**4. Conclusions**

In conclusion, four azo-Keggins macromolecules with hybrid structures in which the photo-responsive azo moieties with different lengths of alkyl linkers and substituents were grafted onto the SiW11 clusters were applied to investigate their photo-responsive conformational variation properties. ESI-MS of compounds **1**–**4** showed that all fragment ions can be well assigned without observing any similar overlapping fashion in each mass envelopes, suggesting that the monomeric state of these azo-Keggins was intact both in the solution and in the gas phase. The 2D IMS/MS spectra further indicated the high stability of the monomeric state which presented as the main peak envelops. Moreover, with the reversible UV and Vis photoirradiation, the subtle conformational change of these azo-Keggins resulted in different CCS values with a higher resolution of 2.44 Å2. Compared with the previously reported data for large assemblies of azo-Andersons, the conformational variation results of monomeric azo-Keggins could more precisely indicate the effect of each group on the shape and stability of isomers: (1) The longer the linker's length was, the greater the conformational variation. (2) Conformational bistability was achieved for compounds **3** and **4**, while for compounds 1 and 2 with electron-withdrawing substituents, higher stability in the *trans* conformation was observed. This work opened up a crucial new characterization dimension for which information on size, shape, charge, mass, and variation can be revealed. Providing deep insight into the shape changes of POMs into areas of self-assembly, catalysis, and responsive behavior from external stimulation such as photo, thermal, magnetic, etc., was helpful.

**Supplementary Materials:** The following supporting information can be downloaded at: https://www. mdpi.com/article/10.3390/molecules27123927/s1, Figures S1–S4: ESI-MS spectrum of compounds **1**–**4** and notable peaks assignment. Figures S5, S7, S9, and S11: 2D IMS/MS spectrum of compounds **1**–**4**

before UV-irradiation. Figures S6, S8, S10 and S12: 2D IMS/MS spectrum of compound **1**–**4** after UV-irradiation. Figures S13–S16: Drift time of compounds [**X1**–**<sup>4</sup>** + 2TBA]2<sup>−</sup> before and after UVirradiation. Tables S1–S4: Change in drift time and CCS of monomeric compound [**X1**–**<sup>4</sup>** + 2TBA]2<sup>−</sup> when switching between the *trans* and the *cis* conformation of the azo bond. Synthesis method of organic compounds used for the synthesis of compounds **1** and **2**. Figures S17–S19: FT-IR, 1H NMR, and 13C NMR spectra of compounds **1** and **2**. Figure S20: (a) 1H NMR spectra of compound **2** in *d*3- CH3CN before (bottom) and after (top) UV irradiation at 365 nm. (b) UV/Vis spectra of compound **2** in CH3CN. (c) The kinetic curve of UV absorbance with increasing time. (d) UV absorbance at 343 nm upon alternating UV irradiation at 365 nm and visible light for five cycles. Figure S21: The models of the azo and nitro-azo groups with *trans-* and *cis-*conformations. The models were constructed by ChemBio3D Software, and the shape difference was roughly calculated by atom distance.

**Author Contributions:** Conceptualization, W.C. and Y.-F.S.; methodology B.Q. and L.J.; validation, B.Q. and S.A.; formal analysis, S.A. and L.J.; investigation, W.C. and B.Q.; writing—original draft preparation, B.Q.; writing—review and editing, Y.-F.S.; visualization, B.Q. and Y.-F.S.; supervision, W.C. and Y.-F.S.; project administration, W.C. and Y.-F.S.; funding acquisition, Y.-F.S. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by Beijing Natural Science Foundation (2202039), the National Nature Science Foundation of China (22101017, 22178019, 21625101, 21808011, and 21521005), the National Key Research and Development Program of China (2017YFB0307303), and the Fundamental Research Funds for the Central Universities (XK1802-6, XK1803-05, XK1902, and 12060093063).

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Conflicts of Interest:** The authors declare no conflict of interest.

**Sample Availability:** Samples of the compounds are available from the authors.
