Amphiphilicity\nof Tetraazaporphyrins Containing Four\nTerminal Carboxylic Acid and Four Alkyl Groups Promotes Face-On Orientation\nin Langmuir Films

Control over the orientation of polycyclic aromatic dyes\nin thin\nfilms is paramount to tailoring their optical, electronic, and mechanical\nproperties. Their supramolecular assembly in films is tuned here by\nconverting the macrocyclic dyes to large amphiphiles. Two octaalkythio-substituted\ntetraazaporphyrins (TAPs) with one 5-carboxypentyl and one pentyl\nor dodecyl chain per pyrrole ring were synthesized as statistical\nmixtures of four regioisomers. The unsymmetrically substituted maleodinitrile\nprecursors were prepared in good yield with a flow reactor. Neither\nthe tetraester precursor TAPs nor the tetraacid TAPs show mesomorphism,\nand both belong to the small class of porphyrin derivatives that display\nisotropic liquid phases at or close to room temperature. Interfacial\nproperties of the two amphiphilic tetraacids were probed using Langmuir\nfilms on aqueous subphases at different pH values, and Langmuir–Blodgett\nfilms were transferred onto mica substrates. The tetraacid with the\nlonger dodecyl chains forms inhomogeneous films comprising a combination\nof monolayer, stacked macrocycles with interdigitated chains, and\n3D structures, with the latter favored at higher subphase pH and higher\nsurface pressures. In contrast, films of the tetraacid with the shorter\npentyl chains yielded relatively homogeneous monolayers. The combination\nof atomic force microscopy imaging and packing correlations elucidated\nby grazing incidence X-ray diffraction suggests that these form spider-like\nconformations, with the macrocycles close packed and oriented parallel\nto the substrate (face-on). This difference in molecular packing is\nattributed to a possible intramolecular mixing of 5-carboxypentyl\nand pentyl chains that gives a better match between the footprints\nof the macrocycle, acid groups, and alkyl groups. The longer dodecyl\nchains are too large for filling the space between 5-carboxypentyl\nchains (mixing) and too small for filling the footprint of a TAP macrocycle.\nWe demonstrate that by judicious tailoring of the chain length and\nsubphase conditions, a desirable homogeneous film of face-on oriented\nmacrocycles can be formed.