Synthesis, characterization and preliminary investigation of the electro redox properties of anthracenyl-functionalized terpyridyl ligands

Article abstract of DOI:10.1016/j.tetlet.2010.11.022

We report the synthesis, characterization and preliminary investigation of the electro redox properties of soluble bromoterpyridine ligand precursors L1, L2 and L3 having one to three anthracenyl modules. The synthetic protocol is based on sequential palladium cross-coupling reactions between 9,10-dibromoanthracene and bromoterpyridine, 9-bromoanthracene-terpyridine or 9-bromo-dianthraceneterpyridine synthons. With increasing numbers of anthracene units on the terpyridine ligand, common absorption wavelengths at the near visible region and enhanced molar absorptivity coefficients were recorded. However, the luminescence intensity decreases with increasing length of π-conjugation relating to aggregation effects in the molecules. The cyclic voltammograms of L1 and L2 display good electro redox activity, being better than that of L3.

Full text of DOI:10.1016/j.tetlet.2010.11.022

Tetrahedron Letters 52 (2011) 274–277
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Synthesis, characterization and preliminary investigation of the electro
redox properties of anthracenyl-functionalized terpyridyl ligands
⇑
Adewale O. Adeloye, Peter A. Ajibade
Department of Chemistry, Faculty of Science and Agriculture, University of Fort Hare, Private Bag X1314, Alice 5700, South Africa
a r t i c l e i n f o
a b s t r a c t
Article history:
We report the synthesis, characterization and preliminary investigation of the electro redox properties of
soluble bromoterpyridine ligand precursors L1, L2 and L3 having one to three anthracenyl modules. The
synthetic protocol is based on sequential palladium cross-coupling reactions between 9,10-dibromoan-
thracene and bromoterpyridine, 9-bromoanthracene-terpyridine or 9-bromo-dianthraceneterpyridine
synthons. With increasing numbers of anthracene units on the terpyridine ligand, common absorption
wavelengths at the near visible region and enhanced molar absorptivity coefficients were recorded. How-
ever, the luminescence intensity decreases with increasing length of p-conjugation relating to aggrega-
tion effects in the molecules. The cyclic voltammograms of L1 and L2 display good electro redox
activity, being better than that of L3.
Received 25 August 2010
Revised 25 October 2010
Accepted 5 November 2010
Available online 11 November 2010
Keywords:
Terpyridine ligands
9,10-Dibromoanthracene
Palladium-catalyzed reactions
Electrochemistry
Ó 2010 Elsevier Ltd. All rights reserved.
Molecular architectures in which highly-efficient photoinduced
energy- or electron-transfer processes can take place over long
reagents for other molecular substitutions or engage in providing
increased supramolecular order in solid state structures. As an
electron transfer function, anthracene has proved to be a good
hole-transport material in organic light emitting diodes.17–20 The
photophysical and preliminary electrochemical behaviour of the
ligands are presented.
distances have attracted significant interest in recent years.1,2
A
variety of potential applications such as artificial photosynthesis,3
photocatalysis,4 molecular photovoltaic cells,5 molecular infor-
matics6 and optoelectronic devices,7,8 are beginning to emerge
from this new field of research. However, in order to favour
vectorial transfer of information over long distances along the
molecular axis, the use of extended and rigid linkages between
two complexation sites is required. Multidentate ligands,
such as 2,3,5,6-tetrapyridinylpyrazine,9 bis-10 or tris-2,20:60,200-
terpyridine,11 3,30,5,50-tetrapyridyl-biphenyl12 or polyphenyl
back-to-back bis-2,20:60,200-terpyridines13 provide suitable molec-
ular frameworks for the study of electronic coupling between
two proximal redox centres. Terpyridine ligands are effective
complexing agents and key building blocks in supramolecular
chemistry, and their 40-substitution is also particularly attractive
because it leads to the construction of linear, rod-like polynuclear
polymers and complexes.14 Recent efforts have been focussed on
the development of new molecules14–16 containing terpyridinyl
ligands with carboxylate or carboxylic acid, or phosphonic acid
functions which could be used in polymer solar cells as well as
sensitizers for dye-sensitizer solar cells.
The traditional method of producing polymerized hydrocarbons
by dehalogenation of hydrocarbons using biscyclooctadienyl
nickel(0) or magnesium–diene complexes as catalysts,21 and
Sonogashira coupling of aryl bromides under palladium-catalysis
is well documented, and have been modified in this work.22,17
Scheme 1 shows the synthetic route for the formation of the
ligands. The first step involves reaction of a stoichiometric ratio
of terpyridine ligand 1a with hydrobromic acid and hydrogen per-
oxide (1:1, v/v) solution in methanol at room temperature using a
modified literature procedure.23 The product, 40-bromoterpyridine
124 was subsequently reacted with 9,10-dibromoanthracene in a
palladium-catalyzed cross-coupling reaction (CH2Cl2/C6H6, 1:1, v/v)
to form ligand L1.25 Reaction of L1 with 9,10-dibromoanthracene
using the same conditions afforded ligand L2.26 Ligand L327 was
obtained by similar reaction of L2 with 9,10-dibromoanthracene. A
stoichiometric reaction in 50% dichloromethane-benzene and the
use of a higher temperature for the synthesis of L1, L2 and L3 limits
the formation of mixtures of compounds as observed when milder
experimental conditions and inappropriate solvents were used. For
example, poor product yields and mixtures were obtained when
the reactions were carried out in ethanol and on using palladium(II)
acetate as the catalyst.
The emphasis of the present work is the synthesis of three new
bromo-substituted anthracenyl terpyridine ligands incorporating
increasing numbers of anthracene units, which on dehalogenation
of the bromine atom on the anthracene could serve as versatile
The infrared spectra of the ligands were compared and assigned
on careful comparison with the starting materials. Conspicuous in
⇑
Corresponding author. Tel.: +27 406 022 055; fax: +27 406 022 094.
E-mail address: pajibade@ufh.ac.za (P.A. Ajibade).
0040-4039/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2010.11.022

A. O. Adeloye, P. A. Ajibade / Tetrahedron Letters 52 (2011) 274–277
275
Br
Br
Br
1b
N
Br
HBr/H
O
(1:1)
2
N
2
(i)
N
N
CH OH, 20 h, rt
3
N
N
CH Cl -C H , Pd/C
2
2
6
6
N
o
1a
110-120 C, 8 h, dark
1
N
N
(Yield = 65%),
L1
Br
Br
Br
CH Cl -C H , Pd/C
2
2
6
6
o
110-120 C, 8 h, dark
+
(ii)
Br
1b
N
N
N
N
L1
N
N
L2
(Yield = 53%)
Br
Br
Br
CH Cl -C H , Pd/C
2
2
6
6
(iii)
o
+
110-120 C, 8 h, dark
Br
1b
N
N
N
L2
N
N
N
L3 (Yield = 79%)
Scheme 1. The synthesis of mono-, di- and tri-anthracenyl terpyridines.
the spectrum was a single broad band at 3400 cmꢀ1 due to an O–H
vibration for the unsubstituted terpyridine ligand which split into
two vibrational peaks of equal intensity at 3475 and 3407 cmꢀ1 for
the 40-bromoterpyridyl derivative. The O–H is assumed to derive
from water vapour. The IR spectra of the respective ligands showed
slight shifts in the frequencies and intensities for the C@N, C@C
and C–Br vibrations due to the increasing anthracene substitution
in the products.
Figure 1. UV–vis absorption spectra of L1, L2 and L3 in DMF.
Figure 2. Emission spectra of L1 (a), L2 (b) and L3 (c) in DMF.

276
A. O. Adeloye, P. A. Ajibade / Tetrahedron Letters 52 (2011) 274–277
Figure 3. Cyclic voltammetry profiles of 1 ꢁ 10ꢀ3 M solutions of L1, L2 and L3 in freshly distilled DMF containing 0.1 M TBABF4 supporting electrolyte. Step potential = 5 mV,
amplitude = 50 mV versus Ag|AgCl, frequency = 10 Hz. Scan rate = 100 mV sꢀ1 versus Ag|AgCl.
The UV–vis absorption spectra of ligands L1–L3 are shown in
Figure 1. The * intra-ligand transitions occur between 290
the origin of the shifting in the emission wavelength to the blue-
green region.28
p
? p
and 420 nm for the anthracene and the terpyridine conjugated
couples. Maxima characteristic of anthracene derivatives are found
at 342, 361, 381 and 403 nm for L3, these are slightly blue-shifted
(ca. 1–2 nm) for L2 and L1. The molar absorptivity coefficient in-
creases with a corresponding increase in the number of anthracenes
on the terpyridyl ligand in the order L3 > L2 > L1. One interesting
observation was found as the wavelength shifted to the red.
The molar absorptivity tends to favour L2 containing a double-
anthracenyl substitution compared to L1 and L3 with mono- and
tri-anthracenyl substitution. This we thought to be indicative of
the optimum number of anthracene units as further increases led
to aggregation thus affecting the energy transfer properties of the
molecules.
The fluorescence spectra of ligands L1–L3 are shown in Figure 2.
A common strong emission wavelength band at 745 nm due to sin-
glet-to-singlet energy transfer at an excitation wavelength of
450 nm was recorded for L1, L2 and L3. The emission spectra show
an inverse relationship between the bathochromic shifts in wave-
length and the intensity of emission. This result is in accord with
the B3LYP/6-31G theoretical calculations for substituted anthra-
cene derivatives in which slight variations are found in the elec-
tronic structures leading to electron accepting ability, and then a
decrease in the HOMO–LUMO energy gap which is believed to be
The cyclic voltammograms of L1, L2 and L3 in DMF, containing
tetrabutylammonium tetrafluoroborate as a supporting electrolyte,
are shown in Figure 3 (Plates 1–4), with their electrochemical data
summarized in Table 1. Irreversible oxidation peaks were found at
+0.65 and +0.73 V, respectively, for L1 and L2. Other redox waves
were observed at E1/2 = ꢀ0.27, ꢀ0.51 and ꢀ0.99 V, (for L1) and
E
1/2 = ꢀ0.36, ꢀ0.87 V (for L2). The cyclic voltammogram of L3 did
not show any appreciable electro redox properties (Plate 4). The
reason for this is not understood at this stage.
In conclusion, we have designed bromoterpyridine ligands
substituted with one to three anthracene units as molecular archi-
tectures in which highly-efficient photoinduced energy or electron
transfer processes can take place over long distances. This work
has provided information on the photophysical, luminescence
and electro redox properties of the ligands which stand to be im-
proved on the premise of further functionalization through
bromine substitution.
Acknowledgements
The authors thank the South African National Energy Research
Institute (SANERI) for a bursary and the University of Fort Hare,
Alice, South Africa. A.O. acknowledges Obafemi Awolowo University,
Nigeria for study leave.
Table 1
Cyclic voltammetric data for ligands L1, L2 and L3
Compound
Epa/V
E1/2/V
Supplementary data
L1
L2
L3
0.65
0.73
NV
ꢀ0.27, ꢀ0.51, 0.99
ꢀ0.36, ꢀ0.87
NV
Supplementary data (IR, 1H and 13C NMR spectra for ligands L1,
L2 and L3) associated with this article can be found, in the online
version, at doi:10.1016/j.tetlet.2010.11.022.
Key: NV = not visible.

A. O. Adeloye, P. A. Ajibade / Tetrahedron Letters 52 (2011) 274–277
277
CH2Cl2–EtOAc (4:1, v/v, 200 mL) to give a yellow-red viscous liquid which was
allowed to air dry, and finally recrystallized from Et2O–EtOH to afford the title
compound 1 (2.74 g, 91%) as a bright yellow solid. Found: C, 57.94; H, 3.54; N,
13.27; C15H10N3Br requires C, 57.71; H, 3.23; N, 13.46; mp 209–211 °C; IR
(KBr): 3475, 3407, 3039, 2806, 1606, 1583, 1562, 1524, 1454, 1442, 1423, 1355,
References and notes
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1289, 1172, 1153, 1078, 1039, 991, 857, 833, 784, 764, 547 cmꢀ1
.
25. 40-(9-Bromoanthracenyl)-terpyridine (L1): A mixture of 4-bromoterpyridine (1)
(1.00 g, 3.20 mmol) and 9,10-dibromoanthracene (1.08 g, 3.20 mmol) was
dissolved in C6H6–CH2Cl2 (1:1, v/v, 160 mL), and the solution stirred for 10 min
until all the solid had dissolved. Next, Et3N (1.0 mL) and Pd/C (0.045 g) were
added. After heating for 8 h at reflux, the mixture was concentrated under
reduced pressure and distilled H2O (30 mL) was added. The organic material
was extracted with CHCl3 (100 mL) and evaporated to dryness. The solid was
recrystallized from EtOH to afford the title compound L1 as a pink-red solid
(yield = 1.36 g, 65%, mp 189–190 °C). IR (KBr): 3419, 3048, 2925, 2852, 1744,
1620, 1581, 1562, 1453, 1437, 1423, 1304, 1256, 1078, 1039, 1028, 989, 926,
3. Gust, D.; Moore, T. A. Science 1989, 244, 35.
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832, 763, 747, 675, 578. UV–vis (kmax nm,
e, , CHCl3): 342
104 Mꢀ1 cmꢀ1
(3.6900); 361 (7.7000); 381 (12.4000); 403 (11.6000). 1H NMR (CDCl3): d 8.69
(d, J = 4.4 Hz, 2H, H-3, 300), 8.60 (d, J = 8.0 Hz, 2H, H-5, 500), 8.56 (dd, J = 3.2,
6.7 Hz, 4H, anthr.), 8.44 (d, J = 7.8 Hz, 1H, H-30), 7.94 (t, J = 7.8 Hz, 2H, H-4, 400),
7.84 (t, J = 7.7 Hz, 2H, H-6, 600), 7.60 (dd, J = 3.1, 6.8 Hz, 4H, anthr.), 7.27 (t,
J = 4.4 Hz, 1H, H-50). 13C NMR (CDCl3): d 156.26, 155.36, 149.14, 137.89, 136.83,
131.06, 128.28, 127.45, 123.73, 123.53, 121.16, 120.99. Found: C, 71.10; H,
3.83; N, 8.48, C29H18N3Br requires, C, 71.32; H, 3.71; N, 8.60.
26. 40-(9-Bromodianthracenyl)-terpyridine (L2): The synthesis of L2 followed the
same procedure as reported for L1. Ligand, L1 (0.90 g, 1.83 mmol) was reacted
with 9,10-dibromoanthracene (0.63 g, 1.83 mmol) to afford the title compound
as a yellow solid. (Yield = 0.80 g, 53%, mp 215–216 °C). IR (KBr): 3473, 3409,
3035, 2926, 2851, 1929, 1620, 1606, 1582, 1561, 1523, 1437, 1423, 1350, 1304,
1289, 1256, 1150, 1077, 1027, 991, 959, 926, 838, 764, 747, 675, 578. UV–vis
(kmax nm,
e
, 104 Mꢀ1 cmꢀ1, CHCl3): 342 (6.6000); 361 (11.8500); 381 (18.7500);
14. Duprez, V.; Biancardo, M.; Spanggaard, H.; Krebs, F. C. Macromolecules 2005, 38,
10436.
1
403 (16.8500). H NMR (CDCl3): d 8.70 (d, J = 4.4 Hz, 2H, H-3, 300), 8.61 (d,
J = 7.96 Hz, 2H, H-5, 500), 8.56 (dd, J = 3.2, 6.8 Hz, 8H, anthr.), 8.44 (d, J = 7.8 Hz,
1H, H-30), 7.95 (t, J = 7.8 Hz, 2H, H-4, 400), 7.85 (t, J = 1.5, 7.7 Hz, 2H, H-6, 600),
7.60 (dd, J = 3.2, 6.9 Hz, 8H, anthr.), 7.41 (t, J = 0.9, 5.1 Hz, 1H, H-50). 13C NMR
(CDCl3): d 156.13, 155.24, 149.05, 137.95, 136.99, 131.06, 128.28, 127.46,
123.80, 123.53, 121.25, 121.09. Found: C, 77.72; H, 3.64; N, 6.76, C43H26N3Br
requires, C, 77.71; H, 3.94; N, 6.32.
15. Duprez, V.; Krebs, F. C. Tetrahedron Lett. 2006, 47, 3785.
16. Duprez, V.; Biancardo, M.; Krebs, F. C. Sol. Energy Mater Sol. Cells 2007, 91, 230.
17. Krzysztof, D.; Huang, T. H.; Lin, J. T. Chem. Mater. 2002, 14, 3860.
18. Yu, M. X.; Duan, J. P.; Lin, C. H. Chem. Mater. 2002, 14, 3958.
19. Godbert, N.; Bryce, M. R.; Dahaoui, S.; Batsanov, A. S.; Howard, J. A. K.;
Hazendonk, P. Eur. J. Org. Chem. 2001, 749.
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Gallardo, I. Polycyclic Aromat. Compd. 2003, 23, 457.
21. Yamamoto, T.; Kitamura, N.; Fukumoto, H. Catalytic dehydrogenation method
for producing polymerized hydrocarbons. U.S. Patent 7138484; Chem. Abstr.
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27. 40-(9-Bromotrianthracenyl)-terpyridine (L3): The synthesis of L3 followed the
same procedure as reported for L1. Ligand, L2 (0.50 g, 0.75 mmol) was reacted
with 9,10-dibromoanthracene (0.25 g, 0.75 mmol) to afford the title compound
as a dark yellow solid after recrystallization from Et2O–EtOH. (Yield = 0.59 g,
79%, mp 218–220 °C). IR (KBr): 3403, 3075, 2926, 2852, 1621, 1605, 1582,
1562, 1523, 1437, 1423, 1304, 1256, 1027, 990, 960, 926, 837, 763, 746, 675,
22. Lipshutz, B. L.; Chung, D. W.; Rich, B. Org. Lett. 2008, 10, 3793.
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604, 578. UV–vis (kmax nm,
e, , CHCl3): 342 (8.9200); 361
104 Mꢀ1 cmꢀ1
(18.0000); 381 (27.4000); 403 (23.3500). 1H NMR (CDCl3): d 8.70 (d, J = 4.1 Hz,
2H, H-3, 300), 8.61 (d, J = 7.92 Hz, 2H, H-5, 500), 8.56 (dd, J = 3.2, 6.8 Hz, 12H,
anthr.), 8.45 (d, J = 7.8 Hz, 1H, H-30), 7.95 (t, J = 7.8 Hz, 2H, H-4, 400), 7.86 (t,
J = 7.7 Hz, 2H, H-6, 600), 7.61 (dd, J = 3.2, 6.9 Hz, 12H, anthr.), 7.33 (t, J = 6.6 Hz,
1H, H-50). 13C NMR (CDCl3): d 156.03, 155.15, 148.99, 137.98, 137.09, 131.06,
128.28, 127.45, 123.83, 123.53, 121.30, 121.15. Found: C, 81.88; H, 4.43; N,
5.36, C57H34N3Br requires, C, 81.42; H, 4.08; N, 5.00.
24. Preparation of 40-Bromoterpyridyl.
A stirred solution of terpyridine (3.0 g,
0.013 mmol) and HBr (1.04 g, 0.7 mL, 0.013 mmol) in CH3OH (60 mL) was
added to a 47% aqueous solution of H2O2 (0.44 g, 0.4 mL, 0.013 mmol) slowly,
over a period of 15 min at room temperature. The mixture was stirred for 20 h
whilst monitoring the reaction progress by TLC. After completion, the solvent
was removed under reduced pressure and the residue dissolved in EtOAc
(150 mL), washed with brine (3 ꢁ 100 mL) and dried over anhydrous Na2SO4.
The product was isolated by careful column chromatography on silica gel using
28. Zhang, P.; Baohui, X.; Yinghui, S.; Bing, Y.; Wenjing, T.; Yue, W.; Guo, Z. Chin.
Sci. Bull. 2006, 51, 2444.