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 1 - 4 ). The as-prepared azo-Keggins showed the general molecular formula of [C 16 H 36 N] 4 [SiW 11 O 40 (Si(CH 2 ) 3 NH-CO(CH 2 ) n O-C 6 H 4 N=NC 6 H 4 -R) 2 ] (R = H, n = 0 ( 1 ); R = NO 2 , 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 Å 2 to 6.91 Å 2 . 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.