Optical Switching of Azophenol Derivatives in Solution and in Polymer Thin Films: The Role of Chemical Substitution and Environment

Design of polymer materials whose properties can be reversibly changed by illumination with light is a technology of particular scientific interest. Such materials contain molecular chromophors, which change their geometry and/or polarity upon absorption of light of a specific wavelength. The most prominent chromophores are azobenzene derivatives. Here, we present a systematic study on azobenzene derivatives in order to quantify the impact of chemical substitution and chemical environment on the dynamics of light-induced trans-cis isomerization (at 368 nm and 355 nm), thermal cis-trans relaxation, and light-induced cis-trans isomerization (at 434 nm). Systems under investigation were 4-hydroxyazobenzene (4-HAB) in acetonitrile (MeCN) solution and in a poly(methylmethacrylate) (PMMA) matrix. These two systems are compared to systems in which 4-HAB is esterified, namely 4-hydroxyazobenzene covalently bound (esterified) to PMMA matrix, and N-(tert-butoxycarbonyl)glycine-4- hydroxyazobenzene (Boc-Gly-4-HAB) in MeCN and in PMMA. Photoisomerization and thermal relaxation kinetics are monitored with UV-vis absorption spectroscopy and accompanied by quantum chemical calculations to shed light into the molecular origin of observed differences in switching properties. We find that the chemical environment (MeCN vs. PMMA) only has minor impacts (~10%) on trans to cis photoisomerization rates. Also, the impact of chemical environment on thermal cis to trans relaxation is small; with relaxation rates in PMMA beeing 400) than the rates of esterified systems. This difference is a clear result of the different substituents on the azobenzene moiety. Quantum chemical calculations suggest that the cis-configuration in the esterified systems is stabilized by an intramolecular H-bond between a carbonyl oxygen on the substituent and an H atom on the phenyl ring. In all systems, the cis to trans isomerization can be significantly accelerated by illumination with 434 nm light. For esterified systems, accelerations by factors of about 5700 – 15500 are observed. In the case of 4-hydroxyazobenzene covalently bound (esterified) to the PMMA matrix, complete light induced transfer from cis to trans is possible. In addition, it features a low thermal cis to trans isomerization rate and acceptable photoinduced trans to cis isomerization properties. With this, the material fulfills the basic requirements of a functional polymer material whose properties can be reversibly changed by illumination with light.