Quantitative real time sensing reveals enhanced sensitivity of polar dendrimer thin films for plastic explosive taggants

Article abstract of DOI:10.1039/c5tc01967b

A method of introducing pulses of analyte vapours has been developed to study the interactions of nitro-containing analytes with fluorescent sensing films. The quenching of the photoluminescence of thin dendrimer films by exposure to sub-saturation concen

Full text of DOI:10.1039/c5tc01967b

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Materials Chemistry C
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Quantitative real time sensing reveals enhanced
sensitivity of polar dendrimer thin films for plastic
explosive taggants†
Cite this:DOI: 10.1039/c5tc01967b
a
a
a
a
Andrew J. Clulow, Hamish Cavaye, Guoqiang Tang, Paul E. Shaw,
bc
a
a
Justin J. Cooper-White, Paul L. Burn* and Paul Meredith
A method of introducing pulses of analyte vapours has been developed to study the interactions of nitro-
containing analytes with fluorescent sensing films. The quenching of the photoluminescence of thin
dendrimer films by exposure to sub-saturation concentration vapour pulses of nitro-containing analytes
is quantified and the reversibility of quenching interactions qualitatively examined. The analysis reveals a
0
linear dependence of the initial/quenched fluorescence ratio (F /F) with quencher concentration akin to
Stern–Volmer behaviour routinely used to quantify quenching efficiency in solution. Detection limits and
quenching efficiencies were calculated in the sub-saturation regime for each of the analytes and trace-
level detection of 1,4-dinitrobenzene, 2,4-dinitrotoluene, 4-nitrotoluene, and nitromethane vapours was
observed. It was found that the sensitivity of first generation bifluorene- and carbazole-cored dendrimers
towards vapours of the nitroaliphatic taggant 2,3-dimethyl-2,3-dinitrobutane was significantly enhanced
by changing the solubilising chains on the core from alkyl to oligo(ethylene oxide) groups. The responses
of the films were observed to be independent of film exposure history making these dendrimers
interesting candidates for deployment in thin film vapour sensors for explosives.
Received 2nd July 2015,
Accepted 6th August 2015
DOI: 10.1039/c5tc01967b
www.rsc.org/MaterialsC
although the focus of these studies has generally been on the
detection of nitroaromatic analytes. One of the reasons why DMNB
Introduction
The need to detect high explosives in current and past conflict is less well studied is that it has a lower electron affinity than the
1–4
13
zones has spurred the development of new sensing technologies.
nitroaromatics and hence is more difficult to detect. Initial
Nitroaromatics are common targets for detection due to the work on fluorescent sensing materials focussed on conjugated
widespread use of 2,4,6-trinitrotoluene (TNT) in military explosive polymers, and indeed polymeric-based materials have remained
5
–8
compositions and landmines.
Nitroaliphatic compounds, the mainstay of materials development. However, more recently
whilst having received comparably less attention than nitro- fluorescent dendrimers have been used for the detection of
2
5–30
aromatics, are also important targets for the detection of explosives both nitroaromatic and nitroaliphatic analytes.
Fluorescent
due to their use as tagging agents [e.g., 2,3-dimethyl-2,3-dinitro- dendrimers are comprised of a core, dendrons, and surface
9
–12
butane (DMNB)] and accelerants in explosive mixtures.
groups, and this modular nature makes them an ideal class of
Amongst the technologies developed for explosives detection, materials to explore different sensing concepts. For example, it
oxidative quenching of fluorescent thin films has come to the has been shown previously that dendrimers with carbazole-
fore as a promising method for portable detection of nitrated containing cores displayed a larger response to DMNB in
organic vapours. As a consequence there has been a steady stream solution compared to a dendrimer with a bifluorenyl core but
7,13–24
29
of new fluorescent organic sensing molecules reported,
the same dendrons and surface groups.
Typically, the organic sensing molecules possess lipophilic
alkyl chains that make the conjugated chromophores soluble in
organic solvents and enable them to be processed to form thin
films. However, nitrated organics are inherently polar molecules
due to the electron withdrawing nature of the nitro groups. In
this work we explore the effect of replacing the non-polar alkyl
chains typically found on fluorescent dendrimers with more
polar groups. 2-(2-Methoxyethoxy)ethyl chains were chosen as the
polar groups as poly(ethyleneoxide)s are used as polar stationary
a
b
c
Centre for Organic Photonics & Electronics, The University of Queensland,
St Lucia, QLD 4072, Australia. E-mail: p.burn2@uq.edu.au
Australian Institute for Bioengineering and Nanotechnology,
The University of Queensland, St Lucia, QLD 4072, Australia
School of Chemical Engineering, The University of Queensland, St Lucia,
QLD 4072, Australia
†
Electronic supplementary information (ESI) available: The experimental details
for the synthesis and characterisation of the dendrimer precursors 1 and 3b is
detailed. See DOI: 10.1039/c5tc01967b
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Fig. 1 The chemical structures of the dendrimers used as sensing materials for nitrated analytes.
3
1–33
phases for gas chromatography.
It was expected that by analysed: 2,4-dinitrotoluene (DNT) and 4-nitrotoluene (pNT) are
limiting the chain length to 2-(2-methoxyethoxy)ethyl the solubility both potential impurities from the manufacture of TNT, and the
of the dendrimers in organic solvents would be maintained. presence of DNT is commonly used an indicator for the presence
16,18,34–38
We compare two dendrimer families comprising carbazole or of TNT;
1,4-dinitrobenzene (DNB) is a TNT surrogate with a
bifluorene cores and first generation biphenyl dendrons (Fig. 1). similar saturated vapour concentration and a near-identical
1
4
In each case we compare the effect of changing the solubilising reduction potential; nitromethane (NM), whilst not inherently
group attached to the core, while keeping the same surface explosive can be mixed with a variety of chemicals to yield
9,11
groups [2-ethylhexyloxy or 2-(2-methoxyethoxy)ethyloxy] in each explosive compositions; and finally, 2,3-dimethyl-2,3-dinitrobutane
pair. The sensitivities of these dendrimer films towards the (DMNB), the volatile tagging agent added to commercially
10,12
nitroaromatic and nitroaliphatic analytes depicted in Fig. 2 were manufactured plastic explosives for detection by trained canines.
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ester-focussed first generation biphenyl dendrons containing
2-(2-methoxyethoxy)ethyloxy surface groups (1) and the dibro-
minated core precursors (3a/b). In contrast, for the carbazole-
cored dendrimers two 2-ethylhexyloxy-surfaced dendrons (2)
were attached to 4a/b to give 6a/b. The yield of each dendrimer
was greater than 50%. Gel-permeation chromatography showed
that the dendrimers were monodisperse and their hydro-
dynamic radii calculated from their M%
5b), 8.4 Å (6a) and 8.4 Å (6b). Differential scanning calorimetry
was used to determine glass transition temperatures (T s),
which were 60 1C, 27 1C, 30 1C and 26 1C for 5a, 5b, 6a and
b, respectively.
v
s were 9.4 Å (5a), 9.7 Å
(
g
6
Photophysical and electrochemical properties
Fig. 2 The structures, first reduction potentials (Ered versus the ferrocenium/
ferrocene couple) and saturated vapour concentrations (csat) of the nitrated
The absorption and photoluminescence (PL) spectra for solutions
and thin films of dendrimers 5a/b and 6a/b are shown in Fig. 3.
The spectra of the dendrimers with the same core unit were
found to be essentially the same, indicating that substitution of
the alkyl groups with 2-(2-methoxyethoxy)ethyl groups on the core
does not greatly influence the optical properties of the chromo-
phores. In the case of the bifluorene dendrimers the absorption
13,14,39
analytes.
Results and discussion
Dendrimer synthesis
The final steps of the syntheses of the four dendrimers are and PL spectra recorded are also consistent with reported bifluorene
depicted in Scheme 1 with detailed methods for the preparation dendrimers possessing biphenyl-based first generation dendrons
2
7,40
of the starting materials in the ESI.† The synthesis of carbazole- and 2-ethylhexyloxy surface groups.
That is, changing the
based dendrimer 6a was recently reported by our group and is surface groups on the dendrons does not change the absorption
2
9
included in the scheme for completeness. The final step in properties of the dendrimers. For the bifluorene-cored dendrimers
the convergent synthesis of each dendrimer was a palladium- (5a/b) the absorption spectra in both dichloromethane solution
catalysed Suzuki coupling. The bifluorene-cored dendrimers and thin films display featureless lowest-energy absorptions with a
5a/b with different solubilising groups on the 9-positions of maximum around 350 nm. A second higher-energy peak was also
the fluorene moieties were prepared by reaction of the boronate observed at 265 nm, which has a significant contribution from the
Scheme 1 Synthesis of dendrimers 5a/b and 6a/b. Conditions and reagents: (i) Pd(PPh
1%; 6a 52%; 6b 59%.
3
)
4
, PhMe, tert-BuOH, aq. K
2
CO
3
, 100 1C, N
2
. Yields: 5a 53%; 5b
7
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Fig. 3 The normalised absorption and PL spectra for dendrimers 5a (A), 5b (B), 6a (C) and 6b (D). The solid lines correspond to solution measurements
dichloromethane) and the dashed lines correspond to measurements of thin films spin-coated on glass substrates. For the PL measurements the
bifluorene dendrimers 5a/b were excited at 350 nm and the carbazole dendrimers 6a/b were excited at 320 nm.
(
41
chromophores in the biphenyl dendrons. The solution PL emission peaks at 375 nm and 395 nm along with shoulders
spectra of the bifluorene-cored dendrimers displayed vibronic around 410 nm and 435 nm. The film absorption and PL spectra
features with peaks around 392 nm and 413 nm, and shoulders of the carbazole dendrimers 6a/b were analogous to their
around 435 nm and 465 nm. The thin film absorption and PL solution counterparts. The carbazole dendrimers 6a and 6b
spectra only had a small red-shift compared to that in solution. were found to have solution PLQYs of 19 ꢀ 1% and 15 ꢀ 1%
Increased overlap of the dendrimer absorption and PL causes the with thin film PLQYs of 27 ꢀ 6% and 25 ꢀ 6%, respectively. The
observed reduction in the PL intensity for the (0–0) vibronic PL lifetimes of 6a/b measured at 400 nm in toluene solution
transition relative to the (0–1) transition in the thin film emission were 7.4 ꢀ 0.1 ns and 7.2 ꢀ 0.1 ns, which are an order of
profiles. The solution PLQY of dendrimers 5a and 5b were found magnitude longer than the bifluorene-cored dendrimers and a
to be 81 ꢀ 1% and 85 ꢀ 1%, respectively, showing that radiative reason why these and similar dendrimers show enhanced
2
9
decay of the fluorophore is a highly efficient process. The thin film collisional quenching in solution. The higher film PLQYs are
PLQYs were found to be similar to those in solution with average consistent with previous reports that 3,6-disubstituted carbazole
values of 83 ꢀ 6% and 71 ꢀ 5% for 5a and 5b, respectively. These fluorophores with biphenyl type substituents undergo emission
film PLQYs are significantly higher than a similar bifluorene- enhancement in the solid state due to deactivation of non-radiative
43
cored dendrimer that had 2-ethylhexyloxy surface groups and decay pathways by restriction of intramolecular rotations, and
n-propyl units on the 9-positions of the fluorenyl moieties that the first-generation dendrons attached directly to the carbazole
2
7
(
PLQY = 49 ꢀ 5%). The fact that the solution and film PLQYs unit provide enough steric shielding of the emissive fluorophores
for the glycolated dendrimers are very similar indicates that the to prevent significant aggregation quenching of fluorescence in
first-generation biphenyl dendrons, surface groups and substitu- the solid state. Despite having lower PLQYs than the bifluorene
ents on the 9-positions of the fluorene units provide sufficient dendrimers 5a/b the emission intensity of the carbazole
separation of the emissive chromophores to avoid significant dendrimers 6a/b was sufficient for the materials to be used in
42
aggregation-induced quenching in the solid state. The PL lifetimes sensing applications.
of dendrimers 5a/b were measured at 395 nm in tetrahydrofuran
We used cyclic voltammetry (CV) in combination with the
solution and were found to be 0.77 ꢀ 0.01 ns and 0.78 ꢀ 0.01 ns for optical gap derived from Fig. 3 to determine the oxidation and
dendrimers 5a and 5b, respectively. Such short PL lifetimes are estimate the reduction potentials of the dendrimers in solution.
40
consistent with previously reported bifluorene dendrimers.
The cyclic voltammograms of the bifluorene dendrimers 5a and
The optical properties of dendrimer 6a have been reported 5b both had a single chemically reversible oxidation within the
2
9
previously, and those of the glycolated dendrimer 6b were measurable potential window (Fig. 4). The E1/2s for the first
+
found to be analogous. The dichloromethane solution absorption oxidations were 0.9 V versus the ferrocenium/ferrocene (Fc /Fc)
spectra of dendrimers 6a and 6b had peaks close to the isolated couple for both 5a and 5b. The E1/2s for the first reductions were
dendron absorption maximum at around 265 nm and displayed too close to the edge of the electrochemical window and so we
a number of shoulders leading to the absorption onset of estimated the reduction potentials by adding the energy of
4
4
around 375 nm. The carbazole dendrimer PL spectra displayed the optical gaps, which were calculated by a standard method.
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Fig. 4 The oxidation cyclic voltammograms for dendrimers 5a/b (A) and 6a/b (B) in dichloromethane and tetrahydrofuran, respectively. Measurements
were referenced to the ferrocenium/ferrocene oxidation E1/2 recorded on the day of testing and were performed with a silver/silver nitrate reference
electrode in acetonitrile, a glassy carbon working electrode and a platinum counter electrode.
The absorption and PL spectra were normalised in energy by feasible in these cases. The estimated electron affinities of the
dividing by the photon energy and photon energy cubed, bifluorene dendrimers 5a/b are only 0.2 eV less than those of the
respectively. The rescaled spectra were then normalised to the nitroaliphatic analytes and fluorescence quenching might therefore
first absorption and emission features and the intercept be expected to be weaker or absent. Despite this, quenching was
between the two curves was taken as the optical energy gap. observed when bifluorene dendrimer films were exposed to DMNB
For both dendrimers 5a and 5b the optical energy gap was found and NM and furthermore quenching by DMNB could be enhanced
to be around 3.3 eV giving estimated reduction potentials of by the choice of core substituent.
+
ꢁ
2.4 V versus Fc /Fc by subtracting this energy from the
Quantitative real time thin film quenching measurements
observed oxidation potentials.
A chemically reversible oxidation was also observed for the Film-based fluorescence quenching measurements have generally
carbazole dendrimers 6a/b. The E1/2s of the first oxidations involved exposure to saturated vapours of the analyte with recovery
+
were found to be 0.7 V versus Fc /Fc for both dendrimers. or cycling achieved by removal of the film from the analyte
Voltage sweeps beyond the first oxidation to potentials more vapours followed by purging and/or heating of the film to
+
14,15,19,23,35
positive than 1.0 V vs. Fc /Fc resulted in a second chemically remove the absorbed analyte.
A problem with this
irreversible oxidation process. The E1/2s for the first reductions method is that it does not allow accurate determination of the
lay outside of the electrochemical window and so we again initial quenching response for rapidly diffusing and high electron
estimated the reduction potentials by subtracting the optical affinity analytes as substrate immersion takes place prior to the
gaps of 3.4 eV from the oxidation potentials for dendrimers initiation of the PL measurement. The initial rapid adsorption
+
6
a/b, which gave a value of ꢁ2.7 V versus Fc /Fc.
and diffusion of the analytes into fluorescent dendrimer films
+
The reduction potentials versus Fc /Fc for the higher electron under steady state conditions has been shown by quartz crystal
2
5
affinity nitroaromatic analytes have been reported to be ꢁ1.2 V microbalance with in situ PL measurements. Analyte vapour
+
14
for DNB, ꢁ1.0 V for DNT and ꢁ1.7 V for pNT versus Fc /Fc. The pulses have been reported to be detectable by using a capillary
lower electron affinity nitroaliphatic DMNB has a reduction coated with the sensing material and a saturated vapour
+
13
13,23,47
potential of ꢁ2.2 V versus Fc /Fc. The first reduction potential source.
This procedure, whilst useful for showing a sensors
of NM cannot be measured directly by CV as it decomposes efficacy, defies accurate quantification of film responses towards
4
5
to liberate methylhydroxyamine at the electrode surface.
the analytes as the concentration entering the capillary is
However, it is reasonable to expect that the reduction potential unknown. It is unlikely that either saturated analyte vapours or
for NM would be similar to that of DMNB as both are unconjugated sustained steady state analyte concentrations would be present
nitroaliphatic molecules. In order for oxidative quenching of the for sensing in the field and to this end we have developed a
dendrimer fluorescence to occur the difference between the method to quantify responses to sub-saturated analyte vapour
analyte and dendrimer reduction potentials should be greater pulses in real time for thin films. Thin films of the dendrimers
ꢁ
3
than the exciton binding energy of the fluorophore, which has were spin-coated onto fused silica substrates from 10 mg cm
been reported to be around 0.5 eV for organic semiconductors.
4
6
solutions in toluene at 2000 rpm. Under these conditions the
The energy difference between the reduction potentials of the resulting films were found to be 40–55 nm thick. The substrates
target analytes and those estimated for the dendrimers is Z0.5 eV were clamped in a custom made sample chamber in a fluoro-
for most of the dendrimer/analyte combinations suggesting that meter and a constant flow of nitrogen was passed over the film
oxidative quenching of the dendrimer fluorescence was energetically via a mass flow controller. Syringes were charged with analyte
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and plugged with sufficient cotton wool to prevent injection of and 6b was more strongly quenched than that of their alkylated
the solids. The syringes were primed with nitrogen to eliminate counterparts 5a and 6a by the nitroaliphatic tagging agent
additional quenching by oxygen and stoppered to allow the DMNB. The ability of a sensing element to recover after a
atmosphere to saturate with analyte vapour for at least 16 hours quenching interaction is not a ubiquitous property of all
at ambient temperature prior to analyte injection. Known sensing fluorophores with some requiring thermal or chemical
1
9,28,30
volumes of analyte-saturated nitrogen were manually injected reactivation after exposure to enable further sensing.
into the carrier gas flow over a one second period to introduce Therefore, the recovery of the majority of the film PL is an important
pulses of analyte into the nitrogen stream. The PL of the films at result for real time sensing. It is interesting to note that the initial
400 nm was monitored before, during, and after exposure. The rates of recovery after injection were consistent with the relative
maximum concentration of analyte vapour that could be introduced volatilities of the analytes. For example, slow recovery was observed
into the cell was limited by the saturated vapour concentrations of after injection of the lower volatility analytes DNB and DNT, despite
13,14,39
3
ꢁ1
the analytes (Fig. 1).
The analyte concentrations could be using the higher carrier gas flow rate of 4000 cm min . The fact
varied by either altering the carrier gas flow rate or by injecting that the PL does not fully recover suggests that some analyte
3
0
different volumes of saturated analyte vapour.
remains trapped within the film. It is also worthy of note that
Initial experiments with bifluorene dendrimer 5a revealed DMNB was the only analyte that demonstrated a rapid and
that no responses occurred for the lower electron affinity complete recovery of the film PL in all cases.
3
ꢁ1
analyte DMNB with a carrier gas flow of 4000 cm min , even
The Stern–Volmer equation (1) is commonly used to char-
3
ꢁ1
44
when injecting up to 24 cm s of saturated analyte vapour acterise solution-based quenching interactions,
(
exposure concentration = 0.58 ꢀ 0.11 ppm). However, a
F0
measurable reduction in the fluorescence was seen for the
higher electron affinity analytes and the highly volatile NM at
this flow rate. Reducing the carrier gas flow to 1000 cm min
decreased the dilution of the analyte vapour and enabled the constant that serves as a measure of the strength of the
¼ 1 þ K½Qꢂ
(1)
F
3
ꢁ1
where [Q] is the quencher (analyte) concentration and K is a
3
ꢁ1
presence of DMNB vapour to be detected. Thus a 4000 cm min
quenching interactions occurring – the larger the value of K,
carrier gas flow rate was used for the higher electron affinity the stronger the interaction. In order to quantify the thin film
3
ꢁ1
analytes DNB, DNT and pNT, and a 1000 cm min flow rate was quenching efficiencies we undertook a ‘Stern–Volmer’ like
used with the lower electron affinity analytes DMNB and NM. It analysis whereby the PL intensity immediately prior to analyte
should be noted that varying the carrier gas flow rate should affect exposure (F ) was divided by the PL at the base of the quenching
0
the rate of film fluorescence recovery and hence we do not directly trough (F) and the values of F /F plotted against the injected
0
compare recovery for measurements at different flow rates.
0
analyte concentrations (Fig. 6). Taking the value of F from just
All of the measurements were performed at ambient prior to an injection renormalised each response to account for
temperature, repeated on separate dendrimer films and each any analyte trapped in the film from previous injections or loss
film was used for multiple measurements. To confirm the of initial PL intensity due to photodegradation. The analysis
reproducibility of the results the analytes injected during a given is valid if a steady-state excited fluorophore population is
quenching experiment were varied along with the concentrations established under constant illumination and if the quenching
4
4
injected. By randomising the order of analyte injections between interactions are rapid over the one second exposure time.
runs the influence of the film history on responsiveness could be Within the error of the measurements linear correlations of F /F
0
investigated. The carrier flow rate was not altered within a given with analyte concentration were observed for each of the
measurement. Representative thin film quenching traces for each of dendrimer films. The data collected were consistent between
the dendrimers and analytes are given in Fig. 5. The differences in multiple quenching runs regardless of the order of analyte
the shape of the PL baseline signal are due to the observed time injection and, as such, the exposure history of a film appeared
slices occurring at different points in the hour-long monitoring to have little influence on the magnitude of subsequent quenching
period. That is, the films typically had a ‘burn in’ period that showed interactions. Such behaviour is advantageous for long-term use of
faster initial PL degradation than at later times during the scan. the sensing element in a portable detection system for continuous
However, it is important to note that the rate of background PL monitoring. All the analytes except DMNB showed a first order
decay was very much less than the changes in PL observed on dependence on analyte concentration with intercepts close to the
introduction of the analyte and were ignored in the analysis.
theoretical value of 1.0 for a standard Stern–Volmer analysis.
Rapid quenching was observed after injection of the analytes For DMNB the intercepts were o1.0 suggesting that additional
into the carrier gas flow with responses reaching a maximum physical factors hindering quenching are leading to higher
within 1–2 s after injection. Carrier gas volumes equivalent to threshold concentrations for a response to be observed.
3
the internal volume of the sample chamber (B40 cm ) flow
The quenching constants K determined from Fig. 6 and the
every 1–3 s at the flow rates used and we have assumed that the corresponding detection limit concentrations at F /F = 1.01
0
analyte pulses were rapidly purged from the cell after this time. (i.e., the concentration corresponding to a 1% change in
There are some clear differences between the dendrimers in fluorescence) are given in Fig. 7 (values are tabulated in the
their behaviour towards the analytes, the most prominent ESI†). In general the bifluorene dendrimers 5a/b were found
being that the fluorescence of the glycolated dendrimers 5b to be more sensitive to the nitroaromatics with DNT, DNB
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Fig. 5 Thin film quenching traces for the bifluorene dendrimers 5a (A) and 5b (B) and the carbazole dendrimers 6a (C) and 6b (D). All plots are on the same
PL and time scales and the traces are offset for clarity. Numbers in parentheses indicate the range of vapour concentrations injected into the cell. The order
of injected analyte concentrations was randomised and in each case the magnitude of the decrease in PL increases with the amount of analyte injected.
and pNT having larger quenching constants and thereby DNT, DNB, pNT and NM. Dendrimers 5a and 6b with a mix of
lower detection limits for these analytes than the carbazole oligo(ethylene oxide) and alkyl solubilising groups on the core
dendrimers 6a/b (note the log scale on the Y-axes visually and the dendrons showed slightly greater sensitivities towards
compresses these differences). The detection limits of the DNT, DNB and NM, whilst dendrimers 5b and 6a with all polar
ꢁ3
bifluorene dendrimers for DNB of (0.95 ꢀ 0.19) ꢃ 10 ppm or all non-polar solubilising groups showed greater sensitivity
ꢁ
3
(
5a) and (1.21 ꢀ 0.24) ꢃ 10 ppm (5b) were around 4% of the towards pNT. This suggests that the interplay between the
saturated analyte vapour concentration. Similarly, the detection polarity of the substituents on the core and dendrons exerts
limit concentrations of the bifluorene dendrimers of (1.82 ꢀ some influence on the degree of quenching by the different
ꢁ
3
ꢁ3
0
.36) ꢃ 10 ppm (5a) and (2.35 ꢀ 0.47) ꢃ 10 ppm (5b) for analytes in the thin films. However, as a result of having
DNT were around 1% of the saturated concentration showing oligo(ethylene oxide) chains on the core units the sensitivity
the strong sensitivity of these dendrimers towards traces of the of DMNB detection is significantly increased. That is, both
high electron affinity nitroaromatics. The sensitivity of the dendrimers 5b and 6b showed enhanced sensitivity towards the
carbazole dendrimers 6a/b was around 4–5 times less than that nitroaliphatic tagging agent when compared with their alky-
of the bifluorene dendrimers, with detection limits around 4% lated counterparts. The most significant enhancement in
of the saturated concentration of DNT and 18% of the saturated quenching constant was a factor of 5 increase in sensitivity
concentration of DNB. The quenching constants for the higher when the four n-propyl chains of 5a were exchanged for 2-(2-
volatility and lower electron affinity analytes NM and pNT were methoxyethoxy)ethylchains on the bifluorene core to give 5b.
lower than for DNT and DNB. However, all of the dendrimers The corresponding sensitivity enhancement when exchanging
gave detection limit concentrations that were o1% of the the single n-hexyl chain of 6a for a 2-(2-methoxyethoxy)ethyl
saturated vapour concentrations of NM and pNT, showing that chain on the carbazole core of dendrimer 6b was only a factor
these volatile analytes can be detected at high dilution.
of 2. This suggests that the influence of the oligo(ethylene oxide)
The polarity of the core solubilising groups was found to chains is additive, with greater sensitivity enhancement observed
have a subtle influence on the dendrimer sensitivities towards for the dendrimers with more groups attached to the core.
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3
ꢁ1
Fig. 6
F /F versus analyte concentration plots for thin films of dendrimers 5a/b (A) and 6a/b (B) using nitrogen carrier gas flows of 4000 cm min for DNT, DNB
o
3
ꢁ1
and pNT and 1000 cm min for NM and DMNB. Individual points indicate recorded experimental data and a description of the error analysis is given in the
Experimental details. In both cases black indicates alkyl core substituents, red indicates glycol core substituents and dashed lines indicate straight line fits to the data.
Fig. 7 The quenching constants K (A) and detection limit concentrations (B) for the analytes tested. The quenching constants and associated errors were
determined from the gradient of the straight line fits in Fig. 6. The detection limit concentrations represent the minimum analyte concentration that
0
produces a 1% (F /F = 1.01) fluorescence quenching response over a one second exposure. Horizontal red lines in (B) indicate the saturated vapour
concentration of each analyte given in Fig. 2.
Conclusions
explosive sensing. Reversible fluorescence quenching inter-
In summary, we have developed bifluorene- and carbazole-cored actions were observed upon exposure of dendrimer thin films
dendrimers with first generation biphenyl-based dendrons for to sub-saturation vapour concentrations of nitroaromatic and
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nitroaliphatic vapours, showing their ability to detect traces of
Gel permeation chromatography was performed on a Polymer
these target analytes. The quenching responses were quantified Laboratories PL GPC 50 using PLgel 3 mm mixed-E columns
as a function of analyte concentration using a Stern–Volmer-like (2 ꢃ 300 mm lengths, 7.5 mm diameter) from Polymer Laboratories
analysis akin to that used for solution-based quenching analysis. calibrated against polystyrene narrow standards [ M% = (162–3.8) ꢃ
p
4
The inclusion of 2-(2-methoxyethoxy)ethyl substituents on both 10 ] in tetrahydrofuran containing 0.01% toluene as a flow marker.
the bifluorene and the carbazole units was found to increase the The eluent was degassed with helium and pumped at a rate of
3
ꢁ1
sensitivity towards the nitroaliphatic tagging agent DMNB. 0.5 cm min at 39.3 1C. UV absorption at 256 nm was used to
Detection of this nitroaliphatic tagging agent by oxidative detect the eluting species.
fluorescence quenching is normally challenging, and the ability
g
Glass transition temperatures (T ) were recorded on a Perkin
to molecularly engineer enhanced sensitivity by simple chemical Elmer Diamond DSC Differential Scanning Calorimeter within
substitutions is a strength of dendritic sensing materials. The the temperature range ꢁ150 to 200 1C. Scan rates of between
ꢁ
1
quenching interactions were also independent of film exposure 20 and 100 1C min were used and the heating and cooling
history under continuous illumination, making these molecules rates were identical during each scan. Heating/cooling rates
g
strong candidates for deployment in field portable vapour are quoted with reported T s. Decomposition temperatures
sensors for continuous monitoring of nitroaliphatic and nitro- (T5% dec) are quoted for 5% weight loss and were recorded on
aromatic vapours.
a Perkin Elmer STA 6000 Simultaneous Thermal Analyser.
ꢁ
1
Samples were heated from 30–800 1C at 10 1C min
for
T
5% dec determination. Melting points were measured using a
B u¨ chi Melting Point B-545 and are corrected.
Experimental details
General experimental methods and synthesis of organic
Cyclic Voltammetry (CV) was recorded on a BASi EpsilonEC.
The potentials quoted are referenced to the ferrocenium/ferrocene
compounds
+
(
Fc /Fc) couple measured on the day of testing. A silver/silver
Unless otherwise stated, chemicals were obtained from commercial nitrate reference electrode in acetonitrile, a platinum counter
sources and used as received. Solvents were distilled by rotary electrode and a glassy carbon working electrode were used. Scan
ꢁ1
evaporation under reduced pressure before use. Anhydrous rates between 100 and 1000 mV s
tetrahydrofuran used for reactions was dried over sodium measurements. The measured potentials are denoted E1/2 (redox
benzophenone and distilled immediately prior to use. process, solvent, working electrode). For electrochemical
were used for the CV
Thin layer chromatography was performed on Aldrich measurements dichloromethane was distilled over calcium
aluminium plates coated with silica gel 60 P254. Column hydride and tetrahydrofuran was distilled over sodium/benzo-
chromatography was performed with Merck silica gel (0.063– phenone and then distilled over lithium aluminium hydride.
ꢁ3
0.200 mm). Chromatotron separations were performed using
Thin films were formed by spin-coating 10 mg cm dendrimer
plates of Silica Gel 60 PF254 containing gypsum on a T-squared solutions in spectrophotometric grade toluene using a Speciality
Technology Chromatotron 7924T. When solvent mixtures were Coating Systems (SCS) G3-8 Spincoater. Glass and fused silica
used as eluent the proportions are given by volume. K u¨ gelrohr substrates were cleaned by ultrasonication in acetone, 2-propanol
distillations were performed using a B u¨ chi B-585 K u¨ gelrohr.
and the deposition solvent prior to film spinning. Surface contact
H and C NMR spectra were recorded on Bruker AV300, profilometry to determine film thickness was performed on a
AV400 or AV500 spectrometers. Chemical shifts are reported in Veeco Dektak 150.
parts per million (ppm) and are referenced to the residual 2-(3,5-Bis{4-[2-(2-methoxyethoxy)ethoxy]phenyl}phenyl)-4,4,5,5-
solvent peak [d (CHCl ) = 7.26, d (CDCl ) = 77.0]. Coupling tetramethyl-1,3,2-dioxaborolane (1). 9 (0.424 g, 0.777 mmol) was
1
13
H
3
C
3
3
constants, J, are reported in Hertz (Hz) to the nearest 0.5 Hz. dissolved in anhydrous tetrahydrofuran (2.5 cm ) under argon
Peak multiplicities are labelled in the following manner: singlet and the resulting solution was chilled in a dry ice/acetone bath.
3
(
s), doublet (d), triplet (t), multiplet (m) and broad (br); A solution of n-butyl lithium in hexanes (1.6 M, 2.0 cm ) was
aromatic peak identities are abbreviated as Ar (aryl), Ph added and the mixture was stirred whilst chilled for a further
phenyl), Fl (fluorenyl) and Cbz (carbazolyl). 15 min. 2-Iso-propoxy-4,4,5,5-tetramethyl-1,3,2-dioxaborolane
MALDI-TOF mass spectra were recorded on an Applied (0.70 cm , 3.4 mmol) was added and the mixture was stirred
Biosystems Voyager MALDI-TOF mass spectrometer using in the dry ice/acetone bath for a further 10 min. The mixture was
,8,9-anthracenetriol as the matrix. ESI mass spectra were allowed to warm to room temperature and was stirred for 3 d.
(
3
1
3
recorded on a Bruker HCT 3D Ion Trap using methanol/ The mixture was quenched with water (20 cm ) and diluted with
dichloromethane as the solvent. ESI mass spectra and elemental diethyl ether (20 cm ). The layers were separated and the
3
3
microanalysis were performed by the respective mass spectrometry aqueous layer was extracted with diethyl ether (3 ꢃ 5 cm ).
3
and microanalysis facilities at the School of Chemistry and The combined organics were washed with water (2 ꢃ 5 cm )
3
Molecular Biosciences, UQ.
and brine (5 cm ). The organics were dried over anhydrous
UV-visible absorption spectra were measured on a Varian magnesium sulfate and filtered. The magnesium sulfate was
Cary 5000 UV-Vis-NIR spectrophotometer in spectrophotometric washed with additional diethyl ether and the filtered organics
grade solvents. lmax values are quoted in nm and shoulders combined before the volatiles were removed in vacuo. The crude
denoted by sh.
residue was separated in two stages: firstly residual boronated
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J. Mater. Chem. C

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Paper
Journal of Materials Chemistry C
impurities were removed by short-path distillation using a Found 894.5 (45%), 895.5 (21%), 896.5 (100%), 897.5 (44%),
K u¨ gelrohr distillation apparatus at 0.4 mbar and 100 1C and finally 898.5 (61%), 899.5 (18%).
debrominated by-products in the residue were crystallised from
diethyl ether at 0 1C and removed from the product by filtration. Dibromocarbazole (1.50 g, 4.62 mmol) was dissolved in
Removal of the volatiles from the filtrate in vacuo yielded 1 as an dimethylsulfoxide (5.0 cm ) and aqueous potassium hydroxide
off-white solid (0.382 g, 83%), mp = 79–84 1C. Found C, 68.8; H, 7.8; (50% w/w, 5.0 cm ) was added. 1-Bromo-2-(2-methoxyethoxy)ethane
3,6-Dibromo-9-[2-(2-methoxyethoxy)ethyl]carbazole (4b). 3,6-
4
8
3
3
3
C
34
H
45BrO
8
ꢁ1
requires C, 68.9; H, 7.7. lmax (CH
2
Cl
2
/nm): 267 (log (0.93 cm , 6.9 mmol) was added to the mixture, which was stirred at
3
ꢁ1
e/dm mol cm 4.60). d
ArB[OC(CH ), 3.40 (6H, s, ArOC
m, ArOC OCH CH OMe), 3.73–3.76 (4H, m, ArOC
CH OMe), 3.87–3.91 (4H, m, ArOCH CH OC H OMe), 4.18–4.21 acetate (2 ꢃ 50 cm ) and the combined organics were washed with
H
3
(300 MHz, CDCl ): 1.37 (12H, br s, room temperature for 4 h. The mixture was diluted with diethyl
3
3
3
3
)
2
]
2
2
H
4
OC OCH
2
H
4
3
), 3.58–3.61 (4H, ether (150 cm ), ethyl acetate (50 cm ) and water (100 cm ), and the
OCH layers were separated. The aqueous layer was extracted with ethyl
2
H
4
2
2
2
H
4
2
-
3
2
2
2
2 4
0
0
3
3
(
4H, m, ArOCH CH OC H OMe), 6.99 and 7.60 (8H, AA BB , water (2 ꢃ 50 cm ) and brine (50 cm ). The organics were dried over
2
2
2 4
surface PhH), 7.81 (1H, dd, J = 2.0 Hz, J = 2.0 Hz, branching anhydrous magnesium sulfate and decanted. The magnesium
PhH), 7.94 (2H, d, J = 2.0 Hz, branching PhH). d (125 MHz, sulfate was washed with additional ethyl acetate and the decanted
CDCl ): 24.8, 59.1, 67.4, 69.8, 70.7, 71.9, 83.9, 114.8, 128.1, 128.3, organics combined before the volatiles were removed in vacuo.
31.5, 133.8, 140.6, 158.3. m/z [ESI] anal. calcd for C34 The crude residue was separated by column chromatography
91.3 (25%), 592.3 (100%), 593.3 (37%). Found 614.3 (17%, M + over silica using an ethyl acetate/n-hexane mixture (2 : 3)
C
3
1
5
8
H45BO :
+
+
+
Na ), 615.3 (100%, M + Na ) and 616.3 (28%, M + Na ). Note – the as eluent to give 4b as a white powder (1.78 g, 90%), mp =
product isolated was observed to degrade by hydrolysis if exposed 115–116 1C. Found C, 47.7; H, 4.1; N, 3.2; C H NO Br requires
1
7
17
2
2
3
ꢁ1
to wet solvents or atmospheric moisture. The data quoted is for C, 47.8; H, 4.0; N, 3.3. l
(CH Cl /nm): 236 (log e/dm mol
2 2
max
1
ꢁ1
freshly isolated 1 but over time the H NMR peak at 1.37 ppm was cm 5.35), 242 (5.34), 251 (5.13), 273 (5.12), 292sh (4.62) 298sh
found to decrease in intensity and additional aromatic resonances (4.71), 304 (4.92), 334sh (4.03), 345 (4.21), 345sh (4.16) and 360
in the range 7.4–7.6 ppm were observed.
(4.21). d
H
(500 MHz, CDCl
3
): 3.29 (3H, s, N(C
)OCH CH OCH ), 3.47–3.49 (2H, m,
), 3.83 (2H, t, J = 6.0 Hz, NCH CH O-
), 4.44 (2H, t, J = 6.0 Hz, NCH CH O(C )OCH ),
15 cm ) and a small iodine crystal was added. Light was 7.33 (2H, d, J = 8.5 Hz, CbzH), 7.54 (2H, dd, J = 8.5 Hz and 2.0 Hz,
excluded and a solution of bromine (0.583 g, 3.65 mmol) in CbzH), 8.12 (2H, d, J = 2.0 Hz, CbzH). d (125 MHz, CDCl ): 43.5,
2 4 2 4 3
H )O(C H )OCH ),
0
0
0
0
0
7
,7 -Dibromo-9,9,9 ,9 -tetrakis[2-(2-methoxyethoxy)ethyl]-2,2 - 3.38–3.40 (2H, m, N(C
H
2 4
2
2
3
0
0
bifluorene (3b). 9,9,9 ,9 -Tetrakis[2-(2-methoxyethoxy)ethyl]-2,2 - N(C
bifluorene 11b (1.12 g, 1.52 mmol) was dissolved in chloroform (C
2
H
4
)OCH
)OCH
2
CH
2
OCH
3
2
2
2
H
4
3
2
2
H
2 4
3
3
(
C
3
3
chloroform (10 cm ) was added. The mixture was stirred at room 59.0, 69.3, 70.9, 71.9, 110.7, 112.2, 123.1, 123.5, 129.0, 139.5. m/z
temperature for 3 d before it was quenched with aqueous [ESI] anal. calcd for C17 Br 425.0 (51%), 427.0 (100%),
H
17NO
2
3
+
+
sodium metabisulfite (0.5 M, 30 cm ). After the loss of all 429.0 (49%). Found 426.0 (18%, M + H ), 428.0 (35%, M + H ),
+
+
bromine/iodine colouration the mixture was diluted with 430.0 (16%, M + H ), 447.9 (53%, M + Na ), 449.9 (100%, M +
3
3
+
+
+
chloroform (30 cm ) and water (20 cm ) and the layers were Na ), 451.9 (48%, M + Na ), 463.9 (43%, M + K ), 465.9 (73%,
+
+
separated. The aqueous layer was extracted with chloroform M + K ), 467.9 (41%, M + K ).
3
0
(
3 ꢃ 10 cm ) and the combined organics were washed with
7,7 -Bis(3,5-bis{4-[2-(2-methoxyethoxy)ethoxy]phenyl}phenyl)-
3
3
0
0
0
49
water (2 ꢃ 30 cm ) and brine (2 ꢃ 30 cm ). The organics were 9,9,9 ,9 -tetra-n-propyl-2,2 -bifluorene (5a). 3a (0.500 g, 0.762 mmol)
3
dried over anhydrous magnesium sulfate and filtered. The and 1 (1.13 g, 1.90 mmol) were dissolved in toluene (40 cm ).
3
magnesium sulfate was washed with additional chloroform Aqueous potassium carbonate (2.0 M, 13 cm ) and tert-butanol
and the filtered organics combined before the volatiles were (13 cm ) were added and the mixture was deoxygenated by sparging
3
removed in vacuo. The crude residue was purified by column with nitrogen for 40 min. Tetrakis(triphenylphosphine)palladium(0)
chromatography over silica using ethyl acetate as eluent yielding (0.088 g, 0.076 mmol) was added and the mixture was deoxygenated
3b as a yellow oil that slowly crystallised into a pale yellow solid by sparging with nitrogen for 30 min. The mixture was stirred at
(
0.943 g, 69%), mp = 88–89 1C. Found C, 61.5; H, 6.2; 100 1C under nitrogen for 3 d before it was allowed to cool to room
3
C H Br O requires C, 61.6; H, 6.3. l
(CH Cl /nm): 268sh temperature. The mixture was diluted with toluene (60 cm ) and
2 2
4
6
56
2
3
8
max
ꢁ1
ꢁ1
3
(log e/dm mol cm 3.72), 280sh (3.97), 310sh (4.53) and 335 water (30 cm ) and the layers were separated. The aqueous layer was
3
(
4.81). d
OMe), 2.76–2.88 (8H, m, FlCH
8H, m, FlC OCH CH OMe), 3.27 (12H, s, FlC
OCH ), 3.31 (8H, t, J = 4.5 Hz, FlC OCH CH OMe), 7.49 (2H, filtered. The magnesium sulfate was washed with additional
dd, J = 8.0 Hz and 1.5 Hz, FlH), 7.58 (2H, d, J = 8.0 Hz, FlH), 7.59 toluene and the filtered organics combined before the volatiles
2H, d, J = 1.5 Hz, FlH), 7.63 (2H, dd, J = 7.5 Hz and 1.5 Hz, FlH), were removed in vacuo. The crude residue was initially separated
.65 (2H, br s, FlH), 7.73 (2H, d, J = 7.5 Hz, FlH). d (125 MHz, by column chromatography over silica using ethyl acetate/
H
(500 MHz, CDCl
3
): 2.42–2.52 (8H, m, FlCH
CH OC
OMe), 3.17–3.26 were washed with water (2 ꢃ 20 cm ) and brine (20 cm ). The
OC organics were dried over anhydrous magnesium sulfate and
2
CH
2
O- extracted with toluene (6 ꢃ 20 cm ) and the combined organics
3 3
C
(
H
2 4
2
2
2 4
H
H
2 4
2
2
2
H
4
2 4
H -
3
H
2 4
2
2
(
7
C
CDCl ): 39.7, 51.7, 59.0, 66.9, 69.9, 71.7, 120.2, 121.2, 121.3, n-hexane/methanol mixtures (1 : 1 : 0–19 : 0 : 1) as eluent. Fractions
3
1
[
(
21.6, 126.7, 126.8, 130.5, 138.8, 139.1, 140.7, 149.4, 151.4. m/z containing 5a were combined and purified by chromatotron
MALDI-TOF] anal. calcd 894.2 (47%), 895.2 (25%), 896.2 chromatography in three stages: firstly using ethyl acetate/n-hexane
100%), 897.2 (50%), 898.2 (58%), 899.2 (24%), 900.2 (8%). mixtures (1 : 1–3 : 1) as eluent; secondly using a chloroform/ethanol
J. Mater. Chem. C
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Journal of Materials Chemistry C
Paper
mixture (99 : 1) as eluent and finally using dichloromethane/ ArOC
2
H
4
OC
OMe), 3.75–3.77 (8H, m, ArOC
purifications yielded 5a as a colourless glassy solid (0.579 g, 3.93 (8H, m, ArOCH CH OC H OMe), 4.22–4.24 (8H, m,
2
H
4
OCH
3
), 3.60–3.62 (8H, m, ArOC
2 4
H OCH2-
methanol mixtures (1 : 0–49 : 1) as eluent. This sequence of CH
2
2
H
4
OCH CH OMe), 3.91–
2
2
2
2
2
4
0 0
requires C, 79.1; H, 7.5. ArOCH CH OC H OMe), 7.06 and 7.67 (16H, AA BB , surface
106 12 2 2 2 4
5
3%). Found C, 79.4; H, 7.8; C H
O
9
4
3
ꢁ1
ꢁ1
lmax (CH Cl /nm): 253sh (log e/dm mol
cm
(500 MHz, CDCl
), 0.77–0.87 (8H, m, FlCH CH
CH ), 3.42 (12H, s, ArOC
), 3.60–3.62 (8H, m, ArOC OCH CH OMe), 3.75–3.77 120.2, 121.6, 121.9, 124.1, 124.4, 126.68, 126.72, 128.4, 133.9,
8H, m, ArOC OCH CH OMe), 3.92 (8H, t, J = 5.0 Hz, ArOCH2- 139.4, 139.5, 140.49, 140.53, 141.9, 142.2, 149.86, 149.94, 158.5.
CH OC H OMe), 4.23 (8H, t, J = 5.0 Hz, ArOCH CH OC H OMe), m/z [MALDI-TOF] anal. calcd 1666.9 (89%), 1667.9 (100%),
4.80), 265 PhH), 7.70–7.74 (8H, m, overlapping FlH), 7.77 (2H, br d,
): 0.73 branching PhH), 7.78 (4H, d, J = 1.5 Hz, branching PhH), 7.82
CH ), (4H, dd, J = 1.5 Hz and 8.0 Hz, FlH). d (126 MHz, CDCl ): 39.9,
OC 51.5, 58.9, 59.1, 67.1, 67.5, 69.8, 69.9, 70.8, 71.7, 71.9, 115.0,
2
2
(
(
2
H
4.88), 275sh (4.83) and 353 (4.93). d
12H, t, J = 7.0 Hz, FlCH CH CH
.07–2.14 (8H, m, FlCH CH
OCH
H
3
2
2
3
2
2
3
C
3
2
2
3
2
H
4
2
-
4
3
2
H
4
2
2
(
2
H
4
2
2
2
2
4
2
2
2 4
0
0
7
.06 (8H, 1/2AA BB , surface PhH), 7.65–7.72 (18H, m, overlapping 1668.9 (59%), 1669.9 (24%) and 1670.9 (8%). Found 1667.1
branching PhH, surface PhH and FlH), 7.77 (4H, d, J = 1.5 Hz, (88%), 1668.1 (100%), 1669.1 (62%), 1670.1 (25%), 1671.1
branching PhH), 7.83 (4H, br dd, J = 8.0 Hz, J = 2.0 Hz, FlH). d
(9%). PLQY (solution) = 0.85 ꢀ 0.01, PLQY (film) = 0.71 ꢀ 0.05.
125 MHz, CDCl ): 14.6, 17.3, 42.8, 55.6, 59.1, 67.5, 69.8, 70.8, 72.0,
15.0, 120.0, 121.4, 121.7, 124.3, 124.4, 126.2, 126.3, 128.4, 134.0, below 200 1C). T5% dec = 399 1C. E1/2 (Ox-1, CH
C
ꢁ
1
(
1
3
T
g
(100 1C min ) = 27 ꢀ 1 1C (no melting transitions observed
2
Cl , Glassy C) = 0.87 V.
2
1
39.9, 140.17, 140.25, 140.5, 141.9, 142.7, 151.77, 151.79, 158.5. GPC: M%
= 2026, M%
= 2037, M%
= 2035.
n
w v
m/z [MALDI-TOF] anal. calcd 1426.8 (95%), 1427.8 (100%),
3,6-Bis{3,5-bis[4-(2-ethylhexyloxy)phenyl]phenyl}-9-[2-(2-methoxy-
1428.8 (54%), 1429.8 (20%), 1430.8 (6%). Found 1426.9 (98%), ethoxy)ethyl]carbazole (6b). 3,6-Dibromo-9-[2-(2-methoxyethoxy-
1427.9 (100%), 1428.8 (53%), 1429.9 (36%), 1430.9 (10%). )ethyl]carbazole 4b (0.070 g, 0.136 mmol) and 2-{3,5-bis[4-(2-
PLQY (solution) = 0.81 ꢀ 0.01, PLQY (film) = 0.83 ꢀ 0.06. ethylhexyloxy)phenyl]phenyl}-4,4,5,5-tetramethyl-1,3,2-dioxaborolane
ꢁ
1
41
3
T
g
(100 1C min ) = 60 ꢀ 1 1C (no melting transitions observed
2
(0.250 g, 0.408 mmol) were dissolved in toluene (3.0 cm ).
3
below 200 1C). T5% dec = 383 1C. E1/2 (Ox-1, CH Cl
2
2
, Glassy C) = 0.86 V. Aqueous potassium carbonate (2.0 M, 1.0 cm ) and tert-butanol
3
GPC: M%
= 1954, M%
= 1962, M%
= 1961.
(1.0 cm ) were added and the mixture was deoxygenated by sparging
n
w
v
0
7
,7 -Bis(3,5-bis{4-[2-(2-methoxyethoxy)ethoxy]phenyl}phenyl)- with nitrogen for 10 min. Tetrakis(triphenylphosphine)palladium(0)
,9,9 ,9 -tetrakis[2-(2-methoxyethoxy)ethyl]-2,2 -bifluorene (5b). (0.009 g, 0.008 mmol) was added and the mixture was deoxygenated
b (0.074 g, 0.083 mmol) and 1 (0.122 g, 0.207 mmol) were by sparging with nitrogen for 10 min. The mixture was stirred at
0
0
0
9
3
3
dissolved in toluene (5.0 cm ). Aqueous potassium carbonate 100 1C under nitrogen for 5 h before it was allowed to cool to room
3
3
3
(
2.0 M, 1.5 cm ) and tert-butanol (1.5 cm ) were added and the temperature. The mixture was diluted with toluene (20 cm ) and
3
mixture was deoxygenated by sparging with nitrogen for 30 min. water (20 cm ) and the layers were separated. The aqueous layer was
3
Tetrakis(triphenylphosphine)palladium(0) (0.010 g, 0.009 mmol) extracted with toluene (2 ꢃ 20 cm ) and the combined organics were
3
3
was added and the mixture was deoxygenated by sparging with washed with water (2 ꢃ 20 cm ) and brine (20 cm ). The organics
nitrogen for 20 min. The mixture was stirred at 100 1C under were dried over anhydrous magnesium sulfate and decanted. The
nitrogen for 2 d before it was allowed to cool to room temperature. magnesium sulfate was washed with additional toluene and the
3
The mixture was diluted with ethyl acetate (20 cm ) and water decanted organics combined before the volatiles were removed in
15 cm ) and the layers were separated. The aqueous layer was vacuo. The crude residue was separated by column chromatography
3
(
3
extracted with ethyl acetate (3 ꢃ 5 cm ) and the combined organic over silica in two stages: firstly the mixture was separated using
3
3
layers were washed with water (2 ꢃ 5 cm ) and brine (5 cm ). The dichloromethane/ethyl acetate/n-hexane (1 : 1 : 8) as eluent and
organics were dried over anhydrous magnesium sulfate and secondly using a dichloromethane/toluene mixture (1 : 4) as eluent
filtered. The magnesium sulfate was washed with additional ethyl to yield 6b as an off-white glassy solid (0.120 g, 59%). Found C, 82.5;
acetate and the filtered organics combined before the volatiles H, 8.9; N, 1.2; C85
were removed in vacuo. The crude residue was initially separated (CH Cl /nm): 269 (log e/dm mol cm 5.06), 289sh (4.96), 302sh
by column chromatography over silica using n-hexane/ethyl (4.83), 320sh (4.40) and 350sh (3.34). d (500 MHz, CDCl ): 0.88–1.00
6
H107NO requires C, 82.4; H, 8.7; N, 1.1. lmax
3
ꢁ1
ꢁ1
2
2
H
3
acetate/methanol mixtures (1 : 9 : 0–0 : 19 : 1) as eluent. Fractions (24H, m, overlapping Ethylhexyl CH ), 1.30–1.60 (32H, m, over-
3
containing 5b were combined and purified by chromatotron lapping ethylhexyl CH ), 1.74–1.80 (4H, m, ethylhexyl CH), 3.34
2
chromatography in two stages: firstly using ethyl acetate/methanol (3H, s, N(C
mixtures (1 : 0–97 : 3) as eluent and finally using chloroform/ OCH ), 3.57–3.59 (2H, m, N(C
ethanol/methanol mixtures (99 : 1 : 0–98 : 1 : 1) as eluent. This m, ethyhexyl OCH and NCH
sequence of purifications yielded 5b as a colourless gum J = 6.0 Hz, NCH CH O(C )OCH
0.098 g, 71%). Found C, 73.2; H, 7.3; C102 20 requires C, PhH), 7.59 (2H, d, J = 8.5 Hz, CbzH), 7.66–7.70 (10H, m, overlapping
2
H
4
)O(C
2
H
4
)OCH
3
), 3.46–3.48 (2H, m, N(C
)OCH CH OCH ), 3.91–3.97 (10H,
CH O(C )OCH ), 4.61 (2H, br t,
), 7.03 (8H, 1/2AA BB , surface
2 4 2 2
H )OCH CH -
3
2
H
4
2
2
3
2
2
2
2
H
4
3
0
0
2
2
2
H
4
3
(
7
122
H O
3
ꢁ1
ꢁ1
3.4; H, 7.4. lmax (CH Cl /nm): 254sh (log e/dm mol cm
branching and surface PhH), 7.84–7.85 (6H, m, overlapping
2
2
4.77), 265 (4.83), 278sh (4.77) and 352 (4.87). dH (500 MHz, branching PhH and CbzH), 8.46 (2H, d, J = 1.5 Hz, CbzH). dC
CDCl ): 2.57 (8H, br t, J = 7.5 Hz, FlCH CH OC H OMe), (126 MHz, CDCl ): 11.3, 14.3, 23.2, 24.0, 29.3, 30.7, 39.6, 43.6, 59.2,
3
2
2
2
4
3
2
.86–2.94 (8H, m, FlCH
2
CH
CH
.31 (8H, t, J = 4.5 Hz, FlC
2
OC
OMe and FlC
OCH CH
2
H
4
OMe), 3.21–3.27 (20H, m, 69.5, 70.7, 71.1, 72.1, 109.5, 115.0, 119.2, 123.78, 123.84, 124.5, 125.8,
overlapping FlC
3
2
4
H OCH
2
2
2
H
4
OC OCH ), 128.5, 133.0, 133.8, 140.8, 142.1, 143.1, 159.3. m/z [MALDI-TOF] anal.
2
H
4
3
2
H
4
2
2
OMe), 3.42 (12H, s, calcd 1237.8 (100%), 1238.8 (97%), 1239.8 (47%), 1240.8 (16%),
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Journal of Materials Chemistry C
3
1
241.8 (5%). Found 1237.9 (100%), 1238.9 (90%), 1239.9 (50%), a custom built sample holder (internal volume B40 cm ) in a
1
240.9 (12%), 1241.9 (4%). PLQY (solution) = 0.15 ꢀ 0.01, PLQY Jobin Yvon Horiba Fluorolog Tau fluorometer. The dendrimer
ꢁ1
(film) = 0.25 ꢀ 0.06. T (20 1C min ) = 26 ꢀ 1 1C (no melting film was placed on the opposite side of the substrate to the
g
transitions observed below 200 1C). T
= 389 1C. E (Ox-1, THF, excitation source (excitation beam perpendicular to the substrate
1/2
5% dec
Glassy C) = 0.69 V. GPC: M% n = 1598, M% w = 1605, M% v = 1604.
surface) and the PL was detected in front facing mode. The
dendrimers were excited at 350 nm (bifluorene-based 5a/b) or
Spectroscopy
3
20 nm (carbazole-based 6a/b) and the PL was monitored at
3
ꢁ1
Photoluminescence (PL) spectra were recorded on a Jobin-Yvon 400 nm in all cases. A flow of nitrogen (4000 cm min for DNB,
3
ꢁ1
Horiba Fluorolog Tau or a Jobin-Yvon Horiba Fluoromax 4 DNT, and pNT injections and 1000 cm min for DMNB and NM
using a xenon lamp as an excitation source. Spectroscopic injections) was established into the chamber from an aperture
grade solvents were used for solution measurements. Solution directly facing the film (carrier gas flow perpendicular to the film
PLQY measurements were measured relative to quinine sulfate surface) and held constant throughout each measurement. The
5
0
in 0.5 M aqueous sulfuric acid (PLQY = 0.55). PLQY values flow of nitrogen was controlled by a Bronkhorst EL-FLOW Select
were determined from plots of absorbance vs integrated PL for the mass flow controller calibrated for nitrogen within the flow range
3
ꢁ1
dendrimers relative to that of the quinine sulfate standard (0.01 r 150–7500 cm min . After the apparatus was purged with
A‘ r 0.10). The standard errors in the gradients of these plots nitrogen for 15 min the PL intensity of the films were monitored
calculated by least squares regression were propagated using the for a period of up to 1 h during which analyte vapours were
chain rule to determine the error in PLQY. Thin film PLQY injected. Analytes were introduced by manually injecting a known
3
measurements were performed using the procedure described by volume (between 0.1 and 25 cm ) of nitrogen saturated with
51
Greenham et al. Thin film samples on fused silica substrates were analyte vapour into the nitrogen carrier stream over 1 s. A digital
excited using the 325 nm output of a HeCd laser under nitrogen. metronome was used to aid with timekeeping. Cotton wool was
PL lifetime measurements for the bifluorene-based dendrimers placed inside the syringes at the outlet to prevent the injection of
5a/b were performed in tetrahydrofuran solutions using a particles of analyte into the carrier gas flow. The syringes were
Picoquant time correlated single photon counting (TCSPC) primed with nitrogen and stoppered before they were allowed to
setup. The dendrimers were excited at 375 nm with B70 fs saturate with analyte vapours at 22 1C for at least 16 h prior to
pulses and a repetition rate of 80 MHz with the frequency injection. The approximate concentration of analyte introduced was
13,14,39
doubled output of a Ti:sapphire laser. PL decays were measured estimated from the reported saturated vapour concentrations
at 395 nm with an instrument response function (IRF) of B300 ps and the dilution ratio of the analyte in the nitrogen carrier stream.
FWHM. The corresponding measurements for carbazole-based Thin film quenching constants, K, were determined from
dendrimers 6a/b were performed in toluene solutions using a the gradients of F /F vs. quencher concentration plots and the
0
Jobin-Yvon Horiba Fluorolog FL3-11 with a TCSPC module. The errors quoted in these values are the standard errors for these
dendrimers were excited at 372 nm with 1.2 ns pulses and a gradients calculated by least squares regression. When the
repetition rate of 1 MHz with the output of a Horiba Scientific errors in injection time (ꢀ0.1 s) and injected volume (syringe
NanoLED pulsed diode light source. PL decays were measured at size dependent) were propagated using the chain rule the
400 nm with an IRF of B1.5 ns FWHM. Due to the proximity of the relative error in the injected analyte concentrations was found
detection/excitation wavelengths and the weaker PLQY of the to be rꢀ20%. A relative error of 20% was ascribed to all
carbazole dendrimers, scatter of the excitation beam was observed injected analyte concentrations and thereby to the detection
in the decays. The background signal was recorded using spectro- limit concentrations determined from the plots of F
0
/F vs. [Q].
photometric grade toluene as a scatterer, which was subtracted
from the observed data. The PL decays were fitted to single
exponential decays convoluted with the IRF.
Acknowledgements
This work was supported by the Australian Research Council
DP0986838). AJC, HC and GT were supported by Endeavour
Thin film PL quenching measurements
(
Fused silica substrates (12 mm diameter) were cleaned in
acetone, 2-propanol and toluene under ultrasonication prior
to dendrimer deposition. Dendrimer films were spin coated
International Postgraduate Research Scholarships and University
of Queensland Research Scholarships. PLB is a University of
Queensland Vice Chancellor’s Research Focussed Fellow and PM
the recipient of an Australian Research Council Discovery Out-
standing Researcher Award. We acknowledge funding from the
University of Queensland (Strategic Initiative – Centre for
Organic Photonics & Electronics).
ꢁ
3
from 10 mg cm solutions in spectrophotometric grade toluene.
Dendrimer solutions were pipetted onto the surface of the
cleaned substrates until the surface was covered by the solution
and the films were immediately spun at 2000 rpm for 1 min
ꢁ
1
(
acceleration = 2000 rpm s ). The following film thicknesses
were measured by contact profilometry: 5a, 44 ꢀ 3 nm; 5b,
4
3 ꢀ 5 nm; 6a, 47 ꢀ 4 nm; and 6b, 54 ꢀ 7 nm. These thicknesses
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