Yasuhiro F. Miura

Third-order nonlinear optical properties of didodecyldimethylammonium–Au(dmit)2

The third-order optical nonlinearity of metal(dmit)2 was investigated with femtosecond optical Kerr effect at 830 nm wavelength. The nonresonant second-order hyperpolarizability for the didodecyldimethylammonium–Au(dmit)2 (2C12–Au) molecule was estimated to be as large as 2.2×10−32 esu.

Investigation on ultrafast third-order optical nonlinearity of metal(dmit)2/(mnt)2 charge transfer complexes

We have investigated the transient third-order optical nonlinearity of the solutions of Metal(dmit)2/(mnt)2 salts in acetone by employing femtosecond optical Kerr gate measurement at 830 nm wavelength. An order of enhancement on the second-order hyperpolarizability is found for the above salts with central atoms of Cu and Ni. We have suggested an explanation for the enhancement based on the extension of electronic conjugation by the centring Cu or Ni atom.

Structure and electronic transport of highly conductive Langmuir-Blodgett films of tridecylmethylammonium-Au(dmit)2

Electronic transport measurements are reported for Langmuir-Blodgett (LB) films of tridecylmethylammonium-Au(dmit)2 for 300-4 K. The conductivity after electrochemical oxidation is 30-50 S/cm at room temperature, increases with decreasing temperature, and begins decrease at about 200 K. The conductivity at lower temperatures is explained by two-dimensional variable range hopping (2D VRH). The thermoelectric power (S) is positive and well fitted by a linear combination of the temperature dependence of 2D VRH (Sv\proptT1/3) and that of metal (Sm\proptT). These results can be explained by a model in which highly conductive metallic regions are separated by thin weakly conductive regions. The UV/visible absorption spectra and the X-ray diffraction patterns indicate that a large structural change occurs accompanied by electrochemical oxidation.

Size and molecular configuration of dye aggregates in mixed Langmuir–Blodgett films based on merocyanine dye

We have studied the blue-shifted and the red-shifted bands formed in mixed Langmuir–Blodgett films of the merocyanine dye (MS)-arachidic acid (C20)-n-octadecane (AL18) ternary system with the molar mixing ratio [MS]:[C20]:[AL18]=1:2:x(0.5≦x≦5.0). The formation of the blue-shifted and the red-shifted bands depends on the AL18 content, and shows that the aggregation state can be modulated by changing the AL18 content. The observed overlapping spectra of the blue-shifted and the red-shifted bands are deconvoluted into two original bands. The extended dipole model has been applied to examine the aggregation state of MS referring to the deconvoluted spectra. Thus the estimated minimum aggregation number Nmin and the slip angle α between the long axis of the aggregate and the transition dipole moment are Nmin=40 and α=30° and Nmin=40 and α=50° for fully-developed J- and H-aggregates, respectively, seen for x≦1.5, and Nmin tends to decrease with increasing x.

Self-assembled monolayers of CF3-terminated alkanethiols on gold

Abstract The wettability and friction of self-assembled monolayers (SAMs) of terminally fluorinated alkanethiols, CF3(CH2)nSH with n=9–15, and their nonfluorinated analogs, CH3(CH2)nSH with n=9–15, were examined and compared. Surprisingly, the CF3-terminated monolayers were wetted more by water, glycerol and N,N-dimethylformamide than were the CH3-terminated SAMs. The terminally fluorinated films, however, exhibited significantly lower surface energies. The origin of these trends is discussed. The two types of monolayers were also studied by atomic force microscopy (AFM). Compared to the CH3-terminated SAMs, the fluorinated films revealed an approximately 3-fold increase in friction. A new model for the frictional response, which is based on the difference in size between the methyl and trifluoromethyl groups, is proposed.

Control of aggregate formation in merocyanine Langmuir–Blodgett films

Abstract An H-like band, blue-shifted from an absorption maximum of a monomer band, has been formed in merocyanine–arachidic acid mixed Langmuir–Blodgett (LB) films by adding n -alkane species of different chain lengths (CH 3 (CH 2 ) n −2 CH 3 , n =18 and 28) as a third component. In each ternary system, the blue-shifted band is prominent or is accompanied by a J-band depending on the preparation conditions. The absorption spectra A ‖ and A ⊥ were measured using a normal incident of linearly polarized light with the electric vector either parallel or perpendicular to the dipping direction, respectively. The results of surface pressure–area isotherm measurements have indicated that the alkane species remaining in the monolayer and filling empty spaces near surface-active molecules act as the trigger molecules to induce the H-like aggregates. Comparing these results with the predictions from the theory of flow orientation and the extended dipole model, it is suggested that the present blue-shifted H-like band is of an H-aggregate rather than a dimer.

Wettability and friction of CF3-TERMINATED monolayer films on gold

Abstract Studies of the wettability and friction of self-assembled monolayers (SAMs) generated from the terminally fluorinated alkanethiols (CF 3 (CH 2 ) n SH, n = 9–15) on gold are reported. These data were compared to those of SAMs generated from the non-fluorinated analogs (CH 3 (CH 2 ) n SH, n = 9–15). The CF 3 -terminated SAMs were less wettable by hexadecane and methylene iodide, but surprisingly more wettable by water and glycerol than were the CH 3 -terminated SAMs. Measurements of friction by atomic force microscopy (AFM) revealed an approximate threefold increase in friction on the CF 3 -terminated films compared to the CH 3 -terminated films. A new model for the frictional response of fluorinated SAMs is proposed.

Change in the Layered Structure of Merocyanine Dye Langmuir-Blodgett Films due to N-Octadecane Addition

The layered structure in Langmuir-Blodgett (LB) films of the merocyanine dye (MS)-arachidic acid (C20) binary system and the MS–C20-n-octadecane (AL18) ternary system with low and high C20 contents ([MS]: [C20]:[AL18]=1: 2: x and 1: 5: x, 0x5.0) has been studied by means of surface pressure (π)–area (A) isotherm and X-ray diffraction measurements. The π–A isotherm measurement indicates that AL18 tends to fill the empty space of MS; however, a certain amount of AL18 is found outside of the space of MS even for smaller mixing ratios of AL18. A phase transition is observed in the π–A isotherms of the ternary monolayers with low and high C20 contents for x3.0. Two different regimes can be recognized in the X-ray diffraction patterns of the LB films with low and high C20 contents: 2θ< 9° and 9° < 2θ. The bilayer unit-cell species governing the X-ray diffraction patterns for 2θ<9° changes with increasing molar mixing ratio of C20. X-ray diffraction and π–A isotherm measurements indicate that the increase in AL18 in the empty space of MS causes the expansion of the spacing estimated for 2θ< 9° in the LB films.

Formation and Characterization of H-Like Band in Merocyanine Dye LB Films

Abstract The merocyanine-arachidic acid mixed LB films have been found to exhibit H-like band whose absorbance maximum is blue-shifted from that of monomer band. It is indicated that alkane molecules added to the mixed system induces the formation of H-like band.

Crystal Structure of Dioctadecyldimethylammonium-Au(dmit)2 Salt

The crystal of the dioctadecyldimethylammonium-Au(dmit)2 (2C18-Au(dmit)2) salt has been found to be composed of alternate sheets of Au(dmit)2 anions and dioctadecyldimethylammonium cations with a pronounced interdigitation of the alkyl chains. The planar Au(dmit)2 anions are dimerized by face-to-face stacking and further forming extended columns along the a-axis with short side-by-side contacts by sulfur atoms.