Efficient reversible photoisomerisation with large solvodynamic size-switching of a main chain poly(azobenzene-alt-trisiloxane)

Abstract

A photo-responsive linear poly(azobenzene) with an alternating sequence of azobenzene switching units connected by trisiloxane linkers was synthesised via step-growth polycondensation. By its design, the polymer features a high chromophore load (45 wt%) and a large number (≈130) of photoswitching units per chain. The ensuing photoisomerisation upon UV irradiation with 365 nm light was studied in several solvents and compared to the corresponding free azobenzene siloxane congener. Efficient E → Z isomerisation with high conversion yields was observed for the polymeric and isolated azobenzenes alike. In the slightly polar solvents tetrahydrofuran (THF) and chloroform, this resulted in a photo-induced decrease in solvodynamic volume of 22% (THF) and 32% (chloroform). In n-hexane, however, a light-triggered collapse of the solvated polymer coil lead to a globular non-solvated form with a 70% decrease in volume. Ultrafast transient electronic absorption measurements showed that the underlying excited-state reaction pathways for the azobenzene units in the polymer main chain are essentially similar to those of the analogous single azobenzene in solution. The data suggest a sequential deactivation via ultrafast internal conversion from the initially excited S₂ state to the S₁ state, followed by intramolecular vibrational redistribution in the S₁ state, and subsequent E → Z isomerisation and deactivation to the electronic ground state. These processes are slowed down only moderately for the main chain azobenzene units compared to the free azobenzene in solution and conform to the accepted scenario for unsubstituted azobenzene. The results explain the efficient, nearly unconstrained isomerisation of the embedded molecular switching units that is highly unusual for a main chain azobenzene polymer. Due to the unique combination of a high chromophore load and large number of photo-switchable units in the main chain in a regular alternating architecture, coupled with a high conversion yield, the poly(azobenzene-alt-trisiloxane) should find applications as photo-responsive material, in particular when large switching amplitudes are essential.