Surface-Relief Gratings in Halogen-Bonded Polymer-Azobenzene Complexes: A Concentration-Dependence Study.

Jelle Stumpel,Pierangelo Metrangolo,Arri Priimagi,Marco Saccone,Ossi Lehtonen,Valentina Dichiarante,Matti Virkki

doi:10.3390/molecules22111844

Jelle Stumpel, Pierangelo Metrangolo + Show 5 more

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https://doi.org/10.3390/molecules22111844

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Abstract

In recent years, supramolecular complexes comprising a poly(4-vinylpyridine) backbone and azobenzene-based halogen bond donors have emerged as a promising class of materials for the inscription of light-induced surface-relief gratings (SRGs). The studies up to date have focused on building supramolecular hierarchies, i.e., optimizing the polymer–azobenzene noncovalent interaction for efficient surface patterning. They have been conducted using systems with relatively low azobenzene content, and little is known about the concentration dependence of SRG formation in halogen-bonded polymer–azobenzene complexes. Herein, we bridge this gap, and study the concentration dependence of SRG formation using two halogen-bond-donating azobenzene derivatives, one functionalized with a tetrafluoroiodophenyl and the other with an iodoethynylphenyl group. Both have been previously identified as efficient molecules in driving the SRG formation. We cover a broad concentration range, starting from 10 mol % azobenzene content and going all the way up to equimolar degree of complexation. The complexes are studied as spin-coated thin films, and analyzed by optical microscopy, atomic force microscopy, and optical diffraction arising during the SRG formation. We obtained diffraction efficiencies as high as 35%, and modulation depths close to 400 nm, which are significantly higher than the values previously reported for halogen-bonded polymer–azobenzene complexes.

Highlights

Photoinduced surface patterning is considered to be among the most important macroscopic effects triggered by the photoisomerization reaction in azobenzene-containing polymers [1]
1, appeared as promising for driving we extend studies effect the of concentration on surface-relief gratings (SRGs) formation in efficiency halogenHerein, we our extend our towards studies the towards effect of concentration on SRGefficiency formation bonded supramolecular polymers, an important step towards optimizing the material composition in halogen-bonded supramolecular polymers, an important step towards optimizing the material for SRG-forming supramolecular systems, andsystems, in gaining knowledge on the structure
We have studied the SRG formation in halogen-bonded polymer–azobenzene complexes as a function of complexation degree

Summary

Introduction

Photoinduced surface patterning is considered to be among the most important macroscopic effects triggered by the photoisomerization reaction in azobenzene-containing polymers [1]. It dates back to 1995 when it was observed that, upon irradiating an azopolymer thin film with an optical interference pattern, the polymer starts to migrate and forms a replica of the incident irradiation pattern on the polymer surface in the form of a surface-relief grating (SRG) [2,3] Once inscribed, these diffraction gratings are temporally stable at room temperature, yet can be erased either thermally or Molecules 2017, 22, 1844; doi:10.3390/molecules22111844 www.mdpi.com/journal/molecules fabrication, the SRGs show potential for a plethora of applications in photonics and nanotechnology [6,7]. It is well established that, in polymeric systems, efficient the mechanism and structure-performance relations behind these light-induced macroscopic motions are SRG formation requires specific bonding interactions between the photoactive azobenzene units and not yet perfectly understood. This can take place either through covalent bonding, as in the case of, e.g., poly(disperse ionic bonding [14,15,16], hydrogen bonding [13,17,18], or—as in the case studied in the present paper—

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