Photoisomerization of azobenzene and its substituted derivatives: the effect of protonation

Meral Ari,1 S. Kalthoum,1 and Michael Walter1,2
1Institute of Physics, University of Freiburg, Hermann-Herder-Str. 3, 79104 Freiburg, Germany
2Fraunhofer IWM, MikroTribologie Centrum TC, Wöhlerstrasse 11, D-79108 Freiburg, Germany

One of the most used organic chromophores for optical switching applications are azobenzenes. Azobenzenes exhibit a reversible isomerisation process between its trans and cis isomers of different stability. After a photochemical conversion, the spontaneous thermal back reaction occurs. While the photoinduced trans-to-cis isomerisation reaction can be performed in a few femtoseconds, the rate of the thermal cis-to-trans back reaction depends greatly on the chemical architecture of the system and the environment [1]. While slow thermally back-isomerising azoderivatives are valuable photoactive basic materials for information storage purposes, transmitting at the molecular scale with response times within the nanosecond or picosecond range will enable the potential application of azobenzene-based materials in micropumps and autonomous valves. Novel absorption spectroscopy experiments have shown that the rate of thermal back-isomerization is strongly dependent on pH [2]. With the purpose of understanding the mechanism underlying the isomerization and the changes in reaction paths and energy barriers due to protonation, in collaboration with the group of Andreas Walther from the Institute of Macromolecular Chemistry in Freiburg, we investigated the protonation sites of an azo-derivative and computationally explored the relative energy profiles for the isomerization. The effect of different substitution patterns is investigated also by substitution of a methoxy group.

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  2. N. J. Dunn et al., J. Phys. Chem. A 113, 13144 (2009). doi:10.1021/jp903102u