Electrochemistry at the Air-Water Interface: 2D Crystallization of a Pigment Array Contiguous to a Langmuir Monolayer for Light Harvesting Antenna

The synthesis of novel crystalline molecular materials with tailored physical and electronic properties has increasingly been directed towards the use of organic molecular templates upon which nucleation can be achieved.   The use of organized lipid membrane architectures (e.g., bilayer vesicles, Langmuir monolayers, self-assembled films) as templates for crystal growth is of great importance in both understanding and mimicking organic and inorganic natural biocrystallization processes.

Illustration of molecular organization of DHDP/2,3-TMeAzPc/I monolayer films at the air/water interface, and  the compounds used in this work.  Surface pressure versus molecular area isotherm of DHDP on water and on 1 molar 2,3-TMeAzPc solution.  The changes in the isotherm indicate strong interaction between the charged pigments in the solution and the phosphate head-group.  The complex can then be transferred to solid support using the Langmuir- Blodgett technique.

Reflectivity from DHDP on pure water normalized to the calculated reflectivity for pure water  The green curve is calculated based on the electron density model shown in the inset.  The dashed line in the inset represents electron density profile in the absence of surface roughness
Reflectivity from DHDP on micro molar solution of phthalocyanine   (Pc).   The  green dashed line is a reproduction of the reflectivity from DHDP on pure water.  From the fitting procedure structural parameters regarding the arrangement of this complex  are  determined.
Intensity versus Q scan for DHDP on 1 M 2,3-TMeAzPc ( = 25 mN/m) showing two Bragg reflections: one due the ordering of the lipid chains (Q  1.48 Å-1, ADHDP =  41.51 Å2/molecule), and the other at low Q values due to the crystallization of the pigment-lipid complex (Q = 0.226 Å-1).  The insert shows a rod scan along Qz at the Bragg reflection (Q = 0.226 Å-1 ).