Ryohei Kishi

Synthesis and characterization of quarteranthene: elucidating the characteristics of the edge state of graphene nanoribbons at the molecular level.

The characteristics of the edge state, which is a peculiar magnetic state in zigzag-edged graphene nanoribbons (ZGNRs) that originates from electron-electron correlation in an edge-localized π-state, are investigated by preparing and characterizing quarteranthene molecules. The molecular geometry that was determined from the X-ray analysis is consistent with a zigzag-edge-localized structure of unpaired electrons. The localized electrons are responsible for the peculiar magnetic (room-temperature ferromagnetic correlation), optical (the lowest-lying doubly excited state), and chemical (peroxide bond formation) behaviors. On the basis of these distinguishing properties and a careful consideration of the valence bonding, insight into the edge state of ZGNRs can be gained.

Indeno[2,1‐b]fluorene: A 20‐π‐Electron Hydrocarbon with Very Low‐Energy Light Absorption

Smaller can be better: The first example of meta-quinodimethane embedded in an indenofluorene framework has been synthesized. 10,12-Dimesitylindeno[2,1-b]fluorene exhibits extremely low-energy light absorption, despite the small conjugation space of the molecule, which consists of only 20 π electrons.

Impact of Diradical Character on Two-Photon Absorption: Bis(acridine) Dimers Synthesized from an Allenic Precursor

The two-photon absorption (TPA) of a bis(acridine) dimer (8) having singlet diradical character in its ground state was found to be enhanced by more than 2 orders of magnitude as compared to its closed-shell counterpart (12), which has the same structural backbone and atom composition. The dimer, a tetracation species consisting of two connected acridinium cation moieties with high coplanarity, was obtained during our attempts to synthesize triplet carbenes by double oxidation of an allenic precursor (3b). High conjugation over the two aromatic rings connected by dimerization was revealed by X-ray analysis, and a small HOMO-LUMO gap was found in the visible-near-infrared one-photon absorption spectrum in solution and in the crystalline state, exhibiting that the ground state of 8 has singlet diradical nature. Ab initio molecular orbital calculations of the ground state also suggested that 8 has an intermediate diradical character (y) of 0.685. Interconversion between diradical tetracation dimer 8 and closed-shell dication dimer 12 was achieved by oxidation/reduction in good yields and was accompanied by formation of monoradical trication dimer 13 as an intermediate. TPA measurements at near-infrared wavelengths revealed that diradical dimer 8 has large TPA cross sections (3600 GM at 1200 nm), while closed-shell 12 has TPA cross sections of <21 GM. This result represents a straightforward comparison between the TPA activity of molecules with the same structural backbone and atom composition but with different degree of the diradical character, supporting the theoretical prediction that enhanced TPA intensity can be observed in the intermediate y region (0 < y < 1).

Long-range corrected density functional theory study on static second hyperpolarizabilities of singlet diradical systems

Within the spin-unrestricted density functional theory (DFT) the long-range correction (LC) scheme combined with the Becke-Lee-Yang-Parr exchange-correlation functional, referred to as LC-UBLYP method, has been applied to the calculation of the second hyperpolarizability (gamma) of open-shell singlet diradical systems of increasing complexity and has demonstrated good performance: (i) for the simplest H(2) dissociation model, the gamma values calculated by the LC-UBLYP method significantly overshoot the full configuration interaction result but reproduce qualitatively the evolution of gamma as a function of the diradical character, (ii) for small singlet diradical 1,3-dipole systems, the diradical character dependence of gamma determined by the UCCSD and UCCSD(T) reference methods is reproduced semiquantitatively by the LC-UBLYP method except in the small diradical character region, where the spin-unrestricted solutions coincide with spin-restricted solutions, (iii) the LC-UBLYP method also closely reproduces the UCCSD(T) results on the diradical character dependence of gamma of the p-quinodimethane model system, particularly in the intermediate and large diradical character regions, whereas it shows an abrupt change for a diradical character (y) close to 0.2 originating from the triplet instability, (iv) the reliability of LC-UBLYP to reproduce reference coupled cluster results on open-shell singlet systems with intermediate and large diradical characters has also been substantiated in the case of gamma of 1,4-bis-(imidazol-2-ylidene)-cyclohexa-2,5-diene (BI2Y), then (v), for real systems built from a pair of phenalenyl radicals separated by a conjugated linker, the LC-UBLYP results have been found to closely match the UBHandHLYP values-which, for small systems are in good agreement with those obtained using correlated molecular orbital methods-whereas the UB3LYP results can be much different. These results are not only important from the viewpoint of an efficient determination of the nonlinear optical properties of open-shell singlet systems, but also from the viewpoint of defining new challenges for elaborating improved exchange-correlation functionals.

Origin of the enhancement of the second hyperpolarizability of singlet diradical systems with intermediate diradical character

The origin of the diradical character dependence of the second hyperpolarizability (gamma) of neutral singlet diradical systems is clarified based on the perturbation formula of gamma using the simplest diradical molecular model with different diradical characters, i.e., H2 under bond dissociation. The enhancement of gamma in the intermediate diradical character region turns out to originate from the increasing magnitude of the transition moment between the first and second excited states and the decrease of that between the ground and first excited states, respectively, with the increase in diradical character. This feature confirms that open-shell singlet conjugated molecules with intermediate diradical characters constitute a new class of third-order nonlinear optical systems, whose gamma values can be controlled by the diradical character in addition to the conjugation length.

Remarkable two-photon absorption in open-shell singlet systems

Remarkable enhancement of two-photon absorption (TPA) peak is theoretically predicted in symmetric open-shell singlet diradical systems with intermediate diradical character as compared to closed-shell and pure diradical systems. It is revealed that the largest TPA peak intensities occur for open-shell singlet diradicals having a ferromagnetically coupled ground state and strongly depend on the ratio between damping factors of the excited states. This result confirms that open-shell singlet conjugated molecules with intermediate diradical characters have precedence over conventional closed-shell conjugated systems in resonant third-order nonlinear optical properties.

A new type of organic–inorganic hybrid NLO-phore with large off-diagonal first hyperpolarizability tensors: a two-dimensional approach

We report a novel type of organic-inorganic hybrid material with rare two-dimensional nonlinear optical (NLO) properties. The density functional theory (DFT) calculations combined with the finite-field (FF) method show that the designed molecules (6,9 organo-derivatives of B10H14) could carry the characteristic NLO properties of both organic and inorganic materials. Interestingly, due to their unique V-shaped structures, they have large off-diagonal first hyperpolarizability tensors or nonlinear anisotropy, which is an advantage in their practical applications over conventional donor-π-acceptor (D-π-A) NLO-phores. The systematic substitutions of terminal donor/acceptor groups as well as the extension of π-conjugation along the V-shape in these derivatives have been evaluated to guide a purpose-oriented synthesis of NLO material. All the systems in the present study have been categorized into Set-I and Set-II with D-π-A-π-D and A-π-D-π-A configurations, respectively. These designed derivatives show large amplitudes of βz values. For example, systems 3N (6,9-[(N=C=Ph-NO2)2]-B10H12) and 4N (6,9-[((N=C=Ph)2-NO2)2]-B10H12) have βz amplitudes as large as 34.16 and 276.91 × 10(3) a.u. which are 380 and 3000 times larger than those of a typical NLO molecule of urea, respectively. Remarkably, the substitution of nitrogen atoms with 6,9 hydrogen atoms in decaborane is shown to cause a lone pair back donation to vacant p orbitals of 6,9 boron atoms of the decaborane basket. This p orbital overlapping verticalizes the V-arms of the decaborane derivatives and boosts their nonlinear anisotropies due to their larger off-diagonal tensor components. The nonlinear anisotropy values are significantly larger, ranging from 1 (minimum in system 1) to 31.90 (maximum in system 3A) due to their unique V-shape. Comparison of their efficiencies with standard molecules demonstrates that our designed organic-inorganic hybrid molecules have significant potential as excellent candidates for NLO applications.

Halide Ion Complexes of Decaborane (B10H14) and Their Derivatives: Noncovalent Charge Transfer Effect on Second-Order Nonlinear Optical Properties

Quantum molecular engineering has been performed to determine the second-order nonlinear optical (NLO) properties in different halo complexes of decaborane (B(10)H(14)) and their derivatives using the density functional theory (DFT) method. These decaborane halo complexes of X(-)@B(10)H(14) (X = F, Cl, Br, and I) are found to possess noncovalent charge transfer interactions. The static polarizability (α(0)) and first hyperpolarizability (β(0)) among these complexes increase by moving down the group from F to I, partly due to the increase in size of their anionic radii and the decrease in their electron affinities. A two-level approximation has been employed to investigate the origin of β(0) values in these halo complexes, which show very consistent results with those by the finite-field method. Furthermore, in the same line, two experimentally existing complexes, I(-)@B(10)H(14) and I(-)@2,4-I(2)B(10)H(12), are found to have considerably large β(0) values of 2859 and 3092 a.u., respectively, which are about three times larger than a prototypical second-order NLO molecule of p-nitroaniline, as reported by Soscun et al. [Int. J. Quantum Chem.2006, 106, 1130-1137]. Besides this, the special effects of solvent, counterion, and bottom substitutions have also been investigated. Interestingly, 2,4-alkali metal-substituted decaborane iodide complexes show remarkably large second-order NLO response with β(0) amplitude as large as 62436 a.u. for I(-)@2,4-K(2)B(10)H(12) complex, which are explained in terms of their transition energies, frontier molecular orbitals and electron density difference plots. Thus, the present investigation provides several new comparative insights into the second-order NLO properties of halo complexes of decaborane, which possess not only large first hyperpolarizabilities, but also high tunability to get a robustly large second-order NLO response by alkali metal substitution effects.

Interplay between the Diradical Character and Third‐Order Nonlinear Optical Properties in Fullerene Systems

To reveal new structure-property relationships in the nonlinear optical (NLO) properties of fullerenes that are associated with their open-shell character, we investigated the interplay between the diradical character (y(i)) and second hyperpolarizability (longitudinal component, γ(zzzz)) in several fullerenes, including C(20), C(26), C(30), C(36), C(40), C(42), C(48), C(60), and C(70), by using the broken-symmetry density functional theory (DFT; LC-UBLYP (μ=0.33)/6-31G*//UB3LYP/6-31G*). We found that the large differences between the geometry and topology of fullerenes have a significant effect on the diradical character of each fullerene. On the basis of their different diradical character, these fullerenes were categorized into three groups, that is, closed-shell (y(i)=0), intermediate open-shell (0<y(i)<1), and almost pure open-shell compounds (y(i)=/~1), which originated from their diverse topological features, as explained by odd-electron-density and spin-density diagrams. For example, we found that closed-shell fullerenes include C(20), C(60), and C(70), whereas fullerenes C(26) and C(36) and C(30), C(40), C(42), and C(48) are pure and intermediate open-shell compounds, respectively. Interestingly, the γ(zzzz) enhancement ratios between C(30)/C(36) and C(40)/C(60) are 4.42 and 11.75, respectively, regardless of the smaller π-conjugation size in C(30) and C(40) than in C(36) and C(60). Larger γ(zzzz) values were obtained for other fullerenes that had intermediate diradical character, in accordance with our previous valence configuration interaction (VCI) results for the two-site diradical model. The γ(zzzz) density analysis shows that the large positive contributions originate from the large γ(zzzz) density distributions on the right- and left-extended edges of the fullerenes, between which significant spin polarizations (related to their intermediate diradical character) appear within the spin-unrestricted DFT level of theory.

Third-order nonlinear optical properties of open-shell supermolecular systems composed of acetylene linked phenalenyl radicals.

The third-order nonlinear optical (NLO) properties, at the molecular level, the static second hyperpolarizabilities, γ, of supermolecular systems composed of phenalenyl and pyrene rings linked by acetylene units are investigated by employing the long-range corrected spin-unrestricted density functional theory, LC-UBLYP, method. The phenalenyl based superethylene, superallyl, and superbutadiene in their lowest spin states have intermediate diradical characters and exhibit larger γ values than the closed-shell pyrene based superpolyene systems. The introduction of a positive charge into the phenalenyl based superallyl radical changes the sign of γ and enhances its amplitude by a factor of 35. Although such sign inversion is also observed in the allyl radical and cation systems in their ground state equilibrium geometries, the relative amplitude of γ is much different, that is, |γ(regular allyl cation)/γ(regular allyl radical)| = 0.61 versus |γ(phenalenyl based superallyl cation)/γ(phenalenyl based superallyl radical)| = 35. In contrast, the model ethylene, allyl radical/cation, and butadiene systems with stretched carbon-carbon bond lengths (2.0 Å), having intermediate diradical characters, exhibit similar γ features to those of the phenalenyl based superpolyene systems. This exemplifies that the size dependence of γ as well as its sign change by introducing a positive charge on the phenalenyl based superpolyene systems originate from their intermediate diradical characters. In addition, the change from the lowest to the highest π-electron spin states significantly reduces the γ amplitudes of the neutral phenalenyl based superpolyene systems. For phenalenyl based superallyl cation, the sign inversion of γ (from negative to positive) is observed upon switching between the singlet and triplet states, which is predicted to be associated with a modification of the balance between the positive and negative contributions to γ. The present study paves the way toward designing a variety of open-shell NLO supermolecular systems composed of phenalenyl radical building blocks.