Engineering control over the conformation of the alkyne - Aryl bond by the introduction of cationic charge

Article abstract of DOI:10.1021/ol7016966

A novel arylene - ethynyiene molecule has been synthesized. This molecule is more stable in a coplanar form than in a twisted form as in the cases of typical arylene - ethynyiene molecules. When the cationic charge was introduced into the π-conjugated sys

Full text of DOI:10.1021/ol7016966

ORGANIC
LETTERS
2007
Vol. 9, No. 21
4195-4198
Engineering Control over the
Conformation of the Alkyne
−Aryl Bond
by the Introduction of Cationic Charge
Takashi Terashima, Takuya Nakashima, and Tsuyoshi Kawai*
Graduate School of Materials Science, Nara Institute of Science and Technology,
8916-5 Takayama, Ikoma, Nara 630-0192, Japan
tkawai@ms.naist.jp
Received July 18, 2007
ABSTRACT
A novel arylene−ethynylene molecule has been synthesized. This molecule is more stable in a coplanar form than in a twisted form as in the
cases of typical arylene
−
ethynylene molecules. When the cationic charge was introduced into the -conjugated system, the perpendicularly
π
twisted form became more stable than the coplanar state. The conformational change was controlled by introduction and removal of cationic
charge, confirmed by the absorption and fluorescence spectroscopy and DFT calculation.
Arylene-ethynylene derivatives, such as 1,4-bis(phenyl-
ethynyl)benzene, 9,10-bis(phenylethynyl)anthracene, and the
analogues oligomers and polymers1 are recently attracting
substantial interest due to their fairly high charge transport
capability and highly efficient luminescent properties. A
number of potential applications have been proposed such
as displays,2 sensors,3 and molecular electronic conducting
wires.4 The origin of these fascinating properties is mainly
attributed to their extended linear π-system and consequently
to the relative intramolecular orientation of planar aromatic
rings.
the neighboring aryl groups is the most stable both in the
ground state and in the excited state, which affords the well-
extended π-conjugation system. The energy barrier for the
rotation about the alkyne-aryl bond is relatively low in the
ground state,6 and the energy difference between the fully
coplanar structure and the perpendicularly twisted structure
has been estimated to be less than kT of room temperature.5c
Therefore, the rotation around the ethynyl group and the rapid
exchange between the coplanar form and the twisted form
(3) (a) Moon, J. H.; Deans, R.; Krueger, E.; Hancock, L. F. Chem.
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Dunkhorst, A.; Gilbert, J.; Bunz, U. H. F. Macromolecules 2005, 38, 4560-
4562.
The structure-property relationships of the arylene-
ethynylene derivatives have been studied extensively.5 In
most molecules and polymers, the coplanar structure over
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10.1021/ol7016966 CCC: $37.00
© 2007 American Chemical Society
Published on Web 09/18/2007

are allowed. The conformational change of 1,4-bis(phenyl-
ethynyl)benzene derivatives resulting from the rotation about
the central axis has been reported to modulate the conducting
properties at the single molecular level.7 Several groups have
achieved a certain degree of configurational control between
the coplanar form and the twisted form in the sterically
restricted system such as Langmuir-Blodgett films,8 self-
assembled monolayers,9 intramolecular tethering,10 and liquid
crystalline aggregates.11 From the viewpoint of the molecular
switching materials, stimuli-responsive molecular units whose
coplanarity can be modulated by external stimulation are
worth studying. For example, Yamamoto et al. have prepared
unique π-conjugated polymers based on the arylene-
ethynylene structure containing the imidazole group, and
protonation of the imidazole unit in the main chain has
resulted in the blue shift of the HOMO-LUMO absorption
band. They suggested reversible modulation of the copla-
narity of the main chain with the reversible transformation
between imidazole and imidazolium.12a However, to the best
of our knowledge, none has performed systematic study on
the conformational modulation of the arylene-ethynylene
unit by the reversible protonation-deprotonation of the
arylene groups. In the present study, we report a new
bisimidazolyl-ethynyl anthracene whose π-conjugation sys-
Chart 1. Compounds 1-3
tem is controlled by means of the molecular conformation
along the alkyne-aryl bonds.
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Compound 1 whose chemical structure is shown in
Scheme S-1 (Supporting Information) was synthesized by
the cross-coupling reactions of 9,10-dibromoanthracene and
ethynylated imidazole derivatives.13 Compounds 2 and 3
were also synthesized as the reference compounds. The
molecular structures and optical properties of these com-
pounds are summarized in Chart 1 and Table 1, respectively.
Table 1. Experimental and Calculated Photophysical Properties
1
2
3
observed λabs (nm)
observed ꢀ (M-1‚cm-1
observed λem (nm)
511
490
476
)
4.5 × 104 6.0 × 104 4.8 × 104
555
531
0.80
3.4
523
4°
497
0.80
2.9
emission quantum yield (φ) 0.72
emission lifetime (ns)
calcd λabsa (nm)
calcd dihedral angleb
2.7
567
14°
470
89°
Calcd UV-vis spectra and b calcd dihedral angles between anthracene
and imidazole were estimated by Gaussian03. All other conditions are
presented in the Supporting Information.
a
Compounds 1 and 2 show characteristic absorption bands at
511 and 490 nm and fluorescence emissions at about 555
and 531 nm, respectively. It should be noted that these
molecules show clear fluorescence emission with quantum
yields as high as 80%. The characteristic emission wave-
length and absorption maximum wavelength of the com-
pound 2 are markedly shorter than those of compound 1.
This difference in the HOMO-LUMO gap energy might
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4196
Org. Lett., Vol. 9, No. 21, 2007

fluorescence. These blue shifts in both fluorescence and
absorption spectra indicate that the π conjugation length
becomes short upon the protonation in both the ground and
the excited states. Taking the symmetrical structure into
account from the change in the 1H NMR spectra of 1 caused
by the addition of TFA-d,13 diprotonation of 1 was concluded.
The absorption and fluorescence spectra of the protonated
form of compound 2 in the presence of TFA were also the
same as those of compound 3. That is, the protonated forms
of compounds 1 and 2 have electronic and, thus, geometrical
structures similar to those of compound 3.
To evaluate the origin of the spectral blue shifts in the
imidazolium form, we performed quantum mechanical
calculations of the geometrical and electrical structure and
the basic photochemical properties for compounds 1-3 by
DFT14 and TDDFT15 methods at the B3LYP/DGDZVP level.
For the calculation, the Gaussian0316 suite of programs was
used. From the most stable optimized geometries of 1-3
depicted in Figure 2, the approximate dihedral angles
Figure 1. (A) Absorption spectra of 1 with TFA concentrations
of (a) 0, (b) 0.4, (c) 6.3, (d) 25.0, and (e) 2500 mM and 3 in DMSO.
(B) Fluorescence spectra of 1 with TFA concentration of (a) 0, (b)
0.4, (c) 6.3, (d) 25.0, and (e) 2500 mM and 3 in DMSO excited at
360 nm. [1] ) 5.0 µM. [3] ) 5.0 µM.
be attributed to the restricted π-conjugational expansion in
compound 2 originated from the disconnection of the
π-conjugation at the imidazole groups. Considering the
resonance structure of 3 whose delocalized π-electron
structure seems to be a hybrid of 1 and 2, the intermediate
wavelength is expected. However, both absorption and
emission peaks of 3 appeared at shorter wavelengths than
those of compounds 1 and 2. These results suggest that
π-conjugation length is not necessarily determined by the
π-electron resonance structure on imidazolium under as-
sumption of the coplanar structure. Fluorescence lifetime
studies13 revealed that the emission decay from each
compound exhibited monoexponential decay, confirming the
presence of a single emitting species.
Figure 2. DFT-based optimized structures of 1-3 and dihedral
angles between anthracence and imidazole, respectively.
As shown in Figure 1a, when TFA, as a proton source,
was added, the absorption peak at 511 nm was suppressed
and an another absorption band appeared at the shorter
wavelength continuously. Finally, the absorption peak almost
coincided with that of 3. The resulting spectrum reverted to
the starting spectrum by the addition of triethylamine (TEA)
as a base, indicating the reversible switching in the absorption
spectrum. The presence of quasi-isosbestic points at 360 and
477 nm supported the reversible quasi-two-component equi-
librium reaction, showing difficulty in distinguishing between
the mono- and the diprotonated forms. In a fluorescence
mesurement, the peak at 555 nm decreased and a new
fluorescence band appeared at around 493 nm, finally
coinciding with that of 3. A significant color change from
yellow to greenish blue was visibly observed. The fluores-
cence spectrum also reverted to the starting spectrum by the
addition of TEA, also indicating the reversible switching of
between the anthracene and the imidazole unit were evaluated
to be 14° for 1 and 4° for 2, which correspond to the coplanar
structure. Meanwhile, the dihedral angle for 3 was 89°, which
indicates a perpendicularly twisted form. The energy differ-
ence between the most stable form and the unstable twisted
or coplanar forms was evaluated to be significantly larger
than the kT at room temperature (0.6 kcal/mol) for each
molecule, to shift the population over the conformations.13
(14) (a) Lee, C.; Yang, W.; Parr, R. G. Phys. ReV. B 1988, 37, 785-
789. (b) Becke, A. D. J. Chem. Phys. 1993, 98, 5648-5652.
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1998, 109, 8218-8224. (b) Bauernschmitt, R.; Ahlrichs, R. Chem. Phys.
Lett. 1996, 256, 454-464. (c) Casida, M. E.; Jamorski, C.; Casida, K. C.;
Salahub, D. R. J. Chem. Phys. 1998, 108, 4439-4429.
(16) Gaussian 03, revision B.03; Gaussian, Inc.: Pittsburgh, PA, 2003.
See Supporting Information for the full reference.
Org. Lett., Vol. 9, No. 21, 2007
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The dihedral angles between the phenyl group and the
imidazolium group of compounds 1 and 2 were also
significantly smaller than those of compound 3. HOMO and
LUMO orbitals of compound 1 markedly expand throughout
the molecule, and those of compound 2 expand between
imidazoles. The HOMO-6 and LUMO orbitals of compound
3, which are responsible for the absorption band at 476 nm,
are confined in the ethynyl anthracene unit (Figure 3). It
imidazolium units, the π-orbitals of two imidazolium units
are in conjugate with that of the central ethynylanthracene
unit. Consequently, the calcd λabs of 3 approached those of
compound 1 and 2 in the most stable coplanar conformations.
On the contrary, significant blue shifts are observed for
compounds 1 and 2 when they are assumed to be in the
unstable twisted conformation.13 So the significant twist
around the imidazolium units seems to be the origin of the
spectral blue shift in the optical spectra of compound 3.
Yamamoto et al. have suggested that the repulsive interaction
between the positively charged N-H unit in the imidazolium
group and the partially polarized C-H unit would be
responsible for the twisted geometry.12a The hyperconjugation
between the N+-H σ orbital and the p-orbitals of the
neighboring ethynyl and aryl groups may stabilize the
positive charges in the imidazolium groups in the twisted
form of compound 3.
In conclusion, the engineering control over the molecular
conformation between a coplanar structure and a twisted
structure was achieved by the introduction of cationic charge
into the imidazole unit. These showed significantly high
fluorescence emission quantum yields in both the protonated
and deprotonated forms. The absorption and fluorescence
spectra and DFT calculations revealed that the coplanar
structures are more stable than the twisted one for neutral 1.
When cationic charges were introduced, the perpendicularly
twisted structure became more stable than the coplanar one.
This mechanism would open up a new design concept for
controlling the molecular conformation in the alkyne-aryl
bond of the conjugated molecules and polymers.
Figure 3. Frontier orbitals of compounds 1-3 calculated by the
B3LYP/DGDZVP DFT method.
Acknowledgment. This work was partly supported by a
Grant-in-Aid for Scientific Research in Priority Area “Super-
Hierarchical Structure” and also by a Grant-in-Aid for
Scientific Research (B) from the Ministry of Education,
Culture, Sports, Science and Technology, MEXT, Japan. T.T.
acknowledges JSPS for a Research Fellowship for Young
Scientists. T.K. acknowledges Prof. T. Yamamoto in Tokyo
Institute of Technology and Prof. Y. Aso in Osaka University
for their valuable discussions.
should be noted that the orbitals HOMO∼HOMO-5 were
localized on the iodide ion and that the transitions between
the iodide ion and the molecule showed almost zero oscillator
strengths. The TDDFT calculations predicted the distinct blue
shift of the absorption by the transformation from the
imidazole form to the imidazolium form (Table 1). The calcd
λabs were in good agreement with the experimental data. The
calcd absorption bands shifted to the red by 10-60 nm with
respect to the measured spectra, but the relative position of
the transition gave satisfactory results. In neutral compounds
1 and 2, the π-orbitals of two imidazole units are in conjugate
with that of the central ethynylanthracene unit. Under the
assumption of the coplanar form of 3 obtained by constrain-
ing the dihedral angles of 0° between the anthracene and
Supporting Information Available: Experimental pro-
cedures and data for all new compounds, fluorescence decay
data, NMR spectrum, optimized geometries, their relative
enegies, and molecular orbitals. This material is available
free of charge via the Internet at http://pubs.acs.org.
OL7016966
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