Control of the arrangement of dipolar orientation of pyrimidine inside the conjugated backbone

Article abstract of DOI:10.1021/ol006877k

equation presented Linear molecules with different arrangement of dipolar pyrimidine moieties in the π-conjugated backbone were synthesized by a Pd-catalyzed Sonogashira coupling reaction. Significant reduction of the HOMO-LUMO energy band gap and improvement of fluorescence quantum yield were observed upon incorporating pyrimidine into the conjugated backbone.

Full text of DOI:10.1021/ol006877k

ORGANIC
LETTERS
2001
Vol. 3, No. 2
173-175
Control of the Arrangement of Dipolar
Orientation of Pyrimidine Inside the
Conjugated Backbone
Ken-Tsung Wong* and Chun Che Hsu
Department of Chemistry, National Taiwan UniVersity, Taipei 106, Taiwan, R.O.C.
kenwong@ccms.ntu.edu.tw
Received November 16, 2000
ABSTRACT
Linear molecules with different arrangement of dipolar pyrimidine moieties in the π-conjugated backbone were synthesized by a Pd-catalyzed
Sonogashira coupling reaction. Significant reduction of the HOMO−LUMO energy band gap and improvement of fluorescence quantum yield
were observed upon incorporating pyrimidine into the conjugated backbone.
Linear organic molecules with extended π-conjugation have
received a lot of attention due to their potential use as
molecular wires.1 Many practical synthetic approaches and
testing methods have been developed for constructing and
evaluating molecular wires of different lengths.2 More
interestingly, there has been significant progress in modifying
the primary structure of the conjugated system for developing
functional molecular wires.3 Control of the regioregularity
in polymerization is an important issue that results in the
formation of polymers with interesting properties by com-
parison with those of their irregular counterparts. Regio-
regular poly(pyridine),4 ploy(3-alkylthiophene),5 and oligo-
(thiophene ethynylenes)6 with precise length have been
synthesized and have proven to have very interesting
optoelectronic, nonlinear properties. We wish to report herein
on the synthesis of a new linear π-conjugated system by
manipulating the orientation of the dipolar component in their
backbone. This approach will ultimately change the dipolar
character of a π-conjugated system that may lead to the
development of an interesting method to probe the structure-
property relationship.
(1) Lehn, J.-M. Supramolecular Chemistry: Concepts and PerspectiVes;
VCH: Weinheim, 1995. Petty, M. C.; Bryce, M. R.; Bloor, D. Introduction
to Molecular Electronics; Edward Arnold: London, 1995.
(2) Tour, J. M. Acc. Chem. Res. 2000, 33, 791.
(3) Marsella, M. J.; Wang, Z.-Q.; Mitchell, R. H. Org. Lett. 2000, 2,
2979. Matsuda, K.; Irie, M. J. Am. Chem. Soc. 2000, 122, 7195. Fernandez-
Acebes, A.; Lehn, J.-M. Chem. Eur. J. 1999, 5, 3285. Jones, N. D.; Wolf,
M. O. Organometallics 1997, 16, 1352. Collbert, M. C. B.; Lewis, J.; Long,
N. J.; White, A. J. P.; Willams, D. J. J. Chem. Soc., Dalton Trans. 1997,
99. Lebreton, C.; Touchard, D.; Pichon, L. L.; Darido, A.; Toupet, L.;
Dixneuf, P. H. Inorg. Chim. Acta 1998, 272, 188. Dembinski, R.; Bartik,
T.; Bartik, B.; Jaeger, M.; Gladysz, J. Am. Chem. Soc. 2000, 122, 810.
Bruce, M. I.; Low, P. J.; Costuas, K.; Halet, J.-F.; Best, S. P.; Heath, G. A.
J. Am. Chem. Soc. 2000, 122, 1949. Kheradmandan, S.; Heinze, K.;
Schmalle, H. W.; Berke, H. Angew. Chem., In. Ed. 1999, 111, 2412.
Guillemot, M.; Toupet, L.; Lapinte, C. Organometallics 1990, 9, 1992.
Bruce, M. I.; Ke, M.; Low, P. J. Chem. Commun. 1996, 2405. Alain, V.;
Blachard-Desce, M.; Ledoux-Rak, I.; Zyss, Joseph Chem. Commun. 2000,
353. Slama-Schwok, A.; Blanchard-Desce, M.; Lehn, J.-M. J. Phy. Chem.
1990, 94, 3894.
A highly electronegative pyrimidine ring was utilized as
the dipolar moiety for incorporation into the conjugated
backbone.7 The chemoselectivity of 5-bromo-2-iodopyrimi-
dine 18 toward the Pd-catalyzed coupling reaction with
(4) Yamamoto, T.; Maruyama, T.; Zhou, Z. H.; Ito, T.; Fukuda, T.;
Yoneda, Y.; Begum, F.; Ikeda, T.; Sasaki, S.; Takezoe, H.; Fukuda, A.;
Kubota, K. J. Am. Chem. Soc. 1994, 116, 4832. Gebler, D. D.; Wang, Y.
Z.; Blatchford, J. W.; Jessen, S. W.; Lin, L.-B.; Gustafson, Wang, H. L.;
Swager, T. M.; MacDiarmid, Esptein, A. J. J. Appl. Phys. 1995, 78, 4264.
(5) McCullough, R. D. AdV. Mater. 1998, 10, 93 and references therein.
(6) Pearson, D. L.; Tour, J. M.J. Org. Chem. 1997, 62, 1367.
10.1021/ol006877k CCC: $20.00 © 2001 American Chemical Society
Published on Web 12/22/2000

terminal alkynes (Sonogashira coupling reaction) provides
us with an effective synthetic strategy for controlling the
dipolar orientation of pyrimidine in the conjugated backbone.
A very convenient one-pot consecutive Sonogashira coupling
procedure has been developed to introduce different alkynyl
groups at the 2- and 5-positions of pyrimidine.9 In the
presence of a catalytic amount of Pd(PPh3)4, 2 was synthe-
sized in 74% yield. 1,4-Diiodo-2,5-dioctylbezene was al-
lowed to react with 2 in the presence of Pd(PPh3)4 to provide
3 (54%). This new linear molecule 3 contains five aromatic
rings including two pyrimidine rings with two pairs of
nitrogen directed away from the central aryl linker (Scheme
1).
similar to 3 but with two pairs of pyrimidine nitrogens
directed toward the central aromatic linker, can be prepared
in 71% yield by coupling 4 with 1,3-di-tert-butyl-5-ethyn-
ylbenzene in the presence of the same catalyst at elevated
temperature (Scheme 2).
Scheme 2a
Scheme 1a
i
a (a) 1, Pd(PPh3)4, CuI, Pr2NH, THF, room temperature, 82%
i
(b) 1,3-di-tert-butyl-5-ethynylbenzene, Pd(PPh3)4, CuI, Pr2NH,
THF, reflux, 71%.
Synthetic pathways toward linear molecules possessing the
same arrangement of the dipolar orientation of pyrimidine
in the conjugated backbone are more complicated. First,
1-iodo-4-(trimethylsilylethynyl)-2,5-dioctylbenzene10 was
treated with 2 at 80 °C in the presence of Pd(PPh3)4 to afford
intermediate 6 in 94% yield. Removal of the TMS group
from 6 followed by coupling with 1 afforded 7 in 90% yield.
Further treatment of 7 with 1,3-di-tert-butyl-5-ethynylben-
zene in the presence of Pd catalyst gave the desired
compound 8 in 79% yield (Scheme 3).
a (a) i: 1,3-Di-tert-butyl-5-ethynylbenzene, Pd(PPh3)4, CuI,
iPr2NH, THF, 30 °C, ii: Me3SiCtCH, reflux, iii: 2 N NaOH, THF/
MeOH, 74%; (b) 1,4-diiodo-2,5-dioctylbenzene, Pd(PPh3)4, CuI,
iPr2NH, THF, reflux, 54%.
Treatment of 1 with 1,4-diethynyl-2,5-dioctylbezene in the
presence of Pd(PPh3)4 afforded dibromo compound 4 in 82%
yield after purification on silica gel. Linear molecule 5,
Scheme 3a
(7) Gommper, R.; Mari, H.-J.; Polborn, K. Synthesis 1997, 696. Kanbara,
T.; Kushida, T.; Saito, N.; Kuwajima, I.; Kubota, K.; Yamamoto, T. Chem.
Lett. 1992, 583.
(8) Goodby, J. W.; Hird, M. H.; Lewis, R. A.; Toyne, K. J. Chem.
Commun. 1996, 2719.
(9) One-pot consecutive coupling procedure: Under argon, a solution
of 5-bromo-2-iodopyrimidine (2.85 g, 10.0 mmol), Pd(PPh3)4 (580 mg, 0.5
i
mmol), CuI (100 mg, 0.5 mmol), and Pr2NH (2.67 mL, 19.0 mmol) in
THF (40 mL) was stirred for 3 min, and to this was added dropwisely over
a period of 3 mim 1,3-di-tert-butyl-5-ethynylbenzene (2.36 g, 11.0 mmol)
in THF (10 mL). The mixture was stirred for 2.5 h at 30 °C. Trimethyl-
i
silylacetylene (1.70 mL, 12.0 mmol) and Pr2NH (2.67 mL, 19.0 mmol)
were introduced quickly, and the mixture was heated to reflux for 1.5 h.
After cooling to room temperature, the ammonium salt formed was removed
by filtration through a short column of aluminum oxide and washed with
ether (2 × 20 mL). The combined filtrate was concentrated to dryness under
reduced pressure. The crude product was dissolved in THF (20 mL) and
methanol (10 mL), and to this solution was introduced 2.0 N NaOH (5
mL) to remove the silyl group in 20 min at room temperature. After
concentrating in vacuo, water (20 mL) was added, and the solution was
extracted with ether (2 × 25 mL). The combined organic solution was dried
(MgSO4) and concentrated. The residue was purified by column chroma-
tography on silica gel (EtOAc:hexanes ) 1:12) to afford 2 as pale yellow
solid (2.34 g, 74%): mp 202-204 °C; IR (KBr) 3274 (s), 2957 (m), 2219
(s), 1572 (m), 1414 (s), 1180 (m), 883 (m), 796 (m), 706 (m), 641 (m)
a (a) 1-Ethynyl-4-iodo-2,5-dioctylbenzene, Pd(PPh3)4, CuI, iPr2NH,
THF, room temperature, 94%; (b) i: 2 N NaOH, THF/MeOH, ii:
i
1, Pd(PPh3)4, CuI, Pr2NH, THF, room temperature, 90%; (c) 1,3-
di-tert-butyl-5-ethynylbenzene, Pd(PPh3)4, CuI, iPr2NH, THF, reflux,
1
cm-1; H NMR (CDCl3, 400 MHz) 1.32 (s, 18H), 3.47 (s, 1H), 7.49 (t, J
79%.
) 1.8 Hz, 1H), 7.55 (d, J ) 1.8 Hz, 2H), 8.81 (s, 2H); 13C NMR (CDCl3,
100 MHz) 31.5, 35.1, 85.8, 92.3, 116.1, 120.2, 124.8, 127.4, 151.3, 151.7,
159.9; MS (m/z, FAB+) 316 (100), 301 (15), 261 (9), 245 (6), 224 (4), 154
(15), 136 (11), 89 (4), 58 (25); HRMS cacld for C22H25N2 317.2018, found
317.2028. Anal. Cacld for C22H24N2: C, 83.50; H, 7.64; N, 8.85. Found:
C, 83.69; H, 7.38; N, 8.68.
Linear molecule 10 (Scheme 4) prepared in a similar
manner as that for 5 was employed as a reference compound.
174
Org. Lett., Vol. 3, No. 2, 2001

Scheme 4a
i
a (a) 1-Bromo-4-iodobenzene, Pd(PPh3)4, CuI, Pr2NH, THF,
room temperature, 91%; (b) 1,3-di-tert-butyl-5-ethynylbenzene,
i
Pd(PPh3)4, CuI, Pr2NH, THF, reflux, 92%.
Our synthetic protocol provides a successful control of
the dipolar orientation of the pyrimidine moiety in the
conjugated backbone. The optical properties are demonstrated
in their absorption and photoluminescence (PL) spectra
(Figure 1).
Figure 1. Normalized absorption (CHCl3, 1 × 10-5 M) and
photoluminescence (CHCl3, 5.5 × 10-6 M) spectra of pyrimidine-
containing linear compounds 3, 5, and 8 and model compound 10.
inside a defined conjugated backbone. Significant reduction
of the HOMO-LUMO energy band gap and improvement
of fluorescence quantum yield were observed upon incor-
porating pyrimidine into the conjugated backbone. Further
studies on the dependence of electochemical properties on
the different arrangement of dipolar orientation are in
progress.
Linear molecules 3, 5, and 8 reveal very similar behavior
in their photophysical properties. Significant red shifts of 3,
5, and 8 in their absorption maximum (λmax 371 nm)
compared with that of the corresponding model compound
10 (λmax 311 nm) were observed. By inspecting the edge of
absorption spectra, the energy band gaps for 3, 5, and 8 were
estimated to be ca. 0.2 eV smaller than that of 10. All the
pyrimidine-containing molecules 3, 5, and 8 exhibit strong
blue fluorescence with emission maximum at 406 nm and a
weaker band centered on 428 nm. The photoluminescence
also indicates substantial red shifts by 38 nm relative to that
of 10 (364 nm, 384 nm). The PL efficiency of 3, 5, 8, and
10 are 0.63, 0.61, 0.75, and 0.34, respectively. Apparently,
the incorporation of pyrimidines inside a defined π-conju-
gated system increases the quantum yield of fluorescence.
In conclusion, we have demonstrated methods for control-
ling the arrangement of dipolar orientation of pyrimidine
Acknowledgment. We gratefully thank the National
Science Council (NSC 89-2113-M-002-008) for financial
support.
1
Supporting Information Available: H NMR spectra of
3, 4, 5, 6, 7, 8, 9, and 10 and detailed experimental
procedures, full characterization. This material is available
free of charge via the Internet at http://pubs.acs.org.
(10) Prepared by the literature procedure: Davey, A. P.; Elliott, S.;
O’Connor, O.; Blau, W. J. Chem. Soc., Chem. Commun. 1995, 1433.
OL006877K
Org. Lett., Vol. 3, No. 2, 2001
175