Preparation and characterisation of Langmuir-Blodgett films of 4-(4-(N-octadecyl-N-methylamino) phenylazo)benzoic acid

Abstract

Langmuir films of the title compound have been spread on an aqueous subphase at various values of pH at 10, 15 and 20°C. Increasing pH and increasing temperature favour stable films, but at the higher subphase pH and temperature values the pressure–area isotherms exhibit a transition to aggregated forms. Tilt angles between the alkyl chains and the normal to the subphase increase with increasing subphase pH and temperature. The area per molecule for films deposited at 15 °C decreased steadily with time at a fixed surface pressure, except at the highest pH, indicating poor stability. Langmuir–Blodgett films deposited at a surface pressure of 30 mN m−1 and a subphase temperature of 15 °C were of Y‐type and showed transfer ratios above unity for the lower subphase pH values. UV/visible spectra of the LB films showed features characteristic of the formation of H aggregates for deposition at the higher subphase pH values. Over long periods of time the spectra for high pH showed evidence of increasing aggregation. Small‐angle X‐ray diffraction confirmed molecular tilts larger than those deduced in the floating monolayer. Reflection–absorption infrared spectroscopy of the LB films showed differences from the bulk also indicative of significant tilt, as did surface‐enhanced Raman spectroscopy. The LB films showed weak second‐harmonic generation from 1064 nm radiation consistent with a polar film structure parallel to the substrate. Copyright © 1999 John Wiley & Sons, Ltd.

Aggregation-Induced Fabrication of Fluorescent Organic Nanorings: Selective Biosensing of Cysteine and Application to Molecular Logic Gate

“…The angles α and φ calculated from the area of the above-mentioned absorption and emission spectra for J-aggregation can also be corroborated with the surface pressure−mean molecular area (π−A) isotherms for Langmuir films. If we plot π−A isotherms for Langmuir films (Figure S4) of NPCl monolayer when no aggregation is formed (0.83 μM) on the water surface during compression process, the tilt angle (θ) can be calculated as θ = cos −1 (A/A m ), 32 where A is the cross-sectional area of the molecule obtained from the tangent to the area axis from collapse point, and the area per molecule, A m , in an uncompressed monolayer is estimated by extra-polation of the isotherms to zero pressure. From Figure S4, the value of A was found out from the intercept point of the tangent to the area axis, which is 0.09 nm 2 .…”

Capping-Ligand Effect on the Stability of CdSe Quantum Dot Langmuir Monolayers

“…Since the discovery of Qdots in the early 1980s, many researchers have exploited their size quantization effects and high surface area−volume ratios to prepare materials that could not have been created from their bulk counterparts. − The unique properties of Qdots − led to interest in preparing 2D films of Qdots for many applications such as light-emitting devices and solar cells. , In particular, special attention has been given to Qdots composed of II−VI semiconductors such as CdS, CdSe, and CdTe because of their ease of preparation that enables precise control of their size-dependent optoelectronic properties . Films can be formed by drop-casting Qdot solutions directly onto substrates, by synthesizing Qdots in situ within a Langmuir film, , or by spreading immiscible solutions of Qdots at the air−water interface. ,,− The latter method holds appeal because Qdots can be size-selected prior to use; , the film thickness can be controlled precisely to within one monolayer; , interparticle spacing can be manipulated, which impacts film optical properties; , and the Qdot Langmuir films can be transferred to planar substrates to create organized, layered assemblies with tailored attributes. ,,,,,,, In addition, the surface chemistry ,, of the Qdots can be preselected for compatibility with chosen subphase conditions such as ionic strength, pH, ,, and temperature . However, because Qdots are expected to behave differently than amphiphilic lipid molecules at an air−water interface, , concern has naturally arisen regarding the stability and reproducibility of Qdot Langmuir films. ,,− …”