The racemic mixture of [7]thiaheterohelicene is a chiral π-conjugated system in which heteroatom substitution modulates intermolecular interactions and coupling to metal surfaces. Here, we investigate the coverage-dependent self-assembly and electronic structure of [7]thiaheterohelicene on Au(111) using scanning tunneling microscopy (STM), scanning tunneling spectroscopy (STS), X-ray photoelectron spectroscopy (XPS), and ultraviolet–visible spectroscopy (UV–Vis). Stepwise thermal deposition produces a sequence of supramolecular phases, evolving from isolated molecules and small aggregates at low coverage to extended, densely packed molecular overlayers at high coverage. STM reveals that adsorption is influenced by the Au(111)-(22 × √3) herringbone reconstruction, which acts as an anisotropic template for molecular stabilization, diffusion, and nucleation while remaining largely preserved beneath the assemblies. XPS confirms that rac-[7]TH molecules remain intact and adsorb via weak, site-dependent physisorption without C–S bond cleavage and reveal distinct sulfur environments associated with different adsorption environments on the surface. STS measurements of the compact molecular layer yield an apparent quasiparticle gap of ~4.2 eV, which is larger than the 3.1 eV optical gap measured in solution due to the different nature of tunneling and optical excitations as well as substrate-induced screening effects.

The adsorption and self-assembly of racemic [5]thiaheterohelicene (rac-[5]TH) adsorbed on Ag(111) were investigated using molecular dynamics (MD) simulations and density functional theory (DFT) calculations and compared with scanning tunneling microscopy (STM) results. While the molecule is physisorbed on Ag(111), the results of DFT calculations also exhibit pronounced site and orientation dependence with respect to the substrate lattice. Displacements from preferred adsorption sites and orientations on Ag(111) result in an energy cost nearly an order of magnitude larger than that associated with deviations from the optimal intermolecular distances favoured by simple intermolecular interactions. This indicates that self-assembly is governed primarily by substrate registry, while intermolecular interactions act as secondary constraints. Consequently, the ordered structures observed experimentally can be understood as a balance in which molecules anchor at their preferred adsorption sites and orientations on the substrate, while locally adopting dimer motifs favoured by intermolecular interactions. These findings provide insight into how helical length controls the balance between intermolecular and molecule-substrate interactions in chiral self-assembly on the Ag(111) surfaces.

Triplet excited states in organic molecules are generally optically inactive due to spin-forbidden transitions to the singlet ground state. Recent studies have extensively investigated triplet states to circumvent limitations imposed by spin statistics in organic light-emitting diodes. Room-temperature electrophosphorescence and electrofluorescence have previously been observed in thienyl-substituted phenazines without heavy metals; however, phosphorescence was absent when spin density was localized on the thiophene units, despite the expected heavy-atom effect from the sulfur atom. In this study, we investigate the spin dynamics of thiophene-fused π-systems lacking phenazine units to enable the effective utilization of triplet states in thiophene-containing π-systems. This investigation reveals delayed emission in the excited dimers of fused thiophenes, likely associated with triplet–triplet annihilation. Optical and EPR measurements, together with quantum chemical calculations, indicate that the fused thiophenes form head-to-tail dimers in both the ground and excited states.

We investigated the adsorption and aggregation properties of 2,13-bis(hydroxymethyl)-[7]thiaheterohelicene ([7]TH-diol) on the Ag(111) surface by scanning tunneling microscopy (STM) and molecular dynamics (MD) simulation. STM observation revealed that both racemic and enantiopure [7]TH-diol formed apparently similar "zigzag" chain structures. To elucidate the molecular arrangements in these structures, MD simulation successfully differentiates the formation mechanisms of these structures, demonstrating that hetero-chiral chains are stabilized primarily by van der Waals forces, whereas homo-chiral chains are stabilized through hydrogen bonding. The formation of homo-chiral chains is driven by the alignment of hydroxymethyl groups between the neighboring molecules, whereas the steric hindrance of helical skeletons affects chain growth. These findings highlight the critical role of inter-molecular interactions-particularly hydrogen bonding-in the self-assembly of helicene molecules.

The chiral recognition of a self-assembled structure of enantiopure (M)-type 2,13-diphenyl[7]thiaheterohelicene ((M)-Ph-[7]TH) was investigated on a Ag(111) substrate by scanning tunnelling microscopy (STM) and tip-enhanced Raman spectroscopy (TERS). In contrast to previous research of thiaheterohelicene and its derivatives showing zigzag row formation on the Ag(111) substrate, the hexagonal ordered structure was observed by STM. The obtained TERS spectra of (M)-Ph-[7]TH were consistent with the Raman spectra calculated on the basis of density functional theory (DFT), which suggests that (M)-Ph-[7]TH was adsorbed on the substrate without decomposition. The sample bias voltage dependence of STM images combined with the calculated molecular orbitals of (M)-Ph-[7]TH indicates that a phenyl ring was observed as a protrusion at +3.0 V, whereas the helicene backbone was observed at +0.5 V. From these results, a possible model of the hexagonal structure was proposed. Owing to the phenyl ring, the van der Waals interaction between (M)-Ph-[7]TH and the substrate becomes strong. This leads to the formation of the hexagonal structure with the same symmetry as the substrate.

We have investigated the chiral recognition behavior leading to the formation of self-assembled zigzag chain structures of rac-[7]thiaheterohelicene on the Ag(111) surface by density functional theory (DFT) calculations and molecular dynamics (MD) simulations. The DFT calculations showed a weak interaction between the molecule and the Ag surface, indicating that the molecule was fully physisorbed on the surface. On the other hand, the MD simulations provided meaningful insight into the initial formation process of the ordered structures, which strongly supported the experimental observations. We found that the heterochiral dimer, combining through pi-pi interaction, formed as the crucial building block for long-range zigzag chains, where different enantiomers were arranged alternately. The interaction among neighboring chains was also elucidated, showing the most preferable arrangement of 2D ordered structures.

The crystallization of a racemic mixture of [5] and [7]thiaheterohelicene molecules into two-dimensional highly ordered molecular domains on a Ag(111) surface was investigated by scanning tunneling microscopy (STM) at 80 K. We observed that the crystallization of both thiaheterohelicene molecules on a Ag(111) surface was strongly affected by stereospecific interactions resulting from different numbers of aromatic rings incorporated into the helical skeletons of molecules. [5]Thiaheterohelicene molecules crystallized predominant into a few racemate polymorphic molecular domains, allowing us to obtain high-resolution STM images with the unambiguous identification of the absolute handedness of enantiomeric species. In contrast, the stereochemical chiral interactions among [7]thiaheterohelicene molecules led to the formation of one type of racemate molecular domain consisting of self-assembled zigzagged twin rows. X-ray photoelectron spectroscopy investigation strongly suggests weak interaction between thiaheterohelicene molecules and Ag(111) surface and lack of formation the S-Ag chemical bond. Surface-enhanced Raman spectroscopy combined with theoretical calculation based on a density functional theory was applied to the chemical verification of molecules adsorbed on a plasmonic silver surface for future tip-enhanced Raman spectroscopy.

Light emission from the M-type enantiomer of a helicene derivative (2,13-bis(hydroxymethyl)[7]-thiaheterohelicene) adsorbed on the clean Au(111) and the C60-covered Au(111) surfaces were investigated by tunneling-current-induced light-emission technique. Plasmon-originated light emission was observed on the helicence/Au(111) surface and it was strongly suppressed on the area where the helicene molecules were adsorbed at the edges of the Au(111) terraces. To avoid luminescence quenching of excited helicene molecules and to suppress strong plasmon light emission from the Au(111) surface, C60 layers were used as decoupling buffer layers between helicene molecules and the Au(111) surface. Helicene molecules were adsorbed preferentially on the Au(111) surface rather than on the C60 buffer layers due to the small interaction of the molecules and C60 islands. This fact motivated us to deposit a multilayer of helicene molecules onto the C60 layers grown on the Au(111) surface, leading to the fact that the helicene/C60 multilayer showed strong luminescence with the molecules character. We consider that such strong light emission from the multilayer of helicene molecules has a plasmon origin strongly modulated by the molecular electronic states of (M)-[7]TH-diol molecules.

The coverage-dependent adsorption of a racemic mixture of [7]thiaheterohelicene-2,13-carboxaldehyde on Au(111), Cu(001), and NiAl(110) surfaces was investigated using a scanning tunneling microscope (STM). At a low coverage, the adsorption process for helicene molecules on Au(111) was strongly affected by surface reconstruction. At monolayer saturation coverage, the dominant molecular structure observed on Au(111) characteristically had separated self-assembled twin rows aligned in.112. directions. The helicene molecules within these separated twin rows are preferentialy arranged in a zigzag pattern with alternating (M)-and (P)-enantiomers. With increasing molecular coverage, the molecular structural transition from self-assembled twin rows to self-assembled single rows was observed. STM-induced light emission (STM-LE) investigation of helicene molecules on Au(111) showed the suppression of plasmon light emission over the molecules. The adsorption of helicene molecules on Cu(001) and NiAl(110) was quite different from that observed on Au(111). Neither the formation of self-assembled twin rows nor the molecular arrangement into different domains was observed. The formation of molecular clusters on Cu(001) and NiAl(110) was observed. STM-LE investigation of helicene molecules adsorbed on Cu(001) showed the suppression of plasmon light emission over these molecules. In contrast to metallic Au(111) and Cu(001) substrates, STM-LE investigations revealed the enhancement of light emission above the molecular clusters formed on metallic NiAl(110), suggesting plasmon-enhanced molecular light emission.

1.背景・目的 有機薄膜トランジスタ(OTFT)は溶液プロセスを用いることで低コストに作製できることが期待 されている。われわれは、大気中安定性に優れ、高キャリア移動度が期待できるベンゾジチオフ ェンダイマー(BDT-dimer)誘導体の薄膜を用いた OTFT を検討している。[1]今までに、図 1 に示す BDT-dimer の両端をブチル基で修飾した 2C4-BDT-dimer の真空蒸着膜を用いた OTFT では最大キ ャリア移動度 1.8 cmVsが得られている。本研究ではフローコート法により 2C4-BDT-dimer 薄 膜を形成し、それを用いた OTFT の作製を検討した。 2.実験方法 まず、n+-Si/SiO2基板をフェニルトリクロロシラン(PTS)雰囲気下に 30 分間置き、SiO2絶縁膜表 面の疎水化処理を行った。次にフローコート法を用いて 2C4-BDT-dimer 薄膜を作製した。今回は トルエンと o-ジクロロベンゼンの 2 種類の溶媒を用いて成膜を行った。溶媒にトルエンを用いた 場合は濃度 0.3 wt%の 2C4-BDT-dimer の溶液を調製し、基板加熱温度は 50~80 °Cで成膜した。溶 媒に o-ジクロロベンゼンを用いた場合は濃度0.2 wt%の溶液を調製し、基板加熱温度は 100~160 °C で成膜した。どちらの場合もフローコート速度の範囲は 5~20 μm/s であった。最後にソース・ド レイン電極として金をマスク蒸着した。図 2には作製した OTFT の構造を示す。 3.結果 平均キャリア移動度は、全ての基板加熱温度でフローコート速度が 10 μm/s の時に最も高い値が 得られた。その場合の基板加熱温度と平均キャリア移動度の関係を図 3 に示す。基板加熱温度が 50 °Cのときには平均キャリア移動度は 0.64 cmVs で、基板加熱温度が 100 °Cになると 0.91 cm 2 V -1 s まで向上し、最大キャリア移動度 1.6 cmVsを得ることができた。しかし、基板加熱温 度が 130 °Cでは平均キャリア移動度は 0.58 cmVs、160 °Cでは 0.22 cmVsと低下した。顕微 鏡による 2C4-BDT-dimer 薄膜の観察では、基板加熱温度が 130 °C以上になるとクラックが生じて いることが確認できた。そのため 130 °C以上では、キャリア移動度が低下したと考えられる。

This study shows evidence of nanoscale dehydrogenation occurring during tip-enhanced Raman spectroscopy (TERS) measurements. The near-field TERS spectra obtained locally on a self-assembled monolayer of 2,13-bis(aldehyde)[7]thiaheterohelicene molecules showed vibrational frequencies in good agreement with that predicted by density functional theory calculations, except for the L-mode at ∼2000 cm–1, which is ascribed to a C≡C triple bond, implying that the benzene rings had been dehydrogenated during the experiments. Using peak analysis, incorporated with the molecular adsorption orientation deduced from high-resolution STM imaging, we conclude that one side benzene ring protruding upward was dehydrogenated as a result of pyrolysis, with the Ag tip serving as both local heat source and catalyst.

Benzodithiophene dimers modified with hexyl groups (2C6-BDT-dimer) were investigated as solution-processable organic semiconductors for organic thin-film transistors (OTFTs). Since 2C6-BDT-dimer crystals have an anisotropic shape, flow coating was adopted to grow polycrystalline films. The flow-coated films were inferior to the vacuum-evaporated ones in terms of their crystallinity estimated from X-ray diffraction data. However, the hole mobility of the OTFTs with the flow-coated films, which was 1.7 cm2 V−1 s−1 at maximum, was higher than that of the OTFTs with vacuum-evaporated films because the one-dimensional thin crystals of the flow-coated films were aligned in the flow-coating direction.

われわれは、大気中安定性に優れ、高キャリア移動度が期待できるベンゾジチオフェンダイマ ー(BDT-dimer)誘導体を用い、有機薄膜トランジスタ(OTFT)の作製を試みている。しかし、 BDT-dimer の蒸着膜は 3 次元的に成長してしまい、成膜性が悪くて平坦な膜を得ることが難しい ため、高キャリア移動度を示す TFT の作製には至らなかった。そこで、両端にアルキル鎖を付与 した 2Cn-BDT-dimerを用いたところ、成膜性が改善され、100°C程度の基板加熱をした蒸着膜を用 いて、1.8 cmVsのキャリア移動度を示す TFT が得られた。今回は、BDT-dimer 誘導体におけ るアルキル鎖の有無による成膜性への影響について量子化学計算によって検討を行ったので報告 する。 BDT-dimer では、剥き出しになっているチオフェン環の硫黄原子とその隣の炭素とが電気双極 子を形成しているため、分子同士が縦方向に並んだ場合でも比較的強い相互作用が働き、3 次元 的な薄膜成長をすると考えられる。しかし、2Cn-BDT-dimerでは、アルキル鎖が付与されることで、 その相互作用が弱められ、2 次元的な薄膜成長が促されたと考えられる。しかし、このことは実 験では解明することができないため、量子化学計算を用いて縦方向および横方向に並んだ 2 量体 の生成エネルギーを算出し、それぞれの構造の安定性を比較した。横方向 2 量体と縦方向 2 量体 の概念図を図 2に示す。生成エネルギーの算出は、Gaussian 09 programを用い、CAM-B3LYP/6-31 G(d,p) levelで行った。また、計算に用いた構造座標は X 線単結晶構造解析のデータを使用した。 横方向 2量体の生成エネルギーは、BDT-dimer では-17.4 kJ/mol、2C2-BDT-dimerでは-15.9 kJ/mol、 2C4-BDT-dimerでは-15.2 kJ/molであり、それぞれの材料に大きな差は見られなかった。しかし、 縦方向 2量体の生成エネルギーは、BDT-dimer では-5.1 kJ/mol、2C2-BDT-dimerでは-1.6 kJ/mol、 2C4-BDT-dimerでは-0.82 kJ/molであり、横方向 2量体と縦方向 2量体の生成エネルギーの差を比 較すると BDT-dimer では約 3.4 倍であるのに対し、2C2-BDT-dimer では約 10 倍、2C4-BDT-dimer では約 19倍と縦方向 2量体より横方向 2量体をとる方が圧倒的に有利であることがわかった。ま た、縦方向2量体の生成エネルギーをエレクトロンボルトに換算すると、BDT-dimer では約0.05 eV、 2C2-BDT-dimerでは約0.02 eV、2C4-BDT-dimerでは約0.009 eVとなる。100°Cの熱エネルギーは0.032 eV に相当するので、2C2-BDT-dimer と 2C4-BDT-dimer の縦方向 2 量体は 100°C程度の基板加熱で 容易に解離すると考えられる。 以上の結果より、2C2-BDT-dimer および 2C4-BDT-dimer は BDT-dimer と比べて縦方向 2 量体よ り横方向 2 量体をとる方がエネルギー的に有利であることがわかった。また、2C2-BDT-dimer と 2C4-BDT-dimerは基板加熱によって縦方向 2量体が容易に解離すると考えられる。その結果、2次 元的な薄膜成長が促され、成膜性が良くなったと考えられる。

Adsorption of the M-type enantiomer of 2,13-bis(hydroxymethyl[7]-thiaheterohelicene molecules deposited on Au(111) was investigated by scanning tunneling microscopy (STM) and current imaging tunneling spectroscopy (CITS). We observed. preferential adsorption at step edges, fcc regions, and fcc elbow regions of the herringbone reconstruction at a low coverage after molecular deposition onto a substrate maintained at 150 +/- 20 K. Surprisingly, after molecular deposition onto a substrate maintained at 355 +/- 20 K, highly ordered islands exhibiting molecular self-assembly surrounded by disordered areas were observed. The islands consist of self-assembled zigzagged twin rows with the preferential direction tilted clockwise and anticlockwise from the < 1.1 (2) over bar > direction of the Au(111) substrate by an angle of 12 +/- 2 degrees. A detailed statistical analysis of the twin rows indicated that the unit cell lattice vectors a and b have lengths of 2.4 and 4.4 nm, respectively, with an angle between them of 77 degrees. We postulate an adsorption model without hydrogen bond formation between the hydroxyl groups of neighboring molecules or a strong pi-pi stacking interaction. We emphasize the advantages of this molecular system investigated for STM-induced luminescence experiments in the near future.

The stretching properties of a [7]thiaheterohelicene framework, what we call molecular spring, have not been investigated so far, despite a variety of [7]thiaheterohelicene derivatives having very interesting characteristics due to both the rigidity arising from fused benzene rings and the flexibility like a spring originating from helical structure. In this study, a novel [7]thiaheterohelicene derivative, which has a disulfide moiety for bonding to a gold-coated substrate and a carboxy group for reacting with an amino-modified probe tip at each of its end groups, was synthesized in order to elucidate the elasticity of the [7]thiaheterohelicene framework by atomic force microscopy (AFM). The AFM force measurements were carried out using two carboxy-terminated disulfide derivatives with or without a [7]thiaheterohelicene moiety, and the deviation between two kinds of force-extension curves was related to the stretching originating from the [7]thiaheterohelicene framework here. Furthermore, its elasticity was compared to that of biphenyl, which is generally known as a rigid framework, using electrostatic state calculations.

We have investigated the growth process of benzodithiophene (BDT)-dimer films deposited on ∼1.1-monolayer-thick pentacene layers used as crystallinity control layers (CCLs). It has been found that the BDT-dimer film shows, on their initial stage, two-dimensional growth with the 2nd-layer pentacene islands of the CCLs working as the growth nuclei. Thus, we considerably reduced the density of the 2nd-layer pentacene islands of the CCLs by lowering their growth rate because the density is determined by the migration length of the pentacene molecules on the surfaces. As a result, we obtained BDT-dimer films consists of enlarged grains, and lowered the channel resistance of the pentacene-CCL/BDT-dimer FETs.

A novel co-polymer, P-PBTx, is a promising emitting material for white OLED. Appearance of unexpected yellow emission in EL is a key point, but its origin has not been clarified yet. Photo-luminescence properties of the copolymer were investigated in comparison with those of its monomer as well as two component molecules. An PL yellow band was also observed even in a film of a chromophore molecule at low temperature, depending on the position of an inhomogeneous film. Then, modified molecule center such as a dimer or an excimer of neighboring chromophores is a possible origin of the PL yellow emission. PACS: 33.50.Dq; 33.70.-w; 78.60.Fi;