Suk Joong Lee

Asymmetric catalytic reactions by NbO-type chiral metal–organic frameworks

Chiral metal–organic frameworks (MOFs) constitute a unique class of multifunctional hybrid materials and are envisioned as a versatile tool for various enantioselective applications, including the separation of optical isomers and the promotion of catalytic enantioselective reactions. Despite some pioneering works on catalytic enantioselective reactions promoted by chiral MOFs, there is still a need for practical catalysts and many fundamental issues must be answered; such as pin-pointing the site of the reaction and expedition of the reaction rate to the level of that in homogeneous media. We have designed and synthesized a chiral metal–organic framework, (S)-KUMOF-1 (Cu2(S)-1)2(H2O)2, 1 = 2,2′-dihydroxy-6,6′-dimethyl(1,1′-biphenyl)-4,4′-dicarboxylate) of which a non-interpenetrating NbO type framework provides a spacious pore (2 × 2 × 2 nm3) and is equipped with potential catalytic sites exposed into the pore. Since the functional group on the organic links, biphenols in this MOF, can be modified further on demand, this MOF can serve as a platform for new heterogeneous catalysis. Two reactions, the carbonyl-ene reaction with modified MOF after replacement of the protons on biphenol on the organic links with Zn(II) and the hetero-Diels–Alder reaction with Ti(IV), respectively, were studied. In this manoeuver, we observed that the reaction occurs entirely inside the pores and the reaction rate of the heterogeneous reaction by this specific MOF is comparable to that of its homogeneous counterpart. In addition, it is also observed that the enantioselectivities are significantly improved by extra steric bias provided from the frames of the MOF. These observations reinforce the legitimacy of the strategy of using a chiral MOF as a highly enantioselective heterogeneous catalyst.

Cavity-Tailored, Self-Sorting Supramolecular Catalytic Boxes for Selective Oxidation

Using a steric self-sorting strategy, the assembly of highly ordered and rigid supramolecular boxes possessing catalytic properties has been achieved in one step. The formation of these assemblies, comprising up to 18 porphyrin centers, was readily confirmed by solution X-ray scattering in conjunction with fluorescent spectroscopy. Size-selective and enantioselective oxidation catalysis were both demonstrated.

Growth of Narrowly Dispersed Porphyrin Nanowires and Their Hierarchical Assembly into Macroscopic Columns

The self-assembly of amphiphilic (porphyrin)Sn 1 in an aqueous Pluronic F127 media results in the formation of narrowly dispersed nanowires. Variation of aqueous Pluronic concentrations allows for a systematic variation of porphyrin nanowire lengths while keeping their widths constant. These nanowires can be further assembled into secondary macroscopic columns.

Intramolecular Energy Transfer within Butadiyne-Linked Chlorophyll and Porphyrin Dimer-Faced, Self-Assembled Prisms

The synthesis and photophysical properties of butadiyne-linked chlorophyll and porphyrin dimers in toluene solution and in several self-assembled prismatic structures are described. The butadiyne linkage between the 20-positions of the macrocycles results in new electronic transitions polarized along the long axes of the dimers. These transitions greatly increase the ability of these dimers to absorb the solar spectrum over a broad wavelength range. Femtosecond transient absorption spectroscopy reveals the relative rate of rotation of the macrocycles around the butadiyne bond joining them. Following addition of 3-fold symmetric, metal-coordinating ligands, both macrocyclic dimers self-assemble into prismatic structures in which the dimers comprise the faces of the prisms. These structures were confirmed by small-angle X-ray scattering experiments in solution using a synchrotron source. Photoexcitation of the prismatic assemblies reveals that efficient, through-space energy transfer occurs between the macrocyclic dimers within the prisms. The distance dependence of energy transfer between the faces of the prisms was observed by varying the size of the prismatic assemblies through the use of 3-fold symmetric ligands having arms with different lengths. These results show that self-assembly of discrete macrocyclic prisms provides a useful new strategy for controlling singlet exciton flow in antenna systems for artificial photosynthesis and solar cell applications.

Highly Photosensitive J‐Aggregated Single‐Crystalline Organic Transistors

Charge-transport phenomena and photoinduced charge generation of organic semiconductors have potential applications in many electronic and optoelectronic devices. Organic fi eld-effect transistors (OFETs) are the most promising electronic devices fabricated using either well-defi ned single crystals (SCs) or thin fi lms as the charge-transporting layers. [ 1–3 ] In particular, soluble organic semiconductors are attracting attention for use in the fabrication of low-cost, large-scale, and practical devices. [ 4 , 5 ]

Unusually High‐Performing Organic Field‐Effect Transistors Based on π‐Extended Semiconducting Porphyrins

Highly conjugated porphyrin derivatives, H(2) TP and ZnTP, are synthesized. J-aggregations of the H-aggregated dimeric porphyrin pairs are clearly observed by their single crystal structures that facilitate slip-stacked charge transport phenomenon. In particular, their SC-FETs show the highest field-effect mobilities of 0.85-2.90 cm(2) V(-1) s(-1) . Furthermore, the ZnTP-based OPT displays a dramatic photoinduced current enhancement with a high photoresponsivity of 22 000 A W(-1) under a very low light intensity (5.6 m W cm(-2) ).

Highly sensitive phototransistor with crystalline microribbons from new π-extended pyrene derivative via solution-phase self-assembly

A new pyrene-cored π-conjugated molecule has been synthesized through Sonogashira coupling reaction. The single-crystalline microribbon-based FET exhibited the highest mobility of 0.7 cm(2) V(-1) s(-1) (I(on)/I(off) > 10(6)). Single-crystalline microribbons were employed to operate in an organic phototransistor (OPT) under very low light intensity (I = 5.6 μW cm(-2)).

Organic photovoltaics interdigitated on the molecular scale

Phosphonated porphyrin frameworks that are porous to an electron-accepting perylenediimide are systematically interdigitated with a phosphonated form of the diimide to form bulk heterojunctions. When employed in photovoltaics, these molecularly interlaced heterojunctions provide large junction areas while retaining the phase connectivity of traditional bilayer heterojunctions. Zirconium phosphonate linkages facilitate layer-by-layer chromophore assembly from solution under ambient conditions. The dependence of photovoltaic performance on heterojunction architecture is investigated.

Fast energy transfer within a self-assembled cyclic porphyrin tetramer

The structure of a cyclic self-assembled tetramer of an asymmetric meso-ethynylpyridyl-functionalized Zn(II)-porphyrin was established by solution-phase X-ray scattering and diffraction; femtosecond transient absorption and anisotropy spectroscopies were used to (a) observe rapid energy transfer (3.8 ps(-1)) between porphyrin subunits and (b) establish that the transfer occurs between adjacent units.

Remarkable Solvent, Porphyrin Ligand, and Substrate Effects on Participation of Multiple Active Oxidants in Manganese(III) Porphyrin Catalyzed Oxidation Reactions

The participation of multiple active oxidants generated from the reactions of two manganese(III) porphyrin complexes containing electron-withdrawing and -donating substituents with peroxyphenylacetic acid (PPAA) as a mechanistic probe was studied by carrying out catalytic oxidations of cyclohexene, 1-octene, and ethylbenzene in various solvent systems, namely, toluene, CH(2) Cl(2) , CH(3) CN, and H(2) O/CH(3) CN (1:4). With an increase in the concentration of the easy-to-oxidize substrate cyclohexene in the presence of [(TMP)MnCl] (1a) with electron-donating substituents, the ratio of heterolysis to homolysis increased gradually in all solvent systems, suggesting that [(TMP)Mn-OOC(O)R] species 2a is the major active species. When the substrate was changed from the easy-to-oxidize one (cyclohexene) to difficult-to-oxidize ones (1-octene and ethylbenzene), the ratio of heterolysis to homolysis increased a little or did not change. [(F(20) TPP)Mn-OOC(O)R] species 2b generated from the reaction of [(F(20) TPP)MnCl] (1b) with electron-withdrawing substituents and PPAA also gradually becomes involved in olefin epoxidation (although to a much lesser degree than with [(TMP)Mn-OOR] 2a) depending on the concentration of the easy-to-oxidize substrate cyclohexene in all aprotic solvent systems except for CH(3) CN, whereas Mn(V)=O species is the major active oxidant in the protic solvent system. With difficult-to-oxidize substrates, the ratio of heterolysis to homolysis did not vary except for 1-octene in toluene, indicating that a Mn(V)=O intermediate generated from the heterolytic cleavage of 2b becomes a major reactive species. We also studied the competitive epoxidations of cis-2-octene and trans-2-octene with two manganese(III) porphyrin complexes by meta-chloroperbenzoic acid (MCPBA) in various solvents under catalytic reaction conditions. The ratios of cis- to trans-2-octene oxide formed in the reactions of MCPBA varied depending on the substrate concentration, further supporting the contention that the reactions of manganese porphyrin complexes with peracids generate multiple reactive oxidizing intermediates.