Synthesis and Optical Properties of Imidazole- and Benzimidazole-Based Fused π-Conjugated Compounds: Influence of Substituent, Counteranion, and π-Conjugated System

Article abstract of DOI:10.1021/acs.joc.5b01028

Fused π-conjugated imidazolium chlorides having hydrogen (1-Cl), octyloxy (2-Cl), N,N-dibutylamino (3-Cl), trifluoromethyl (4-Cl), and cyano (5-Cl) groups substituted on the benzene ring at the 2-position of imidazole were prepared. Counteranion exchanges from chloride to bis(trifluoromethanesulfonyl)imidate (2-TFSI) and tetrafluoroborate (2-BF4) were performed. The optical properties of these compounds (absorption and emission wavelengths, fluorescence quantum yield, and solvatochromism) were influenced by both the substituent and anion character, which was investigated by theoretical calculations using the density functional theory (DFT) and symmetry-adapted cluster-configuration interaction (SAC-CI) methods. Fused π-conjugated benzimidazolium chlorides having N,N-dibutylamino (6-Cl) and cyano (7-Cl) groups were also prepared to observe the different solvatochromic shifts.

Full text of DOI:10.1021/acs.joc.5b01028

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Article
Synthesis and optical properties of imidazole and benzimidazole-
based fused pi-conjugated compounds: Influence of
substituent, counter anion, and pi-conjugated system
Koji Takagi, Kazuma Kusafuka, Yohei Ito, Koji Yamauchi, Kaede Ito, Ryoichi Fukuda, and Masahiro Ehara
J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.joc.5b01028 • Publication Date (Web): 23 Jun 2015
Downloaded from http://pubs.acs.org on June 25, 2015
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The Journal of Organic Chemistry
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Synthesis and optical properties of imidazole and benzimidazole-
based fused π-conjugated compounds: Influence of substituent,
counter anion, and π-conjugated system
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Koji Takagi,a,* Kazuma Kusafuka,a Yohei Ito,a Koji Yamauchi,a Kaede Ito,a
Ryoichi Fukuda,b,c,* and Masahiro Eharab,c
aDepartment of Materials Science and Engineering, Graduate School of Engineering, Nagoya Institute
of Technology, Gokiso, Showa, Nagoya, 466-8555 Japan
bResearch Center for Computational Science, Institute for Molecular Science, 38 Nishigo-Naka,
Myodaiji, Okazaki, 444-8585 Japan
cElements Strategy Initiative for Catalysts and Batteries (ESICB), Kyoto University, Kyoto, 615-8520
Japan
*Corresponding author: takagi.koji@nitech.ac.jp (K.T.) and fukuda@ims.ac.jp (R.F.)
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Abstract: Fused π-conjugated imidazolium chlorides having the hydrogen (1-Cl), octyloxy (2-Cl), N,N-
dibutylamino (3-Cl), trifluoromethyl (4-Cl), and cyano (5-Cl) groups-substituted benzene ring at the 2-
position of imidazole were prepared. The counter anion exchanges from chloride to
bis(trifluoromethanesulfonyl)imidate (2-TFSI) and tetrafluoroborate (2-BF4) were performed. The
optical properties of these compounds (absorption and emission wavelengths, fluorescence quantum
yield, and solvatochromism) were influenced by both the substituent and anion character, which was
investigated by the theoretical calculations using the density functional theory (DFT) and symmetry-
adapted cluster-configuration interaction (SAC-CI) methods. Fused π-conjugated benzimidazolium
chlorides having N,N-dibutylamino (6-Cl) and cyano (7-Cl) groups were also prepared to observe the
different solvatochromic shifts.
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Keywords: imidazole / imidazolium / conjugated system / optical properties / theoretical calculation
Table of content
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INTRODUCTION
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π-Conjugated molecules have many fascinating properties such as the light absorption, fluorescence
emission, and charge transport, which leads to optoelectronic device applications including the
photovoltaic cell, light emitting diode, and field-effect transistor. The intrinsic character of π-conjugated
molecules can be tuned by the molecular design, and the advancement in the organic synthetic
chemistry enables the preparation of numerous π-conjugated molecules. Especially, ladder and fused π-
conjugated molecules1 have been attracting much attentions because of the rigid and coplanar structure
with the restricted conformational change that give rise to intriguing properties such as the intense
luminescence2 and remarkable hole/electron mobility.3 In addition to the ladder and fused π-conjugated
molecules comprised solely of the hydrocarbon skeleton, the incorporation of heteroatoms like sulfur
(thiophene unit) influences the environmental stability and molecular packing structure, which
contributes to the improvement of the device performance. Thus the development of new fused
oligothiophene derivatives and the understanding of structure-properties relation are the focus of interest
in the recent material chemistry.4 Other fused π-conjugated molecules including heterole (borole, furan,
silole, and phosphole units) are also attractive materials for optoelectronic applications.
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On the other hand, imidazole is a versatile building block for the selective introduction of aromatic
rings at the 1, 2, 4, and 5-positions.5 The transformation to imidazolium cation through the alkylation at
the 3-position further imparts photophysical,6 chemical,7 self-organization,8 catalytic,9 and ion
transport10 properties. Accordingly, fused π-conjugated imidazolium compounds would be expected as
the novel functional materials. With these backgrounds in our mind, we have previously synthesized
two fused π-conjugated imidazolium compounds with the rigid structure.11 The compound having the
methylene bridge (0-Cl in Figure 1) showed the absorption and emission spectra with the vibronic fine
structure and relatively high fluorescence quantum yields, however, the emission color-tuning was
impossible due to the limitation of the synthetic method.
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We herein describe the synthesis of new fused π-conjugated imidazolium chlorides having the
electron-donating and accepting substituents at the para-position of the benzene ring at the 2-position of
imidazole (1-Cl, 2-Cl, 3-Cl, 4-Cl, and 5-Cl). The exchange of counter anion was also carried out (2-
TFSI and 2-BF4). In addition, fused π-conjugated benzimidazolium chlorides were synthesized (6-Cl
and 7-Cl). The absorption and emission wavelengths, fluorescence quantum yield, and solvatochromism
were investigated in conjunction with the theoretical calculation to examine the emission color-tuning
possibility and understand the structure-properties relationship of these compounds.
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Figure 1. Chemical structure of fused π-conjugated imidazolium and benzimidazolium compounds
including 0-Cl reported previously.
RESULTS AND DISCUSSION
Influence of substituent
Initially, a fused π-conjugated imidazolium chloride (1-Cl) was synthesized from 2-phenylimidazole
by the bromination at the 4,5-positions, Suzuki coupling with 2-hydroxymethylphenylboronic acid,
chlorination using SOCl2, and LiHMDS-promoted cyclization according to our previous report.11 The
double cyclization did not proceed effectively even when the reaction was performed in DMF. The
single cyclization product was contaminated in the crude reaction mixture. Therefore the solution of the
crude product in DMSO was heated to complete the reaction and 1-Cl was finally obtained in 67%
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yield. The UV-vis absorption and fluorescence emission spectra of acyclic 4,5-bis(2’-
hydroxymethylphenyl)-2-phenylimidazole (2) and 1-Cl were measured in MeOH (Figure 2). While the
absorption spectrum of 2 showed a smooth curve, that of 1-Cl showed the vibronic fine structure. Both
the absorption and emission spectra of 1-Cl (λabs at 321 nm and λem at 397 nm) exhibited the
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bathochromic shift from those of 2 (λabs at 294 nm and λem at 385 nm), and the fluorescence quantum
yield was increased from 0.01 (2) to 0.29 (1-Cl). These results indicate the wide conjugation system of
1-Cl. The absence of the vibronic fine structure in the emission spectrum of 1-Cl and the lower
fluorescence quantum yield compared with 0-Cl (0.62) could be ascribed to the C–C bond rotation
around the benzene (2-position) and imidazolium rings leading to the emission from the energetically-
equivalent states and non-radiative decay processes, respectively.
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Wavelength/nm
450
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Figure 2. UV-vis absorption and fluorescence emission spectra of 2 (broken line) and 1-Cl (solid line)
in MeOH.
Subsequently, 2-Cl and 3-Cl bearing the electron-donating octyloxy and N,N-dibutylamino groups
were synthesized from 1,2-bis(2’-(hydroxymethylphenyl)ethyne in five steps (Scheme 1). 4,5-Bis(2’-
hydroxymethylphenyl)-2-arylimidazoles were prepared through the imidazole ring formation from 1,2-
diketone and aryl aldehyde derivatives because the bromination of electron-rich 2-arylimidazole
compounds may suffer from the undesirable reaction on the aryl ring instead of the imidazole one. The
methylene proton signals at the bridge point of 2-Cl (5.88 ppm) and 3-Cl (5.86 ppm) exhibited the
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upfield-shift from 1-Cl (5.93 ppm) suggesting the electron-rich character of the fused π-conjugated
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OTBDMS
OH
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HO
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Ar
Ar
Ar
Cl
Cl
HN
NH
e
HN
N
d
N
N
Cl
Cl
HO
OH
Key: a) TBDMSCl, imidazole, DMF, rt; b) KMnO4, NaHCO3, MgSO4, acetone/H2O, rt; c) ArCHO, NH4OAc, L-proline,
EtOH/DOX, reflux; d) SOCl2, MeCN, rt; e) LiHMDS, DMF, rt then DMSO, 50 °C
Scheme 1. General synthetic route to fused π-conjugated imidazolium compounds.
The UV-vis absorption and fluorescence emission spectra were measured in MeOH (Figure 3A). The
absorption and emission maxima of 2-Cl (λabs at 324 nm and λem at 395 nm) were comparable to those
of 1-Cl, however, the peak maxima of 3-Cl (λabs at 349 nm and λem at 420 nm) were obviously red
shifted. The fluorescence quantum yields of 2-Cl and 3-Cl were 0.62 and 0.40, respectively. The density
functional theory (DFT) calculation was performed using the Gaussian 09W program at the B3LYP/6-
31G(d) level of theory to optimize the ground state structure of the model compounds (2’-Cl and 3’-
Cl).12 The dihedral angles between the imidazolium and benzene (2-position) rings of 2’-Cl and 3’-Cl
were 38.5 ° and 34.4 °, respectively, reflecting the pronounced charge transfer from the strong electron-
donating N,N-dimethylaminophenyl group to the fused imidazolium cation unit in 3’-Cl. The time
dependent (TD)-DFT calculation was then performed at the B3LYP/6-311+G(d,p) level of theory to
estimate the vertical excitation (UV-vis absorption) energy. For 2’-Cl, the one-electron transition with
the largest oscillator strength (f=0.492) was from the highest occupied molecular orbital (HOMO) to the
second lowest unoccupied molecular orbital (LUMO+1) (Figure 4 and Table S1). The frontier
molecular orbital (FMO) of HOMO was spread over the hole molecule and that of LUMO+1 was
located on the fused imidazolium moiety. On the other hand, for 3’-Cl, the important one-electron
transition (f=0.664) was from HOMO to LUMO. The FMOs of HOMO and LUMO were located on the
N,N-dimethylaminophenyl and central imidazolium moieties, respectively, implying the intramolecular
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charge transfer characteristic. The smaller calculated one-electron transition energy of 3’-Cl (3.76 eV)
compared with 2’-Cl (4.17 eV) agreed well with the observed red-shifted absorption spectrum of 3-Cl.
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Figure 3. UV-vis absorption and fluorescence emission spectra of (A) 2-Cl (broken line) and 3-Cl
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–4.83 eV
–5.32 eV
–4.53 eV
–4.96 eV
–5.65 eV
–4.68 eV
–8.29 eV
3’-Cl
–8.85 eV
2’-Cl
–9.23 eV
–9.33 eV
4-Cl
5-Cl
Figure 4. Frontier molecular orbital diagrams involved in the vertical excitation, calculated using TD-
DFT//B3LYP/6-311+G(d,p), based on the optimized ground state geometry. Red, blue, and green lines
indicate energy levels of HOMO, LUMO, and LUMO+1, respectively.
4-Cl and 5-Cl bearing the electron-accepting trifluoromethyl and cyano groups were likewise
synthesized. The methylene proton signals at the bridge point of 4-Cl and 5-Cl (5.97 ppm) showed the
downfield-shifts from 1-Cl suggesting the electron-poor character of the fused π-conjugated system.
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The UV-vis absorption and fluorescence emission spectra were measured in MeOH (Figure 3B). The
absorption maxima at around 305 nm and 320 nm were similarly observed for both compounds, but the
absorbance at around 350 nm was considerably increased for 5-Cl. The emission spectrum of 5-Cl (λem
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at 448 nm) was red shifted compared with 4-Cl (λem at 417 nm). The fluorescence quantum yields of 4-
Cl and 5-Cl were 0.41 and 0.20, respectively. The DFT (geometry optimization at the B3LYP/6-31G(d)
level of theory) and TD-DFT (estimation of the vertical excitation energy at the B3LYP/6-311+G(d,p)
level of theory) calculations were performed for 4-Cl and 5-Cl, and the results are indicated in Figure 4
and Table S1. The one-electron transitions with the largest oscillator strength were always from HOMO
to LUMO+1. Both FMOs were distributed on the fused imidazolium moiety. On the other hand, the
one-electron transitions with the second largest oscillator strength was from HOMO to LUMO, and the
oscillator strength of 5-Cl (f=0.231) was larger than that of 4-Cl (f=0.187). From the FMO topologies,
these HOMO-LUMO one-electron transitions indicate the intramolecular charge transfer induced by the
strong electron-accepting ability of the substituent. Bielawski et al also described that the installation of
the 4-cyanophenyl group at the C1 position of imidazolium compound resulted in the noticeable charge
transfer interacton.6d Accordingly, the theoretical TD-DFT calculation nicely accounts the trend of UV-
vis absorption spectra of these fused π-conjugated imidazolium compounds and is the helpful skill to
predict their optical properties, although there are some discrepancies between the calculated and
experimental values.13
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In order to deeply understand the optical properties of these compounds, the UV-vis absorption and
fluorescence emission spectra of 3-Cl and 5-Cl were measured in 1,4-dioxane (DOX), tetrahydrofuran
(THF), acetone, N,N-dimethylformamide (DMF), dimethylsulfoxide (DMSO), and acetonitrile (AN).
The absorption spectra was affected by the solvent character (Figure S20), however, the influence was
relatively small or no clear relationships were observed against the orientational polarizability14 or
solvent dielectric. On the other hand, the emission wavelength of 5-Cl was found to be more susceptible
to the solvent character and was gradually red shifted with increasing the solvent polarity (Figures 5B).
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Figure 5C shows the plots of Stokes shifts against the Reichardt’s solvent polarity parameters
(ET(30)).15 The larger positive solvatofluorochromic shift of 5-Cl compared with 3-Cl was obtained.
The Stokes shifts were ranged from 4620 cm-1 (DOX) to 5980 cm-1 (AN). This result is in sharp contrast
to 0-Cl that exhibits the negligible solvatochromic character,11 and the intramolecular charge transfer
might be responsible for the bathochromic shift of the emission wavelength in polar solvents (vide
infra). The fluorescence quantum yields were also dependent on the solvent character (Table S2).
However, the results are difficult to explain because the solvent polarity influences not only the
intramolecular charge transfer but also the cation-anion interaction, which is discussed in the next
section.
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Figure 5. Fluorescence emission spectra of (A) 3-Cl and (B) 5-Cl in various solvents (black: DOX,
purple: THF, red: acetone, pink: DMF, orange: AN, and yellow: DMSO). (C) Plots of Stokes shifts
against ET(30) values of solvents for 3-Cl (open circle) and 5-Cl (closed circle).
Influence of counter anion
We also performed the anion exchange reaction from chloride to bis(trifluoromethanesulfonyl)imidate
and tetrafluoroborate. The anion metathesis from chloride to bis(trifluoromethanesulfonyl)imidate was
performed in the CHCl3/water biphasic system giving rise to 2-TFSI simply by evaporating the organic
phase. Et3OBF4 was used for the anion metathesis from chloride to tetrafluoroborate following to the
Bielawski’s protocol.16 By introducing the bulky counter anion, the melting point was decreased from
1
255 °C (2-Cl) to 235 °C (2-BF4) and 126 °C (2-TFSI). Figure S16 shows the H NMR spectra in
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CDCl3 (2 mM, room temperature) of three fused π-conjugated imidazolium compounds. The methylene
proton signal at the bridge point exhibited the downfield-shift from 5.08 ppm (2-Cl) to 5.42 ppm (2-
BF4) and 5.51 ppm (2-TFSI). In the spectrum of 2-Cl, the proton signals were generally broad. These
results likely suggest that the chloride anion, in which the charge is localized on the chlorine atom, has
the strongest interaction with the imidazolium cation.17 The UV-vis absorption and fluorescence
emission spectra were measured in THF (Figure S21). The absorption spectra showed very small shifts
by changing the counter anion and the tailing to the longer wavelength was observed only for 2-Cl.
Because the emission spectra were completely overlapped, the fused π-conjugated imidazolium cation
should be the emissive core. On the other hand, the fluorescence quantum yield was dependent on the
counter anion. The low fluorescence quantum yield of 2-Cl (0.31) was improved upon the introduction
of counter anions with the weak coordination ability, namely, 0.45 for 2-BF4 and 0.62 for 2-TFSI.
Matsumoto et al also reported the similar results for diimidazoquinoxalinium salts.6j The orders of the
methylene proton chemical shift and the fluorescence quantum yield are in good accordance each other.
Therefore it can be considered that the strong interaction between the fused π-conjugated imidazolium
cation and chloride anion in 2-Cl results in the non-radiative decay from the excited state, particularly in
less polar THF. In polar MeOH solution, on the other hand, the ionic species separate by the solvation to
suppress the non-radiative decay recovering the relatively high fluorescence quantum yield (0.62).
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Influence of π-conjugated system
As mentioned above, the fused π-conjugated imidazolium compounds are the potential candidate for
the fluorescent material. The emission wavelength can be tuned by the substituent and the emission
intensity can be controlled by the counter anion. In the third section, the fused benzimidazolium
compounds were synthesized to extend the effective conjugation length and compare the
absorption/emission behavior with the fused imidazolium compounds. Herein, 6-Cl and 7-Cl bearing
N,N-dibutylamino and cyano groups, respectively, were prepared following to Scheme 2. Acyclic
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precursors bearing the 2-hydroxymethylphenyl group were obtained by the Suzuki coupling reaction of
2-aryl-4,7-dibromo-1H-benzimidazole with 2-hydroxymethylphenylboronic acid. After the chlorination
with SOCl2, 2-aryl-4,7-bis(2’-chloromethylphenyl)-1H-benzimidazolium chloride was treated with
LiHMDS to give the fused π-conjugated benzimidazolium chlorides. The chemical shifts of the
methylene proton at the bridge point of 6-Cl and 7-Cl were comparable with 3-Cl and 5-Cl bearing the
same substituent.
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Ar
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HO
OH
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NH2
Br
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a
N
NH
N
NH
Br
Br
Br
Br
Br
Cl
Ar
Ar
Cl
Cl
Cl
e
d
N
N
HN
NH
Key: a) NaBH4, CoCl2, THF/EtOH, reflux; b) ArCHO, ZrCl4, CHCl3, rt; c) 2-(hydroxymethyl)phenylboronic acid,
Pd(PPh3)4, 2M aq. Na2CO3, THF, reflux; d) SOCl2, MeCN, rt; e) LiHMDS, DMF, rt then DMSO, 50 °C
Scheme 2. General synthetic route to fused π-conjugated benzimidazolium compounds.
Figure 6 shows the UV-vis absorption and fluorescence emission spectra of compounds consisting of
the same substituent and different π-conjugated system in MeOH. Both the absorption and emission
spectra of 6-Cl (λabs at 368 nm and λem at 474 nm) showed the bathochromic shift compared with 3-Cl
(Figure 6A). Unexpectedly, the fluorescence quantum yield of 6-Cl was quite low (0.03). From the DFT
calculation of the model compounds (6’-Cl) at the B3LYP/6-31G(d) level of theory, the dihedral angle
between benzimidazolium and benzene (2-position) rings was 47.8 ° which is larger than that of 3’-Cl
(34.4 °). Accordingly, the photoinduced electron-transfer (PET) from the N,N-dibutylaminophenyl
moiety to the emissive benzimidazolium core might be a plausible reason to quench the fluorescence
emission,18 which is carefully discussed using the symmetry-adapted cluster-configuration interaction
(SAC-CI) calculation in the following section. The oscillator strength of fluorescence for 6’-Cl
calculated by the SAC-CI method is distinguishably smaller than that for 3’-Cl with reflecting the long-
distance electron-transfer induced by the photoemission. The absorption spectrum of 7-Cl did not
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exhibit an apparent bathochromic shift from 5-Cl, but the peak at 367 nm with a medium intensity was
red shifted compared with the peak maximum (λabs at 349 nm) of 5-Cl (Figure 6B). The DFT
calculation was performed for 7-Cl to find out that the dihedral angle between benzimidazolium and
benzene (2-position) rings was 57.0 °, which is larger than that of 5-Cl (44.7 °). This result is similar to
the relationship of 3’-Cl and 6’-Cl. The emission spectrum of 7-Cl (λem at 524 nm) was obviously red
shifted compared with 5-Cl and the fluorescence quantum was low (0.03).
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A
B
280
380
480
Wavelength/nm
580
680
280
380
480
580
680
Wavelength/nm
Figure 6. UV-vis absorption and fluorescence emission spectra of (A) 3-Cl (broken line) and 6-Cl
(solid line) and (B) 5-Cl (broken line) and 7-Cl (solid line) in MeOH.
The solvatochromic behaviors of 6-Cl and 7-Cl were examined by collecting the UV-vis absorption
(Figure S22) and fluorescence emission spectra in various solvents. Both 6-Cl and 7-Cl had larger
Stokes shifts compared with the fused π-conjugated imidazolium compounds (3-Cl and 5-Cl), although
the spectra of 7-Cl in DOX and THF could not be obtained due to its low solubility. As indicated in
Figure 7, 6-Cl having the electron-donating N,N-dimethylamino group exhibited a clear positive
solvatofluorochromic shift, which is in contrast to the trend observed for 3-Cl (Figure 5). The Stokes
shifts were ranged from 5140 cm-1 (DOX) to 6770 cm-1 (DMSO).
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The Journal of Organic Chemistry
9500
B
A
1
2
C
AN
DMF
8500
7500
6500
5500
4500
3
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DMSO
acetone
DOX
350
450
550
650
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47
350
450
550
650
ET (30)/kcal・mol-1
Wavelength/nm
Wavelength/nm
Figure 7. Fluorescence emission spectra of (A) 6-Cl and (B) 7-Cl in various solvents (black: DOX,
purple: THF, red: acetone, pink: DMF, orange: AN, and yellow: DMSO). (C) Plot of Stokes shifts
against ET(30) values of solvents for 6-Cl (open circle) and 7-Cl (closed circle).
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SAC-CI calculations
For detailed analyses and deep understanding of the substituent and π-conjugation effects, we
performed highly-accurate quantum chemical calculations by using the SAC-CI method19 for the
cationic frameworks of 3’-Cl, 5-Cl, 6’-Cl, and 7-Cl, where the amino group of 3-Cl and 6-Cl was
replaced by -N(CH3)2. Tables S4 and S5 show the low-lying UV-vis absorptions calculated by the SAC-
CI method. The results in gas phase as well as in solutions (DOX and AN) are shown. The solvent
effects were considered by the polarizable continuum model (PCM) SAC-CI method.20 The stick
absorption spectra are shown in Figure 8. For 3’-Cl, the 11A state calculated around 350 nm corresponds
to the observed absorption. The SAC-CI calculation indicates that this state exhibits a positive
solvatochromic shift due to the charge-transfer excitation character; however, a significant
solvatochromic shift was not observed in the measured spectra. For 5-Cl, the 11A state calculated
around 320 nm corresponds to the observed absorption with negligible solvatochromic shifts. The
observed solvatochromic shift was also small.
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Figure 8. The SAC-CI spectra for 3’-Cl, 5-Cl, 6’-Cl, and 7-Cl in gas phase, 1,4-dioxane, and
acetonitrile.
For 6’-Cl, small absorptions of the 11A state were calculated around 450 nm in DOX and AN solutions.
These states must correspond to the observed peak shoulder around 410 nm although the SAC-CI
calculations underestimated the excitation energy about 0.25 eV. The intensity of this state is small
because the weight of two-electron excitations becomes large. Namely, this state has a singlet biradical
character to some extent. Interestingly, this biradical-like state was not obtained by the SAC-CI
calculation in gas phase. More detailed computations are necessary for understanding this biradical-like
state; however, such study is beyond the purpose of this paper and will be reported in the future. The
main peak of 6’-Cl observed around 350 nm is assignable to the 11B state. For 7-Cl, the 11B state
calculated around 370 nm corresponds to the observed absorption. The SAC-CI calculation indicates
that this state exhibits a negative solvatochromic shift. In comparison with 3’-Cl and 5-Cl, the
absorptions of 6’-Cl and 7-Cl are generally calculated in lower energy with higher intensity. These
findings are the results of extending the π-conjugated systems. Comparing the 5-Cl and 7-Cl results
with 3’-Cl and 6’-Cl ones, we may find the substituent effect of -CN and -N(CH3)2. The 11A states
exhibit a high-energy shift by the substitution with the CN group. As a result, the lowest state of 7-Cl is
changed to the 11B states. These findings result in the substituent effects on the HOMO and LUMO; the
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The Journal of Organic Chemistry
electron withdrawing ability of CN group tends to localize the HOMO and LUMO to imidazolium
(benzimidazolium) and Ar-CN moieties, respectively, as we found in the DFT calculations.
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6
Table 1. Emission energy (EE/eV) and fluorescence wavelength (Flu/nm) of 3’-Cl, 5-Cl, 6’-Cl, and 7-Cl calculated by the SAC-CI method
7
8
for the lowest 1A and 1B states in gas phase, 1,4-dioxane (DOX), and acetonitrile (AN).
9
3’-Cl
5-Cl
6’-Cl
7-Cl
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11A
11B
11A
11B
11A
11B
11A
11B
EE / Flu
EE / Flu
EE / Flu
EE / Flu
EE / Flu
EE / Flu
EE / Flu
EE / Flu
2.99 / 415
(1.286)a
3.88 / 319
(1.425)a
3.23 / 384
(0.774)a
3.98 / /312
(1.700)a
4.27 / 290
(0.223)a
2.84 / 437
(0.411)a
3.42 / 363
(1.139)a
2.41 / 515
(0.262)a
Gas phase
DOX
AN
3.01 / 412
2.77 / 447
3.70 / 335
2.95 / 421
3.11 / 399
2.87 / 433
3.95 / 314
3.76 / 329
3.85 / 322
2.42 / 512
3.06 / 405
3.06 / 405
3.29 / 376
2.19 / 566
2.46 / 504
2.03 / 611
Exp (DOX)
2.89 / 429
2.92 / 425
2.96 / 419
2.81 / 441
2.70 / 460
2.55 / 487
NA
Exp (AN)
2.37 / 524
aOscillator strength.
Table 1 shows the emission energies and fluorescence wavelengths of 3’-Cl, 5-Cl, 6’-Cl, and 7-Cl
calculated by the SAC-CI method for the lowest 1A and 1B states. Both states are optically-allowed, and
therefore the observed fluorescence is emitted from the lower one. For 3’-Cl and 5-Cl, the lowest state
is the 11A, while the lowest state of 7-Cl is the 11B. The calculated oscillator strengths of fluorescence
are 1.286, 0.774, 0.411, and 0.262 for 3’-Cl, 5-Cl, 6’-Cl, and 7-Cl, respectively, in gas phase. Although
the fluorescence quantum yield is given by a competition of several relaxation pathways, the calculated
oscillator strengths indicate the importance of the electron-transfer quenching processes for these series
of compounds. Indeed, the differences of electric dipole momenta between the emissive and ground
states are 2.23, 10.77, 11.33, and 12.30 (Debye) for 3’-Cl, 5-Cl, 6’-Cl, and 7-Cl, respectively,
calculated by the SAC-CI method in gas phase. These values indicate that the electron distribution of 6’-
Cl is significantly changed by the fluorescence due to its charge-transfer (CT) transition character. In
accordance with the electric dipole moment, the CT character of 6’-Cl is as large as those of cyano
substituted 5-Cl and 7-Cl. Those fluorescence transitions are calculated to be the HOMO-LUMO
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transition (the orbital diagram is given in Figure S23). The distributions of HOMO and LUMO are
significantly different; therefore, the transition dipole and oscillator strength of 6’-Cl become small in
comparison with those of 3’-Cl. For 6’-Cl, the 11B is the lowest in gas phase or less polar environment;
however, the 11A shifts to the lowest state in polar solutions. Therefore the emission state of 6-Cl is
altered by the polarity of solvent environment that must be responsible for the significant
solvatofluorochromic shift. The calculated emission energies of 3’-Cl and 5-Cl agree well with the
experimental observation. The calculated results of 6’-Cl and 7-Cl reasonably agree with the
experiment.
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The calculated solvatochromic shifts of fluorescence are shown in Figure 9 with respect to the
logarithm of the dielectric constant (ε) of the solvent: ε = 1.0 for gas phase, ε = 2.21 for DOX, and ε =
35.69 for AN. The calculated solvatochromic shifts correlate better with ε than ET(30) values because
the solvent is modeled by ε in the PCM calculations; ET(30) contains all kind of solvent effects — some
of them are not considered by PCM. Although the calculated number of solvents is limited, we found
the following tendency of solvatofluorochromic shifts. 1) The fluorescence solvatochromic shift of 5-Cl
is larger than that of 3’-Cl (emission from the 11A states: solid line). 2) For 6’-Cl, we obtain the
fluorescence solvatochromic shift whose extent is larger than 5-Cl (dotted line is drawn with the
approximation of two crosses and one circle). These findings agree well with the experiment. According
to the SAC-CI calculations, 7-Cl is expected to show a fluorescence solvatochromic shift similar to 6’-
Cl (emission from the 11B states: broken line), although its low solubility restricts the experimental
measurement. The fluorescence solvatochromic shift of the 11A state is significant for the larger π-
conjugated systems of 6’-Cl and 7-Cl. This indicates that the charge transfer in the 11A state is
enhanced by extending the π-conjugation.
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The Journal of Organic Chemistry
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Figure 9. The solvent dependence of fluorescence wavelengths with respect to the logarithm of the
dielectric constant of solvent (ε).
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The calculated Stokes shifts are shown in Table 2. The SAC-CI results well reproduced the observed
trends. The solvent dependence of the Stokes shift is remarkably small for 3’-Cl because the absorption
maximum and fluorescence energies exhibit similar positive solvatochromic shifts. The absorption
maxima of 5-Cl, 6’-Cl, and 7-Cl exhibit rather small negative solvatochromic shifts, and this enhances
the solvent dependence of the Stokes shifts.
Table 2. Calculated Stokes shifts (cm−1) for 3’-Cl, 5-Cl, 6’-Cl, and 7-Cl in gas phase, 1,4-dioxane (DOX), and acetonitrile
(AN).
3’-Cl
4678
4436
5081
645
5-Cl
5243
5727
8066
2339
6’-Cl
3952
3468b
9275b
5807
7-Cl
7420
7718
11776
4597
Gas phase
DOX
AN
ꢀ(AN − DOX)a
aDifference of the calculated Stokes shifts between AN and DOX. bThe absorption maximum is the 11B state.
Table 3 shows the dihedral angles and bond lengths between imidazolium (benzoimidazolium) and
benzene (2-position) rings of the optimized geometries used for the SAC-CI calculations. Except for the
11A state of 6’-Cl, the dihedral angles in the excited states are smaller than those in the ground state.
The conjugation between imidazolium (benzoimidazolium) and benzene rings is enhanced in the excited
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states. Indeed the bond lengths between imidazolium (benzoimidazolium) and benzene are shortened
about 0.05 Å on going from the ground state to the excited state. In gas phase, the dihedral angle of 6’-
Cl in the 11A state is 87.9 ° indicating that the molecule has a twisted structure. Thus this molecule is
expected to show a twisted intramolecular charge transfer (TICT) phenomenon. The TICT structure is
quite solvent-sensitive. Namely, the twisted structure is significantly relaxed even in less polar DOX
solution. In AN solution, the dihedral angle in the 11A state is smaller than that in the ground state;
therefore, 6’-Cl takes local excited states in AN solution similar to the other molecules. As shown in
Table 1 and Figure 9, the emission state of 6’-Cl is altered by the polarity of solvent. The solvent-
depended TICT structure is the origin of this characteristic findings of the fluorescence of 6’-Cl.
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Table 3. The optimized dihedral angles and bond lengths between imidazolium (benzoimidazolium) and benzene (2-position) rings for
3’-Cl, 5-Cl, 6’-Cl, and 7-Cl in gas phase, 1,4-dioxane (DOX), and acetonitrile (AN) for the ground and lowest excited states.
3’-Cl
5-Cl
6’-Cl
7-Cl
GSa
11A
11B
GSa
11A
11B
GSa
11A
11B
GSa
11A
11B
Dihedral angle / degree
Gas
DOX
AN
34.9
35.7
35.3
21.6
19.4
16.0
32.2
27.4
22.1
43.9
44.0
40.6
25.6
33.3
34.6
48.0
87.9
55.6
42.1
26.5
32.8
41.7
57.3
56.4
57.1
31.8
31.7
30.2
29.9
31.2
32.3
23.3
18.8
47.0
47.0
35.8
Bond length / Å
1.446
Gas
DOX
AN
1.449
1.450
1.451
1.407
1.405
1.401
1.453
1.440
1.431
1.464
1.465
1.464
1.411
1.410
1.407
1.453
1.452
1.453
1.478
1.462
1.440
1.401
1.415
1.437
1.469
1.470
1.470
1.408
1.405
1.400
1.407
1.410
1.415
1.449
1.451
aGround state.
CONCLUSIONS
We have successfully prepared a set of fused π-conjugated imidazolium and benzimidazolium
compounds bearing the electron-donating and accepting substituents. The anion exchange was also
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The Journal of Organic Chemistry
carried out. For a family of imidazolium derivatives, the absorption/emission properties and
fluorescence quantum yield were influenced by the substituents. These results can be nicely reproduced
on the basis of the optimized ground state structure and estimated vertical excitation energy obtained by
the DFT and TD/DFT calculations, respectively. The compound bearing the strong electron-accepting
cyano group exhibited the pronounced positive solvatofluorochromism. The counter anion impacted on
the fluorescence quantum yield. In contrast to the imidazolium derivatives, the benzimidazolium bearing
the strong electron-donating N,N-dibutylamino group demonstrated the marked positive
solvatofluorochromic shift. For the accurate understanding of these curious solvatofluorochromic
behaviors, SAC-CI calculation was conducted both in gas phase and in solutions. The presented
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1
1
molecules have C2 symmetry, and therefore lowest A and B states can be relevant to the absorption
1
1
and fluorescence. For imidazolium compounds, the A state has much lower energies than the B state.
1
For benzimidazolium compounds, on the other hand, the B state is stabilized and is relevant to the
absorption and fluorescence. The electron withdrawing ability of cyano group affects the localized
electron distributions of the valence orbitals. The presented molecules have a positive charge; therefore,
solvent effects significantly stabilize the explicit charge in the initial state. The solvatochromic behavior
may slightly differ from that in neutral molecules in which the solvation of electric dipole is dominant.
EXPERIMENTAL SECTION
4,5-Dibromo-2-phenylimidazole (1)21: To a solution of 2-phenylimidazole (2.0 g, 14 mmol) in DMF (76
mL) was added NBS (4.8 g, 27 mmol) at 0 °C in the dark, and the mixture was stirred overnight at room
temperature. After the solvent was removed, water and ethyl acetate were added in the separating
funnel. An aqueous phase was extracted with ethyl acetate. The combined organic phase was rinsed
with sat. aqueous Na2S2O3 and dried over MgSO4. The solvent was removed and the remaining solid
1
was washed with hexane to obtain 1 (3.0 g, 72% yield). Colorless solid; m.p. 139–140 °C; H NMR
(200 MHz, DMSO-d6) δ ppm 7.29–7.55 (m, 3H), 7.88 (d, J = 7.58 Hz, 2H), 13.61 (brs, 1H).
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4,5-Bis(2’-hydroxymethylphenyl)-2-phenylimidazole (2): To a solution of 1 (0.40 g, 1.3 mmol) and 2-
hydroxymethylphenylboronic acid (0.61 g, 4.0 mmol) in THF (24 mL) were added 2 M aqueous
Na2CO3 (5.7 mL) and Pd(PPh3)4 (77 mg, 67 ꢁmol), and the mixture was heated to reflux overnight.
After the solvent was removed, water and ethyl acetate were added in the separating funnel. An aqueous
phase was extracted with ethyl acetate. The combined organic phase was rinsed with sat. aqueous
NH4Cl and dried over Na2SO4. The solvent was removed and the remaining solid was purified by SiO2
column chromatography (ethyl acetate : hexane = 1 : 1, Rf = 0.3) to obtain 2 (0.33 g, 70% yield).
Colorless solid; m.p. 159–160 °C; 1H NMR (400 MHz, DMSO-d6) δ ppm 4.14–4.28 (m, 2H), 4.46–4.54
(m, 2H), 5.14–5.32 (m, 1H), 6.11–6.21 (m, 1H), 6.92 (d, J = 7.03 Hz, 1H), 7.03–7.10 (m, 1H), 7.22–
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7.26 (m, 1H), 7.32 (brs, 2H), 7.40–7.42 (m, 1H), 7.47–7.60 (m, 5H), 7.97–8.01 (m, 1H); C NMR (50
MHz, DMSO-d6) δ ppm 125.4, 127.1, 129.0, 129.4, 130.3, 145.2; IR (ATR) cm-1 3738.3, 323.07,
3064.3, 2959.2, 2869.7, 2865.3, 1598.7, 1464.8, 1309.4, 1006.7, 963.3, 761.7; Anal Calcd for
C23H20N2O2·0.5H2O: C, 75.60%; H, 5.79%; N, 7.67%; Found: C, 75.88%; H, 5.60%; N, 7.65%.
1,3-Dihydro-4,5-bis(2’-chloromethylphenyl)-2-phenylimidazolium chloride (3): To a solution of 2 (0.27
g, 0.76 mmol) in CH3CN (68 mL) was added SOCl2 (15 mL, 0.21 mol), and the mixture was stirred
overnight at room temperature. After the solvent was removed, the remaining solid was washed with
1
hexane to obtain 3 (0.20 g, 76% yield). Colorless solid; m.p. > 350 °C; H NMR (200 MHz, CDCl3) δ
13
ppm 0.84 (brs, 6H), 1.60 (brs, 4H), 3.53 (brs, 4H), 4.67–4.77 (m, 4H), 7.19–7.57 (m, 12H); C NMR
(50 MHz, DMSO-d6) δ ppm 44.7, 124.2, 127.1, 127.9, 129.1, 129.4, 129.7, 130.8, 131.0, 132.1, 132.3,
137.8, 144.6; IR (ATR) cm-1 3853.1, 3735.4, 3058.6, 2868.6, 2718.2, 2562.0, 2342.1, 1644.0, 1484.0,
1321.0, 1164.0, 1035.6, 764.6, 706.8, 676.9.
Phenyl-substituted fused-imidazolium chloride (1-Cl): To a solution of 3 (0.11 g, 0.25 mmol) in DMF
(25 mL) was added 1.0 M THF solution of LiHMDS (0.5 mL, 0.5 mmol) at 0 °C, and the mixture was
stirred overnight at room temperature. After the solvent was removed, water and CHCl3 were added in
the separating funnel. An aqueous phase was extracted with CHCl3. The combined organic phase was
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rinsed with sat. aqueous NH4Cl and dried over MgSO4. The solvent was removed and the remaining
solid was dissolved in DMSO (16 mL) to be heated to 50 °C for 5 h. After the solvent was removed, the
remaining solid was dissolved in CHCl3 to be poured in ethyl acetate/hexane mixed solvent (CHCl3 :
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3
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5
6
7
8
1
ethyl acetate : hexane = 1 : 2 : 5) to obtain 1-Cl (60 mg, 67% yield). Brown solid; m.p. > 350 °C; H
9
13
NMR (400 MHz, DMSO-d6) δ ppm 5.93 (brs, 4H), 7.63–7.82 (m, 9H), 8.27 (d, J = 6.78 Hz, 4H); C
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NMR (100 MHz, DMSO-d6) δ ppm 44.7, 122.3, 122.7, 124.8, 127.3, 128.2, 129.7, 129.9, 130.2, 133.1,
138.3, 140.9; IR (ATR) cm-1 3445.21, 3053.7, 2912.9, 2847.4, 2217.7, 1664.3, 1579.4, 1140.7, 996.1,
888.1, 764.6, 721.2, 695.2; HRMS (ESI/TOF-Q) m/z: [1-Cl - Cl]+ Calcd for C23H17N2 321.1386; Found:
321.1411; Anal Calcd for C23H17ClN2·2.9H2O: C, 67.53%; H, 5.62%; N, 6.85%; Found: C, 67.84%; H,
5.32%; N, 7.08%.
1,2-Bis[2’-(t-butyldimethylsiloxymethyl)phenyl]ethyne (4): To
a
solution of 1,2-bis[2’-
(hydroxymethyl)phenyl]ethyne2 (1.9 g, 7.7 mmol) in DMF (30 mL) were added imidazole (3.0 g, 31
mmol) and t-butyldimethylsilyl chloride (2.1 g, 20 mmol), and the mixture was stirred overnight at room
temperature. After the solvent was removed, water and ethyl acetate were added in the separating
funnel. An aqueous phase was extracted with ethyl acetate. The combined organic phase was dried over
MgSO4. The solvent was removed and the remaining solid was extracted with hexane to obtain 4 (3.6 g,
1
quantitative yield). Colorless oil; H NMR (200 MHz, CDCl3) δ ppm 0.13 (s, 12H), 0.97 (s, 18H), 4.98
(s, 4H), 7.26–7.64 (m, 8H).
1,2-Bis[2’-(t-butyldimethylsiloxymethyl)phenyl]ethane-1,2-dione (5): To a mixture of 4 (3.7 g, 8.2
mmol), NaHCO3 (1.1 g, 14 mmol), MgSO4 (2.0 g, 17 mmol) in acetone (240 mL) and water (160 mL)
was added KMnO4 (5.1 g, 32 mmol), and the mixture was stirred overnight at room temperature. An
aqueous phase was extracted with CH2Cl2 and the combined organic phase was dried over MgSO4. The
solvent was removed and the remaining solid was rinsed with CH3OH to obtain 5 (1.8 g, 43% yield).
Colorless solid; m.p. 135–136 °C; 1H NMR (200 MHz, CDCl3) δ ppm 0.16 (s, 12H), 0.99 (s, 18H), 5.25
(s, 4H), 7.21–7.42 (m, 2H), 7.57–7.74 (m, 4H), 7.99 (d, J = 7.83 Hz, 2H); 13C NMR (50 MHz, CDCl3) δ
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ppm 18.5, 26.1, 63.5, 126.5, 126.9, 129.3, 133.1, 134.4, 146.0, 196.6; IR (ATR) cm-1 2948.6, 2925.5,
2854.1, 1656.6, 1600.6, 1570.7, 1458.9, 1251.6, 1199.5, 809.0, 777.3, 724.1, 668.2, 648.9; HRMS
(ESI/TOF-Q) m/z: [5 + Na]+ Calcd for C28H42NaO4Si2 521.2519; Found: 521.2519.
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8
9
4,5-Bis[2’-(hydroxymethyl)phenyl]-2-(4”-octyloxyphenyl)imidazole (6): A mixture of 5 (0.30 g, 0.60
mmol), 4-octyloxybenzaldehyde (0.17 mL, 0.72 mmol), L-proline (10 mg, 0.09 mmol), and ammonium
acetate (1.4 g, 18 mmol) in EtOH (2.4 mL) and 1,4-dioxane (1.2 mL) was heated to reflux overnight.
Water was added and an aqueous phase was extracted with ethyl acetate. The combined organic phase
was rinsed with sat. aqueous NH4Cl and dried over MgSO4. The solvent was removed and the remaining
solid was dissolved in CHCl3 to be poured in hexane to obtain 6 (0.21 g, 72% yield). Colorless solid;
m.p. 163–164 °C; 1H NMR (200 MHz, DMSO-d6) δ ppm 0.85–0.94 (m, 3H), 1.31 (brs, 10H), 1.73–1.84
(m, 2H), 3.98 (t, J = 6.44 Hz, 2H), 4.69 (s, 4H), 6.95 (d, J = 8.59 Hz, 2H), 7.07–7.20 (m, 4H), 7.29 (brs,
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13
2H), 7.44 (d, J = 7.58 Hz, 2H), 7.77 (d, J = 8.59 Hz, 2H); C NMR (50 MHz, CD3OD) δ ppm 13.1,
22.4, 25.8, 29.0, 29.1, 31.6, 62.5, 67.8, 114.6, 122.1, 126.5, 127.1, 128.5, 130.2, 131.4, 139.6, 145.8,
160.0; IR (ATR) cm-1 3066.3, 2926.5, 2855.1, 1658.5, 1601.6, 1571.7, 1485.9, 1248.7, 1199.5, 1119.5,
1079.9, 1006.7, 970.0, 891.9, 835.0, 754.0, 725.1, 682.7, 603.6; HRMS (ESI/TOF-Q) m/z: [6 + H]+
Calcd for C31H37N2O3 485.2799; Found: 485.2804.
1,3-Dihydro-4,5-bis[2’-(chloromethyl)phenyl]-2-(4”-octyloxyphenyl)imidazolium chloride (7): To a
solution of 6 (40 mg, 83 ꢁmol) in CH3CN (2 mL) was added SOCl2 (30 ꢁL, 0.42 mmol), and the
mixture was stirred overnight at room temperature. After the solvent was removed, the remaining solid
was dissolved in CHCl3 to be poured in hexane to obtain 7 (55 mg, quantitative yield). Colorless solid;
m.p. 112–113 °C; 1H NMR (200 MHz, CDCl3) δ ppm 0.79–1.06 (m, 3H), 1.34–1.58 (m, 10H), 1.86 (d,
J = 6.57 Hz, 2H), 4.03 (t, J = 5.94 Hz, 2H), 4.68 (brs, 4H), 6.64–7.15 (m, 10H), 8.27 (d, J = 7.58 Hz,
2H), 14.33 (brs, 2H); 13C NMR (50 MHz, CDCl3) δ ppm 14.2, 22.7, 26.0, 29.4, 31.8, 44.8, 114.3, 115.0,
125.3, 128.0, 128.3, 130.5, 131.7, 137.2, 144.1, 162.4; IR (ATR) cm-1 2925.5, 2855.1, 2696.0, 2617.9,
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2586.1, 1643.1, 1611.2, 1504.2, 1474.3, 1297.9, 1019.2, 814.8, 764.6, 735.7, 677.9; HRMS (ESI/TOF-
Q) m/z: [7 - Cl]+ Calcd for C31H35Cl2N2O 521.2126; Found: 521.2121.
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7
8
4-Octyloxyphenyl-substituted fused-imidazolium chloride (2-Cl): To a solution of 7 (50 mg, 90 ꢁmol)
in DMF (10 mL) was added 1.0 M THF solution of LiHMDS (0.18 mL, 0.18 mmol) at 0 °C, and the
mixture was stirred overnight at room temperature. After the solvent was removed, water and CHCl3
were added in the separating funnel. An aqueous phase was extracted with CHCl3. The combined
organic phase was rinsed with sat. aqueous NH4Cl and dried over MgSO4. The solvent was removed and
the remaining solid was dissolved in DMSO (10 mL) to be heated to 50 °C for 6 h. After the solvent
was removed, the remaining solid was dissolved in CHCl3 to be poured in ethyl acetate to obtain 2-Cl
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(17 mg, 39% yield). Colorless solid; m.p. 254–256 °C; H NMR (400 MHz, DMSO-d6) δ ppm 0.83–
0.94 (m, 3H), 1.23–1.37 (m, 8H), 1.42–1.48 (m, 2H), 1.68–1.85 (m, 2H), 4.16 (t, J = 6.40 Hz, 2H), 5.88
(s, 4H), 7.31 (d, J = 8.78 Hz, 2H), 7.51–7.68 (m, 4H), 7.76 (d, J = 7.53 Hz, 2H), 8.08–8.38 (m, 4H); 13C
NMR (100 MHz, DMSO-d6) δ ppm 14.5, 22.6, 26.0, 29.0, 29.2, 31.7, 53.9, 68.7, 114.4, 116.0, 122.1,
124.7, 127.3, 129.1, 129.6, 129.7, 130.2, 138.4, 140.8, 162.2; IR (ATR) cm-1 3389.3, 2926.5, 2847.4,
1599.7, 1463.7, 1270.9, 1187.9, 1003.8, 838.9, 756.9, 730.9, 715.5, 678.8, 645.1; HRMS (ESI/TOF-Q)
m/z: [2-Cl - Cl]+ Calcd for C31H33N2O 449.2587; Found: 449.2593; Anal Calcd for C31H33ClN2O·2H2O:
C, 71.45%; H, 7.16%; N, 5.38%; Found: C, 71.10%; H, 6.93%; N, 5.35%.
4,5-Bis[2’-(hydroxymethyl)phenyl]-2-(4”- N,N-dibutylaminophenyl)imidazole (8): From 5 (0.20 g, 0.40
mmol) and 4-N,N-dibutylaminobenzaldehyde (0.17 g, 0.72 mmol), the compound was synthesized in the
similar manner of 6 (0.10 g, 53% yield). Colorless solid; m.p. 75–76 °C; 1H NMR (200 MHz, CDCl3) δ
ppm 0.89–0.97 (m, 6H), 1.32 (dd, J = 14.53, 7.20 Hz, 4H), 1.53 (d, J = 6.32 Hz, 4H), 3.18–3.33 (m, 4H),
13
4.54 (s, 4H), 6.62 (d, J = 8.34 Hz, 2H), 7.00–7.32 (m, 8H), 7.68 (d, J = 8.34 Hz, 2H), 9.59 (s,1H); C
NMR (50 MHz, CDCl3) δ ppm 14.0, 22.3, 29.5, 50.8, 64.5, 110.8, 111.7, 116.2, 126.5, 127.9, 130.8,
133.0, 138.6, 146.1, 148.6, 152.8; IR (ATR) cm-1 3063.4, 2954.4, 2929.3, 2859.9, 1613.2, 1501.3,
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1455.1, 1402.0, 1365.4, 1286.3, 1198.5, 1110.8, 1002.8, 815.7, 761.7, 613.3; HRMS (ESI/TOF-Q) m/z:
[8 + H]+ Calcd for C31H38N3O2 484.2964; Found: 484.2954.
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5
6
1,3-Dihydro-4,5-bis[2’-(chloromethyl)phenyl]-2-(4”-N,N-dibutylaminophenyl)imidazolium
chloride
7
8
(9): The compound was synthesized from 8 (80 mg, 0.17 mmol) in the similar manner of 7 (75 mg, 80%
9
1
yield). Brown solid; m.p. 115–117 °C; H NMR (200 MHz, CDCl3) δ ppm 0.94 (brs, 6H), 1.31 (brs,
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4H), 1.59 (brs, 4H), 3.37 (brs, 4H), 4.74 (brs, 4H), 7.07–7.54 (m, 10H), 8.53 (brs, 2H), 14.55 (brs, 1H);
13C NMR (50 MHz, CD3OD) δ ppm 12.7, 19.6, 27.9, 43.2, 62.0, 126.1, 128.7, 129.0, 130.7, 131.6,
137.6; IR (ATR) cm-1 2959.2, 2925.5, 2870.5, 2702.7, 2572.6, 1642.1, 1606.4, 1509.1, 1259.3, 1143.6,
1052.9, 816.7, 765.6, 678.8, 606.5; HRMS (ESI/TOF-Q) m/z: [9 - Cl]+ Calcd for C31H36Cl2N3
520.2286; Found: 520.2284.
4-(N,N-Dibutylamino)phenyl-substituted fused-imidazolium chloride (3-Cl): The compound was
synthesized from 9 (40 mg, 70 ꢁmol) in the similar manner of 2-Cl (14 mg, 40% yield). Brown solid;
m.p. 175–177 °C; 1H NMR (400 MHz, DMSO-d6) δ ppm 0.96 (t, J = 7.28 Hz, 6H), 1.33–1.41 (m, 4H),
1.54–1.61 (m, 4H), 3.45 (t, J = 7.40 Hz, 4H), 5.86 (s, 4H), 6.92 (d, J = 9.03 Hz, 2H), 7.59–7.68 (m, 4H),
13
7.75 (d, J = 7.53 Hz, 2H), 8.00 (d, J = 9.03 Hz, 2H), 8.23 (d, J = 7.53 Hz, 2H); C NMR (100 MHz,
CD3OD) δ ppm 12.9, 19.8, 29.0, 50.3, 53.5, 106.8, 111.5, 121.2, 123.8, 127.2, 128.5, 128.9, 129.0,
139.3, 140.0, 150.8; IR (ATR) cm-1 3366.1, 2961.2, 2932.2, 2871.5, 2716.3, 2587.0, 1640.2, 1605.5,
1509.1, 1374.0, 1259.3, 1212.1, 1157.1, 950.7, 816.7, 679.8, 620.1, 608.4; HRMS (ESI/TOF-Q) m/z:
[3-Cl - Cl]+ Calcd for C31H34N3 448.2753; Found: 448.2780; Anal Calcd for C31H34ClN3·4H2O C,
66.95%; H, 7.61%; N, 7.56%; Found: C, 66.69%; H, 7.44%; N, 7.64%.
4,5-Bis[2’-(hydroxymethyl)phenyl]-2-(4”-trifluoromethylphenyl)imidazole (10): From 5 (0.30 g, 0.60
mmol) and 4-trifluoromethylbenzaldehyde (0.20 g, 1.2 mmol), the compound was synthesized in the
1
similar manner of 6 (80 mg, 32% yield). Colorless solid; m.p. 205–206 °C; H NMR (200 MHz,
DMSO-d6) δ ppm 4.26 (brs, 2H), 4.56 (brs, 2H), 5.25 (brs, 1H), 5.88 (brs, 1H), 6.84–7.18 (m, 2H),
13
7.27–7.65 (m, 6H), 7.92 (d, J = 8.08 Hz, 2H), 8.24 (d, J = 8.08 Hz, 2H), 13.23 (brs, 1H); C NMR (50
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MHz, DMSO-d6) δ ppm 65.2, 125.8, 126.5, 128.6, 134.1, 143.7; IR (ATR) cm-1 3168.5, 3073.0, 3053.7,
2940.0, 1621.8, 1589.1, 1461.8, 1436.7, 1323.9, 1251.6, 1174.4, 1108.9, 1065.5, 1012.5, 983.5, 849.5,
758.9; HRMS (ESI/TOF-Q) m/z: [10 + H]+ Calcd for C24H20F3N2O2 447.1477; Found: 447.1496.
1,3-Dihydro-4,5-bis[2’-(chloromethyl)phenyl]-2-(4”-trifluoromethylphenyl)imidazolium chloride (11):
The compound was synthesized from 10 (70 mg, 0.22 mmol) in the similar manner of 7 (80 mg, 94%
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yield). Colorless solid; m.p. 260–261 °C; H NMR (200 MHz, DMSO-d6) δ ppm 4.88 (brs, 4H), 7.30–
7.39 (m, 4H), 7.47 (t, J = 6.97 Hz, 2H), 7.62 (d, J = 7.58 Hz, 2H), 8.00 (d, J = 8.31 Hz, 2H), 8.38–8.45
(m, 2H); 13C NMR (50 MHz, DMSO-d6) δ ppm 44.7, 126.6, 127.4, 129.2, 131.2, 131.8, 137.3, 143.6; IR
(ATR) cm-1 3853.1, 3734.5, 3710.4, 3648.7, 3628.4, 3053.7, 2866.7, 2836.8, 2533.0, 1748.1, 1620.9,
1508.1, 1399.1, 1174.4, 1017.3, 816.8, 681.7; HRMS (ESI/TOF-Q) m/z: [11 - Cl]+ Calcd for
C24H18Cl2F3N2 461.0799; Found: 461.0794.
4-Trifluoromethylphenyl-substituted fused-imidazolium chloride (4-Cl): The compound was
synthesized from 11 (80 mg, 0.16 mmol) in the similar manner of 2-Cl (14 mg, 40% yield). Colorless
1
solid; m.p. 313–315 °C; H NMR (400 MHz, DMSO-d6) δ ppm 5.97 (s, 4H), 7.59–7.74 (m, 4H), 7.79
(d, J = 7.28 Hz, 2H), 8.18 (d, J = 8.28 Hz, 2H), 8.31 (d, J = 7.53 Hz, 2H), 8.49 (d, J = 8.28 Hz, 2H); 13C
NMR (100 MHz, DMSO-d6) δ ppm 54.1, 122.4, 124.8, 127.2, 129.4, 129.7, 130.1, 130.2, 137.0, 140.8;
IR (ATR) cm-1 3524.7, 3396.3, 3360.4, 3017.1, 2926.5, 1711.5, 1619.9, 1476.3, 1170.7, 1009.6, 946.9,
892.9, 857.2, 780.1, 731.9, 722.2, 658.6; HRMS (ESI/TOF-Q) m/z: [4-Cl - Cl]+ Calcd for C24H16F3N2
389.1260; Found: 389.1268; Anal Calcd for C24H16ClF3N2·2.5H2O: C, 61.35%; H, 4.50%; N, 5.96%;
Found: C, 61.10%; H, 4.33%; N, 5.80%.
4,5-Bis[2’-(hydroxymethyl)phenyl]-2-(4”-cyanophenyl)imidazole (12): From 5 (0.15 g, 0.30 mmol) and
4-cyanobenzaldehyde (50 mg, 0.39 mmol), the compound was synthesized in the similar manner of 6
(70 mg, 31% yield). Brown solid; m.p. 125–127 °C; 1H NMR (400 MHz, CDCl3) δ ppm 4.69 (brs, 4H),
7.15 (brs, 4H), 7.27–7.33 (m, 2H), 7.45 (d, J = 7.53 Hz, 2H), 7.69 (d, J = 8.53 Hz, 2H), 7.90 (s, 2H); 13C
NMR (50 MHz, CDCl3) δ ppm 64.7, 111.3, 111.7, 125.3, 127.4, 128.1, 128.3, 131.8, 132.9, 133.3,
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142.8, 143.5; IR (ATR) cm-1 3194.5, 2938.9, 2847.4, 2226.4, 1609.3, 1486.9, 1440.5, 1220.5, 1001.8,
972.9, 842.7, 763.7, 695.2, 607.5; HRMS (ESI/TOF-Q) m/z: [12 + Na]+ Calcd for C24H19N3NaO2
404.1375; Found: 404.1380.
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1,3-Dihydro-4,5-bis[2’-(chloromethyl)phenyl]-2-(4”-cyanophenyl)imidazolium chloride (13): The
compound was synthesized from 12 (80 mg, 0.22 mmol) in the similar manner of 7 (65 mg, 64% yield).
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Brown solid; m.p. 189–191 °C; H NMR (200 MHz, CD3OD) δ ppm 4.65 (s, 4H), 7.52–7.68 (m, 8H),
13
8.08–8.17 (m, 2H), 8.23–8.30 (m, 2H); C NMR (50 MHz, CD3OD) δ ppm 43.2, 115.6, 117.2, 125.7,
127.7, 129.0, 130.8, 131.6, 133.3, 137.7, 143.1; IR (ATR) cm-1 2866.7, 2843.5, 2719.1, 2374.9, 2342.1,
1717.3, 1642.1, 1610.3, 1399.1, 1264.1, 902.5, 842.7, 767.5, 603.6; HRMS (ESI/TOF-Q) m/z: [13 - Cl]+
Calcd for 418.0872; Found: 418.0869.
4-Cycnophenyl-substituted fused-imidazolium chloride (5-Cl): The compound was synthesized from 13
(55 mg, 0.12 mmol) in the similar manner of 2-Cl (10 mg, 22% yield). Brown solid; m.p. 335–337 °C;
1H NMR (400 MHz, DMSO-d6) δ ppm 5.97 (s, 4H), 7.63–7.73 (m, 4H), 7.79 (d, J = 7.28 Hz, 2H), 8.29–
13
8.35 (m, 4H), 8.45 (d, J = 8.53 Hz, 2H); C NMR was not available due to low solubility; IR (ATR)
cm-1 3092.3, 3053.7, 2696.0, 2533.1, 2475.2, 1634.4, 1499.4, 1387.5, 1127.2, 1043.3, 898.7, 764.6,
753.1, 670.1, 610.4; HRMS (ESI/TOF-Q) m/z: [5-Cl - Cl]+ Calcd for C24H16N3 346.1339; Found:
346.1339; Anal Calcd for C24H16ClN3·1.6H2O: C, 70.19%; H, 4.71%; N, 10.23%; Found: C, 70.03%; H,
4.64%; N, 10.19%.
4-Octyloxyphenyl-substituted fused-imidazolium bis(trifluoromethanesulfonyl)imidate (2-TFSI): To a
solution of 1-Cl (10 mg, 21 ꢁmol) in CHCl3 (1.2 mL) was added a solution of LiTFSI (0.46 g, 1.6
mmol) in distilled water (1.6 mL), and the mixture was stirred overnight at room temperature. An
aqueous phase was extracted with CHCl3. The combined organic phase was dried over MgSO4 to be
1
poured in hexane to obtain 2-TFSI (15 mg, quantitative yield). Colorless solid; m.p. 126–127 °C; H
NMR (400 MHz, CDCl3) δ ppm 0.82–0.99 (m, 3H), 1.25–1.46 (m, 10H), 1.77 (d, J = 6.57 Hz, 2H), 3.98
(t, J = 6.44 Hz, 2H), 5.51 (s, 4H), 7.14 (d, J = 8.84 Hz, 2H), 7.40–7.67 (m, 6H), 7.77 (d, J = 7.33 Hz,
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2H), 7.88 (d, J = 8.59 Hz, 2H); 13C NMR (100 MHz, CD3OD) δ ppm 13.0, 22.3, 25.7, 28.8, 29.0, 29.1,
31.6, 53.2, 68.3, 115.6, 121.5, 123.8, 127.3, 129.1, 140.0; IR (ATR) cm-1 2939.9, 2857.1, 1603.5,
1475.3, 1336.4, 1313.3, 1267.1, 1182.2, 1128.2, 760.8, 727.1, 661.5; HRMS (ESI/TOF-Q) m/z: [TFSI]-
Calcd for C2F6NO4S2 279.9173; Found: 279.9178; Anal Calcd for C33H33F6N3O5S2: C, 54.31%; H,
4.56%; N, 5.76% S, 8.79%; Found: C, 54.14%; H, 4.60%; N, 5.43% S, 8.95%.
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4-Octyloxyphenyl-substituted fused-imidazolium tetrafluoroborate (2-BF4): To a solution of 1-Cl (30
mg, 62 ꢁmol) in AN (1.9 mL) was added a 1 M CH2Cl2 solution of Et3OBF4 (62 ꢁL, 62 ꢁmol), and the
mixture was stirred overnight at room temperature. After the solid was filtered, the filtrate was
evaporated to give yellow solid that was recrystallized from CHCl3/hexane to obtain 2-BF4 (20 mg,
60% yield). Pale yellow solid; m.p. 235–236 °C; 1H NMR (400 MHz, CDCl3) δ ppm 0.89–0.93 (m, 3H),
1.33 (brs, 10H), 1.67–1.84 (m, 2H), 3.75 (t, J = 6.53 Hz, 2H), 5.42 (s, 4H), 6.98 (d, J = 8.53 Hz, 2H),
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7.30–7.45 (m, 4H), 7.62 (dd, J = 16.82, 6.53 Hz, 4H) 7.90 (d, J = 8.53 Hz, 2H); C NMR (100 MHz,
CDCl3) δ ppm 14.1, 22.7, 25.9, 29.0, 29.3, 29.4, 31.6, 31.8, 54.1, 68.6, 112.9, 116.0, 121.2, 124.8,
126.7, 129.2, 129.4, 129.7, 139.5, 162.5; IR (ATR) cm-1 2923.6, 2840.7, 1606.4, 1488.8, 1463.7,
1445.4, 1310.4, 1291.1, 1263.2, 1180.2, 1029.8, 896.7, 843.7, 809.9, 776.3, 727.1, 679.8; HRMS
(ESI/TOF-Q) m/z: [BF4]- Calcd for BF4 87.0029; Found: 87.0029; Anal Calcd for C31H33BF4N2O·H2O:
C, 67.16%; H, 6.36%; N, 5.05%; Found: C, 67.42%; H, 6.19%; N, 5.11%.
4,7-Dibromo-2-(4’-N,N-dibutylaminophenyl)-1H-benzimidazole (14): To a solution of 3,6-dibromo-1,2-
phenylenediamine (1.7 g, 6.4 mmol) and N,N-dibutylbenzaldehyde (2.0 g, 8.3 mmol) in CHCl3 (70 mL)
was added ZrCl4 (0.14 g, 0.64 mmol), and the mixture was stirred for 4 d at room temperature. After
solid was filtered, the filtrate was evaporated to give yellow solid that was recrystallized from
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CHCl3/AN to obtain 14 (1.5 g, 43% yield). Colorless solid; m.p. 239–240 °C; H NMR (200 MHz,
DMSO-d6) δ ppm 0.83–0.98 (m, 6H), 1.32–1.68 (m, 8H), 6.76 (d, J = 8.84 Hz, 2H), 7.28 (s, 2H), 8.10
(d, J = 8.59 Hz, 2H); 13C NMR (100 MHz, DMSO-d6) δ ppm 14.3, 20.1, 29.5, 50.3, 111.4, 115.1, 125.9,
129.5, 150.0, 154.6; IR (ATR) cm-1 3901.3, 3735.4, 3073.1, 2955.4, 2869.6, 2777.9, 1607.4, 1494.6,
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1470.5, 1305.6, 1169.6, 1110.8, 927.6, 757.9, 700.1; HRMS (ESI/TOF-Q) m/z: [14 + H]+ Calcd for
C21H26Br2N3 478.0493; Found: 478.0493.
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2-(4”-N,N-Dibutylaminophenyl)-4,7-bis(2’-hydroxymethylphenyl)benzimidazole (15): To a solution of
14 (1.3 g, 2.7 mmol) and 2-hydroxymethylphenylboronic acid (0.9 g, 5.9 mmol) in THF (100 mL) were
added 2 M aqueous Na2CO3 (9.0 mL) and Pd(PPh3)4 (0.47 g, 0.41 mmol), and the mixture was heated to
reflux overnight. After the solvent was removed, water and ethyl acetate were added in the separating
funnel. An aqueous phase was extracted with ethyl acetate. The combined organic phase was rinsed
with sat. aqueous NH4Cl and dried over MgSO4. The solvent was removed and the remaining solid was
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recrystallized from CHCl3/AN to obtain 15 (0.38 g, 26% yield). Colorless solid; m.p. 169–170 °C; H
NMR (200 MHz, CDCl3) δ ppm 0.82–0.91 (m, 6H), 1.22–1.50 (m, 8H), 3.19 (t, J = 7.20 Hz, 4H), 4.51
(brs, 4H), 6.47 (d, J = 8.34 Hz, 2H), 7.26 (s, 2H), 7.46 (brs, 6H), 7.55–7.67 (m, 4H); 13C NMR (100
MHz, DMSO-d6) δ ppm 14.3, 20.1, 29.4, 50.2, 79.7, 111.3, 116.2, 123.4, 127.3, 128.0, 129.0, 130.9,
141.0, 149.5, 153.7; IR (ATR) cm-1 3853.1, 3734.5, 3063.4, 2955.4, 2859.9, 1609.3, 1490.7, 1398.1,
1363.4, 1285.3, 1200.5, 1001.8, 926.6, 819.6, 756.9; HRMS (ESI/TOF-Q) m/z: [15 + H]+ Calcd for
C35H40N3O2 534.3121; Found: 534.3121.
1,3-Dihydro-4,7-bis(2’-chloromethylphenyl)-2-(4”-N,N-dibutylaminophenyl)benzimidazolium chloride
(16): To a solution of 15 (0.33 g, 0.62 mmol) in CH3CN (50 mL) was added SOCl2 (0.22 mL, 3.1
mmol), and the mixture was stirred overnight at room temperature. After the solvent was removed, the
remaining solid was dissolved in CHCl3 to be poured in hexane to obtain 16 (0.28 g, 74% yield). Brown
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solid; m.p. 135–136 °C; H NMR (200 MHz, CDCl3) δ ppm 0.93–1.00 (m, 6H), 1.33 (brs, 8H), 3.34
(brs, 4H), 4.63 (brs, 4H), 6.86 (brs, 2H), 7.42–7.76 (m, 10H), 8.44 (brs, 2H); 13C NMR (100 MHz,
DMSO-d6) δ ppm 14.3, 20.0, 29.3, 45.0, 50.4, 111.4, 125.6, 126.7, 129.4, 129.6, 131.1, 131.3, 131.6,
136.3, 136.7, 152.4; IR (ATR) cm-1 3854.1, 3750.9, 3676.6, 3648.7, 3023.8, 2956.3, 2925.5, 2859.9,
2637.2, 2568.7, 2500.3, 1604.5, 1509.1, 1457.9, 1289.2, 1208.2, 926.6, 819.6, 669.2; HRMS
(ESI/TOF-Q) m/z: [16 - Cl]+ Calcd for C35H38Cl2N3 570.2437; Found: 570.2443.
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The Journal of Organic Chemistry
4-(N,N-Dibutylamino)phenyl-substituted fused-benzimidazolium chloride (6-Cl): To a solution of 16
(0.10 g, 0.17 mmol) in DMF (22 mL) was added 1.0 M THF solution of LiHMDS (0.38 mL, 0.38
mmol) at 0 °C, and the mixture was stirred overnight at room temperature. After the solvent was
removed, water and CHCl3 were added in the separating funnel. An aqueous phase was extracted with
CHCl3. The combined organic phase was rinsed with sat. aqueous NH4Cl and dried over MgSO4. The
solvent was removed and the remaining solid was dissolved in DMSO (12 mL) to be heated to 50 °C for
6 h. After the solvent was removed, the remaining solid was washed with ethyl acetate to obtain 6-Cl
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(50 mg, 59% yield). Brown solid; m.p. 229–231 °C; H NMR (400 MHz, CD3OD) δ ppm 1.06 (t, J =
7.40 Hz, 6H), 1.44–1.56 (m, 4H), 1.72 (quin, J = 7.59 Hz, 4H), 3.49–3.58 (m, 4H), 5.86 (s, 4H), 7.05 (d,
J = 9.03 Hz, 2H), 7.38–7.52 (m, 6H), 7.83–7.91 (m, 4H), 8.04 (d, J = 6.78 Hz, 2H); 13C NMR (100
MHz, DMSO-d6) δ ppm 14.4, 20.1, 29.4, 48.1, 50.4, 79.8, 105.4, 111.8, 118.9, 119.9, 123.8, 126.4,
126.9, 128.6, 129.0, 130.0, 131.5, 148.5, 151.3; IR (ATR) cm-1 3854.1, 3819.3, 3750.9, 3711.3, 3676.6,
3648.7, 2948.6, 2929.3, 2861.8, 1716.3, 1558.2, 1465.6, 1204.3, 879.4, 825.4, 744.4, 695.2; HRMS
(ESI/TOF-Q) m/z: [6-Cl - Cl]+ Calcd for C35H36N3 498.2904; Found: 498.2927; Anal Calcd for
C35H36ClN3·4H2O: C, 69.35%; H, 7.32%; N, 6.93%; Found: C, 69.47%; H, 7.26%; N, 6.83%.
4,7-Dibromo-2-(4’-cyanophenyl)-1H-benzimidazole (17): From 3,6-dibromo-1,2-phenylenediamine3
(1.5 g, 5.6 mmol) and 4-cyanobenzaldehyde (0.87 g, 6.7 mmol), the compound was synthesized in the
similar manner of 14 (1.7 g, 81% yield). Colorless solid; 1H NMR (200 MHz, DMSO-d6) δ ppm 7.47 (s,
2H), 8.09 (d, J = 8.34 Hz, 2H), 8.55 (d, J = 8.34 Hz, 2H); 13C NMR (100 MHz, DMSO-d6) δ ppm 79.7,
113.0, 118.9, 127.1, 128.5, 133.1, 133.6, 151.5; IR (ATR) cm-1 3902.3, 3854.1, 3819.3, 3750.9, 3711.3,
3675.7, 3215.7, 2240.9, 1698.1, 1558.2, 1540.9, 1488.8, 1473.4, 1298.8, 1176.4, 842.7, 756.9, 668.2;
HRMS (ESI/TOF-Q) m/z: [17 + H]+ Calcd for C14H8Br2N3 377.9085; Found: 377.9068.
2-(4”-Cyanophenyl)-4,7-bis(2’-hydroxymethylphenyl)-1H-benzimidazole (18): The compound was
synthesized from 17 (1.0 g, 2.7 mmol) in the similar manner of 15 (0.50 g, 43% yield). Colorless solid;
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m.p. 171–172 °C; H NMR (200 MHz, DMSO-d6) δ ppm 4.38–4.52 (m, 4H), 5.09–5.30 (m, 2H), 7.20
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