Abstract

Three series of ionic self-assembled materials based on anionic azo-dyes and cationic benzalkonium surfactants were synthesized and thin films were prepared by spin-casting. These thin films appear isotropic when investigated with polarized optical microscopy, although they are highly anisotropic. Here, three series of homologous materials were studied to rationalize this observation. Investigating thin films of ordered molecular materials relies to a large extent on advanced experimental methods and large research infrastructure. A statement that in particular is true for thin films with nanoscopic order, where X-ray reflectometry, X-ray and neutron scattering, electron microscopy and atom force microscopy (AFM) has to be used to elucidate film morphology and the underlying molecular structure. Here, the thin films were investigated using AFM, optical microscopy and polarized absorption spectroscopy. It was shown that by using numerical method for treating the polarized absorption spectroscopy data, the molecular structure can be elucidated. Further, it was shown that polarized optical spectroscopy is a general tool that allows determination of the molecular order in thin films. Finally, it was found that full control of thermal history and rigorous control of the ionic self-assembly conditions are required to reproducibly make these materials of high nanoscopic order. Similarly, the conditions for spin-casting are shown to be determining for the overall thin film morphology, while molecular order is maintained.

Highlights

  • To build a device from functional molecular materials, one has to rely on the materials to: (i) crystallize in a structure suitable for the fabrication of devices [1]; (ii) appropriately self-assemble or self-organize on the device substrate [2,3,4,5,6]; or you have to (iii) develop a tailor made processing platform such as the zone casting method that can order pentacene and hexabenzocoronene on substrates [7,8,9,10,11,12]

  • Building on the seminal work of Faul and Antonietti [24,36], we have developed a ionic self-assembly (ISA) system based on benzalkonium surfactants BZK that allow for facile formation of ordered materials and thin films upon ISA with polyanionic functional units [37]

  • SynthIoesniisc self-assembled (ISA) materials are often made by mixing a solution of a functional building block with a solution of a suitable surfactant, and collecting the precipitating nanomateriallf[-2a2s,s2e3m,25b,l3e7d]. (IInSAth)emparotecreisaslsofarpereopfaterinngmmadaetebriyalms firxoimngbaensozalulktoionniuomf asufrufnaccttaiontnsaBl ZbuKilding blockwiwthitthailalesnogltuhtsiofrnomoftean stoueitiagbhlteeesnu, ir.fea. cBtZaKnt1,0–aBnZdKt1h8e,nwecoolblesecrtvinedg tthhaet tphreepcriepciitpaittiantigonnkainnoetmicsaterial

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Summary

Introduction

To build a device from functional molecular materials, one has to rely on the materials to: (i) crystallize in a structure suitable for the fabrication of devices [1]; (ii) appropriately self-assemble or self-organize on the device substrate [2,3,4,5,6]; or you have to (iii) develop a tailor made processing platform such as the zone casting method that can order pentacene and hexabenzocoronene on substrates [7,8,9,10,11,12]. ISA happens when two solutions of a positively and a negatively charged water soluble building blocks are mixed. By choosing building blocks of the correct shape, lamellar structures can be precipitated directly following the ISA process [36]

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