Polymer-azobenzene complexes: from supramolecular concepts to efficient photoresponsive polymers

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Doctoral thesis (article-based)
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TKK dissertations, 183
Linear and nonlinear optical properties of azobenzene-containing polymers give rise to a range of exciting optical phenomena due to the unique response of the azobenzene chromophores to light fields. The light-induced motions of the azobenzene molecules can be used to inscribe large and stable in-plane anisotropy, nonlinear optical response, and surface-relief structures into the material system, showing great potential for various photonics applications. This thesis deals with a common drawback often encountered in amorphous azobenzene-containing polymers: when the chromophores are dissolved in the polymer matrix, they tend to form aggregates even at moderate doping levels, which is generally detrimental to the optical response of the system. Aggregation can be suppressed by covalently attaching the chromophores to the polymer backbone, which improves the optical performance but requires a laborious organic synthesis process for each polymer-chromophore combination. We have addressed this issue by using supramolecular concepts: instead of covalent functionalization we use spontaneous non-covalent interactions to attach the chromophores to the polymer backbone to form polymer-azobenzene complexes. In the thesis we systematically explore the connection between the aggregation of strongly dipolar chromophores and the photoresponse of the material system. In particular, we show that the aggregation can be suppressed by properly choosing the polymer matrix. Moreover, by using phenol-pyridine hydrogen bonding to attach the chromophores to the polymer backbone, the chromophore content can be increased up to the point where each polymer repeat unit is occupied, yielding photoresponsive polymers with (i) high and stable photoinduced anisotropy and (ii) efficient formation of photoinduced surface-relief gratings. Hydrogen bonding makes a difference, and due to the generality of the supramolecular concepts, we anticipate that the proposed method provides a pathway to enhancing a wide range of optical phenomena where chromophore aggregation or phase separation is a limiting factor for the system performance.
azobenzene, aggregation, hydrogen bonding, photoinduced anisotropy, surface-relief grating
Other note
  • [Publication 1]: Arri Priimagi, Stefano Cattaneo, Robin H. A. Ras, Sami Valkama, Olli Ikkala, and Martti Kauranen. 2005. Polymer–dye complexes: a facile method for high doping level and aggregation control of dye molecules. Chemistry of Materials, volume 17, number 23, pages 5798-5802.
  • [Publication 2]: Arri Priimagi, Matti Kaivola, Francisco J. Rodriguez, and Martti Kauranen. 2007. Enhanced photoinduced birefringence in polymer-dye complexes: Hydrogen bonding makes a difference. Applied Physics Letters, volume 90, number 12, 121103.
  • [Publication 3]: Arri Priimagi, Jaana Vapaavuori, Francisco J. Rodriguez, Charl F. J. Faul, Markku T. Heino, Olli Ikkala, Martti Kauranen, and Matti Kaivola. 2008. Hydrogen-bonded polymer–azobenzene complexes: enhanced photoinduced birefringence with high temporal stability through interplay of intermolecular interactions. Chemistry of Materials, volume 20, number 20, pages 6358-6363.
  • [Publication 4]: Arri Priimagi, Klas Lindfors, Matti Kaivola, and Paul Rochon. 2009. Efficient surface-relief gratings in hydrogen-bonded polymer–azobenzene complexes. ACS Applied Materials & Interfaces, volume 1, number 6, pages 1183-1189.
  • [Publication 5]: Arri Priimagi, Stefano Cattaneo, and Martti Kauranen. 2006. Real-time monitoring of all-optical poling by two-beam second-harmonic generation. Optics Letters, volume 31, number 14, pages 2178-2180.