3318
J. Phys. Chem. B 1997, 101, 3318-3323
Optical Rotatory Silica Materials Prepared via Sol-Gel Processes Yen Wei,* Danliang Jin, and Tianzhong Ding Department of Chemistry, Drexel UniVersity, Philadelphia, PennsylVania 19104 ReceiVed: August 20, 1996; In Final Form: December 12, 1996X
A series of transparent, monolithic silica materials doped with 5-20 wt % of (-)-dibenzoyl-L-tartaric acid (DBTA) and (+)-D-glucose have been prepared via the acid-catalyzed sol-gel reactions of tetraethyl orthosilicate in the presence of the organic dopants. The materials show optical rotations, which can be directly measured on a common polarimeter. The optical rotation was found to be proportional to the dopant concentration and to the thickness of the samples. The specific rotations measured at 589 nm at 25 °C are +53° for the D-glucose-doped and -112° for the DBTA-doped silica samples, which are close to those for the same compounds measured in liquid solutions. Both the DBTA- and D-glucose-doped silicas exhibit a simple normal dispersion with negligible concentration effects. The organic compounds retain their stereochemical structures during the sol-gel reactions and, therefore, their optical activities. These materials can be regarded as nanocomposites in which the organic dopants uniformly distribute in the amorphous silica networks.
Introduction Recent progress in doping organic compounds into inorganic oxides using the sol-gel process1-3 opens a broad range of possibilities for the preparation of optical materials, whose properties can be tuned by selecting organic components (dopants) with appropriate photoactivities, bioactivities, and chemical activities.4-13 As examples, organic and organometallic photoactive compounds have been employed to probe the physicochemical structural changes during the sol-gel reactions.14-28 Laser action,8,29 photochromism,30-32 and nonlinear optical effects33,34 were introduced into silica or other metal oxides by doping with appropriate organic dyes. Photocatalysts,35 waveguides, and hole-burning materials36 have been made similarly. Optically based chemical and biochemical sensors were prepared by encapsulation of pH indicators,37,38 selective colorimetric reagents,37,39 enzymes, and other proteins40-51 in inorganic sol-gel networks. Electroactive films were fabricated by incorporating polyaniline into silica matrices.52 The preparation of all these materials or devices becomes possible because the sol-gel reactions can proceed to afford the oxide networks at relatively low temperatures without decomposition or chemical modification of the organic dopants. Under controlled reaction conditions, the organic compounds could be dispersed uniformly in the inorganic network and fabrication of transparent bulk objects, films, and fibers can be achieved.1-3 On the other hand, since the discovery of rotation of the planepolarized light, the optical rotations have been commonly observed under two conditions:53,54 (1) in solution for organic enantiomers that have dissymmetric molecular structures and (2) in the solid state for macroscopic single crystals such as quartz, which are optically active either because of the small chiral molecules composing the crystals or because of the dissymmetry of the crystal structures. An interesting method has also been developed for measuring the optical rotations in powder suspensions of optically active macromolecules such as helical polymers and biological macromolecules.55 We have been studying the polymer-modified silica or other inorganic oxide materials prepared by chemically or photochemically X
Abstract published in AdVance ACS Abstracts, April 1, 1997.
S1089-5647(96)02567-9 CCC: $14.00
catalyzed sol-gel reactions.56-58 Recently, we are particularly interested in the synthesis of optically active sol-gel materials and the basic behavior of chiral molecules in inorganic networks. Since the organic compounds could retain their chemical and stereochemical structures when encapsulated in the networks via the sol-gel process at relatively low temperatures, it is reasonable to assume that the trapped chiral organic compounds would still be optically active. Furthermore, the inorganic matrices such as silica are amorphous and have largely isotropic refractive indexes and excellent optical transparency. Hence, we should be able to obtain new transparent, optically active solid-state materials by the sol-gel process. In this article, we demonstrate that the optically active silica can be prepared by doping (-)-dibenzoyl-L-tartaric acid (DBTA) or D-glucose into a silica network through the acid-catalyzed sol-gel reactions of tetraethyl orthosilicate (TEOS) in the presence of DBTA or D-glucose. The synthetic procedures have been designed to obtain monolithic pieces of the doped silica having excellent transparency and constant thickness so that the optical rotation can be measured directly with a common polarimeter. We have monitored the optical rotations through the entire sol-gel process from initial solutions (sols) to gels and to the final dry products. We have investigated the effects of the concentration of the organic components and the thickness of the samples on the optical rotation and determined the specific rotations of the materials based on Biot’s equation. The specific rotations of the doped silicas are compared with those measured in liquid solutions of DBTA and D-glucose. The effects of dopant concentration on the specific rotation and rotatory dispersion have also been studied. Experimental Section Materials and Instrumentation. Tetraethyl orthosilicate (TEOS, Aldrich) was purified by distillation. (-)-DibenzoylL-tartaric acid (DBTA, monohydrate), (+)-D-glucose, and ethanol (98%) were used as received from Aldrich without further purification. Polystyrene containers (Cell Well from Corning) were used as molds in the preparation of the final products. The optical rotation measurements were conducted on a PerkinElmer 241 polarimeter at a temperature of 25 ( 1 °C. For the measurements of the solids, the doped silica disk was placed in © 1997 American Chemical Society
Optical Rotatory Silica Materials
Figure 1. General sol-gel processing diagram for the preparation of optically active compound doped silica materials.
the polarimeter with the surface of the sample aligned perpendicular to the light pass. The thickness of the disk was measured with a micrometer at five different positions. The variation in the thickness was less than 0.02 mm. For the liquid solutions, the optical rotations were measured with water as solvent using a standard polarimetry cell with a path length of 100 mm. Error margins were estimated to be about (0.002° based on at least three parallel measurements. Thermogravimetric analysis (TGA) was performed on a DuPont 9900 Thermal Analysis System equipped with a TGA951 module under nitrogen or air at a programmed heating rate of 10 °C/min. The density of the bulk monolithic sol-gel materials was determined using Archimede’s method with carbon tetrachloride as the measurement medium at 25 °C within an error margin of (0.02 g/cm3. The BET specific surface areas of the powder samples were determined on a Micromeritics ASAP2010 Micropore Analysis System using the nitrogen adsorption-desorption isotherm method. The samples were degassed for 3-4 days at ambient temperature under a reduced pressure (7 µmHg) prior to the measurements. Optical transparency of the materials was measured on a PerkinElmer Lambda 2 spectrophotometer. Infrared spectra of the organic-doped silica samples as powder-pressed KBr pellets were recorded on a Perkin-Elmer 1600 FTIR spectrometer. Preparation of Optically Active Silica Doped with DBTA or D-glucose. The synthesis was carried out by following similar procedures reported in the literature.1-3 As illustrated in Figure 1, a general synthetic procedure consists of the following four steps: (1) preparation of prehydrolyzed/ condensed TEOS solution by treating TEOS with distilled H2O and EtOH in the presence of HCl(aq) at 60 °C for 1-2 h, during which the initial organic-inorganic phase separation disappeared and a homogeneous solution was obtained, followed by cooling the homogeneous solution to room temperature; (2) preparation of aqueous solutions of D-glucose (or ethanol solution of DBTA) at various concentrations by dissolving different amounts of D-glucose (or DBTA) in the same amount of distilled H2O (or EtOH for DBTA); (3) mixing a portion of the solution from step 1 with one of the solutions from step 2 under stirring to afford a homogeneous combined solution; (4) casting the solution obtained in step 3 into a plastic mold, followed by sealing the mold with a couple of pinholes on the cover to allow the evaporation of solvents and small molecule byproducts of the sol-gel reactions. Finally, the reaction system was allowed to stand at room temperature for gelation and slow drying for
J. Phys. Chem. B, Vol. 101, No. 17, 1997 3319 15-20 days to afford a colorless, highly transparent (to visible light) and monolithic disk of the optically active silica materials. The thickness of the disks was controlled by the amount of solution cast into the molds. For a typical procedure (sample DBTA-10), 10.40 g (0.05 mol) of TEOS, 6.9 g (0.15 mol) of ethyl alcohol, 3.6 g (0.20 mol) of distilled H2O, and 0.25 g of 2 M HCl(aq) were mixed together at room temperature in a 50-mL three-necked round bottomed flask that was equipped with a thermometer and a condenser. The two-phase mixture was stirred at room temperature under nitrogen for about 15 min until the phase separation disappeared and the solution became clearly homogeneous. The solution was heated at 60 °C for 2 h with stirring on a hot-water bath, followed by cooling down to room temperature. To the above solution was added a solution of 0.333 g (0.93 mmol) of (-)-dibenzoyl-L-tartaric acid in 2 mL of ethyl alcohol dropwise with stirring. After stirring for 30 min, the resultant solution was cast into several round polystyrene molds (22 mm in internal diameter and 20 mm in height). The molds containing various amounts of the solution were covered with polystyrene caps having 2-3 pinholes and were allowed to stand at room temperature for 15-20 days. Upon drying, transparent monolithic DBTA-doped silica disks (ca. 16 mm in diameter) were obtained in a variety of thicknesses of 0.5-2.3 mm. The content of DBTA was found to be 8.1 wt % from TGA measurements, which was quite close to the value (10 wt %) calculated from the stoichiometry of the starting materials. Results and Discussion The optically active silica materials were prepared by HClcatalyzed sol-gel reactions of tetraethyl orthosilicate (TEOS) in the presence of various amounts of enantiomerically pure compound D-glucose or DBTA. The compositions of the starting materials and other synthetic parameters are summarized in Table 1. The data for the synthesis of pure sol-gel silica as a control experiment are also included in the table for comparison. The final products were characterized by using FTIR spectroscopy and BET surface area analysis. The FTIR spectra confirm the formation of the silica network (e.g. SiO-Si stretching at 1000-1200 cm-1) and the presence of D-glucose (e.g. C-H bending at 2938 cm-1) or DBTA (e.g. CdO stretching at 1739 cm-1). The total surface areas of the materials (e.g. 2.7 m2/g for the 20% DBTA-doped silica) were found to be much smaller than that (ca. 70-100 m2/g) of the pure sol-gel silica prepared under the same conditions. The synthesis experiments should be carefully designed and performed in order to obtain the products that are suitable for the optical rotation measurements. There are a number of general considerations in the system design. First, the organicdoped silica samples must be homogenous and transparent to the light employed in the optical rotation measurements. Second, the thickness of the sample should be as constant as possible in order to minimize the reflection and refraction of the light. Finally, any physical defects, such as small bubbles and cracks, should be avoided. The optically active organic compounds should have good compatibility with (or solubility in) the sol-gel reaction systems to ensure the homogeneity and optical transparency of the products. We have screened a variety of the organic compounds and found that the compounds with reasonable solubility in water and alcohols are suitable candidates. A large specific rotation is also preferred for more accurate measurements. The compatibility with water and alcohols is of great importance particularly in the later stages of the sol-gel process because water and alcohols are the reaction byproducts. Poor compatibility would result in a
3320 J. Phys. Chem. B, Vol. 101, No. 17, 1997
Wei et al.
TABLE 1: Compositions and Other Synthetic Parameters for the Preparation of the Optical Rotatory Silica Materials via the HCl-Catalyzed Sol-Gel Reactions of Tetraethyl Orthosilicate (TEOS) in the Presence of (+)-D-Glucose and (-)-Dibenzoyl-L-Tartaric Acid (DBTA) sample code
silica
DBTA-5
DBTA-10
DBTA-20
DG-5
DG-10
DG-20
TEOS (mol) C2H5OH (mol) H2O (mol) HCl (mmol) DBTA (mmol) C2H5OH (mL) D-Glucose (mmol) H2O (mL) organic dopant content (wt %) calca expt (TGA)b [R]D (25°C) Td (°C)c
0.05 0.15 0.20 0.50 0.00
0.05 0.15 0.20 0.50 0.439 5
0.05 0.15 0.20 0.50 0.930 5
0.05 0.15 0.20 0.50 2.095 8
0.05 0.15 0.20 0.50
0.05 0.15 0.20 0.50
0.05 0.15 0.20 0.50
0.872 2 5.0 3.8 +50° 209
1.850 3 10.0 9.3 +54° 202
4.167 5 20.0 15.7 +57° 198
0.00 0.0 0.0 0
5.0 3.7 -111° 252
10.0 8.1 -111° 259
20.0 16.2 -116° 255
a The dopant contents were calculated on the basis of the compositions of the starting materials assuming that all the ethoxy groups were hydrolyzed and all the volatile compounds were removed from the final materials. b The experimental dopant contents were determined from TGA curves at 750 °C in air. c The onset decomposition temperature (Td) was determined from TGA curves in air.
macroscopic phase separation between inorganic and organic components during the gelation and drying, yielding translucent or totally opaque materials. DBTA and D-glucose were, therefore, selected for this study. The choice of materials of the casting molds appeared to be important in order to get large crack-free monolithic pieces with a reasonably constant thickness. Common silicate glass or other ceramic containers as the molds were found to have a tendency to deform the samples or cause the formation of cracks, probably because of the development of unevenly distributed stress in the materials caused by the strong affinity between the glass/ceramic wall and the solgel silica materials. Hydrophobic polystyrene cells as the molds gave satisfactory results in this study. As listed in Table 1, the composition of the starting materials was so designed that the percentage of the optically active organic component in the final dried products would be around 5, 10, and 20% by weight. The actual percentages of the organic component in the final products were determined by TGA experiments performed in a temperature range of 25-800 °C under an air atmosphere. Thermal decomposition of DBTA and D-glucose occurred at about 250 and 200 °C, respectively. At 750 °C, the organic component was completely decomposed and vaporized, leaving behind only the inorganic silica, as evidenced by the disappearance of C-H bands in the IR spectra. Thus, the organic content can be calculated from the amount of silica residue at 750 °C. The experimental values of the organic contents are close to, though a little lower than, those calculated from the reactant compositions. The small discrepancy is mainly attributed to the presence of residual volatile compounds in the materials. It should be noted that a very slow drying process (i.e. g20 days at room temperature) was employed in our preparation to ensure the high quality (e.g. crack-free) of samples. Facilitating the drying by heating often leads to the occurrence of cracks and deformation of the samples. The samples prepared without heating contained some residual solvents and byproducts of the sol-gel reactions (
