The Langmuir-Blodgett process is an efficient way of obtaining thin films of monomolecular thickness. The LEPO team at SPEC has produced layers of perylene-diimide (PDI), a dye with attractive optical properties. This type of molecule has high light absorption in the visible range, opening up a wide range of possibilities as an active element in optoelectronic processes and photovoltaics.
By the present study, the SPEC/LEPO team has established the link between the resulting layer structure and its optical properties, which is essential for developing new photonic materials with adjustable properties.
When organic molecules condense together, they acquire optical properties quite distinct from those observed for the individual molecule or in solution. These changes are linked to intermolecular interactions, depending on the distance between molecules and their relative orientation, which modify their electronic energy levels. The spatial organization of molecules within a material thus plays a crucial role in determining its optical properties.
Among these molecules, perylene diimide (PDI) dye molecules are particularly well studied for their absorption in the visible range, their high photo-stability and their potential for numerous photonics applications. PDI molecules are flat and elongated, and their orientation with respect to a substrate and the distance between them are key parameters for understanding their properties.
In order to study the influence of this molecular organization on the optical properties of PDIs, the SPEC/LEPO team has elaborated monolayers of these molecules on a glass substrate using the Langmuir-Blodgett technique. This method enables a monomolecular layer to be obtained on the surface of water, then transferred to a solid support, glass in this case. The transferred molecules can then have one of three orientations: “flat”, “edge-on” or “head-on” – see Figure 1.a.
Figure 1. a) The 3 possible orientations of PDI molecules on a glass surface. b) After a local laser excitation of the PDI monolayer, the photoluminescence is collected by an objective lens and recorded in the focal plane. The image obtained (b) gives the orientation of the emitting dipole, linked to the orientation of the molecule. c) Simulated image for a single dipole parallel to the surface. The agreement obtained shows that the orientation of adsorbed molecules is preferentially “edge on”.
For this study, different optical techniques were combined:
- Micro-absorption: this technique reveals the presence of an absorption band characteristic of strong π-π interaction between molecules, suggesting a strong interaction between close molecules. This observation rules out a “flat” organisation.
- Photoluminescence: After local light excitation at 532 nm, a broad emission band (600- 850 nm) is observed. A photoluminescence pattern is recorded in the focal plane of a collecting lens. By comparing the photoluminescence pattern obtained (Figure 1.b) for this PDI monolayer with the calculated theoretical pattern (Figure 1.b and c), it can be concluded that the preferred orientation of the emitting dipole, and therefore of the principal axis of the adsorbed molecules, is “edge-on”.
- Polarisability: measurements of absorption as a function of angle of incidence show that the polarizability of the molecules is maximal in the direction parallel to the surface, confirming the “edge-on” orientation of the molecules.
These consistently obtained results show that optical properties depend on the orientation of PDI molecules within the Langmuir-Blodgett monolayer transferred onto a glass plate. This result opens up new prospects for the design of photonic materials with tunable properties, by playing on molecular organization.
Reference:
N. Fabre, R. Trojanowicz, L. Moreaud, C. Fiorini-Debuisschert, S. Vassant, and F. Charra, Langmuir 39 (50) (2023) 18252.
Contact CEA: Nicolas Fabre, Université Paris-Saclay SPEC/LEPO – UMR CEA-CNRS.