Synthesis of Novel Periodic Mesoporous Organosilicas Containing 1,4

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Synthesis of novel periodic mesoporous organosilicas containing 1,4,5,8-naphthalenediimides within the pore walls and their reduction to generate wall-embedded free radicals Bruna Castanheira, Eduardo Rezende Triboni, Luana dos Santos Andrade, Fabiane de Jesus Trindade, Larissa Otubo, Antonio Carlos Silva Costa Teixeira, Mario J. Politi, Thiago Branquinho de Queiroz, and Sergio Brochsztain Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b00220 • Publication Date (Web): 16 Jun 2018 Downloaded from http://pubs.acs.org on June 20, 2018

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Synthesis of novel periodic mesoporous organosilicas containing 1,4,5,8naphthalenediimides within the pore walls and their reduction to generate wallembedded free radicals

Bruna Castanheira,a Eduardo Resende Triboni,b Luana dos Santos Andrade,c Fabiane de Jesus Trindade,a Larissa Otubo,d Antonio Carlos Silva Costa Teixeira,a Mário José Politi,e Thiago Branquinho de Queiroz,f Sergio Brochsztain*c

a

Departamento de Engenharia Química, Universidade de São Paulo. bEscola de Engenharia de Lorena, Universidade de São Paulo. cCentro de Engenharia, Modelagem e Ciências Sociais Aplicadas, Universidade Federal do ABC. dInstituto de Pesquisas Energeticas e Nucleares. e

Instituto de Química, Universidade de São Paulo. fCentro de Ciências Naturais e Humanas, Universidade Federal do ABC.

* Corresponding author: Avenida dos Estados, 5001, 09210-170, Santo Andre, Brazil. Phone: 55-11-49963166. E-mail: [email protected].

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Abstract Novel

periodic

mesoporous

organosilicas

(PMO)

containing

1,4,5,8-

naphthalenediimide (NDI) chromophores as integral part of the pore walls were synthesized in acidic conditions, in the presence of inorganic tetraethyl orthosilicate, using triblock copolymer surfactant Pluronic P-123 as a template. The NDI precursor, the bridged silsesquioxane N,N´-bis-(3-triethoxysilylpropyl)-1,4,5,8-naphthalenediimide, was synthesized by

reaction

of

1,4,5,8-naphthalenetetracarboxylic

dianhydride

with

excess

2-

aminopropyltriethoxysilane. A series of samples containing up to 19% (weight%) of NDI was prepared (the materials were labeled PMONDIs).

13

C and

29

Si solid state nuclear magnetic

resonance revealed that the NDI moiety was intact in the PMONDIs and efficiently grafted to the silica network. Samples with up to 16% NDI load presented ordered 2D-hexagonal mesoscopic structure, according to small angle X-ray scattering, transmission electron microscopy and nitrogen adsorption isotherms. Fluorescence spectra of the PMONDIs showed excimer formation upon excitation, suggesting high flexibility of the organic moieties. Reduction of PMONDIs with aqueous sodium dithionite led to the formation of wall-embedded NDI anion radicals, as observed by the appearance of new visible/nearinfrared absorption bands. The PMONDIs were also shown to be efficient photocatalysts in the degradation of sulfadiazine, an antibiotic selected here as a model pollutant, which is usually present in water bodies and wastewater.


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Introduction The 1,4,5,8-naphthalenediimides (NDI) constitute a class of very stable aromatic molecules with outstanding photophysical, photochemical and electrochemical properties.1-4 Due to the electron deficient character of the aromatic core, the NDI can be easily reduced to give stable organic radicals,5-7 placing them among the best known n-type semiconductors, with wide applications in organic electronics.8-11 These properties prompted researchers to dope a variety of functional materials with NDI radicals, including peptide nanotubes,12 thin films,13,14 metal organic frameworks,13-16 carbon quantum dots17 and polyoxometalates.18 In this context, periodic mesoporous organosilicas (PMOs) are very attractive materials for the incorporation of NDI and their radicals. There are no reports in the literature, to our knowledge, of NDI-containing PMOs. Furthermore, PMOs containing wall-embedded free radicals are of great current interest.19-21 PMOs are hybrid organic-inorganic mesoporous materials synthesized from bissilylated bridging organosilanes of the form (RO)3Si-R´-Si(OR)3.22-25 The presence of the two oppositely directed silane groups results in the insertion of the organic moiety as integral part of the pore walls, differently from other techniques, like post-synthetic grafting or cocondensation with mono-silylated precursors, where the organic groups are placed in the pore interiors, anchored to the walls.22-25 In some cases, it has been possible to construct PMOs with 100% of the bridging organosilane, without the presence of an inorganic silica source. PMOs containing phenyl,26 biphenyl,27 2,2´-bipyridine28 and naphthalene,29 for instance, were made in this way. With bulkier organic molecules, however, it has not been possible to obtain PMOs with the pure bis-silylated organosilane. Loss of mesostructure is usually observed when the bridging organosilane is a polycyclic aromatic molecule, and therefore it is necessary to perform a co-condensation in the presence of an inorganic silica source, usually tetraethyl orthosilicate (TEOS). This was the case with 9,10-diphenylanthracene,30


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oligo(phenylenevinylene),31 tetraphenylpyrene,32 rhodamine33 and perylenediimides.34 Besides the bulkiness of the organic fragment, the pH of the synthesis is also crucial. PMOs with 100% organic precursor displaying crystal-like order in the pore walls have been obtained using strongly basic conditions (pH > 11).29,33 For PMOs prepared in acidic conditions, in contrast, less organized structures are obtained,29 and co-condensation with TEOS is usually required to obtain organized mesostructures.30 In the present work, NDI molecules were incorporated into PMOs, as integral part of the pore walls, and subsequently reduced to generate PMOs with wall-embedded NDI radicals. Recently, a few reports on the incorporation of aromatic monoimides35-40 and diimides34,41-43 into mesoporous silicas appeared, using either grafting or co-condensation with monosilylated precursors. In those cases, however, the materials were obtained with the aromatic imides in the pore interiors, as expected for the methods employed, and not as integral part of the pore walls. Mizoshita et al succeeded to prepare PMO containing perylenediimides within the pore walls using basic conditions and a cationic surfactant.44 In that case, however, the PMO obtained was unstable and collapsed upon surfactant extraction. A template-free PMO could be obtained only after stabilization by coating the material with an inorganic silica layer.44 For that purpose, we first reacted 1,4,5,8-naphthalenetetracarboxylic dianhydride (NTCA) with excess 3-aminopropyltriethoxysilane (APTES), giving the corresponding bridged silsesquioxane, N,N´-bis-(3-triethoxysilylpropyl)-1,4,5,8-naphthalenediimide (TESPNDI), which is the required precursor to incorporate the NDI chromophore in the PMOs (Scheme 1). We then synthesized NDI-containing PMOs (labelled heretofore as PMONDIs) using a well-known procedure reported for inorganic SBA-15, using triblock copolymer surfactant (Pluronic P-123) as the template in acidic conditions.45 It should be stressed that aromatic imides are readily hydrolyzed in alkaline pHs, and therefore the only possibility to


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prepare PMONDIs is the acidic route, using co-condensation with TEOS (Scheme 1). Finally, PMONDIs doped with NDI anion radicals were obtained by chemical reduction with sodium dithionite. The presence of the NDI moiety in the PMONDIs was confirmed by solid state nuclear magnetic resonance (NMR) and Fourier-transform infrared (FTIR) studies. The new materials presented highly ordered 2D-hexagonal mesoscopic structure, as probed by small angle X-ray scattering (SAXS), transmission electron microscopy (TEM) and N2 adsorption isotherms. Moreover, the materials presented quite interesting optical properties, such as excimer formation, as well as visible and near infrared absorption when reduced to the anion radical form. The possibility of doping the pores of PMONDIs with different guest molecules shall open the doors for a great variety of applications, such as sensors, photocatalysis, lightharvesting materials and solar cells. In order to demonstrate further the potential of PMONDIs for advanced applications, we tested the new materials as heterogeneous catalysts for the photodegradation of pollutants. We have recently shown that mesoporous organosilicas functionalized with 1,8naphthalimides are efficient heterogeneous catalysts for the photodegradation of methylene blue.40 Here, we show the photocatalytic effect of PMONDIs in the degradation of sulfadiazine (SDZ), an antibiotic present in water bodies and that is not removed from industrial and hospital effluents in wastewater treatment plants. The tests were carried out using a continuous flow photocatalytic reactor and showed a high efficiency in SDZ degradation.


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Scheme 1. Synthesis of TESP-NDI and PMONDIs.


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Results and Discussion Synthesis of TESP-NDI and PMONDIs. The detailed procedures for the synthesis of TESP-NDI and of PMONDIs (Scheme 1) are given as Supporting Information. We present here, however, a summary of the important new synthetic developments. The best procedure that we found for the synthesis of TESP-NDI was heating NTCA with excess APTES, where the APTES works as both reagent and solvent. The excess APTES was then removed by washing the product with petroleum ether. The procedure was adapted from the method reported for the structurally analogous 3,4,9,10-perylenetetracarboxylic dianhydride reacting with APTES.34,46-48 The reaction of pyromellitic dianhydride with APTES has also been reported.49 Surprisingly, no reports were found in the literature concerning the reaction of the corresponding naphthalene dianhydride with APTES. Therefore, to our knowledge, this is the first reported synthesis of TESP-NDI. The structure of TESP-NDI was confirmed by UV-vis and liquid state 1H- and

13

C-NMR spectra (Figures S1-S3). The compound was relatively

stable in CDCl3 solution (1H-NMR spectrum of the solution after a week was nearly unchanged, Figure S2). The solid left after solvent evaporation, however, could not be dissolved again, attesting a strong trend of TESP-NDI in the solid state towards condensation. Because of this inherent instability, the precursor was always freshly prepared for PMO synthesis (it was used within half an hour after preparation). PMONDIs with increasing organic content were prepared by varying the amount of the precursor TESP-NDI added to the Pluronic P-123 templated hydrothermal synthesis (in acidic conditions), maintaining constant the amount of TEOS (Scheme 1, Table 1). TEOS was added at once to the reaction mixture, followed by TESP-NDI, which was added dropwise over 30 min, dissolved in 10 ml of acetone. In one of the samples, an extra portion of TESPNDI was added to the reaction dissolved in 3 ml of chloroform, in order to increase the NDI uptake in the material (last entry in Table 1). The as-synthesized PMONDIs were Soxhlet-


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extracted with ethanol, resulting in complete template removal, according to 13C-NMR spectra (see below). The presence of NDI in the obtained samples was confirmed by elemental analysis (Table S1), solid state

13

C- and

29

Si-NMR (Figure 1) and FTIR spectra (Figure 2). The NDI

content in the samples, as calculated from the percent of carbon found in the elemental analyses, was in good agreement with the expected, based on the amount of TESP-NDI added to the syntheses (Table 1), except for sample PMONDI-16, where the amount added was probably overestimated. The samples were then labeled as PMONDI-8, PMONDI-15, PMONDI-16 and PMONDI-19, based on the weigth% of NDI found by elemental analysis. We believe that the elemental analysis results are reliable, since all other measurements throughout the article point to NDI increasing in this order (see below).


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Table 1. Amounts of organic and inorganic precursors employed in the PMONDI synthesis. Sample

a

TESP-NDI

TEOS

(mmol)a

(mmol)

NDI content (added)

NDI content .

(found)

mol%

weight%b

weight%c

PMONDI-8

0.45

41

1.1 %

7.7 %

8.3 %

PMONDI-15

0.93

41

2.2%

15.1 %

15.1 %

PMONDI-16

1.3

41

3.1 %

19.4 %

16.5 %

PMONDI-19d

1.3

41

3.1 %

19.4 %

19.0 %

TESP-NDI was added to the reaction mixture dissolved in 10 ml of acetone. bCalculated

considering full hydrolysis and condensation. cFrom carbon content in elemental analysis (Table S1). dIn this sample, an extra portion of TESP-NDI was added in 3 ml of of CHCl3, following the acetone solution. The given amount is the sum of the two solutions.


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Characterization of the PMONDIs by solid state NMR and FTIR spectra. Figure 1A shows the 13C solid state NMR spectra of PMONDI-19 and PMONDI-16, compared to the 13

C NMR spectrum of the precursor TESP-NDI in CDCl3 (peak assignment is shown in

Figure S3). All samples showed the imide carbonyl signals at 163 ppm, with the remaining aromatic signals at 131 and 127 ppm. Signals due to the propyl chains appear at 43, 22 and 8 ppm (N-CH2, -CH2- and CH2-Si, respectively). These results show that the intact NDI moiety was successfully incorporated into the PMOs, resisting the hydrothermal synthetic conditions. Note that the PMONDI samples did not present signals from the Pluronic surfactant, that would appear at 76, 71 and 17 ppm, showing that Soxhlet extraction with ethanol was rather efficient in removing the template (see NMR spectra before and after template extraction in Figure S4). The bands at 58 and 18 ppm seen in the solution spectrum of TESP-NDI are due to the ethoxy groups, which should not be present in the PMONDIs, since they are released as ethanol during hydrolysis. The presence of these bands in PMONDI-16 (Figure 1A) could indicate incomplete hydrolysis in this sample. However, it is more likely that the bands are due to residual ethanol remaining after Soxhlet extraction, considering that the hydrothermal conditions employed are too harsh for incomplete hydrolysis. Figure 1B shows 29Si solid state NMR spectra of the PMONDI samples, with a large line broadening cutoff of 1 kHz, in order to increase the perception of small amounts of organosilica groups in the less concentrated samples. The signals at -65 and -60 ppm correspond to T3 species, C-Si*(OSi)3, and T2 species, C-Si*(OSi)2(OH), respectively (asterisks indicate the observed silicon). The peaks at -110 ppm correspond to fully condensed Q4 species, Si*(OSi)4, while the shoulders at -100 and -90 ppm can be assigned to Q3, Si*(OSi)3(OH), and Q2, Si*(OSi)2(OH)2 species, respectively.


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Table 2 presents the contributions of each species to the

29

Si SP-MAS spectra,

obtained from a careful spectral deconvolution (Figure S5). As seen in the table, organic silicon corresponds to 2% and 4% of total silicon, respectively, for samples PMONDI-8 and PMONDI-15, although signal intensity was close to the signal-ratio of the measurement for these samples. For samples PMONDI-16 and PMONDI-19, on the other hand, Tn bands were quite prominent and represent 7 and 12% of total silicon, respectively. The presence of T signals in the spectra confirms the incorporation of organic silicon (C-Si) into the samples. It can be noticed that the NDI load, as indicated by the amount of organic silicon (Table 2), increased in the same order as found by elemental analysis (Tables 1 and S1). Note the absence of detectable C-Si(OH)3 or any species resulting from unreacted TESP-NDI (T0), which would appear at chemical shifts higher than -50 ppm.49 This means that the NDI ligand was double bounded into the silica network, confirming that it was truly embedded in the pore walls. In addition, the increase in the amount of Tn species with increasing NDI took place at expenses of Q3 + Q2 species, rather than Q4. Since Q3 and Q2 species are exposed on the surface, in contrast to Q4 species, which are inserted in the bulk of the silica (Scheme 2), it seems that most of the NDI ligands are located close to the wall/pore interface, lining the pore walls, which is a desired characteristic for most applications.

Table 2. Tn and Qn contributions to the 29Si SPMAS spectra of the PMO-NDI samples. sample

T2 %

T3 %

Q2 %

Q3 %

Q4 %

PMONDI-8

2

0

3

45

50

PMONDI-15

4

0

1

34

61

PMONDI-16

3

4

7

32

54

PMONDI-19

4

8

3

21

64


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A p

PMONDI-19

p

p

C=O Ar 200

180

160

140

120

100

80

60

PMONDI-16

40

p

20

0

p p

C=O Ar 200

180

160

140

TESP-NDI in CDCl3 C=O

200

180

160

Figure 1. (A) From bottom to top:

140

13

120

100

Ar Ar

120

80 CDCl 3

60

40

p

100

80

20

ethoxy

60

40

0

ethoxy p

20

p TMS

0

C-NMR spectrum of TESP-NDI in CDCl3; solid state

13

C-NMR spectrum of PMONDI-16; 13C-NMR spectrum of PMONDI-19. The bands marked

with “p” correspond to the propyl chain carbons (see Figure S3). (B) Solid state 29Si-NMR of PMONDI samples. In order to enhance the Tn species, which have intensities comparable to the noise, Fourier transform was done after a line broadening cutoff of 1000 Hz.


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Scheme 2. Different possibilities for the location of the NDI chromophore within the PMO walls. (A) NDI insertion in the core of the walls. (B) NDI insertion near the wall surfaces.

The presence of NDI in the samples was further confirmed by infrared spectroscopy. FTIR spectra of the PMONDI samples (Figure 2 and S6) showed the characteristic NDI symmetric (1706 cm-1) and asymmetric (1664 cm-1) carbonyl stretches. These bands were superimposed to an absorption of the silica framework (the band at 1635 cm-l is due to the overtone of a Si-O lattice vibration at lower frequency), but they became preponderant with increasing NDI load (Figure 2). Other bands at 1584, 1460, 1383 and 1345 cm-1, which are characteristic of the NDI aromatic system, can be clearly seen. Bands attributable to the C-H stretches of the propyl chains (2920 and 2970 cm-1) can also be observed (Figure S6).


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14

100

80

60

%T

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

40

* *

20

PMONDI-8 PMONDI-15 PMONDI-16 PMONDI-19

0 2000 1900 1800 1700 1600 1500 1400 1300 1200 1100 1000

Wavenumber (cm-1)

Figure 2. FTIR spectra of PMONDI-8, PMONDI-15, PMONDI-16 and PMONDI-19, highlighting the carbonyl region of the spectra (for full spectra see Figure S6). The symmetric (1706 cm-1) and asymmetric (1664 cm-1) NDI carbonyl stretches are marked with a star.


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Mesoscopic structure of the PMONDIs. Figure 3 shows small angle X-ray scattering (SAXS) patterns for Soxhlet extracted PMONDIs (data for as-synthesized samples are shown in Figure S7). The samples with lower NDI loads, PMONDI-8 and PMONDI-15, show diffraction patterns similar to SBA-15, typical of 2D-hexagonal structures (P6mm symmetry group), composed of the main (100) reflection (2θ in the range 0.7-0.80) and well defined (110) and (220) reflections with 2θ between 1.2 and 1.70 (higher reflections can also be seen in the case of PMONDI-8). In PMONDI-16, a well pronounced (100) peak was still present, but the (110) and (220) peaks were broader than the samples with lower NDI content. Ordered structure was completely lost in PMONDI-19, which showed no X-ray diffraction peaks. Thus, loss of structure was evident as going to higher NDI loads, as usually observed with PMOs containing bulky aromatic molecules. Wahab et al.,34 for instance, prepared PMOs containing perylenediimides co-condensed with 1,2-bis(triethoxysilyl)ethane and noticed the absence of SAXS diffraction peaks when the dye content reached 13 weight%. Compared to their results, a higher load of NDI could be reached in the present work (up to 16 weight%), while still maintaining structural order. It can also be noticed in Figure 3 that peak positions were shifted to lower 2-theta values with increasing NDI (Table 3), indicating an increase in the hexagonal mesoporous parameter a0 (which is the sum of pore diameter plus wall thickness) from 12.9 nm in PMONDI-8 to 13.4 nm in PMONDI-15 and 14.0 nm in PMONDI-16. The parameter for the PMONDIs was also significantly higher than the value a0 = 12.1 nm found for inorganic SBA-15 synthesized in the same conditions (Table 3). These findings suggest that the precursor TESP-NDI acted concomitantly as organosilica source and swelling agent. This can be rationalized assuming that TESP-NDI concentrated at the outer interface of the Pluronic micelles during synthesis, increasing micellar size.


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Nitrogen adsorption isotherms of the PMONDI samples are shown in Figure 4A. Figure 4B shows the corresponding Barrett-Joyner-Halenda (BJH) pore size distributions (PSD), while textural parameters are summarized in Table 3. Except for the sample with highest NDI load, type IV(a) isotherms were obtained, with H1 hysteresis, which are typical of 2D-hexagonal SBA-15 materials.50 The shape of the isotherms suggests a decrease in structural order with increasing NDI load, in agreement with the SAXS results. The isotherm of PMONDI-16, for instance, did not reach a plateau at high P/P0 values. Furthermore, the isotherm of PMONDI-19 presented a completely different shape, with a steep desorption branch. The hysteresis loop in this case can be classified as H2(a), and is characteristic of disordered materials with partial pore blocking (or with ink-bottle shaped pores).50 In this case, the steep desorption at lower than expected P/P0 values represent evaporation of liquid nitrogen from the narrower pore entrances. As seen in Table 3, PMONDI samples presented high surface areas and pore volumes. In the case of PMONDI-8, the surface area was 562 m2/g and the pore volume 1.08 cm3/g, which were comparable to inorganic SBA-15. On the other hand, surface areas and pore volumes both decreased with increasing NDI load, but even the pore blocked sample PMONDI-19 displayed considerable high surface area (464 m2/g) and pore volume (0.44 cm3/g). Thus, it is remarkable that PMONDI-19, in spite of losing the 2D-hexagonal mesostructure, still retained a large mesopore volume. Materials with partially blocked mesopores are very interesting for several applications, such as controlled drug release. Furthermore, the presence of NDI in PMONDI-8 and PMONDI-15 resulted in the formation of materials with wider pores than SBA-15 (Table 3, Figure 4B), in agreement with the SAXS results, reinforcing the idea that the precursor TESP-NDI acted as a micelle swelling agent. However, pore sizes obtained from BJH analysis of the desorption branch seem to be overestimated (BJH pore diameters are of the same order as the parameter a0,


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which is the sum of pore diameter plus wall thickness, leading to underestimated wall thickness). This inconsistency could be caused by non-equilibrium conditions in the adsorbate-adsorbent system during the measurements.50 For higher NDI loads, as in PMONDI-16, the pore size decreased, while a0 increased, meaning that the walls became thicker, as expected for NDI-embedded walls. The pore size of PMONDI-19 was found to be 4.6 nm, much narrower than the other samples. In the case of pore blocked materials, however, BJH pore sizes obtained from the steep desorption branch represent the size of the narrow pore entrances, rather than the pore interiors.50 The structural order of PMONDIs was further studied by high resolution transmission electron microscopy (HRTEM). The images seen in Figure 5 (additional images are shown in Figure S8) revealed ordered materials composed of rod-shaped particles with parallel mesoporous channels running along the long axis. The rods were typically 200-400 nm wide and about 1.0 µm long in the case of PMONDI-8 and PMONDI-15. However, narrower rods (80-150 nm width) were obtained with PMONDI-16. When the rod ends were examined with higher magnification, some of them showed clearly a hexagonal shape. The hexagonal arrangement of the mesopores could be readily observed in high-magnification images (Figure 5 and S8). It can be noticed that PMONDI-8 showed higher degree of order in comparison to the PMONDIs with higher NDI load, in agreement with the other methods. HRTEM images of PMONDI-19 (Figure S8B), on the other hand, did not show any sign of hexagonal mesoscopic order, as expected from the absence of diffraction peaks in the SAXS measurements.


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18

(220)

(100)

(110)

15000

0.73

10000

intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

0.76 0.79

1.0

1.5

2.0

5000

2.5

3.0


0 0.5

1.0

1.5

2.0

2.5

3.0

2θ (degrees)

Figure 3. SAXS of the PMONDI samples.

Table 3. SAXS and nitrogen isotherms parameters for PMONDI samples. d 2θ100 d100 (Å) a0 (Å)a SBET (m2/g) Vpore (cm3/g)b ps (Å)c wt (Å)

sample SBA-15

0.84

105

121

631

1.07

102

19

PMONDI-8

0.79

112

129

562

1.08

126

3

PMONDI-15

0.76

116

134

485

0.76

125

9

PMONDI-16

0.73

121

140

441

0.64

102

38

PMONDI-19

e

e

e

464

0.44

46

e

a

a0 = 2d100 / 3 .

b

Calculated at P/P0= 0.97. cBJH pore size (desorption branch). d Wall thickness

(wt = a0 - ps). e Diffraction peaks were absent.


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19

PMONDI-8 (offset = +120) PMONDI-15 (offset = +60) PMONDI-16 (no offset)

800 700

PMONDI-19 (offset = -70)

600 500 3

cm /g

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

400 300 200 100

A

0 0.0

0.2

0.4

0.6

0.8

1.0

P/P0

B

0.46 nm 12.6 nm 10.2 nm 12.5 nm PMONDI-8

PMONDI-15 PMONDI-16 PMONDI-19

50

100

150

200

250

pore diameter (Å)

Figure 4. (A) Nitrogen adsorption isotherms of PMONDI-8, PMONDI-15, PMONDI-16 and PMONDI-19. The isotherms were vertically offset as indicated for sake of clarity. (B) Pore size distributions derived from the isotherms (BJH method from desorption branches).


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Figure 5. HRTEM images of PMONDI-8 (A, B); PMONDI-15 (C, D); PMONDI-16 (E, F).


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Optical properties of PMONDIs. Diffuse reflectance spectra of the PMONDI samples (Figure 6A) showed the typical absorption band of NDI derivatives (λmax = 380 nm), as compared to the solution absorption spectrum of the precursor TESP-NDI. The broad band seen between 450-600 nm can be attributed to charge-transfer (CT) between neighboring NDI units, as usually observed in NDI-based solid materials.3,18 Mizoshita et al have observed similar CT transitions in the absorption spectra of PMO containing perylenediimides.44 Figure 6B shows excitation and emission spectra of PMONDI powders dispersed in chloroform. Excimer-like emission spectra were found for the PMONDIs, with broad, structureless bands with maxima between 480 – 500 nm. When excitation spectra of the PMONDIs were registered with the emission fixed at the excimer band, spectra typical of monomeric NDI were obtained, confirming that the broad emission band was due to true excimers rather than to ground-state dimers. The excitation spectra of the PMONDIs were in fact mirror-images of the emission of a monomeric NDI in solution, as seen in Figure S9A. These results imply in a great flexibility of the organic chains, allowing excimer formation within the PMO walls. These conclusions are supported by literature studies showing that organic groups inserted in PMO walls have a high degree of motional freedom.29,51-54 For instance, phenyl,51 biphenyl52,53 and naphthalene29 groups inserted in amorphous PMO walls were shown by fluorescence studies to form excimers. Bracco et al52 reported that biphenylene moieties within PMO walls behaved as molecular rotors, with high mobility. The precursor TESP-NDI also presented excimer-like emission in homogeneous solution (Figure S9B). In this case, however, excitation with the emission fixed in the excimer band resulted in a broad band with maximum at 433 nm, in addition to the monomer bands, suggesting that a ground-state dimer was involved.


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Absorbance

1.0

A

PMONDI-8 PMONDI-15 PMONDI-16 PMONDI-19 TESP-NDI in CHCl3

0.5

0.0 300

400

500

600

Wavelength (nm)

1.0

normalized intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


B

0.8

0.6

0.4

0.2

0.0 350

400

450

500

550

600

wavelength (nm)

Figure 6. (A) Diffuse reflectance spectra (converted to absorbance units) of PMONDI samples. The absorption spectrum of TESP-NDI in homogeneous solution (chloroform) was added for comparison. (B) Normalized excitation (dashed lines) and emission (solid lines) (λex = 330 nm) spectra of PMONDI-8, PMONDI-15 (as-synthesized sample), PMONDI-16 and PMONDI-19 dispersed in chloroform. Emission spectra were recorded with λex = 330 nm. Excitation spectra were recorded with λem = 485 nm (PMONDI-19), λem = 490 nm (PMONDI-15) and λem = 500 nm (PMONDI-8 and PMONDI-16).


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Radical generation within PMO walls. When the solid PMONDI materials were treated with a droplet of 0.02 M sodium dithionite solution (in borate buffer, pH = 8), the white powders immediately became brownish (Figure 7), evidencing the reduction of the NDI moieties within the pore walls to the anion radical form. UV/Vis/NIR diffuse reflectance spectra of PMONDIs before and after treatment with dithionite are shown in Figure 7 and S10. It can be clearly seen the appearance of a new series of bands in the visible/NIR region, not present in the materials before dithionite addition. The new bands at 494, 616, 727 and 980 nm are the typical signature of NDI anion radicals, as reported in the literature,6,14,55-58 showing that wall-embedded NDI radicals were generated in PMONDIs. Mizoshita et al44 have reported on the reduction of perylenediimides incorporated in PMOs by chemical doping with hydrazine to give wall-embedded anion radicals. The radical-doped PMONDIs were stable in air for several minutes, being oxidized back to NDI in ca 15 min (Figure S11A). Furthermore, the absorption spectrum of the NDI was restored after five reduction-reoxidation cycles with dithionite (Figure S11B), indicating a great stability of the PMONDI materials. No desorption of NDI was observed when PMONDI powders were dispersed in aqueous sodium dithionite solutions (Figure S12). The broad CT band in the NIR region (950 – 1030 nm) (Figure 7 and S10) has been observed by other research groups59-61 and can be attributed to the pairing of NDI radicals to form radical π-dimers. This low energy CT band (∆E ≈ 1.1 eV) is related to the semiconducting properties of NDI and other aromatic imides,59-61 attesting the great potential of the PMONDIs in organic electronics. π-Dimer formation is in agreement with the fluorescence results, confirming the great flexibility of the imide moieties embedded into the PMONDI walls.


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2.5

494 nm Na2S2O4 2.0

Absorbance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

PMONDI-8 + Na2S2O4

1.5

PMONDI-15 + Na2S2O4

616 nm

PMONDI-16 + Na2S2O4 1.0

PMONDI-19 + Na2S2O4 727 nm

0.5

980 nm

0.0 400

500

600

700

800

900

1000

Wavelength (nm) Figure 7. Diffuse reflectance spectra (converted to absorbance units) of PMONDI powders after the addition of a drop of sodium dithionite solution. Inset: Photographs of PMONDI-8 powder before and after treating with sodium dithionite.


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Photocatalytic degradation of sulfadiazine by PMONDIs. Aromatic imides are known as efficient photosensitizers for the degradation of pollutants.40 We therefore studied the PMONDIs as heterogeneous catalysts for the photodegradation of sulfadiazine (SDZ), a sulfonamide antibiotic usually found in waterbodies and wastewater.62 The presence of pharmaceutical products in the environment (regarded as emerging pollutants) has become a reason of great concern recently.63 Antibiotics can get to the environment through human and veterinary excretions, as well as from the effluent of hospitals and the pharmaceutical industry, since they are not efficiently removed in wastewater treatment plants. They are toxic to the aquatic environment and can induce bacterial resistance, stressing the importance of degrading antibiotics in water.62,63 For the photocatalytic tests, the PMONDIs were packed into capillary tubes as micropacked bed reactors (µPBR) (Figure S13A). A 100 W UV LED chip (λemmax = 370 nm) was employed for the irradiation (Figure S13B). The lamp output matched closely the absorption spectra of the PMONDIs, with no overlap with the SDZ absorption spectrum (Figure S13C), assuring that only the NDI was excited. A SDZ aqueous solution was then delivered to the µPBR, first in the dark, to check whether SDZ would adsorb into the PMONDIs, then with the lamp on, to study photodegradation. More experimental details about the µPBR assembly are given as supporting information. As seen in Figure 8, SDZ concentration was unchanged after passing by the µPBR reactor packed with PMONDIs in the dark, showing that no adsorption occurred. After the lamp was turned on, SDZ was rapidly depleted from the reactor effluent, demonstrating the photocatalytic activity of the PMONDIs. In a control experiment with inorganic SBA-15, no degradation occurred (Figure 8). Straight irradiation of the SDZ solution (photolysis) resulted in no degradation as well, confirming that wall-embedded NDIs acted as photosensitizers for SDZ degradation. It is known that the NDI when irradiated in aerated media generate singlet


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oxygen (and other reactive oxygen species) via energy transfer from NDI triplet states.64 These reactive oxygen species can then degrade a variety of molecules in solution. Therefore, this is the most likely mechanism involved here for SDZ degradation (further studies are on the way to elucidate the detailed mechanism). A high concentration of reactive oxygen species is expected within the confined pore space.

1.0

0.8

C/C0

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

0.6

0.4

photolysis SBA-15 PMONDI-8 PMONDI-15 PMONDI-16

0.2

lamp on

dark 0.0 -200

-150

-100

-50

0

50

100

150

200

250

300

t (min)

Figure 8. SDZ degradation by photocatalysis with PMONDIs in the µPBR as a function of irradiation time, including control experiments by direct photolysis and with pristine SBA-15. In the graphic, C is the SDZ concentration at time t and C0 is the inlet SDZ concentration. The dashed line marks the moment the lamp was switched on (negative time values correspond to concentration profiles in the dark).


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During the photo irradiated step, it is worth observing that after a fast SDZ degradation in the micro packed-bed reactor, leading to C/C0 in the 0.2-0.3 range, a small increase in non-decomposed SDZ was observed (Figure 8). For the samples with lower NDI content (PMONDI-8 and PMONDI-15), the time of maximum decomposition was around 50 min, while for PMONDI-16 it was around 150 min. After this point, a steady-state SDZ concentration was reached at around 50% of C0 for PMONDI-8 and PMONDI-15 and 30% of C0 for PMONDI-16. These effects are related to the setup used to evaluate the photodegradation, carried out at constant inlet SDZ concentration and flow rate (i.e., constant space time). In fact, the SDZ solution was continuously percolated through the sample, meaning that a continuous amount of SDZ was offered to the NDI sites. In this way, SDZ photodegradation was, in a first moment, very effective, due to the fact that all active sites were available. In a second moment, an equilibrium between the SDZ site occupancy and photodegradation reaction, followed by subproducts release, is expected. However, partial catalyst deactivation, probably due to the irreversible adsorption of transformation products at NDI sites, resulted in a slight decrease in catalyst efficiency, leading to steady-state outlet SDZ concentrations in the range 30-50% of C0. The time of maximum degradation is expectedly shorter in samples with a lower density of active sites, and longer in samples with a higher active site density, as observed for the PMONDIs investigated. In fact, 13C NMR shows that the NDI sites of samples PMONDI-8 and PMONDI-15 are considerably rarer in comparison with PMONDI-16. The latter therefore exhibits the best performance, with about 70% SDZ removal, being a promising candidate as a photocatalytic material.


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Conclusions Periodic mesoporous organosilicas with remarkable mesoscopic order containing 1,4,5,8-naphthalenediimides as integral part of the pore walls were synthesized and thoroughly characterized. Well-structured PMONDI materials were successfully prepared containing up to 16% (weight %) NDI. The NDI moieties showed a great flexibility within the amorphous silica framework, allowing for excimer formation upon excitation of the NDI chromophore. Framework flexibility also accounts for radical pairing after chemical reduction of the NDI. The NDI units were found to concentrate near the pore/wall interface, lining the pore walls, what could facilitate interactions with guest molecules. The large pores of PMONDIs shall allow the incorporation of a great variety of guest molecules, enhancing the potential for advanced applications, such as energy transfer processes for light-harvesting devices or electron transfer processes for photovoltaics. We have also shown that the PMONDIs have a great potential as photocatalysts in the degradation of pollutants.

Acknowledgments S. B. and E. R. T. acknowledge the support of FAPESP (S. B.: grant No 2016/054962; E. R. T.: grant No 2015/06064-6 and 2017/13839-0). T. B. Q. acknowledges the support of CNPq (Universal 404951/2016-3). We thank Prof. Marcia C.A. Fantini, responsible for the Crystallography Laboratory at University of Sao Paulo (USP), for the use of the SAXS equipment. The authors are grateful to the Multiuser Central Facilities (UFABC) for the experimental support.


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