Syntheses and Tunable Emission properties of 2-alkynyl azulenes

Article abstract of DOI:10.1021/ol300327b

Various substituted 2-azulenes have been synthesized via Sonogashira coupling. Doping with superacids allows tunable emission from 443 to 750 nm depending on the substitution. The proton doped compounds are the first azulene alkyne based systems that show emission originating only from the S₁ excited state.

Full text of DOI:10.1021/ol300327b

ORGANIC
LETTERS
2012
Vol. 14, No. 6
1580–1583
Syntheses and Tunable Emission
Properties of 2-Alkynyl Azulenes
Michael Koch, Olivier Blacque, and Koushik Venkatesan*
Institute of Inorganic Chemistry, University of Zurich, Winterthurerstrasse 190,
CH-8057 Zurich, Switzerland
venkatesan.koushik@aci.uzh.ch
Received February 9, 2012
ABSTRACT
Various substituted 2-azulenes have been synthesized via Sonogashira coupling. Doping with superacids allows tunable emission from 443 to
750 nm depending on the substitution. The proton doped compounds are the first azulene alkyne based systems that show emission originating
only from the S1 excited state.
In recent years pushꢀpull chromophores have been exten-
sively studied owing to their promising optoelectronic pro-
perties,1 especially second- and third-order optical nonlinea-
Scheme 1. Synthesis of 2-Bromoazulene 3
rities (NLO).2,3 A typical organic pushꢀpull chromophore
consists of an electron acceptor and a donor bridged by a π-
conjugated spacer.4 This arrangement facilitates efficient
intramolecular charge transfer and enables further tuning
of the polarizability of the chromophore. Additionally, a
planar structure ensures the π-conjugation is retained over
S2 state tothe S0 state, due tothe violation of Kasha’s rule.8
the entire donorꢀspacerꢀacceptor system.5
The emission from S1 to S0 can only be observed with very
Azulene is known for its intense blue color which is a
little intensity.9 Since the S2 to S0 emission is batho-
result of its interesting electronic properties. Despite the
chromically shifted with respect to the S1 to S0 emission, it is
fact that azulene possesses donorꢀacceptor character, its
desirable to alter the azulene structure in such a way that
optoelectronic properties have been rarely explored.6,7
the S1 to S0 transition becomes the dominant emission
This is due to azulene’s low emission quantum yields
pathway and which is expected to allow the preparation of
owing to the domination of fluorescence from the excited
NIR emitters based on azulene. 1- and 1,3-disubstituted
alkynyl azulenes are widely known, and their properties
have been extensively investigated in contrast to the 2-
alkynyl azulenes that have been explored only rarely.10,11
(1) Ito, S.; Morita, N. Eur. J. Org. Chem. 2009, 4567–4579.
(2) Szablewski, M.; Thomas, P. R.; Thornton, A.; Bloor, D.; Cross,
ꢀ
G. H.; Cole, J. M.; Howard, A. K.; Malagoli, M.; Meyers, F.; Bredas,
J.-L.; Wenseleers, W.; Goovaerts, E. J. Am. Chem. Soc. 1997, 119, 3144–
3154.
(8) Turro, N. J. Modern Molecular Organic Photochemistry; Univer-
sity Science Books: Sausalito, CA, 1991.
(9) Kim, S. Y.; Lee, G. Y.; Han, S. Y.; Lee, M. Chem. Phys. Lett.
(3) Pasini, D.; R.ighetti, P. P.; Rossi, V. Org. Lett. 2002, 4, 23–26.
(4) Kival, M.; Diederich, F. Acc. Chem. Res. 2009, 42, 235–248.
(5) Kato, S.; Diederich, F. Chem. Commun. 2010, 46, 1994–2006.
(6) Lacroix, P. G.; Malfant, I.; Iftime, G.; Razus, A. C.; Nakatani,
K.; Delaire, J. A. Chem.;Eur. J. 2000, 6, 2599–2608.
(7) Gompper, R.; Wagner, H.-U. Angew. Chem., Int. Ed. Engl. 1988,
27, 1437–1455.
2000, 318, 63–68.
(10) Fabian, K. H. H.; Elwahy, A. H. M.; Hafner, K. Eur. J. Org.
Chem. 2006, 791–802.
(11) Fabian, K. H. H.; Elwahy, A. H. M.; Hafner, K. Tetrahedron
Lett. 2000, 41, 2855–2858.
r
10.1021/ol300327b
Published on Web 03/08/2012
2012 American Chemical Society

Scheme 2. Sonogashira Coupling and Deprotection of 4
Ithasbeenpreviouslyshownthatproton doping of azulene
leads to an azulenium cation with the proton at the
1-position.12 It was expected that substitution with alkyne
at the 2-position of the azulene would not alter the con-
jugation after doping and enhance the luminescence pro-
perties by switching the emission from the S1 to S0 state.
Also the 2-substitution should retain the linearity of the
molecule, which should favor charge delocalization over
the entire molecule, since the dipole moment is expected be
maximum in this configuration. One of the main reasons
for the rare exploration of the 2-alkynyl azulene moiety is
due to the daunting synthetic access associated with this
class of molecules. In this work, we report a stepwise pro-
tocol for the good to high yielding synthesis of 2-alkynyl
azulenes starting from the easily accessible diethyl 2-
hydroxyazulene-1,3-dicarboxylate. Further, detailed photo-
physics in conjunction with TD-DFT calculations of the
various 2-alkynyl azulenes have been examined in both the
doped and undoped state.
with aryl halides. Although the reaction times were longer
and the yields of this second approach were generally lower
thanthose for thecouplingofthe 2-bromoazulene, thiswas
a convenient procedure for substituents whose corre-
sponding alkynes are not commercially available or the
synthesis comprises tedious steps. Single crystal X-ray
diffraction studies carried out on6, 7, 8, 10, and 15revealed
a linear planar configuration.14 In all cases the aromatic
ring directly attached to the 2-ethynylazulene shows alter-
nating bond distances15 which is suggestive of the π
electrons from azulene being pushed into the π* orbital
of the attached ring. This is strongly indicative of the
electronic nature of the azulene alkynes behaving as a
pushꢀpull system.
Diethyl 2-hydroxyazulene-1,3-dicarboxylate 1 was ther-
mally dealkoxycarboxylated in a DMF/water mixture in
the presence of an excess of LiCl to give 2-hydroxyazulene
2 in 91% yield (Scheme 1). 2 was further reacted with PBr3
in toluene at 95 °C to give 2-bromoazulene 3 in 71% yield
as a violet powder.13 This was utilized as the key starting
materialtoprepare the corresponding 2-alkynylazulene by
reacting in Sonogashira coupling conditions that include
an alkyne with catalytic amounts of CuI and Pd(PPh3)2Cl2
in the presence of an excess amount of Et3N (general
coupling procedure I). Various substituted 2-ethynylazulenes
have been synthesized starting from 2-bromoazulene
(Scheme 2). The coupling product 4 was deprotected
almost quantitatively to give 5, which then was further
used as a starting material for the reactions using Sonoga-
shira coupling conditions (general coupling procedure II)
Figure 1. Thermal ellipsoid plot of 8 (30% probability level of
thermal ellipsoids) with selective atomic numbering scheme.
˚
Selected bond distances (A) and angles (deg): C4ꢀC5 1.427(2),
C5ꢀC6 1.191(2), C6ꢀC7 1.423(2), C4ꢀC5ꢀC6 180.0,
C5ꢀC6ꢀC7 180.0.
TD-DFT calculations performed at the PBE1PBE/
6-31þG(d) level15 indicate that the absorption bands of 8
(Figure 1) at 394 and 310 nm correspond to the singletꢀ
singlet excitations from its electronic ground state S0 to
the higher electronic states S2 (386 nm, f = 0.911) and
(12) Wang, X.; Ng, J. K.-P.; Jia, P.; Lin, T.; Cho, C. M.; Xu, J.; Lu,
X.; He, C. Marcromolecules 2009, 42, 5534–5544.
(13) Ito, S.; Nomura, A.; Morita, N.; Kabuto, C.; Kobayashi, H.;
Maejima, S.; Fujimori, K.; Yasunami, M. J. Org. Chem. 2002, 67, 7295–
7302.
(14) CCDC 858052ꢀ858056 contain the supplementary crystallo-
graphic data for this paper. These data can be obtained free of charge
via www.ccdc.cam.ac.uk.
(15) See Supporting Information.
Org. Lett., Vol. 14, No. 6, 2012
1581

S4 (301 nm, f = 1.400), respectively. The major contribu-
tion to S0fS2 is from the HOMO-1fLUMO transi-
tion with the orbitals delocalized all over the molecule
(πallfπ*all), while S0fS4 is mainly from the HOMO-
fLUMOþ1 transition with molecular orbitals exclusively
located on the azulene moiety (πazulenefπ*azulene). The
lowest excitation S0fS1 calculated at 537 nm from
HOMOfLUMO (πazulenefπ*all) exhibits a quite small
oscillator strength f of 0.017 and is not experimentally
observed. The wavelength of the absorption maximum can
be tuned from 372 nm (5) to 442 nm (13) using different
substituents at the 2-position of the azulene resulting in
emission wavelengths between 400 nm (5) and 474 nm (13)
(Table 1). In the emission spectra only a weak fluorescence
Figure 3. Doping of 8 with superacids; DFT calculations of 8
and 8-Hþ.
is no longer observed in 8-Hþ (Figure 3). According to
Kasha’s rule, which states that the fluorescence emission is
expected in appreciable yield only from the lowest excited
state S1, the proton doping of 8 resulted in an increase in
the fluorescence intensity by a direct S1 to S0 emission. To
the best of our knowledge the obtained azulene alkynes in
the proton doped state are the first examples of azulene
alkyne based small molecules that strongly emit only from
the S1 to S0 state. Proton doping showed a huge effect on
the absorption properties of the alkyne azulenes.
The intensity of the HOMOꢀLUMO band increased
significantly while the HOMO-1-LUMO band disap-
peared (Figure 4). This is indicative of the absence of the
S2 state in the protonated molecule. This results in an
emission solely from the S1 stateand theincreaseinintensity
depending strongly on the substitution, which results in a
Figure 2. Emission spectra of 8 in CH2Cl2 (black line). After
treatment: with TBAF (blue line), with nPr4NBF4 (green line),
and with TFA (red line).
was observed when excited at the wavelength of the HOMOꢀ
LUMO transition. Doping with an anion resulted in an
increase in the fluorescence intensity without altering the
emission wavelength (Figure 2). In the case of 8 the presence
of fluorine functional groups even roughly doubled the in-
tensity, while the presence of BF4ꢀ ions increased the intensity
by a factor of ∼12.
Recently it has been shown that doping with superacids
leads to a bathochromic shift of the emission band
in oligomers of azulene.16 After MeSO3H doping, the
absorption spectrum of the cationic species 8-Hþ shows a
single red-shifted band (in comparison with 8) at 440 nm.17
This band corresponds to the lowest singletꢀsinglet ex-
citation S0fS1 (437 nm, f = 1.238) and consists of an
intramolecular charge transfer from the HOMO to the
LUMO with the two orbitals delocalized all over the
molecule (πallfπ*all). It is important to note that the
πazulenefπ*azulene transition (HOMOfLUMOþ1 for 8)
(16) Amir, E.; Amir, R. J.; Campos, L. M.; Hawker, C. J. J. Am.
Chem. Soc. 2011, 133, 10046–10049.
(17) Heilbronner, E.; Simonetta, M. Helv. Chim. Acta 1952, 35, 1049–
Figure 4. UVꢀvis absorption spectra of 8 in CH2Cl2. Black line
undoped; gray line treated with MeSO3H.
1065.
1582
Org. Lett., Vol. 14, No. 6, 2012

Table 1. Absorption and Emission Data of Undoped and
Doped Species in CH2Cl2 at rt
undoped
max(abs) max(emm)
dopeda
max(abs) max(emm)
λ
λ
λ
λ
compd
[nm]
[nm]
[nm]
[nm]
azulene
340
381
372
401
400
394
402
412
413
414
442b
394
414
376
410
400
424
424
424
430
425
424
426
474
424
422
355
417
394
471
473
440
452
488
450
506
608
412
509
422
467
463
533
539
487
497
535
490
649
750c
443
592
4
5
6
7
8
9
10
11
12
13
14
15
Figure 5. Emission spectra in CH2Cl2 after treatment with
MeSO3H of 6 (black), 8 (red), 12 (blue), 14 (green), and 15
(gray).
a Doping with MeSO3H. b Shoulder. c Very weak intensity and very
broad band.
upon doping with superacids, these compounds showed a
strong shift from blue to red emission in the electromag-
netic spectrum originating from the S1 state. Strong emis-
sion from this state has been unprecedented for azulene
alkyne based compounds to date. Based on their facile
synthesis and interesting emission properties, azulenes
are interesting candidates for organic optoelectronic
materials.
factor of roughly up to 100 (Supporting Information). Tun-
ing of the emission properties over a wide range of wave-
lengths from 443 to 750 nm was achieved with the electron-
donating or extended conjugation leading to emission with
longer wavelengths, while the electron-withdrawing sub-
stituents result in shorter emission wavelengths (Figure 5).
It is worth noting that the doping process is reversible and
upon neutralization with Et3N the absorption and emission
properties of the undoped species completely recovered.
According to expectations the doped 8 shows the most
relative emission intensity. The huge change in the emission
wavelength upon doping as well as the significant increase in
its intensity makes this system amenable to use in various
scaffold in sensors.
Acknowledgment. This work was supported by the
Swiss National Science Foundation NRP 62 Smart Mate-
rials Program (Grant No. 406240-126142). Support also
€
from the University of Zurich and Prof. H. Berke (Institute
of Inorganic Chemistry, University of Zurich) is gratefully
€
acknowledged.
In conclusion, we have described a facile route to a series
of alkyne substituted azulenes synthesized via Sonogashira
coupling by utilizing two different approaches. The ob-
tained azulene alkynes possess high molar absorption co-
efficients and exhibit weak luminescence. The wavelength
of the emission maximum does not change upon anion
doping, but its intensity increases significantly. However,
Supporting Information Available. Experimental pro-
cedures, spectral characterization data, computational
details, crystallographic data and CIF files for com-
pounds 6, 7, 8, 10, and 15. This material is available free
of charge via the Internet at http://pubs.acs.org.
The authors declare no competing financial interest.
Org. Lett., Vol. 14, No. 6, 2012
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