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The hole-carrier transport of organic semiconductors is widely known to occur via intermolecular orbital overlaps of the highest occupied molecular orbitals (HOMO), though the effect of other occupied molecular orbitals on charge transport is rarely investigated. In this work, we first demonstrate evidence of a mixed-orbital charge transport concept in the high-performance N-shaped decyl-dinaphtho[2,3- d :2',3'- d ']benzo[1,2- b :4,5- b ']dithiophene (C 10 -DNBDT-NW), where electronic couplings of the second HOMO (SHOMO) and third HOMO (THOMO) also contribute to the charge transport. We then present the molecular design of an N-shaped bis(naphtho[2',3':4,5]thieno)[2,3- b :2',3'- e ]pyrazine (BNTP) π-electron system to induce more pronounced mixed-orbital charge transport by incorporating the pyrazine moiety. An effective synthetic strategy for the pyrazine-fused extended π-electron system is developed. With substituent engineering, the favorable two-dimensional herringbone assembly can be obtained with BNTP, and the decylphenyl-substituted BNTP (C 10 Ph-BNTP) demonstrates large electronic couplings involving the HOMO, SHOMO, and THOMO in the herringbone assembly. C 10 Ph-BNTP further shows enhanced mixed-orbital charge transport when the electronic couplings of all three occupied molecular orbitals are taken into consideration, which results in a high hole mobility up to 9.6 cm 2 V -1 s -1 in single-crystal thin-film organic field-effect transistors. The present study provides insights into the contribution of HOMO, SHOMO, and THOMO to the mixed-orbital charge transport of organic semiconductors.

Operational stability, such as long-term ambient durability and bias stress stability, is one of the most significant parameters in organic thin-film transistors (OTFTs). The understanding of such stabilities has been mainly devoted to energy levels of frontier orbitals, thin-film morphologies, and device configuration involving gate dielectrics and electrodes, whereas the roles of molecular and aggregated structural features in device stability are seldom discussed. In this Letter, we report a remarkable enhancement of operational stability, especially bias stress, of n-channel single-crystal OTFTs derived from a replacement of phenyl with perfluorophenyl groups in the side chain. Because of the several-molecule-thick single-crystal nature employed for the OTFTs, the crystal-surface properties are thought to be critical, where the surface structure composed of perfluorophenyl moieties could suppress interactions between environmental species and field-induced carriers owing to increased hydrophobicity and steric protection of π-conjugated units.

Toward the development of high-performance organic semiconductors (OSCs), carrier mobility is the most important requirement for next-generation OSC-based electronics. The strategy is that OSCs consisting of a highly extended π-electron core exhibit two-dimensional (2D) aggregated structures to offer effective charge transport. However, such OSCs, in general, show poor solubility in common organic solvents, resulting in limited solution processability. This is a critical trade-off between the development of OSCs with simultaneous high carrier mobility and suitable solubility. To address this issue, herein, five-membered ring-fused selenium-bridged V-shaped binaphthalene with decyl substituents (C 10 -DNS-VW) is developed and synthesized by an efficient method. C 10 -DNS-VW exhibits significantly high solubility for solution processes. Notably, C 10 -DNS-VW forms a one-dimensional π-stacked packing motif (1D motif) and a 2D herringbone (HB) packing motif (2D motif), depending on the crystal growth condition. On the other hand, the fabrication of thin films by means of both solution process and vacuum deposition techniques forms only the 2D HB motif. External stress tests such as heating and exposure to solvent vapor indicated that 1D and 2D motifs could be synergistically induced by the total balance of intermolecular interactions. Finally, the single-crystalline films of C 10 -DNS-VW by solution process exhibit carrier mobility up to 11 cm 2 V -1 s -1 with suitable transistor stability under ambient conditions for more than two months, indicating that C 10 -DNS-VW is one of the most promising candidates for breaking the trade-off in the field of solution-processed technologies.

Bis(phenoxy-imine) Ti complexes bearing a phenyl group ortho to the phenoxy-O can mediate the copolymerization of ethylene and 5-hexene-1-yl-acetate though they are group 4 transition metal catalysts.

The molecular and electronic structures of the Rieske iron-sulfur [2Fe-2S] cluster with an imidazolate and imidazole were investigated by using usual unrestricted and broken symmetry B3LYP methods for the highest and lowest spin states, respectively. The electronic structures of the lowest spin states were determined by the spin contaminations and natural orbital analyses. It was shown that the spin contamination presents the number of pairs of the antiferromagnetic spin couplings. The oxidation mechanism of the ubquinol at the Q(p) site of the cytochrome bc(1) complex was also examined by the broken symmetry B3LYP methods. In the [2Fe-2S] clusters with an imidazolate, the oxidized and lowest spin state, [(Imz(-))FeS](ox)LS, was lowest in energy among four possible states, consistent with experimental observations. In the examination of the mechanism of the ubquinol oxidation, it was confirmed that the ubiquinol docks between the imidazolate of [2Fe-2S] clusters and Glu272(-) of cytochrome b by the hydrogen bonds before the oxidation proceeds, consistent with the experimental proposals. Our results support a "Glu272-first" mechanism that Glu272 serves as an acceptor of the first proton from the ubiquinol and subsequently the proton-coupled electron transfer (PCET) occurs from the ubisemiquinone anion to the Rieske iron-sulfur [2Fe-2S] cluster.

We report the geometrical features and electronic structures of di-mu-oxo-bridged Mn-Mn binuclear complexes with H2O ligands [Mn2O2(H2O)8]q+ in the iso- and mixed-valence oxidation states. All of the combinations among Mn(II), Mn(III), and Mn(IV) ions are considered the oxidation states of the Mn-Mn center, and the changes in molecular structure induced by the different electron configurations of Mn-based orbitals are investigated in relation to the oxygen-evolving complex (OEC) of photosystem II. The stable geometries of complexes are determined by using the hybrid-type density functional theory for both of the highest- and lowest-spin couplings between Mn sites, and the lowest-spin-coupled states are energetically more favorable than the highest-spin-coupled states except in the case of the complexes with the Mn(II) ion. The coordination positions of H2O ligands at the Mn(II) site tend to shift from the octahedral positions in contrast to those at the Mn(III) and Mn(IV) sites. The shape of the Mn2O2 core and the distances between the Mn ions and the H2O ligands vary depending on the electron occupations of the octahedral eg orbitals on the Mn site with an antibonding nature for the Mn-ligand interactions, indicating the trend as Mn(II)-O > Mn(III)-O and Mn(IV)-O, O-Mn(II)-O > O-Mn(III)-O > O-Mn(IV)-O among the iso-valence Mn2O2 cores, and O-Mn(lower)-O < O-Mn(higher)-O within the mixed-valence Mn2O2 core, and as Mn(II)-OH2 and Mn(III)-OH2 > Mn(IV)-OH2 for the axial H2O ligand. The optimized geometries of model complexes are compared with the X-ray structure of the OEC, and it is suggested that the cubane-like Mn cluster of the active site may not contain a Mn(II) ion. The effective exchange integrals are estimated by applying the approximate spin projection to clarify the magnetic coupling between Mn sites, and the superexchange pathways through the di-mu-oxo bridge are examined on the basis of the singly occupied magnetic orbitals derived from the singlet-coupled natural orbitals in the broken-symmetry state. The comparisons of the calculated results between [Mn2O2(H2O)8]q+ in this study and [Mn2O2(NH3)8]q+ reported by McGrady et al. suggest that the symmetric pathways are dominant to the exchange coupling constant, and the crossed pathway would be less important for the former than it would for the latter in the Mn(III)-Mn(III), Mn(IV)-Mn(IV), and Mn(III)-Mn(IV) oxidation states.

Bis(pyrrolide-imine) Ti complexes in conjunction with methylalumoxane (MAO) were found to work as efficient catalysts for the copolymerization of ethylene and norbornene to afford unique copolymers via an addition-type polymerization mechanism. The catalysts exhibited very high norbornene incorporation, superior to that obtained with Me(2)Si(Me(4)Cp)(N-tert-Bu)TiCl(2) (CGC). The sterically open and highly electrophilic nature of the catalysts is probably responsible for the excellent norbornene incorporation. The catalysts displayed a marked tendency to produce alternating copolymers, which have stereoirregular structures despite the C(2) symmetric nature of the catalysts. The norbornene/ethylene molar ratio in the polymerization medium had a profound influence on the molecular weight distribution of the resulting copolymer. At norbornene/ethylene ratios larger than ca. 1, the catalysts mediated room-temperature living copolymerization of ethylene and norbornene to form high molecular weight monodisperse copolymers (M(n) > 500,000, M(w)/M(n) < 1.20). (13)C NMR spectroscopic analysis of a copolymer, produced under conditions that gave low molecular weight, demonstrated that the copolymerization is initiated by norbornene insertion and that the catalyst mostly exists as a norbornene-last-inserted species under living conditions. Polymerization behavior coupled with DFT calculations suggested that the highly controlled living polymerization stems from the fact that the catalysts possess high affinity and high incorporation ability for norbornene as well as the characteristics of a living ethylene polymerization though under limited conditions (M(n) 225,000, M(w)/M(n) 1.15, 10-s polymerization, 25 degrees C). With the catalyst, unique block copolymers [i.e., poly(ethylene-co-norbornene)(1)-b-poly(ethylene-co-norbornene)(2), PE-b-poly(ethylene-co-norbornene)] were successfully synthesized from ethylene and norbornene. Transmission electron microscopy (TEM) indicated that the PE-b-poly(ethylene-co-norbornene) possesses high potential as a new material consisting of crystalline and amorphous segments which are chemically linked.

The propylene polymerization behavior of a series of Ti complexes featuring fluorine-containing phenoxy-imine chelate ligands is reported. The Ti complexes combined with methylalumoxane (MAO) can be catalysts for living and, at the same time, stereospecific polymerization of propylene at room temperature or above. DFT calculations suggest that the attractive interaction between a fluorine ortho to the imine nitrogen and a beta-hydrogen of a growing polymer chain is responsible for the achievement of room-temperature living propylene polymerization. Although the Ti complexes possess C(2) symmetry, they are capable of producing highly syndiotactic polypropylenes. (13)C NMR is used to demonstrate that the syndiotacticity is governed by a chain-end control mechanism and that the polymerization is initiated exclusively via 1,2-insertion followed by 2,1-insertion as the principal mode of polymerization. (13)C NMR spectroscopy also elucidated that the polypropylenes produced with the Ti complexes possess regio-block structures. Substitutions on the phenoxy-imine ligands have profound effects on catalytic behavior of the Ti complexes. The steric bulk of the substituent ortho to the phenoxy oxygen plays a decisive role in achieving high syndioselectivity for the chain-end controlled polymerization. Over a temperature range of 0-50 degrees C, Ti complex having a trimethylsilyl group ortho to the phenoxy oxygen forms highly syndiotactic, nearly monodisperse polypropylenes (94-90% rr) with extremely high peak melting temperatures (T(m) = 156-149 degrees C). The polymerization behavior of the Ti complexes can be explained well by the recently proposed site-inversion mechanism for the formation of syndiotactic polypropylene by a Ti complex having a pair of fluorine-containing phenoxy-imine ligands.

A fluorine- and trimethylsilyl-containing phenoxy-imine titanium complex was synthesized and the structure was determined by an X-ray analysis. The complex on activation with MAO initiates highly controlled syndiospecific living propylene polymerization to form extremely high Tm syndiotactic polypropylenes (Mw/Mn = 1.05-1.08, Tm = 156-152 degrees C) at 0 or 25 degrees C. Moreover, at 50 degrees C, the complex afforded monodisperse syndiotactic polypropylene with very high Tm's of 149, 150 degrees C. In contrast, complexes having a t-Bu group instead of the silyl group provided lower tacticity polymers with much lower Tm's. In addition, we revealed the substituent effect that plays a key role for the highly controlled syndiospecific polymerization displayed by the catalyst.

Seven titanium complexes bearing fluorine-containing phenoxy-imine chelate ligands, TiCl(2)[eta(2)-1-[C(H)=NR]-2-O-3-(t)Bu-C(6)H(3)](2) [R = 2,3,4,5,6-pentafluorophenyl (1), R = 2,4,6-trifluorophenyl (2), R = 2,6-difluorophenyl (3), R = 2-fluorophenyl (4), R = 3,4,5-trifluorophenyl (5), R = 3,5-difluorophenyl (6), R = 4-fluorophenyl (7)], were synthesized from the lithium salt of the requisite ligand and TiCl(4) in good yields (22%-76%). X-ray analysis revealed that the complexes 1 and 3 adopt a distorted octahedral structure in which the two phenoxy oxygens are situated in the trans-position while the two imine nitrogens and the two chlorine atoms are located cis to one another, the same spatial disposition as that for the corresponding nonfluorinated complex. Although the Ti-O, Ti-N, and Ti-Cl bond distances for complexes 1 and 3 are very similar to those for the nonfluorinated complex, the bond angles between the ligands (e.g., O-Ti-O, N-Ti-N, and Cl-Ti-Cl) and the Ti-N-C-C torsion angles involving the phenyl on the imine nitrogen are different from those for the nonfluorinated complex, as a result of the introduction of fluorine atoms. Complex 1/methylalumoxane (MAO) catalyst system promoted living ethylene polymerization to produce high molecular weight polyethylenes (M(n) > 400 000) with extremely narrow polydispersities (M(w)/M(n) < 1.20). Very high activities (TOF > 20 000 min(-1) atm(-1)) were observed that are comparable to those of Cp(2)ZrCl(2)/MAO at high polymerization temperatures (25, 50 degrees C). Complexes 2-4, which have a fluorine atom adjacent to the imine nitrogen, behaved as living ethylene polymerization catalysts at 50 degrees C, whereas complexes 5-7, possessing no fluorine adjacent to the imine nitrogen, produced polyethylenes having M(w)/M(n) values of ca. 2 with beta-hydrogen transfer as the main termination pathway. These results together with DFT calculations suggested that the presence of a fluorine atom adjacent to the imine nitrogen is a requirement for the high-temperature living polymerization, and the fluorine of the active species for ethylene polymerization interacts with a beta-hydrogen of a polymer chain, resulting in the prevention of beta-hydrogen transfer. This catalyst system was used for the synthesis of a number of unique block copolymers such as polyethylene-b-poly(ethylene-co-propylene) diblock copolymer and polyethylene-b-poly(ethylene-co-propylene)-b-syndiotactic polypropylene triblock copolymer from ethylene and propylene.

The refolding kinetics of alpha-lactalbumin at different concentrations of guanidine hydrochloride have been investigated by means of kinetic circular dichroism and stopped-flow absorption measurements. The refolding reaction consists of at least two stages, the instantaneous accumulation of the transient intermediate that has peptide secondary structure and the subsequent slow process associated with formation of tertiary structure. The transient intermediate is compared with the well-characterized equilibrium intermediate observed during the denaturant-induced unfolding. Stabilities of the secondary structures against the denaturant, affinities for Ca2+, and tryptophan absorption properties of the transient and equilibrium intermediates were investigated. In all of these respects, the transient intermediate is identical with the equilibrium one, demonstrating the validity of the use of the equilibrium intermediate as a model of the folding intermediate. Essentially the same transient intermediate was also detected in the folding of lysozyme, the protein known to be homologous to alpha-lactalbumin but whose equilibrium unfolding is represented as a two-state reaction. The stability and cooperativity of the secondary structure of the intermediate of lysozyme are compared with those of alpha-lactalbumin. The results show that the protein folding occurring via the intermediate is not limited to the proteins that show equilibrium intermediates. Although the unfolding equilibria of most proteins are well approximated as a two-state reaction, the two-state hypothesis may not be applicable to the folding reaction under the native condition. Two models of protein folding, intermediate-controlled folding model and multiple-pathway folding model, which are different in view of the role of the intermediate in determining the pathway of folding, are also discussed.