The doctoral dissertations of the former Helsinki University of Technology (TKK) and Aalto University Schools of Technology (CHEM, ELEC, ENG, SCI) published in electronic format are available in the electronic publications archive of Aalto University - Aaltodoc.
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Dissertation for the degree of Doctor of Science in Technology to be presented with due permission of the Faculty of Information and Natural Sciences for public examination and debate in Auditorium F239a at Helsinki University of Technology (Espoo, Finland) on the 30th of October, 2009, at 12 noon.
Overview in PDF format (ISBN 978-952-248-101-6) [2214 KB]
Dissertation is also available in print (ISBN 978-952-248-100-9)
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.
This thesis consists of an overview and of the following 5 publications:
Keywords: azobenzene, aggregation, hydrogen bonding, photoinduced anisotropy, surface-relief grating
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© 2009 Helsinki University of Technology