Detecting the Subtle Photo-Responsive Conformational Bistability of Monomeric Azobenzene Functionalized Keggin Polyoxometalates by Using Ion-Mobility Mass Spectrometry.

Accurately characterizing the conformational variation of novel molecular assemblies is important but often ignored due to limited characterization methods. Herein, we reported the use of ion-mobility mass spectrometry (IMS/MS) to investigate the conformational changes of four azobenzene covalently functionalized Keggin hybrids (azo-Keggins, compounds -). The as-prepared azo-Keggins showed the general molecular formula of [CHN][SiWO(Si(CH)NH-CO(CH)O-CHN=NCH-R)] (R = H, = 0 (); R = NO, = 0 (); R = H, = 5 (); R = H, = 10 ()). The resultant azo-Keggins maintained stable monomeric states in the gas phase with intact molecular structures. Furthermore, the subtle photo-responsive trans-cis conformational variations of azo-Keggins were clearly revealed by the molecular shape-related collision cross-section value difference ranging from 2.44 Å to 6.91 Å. The longer the alkyl chains linkers were, the larger the conformational variation was. Moreover, for compounds and , higher stability in -conformation can be observed, while for compounds and , bistability can be achieved for both of them.