Finding the Sweet Spot of Photocatalysis─A Case Study Using Bipyridine-Based CTFs [original]

Marcelo Alves Fávaro, Daniel Ditz, Jin Yang, Sebastian Bergwinkl,

Ashta C. Ghosh, Michael Stammler, Chantal Lorentz, Jérôme

Roeser, Elsje Alessandra Quadrelli, Arne Thomas, Regina Palkovits,

Jérôme Canivet, Florian M. Wisser

Finding the Sweet Spot of PhotocatalysisA Case

Study Using Bipyridine-Based CTFs

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Alves Fávaro, M., Ditz, D., Yang, J., Bergwinkl, S., Ghosh, A. C., Stammler, M., Lorentz, C., Roeser, J.,

Quadrelli, E. A., Thomas, A., Palkovits, R., Canivet, J., & Wisser, F. M. (2022). Finding the Sweet Spot of

PhotocatalysisA Case Study Using Bipyridine-Based CTFs. In ACS Applied Materials amp; Interfaces (Vol. 14,

Issue 12, pp. 14182–14192). American Chemical Society (ACS). https://doi.org/10.1021/acsami.1c24713.

This document is the Accepted Manuscript version of a Published Work that appeared in final form in ACS

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1Finding the Sweet Spot of PhotocatalysisA Case Study Using

2Bipyridine-Based CTFs

3Marcelo Alves Fávaro,

Daniel Ditz,

Jin Yang, Sebastian Bergwinkl, Ashta C. Ghosh, Michael Stammler,

4Chantal Lorentz, Jérome Roeser, Elsje Alessandra Quadrelli, Arne Thomas, Regina Palkovits,*

5Jérome Canivet,*and Florian M. Wisser*

6ABSTRACT: Covalent triazine frameworks (CTFs) are a class of porous organic polymers

7that continuously attract growing interest because of their outstanding chemical and physical

8properties. However, the control of extended porous organic frameworks’structures at the

9molecular scale for a precise adjustment of their properties has hardly been achieved so far.

10 Here, we present a series of bipyridine-based CTFs synthesized through polycondensation, in

11 which the sequence of speciﬁc building blocks is well controlled. The reported synthetic

12 strategy allows us to tailor the physicochemical features of the CTF materials, including the

13 nitrogen content, the apparent speciﬁc surface area, and optoelectronic properties. Based on a

14 comprehensive analytical investigation, we demonstrate a direct correlation of the CTF

15 bipyridine content with the material features such as the speciﬁc surface area, band gap, charge

16 separation, and surface wettability with water. The entirety of these parameters dictates the

17 catalytic activity as demonstrated for the photocatalytic hydrogen evolution reaction (HER).

18 The material with the optimal balance between optoelectronic properties and highest

19 hydrophilicity enables HER production rates of up to 7.2 mmol/(h g) under visible light irradiation and in the presence of a

20 platinum cocatalyst.

21 KEYWORDS: covalent triazine framework, photocatalysis, hydrogen evolution reaction, molecular control, bipyridine

22 ■INTRODUCTION

23 A current challenge in materials science is to keep rigorous

24 control over the increasingly complex assembly of active

25 building blocks in extended porous frameworks while retaining

26 predictive insight into their ultimate performance.

The ability

27 to introduce various active building blocks into one porous

28 framework has already allowed the modulation and prediction of

29 physical and chemical properties, improving the capacity to tune

30 fundamental properties including hydrophobicity,

porosity,

3,4

31 optical response,

5−8

and catalytic activity.

6,9−11

The capacity to

32 extend the control over optoelectronic properties to the

33 molecular level while retaining the control over other

34 physicochemical and structural properties becomes a key in

35 the context of material applications to renewable energy

36 utilization; the capacity to control optoelectronic properties

37 will, in turn, drive the materials’photocatalytic performances.

38 While some interesting results have been achieved in

39 photocatalysis,

12−16

typically, by adding electron acceptor (p-

40 doping) or electron donor atoms or moieties (n-doping) in the

41 ﬁnal composition of the material,

6,13,17,18

the correlation of the

42 material’s structure with the observed catalytic performance

43 remains phenomenological,

17,19,20

as the observed changes in

44 the electronic properties, geometry, and morphology seemed

45 too intertwined to rationalize the changes in catalytic

46 activity.

21,22

One frontier in preparing photoactive multifunctional

materials by modular design thus lies in the control and

comprehension from the material’s molecular structure up to the

macroscopic scale, understanding the material’s performances in

its entirety, beyond the description of its molecular units. To

address these challenges, a regular, alternating assembly between

building blocks containing diﬀerent functional groups is a route

to yield a periodic, sequential copolymer

with a uniform

55integration of the dopant.

Here, we report (i) a synthetic strategy for the structural

control at the molecular level for a series of periodic copolymers,

namely, bipyridine-containing covalent triazine frameworks

(CTFs), (ii) a general protocol for the consistent measurement

of their resulting electronic properties, e.g., frontier orbital

position alignment, a feat not described yet, and (iii) their

62correlation with the obtained catalytic performances in the

63 photochemical hydrogen evolution reaction (HER), which

64 reaches 7.2 mmol/(h gcat).

65 ■RESULTS AND DISCUSSION

66 Synthesis and Characterization of the Materials. Four

67 CTFs were synthesized via condensation of aromatic diamidine

68 bromides and aromatic dialdehydes. A series of periodic

69 copolymers with the desired quantity and positioning of the

70 dopant in the material were adjusted by the stoichiometric ratio

71 of diamidine and dialdehyde in a 2:1 ratio for in situ

f1 72 polycondensation of the triazine linkage (Figure 1a). This

73 polycondensation approach, recently reported by Cooper, Tan,

74 and co-workers,

provides superior control compared with the

75 well-established ionothermal CTF synthesis,

paving the way to

76 master the desired building block sequence in the bipyridine-

77 containing CTF family by applying the principle of orthogonal

78 chemistry.

26,27

However, so far, only a few CTFs obtained by

79 polycondensation are reported, mostly limited to functionalized

80 aromatic aldehydes and nonfunctionalized terephthalamidine or

81 [1,1′-biphenyl]-4,4′-bis(carboximidamide).

28−32

Here, we em-

82 ployed bipyridine and biphenyl aromatic cores as n-doping and

83 nondoping moieties, respectively (Figure 1). Since the aromatic

84 dialdehydes are commercially available, the synthesis of CTFs

85 started with preparing 5,5′-diamidine-2,2′-bipyridine dihalide

86 and 4,4′-biphenyldiamidine dihalide, adapting the previously

87 established procedure.

88 As the synthesis of amidine chloride produces a large excess of

89 NH4Cl, its puriﬁcation is a crucial step. While nonfunctionalized

90 terephthalamidine dichloride could be obtained in good purity,

91 following the procedure established by Shu et al.,

all attempts

92 to suﬃciently obtain pure 5,5′-diamidine-2,2′-bipyridine by

93 washing or recrystallization from ethanol/acetone mixtures were

94 unsuccessful. Thus, we applied a counterion exchange strategy

95 to transfer the amidine ﬁrst into tetraphenylborate, which is

96 soluble in acetone, and then into the corresponding bromide

97 (for details, see Supporting Information (SI) Section 2.1). Note

98 that using terephthalamidine dibromide in the synthesis of CTF-

99 1did not aﬀect porosity, band gap, or crystallinity with respect to

100 the synthesis procedure described by Cooper and co-workers,

101 applying terephthalamidine dichloride (see SI Section 3).

102 Since two molecules of amidine bromide and one of aldehyde

103 are needed for the synthesis of a triazine core, a series of

104 materials containing 0, 33, 66, and 100% of bipyridine building

105 units were realized (Figure 1b). The materials were named

106

CTF-02,

CTF-02-Bpy0.33,CTF-02-Bpy0.66, and CTF-02-

107

Bpy, following the increase in the bipyridine content within the

108

CTF structure. The successful copolymerization was assessed by

109

IR spectroscopy, elemental analysis, and quantitative 13C

110

composite-pulse multiple cross-polarization (ComPmultiCP)

111

MAS NMR spectroscopy, while thermal stability was assessed by

112

thermogravimetric analysis (TGA). TGA under synthetic air

113

reveals that all materials are stable up to 200 °C. Above 200 °C, a

114

slight mass loss is observed for all materials while complete

115

decomposition starts above 400 °C for bipyridine-containing

116

CTFs and above 480 °C for pure biphenyl-based CTF-02

117

(Figure S9). The lower stability of bipyridine-containing CTFs

118

compared to the biphenyl-based CTF-02 might be explained by

119the lower stability of the nitrogen-containing linkers.

35,36

120

In IR spectra, copolymerization can be followed qualitatively,

121

as the characteristic bands of biphenyl moieties at 1416 and

122

1000 cm−1decrease in intensity when going from CTF-02 to

123

CTF-02-Bpy (Figure S10). At the same time, the characteristic

124

bands of bipyridine moieties at 1467, 1050, and 1030 cm−1

125

appear. Moreover, the characteristic vibration of the triazine

126

moiety around ∼1500 cm−1is indicative of the triazine ring

127

formation in the CTF-02 series.

Finally, the characteristic

128

amidine N−H stretching vibration bands between 3090 and

129

3340 cm−1as well as the characteristic aldehyde C−H stretching

130

vibration bands between 2740 and 2860 cm−1of the monomers

131

are no longer detectable in CTFs (Figure S11), indicative of a

132

complete conversion of the reactive end-groups into triazine

133moieties.

134

To obtain a detailed and quantitative insight into the

135

composition of CTFs, 13C ComPmultiCP MAS NMR spectra

136

were recorded.

14,38,39

The signal around 170 ppm can be

137

attributed to the triazine core carbon atom, conﬁrming the

138

successful condensation and oxidation of amidine and aldehydes

139

to the triazine core in all materials (Figure 2a). The absence of

140

any signal in the 13C NMR spectra at about 190 and 163 ppm

141

highlights the complete transformation of aldehyde and amidine

142

moieties in all materials (see Figure S20). In the spectrum of the

143

biphenyl-based material CTF-02, signals at 143.0, 135.1, and

144

128.0 ppm in an integrated ratio of 1:1:4 of the biphenyl moiety

145

occur, while in the spectrum of CTF-02-Bpy,ﬁve diﬀerent

146

resonances at 156.9, 149.6, 136.9, 130.5, and 121.6 ppm appear

147

with equal intensities (Figure 2a). In the spectra of the mixed

148

CTFs, containing both biphenyl- and bipyridine-based mono-

149mers, all those signals are present, and their integrated ratio is in

Figure 1. (a) Schematic representation of triazine formation in the CTF-02 series (X,Y= CH/N) synthesized via polycondensation. (b) Repeating

units of the four diﬀerent CTFs: CTF-02 (X,Y= CH), CTF-02-Bpy0.33 (X=N,Y= CH), CTF-02-Bpy0.66 (X= CH, Y= N), and CTF-02-Bpy (X,Y=

N). Color code: blue, triazine; green, pyridine; and gray, phenyl moieties. (c) Idealized structural model of CTF-02 stacking along the crystallographic

c-direction. Color code: gray, carbon; blue, nitrogen; and white, hydrogen atoms. All materials are partially ordered, characteristic for materials

prepared by the polycondensation approach (Figures S23 and S24).

150 line with a 1−2 and 2−1 composition (see Table 1 and SI

151 Section 4.5 for further discussion). This perfect stoichiometric

152 control is further evidenced by elemental analysis. The nitrogen

153 content of these materials increases linearly from 12.8% for

154 CTF-02 to 22.1% for CTF-02-Bpy, while the molar ratio of

155 carbon to nitrogen is in line with the theoretical composition

156 (Table S1). The signal of the triazine carbon atom shifts from

157 170.6 ppm in CTF-02 to 169.1 ppm in CTF-02-Bpy0.66 and

158 CTF-Bpy. This shielding eﬀect reﬂects the increase in electron

159 density on the triazine moiety with an increasing number of

160 bipyridine ligands, reﬂecting the enhanced donor capability of

161 the n-doped building block. A further increase from 66%

162 bipyridine to 100% bipyridine did not result in a further

163 shielding eﬀect on the triazine moiety acting as an electron

164 acceptor.

f2t1 165 All materials show a permanent porosity toward nitrogen at 77

166 K, together with the characteristic swelling behavior for CTFs

167 prepared by this methodology (Figure 2b).

24,40

The hysteresis

168 between absorption and desorption branch of the isotherms

169 increases with increasing bipyridine content, indicative of a more

170pronounced swelling of the ﬂexible network structure in the

171CTF-02 series.

41−43

The purely biphenyl-based CTF-02 has the

172highest apparent surface area within the series, achieving 570

173m2/g. As for other CTFs prepared by the condensation

174approach,

the apparent surface area of CTF-02 is signiﬁcantly

175lower than the apparent surface areas obtained under

176ionothermal conditions (1880−2480 m2/g)

25,34

but compara-

Figure 2. (a) 13C ComPmultiCP MAS NMR spectra and corresponding deconvolution for CTF-02,CTF-02-Bpy0.33,CTF-02-Bpy0.66, and CTF-02-

Bpy from bottom to top. In all spectra, a line broadening of 100 Hz was applied. Color code for deconvoluted signals: blue, triazine; green, bipyridine;

and gray, biphenyl carbon atoms (see also Figure 1). Complete NMR spectra are provided in the SI (Figure S20). Signal assignment was done

according to the literature.

11,37

(b) N2physisorption isotherms measured at 77 K and (c) water vapor physisorption isotherms measured at 298 K of

CTF-02 (black), CTF-02-Bpy0.33 (blue), CTF-02-Bpy0.66 (green), and CTF-02-Bpy (red). (d) Experimentally observed evolution of composition

from 13C ComPmultiCP MAS NMR spectroscopy (black), Henry constant (KH, blue), and hydrophobicity (α, gray) obtained from water adsorption

isotherms as a function of monomer fraction. Dashed lines are guidelines for the eye.

Table 1. Monomer Feed Ratios and 13C ComPmultiCP

Integrals of Repeating Units

monomer feed monomer ratio from NMR

polymer biphenyl bipyridine biphenyl bipyridine

CTF-02 3 0 2.9 ±0.2 0

CTF-02-Bpy0.33 2 1 1.9 ±0.2 1.0 ±0.1

CTF-02-Bpy0.66 1 2 1.0 ±0.1 1.9 ±0.2

CTF-02-Bpy 0 3 0 2.8 ±0.3

Ratios are calculated from the sum of the integrals of all resonances

for each repeating unit, normalized to the triazine resonance (three

carbon atoms) and divided by the number of carbon atoms per

repeating unit.

177 ble to the apparent surface area of materials prepared by

178 triﬂuoromethanesulfonic acid (TfOH)-mediated polyconden-

179 sation (560 m2/g).

5,44

The observed lower surface area might be

180 explained by avoiding the partial carbonization that occurs

181 during ionothermal synthesis in molten ZnCl2, which may

182 increase the surface area. ZnCl2may also act as a porogene.

45,46

183 With increasing bipyridine content, and thus with increasing

184 surface polarity (vide infra), the apparent surface area decreases

185 to approx. 400, 370, and 240 m2/g for CTF-02-Bpy0.33,CTF-

t2 186 02-Bpy0.66, and CTF-02-Bpy, respectively (Table 2). A possible

187 explanation for the decrease in the apparent surface area might

188 include a nonfavorable interaction of N2with the more polar

189 surface of the bipyridine-containing materials, diﬀerent

190 orientation of the quadrupolar N2on the surface that depends

191 on the composition (bipyridine or biphenyl),

or a denser state

192 of the ﬂexible bipyridine-containing CTFs after activation and

193 cooling to 77 K.

Another explanation might be that the

194 grinding process, to achieve similar particle sizes and thus similar

195 light scattering of the suspension used in photocatalysis (vide

196 infra), may result in delamination or displacement of the two-

197 dimensional (2D) stacks in CTFs. Such delamination causes a

198 broadening or vanishing of peaks in the pXRD patterns (Figure

199 S23) and seems to be stronger for the bipyridine-containing

200 CTFs (for further discussion, see SI Section 4.8). It has been

201 demonstrated by Banerjee and co-workers for similar 2D

202 covalent organic frameworks that delamination results in a

203 reduced apparent surface area.

204 Speciﬁcally, in heterogeneous catalysis, a high speciﬁc surface

205 area in an open material is a key feature to assure accessibility of

206 the active sites. As we aim for aqueous liquid-phase catalysis, not

207 only the apparent surface area but also the interaction and

208 porosity toward a given solvent have to be considered. Water

209 vapor physisorption isotherms revealed a high water vapor

210 uptake at high partial pressure for all materials (Figure 2c). The

211 solid−solvent interaction of the ﬁrst water molecules with the

212 solid surface, and thus, the wettability of the surface, is reﬂected

213

by the material’s Henry constant (KH), determined at low p/

214

p0.

In this series, KHincreases linearly with an increasing

215

amount of bipyridine from 1.5 ×10−6mol/(g Pa) for CTF-02,

216

providing the weakest solid−solvent interaction, to 5.2 ×10−6

217

mol/(g Pa) for CTF-02-Bpy (Table 2 and Figure 2d). In the

218

same way, the degree of pore ﬁlling (Ξ)

increases from 0.7 to

219

3.1 (Figure S21). Thus, in all bipyridine-CTFs, the pores are

220

completely ﬁlled with water, a beneﬁcial characteristic for the

221

intended application in aqueous HER. In the case of CTF-02-

222

Bpy0.66 and CTF-02-Bpy, the H2O-accessible pore volume even

223

strongly increased compared to N2accessible pore volume,

224

highlighting the pronounced swelling of the material and

225

therefore good accessibility of active sites even in the center of

226

the material. An important parameter to evaluate hydrophilicity

227

is the relative partial pressure αat which the capacity for water

228

uptake reaches 50%. In contrast to the Henry constant, which is

229

evaluated at the initial stage of the isotherm, αis determined

230

based on the total water capacity, thus considering the whole

231

isotherm. It describes the hydrophilic/hydrophobic character of

232

a porous solid based on its ﬁlling with water: the lower αis, the

233

more hydrophilic is the material.

In this series, the highest

234

hydrophilicity is observed for CTF-02-Bpy0.66 (α= 0.63), and

235

the highest hydrophobicity for CTF-02 (α= 0.85, Table 2 and

236

Figure 2d). The higher hydrophilicity of CTF-02-Bpy0.66 when

237

compared to CTF-02-Bpy (α= 0.69) may be explained by its

238

lower total water capacity and degree of pore ﬁlling, causing an

239easier, more eﬃcient ﬁlling with water.

240

Optical Properties of the Materials. The optical proper-

241

ties of the CTF series were investigated by diﬀuse reﬂectance

242 f3

UV−vis spectroscopy (Figure 3a). Following the increase in the

243

bipyridine content within the series, and consequently, increase

244

in n-doping, the light absorbance shifts toward higher wave-

245

lengths, in line with predictions obtained from DFT calculations

246

by Van Speybroeck and co-workers.

In the series, the direct

247

band gap depends on the molecular composition, e.g., CTF-02,

248without any bipyridine moieties, holds the largest direct band

Table 2. Porosity and Optical Properties in the Series of CTFs

polymer SBET/m2/g Vtot

/cm3/g Vtot

/cm3/g KH/mol/(g Pa) α/1 Edir

/eV Eindir

/eV

CTF-02 570 0.71 0.47 1.5 ×10−60.85 3.05 2.86

CTF-02-Bpy0.33 400 0.40 0.39 2.5 ×10−60.77 2.98 2.73

CTF-02-Bpy0.66 370 0.25 0.39 4.0 ×10−60.63 2.90 2.64

CTF-02-Bpy 240 0.18 0.56 5.2 ×10−60.69 2.87 2.37

etermined at 0.95 p/p0from N2physisorption isotherms measured at 77 K.

Determined at 0.95 p/p0from H2O vapor physisorption isotherms

measured at 298 K.

irect.

Indirect optical band gaps determined via the Tauc plot method (Figure S13).

Figure 3. (a) Solid-state UV−vis spectra and (b) solid-state and steady-state emission spectra after excitation at λexc = 378 nm of CTF-02 (black),

CTF-02-Bpy0.33 (blue), CTF-02-Bpy0.66 (green), and CTF-02-Bpy (red).

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