Tripodal naphthalene ether ligand: Solid-state anion recognition and fluorescence studies

Article abstract of DOI:10.1016/j.molstruc.2007.08.016

The simple tripodal amine ligand Tris-[2-(naphthalen-1-yloxy)-ethyl]-amine (L₁) was screened for anion recognition. Four crystal structures confirmed the inorganic as well as organic anion recognition in the solid state. Solid-state structures are results of supramolecular self-assembly and 3D molecular network involves C-H?O and C-H?π bonding in the crystal lattice. In the solid state, it forms a strong C-H?Cl and C-H?O type interactions with the anions. This anion recognition was also confirmed by steady state fluorescence spectroscopy. In complex 4, L₁ is confined between 2D hydrogen bonded sheet formed by pyromellitic acid anion. L₁ shows unusually high selectivity toward nitrate in solution resulting in both a dramatic color change and a concomitant quenching of luminescence.

Full text of DOI:10.1016/j.molstruc.2007.08.016

Available online at www.sciencedirect.com
Journal of Molecular Structure 879 (2008) 88–95
www.elsevier.com/locate/molstruc
Tripodal naphthalene ether ligand: Solid-state anion recognition
and fluorescence studies
*
Avijit Pramanik, Mouchumi Bhuyan, Rajib Choudhury, Gopal Das
Department of Chemistry, Indian Institute of Technology, Guwahati, Assam 781039, India
Received 21 June 2007; received in revised form 17 August 2007; accepted 17 August 2007
Available online 28 August 2007
Abstract
The simple tripodal amine ligand Tris-[2-(naphthalen-1-yloxy)-ethyl]-amine (L1) was screened for anion recognition. Four crystal
structures confirmed the inorganic as well as organic anion recognition in the solid state. Solid-state structures are results of supra-
molecular self-assembly and 3D molecular network involves C–Hꢀ ꢀ ꢀO and C–Hꢀ ꢀ ꢀp bonding in the crystal lattice. In the solid state, it
forms a strong C–Hꢀ ꢀ ꢀCl and C–Hꢀ ꢀ ꢀO type interactions with the anions. This anion recognition was also confirmed by steady state
fluorescence spectroscopy. In complex 4, L1 is confined between 2D hydrogen bonded sheet formed by pyromellitic acid anion. L1
shows unusually high selectivity toward nitrate in solution resulting in both a dramatic color change and a concomitant quenching
of luminescence.
ꢀ 2007 Elsevier B.V. All rights reserved.
Keywords: Tripodal host; Structural studies; Anion recognition; Anion sensor; Fluorescence
1. Introduction
We have been focusing on the discovery of new supramo-
lecular fluorescent sensor molecules [8]. In the present
paper, we report a simple and novel fluorescent anion sen-
sor, Tris-[2-(naphthalen-1-yloxy)-ethyl]-amine (L1). The
binding of the hosts to a series of anions was investigated
by fluorescence titration and 1H NMR techniques. The
anion binding properties was investigated by monitoring
the changes in the fluorescence intensity of dry THF solu-
Anion sensing has been of great interest in biological
and environmental sciences for several decades. Various
fluorescent sensors have been developed for sensitive and
simple detections of anions [1]. The rapid detection of anio-
nic species is of great significance in the environment and in
physiological systems [1]. As signaling mechanisms, photo-
induced electron transfer (PET) [2], intramolecular charge
transfer (ICT) [3], excited-state proton transfer [4], metal
to ligand charge transfer [5], excimer/exciplex formation
[6] and competitive binding [7] have been reported.
Although some excellent examples of compounds capable
of anion recognition and sensing are reported [1]. However,
these approaches have often involved the synthesis of struc-
turally complicated hosts.
tion of L1 at 298 K upon addition of Fꢁ, Clꢁ, Brꢁ, NO
,
ꢁ
3
ClO4ꢁ, CF3COOꢁ and Pyromellitate as their tetrabutylam-
monium (TBA) salt. The ligand L1 shows a drastic change
in the fluorescence intensity after anion binding.
Therefore, the discovery and/or development of new
simple and sensitive anion sensors are strongly desired.
*
Corresponding author. Tel.: +91 361 2582313; fax: +91 361 2582349.
E-mail address: gdas@iitg.ernet.in (G. Das).
0022-2860/$ - see front matter ꢀ 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2007.08.016

A. Pramanik et al. / Journal of Molecular Structure 879 (2008) 88–95
89
2. Results and discussion
2.1. Synthesis
Tris-[2-(naphthalen-1-yloxy)-ethyl]-amine (L1) has been
synthesized following literature method [9]. Synthesis of
all the anionic complexes their characterization data are
all given in the experimental section. IR and NMR spectral
data and the results of chemical analyses are in order and
in complete agreement with the molecular formulae of
the compound. Crystals suitable for X-ray studies of these
anionic complexes were obtained by slow crystallization
from methanol:ethyl acetate (1:3) mixture at room
temperature.
Fig. 1. Hydrophobic pocket in L1.
˚
diameter of ꢂ5 A (Fig. 1). However, the cavity is exclu-
sively hydrophobic in nature. Hence, in the solid state L1
cannot be an endo-receptor for hydrophilic anions. L1
encapsulate these anions in between space in the solid-
state.
Attempts were made to provide solid-state evidence for
anion encapsulation by setting up a series of crystallization
experiments of L1 with different anion. We have shown the
crystal structure analyseꢁs of the following complꢁexes:
½HL1þꢃ½Brꢁꢃð1Þ, ½HL1þꢃ½Cl ꢃ ꢀ CH3OHð2Þ, ½HL1þꢃ½NO3 ꢃð3Þ
and ½HLþ1 ꢃ½Pyromellitateꢁꢃ ꢀ CHCl3ð4Þ. We have crystal-
lized L1 in presence of CF3COOꢁ also, but structural anal-
ysis of these crystals has met with limited success. The
resolution of the structure was poor [10]. CF3COOꢁ unit
is not behaved properly during refinement. However,
ligand structure is properly solved.
2.2. Crystal structure analyses
Some of our main concerns have been to ascertain the
recognition of anion in the solid-state and the conse-
quences of weak hydrogen bonding in the intermolecular
network structure. Accordingly, X-ray diffraction analyses
of the four anionic complexes were done and supramolec-
ular organization in the crystals determined and pertinent
structural data are provided in Table 1.
The bridgehead tertiary nitrogen of the tripodal ligand
L1 exists in monoprotonated form in presence of acid.
The mono-protonated charge of the tripodal ligand is sat-
isfied by one external counter anions. The ligand has a C3
axis of symmetry in the solid state [9]. Space-filling model
of the ligand shows the presence of a cavity of average
Table 1
Crystallographic data for 1,2,3 and 4
1
2
3
4
CCDC No.
Chemical formula
Fw
644569
644570
C38H40ClNO4
610.16
644571
C36H34N2O6
590.6
646021
C47H40Cl3NO11
901.15
C
36H34BrNO3
608.55
Crystal system
Crystal size (mm)
Space group
Cubic
0.23 · 0.19 · 0.11
P2(1)3
14.3769(17)
14.3769(17)
14.3769(17)
90.00
90.00
90.00
2971.6(6)
4
1.360
Triclinic
0.22 · 0.19 · 0.16
Pꢁ1
Cubic
0.21 · 0.16 · 0.12
P2(1)3
14.4685(5)
14.4685(5)
14.4685(5)
90.00
90.00
90.00
3028.80(18)
4
1.295
Monoclinic
0.41 · 0.37 · 0.31
P2(1)/n
13.1145(6)
24.7506(12)
13.6777(7)
90
90.19(4)
90
4439.7(4)
4
˚
a (A)
11.526(3)
11.805(3)
14.313(6)
112.108(10)
108.979(10)
96.360(7)
1644.7(10)
2
˚
b (A)
˚
c (A)
a (ꢁ)
b (ꢁ)
c (ꢁ)
3
˚
V (A )
Z
dcald (mg/m3)
T (K)
1.239
273(2)
0.150
596
1.341
273(2)
0.271
2255
273(2)
1.420
1264
273(2)
0.088
1248
l (mmꢁ1
F(000)
)
Final R indices [I > 2r]
R1
0.0509
0.1239
0.0603
0.2218
0.0647
0.1658
0.0556
0.1948
wR2
R indices (all data)
R1
0.0512
0.1840
0.829
0.0901
0.2460
1.058
0.948
0.1942
1.002
0.726
0.2111
1.071
wR2
Goodness of fit

90
A. Pramanik et al. / Journal of Molecular Structure 879 (2008) 88–95
Table 2
It appears that conformational freedom of the ligand in
˚
Hydrogen bond distances (A) and angles (ꢁ) in 1, 2, 3 and 4
the crystal lattice is not so restricted thereby allowing dif-
ferent packing options. Thus, there is a significant degree
of reciprocal intermolecular hydrogen bonding in the
solid-state between molecules. Crystal structure analysis
shows that L1 acts as an exo-receptor for the anions. In
the solid state, anions are present within a hydrophobic
channel out side the ligand cavity. Protonated bridgehead
N atom is buried inside the hydrophobic region. Moreover,
the proton attached to the apical nitrogen is in endo orien-
tation. Therefore, it cannot form N–Hꢀ ꢀ ꢀX type hydrogen
bonding. All the C–N and C–O bond lengths in these salts
are longer than the corresponding free L1. This is because
of the repulsion of the charged endo oriented N–H bond.
The mean value of C–N–C bond angles in these salts are
smaller than those in free L1 (113.6(2)ꢁ), which also indi-
cates the repulsion.
The bromide complex 1 crystallizes in a highly symmet-
ric cubic space group with one molecule in the asymmetric
unit (Fig. 2). Ligand posses C3 axis of symmetry in the
solid state. Counter anion is outside the ligand cavity,
which is true for all other complexes also. The ligand moi-
eties are organized via intermolecular C–Hꢀ ꢀ ꢀp interactions
(Table 2). Ligand does not have any kind of strong or weak
interactions with the bromide ion in the solid-state. It
forms alternate up-down 3D network of dumbbell shape
boxes (Fig. 3). Each naphthalene unit is parallel to the
plane of the unit cell. Bromide ion is present in a triangular
channel in the lattice formed by the self-assembly of the
ligand (Fig. 4).
˚
˚
Complex D–Hꢀ ꢀ ꢀA
Hꢀ ꢀ ꢀA (A) Dꢀ ꢀ ꢀA (A) \D–Hꢀ ꢀ ꢀA (ꢁ)
1
C2-H2Aꢀ ꢀ ꢀpa
3.080
3.572
3.498
3.404
4.443
4.267
101.20
157.02
141.66
C4-H4ꢀ ꢀ ꢀpa
C9-H9ꢀ ꢀ ꢀpa
2
C5-H5ꢀ ꢀ ꢀO13
2.961
2.707
2.522
2.731
2.929
2.921
2.823
2.274
3.420
3.572
3.498
3.108
3.700
3.470
3.439
3.529
3.613
3.756
3.594
3.053
4.166
4.443
4.267
4.056
137.49
139.83
168.56
139.92
128.46
150.06
137.01
137.71
136.30
157.02
141.66
166.72
C18-H18ꢀ ꢀ ꢀO13
C28-H28ꢀ ꢀ ꢀO13
C13-H13Aꢀ ꢀ ꢀCl1
C13-H13Bꢀ ꢀ ꢀCl1
C20-H20ꢀ ꢀ ꢀCl1
C25-H25Aꢀ ꢀ ꢀCl1
C37-H37Aꢀ ꢀ ꢀCl1
C1-H1Aꢀ ꢀ ꢀpb
C4-H4ꢀ ꢀ ꢀpb
C9-H9ꢀ ꢀ ꢀpb
C25-H25Bꢀ ꢀ ꢀpc
3
4
C6-H6ꢀ ꢀ ꢀO2
2.393
2.547
3.227
3.429
3.448
3.380
3.461
4.408
167.87
145.50
92.39
C12-H12Bꢀ ꢀ ꢀO2
C11-H11Bꢀ ꢀ ꢀpd
C10-H10ꢀ ꢀ ꢀpd
155.26
C2-H2Aꢀ ꢀ ꢀO10
C13-H13Aꢀ ꢀ ꢀO5
C13-H13Aꢀ ꢀ ꢀO7
C25-H25Aꢀ ꢀ ꢀO7
2.614
2.600
2.572
2.623
3.266
3.293
3.432
3.217
3.244
3.377
3.239
3.843
124.83
128.67
147.85
119.90
124.74
165.91
121.43
140.89
C25-H25Bꢀ ꢀ ꢀO11 2.592
C28-H28ꢀ ꢀ ꢀO4
C32-H32ꢀ ꢀ ꢀO7
C18-H18ꢀ ꢀ ꢀpe
2.468
2.654
3.077
a
b
c
Centroid of ring C3–C12.
Centroid of ring C3–C12.
Centroid of ring C31–C36.
Centroid of ring C1–C10.
Centroid of ring C37–C42.
d
e
When we replace the larger bromide ion with smaller
chloride ion, the symmetry of the crystal is destroyed.
Chloride complex 2 crystallizes in triclinic space group.
Asymmetric unit contains one CH3OH molecule (Fig. 5).
We have observed several other differences in the solid-
state structure in comparison to the bromide complex 1.
The ligand moieties self-assembled via intermolecular
C–Hꢀ ꢀ ꢀp interactions. Unlike complex 1, the ligand forms
some addition weak hydrogen bond interactions. In com-
plex 2, ligand form intermolecular C–Hꢀ ꢀ ꢀO interactions
with oxygen atom of solvent molecule (Table 2). Several
strong C–Hꢀ ꢀ ꢀClꢁ interactions are also present in the chlo-
ride complexes [11]. Larger Brꢁ ion does not show this kind
of interaction. All the chloride ions are lined up along crys-
tallographic a axis (Fig. 6). Ligands are organized as a
bowl in alternated up and down manner in the solid-state.
Chloride ion is situated in a rectangular channel formed by
the self-assembled ligand in the crystal (Fig. 7). In the lat-
tice, it is sandwiched between two naphthalene unit.
Crystal structure of nitrate complex 3 is similar that of
the bromide complex 1. It also crystallizes in a highly sym-
metric cubic space group. Asymmetric unit contains one
molecule (Fig. 8). Here ligand and anion has the similar
3D arrangement like complex 1. The packing pattern of
the bromide and nitrate complex are exactly same. How-
ever, in addition to the intermolecular C–Hꢀ ꢀ ꢀp interac-
tions ligand forms intermolecular C–Hꢀ ꢀ ꢀO interactions
with oxygen atom of anion (Table 2). Nitrate anion occu-
pies the triangular hydrophobic void space in the crystal
lattice, which is same as of bromide complex (Fig. 9).
Fig. 2. ORTEP plot of complex1. Hydrogen atoms are omitted for clarity.

A. Pramanik et al. / Journal of Molecular Structure 879 (2008) 88–95
91
Fig. 5. ORTEP plot of complex 2. Hydrogen atoms are omitted for
clarity.
Fig. 3. Packing diagram of complex 1.
Fig. 6. Packing diagram of complex 2.
Fig. 4. Bromide recognition in the solid state.
Ligand in presence of pyromellitic acid crystallizes in
monoclinic space group. Asymmetric unit contains one
CHCl3 molecule (Fig. 10). Unlike CH3OH molecule in 2,
CHCl3 molecule does not form any type of weak interac-
tions with the ligand or acid molecules. All the pyromelli-
tate ions are lined up along crystallographic c axis
(Fig. 11). Ligand and CHCl3 molecules are arranged in in
alternate up–down manner in between two hydrogen
bonded sheets. Each ligand is connected to the sheet via
several C–Hꢀ ꢀ ꢀO interactions [10]. Geometry of the ligand
does not allow two naphthalene units to come within the
non-bonded distances in the solid-state. Hence, no p–p
interactions are observed in the solid state within the ligand
molecules. Electron deficient pyromellitic acid also does
not show p–p interactions with electron rich ligand.
Fig. 7. Chloride recognition in the solid state.
Pyromellitic acid unit form a 2D hydrogen bonded
sheet along ac plane of the crystal. It forms a R4 ð29Þ pat-
4
tern hydrogen bonded ring (Fig. 12). Another notable fea-
ture of the crystal structure of 4 is the formation of intra as

92
A. Pramanik et al. / Journal of Molecular Structure 879 (2008) 88–95
Fig. 10. ORTEP plot of complex 4. Hydrogen atoms are omitted for
clarity.
Fig. 8. ORTEP plot of complex 3. Hydrogen atoms are omitted for
clarity.
Fig. 11. Packing diagram of complex 4.
Fig. 9. Nitrate recognition in the solid state.
well as intermolecularsix membered intermolecular hydro-
gen bonded ring.
In the case of the solution-state study by 1H NMR,
when HCl was added to the DMSO-d6 solution of L1, the
1H NMR spectrum shows a shift in the position of the tren
NCH2 proton from d 3.41 ppm to the higher d value of
3.64 ppm. This shift indicates the influence of the proton-
ated apical nitrogen on the neighboring proton. Similar
changes observed in the solution phase with other anions
also.
2.3. Absorption spectroscopy
UV–visible absorption spectra of L1 in dry THF at
1
298 K shows the La ‹ 1A transitions of the naphthalene
Fig. 12. 2D hydrogen bonded sheet structure of pyromellitic acid along ac
plane in 4.
unit in the region 250–300 nm with e = 1905 Mꢁ1 cmꢁ1

A. Pramanik et al. / Journal of Molecular Structure 879 (2008) 88–95
93
1
and lower energy Lb ‹ 1A transitions in the region 300–
350 nm with e = 932 Mꢁ1 cmꢁ1 [8]. These transitions are
found to be non-solvatochromic in nature. Changes in
the UV–visible absorption spectra were monitored upon
successive addition of anions to sensor L1. A very weak
hypsochromic shift observed only when trifluoro acetic
acid was added to L1 (Fig. 13). All other sensor and inor-
ganic anion combinations afforded almost no noticeable
shift in the peak positions. These results suggest a different
mode of binding of L1 to CF3COOꢁ. During titration with
pyromellitic acid we got an additional peak at 295 nm due
to pyromellitic acid [10].
2.4. Fluorescence spectroscopy
The emission spectra of L1 in dry THF at 298 K shows a
locally excited (LE) broad emission of naphthalene with
emission maximum at 410 nm (kex = 330 nm) (Fig. 14).
LE emission remains virtually unchanged with respect to
solvent polarity. The excitation spectra monitoring kmax
of the emission band is identical with the absorption spec-
tra of the free ligand in dry THF at 298 K [10]. With
increasing concentration of the ligand, the total intensity
of fluorescence emission decreases significantly without
the appearance of any broad band in the higher wavelength
region. This suggests that self-quenching takes place with-
out formation of any excimer at higher concentration [12].
Geometry of the ligand does not allow two naphthalene
chromophores to come in contact with each other, which
is similar to the reported observation [8]. The concentra-
tion of the ligand was maintained at 10ꢁ6 M throughout
the study as the emission intensity is found to be maximum
at this concentration.
Fig. 14. Emission spectrum of L1 (1 · 10ꢁ6 M in dry THF) during the
titration with nitric acid from 0 to 20 equivalent.
When THF solutions of acids were added gradually to
THF solution of L1 the fluorescence was quenched to vary-
ing degrees (Figs. 14 and 15). The best results were
observed for additions of nitric acid to L1, where 97%
quenching was observed after 16 equiv of anion had been
added (Fig. 14 inset). Ligand L1 exhibits high selectivity
toward nitrate anion over other anions present in solution.
The linear Stern–Volmer response with nitrate as quencher
is consistent with well-behaved fluorescence quenching sys-
tems [13]. The absence of the red shifted band in these sys-
tems excludes the possibility of an emission from a charge
transfer state involving the protonated amine/ether oxygen
and the naphthalene unit [14]. To demonstrate the selectiv-
ity of L1 toward nitrate, we have monitored the change in
fluorescence quantum yields in the presence of different
anions. Fig. 15 clearly shows that L1 has a remarkably high
selectivity toward nitrate anion in terms of change of fluo-
rescence quantum yield. The dissociation constant (Kd) of
4.8 lm was calculated from this fluorescence titration
[15]. The spectral characteristics of L1 in the solid state
are consistent with its solution phase behavior. In the solid
state, L1 exhibits two broad absorption bands at 275 and
320 nm. It shows a broad emission band at 415 nm.
3. Experimental
3.1. Physical measurements
The absorption spectra were recorded on a Perkin Elmer
Lambda- 25 UV–Visible Spectrometer at 298 K. The fluo-
rescence spectra were recorded on a Varian Cary-Bio spec-
trofluorimeter and corrected for emission. NMR spectra
Fig. 13. Changes in UV–visible absorption spectra upon addition of
CF3COOꢁ to a THF solution of L1 at 298 K.

94
A. Pramanik et al. / Journal of Molecular Structure 879 (2008) 88–95
butyl ammonium salt of corresponding anion (1 mmol).
The resulting mixture was stirred at room temperature
for 30–60 min. Precipitate (if any) was filtered and filtrate
was kept at room temperature without any mechnical
disturbance.
Complex 1: yellow plates, yield (88%), C36H34NO3Br:
calcd. C 71.05, H 5.63, N 2.30, found C 70.87, H 5.49, N
2.16. Complex 2: pale yellow plates, yield (92%),
C38H40NO4Cl: calcd. C 74.79, H 6.60, N 2.29, found C
74.83, H 6.68, N 2.26. Complex 3: white needles, yield
(96%), C36H34N2O6: calcd. C 73.20, H 5.80, N 4.74, found
C 73.57, H 5.87, N 4.81. Complex 4: white plates, yield
(71%), C47H30NO11Cl3: calcd. C 62.64, H 4.47, N 1.55,
found C 62.72, H 4.52, N 1.51.
4. Conclusion
In conclusion, a simple tripodal naphthalene ether
ligand L1 is reported as chromogenic anion sensor. It can
selectively capture nitrate anion in solution. L1 form self-
assembled structures in the solid state. In the solid-state
different anions are encapsulated in the channel formed
by the ligand. In the solid-state they are assembled via sev-
eral week non-covalent interactions. The spectral features
of the ligand as well as the anionic complexes are similar
in solution phase and solid state. Anions quench the fluo-
rescence intensity of the free L1. Designing of supramolec-
ular host guest fluorescence signaling systems for other
type of guests is in progress in our laboratory.
Fig. 15. Schematic representation showing the change of fluorescence
quantum yield (UF–Uq) of L1 upon addition of the anions. UF and Uq are
quantum yields of L1 in absence and presence of anion, respectively. Inset:
Sterni–Volmer plot for fluorescence quenching of L1(1 · 10ꢁ6 M) on
gradual addition of nitrate (1 · 10ꢁ6 M) in dry THF.
were recorded on a Varian FT-400 MHz instrument. The
chemical shifts were recorded in parts per million (ppm)
on the scale using tetramethylsilane (TMS) as a reference.
Elemental analyses were carried out on a Perkin-Elmer
2400 automatic carbon, hydrogen and nitrogen analyzer.
3.2. X-ray structural determination
Acknowledgements
The intensity data were collected using a Bruker
SMART APEX-II CCD diffractometer, equipped with a
fine focus 1.75 kW sealed tube MoKa radiation
Financial support from the Council of Scientific and
Industrial Research (CSIR), New Delhi, India (Grant
No. 01(1948)/04/EMR-II) is gratefully acknowledged.
A.P. thanks CSIR for JRF. Thanks are due to Chemistry
Department IIT Guwahati and DST FIST for XRD
facility.
˚
(k = 0.71073 A) at 273(3) K, with increasing x (width of
0.3ꢁ per frame) at a scan speed of 3 s/frame. The SMART
software was used for data acquisition. Data integration
and reduction were undertaken with SAINT and XPREP
[16] software. Multi-scan empirical absorption corrections
were applied to the data using the program SADABS
[17]. Structures were solved by direct methods using SHEL-
XS-97 and refined with full-matrix least squares on F2
using SHELXL-97 [18]. All non-hydrogen atoms were
refined anisotropically. The hydrogen atoms were located
from the difference Fourier maps and refined. Structural
illustrations have been drawn with ORTEP-3 for Windows
[19]. CCDC-607228, 607229 and 607230 contain the sup-
plementary crystallographic data for this paper. The data
can be obtained free of charge from The Cambridge Crys-
tallographic Data Centre via www.ccdc.cam.ac.uk/
data_request/cif.
Appendix A. Supplementary data
Supplementary data associated with this article can be
found, in the online version, at doi:10.1016/
j.molstruc.2007.08.016.
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