Tunable star-shaped triphenylamine fluorophores for fluorescence quenching detection and identification of nitro-aromatic explosives

Article abstract of DOI:10.1039/c2cc34008a

Triphenylamine-based fluorophores containing pyrene or corannulene show variable fluorescence quenching sensitivity toward nitro explosives. The most sensitive fluorophore is capable of detecting TNT on the ng cm^-2 scale; the array is useful for identifying nitro aromatics.

Full text of DOI:10.1039/c2cc34008a

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Tunable star-shaped triphenylamine fluorophores for
fluorescence quenching detection and identification
of nitro-aromatic explosives†
Cite this: DOI: 10.1039/c2cc34008a
Received 5th June 2012,
Nakorn Niamnont,ab Nattaporn Kimpitak,a Kanet Wongravee,a
Paitoon Rashatasakhon,a Kim K. Baldridge,c Jay S. Siegel*c and
Mongkol Sukwattanasinitt*a
Accepted 4th December 2012
DOI: 10.1039/c2cc34008a
www.rsc.org/chemcomm
Triphenylamine-based fluorophores containing pyrene or corannulene
show variable fluorescence quenching sensitivity toward nitro explo-
sives. The most sensitive fluorophore is capable of detecting TNT on the
ng cmꢀ2 scale; the array is useful for identifying nitro aromatics.
Detection of trace explosives, such as 2,4,6-trinitrotoluene (TNT), is of
great social concern.1 Fluorescence quenching provides a mecha-
nism suitable for a real-time TNT sensor.2 The demand for detectors
of higher sensitivity and accuracy requires agents for selective and
sensitive TNT sensing. To address these issues, four triskelion-
shaped fluorophores were designed containing an electron donating
triphenylamine core coupled, by either ethynylene or triazole linkers,
to a fluorescent polynuclear aromatic hydrocarbon (PAH) unit
(Fig. 1). Specifically, compounds bearing the bowl-shaped corannu-
lene ring (TEC; TAC) were compared with analogs bearing a planar
Fig. 1 Structures of fluorophores.
polyaromatic pyrene ring (TEP; TAP). Their sensing properties for the incorporation of multiple fluorescent units, which interact
detection of TNT and analogous compounds are reported herein.
through efficient energy and electron transfer.5b,6 Cooperative
Fluorescence quenching efficiency is normally rated by the p–p interactions with TNT can also enhance sensitivity.7
Stern–Volmer constant (Ksv).3 Such quenching, between an
The ethynylene linked triskelion branched fluorophores TEP
electron rich fluorophore and electron poor TNT, is associated and TEC were synthesized from 4,40,400-triiodotriphenylamine8
with an intermolecular photoinduced electron transfer.4 This via Sonogashira cross coupling reaction with ethynylpyrene9
electron transfer efficiency relies heavily on the p–p interaction and ethynylcorannulene.10 The triazole linked TAP and TAC
between TNT and the fluorophore; in addition, the LUMO were obtained from the click reaction between 4,40,400-triethynyl-
energy level of the fluorophore has to be higher than the HOMO triphenylamine11, 1-azidopyrene12 and 1-azidocorannulene.13 The
energy level of TNT. Indeed, pyrene derivatives, with large detailed synthetic procedures and spectroscopic data for all
delocalized p systems and well-matched orbital energy levels, fluorophores are available in the ESI.†
show good selectivity and sensitivity for the detection of TNT.5
The UV-Vis absorption spectrum of each fluorophore in chloro-
Amplification of fluorescence quenching has been achieved by form showed a more structured absorption band at higher energy
(lmax o 300 nm) corresponding to a local p–p* local transition and a
a Organic Synthesis Research Unit, Department of Chemistry, Faculty of Science,
broad absorption (lmax > 300 nm) associated with the p–p* electronic
Chulalongkorn University and Nanotec-CU Center of Excellence on Food and
transition of the whole conjugated system (Fig. S5.1, ESI†). The latter
Agriculture, Bangkok 10330, Thailand. E-mail: smongkol@chula.ac.th;
lmax is thus related to the p-electron delocalization in the HOMO
level. TEP and TEC possess significantly longer lmax (415 and
421 nm) than those of TAP and TAC (346 and 329 nm), which
suggests greater electron delocalization in TEC and TEP compared
to TAP and TAC. The lem of TEP and TEC (484 and 494 nm)
appeared at considerably higher energy than those of TAP and
TAC (506 and 537 nm) and the large Stokes shifts (B200 nm)
Fax: +66 2 2187598; Tel: +66 2 2187620
b Department of Chemistry, Faculty of Science, King Mongkut’s University of
Technology Thonburi, Bangkok 10140, Thailand
c Organic Chemistry Institute, University of Zurich, Winterthurerstrasse 190, Zurich,
8057, Switzerland. E-mail: jss@oci.uzh.ch; Fax: +41 44 635 6888;
Tel: +41 44 635 4201
† Electronic supplementary information (ESI) available: Synthesis and
characterization data. See DOI: 10.1039/c2cc34008a
c
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Fig. 2 HOMO–LUMO of TAP and TEP.
for TAC and TAP suggest a nuclear or electronic structure in
their excited states (i.e. a conformational change like flattening
or more likely an electronic redistribution like charge trans-
fer).14 The lower fluorescence quantum yields (FF) of TAP and
TAC (4.6% and 9.6%) compared to TEP and TEC (24% and 19%)
further support this hypothesis.
B97D/Def2-TZVPP calculations15 on all four fluorophores
depict the greater delocalization of the MOs in TEP and TEC
in comparison with TAP and TAC as well as the shift from the
core to the periphery for the HOMO vs. the LUMO of TAP and
TAC (Fig. 2). Calculations further confirmed that the TNT
LUMO level (ꢀ4.27 eV) is located between the HOMO and
LUMO levels of TAP and TAC. Indeed, predicted IP values
(DSCF) for TEP, TEC, TAP, TAC are 5.42, 5.65, 5.62, 5.75 eV,
respectively, which support their electron donor character.
The emission spectra of TAP in the presence of various con-
centrations of TNT (Fig. 3, left) demonstrated efficient fluorescence
quenching. The quenching efficiency by TNT of each fluorophore
was determined from the Stern–Volmer plot4 in comparison with
the underivatized corannulene (Co) and pyrene (Py) (Fig. 3, right).
The plots clearly showed that the fluorescence quenching of TAP
and TAC was much more sensitive than their parent fluorophores,
pyrene and corannulene. Despite having the same number of
fluorogenic units, TEP and TEC displayed significantly lower
quenching efficiencies (Ksv = 8.4 ꢁ 102 and 7.2 ꢁ 102 Mꢀ1) than
those of TAP and TAC (Ksv = 1.7 ꢁ 104 and 6.4 ꢁ 103 Mꢀ1). In fact
TEP and TEC hardly showed any signal quenching amplification.
The results suggested that the triazole bridge must play an
important role in enhancing quenching.
Fig. 4 1H NMR of TAP (0.1 mM in CDCl3) in the absence and presence of TNT. The
top diagram is the proposed TAP–TNT interaction geometry.
1
through a dynamic process. The H NMR shows a commensu-
rate shift with increasing addition of TNT and no signals for the
fluorophore independent of the fluorophore–TNT complex are
evident. The Ksv values obtained from the quenching experi-
ments at 5 1C to 45 1C were 1.4 ꢁ 104 Mꢀ1 and 2.0 ꢁ 104 Mꢀ1
,
respectively (Fig. S7.2, ESI†). The increase in Ksv values with
temperature supports the dynamic quenching mechanism.4d
Underivatized corannulene and TEC (Ksv = 6.7 ꢁ 102 and
7.2 ꢁ 102
M
ꢀ1) exhibited slightly lower Ksv values than those
for the corresponding pyrene systems (Ksv = 7.8 ꢁ 102 and 8.4 ꢁ
102
M
ꢀ1). More significantly, TAC gave three times lower
quenching efficiency than TAP (Ksv = 6.4 ꢁ 103 vs. 1.7 ꢁ
104 Mꢀ1). We attribute the lower sensitivity of the corannulene
to its bowl shape geometry, which probably does not well
accommodate the p–p interaction with TNT, required for an
efficient photoinduced electron transfer quenching process.
To evaluate the selectivity of the fluorophores, nine aromatic
compounds (phenol, benzoic acid, benzophenone, nitrobenzene,
3-nitrophenol, 2,4-dinitrophenol (DNP), nitrotoluene, 2,4-dinitro-
toluene and TNT) were tested as quenchers. The Ksv value of each
quencher–fluorophore pair was determined. Overall, TAP showed the
highest sensitivity toward TNT (Ksv = 1.7 ꢁ 104 Mꢀ1) and a TNT : DNP
selectivity ratio of 4.3 (Fig. S8.2, ESI†). TAC exhibited lower sensitivity
(6.4 ꢁ 103 Mꢀ1) as well as selectivity (TNT : DNP B 1.6).
1H NMR spectroscopy of TAP and TNT showed that the proton
signals of the pyrene and triazole units were clearly shifted upon
the addition of 100 equiv. of TNT (Fig. 4). Notably, the downfield
shift of the triazole proton (Hj) singlet signal suggests an interaction
between Hj and the nitro group. The upfield shifts of pyrene signals
(a–i) are consistent with the p–p interaction between the pyrene and
nitroaromatic ring.5a
All the fluorophores generally exhibited greater fluorescence
quenching with electron deficient nitroaromatic compounds than
with other aromatic compounds; nonetheless, the variation in
quenching profiles provided patterns useful for identification of
the quencher. The change in fluorescence intensities of the four
fluorophores (DI = I0 ꢀ I) in the absence (I0) and presence (I) of
each aromatic compound, at fixed absorbance (A(lmax) = 0.01),
was acquired. Using a non-supervised multivariate principal
component analysis (PCA) method,16 the total intensity changes
(430–600 nm) were transformed into principal components (PCs).
The PCA score plot in 2D space of PC1 and PC2 statistically
demonstrated nine identifiable clusters of data, obtained from 4
fluorophores ꢁ 9 aromatic samples ꢁ 9 measurements, with a total
variance of 86.7%. A cross validation by linear discriminant analysis
(LDA),17 a supervised method,18 on the PC scores gave 96.3%
The supramolecular complexation, and thus the fluores-
cence quenching, between our analytes and TNT likely occurs
Fig. 3 (left) Emission spectra of TAP (0.1 mM in CHCl3; lex = 345 nm) in the
presence of TNT at various concentrations. (right) Stern–Volmer plots for
quenching of the fluorophores with TNT at 25 1C.
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containing multiple pyrenes to the level that visual detection of
TNT on the ng mmꢀ2 scale was possible. The series of triphenyl-
amine based fluorophores containing multiple pyrene and
corannulene was capable of identifying electron deficient aro-
matic compounds including TNT and DNT.
We would like to thank the Thailand Nanotechnology Center
(NANOTEC), the IIAC Chulalongkorn University Centenary
Academic Development Project, the National Research Univer-
sity Project of Thailand, Office of the Higher Education Com-
mission (AM1006A), the 90th Anniversary Fund of CU and the
Swiss National Foundation for financial support.
Notes and references
Fig. 5 PC score plot of DI of the fluorophores (A = 0.01 @ 500 nm).
1 (a) A. W. Czarnik, Nature, 1998, 394, 417; (b) K. J. Albert, N. S. Lewis,
C. L. Schauer, G. A. Sotzing, S. E. Stitzel, T. P. Vaid and D. R. Walt,
Chem. Rev., 2000, 7, 2595.
2 (a) A. Rose, Z. G. Zhu, C. F. Madigan, T. M. Swager and V. Bulovic,
Nature, 2005, 434, 876; (b) E. R. Goldman, I. L. Medintz, J. L. Whitley,
A. Hayhurst, A. R. Clapp, H. T. Uyeda, J. R. Deschamps,
M. E. Lassman and H. Mattoussi, J. Am. Chem. Soc., 2005, 127, 6744.
3 (a) J. S. Yang and T. M. Swager, J. Am. Chem. Soc., 1998, 120, 11864;
(b) S. J. Toal, D. Magde and W. C. Trogler, Chem. Commun., 2005,
5465; (c) G. V. Zyryanov, M. A. Palacios and P. J. Anzenbacher, Org.
Lett., 2008, 10, 3681; (d) Y. Y. Long, H. B. Chen, Y. Yang, H. M. Wang,
Y. F. Yang, N. Li, K. A. Li, J. Pei and F. Liu, Macromolecules, 2009,
42, 6501; (e) W. Chen, N. B. Zuckerman, J. P. Konopelski and S. Chen,
Anal. Chem., 2010, 82, 461; ( f ) H. Nie, Y. Zhao, M. Zhang, Y. Ma,
M. Baumgarten and K. Mullen, Chem. Commun., 2011, 47, 1234.
4 (a) S. J. Toal and W. C. Trogler, J. Mater. Chem., 2006, 16, 2871;
(b) S. W. Thomas, G. D. Joly and T. M. Swager, Chem. Rev., 2007,
107, 1339; (c) M. E. Germain and M. J. Knapp, Chem. Soc. Rev., 2009,
38, 2543; (d) J. R. Lakowicz, Principles of fluorescence spectroscopy,
John Wiley & Sons, Inc., Kluwer, 3rd edn, 2006.
5 (a) Y. H. Lee, H. Liu, J. Y. Lee, S. H. Kim, S. K. Kim, J. L. Sessler,
Y. Kim and J. S. Kim, Chem.–Eur. J., 2010, 16, 5895; (b) H. Wang,
L. Ding, S. Yin and Y. Fang, Macromolecules, 2011, 44, 4759.
6 H. Cavaye, P. E. Shaw, X. Wang, P. L. Burn, S.-C. Lo and P. Meredith,
Macromolecules, 2010, 43, 10253.
Fig. 6 (a) TNT stains on a filter paper sprayed with TAP (20 mM in CHCl3–toluene
(1/3 v/v)); fluorescence microscopy images of areas with (top right) and without
(top left) TNT. (b–e) Varying amounts of TNT.
7 (a) D. A. Olley, E. J. Wren, G. Vamvounis, M. J. Fernee, X. Wang,
P. L. Burn, P. Meredith and P. E. Shaw, Chem. Mater., 2011, 23, 789;
(b) G. Q. Tang, S. S. Y. Chen, P. E. Shaw, K. Hegedus, X. Wang,
P. L. Burn and P. Meredith, Polym. Chem., 2011, 2, 2360.
8 N. Niamnont, W. Siripornnoppakhun, P. Rashatasakhon and
M. Sukwattanasinitt, Org. Lett., 2009, 11, 2768.
9 (a) S. Kajigaeshi, T. Kakinami, M. Moriwaki, T. Tanaka, S. Fujisaki
and T. Okamoto, Bull. Chem. Soc. Jpn., 1989, 62, 439; (b) F. Liu,
C. Tang, Q. Q. Chen, S. Z. Li, H. B. Wu, L. H. Xie, B. Peng, W. Wei,
Y. Cao and W. Huang, Org. Electron., 2009, 10, 256.
10 (a) Y. T. Wu and J. S. Siegel, Chem. Rev., 2006, 106, 4843; (b) J. Mack,
P. Vogel, D. Jones, N. Kavala and A. Sutton, Org. Biomol. Chem., 2007,
5, 2448.
11 L. Porres, O. Mongin, C. Katan, M. Charlot, T. Pons, J. Mertz and
M. Blanchard-Desce, Org. Lett., 2004, 6, 47.
accuracy of classification of the nine aromatic samples. Cross
validation, performed at each wavelength within the 480–
530 nm range, revealed that correct classification was achieved
from the data at individual wavelengths; results from DI at
500 nm give the total variance of the first two PCs as high as
90.8% with 100% classification accuracy (Fig. 5).
A PCA loading plot identified TAC and TAP as the most
dominant fluorophore contributors to PC1 and PC2, respectively;
however, the PCA score plot of these two fluorophores at 500 nm
showed poor separation among data groups and the classification
accuracy dropped to 95.06%. A classification accuracy of 100% was
obtained only from the combination of all fluorophores; thus, all
four fluorophores are needed to identify these nine electron
deficient aromatic compounds uniquely.
Visual detection of trace explosives is very convenient for security
screening. Spraying TAP solution on top of a filter paper tainted with
trace TNT (50 mM in ethanol) clearly revealed a darker pattern under
common black light illumination (Fig. 6). Under a fluorescence
microscope, the TNT mark appeared dark providing high contrast
with the blue luminescence of the untainted area; the technique rea-
12 (a) A. K. Schrock and G. B. Schuster, J. Am. Chem. Soc., 1984,
106, 5234; (b) T. Romero, A. Caballero, A. Tarraga and P. Molina,
Org. Lett., 2009, 11, 3466.
13 (a) T. J. Seiders, K. K. Baldridge and J. S. Siegel, J. Am. Chem. Soc., 1996,
118, 2754; (b) T. J. Seiders, E. Elliott, G. Grube and J. S. Siegel, J. Am. Chem.
Soc., 1999, 121, 7804; (c) M. C. Stuparu, Tetrahedron, 2012, 68, 3527.
14 (a) A. O. Doroshenko, A. V. Kirichenko, V. G. Miting and O. A.
Ponomaryov, J. Photochem. Photobiol., A, 1996, 94, 15; (b) R. Karpicz,
S. Puzinas, S. Krotkus, K. Kazlauskas, S. Jursenas, J. V. Grazulevicius,
S. Grigalevicius and V. Gulbinas, J. Chem. Phys., 2011, 134, 204508.
15 Computational methods and references in ESI†.
16 N. Niamnont, R. Mungkarndee, I. Techakriengkrai, P. Rashatasakhon
and M. Sukwattanasinitt, Biosens. Bioelectron, 2010, 26, 863.
dily allowed visual detection of TNT down to 0.58 ng mmꢀ2, which is 17 M. E. Germain and M. J. Knapp, J. Am. Chem. Soc., 2008, 130, 5422.
about ten times lower than the lowest number previously reported.19
18 F. Samain, S. Ghosh, Y. N. Teo and E. T. Kool, Angew. Chem., Int. Ed.,
2010, 49, 7025.
In conclusion, the triazole unit can be used to increase the
19 K. Zhang, H. Zhou, Q. Mei, S. Wang, G. Guan, R. Liu, J. Zhang and
TNT quenching sensitivity of triphenylamine based fluorophores
Z. Zhang, J. Am. Chem. Soc., 2011, 133, 8424.
c
This journal is The Royal Society of Chemistry 2012
Chem. Commun.