Young Mo Sung

Stable Tetrabenzo-Chichibabin’s Hydrocarbons: Tunable Ground State and Unusual Transition between Their Closed-Shell and Open-Shell Resonance Forms

Stable open-shell polycyclic aromatic hydrocarbons (PAHs) are of fundamental interest due to their unique electronic, optical, and magnetic properties and promising applications in materials sciences. Chichibabins hydrocarbon as a classical open-shell PAH has been investigated for a long time. However, most of the studies are complicated by their inherent high reactivity. In this work, two new stable benzannulated Chichibabins hydrocarbons 1-CS and 2-OS were prepared, and their electronic structure and geometry in the ground state were studied by various experiments (steady-state and transient absorption spectra, NMR, electron spin resonance (ESR), superconducting quantum interference device (SQUID), FT Raman, X-ray crystallographic etc.) and density function theory (DFT) calculations. 1-CS and 2-OS exhibited tunable ground states, with a closed-shell quinoidal structure for 1-CS and an open-shell biradical form for 2-OS. Their corresponding excited-state forms 1-OS and 2-CS were also chemically approached and showed different decay processes. The biradical 1-OS displayed an unusually slow decay to the ground state (1-CS) due to a large energy barrier (95 ± 2.5 kJ/mol) arising from severe steric hindrance during the transition from an orthogonal biradical form to a butterfly-like quinoidal form. The quick transition from the quinoidal 2-CS (excited state) to the orthogonal biradicaloid 2-OS (ground state) happened during the attempted synthesis of 2-CS. Compounds 1-CS and 2-OS can be oxidized into stable dications by FeCl(3) and/or concentrated H(2)SO(4). The open-shell 2-OS also exhibited a large two-photon absorption (TPA) cross section (760 GM at 1200 nm).

Pushing extended p-quinodimethanes to the limit: Stable tetracyano-oligo(N-annulated perylene)quinodimethanes with tunable ground states

p-Quinodimethane (p-QDM) is a fundamental building block for the design of π-conjugated systems with low band gap and open-shell biradical character. However, synthesis of extended p-QDMs has usually suffered from their intrinsic high reactivity and poor solubility. In this work, benzannulation together with terminal cyano-substitution was demonstrated to be an efficient approach for the synthesis of a series of soluble and stable tetracyano-oligo(N-annulated perylene)quinodimethanes nPer-CN (n = 1-6), with the longest molecule having 12 para-linked benzenoid rings! The geometry and electronic structures of these oligomers were investigated by steady-state and transient absorption spectroscopy, nuclear magnetic resonance, electron spin resonance, superconducting quantum interference device, and FT Raman spectroscopy assisted by density functional theory calculations. They showed tunable ground states, varying from a closed-shell quinoidal structure for monomer, to a singlet biradical for dimer, trimer, and tetramer, and to a triplet biradical for pentamer and hexamer. Large two-photon absorption cross-section values were observed in the near-infrared range, which also exhibited a clear chain-length dependence.

Reversal of Hückel (anti)aromaticity in the lowest triplet states of hexaphyrins and spectroscopic evidence for Baird's rule

The reversal of (anti)aromaticity in a molecules triplet excited state compared with its closed-shell singlet ground state is known as Bairds rule and has attracted the interest of synthetic, physical organic chemists and theorists because of the potential to modulate the fundamental properties of highly conjugated molecules. Here we show that two closely related bis-rhodium hexaphyrins (R26H and R28H) containing [26] and [28] π-electron peripheries, respectively, exhibit properties consistent with Bairds rule. In the ground state, R26H exhibits a sharp Soret-like band and distinct Q-like bands characteristic of an aromatic porphyrinoid, whereas R28H exhibits a broad absorption spectrum without Q-like bands, which is typical of an antiaromatic porphyrinoid. In contrast, the T–T absorption of R26H is broad, weak and featureless, whereas that of R28H displays an intense and sharp Soret-like band. These spectral signatures, in combination with quantum chemical calculations, are in line with qualitative expectations based on Bairds rule. In the ground state, aromatic rings contain [4n + 2] π electrons whereas antiaromatic systems have [4n] π-electrons. Bairds rule states that this situation is reversed in the lowest triplet excited state. It has now been shown using a combination of spectroscopy and quantum chemical calculations that two closely related bis-rhodium hexaphyrins exhibit properties consistent with Bairds rule.

Ion-Regulated Allosteric Binding of Fullerenes (C60 and C70) by Tetrathiafulvalene-Calix[4]pyrroles

The effect of ionic species on the binding of fullerenes (C60 and C70) by tetrathiafulvalene-calix[4]pyrrole (TTF-C4P) receptors and the nature of the resulting supramolecular complexes (TTF-C4P + fullerene + halide anion + tetraalkylammonium cation) was studied in the solid state through single crystal X-ray diffraction methods and in dichloromethane solution by means of continuous variation plots and UV-vis spectroscopic titrations. These analyses revealed a 1:1 stoichiometry between the anion-bound TTF-C4Ps and the complexed fullerenes. The latter guests are bound within the bowl-like cup of the C4P in a ball-and-socket binding mode. The interactions between the TTF-C4P receptors and the fullerene guests are highly influenced by both the nature of halide anions and their counter tetraalkylammonium cations. Three halides (F(-), Cl(-), and Br(-)) were studied. All three potentiate the binding of the two test fullerenes by inducing a conformational change from the 1,3-alternate to the cone conformer of the TTF-C4Ps, thus acting as positive heterotropic allosteric effectors. For a particular halide anion, the choice of tetraalkylammonium salts serves to modulate the strength of the TTF-C4P-fullerene host-guest binding interactions and, in conjunction with variations in the halide anion, can be exploited to alter the inherent selectivity of the host for a given fullerene. Differences in binding are reflected in the excited state optical properties. Overall, the present four-component system provides an illustration of how host-guest binding events involving appropriately designed artificial receptors can be fine-tuned via the addition of simple ionic species as allosteric modulators.

A p‐Quinodimethane‐Bridged Porphyrin Dimer

A p-quinodimethane (p-QDM)-bridged porphyrin dimer 1 has been prepared for the first time. An unexpected Michael addition reaction took place when we attempted to synthesize compound 1 by reaction of the cross-conjugated keto-linked porphyrin dimers 8a and 8b with alkynyl/aryl Grignard reagents. Alternatively, compound 1 could be successfully prepared by intramolecular Friedel-Crafts alkylation of the diol-linked porphyrin dimer 14 with concomitant oxidation in air. Compound 1 shows intense one-photon absorption (OPA, λ(max)=955 nm, ε=45400 M(-1) cm(-1)) and a large two-photon absorption (TPA) cross-section (σ((2))(max)=2080 GM at 1800 nm) in the near-infrared (NIR) region due to its extended π-conjugation and quinoidal character. It also exhibits a short singlet excited-state lifetime of 25 ps. The cyclic voltammogram of 1 displays multiple redox waves with a small electrochemical energy gap of 0.86 eV. The ground-state geometry, electronic structure, and optical properties of 1 have been further studied by density functional theory (DFT) calculations and compared with those of the keto-linked dimer 8b. This research has revealed that incorporation of a p-QDM unit into the porphyrin framework had a significant impact on its optical and electronic properties, leading to a novel NIR OPA and TPA chromophore.

Synthesis of Thieno-Bridged Porphyrins: Changing the Antiaromatic Contribution by the Direction of the Thiophene Ring

Two types of thieno-bridged porphyrins were synthesized by incorporating a thiophene group across their meso and β positions with different directions of the thiophene ring to investigate the aromaticity of these porphyrins with extended π-systems. The 2,3-thieno-bridged porphyrin showed a larger antiaromatic contribution than did the 3,4-thieno-bridged porphyrin. In the former, the antiaromatic contribution is based on a 20-π-electron conjugated circuit. The two thieno-bridged porphyrins were characterized by calculations of nucleus-independent chemical shift and anisotropy of the induced current density as well as by X-ray crystallography, NMR spectroscopy, UV-vis-NIR absorption spectroscopy, electrochemical studies, time-resolved excited-state analysis, and two-photon absorption cross section measurements. Chemical derivatization of the 2,3-thieno-bridged porphyrin was also demonstrated.

Control and Switching of Aromaticity in Various All-Aza-Expanded Porphyrins: Spectroscopic and Theoretical Analyses

Modification of aromaticity is regarded as one of the most interesting and important research topics in the field of physical organic chemistry. Particularly, porphyrins and their analogues (porphyrinoids) are attractive molecules for exploring various types of aromaticity because most porphyrinoids exhibit circular conjugation pathways in their macrocyclic rings with various molecular structures. Aromaticity in porphyrinoids is significantly affected by structural modification, redox chemistry, NH tautomerization, and electronic states (singlet and triplet excited states). Conversely, aromaticity significantly affects the spectroscopic properties and chemical reactivities of porphyrinoids. In this context, considerable efforts have been devoted to understanding and controlling the aromaticity and antiaromaticity of porphyrinoids. Thus, a series of porphyrinoids are in the limelight, being expected to shed light on this field because they have some advantages to demonstrate the switching of aromaticity; it is possible to control the aromaticity by lowering the temperature, adding and removing the protons of expanded porphyrins, changing the chemical environment, and switching the electronic states (triplet and singlet excited states) by photoexcitation. In this regard, this Review describes the control of aromaticity in various expanded porphyrins from the spectroscopic point of view with assistance from theoretical calculations.

Switching between Aromatic and Antiaromatic 1,3-Phenylene-Strapped [26]- and [28]Hexaphyrins upon Passage to the Singlet Excited State

We have demonstrated aromaticity reversal in the singlet excited states of internally 1,3-phenylene-strapped [26]- and [28]hexaphyrins (P26H and P28H). P26H displays a broad and reduced singlet-excited-state absorption spectrum, whereas P28H exhibits a sharp and intense singlet-excited-state absorption spectrum; both are in contrast to the ground-state absorption spectra, strongly indicating aromaticity reversal in the singlet excited state. Furthermore, magnetic and topological indices of aromaticity such as nucleus-independent chemical shift and harmonic oscillator model of aromaticity values for P26H and P28H also suggest that their singlet excited states become antiaromatic and aromatic, respectively.

PdIIComplexes of [44]- and [46]Decaphyrins: The Largest Hückel Aromatic and Antiaromatic, and Möbius Aromatic Macrocycles

Reductive metalation of [44]decaphyrin with [Pd2(dba)3] provided a Hückel aromatic [46]decaphyrin Pd(II) complex, which was readily oxidized upon treatment with DDQ to produce a Hückel antiaromatic [44]decaphyrin Pd(II) complex. In CH2Cl2 solution the latter complex underwent slow tautomerization to a Möbius aromatic [44]decaphyrin Pd(II) complex which exists as a mixture of conformers in dynamic equilibrium. To the best of our knowledge, these three Pd(II) complexes represent the largest Hückel aromatic, Hückel antiaromatic, and Möbius aromatic complexes to date.

Antiaromatic Hexaphyrins and Octaphyrins Stabilized by the Hydrogen‐Bonding Interactions of meso‐Imidazolyl Groups

Stable antiaromatic expanded porphyrins were designed by the judicious implementation of meso-imidazolyl groups, which cause stabilization through the creation of a hydrogen-bonding network that overcomes antiaromatic electronic destabilization. Both the [28]hexaphyrin 1 and the [36]octaphyrin 2, which contain imidazolyl groups at two opposite meso positions, are shown to be stable Hückel antiaromatic species.