Rigid π-Conjugated mono-, bis-, and tris(2,2′:6′,2″- terpyridines)

Article abstract of DOI:10.1021/ol0707261

A set of rigid, π-conjugated linear mono- and bisterpyridines and star-shaped tristerpyridines have been synthesized using Pd(0)-catalyzed coupling reactions and the Horner -Wadsworth-Emmons reaction. The terpyridyl ligands obtained feature strong emissio

Full text of DOI:10.1021/ol0707261

ORGANIC
LETTERS
2007
Vol. 9, No. 12
2345-2348
Rigid
Tris(2,2
π-Conjugated Mono-, Bis-, and
′:6′,2′′-terpyridines)
Andreas Winter,† Daniel A. M. Egbe,† and Ulrich S. Schubert*,†,‡
Laboratory for Macromolecular Chemistry and Nanoscience, EindhoVen UniVersity of
Technology, P.O. Box 513, 5600 MB EindhoVen, The Netherlands and
Center for Nanoscience (CeNS), LMU Mu¨nchen, Geschwister-Scholl-Platz 1,
80333 Munich, Germany
u.s.schubert@tue.nl
Received March 24, 2007
ABSTRACT
A set of rigid,
coupling reactions and the Horner
π
-conjugated linear mono- and bisterpyridines and star-shaped tristerpyridines have been synthesized using Pd(0)-catalyzed
Wadsworth Emmons reaction. The terpyridyl ligands obtained feature strong emission in the blue range
−
−
with high quantum yields in dilute solution and thin films.
Supramolecular chemistry has evolved into one of the most
active areas of research in modern chemistry. Both the design
and effective preparation of new functional ligands and their
incorporation into supramolecular assemblies by transition
metal complexation mark the key challenges in this field.1
In recent years, oligopyridyl ligands and especially their
transition metal complexes have found numerous applications
as luminescent sensors in molecular biology and medical
diagnostics, in photocatalysis, as active materials in self-
assembled molecular devices, and as photoactive molecular
wires.2 Furthermore, these complexes have been introduced
effectively in electroluminescent materials and for storage
applications in molecular electronics and photonics.3
(1) (a) Lehn, J.-M. Supramolecular Chemistry - Concepts and Chemistry;
VCH: Weinheim, Germany, 1995. (b) Schubert, U. S.; Hofmeier, H.;
Newkome, G. R. Modern Terpyridine Chemistry; Wiley-VCH: Weinheim,
2006. (c) Schubert, U. S.; Eschbaumer, C. Angew. Chem., Int. Ed. 2002,
41, 2892-2926. (d) Newkome, G. R.; He, E.; Moorefield, C. N. Chem.
ReV. 1999, 99, 1689-1746. (e) Newkome, G. R.; Wang, P.; Moorefield,
C. N.; Cho, T. J.; Mohapatra, P. P.; Li, S.; Hwang, S.-H.; Lukoyanova, O.;
Echegoyen, L.; Palagallo, J. A.; Lancu, V.; Hla, S.-W. Science 2006, 312,
1782-1785. (f) Bonnet, S.; Collin, J.-P.; Koizumi, M.; Mobian, P.; Sauvage,
J.-P. AdV. Mater. 2006, 18, 1239-1250. (g) Flores-Torres, S.; Hutchison,
G. R.; Soltzberg, L. J.; Abrun˜a, H. D. J. Am. Chem. Soc. 2006, 128, 1513-
1522.
† Eindhoven University of Technology.
‡ LMU Mu¨nchen.
10.1021/ol0707261 CCC: $37.00
© 2007 American Chemical Society
Published on Web 05/11/2007

The remarkably high binding affinity toward most transi-
tion metal ions by dπfpπ* bonding, together with their
chelation properties, make terpyridines attractive building
blocks for the construction of supramolecular assemblies.
2,2′:6′,2′′-Terpyridines bearing π-conjugated substituents at
the 4′-position are known to present interesting photophysical
and electrochemical properties.1b,2e,4 The electronic com-
munication between the metal-complexed units of bis- and
tristerpyridines is another fascinating feature pointing out
their potentials in the design of new functional materials.2-4
Light-emitting ligands are considered to be important
structures as hosts for photoinduced energy and electron
transfers in supramolecular chemistry.4 However, unlike for
their 2,2′-bipyridyl counterparts,5 the applications of π-con-
jugated terpyridines in these fields have not been elaborated
so far and only a few publications deal with the synthesis
and properties of rigid-linear or star-shaped derivatives.6
rigid π-conjugated mono-, bis-, and tris(2,2′:6′,2′′-terpyrid-
ines). The structures shown in this contribution bearing
various conjugated linkagessincluding alkene, alkyne, and
phenyl moietiessand solubilizing alkoxy chains are promis-
ing with respect to the design of new supramolecular
assemblies, molecular wires, or dendritic structures.
A set of 4′-substituted 2,2′:6′,2′′-terpyridines were chosen
as the starting point for the construction of the rigid mono-,
bis-, and tristerpyridines via Pd(0)-catalyzed coupling reac-
tions and the Horner-Wadsworth-Emmons reaction (HWE).
For this purpose, the Kro¨hnke-type derivatives 1 and 2 were
prepared according to methods published previously.9 Scheme
1 summarizes the straightforward syntheses of the key
substrates 4 and 5 for the HWE reactions.10
Scheme 1. Synthesis of 4′-Substituted 2,2′:6′,2′′-Terpyridines
In continuation of prior work in the field of terpyridines
and their applications in supramolecular chemistry,1b,c,7 as
well on the design of PPV/PPE hybrid polymers,8 we now
present the synthesis and photophysical investigations of new
(2) (a) Erkkila, K. E.; Odom, D. T.; Barton, J. K. Chem. ReV. 1999, 99,
2777-2795. (b) Barigeletti, F.; Flamigni, L. Chem. Soc. ReV. 2000, 29,
1-12. (c) Ott, S.; Kritikos, M.; A° kermark, B.; Sun, L. Angew. Chem., Int.
Ed. 2003, 42, 3285-3288. (d) Andres, P. R.; Schubert, U. S. AdV. Mater.
2004, 16, 1043-1068. (e) Hofmeier, H.; Schubert, U. S. Chem. Soc. ReV.
2004, 33, 373-399. (f) Bianke´, G.; Ha¨ner, R. ChemBioChem 2004, 5,
1063-1068. (g) Schmittel, M.; Kalsani, V.; Mal, P.; Bats, J. W. Inorg.
Chem. 2006, 45, 6370-6377. (h) Hwang, S. H.; Moorefield, C. N.; Dai,
L.; Newkome, G. R. Chem. Mater. 2006, 18, 4019-4022.
(3) (a) Ziessel, R.; Hissler, M.; El-ghayoury, A.; Harriman, A. Coord.
Chem. ReV. 1998, 178-180, 1251-1598. (b) Balzani, V.; Ceroni, P.; Juris,
A.; Venturi, A.; Venturi, M.; Campagna, S.; Puntoriero, F.; Serroni, S.
Coord. Chem. ReV. 2001, 219-221, 545-572. (c) Flood, A. H.; Stoddart,
J. F.; Steuerman, D. W.; Heath, J. R. Science 2004, 306, 2055-2056. (d)
Ciofini, I.; Laine´, P. P.; Bedioui, F.; Adamo, C. J. Am. Chem. Soc. 2004,
126, 10763-10777. (e) Duprez, V.; Biancardo, M.; Spanggaard, H.; Krebs,
F. C. Macromolecules 2005, 38, 10436-10448. (f) Hagemann, O.;
Jørgensen, M.; Krebs, F. C. J. Org. Chem. 2006, 71, 5546-5559. (g)
Benniston, A.; Harriman, A.; Li, P.; Patel, P. V.; Sams, C. A. J. Org. Chem.
2006, 71, 3481-3493.
The Pd(0)-catalyzed Sonogahira cross-coupling reaction
of the ethynyl-functionalized terpyridine 2 (see Supporting
Information for details) with bis(alkoxy)-arylbromides in the
presence of Pd(PPh3)4/CuI as catalyst directly provided the
(4) (a) Barigeletti, F.; Flamigni, L.; Balzani, V.; Collin, J.-P.; Sauvage,
J.-P.; Sour, A.; Constable, E. C.; Cargill Thompson, A. M. W. J. Am. Chem.
Soc. 1994, 116, 7692-7699. (b) Barigeletti, F.; Flamigni, L.; Calogero,
G.; Hammarstro¨m, L.; Sauvage, J.-P.; Collin, J.-P. Chem. Commun. 1998,
2333-2334. (c) Hissler, M.; El-ghayoury, A.; Harriman, A.; Ziessel, R.
Angew. Chem., Int. Ed. 1998, 37, 1717-1720. (d) Berhard, S.; Goldsmith,
J. I.; Takada, K.; Abrun˜a, H. D. Inorg. Chem. 2003, 42, 4389-4393. (e)
Duncan, T. V.; Rubtson, I. V.; Uyela, H. T.; Therien, M. J. J. Am. Chem.
Soc. 2004, 126, 9474-9475. (f) Barbieri, A.; Ventura, B.; Barigeletti, F.;
de Nicolar, A.; Quesada, M.; Ziessel, R. Inorg. Chem. 2004, 43, 7359-
7368.
Scheme 2. Sonogashira Coupling of Ethynyl Terpyridine 2
(5) Newkome, G. R.; Patri, A. K.; Holder, E.; Schubert, U. S. Eur. J.
Org. Chem. 2004, 235-254.
(6) (a) Chen, Y.-Y.; Tao, Y.-T.; Lin, H.-C. Macromolecules 2006, 39,
8559-8566. (b) Yuan, S.-C.; Chen, H.-B.; Pei, J. Org. Lett. 2006, 8, 5701-
5704. (c) Winter, A.; Hummel, J.; Risch, N. J. Org. Chem. 2006, 71, 4862-
4871. (d) Constable, E. C.; Cargill Thompson, A. M. W. J. Chem. Soc.,
Dalton Trans. 1992, 3467-3475. (e) Constable, E. C. Chem. Commun. 1997,
1073-1080.
(7) (a) Lohmeijer, B. G. G.; Schubert, U. S. J. Polym. Sci., Part A:
Polym. Chem. 2004, 42, 4016-4027. (b) Hofmeier, H.; Hoogenboom, R.;
Wouters, M. E. L.; Schubert, U. S. J. Am. Chem. Soc. 2005, 127,
2913-2921. (c) Meier, M. A. R.; Wouters, D.; Ott, C.; Guillet, P.;
Fustin, C.-A.; Gohy, J.-F.; Schubert, U. S. Macromolecules 2006, 39, 1569-
1576.
(8) (a) Egbe, D. A. M.; Bader, C.; Klemm, E.; Ding, L.; Karasz, F. E.;
Grummt, U.-W.; Birckner, E. Macromolecules 2003, 36, 9303-9312. (b)
Ding, L.; Egbe, D. A. M.; Karasz, F. E. Macromolecules 2004, 37, 6124-
6131. (c) Kietzke, T.; Egbe, D. A. M.; Hoerhold, H.-H.; Neher, D.
Macromolecules 2006, 39, 4018-4022. (d) Tekin, E.; Wijlaars Holder, E.;
Egbe, D. A. M.; Schubert, U. S. J. Mater. Chem. 2006, 16, 4294-4298.
2346
Org. Lett., Vol. 9, No. 12, 2007

Scheme 3. HWE Reaction of the Aldehyde-Functionalized
Terpyridines 4 and 6
Table 1. Overview of the HWE Reaction of
Phosphonate-Functionalized Terpyridine 5 with Various
Aromatic Dialdehydes
corresponding mono- and bisterpyridines (6 and 7) in
moderate to good yields as depicted in Scheme 2.6a
In a second approach, the HWE reaction of terpyridines
4-6 using KOtBu as a base afforded the desired C-C double
bonds with high E-selectivity, which was proven by 1H NMR
spectroscopy. The general protocol was successfully applied
to synthesize the π-conjugated mono- and bisterpyridines
8-16 in yields higher than 80% (Schemes 3 and 4, Table
1).
Additionally, the method was successfully extended to the
synthesis of a star-shaped π-conjugated tristerpyridine 17
(yield 66%) starting from the phosphonate-functionalized
terpyridine 5 and an aromatic trialdehyde (Scheme 5).11
All terpyridine derivatives shown in this contribution were
fully characterized by NMR spectroscopy, MALDI-TOF
mass spectrometry, and elemental analysis (see Support-
Scheme 4. HWE Reaction of the Phosphonate-Functionalized
Terpyridine 5
Scheme 5. Synthesis of the Star-Shaped Tristerpyridine 17
In comparison, the Horner-Wadsworth-Emmons reaction
was found to be the method of choice for the attachment of
π-conjugated spacers to the terpyridine moiety, as the starting
materials are easily accessible via known procedures and the
yields are significantly higher than for the Pd(0)-catalyzed
coupling reactions.
(9) (a) Wang, J.; Hanan, G. S. Synlett 2005, 1251-1254. (b) Smith, C.
B.; Raston, C. L.; Sobolev, A. N. Green Chem. 2005, 7, 650-654. (c)
Winter, A.; van den Berg, A. M. J.; Hoogenboom, R.; Kickelbick, G.;
Schubert, U. S. Synthesis 2006, 2873-2878.
(10) (a) Jørgensen, M.; Krebs, F. C. J. Org. Chem. 2005, 70, 6004-
6017. (b) Collin, J.-P.; Heitz, V.; Sauvage, J.-P. Tetrahedron Lett. 1991,
32, 5977-5980.
Org. Lett., Vol. 9, No. 12, 2007
2347

ing Information). Because of the long alkoxy chains, an
improved solubility in organic solvents of medium polarity
in comparison to unsubstituted analogous substrates was
achieved.
The photophysical properties of the π-conjugated ter-
pyridines as summarized in Table 2 were investigated by
absorption and emission spectroscopy.
Table 2. Photophysical Properties of Terpyridines 6-17
entry
6
7
8
9
10
11
λabs/nm
λem/nm
Φfl/%
348
440
28
407
478
68
417
480
82
394
459
81
358
437
21
414
479
78
entry
12
13
14
15
16
17
λabs/nm
λem/nm
Φfl/%
392
444
85
424
487
54
423
471
74
419
490
68
406
458
60
433
461
59
As illustrated in Figure 1, strong absorption around 350
nm was observed for the π-conjugated terpyridines 6 and
10. With increasing conjugation, a distinct red-shift of the
absorption maximum (between 60 and 80 nm), together with
a strong increase in the molar extinction coefficient, for all
bis- and tristerpyridines was found. This effect is most
pronounced for the bisterpyridines 13-15 indicating the very
high effective conjugation length of these derivatives.
Figure 1. Normalized absorption (top) and emission spectra
(bottom) of terpyridines in dilute CHCl3 solution (10-6 mol/L).
Emission spectra were obtained upon excitation at the absorption
maximum.
The emission maxima of the terpyridines cover the whole
blue range (425-500 nm). In comparison to the terpyridines
6 and 10 and in similarity to their absorption behavior, the
emission maximum of the higher conjugated derivatives is
also red-shifted. High quantum yields up to 85% were
determined. As a result, the conjugated terpyridines shown
here might be promising candidates for the design of
functional supramolecular materials.
These compounds are suitable building blocks for the design
of supramolecular assemblies, including block copolymers
or chain-extended polymers, by metal complexation. Strong
blue emission with high quantum yields has been observed
for the solutions and thin films. These properties reveal the
potential of these terpyridine systems with respect to potential
applications as functional materials.
We also compared the emission data obtained for dilute
solutions to those of spin-coated thin films. Because of strong
π-π stacking in the soild state, the emission is quenched
significantly. Thin films were obtained by spin-coating the
materials onto glass slides. In comparison to the solutions,
a red-shifting of the emission maxima of approximately 30
nm with quantum yields of about 25% was observed.
Acknowledgment. Financial support of this work by the
Dutch Polymer Institute, the Nederlandse Organisatie voor
Wetenschappelijk Onderzoek (VICI award for U.S.
Schubert), and the Fonds der Chemischen Industrie is
kindly acknowledged.
In conclusion, a library of π-conjugated terpyridines with
various linkages and geometries have been synthesized.
Supporting Information Available: Experimental pro-
cedures and characterization data for all compounds shown
in this contribution. This material is available free of charge
via the Internet at http://pubs.acs.org.
(11) (a) Weber, E.; Hecker, M.; Koepp, E.; Orlia, W.; Czugler, M.;
Cso¨regh, I. J. Chem. Soc., Perkin Trans. II 1988, 1251-1257. (b) Xu, Z.;
Kiang, Y.-H.; Lobkovsky, E. B.; Emmott, N. J. Am. Chem. Soc. 2000, 122,
8376-8391.
OL0707261
2348
Org. Lett., Vol. 9, No. 12, 2007