-phosphole couplers exhibit the strongest intramolecular ferromagnetic and antiferromagnetic interactions among 15 aromatic couplers when linked to diverse bisradicals.

Scaling approach for intramolecular magnetic coupling constants of organic diradicals.

The intramolecular magnetic coupling constants (J) of 9 diradicals (i-ix) coupled with an aromatic ring were investigated by means of unrestricted density functional theory (DFT) calculations [UB3LYP/6-311++G(d,p)]. For these diradicals, a remarkable linear relationship between the calculated and experimental J values was found. In this study, we suggest that the slope (0.380) of the linear relationship can be utilized as a scaling factor for estimating J values. By applying this scaling factor and calculating J values, we could predict the reliable J values of four dithiadiazolyl (DTDA) diradicals coupled with an aromatic ring. It was also found that this scaling scheme shows a dependence on the length of a coupler. Nevertheless, this scaling approach could be used to estimate J values for diverse diradical systems coupled with a particular coupler by DFT calculations.