Molecular Orientation and Structure Formation of BOPHY Chromophores in Langmuir Films: A Combined Experimental and Theoretical Study

The uniform orientation of 𝝅-conjugated chromophores in thin films is critical for anisotropic charge, energy, and mass transport, with direct implications for optoelectronic and sensing applications. In this study, the molecular orientation and supramolecular organization of a linear amphiphilic BOPHY dye in Langmuir monolayers at the air-water interface are investigated. By combining in situ X-ray reflectivity (XRR), grazing incidence X-ray diffraction (GIXD), and classical molecular dynamics (MD) simulations, the structural evolution of the monolayer through two distinct condensed phases is elucidated. GIXD reveals that the alkyl chains primarily determine the packing density, forming short-range hexagonal domains upon compression, while the chromophores remain locally ordered but lack long-range crystallinity. These results indicate that molecular tilting and bending arise from asymmetries between the BOPHY chromophore and the three alkyl chains. MD simulations further reveal coplanar 𝝅-stacked BOPHY dimers or small aggregates with angular distributions that differ between the two phases. The strong agreement between experiment and simulation provides a comprehensive understanding of the alignment of the linear chromophores at the air-water interface and establishes this integrated approach as a powerful framework for predicting and designing functional chromophore assemblies in 2D molecular films.