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 irradiation, limiting their use in photoenergy conversion. We propose integrating the roles of plasticizer and photosensitizer into a single molecule to produce homogeneous, continuous, and light-harvesting membranes. Twisted perylenes can fulfill this dual function, and simple theoretical models can predict the densest packing of π-electron systems. We fabricated molecular monolayers comprising an amphiphilic π-conjugated oligo(phenylene ethynylene) derivative (OPE-NH2) and a twisted perylene dye (PMIDA-C12) exhibiting broad visible-light absorption. Monolayer homogeneity was assessed by Brewster-angle microscopy and atomic force microscopy across a range of mixing ratios, and optical properties were probed by using photothermal deflection spectroscopy. Experimentally derived packing densities were 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 the 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.