Synthesis, thermal stability and photoresponsive behaviors of azobenzene-tethered polyhedral oligomeric silsesquioxanes

Abstract

A series of azobenzene-tethered polyhedral oligomeric silsesquioxane (POSS) derivatives, i.e.monoazobenzene-substituted POSS (MonoAzo-POSS), bisazobenzene-substituted POSS (BisAzo-POSS) and triazobenzene-substituted POSS (TriAzo-POSS), were synthesized through the amidation acidylation of aminopropylisobutyl POSS and benzoic acid derivatives (AzoMs) with one, two and three azobenzene groups (AzoM1, AzoM2 and AzoM3). Their structures were characterized by FT-IR, ¹H NMR, ¹³C NMR and mass spectra, and their thermal stability and photoresponsive behaviors in DMF solutions were evaluated with TGA, XRD and UV-vis spectra, respectively. The results indicated that the thermal stability and photoisomerization of azobenzenes could be effectively controlled by their molecular structure. In MonoAzo-POSS, the large steric hindrance of POSS destroys the molecular ordering and limits the molecular packing, contributing to its poor thermal stability. And the low molecular ordering of MonoAzo-POSS offers an azo group with large free space, and its trans–cisphotoisomerization rate increases accordingly. But, in BisAzo-POSS and TriAzo-POSS, the incorporation of POSS units does not impact on the regularity of azobenzenes obviously, and the hindrance effect of nanosize POSS on the molecular motion plays a primary role in increasing their high thermal stability. Their photoisomerization rates decrease due to the steric hindrance of POSS and the unfolding structure of the azo moieties in BisAzo-POSS and TriAzo-POSS.