Assessing the Nanoscale Structure and Mechanical Properties of Polymer Monoliths used for Chromatography

Concerning polymeric monolithic materials utilized in separation science, the structural and mechanical characteristics from the nanoscopic to the macroscopic scale remain of great interest. Suitable analytical tools are urgently required to understand the polymer monolith’s constituent structure, particularly in the case of nanoscale polymer properties that tend to develop gel porosity in contact with a mobile phase ultimately affecting the chromatographic performance. Herein described are our first findings from a characterization of commercially available analytical polymer monoliths based on styrene/divinylbenzene and methacrylate chemistries utilizing confocal Raman spectroscopy imaging and atomic force microscopy (AFM). Confocal Raman spectroscopy can be used to generate a three-dimensional representation of monoliths in both dry state and in contact with solvent. AFM force–indentation measurements on individual cross-sectioned globular features permit detailed assessment of mechanical properties of the stationary phase. This approach allowed so far unprecedented insight and identification of a heterogeneous cross-link density distribution of polymer material within individual globular features on a submicrometer scale

Photocatalytic Reduction of Artificial and Natural Nucleotide Co-factors with a Chlorophyll-Like Tin-Dihydroporphyrin Sensitizer

An efficient photocatalytic two-electron reduction and protonation of nicotine amide adenine dinucleotide (NAD⁺), as well as the synthetic nucleotide co-factor analogue <i>N</i>-benzyl-3-carbamoyl-pyridinium (BNAD⁺), powered by photons in the long-wavelength region of visible light (λ_irr > 610 nm), is demonstrated for the first time. This functional artificial photosynthetic counterpart of the complete energy-trapping and solar-to-fuel conversion primary processes occurring in natural photosystem I (PS I) is achieved with a robust water-soluble tin­(IV) complex of <i>meso</i>-tetrakis­(<i>N</i>-methylpyridinium)-chlorin acting as the light-harvesting sensitizer (threshold wavelength of λ_thr = 660 nm). In buffered aqueous solution, this chlorophyll-like compound photocatalytically recycles a rhodium hydride complex of the type [Cp*Rh­(bpy)­H]⁺, which is able to mediate regioselective hydride transfer processes. Different one- and two-electron donors are tested for the reductive quenching of the irradiated tin complex to initiate the secondary dark reactions leading to nucleotide co-factor reduction. Very promising conversion efficiencies, quantum yields, and excellent photosensitizer stabilities are observed. As an example of a catalytic dark reaction utilizing the reduction equivalents of accumulated NADH, an enzymatic process for the selective transformation of aldehydes with alcohol dehydrogenase (ADH) coupled to the primary photoreactions of the system is also demonstrated. A tentative reaction mechanism for the transfer of two electrons and one proton from the reductively quenched tin chlorin sensitizer to the rhodium co-catalyst, acting as a reversible hydride carrier, is proposed