Face- and edge-on orientations of octa-acid and -alcohol substituted tetraazaporphyrins in Langmuir and Langmuir–Blodgett monolayers

Abstract

Polyaromatic dyes, such as phthalocyanines and tetraazaporphyrins (TAPs), arrange with an edge-on orientation of the macrocycle in almost all Langmuir and Langmuir–Blodgett (LB) monolayers, especially when condensed at higher surface pressures. Presented here is a new amphiphilic TAP that contains eight terminal carboxylic acid groups attached to the macrocycle via decyl spacers, which forms stable Langmuir and LB monolayers with a flat-on orientation of the macrocycle. A spider-like molecular conformation is proposed based on the measured surface area per molecule and film thickness determined by ellipsometry and AFM. This conformation allows all eight terminal carboxylic acid and carboxylate groups to attach to the polar interface while the aromatic macrocycle sits on top of the orthogonal decyl chains to generate a micro-segregated layer structure. Octa-acid TAPs with shorter alkyl spacers form edge-on structures in Langmuir monolayers upon compression and are not free of 3-dimensional aggregates based on observations by Brewster angle microscopy (BAM). All octa-acid TAPs are prone to spontaneous self-assembly that is attributed to strong interactions between carboxylic acid groups. Aggregation and spontaneous self-assembly can be minimized by deprotonation of the carboxylic acid groups but all octa-acid TAPs become water soluble when more than four carboxylic acid groups are deprotonated at a pH higher than 8. Cu(II) chelates of the octa-acid TAPs were also prepared but are unsuitable for Langmuir monolayer formation because of insufficient solubility in spreading solvents. Langmuir and LB monolayers of octa-hydroxy TAPs with propyl and undecyl spacers were also studied for comparison. An edge-on orientation of the macrocycle is obtained at higher surface pressures for both TAPs, although a flat-on orientation of the macrocycle in a spider-like confirmation may be present at low surface pressure for the derivative with undecyl spacers. No mesophases are formed by any of the octa-acid and -hydroxy TAPs as confirmed by polarized optical microscopy and thermal analysis measurements. In fact, of the octa-acid TAPs, only the derivative with the longest spacer (decyl) melts at a temperature below the onset of thermal decomposition.