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

**Bo Qi, Luran Jiang, Sai An, Wei Chen \* and Yu-Fei Song \***

State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029, China; bqi@mail.buct.edu.cn (B.Q.); lr\_jiang2021@163.com (L.J.); ansai@mail.buct.edu.cn (S.A.) **\*** Correspondence: chenw@mail.buct.edu.cn (W.C.); songyf@mail.buct.edu.cn (Y.-F.S.)

**Abstract:** 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 **1**–**4**). The asprepared azo-Keggins showed the general molecular formula of [C16H36N]4[SiW11O40(Si(CH2)3NH– CO(CH2)nO–C6H4N=NC6H4–R)2] (R = H, *n* =0(**1**); R = NO2, *n* =0(**2**); R = H, *n* =5(**3**); R=H, *n* = 10 (**4**)). 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 Å2 to 6.91 Å2. The longer the alkyl chains linkers were, the larger the conformational variation was. Moreover, for compounds **1** and **2**, higher stability in *trans*conformation can be observed, while for compounds **3** and **4**, bistability can be achieved for both of them.

**Keywords:** polyoxometalates; azobenzene; photo-responsive; ion-mobility mass spectrometry; shape characterization
