Brewster Angle Microscopic Observations of the Langmuir Films of Amphiphilic Spiropyran during Compression and under UV Illumination

Abstract

The structure of the Langmuir film of an amphiphilic spiropyran, 1',3'-dihydro-3',3'-dimethyl-6-nitro-l'-octadecyl-8-(docosanoyloxyme thyl)spiro[2H-1-benzopyran-2,2'-(2H)-indole] (SP), is investigated using Brewster angle microscopy (BAM). The BAM observations show that the Langmuir film of SP can be roughly categorized into three regimes: a low-temperature regime at 7-13 degrees C; a medium-temperature regime at 23-30 degrees C; a high-temperature regime at 40 degrees C. The low-temperature regime is characterized both by the domains that are formed just after the spreading and by the onset of the surface pressure when the domains are merged together to form continuous trilayers. In the medium-temperature regime, a continuous monolayer film is formed after the solvent evaporation, followed by the growth of "embryos" with compression. Around the phase transition point, the "embryos" serve as the "nucleation sites" of the circular trilayer domains. The characteristic features of the high-temperature regime are similar to the ones of the medium-temperature regime except for the absence of a steep rise in surface pressure after the plateau region and the absence of the circular trilayer domains. UV illumination of the Langmuir films leads to the isomerization of SP into merocyanine (MC). However, J-aggregates of MC are formed only when the circular trilayer domains are present. On the basis of the above results, we present a phase diagram of the Langmuir film of SP. The structure and photoreaction depend strongly on the phase of the Langmuir film, indicating that the area/molecule is not the only decisive parameter.

Structures of Langmuir-Gibbs Films Consisting of Long-Chain Fatty Acid and Water-Soluble Surfactants

“…In the region c SDS ≥ 1.5 mM, much brighter island-like domains were occasionally observed. It is suggested that these island-like domains have multilayer structures because these domains were much brighter than the condensed phases in the Langmuir films of D19A at the air-water interface not at the air-solution interface and because multilayer are much brighter than monolayers 37 . In regime B, the D19A-…”

Photoreactions and lateral patterning in Langmuir and Langmuir–Blodgett films

“…Room‐temperature UV illumination of mixed LB multilayers of SP and octadecane causes photoisomerization to MC only, whereas UV illumination of the films at temperatures equal to or higher than 35°C promotes the J‐aggregation of MC 10. We have started to study the Langmuir and LB films of SP alone because they were not satisfactorily characterized 11–14…”

“…The isotherm shows a steep rise at low temperatures. At high temperatures, the molecular area is large in the low surface pressure region and the isotherm shows a phase transition 11,12. Structural changes of the Langmuir film during compression were monitored using a Brewster angle microscope (BAM).…”

Effect of Fabrication Conditions on the Structures and Photoreactions of Langmuir-Blodgett Films of Amphiphilic Spiropyran

“…J-aggregates of dye molecules have been investigated extensively from the viewpoint of the applications to the spectral sensitization, optical storage, and optical switching [2]. J-aggregates in Langmuir and LB films are particularly interesting because of the low-dimensional structures of the films [3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18]. The absorption bands of }-aggregates are red-shifted and very narrow compared to the monomer bands.…”