Real-time optical and electronic sensing with a β-amino enone linked, triazine-containing 2D covalent organic framework

Fully-aromatic, two-dimensional covalent organic frameworks (2D COFs) are hailed as candidates for electronic and optical devices, yet to-date few applications emerged that make genuine use of their rational, predictive design principles and permanent pore structure. Here, we present a 2D COF made up of chemoresistant β-amino enone bridges and Lewis-basic triazine moieties that exhibits a dramatic real-time response in the visible spectrum and an increase in bulk conductivity by two orders of magnitude to a chemical trigger - corrosive HCl vapours. The optical and electronic response is fully reversible using a chemical switch (NH3 vapours) or physical triggers (temperature or vacuum). These findings demonstrate a useful application of fully-aromatic 2D COFs as real-time responsive chemosensors and switches.


Supplementary Figure 1. Synthesis of PBHP (2) and TAPT (4) monomers
Synthesis of (2Z,2'Z)-1,1'-(1,4-phenylene)bis(3-hydroxyprop-2-en-1-one) (2) (PBHP): A solution of 1,4-diacetylbenzene (1 g, 6.16 mmol) in 80 ml of anhydrous THF was added drop-wise to tBuOK (3.45 g, 30.82 mmol in 100 ml of anhydrous THF) at -78 ºC under argon atmosphere. After stirring at the same temperature for additional 1 hr a light yellow solution was obtained. To this ethyl formate (4.56 g, 4.95 mL, 61.65 mmol) was added and allowed the reaction mixture to warm up to room temperature and continued the stirring for 12 h. The reaction progress was monitored by TLC. After completion the reaction was quenched with 3N HCl (60 ml) and the organic layer were extracted with ethyl acetate (4 x100 mL) dried over MgSO4 and concentrated under reduced pressure. The resultant solid obtained was triturated with 150 mL of diethyl ether, filtered, and washed with diethyl ether to obtain pure PBHP monomer as a yellow solid 1.3gm (5.3mmol, 89% yield). Spectral data was identical to previous report. 1 1  Synthesis of of 1,3,5-tris(4-aminophenyl)triazine (4) (TAPT): 1,3,5-tris-(4-aminophenyl)triazine was synthesized via trimerization catalyzed by trifluromethanesulphonic acid. In a typical synthesis, 2.0 g (16.94 mmol) 4-aminobenzonitrile was taken in a round bottom flask at 0 ºC. Then 4.48 mL (50.82 mmol) trifluoromethanesulfonic acid was added dropwise at 0 ºC. The resultant mixture was stirred for 24 h at room temperature in inert atmosphere. After completion the reaction mixture was neutralized using 2 N NaOH, The resultant white solid obtained was filtered and washed several times with water to obtain pure TAPT monomer as white solid 1.64 gm (4.6 mmol, 89% yield). The spectral data obtained was identical to previous reports. 2  Synthesis of (Z)-1-phenyl-3-(phenylamino)prop-2-en-1-one (KE): A solution of Aniline (0.1 g, 1.07 mmol) dissolved in 2 ml of anhydrous ethanol was added drop-wise to a solution of (Z)-3-hydroxy-1-phenylprop-2-en-1-one (mono-keto-enol , 0.159 g, 1.07 mmol) in 20ml ethanol and 6N AcOH (0.2ml) at room temperature. After stirring for 10 min, the reaction mixture was refluxed at 80 o C for 12 h under Argon atmosphere. After completion, the reaction mixture was cooled to room temperature and the solid product was filtered off and washed with ethanol several times, to obtain pure keto-enamine (KE) as a yellow solid 0.215 gm (0.9 mmol, 90% yield). The spectral data obtained was identical to previous reports. 3  Synthesis of 2,4,6-triphenyl-1,3,5-triazine (TPT): In a typical synthesis 1.0 g (9.6 mmol) benzonitrile was taken in a round bottom flask at 0ºC. Then 2.53 mL (28.8 mmol) trifluoromethanesulfonic acid was added dropwise at 0 ºC. The resultant mixture was stirred for 24 h at room temperature in inert atmosphere. After completion the reaction mixture was neutralized using 2M NaOH, The resultant white solid obtained was filtered and washed several times with water to obtain pure TPT monomer as white solid 0.82gm, (2.6mmol, 82% yield). The spectral data obtained was identical to previous reports. 4

Supplementary Figure 3. Synthesis of PBHP-TAPT COF
Triazine containing 2D covalent organic frameworks was synthesized according to method described by Perepichka et al. 1 All the trails for synthesis of COFs were carried out in a Pyrex ampule of OD: 2.6 mm, ID: 2.4 mm. In a typical synthesis mixture of TAPT (71 mg, 0.2 mmol) and PBHP (65 mg, 0.3 mmol) and 6 M aq. acetic acid (0.5 mL) were suspended in a degassed mixture of Mesitylene and Dioxane. The mixture was sonicated for 10 min, flash frozen in liquid N2 and degassed for 10 min. The ampule was then sealed and heated to 110 °C for 3 days. After the reaction the ampule was broken and the contents were filtered and washed thoroughly with ethanol, dioxane and methanol to get rid of any oligomer impurities as well as unreacted monomers. Furthermore, the solid was then subjected to soxhlet extraction using methanol for 72 h, which gave an orange powder PBHP-TAPT COF 98 mg, (0.0780mmol calculated for unit cell C(78)H(54)N(12)O(6), 79.9% yield). The isolated solid PBHP-TAPT COF was insoluble in water and common organic solvents such as acetone, ethanol, and N,N-dimethylformamide.

Supplementary Note 1.
Colorimetric detection limit of PBHP-TAPT COF towards HCl vapors was carried out using a glass solution bottle with a stopper COF was exposed to vapours of hydrochloric acid in temperature controlled environment by keeping the reaction vial in the oil bath (25 o C monitored with a thermometer). Smaller vial with sample was introduced inside the reaction vial containing necessary amount of HCl. Samples were exposed for 10 s and further subjected to solid-state UV-Vis measurements without any delays. HCl vapour concentration in ppm was calculated by following equation:

a) b)
Supplementary Note 2. Figure 20), using the same models and methods as described above in computational details in the main text. Protonation sites on the N atom of triazine and on the N atom of the ketoenamine linker were considered. The DFT results show that the protonation on triazine is preferred by 70 kJ mol -1 over protonation on the keto-enamine linker. The proton added on the N atom of triazine is not involved in any H-bonding. On the contrary, the proton added to the N atom of the keto-enamine linker is stabilized by the formation of the H-bond with the O atom of the linker at the adjacent layer. At the minimum energy structure, the proton is shifted from N to O atom of the keto-enamine linker. We carried out the same calculations using cluster model (Supplementary Figure 21)

Supplementary Note 3.
In order to further confirm the protonation site, we performed solid-state 13 C CP-MAS NMR on the HCl-activated PBHP-TAPT COF. 0.2 g of pristine sample was protonated using a steady stream of HCl gas for 10 s, as in all previous protonation experiments. 13 C CP-MAS solid-state NMR spectra of PBHP-TAPT COF were recorded in 3.2 mm rotors at 13 kHz, and protonated PBHP-TAPT COF spectra were obtained in 4 mm rotors at10 kHz. The 13 C signals were recorded for 12 h. It should be noted, that the 4 mm rotor was not absolutely air-tight, as we observed a color change of the protonated PBHP-TAPT COF from deep red to dark orange over the course of 12 h. Upon protonation, most peaks experience an upfield shift, with the notable exceptions of the aryl sp 2 carbon environment (e) and the keto-enamine carbon (a). This corresponds best to the scenario that the protonation site is preferentially at the ringnitrogen of the triazine sub-unit (see Supplementary Figure 28).

Supplementary Figure 28.
Observed and predicted 13 C signals of several protonation sites.

Supplementary Note 4.
The figure below shows the predicted chemical shifts of carbon signals upon protonation; all three different possibilities were considered and are compared in the table below: 1) protonation only at the triazine, 2) protonation at the triazine and keto-enamine bridge, and 3) protonation only at the KE. Based on the comparisons the 13 C NMR suggests that the triazine core is the preferred protonation site.