Note: Descriptions are shown in the official language in which they were submitted.
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INORGANIC POLYMERS AND USE OF INORGANIC POLYMERS FOR
DETECTING NITROAROMATIC COMPOUNDS
TECHNICAL FIELD
A field of the invention is analyte detection. The instant
invention is directed to inorganic polymers and use of inorganic polymers,
namely photoluminescent metallole-containing polymers and copolymers, for
detection of nitroaromatic compounds based on photoluminescence quenching.
BACKGROUND ART
Use of chemical sensors to detect ultra-trace analytes from
explosives has been the focus of investigation in recent years owing to the
critical importance of detecting explosives in a wide variety of areas, such
as
mine fields, military bases, remediation sites, and urban transportation
areas.
Detecting explosive analytes also has obvious applications for homeland
security and forensic applications, such as the examination of post-blast
residue. Typically these chemical sensors are small synthetic molecules that
produce a measurable signal upon interaction with a specific analyte.
Chemical sensors are preferable to other detection devices, such
as metal detectors, because metal detectors frequently fail to detect
explosives,
such as in the case of the plastic casing of modem land mines. Siunilarly,
trained dogs are both expensive and difficult to maintain. Other detection
methods, such as gas chromatography coupled with a mass spectrometer,
surface-enhanced Raman, nuclear quadrupole resonance, energy-dispersive X-
ray diffraction, neutron activation analysis and electron capture detection
are
highly selective, but are expensive and not easily adapted to a small, low-
power
package.
Conventional chemical sensors have drawbacks as well. Sensing
TNT and picric acid in groundwater or seawater is important for the detection
of buried, unexploded ordnance and for locating underwater mines, but most
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chemical sensor detection methods are only applicable to air samples because
interference problems are encountered in complex aqueous media. Thus,
conventional chemical sensors are inefficient in environmental applications
for
characterizing soil and groundwater contaminated with toxic TNT at military
bases and munitions production and distribution facilities. Also, conventional
chemical sensors, such as highly 7r-conjugated, porous organic polymers, are
commonly used as chemical sensors and can be used to detect vapors of
electron deficient chemicals, but require many steps to synthesize and are not
selective to explosives.
Furthermore, many conventional chemical sensing methods are not
amenable to manufacture as inexpensive, low-power portable devices.
Additionally, these methods are limited to vapor phase detection, which is
disadvantageous given the low volatility of many explosives. For example, the
vapor pressure of TNT, which is approximately 5 ppb at room temperature, may
be up to six times lower when enclosed in a bomb or mine casing, or when
present in a mixture with other explosives.
Additionally, current routes for synthesis of polymetalloles use
hazardous reagents and are of low efficiency. For example,
poly(tetraphenyl)silole has been synthesized by Wurtz-type polycondensation,
but the reaction yields are low.
DISCLOSURE OF INVENTION
An embodiment of the present invention is a directed device and
method for detecting solid-state, vapor phase and solution phase nitroaromatic
compounds using an inorganic polymer sensor, namely photoluminescent
metallole-containing polymers and copolymers. The invention also includes a
method for synthesizing an inorganic polymer sensor, namely
photoluminescent metallole-containing copolymers.
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BRIEF DESCRIPTION OF THE DRAWINGS
FIGURE 1 is a model of a polysilole molecule;
FIG. 2 illustrates a pair of equations for the synthesis of
polygermole and polysilole according to an embodiment of the invention;
FIG. 3 illustrates a pair of equations for the synthesis of a silole-
germole copolymer according to an embodiment of the invention;
FIG. 4 illustrates a pair of equations for the synthesis of silole-
silane alternating copolymers according to an embodiment of the invention;
FIG. 5 is a table of the absorption and fluorescence spectra
observed in one embodiment of the instant invention and taken at the
concentrations of 2 mg/L in THF and 10 mg/L in toluene, respectively;
FIG. 6 is a schematic energy level diagram illustrating energy-
levels for polymetalloles and metallole-silane copolymers;
FIG. 7 is a graphical representation of UV-vis absorption spectra
-in THF (solid line) and fluorescence spectra in toluene (dotted line) for (A)
poly(tetraphenyl) germole 2. (B) silole-silane copolymer 4, and (C) germole-
-,silane copolymer 9;
FIGs. 8A and 8B illustrate a HOMO (A) and LUMO (B) of 2.5-
,diphenylsilole, Ph2C4SiH2 from the ab initio calculations at the HF/6-31 G*
level;
FIG. 9 is a graphical representation of the fluorescence spectra of
polysilole 1 in toluene solution (solid line) and in thin solid film (dotted
line);
FIG. 10 is a graphical representation of the quenching of
photoluminescence spectra of silole-silane copolymer 5 with (A) nitrobenzene,
from top 2.0 x 10-5 M, ; 3.9 x 10"5 M, 7.8 x 10"5 M, and 11.5 x 10"5 M, (B)
DNT, from top 1.4 x 10-5 M, 3.9 x 10-5 M, 7.8 x 10-5 M, and 12.4 x 10"5 M, (C)
TNT, from top 2.1 x 10"5 M, 4.2 x 10-5 M, 8.1 x 10"5 M, and 12.6 x 10-5 M, (D)
picric acid, from top 2.1 x 10"5 M, 4.2 x 10"5 M, 8.0 x 10"5 M, and 12.6 x
10"5
M;
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FIGs. 11A, 11B and 11C are Stern-Volmer plots; from top
polysilole 1, polygermole 2, and silole-silane copolymer 8; =(picric acid), ~
(TNT), =(DNT), =(nitrobenzene); the plots of fluorescence lifetime (tio/'c),
shown as inset, are independent of added TNT;
FIG. 12 illustrates fluorescence decays of polysilole 1 for
different concentrations of TNT: 0 M, 4.24 x 10-5 M, 9.09 x 10-5 M, 1.82 x 10-
4
M;
FIG. 13 illustrates Stern-Volmer plots of polymers =(polymer
1), ~ (polymer 5), = (polymer 4), = (polymer 6), 0 (polymer 2), and -
(organic pentiptycene-derived polymer 13), for TNT;
FIG. 14 illustrates a structure of the pentiptycene-derived
polymer;
FIG. 15 illustrates, from left to right, highest and lowest
photoluminescence quenching efficiency for picric acid (left-most two lines),
TNT (two lines immediately to the right of picric acid), DNT (two lines
immediately to the right of TNT), and nitrobenzene, (right-most two lines)
showing how the varying polymer response to analyte could be used to
distinguish analytes from each other;
FIG. 16 illustrates a comparison of the photoluminescence
quenching constants (from Stern-Volmer plots) of polymers 1-12 with different
nitroaromatic analytes;
FIG 17 illustrates a plot of log K vs reduction potential of
analytes: = (polymer 1), ~ (polymer 2), = (polymer 3), = (polymer 4), 0
(polymer 5), and (polymer 10);
FIG. 18 illustrates a schematic diagram of electron-transfer
mechanism for quenching the photoluminescence of polymetallole by analyte;
FIG. 19 illustrates an absence of quenching of photoluminescence
by polysilole 1 with 4 parts per hundred of THF; and
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FIG. 20 illustrates an equation for a catalytic dehyrdocoupling
method for synthesizing metallole polymers according to one embodiment of
the invention.
FIGs. 21a, 21b and 21c illustrate various copolymers as well as
their syntheses, namely PDEBSi, PDEBGe, PDEBSF, PDEBGF, PSF and PGF;
FIG. 22 is a table summarizing the detection limits of TNT, DNT,
and picric acid using the five metallole-containing polymers synthesized, PSi,
PDEBSi, PGe, PDEBGe, and PDEBSF;
FIG. 23 are black and white images, of the luminescence
quenching of three polymers, PSi, PDEBSi, and PGe, by 200, 100, 50, and 10
ng TNT on porcelain plates as observed on a porcelain plate; and
FIG. 24 are exemplary black and white images of the
luminescence quenching of polysilole by each analyte at different surface
concentrations.
BEST MODE FOR CARRYING OUT THE INVENTION
Solid state sensing may be especially desirable for trace residue
detection on surfaces believed to be contaminated, such as, for example, where
filter paper is used to swab or wipe a surface of interest and the filter
paper is
subsequently subjected to analysis. Conventional solid state detection kits,
such as that manufactured under the brand name ExPray by Plexus Scientific
Corporation of Alexandria, VA are able to detect various explosive through a
color change, with sensitivity down to the tens of nanogram level.
The vapor pressure of TNT, for example, which is approximately
5 ppb at room temperature, may be up to 6 times lower when enclosed in a
bomb or mine casing or when present in mixtures with other explosives.
Accordingly, embodiments of the invention include the solid-state detection of
trace residue of nitroaromatics, such as picric acid (PA, 2,4,6-trinitrophenol
or
C6H2(NO2)3OH), nitrobenzene (NB or C6H5NO2), 2,4-dinitrotoluene (DNT or
C7H6N204) and 2,4,6-trinitrotoluene (TNT or CJH5N306), using thin films of
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luminescent metallole-containing polymers. Advantageously, detection limits
as low as 5 ng are obtained. Polymetalloles and= copolymers have the
advantage of being inexpensive, easily prepared, and readily fielded for on-
site
explosives detection.
For example, one preferred embodiment includes a method for detecting
an analyte that may be present in ambient air, bound to a surface or as part
of
complex aqueous media that includes a metallole-containing polymer or
copolymer being exposed to a system suspected of containing the analyte, such
as on a solid surface or in an aqueous medium. By subsequently measuring the
photoluminescence of the metallole-containing polymer or copolymer, the
presence, absence and approximate quantity may' be determined with great
sensitivity. By illuminating the polymer or copolymer with light having a
wavelength of between 250 nm and 420 nm, photoluminescence quenching
may be observed.
Another preferred embodiment includes a metallole-containing
polymer sensor for sensing trace amounts of nitroaromatic compounds that
includes a metallole-containing polymer cast, sprayed or otherwise deposited
on
a surface suspected of containing the nitroaromatic compounds. It is
contemplated that the solid surface on which detection may occur may include a
virtually boundless number of surfaces, such as glass, paper, plastic, wood,
porcelain or metal, to name a few.
Additionally, embodiments of the invention include the synthesis
and use of inorganic polymers, namely photoluminescent metallole-containing
polymers and copolymers, in solid state or solution for detection of
nitroaromatic compounds based on photoluminescence quenching. Inorganic-
organic polymers may be prepared by catalytic hydrosilation or
hydrogermylation with dihydrosilole or dihydrogermole compounds and organic
diynes or dialkenes. The invention includes an inexpensive and highly
efficient
inorganic or inorganic-organic polymer sensor that can detect the existence of
an
analyte, namely nitroaromatic compounds such as picric acid, nitrobenzene,
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DNT and TNT in air, water, on surfaces, organic solution, or other complex
aqueous media.
Photoluminescent metallole polymers are stable in air, water,
acids, common organic solvents, and even seawater containing bioorganisms.
Therefore, the inorganic polymer sensor of the instant invention includes the
metallole copolymers for detection of analytes in these media. Importantly,
the
inorganic polymer sensors of the instant invention are insensitive to organic
solvents and common environmental interferents, allowing the use of the
sensor in a wide variety of environments and applications.
Metalloles are silicon (Si) or germanium (Ge)-containing
metallocyclopentadienes that include one-dimensional Si-Si, Ge-Ge, or Si-Ge
wires encapsulated with highly conjugated organic ring systems as side chains.
Silole and germole dianions (RC)4Si2" and (RC)4Ge2", where R=Ph or Me, have
been studied by X-ray crystallography and found to be extensively delocalized.
Siloles and germoles are of special interest because of their unusual
electronic
and optical properties, and because of their possible application as electron
transporting materials in devices. Polysilanes and polygermanes containing a
metal-metal backbone emit in the near UV spectral region, exhibit high hole
mobility, and show high nonlinear optical susceptibility, which makes them
efficient photoemission candidates for a variety of optoelectronics
applications.
These properties arise from a 6-6* delocalization along the M-M backbones
and confinement of the conjugated electrons along the backbone.
Polymetalloles and metallole-silane copolymers are unique in
having both a M-M backbone as well as an unsaturated five-membered ring
system. These polymers are highly photoluminscent, and are accordingly
useful as light emitting diodes (LEDs) or as chemical sensors. Characteristic
features of polymetalloles and metallole-silane copolymers include a low
reduction potential and a low-lying lowest unoccupied molecular orbital
(LUMO) due 6*-n* conjugation arising from the interaction between the a*
orbital of silicon or germanium and the 71* orbital of the butadiene moiety of
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the five membered ring. In addition, the M-M backbones exhibit 6*-6*
delocalization, which further delocalizes the conjugated metallole it
electrons
along the backbone. Electron delocalization in these polymers provides a
means of amplification, because interaction between an analyte molecule and
any position along the polymer chain is communicated throughout the
delocalized chain.
Detection may be accomplished by measurement of the
quenching of photoluminescence of metallole copolymers by the analyte.
Sensitivity of metallole copolymers to the analytes picric acid, TNT, DNT and
NB is as follows: PA > TNT > DNT > NB. A plot of log K versus the
reduction potential of analytes (NB, DNT, and TNT) for each metallole
copolymer yields a linear relationship, indicating that the mechanism of
quenching is attributable to electron transfer from the excited metallole
copolymers to the lowest unoccupied orbital of the analyte.
Excitation may be achieved with electrical or optical stimulation.
If optical stimulation is used, a light source containing energy that is
larger than
the wavelength of luminescence emission of the polymer is preferably used.
This could be achieved with, for example, a mercury lamp, a blue light
emitting
diode, or an ultraviolet light emitting diode.
FIG. 1 illustrates a space filling model structure of polysilole 1,
which features a Si-Si backbone inside a conjugated ring system of side chains
closely packed to yield a helical arrangement. FIG. 2 illustrates polymers 1
and 2, FIG. 3 illustrates polymer 3, and FIG. 4 illustrates copolymers 4-12.
FIGs. 21 a through 21 c illustrate additional copolymers as well as their
syntheses, Poly(1,4-diethynylbenzene)2,3,4,5-tetraphenylsilole (PDEBsilole),
Poly(1,4-diethynylbenzene)2,3,4,5-tetraphenylgermole (PDEBgermole),
Poly(1,4-diethynylbenzene)silafluorene (PDEBSF), Poly(1,4-
diethynylbenzene)germafluorene (PDEBGF), Polysilafluorene (PSF) and
Polygermafluorene (PGF). A similar means of amplification is available to
quantum-confined semiconductor nanocrystallites, via a three-dimensional
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crystalline network, where the electron and hole wave functions are
delocalized
throughout the nanocrystal.
A conventional method for preparing polymetalloles and
metallole copolymers is Wurtz-type polycondensation. The syntheses of
polygermole and polysiloles, and other copolymers are analogous to one
another, as illustrated in equation 1 in FIG. 2, and employ the Wurtz-type
polycondensation. However, yields from this method of synthesis are low (ca.
-30%). Thus, Wurtz-type polycondensation is not well-suited to large-scale
production.
Embodiments of the instant invention include alternative methods
for synthesizing polymetalloles that use catalytic dehydrocoupling of
dihydrosiloles with a catalyst as an attractive alternative to Wurtz-type
polycondensation. Bis(cyclopentadienyl) complexes of Group 4 have been
extensively studied and shown to catalyze the dehydrocoupling of hydrosilanes
to polysilanes for the formation of Si-Si bonds. However, only the primary
organosilanes react to give polysilane. Secondary and tertiary silanes give
dimers or oligomers in low yield. It has been reported that the reactivity
decreases dramatically with increasing substitution at the silicon atom, since
reactions catalyzed by metallocenes are typically very sensitive to steric
effects. Mechanisms for dehydrogenative coupling of silanes have also been
extensively investigated, which involves 6-bond metathesis.
Embodiments of the instant invention include catalytic
dehydrocoupling of dihydrosiloles and dihydrogermoles with a catalyst. In one
embodiment, the invention includes catalytic dehydrocoupling
polycondensation of dihydro(tetraphenyl)silole or dihydro(tetraphenyl)germole
with 1-5 mol % of Wilkinson's catalyst, Rh(PPh3)3C1, or Pd(PPh3)4, as
illustrated in FIG. 2, or 0.1-0.5 mol % of HZPtC16-xH2O in conjuction with 2-5
equivalents of allylamine, or other alkene, such as cyclohexene, for example,
as
illustrated in FIG. 20. The latter reactions produce the respective polysilole
or
polygermole in high yield (ca. 80-90%). By 'H NMR spectroscopy, the
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monomer, dihydrometallole, was completely consumed in the reaction.
Molecular weights (Mw) of 4000-6000 are obtained, similar to those obtained
by the Wurtz-type polycondensation (ca. -30%).
Turning now to FIG. 3, silole-germole alternating copolymer 3, in
which every other silicon or germanium atom in the polymer chain is also part
of a silole or germole ring, was synthesized from the coupling of
dichloro(tetraphenyl)germole and dilithio(tetraphenyl)silole. The latter is
obtained in 39% yield from dichlorotetraphenylsilole by reduction with
lithium,
as illustrated in the equation of FIG. 3. The molecular weight of the silole-
germole copolymer, M,, = 5.5 X 103, Mõ = 5.0 X 103 determined by SEC (size
exclusion chromatography) with polystyrene standards, is similar to that of
polysiloles or polygermoles. All of the polymetalloles are extended oligomers
with a degree of polymerization of about 10 to 16, rather than a true high MW
polymer; however, they can be cast into a thin film from solution and show
polymer-like properties.
Also illustrated in FIG. 4 are silole-silane alternating copolymers
4, 5, 6, 7, 8, which were also prepared from coupling of the silole dianion
(Ph4C4Si)Li2 with the corresponding silanes. Germole-silane alternation
copolymers 9, 10, 11, 12 were also synthesized from the coupling of the
germole dianion (Ph4C4Ge)Li2 with the corresponding silanes, as illustrated in
FIG. 4. These reactions generally employ reflux conditions in tetrahydrofuran
under an argon atmosphere for about 72 hours. Some silole-silane copolymers
have been synthesized previously and shown to be electroluminescent.
Metallole-silane copolymers were developed so that they could be easily
functionalized along the backbone by hydrosilation. The molecular weight of
metallole-silane copolymers, M, = 4.1 X 103 - 6.2 X 103, Mõ = 4.1 X 103 - 5.4
X 103 determined by SEC, is similar to that of the polymetalloles.
The molecular weights and polydisperity indices (PDI) of
polymers 1-12 (FIG. 4) determined by gel permeation chromatography (GPC)
are illustrated in Table 1 of FIG. 5.
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Inorganic-organic poly(1,4-diethynylbenzene)metallole (DEB)
type polymers may be obtained by hydrosilation of an dialkyne, specifically
DEB, with a dihydrometallole using a catalyst such as chloroplatinic acid.
FIGs. 21 a-21 c illustrate the reaction whereby the DEB type polymers are
obtained according to embodiments of the invention. A reasonable extension
of this principle includes hydrosilation and hydrogermylation of any organic
diyne. A reasonable interpolation of this principle includes hydrosilation and
hydrogermylation of organic dialkenes to obtain less conjugated polymers.
Absorption and Fluorescence
The UV-vis absorption and fluorescence spectral data for
polymers 1-12 are also illustrated in Table 1 of FIG. 5. The
poly(tetraphenyl)metalloles 1-3 and tetraphenylmetallole-silane copolymers 4-
12 exhibit three absorption bands, which are ascribed to the a-70 transition
in
the metallole ring and the 6-(6*+ 7r*) and 6-a* transitions in the M-M
backbone. FIG. 6 illustrates a schematic energy-level diagram for
polymetalloles and metallole-silane copolymers.
UV-vis absorption in THF (solid line) and fluorescence spectra in
toluene (dotted line) for poly(tetraphenygermole) 2, silole-silane copolymer 4
and germole-silane copolymer 9 are shown in FIG. 7. Absorptions at a
wavelength of about 370 nm for the poly(tetraphenylmetallole)s 1-3 and
tetraphenylmetallole-silane copolymers 4-12 are ascribed to the metallole a-a*
transition of the metallole moiety, which are about 89 to 95 nm red-shifted
relative to that of oligo[1,1- (2,3,4,5-tetramethylsilole)] (kmaX = 275 nm)
and
are about 75 to 81 nm red-shifted relative to that of oligo[1,1-(2,5-dimethyl-
3,4-diphenylsilole)] N,,aX = 289 nm). These red shifts are attributed to an
increasing main chain length and partial conjugation of the phenyl groups to
the silole ring.
FIG. 8 shows the HOMO (A) and LUMO (B) of 2,5-
diphenylsilole, Ph2C4SiH2, from the ab initio calculations at the HF/6-31G*
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level. Phenyl substituents at the 2,5 metallole ring positions may 7c-
conjugate
with the metallole ring LUMO. Second absorptions at wavelengths of 304 to
320 nm for the poly(tetraphenylmetallole)s 2-3 and tetraphenylmetallole-silane
copolymers 4-12 are assigned to the 6-(62* +a*) transition, which parallels
that of the poly(tetraphenyl)silole 1.
Polymetallole 1-2 and silole-silane copolymers 4-7 exhibit one
emission band (k,,,a, 486 to 513 nm) when excited at 340 nm, whereas the
others exhibit two emission bands with k,,,aX of 480-510 nm and 385-402 nm.
The ratios of the two emission intensities are not concentration dependent,
which indicates that the transition does not derive from an excimer. Emission
peaks for germole-silane copolymers 9-12 are only 2 to 33 nm blue-shifted
compared to the other polymers. FIG. 9 shows fluorescence spectra of the
poly(tetraphenyl)silole in toluene solution (solid line) and in the solid
state
(dotted line). The bandwidth of the emission spectrum in solution is slightly
larger than in the solid state. There is no shift in the maximum of the
emission
wavelength. This suggests that the polysilole exhibits neither a -stacking of
polymer chains nor excimer formation.
The angles of C-M-C of dihydro(tetraphenyl)silole and
dihydro(tetraphenyl)germole are 93.11 on C-Si-C and 89.76 on C-Ge-C,
respectively. Polymerization might take place, since the tetraphenylmetalloles
have small angles at C-M-C in the metallocyclopentadiene ring, which results
in less steric hindrance at the metal center. In addition, the bulky phenyl
groups of silole might prevent the formation of cyclic hexamer, which is often
problematic in polysilane syntheses. Cyclic polymetallole product formation
was not observed.
Fluorescence Quenching With Nitroaromatic Analytes
The method of detection of the instant invention includes using a
chemical sensor, namely a variety of photoluminscent copolymers having a
metalloid-metalloid backbone such as Si-Si, Si-Ge, or Ge-Ge, or altelrnatively
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an inorganic-organic metallole-containing copolymer. While polymetalloles in
various forms may be used to detect analytes, one embodiment includes casting
a thin film of the copolymers is employed in detecting.the analyte, e.g.,
picric
acid, DNT, TNT and nitrobenzene. Detection is achieved by measuring the
quenching of the photoluminescence of the copolymer by the analyte.
Accordingly, the instant invention contemplates use of the polymetallole
polymers and copolymers in any form susceptible to measurement of
photoluminescence quenching. For example, since it is possible to measure
fluorescence of solutions, other embodiments of the instant method of
detection
may optionally include a polymetallole in solution phase, where powdered bulk
polymer is dissolved in solution. Yet another embodiment includes producing
a colloid of the polymer, which is a liquid solution with the polymer
precipitated and suspended as nanoparticles.
The detection method involves measurement of the quenching of
photoluminescence of the polymetalloles 1-3 and metallole-silane copolymers
4-12 by the analyte, such as a toluene solution (using a Perkin-Elmer LS 50B
fluorescence spectrometer, 340 nm excitation wavelength). For example,
turning now to FIG. 10, when used to detect TNT, fluorescence spectra of a
toluene solution of the metallole copolymers were obtained upon successive
addition of aliquots of TNT. Photoluminescence quenching of the polymers 1-
12 in toluene solutions were also measured with nitrobenzene, DNT, TNT and
nitrobenzene. The relative efficiency of photoluminescence quenching of
metallole copolymers is unique for TNT, DNT, and nitrobenzene, respectively,
as indicated in FIG. 10 by the values of K determined from the slopes of the
steady-state Stern-Volmer plots. FIG. 10 demonstrates that each copolymer
has a unique ratio of quenching efficiency to the corresponding analyte.
The purity of the TNT sample was found to be important to
obtain reproducible results. It was synthesized by nitration of dinitrotoluene
and recrystallized twice from methanol. A third recrystallization produces the
same results as the twice-recrystallized material. When the quenching
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experiment was undertaken without recrystallization of TNT, higher (ca. 10 x)
quenching percentages are obtained. Presumably, impurities with higher
quenching efficiencies are present in crude TNT.
The Stem-Volmer equation, which is (Io/I)-1 = Ksv[A], is used to
quantify the differences in quenching efficiency for various analytes. In this
equation, Io is the initial fluorescence intensity without analyte, and I is
the
fluorescence intensity with added analyte of concentration [A], and Ksv is the
Stern-Volmer constant.
FIG. 11 shows the Stern-Volmer plots of polysilole 1,
polygermole 2, and silole-silane copolymer 8 for each analyte. A linear Stern-
Volmer relationship was observed in all cases, but the Stern-Volmer plot for
picric acid exhibits an exponential dependence when its concentration is
higher
than 1.0 x 10"4 M. A linear Stern-Volmer relationship may be observed if
either static or dynamic quenching process is dominant. Thus, in the case of
higher concentrations of picric acid, the two processes may be competitive,
which results in a nonlinear Stern-Volmer relationship. This could also arise
from aggregation of analyte with chromophore.
Photoluminescence may arise from either a static process, by the
quenching of a bound complex, or a dynamic process, by collisionally
quenching the excited state. For the former case, Ksv is an association
constant
due to the analyte-preassociated receptor sites. Thus, the collision rate of
the
analyte is not involved in static quenching and the fluorescence lifetime is
invariant with the concentration of analyte. With dynamic quenching, the
fluorescence lifetime should diminish as quencher is added.
A single "mean" characteristic lifetime (ti) for polymetalloles and
metallole-silane copolymers 1-12 has been measured and summarized in Table
1 of FIG. 5. Luminescence decays were not single-exponential in all cases.
Three lifetimes were needed to provide an acceptable fit over the first few
nanoseconds. The amplitudes of the three components were of comparable
importance (the solvent blank made no contribution). These features suggest
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that the complete description of the fluorescence is actually a continuous
distribution of decay rates from a heterogeneous collection of chromophore
sites. Because the oligomers span a size distribution, this behavior is not
surprising. The mean lifetime parameter reported is an average of the three
lifetimes determined by the fitting procedure, weighted by their relative
amplitudes. This is the appropriate average for comparison with the "amount"
of light emitted by different samples under different quenching conditions, as
has been treated in the literature. Given this heterogeneity, possible long-
lived
luminescence that might be particularly vulnerable to quenching has been a
concerri. However, measurements with a separate nanosecond laser system
confirmed that there were no longer-lived processes other than those captured
by the time-correlated photon counting measurement and incorporated into
Table 1 of FIG. 5.
It is notable that polysilole 1 and silole-silane copolymers 4-8
have about 3 to 11 times longer fluorescence lifetimes than polygermole 2 and
germole-silane copolymers 9-12. Fluorescence lifetimes in the thin films
(solid
state) for polysilole 1 and polygermole 2 are 2.5 and 4.2 times longer than in
toluene solution, respectively. The fluorescence lifetimes as a function of
TNT
concentration were also measured and are shown in the inset of Figure 11 for
polymers 1, 2, and 8. No change of mean lifetime was observed by adding
TNT, indicating that the static quenching process is dominant for
polymetalloles and metallole-silane copolymers 1-12 (FIG. 12). Some issues
with such analyses have been discussed in the literature. This result suggests
that the polymetallole might act as a receptor and a TNT molecule would
intercalate between phenyl substituents of the metallole moieties (FIG. 1).
For chemosensor applications, it is useful to have sensors with
varied responses. Each of the 12 polymers exhibits a different ratio of the
photoluminescence quenching for picric acid, TNT, DNT, and nitrobenzene
and a different response with the same analyte. The use of sensor arrays is
inspired by the performance of the olfactory system to specify an analyte.
FIG.
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13 displays the Stem-Volmer plots of polymers 1, 2, 4, 5, and 6 for TNT,
indicating that the range of photoluminescence quenchiag efficiency for TNT is
between 2.05 x 103 and 4.34 x 103 M"1. The relative efficiencies of
photoluminescence quenching of poly(tetraphenylmetallole)s 1-3 and
tetraphenyl-metallole-silane copolymers 4-12 were obtained for picric acid,
TNT, DNT, and nitrobenzene, as indicated by the values of Ksv determined
from the slopes of the steady-state Stern-Volmer plots and summarized in
Table 1 of FIG. 5. Polymer 13, which is illustrated in FIG. 14, is an organic
pentiptycene-derived polymer for comparison. The metallole copolymers are
more sensitive to TNT than the organic pentiptycene-derived polymers in
toluene solution. For example, polysilole 1 (4.34 x 103 M-1) has about a 370%
better quenching efficiency with TNT than organic pentiptycene-derived
polymer (1.17 x 103 M"1).
The trend in Stern-Volmer constants usually reflects an enhanced
charge-transfer interaction from metallole polymer to analyte. For example,
the relative efficiency of photoluminescence quenching of polysilole 1 is
about
9.2:3.6:2.0:1.0 for picric acid, TNT, DNT, and nitrobenzene, respectively.
Although polysilole 1 shows best photoluminescence quenching efficiency for
picric acid and TNT, polymer 9 and 5 exhibit best quenching efficiency for
DNT and nitrobenzene, respectively. (FIG. 15) Polygermole 2 has the lowest
quenching efficiency for all analytes. Since the polymers 1-12 have similar
molecular weights, the range of quenching efficiencies with the same analyte
would be expected to be small. Polysilole 1 (11.0 x 103*M"1 and 4.34 x103 M"1)
exhibits 164% and 212% better quenching efficiency than polygermole 2 (6.71
x 103 M'1 and 2.05 x 103 M-1 ) with picric acid and TNT, respectively. Polymer
9 (2.57 x 103 M"1) has 253% better quenching efficiency than polymer 2 (1.01
X 103 M"1) with DNT. Polymer 5 (1.23 x 103 M-') has 385% better quenching
efficiency than metallole polymer 2 (0.32 x 103 M-1) with nitrobenzene. FIG.
16 illustrates how an analyte might be specified using an array of multi-
sensors.
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FIG. 17 shows a plot of log Ksv vs. reduction potential of
analytes. All metallole polymers exhibit a linear relationship, even though
they
have different ratios of photoluminescence quenching efficiency to analytes.
This result indicates that the mechanism of photoluminescence quenching is
primarily attributable to electron transfer from the excited metallole
polymers
to the LUMO of the analyte. Because the reduction potential of TNT (-0.7 V
vs NHE) is less negative than that of either DNT (-0.9 V vs NHE) or
nitrobenzene (-1.15 V vs NHE), it is detected with highest sensitivity. A
schematic diagram of the electron-transfer mechanism for the quenching of
photoluminescence of the metallole polymers with analyte is shown in FIG. 18.
Optical excitation produces an electron-hole pair, which is delocalized
through
the metallole copolymers. When an electron deficient molecule, such as TNT
is present, electron-transfer quenching occurs from the excited metallole
copolymer to the LUMO of the analyte. The observed dependence of Ksv on
analyte reduction potential suggests that for the static quenching mechanism,
the polymer-quencher complex luminescence intensity depends on the electron
acceptor ability of the quencher. An alternative explanation would be that the
formation constant (Ksv) of the polymer-quencher complex is dominated by a
charge-transfer interaction between polymer and quencher and that the
formation constant increases with quencher electron acceptor ability.
An important aspect of the metallole copolymers is their relative
insensitivity to common interferents. Control experiments using both solutions
and thin films of metallole copolymers (deposited on glass substrates) with
air
displayed no change in the photoluminescence spectrum. Similarly, exposure
of metallole copolymers both as solutions and thin films to organic solvents
such as toluene, THF, and methanol or the aqueous inorganic acids H2SO4 and
HF produced no significant decrease in photoluminescence intensity. Figure 19
shows that the photoluminescence spectra of polysilole 1 in toluene solution
display no quenching of fluorescence with 4 parts per hundred of THF. The
ratio of quenching efficiency of polysilole 1 with TNT vs benzoquinone is
17
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much greater than that of polymer 13. The Ksv value of 4.34 x 103 M-1 of
polysilole 1 for TNT is 640% greater than that for benzoquinone (Ksv = 674 M-
I). The organic polymer 13, however, only exhibits a slightly better quenching
efficiency for TNT (Ksv = 1.17 x 103 M"1) (ca. 120%) compared to that (Ksv =
998 M-1) for benzoquinone. This result indicates that polysilole 1 exhibits
less
response to interferences and greater response to nitroaromatic compounds
compared to the pentiptycene-derived polymer 13.
Statistical Estimates of Detection Limit from Extrapolation of Stern-
Volmer Quenching Data:
From Stern-Volmer Quenching Data:
Of log(Io/I) - 1 vs [TNT] in ppb.
This corresponds to an extrapolated detection limit of -1.5 ppt for
instant detection with our fluorescence spectrometer at the 95% confidence
limit. Of course, this is for solution data and with a spectrometer, which is
not
optimized for detection at a single wavelength.
Example
All synthetic manipulations were car'ried out under an
atmosphere of dry dinitrogen gas using standard vacuum-line Schlenk
techniques. All solvents were degassed and purified prior to use according to
standard literature methods: diethyl ether, hexanes, tetrahydrofuran, and
toluene purchased from Aldrich Chemical Co. Inc. were distilled from
sodium/benzophenone ketal. Spectroscopic grade of toluene from Fisher
Scientific was used for the fluorescent measurement. NMR grade
deuteriochloroform was stored over 4 A molecular sieves. All other reagents
(Aldrich, Gelest) were used as received or distilled prior to use. NMR data
were collected with Varian Unity 300, 400, or 500 MHz spectrometers (300.1
MHz for 1H NMR, 75.5 MHz for 13C NMR and 99.2 MHz for 29Si NMR) and
all NMR chemical shifts are reported in parts per million (8 ppm); downfield
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shifts are reported as positive values from tetramethylsilane (TMS) as
standard at 0.00 ppm. The 'H and 13C chemical shifts are reported relative to
CHC13 (S 77.0 ppm) as an internal standard, and the 29Si chemical shifts are
reported relative to an external TMS standard.
NMR spectra were recorded using samples dissolved in CDC13,
unless otherwise stated, on the following instrumentation. 13C NMR were
recorded as proton decoupled spectra, and 29Si NMR were recorded using an
inverse gate pulse sequence with a relaxation delay of 30 seconds. The
molecular weight was measured by gel permeation chromatography using a
Waters Associates Model 6000A liquid chromatograph equipped with three
American Polymer Standards Corp. Ultrastyragel columns in series with
porosity indices of 103, 104, and 105 A, using freshly distilled THF as
eluent.
The polymer was detected with a Waters Model 440 ultraviolet
absorbance detector at a wavelength of 254 nm, and the data were
manipulated using a Waters Model 745 data module. Molecular weight was
determined relative to calibration from polystyrene standards. Fluorescence
emission and excitation spectra were recorded on a Perkin-Elmer
Luminescence Spectrometer LS 50B. Monomers, 1,1-dichloro-2,3,4,5-
tetraphenylsilole, 1,1-dichloro-2,3,4,5-tetraphenylgermole, 1, 1 -dilithio-
2,3,4,5-tetraphenylsilole, and 1,1-dilithio-2,3,4,5-tetraphenylgermole were
synthesized by following the procedures described in the literature. All
reactions were performed under Ar atmosphere.
Polymetalloles 1, 2, and 3 were synthesized by following the
procedures described in the literature.
Preparation of silole-silane copolymers, (silole-SiR1R2),,:
Stirring of 1,1-dichloro-2,3,4,5-tetraphenylsilole (5.0 g, 11.0 mmol) with
lithium (0.9 g, 129.7 mmol) in THF (120 mL) for 8 h at room temperature
gave a dark yellow solution of silole dianion. After removal of excess
lithium, lmol equiv of corresponding silanes, R1R2SiC12(11.0 mmol) was
added slowly to a solution of tetraphenylsilole dianion, and stirred at room
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temperature for 2 hours. The resulting mixture was refluxed for 3 days. The
reaction mixture was cooled to room temperature and quenched with
methanol. Then the volatiles were removed under reduced pressure. THF (20
mL) was added to the residue and polymer was precipitated by slow addition
of the solution into 700 mL of methanol. The third cycle of dissolving-
precipitation followed by freeze-drying gave the polymer as yellow powder.
For (silole)õ(SiMeH),,,(SiPhH)o, each 5.5 mmol of SiMeHC12
and SiPhHC12 were slowly added into a THF solution of silole dianion. In
case of (silole-SiH2),,,, after addition of the xylene solution of SiH2C12
(11.0
mmol), the resulting mixture was stirred for 3 days at room temperature
instead of refluxing.
Selected data for (silole-SiMeH),,, 4; Yield = 2.10 g (44.5%);
1H NMR (300.134 MHz, CDC13): 8=-0.88-0.60 (br. 3H, Me), 3.06-4.89 (br.
1H, SiH), 6.16-7.45 (br. 20H, Ph); 13C{H} NMR (75.469 MHz, CDC13): S=
0.61-1.69 (br. Me), 123.87-131.75, 137.84-145.42, 153.07-156.73 (br. m,
Ph); 29Si NMR (71.548 MHz, inversed gated decoupling, CDC13): 5=-29.22
(br. silole), -66.61 (br. SiMeH). GPC: Mw = 4400, Mw/Mn = 1.04.
Fluorescence (conc. = l Omg/L); kem = 492 nm at keX = 340 nm.
Selected data for (silole-SiPhH),,, 5; Yield = 2.00 g (37.0%); 'H
NMR (300.134 MHz, CDC13): 8= 3.00-4.00 (br. 1H, SiFl), 6.02-7.97 (br.
20H, Ph); 13C{H} NMR (75.469 MHz, CDC13): S= 123.64-143.98, 152.60-
157.59 (br. m, Ph); 29Si NMR (71.548 MHz, inversed gated decoupling,
CDC13): 8 =-37.51 (br. silole), -71.61 (br. SaPhH). GPC: Mw = 4500, Mw/Mn
= 1.09, determined by SEC with polystyrene standards; Fluorescence (conc.
= 10mg/L); kem = 487 nm at ke,t = 340 nm.
Selected data for (silole)õ(SiMeH)0.5õ(SiPhH)0.5,,, 6; Yield =
2.10 g (41.5%); 'H NMR (300.134 MHz, CDC13): 5 =-0.67-0.40 (br. 3H,
Me), 3.08-4.98 (br. 2H, SiH), 6.00-7.82 (br. 55H, Ph); 13C{H} NMR (75.469
MHz, CDC13): 8 =-0.85-1.76 (br. Me), 122.06-147.25, 153.11-157.26 (br. m,
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Ph); 29Si NMR (71.548 MHz, inversed gated decoupling, CDC13): 8 = -28.61
(br. silole), -59.88 (br. Sa1VIeH and SaPhH). GPC: Mw = 4800, Mw/Mn = 1.16,
determined by SEC with polystyrene standards; Fluorescence (conc. =
l Omg/L); 2em = 490 nm at ke,t = 340 nm.
Selected data for (silole-SiH2),,, 8; Yield = 2.05 g (44.9%); 1H
NMR (300.134 MHz, CDC13): S= 3.00-4.96 (br. 2H, SiH ), 6.12-7.72 (br.
20H, Ph); 13C{H} NMR (75.469 MHz, CDC13): 8= 122.08-132.78, 136.92-
146.25, 152.81-160.07 (br. m, Ph); 29Si NMR (71.548 MHz, inversed gated
decoupling, CDC13): 8=-30.95 (br. silole), -51.33 (br. SiH2). ratio of n : m =
1.00 : 0.80; GPC: Mw = 4600, Mw/Mn = 1.14, determined by SEC with
polystyrene standards; Fluorescence (conc. = l Omg/L); ken, = 499 nm at ke,,
=340nm.
Selected data for (silole-SiPh2),,, 7; Yield = 2.93 g (47.0%); 1H
NMR (300.134 MHz, CDC13): S= 6.14-7.82 (br. 20H, Ph); 13C{H} NMR
(75.469 MHz, CDC13): S= 122.08-146.25 (br. m, Ph), 152.81-160.07 (silole
ring); GPC: Mw = 5248, Mw/Mn = 1.05, determined by SEC with
polystyrene standards; Fluorescence (conc. = 10mg/L); ~ em = 492 nm at ke,,
= 340 nm.
Preparation of germole-silane copolymers, (germole-
SiR1R2)n: The procedure for synthesizing all germole-silane copolymers
was similar to that for silole-silane copolymers. For
(germole)õ(SiMeH)0.5i(SiPhH)o.5n, each 5.0 mmol of SiMeHC12 and
SiPhHC12 were added slowly into a THF solution of germole dianion. The
resulting mixture was stirred for 3 days at room temperature.
Selected data for (germole-SiMeH),,, 9; Yield = 2.03 g (43%);
'H NMR (300.134 MHz, CDC13): 5 =-0.21-0.45 (br. 2.4H, Me), 5.14-5.40
(br. 0.8H, SiH), 6.53-7.54 (br. 20H, Ph); 13C{H} NMR (75.469 MHz,
CDC13): 8 =-9.70 - -8.15 (br. Me), 125.29-130.94, 139.08-148.12, 151.29-
152.88 (br. m, Ph); 29Si NMR (71.548 MHz, inversed gated decoupling,
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CDC13): 8 =-50.40 (br. SiMeH); GPC: Mw = 4900, Mw/Mn = 1.12,
determined by SEC with polystyrene standards; UV (conc. = l Omg/L); 8abs =
296, 368 nm; Fluorescence (conc. = 10mg/L); ke1,, = 401, 481 nm at kex =
340 nm.
Selected data for (germole-SiPhH)õ 10; Yield = 2.13 g (40%);
'H NMR (300.134 MHz, CDC13): b= 4.71 (br. l.OH, SiH), 6.30-7.60 (br.
25H, Ph); 13C{H} NMR (75.469 MHz, CDC13): S= 125.50-144.50, 151.50-
153.00 (br. m, Ph); 29Si NMR (71.548 MHz, inversed gated decoupling,
CDC13): 8 =-56.81 (br. SiPhH).; GPC: Mw =~ 4400, Mw/Mn = 1.06,
determined by SEC with polystyrene standards; UV (conc. = l Omg/L); kabs =
294, 362 nm; Fluorescence (conc. = lOmg/L); ~em = 401, 486 nm at 1%eX =
340 nm.
Selected data for (germole)õ(SiMeH)o.sn(SiPhH)o.sn, 11; Yield =
2.01 g(40%); 'H NMR (300.134 MHz, CDC13): 8=-0.04-0.42 (br. 3H, Me),
4.94 (br. 2H, SiH), 6.33-7.66 (br. 25H, Ph); 13C{H} NMR (75.469 MHz,
CDC13): 8= 124.31-130.66, 138.43-152.54 (br. m, Ph); 29Si NMR (71.548
MHz, inversed gated decoupling, CDC13): 5 =-63.01 (br. Sa1VIeH and SaPhH):
0.71; GPC: Mw = 4100, Mw/Mn = 1.06, determined by SEC with polystyrene
_
standards; UV (cone. = 10mg/L); kabs = 290, 364 nm; Fluorescence (cone.
lOmg/L); kem = 399, 483 nm at ,%eX = 340 nm.
Selected data for (germole-SiPh2),,, 12;Yield = 3.23 g (48%);
1H NMR (300.134 MHz, CDC13): 6 = 6.21-7.68 (br. 30H, Ph); 13C{H} NMR
(75.469 MHz, CDC13): 8= 125.15-141.40 (br. m, Ph), 151.12-153.99
(germole ring carbon); GPC: Mw = 5377, Mw/Mn = 1.09, determined by SEC
with polystyrene standards; UV (conc. = lOmg/L); 21abs = 298, 366 nm;
Fluorescence (conc. = lOmg/L); kem = 400, 480 nm at /%eX = 340 nrn.
Preparations for other metallole-silane and metallole-germane
copolymers such as tetraalkylmetallole -silane copolymers and
tetraarylmetallole-germane copolymers can be prepared by the above method
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described.
Preparation of Poly(tetraphenyl)silole and
Poly(tetraphenyl)germole by Catalytic Dehydrocoupling - Preparation of
polymetallole: 1,1-dihydro-2,3,4,5-tetraphenylsilole or germole were prepared
from the reduction of 1,1-dichloro-2,3,4,5-tetraphenylsilole or germole with
lmol equiv of LiAlH4. Additionally, an alternate method to prepare the
dihydrometallole is to add dichlorosilane (25% in xylenes) to an solution of
tetraphenylbutadiene dianion in ether, as described in the literature.
Reaction
conditions for preparing the polygermole are the same as those for polysilole.
1,1-dihydro-2,3,4,5-tetraphenylsilole (1.0 g, 2.59 mmol) and 1-5 mol % of
RhCI(PPh3)3 or Pd(PPh3)4 in toluene (10 mL) were placed under an Ar
atmosphere and degassed through 3 freeze-pump-thaw cycles. The reaction
mixture was vigorously refluxed for 72 h. The solution was passed rapidly
through a Florisil column and evaporated to dryness under Ar atmosphere. 1
mL of THF was added to the reaction mixture and the resulting solution was
then poured into 10 mL of methanol. Poly(tetraphenyl)silole, 1, was obtained
as a pale yellow powder after the third cycle of dissolving-precipitation
followed by freeze-drying. An alternative method for poly(tetraphenyl)silole
preparation is as follows. 1,1-dihydro-2,3,4,5-tetraphenylsilole (1.0 g, 2.59
mmol) and 0.1-0.5 mol % H2PtCl6-xH2O and 2-5 mol equivalents of allylamine
in toluene (10 mL) were vigorously refluxed for 24 hours. The solution was
passed through a sintered glass frit and evaporated to dryness under an Ar
atmosphere. Three dissolving-precipitation cycles with THF and methanol
were performed as stated above to obtain 1. The, molecular weights of
polymers were obtained by GPC. 1,1-dihydro-2,3,4,5-tetraphenylsilole with
RhCI(PPh3)3, 1: isolated yield = 0.81 g, 82%, M, = 4355, MIMõ = 1.02,
determined by SEC with polystyrene standards; 1,1-dihydro-2,3,4,5-
tetraphenylsilole with Pd(PPh3)4, 1: 0.84 g, 85%, MW = 5638, MW/Mn = 1.10).
1,1-dihydro-2,3,4,5-tetraphenylgermole with RhCI(PPh3)3,
poly(tetraphenyl)germole: 0.80 g, 81%, MW = 3936, Mw/Mn = 1.01; 1,1-
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dihydro-2,3,4,5-tetraphenylgermole with Pd(PPh3)4, poly(tetraphenyl)germole:
0.81 g, 82%, M, = 4221, MWIMn = 1.02) 1H NMR (300.133 MHz, CDC13): 6 =
6.30-7.90 (br, m, Ph); 13C{H} NMR (75.403 MHz, CDC13 (8 = 77.00)): 8=
124-130 (br, m, Ph), 131-139 (germole carbons). If less vigorous reflux
conditions are used, with the RhC1(PPh3)3 and Pd(PPh3)4 catalysts, then
corresponding dimers form along with lesser amounts of polymer. The dimer is
less soluble and crystallizes from toluene.
Preparation of Poly(1,4-diethynylbenzene)2,3,4,5-tetraphenylsilole
(PDEBsilole):
1,1 dihydro-2,3,4,5-tetraphenylsilole (250 mg, 0.65 mmol), 1,4-
diethynylbenzene (100 mg, 0.80 mmol), and 0.1-0.5 mol % H2PtC16=xH2O
were vigorously refluxed in toluene (10 mL), under argon for 4 hours. The dark
orange solution was passed through a sintered glass frit and evaporated to
dryness. The remaining solid was dissolved in 1 ml of THF, precipitated with
10 ml of methanol, and collected by filtration on a sintered glass frit. The
precipitation was repeated twice more and the polymer was obtained as a
yellow solid (0.17 g, 51%). The molecular weight of the polymer was
determined by GPC with polystyrene standards. MN, = 6,198, M,/M7z = 1.822;
'H NMR (300.075 MHz, CDC13): S 6.60 - 7.20 (br, 24H, silole Ph, =CH-Si,
and =CH-Ph), 6 7.40 (br, 4H, phenylene Ph); UV (cone. = 20 mg/L); ~,bs = 302,
378 nm; Fluorescence (conc. 20 mg/L); ~,m = 500 nm (ke, = 360 nm).
Preparation of Poly(1,4-diethynylbenzene)2,3,4,5-tetraphenylgermole
(PDEBgermole):
1,1-dihydro-2,3,4,5-tetraphenylgermole (100 mg, 0.23 mmol),
1,4-diethynylbenzene (34 mg, 0.26 mmol), and 0.1-0.5 mol % H2PtC16=xH2O
were vigorously refluxed in toluene (10 mL), under argon for 12 hours. The
catalyst was removed by filtration, and the filtrate then evaporated to
dryness.
The remaining solid was dissolved in THF (1 mL) and precipitated by
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subsequent addition of methanol (10 mL). The polymer was collected by
filtration and dried to afford the yellow powder (0.095 g, 73%). Molecular
weights determined by GPC: M,,, = 4800, M,,,/Mn = 1.6; 1H NMR (300.075
MHz, CDC13): S 6.50 - 7.60 (br, silole Ph, =CH-Ge, and =CH-Ph, phenylene
H); UV-Vis (Toluene): kabs = 290, 362 nm; Fluorescence (Toluene): Xe11, = 475
nm (keX = 360 nm).
Preparation of Poly(1,4-diethynylbenzene)silafluorene (PDEBSF):
1,1 dihydrosilafluorene (0.25 g, 1.37 mmol), 1,4-
diethynylbenzene (0.19 g, 1.51 mmol), and 0.1-0.5 mol % H2PtC16=xH2O were
vigorously refluxed in toluene (3 mL), under argon for 24 hours. The dark
orange/red solution was filtered and evaporated to dryness. The remaining
solid
was dissolved in 4 ml of THF, precipitated with 40 ml of methanol. The white
solid (0.17 g, 34%) was collected by filtration on a sintered glass frit. The
molecular weight of the polymer was determined by GPC with polystyrene
standards. Mti,, = 1,957, Mõ/M, = 1.361; 1H NMR (300.075 MHz, CDC13): b
6.00 - 8.00 (br, 16H, silafluorene H-Ph, =CH-Si, and =CH-Ph); UV (conc. =
mg/L); kabs = 292 nm; Fluorescence (conc. 0.2 mg/L); XeM = 341, 353 nm at
?IeJt = 292 nm.
Preparation and characterization of Polysilafluorene (PSF):
20 The high energy of the excited state in the UV luminescent
polysilafluorene offers an increased driving force for electron transfer to
the
explosive analyte and improved detection limits by electron;transfer
quenching,
which should be applicable for any UV emitting conjugated organic or
inorganic polymer.
1,1-dihydrosilafluorene (500 mg, 2.7 mmol) and 0.5 mol %
H2PtC16=xH2O were stirred in toluene (3 mL) at 80 C under argon for 24
hours. The orange-brown solution was filtered while warm and evaporated to
dryness. The remaining solid was dissolved in 3 mL of THF and precipitated
with the addition of 30 mL of methanol. The resulting light orange-white solid
CA 02620238 2008-02-22
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was collected by vacuum filtration (0.101 g, 20%). The molecular weight of the
polymer was determined by GPC with polystyrene standards. M,,, = 576,
M,M/M7z = 1.074; 'H NMR (300.075 MHz, CDC13): 8 6.60 - 7.90 (br, 8H,
silafluorene H-Ph), S 4.62 (weak s, terminal Si-H); UV (conc. = 20 mg/L); kabs
= 392 nm; Fluorescence (conc. 0.2 mg/L); ?~em = 342, 354 nm, at XeX = 292 nm.
Detection limits of trinitrotoluene (TNT), dinitrotoluene (DNT),
picric acid (PA), 2,2'-dimethyl-2,2'-dinitrobutane (DMNB),
orthomononitrotoluene (OMNT), and paramononitrotoluene (PMNT) were
detemined by fluorescence quenching of polysilole, polyDEBsilole,
polygermole, polyDEBgermole, PSF, po1yDEBSF, and ExPray. (DEB =
diethynylbenzene.) The emission of PSF is centered in the UV, so detection
limits with a UV camera are expected to be even better than those determined
visually.
Preparation and characterization of Polygermafluorene (PGF):
1,1-dihydrogermafluorene (0.1 g, 0.44 mmol) and 0.5 mol %
H2PtC16=xH2O were refluxed in toluene (4 mL) under argon for 24 hours. The
thick orange solution was filtered while warm and evaporated to dryness. The
remaining solid was dissolved in 2 mL of THF and precipitated with 22 mL of
methanol. The resulting light orange-white solid was collected by vacuum
filtration (0.010g, 10%). The molecular weight of the polymer was determined
by GPC with polystyrene standards. M,,, = 890, MIM7z = 1.068; 1H NMR
(300.075 MHz, CDC13): S 6.40 - 7.90 (br, 8H, silafluorene H-Ph).
Preparation and characterization of Poly(1,4-
diethynylbenzene)germafluorene (PDEBGF):
1,1 dihydrogermafluorene (0.15 g, 0.66 mmol), 1,4-
diethynylbenzene (0.092 g, 0.73 mmol), and 0.1-0.5 mol % H2PtC16=xH2O were
vigorously refluxed in toluene (4 mL), under argon for 24 hours. The dark
orange-red solution was filtered and evaporated to dryness. The remaining
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solid was dissolved in 4 ml of THF and precipitated with 40 ml of methanol.
The light orange solid (0.021 g, 15%) was collected by filtration on a
sintered
glass frit. The molecular weight of the polymer was determined by GPC with
polystyrene standards. M., = 1,719, M/Mõ = 1.872; 'H NMR (300.075 MHz,
CDC13): S 6.00 - 8.00 (br, 16H, germafluorene H-Ph, =CH-Si, and =CH-Ph).
Experimental Results and Data
The method of explosives detection is through luminescence
quenching of the metallole-containing polymers by the nitroaromatic analyte.
Three common explosives were tested, Trinitrotoluene (TNT), 2,4-
dinitrotoluene (DNT), and picric acid (PA). Stock solutions of the explosives
were prepared in toluene. Aliquots (1-5 L) of the stock (containing 5 to 100
ng analyte) were syringed onto either Whatman filter paper or a CoorsTek
porcelain spot plate and allowed to dry completely. The spots were between 3
and 10 mm in diameter, producing a surface concentration of not more than 64
ng/cm2 and not less than 17 ng/cm2. Solutions of the polymers (0.5-1 % w:v)
were prepared in acetone (PSi, PGe), 1:1 toluene:acetone (PDEBGe), 2:1
toluene:acetone (PDEBSi), or toluene (PDEBSF). A thin film of a polymer
was applied to the substrate by spray coating a polymeric solution onto the
substrate and air drying. The coated substrates were placed under a black
light
to excite the polymer fluorescence. Dark spots in the film indicate
luminescence quenching of the polymer by the analyte. The process was
carried out for each of the three explosive analytes with each of the six
polymers on both substrates.
Results and Discussion
Nitroaromatic explosives may be visually detected in nanogram
quantities by fluorescence quenching of photoluminescent metallole-containing
polymers. Detection limits depend on the nitroaromatic analyte as well as on
the polymer used.
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FIG. 22 summarizes the detection limits of TNT, DNT, and picric
acid using the five metallole-containing polymers synthesized, PSi, PDEBSi,
PGe, PDEBGe, PSF and PDEBSF.
In all cases, the detection limit of the explosives was as low or
lower on the porcelain than on paper, likely because the solvated analyte may
be carried deep into the fibers of the paper during deposition, thus lowering
the
surface contamination after solvent evaporation. Less explosive would be
present to visibly quench the thin film of polymer on the surface. This
situation is less pronounced in actuality when explosives are not deposited
via
drop-casting from an organic solution, but handled as the solid. Illumination
with a black light (keX - 360 nm) excites the polymer fluorescence near 490 -
510 nm for the siloles, 470 - 500 for germoles. The silafluorene luminescence,
which peaks at 360 nm, is very weak in the visible region, but it is
sufficient for
visible quenching.
FIG. 23 shows a sample black and white images of the
luminescence quenching of three polymers, PSi, PDEBSi, and PGe, by 200,
100, 50, and 10 ng TNT on porcelain plates as observed on a porcelain plate.
FIG. 24 shows sample black and white images of the luminescence quenching
of polysilole by each analyte at different surface concentrations.
The method of detection is through electron-transfer
luminescence quenching of the polymer luminescence by the nitroaromatic
analytes. Consequently, the ability of the polymers to detect the explosives
depends on the oxidizing power of the analytes. The oxidation potentials of
the
analytes follow the order TNT > PA > DNT. Both TNT and PA have three
nitro substituents on the aromatic ring which account for their higher
oxidizing
potential relative to DNT, which has only two nitroaromatic substituents. PA
has a lower oxidation potential than TNT due to the electron donating power of
the hydroxy substituent. The molecular structure accounts for the lowest
detection limit for TNT, followed by PA and DNT.
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Luminescence quenching is observed immediately upon
illumination. The polymers are photodegradable, however, and luminescence
begins to fade after a few minutes of continual UV exposure. Nevertheless,
these polymers present an inexpensive and simple means to detect low
nanogram level of nitroaromatic explosives.
While various embodiments of the present invention have been
shown and described, it should be understood that modifications,
substitutions,
and alternatives are apparent to one of ordinary skill in the art. Such
modifications, substitutions, and alternatives can be made without departing
from the spirit and scope of the invention, which should be determined from
the appended claims.
Various features of the invention are set forth in the appended
claims.
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