Search

Raman scattering signals recorded by microscopy from organic self-assembled monolayers (thin nanometric films of calibrated thickness) on silica substrates were found to be much stronger than those obtained from identical films assembled on bulk silicon substrates. This effect, observed in the backscattering geometry, is shown to result from interferences between the direct and reflected beams (including both the excitation and scattered radiation) in front of a smooth reflecting surface. Strong dependence of the effect on the distance between the sampled monolayer and the bulk silicon substrate allows enhancement of the Raman signals of organic monolayer films on silicon by factors up to approximately 70 by using appropriate silica spacers. The dependence of the Raman signal intensity on film thickness was also studied for thicker nanometric films comprising a series of self-assembled organosilane multilayers on bulk silicon and fused silica substrates, and the predicted deviation from linearity in the case of the silicon substrate is experimentally confirmed.

A new route for the elaboration of the polypropylene surface directly modified by 4-amino-4‘-nitroazobenzene (Disperse Orange 3) was studied. The synthesis consists of a cold carbon dioxide-plasma treatment followed by a mixed anhydride reaction and Disperse Orange 3 fixation. The quantity of active acid introduced by the plasma treatment was estimated to be 0.7 groups/nm2 by fluorescence labeling with thionine acetate. The azo-modified surfaces were characterized by Fourier transform infrared, resonance Raman, and X-ray photoelectron spectroscopies. Confocal resonance Raman spectrometry has shown that Soxhlet extraction is effective to eliminate diffused molecules from the material. It was shown that the azo dye molecules are chemically fixed onto the polymer surface.

The structure of a series of alcohol-terminated bifunctional long-tail organosilane films with varying numbers of superimposed monolayers (between 1 and 11), prepared on smooth, hydrophilic silicon substrates by the layer-by-layer self-assembly approach, has been investigated with the purpose of elucidating details of the molecular organization and the intra- and interlayer modes of binding in such films. To this end, experimental results obtained by synchrotron X-ray scattering, micro-Raman, and Fourier transform infrared (FTIR) spectroscopic techniques have been combined and compared. A comprehensive analysis of all data demonstrates that the studied multilayer films consist of stacks of uncorrelated discrete monolayers, the inner molecular order of which is preserved with the growing total number of superimposed layers. Similar to self-assembled films of long-tail silanes with terminal −COOH groups,1 the intermolecular binding in the present films is characterized by partial intra- and interlayer coval...

Using temperature-dependent Fourier transform infrared (FTIR) spectroscopy, we probe the molecular level, chain-structural dynamics associated with solid−solid transitions between 25 and 250 °C in a layered inorganic−organic silver dodecanethiolate, AgS(CH2)11CH3. Spectroscopic evidence presented here establishes two major transitions: the transition occurring at ∼130 °C is characterized by an abrupt, but fully reversible, change in the chain conformational order from an initial all-trans state to the one characterized by mixed or partial chain disorder. The observation of this phase transition is consistent with the previous predictions of a rapid and drastic change in the structural motif from an initial bilayer to the final micellar state. The second transition at about 190 °C, which is consistent with the previous assignment of micellar amorphous transition, is furthermore irreversible and represents thermal degradation of the material. Implications of these results for the general family of chain mo...

Engineering highly nonlinear optically (NLO) active assemblies can lead to thin films with potentially useful properties. For instance, materials with electron donor and acceptor groups bridged by a conjugated system have potential application as molecular wires, where electrons can tunnel or hop through the conjugated bridge. Phenothiazine and its derivatives are of widespread chemical interest. For example, they have useful pharmacological properties and can potentially be used in solar energy conversion. There is also interest in these systems as charge-transfer complexes with several acceptors due to their low ionization potentials. 4-(trans-4-(4-(10-phenothiazinyl)butyramido)styryl)pyridine (PBSP) has been synthesized and covalently bound to a coupling layer of (p-(chloromethyl)phenyl)trichlorosilane (CPTS) on a quartz substrate to produce a polar-ordered thin film. The addition of the PBSP molecules on the coupling layer was monitored by FTIR spectroscopy, UV--visible spectroscopy, AFM, and second harmonic generation (SHG). The observation of an amine band and strong methylene bands in the infrared spectra indicates the presence of the PBSP layer. The UV-visible spectrum shows a red shift in the charge-transfer band, consistent with the addition of the PBSP layer on the coupling layer. The nonlinear optical properties of the PBSP monolayers were measured by SHG. The second harmonic (SH) signal increasedmore » by 1 order of magnitude after the addition of the PBSP chromophore layer. The dependence of the SH intensity on incident angle and input polarization was modeled. The average molecular orientation angle of the chromophore was determined to be 55{degree}, and a value of {chi}{sub zzz}= {chi}{sub zxx} = 2.8 x 10 {sup {minus}8} esu was calculated.« less

The structure described is a new polymorph of the one-dimensional material [Pt(en)2I2][Pt(en)2]ClO4 (en = 1,2-diaminoethane) that occurs when the en ligands are fully deuterated. This structure is novel in that it contains ordered MX chains, a structural feature absent in all MX structures to date. It also represents a rare case of a structural change induced by isotopic substitution. This system presents a unique opportunity to study the interchain interactions that might lead to a fully ordered MX system and the behavior of ordered versus disordered chain systems.