Hybrid mesoporous silica grafted with photoisomerizable 2-hydroxychalcones

Microporous and Mesoporous Materials 180 (2013) 40–47

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Hybrid mesoporous silica grafted with photoisomerizable 2-hydroxychalcones Sandra Gago ⇑, Isabel M. Fonseca, A. Jorge Parola ⇑ REQUIMTE, Departamento de Química, Faculdade de Ciências e Tecnologia, Universidade Nova de Lisboa, 2829-516 Caparica, Portugal

a r t i c l e

i n f o

Article history: Received 28 March 2013 Received in revised form 31 May 2013 Accepted 1 June 2013 Available online 15 June 2013 Keywords: Flavylium Chalcone Photochromism Mesoporous materials Photochromic materials

a b s t r a c t Hybrid photochromic mesoporous materials based on MCM-41 and SBA-15 were synthesized by covalent attachment of 30 -butoxy-7-hydroxyflavylium (Fl-OH) and 30 -butoxy-7-metoxyflavylium (Fl-OCH3) hydrogensulfates. The pristine materials were initially grafted with 3-chloropropyl groups, reacted with 30 -hydroxyacetophenone and finally condensed with appropriate salicylaldehydes to yield the new hybrids MCM-41-Fl-OH and SBA-15-Fl-OCH3. The materials were characterized by powder X-ray diffraction, N2 adsorption, solid-state 13C CPMAS NMR spectroscopy, and thermogravimetric and elemental analyses, which confirm the successful covalent bonding of the flavylium moieties with loadings of 16.90 ± 0.05% and 11.78 ± 0.04% (w/w) for MCM-41-Fl-OH and SBA-15-OCH3, respectively. Flavylium compounds originate in solution a multiequilibria reaction network than can be actuated by pH and light, defining pH-coupled photochromic systems. The new hybrids show pH-dependent reflectance spectra resembling those observed in solution, but shifted to higher pH ranges, indicating a higher stability of the grafted flavylium cations. Irradiation of these materials equilibrated at adequate pH values where the photoisomerizable trans-chalcones predominate shows formation of the respective flavylium cations that recover back to the initial compositions upon standing in the dark, leading these new organic–inorganic hybrids as pH-dependent photochromic materials. Ó 2013 Elsevier Inc. All rights reserved.

1. Introduction Flavylium compounds belong to an important family of molecules that comprises anthocyanins, anthocyanidins and deoxyanthocyanins, whose basic structure is the 2-phenyl-1-benzopyrylium (flavylium) cation. These dyes are responsible for the bright colors of flowers, fruits and leaves and found applications in the food and cosmetic industries, as models for optical memories and even as light absorbers in solar cells aiming at a greener solar energy conversion [1,2]. A variety of natural and synthetic flavylium derivatives are known, according to the nature and position of the substituents on the basic ring system. In aqueous solutions, the flavylium salts undergo a complex network of reactions whose distribution of species can be controlled by external stimuli such as pH and light, Scheme 1. Key species of these networks are the trans-2hydroxychalcones, that are able to undergo photoisomerization to the cis isomer, allowing to exploit flavylium-based systems for the development of photochromic materials [1,3]. Although these systems have been extensively studied in homogeneous and microheterogeneous micellar media, [4–8] to aim an application of their photochemistry it is important their

⇑ Corresponding authors. Tel.: +351 212948300 (S. Gago). E-mail addresses: [email protected] (S. Gago), [email protected] (A.J. Parola). 1387-1811/$ - see front matter Ó 2013 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.micromeso.2013.06.001

heterogenization and the resultant new hybrid materials can own new properties. Concerning inorganic matrices, several systems including zeolites [9,10], clays [11] and mesoporous silica materials [12] have been explored to host flavylium derivatives. Kohno et al. have shown that these dyes have enhanced thermal and photochemical stability when encapsulated into the pores of zeolites, and that there is a larger stabilization when the pore is small enough to tightly fit the cation [10]. Our group encapsulated the 40 ,7-dihydroxyflavylium as well as the respective trans-chalcone form in the channels of zeolite L and have shown for the first time that the trans-chalcone can be isomerized inside zeolite channels, which is a step forward in the development of solid state memories [9]. Nevertheless, zeolites, being negatively charged, strongly stabilize the cationic flavylium species, preventing the occurrence of trans-chalcone/flavylium photochromic cycling. Changing the host matrix to neutral mesoporous silica would in principle avoid this drawback. Kohno et al. [10,12a,b] have reported the stabilization of flavylium dyes upon incorporation into several mesoporous silicas (FSM-16, MCM-41 and HMS). They found that the mesopore structure slows down oxygen diffusion leading to increased photostability of the dye and that the photostability is enhanced when Fe3+ or Al3+ acidic sites are included in the mesoporous silica structure. The same group was able to observe photochromism of trans-2-hydroxychalcones in these silicas

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S. Gago et al. / Microporous and Mesoporous Materials 180 (2013) 40–47 OH

OH

Kh

O+

HO

HO

O

OH

pH

Ki

h

isomerization

OH HO

O

OH O

cis-chalcone, Cc

hemiketal, B

Ka proton transfer

HO

ring-chain tautomerization

hydration flavylium, AH+

OH Kt

OH

OH

O

quinoidal base, A

O trans-chalcone, Ct

Scheme 1. Chemical reaction network established in solution by flavylium cations, exemplified for 40 ,7-dihydroxyflavylium. The species AH+ and A are strongly colored while the other species are essentially colorless. The Ct species is photoisomerizable yielding Cc that evolves to AH+ or A (depending on pH), defining pH-dependent photochromic systems.

[12c,13] as well as in modified clay interlayers [11a] with very slow conversions (days) in the former and faster conversions (hours) in the latter. However, in these previous works the incorporation of the organic guest in mesoporous materials was performed by diffusion from a solution, which often leads to weakly bound physisorbed molecules. The immobilization by covalent grafting would in principle lead to more stable materials with no leaching of the guest molecules. This work reports the covalent attachment of two flavylium compounds, 30 -butoxy-7-hydroxyflavylium hydrogensulfate (Fl-OH) and 30 -butoxy-7-metoxyflavylium hydrogensulfate (FlOCH3), to the mesoporous materials MCM-41 and SBA-15. These matrices are characterized by highly ordered hexagonal arrays of channels, large surface area and narrow pore size distribution, with pore sizes in the range 1.5–10 nm for MCM-41 and in the range 5– 30 nm for SBA-15 [14,15]. The characteristic pH dependent flavylium reaction network operates in these hybrid materials allowing to exploit them as solid state photochromic materials with relatively fast kinetics for color change (minutes under our working conditions). 2. Experimental 2.1. General All reagents and solvents were of analytical grade. Elemental analyses were performed in a Thermofinnigan Flash EA 112 series. The solid state 13C CPMAS NMR spectra were acquired with a CPMAS-HPD sequence on a BRUKER AVANCE III spectrometer, at 75.4 MHz. The measurements were performed with a spinning rate of 10 kHz at the magic angle, a proton direct excitation pulse of 2.5 ls at 99.8 W, a cross polarization contact time of 1.5 ms and cross polarization pulses of 57 W for the 13C and a variable ramp pulse with a maximum power level of 39.7 W for the 1H. The high power decoupling was applied with a tppm composite pulse decoupling during an acquisition time of 43 ms and a proton power level of 99.8 W. A recycling delay of 2 s has been used. The chemical shifts were referenced to glycine.

30 mL H2O and added slowly to a stirred solution of 7.50 g of [(C16H33)NMe3]Br in 80 mL H2O. The precipitate formed was stirred at ambient temperature for 30 min. Dilute sulfuric acid (2 M) was then added dropwise to adjust the pH of the gel to 10.0. After stirring for a further 30 min, the pH was readjusted to 10.0. The mixture was then transferred for an autoclave and heated at 100 °C under static conditions for 48 h. The solid was recovered by filtration, washed with water, dried at 60 °C and calcined at 560 °C for 6 h (using a heating ramp of 1 °C min1 under air) to remove the surfactant. In the case of SBA-15, 2.0 g of Pluronic P123 was dissolved in 60 mL of 2 M HCl and 15 mL of H2O under stirring and at ambient temperature. Then 4.4 g of TEOS was added dropwise and the mixture stirred for 24 h at 40 °C, transferred for an autoclave and heated at 100 °C for 24 h. The solid was filtered, washed with water and calcined at 500 °C for 5 h. The functionalization with 3-chloropropyl groups to obtain MCM-41-Cl and SBA-15-Cl was carried out following the procedure described in reference [16]. 1.0 g of MCM-41 or SBA-15 was refluxed with 3-chloropropyltriethoxysilane (1 mL) in dried toluene (20 mL) under argon for 24 h. The solid was filtered, washed several times with dichloromethane and dried under vacuum. All synthesized materials were characterized by 13C-CPMAS NMR and X-ray powder diffraction; the final materials were further characterized by elemental analysis, thermogravimetric analysis and textural characterization (see below). 2.3. Synthesis of MCM-41-AcPh and SBA-15-AcPh A mixture of MCM-41-Cl or SBA-15-Cl (0.5 g), 30 -hydroxyacetophenone (0.45 g, 3.3 mmol), 1.5 equiv. of potassium carbonate (0.68 g, 4.95 mmol) and a catalytic amount of potassium iodide (0.05 g), in dried DMF (20 mL) was heated at 90 °C, under Argon, for 2 days. After cooling down to room temperature, the solid was filtered, washed with ethanol until no acetophenone was observed on TLC, with water and then dried under vacuum. MCM41-AcPh: 13C-CPMAS NMR: d (ppm) = 12.5; 26.6, 29.9, 49.7, 68.5, 124.9, 134.4, 142.6, 163.9, 200.6; SBA-15-AcPh: 13C-CPMAS NMR: d (ppm) = 12.3, 26.6, 29.8, 63.6, 68.6, 74.6, 118.3, 124.3, 133.2, 142.9, 164.0, 204.0.

2.2. Synthesis of the mesoporous silica matrices MCM-41 [16] and SBA-15 [17] were prepared according to literature procedures, using [(C16H33)NMe3]Br and Pluronic P123, respectively, as the templating agent. In the case of MCM-41, the material was synthesized from a gel with the molar composition SiO2:0.29Na2O:0.50[(C16H33)NMe3]Br:150H2O. Specifically, 10.0 g sodium silicate solution (8% Na2O, 27% SiO2) were diluted with

2.4. Synthesis of MCM-41 functionalized with 30 -butoxy-7hydroxyflavylium (MCM-41-Fl-OH) A mixture of MCM-41-AcPh (0.2 g) and 2,4-dihydroxybenzaldehyde (0.15 g, 1.1 mmol), in glacial acetic acid/concentrated sulfuric acid (8 mL:2 mL), was stirred at room temperature for 24 h. The resulting yellow solid was filtered, washed with diethyl ether until

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S. Gago et al. / Microporous and Mesoporous Materials 180 (2013) 40–47

no salicylaldehyde could be observed by TLC and dried under vacuum. 13C-CPMAS NMR: d (ppm) = 12.5; 22.9, 26.1, 30.2, 33.5, 50.3, 70.3, 107.4, 117.5, 125.7, 134.3, 141.9, 164.1, 176.9. Elemental analysis (%) found: C 9.03, H 1.47. 2.5. Synthesis of SBA-15 functionalized with 30 -butoxy-7methoxyflavylium (SBA-15-Fl-OCH3) The same procedure was followed using as precursor material SBA-15-AcPh and 2-hydroxy-4-methoxybenzaldehyde. A dark orange solid was obtained. 13C-CPMAS NMR: d (ppm) = 14.0; 24.7, 27.6, 30.9, 42.4, 49.1, 61.8, 70.8, 105.3, 120.3, 127.9, 136.5, 159.8, 162.1, 170.0, 178.01, 201.1. Elemental analysis (%) found: C 6.41, H 1.38. 2.6. Characterization The room temperature powder XRD data of MCM-41 type materials were collected on a Rigaku, model MiniFlex II, diffractometer with a Cu Ka (k = 1.5418 Å) radiation source (30 kV, 15 mA). Measurements were step-scanned in 0.02° 2h steps in the 1–12° range, with a scan speed of 0.5°/min. The powder XRD patterns of SBA-15 samples were obtained at room temperature in a Pan’Analytical PW3050/60X’Pert PRO (h/2h) equipped with X’Celerator detector and with automatic data acquisition (X’Pert Data Collector (v2.0b) software) using a monochromatized Cu Ka radiation as incident beam (40 kV, 30 mA). Measurements were obtained by continuous scanning in the 0.3–10° 2h range with a step size of 0.03° and a time per step of 99.7 s. The textural characterization of the MCM and SBA materials was obtained from physical adsorption of nitrogen at 77 K, using a Micrometrics ASAP 2010 V4 instrument. The BET surface area was calculated by using the relative pressure data in the range 0.04–0.2. The total pore volume, Vp, was evaluated on the basis of the amount adsorbed at a relative pressure of about 0.97. The pore size distributions were obtained from the adsorption branches of the isotherms, applying the BJH method with the modified Kelvin equation and a correction for the statistical film thickness of the pore walls. The statistical film thickness was calculated using the Harkins–Jura equation, in the p/p0 range of 0.1–0.95. Thermogravimetric analyses were performed using TA Instruments SDT 2960 simultaneous DSC–TGA at a heating rate of 10 °C min1 from room temperature to 700 °C. 2.6.1. Preparation of the samples at different pH values The materials were stirred at room temperature overnight in buffered [18] water/ethanol (70:30) solutions at specific pH values. The suspensions were centrifuged, decanted and finally dried under vacuum. Typically, 5.00 mg of the material were suspended in 1 mL of the appropriate buffered solution. For diffuse reflectance spectra, the samples were eventually diluted 1:30 (w/w) with barium sulfate. 2.6.2. Measurements pH values were measured with a MeterLab pHM240 pH meter from Radiometer Copenhagen. Diffuse reflectance UV–Vis spectra were acquired in a Shimadzu UV-2501PC equipped with an integrating sphere. The powdered samples were smashed between two quartz lamellae, accommodated on a BaSO4 filled support and the spectra run using an identical BaSO4 filled support as blank. The remission function, F(R), was calculated using the Kubelka–Munk equation for optically thick samples. Irradiation experiments were carried out in a spectrofluorimeter (Spex Fluorolog 3.22) equipped with a 150 W Xe–Hg lamp as light source and using maximum slit width; in these conditions, I0 values around 107 Einstein/min were obtained at the selected wavelengths.

3. Results and discussion 3.1. Synthesis and structural characterization The mesoporous siliceous materials MCM-41 and SBA-15 were prepared according to literature procedures [16,17]. The immobilization of two different flavylium compounds through covalent bonding was achieved by the surface functionalization of the materials in three steps (Scheme 2). The first one was performed by modification of the materials surfaces with chloropropyl groups leading to the materials MCM-41-Cl and SBA-15-Cl, upon reaction with 3-chloropropyltriethoxysilane [16]. The introduction of acetophenone moieties was achieved by reacting these materials with excess 30 -hydroxyacetophenone, in DMF, using potassium iodide as catalyst, leading to the materials designated as MCM-41-AcPh and SBA-15-AcPh. Finally, the acetophenone groups were condensed under acidic conditions with the appropriate salicylaldehydes (in excess) to yield the hybrid materials MCM-41-Fl-OH and SBA-15-Fl-OCH3 composed by mesoporous silica with covalently attached flavylium units and respective counterions. The synthesized materials were characterized by Powder X-ray Diffraction. The XRD patterns for pristine MCM-41 and SBA-15 starting materials and for the final materials with flavylium units are shown in Fig. 1. Both pristine materials show well-defined peaks corresponding to the typical basal reflections indexed assuming a hexagonal cell [14a,15b]. The same reflections are also observed for the materials grafted with flavylium units which are indicative of the preservation of the long range hexagonal symmetry of the mesoporous hosts. For MCM-41-Fl-OH an attenuation of the X-ray peaks is observed that may be attributed to a reduction in the X-ray scattering contrast between the silica walls and the pore filling material and not to a loss of crystallinity [19,20]. Combined data from N2 adsorption experiments and powder diffractograms (Table 1) show a simultaneous decrease in the surface area (SBET), pore volume (Vp) and pore diameter (Dp) when going from the pristine material to the final MCM-41-Fl-OH, consistent with the occupation of the pore channels with flavylium moieties (and respective counterions). The wall thickness increases from 1.08 to 1.68 nm, which is in accordance with the presence of immobilized flavylium salts [21]. As expected, in the case of SBA-FlOCH3, a simultaneous decrease in the surface area and in the pore volume is also observed relative to the pristine material, indicating the presence of attached flavylium units. However, the pore diameter increases as revealed from N2 adsorption experiments. This data, combined with the observed shift of the (1 0 0) reflection to lower 2h values, leads also to a decrease of the wall thickness (dw). Pore enlargement upon encapsulation of guests (either by adsorption or covalent grafting) has been previously reported for SBA-15 but no explanation was then given [21–23]. These surprising results can be explained by the strong acidic medium used during the final condensation step that leads to flavylium formation. Subjecting mesoporous silica to strongly acidic conditions may lead to the deterioration of the pores with a corresponding increase of the pore diameter and decrease of the wall thickness [24]. In particular, this was observed for SBA-15 upon exposure to 3 M H2SO4 (close to our experimental conditions) while in the same conditions, MCM-41 remain essentially unaffected [24]. The mesoporous silica matrices bearing organic groups were characterized by solid state 13C CPMAS NMR (Fig. 2). The spectra of MCM-41-Cl and SBA-15-Cl are quite similar presenting five peaks assigned to the corresponding carbons of the chloropropyl groups and to the residual ethoxy groups that have not undergo condensation with the surface silanol groups (Fig. 2I-(a) and II(a)). The intermediate materials MCM-41-AcPh and SBA-15-AcPh show one additional peak at around 23 ppm assigned to the methyl

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S. Gago et al. / Microporous and Mesoporous Materials 180 (2013) 40–47

Cl

EtO

O

Si OH

OH

Si

Si

O

EtO

+

O

EtO Si

O

Cl

Si

reflux, toluene

OEt

O O

+

Si

O

MCM-41-Cl or SBA -15-Cl

MCM-41 or SBA-15 R

HO KI, K2CO3

DMF, 100 ˚C 2 days

HSO4O O

R

+

OH

O

H2SO4 /CH 3COOH (2:8, v/v), RT

O

O R= OH, OCH3

EtO

EtO

Si

Si

O

O

O

Si

Si

Si

O

O

R= OH: MCM-41-Fl-OH R= OCH3: S BA-15-Fl-OCH3

O O

Si

O

MCM-41-AcP h or S BA-15-AcP h

Scheme 2. Synthetic strategy leading to the hybrid materials MCM-41-Fl-OH and SBA-15-Fl-OCH3.

A

B

(100)

(110) (200)

Intensity (a.u)

Intensity (a.u)

(100)

(b)

(110) (200)

(b)

(a)

(a) 1

2

3

4

5

6

7

8

0

2 /degrees

1

2

3

4

5

6

7

8

2 /degrees

Fig. 1. Powder XRD patterns of A – pristine MCM-41 (a) and MCM-41-Fl-OH (b); B – pristine SBA-15 (a) and SBA-15-Fl-OCH3 (b).

Table 1 Textural properties of the pristine and final hybrid materials.

a

Materials

d (1 0 0) (nm)

a (nm)

SBET (m2/g)

Dp (nm)

dwa (nm)

Vp (cm3/g)

MCM-41 MCM-Fl-OH SBA-15 SBA-Fl-OCH3

4.16 3.98 9.67 10.06

4.81 4.59 11.16 11.62

1237 813 1089 529

3.73 2.91 5.19 6.59

1.08 1.68 5.97 5.02

1.15 0.59 1.41 0.87

dw = a  Dp.

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S. Gago et al. / Microporous and Mesoporous Materials 180 (2013) 40–47

Fig. 2. Solid state 13C CPMAS NMR spectra of I – (a) MCM-41-Cl; (b) MCM-41-AcPh; (c) MCM-41-Fl-OH; (d) model compound Fl-OH; II – (a) SBA-15-Cl; (b) SBA-15-AcPh; (c) SBA-15-Fl-OCH3; solvent peaks are marked with an .

group of the ketone and five new peaks in the range 120–200 ppm corresponding to the ring carbon atoms (Fig. 2I-(b) and Fig. 2II-(b)). In the final material, MCM-41-Fl-OH, new peaks in the aromatic region are observed that could be assigned to the flavylium moiety, as confirmed by comparison with the solid state spectrum of the model compound Fl-OH (Fig. 2I-(c) and (d)). A similar behavior is observed for the spectrum of SBA-15-Fl-OCH3 where the peak around 57 ppm, assigned to the methoxy group, further confirms the grafting of the 30 -butoxy-7-methoxyflavylium unit to the silica matrix (Fig. 2II-(c)). The TGA curves for the final materials exhibit a weight loss of ca. 3.3% for MCM-41-Fl-OH, and 6.8% for SBA-15-OCH3 below 150 °C, which corresponds to the removal of adsorbed water and the release of solvent used in the reactions. In the range 150–550 °C the thermal decomposition of the organic molecules occurred with weight losses of 16.8% for MCM-41-Fl-OH and 12.8% for SBA-15-OCH3. These results are in agreement with the %C values obtained from elemental analysis. In the materials resulting from the initial functionalization of the siliceous matrices with 3-chloropropyltriethoxysilane, there may exist 3-chloropropylsilane groups with two, one or no free ethoxyl groups. Considering that all 3-chloropropylsilane groups were converted into the desired flavylium species and taking the situations where two or no free ethoxyl groups remain allowed us obtaining the upper and lower limits of flavylium loadings in the final materials, respectively. Therefore, the flavylium loading is in the range 0.42–0.34 mmol/g (16.90 ± 0.05%, w/w) for MCM-41-Fl-OH and 0.28–0.23 mmol/g (11.78 ± 0.04%, w/w) for SBA-15-OCH3, values that are close to the ones obtained from TGA. These loadings of covalently attached flavylium units confirm that our encapsulation approach using covalent grafting allows a much more efficient loading of mesoporous silica when compared with previous work [12b] that used adsorption as encapsulation method and where amounts of flavylium guests in the range 0.5–0.8% (w/w) were achieved.

Scheme 3. Model compounds 30 -butoxy-7-hydroxyflavylium (Fl-OH) hydrogensulfate and 30 -butoxy-7-metoxyflavylium (Fl-OCH3) hydrogensulfate.

3.2. Flavylium reactivity and photochromism in the hybrid materials The model compounds 30 -butoxy-7-hydroxyflavylium hydrogensulfate (Fl-OH) and 30 -butoxy-7-methoxyflavylium hydrogensulfate (Fl-OCH3) were available from previous studies [25] (Scheme 3) and used here for comparison purposes. Fig. 3 shows the diffuse reflectance UV–Vis absorption spectra of MCM-41-Fl-OH and SBA-15-Fl-OCH3 at different pH values. The materials were previously equilibrated at each pH value, centrifuged and pump dried, as described in Section 2. The spectra at more acidic pH values show absorption maxima at ca. 447 nm for both materials. These maxima correspond to the presence of the flavylium cations and compare with kmax = 438 nm and 444 nm for the model compounds Fl-OH and Fl-OCH3 in acidic ethanol:water 30:70 (v/v), respectively. The small red shifts in the absorption maxima on passing from solution to the mesoporous materials suggest that the microenvironment inside the materials pores is different. Flavylium cations usually display negative solvatochromism, i.e., decreasing the polarity of the medium leads to red shifts in kmax, as expected for positively charged heterocycles characterized by p–p⁄ transitions with strong charge-transfer character [26]. Taking this into account, the observed red shifts suggest a less polar environment inside the pores than in solution, indicating that the solvation of the ground state of the flavylium cation is more difficult inside the pores. When the pH is raised new bands appear indicating that other species from the flavylium reaction network are formed. In the case of MCM-41-Fl-OH, a new band at 485 nm and a shoulder at

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S. Gago et al. / Microporous and Mesoporous Materials 180 (2013) 40–47

6

8 pH = 2.5

F(R)

8

F(R)

6

pH = 2.6

pH = 7.2

2

3

5

0

2

4

6

F(R)

F(R)

4

8

pH

2

4

0

2

4

6

8

pH

pH = 8.29

A

B

0

0 300

400

500

600

700

Wavelength (nm)

300

400

500

600

700

Wavelength (nm)

Fig. 3. Diffuse reflectance UV–Vis absorption spectra of the materials. A – MCM-41-Fl-OH in the range 2.6 < pH < 8.3; inset: pH dependence of F(R) at 447 nm (d) and 485 nm (s). B – SBA-15-Fl-OCH3 in the range 2.5 < pH < 7.2; inset: pH dependence of F(R) at 447 nm (d) and 380 nm (s).

ca. 380 nm indicate the formation of the quinoidal base, A, and of the trans-chalcone, Ct, respectively, as corroborated by absorption maxima very close to those observed for the model compound FlOH in solution (484 nm and 378 nm, respectively) [25]. A small band at 575 nm also appears that could not be clearly assigned. The fact that this band decreases with increasing pH suggests that it may arise from flavylium aggregation, a known phenomenon in flavylium chemistry, particularly in anthocyanins [27]. Similar bands were observed for flavylium cations physically adsorbed onto mesoporous silica [12c]; the authors have assigned this band to an elusive and poorly characterized intermediate species between flavylium and chalcone [12c,28]. The overall spectral changes of the hybrid MCM-41-Fl-OH material with pH (Fig. 3A) show that the characteristic flavylium reaction network (Scheme 1) is established in these hybrid materials. However, there are significant differences relatively to the behavior of the model compound Fl-OH in solution (ethanol:water 30:70 (v/v)). In particular, there is a shift in the apparent pK 0a value from 1.16 in solution to higher values in the hybrid material (pK a  5; a precise value cannot be given since the pH environment of the inner pore after drying is not the same as that of the buffer solution used in each case). The multiequilibria reaction network originated by flavylium compounds (Scheme 1) is adequately characterized by an apparent acidity constant, K 0a , corresponding to the equilibrium between AH+ and a conjugate base, CB, see Eq. (1). CB comprises all the other network species, A, B, Cc and Ct, Eq. (2), and the apparent acidity constant K 0a is related with the equilibrium constants of the network reactions (3)–(6) through Eq. (7) [1]:

AHþ þ 2H2 O ¢ CB þ H3 Oþ ;

K 0a

ð1Þ

½CB ¼ ½A þ ½B þ ½Cc þ ½Ct

ð2Þ

AHþ þ H2 O ¢ A þ H3 Oþ ;

ð3Þ

AHþ þ 2H2 O ¢ B þ H3 Oþ ;

Ka Kh

ð4Þ

B ¢ Cc; K t

ð5Þ

Cc ¢ Ct; K i

ð6Þ

K 0a ¼ K a þ K h þ K h K t þ K h K t K i

ð7Þ

The increase in pK 0a in almost four pH units shows that inside the pores, it is more difficult for AH+ to react either by deprotonation to form A (Eq. (3)) or through hydration to yield B (Eq. (4)). Apparently, the water activity is decreased in the confined microenvironment of the pores, which may arise from both the final drying step that decreases the water content and from the expected decreased mobility of water due to hydrogen bonding with the silanol groups of the pore walls. On the other hand, the distribution of species in MCM-41-Fl-OH in the final equilibrium mixture corresponding to the plateau in the neutral/basic pH region differs substantially from that observed in solution: while in the former there is some Ct but the quinoidal base A is clearly predominant (Fig. 3A, spectrum at pH = 8.29), in ethanol:water 30:70 (v/v) only Ct is present [25]. This is compatible with low availability of water molecules in the pores since the hydration reaction to form B is more water demanding than the deprotonation to form A: deprotonation involves only one water molecule to capture a proton while the hydration reaction requires one water molecule to attack C2 and form the protonated hemiketal and a second water molecule to remove the proton. In the case of SBA-15-Fl-OCH3, Fig. 3B, increasing the pH leads to new bands centered at 380 nm and 285 nm, respectively assigned to the trans-chalcone (Ct) and to the hemiketal (B), on the basis of the similarity of kmax of the model compound Fl-OCH3 in ethanol:water 30:70 (v/v) [25]. In this compound, there are no phenol groups to deprotonate and form a quinoidal base, so the water that is present is used to form the hemiketal that subsequently equilibrates with Cc and Ct (Eqs. (4)–(6)). Similarly to what was observed with MCM-41-Fl-OH, the pK 0a value increases from 0.91 to 4 on passing from solution to the solid hybrid material. Irradiations of the hybrid materials carried out near the absorption maxima of the Ct species (380 nm) were followed by diffuse reflectance UV–Vis spectroscopy, Fig. 4. MCM-41-Fl-OH was irradiated at pH = 3.90, a pH near pK 0a where Ct coexists with AH+ (Fig. 4A), and at pH = 7.37 where A coexists with a short amount of Ct (Fig. 4B). At the acidic pH, irradiation leads to conversion of Ct into AH+ reaching a photostationary state that contains essentially AH+ and apparently also a short amount of A as suggested by the small increase in absorbance around 500 nm. The flavylium cation fully recovers in the dark back to Ct while the small amount of species absorbing in the 500 nm region does not recover (black

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A

B

t= 20 min

8

t= 11 min

4 t= 0 min

F(R)

F(R)

t= 0 min

0

0 300

400

500

600

700

300

400

500

600

700

Wavelength (nm)

Wavelength (nm)

C t= 20 min

F(R)

8

t= 0 min 4

0 300

400

500

600

700

Wavelength (nm) Fig. 4. Irradiation at 380 nm of MCM-41-Fl-OH previously equilibrated at pH = 3.90 (A) and at pH = 7.37 (B); and of SBA-15-Fl-OCH3 at pH = 3.80 (C). Thermal recovery in the dark at room temperature (black traces) occurred in the time scale of a few hours.

trace in Fig. 4A). Irradiation of Ct at pH = 7.37 leads to the formation of A that recovers back to Ct in the dark. The MCM-41-Fl-OH hybrid material is then characterized by a pH-dependent photochromism in the solid state: at acidic pH the color alternates between those of Ct and AH+ while at neutral pH the color alternates between those of Ct and A. In the case of SBA-15-Fl-OCH3 (Fig. 4C), irradiation of Ct at pH = 3.80 leads to a photostationary state that contains both AH+ and hemiketal B (kmax  280 nm). The process is reversible in the dark with full recovery of the initial absorbance of the flavylium. However, the chalcone is not fully recovered and the higher absorbance at 280 nm relatively to the initial spectrum suggests that it remained in the hemiketal form. The overall results show that the synthesized hybrid materials show up as pH-dependent photochromic materials, a behavior characteristic of the flavylium/2-hydoxychalcone system.

fates, respectively. Characterization of the materials by powder X-ray diffraction, N2 adsorption, solid-state 13C CPMAS NMR spectroscopy, and thermogravimetric and elemental analyses, confirm the covalent bonding of the flavylium moieties with loadings of 16.90 ± 0.05 and 11.78 ± 0.04% (w/w) for MCM-41-Fl-OH and SBA-15-OCH3, respectively. The new hybrids show pH-dependent reflectance spectra resembling those observed in solution for the grafted compounds, but shifted to higher pH ranges, indicating a higher stability of the flavylium cations. Irradiation of these materials equilibrated at adequate pH values where the photoisomerizable trans-chalcones predominate shows formation of the respective flavylium cations that recover back to the initial compositions upon standing in the dark, leading these new organic–inorganic hybrids as pH-dependent photochromic materials. Acknowledgments

4. Conclusions Two new hybrid photochromic mesoporous materials MCM-41Fl-OH and SBA-15-Fl-OCH3 based on MCM-41 and SBA-15 were synthesized by covalent attachment of 30 -butoxy-7-hydroxyflavylium (Fl-OH) and 30 -butoxy-7-metoxyflavylium (Fl-OCH3) hydrogensul-

This work was supported by European project NMP4-SL-2012310651 under FP7-NMP-2012-SMALL-6 and by Fundação para a Ciência e Tecnologia through the National Portuguese NMR Network, grant PEst-C/EQB/LA0006/2011 and projects PTDC/QUI-QUI/104129/2008 and PTDC/QUI-QUI/119932/2010.

S. Gago et al. / Microporous and Mesoporous Materials 180 (2013) 40–47

Dr. A. Coelho and Prof. F. Lemos (IST-UTL) are acknowledged for TGA and powder X-Ray diffraction data. [15]

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