The structure and growth of functionalized thiol self-assembled monolayers (SAMs) on Au(111) derived from a new compound, 12-(4-((4-hexylphenyl)azo)phenoxy)dodecane-1-thiol (“hexyl azobenzene thiol”), were examined by atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), dynamic contact angles, Fourier transform infrared-reflection absorption spectroscopy (FTIR-RAS), and reflection UV−vis absorption spectroscopy. The hexyl azobenzene thiol SAM showed adsorption kinetics comparable to that of alkanethiols. At the initial stage of SAM growth, formation of nanosize domains resulting from the rearrangement of molecules on the surface was confirmed in a similar manner as for alkanethiol SAMs with long alkyl chains. The hexyl azobenzene thiol SAM exhibited a unique molecular ordering due to the intermolecular interaction between tail groups. Molecular resolution AFM images revealed an expanded lattice (nearest neighbor spacings, a = 5.3 ± 0.1 Å, b = 5.6 ± 0.1 Å; angle between the two axes, φ = 85 ± 1°) relative to that of simple azobenzene-terminated SAMs, due to the influence of the hexyl-terminal groups attached to the para-position of the azobenzene moieties. Average molecular tilt and twist angles (α = 14°, β = 42°) were determined by quantitative analysis of C−H stretching mode for FTIR-RAS data. The reflection UV−vis absorption spectra implied that the long axes of the trans-azobenzene moieties were aligned normal to the plane of surface in fully covered SAMs, while those were more tilted (aligned more parallel to the surface plane) in the submonolayers.

Thiophene/phenylene co-oligomers that include the thiophenes and phenylenes with various arrangements in the molecule were investigated spectroscopically in solutions and compared with oligothiophenes and oligophenylenes. The electronic spectra are influenced by both the molecular size and the difference in the thiophene/phenylene arrangements. In the dilute regime (∼10 -5 M) monomeric emissions prevail. Quantum efficiencies of the fluorescence emissions of the co-oligomers are estimated in this regime. High quantum efficiencies of 0.79 and 0.74 were recorded for 4,4′-bis(2-thienyl)biphenyl (T2P) and 1,4-bis(5-phenylthiophene-2-yl)benzene (AC5), respectively. The experimental data in the dilute regime are compared with the results of the quantum chemical calculations at semiempirical levels. Intermolecular interactions increasingly play a role for higher concentrations. The spectroscopic characteristics in this regime are closely inspected using 2,5-diphenylthiophene (P1T), 5,5′-diphenyl-2,2′-bithiophene (P2T), and 1,4-bis(2-thienyl)benzene (T1P). By increasing the concentration, these co-oligomers show well-structured emissions that are red-shifted relative to the monomeric emissions. New absorption shoulders appear in the longer wavelengths region due to the intermolecular interaction. These features are assigned to the intermolecular ground-state complexes with fully overlapped π-π groups. Besides the above spectral features, T1P exhibits a long tail toward ∼700 nm in the absorption spectra at higher concentrations and a broad emission band around 520 nm replaces the strong band at 450 nm. We referred these features to the intermolecular charge-transfer from a thiophene to a phenylene ring. The fluorescence emissions of the co-oligomer thin films are also red-shifted relative to the monomeric emissions. The origin of these emissions in the solid state is briefly discussed.

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A new type of organic photoreaction, a water-participation reaction in the crystalline state, has been investigated. The photoirradiation of p-formylcinnamic acid (p-FCA) crystals in the presence of water leads to 4,4′-diformyl-β-truxinic acid (p-FCA dimer) which has one molecule of crystal water. A comparison of the photodimerization carried out in the presence/absence of water revealed the effects of the water molecule on the reaction. That is, the water molecule is incorporated into the p-FCA dimer during the photodimerization process. Furthermore, the water accelerates the reaction rate of photodimerization, leading to the formation of the crystalline dimer, whereas only the water-free amorphous dimer results in the absence of water. In addition, by heating at 100–110 °C, the p-FCA dimer with a water molecule is converted into a water-free crystalline dimer, which is not attainable from a water-free amorphous dimer. The photodimerization of several β-form crystals of cinnamic acid derivatives has been examined from the viewpoint of water-participation: p-nitro, p-chloro, and 3,4-dichlorocinnamic acid were found to photodimerize in the same manner as p-FCA.

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