Metal ion mediated mesomorphism and thin film behaviour of amphitropic tetraazaporphyrin complexes

Abstract

Octa-substituted tetraazaporphyrins with amphiphilic 3,6-dioxaheptylthio and 3,6,9-trioxadecylthio chains and their metal complexes (Co(II), Ni(II), Cu(II), Zn(II)) were synthesised in order to compare their thermotropic and lyotropic mesomorphism as well as their thin film properties with related tetraazaporphyrins, phthalocyanines and triphenylenes, which have been reported previously. The Co, Ni, and Cu complexes melted into hexagonal columnar mesophases, whereas the Zn(II) complexes and the free-base porphyrins did not display mesomorphism. In fact, the zinc complexes did not crystallise on cooling, rather giving highly viscous isotropic oils at room temperature that solidified below 250 ‡C to glasses. This unusual behaviour may be caused by axial interactions between the zinc ion and the oxygen of the polyether groups. All 3,6,9-trioxadecylthio substituted derivatives solidified far below 0 ‡C giving rise to columnar hexagonal mesophases at room temperature for the Co(II), Ni(II), and Cu(II) complexes. The columnar hexagonal mesophases of the long chain derivatives spontaneously align homeotropically when sandwiched between two substrates, in contrast to the short chain analogues. Both the long chain and the short chain Co(II) complexes displayed a rather disordered hexagonal columnar packing as revealed by X-ray diffraction. Binary mixtures of the short chain derivatives with non-polar or polar organic solvents did not display additional lyotropic mesophases. However, the transition temperatures and enthalpies, as well as the texture of the hexagonal columnar mesophase were affected, assuming a solubility of the solvent in the thermotropic mesophase as shown for the Cu(II) derivative. All long chain derivatives were also soluble in water but, again, lyotropic mesomorphism was observed neither with water nor with organic solvents. The film forming properties of some derivatives were investigated by spin-coating and the Langmuir-Blodgett (LB) technique. Homogeneous films of the mesomorphic short chain copper complex were obtained by the spin-coating method. The films displayed a layer structure with edge-on orientation at ambient temperature, although they were of crystalline morphology, as shown by X-ray reflectivity measurements and polarised UV/VIS spectroscopy. Only monolayer films of short and long chain derivatives could be transferred by the LB method. Again, determination of the thickness of the monolayer by X-ray agreed with an edge-on orientation of the tetraazaporphyrins. In contrast, the extrapolated molecular areas on the water surface suggested a flat-on orientation of the macrocycle with the oligo(oxyethylene) chains being dissolved in the sub-phase.

Efficient Light‐Harvesting Layers of Homeotropically Aligned Porphyrin Derivatives

“…To the knowledge of the authors, homeotropic alignment of porphyrin derivatives has only been reported in one previous study. [27] However, in the work of Eichhorn et al [27] the homeotropic alignment was not uniformly present over the entire layer and was not achieved spontaneously upon film deposition. The present study is the first to show a spontaneous uniform homeotropic molecular alignment for a porphyrin derivative.…”

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

“…Most solution based attempts of generating mono-and multi-layers of Pcs or TAPs with a face-on orientation of their macrocycles have relied on the LB technique 12,13 although some studies based on self-assembled monolayers have also been reported. 14 A at-on anchoring of the rather hydrophobic Pc macrocycle requires the attachment of strongly polar groups either to the centre or the perimeter of the macrocycle to overcome the strong co-facial interactions between Pcs.…”

“…The only attempt of preparing TAP LB monolayers with at-on orientation of the macrocycle we are aware of was based on oligo(ethylene oxide) substituted TAPs. 13 Their copper chelates form stable Langmuir monolayers at the air-water interface and the macrocycle is likely oriented at-on with the oligo-ether groups being dissolved in the subphase. However, transfer onto silicon wafers produces LB monolayers of edge-on orientation.…”

“…All TAPs are characterized by IR, UV-Vis spectroscopy, elemental analysis, and mass spectrometry. The metal-free TAPs 6-8 and C3OH are also characterized by 1 H and 13 C NMR, whereas TAPs C3COOH, C5COOH, C10COOH, and C11OH are studied only by 1 H NMR because of their lower solubility. Crystals of the metal-free and copper metallated octa-acid TAPs contain 2-4 water molecules as conrmed by elemental analysis and TGA (see below).…”

“…A higher solubility is commonly found for metallated TAPs and similar macrocycles when compared to their metal-free analogues because of axial interactions between the chelated metal ion and solvent molecules. 13 The low solubility of CuC3,5,10COOH may be the result of axial interactions between Cu(II) and carboxylic acid groups that generate networks of low solubility.…”