**1. Introduction**

Azobenzene (Azo) and its derivatives have been widely investigated as promising photochemical systems, owing to their properties of reversible *trans-cis* isomerization upon light irradiation, which further results in reversibly changed physical and optical properties such as the solubility, dipole moment, surface free energy, and mechanical actuation behaviors [1–6]. By attaching azo groups, photochemical properties could be introduced to new functional materials [7]. In recent years, polyoxometalates (POMs), a class of anionic molecular metal oxide clusters with intriguing physical properties and nanometer size [8–14], have been applied to combine with azo groups. The resultant azobased polyoxometalates (azo-POMs) have shown fascinating photo-responsive behaviors and have been applied for to a broad area, such as lyotropic or thermotropic liquid crystal, supramolecular self-assembly, and catalysis [15–19]. Different from small molecular azo, macromolecular azo-POMs show relatively complicated physical properties because of the introduction of more parameters such as large molecular size, multiple charges, steric hindrance effect, electronic effect, etc. [20]. Although the reversible *trans-cis* conformational change of molecules was studied, the detection means were still limited to UV-Vis or NMR spectroscopy [19,21]. Moreover, the shape's variation was inevitably involved during conformational changes; however, this factor is often ignored. Therefore, it is important to develop novel characterization methods for obtaining more structural information about these azo-POM macromolecules.

**Citation:** Qi, B.; Jiang, L.; An, S.; Chen, W.; Song, Y.-F. Detecting the Subtle Photo-Responsive Conformational Bistability of Monomeric Azobenzene Functionalized Keggin Polyoxometalates by Using Ion-Mobility Mass Spectrometry. *Molecules* **2022**, *27*, 3927. https:// doi.org/10.3390/molecules27123927

Academic Editor: Xiaobing Cui

Received: 25 May 2022 Accepted: 16 June 2022 Published: 19 June 2022

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The technique of ion mobility mass spectrometry (IMS/MS) has been demonstrated to be useful for larger and more complex biological molecules as early as the 1990s [22–25]. With the development of ion mobility separation technology and high-resolution electrospray ionization mass spectrometry systems, the applications of IMS/MS were expanded to the investigation of polymers, proteomics, protein digest mixture, and carbon clusters as well as the supramolecular assembly of POMs [26–32]. By coupling MS instruments that separate ions on the basis of mass-to-charge ratio and IMS instruments that separate ions based on size-to-charge ratio, a two-dimensional mobility-mass spectrum could be obtained with information on size, charge, shape, and mass dimensions. In particular, the isomers or conformers have different shapes while very similar mass information can be separated in the mobility space [18,29]. During the past five years, the conformational variation of azobenzene was studied by using IMS/MS. However, most related studies were focused on the organic azo-based small molecules with CCS values less than 277 Å2 [33,34]. In contrast, few works were reported for investigating the azo-based large macromolecules, which may bring a more complex while fascinating isomerization behavior. Therefore, it should be interesting to study azo-based POMs by using IMS/MS, due to their 1–10 nm molecular sizes, multiple negative charges, and redox properties, which were quite different with small azo molecules.

To best of our knowledge, the only work for studying the azo-POMs by IMS/MS was reported by Song, Cronin, and co-workers [18]. Taking the advantage of IMS/MS, the high CCS value of 600 Å<sup>2</sup> and the bistability conformational variations of azo-modified Mn-Anderson POMs (denoted as azo-Andersons) were observed. A significant difference in the collision cross-section (CCS) value of the oligomeric compounds was observed between the *trans*- and *cis*-conformation of the azo groups. However, these oligomeric structures involved the influence of intermolecular arrangement, which may overestimate shape differences caused by the isomerization of the single molecule itself. Therefore, applying the novel azo-POM systems with relatively stable monomeric molecules is crucial for detecting the bistability nature of azo-based macromolecules with the more subtle and intriguing reversible variation. Moreover, it was also intriguing to develop the substituents effect on the stability of *trans-* and *cis-*isomers.
