Cruciform-silica hybrid materials

Article abstract of DOI:10.1002/asia.200800316

Preserving the solution properties of functional fluorophores upon incorporation into solid state sensory schemes remains a significant challenge. To address this concern, a silica scaffold was employed to support functional fluorophores in the solid stat

Full text of DOI:10.1002/asia.200800316

FULL PAPERS
DOI: 10.1002/asia.200800316
Cruciform-Silica Hybrid Materials
Anthony J. Zucchero,[a] Rebecca A. Shiels,[b] Psaras L. McGrier,[a] M. Alicia To,[b]
Christopher W. Jones,[b] and Uwe H. F. Bunz*[a]
Abstract: Preserving the solution prop-
erties of functional fluorophores upon
incorporation into solid state sensory
mesoporous silica particles. By employ-
ing surface-functionalized mesoporous
SBA-15 silica, novel fluorescent cruci-
form-silica hybrid materials are gener-
ated which retain the desirable solution
properties of cruciforms in the solid
state. Organic surface functionalities,
such as acidic, basic, and hydrophobic
groups employed on the silica scaffold,
modulate the observed emissions of
the resulting solid state materials. The
potential of these XF-silica hybrid ma-
terials to display sensory responses to
representative vapor-phase analytes is
demonstrated.
schemes remains
a significant chal-
lenge. To address this concern, a silica
scaffold was employed to support func-
tional fluorophores in the solid state.
Herein, we report an effort to support
1,4-distyryl-2,5-bisarylethynylbenzene
cruciforms (XFs) using functionalized
Keywords: cruciforms · functional
fluorophores · materials science ·
mesoporous materials · sensors
Introduction
secondary functions such as suppressing aggregation/excimer
formation or aiding in preconcentration of analytes. An ele-
gant example of this approach is the work by Rakow and
Suslick, who investigated the response of an array of immo-
bilized porphyrin dyes towards a battery of different ana-
lytes.[2] The success of their colorimetric approach was
rooted in the immobilization of their dyes onto hydrophobic
silanized silica gel which helped to pre-concentrate gaseous
or liquid analytes either from the gas phase or from the
aqueous phase onto their solid support, where they could
react with the dye under consideration.
We are interested in fluorophores where the frontier mo-
lecular orbitals (FMOs), HOMO and LUMO, are spatially
separated, as is achieved in 1,4-distyryl-2,5-bisarylethynyl-
benzenes by equipping the two differentiated molecular
axes of these materials with donor and/or acceptor substitu-
ents.[3–5] These cruciforms (XF) display exceptionally large
red or blue shifts in absorption and emission upon exposure
to acids or metal salts. Other examples of FMO-separated
species include Haleyꢀs tetraethynylbenzenes,[6] Scherfꢀs
swivel cruciforms,[7] and Fahrniꢀs triarylpyrazolines.[8]
Functional chromophores and fluorophores are attractive as
sensory and responsive materials in biology, materials sci-
ence, organic electronics, and analytical chemistry.[1] For de-
ployment in biological applications such as the targeted
staining of cell compartments, water soluble fluorophores
appended with binding elements are highly desirable and
necessary. To enable charge transport for applications in or-
ganic electronics, chromophores/fluorophores must be capa-
ble of forming high quality, ordered thin films. For many en-
vironmental and biodiagnostic sensory applications, it is de-
sirable if the fluorophores or chromophores utilized for
analysis are immobilized—temporarily or permanently—on
a solid support. Such solid supports can either be just a scaf-
fold for the dye(s) under consideration, or they can perform
[a] A. J. Zucchero, P. L. McGrier, Prof. U. H. F. Bunz
School of Chemistry and Biochemistry
Georgia Institute of Technology
Coordination of metal cations to XFs results in an either
red- or blue-shifted emission if pyridines or dialkylanilines
are incorporated.[3a,b] If both are present, a two-stage mecha-
nism, where there is first a blue shift followed by a red shift,
is observed that results from the complexation of an XF
such as 5 with increasing amounts of zinc or magnesium
ions. If we incorporate hydroxyl groups into the p-system of
these functional fluorophores, we observe fluorescence shifts
Atlanta, GA 30332 (USA)
Fax : (+1)404-385-1795
E-mail: uwe.bunz@chemistry.gatech.edu
[b] R. A. Shiels, M. A. To, Prof. C. W. Jones
School of Chemical and Biomolecular Engineering
Georgia Institute of Technology
Atlanta, GA 30332 (USA)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/asia.200800316.
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Chem. Asian J. 2009, 4, 262 – 269

upon deprotonation. These compounds can serve as fluores-
cent probes for the differentiation of amine bases.[5]
Results and Discussion
Synthesis
In this contribution we describe the interaction of XFs 1–
7 with mesoporous SBA-15 silica materials A–D containing
acidic sites (A), basic sites (B), hydrophobic trimethylsilyl
sites (C), and bare, unfunctionalized silica containing sila-
nols (D) (Scheme 1). We investigate the resulting cruciform-
XF fluorophores consist of a tetrasubstituted benzene core
with two arylethynyl and two distyryl branches affixed in a
1,4-2,5-relationship. The synthesis of XFs 1–5 has been pre-
viously reported;[3a,b,f,g] the synthesis and characterization of
XFs 6 and 7 are detailed in the Supporting Information. A
generalized synthetic methodology permitting access to the
cruciforms is outlined in Scheme 2. A Horner coupling of 8
Scheme 2. General synthetic pathway providing access to XFs 1–7.
with aryl aldehydes 9a–g provides for attachment of the dis-
tyryl branch of the cruciforms and gives 10a–g in good
yields.[9] A subsequent Sonogashira coupling is utilized to
affix the arylethynyl substituents.[10] This pathway permits
the synthesis of XFs 1–7 in yields of 53–91%. Incorporation
of hydroxy functionality in the case of 6 and 7 requires tet-
rahydropyran (THP) protection of 4-hydroxybenzaldehyde
prior to the Horner olefination. Following the Sonogashira
coupling, deprotection with trifluoroacetic acid readily
yields 6 and 7 from their THP-protected precursors in 88%
and 91%, respectively.
Mesoporous silica SBA-15 was identified as a good candi-
date for a porous host material.[11] SBA-15 can be easily pre-
pared by block copolymer templating methods and the size
of the mesopores can be controlled. In this work, SBA-15
with an average pore diameter of 57 ꢂ and a surface area of
~700 m2 g was prepared using standard methods (Supporting
Information).[12] After calcination to remove the block copo-
lymer template, the material was functionalized by standard
silane grafting techniques to introduce Lewis basic amino-
propyl groups,[13] Brønsted acidic sulfonic acid groups,[14] or
hydrophobic trimethylsilyl groups.[15] Changes in surface
properties were verified by nitrogen physisorption and ther-
mogravimetric analysis.
Scheme 1. Structure of XFs 1–7 and a schematic representation of the
surface functionality of silicas A–D.
silica hybrid materials by optical and fluorescent spectros-
copies. It was of great interest to examine the interactions
between the various XFs and the different mesoporous
silica samples, establish what emission responses would be
observed, whether support of XFs on silica would allow the
XFs to maintain their fluorescence properties in the solid
state, and if these solid-state adsorbed XFs could be used to
detect amines or organic acids in the gas phase.
Spectroscopic Properties of the Cruciforms 1–7 in the
Presence of Microstructured Functionalized Silica Supports
XFs 1–7 emit vibrantly in organic solutions. We have de-
tailed their sensory responses towards metal cations, pro-
tons, and amines.[3a,b;5] Emissions of the XFs in the solid
state are generally red shifted, broadened, and less intense,
limiting their potential use as sensory materials in the solid
state (Figure 1). A possible method to overcome these limi-
tations is to employ the fluorophore immobilized on a solid
support for potential environmental and biodiagnostic appli-
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U. H. F. Bunz et al.
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Figure 2. Vials containing XFs 1–7 in toluene incubated with silicas (D=
Bare silica, C=Capped Silica, A=Acidic Silica, B=Basic Silica) for
24 h. For comparison, column F shows XFs 1–7 in toluene exposed to tri-
fluoroacetic acid (1–5) or n-hexylamine (6,7). Column E shows XFs 1–7
in a toulene solution. Photos were taken under blacklight ex=365 nm)
and photographed using a Canon EOS Digital Camera equipped with an
EFS 18–55 mm lens.
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-
Figure 1. Normalized emission spectra of 1–7 in toluene (top, -&- 1, -
2, -x- 3, -*- 4, -~- 5, -~- 6, -&- 7) and the solid state (bottom -&- 1,
^
^
-
- 2, -x- 3, - - 4, -~- 5, -~- 6, -&- 7). In the solid state, spectra are
(XF 7) to 548 (XF 4) nm. In addition, the intensity and
shape of the observed emissions are reminiscent of those ob-
served in solution, not those observed in the solid state.
Thus, mesoporous SBA-15 silica appears to be a promising
platform to enhance and/or modulate XF fluorescence.
XFs 1–5—all of which possess Lewis base moieties—show
large shifts in fluorescence upon exposure to acidic silica.
These shifts can be rationalized by assuming protonation of
1–5 occurs upon exposure to the sulfonic acid moieties pres-
ent on these silica particles. We have previously established
that upon donor and/or acceptor substitution, XFs can dis-
play spatially separated FMOs. In the case of 1 and 2, the
LUMO is localized primarily on the acceptor-substituted
axis of the molecule while the HOMO resides on the ꢃnon-
substitutedꢀ branch of the XF. Upon protonation of the pyri-
dine, the LUMO is stabilized while the HOMO remains
largely unaffected, resulting in large bathochromic shifts in
1 (434 to 537 nm) and 2 (446 to 555 nm) in emission (Fig-
ure 4A).
broadened, redshifted, and of dramatically decreased intensity compared
to in solution. Spectra of XFs in the solid state are noisy due to scattering
off of the powdered solid as well as relatively low fluorescence intensity.
cations. Solid supports serve as scaffolds for the dye(s)
under consideration; they may also suppress aggregation/ex-
cimer formation or preconcentrate analytes.
XFs 1–7 were dissolved in toluene and dry mesoporous
silica was added. The resulting suspensions were incubated
in the dark for 24 h, at which point the samples were photo-
graphed under UV light (ex=365 nm) to qualitatively exam-
ine the resulting fluorescence of the cruciform-silica hybrid
materials. As Figure 2 shows, the solid silica settled to the
bottom of the vials and was highly fluorescent. To more
quantitatively assess the fluorescence of these XF-silica
hybrid materials, we recorded the fluorescence spectra of
suspensions of these hybrid materials in toluene using a tri-
angular cuvette to minimize scattering (Figure 3 and
Table 1). When compounds 1–7 are exposed to capped
silica, the emission of the XF-silica hybrids ranges from 424
In the case of 3 and 4, we observe large hypsochromic
shifts upon protonation. Upon incubation with acidic silica,
we observe blue shifts in the
emission of 3 (492 to 427 nm)
Table 1. Tabulated emission data of XFs 1–7 in the solid state, solution, and complexed with functionalized
silica. For reference, emissions of 1–7 upon exposure to trifluoroacetic acid and n-hexylamine in toluene solu-
tion. All lmax emission values are reported in nm.
and 4 (547 to 433 nm). This is a
consequence of the FMO struc-
ture of these donor/acceptor-
substituted XFs. In compounds
3 and 4, the HOMO is localized
on the electron-rich distyryl
axis of the XF while the
LUMO lies on the arylethynyl
arms. Protonation of the alkyl-
XF
Solid
Toluene
Bare
Capped
Acidic
Basic
TFA
Hexylamine
1
2
3
4
5
6
7
515
515
615
625
605
515
550
434
446
492
547
531
473
424
508
513
426, 492
428, 547
468
434
446
492
548
531
475
424
537
555
427
433
523
473
426
434
446
492
546
531
476, 550
513
530
555
424
432
532
n/a
n/a
n/a
n/a
n/a
n/a
n/a
561
454, 497
475
425
AHCTUNGTRENNaGUN niline functionalities stabilizes
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Cruciform-Silca Hybrid Materials
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^
Figure 3. Normalized emission spectra of 1–7 supported on bare (-&-), capped (- -), acidic (- -), and basic (-x-) silica. For comparison, the emission of
XFs 1–7 in toluene (-&-), 1–5 with trifuoroacetic acid (-~-), and 6–7 with n-hexylamine (-~-) are shown. Spectra were taken of the suspended silica par-
ticles in toluene using a triangular cuvette. Emission maxima are shown in Table 1.
the HOMO while the LUMO remains unaffected, resulting
in a blue shift (Figure 4B).
We also observe a small blue shift (531–523 nm) upon in-
cubation of 5 with acidic silica. We are able to rationalize
this slight blue shift as the consequence of the two-stage
fluoresence response previously observed upon reaction of 5
with trifluoroacetic acid.[3a,b] In the case of 5, the HOMO
lies on the donor-substituted distyryl axis of 5, while the
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U. H. F. Bunz et al.
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bly because the acidic support adsorbs all the basic XFs
from solution. In all cases, the emissions observed for com-
plexes of 1–5 with A are similar to emissions recorded upon
addition of excess trifluoroacetic acid to 1–5; 6 and 7 show
no change in emission upon exposure to A. This can be ra-
tionalized by assuming that the hydroxy functionalities pres-
ent in these XFs do not react with the acidic functional
groups of the silica particles. As a result, the emission of the
resulting composites are roughly identical to the emissions
observed upon complexation with capped silica.
Upon exposure of 1–7 to basic silica, an opposite response
is observed. The composites of Lewis basic substituted XFs
1–5 with basic silica B display the same emission as 1–5 with
C. This is readily rationalized by assuming that the basic sur-
face functionality of B does not interact with these XFs and
affect the photophysics of 1–5. In the case of 6 and 7, the
chromophores possess hydroxy substituents which interact
with the amino-functionalized surface of B. We have previ-
ously reported that hydroxy-functionalized XFs such as 6
and 7 can display shifts in emission upon exposure to
amines and other bases. Similar effects are observed here
upon complexation of 6 and 7 with B. Reaction with B de-
protonates the hydroxy functionalities destabilizing the
HOMO of 6 and 7 while the LUMO remains relatively un-
perturbed (Figure 4C). As a result, bathochromic shifts are
observed upon complexation of hydroxy-functionalized XFs
with B. In the case of 6, a large redshift is readily visible in
Figure 2; this is observed as a large shoulder in the emission
of 6·B centered near 550 nm. Some of the unreacted XF 6
also remains in the silica, which appears dominant because
of the relatively low emission intensity of the sample as well
as the higher quantum yield of the blue species relative to
the red species. Upon exposure of 7 to B, we observe a simi-
lar redshift from 424 nm to 513 nm.
Complexation of XFs 1–7 with bare silica D also gener-
ates fluorescent hybrid materials. The surface chemistry of
D is mildly acidic; therefore, one might expect to observe
similar responses to those observed for the sulfonic-acid
functionalized silica A. Upon exposure of 6 and 7 to bare
silica, solids are formed which retain the fluorescence of 6
and 7 in solution. As in the case of A, no large shifts in
emission are observed upon formation of 6·D and 7·D. In
the case of the complex of 3 with D, we observe little
change in emission qualitatively. Spectroscopic examination
of 3·D reveals a small amount of a blueshifted species pres-
ent in the hybrid material at 426 nm, corresponding to the
emission of the protonated XF 3. However, the majority of
the XF is deposited in the complex as the native unprotonat-
ed 3, responsible for the dominant emission at 492 nm. A
similar result is observed in the case of 4·D. Here we ob-
serve a dominant emission at 547 nm originating from un-
protonated 4; however, a small blueshifted band is observed
at 428 nm, contributed by protonated 4.
Figure 4. Schematic representation of the effect of protonation upon the
FMOs and emission of XFs 1 (A, top left) and 3 (B, top right). C
(bottom) shows the effect of deprotonation on the FMOs and emission
of 6.
LUMO is localized primarily on the arylethynyl branch of
the XF. Upon exposure to acidic silica, the protonation of
all four nitrogens stabilizes both the HOMO and the
LUMO, resulting in a slight net blue shift. As the digital
photograph indicates, the toluene supernatant was complete-
ly non-fluorescent upon incubation of 1–5 with A, presuma-
Upon reaction of XF 5, containing both alkylamino sub-
stituents and pyridyl substituents, with bare silica particles,
we observe a large hypsochromic shift from 531 nm to
468 nm. This emission is attributed to the bisprotonated
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Cruciform-Silca Hybrid Materials
state of 5 and is consistent with the emission observed in
previous titrations of 5 with trifluoroacetic acid.
When 1 and 2 are exposed to bare silica, bathochromic
shifts are observed upon formation of hybrids 1·D and 2·D.
In the case of 1, a shift from 434 to 508 nm is observed; in 2,
the emission shifts from 446 to 513 nm. These bathochromic
shifts are consistent with an interaction which stabilizes the
LUMOs of the XFs while leaving the HOMOs unperturbed
(i.e., protonation); however, the magnitude of the shift is
considerably smaller in both cases as compared with those
shifts observed upon addition of sulfonic-acid functionalized
silica or trifluoroacetic acid. We attribute this to hydrogen
bond formation rather than true protonation. It is interest-
ing to note that while the alkylamino functionalities are con-
siderably more basic than the pyridine moieties, the experi-
mental results suggest that protonation of the pyridine nitro-
gens in 1·D and 2·D appears more favorable than protona-
tion of the alkylamino nitrogens in 3·D and 4·D. We attri-
bute this to the steric effects of the dibutyl chains which
limit the interaction of the aniline nitrogens with the silica
surface.
Sensory Responses of XF-Functionalized Silica
Microstrucures Towards Representative Volatile Organic
Compounds (VOCs)
Functionalized mesoporous silica structures provide an at-
tractive platform for the solid-phase support of XFs. We
were anxious to assess the potential of these fluorophores to
respond to the presence of vapor-phase analytes. We ex-
posed 1 supported on all four functionalized silicas to repre-
sentative vapor phase analytes of interest. This proof-of-
principle sensing experiment was conducted using dried XF-
silica hybrids. After incubation of the desired XF dye with
the functional silica scaffold of choice, evaporation of the
solvent in vacuo yields dry, vibrantly fluorescent solids (Fig-
ure 5A).
Figure 5 shows the responses observed upon exposure of
1 (A) to triethylamine (B) and trifluoroacetic acid (C)
vapors. In the dry solid state, the hybrid materials resulting
from the exposure of XF 1 to both basic and capped silica
display emission at approximately 460 nm. Incorporation of
1 into/onto bare and acidic particles generates materials
with emission at 550 and 555 nm respectively. Upon expos-
ing these solids to NEt3 vapors for five minutes, large hypso-
chromic shifts in the emission of the acidic and bare hybrid
materials are observed while the emission of the capped and
basic materials remain largely unchanged; the result is
nearly identical emissions at between 460 and 465 nm for all
materials. Upon exposure to trifluoroacetic acid, a large red
shift in the emission of the capped and basic hybrid materi-
als is observed. However, the emission of the acidic and
bare composites remain largely intact, resulting in similar
emissions—ranging from 560 to 580 nm—for all four cases.
These responses can be rationalized by considering the
protonation states of XF 1 when deposited on silica scaf-
folds and when exposed to vapor-phase analytes. The emis-
Figure 5. Fluorescence response of 1 supported on functionalized silica
scaffold upon exposure to vapor analytes. The top spectra displays the
^
^
emission of 1 supported on bare (- -), capped (-&-), acidic (- -), and
basic (-~-) silicas. Upon exposure to NEt3 (middle) and trifluoroacetic
acid (bottom) vapors, notable fluorescence responses are observed.
sions of hybrids 1·C and 1·B centered at 460 nm indicate the
presence of the nonprotonated XF 1. Emissions at 550 and
555 nm recorded for 1·D and 1 A, respectively, correspond
to the expected protonated form of 1. Upon exposure to
ambient NEt3 vapors, we observe large bathochromic shifts
in 1 A and 1·D while the emissions of the capped and basic
hybrids remain unchanged; after exposure, the emissions of
all four species appear between 460 and 465 nm. This can be
explained by assuming exposure to NEt3 vapor causes the
deprotonation of 1 supported in/on 1 A and 1·D, restoring
their emission to the native form. A similar but opposite
effect is observed upon exposure to trifluoroacetic acid
vapors. Upon exposure, the bathochromic shift is observed
in the case of 1·B (460–570 nm) and 1·C (460–560 nm) while
acidic and bare hybrids of 1 remain unchanged. This finding
is consistent with the protonation of 1 in the basic and
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U. H. F. Bunz et al.
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were acquired using a Shimadzu RF-5301PC spectrofluorophotometer or
a PTI QuantaMaster spectrofluorophotometer outfitted with a xenon arc
lamp and series 814 PMT detector. To minimize scattering, spectra of
silica suspensions were obtained using a triangular cuvette. Scattering
peaks were removed by subtracting a fluorescence spectra of suspended
silica with no added fluorophores from all spectra. Solid state emission
spectra of XFs and dried functionalized silica materials were acquired
using a Spectra Max M2 plate reader from Molecular Devices.
capped hybrids, resulting in the observed redshift in these
samples.
The shifts observed upon exposure of these XF-silica hy-
brids are not readily reversed upon incubation of the react-
ed solids under a flow of air. Over 1 hour, no reversal of
these shifts is observed in the emission spectra of the react-
ed hybrids (see Supporting Information). In this application,
the silica scaffolds serve two functions. First, the porous par-
ticles preserve the desirable solution properties of the XFs
in the solid state hybrids, rendering them potentially useful
for a wider variety of environmental and biodiagnostic
assays. In addition, the functionality of these particles modu-
lates the photophysics of the XFs as well as their reactivity
towards the simple VOCs employed in this proof-of-princi-
ple assay.
Synthesis
Mesoporous Silica Materials. SBA-15 was prepared similarly to reported
literature procedures.[12] A copolymer template of poly(ethylene oxide)-
block-poly(propylene oxide)-block-poly(ethylene oxide) (18 g) was dis-
solved in a solution of cHCl (103.5 g) and deionized water (477 g). Tet-
raethyl orthosilicate (38.4 g) was added to the solution which was subse-
quently stirred for 20 h at 358C, heated to 808C, and held for 24 h at
808C. At the end of this period, the reaction was quenched with deion-
ized water, and the solid was filtered and washed with several portions of
deionized water to remove residual copolymer and give SBA-15 as a
white powder. The material was dried for 3 h at 508C and then calcined
as follows: ramp to 2008C at 1.28CminÀ1, hold at 2008C for 1 h, ramp to
5508C at 1.28CminÀ1, and hold at 5508C for 6 h. The calcined SBA-15
was then heated under vacuum at 2008C for three hours and yielded ap-
proximately 12 g of SBA-15. Nitrogen physisorption experiments showed
the material to have a BET surface area of 687 m2 g and a BJH adsorp-
tion pore diameter of 57 ꢂ.
Conclusions
Mesoporous silicas possessing varied functionalities were
successfully employed as scaffolds for the support of XFs.
Whereas crystalline XFs frequently display weak emission
in the solid state, immobilization of XFs in/on these particles
yields solids which retain the highly fluorescent character of
the parent cruciforms. Functionality integrated into the
silica scaffold can be utilized to modulate the photophysical
behavior of the incorporated dyes. The resulting XF-silica
hybrid materials display reactivity towards representative
amines and organic acids which is modulated by the func-
tionalization present on the silica scaffold. Future contribu-
tions will more thoroughly examine the potential of silica-
supported XFs—as well as the hybrid materials generated
from the XF’s metallated and protonated analogues—as
fluorescent dyes for the detection of a variety of volatile or-
ganic compounds. Such materials may prove useful in the
future development of fluorescent differential sensory
arrays for the detection of VOCs in the gas phase as well as
in aqueous solution.
Capped SBA-15. In order to remove surface silanol groups and reduce
surface acidity, 1,1,1,3,3,3-hexamethyldisilazane (1.0 g) was added to a so-
lution of calcined SBA-15 (1.0 g) in hexanes. The solution was stirred
overnight and then filtered. The solid material was washed with copious
amounts of hexanes and dried under vacuum at 508C. Thermogravimetric
analysis indicated a capping of 1.6 mmol silanols per gram of SiO2. Nitro-
gen physisorption experiments showed the material to have a BET sur-
face area of 332 m2 g and a BJH adsorption pore diameter of 49 ꢂ.
Sulfonic acid functionalized SBA-15. The sulfonic acid functionalized
SBA-15 was prepared similarly to reported literature procedures.[13] 3-
mercaptopropyltrimethoxysilane (1.0 g) was added to a solution of cal-
cined SBA-15 (1.0 g) in toluene. The solution was stirred overnight and
then filtered. The solid material was washed with copious amounts of tol-
uene and hexanes and dried under vacuum at 508C. Thermogravimetric
analysis indicated a loading of 0.57 mmol SH per gram of SiO2. The re-
sidual surface silanols groups on the thiol functionalized SBA-15 were
capped by adding the material (1.0 g) to 1,1,1,3,3,3-hexamethyldisilazane
(1.0 g) in hexanes and stirring overnight. The capped, thiol functionalized
material was then filtered, washed with hexanes, and dried under vacuum
at 508C. Thermogravimetric analysis indicated a capping of 0.55 mmol si-
lanols per gram of SiO2. Finally, the capped, thiol functionalized material
(1.0 g) was oxidized by adding it to a solution of methanol (10 g) and
30% H2O2 (20 g). The solution was stirred overnight and filtered. The
solid material was washed with deionized water and dried under vacuum
at 508C. Nitrogen physisorption experiments showed the material to
have a BET surface area of 450 m2 g and a BJH adsorption pore diameter
of 50 ꢂ.
Experimental Section
General
All chemicals were purchased from Aldrich Chemical, Acros, TCI Amer-
ica, or Fischer Scientific and used without purification unless otherwise
specified. Column chromatography was performed using Standard Grade
silica gel 60 ꢂ, 32–63 mm (230ꢄ450 mesh) from Sorbent Technologies
and the indicated eluent. Elution of cruciforms was readily monitored
using a handheld UV lamp (365 nm). Melting points were obtained using
a Mel-Temp apparatus fitted with a Fluke 51 K/J digital thermometer.
All IR spectra were obtained using a Simadzu FTIR-8400 s spectrometer.
Unless otherwise specified, NMR spectra were recorded at 298 K on a
Varian Mercury spectrometer (300 MHz). Chemical shifts are reported in
parts per million (ppm), using residual solvent (chloroform-d) as an inter-
nal standard. Data reported as follows: chemical shift, multiplicity (s=
singlet, d=doublet, t=triplet, q=quartet, m=multiplet), coupling con-
stant, and integration. Mass spectral analyses were provided by the Geor-
gia Institute of Technology Mass Spectrometry Facility.
Amine functionalized SBA-15. The amine functionalized SBA-15 was
prepared similarly to reported literature procedures.[16,17] 3-aminopropyl-
trimethoxysilane (1.0 g) was added to a solution of calcined SBA-15
(1.0 g) in toluene. The solution was stirred overnight and then filtered.
The solid material was washed with copious amounts of toluene and hex-
anes and dried under vacuum at 508C. Thermogravimetric analysis indi-
cated a loading of 1.7 mmol NH2 per gram of SiO2. Nitrogen physisorp-
tion experiments showed the material to have a BET surface area of
180 m2 g and a BJH adsorption pore diameter of 38 ꢂ.
Silica Material Characterization
Thermogravimetric analyses (TGA) were conducted on
a Netzsch
STA409. Samples were heated from 308C to 9008C at 108CminÀ1 under
an air blanket. The organic loading was determined from weight loss oc-
curring between 2008C and 7508C. Nitrogen physisorption measurements
All absorption spectra were collected using a Shimadzu UV-2401PC
spectraphotometer. The emission spectra of solutions and suspensions
268
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ꢁ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Asian J. 2009, 4, 262 – 269

Cruciform-Silca Hybrid Materials
were performed on a Micromeritics ASAP 2010 at 77 K. SBA-15 samples
were degassed at 1508C under vacuum overnight prior to analysis, and
functionalized SBA-15 samples were degassed at 508C under vacuum
overnight prior to analysis. Analysis of the porosity of the organic–inor-
ganic hybrid materials before and after XF adsorption showed minimal
loss of porosity, indicating that the XFs adsorbed primarily on the outer
surface of the particles or in the pore mouths.
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Acknowledgements
We thank the National Science Foundation (NSF-CHE 0750275) for fi-
nancial support. AJZ acknowledges the Center for Organic Photonics
and Electronics at Georgia Tech for a graduate research fellowship. CWJ
thanks ChBE at GT for the J. Carl & Shiela Pirkle Faculty Fellowship.
The authors also acknowledge Jeff Drese for assistance with nitrogen
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Received: August 15, 2008
Published online: November 12, 2008
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