Azulene-containing organic chromophores with tunable near-IR absorption in the range of 0.6 to 1.7 μm

Article abstract of DOI:10.1039/c2jm31098h

Near-infrared (NIR) absorption in the range of 0.75-2.5 μm is in great demand for a variety of applications but currently the majority of commercial NIR devices are based on inorganic materials due to the limited number of organic materials that are active in this range. Here we present the preparation and optical studies of a new series of stable azulene-containing NIR chromophores whose absorption can be tuned in the range of 0.6-1.7 μm. DFT calculation revealed that the protonation-induced low excitation gap was attributable to the substantial intramolecular charge transfer. The Royal Society of Chemistry 2012.

Full text of DOI:10.1039/c2jm31098h

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COMMUNICATION
Azulene-containing organic chromophores with tunable near-IR absorption in
the range of 0.6 to 1.7 mm†
ab
a
Ting Ting Lin, Chaobin He,
ac
Hong Chi,ab Tao Tangac and Yee-Hing Laib
*
*
Fuke Wang,
Received 22nd February 2012, Accepted 8th April 2012
DOI: 10.1039/c2jm31098h
simply by controlling the substitution of the molecule. Most impor-
tantly, the obtained azulene-containing NIR chromophores are very
stable in air, making them promising for the fabrication of organic-
based NIR photonic devices.
Near-infrared (NIR) absorption in the range of 0.75–2.5 mm is in
great demand for a variety of applications but currently the majority
of commercial NIR devices are based on inorganic materials due to
the limited number of organic materials that are active in this range.
Here we present the preparation and optical studies of a new series
of stable azulene-containing NIR chromophores whose absorption
can be tuned in the range of 0.6–1.7 mm. DFT calculation revealed
that the protonation-induced low excitation gap was attributable to
the substantial intramolecular charge transfer.
Azulene has recently been named as an ‘‘aromatic chameleon’’
because of its unique structural and electronic properties.12 Azulene is
a polar (Pe ¼ 1.08 D),13 resonance-stabilized nonalternant aromatic
hydrocarbon. The large dipole moment arises from the electron drift
from its seven-membered ring to its five-membered ring, which leads
to its aromatic delocalization energy being 5 times lower than that of
benzene.14 Upon release or gain of a charge, totally different new
resonance-stabilized species – azulenylium ions – are formed.
Azulenylium ions are quite stable due to the formation of either the
aromatic cyclopentadienyl anion or cycloheptatrienyl (tropylium)
cation.15 The confinement of charges on the azulene ring in azulene-
containing conjugated compounds is expected to produce additional
stability for the resultant ions. Four azulene-containing compounds
and their corresponding ions with different substituents and conju-
gation length are thus designed and synthesized in this work.
The coupling of azulene with thiophene derivatives was achieved
through the nickel-catalyzed Grignard reaction of 1,3-dibromoazu-
lene with 2-bromo-3-methylthiophene and 2-bromo-3-phenyl-
thiophene to obtain compounds 1 (yield: 78%) and 2 (yield: 62%),
respectively (Fig. 1). To further extend the conjugation length, the
Near-infrared (NIR) absorption in the range of 0.75–2.5 mm is in
great demand for a variety of applications such as optical commu-
nications, energy, optoelectronics, bio-imaging, biosensors, and
thermal therapy.1–5 Currently, the majority of commercial NIR
devices are based on inorganic materials such as PbS, PbSe, and
InGaAs nanoparticles as they show absorption that can be tuned in
the range of 0.7–2.0 mm.6 These devices are, however, only suitable
for small-area applications due to the high cost of epitaxially grown
inorganic devices. Organic materials that are optically and thermally
active in the NIR region can in principle be used in NIR devices with
advantages of low-cost fabrication, potential to produce large-area
devices, light weight and flexibility. Currently, however, commercially
available organic NIR optoelectronic devices operating in the range
of 1–2 mm are extremely rare because of the limited number of
organic materials that are active in this range.5,7 Recently, various
novel NIR chromophores like phthalocyanine derivatives,8 porphy-
rins,9 squaraines,10 and polyaromatic dyes11 have been developed but
their structural complexity is generally associated with tedious
multistep synthesis and their absorption is typically less than 1 mm.
We here report the synthesis of azulene-containing organic chromo-
phores, whose absorption can be tuned in the range of 0.6–1.7 mm
aInstitute of Materials Research and Engineering, A*STAR (Agency for
Science, Technology and Research), 3 Research Link, Singapore 117602.
E-mail: wangf@imre.a-star.edu.sg; cb-he@imre.a-star.edu.sg; msehc@
nus.edu.sg; Fax: +65 6872-0785; Tel: +65 6874-8111
bDepartment of Chemistry, National University of Singapore, 3 Science
Drive 3, Singapore 117543
cDepartment of Materials Science and Engineering, National University of
Singapore, 9 Engineering Drive 1, Singapore 117576
† Electronic supplementary information (ESI) available: Detailed
experimental methods, NMR spectra, and UV-vis-NIR study. See
DOI: 10.1039/c2jm31098h
Fig. 1 Synthetic routes for neutral compounds 1–4.
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obtained compound 1 was brominated with NBS to give the dibro-
minated intermediate (yield: 86%); this was then reacted with excess
aromatic tins such as phenyl tri-n-butyltin and 5-phenylthienyl
tri-n-butyltin. In the presence of a catalytic amount of tetrakis-
(triphenylphosphine)palladium(0) (Pd(PPh3)4) and the electron-
donating ligand triphenylarsine (Ph3As), we obtained target
compounds 3 and 4 with yields of 61% and 56%, respectively.16 All
the obtained neutral compounds are soluble in organic solvents such
as chloroform, THF, and dichloromethane. Their structures and
1
purity were characterized by H NMR, 13C NMR and mass spec-
troscopy (see ESI†). The conversion of the neutral compounds to
their corresponding ions was achieved easily by protonation with
trifluoroacetic acid (TFA) (30%) in chloroform solution (Fig. 2).
Addition of TFA to solutions of the neutral compounds induces an
immediate color change. The conversion process can also be accel-
erated by water bath heating at 50 ꢀC.
Fig. 3 UV-vis spectra of neutral compounds in chloroform.
Due to the polarized and nonalternant p-electron system of azu-
lene, modest perturbation is expected to induce bond alternations in
azulene and significant changes of p-electron distribution on azulene
rings and thus of optical properties.17 The sensitivity of azulene
p-electrons to different substituents can be clearly seen from the
chemical shifts of its protons. A comparison of the proton chemical
shifts of azulene protons in compounds 1–4 shows remarkable
chemical shifts induced by the different substituents. The sensitivity of
azulene p-electron distribution to substitution allows the optical
properties of its derivatives to be easily tuned. As shown in Fig. 3, the
absorption maxima (lmax) of compounds 1, 2, 3, and 4 in chloroform
are observed at 299 (3 ¼ 5.9 ꢁ 104 Mꢂ1 cmꢂ1), 310 (3 ¼ 6.3 ꢁ
104 Mꢂ1 cmꢂ1), 326 (3 ¼ 7.1 ꢁ 104 Mꢂ1 cmꢂ1), and 375 (3 ¼ 5.7 ꢁ
104 Mꢂ1 cmꢂ1) nm, respectively (Fig. 3). This increasing red shift from
compound 1 to 4 is attributed to the increase of effective conjugation
length of these compounds induced by the substituents.18
conjugation backbone (Fig. 4a). In addition to the remarkable
bathochromic shift of spectra, protonation also leads to significant
enhancement of the molar extinction coefficient (3) of these azulene-
containing compounds. As shown in Fig. 4b, addition of TFA to the
neutral compound solutions not only induces solution color change,
but also remarkably increases solution optical density. The high
optical densities of protonated compounds make their solutions
darken and further dilution is required to see the real color of the
solution by eye (Fig. 4b). The high optical densities of protonated
compounds suggest that they exhibit light-harvesting ability as
compared to neutral ones. The measured molar extinction coeffi-
cients for protonated compounds 1A, 2A, 3A, and 4A are 2.76 ꢁ 105
(lmax ¼ 548 nm), 3.14 ꢁ 105 (lmax ¼ 996 nm), 5.59 ꢁ 105 (lmax
¼
1.3 mm), and 1.91 ꢁ 105 (lmax ¼ 1.7 mm) Mꢂ1 cmꢂ1, respectively. The
enhanced molar extinction coefficient induced by protonation could
be attributed to the higher polarity of the protonated compounds,
which are expected to have a faster charge transfer from HOMO to
LUMO. The thermal stability of the protonated compounds was
investigated by incubation of the solutions in sealed vials for half
a year at room temperature. UV-vis-NIR spectra of protonated
The tuning of the absorption by substituents was more significant
when the neutral compounds were protonated by TFA. Protonation
of azulene-containing compounds leads to remarkable changes in the
spectra and the appearance of new peaks in the visible and NIR
regions (Fig. 4a). For example, upon protonation, the methyl-
substituted compound 1A shows a new peak at 548 nm, while the
protonated phenyl-substituted compound 2A shows absorption at
996 nm. The change from the methyl group to phenyl group induced
a red shift of about 450 nm. The tuning can also be controlled by the
substituent positions. The change of phenyl substituent position from
C2 on thiophene ring (as in compound 2A) to C5 (as in compound
3A) leads to a change of the maximum absorption from 1.0 to 1.3 mm.
The NIR region absorption can be further extended by increasing the
conjugation length. We can adjust the maximum absorption to about
1.7 mm (compound 4A) if one more thiophene ring is inserted into the
Fig. 4 Optical spectra and photos of protonated compounds 1A–4A in
30% TFA–chloroform solution. (a) UV-vis-NIR spectra of protonated
compounds 1A–4A. (b) Photos of neutral and protonated compounds;
solutions in spectrophotometer cuvettes are corresponding protonated
compounds with further dilution.
Fig. 2 The protonation of neutral compounds to give ions 1A–4A.
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compounds show no apparent spectral change as shown in the ESI†.
This indicates that these azulene-containing organic chromophores
are promising candidates for NIR device applications.
structure. They are, however, split into two double peaks at 9.78 ppm
and 8.84 ppm in the protonated compound 1A. Similarly, peaks of
H-3 and H-5 change from one triple peak in neutral compound 1 to
two triple peaks in the protonated compound 1A with large down-
field shift (Fig. 5). All these observations suggest that only monop-
rotonation of the azulene-containing compounds occurs, and the
monoprotonation at position 1 or 3 induces an asymmetric structure
of the protonated compounds (Fig. 2). This is consistent with
previous reports, where it was established that only monoprotonation
of the hydrocarbon derivatives of azulene can occur.25
The newly appearing peaks in the vis–NIR region can be attributed
to the intramolecular charge transfer (ICT) transition in the
protonated azulene-containing compounds.19 Protonation of azulene
derivatives and azulene-containing polymers has been extensively
studied by our group20,21 and others,22 and protonation has been
employed to ‘‘switch on’’ the conductivity and luminescence of azu-
lene-containing organic materials. Both quantum chemical calcula-
tion and experimental results have unambiguously demonstrated that
protonation occurred predominately at the C-1 and C-3 atoms of
azulene (Fig. 2), which are by far the most stable isomers.23 Due to the
large dipole moment of azulene, the protonation is expected to induce
p-electron drift from the seven-membered ring of azulene to the five-
membered ring, forming the stable cycloheptatrienyl (tropylium)
cation. The delocalization of the charge on the azulene ring induces
charge separation in the protonated compounds, forming a stronger
electron-donating (D) and electron-accepting (A) structure.24
Protonation thus has a profound impact on the geometry of azulene
compounds and, as a consequence, significant effects on their optical
properties.
To better understand the ICT transition of the protonated azulene-
containing chromophores, the geometry of neutral and protonated
azulene-containing chromophores was optimized by density func-
tional theory (DFT) calculations using compounds 3 and 3A (with
trifluoroacetate as counter ion) as examples. Conformational prop-
erties and dipole moments of neutral compound 3 and protonated
compound 3A were theoretically calculated using B3LYP/6-31G*
geometry and natural bond orbital (NBO) population analysis. As
expected, protonation leads to a large increase in the dihedral angle
between the azulene ring and thiophene ring. The calculated angle of
protonated compound 3A (87.1ꢀ) is much larger than that of the
corresponding neutral compound 3 (44.5ꢀ), which is due to the
change of the sp2 carbon to sp3 carbon by protonation. Most
importantly, the calculated HOMOs and LUMOs show a remark-
ably different ICT interaction between the neutral and the protonated
compounds (Fig. 6). The HOMOs in the neutral compound 3 are
delocalized over the azulene ring and thiophene rings, while the
LUMOs are located mainly on the azulene unit. However, the
HOMOs of protonated compound 3A are all located on the thio-
phene rings and benzene rings, and the LUMOs are all localized on
the azulene ring. The distinct charge separation between HOMO and
LUMO in the protonated compound 3A indicates that substantial
charge transfer from the donor moiety (thiophene and benzene units)
to the accepting moiety (azulene unit) occurs when the molecules are
irradiated by light. The HOMO–LUMO transition can be considered
a charge-transfer transition. The significant intramolecular charge
transfer in the protonated compound 3A is also confirmed by the
dipole moment change. The calculated dipole moment for neutral
compound 3 is found to be 0.61 D, while the calculated dipole
moment for the protonated compound 3A is found to be 11.45 D.
In summary, we here presented a new series of azulene-containing
NIR organic chromophores with a tunable absorption range of 0.6 to
1.7 mm. The tuning of the maximum absorption can be easily adjusted
by the substituent groups, positions, and conjugated length. The NIR
The protonation was further studied by using nuclear magnetic
resonance (NMR) spectral analysis, for which compounds were
protonated by deuterated TFA (TFA-d). Typical 1H NMR spectra of
compound 1 before and after protonation in 30% TFA-d/CDCl3 are
shown in Fig. 5. Upon protonation, a remarkable downfield shift of
proton signals of azulene can be observed, with the largest downfield
shift up to 1.4 ppm. The large downfield shift of proton signals of
azulene indicates strong deshielding effects of the localized positive
charge, suggesting that the positive charge from protonation is
mainly localized on the azulene ring. Furthermore, protonation
changes the neutral compounds’ symmetric structure to become
asymmetric in protonated compounds. For instance, H-4 and H-8
(see the definition of proton numbers in Fig. 2 and 5) in neutral
compound 1 show one double peak at 8.38 ppm due to the symmetric
Fig. 5 1H NMR spectral change of compound 1 before and after
protonation in 30% TFA-d/CDCl3. Inset shows the splitting of methyl
peaks after protonation.
Fig. 6 Spatial distributions of the calculated HOMOs and LUMOs of
compounds 3 and 3A.
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12 H. Mollerstedt, M. C. Piqueras, R. Crespo and H. Ottosson, J. Am.
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transfer transition, which was illustrated clearly by theory calculations.
Because of the ease of preparation and manipulation of the molecular
structures of these azulene-containing chromophores, they show great
potential as promising NIR dye materials, whose ‘‘color’’ can be varied
over a wide range whilst maintaining their stability.
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