Embedding an Amphiphilic 4-Hydroxy Thiazole Dye in Langmuir Matrices: Studying Miscibilities with Arylic and Alkylic Matrix Amphiphiles via Langmuir Isotherms and Photo-induced Force Microscopy

We present here a fundamental study on the miscibility between a prototype amphiphilic dye and alkylic and arylic Langmuir monolayers. Embedding dyes in such matrices is crucial for utilizing dyes in any photo-energy conversion process if the involved dyes form aggregates that provide thermal deactivation channels. Because miscibility in Langmuir matrices depends on the blending ratio between the dye and matrix and on the Langmuir film density, as characterized via the surface pressure and the mean molecular area, we employ Langmuir miscibility studies to identify ideal miscibility parameters for each matrix. Atomic force microscopy (AFM) results support miscibility between the dye and both matrix materials at low surface pressures, where smooth and homogeneous films are obtained. AFM and photo-induced force microscopy (PiFM) reveal phase separation if the Langmuir monolayers are deposited at surface pressures above 8 mN/m at which reorientation of the chromophores has been reported. The nanoscale chemical fingerprint mapping enabled by PiFM enables assigning segregated spots to small stearic acid (SA)-enriched domains that have not been detected via AFM, thus demonstrating the value of the IR-spectroscopic contrast provided by PiFM. In this work, we have presented a so far unexploited matrix material (terphenylene carboxylic acid; TPCA) and found it equally suitable for embedding dyes as the standard amphiphile SA. In contrast to SA, TPCA is composed of rigid and electrically conducting π-electron systems, hence, being predestined for aligning dyes in Langmuir matrices and for application in optoelectronic systems.