Monitoring adsorption induced structural transformations of MOFs at a molecular level by in situ EPR spectroscopy

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Among flexible coordination polymers switchable MOFs are a subclass of porous materials with fascinating properties such as pressure gating, responsiveness, and selective molecular recogni-tion. The understanding and ultimately the control of these properties in gate opening processes are important objectives in MOF research today. A particular interesting functionality of responsive flex-ible MOFs is the selective adsorption from gaseous or liquid mixtures. A number of open questions needs to be addressed in this context such as what are the specific properties of gases or liquids in a mixture that are responsible for the switching behaviour (QP4, QF1), are there preferred adsorp-tion sites for the active component (QP4), what is the influence of the inactive component in a mix-ture on the gating process (QF1), does the inactive component enter the pores and if yes, how is it distributed across the pore system (QF1), and finally, how do mesoscopic parameters for instance in core/shell systems composed of flexible and rigid frameworks modulate the switching behaviour of the flexible component (QP3). Continuous wave (cw) and pulsed electron paramagnetic reso-nance (EPR) spectroscopy can be used to characterize and explore the switching of the MOF framework and the properties of the adsorbed molecules on a local molecular level if suitable par-amagnetic probes are available. We will employ paramagnetic framework ions (Cu2+, Mn2+) and nitroxide radicals (TEMPO) located in the pores as local probes to investigate gating processes and selective molecular recognition for the adsorption of gaseous and liquid mixtures (QF1, QF2) over the flexible MOF materials DUT-8(Ni) (S2), DUT-128(Ni) (S2), and [M11-xM2x(fu1-bdc′)2-y(fu2-bdc″)yP]n (S1). In cw in situ EPR adsorption experiments of mixtures ( QP2) the paramagnetic ions will provide information about the structural changes of the framework at the paddle wheel units caused by the gating process, whereas the tumbling motion of the TEMPO radicals will serve as a sensitive indicator for the opening and closing of the pores (T2). Cu2+ and Mn2+ spin probes are also used to study the adsorption-stimulated gating for mesoscopic core/shell systems (QP3) composed of switchable and rigid DUT-8(Ni)/DUT-8(Zn) derivatives (S2). Besides cw EPR, HYSCORE and pulsed ENDOR spectroscopy, supported by quantum chemical calculations (T1), will be applied to investigate the distribution of the two components, active and inactive in the switching process, of binary gas and liquid mixtures across the pores using local paramagnetic probes.