Impact of 2,6-connectivity in azulene: Optical properties and stimuli responsive behavior

Article abstract of DOI:10.1039/c3tc31610f

The possible incorporation of the redox active, small donor acceptor molecule azulene as a core structure for potential optoelectronic applications has been evaluated. The synthesis and characterization of different isomers of di(phenylethynyl)azulene 1-4 have been successfully carried out. The photophysical properties as well as their stimuli responsive behavior reflecting their corresponding electronic properties were also investigated. The experimental observations show that 2,6-connection (3) exhibits intense luminescence and shows the strongest stimuli responsive behavior upon acid treatment. DFT calculations of 3 reveal the highest contributions of the alkyne substituents at the 2- and 6-position of the azulene to the ground- and excited state that makes the 2,6-connectivity at azulene a very promising candidate for low band-gap conjugated materials.

Full text of DOI:10.1039/c3tc31610f

Journal of
Materials Chemistry C
View Article Online
View Journal | View Issue
PAPER
Impact of 2,6-connectivity in azulene: optical properties
and stimuli responsive behavior†
Cite this: J. Mater. Chem. C, 2013, 1,
7400
*
Michael Koch, Olivier Blacque and Koushik Venkatesan
The possible incorporation of the redox active, small donor acceptor molecule azulene as a core structure
for potential optoelectronic applications has been evaluated. The synthesis and characterization of
different isomers of di(phenylethynyl)azulene 1–4 have been successfully carried out. The photophysical
properties as well as their stimuli responsive behavior reflecting their corresponding electronic
properties were also investigated. The experimental observations show that 2,6-connection (3) exhibits
intense luminescence and shows the strongest stimuli responsive behavior upon acid treatment. DFT
calculations of 3 reveal the highest contributions of the alkyne substituents at the 2- and 6-position of
the azulene to the ground- and excited state that makes the 2,6-connectivity at azulene a very
promising candidate for low band-gap conjugated materials.
Received 15th August 2013
Accepted 16th September 2013
DOI: 10.1039/c3tc31610f
www.rsc.org/MaterialsC
at the unsubstituted 1-position with the concomitant
increase in luminescence intensity. This is accompanied by a
Introduction
Design and synthesis of novel conjugated polymers have
been intensely investigated over the last two decades due to
their extensive application as optoelectronic materials.1–9 In
particular, conjugated polymers have been utilized in a wide
variety of transduction schemes to realize sensitive and
highly selective detection of chemical and biological analy-
tes by taking advantage of their stimuli responsive
behavior.10–17 Due to azulene's unique electronic and optical
properties such as large permanent dipole moment, intense
blue color and the domination of uorescence from S2 / S0
with low emission intensity, due to the violation of Kasha's
rule,18–20 it has been long considered to be an interesting
candidate for application in optoelectronic materials
(see Fig. 1).21,22
bathochromic shi of the emission wavelength especially in
the case of ethynyl spacer bearing groups. The S1 / S0
transition becomes the dominant radiative decay pathway of
the protonated species.24–26 Such a response of azulene
towards superacids not only allows one to control the emis-
sion properties, but also permits to tailor the conducting
properties of the azulene as recently demonstrated by
Hawker and co-workers.27
Despite quite a large number of theoretical calculations on
the inuence of the functional bis-substitution pattern of azu-
lenes on their electronic properties, the experimental investi-
gations have been rather limited.28,29 Based on theoretical
calculations, the 2,6-connected polyazulenes are expected to be
most unique compared with other substitution patterns. The
p-electrons can easily delocalize along the direction of
the dipolar C2v axis of azulene resulting in a large dipole
moment and large hyperpolarizability. This is especially sup-
ported by the ability to form an expected quinoid structure.
2,6-connected azulene systems would thus possess a lower band
gap than the otherwise connected azulene polymers, which is
also highly interesting for preparing low band-gap conducting
However, the evaluation of azulene as a core structure for
optoelectronic applications remains scarce, although calcu-
lated theoretical results using various aromaticity theories
suggest that azulene possesses a much lower delocalization
energy (4.2 kcal molꢀ1) in comparison with benzene (20 kcal
molꢀ1), thiophene (16.1 kcal molꢀ1
) and naphthalene
(30.5 kcal molꢀ1).23 Recently Hawker's group and our group
reported on the stimuli responsive behavior of azulene
derivatives. The treatment of azulene derivatives with
superacids results in an azulenium cation upon protonation
Institute of Inorganic Chemistry, University of Zurich, Winterthurerstrasse 190,
CH-8057 Zurich, Switzerland. E-mail: venkatesan.koushik@aci.uzh.ch; Fax: +41 44-
635-68-03; Tel: +41 44-635-46-94
† Electronic supplementary information (ESI) available: Contains experimental
details, characterization data and various NMR spectra. Additional gures
showing molecular orbitals, calculated spectra and Cartesian coordinates are
also provided. See DOI: 10.1039/c3tc31610f
Fig.
1
Mesomeric structure of azulene with the numbering scheme and
permanent dipole moment.
7400 | J. Mater. Chem. C, 2013, 1, 7400–7408
This journal is ª The Royal Society of Chemistry 2013

View Article Online
Paper
Journal of Materials Chemistry C
polymers for application in organic solar cells.29,30 In these
polymers, the highest symmetry of C2v will be preserved in the
planar form, while in all the other possible polymers of azulene
a much lower symmetry is expected because in the latter cases
the monomer symmetry axis does not equal the polymeric
Synthesis
Since the synthesis of 1 has been already known in the litera-
ture,32 our detailed discussion pertains only to the synthesis and
properties of the new compounds 2, 3 and 4. As previously
mentioned, we utilize different approaches as shown in Scheme
1.46 1 was synthesized starting from the dark green colored
1,3-diiodoazulene 6, which in turn was prepared in excellent
yields (98%) by reacting the commercially available compound
5 with 2 eq. N-iodosuccinimide (NIS) in dry CH2Cl2. Subsequent
application of Sonogashira coupling conditions consisting of an
excess amount of phenyl acetylene and catalytic amounts of
CuI, PPh3 and Pd(PPh3)2Cl2 in the presence of an excess amount
of Et3N at elevated temperature gave 1 as a blue green colored
solid in very good yield (91%).
Compound 2 was prepared from the known 2-phenyl-
ethynylazulene 7.26 The mono-alkyne 7 was treated with 1 eq.
NIS in dry toluene below 10 ꢁC to minimize diiodination, to give
green 1-iodo-2-phenylethynylazulene 8 which decomposes in
solid state and is stable in solution only for a couple of days.
Therefore the ltered reaction mixture was directly utilized in
the Sonogashira coupling reaction leading to green 2 in a good
overall yield (91%).
Despite the synthesis of 3 involving many steps due to the
challenging substitution pattern, we were able to obtain the
targeted compound in an overall yield of 21%. A direct synthesis
of the 2,6-pattern is not feasible since it requires an electro-
philic attack at the electron rich ve membered ring for the
substitution of the 2-position which is more favored at the more
nucleophilic 1- and 3-positions, while on the other hand a
mandatory nucleophilic attack at the electron poor seven
membered ring will be directed to the 4- and 8-position and not
result in the desired 6-substitution. Therefore the parent azu-
lene 5 cannot be directly used as a precursor for the synthesis of
2,6-dialkyne azulene, but instead starting material (9), based on
the work of Nozoe and co-workers and the work of Ito and co-
workers,39,47,48 was utilized in which the 1- and 3-positions
bearing ethyl ester protected groups shielded the 4- and 8-
position from a nucleophilic attack.
Starting from the orange colored diethyl 2-hydroxyazulene-
1,3-dicarboxylate 949 the hydroxyl-group was alkylated with
methyl iodide to give the known bright red product 10 in very
good yields (91%). This product was brominated at the
6-position to result in the known orange product 11 in 85%
yield. The ester-functionalities act as protection groups in this
step. Due to sterical hindrance, the nucleophilic bromination
at 4- and 8-positions is blocked resulting in an exclusive
bromination at the 6-position. Sonogashira coupling with
phenyl acetylene gave the mono-alkyne product 12 in 83%
yield, which was subsequently thermally dealkoxycarboxylated
to give 2-hydroxy-6-phenylethynyl-azulene 13 in 56% yield.
Conversion of the hydroxyl-group with PBr3 to 14 (68%) fol-
lowed by Sonogashira coupling gave eventually 3 as a dark
green product in 96% yield.
symmetry axis. Also, the polymer consisting of only
a
2,6-connectivity will show a head-to-tail alignment of the
monomeric dipole moments.
While there are many examples of mono-alkyne substituted
azulenes reported in the literature, dialkyne substituted azu-
lenes remain scarce except for the ones exclusively bearing the
alkynes at the 1- and 3-position of the azulene.26,31–38 The only
known 2,6-bissubstituted compounds either bear an alkyne or
an alkyl chain at the 6 position, or they are connected to a metal
center as a bridging ligand or, are part of a 6,6'-connected
azulene dimer.39–42
In this work, we report the systematic synthesis of 2,6-
diphenyl acetylene substituted azulene and investigation of its
stimuli responsive behavior as a model compound for macro-
molecules. Di(phenylethynyl) substitution was particularly
chosen in order to allow for easy correlation of the properties as
model compounds. For comparison reasons, we have also
prepared the three other isomers of di(phenylethynyl)azulenes
1, 2 and 4 (see Fig. 2).43
Synthesis of specic substitution patterns on azulene is
quite challenging and difficult to achieve,44,45 and since there
is no common starting material available to prepare the
target molecules, different approaches had to be sought to
accomplish the desired molecules (see Scheme 1). Given the
unique interesting properties expected to arise from the
2,6-connectivity in azulene, we set out to develop a synthetic
strategy that would not only allow one to achieve the tar-
geted 2,6-diphenyl substituted azulene, but would also offer
considerable modularity in synthesis, starting from
2-bromo-6-phenylethynyl azulene (14). To the best of our
knowledge, this work represents the rst such experimental
study combined with a deeper theoretical insight provided
by DFT calculations into the optoelectronic behavior of an
azulene core including the targeted synthesis and photo-
physical evaluation of the elusive 2,6-alkyne substituted
azulene 3.
4 was synthesized as a blue colored product in very good
yields (93%) starting from the known 4,7-dibromoazulene 15
under Sonogashira conditions.25
Fig. 2 Structures of di(phenylethynyl)azulenes 1, 2, 3 and 4.
This journal is ª The Royal Society of Chemistry 2013
J. Mater. Chem. C, 2013, 1, 7400–7408 | 7401

View Article Online
Journal of Materials Chemistry C
Paper
Scheme 1 Synthesis of di(phenylethynyl)azulenes 1, 2, 3 and 4.
7402 | J. Mater. Chem. C, 2013, 1, 7400–7408
This journal is ª The Royal Society of Chemistry 2013

View Article Online
Paper
Journal of Materials Chemistry C
Table 1 Absorption and emission data (nm) of unprotonated and protonated
UV-Vis studies
species in CH2Cl2 at rt
The UV-Vis absorption spectra of 1–4 can be classied into three
regions: the rst region for l > 500 nm which is composed of the
S0 / S1 transitions, the second region for 370 nm < l < 500 nm
showing the S0 / S2 transitions and the third region for l < 370
nm comprising the S0 / S3 transitions.
Unprotonated
Protonateda
lmax
lmax
lmax
lmax
Compd.
(abs)
(em)
(abs)
(em)
5b
7b
1
2
3
340
401
423
418d
446
409
376
424
465
432
484
416
355
471
703
689e
525
490
422
533
722c
733
571
537
The UV-Vis absorption spectra of 1–4 exhibit high molar
extinction coefficients in the region l < 500 nm (see Fig. 3). In
the region of l > 500 nm compound 4 possesses the most broad
absorption band between 520 nm and 800 nm among the
synthesized compounds; however the intensity of the band was
low and similar to that found for the parent azulene (5).
A similar band could only be detected at much higher concen-
trations in the spectra of 1, 2 and to an even lesser extent for 3.
This difference can be ascribed to the fact that the desired
electronic contribution of the substituents to the transition is
less pronounced for 4 while it is strongest for 3 (see also DFT
calculations). In the region that is assigned to the S0 / S2
transition (ca. 370–500 nm) 1 and 2 reveal only a small molar
extinction coefficient in comparison to 3 and 4. It is evident
from this observation that the position of the substitution alters
the probability of the two possible transitions that govern azu-
lene's absorption behavior. This is especially the case for 3 that
shows similar intensities for both transitions. Additionally 3
shows the highest molar extinction coefficient in this red shif-
ted region (44000 Mꢀ1 cmꢀ1 compared to 10400 Mꢀ1 cmꢀ1 (1),
14000 Mꢀ1 cmꢀ1 (2), 34000 Mꢀ1 cmꢀ1 (4)). Since azulene's
uorescence results from excited state S2, a transition in this
wavelength region with a high molar absorption coefficient is
crucial (see Table 1). Furthermore tuning of this band to longer
wavelength is desired due to the fact that this correlates with a
smaller energy gap of the transition and better polarizability of
the molecule. In the region assigned to the S0 / S3 transition
(l < 370 nm), the extinction coefficients as well as the absorp-
tion wavelength vary only to a certain extent.
4
Treated with MeSO3H b Literature values.26 Extremely weak emission.
Shoulder. e Very broad band with low intensity.
a
c
d
Fig. 4 Plausible structures of di(phenylethynyl)-azulenes after treatment with
superacids.
Fig. 5 Left: Photo of 3 (CH2Cl2, treated with MeSO3H, treated with MeSO3H (exc.
at 365 nm)). Right: Photo of 4 (CH2Cl2, treated with MeSO3H, treated with
MeSO3H (exc. at 365 nm)).
Azulene's optical behavior is tremendously changed upon
protonation (see Fig. 4 and 5), since the emission of the
protonated species results from the S1 states. The correspond-
ing S0 / S1 transition is observed in the UV-Vis absorption
spectra in the region of ca. 420–580 nm for 1-H+–4-H+. Aer
protonation, signicant changes in the UV-Vis absorption
spectra were observed.50
4-H+ showed a very distinct absorption band centered
around 490 nm with a high molar extinction coefficient
(3 ¼ 57800 Mꢀ1 cmꢀ1, see Fig. 6). 3-H+ exhibited an even more
distinct absorption band combined with the highest molar
extinction coefficient in comparison with the rest of the
protonated isomers (525 nm, 3 ¼ 77100 Mꢀ1 cmꢀ1) and in fact
even higher than the extinction coefficients of the untreated
isomers 1–4. Although compounds 1-H+ and 2-H+ also display
an absorption band in this region, they are not distinct and have
signicantly lower intensities, especially in the case of 1-H+.
1-H+ and 2-H+ display aer protonation an additional broad but
low intensity band with a bathochromic shi at 703 nm (1-H+)
and 689 nm (2-H+) respectively (see Fig. 6), which is neither
Fig. 3 UV-Vis absorption spectra of 1 (black line), 2 (red line), 3 (green line) and 4
(blue line) in CH2Cl2.
This journal is ª The Royal Society of Chemistry 2013
J. Mater. Chem. C, 2013, 1, 7400–7408 | 7403

View Article Online
Journal of Materials Chemistry C
Paper
Fig. 8 UV-Vis emission spectra of 1-H+ (black line), 2-H+ (red line), 3-H+ (green
line) and 4-H+ (blue line) in CH2Cl2.50
Fig. 6 UV-Vis absorption spectra of 1-H+ (black line), 2-H+ (red line), 3-H+ (green
line) and 4-H+ (blue line) in CH2Cl2.
observed in the 3-H+ and 4-H+ nor in the protonated 2-phenyl- candidates for stimuli responsive materials. The emission
ethynylazulene (7).
wavelengths of 2-H+ (733 nm) and 1-H+ (722 nm) are strongly
The emission spectra reveal the intensity of 1 to be very weak, red-shied. This can be explained based on the dipole-moment
while 2 is only slightly stronger; 3 and 4 show a substantial of the excited state of the protonated compounds, which is
uorescence intensity although much stronger in the case of 4 diametrically opposed to the ground-state dipole-moment, and
but more red shied in the former case (see Fig. 7). The low therefore points in the direction of the seven-membered ring.
emission intensity of 1 and 2 can be caused by the transfer of 3-H+ and to a lesser extent 4-H+ display strongly increased
the additional electron density into the ve membered ring and emission intensity making both connection patterns promising
the limited contribution of the phenylethynyl substituents to candidates for stimuli responsive materials. Nevertheless the
states relevant for the transition. The emission bands were uorescence wavelengths were found to be less red shied than
found to be located in the range of ca. 70 nm (416 nm to in the case of 1-H+ and 2-H+. In this context 3-H+ is more
484 nm). In this context 3 showed the most red shied emis- bathochromically shied than 4-H+ and exhibits a signicantly
sion band among all the isomers although it is not as intense stronger stimuli response than all other isomers. The smaller
as that of 4 but is the most broad band (ca. from 430 nm to shi can be explained by the fact that the distance of the
650 nm). 4 shows a more signicant hypsochromic shi in the charge-transfer is longer in 1-H+ and 2-H+.
emission wavelength than the mono-alkyne compound
2-phenylethynylazulene 7 (424 nm). Upon protonation the
emission intensity of azulene compounds was expected to
All calculations were performed with the Gaussian 09
increase substantially as it was shown for other related
DFT calculations
program package52 using the hybrid functional PBE1PBE53 in
conjunction with the cc-pVDZ basis set.54 Full geometry
optimizations without symmetry constraints were carried
out in the gas phase for the singlet ground states (S0). The
optimized geometries were conrmed to be potential
energy minima by vibrational frequency calculations at the
same level of theory, as no imaginary frequency was found.
The rst twenty singlet–singlet transition energies were
computed at the optimized S0 geometries by using the time-
dependent DFT (TD-DFT) methodology.55–57 Solvent effects
were taken into account using the conductor-like polarizable
continuum model (CPCM)58,59 with dichloromethane as
solvent for the TD-DFT calculations on all optimized gas-
phase geometries. The excited state structures were opti-
mized by TD-DFT calculations, and the rst four electronic
transitions were computed taking into account the state-
specic equilibrium solvation of each excited state at its
equilibrium geometry.
compounds.25,26 Nevertheless in the case of 2-H+, only a
slightly more intense emission was observed (see Fig. 8) and in
the case of 1-H+, the emission was even barely detectable
revealing that both connection patterns are not promising
The TD-DFT calculations performed at the PBE1PBE/cc-
pVDZ level on the DFT optimized ground state geometries of the
di(phenylethynyl)azulene compounds 1–4 (see Fig. S31–S35,
Fig. 7 UV-Vis emission spectra of 1 (black line), 2 (red line), 3 (green line) and 4
(blue line) in CH2Cl2.51
7404 | J. Mater. Chem. C, 2013, 1, 7400–7408
This journal is ª The Royal Society of Chemistry 2013

View Article Online
Paper
Journal of Materials Chemistry C
ESI†) conrm that the absorption spectra can be divided into coefficient. This observation holds well only in the case of 3
three regions: where the S2 excited state arises directly from the HOMO and
(1) l > 550 nm: region with very weak absorption bands cor- LUMO orbitals. In all cases, the occupied MOs involved in the
responding to the S0 / S1 singlet–singlet transition. The S1 excited S0 / S2 transitions are of p character mainly located on the
states are derived from the HOMO / LUMO excitation for 1, 2 and azulene core and to a lesser extent on one or both phenylethynyl
4, and from the HOMOꢀ1 / LUMO excitation for 3. The calcu- substituents with a p bonding character of the C^C triple
lated oscillator strengths are very small in the range of 0.007–0.025, bond(s) (see Fig. 9 and 10). Except for the LUMO of 2 which is
except for 4 which shows a signicant value f of 0.159;
only delocalized over the azulene moiety, LUMO and LUMO+1
(2) 375 < l < 550 nm: region with moderate to intense show very similar compositions as HOMOꢀ1 and HOMO,
absorption bands (0.292 < f < 1.877) corresponding to the S0 / S2 respectively, notably with an antibonding p* character of the
singlet–singlet transition. The singlet S2 excited states are C^C triple bond(s). The absorption data calculated for the
derived from a combination of the HOMO / LUMO+1 and protonated cationic species 1-H+–4-H+ are not as accurate as
HOMOꢀ1 / LUMO excitations for 1, 2 and 4, and from the the values obtained for the neutral species (see Table 2). Indeed,
HOMO / LUMO excitation for 3 which shows the highest the TD-PBE1PBE calculations failed to reproduce the additional
oscillator strength for this transition;
broad bathochromic bands at 702 and 688 nm for 1-H+ and
(3) 300 < l < 375 nm: region with moderate to intense 2-H+, respectively. Nevertheless, it appears that upon proton-
absorption bands (0.137 < f < 1.805) corresponding to the S0 / S3 ation the S0 / S1 transition exhibits the largest oscillator
singlet–singlet transition. The S3 excited state is derived from the strength of the rst singlet–singlet transitions and corresponds
HOMO / LUMO+2 excitation for 1, the HOMO / LUMO+1 to the HOMO / LUMO excitation (see Fig. 11).
excitation for 2, HOMOꢀ2 / LUMO+2 for 3, and mainly from
Despite the mismatch of both the trends of the calculated
absorption wavelengths, the S0 / S1 oscillator strengths mimic
the HOMOꢀ1 / LUMO excitation for 4.
The S0 / S1 transitions are experimentally observed but well the relative molar extinction coefficients with f ¼ 0.019
appear as very weak and broad bands. The wavelength maxima (1-H+) < 0.612 (2-H+) < 1.467 (4-H+) < 2.069 (3-H+). The main
of the S0 / S2 transitions are quite similar for the di(phenyle- difference with the unprotonated species is that the LUMOs are
thynyl)azulene compounds except for 3 which shows a signi- of p/p* character on the azulene rings but exhibit mainly
cantly red shied band with the highest molar extinction nonbonding orbitals on the phenylethynyl substituents,
Fig. 9 Spatial plots of the HOMO (left) and LUMO (right) orbitals of the ground-state of 3.
Fig. 10 Spatial plots of frontier orbitals of the ground-states of 1 (top), 2 (middle) and 4 (bottom).
This journal is ª The Royal Society of Chemistry 2013
J. Mater. Chem. C, 2013, 1, 7400–7408 | 7405

View Article Online
Journal of Materials Chemistry C
Paper
Table 2 Selected singlet–singlet (S0–Sn) transition energies (nm) and oscillator strengths f (in italics) for protonated 1-H+–4-H+ and unprotonated 1–4 species
1
2
3
4
1-H+
2-H+
3-H+
4-H+
exp abs lmax
exp em lmax
423
465
418
432
446
484
409
416
703
722
689
733
525
571
490
537
TD-DFT on ground-state structures
S0 / S1
S0 / S2
S0 / S3
614.0
0.007
419.4
0.292
353.1
1.345
600.2
0.025
418.8
0.331
358.9
1.805
574.6
0.012
463.8
1.877
347.9
0.137
603.7
0.159
414.6
0.972
360.2
0.401
492.6
0.019
475.0
0.026
445.7
0.023
536.7
0.612
494.3
0.039
394.7
0.481
518.4
2.069
456.6
0.022
398.4
0.004
505.6
1.467
448.5
0.170
386.0
0.078
TD-DFT on excited-state structures
S1 / S0
1317.5
0.004
754.6
0.038
764.4
0.131
677.4
0.019
560.9
1.006
490.6
0.945
517.1
1.257
450.5
0.097
S2 / S0
459.8
0.222
457.6
0.424
506.5
1.680
445.0
1.244
Fig. 11 Spatial plots of the HOMO (left) and LUMO (right) of the ground states of 1-H+–4-H+.
7406 | J. Mater. Chem. C, 2013, 1, 7400–7408
This journal is ª The Royal Society of Chemistry 2013

View Article Online
Paper
Journal of Materials Chemistry C
suggesting a lesser participation of the latter in the absorption
and uorescence properties.
Acknowledgements
This work was supported by the Swiss National Science Foun-
dation NRP 62 Smart Materials Program (Grant no. 406240-
126142). Support from the University of Zurich and Prof. Heinz
Berke is gratefully acknowledged.
The rst excited state (S1) for the protonated species and the
second excited state (S2) for the neutral compounds were
successfully optimized by TD-DFT at the same level of theory
used for the DFT calculations (PBE1PBE/cc-pVDZ). Fluores-
cence wavelengths, energies and oscillator strengths were
computed taking into account the state-specic equilibrium
solvation of each excited state at its equilibrium geometry (see
Table 2). The structural changes between the ground state and
excited state structures are especially observed on the ve-
membered ring as well as on the alkyne substituents for the
neutral compounds and, in contrast, only on the seven-
membered ring for the protonated species (see ESI†,
Fig. S21–S22). The S2 / S0 uorescence of 1–4 corresponds
exactly to the S0 / S2 absorption. The predicted wavelengths of
the uorescence maxima respectively of 460 (ꢀ5 nm compared
to the exp. data of 465 nm), 458 (+26), 507 (+23), 445 (+29) nm
are acceptable and interestingly the calculated oscillator
strengths of 0.222, 0.424, 1.6803, and 1.244 reproduce well the
relative uorescence intensity of 1–4, with 3 and 4 showing
the most intense emission bands. For the protonated species,
the same conclusion can be arrived at with the uorescence
maxima tting correctly with the experimental data 755 (+33),
764 (+31), 561 (ꢀ10) and 517 (ꢀ20) for 1-H+ to 4-H+ respectively
(except that in the case of 1-H+ it corresponds to the S2 / S0
transition instead of S1 / S0 for the other species) and also the
oscillator strengths of 0.038 and 0.131 for 1-H+ and 2-H+, and of
1.006 and 1.257 for 3-H+ and 4-H+, corresponding well with the
experimental features (see Fig. 8).
Notes and references
1 V. Kamm, G. Battagliarin, I. A. Howard, W. Pisula,
¨
A. Mavrinskiy, C. Li, K. Mullen and F. Laquai, Adv. Energy
Mater., 2011, 1, 297–302.
2 R. Mauer, I. A. Howard and F. Laquai, J. Phys. Chem. Lett.,
2010, 1, 3500–3505.
3 S. W. Thomas, G. D. Joly and T. M. Swager, Chem. Rev., 2007,
107, 1339–1386.
4 V. Coropceanu, J. Cornil, D. A. da Silva Filho, Y. Olivier,
´
R. Silbey and J.-L. Bredas, Chem. Rev., 2007, 107, 926–
952.
5 A. Heckmann and C. Lambert, Angew. Chem., Int. Ed., 2012,
51, 326–392.
6 M. A. Rahman, P. Kumar, D. S. Park and Y. B. Shim, Sensors,
2008, 8, 118–141.
7 E. J. Feldmeier and C. Melzer, Org. Electron., 2011, 12, 1166–
1169.
8 C. Zhu, L. Liu, Q. Yang, F. Lv and S. Wang, Chem. Rev., 2012,
112, 4687–4735.
9 I. A. Howard, R. Mauer, M. Meister and F. Laquai, J. Am.
Chem. Soc., 2010, 132, 14866–14876.
10 A. P. de Silva, H. Q. N. Gunaratne, T. Gunnlaugsson,
A. J. M. Huxley, C. P. McCoy, J. T. Rademacher and
T. E. Rice, Chem. Rev., 1997, 97, 1515–1566.
11 S. Malashikhin and N. S. Finney, J. Am. Chem. Soc., 2008, 130,
12846–12847.
Conclusions
In summary, we have successfully synthesized four di(pheny-
lethynyl)azulene isomers and experimentally demonstrated that 12 S. A. Malashikhin, K. K. Baldridge and N. S. Finney, Org.
2,6-connection in azulene exhibits quite interesting and unique Lett., 2010, 12, 940–943.
optical and physical properties in comparison with the other 13 A. P. de Silva and T. E. Rice, Chem. Commun., 1999, 163–164.
isomers of di(phenylethynyl)azulenes. UV-Vis absorption and 14 B. Esser and T. M. Swager, Angew. Chem., Int. Ed., 2010, 49,
luminescence properties were investigated to evaluate possible
different incorporation positions of azulene for use as opto- 15 J. Bouffard, Y. Kim, T. M. Swager, R. Weissleder and
electronic materials. Among the investigated compounds only S. A. Hilderbrand, Org. Lett., 2008, 10, 37–40.
3-H+ and to a lesser extent 4-H+ showed a signicant increase in 16 T. Gupta and M. E. van der Boom, J. Am. Chem. Soc., 2006,
emission intensity upon protonation. While 4-H+ displayed a
128, 8400–8401.
slightly higher intensity, the emission wavelength of 3-H+ was 17 T. Gupta, R. Cohen, G. Evmenenko, P. Dutta and M. E. van
found to be substantially more red-shied. DFT calculations der Boom, J. Phys. Chem. C, 2007, 111, 4655–4660.
were carried out to explain the experimental observations. 18 N. J. Turro, Modern Molecular Organic Photochemistry,
These results led to the conclusion that azulene incorporated as University Science Books, Sausalito, CA, 1991.
a core structure in optoelectronic materials exhibits very weak 19 P. G. Lacroix, I. Malfant, G. Iime, A. C. Razus, K. Nakatani
S0 / S2 and corresponding S2 / S0 transitions if both substi- and J. A. Delaire, Chem.–Eur. J., 2000, 6, 2599–2608.
tutions are situated at the ve-membered ring. It is required 20 D. R. Mitchell and G. D. Gillispie, J. Phys. Chem., 1989, 93,
that at least one substitution be present in the seven-membered 4390–4393.
ring in order to obtain uorescence materials that display 21 Y. Yamaguchi, Y. Maruya, H. Katagiri, K. Nakayama and
signicant stimuli response behavior. Based on these ndings, Y. Ohba, Org. Lett., 2012, 14, 2316–2319.
studies involving the incorporation of 2,6-connected azulene 22 K. Kurotobi, K. S. Kim, S. B. Noh, D. Kim and A. Osuka,
into macromolecules and subsequent evaluation of the stimuli Angew. Chem., Int. Ed., 2006, 45, 3944–3947.
responsive behavior as well as its potential in opto-electronic 23 P. Wang, P. Zhu, C. Ye, A. E. Asato and R. S. H. Liu, J. Phys.
8872–8875.
applications are currently under way.
Chem. A, 1999, 103, 7076–7082.
This journal is ª The Royal Society of Chemistry 2013
J. Mater. Chem. C, 2013, 1, 7400–7408 | 7407

View Article Online
Journal of Materials Chemistry C
Paper
24 R. S. H. Liu, R. S. Muthyala, X.-s. Wang, A. E. Asato, P. Wang 45 S. Carret, A. Blanc, Y. Coquerel, M. Berthod, A. E. Greene and
´
and C. Ye, Org. Lett., 2000, 2, 269–271. J.-P. Depres, Angew. Chem., Int. Ed., 2005, 44, 5130–5133.
25 E. Amir, R. J. Amir, L. M. Campos and C. J. Hawker, J. Am. 46 See ESI.†
Chem. Soc., 2011, 133, 10046–10049.
26 M. Koch, O. Blacque and K. Venkatesan, Org. Lett., 2012, 14,
1580–1583.
27 M. Murai, E. Amir, R. J. Amir and C. J. Hawker, Chem. Sci.,
2012, 3, 2721–2725.
28 S. Dutta, S. Lakshmi and S. K. Pati, Bull. Mater. Sci., 2008, 31,
353–358.
29 J. R. Dias, J. Phys. Org. Chem., 2007, 20, 395–409.
47 T. Nozoe, T. Asao and M. Oda, Bull. Chem. Soc. Jpn., 1974, 47,
681–686.
48 T. Nozoe, K. Takase and N. Shimazaki, Bull. Chem. Soc. Jpn.,
1964, 37, 1644–1648.
49 R. Brettle, D. A. Dunmur, S. Estdale and C. M. Marson,
J. Mater. Chem., 1993, 3, 327–331.
50 It has to be mentioned that for compound 1 higher
concentrations of superacid were needed for full
conversion into the protonated species 1-H+.
¨
30 H. Kim, N. Schulte, G. Zhou, K. Mullen and F. Laquai, Adv.
Mater., 2011, 23, 894–897.
51 For 3 and 4 concentrations resulting in an O.D. of 0.1 a.u. in
the UV-vis absorption spectra were chosen for the
measurements of the UV-vis emission spectra. In the case
of 1 and 2 concentrations resulting in an O.D. of 0.3 a.u.
were chosen to determine the uorescence properties.
52 M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria,
M. A. Robb, J. R. Cheeseman, G. Scalmani, V. Barone,
B. Mennucci, G. A. Petersson, H. Nakatsuji, M. Caricato, X. Li,
H. P. Hratchian, A. F. Izmaylov, J. Bloino, G. Zheng,
J. L. Sonnenberg, M. Hada, M. Ehara, K. Toyota, R. Fukuda,
J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao,
H. Nakai, T. Vreven, J. A. Montgomery, Jr, J. E. Peralta,
F. Ogliaro, M. Bearpark, J. J. Heyd, E. Brothers, K. N. Kudin,
V. N. Staroverov, T. Keith, R. Kobayashi, J. Normand,
K. Raghavachari, A. Rendell, J. C. Burant, S. S. Iyengar,
J. Tomasi, M. Cossi, N. Rega, J. M. Millam, M. Klene,
J. E. Knox, J. B. Cross, V. Bakken, C. Adamo, J. Jaramillo,
R. Gomperts, R. E. Stratmann, O. Yazyev, A. J. Austin,
R. Cammi, C. Pomelli, J. W. Ochterski, R. L. Martin,
K. Morokuma, V. G. Zakrzewski, G. A. Voth, P. Salvador,
J. J. Dannenberg, S. Dapprich, A. D. Daniels, O. Farkas,
J. B. Foresman, J. V. Ortiz, J. Cioslowski and D. J. Fox,
Gaussian 09, Revision B.01, Gaussian, Inc., Wallingford CT, 2010.
31 N. C. Thanh, M. Ikai, T. Kajioka, H. Fujikawa, Y. Taga,
Y. Zhang, S. Ogawa, H. Shimada, Y. Miyahara, S. Kuroda
and M. Oda, Tetrahedron, 2006, 62, 11227–11239.
32 X.-Y. Chen, C. Barnes, J. R. Dias and T. C. Sandreczki, Chem.–
Eur. J., 2009, 15, 2041–2044.
33 K. H. H. Fabian, A. H. M. Elwahy and K. Hafner, Eur. J. Org.
Chem., 2006, 791–802.
34 V. V. Filichev, M. C. Nielsen, N. Bomholt, C. H. Jessen and
E. B. Pedersen, Angew. Chem., Int. Ed., 2006, 45, 5311–5315.
35 S. Ito, H. Inabe, T. Okujima, N. Morita, M. Watanabe and
K. Imafuku, Tetrahedron Lett., 2000, 41, 8343–8347.
36 S. Ito, H. Inabe, T. Okujima, N. Morita, M. Watanabe,
N. Harada and K. Imafuku, J. Org. Chem., 2001, 66, 7090–
7101.
37 K. H. H. Fabian, A. H. M. Elwahy and K. Hafner, Tetrahedron
Lett., 2000, 41, 2855–2858.
38 M. V. Barybin, M. H. Chisholm, N. S. Dalal, T. H. Holovics,
N. J. Patmore, R. E. Robinson and D. J. Zipse, J. Am. Chem.
Soc., 2005, 127, 15182–15190.
39 S. Ito, M. Ando, A. Nomura, N. Morita, C. Kabuto, H. Mukai,
K. Ohta, J. Kawakami, A. Yoshizawa and A. Tajiri, J. Org.
Chem., 2005, 70, 3939–3949.
40 T. R. Maher, A. D. Spaeth, B. M. Neal, C. L. Berrie, 53 C. Adamo and V. Barone, J. Chem. Phys., 1999, 110, 6158–
W. H. Thompson, V. W. Day and M. V. Barybin, J. Am.
Chem. Soc., 2010, 132, 15924–15926.
6170.
54 J. T. H. Dunning, J. Chem. Phys., 1989, 90, 1007–1023.
41 K. Nakagawa, T. Yokoyama, K. Toyota, N. Morita, S. Ito, 55 R. E. Stratmann, G. E. Scuseria and M. J. Frisch, J. Chem.
S. Tahata, M. Ueda, J. Kawakami, M. Yokoyama, Y. Kanai
and K. Ohta, Tetrahedron, 2010, 66, 8304–8312.
42 T. C. Holovics, R. E. Robinson, E. C. Weintrob, M. Toriyama,
Phys., 1998, 109, 8218–8224.
56 R. Bauernschmitt and R. Ahlrichs, Chem. Phys. Lett., 1996,
256, 454–464.
G. H. Lushington and M. V. Barybin, J. Am. Chem. Soc., 2006, 57 M. E. Casida, C. Jamorski, K. C. Casida and D. R. Salahub,
128, 2300–2309. J. Chem. Phys., 1998, 108, 4439–4449.
43 M. Kivala and F. o. Diederich, Acc. Chem. Res., 2009, 42, 235– 58 V. Barone and M. Cossi, J. Phys. Chem. A, 1998, 102, 1995–
248. 2001.
44 D. Balschukat and E. V. Dehmlow, Chem. Ber., 1986, 119, 59 M. Cossi, N. Rega, G. Scalmani and V. Barone, J. Comput.
2272–2288.
Chem., 2003, 24, 669–681.
7408 | J. Mater. Chem. C, 2013, 1, 7400–7408
This journal is ª The Royal Society of Chemistry 2013