Electronic structure modulation of metal-free graphdiyne for acidic oxygen evolution reaction

Developing high-performance metal-free electrocatalysts for acidic oxygen evolution reaction (AOER) is highly desirable but remains great challenge. Here we report a rationally substituting sp-C strategy for the synthesis of methyl- and hydrogen-substituted graphdiyne (MGDY, HGDY) nanowires arrays as 3D porous flexible metal-free electrodes for AOER. Methyl group in MGDY with stronger electron-pushing effect makes electrons around the acetylenic carbon atoms more delocalized, resulting in more uneven distributed surface charge, and higher intrinsic catalytic activities for AOER than HGDY, with the smaller overpotential of 406 mV at 10 mA cm−2 than HGDY and previously reported metal-free electrocatalysts. Our results reveal that the modulation of the electronic structure of GDY by selectively substituting sp-C allows for facilitating charge transfer kinetics, improving adsorption of reaction intermediate, and thereby accelerating the sluggish kinetics of AOER. This work provides us an ideal opportunity for studying the exact HER/OER mechanisms of metal-free carbon materials.

Triplet Dynamic Nuclear Polarization of Guest Molecules through Induced Fit in a Flexible Metal-Organic Framework

Dynamic nuclear polarization utilizing photoexcited triplet electrons (triplet-DNP) has great potential for room-temperature hyperpolarization of nuclear spins. However, the polarization transfer to molecules of interest remains a challenge due to the fast spin relaxation and weak interaction with target molecules at room temperature in conventional host materials. Here, we demonstrate the first example of DNP of guest molecules in a porous material at around room temperature by utilizing the induced-fit-type structural transformation of a crystalline yet flexible metal-organic framework (MOF). In contrast to the usual hosts, 1H spin-lattice relaxation time becomes longer by accommodating a pharmaceutical model target 5-fluorouracil as the flexible MOF changes its structure upon guest accommodation to maximize the host-guest interactions. Combined with triplet-DNP and cross-polarization, this system realizes an enhanced 19F-NMR signal of guest target molecules at around room temperature.