Structural Elucidation of Amorphous Photocatalytic Polymers from Dynamic Nuclear Polarization Enhanced Solid State NMR

Nick J Brownbill,Fabien Aussenac,Baltasar Bonillo,Andrew I Cooper,Frédéric Blanc,Shane Pawsey,Alistair J Fielding,Reiner Sebastian Sprick

doi:10.1021/acs.macromol.7b02544

Abstract

Dynamic nuclear polarization (DNP) solid-state nuclear magnetic resonance (NMR) offers a recent approach to dramatically enhance NMR signals and has enabled detailed structural information to be obtained in a series of amorphous photocatalytic copolymers of alternating pyrene and benzene monomer units, the structures of which cannot be reliably established by other spectroscopic or analytical techniques. Large 13C cross-polarization (CP) magic angle spinning (MAS) signal enhancements were obtained at high magnetic fields (9.4–14.1 T) and low temperature (110–120 K), permitting the acquisition of a 13C INADEQUATE spectrum at natural abundance and facilitating complete spectral assignments, including when small amounts of specific monomers are present. The high 13C signal-to-noise ratios obtained are harnessed to record quantitative multiple contact CP NMR data, used to determine the polymers’ composition. This correlates well with the putative pyrene:benzene stoichiometry from the monomer feed ratio, enabl...

Highlights

Organic polymeric photocatalysts that generate hydrogen from water under electromagnetic irradiation have become an area of great interest in materials chemistry in recent years due to their potential greater diversity than classic metal based photocatalysts.[1−3] Organic photocatalysts such as graphitic carbon nitride[4] and poly(p-phenylenes)[5−7] are primarily active when irradiated with light in the UV region of the electromagnetic spectrum.[8]
We show that the high signal obtained facilitates the acquisition of a two-dimensional 13C−13C through-bond correlation spectrum at natural abundance of amorphous macromolecules; such a spectrum could not be obtained without Dynamic nuclear polarization (DNP)
CP-CMP9 displays reasonable photocatalytic water splitting under visible light, and increasing pyrene concentration in the monomer feed by 25% (CP-CMP10, Figure 1c) leads to a substantial increase in photocatalytic water splitting likely resulting from slight decrease of the optical bandgap

Summary

INTRODUCTION

Organic polymeric photocatalysts that generate hydrogen from water under electromagnetic irradiation have become an area of great interest in materials chemistry in recent years due to their potential greater diversity than classic metal based photocatalysts.[1−3] Organic photocatalysts such as graphitic carbon nitride[4] and poly(p-phenylenes)[5−7] are primarily active when irradiated with light in the UV region of the electromagnetic spectrum (just 3% of the solar light on the Earth’s surface).[8] Recent developments in the field have targeted polymers active in the visible light region, the predominant wavelengths of radiation present in solar light, with tunable band gaps to optimize photocatalytic activity.[7−17] One of these families of polymers (Figure 1), which belongs to the conjugated microporous polymers (CMPs),[18] is investigated in this work due to both their remarkable hydrogen evolution rates under visible light (Figure 1) and their large accessible surface areas (Brunauer−Emmett− Teller surface areas (SABET) up to ∼1200 m2 g−1, Table S1, pore size distributions, Figure S1). DNP enhancements with the quantitative multiCP experiment,[49] we were able to determine the polymers’ compositions, estimate the average polymeric structure and compare with the monomer feed ratio, a result that cannot be obtained by any other spectroscopic or analytical method

EXPERIMENTAL PROCEDURES

RESULTS AND DISCUSSION

CONCLUSION

■ ACKNOWLEDGMENTS

■ REFERENCES