Synthesis and characterization of new derivatives of azulene, including experimental and theoretical studies of electronic and spectroscopic behavior

Article abstract of DOI:10.1002/poc.2934

Four new azulene derivatives featuring two π-conjugated and sulfur terminated rigid side arms in the 1,3-positions of the azulene core were designed, synthesized, and characterized. The syntheses were carried out on the basis of a palladium cross-coupling procedure in the key reaction step. The crystal structures of the target compound 3 and of the intermediate compounds 7 and 9 have been studied. Supporting density functional theory calculations reveal insights into the electronic properties of the particular azulenes. The comparison of measured UV-Vis, infrared, and Raman data to calculated values allowed assignment of major spectral features to the particular structural differences of the new compounds and comparison to plain azulene. Cyclic voltammetry measurements were performed showing the redox characteristics of the new azulene derivatives. Copyright

Full text of DOI:10.1002/poc.2934

Research Article
Received: 11 November 2011,
Revised: 27 January 2012,
Accepted: 2 February 2012,
Published online in Wiley Online Library: 2012
(wileyonlinelibrary.com) DOI: 10.1002/poc.2934
Synthesis and characterization of new
derivatives of azulene, including experimental
and theoretical studies of electronic and
spectroscopic behavior
Sebastian Förstera, Torsten Hahnb, Claudia Looseb, Christian Röderb,
Simon Liebingb, Wilhelm Seichtera, Frank Eißmanna, Jens Kortusb*
and Edwin Webera*
Four new azulene derivatives featuring two p-conjugated and sulfur terminated rigid side arms in the 1,3-positions
of the azulene core were designed, synthesized, and characterized. The syntheses were carried out on the basis of a
palladium cross-coupling procedure in the key reaction step. The crystal structures of the target compound 3 and of
the intermediate compounds 7 and 9 have been studied. Supporting density functional theory calculations reveal
insights into the electronic properties of the particular azulenes. The comparison of measured UV–Vis, infrared,
and Raman data to calculated values allowed assignment of major spectral features to the particular structural
differences of the new compounds and comparison to plain azulene. Cyclic voltammetry measurements were
performed showing the redox characteristics of the new azulene derivatives. Copyright © 2012 John Wiley & Sons, Ltd.
Supporting information may be found in the online version of this paper.
Keywords: azulene derivatives; cyclic voltammetry; DFT calculation; optical spectroscopy; single-crystal X-ray diffraction study
different types of sulfur-containing end groups in the structures
of the molecules. Structures of the respective compounds
INTRODUCTION
The naphthalene isomer azulene is a blue nonbenzenoid
aromatic hydrocarbon. Its small highest occupied molecular
orbital–lowest unoccupied molecular orbital (HOMO–LUMO)
gap leads to remarkable electronic and optical properties, which
result in potential applications in the fields of organic semicon-
ductors,[1,2] molecular switches,[3,4] and organic photovoltaics.[5,6]
The ability to shift electronic density from the seven-membered
ring toward the five-membered ring, which leads to a dipole
moment of around 1.0D,[7] is likely to stabilize cations as well as
anions (Scheme 1). Hence, after single oxidation or reduction
one ring of the radical ions maintains aromatic (cyclopentadienyl
anion or tropylium cation). Furthermore, the radical in the other
ring is mesomerically stabilized.
together with a blueprint of their construction are illustrated in
Scheme 2.
We report the synthesis of these new 1,3-disubstituted
azulenes (1–4), show experimentally their properties in cyclic
voltammetry (CV), UV–Vis and Raman spectroscopy, and discuss
their behavior in these respects as compared with the results
of corresponding density functional theory (DFT) calculations
that have also been carried out including basic azulene.
Moreover, the crystal structure of one of the target com-
pounds (3) and two synthetic intermediates (7, 9) have been
solved, giving indication of geometric parameters typical of
the compound class.
As indicated earlier, aromatic compounds and especially
derivatives of azulene are possible candidates for molecular
electronic devices.[8] Following this line, a systematic approach
to enable connection of the azulene to suitable electrodes in
order to close the electric circuit between two electrodes and
the azulene moiety might possibly be developed. This involves
attachment of linear p-conjugated spacer units to the azulene
with the spacers bearing specific end groups that show affinity
to the electrode material. Because an electrode coated with gold
was thought to be advisable, an end group comprising sulfur as
the effective part for the contact was considered. However, it is
known that current–voltage characteristics do not only depend
on the molecular properties but also largely on the microscopic
contact between the molecule and a metallic electrode.[9] Hence,
the decision was made to use aside from different spacers also
* Correspondence to: E. Weber, Institut für Organische Chemie, Technische
Universität Bergakademie Freiberg, Leipziger Strasse 29, D-09596 Freiberg,
Germany.
E-mail: edwin.weber@chemie.tu-freiberg.de
* Correspondence to: J. Kortus, Institut für Theoretische Physik, Technische
Universität Bergakademie Freiberg, Leipziger Strasse 23, D-09596 Freiberg,
Germany.
E-mail: Jens.Kortus@physik.tu-freiberg.de
a S. Förster, W. Seichter, F. Eißmann, E. Weber
Institut für Organische Chemie, Technische Universität Bergakademie Freiberg,
Leipziger Straße 29, D-09596 Freiberg, Germany
b T. Hahn, C. Loose, C. Röder, S. Liebing, J. Kortus
Institut für Theoretische Physik, Technische Universität Bergakademie Freiberg,
Leipziger Straße 23, D-09596 Freiberg, Germany
J. Phys. Org. Chem. (2012)
Copyright © 2012 John Wiley & Sons, Ltd.

S. FÖRSTER ET AL.
sulfide (8),[12] 1,3-bis[(trimethylsilyl)ethynyl]azulene (9),[13] and 4-(acetylthio)
iodobenzene (10)[14] were prepared as described in the literature.
General procedure. Synthesis of compounds 1–3
1,3-Diiodoazulene (5) (0.60g, 1.58mmol), the respective ethynylic coupling
component (3.47 mmol), bis(triphenylphosphane)palladium(II) chloride
(50 mg, 0.07mmol), copper(I) iodide (27 mg, 0.14mmol) and triphenylpho-
sphane (37mg, 0.14 mmol) were dissolved in oxygen free diisopropylamine
(50 mL) under an argon atmosphere. The solution (soln) was stirred at rt for
24 h and then at 50 ꢁC for 10 h. After removal of the solvent, the residue was
purified by column chromatography on SiO2 to yield the pure products.
Details for the individual compounds are given as follows.
Scheme 1. Azulene and its stabilized single reduced and oxidized forms
(a)
A
C
C
B
B
R
(b)
S
1
2
3
4
1,3-Bis(3-thienylethynyl)azulene (1)
S
3-Ethynylthiophene (6) (0.38 g, 3.47 mmol) was reacted, and for chroma-
tography, hexane–ethyl acetate 20:1 (v/v) was used as the eluent to yieꢀld1
0.42 g (78%) dark green solid; mp = 120–122 ꢁC; IR (KBr): n = 3104 cm
(thiophene C–H), 2195 (CꢂC), 1635, 1572, 1448, 1407, 1356, 862, 780,
748, 625; 1H NMR (500 MHz, CDCl3): d = 7.26–7.31 (m, 4H, azulene–H and
R
R
S
S
O
3
4
thiophene–H), 7.32 (dd, JHH = 4.9 Hz, JHH = 3.0 Hz, 2H, thiophene–H),
4
4
3
7.56 (dd, JHH = 3.0 Hz, JHH = 1.1 Hz, 2H, thiophene–H), 7.66 (t, JHH = 9.9
3
Hz, 1H, azulene–H), 8.09 (s, 1H, azulene–H), 8.55 (d, JHH = 9.3 Hz, 2H,
1 - 4
azulene–H) ppm; 13C NMR (125 MHz, CDCl3) d = 84.2 (CꢂC), 88.9 (CꢂC),
110.5, 122.8, 125.3, 125.5, 127.9, 129.9, 137.1, 140.0, 141.3, 141.8 ppm;
MS (APCI): m/z = 341.1 [M + H]+ C22H12S2 (340.04).
Scheme 2. (a) Basic design of the studied compound type involving a con-
nection of azulene (A), gold affine sticky end groups (B) and p-conjugated
connection pieces (C). (b) Formula structures of the molecules 1–4
1,3-Bis{[4-(3-thienylethynyl)phenyl]ethynyl}azulene (2)
3-[(4-Ethynylphenyl)ethynyl]thiophene (7) (1.0 g, 4.80 mmol) was reacted
and for chromatography hexane–ethyl acetate 2:1 (v/v) was used as the
eluent to yield 0.54 g (63%) green solid; mp = 211 ꢁC; IR (KBr): n = 3104
cmꢀ1 (thiophene C–H), 2189 (CꢂC), 1568, 1527, 1486, 1445, 1363, 837
EXPERIMENTAL
Methods and material
3
(ArC–H), 780, 742; 1H NMR (500 MHz, CDCl3) d = 7.21 (dd, JHH = 5.0 Hz,
Melting points were determined with a Kofler melting point microscope
and are uncorrected. The infrared (IR) measurements were carried out
with a Nicolet-FT/IR-Spectrometer (Madison, WI, USA). The Raman mea-
surements were performed at room temperature (rt) in backscattering
geometry using a Labram HR 800 Horiba Jobin Yvon (Bensheim, Hessen,
3
4
4JHH = 1.1 Hz, 2H, thiophene–H), 7.32 (dd, JHH = 5.0 Hz, JHH = 3.0 Hz, 2H,
thiophene–H), 7.37 (t, 3JHH = 9.9 Hz, 2H, azulene–H), 7.52 (d, 3JHH = 8.4 Hz,
4H, Ar–H), 7.55 (dd, 4JHH = 3.0 Hz, 4JHH = 1.1 Hz, 2H, thiophene–H), 7.59 (d,
3JHH = 8.4 Hz, 4H, Ar–H), 7.74 (t, 3JHH = 9.9 Hz, 1H, azulene–H ), 8.14 (s, 1H,
3
azulene–H), 8.61 (d, JHH = 9.3 Hz, 2H, azulene–H) ppm; 13C NMR
Germany) spectrometer with
a charge-coupled device detector. As
excitation wavelength, the 532-nm (2.33 eV) line of a frequency-doubled
Nd:YAG laser was applied. In order to avoid thermal decomposition or laser-
induced degradation of the specimen, the applied laser power density was
carefully adjusted and kept very low (16 W cmꢀ2). That is why acceptable
Raman spectra were obtained within 10–20 min. Nuclear magnetic resonance
(NMR) spectra were recorded on a BRUKER (Billerica, Massachusetts, United
States) AVANCE DPX 500 with tetramethylsilane as internal standard. For
compound 1–4, the 1H and 13C Spectra can be found in the Supporting Infor-
mation. The CV studies were carried out using a standard three-electrode
setup: platinum working electrode, platinum counter electrode, and an Ag/
AgCl reference electrode. Anhydrous acetonitrile containing 0.1 M tetrabuty-
lammonium hexafluorophosphate and 0.5 mM analyte was used. UV–Vis
analyses were carried out with a JASCO (Groß-Umstadt, Hessen, Germany)
V630 in dichloromethane. The liquid chromatography/mass spectrometry
(MS) measurements were executed using a Varian (Böblingen, Baden-
Württemberg, Germany) 3200Q-TRAP. For thin-layer chromatography anal-
ysis, aluminum sheets precoated with silica gel 60 F254 (Merck) (Darmstadt,
Hessen, Germany) were used. Column chromatography was performed
with silica gel (63–100 mm, Merck). The solvents used were purified applying
standard methods and for the coupling reactions degassed by ultrasonic
treatment prior to use. Trimethylsilylethyne, triphenylphosphane, bis(tri-
phenylphosphane)palladium(II) chloride, and copper(I) iodide were pur-
chased from commercial sources.
(125 MHz, CDCl3) d = 86.3 (CꢂC), 86.8 (CꢂC), 88.8 (CꢂC), 94.1 (CꢂC),
110.5, 122.1, 122.6, 123.6, 125.5, 126.0, 128.8, 129.8, 131.3, 131.5, 137.2,
140.2, 141.5, 142.1 ppm; MS (APCI): m/z = 540.2 [M]+ C38H20S2 (540.10).
1,3-Bis[(4-tert-butylthiophenyl)ethynyl]azulene (3)
4-Ethinylphenyl tert-butyl sulfide (8) (0.66 g, 3.47 mmol) was reacted, and
for chromatography, hexane was used as the eluentꢀt1o yield 0.74 g (93%)
dark green solid; mp = 102 ꢁC; IR (KBr): n = 2959 cm (CH3), 2917, 2857,
1
2192 (CꢂC), 1572, 1363, 1477, 1435, 1394, 1166, 831; H NMR (500 MHz,
3
CDCl3) d = 1.32 (s, 18H, CH3), 7.35 (t, JHH = 9.9 Hz, 2H, azulene–H), 7.56
3
(m, 8H, Ar–H), 7.73 (t, JHH = 9.9 Hz, 1H, azulene–H), 8.14 (s, 1H, azulene–
H), 8.61 (d, 3JHH = 9.3 Hz, 2H, azulene–H) ppm; 13C NMR (125 MHz, CDCl3)
d = 31.0 (CH3), 46.5 (Ctert), 86.4 (CꢂC), 93.7 (CꢂC), 110.5, 124.2, 125.9,
131.3, 132.8, 137.2, 137.3, 140.2, 141.6, 142.1; MS (ESI): m/z = 504.5 [M]+
C34H32S2 (504.19).
1,3-Bis[(4-acetylthiophenyl)ethynyl]azulene (4)
The mixture of 1,3-bis[(trimethylsilyl)ethynyl]azulene (9) (0.93 g,
2.90 mmol) in methanol (50 mL) and potassium hydroxide (0.34 g,
6.0 mmol) in H2O (6 mL) was stirred for 5 h at 0 ꢁC. The soln is extracted
three times with dichloromethane. The combined organic phases are
washed with water and dried over Na2SO4. To the filtrate 4-(acetylthio)
iodobenzene (10) (0.81 g, 2.91 mmol), bis(triphenylphosphane)palla-
dium(II) chloride (31 mg, 0.04 mmol), copper(I) iodide (17 mg, 0.09 mmol),
triphenylphosphane (23 mg, 0.09 mmol), and diisopropylamine (5 mL) are
added, and the soln is stirred for 18 h at rt. The solvent is removed and
Synthesis
The intermediate compounds 1,3-diiodoazulene (5),[6] 3-ethynylthiophene
(6),[10] 3-[(4-ethynylphenyl)ethynyl]-thiophene (7),[11] ethinylphenyl tert-butyl
wileyonlinelibrary.com/journal/poc
Copyright © 2012 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. (2012)

NEW DERIVATIVES OF AZULENE
the residue purified by column chromatography (SiO2) with hexane–
ethyl acetate 2:1 (v/v) as the eluent to yield 0.21 g (15%) dark green
powder; mp = 89 ꢁC; IR (KBr): n = 2923 cmꢀ1 (CH3), 2854, 2198 (CꢂC),
1
1698 (C=O), 1568, 1483, 1442, 1394, 1359, 1119, 827; H NMR (500 MHz,
3
CDCl3) d = 2.45 (s, 6H, CH3), 7.38 (t, JHH = 9.9 Hz, 2H, azulene–H), 7.43 (d,
3JHH = 8.4 Hz, 4H, Ar–H), 7.64 (d, 3JHH = 8.3 Hz, 4H, Ar–H), 7.73 (t, 3JHH = 9.7
3
Hz, 1H, azulene–H), 8.14 (s, 1H, azulene–H), 8.60 (d, JHH = 9.4 Hz, 2H,
azulene–H) ppm; 13C NMR (125 MHz, CDCl3) d = 30.3 (CH3), 86.6 (CꢂC),
93.5 (CꢂC), 110.3, 125.1, 126.1, 127.5, 131.9, 134.3, 137.2, 140.2, 141.7,
142.2, 193.6 (C=O) ppm; MS (APCI): m/z = 476.2 [M]ꢀ C30H20O2S2 (476.09).
Crystallography
Crystals suitable for single-crystal X-ray diffraction studies were obtained by
slow evaporation of soln of 3 (acetone) 9 (ether) and 11 (ether). Information
concerning the crystallographic data and the structure refinement calcula-
tions of the three compounds is summarized in Table S1. The intensity data
were collected on a Kappa/APEX II (Bruker AXS) diffractometer, with Mo Ka
radiation (l = 7.1073 nm) using o and ’ scans. Reflections were corrected
for background, Lorentz, and polarization effects. Preliminary structure
models were derived by application of direct methods[15] and were refined
by full-matrix least squares calculations on the basis of F2 for all reflec-
tions.[16] The hydrogen atoms were included in the models in the calculated
positions and were refined as constrained to bonding atoms.
Scheme 3. Reaction scheme for the synthesis of 1, 2, and 3
Electronic structure calculations
The DFT calculations on individual, free molecules were carried out by
using the Naval Research Laboratory Molecular Orbital Library (NRLMOL)
program,[17–23] which is an all-electron implementation of DFT. NRLMOL
combines large Gaussian orbital basis sets, numerically precise variational
integration and an analytic soln of Poisson’s equation in order to
accurately determine the self-consistent potentials, secular matrix, total
energies, and Hellmann–Feynman–Pulay forces.[24] The GGA/PBE[25]
functional was used to describe the exchange-correlation effects of the
electrons. Because no structure data were available (except for
compound 3), we generated starting geometries by AVOGADRO,[26] and
geometry optimization was performed using the NRLMOL package. All
results are based on these geometries. To improve the accuracy of the
calculated spectroscopic properties, we utilized the ORCA[27,28] program
together with the B3LYP[29,30] functional and the 6-31G*[31] basis set. Note
that the Fermi energy is defined similar to the standard definition of
semiconductor physics within this paper as EF = (ELUMO ꢀ EHOMO)/2.
Scheme 4. Reaction scheme for the synthesis of 4
ethynyl]azulene (9)] with 4-(acetylthio)iodobenzene (10) (Scheme 4).
Nevertheless, this alternative coupling reaction gave only a mod-
erate yield (15%) of 4, possibly attributable to the instability of
the deprotected intermediate compound 1,3-diethynylazulene.
RESULTS AND DISCUSSION
Structural design and synthesis of compounds
Considering the requirements stated at the beginning, the
Crystal structures
azulene-based target compounds (1–4) exhibit
a specific
connection mode of building units as shown in Scheme 2. This
involves three structural components: (1) The azulene unit (A)
as a redox element; (2) end groups (B) acting as alligator clips
for the purpose of connecting to an electrode; and (3) rigid and
p-conjugated pieces (C) combining the azulene with the end
groups. These three parts were linked together using a process
known as the Sonogashira–Hagihara cross-coupling[32] as the
key reaction step. Hence, corresponding aryl halides and terminal
ethynes are the appropriate educts.
As specified in Scheme 3, 1,3-diiodoazulene (5) reacts with 3-
ethynylthiophene (6), 3-[(4-ethynylphenyl)ethynyl]-thiophene
(7), or 4-ethinylphenyl tert-butyl sulfide (8) to yield 1, 2, and 3,
respectively. However, owing to problems in regard to the
analogous coupling between 5 and 4-ethynyl(acetylthio)benzene
for the synthesis of 4, a different course was adopted by reaction
of 1,3-diethynylazulene [via deprotection from 1,3-bis[(trimethylsilyl)
Crystals suitable for X-ray diffraction analysis have been obtained
for the intermediate compounds 7 and 9 as well as for the
azulene derivative 3. Crystal data for these compounds, struc-
tural features of the appertaining molecules, including torsion
angles and selected bond lengths, and geometric parameters
of possible intermolecular hydrogen bond type contacts are
summarized in Tables S1–S2 (ESI), respectively. Further crystal
data and geometric parameters have been deposited at the
Cambridge Crystallographic Data Centre (CCDC) with the
reference numbers CCDC 826959, CCDC 826960, and CCDC
826961 for 9, 3, and 11, respectively.
The yellow crystals of 7, grown from soln in diethylether, show
the monoclinic space group Cc with the asymmetric unit cell
containing one molecule. The molecule deviates slightly from
planarity, which is reflected by a twist angle of 2.9(1)ꢁ between
the aromatic rings [Fig. 1(a)]. Taking into account the dominant
J. Phys. Org. Chem. (2012)
Copyright © 2012 John Wiley & Sons, Ltd.
wileyonlinelibrary.com/journal/poc

S. FÖRSTER ET AL.
Figure 1. (a) Molecular structure of 7 including the atom numbering
scheme. Thermal displacement ellipsoids are drawn at 50% probability
level. (b) Packing diagram of 7 viewed down the crystallographic a-axis
non-covalent interaction, the crystal of 7 can be regarded as be-
ing composed of molecular zigzag strands [Fig. 1(b)] in which
consecutive molecules are connected by a relatively strong C–
H⋯p hydrogen bond[33] formed between the acidic ethynyl
hydrogen and the thiophene ring [C1–H1⋯ring center A
26.6 nm, 161ꢁ]. Interstrand association is accomplished by
p⋯p interactions[34] between the phenylene and thiophene
rings as well as weak C–H⋯p contacts with the phenylene
ring [C13–H13⋯ring center B 27.0 nm, 138ꢁ] and the terminal
ethynyl unit [d(H⋯p) 27.8–28.6 nm] acting as acceptors.
The azulene derivative 9 crystallizes as green rods in the
monoclinic space group C2/c with one half of the molecule in
the asymmetric entity of the unit cell. The molecule is situated
on a crystallographic twofold axis passing through the atoms
C1 and C6 [Fig. 2(a)]. The crystal structure of 9 is characterized
by the formation of molecular stacks extending along the
crystallographic c-axis [Fig. 2(b)]. Within a given stack, neighboring
molecules are arranged in a head-to-tail fashion, which can be
explained by the inherent dipole character of the azulene ring
system. Because of the hydrophobic nature of the trimethylsilyl
groups, the crystal structure is additionally stabilized by van der
Waals forces.
Crystallization of 3 from acetone yields green crystals of the
monoclinic space group C2/c. Because of packing effects, the
planes of the azulene and phenylene units are considerably
distorted against each other amounting to 56.16ꢁ and 60.19ꢁ,
respectively, thus differing significantly from the expected
planarity [Fig. 3(a)]. The molecules in the crystal of 3 are stacked
along the b-axis, while the structure in the a and c direction is
stabilized by C–H⋯p hydrogen bonding [C1–H1⋯centroid(C23/
C24), 27.6 nm, 169.3ꢁ] and van der Waals interactions involving the
tert-butyl group [Fig 3(b)].
Figure 2. (a) Molecular structure of 9 including the atom numbering
scheme. Thermal displacement ellipsoids are drawn at 50% probability
level. (b) Packing diagram of 9 viewed down the crystallographic c-axis
ionization potential (IP), and the electron affinity (Eaff) are sum-
marized in Table 1. Figure 4 shows the calculated energy level
diagrams together with plots of the HOMO and LUMO orbitals
of the respective molecules.
In accordance to the literature,[35,36] the HOMO (a 2a2 state) and
LUMO (a 4b1 state) of azulene are ring-derived p-states. The calcu-
lated IP = 7.46 eV of azulene is in good agreement with the litera-
ture,[37] whereas the calculated Eaff = 0.53 eV is about 30% too small
compared with other calculations and experimental data.[38,39] The
attached linker units have the general effects of narrowing the
HLG, increasing Eaff, and (slightly) decreasing the IP compared with
the basic azulene. The position of the Fermi level is barely affected
by the different linker units and remains at approx. ꢀ3.7 (ꢃ0.1) eV.
The HOMO of the azulene derivatives is delocalized over the whole
molecule, while the LUMO is always located at the azulene core.
The latter effect is well known[40] and results from the very low
electron density at the substitution sites.
The structure of the density of states (DOS) around the
Fermi level determines mainly the characteristics of the elec-
tron transport through a molecular device. Because of their
very similar electronic properties, the DOS of the azulene deriva-
tives are also very similar. Figure 5 shows the calculated DOS for
compound 1. The states below and above EF are dominated by
carbon p-states. Because of the integration of sulfur into the
conjugated electronic system of the thiophene units, a small
but reasonable amount of sulfur states can be found at the
HOMO and the levels above the LUMO.
Electronic properties
Density functional theory calculations for the basic azulene and
the present derivates 1–4 were carried out to investigate the
electronic structure of the molecules. Some fundamental proper-
ties such as HOMO–LUMO gap (HLG), Fermi level (EF), the
wileyonlinelibrary.com/journal/poc
Copyright © 2012 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. (2012)

NEW DERIVATIVES OF AZULENE
Az
1
2
3
4
E
F
Az
1
2
3
4
Figure 3. (a) Molecular structure of 3 including the atom number-
ing scheme. Thermal displacement ellipsoids are drawn at 50%
probability level. (b) Packing diagram of 3 viewed down the crystal-
lographic c-axis
HOMO
LUMO
Figure 4. Calculated energy level diagram aligned to EF for compounds
1–4 and azulene (Az) free molecules. The molecular orbital density
contours are depicted
With CV measurements, we are able to gain insights into the
redox characteristics of the new azulene derivatives 1–4 com-
pared with plain azulene. A voltammogram of 4 is exemplary
illustrated in Fig. 6. The new compounds feature two oxidation
and one reduction wave (Table 2), all being irreversible. A
possible explanation is the dimerization in the 6-position of
the azulene unit.[41,42] Nevertheless, it is shown that the intro-
duction of electron-withdrawing ethynylene groups to the
azulene in the 1,3-positions leads to an increase of the oxidation
potentials and the reduction potential. Because of the more
extended p-conjugated system, compound 2 shows a smaller
absolute value of E1ox and E1red, being in conformance with the
trends of the calculated IP and Eaff.
Table 1. Calculated electronic properties of the different
azulene compounds (Fermi level, EF; HOMO–LUMO gap,
HLG; electron affinity, Eaff; ionization potential, IP)
Compound
Azulene
EF [eV]
HLG [eV]
Eaff [eV]
IP [eV]
ꢀ3.74
ꢀ3.78*
ꢀ3.75
ꢀ3.74*
ꢀ3.69
ꢀ3.65*
ꢀ3.91
ꢀ3.90*
ꢀ3.89
ꢀ3.87*
3.24
3.34*
2.59
2.65*
2.38
2.53*
2.64
2.79*
2.63
2.73*
ꢀ0.53
ꢀ0.44*
ꢀ1.27
ꢀ1.23*
ꢀ1.73
ꢀ1.68*
ꢀ1.50
ꢀ1.48*
ꢀ1.56
ꢀ1.48*
7.49
7.16*
6.21
6.21*
5.88
5.99*
6.15
6.09*
6.30
6.28*
1
2
3
4
UV–Vis spectroscopy
Figure 7 shows the measured UV–Vis spectra in dichloromethane
of compounds 1 and 3 compared with reference measurement
Values marked with * are ORCA B3LYP results.
J. Phys. Org. Chem. (2012)
Copyright © 2012 John Wiley & Sons, Ltd.
wileyonlinelibrary.com/journal/poc

S. FÖRSTER ET AL.
Figure 7. UV–Vis spectra of 1 and 3 compared with azulene. The
spectra of 2 and looks very similar and are not shown for clarity
Figure 5. Density of states (DOS) of 1 relative to the Fermi level EF
(red, total DOS; blue, carbon DOS; orange, sulfur DOS)
IR/Raman spectroscopy
Several investigations studying the vibrational properties of
azulene by resonant Raman scattering were carried out in the
past.[43–45] FT-Raman and FT-IR spectra of the azulene single
crystal in all possible polarizations at rt were reported by Bree
et al.[46] First calculations of the vibrational frequencies of
azulene were presented by Steele.[47] The work of Kozlowski
et al.[48] summarizes potential symmetry breaking as well as
structure and definite vibrational assignment for azulene com-
paring both several numerical approaches and experimental
results with each other.
Figure 8 shows the measured and DFT-calculated (shown are
the results of the ORCA program together with the 6-31G* basis
set and the B3LYP functional) IR spectra of the respective
azulene derivatives. The main features are the strong CꢂC signal
(~2195 cmꢀ1), the C–H signal (~3104 cm–1) originated by the
thiophene linker (only 1 and 2), and a large fingerprint region
Figure 6. Cyclic voltammogram of 4 (0.5 mM) with scan rate of 25 mV/s
Table 2. Redox potentials of 1–4 and azulene measured by
cyclic voltammetry, scan rate of 25 mV/s, V vs Ag/AgCl
Compound
E1ox [V]
E2ox [V]
E1red [V]
Azulene
0.67
0.92
0.79
0.94
0.97
1.33
1.83
1.73
1.67
1.80
ꢀ1.68
ꢀ1.28
ꢀ1.16
ꢀ1.21
ꢀ1.37
1
2
3
4
of plain azulene. The narrowing of the HLG of 1 and 3 compared
with azulene is qualitatively reflected by the shift of the weak
HOMO–LUMO transition from 583 nm to approx. 630 nm. The
typical HOMO–LUMO + 1 transition of azulene (340 nm) can be
found at about 420 nm, which again reflects the calculated
changes of the electronic properties (Table 1) by connecting
the different side arms to the azulene core. The HOMO
1 ! LUMO transition is the most intensive band. For compound
1, it can be found at 323 nm with an absorption coefficient of
Figure 8. Comparison of calculated (dashed) and measured (solid)
infrared (IR) spectra of the investigated azulene compounds at room
temperature. The intensities of the calculated spectra were scaled to
match the measured ones
1425 L molꢀ1 cmꢀ1
.
wileyonlinelibrary.com/journal/poc
Copyright © 2012 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. (2012)

NEW DERIVATIVES OF AZULENE
between 500 and 1500 cmꢀ1. The visible splitting of the CꢂC sig-
nal in 2 shows the two chemically different CꢂC bonds in the
unaffected, changing the substituent. Compared with that of
plain azulene, the frequency of the considered Raman mode
is blueshifted. The feature vib2 can be assigned to asymmetric
C–C stretching of the seven-membered ring of the azulene
core, which is only slightly affected by the attached linkers.
The mode marked by vib3 is attributed to a C–C stretch
vibration of the benzene ring of the substituents. Because of
the lack of this structural feature in the linker group of
compound 1 (Scheme 2), this mode cannot be observed in
the respective Raman spectrum. Vib4 (~2200 cmꢀ1) can be
assigned to the CꢂC triple bonds of the linker units. The
measured and DFT calculated values of selected modes are
collected in Table 3. The calculated values of the frequencies
are slightly too high but a clear assignment to the measured
modes can be made.
side arms. Further differences arise from the CH3 (2923 cmꢀ1
)
groups (3 and 4) and the strong C=O (1698 cmꢀ1) stretching
mode (4). The comparison of the calculated spectra to the mea-
sured data underpins the signal assignment. However, a clear
resolution of the fingerprint area would need a far more detailed
investigation. Although the positions of the main signals are in
good agreement with the measured data, the respective intensi-
ties do not match very well.
The synthesized new azulene derivatives were examined by
Raman spectroscopy at rt. Polarization-dependent measure-
ments could not be carried out as no suitable single crystals
of the samples were available. In Fig. 9, the obtained Raman
spectra of the azulene derivatives are presented in the
relevant spectral range. Obviously, the spectra were found to
be very similar as one might expect because of only little
structural differences of the linker groups. The results of ab
initio calculations discussed earlier also predict only minor
differences in the electronic properties of the azulene deriva-
tives. The common Raman feature of all spectra at about
700 cmꢀ1 (marked as vib1) can be assigned as “breathing
mode” (C–C stretch) of the seven-membered ring of the
azulene core. As the substituent groups are linked to the
five-membered ring of the azulene, this mode stays
CONCLUSION
In summary, four new azulene-based compounds 1–4 featuring
two p-conjugated side arms with different gold affine end
groups have been synthesized and studied by means of single-
crystal X-ray diffraction, spectroscopic analysis, and CV, giving in-
sight into their structural, vibrational, and electronic properties.
Weak supramolecular interactions such as C–H⋯p hydrogen
bonding as well as p⋯p stacking interactions dominate the
packing of the crystal structures. The experimental IR and Raman
spectra of the target compounds together with the supporting
DFT calculations ensure the success of the syntheses and are in
good agreement with related literature data. The CV investiga-
tions of the azulenes in soln reveal that the radicals formed are
likely to undergo consecutive reactions. The effects of the
protection groups of compounds 3 and 4 are shown to be
negligible in this regard, and one might also expect that the
introduction of other sulfur end groups will not essentially
change the electronic property of the molecule. This suggests
that further tuning of the electronic property will require
additional substitution to the azulene core including electron
donor and acceptor groups both to the five-membered and
seven-membered ring segments of the azulene as well as the
fixation of a protection group to the 6-position of the azulene,
preventing radical dimerization. Nevertheless, for an intended
use of this type of compounds in a single molecule electronic
device such as an electronic switching element between two gold
electrodes, the stability of the radicals in the solid state is of more
importance and remains to be investigated aside from the ability
of the molecules regarding self-assembly to a gold surface. These
are the tasks we are concerned with in the near future.
Figure 9. Raman spectra of the investigated azulene derivatives recorded
in backscattering geometry at room temperature (selected vibrations
discussed explicitly in the text are depicted by the dashed lines)
Table 3. Measured and calculated values (cmꢀ1) of selected spectral features
Compound
Raman
Vib1
Vib2
Vib3
Vib4
Calc.
Exp.
Calc.
Exp.
Calc.
Exp.
Calc.
Exp.
1
2
3
4
720
715
720
720
700
700
700
700
1648
1646
1645
1645
1570
1573
1572
1575
—
—
2318
2304
2311
2311
2197
2188
2200
2197
1663
1646
1648
1598
1590
1592
J. Phys. Org. Chem. (2012)
Copyright © 2012 John Wiley & Sons, Ltd.
wileyonlinelibrary.com/journal/poc

S. FÖRSTER ET AL.
[17] A. Briley, M. Pederson, K. Jackson, D. Patton, D. C. Porezag, Phys. Rev.
B. 1998, 58, 1786.
Acknowledgements
[18] K. Jackson, M. Pederson, Phys. Rev. B. 1990, 42, 3276.
[19] M. Pederson, K. Jackson, Phys. Rev. B. 1990, 41, 7453.
[20] M. Pederson, K. Jackson, Phys. Rev. B. 1990, 41, 7312.
[21] M. Pederson, D. Porezag, J. Kortus, D. Patton, Phys. Status Solidi
b – Basic Res. 2000, 217, 197.
[22] D. Porezag, M. Pederson, Phys. Rev. B. 1996, 54, 7830.
[23] A. Quong, M. Pederson, J. Feldman, Solid State Commun. 1993, 87,
535.
This work was performed within the Cluster of Excellence “Struc-
ture Design of Novel High-Performance Materials via Atomic De-
sign and Defect Engineering (ADDE)” that is financially supported
by the European Union (European regional development fund)
and by the Ministry of Science and Art of Saxony (SMWK). Further
financial support within the Forschergruppe FOR 1154 (project
KO1924/5) and within the ESF project 080945373 is gratefully
acknowledged. We would also like to thank the ZIH Dresden for
computational support.
[24] D. Porezag, M. Pederson, Phys. Rev. B. 1990, 60, 2840.
[25] J. Perdew, K. Burke, M. Ernzerhof, Phys. Rev. B. 1996, 77, 3865.
[26] http://avogadro.openmolecules.net/
[27] F. Neese, J. Chem. Phys. 2003, 119, 9428.
[28] F. Neese, Int. J. Quantum Chem. 2001, 83, 104.
[29] A. D. Becke, J. Chem. Phys. 1993, 98, 5648.
[30] P. J. Stephens, F. J. Devlin, C. F. Chabalowski, M. J. Frisch, J. Phys.
Chem. 1994, 98, 11623.
[31] V. A. Rassolov, M. A. Ratner, J. A. Pople, P. C. Redfern, L. A. Curtiss, J.
Comp. Chem. 2001, 22, 976.
[32] K. Sonogashira, Y. Tohda, N. Hagihara, Tetrahedron Lett. 1975, 50,
4467.
[33] M. Nishio, Y. Umezawa, K. Honda, S. Tsuboyama, H. Suezawa, Cryst.
EngComm 2009, 11, 1757.
REFERENCES
[1] A. Strachota, V Cimrová, E. Thorn-Csányi, Macromol. Symp. 2008,
268, 66.
[2] L. Farrand, M. Findlater, M. Giles, M. Heeney, S. Tierney, M. Thompson,
M. Shkunov, D. Sparrowe, I. McCulloch, EP 1318183 (A1), 2003.
[3] V. De Waele, U. Schmidhammer, T. Mrozek, J. Daub, E. Riedle, J. Am.
Chem. Soc. 2002, 124, 2438.
[4] S. Ito, S. Kikuchi, T. Okujima, N. Morita, T. Asao, J. Org. Chem. 2001,
66, 2470.
[34] L. M. Salonen, M. Ellermann, F. Diederich, Angew. Chem. Int. Ed.
2011, 50, 4808.
[35] H. Korichi, F. Zouchoune, S.-M. Zendaoui, J.-Y. Saillard, Organometal-
lics 2010, 29, 1693.
[36] Y. Amatatsu, Y. Komura, J. Chem. Phys. 2006, 125, 174311.
[37] M. S. Deleuze, J. Chem. Phys. 2002, 116, 7012.
[38] G. Malloci, G. Mullas, G. Cappellini, V. Fiorentini, I. Porceddu, Astron.
Astrophys. 2005, 432, 585.
[39] E. C. M. Chen, E. S. Chen, M. S. Milligan, W. E. Wentworth, J. R. Wiley,
J. Phys. Chem. 1992, 96, 2385.
[40] S. Shevyakov, H. Li, R. Muthyala, AE. Asato, J. C. Croney, D. M. Jame-
son, R. S. H. Liu, J. Phys. Chem. A 2003, 107, 3295.
[41] H. Bock, C. Arad, C. Nather, L. Gobel, Helv. Chim. Acta 1996, 79, 92.
[42] J. Daub, J. Salbeck, Chem. Ber. 1989, 122, 727.
[43] R. Liang, O. Schnepp, A. Warshel, Chem. Phys. Lett. 1976, 44, 394.
[44] R. Liang, O. Schnepp, A. Warshel, Chem. Phys. 1978, 34, 17.
[45] O. Brafman, C. K. Chan, B. Khodadoost, J. B. Page, C. T. Walker, J.
Chem. Phys. 1984, 80, 5406.
[46] A. Bree, A. J. Pal, C. Taliani, Spectrochim Acta A. 1990, 46, 1767.
[47] D. Steele, J Mol Spectrosc 1964, 15, 333.
[48] P. M. Kozlowski, M. Z. Zgierski, P. Pulay, Chem. Phys. Lett. 1995, 247,
379.
[5] R. Latonen, C. Kvarnström, A. Ivaska, J. Appl. Electrochem. 2010, 40,
1583
[6] G. Nöll, J. Daub, M. Lutz, K. Rurack, J. Org. Chem. 2011, 76, 4859.
[7] A. G. Anderson, B. M. Stecker, J. Am. Chem. Soc. 1959, 81, 4941.
[8] K.-G. Zhou, Y.-H. Zhang, L.-J. Wang, K.-F. Xie, Y.-Q. Xiong, H.-L. Zhang,
C.-W. Wang, Phys. Chem. Chem. Phys. 2011, 12, 15882.
[9] G. Cuniberti, G. Fagas, K. Richter, Introducing Molecular Electronics,
Springer, Berlin, New York, 2005.
[10] I. Novak, S. C. Ng, J. Fang, C. Y. Mok, H. H. Huang, J. Phys. Chem.
1994, 98, 748.
[11] A. Dube, A. R. Chadeayne, M. Sharma, P. T. Wolczanski, T. Peter, J. R.
Engstrom, J. Am. Chem. Soc. 2005, 127, 14299.
[12] N. Stuhr-Hansen, J. K. Soerensen, K. Moth-Poulsen, J. B. Christensen,
T. Bjoernholm, M. B. Nielsen, Tetrahedron 2005, 61, 12288.
[13] K. H. H. Fabian, A. H. M. Elwahy, K. Hafner, Tetrahedron Lett. 2000, 41,
2855.
[14] D. Taher, B. Walfort, H. Lang, Inorg. Chim. Acta 2006, 359, 1899.
[15] G. M. Sheldrick, SHELXS-97, Program for Solution of Crystal
Structures, University of Göttingen, Germany, 1997.
[16] G. M. Sheldrick, SHELXL-97, Program for Refinement of Crystal
Structures, University of Göttingen, Germany, 1997.
wileyonlinelibrary.com/journal/poc
Copyright © 2012 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. (2012)