Macroscopic and nanoscopic current−voltage measurements reveal asymmetries in the DC electrical conductivity through Langmuir−Blodgett multilayers and even monolayers of γ-(n-hexadecyl)quinolinum tricyanoquinodimethanide, C16H33Q-3CNQ (5). These asymmetries are due to a transition of the ground-state zwitterion to an excited-state conformer which is probably neutral. Unimolecular electrical rectification by monolayers of 5 is unequivocally confirmed.

The research on ferroelectric materials-mostly inorganic compounds or organic polymers-is increasingly motivated by both basic scientific concerns and the potential for practical applications in electronics and optics. Ferroelectricity in organic solids would be important for the development of all-organic electronic and photonic devices. The conventional approach to making organic ferroelectrics is based on the use of polar molecules. Here we report that through supramolecular assembly of nonpolar conjugated molecules, a remarkable ferroelectric response can be obtained in co-crystals of low-molecular-weight organic compounds. Co-crystals of phenazine and chloranilic acid reveal large spontaneous polarization and sizable room-temperature dielectric constants exceeding 100. The present findings provide an approach to making potentially useful organic ferroelectric materials.

An amphiphilic anionic azobenzene derivative, soluble in water, formed a monolayer on an aqueous subphase containing a water-soluble polycation. The monolayers were transferred onto solid substrate by the Langmuir−Blodgett (LB) technique. XPS measurements showed that ion exchange reaction proceeded almost completely at the air−water interface and that the ratio of the monomer unit of the polycation to the azobenzene was almost unity. UV/vis absorption and IR measurements indicated that the azobenzene photoisomerized reversibly in the LB films on alternate illumination with UV and vis light. Furthermore, a reversible morphological change induced by light was observed in the LB films with AFM. Before illumination, the surface of the single-layer LB film was very smooth with a surface undulation of less than 1 nm. On illumination with UV light, however, a number of hills, with the height of ca. 5 nm and the diameter of the base of ca. 100 nm, appeared on the film surface. These structures almost disappeared on illumination with vis light. This indicates that the widely accepted assumption that photoisomerization should not change the two-dimensional structures significantly does not hold in the present case.

No abstract

No abstract

No abstract

The modulus and phase of the complex third-order nonlinear optical (NLO) susceptibility X 3 ( -3to\co,co,co) have been measured on an all-trans silicon polymer, poly(dihexylsilane), over a wide photon energy range. Several distinct structures observed in the f (3) spectra are in excellent agreement with the theoretical results of a one-dimensional (ID) exciton model. Important aspects of the ID Wannier-like exciton structure in polysilanes are elucidated in relation to the NLO processes.PACS numbers: 73.20.Dx, 42.70.Nq, 78.65.He Much interest has been focused on the quasi-onedimensional (ID) semiconductors. In a-bonded inorganic semiconductors, nanoscopic structures called quantum wires are fabricated by elaborate techniques of crystal growth and microlithography [1]. Meanwhile, various organic polymers are synthesized to utilize the delocalized ^•-conjugated electronic states on the linear polymer chains [2]. In such quasi-lD inorganic as well as organic semiconductors, the lowest dipole-allowed exciton is predicted to have a very large binding energy and an unusually strong oscillator strength [3][4][5], due to the singular character of the ID Coulomb potential for excitons [6]. This problem has attracted considerable interest from the viewpoint of solid-state physics, but the details remain unexplored as of yet. In this paper, we report the thirdorder nonlinear optical (NLO) spectra of a silicon polymer, polysilane, demonstrating that the characteristic features of quasi-ID (Wannier-like) excitons can be fully revealed by nonlinear optical spectroscopy.Among the various kinds of polysilanes (TSi/?/?T, R and R' being organic sidegroups) with different conformations [7], we have chosen poly(dihexylsilane) (PDHS; R,R' =C 6 H 13) with an all-trans planar structure of obonded Si chains (with a djh symmetry), which may be regarded as a model system of the 1 D analog (or "an ultimate quantum wire") of a 3D silicon crystal. This polymer is predicted to have a direct gap between the obond-like valence band and the a* -bond-like conduction band at the T point [8]. The sharp absorption peak at 3.30 eV is assigned to the lowest allowed X B U exciton, while the two-photon absorption peak at 4.19 eV [9-1 1] is assigned to the second forbidden ] A g exciton. However, the whole profile of the ID excitonic structure has not yet been disclosed.To elucidate the quasi-lD excitonic structure in PDHS, we have undertaken extensive NLO spectroscopy over a wide photon energy range, as recently attempted on various ^--conjugated polymers [12]. In the sample preparation, PDHS powders with molecular weight of about 2.44xl0 5 were dissolved in heptane, and the solution was spin casted onto synthetic fused silica plates to form unoriented film samples. The films were dried at 380 K for 15 min and allowed to stand at room temperature overnight, before optical measurements.In the measurements of x 3 (~~3(o,co,co,a)) three kinds of tunable laser systems were used: a dye laser driven by a 15-ns-wide Xe-Cl excimer laser with use of thirteen ...

No abstract