Click-fluors : Synthesis of a family of π-conjugated fluorescent back-to-back coupled 2,6-Bis(triazol-1-yl)pyridines and their self-assembly studies

Article abstract of DOI:10.1021/jo100598q

(Figure Presented) This paper presents an eight-step synthetic method for the preparation of a series of new back-to-back coupled 2,6-bis(triazol-1-yl) pyridine (btp) molecules (L1-L3). These L1-L3 molecules displayed fluorescent emission at 381, 381, and 457 nm, respectively. We also demonstrated the self-assembly of these fluorescent molecules with Eu³⁺ ions to obtain Eu³⁺ centered red-orange luminescent solids via antenna effect .

Full text of DOI:10.1021/jo100598q

pubs.acs.org/joc
medicinal chemistry, bioinorganic chemistry, magnetochem-
“Click-Fluors”: Synthesis of a Family of
π-Conjugated Fluorescent Back-to-Back Coupled
2,6-Bis(triazol-1-yl)pyridines and Their
Self-Assembly Studies
istry, materials chemistry, and more recently in nanoscience
and technology.1-6 A wide range of functional groups also
can be introduced as substituents and used to control self-
assembly processes and/or the properties of the complexes
which result upon coordination to metal centers. Recently,
the groups of Flood and Hecht5a,b have independently intro-
duced a new member to the tridentate ligand family, i.e., 2,6-
bis(1,2,3-triazol-4-yl)pyridine (btp), synthesized via Sharp-
less click reaction condition.8 But until now, no multitopic
BTB-btp derivatives have been synthesized in contrast to its
structurally analogous counterparts, namely, BTB-terpy1g-i
and BTB-bpp,3b,7 due to the lack of reactive site on the
readily accessible C-4 position of the pyridine unit. In
contrast to both BTB-terpy and BTB-bpp, synthetically
BTB-btp molecules offers tunable solubility provided by
the readily available four alkyl chains obtained via Click
chemistry (after 1,2,3-triazole ring formation). The emission
range of the molecule also can be fine-tuned by varying
π-conjugation lengths of the bridging units linking the two
btp units.
Naisa Chandrasekhar and Rajadurai Chandrasekar*
School of Chemistry, University of Hyderabad,
Prof. C. P. Rao Road, Gachi Bowli, Hyderabad- 500046, India
rcsc@uohyd.ernet.in; chandrasekar100@yahoo.com
Received March 30, 2010
This paper presents an eight-step synthetic method for the
preparation of a series of new back-to-back coupled 2,6-
bis(triazol-1-yl)pyridine (btp) molecules (L1-L3). These
L1-L3 molecules displayed fluorescent emission at 381,
381, and 457 nm, respectively. We also demonstrated the
self-assembly of these fluorescent molecules with Eu3þ
ions to obtain Eu3þ centered red-orange luminescent
solids via “antenna effect”.
Furthermore due to the presence of chelating tridentate
N-donors in the front and back positions of L1-L3 they
have the ability to shield Ln3þ ion completely from solvent
molecules, especially water and alcohols, which are efficient
(2) (a) Halcrow, M. A. Coord. Chem. Rev. 2005, 249, 2880. (b) Halcrow,
M. A. Coord. Chem. Rev. 2009, 253, 2493.
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coupled 2,20:60,200-terpyridine (BTB-terpy) and 2,6-bis-
(pyrazolyl)pyridine (BTB-bpp) molecules have contributed
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4852 J. Org. Chem. 2010, 75, 4852–4855
Published on Web 06/17/2010
DOI: 10.1021/jo100598q
r
2010 American Chemical Society

Chandrasekhar and Chandrasekar
JOCNote
SCHEME 1. Synthetic Route to Back-to-Back Coupled 2,6-Bis-
(triazol-1-yl)pyridine Ligands (L1-L3)a
FIGURE 1. ORTEP diagram of L2 at 293 K with atomic labels (top
figure) and its side view (bottom figure). Hydrogen atoms are
omitted for clarity.
ester was effected as reported.9 Conversion of 2,6-dibromo-
isonicotinicacidmethyl ester into 2 was effected in 72% yield
via Sonogashira coupling conditions by using trimethylsilyl-
acetylene (TMSA) and Pd(PPh3)4 catalyst in Et3N/THF.
Deprotection of 2 in the presence of MeOH/KF gave the
2,6-diethynyl Click precursor 3 in a quantitative 99% yield.
Formation of five-member triazole units was effected on 3
by using Click reaction conditions with Cu(SO4)2/sodium
ascorbate in the presence of butylazide to synthesize the ester
derivative of btp 4 in 97% yield. The saponification reaction
quantitatively converted 4 into its carboxylic acid derivative
5 in 98% yield. Furthermore, sequential conversion of the
carboxylic acid in 5 into acyl azide followed by a thermal
Curtius rearrangement and succeeding hydrolysis of the
trifluoroacetamide provided the amino derivative 6 in 78%
yield as a white powder. From this compound 6, the 4-iodo-
2,6-ditriazolylpyridine 7 was synthesized in a modest 36%
yield by diazotization and followed by reaction with KI.
Treatment of compound 7 with bispinacalatodiboron under
Suzuki cross-coupling condition formed a directly BTB
coupled btp L1 in a decent 41% yield. Suzuki cross-coupling
reactions of compound 7 with 1,4-phenyldiboronic acid and
4,40-biphenyldiboronic acid provided the target molecules
L2 and L3 in ca. 50% yield. The structures of the molecules
aReagents and conditions: (a) TMSA/[Pd(PPh3)4]/CuI/THF/(C2H5)3-
N/4 h/rt; (b) KF/methanol/30 min/rt; (c) butylazide/sodium ascorbate/
copper(II) sulfate pentahydrate/10 h/rt; (d) LiOH/THF/HCl (2 N); (e)
C2O2Cl2/NaN3/TFA/K2CO3; (f) NaNO2/HCl/aqueous KI; (g) [Pd(PPh3)4]/
bispinacalatodiboron/DMF/K2CO3/3 d/70 °C; (h) [Pd(PPh3)4]/1,4-phe-
nylenebisboronic acid/1,4-dioxane/Na2CO3/3 d/70 °C; (i) [Pd(PPh3)4]/
4,40-biphenyldiboronic acid/1,4-dioxane/Na2CO3/3 d/70 °C.
quenchers of Ln3þ luminescence.5 It is also expected that the
significant absorption of L1-L3 in the UV region might help
to antennae their excitation energy efficiently to the excited
state(s) of the Ln3þ ions to obtain sensitized light emission.
This is in fact useful to design and fabricate efficient photonic
devices based on Ln3þ complexes. Moreover due to the BTB
coupled orientation of each btp unit in L1-L3, they are ex-
pected to form rare Eu3þ containing luminescent 3D metallo-
supramolecular networks [(Ln(L1 or L2 or L3)3)3þ]n via self-
assembly.
In this paper, for the first time, we report our novel
methodology for the synthesis of a series of π-conjugated
fluorescent (solution and solid state) BTB coupled btp mole-
cules L1-L3 (without bridging unit (n=0),withphenyl(n=1),
and biphenyl (n = 2) bridging units, respectively; where n is
the number of phenyl linkers), their characterizations, the
crystal structure of L2, and their potential use in preparing
luminescent solids via self-assembly with Eu3þ metal ion.
For the synthesis of BTB coupled btp (L1-L3) molecules,
a new eight-step synthetic protocol from the low-priced
citrazinic acid was developed (Scheme 1). Transformation
of citrazinic acid into 2,6-dibromoisonicotinicacidmethyl
1
L1-L3 were characterized unambiguously by H and 13C
NMR, LCMS, and elemental analyses (see the Support-
ing Information). Compound L2 was successfully crystal-
lized as colorless plates in chloroform by slow evaporation
technique.
Single-crystal X-ray analysis of L2 revealed the monocli-
nic P2(1)/c space group (Figure 1). The molecule is near
planar and the torsion angle between the bridging phenyl
ring and the pyridine unit is 26.28° (C5-C2-C3-C6). The
N7 N70 distance between the two pyridine units is 1.13 nm.
3 3 3
Per unit cell four molecules of L2 were found. In L2 the two
triazole rings in the btp cores adopt anti-anti conformation
due to the repulsive interaction of the lone pairs of the ring
(9) Amb, C. M.; Rasmussen, S. C. J. Org. Chem. 2006, 71, 4696.
J. Org. Chem. Vol. 75, No. 14, 2010 4853

Chandrasekhar and Chandrasekar
JOCNote
FIGURE 2. Top: UV-vis absorbance and emission properties of
L1-L3 in DCM. Bottom: Confocal fluorescence microscope images
of L1-L3.
TABLE 1. Photophysical Properties of L1-L3 in Dichloromethane
compd
λ
abs [nm] (ε [M-1 cm-1])
λ
emission [nm]
Φfa
FIGURE 3. Top: UV-vis emission changes during the titration of
L2 (c = 1.053 ꢀ 10-5 M; DCM) with Eu(NO3)3 solution (c = 0.72 ꢀ
10-3 M; ACN). The arrow facing down (V) shows the decreasing
fluorescent intensity of the ligand and the arrow facing up (v) shows
the increasing Eu3þ emission. Bottom: Solution and solid state
emission of L2 and 1.
L1
L2
L3
265 (50167); 319 (15164)
271 (35445); 327 (45104)
274 (31754); 358 (65582)
381
381
457
0.125
0.374
0.647
aQuinine sulfate as reference (Qref = 0.577) and excitation at 331 nm
for all samples.
ion to the alkyl chains, in addition to the unshifted alkyl chain
proton peaks of the uncoordinated terminal btp cores (no spin
density delocalization). Since the bulk of the compound of 1 is
not soluble in the same acetonitrile solvent, compound that
remains insoluble is supposed to represent 3D higher chain
homologues. It is evident from the recent examples that the
monomeric btp ligand formed a nine-coordinated Eu3þ com-
plex with a trigonal prismatic geometry.5a,b On the basis of
this report, it is anticipated that 1 also adopts a similar type
of coordination environment with 3D polymeric nature
[[Eu(L2)3](NO3)3]n.
nitrogen atoms (N2, N7, and N5) and C-H N hydrogen
3 3 3
bonding interactions.5 The UV-vis absorption studies of
L1-L3 in DCM displayed absorption maxima at λmax 319,
327, and 358 nm, respectively (Figure 2). Both L1 and L2
showed a violet fluorescence with emission maxima at λmax
=
381 nm (λex = 291 nm), while L3 exhibited a red-shifted blue
emission at λmax = 457 nm (λex = 358 nm). The quantum
yield (Φf) of L1-L3 increased in the following order: 0.125,
0.374, and 0.647, respectively. Interestingly, L1-L3 showed
remarkable fluorescence also in the solid states (powder,
crystalline), when exposed to 365 nm UV light (Table 1).
The self-assembly efficiency of L2 and L3 was examined
with Eu3þ ions. The corresponding luminescent property of
the self-assembled product was also investigated. For exam-
ple, treatment of Eu(NO3)3 with a 3-fold excess of ligands L2
formed an air stable complex [[Eu(L2)3](NO3)3]n 1 as a white
precipitate, which exhibits red-orange fluorescent under UV
light (λex = 365 nm). The 1H NMR spectrum of the solu-
ble smaller fragments of complex 1 in CD3CN-d3 solvent
showed substantial broadening and enormous paramagnetic
shifted NMR peaks (Supporting Information, Figure S3).
Interestingly, the aromatic region of the spectra showed three
peaks for L2 and four peaks for 1. An additional paramagne-
tically shifted peak in the aromatic region of 1 possibly arises
from the resonance of the uncoordinated btp core protons of
the 3D polymer chain ends. The aliphatic region of the spec-
trum of 1 further supported this observation by exhibiting
several paramagnetically shifted alkyl chain proton peaks
due to an efficient spin density propagation from the Eu3þ
In the photoluminescence studies (Figure 3), compound 1
exhibited highly red-orange luminescence under UV light
both in the powder state and in solution. Titration of an aceto-
nitrile solution of Eu(NO3)3 with a DCM solution of L2
showed a gradual decrease of the ligand fluorescence and
enhancement of complex 1 emission bands. The complete
disappearance of the broad violet emission band observed at
381 for L2 indicated the completion of metal-ligand self-
assembly. The ligand centered excitation produced Eu3þ
5
7
centered D0 f FJ (J = 0-4) emission bands for 1. This
observation points out an efficient ligand-to-metal energy
transfer process called the “antenna effect”. The emission
spectra are typical of species with low metal ion site symme-
try, and they are dominated by the hypersensitive 5D0 f 7F2
transition.
In conclusion we presented a novel eight-step synthetic
protocol for the preparation of three back-to-back coupled
btp molecules L1-L3. The solid state structure of the ligand L2
4854 J. Org. Chem. Vol. 75, No. 14, 2010

Chandrasekhar and Chandrasekar
JOCNote
showed the near planar structure and adoption of anti-anti
conformation of the btp units. Interestingly molecules
L1-L3 displayed light violet, violet, and blue fluorescence,
respectively, in the solid state (powder and crystal) and in
solution due to the different π-conjugation lengths. Due to
the ditopic nature of the ligand L2, it self-assembled effi-
ciently with Eu3þ ions forming a red-orange (solid and solu-
tion state) emitting 1. This was further supported by the f T f
transitions in the UV-vis region. Detailed photophysical
properties are under investigation.
0.016 mmol, 0.05 equiv) were suspended in degassed 1,4-dioxane
(30 mL). Na2CO3 (2 M, 5 mL) was added to the mixture and
heated at 70 °C for 3 d under nitrogen atmosphere. The course of
the reaction was monitored by TLC (1:19 methanol:DCM).
After disappearance of the starting material the reaction mix-
ture was cooled to rt and then 1,4-dioxane was removed with a
rotary evaporator. The resulting residue was treated with water
and extracted with CH2Cl2 solvent. The separated organic layer
was dried over MgSO4 and the solvent was removed on a rotary
evaporator. The orange residue was washed with a small
amount of petroleum ether (bp 60 °C) to remove the colored
impurities and to afford compound L2 as a white powder in 48%
yield (0.105 g). UV-vis (CH2Cl2) λabs (ε) 271 (35445), 327
Experimental Section
1
(45104); λemis = 381. H NMR (400 MHz, CDCl3) δ 8.46 (s,
4H, 50), 8.26 (s, 4H, 3), 8.04 (s, 4H, 200), 4.47 (t, 3J = 7.26 Hz, 8H,
100), 1.97 (q, 3J = 7.46 Hz, 8H, 200), 1.46-1.36 (m, 8H, 300), 0.99
(t, 3J = 7.36 Hz, 12H, 400). 13C NMR (100 MHz, CDCl3) δ 150.7,
149.2, 148.3, 138.7, 127.1, 127.1, 117.0, 50.3, 32.3, 19.7, 13.5.
FTIR (KBr disk; ν in cm-1) 3483, 2928, 1612, 1460, 1381, 1041,
790. LC-MS m/z calcd 723.90, found 724.42. Anal. Calcd for
C40H48N14: C, 66.27; H, 6.67; N, 27.05. Found: C, 66.17; H,
6.71; N, 26.85.
2,6-Bis(1-butyl-1H-1,2,3-triazol-4-yl)-4-iodopyridine (7). 2,6-
Bis(1-butyl-1H-1,2,3-triazol-4-yl)pyridin-4-amine (6) (0.075 g,
0.22 mmol, 1.0 equiv) was suspended in concd HCl (5 mL) and
stirred for 24 h at rt. The mixture was cooled in an ice bath and
NaNO2 (0.032 g, 0.44 mmol, 2.0 equiv) dissolved in a minimum
amount of water (1 mL) was added dropwise. To this KI in water
(5 mL) was slowly added and the solution was stirred for 5 min.
THF (10 mL) was added into the reaction mixture and the
solution was neutralized by adding solid NaHCO3. The product
was extracted with ether and washed with Na2S2O3 solution.
The collected organic layers were dried over MgSO4 and the
solvent was evaporated on a vacuum evaporator to obtain a pale
yellow crude product, which upon washing with petroleum ether
gives 7 as white color powder in 36% yield (0.037 g). 1H NMR
(400 MHz, CDCl3) δ 8.49 (s, 2H), 8.13 (s, 2H), 4.44 (t, 3J = 7.64
Hz, 4H), 1.96 (q, 3J = 7.64 Hz, 4H), 1.45-1.36 (m, 4H), 0.98 (t,
3J = 7.35 Hz, 6H). 13C NMR (100 MHz, CDCl3) δ 150.5, 147.1,
128.3, 122.3, 106.9, 50.4, 32.3, 19.8, 13.6. LC-MS m/z calcd
451.25, found 451.10. Anal. Calcd for C17H22IN7: C, 45.24; H,
4.91; N, 21.73. Found: C, 45.10; H, 4.85; N, 21.87.
4000,4000-(Biphenylene)bis(2,6-bis(1-butyl-1H-1,2,3-triazol-4-yl))-
pyridine (L3). Compound 7 (0.090 g, 0.2 mmol), 4,40-dipheny-
lenebisboronic acid (0.24 g, 0.095 mmol), and [Pd(PPh3)4] (0.012 g,
0.01 mmol) were suspended in degassed 1,4-dioxane (30 mL).
Na2CO3 (2 M, 5 mL) was added and the mixture was heated at
70 °C for 3 d under nitrogen atmosphere. The course of the
reaction was monitored by TLC (1:19 methanol:DCM). After
disappearance of the starting material the reaction mixture was
cooled to rt and then 1,4-dioxane was removed with a rotary
evaporator. The resulting residue was treated with water and
extracted with CH2Cl2 solvent. The separated organic layer was
dried over MgSO4 and the solvent was removed on a rotary
evaporator. The orange residue was washed with a small amount
of petroleum ether to remove the colored impurities and to afford
compound L2 as a white powder in 51% yield (0.082 g). UV-vis
(CH2Cl2) λabs (ε) 274 (31754), 358 (65582); λemis = 457. 1H
NMR (400 MHz, CDCl3) δ 8.45 (s, 4H, 50), 8.230 (s, 4H, 3), 7.99
(dd, 8H, 200),7.84 (dd, 8H, 300), 4.48 (t, 3J = 7.38 Hz, 8H, 100), 2.20
(q, 3J = 7.60 Hz, 8H, 200), 1.47-1.37 (m, 8H, 300), 1.01 (t, 3J =
7.38 Hz, 12H, 400). 13C NMR (100 MHz, CDCl3) δ 150.8, 149.6,
148.5, 146.2, 127.8, 122.15, 117.0, 50.4, 32.4, 19.8, 13.5. FTIR
(KBr disk; ν in cm-1) 3123, 2959, 2930, 2866, 1703, 1614, 1568,
1458, 1379, 1205, 1043, 821. LC-MS m/z calcd 801.10, found
801.00. Anal. Calcd for C46H52N14: C, 68.98; H, 6.54; N, 24.48.
Found: C, 68.87; H, 6.49; N, 24.35.
2,20,6,60-Tetrakis(1-butyl-1H-1,2,3-triazolyl)-4,40-bipyridine
(L1). Compound 7 (0.10 g, 0.221 mmol), bispinacalatodiboron
(0.061 g, 0.24 mmol), K2CO3 (0.092 g, 0.665 mmol), and
[Pd(PPh3)4] (0.018, 0.016 mmol, 0.05 equiv) were suspended in
degassed DMF (30 mL). The reaction mixture was heated at
70 °C for 3 d under nitrogen atmosphere. The course of the re-
action was monitored by TLC (1:19 methanol:DCM). After
disappearance of the starting material the reaction mixture was
cooled to rt and then DMF was removed with a rotary eva-
porator. The resulting residue was treated with water and
extracted with CH2Cl2 solvent. The separated organic layer
was dried over MgSO4 and the solvent was removed on a rotary
evaporator. The yellow residue was obtained and purified by
column chromatography (silica, 1:50 methanol:DCM). The
obtained first fraction was collected, and rotary evaporated to
afford L1 as a white powder in 41% yield (0.059 g). UV-vis
(CH2Cl2) λabs (ε) 265 (50167), 319 (15164); λemis 381. 1H NMR
(400 MHz, CDCl3) δ 8.50 (s, 4H, 50), 8.25 (s, 4H, 3), 4.46 (t, 3J =
7.06 Hz, 8H, 100), 1.98 (q, 3J = 7.32 Hz, 8H, 200), 1.46-1.36 (m, 3J
= 7.57 Hz, 8H, 300), 0.99 (t, 3J = 7.32 Hz, 12H, 400). 13C NMR
(100 MHz, CDCl3) δ 150.3, 148.1, 147.6, 122.3, 117.3, 50.4, 32.4,
19.8, 13.6. LC-MS m/z calcd 648.81, found 648.90.
Acknowledgment. We are grateful to DST (Fast Track
Scheme No. SR/FTP/CS-115/2007), New Delhi for financial
support. N.C. thanks CSIR for a JRF fellowship.
Supporting Information Available: General experimental
methods, synthesis of (1-6), characterization data (2-7,
L1-L3), crystallographic data/CIF file of L2 and UV-vis
absorption/fluorescent titration data of L3 with Eu(III). This
material is available free of charge via the Internet at http://
pubs.acs.org.
1,4-Bis(1,20:60,100-bis(3-butyl-1H-3,4,5-triazolyl)pyridin-40-yl)-
benzene (L2). Compound 7 (0.14 g, 0.31 mmol), 1,4-phenylene-
bisboronic acid (0.028 g, 0.16 mmol), and [Pd(PPh3)4] (0.018,
J. Org. Chem. Vol. 75, No. 14, 2010 4855