Sangsu Lee

Dibenzoheptazethrene Isomers with Different Biradical Characters: An Exercise of Clar’s Aromatic Sextet Rule in Singlet Biradicaloids

Clars aromatic sextet rule has been widely used for the prediction of the reactivity and stability of polycyclic aromatic hydrocarbons with a closed-shell electronic configuration. Recent advances in open-shell biradicaloids have shown that the number of aromatic sextet rings plays an important role in determination of their ground states. In order to test the validity of this rule in singlet biradicaloids, the two soluble and stable dibenzoheptazethrene isomers DBHZ1 and DBHZ2 were prepared by different synthetic approaches and isolated in crystalline form. These two molecules have different numbers of aromatic sextet rings in their respective biradical resonance forms and thus are expected to exhibit varied singlet biradical character. This assumption was verified by different experimental methods, including nuclear magnetic resonance (NMR), electron spin resonance (ESR), superconducting quantum interference device (SQUID), steady-state and transient absorption spectroscopy (TA), and X-ray crystallographic analysis, assisted by unrestricted symmetry-broken density functional theory (DFT) calculations. DBHZ2, with more aromatic sextet rings in the biradical form, was demonstrated to possess greater biradical character than DBHZ1; as a result, DBHZ2 exhibited an intense one-photon absorption (OPA) in the near-infrared region (λabs(max) = 804 nm) and a large two-photon absorption (TPA) cross-section (σ((2))max = 2800 GM at 1600 nm). This investigation together with previous studies indicates that Clars aromatic sextet rule can be further extended to the singlet biradicaloids to predict their ground states and singlet biradical characters.

Tetracyanoquaterrylene and Tetracyanohexarylenequinodimethanes with Tunable Ground States and Strong Near‐Infrared Absorption

Biradicaloids based on quinoidal rylenes! Soluble and stable tetracyanoquaterrylenequinodimethane (QR-CN) and tetracyanohexarylenequinodimethane (HR-CN) were synthesized. QR-CN has a closed-shell quinoidal structure in the ground state, whereas HR-CN has a singlet biradical ground state. Both compounds showed very strong one-photon and two-photon absorption in the NIR range

Push-Pull Type Oligo(N-annulated perylene)quinodimethanes: Chain Length and Solvent-Dependent Ground States and Physical Properties.

Research on stable open-shell singlet diradicaloids recently became a hot topic because of their unique optical, electronic, and magnetic properties and promising applications in materials science. So far, most reported singlet diradicaloid molecules have a symmetric structure, while asymmetric diradicaloids with an additional contribution of a dipolar zwitterionic form to the ground state were rarely studied. In this Article, a series of new push-pull type oligo(N-annulated perylene)quinodimethanes were synthesized. Their chain length and solvent-dependent ground states and physical properties were systematically investigated by various experimental methods such as steady-state and transient absorption, two-photon absorption, X-ray crystallographic analysis, electron spin resonance, superconducting quantum interference device, Raman spectroscopy, and electrochemistry. It was found that with extension of the chain length, the diradical character increases while the contribution of the zwitterionic form to the ground state becomes smaller. Because of the intramolecular charge transfer character, the physical properties of this push-pull system showed solvent dependence. In addition, density functional theory calculations on the diradical character and Hirshfeld charge were conducted to understand the chain length and solvent dependence of both symmetric and asymmetric systems. Our studies provided a comprehensive understanding on the fundamental structure- and environment-property relationships in the new asymmetric diradicaloid systems.

Toward Tetraradicaloid: The Effect of Fusion Mode on Radical Character and Chemical Reactivity

Open-shell singlet diradicaloids display unique electronic, nonlinear optical, and magnetic activity and could become novel molecular materials for organic electronics, photonics, and spintronics. However, design and synthesis of diradicaloids with a significant polyradical character is a challenging task for chemists. In this Article, we report our efforts toward a tetraradicaloid system. A series of potential tetraradicaloids by fusion of two p-quinodimethane (p-QDM) units with naphthalene or benzene rings in different modes were synthesized. Their model compounds containing one p-QDM moiety were also prepared and compared. Their ground-state structures, physical properties, and chemical reactivity were systematically investigated by various experimental methods such as steady-state and transient absorption, two-photon absorption, X-ray crystallographic analysis, electron spin resonance, superconducting quantum interference device, and electrochemistry, assisted by density functional theory calculations. It was found that their diradical and tetraradical characters show a clear dependence on the fusion mode. Upon the introduction of more five-membered rings, the diradical characters greatly decrease. This difference can be explained by the pro-aromaticity/antiaromaticity of the molecules as well as the intramolecular charge transfer. Our comprehensive studies provide a guideline for the design and synthesis of stable open-shell singlet polycyclic hydrocarbons with significant polyradical characters.

Oxidative Fusion Reactions of meso‐(Diarylamino)porphyrins

In recent years, meso-aminoporphyrins have received increasing attention owing to their potential for vastly improving the economic and environmental validity of dye-sensitized solar cells (DSSC). As a prime example, Gr tzel and co-workers have recently reported a co-sensitized DSSC device incorporating a meso-aminoporphyrin dye that is capable of a power conversion efficiency of 12.3% under simulated air mass 1.5 global sunlight conditions. Beside this, meso-aminoporphyrins have been explored as scaffolds for realizing novel nitrogen-connected dimers, mixed-valent cation radicals, inter-valence charge-transfer absorption, photo-induced reductive meso–meso dimerization, and light-harvesting supramolecular aggregates. Although the amino groups of meso-(diarylamino)porphyrins raise the porphyrin HOMO level, electronic perturbation is only moderate because the diarylamino groups twist out of the bulky porphyrin plane, minimizing conjugation. 7] Our continued interest in conjugated porphyrinoids led us to envision the fusion of a meso-diarylamino group onto the periphery of porphyrins as the expected fused porphyrin products would be interesting in their own right, because they would be structurally similar to azagraphene-type porphyrinoids. To the best of our knowledge, there are no reports of oxidative fusion of meso-aminoporphyrins, which would enable effective pexpansion by coplanarization of the peripheral nitrogen atom lone pair and porphyrin p-electronic system. meso-Phenoxazinoporphyrin 2a (Scheme 1)was prepared in 93 % yield using a Buchwald–Hartwig amination 11] of meso-bromo Ni-porphyrin 1Ni with phenoxazine using PdPEPPSI-IPent as the catalyst. By following previously reported procedures, 13] oxidative fusion of 2 a was attempted by subjecting 2a to 20 equivalents of FeCl3 and 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) for 1.5 hours in CH2Cl2/MeNO2 at room temperature. This reaction afforded an unexpected fused porphyrin 3 a as a sole product in 48 % yield, in which the two 3,5-di-tertbutylphenyl groups had fused to the porphyrin periphery and not the meso-phenoxazine moiety. Although we examined numerous fusion reactions of 2a by changing the oxidants, solvents, and temperature, we could not find reaction conditions that gave us the desired phenoxazine-fused porphyrins. The ESI mass spectrum of 3a showed a parent ion peak at m/z 1107.5374 (calcd for C74H75N5NiO, m/z 1107.5320 [M]), supporting that four protons were removed as a result of oxidation. The H NMR spectrum of 3a in CDCl3 shows a singlet signal owing to the b-protons adjacent to the fused positions at 7.78 ppm. The signals related to the phenoxazine moiety do not show particularly pronounced changes. X-ray crystallographic analysis unambiguously determined the structure of 3a (Figure 1). Interestingly, the fusion reaction took place regioselectively at the bpositions, apart from the phenoxazine moiety. The UV/Vis absorption spectrum of 3a displays broad, red-shifted bands reaching out to around 880 nm, which is similar to that of closely related fused porphyrins that are not functionalized with diarylamino moieties (Figure 2). 14] The electrochemical properties of 2a and 3 a have been studied by cyclic voltammetry (see Supporting Information, Figures S40,S42). The fused porphyrin 3a displays characteristic negative shifts of Eox2 and positive shifts of Ered1, while Eox1 is only slightly affected. As a result, the electrochemical HOMO–LUMO gap of 3a (1.80 eV) is smaller than that of 2a (2.14 eV). Scheme 1. Formulas of porphyrins 1–6. Ar = 3,5-di-tert-butylphenyl.

Effects of air plasma spraying parameters on the Al–Cu–Fe quasicrystalline coating layer

Abstract The focus of this study has been the effect of air plasma spray processing parameters on the characteristics of the Al–Cu–Fe quasicrystalline coatings. Key variables controlling the particle temperature and velocity and substrate temperature, such as the arc current, spraying distance, powder flow rate, and preheating of the substrate surface, were of primary interest. Significant effects of the process parameters on the microstructure and subsequent mechanical properties of the coatings were observed. Process conditions favorable for the formation of coating layers with larger amount of quasicrystalline phase led to the higher values of the microhardness, although it resulted in a relatively large porosity level. The bonding strength was found to be slightly dependent on the spraying distance, powder feeding rate, and preheating of the substrate. However, the predominant variable appeared to be the coating thickness, where a significant reduction of the bonding strength was reported as the coating thickness was increased up to 100 μm .

para‐Quinodimethane‐Bridged Perylene Dimers and Pericondensed Quaterrylenes: The Effect of the Fusion Mode on the Ground States and Physical Properties

Polycyclic hydrocarbon compounds with a singlet biradical ground state show unique physical properties and promising material applications; therefore, it is important to understand the fundamental structure/biradical character/physical properties relationships. In this study, para-quinodimethane (p-QDM)-bridged quinoidal perylene dimers 4 and 5 with different fusion modes and their corresponding aromatic counterparts, the pericondensed quaterrylenes 6 and 7, were synthesized. Their ground-state electronic structures and physical properties were studied by using various experiments assisted with DFT calculations. The proaromatic p-QDM-bridged perylene monoimide dimer 4 has a singlet biradical ground state with a small singlet/triplet energy gap (-2.97 kcal mol(-1)), whereas the antiaromatic s-indacene-bridged N-annulated perylene dimer 5 exists as a closed-shell quinoid with an obvious intramolecular charge-transfer character. Both of these dimers showed shorter singlet excited-state lifetimes, larger two-photon-absorption cross sections, and smaller energy gaps than the corresponding aromatic quaterrylene derivatives 6 and 7, respectively. Our studies revealed how the fusion mode and aromaticity affect the ground state and, consequently, the photophysical properties and electronic properties of a series of extended polycyclic hydrocarbon compounds.

meso–meso Linked Porphyrin–[26]Hexaphyrin–Porphyrin Hybrid Arrays and Their Triply Linked Tapes Exhibiting Strong Absorption Bands in the NIR Region

We describe the synthesis and characterization of directly meso-meso linked porphyrin-[26]hexaphyrin-porphyrin hybrid oligomers and their triply linked (completely fused) hybrid tapes. meso-meso Linked Ni(II) porphyrin-[26]hexaphyrin-Ni(II) porphyrin trimers were prepared by methanesulfonic acid-catalyzed cross-condensation of meso-formyl Ni(II) porphyrins with a 5,10-diaryltripyrrane followed by oxidation with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ). The Ni(II) porphyrin moieties were converted to Zn(II) porphyrins via an indirect route involving reduction of the [26]hexaphyrin to its 28π congener, acid-induced denickelation, oxidation of the [28]hexaphyrin, and finally Zn(II) ion insertion. Over the course of these transformations, porphyrin-[28]hexaphyrin-porphyrin trimers have been revealed to take on a Möbius aromatic twisted structure for the [28]hexaphyrin segment. Oxidation of meso-meso linked hybrid trimer bearing 5,15-diaryl Zn(II) porphyrins with DDQ/Sc(OTf)3 under mild conditions resulted in meso-meso coupling oligomerization, affording the corresponding dimeric (hexamer), trimeric (nonamer), and tetrameric (dodecamer) oligomers. On the other hand, oxidation of a meso-meso linked hybrid trimer bearing 5,10,15-triaryl Zn(II) porphyrin terminals with DDQ/Sc(OTf)3 under harsher conditions afforded a meso-meso, β-β, β-β triply linked hybrid porphyrin tape, which displays a sharp and intense absorption band at 1912 nm. Comparison of this extremely red-shifted absorption band with those of Zn(II) porphyrin tapes suggests that the bathochromic-shifting capability of a [26]hexaphyrin unit is large, almost equivalent to that of four individual Zn(II) porphyrin units. As demonstrated, the fusion of porphyrins to [26]hexaphyrin offers an efficient means to expand their conjugation networks, significantly expanding the capabilities attainable for these chromophores.

Antiaromatic bisindeno-[n]thienoacenes with small singlet biradical characters: syntheses, structures and chain length dependent physical properties

Recent studies demonstrated that aromaticity and biradical character play important roles in determining the ground-state structures and physical properties of quinoidal polycyclic hydrocarbons and oligothiophenes, a kind of molecular materials showing promising applications for organic electronics, photonics and spintronics. In this work, we designed and synthesized a new type of hybrid system, the so-called bisindeno-[n]thienoacenes (n = 1–4), by annulation of quinoidal fused α-oligothiophenes with two indene units. The obtained molecules can be regarded as antiaromatic systems containing 4n π electrons with small singlet biradical character (y0). Their ground-state geometry and electronic structures were studied by X-ray crystallographic analysis, NMR, ESR and Raman spectroscopy, assisted by density functional theory calculations. With extension of the chain length, the molecules showed a gradual increase of the singlet biradical character accompanied by decreased antiaromaticity, finally leading to a highly reactive bisindeno[4]thienoacene (S4-TIPS) which has a singlet biradical ground state (y0 = 0.202). Their optical and electronic properties in the neutral and charged states were systematically investigated by one-photon absorption, two-photon absorption, transient absorption spectroscopy, cyclic voltammetry and spectroelectrochemistry, which could be correlated to the chain length dependent antiaromaticity and biradical character. Our detailed studies revealed a clear structure–aromaticity–biradical character–physical properties–reactivity relationship, which is of importance for tailored material design in the future.

A Diradical Approach towards BODIPY‐Based Dyes with Intense Near‐Infrared Absorption around λ=1100 nm

A diradical approach to obtain stable organic dyes with intense absorption around λ=1100 nm is reported. The para- and meta-quinodimethane-bridged BODIPY dimers BD-1 and BD-2 were synthesized and were found to have a small amount of diradical character. These molecules exhibited very intense absorption at λ=1088 nm (ɛ=6.65×10(5)  M(-1)  cm(-1) ) and 1136 nm (ɛ=6.44×10(5)  M(-1)  cm(-1) ), respectively, together with large two-photon-absorption cross-sections. Structural isomerization induced little variation in their diradical character but distinctive differences in their physical properties. Moreover, the compounds showed a selective fluorescence turn-on response in the presence of the hydroxyl radical but not with other reactive oxygen species.