A Dual-Role Amphiphilic Photosensitizer: Enhancing Structural Uniformity and Optical Properties of Langmuir Monolayers

Amphiphilic π-electron systems form two-dimensional crystalline domains at fluid interfaces, resulting in gaps between the domains. Most such systems are based on linear π-electron backbones that absorb only in the UV, limiting their use in photoenergy conversion. We propose integrating the roles of plasticizer and photosensitizer into a single molecule to produce homogeneous, continuous, light-harvesting membranes. We hypothesize that twisted perylenes can fulfill this dual function and that simple theoretical models can predict the densest packing of π-electron systems. We fabricated molecular monolayers comprising amphiphilic π-conjugated oligo(phenylene ethynylene) derivatives (OPE-NH₂) and a twisted perylene dye (PMIDA-C12) with broad visible-light absorption. Monolayer homogeneity was assessed by Brewster-angle and atomic force microscopy across a range of mixing ratios. Optical properties were probed using photothermal deflection spectroscopy. Experimentally derived packing densities were then compared with cross-sectional areas and aggregate structures predicted by quantum-chemical calculations. OPE-NH₂ monolayers accommodated up to 14 mol% PMIDA-C12 while maintaining homogeneity and exhibiting a marked increase in visible-range absorption. At higher dye loadings, self-aggregation disrupted layer uniformity. These results demonstrate that our twisted amphiphilic dye can act simultaneously as a plasticizer and a photosensitizer. In addition, we show that π-stacking in Langmuir monolayers can be quantified and predicted by combining image binarization with simple theoretical models.