Molecular sensing and discrimination by a luminescent terbium-phosphine oxide coordination material

Article abstract of DOI:10.1039/c3cc44575e

PCM-15 is a robust and recyclable sensor for the effective discrimination of a wide range of small molecules. Sensing is achieved by direct attenuation of the luminescence intensity of Tb(iii) ions within the material. A competition study involving trace amounts of NH₃ in H₂ gas shows that PCM-15 can be used to quantitatively detect trace analytes.

Full text of DOI:10.1039/c3cc44575e

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COMMUNICATION
Molecular sensing and discrimination by a luminescent
terbium–phosphine oxide coordination material†
Cite this: DOI: 10.1039/c3cc44575e
a
a
bc
b
Ilich A. Ibarra, Travis W. Hesterberg, Jong-San Chang, Ji Woong Yoon,
Received 18th June 2013,
Accepted 2nd July 2013
a
a
Bradley J. Holliday and Simon M. Humphrey*
DOI: 10.1039/c3cc44575e
www.rsc.org/chemcomm
PCM-15 is a robust and recyclable sensor for the effective discrimination examples of luminescent Ln(III)-based PCPs where the metal sites are
of a wide range of small molecules. Sensing is achieved by direct coordinated exclusively to organic ligand anions, the absorption and
attenuation of the luminescence intensity of Tb(III) ions within the emission wavelengths/intensities do not vary significantly due to
3 2
material. A competition study involving trace amounts of NH in H gas guest species adsorbed in the pores. Vibrationally-coupled lumines-
shows that PCM-15 can be used to quantitatively detect trace analytes. cence quenching is a short-range phenomenon that diminishes as a
6
13
function of R (R = donor–acceptor separation). So, in order to
Porous coordination polymer (PCP) materials attract wide-spread utilize relative luminescence in a PCP as a sensor to identify and
1
attention, particularly for small molecule storage and separation.
discriminate between adsorbed species, it is necessary to incorporate
PCPs are thermally and chemically robust, while the physical labile ligands on the metal centres. Post-synthetic removal thus
properties such as size, shape and chemical composition of the generates vacant coordination sites and allows guest molecules to
pores can be tailored towards selective adsorption of a wide range interact closely with the luminescent metal sites, thus facilitating
2
of guest molecules. Chemical sensing by PCPs is an intriguing quantifiable luminescence quenching.
application that has been suggested, but remains much less
We recently reported a Tb(III)-based phosphine oxide coordination
extensively studied. A useful PCP-based sensor would exhibit a material, [Tb(tctpo)(OH O (PCM-15; tctpo = P (Q O)(C
)]Á2dmfÁH
measurable and reversible change, induced by host–guest chemical CO ), which is a highly robust three-dimensional coordination
3
2
2
6 4
H –
2 3
)
14
interactions inside the pores, and would be reversible over many material with a two-dimensional pore network. The largest pore
cycles. The inherently high surface areas exhibited by PCPs should windows in PCM-15 measure 14.2 Å (diagonal distance Tb–P; Fig. S1,
4
also allow for very small amounts of material to provide a sufficient ESI†), permitting interior access to a broad range of small molecules.
sensor response in implemented devices.
In addition to seven ligand-based donors, each Tb(III)ionhasaneighth
ligand, which can
saturated metal cations and therefore do not offer a convenient be removed by heating at 423 K at 1 Â 10 Torr for 1 h to obtain the
spectroscopic handle for guest adsorbate sensing. PCP sensors might ‘activated’ form of the material (Scheme 1). Each OH ligand projects
be prepared by: (i) incorporation of metal ions that show a measur- into the pore, so the resulting vacant coordination sites that are
The vast majority of known PCP materials contain coordinatively- coordination site that is occupied by a terminal OH
2
À8
2
5
6
able response to external stimulus (e.g., light, magnetism, tempera- generated upon activation are accessible to guests. We previously
7
ture ); (ii) direct, or post-synthetic incorporation of guest-responsive showed that the photoluminescence intensity of Tb(III) sites in PCM-15
8
organic moieties; and, (iii) exploitation of bulk guest-induced underwent a reversible two-fold increase upon dehydration of the
9
properties of PCP single crystals (e.g., vapochromism). Route (i) crystalline material, thus acting as an efficient and direct probe for the
14
has attracted the most significant attention, primarily because a hydration state of individual Tb(III)sites. In the present work, we have
number of metals that are photoluminescent can be directly incor- studied in detail the luminescence quenching due to adsorption of
10,11
porated into PCPs as the framework cations.
The photolumines- fifteen different small molecule adsorbates, and have successfully
cence of Ln(III)-based materials is known to be sensitive to the performed competition experiments that demonstrate the ability of
12
coordination environment of the metal ion. However, in reported PCM-15 to sense low-level impurities.
Crystalline samples of PCM-15 were prepared in gram
quantities by a low-temperature (358 K) reaction of Tb(NO )
a
Department of Chemistry & Biochemistry, The University of Texas at Austin,
Welch Hall 2.204, 105 E. 24th St. A5300, Austin, TX 78712-1224, USA.
E-mail: smh@cm.utexas.edu; Fax: +1 (512)471-6835; Tel: +1 (512)471-0312
Catalysis Center for Molecular Engineering, Korea Research Institute of Chemical
Technology, PO Box 107, Yusung, Daejeon, 305-600, Korea
3
3
1
4
and tctpoH in DMF–THF–OH solvent. Thermogravimetric
3
2
analyses (TGA; Fig. S2–S5, ESI†) and bulk powder X-ray diffrac-
tion (PXRD; Fig. S6 and S7, ESI†) patterns of as-synthesized,
desolvated and resolvated PCM-15 samples confirmed that the
b
c
Department of Chemistry, Sungkyunkwan University, Suwon 440-476, Korea
†
Electronic supplementary information (ESI) available. See DOI: 10.1039/c3cc44575e material consistently retained its structural integrity upon
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Scheme 1 Left: expanded view of the asymmetric unit in as-synthesized PCM-
1
5; right: after removal of the single OH
2
ligand from the Tb(III) coordination
sphere. Tb = cyan; P = magenta; OH = yellow.
2
exposure to a range of gas and vapour adsorbates, and after
reactivation in vacuo.
2
À1
PCM-15 has a bulk surface area of 1187 m g (BET method;
CO ). A custom cell (Fig. S8, ESI†) designed to be directly inter-
2
changeable between the gas adsorption analyser apparatus and the
spectrophotometer cavity was employed, which allowed each sample
of PCM-15 to be activated under vacuum, directly exposed to
adsorbates in situ, and studied spectrophotometrically over many
Fig. 1 Summary of the measured luminescence intensities and associated errors
cycles without physical manipulation or exposure to air. In each for activated PCM-15 in the presence of atmospheric pressures of various mole-
study, a freshly-synthesized batch of PCM-15 (30–50 mg) was acti- cular adsorbates. All data is normalized versus the activated luminescence intensity.
vated as described above and the luminescence quantum yield was
measured to verify complete dehydration of the Tb(III) centres. Next, (Fig. 1). The H O–D O data yield a quenching constant for water
2 2
15b
activated samples were exposed to 1 atm of each guest for 30 min. (qcorr = 1.20) which is internally consistent with the activation studies
For gaseous adsorbates, the sample chamber was purged with ultra- (vide supra) and the structural and TGA analyses.
high purity (UHP) gas; alternatively, anhydrous, degassed liquid
adsorbates were vaporized with flowing UHP N
using an in-line dimethylsulfoxide (DMSO) and n-hexane were found to be more
bubbler. The resulting total luminescence quantum yields were then effective quenching agents than H O. The quenching effectiveness
recorded and used to calculate the emissive lifetimes for each of of NH is much higher than that observed in molecular systems,
fifteen different adsorbates; the data is summarized in Fig. 1 and where it has been shown that each N–H oscillator is 52% as effective at
3
Of the other fourteen adsorbates studied in this work, only NH ,
2
2
3
16
Table S1 (ESI†). Each measurement was repeated three or more quenching Tb(III) emission when compared to O–H oscillators. N–H
times using freshly-prepared samples of PCM-15 to provide error and O–H oscillators were observed to be equally effective quenchers
ranges as shown. It was possible to reactivate samples and recover with the difference in measured intensity coming from the increased
the original luminescence behaviour using the standard activation number of oscillators per NH
conditions for all of the adsorbates studied.
absolute uptake of a single equivalent of NH
Guest molecules that become adsorbed inside the pores of pre- 15 was confirmed by elemental analysis. The highly effective nature of
activated PCM-15 could act as quenching agents, provided that: (i) NH as a quenching guest can be emphasized by calculating the
they gain close proximity to the unsaturated Tb(III) centres in the apparent number of N–H oscillators using the method developed
3
molecule bound to the Tb(III) ions. The
per Tb in activated PCM-
3
3
16
pore walls (either via formal dative coordination to Tb, or via by Wang et al. This predicts a quenching due to the presence of E30
favourable dipolar interactions; see Scheme 1); or, (ii) there is N–H oscillators based on comparison to molecular species in solution.
sufficient Frank–Condon overlap between the corresponding over- This is consistent with the presence of a completely open coordination
tone of the vibrational oscillator and the excited state of the Tb(III) site per Tb with no requirement for competitive ligand exchange.
ions. To confirm this hypothesis, the luminescence quenching ability DMSO is also a favourable ligand for Ln(III) ions via (H
C) SQO–Tb
3 2
of a broad range of guest adsorbates was studied in PCM-15 (Fig. 1). coordination and the higher quanta of the C–H and SQO stretching
15a
Perhaps the most immediately striking trend is the large variation of vibrations can promote effective luminescence quenching.
total luminescence quenching that was observed, in which only three
The observed quenching effect of anhydrous n-hexane was more
of the fifteen adsorbates studied resulted in more quenching than surprising since hydrocarbon solvents do not usually quench Ln(III)
the fully solvated parent material (Fig. 1, blue dotted line). The qcorr luminescence. Sorption–desorption isotherms were collected for
value for H
O deactivation of the Tb(III) excited state in this system n-hexane in activated PCM-15 which revealed reversible type-I sorp-
2
15a
was determined to be 1.65 by the method of Beeby et al. This is tion behavior and a capacity of 10.2 wt% (p/p
0
= 0.94) or 0.67
consistent with the structural and TGA data used to determine that n-hexane molecules per PCM-15 formula unit (Fig. S8, ESI†). The
there are two water molecules per asymmetric unit in PCM-15 (a confirmation that n-hexane was adsorbed into the pores of PCM-15
single bound water molecule and a second solvate H
2
O closely led us to believe that quenching was being observed, presumably via
associated with each Tb(III) site). As a control study, an activated C–H vibrational modes of alkanes in close proximity of unsaturated
PCM-15 sample was exposed to an atmosphere of D O in N gas. The Tb(III) sites. In support of this assumption, sorption of anhydrous
2
2
resulting luminescence intensity was only found to be partially
d14-hexane resulted in only minimal quenching (Fig. 1). By compar-
reduced since the vibrational frequency of O–D bonds are lower ison, the n-hexane and d14-hexane rate constants for depopulation of
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a broad range of small molecule guest species, determined by
relative luminescence quenching of Tb(III) sites. The measured
quenching was shown to be directly proportional to amount of
guest analyte within the pores. Due to the high density of Tb(III)
sites in PCM-15, the luminescence intensity was easily detectable
using only milligram quantities of sample; so, it could function as
an effective sensor when incorporated into devices in dilute form
(e.g., impregnation into an inert matrix or membrane). The
authors thank Dr V. M. Lynch (X-ray; Austin), the Welch Founda-
tion (F-1738 & F-1631), The NSF (CHE-0847763; B.J.H.), and the
Institutional Research Program (ISTK, SK-1210 and KK-1301-F0;
J.W.Y. & J.-S.C.) for funding.
3
Fig. 2 Relative luminescence quenching observed as a function of NH concen-
tration in an H
2
-saturated sample of PCM-15; inset: calibration curve showing Notes and references
2
linear response (R = 0.99).
1
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emission. CH and H adsorption isotherms for activated PCM-15
4
2
showed that both gases were fully adsorbed inside the pores, with
modest total uptakes at 1.0 bar. It was also possible to confirm that the
larger aromatic and aliphatic cyclic hydrocarbons were adsorbed
inside PCM-15 (Fig. S8, ESI†). The total uptake of both toluene and
cyclohexane at 0.95 bar (0.83 and 0.79 molecules per formula unit,
respectively) and the observation of marked hysteresis in the
desorption step confirmed that these were preferentially adsorbed
inside the pores of PCM-15. However, neither facilitated luminescence
quenching. Adsorption of methanol or ethanol into activated PCM-15
4
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8
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2
resulted in significant quenching, similar to that observed by H O.
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weakly dative interactions to the Tb(III) centre, bringing O–H oscilla-
tors into close proximity. The adsorption–desorption profile of ethanol
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17
had a defined hysteresis, as observed in other PCPs (Fig. S8, ESI†).
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of small quantities of impurities was proven in a model study, in
9
J. Lefebvre, R. J. Batchelor and D. B. Leznoff, J. Am. Chem. Soc., 2004,
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which trace amounts of a strong quencher (NH
non-quenching gas (H ). A pre-activated sample was initially purged
with H gas at 298 K and the resulting photoluminescence intensity
was recorded (Fig. 2, red line). Small aliquots of NH (4.5 mmol) were
then injected in situ and after each injection the relative change in
luminescence intensity was recorded. As shown in Fig. 2, a clear
3
) were dosed into a 10 (a) G. Lu and J. T. Hupp, J. Am. Chem. Soc., 2010, 132, 7832;
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1
3
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3 3
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significantly below saturation). The calibration curve obtained by
integration of normalized photoluminescence intensity versus
1
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1
amount of added NH
NH
/Tb r 0.25 (Fig. 2, inset). The preferential and irreversible
binding of NH in H -loaded PCM-15 was also confirmed by treating
3
confirmed a linear response in the region
3
1
3
2
an NH -loaded sample with H gas, which did not result in NH
3
3
2
displacement (Fig. S9, ESI†). This study illustrates how PCM-15 may
be utilized to quantitatively detect impurity levels in certain gas or
vapour mixtures.
In summary, the Tb(III)–phosphine oxide coordination material
PCM-15 can be utilized as an effective sensor for discrimination of
1
1
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Chem. Commun.