Langmuir 2007, 23, 2261-2268
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Tethering of Modified Reichardt’s Dye on SBA-15 Mesoporous Silica: The Effect of the Linker Flexibility Sonia Fiorilli,†,⊥ Barbara Onida,†,# Claudia Barolo,‡ Guido Viscardi,‡ Daniel Brunel,§ and Edoardo Garrone*,†,# Dipartimento di Scienza dei Materiali e Ingegneria Chimica, Politecnico di Torino, Corso Duca degli Abruzzi 24, 10129 Torino, Italy, NIS Centre of Excellence and Dipartimento di Chimica Generale ed Organica Applicata, UniVersita` di Torino, Corso M. d’Azeglio 48, Torino, Italy, and Laboratoire des Mate´ riaux Catalytiques et Catalyse en Chimie Organique - UMR-CNRS 5618, ENSCM- 8 rue Ecole Normale, F- 34296 Montpellier, Cedex 5, France ReceiVed September 26, 2006. In Final Form: NoVember 30, 2006 Solvatochromic Reichardt’s dye has been covalently anchored to both aniline-functionalized and propylaminefunctionalized SBA-15 mesoporous silicas. The former offers a rigid linker to the surface; the latter offers a flexible one. The optical properties of immobilized dye in the presence of various vapors and gases were investigated by means of in situ diffuse reflectance UV-visible spectroscopy. The nature of the linker (rigid or flexible), used to covalently immobilize the dye, was found to play a significant role in determining the solvatochromic response of the chromophore to molecules. The use of the rigid linker, which reduces dye-support secondary interactions, represents a significant improvement in view of sensing applications, due to the stronger effects of the interaction with molecules from the gas or vapor phase on the visible absorption spectrum. This study provides a direct observation of the effect of linker flexibility on the behavior of anchored species.
1. Introduction Ordered mesoporous silicas, because of their highly uniform and versatile porosity, are excellent hosts for sensing molecules. The transparency in the UV-visible (UV-vis) spectrum, typical of these materials, renders them particularly interesting for the synthesis of chemical sensors with optical response.1,2 To this purpose, the use of solvatochromic dye molecules, such as betaines, is promising,3-5 as exposure to several gaseous molecules leads to easily observable changes in the band intensity and position. Within the class of solvatochromic dyes, Reichardt’s betaine dye [2,6-diphenyl-4-(2,4,6-triphenyl-N-pyridinio)-phenolate, referred to hereafter as RD] exhibits one of the largest negative solvatochromic effects,3 with a hypsochromic shift of the visible absorption band of approximately 350 nm when changing the solvent from tetrahydrofuran to methanol. The transition corresponds to an intramolecular electron transfer from the phenolate ring of the molecule to the pyridinium ring, as shown in Scheme 1.6 The extreme sensitivity of RD solvatochromic shift is due to the large dipole moment change between the ground and lowest excited states. Polar solvents stabilize the * Corresponding author. E-mail:
[email protected]; fax: (+39)011-5644699. † Politecnico di Torino. ‡ Universita ` di Torino. § Laboratoire des Mate ´ riaux Catalytiques et Catalyse en Chimie Organique - UMR-CNRS 5618. ⊥ LaTEMAR, Centre of Excellence funded by MIUR (Italian Ministry for Education, University and Research). # INSTM, Unita ` di Ricerca Torino Politecnico. (1) Scott, B. J; Wirnsberger, G.; Stucky, G. D. Chem. Mater. 2001, 13, 3140. (2) Schulz-Ekloff, G.; Wo¨hrle, D.; van Duffel, B. R.; Schoonheydt, A. Microporous Mesoporous Mater. 2002, 51, 91. (3) Reichardt, C. SolVents and SolVent Effects in Organic Chemistry; VCH: Weinheim, Germany, 1988. (4) Krech, J. H.; Rose-Pehrsson, S. L. Anal. Chim. Acta 1997, 341, 53. (5) Dickert, F. L.; Geiger, U.; Lieberzeit, P.; Reutner, U. Sens. Actuators, B 2000, 70, 263. (6) Kovalenko, S. A.; Eilers-Ko¨nig, N.; Senyushkina, T. A.; Ernsting, N. P. Phys. Chem. A 2001, 105, 4834.
Scheme 1. Structure of the Ground State and First Excited State of RD6
ground state, because of the larger dipole moment, more than they do with the excited state, thus increasing the energy of transition. RD has already been immobilized in polymeric substrates, silica, and glasses7-8 to obtain optochemical sensors for the detection of gas species, including humidity. Also, several authors have used RD to study the local polarity and the hydrogen-bond donor properties of different surfaces, including amorphous silica and MCM-41;9-15 the Brønsted acid/base properties of the dye molecule have been successfully employed for the synthesis of material responding to ammonia or amine vapors.16,17 (7) Blum, P.; Mohr, G. J.; Matern, K.; Reichert, J.; Spichiger-Keller, U. E. Anal. Chim. Acta 2001, 432, 269. (8) Crowther D.; Liu, X. Chem. Commun. 1995, 2445. (9) Chronister, C. W.; Drago, R. S. J. Am. Chem. Soc. 1993, 115, 4793. (10) Macquarrie, D. J.; Tavener, S. T.; Gray, G. W.; Heath, P. A.; Rafelt, J. S.; Saulzet, S. I.; Hardy, J. J. E.; Clark, J. H.; Sutra, P.; Brunel, D.; Di Renzo, F.; Fajula, F. New J. Chem. 1999, 23, 725. (11) Spange, S.; Zimmermann, Y.; Graeser, A. Chem. Mater. 1999, 11, 3245. (12) Spange, S.; Vilsmeier, E. Colloid Polym. Sci. 1999, 277, 687. (13) Spange, S.; Vilsmeier, E.; Zimmermann, Y. J. Phys. Chem. B 2000, 104, 6417. (14) Voigt, I.; Simon, F.; Estel, K.; Spange, S. Langmuir 2001, 17, 3080. (15) Macquarrie, D. J.; Tavener, S. T.; Gray, G. W.; Heath, P. A.; Rafelt, J. S.; Saulzet, S. I.; Hardy, J. J. E.; Clark, J. H.; Sutra, P.; Brunel, D.; Di Renzo, F.; Fajula, F. New J. Chem. 1999, 23, 725. (16) Onida, B.; Fiorilli, S.; Borello, L.; Viscardi, G.; Macquarrie, D.; Garrone, E. J. Phys. Chem. B 2004, 108, 16617.
10.1021/la062829i CCC: $37.00 © 2007 American Chemical Society Published on Web 01/12/2007
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Covalent attachment of RD to silica substrate was first carried out by Crowther et al.,8 who reported the immobilization of an amino-derivative of the dye via thiourea and amide linkage with retention of solvatochromic behavior. In this contribution, RD has been covalently anchored to the surface of large-pored SBA-15 mesoporous silica. The aim of this work is to investigate the effect of the linker, in terms of flexibility, on the behavior of the tethered dye. Tethering via both flexible and rigid linkers has been reported in the past for chiral catalyst in MCM-41, the use of the latter being meant to reduce the interactions between the tethered catalyst and the surface, thus influencing the catalytic performances.18 In the present work, RD has been covalently anchored by postsynthesis modification on SBA-15 silica, using either a flexible linker, propylamine (-(CH2)3NH2), and a rigid one, aniline (-p-C6H6-NH2), which is expected to reduce secondary interactions between the dye and the surface. Preparation was carried out through three successive steps: (i) modification of RD synthesis to produce a carboxylic derivative of the dye able to react with the organic species on silica substrates; (ii) postsynthesis tethering of a proper organosilane agent on surfactant-free SBA-15 silica; and (iii) coupling reaction between the functionalized silica and the modified chromophore. The solids were characterized by means of X-ray powder diffraction (XRD), nitrogen adsorption analysis and Fourier transform infrared (FT-IR) spectroscopy. Optical behavior was investigated by means of Diffuse reflectance UV-vis spectroscopy in the presence of vapors. 2. Experimental 2.1. Materials. Acetophenone was distilled under reduced pressure. Anhydrous solvents, 4-cyanobenzaldehyde, 42% aqueous tetrafluoroboric acid, sodium tetrafluoroborate, concentrated aqueous hydrochloric acid, glacial acetic acid, and sodium methoxide were purchased from Sigma Aldrich and used as received. 4-Amino2,6-diphenylphenol was prepared according to the literature.19,20 Aminopropyltriethoxysilane, p-aminophenyltriethoxysilane, and trimethysilylimidazole were purchased from Sigma Aldrich and used as received. 2.2. Synthesis of Acyl Chloride RD. Compound 4 was prepared according to Scheme 1. 1H NMR, IR, mass spectroscopy (MS), and melting point (mp) were in agreement with the literature data.21,22 Details and yields follow: 3-(4-(Cyanophenyl)-1-phenyl-2-propen-1-one (1). Compound 1 was prepared according to the literature.19 The yield was 74%. 1H NMR (CDCl3): δ 7.52 (t, H4), 7.60 (t, H5), 7.62 (d, H3), 7.61-7.73 (2d, Ha e Hb), 7.65 (d, H2), 8.01 (d, H1). IR (KBr) 2221 cm-1 (CtN), 1665 cm-1 (CdO). MS (EI): 233 (M+), 204, 156, 128, 105, 77, 51, 40. 4-(4-(Cyanophenyl)-2,6-diphenylpyrylium Tetrafluoroborate (2). To a warm (70 °C) solution of 1 (5 g, 21 mmol) and acetophenone (1.29 g, 10 mmol) in DCE (20 mL) was added BF3‚OEt2 (12 mL) dropwise under nitrogen atmosphere. The mixture was stirred at 90 °C for 6 h. After the solution was cooled to room temperature and the precipitate was filtered and dried, the crude product was crystallized from methanol. Yield: 36%. 1H NMR (DMSO-d6): δ 7.53-7.71 (m, 6H), 8.03-8.07 (m, 2H, Ha o Hb), 8.60-8.71 (m, 6H), 9.24 (s, 2H, H3 o H5); (CD3COCD3): 7.79-7.94 (m, 6H), (17) Onida, B.; Borello, L.; Fiorilli, S.; Bonelli, B.; Otero Area´n C.; Garrone, E. Chem. Commun. 2004, 2496. (18) Hultman, H. M.; De Lang, M.; Nowotny M.; Arends, E.; Hanefeld, U.; Sheldon, R. A.; Maschmeyer, T. J. Catal. 2003, 217, 264. (19) Johnson, B. P.; Gabrielsen, B.; Matulenko, M.; Dorsey, J. G.; Reichardt, C. Anal. Lett. 1986, 19, 939. (20) Osterby, B. R.; McKelvey, R. D. J. Chem. Educ. 1996, 73, 260. (21) Reichardt, C.; Harbush-Go¨rnert, E.; Scha¨fer, G. Liebigs Ann. Chem. 1988, 839. (22) Imai, Y.; Chujo, Y. Macromolecules 2000, 33, 3059.
Fiorilli et al. 8.17-8.21 (m, 2H, Ha o Hb), 8.66-8.74 (m, 6H), 9.29 (s, 2H, H3H5). IR (KBr): 2227 cm-1 (CtN), 1083 cm-1 (BF4). MS (EI): 334 (M+), 304, 227, 202, 167, 105, 77, 44, 40. 4-(4-(Cyanophenyl)-1-(4-hydroxy-3,5-diphenylphenyl)-2,6-diphenylpyridinium Tetrafluoroborate (3). Compound 3 was prepared under anhydrous conditions. A hot (reflux) solution of 2 (2 g, 4.7 mmol) and 4-amino-2,6-diphenylphenol (1.43 g, 5.5 mmol) in ethanol (25 mL) was stirred for 5 h. A dark red color was evidenced. After the solution was cooled to room temperature and tetrafluoroboric acid (6.5 mL) was added, the slightly brown precipitate was filtered and collected. The yield after recrystallization from chloroform was 65%. 1H NMR (CDCl3): δ 5.55 (s,1H,OH), 7-7.6 (2 m,20 H, phenyl), 7.12 (s, 2H, phenol), 7.75-7.95 (2d,Ha-Hb), 8.06 (s,2H, pyridin). IR (KBr): 2227 cm-1 (CtN), 1085 cm-1 (BF4). MS (EI): 576, 575 (M+), 332, 246, 227, 105, 77, 49. 4-(4-(Carboxyphenyl)-1-(4-hydroxy-3,5-diphenylphenyl)-2,6-diphenylpyridinium Tetrafluoroborate (4). Compound 3 (3 g, 4.5 mmol) in a mixture of concentrated hydrochloric acid (240 mL), glacial acetic acid (150 mL), and water (75 mL) was refluxed for 4h. After the solution was cooled to room temperature and 30% aqueous NaBF4 (300 mL) was added, it was left for 3 days in a refrigerator. The yield after recrystallization from ethanol and water was 71%. 1H NMR (CDCl3): δ 5.50 (s,2H,OH-COOH), 6.86 (s, 2H, phenol), 6.906.95 and 7.17-7.31 (2 m, 20 H, phenyl), 7.87-8.08 (2d,H1), 8.14 (s,2H, pyridine). IR (KBr): 1710 cm-1 (CdO), 1085 cm-1 (BF4). IR spectroscopy was employed to investigate the occurrence of RD modification with a carboxylic group. IR spectra of COOHRD shows a band at around 1620 cm-1, typical of RD dye, and an intense mode at around 1720 cm-1 associated with the CdO stretching vibration of carboxylic group, confirming the presence of the -COOH group in the modified chromophore. The retention of solvatochromic behavior from carboxy-derivatized RD was investigated by observing UV-vis spectra of compound 4 (properly treated with a methanol-sodium methoxide solution to obtain the phenolate form) in various solvents: the magnitude of the solvatochromic shift was found to be retained. 4-(4-((Chlorocarbonyl)phenyl)-1-(4-hydroxy-3,5-diphenylphenyl)2,6-diphenylpyridinium Tetrafluoroborate (5). The conversion of the COOH-RD in the correspondent acyl chloride was carried out by refluxing the compound with SOCl2 (previously distilled) for 1 h at 80 °C. Acyl chloride-RD was used for the anchoring reaction without any further purification. 2.3. Preparation of Silica-Based Materials. SBA-15 silica was prepared according to the procedure reported by Zhao and coworkers.23 Pluronic P123 triblock copolymer (EO20-PO70-EO20, BASF) was used as the surfactant template, and the molar composition of the gel was 1 SiO2:0.017 P123:2.9 HCl:202.6 H2O. The surfactant was dissolved into the a mixture of distilled water (Carlo Erba) and HCl (Merck) at 40 °C. Tetraethylorthosilicate (Aldrich, 98%) was then added to the surfactant solution and stirred for 24h. The mixture was then transferred to a Teflon bottle and aged at 90 °C for 48 h. The product was filtered, washed with water, and dried at 90 °C. Removal of the template was carried out by treating the powder with a 48% H2SO4 solution and by keeping the mixture at 95 °C for 1 day. The product was washed with water until the eluent became neutral, then it was washed with acetone. Aniline-functionalized and aminopropyl-functionalized (referred hereafter as SBA-15-a and SBA-15-p, respectively) samples were prepared by treating surfactant-free SBA-15 silica with alkoxysilane agents (anilinetriethoxysilane and aminopropyltriethoxysilane, respectively) in toluene.24 The reaction conducted in nonpolar solvents favors the uniform distribution of groups onto the surface,24 unlike the tethering of the alkoxysilane agent performed in acid aqueous solutions, which leads to an island-type distribution due to the condensation and clustering of the silylation agent. Surfactant-free silica powders were outgassed in vacuo at 423 K overnight to activate (23) Zhao, D.; Feng, J.; Huo, Q.; Melosh, N.; Fredrickson, G. H.; Chmelka, B. F.; Stucky, G. D. Science 1998, 548, 279. (24) Brunel, D.; Cauvel, A.; Di Renzo, F.; Fajula, F.; Fubini, B.; Onida, B.; Garrone, E. New J. Chem. 2000, 24, 807.
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Scheme 2. Schematic Representation of the Steps Occurring during Sample Preparation
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Fiorilli et al. Table 1. Nitrogen Contents in Amino-modified Samples and Final Dye-containing Samples sample
N content by EA (mmol/g)
N content by TG (mmol/g)
SBA-15-a SBA-15-p SBA-15-a-RD SBA-15-p-RD
1.52 1.73 1.81 2.09
1.62 1.85
Table 2. Textural Feature Properties of SBA-15, SBA-15-a, SBA-15-a-protected, SBA-15-p, and SBA-15-p-protected
Figure 1. XRD patterns of SBA-15, SBA-15-a, and SBA-15-aprotected.
Figure 2. Adsorption-desorption isotherms of SBA-15, SBA-15a, and SBA-15-a-protected. the surface before functionalization. Both aminopropyltriethoxysilane and p-aminophenyltriethoxysilane were added to a stirred suspension of freshly activated SBA-15 silica in 20 mL of toluene at 298 K, under flowing nitrogen. After 30 min of stirring, water in a stechiometric quantity with organosilane and p-toluensulfonic acid was added to the suspension. The latter was stirred at 25 °C for 1h, then the temperature was raised to 60 °C for 6 h and subsequently to 120 °C for 1 h. p-Toluensulfonic acid acts as an acid catalyst, promoting the hydrolysis of the alkoxysilane molecules with respect to condensation, thus reducing the extent of undesired polymerization of organosilane, due to water, at the expense of the reaction with the silica surface.25,26 SBA-15-a and SBA-15-p were recovered by filtration and washed with toluene and dimethylformamide (DMF). Samples were then washed in a Soxhlet apparatus with a 1:1 diethyl ether and dichloromethane mixture, then dried at 120 °C overnight. To avoid undesired reactions between silica and RD during the coupling step, residual silanols were protected by a reaction with trimethysilylimidazole, yielding trimethylsilyl groups on the surface. (25) Martin, T.; Galarneau, A.; Brunel, D.; Izard, V.; Hulea, V.; Blanc, A. C.; Abramson, S.; Di Renzo, F.; Fajula, F. Stud. Surf. Sci. Catal. 2001, 135, 178. (26) Lindlar, B.; Lu¨chinger, M.; Ro¨thlisberger, A.; Haouas, M.; Pirngruber, G.; Kogelbauer, A.; Prins, R.; J. Mater. Chem. 2002, 12, 528.
sample
surface area (m2/g-1)
BdB pore diametera (nm)
pore volumeb (cm3/g)
SBA-15 SBA-15-a SBA-15-a-protected SBA-15-p SBA-15-p-protected
551 439 326 459 360
10.4 9.2 8.6 9.6 8.2
1.24 0.90 0.74 0.87 0.68
a Calculated on the desorption branch. b Pore volume standardized versus pure silica weight.
Trimethysilylimidazole was found to be a suitable agent for the silanization of aminated-silica because it reacts selectively with silanol groups.27 Protected samples were prepared by keeping amino-modified SBA15 silicas, previously outgassed at 423 K overnight, in contact with trimethysilylimidazole in toluene; the suspensions were stirred overnight at 60 °C. Solid phases were recovered by filtration and washed with toluene and DMF. Samples were washed in a Soxhlet apparatus with a 1:1 diethyl ether and dichloromethane mixture, then dried at 120 °C overnight. Functionalized and protected samples were coupled with modified RD by following the procedure described in the next paragraph. 2.4. Silica-Dye Coupling Reaction. COCl-RD (0.93 mmol/g of silica) was dissolved in anhydrous DMF under nitrogen atmosphere, and 1,8-diazobicyclounde-7-ene (DBU) was added to trap HCl gas, which develops during reaction with NH2 functionalities. When the solution was clear, functionalized and protected silica was added, and the resulting suspensions were left refluxing overnight. This procedure was followed both for SBA-a and SBA-p, obtaining SBAa-RD and SBA-p-RD, respectively. Solids were filtered off and washed several times with DMF, EtOH, and diethyl ether. Overnight treatment in a Soxhlet apparatus with a 1:1 diethyl ether and dichloromethane mixture was then carried out. The obtained solids were colorless, as expected, because anchored RD molecules were in the phenolic form (colorless). To convert the latter in the phenolate and thus re-establish the characteristic absorption band in the Visible region, samples were put in contact with a methanol-sodium methoxide solution: powders immediately turned to a strong purple color, typical of RD in polar solvents. All the steps occurring in the samples preparation are reported in Scheme 2. 2.5. Instrumentation. Organic intermediates and dyes were characterized by means of 1H NMR and MS. 1H NMR spectra were performed on a Bruker Avance 200 using d6-DMSO with TMS as the internal standard. NMR signals are described by use of s for singlet, d for doublet, t for triplet, and m for multiplet, and are expressed in δ. MS data were collected by a Thermo Finnigan (EI) Instrument. Samples were characterized by XRD (Philips X’pert, CuKR radiation). Nitrogen adsorption-desorption measurements were conducted at 77 K on a Micrometrics ASAP2000 sorptometer; the Brunauer-Emmett-Teller (BET) specific surface area was calculated in the relative pressure range of 0.04-0.1. The pore size diameter was calculated by using Broekhoff-de Boer (BdB) method on the desorption branch of the isotherm. (27) McMurtrhey, K. J. Liq. Chromatogr. 1988, 11, 3375.
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Figure 3. IR spectra of extracted SBA-15 and SBA-15-a-protected high-frequency region (panel on left) and low-frequency region (panel on right).
Figure 4. (Right) IR spectra of aniline-functionalized SBA-15 before (curve a) and after (curve b) a coupling reaction with RD; (left) propylamine-functionalized SBA-15 before (curve c) and after (curve d) a coupling reaction with RD. Thermogravimetric (TG) analyses were carried out on a Mettler Toledo TG analyzer with a heating speed of 10 K/min under air in a flow of 50 mL/min. Elemental analysis (EA) was performed on a CHNS elemental analyzer by REDOX snc. The FT-IR characterization was carried out on a Bruker FTIR Equinox 55 spectrometer, equipped with a MCT cryodetector. Silica powders were pressed into self-supporting discs and placed in a cell allowing outgassing (residual pressure
