Covalent organic frameworks (COFs) are coveted for their porosity. This feature makes the polymeric compounds useful systems for detecting volatile analytes such as water vapor. Researchers have reported COF films that sense humidity via a color change, but the optical response is modest. Now a team of scientists, with results from investigations at the U.S. Department of Energy’s Advanced Photon Source (APS), have demonstrated that a diiminol-based COF can act as a rapid humidity sensor with an easily visible color change. These findings, published in the Journal of the American Chemical Society, provide an important proof-of-concept for using tautomerization-induced changes in COFs to design rapid and reversible sensing systems. The team expects that demonstrating the viability of practical tautomeric sensing will inspire engineered devices capable of complex sensing responses, and that these materials might find interest as components in internet of things systems where passive and long-term-stable sensors are a necessity.

To detect water vapor, the diiminol-based COF undergoes a tautomerization reaction, whereby a proton shifts from one atom in the compound to another, to change from an isomer that appears orange to an isomer that appears black when wet. Notably, this structural rearrangement is reversible, which the team observed through x-ray diffraction experiments as they watched the COF expand and contract as water molecules were added or removed from the atmosphere. The team demonstrated that the diiminol-based COF film was stable up to two months and was responsive even to slight changes in humidity, such as that produced by breathing on the film.

Switching between isomers in tautomerization reactions often takes place quickly and in response to changes in the surrounding environment. For this reason, many tautomeric compounds have been explored as sensing agents, including tautomeric COFs.

One such well-known tautomeric COF is TAPB-TFP, made from the condensation between 2,4,6-triformylphloroglucinol and 1,3,5-tris(4-aminophenylbenzene). In the presence of water, TAPB-TFP completely tautomerizes from a triiminol compound to a β-ketoenamine compound. However, the β-ketoenamine isomer is highly stable, which means the reaction is only reversible at raised temperatures.

Now, the researchers in this study from the Georgia Institute of Technology and Northwestern University have found that a similar compound, which swaps 2,4,6-triformylphloroglucinol for 2,5-dihydroxyterephthaldehyde (PDA-OH) to form the diiminol TAPB-PDA-OH, also undergoes a water-induced tautomerization reaction but reversibly. Based on the molecule’s resonance structure, TAPB-PDA-OH would only tautomerize partially on one side of the molecule, to form a ketoenamine that exists in dynamic equilibrium with the diiminol. In the diiminol form, the compound is an orange powder while the ketoenamine appears black, which makes TAPB-PDA-OH an attractive material for colorimetric sensors. A previously reported tautomeric COF that was used to sense humidity changes did so through solvatochromatism, which relies on a limited optical response that came from water stabilizing an intermediate.

The team synthesized a model compound to confirm their proposed mechanism and found that the compound indeed exhibited the water-dependent equilibrium between iminol and ketoenamine forms, which display distinct absorption patterns. In control experiments, the scientists synthesized analogs of TAPB-PDA-OH that were incapable of tautomerization and confirmed that these analogs did not display optical changes. Data from computational DFT studies further supported that COF’s color change was attributable to tautomerization and not solvatochromatism.

To study the structural changes to the polymer during tautomerization, the researchers turned to synchrotron x-ray diffraction analysis. Small- and wide-angle x-ray scattering diffraction patterns were collected at the DuPont-Northwestern-Dow Collaborative Access Team 5-ID-D x-ray beamline at the APS, and grazing incidence diffraction patterns were obtained at the X-ray Science Division 8-ID-E beamline also at the APS (the APS is an Office of Science user facility at Argonne National Laboratory). The x-ray diffraction data revealed that the local tautomerization induced a larger structural change to the COF, essentially letting the researchers watch the COF “breathe” as it became wet or dry (Fig. 1).

To test TAPB-PDA-OH’s ability to sense humidity, the team made a detector using a thin film of the COF, which was monitored using UV-vis spectroscopy. They found that when the environment was switched from dry to humid, the sensor responded in 9 sec and even as fast as 1 sec when conditions were reversed. After multiple cycles and more than a month of storage, the sensors demonstrated the same level of performance, suggesting that these materials could be valuable long-term sensors.  

― Tien Nguyen

See: Samik Jhulki¹, Austin M. Evans², Xue-Li Hao¹, Matthew W. Cooper¹, Cameron H. Feriante¹, Johannes Leisen¹, Hong Li¹, David Lam², Mark C. Hersam², Stephen Barlow¹, Jean-Luc Brédas¹, William R. Dichtel²*, and Seth R. Marder¹**, “Humidity Sensing through Reversible Isomerization of a Covalent Organic Framework,” J. Am. Chem. Soc. 142, 783 (2020). DOI: 10.1021/jacs.9b08628

Author affiliations: ¹Georgia Institute of Technology, ²Northwestern University

Correspondence: *wdichtel@northwestern.edu, **seth.marder@chemistry.gatech.edu

Note: Congratulations to co-author William R. Dichtel (Northwestern Univ.) who has been named the National Laureate in Chemistry by the 2020 Blavatnik National Awards for Young Scientists. Read the APS/User News article here.

We thank the United States Army Research Office for a Multidisciplinary University Research Initiative (MURI) award under grant number W911NF-15-1-0447. S.J. thanks the United States-India Educational Foundation (USIEF, India) and the Institute of International Education (IIE, USA) for a Fulbright-Nehru Postdoctoral Fellowship (grant no. 2266/FNPDR/2017). A.M.E. is supported by the National Science Foundation Graduate Research Fellowship under grant no.(DGE-1324585). H.L. and J.-L.B. acknowledge funding of this work by the United States Army Research Office under award W911NF-17-10339. D.L. and M.C.H. acknowledge the U.S. Department of Energy (DOE) (grant DE-SC0019356) for support of the Raman spectroscopy characterization. The DuPont-Northwestern-Dow Collaborative Access Team is supported by Northwestern University, The Dow Chemical Company, and DuPont de Nemours, Inc. This research used resources of the Advanced Photon Source, a U.S. DOE Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under contract no. DE-AC02-06CH11357.

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