V-shaped fluorophores with a 1-methyl-4,5-bis(arylethynyl)imidazole skeleton displaying solid-state fluorescence, acid responsiveness, and remarkable fluorescence solvatochromism

Article abstract of DOI:10.1039/d0nj05323f

Novel V-shaped fluorophores with an imidazole skeleton composed of arylethynyl moieties were developed. Their HOMO-LUMO energy gaps were well controlled by the introduction of electron-withdrawing or -donating groups in the aryl moiety. Compounds with different substituents at the 4- and 5-positions were selectively synthesized exploiting the difference in reactivity between the iodo and bromo groups. All compounds synthesized displayed fluorescence not only in solution but also in the solid state. The compound with a donor-acceptor structure displayed a fluorescence maximum with a wavelength over 500 nm and a large Stokes shift of about 150 nm, due to its intramolecular charge transfer character. In the solid state, the synthesized compounds have planar structures, indicating an expansion of π-conjugation. Upon exposure of the compounds in the solid state to acid vapor, the fluorescence of the compounds with acceptor groups was quenched, and that of the others shifted to longer wavelengths.

Full text of DOI:10.1039/d0nj05323f

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V-shaped fluorophores with a 1-methyl-4,5-
bis(arylethynyl)imidazole skeleton displaying
solid-state fluorescence, acid responsiveness,
and remarkable fluorescence solvatochromism†
Cite this:DOI: 10.1039/d0nj05323f
Youhei Miura, * Kotaro Kobayashi and Naoki Yoshioka
Novel V-shaped fluorophores with an imidazole skeleton composed of arylethynyl moieties were
developed. Their HOMO–LUMO energy gaps were well controlled by the introduction of electron-
withdrawing or -donating groups in the aryl moiety. Compounds with different substituents at the
4- and 5-positions were selectively synthesized exploiting the difference in reactivity between the iodo
and bromo groups. All compounds synthesized displayed fluorescence not only in solution but also in
the solid state. The compound with a donor–acceptor structure displayed a fluorescence maximum with
a wavelength over 500 nm and a large Stokes shift of about 150 nm, due to its intramolecular charge
transfer character. In the solid state, the synthesized compounds have planar structures, indicating an
expansion of p-conjugation. Upon exposure of the compounds in the solid state to acid vapor, the
fluorescence of the compounds with acceptor groups was quenched, and that of the others shifted to
longer wavelengths.
Received 30th October 2020,
Accepted 3rd December 2020
DOI: 10.1039/d0nj05323f
rsc.li/njc
have been published on fluorescent dyes containing the imidazole
moiety in the absence of annulation,6 although several annu-
Introduction
Fluorescent organic compounds have attracted much research lated imidazole derivatives, such as phenanthroimidazole7 and
attention, as they are used in various applications in the fields pyridoimidazole,8 have been reported as organic fluorophores.
of functional materials and bio-probes.1 Organic fluorophores In fact, the extent of p-conjugation is limited in imidazole, and
are often characterized by large p-conjugated skeletons like this compound is not fluorescent on its own. However, as in the
naphthalene, pyrene, perylene, and azaacene.2,3 These skele- case of 2,4,5-triphenylimidazole, known as lophine,9 which has
tons are especially desirable in fluorophores, because they have been found to be chemoluminescent and fluorescent,10 the
fluorescence properties themselves; furthermore, substituents imidazole skeleton can be made to acquire fluorescence proper-
introduced in these skeletons may act as auxochromes. Notably, ties via the introduction of appropriate substituents.
however, large p-conjugate systems often display a tendency
The carbon atoms in the 4- and 5-positions of imidazole are
toward aggregation, which may induce fluorescence quenching.4 characterized by different electronic densities (Scheme 1),
Therefore, fluorophores characterized by small p-conjugate struc- because the nitrogen atoms linking them are of different types;
tures represent desirable target compounds when designing fluo- one is a sp3 hybridization, whereas the other is imine-type sp2
rescent functional materials. Imidazole is one of the smallest hybridization. Although the electron density of the highest
neutral aromatic compounds; it has both hydrogen-bond- occupied molecular orbital (HOMO) at C4 is almost equal to
donating and hydrogen-bond-accepting characters, as well as an that at C5, the electron density of the lowest unoccupied
ability to form coordination bonds. These features have often been molecular orbital (LUMO) is three times larger at C5 than that
exploited in the construction of supramolecular species and to at C4. Nevertheless, the differences between these carbon
obtain proton-conducting materials.5 Nevertheless, few reports atoms are often not taken into consideration, as imidazole
easily isomerizes in solution via hydrogen migration. Notably,
Department of Applied Chemistry, Faculty of Science and Technology,
placing a substituent on the nitrogen at the 1-position of
imidazole is the easiest way to prevent the mentioned isomeri-
zation. This synthetic transformation does not affect the atomic
orbital coefficients of HOMO and LUMO in imidazole and
allows the difference in the electronic states between C4 and
C5 to be observed. However, the high-selectivity synthesis of
Keio University, Yokohama, 223-8522, Japan. E-mail: y-miura@applc.keio.ac.jp
† Electronic supplementary information (ESI) available: Synthetic method, ana-
lytical and spectral data, and details of computational data are available. CCDC
1950075 (1a), 1950076 (1b), 2025902 (1c), 1950078 (1d), 1950077 (1e), 2025951 (1a-
TFA salt), 2025950 (1d-TFA salt) and 2025952 (1e-TFA salt). For ESI and crystal-
lographic data in CIF or other electronic format see DOI: 10.1039/d0nj05323f
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increasing its bulkiness only modestly; additionally, the arylethynyl
group is stable, and it is easily introduced into the aryl skeleton via
Sonogashira coupling. Notably, the substitution effects on the aryl
rings of 4,5-diarylethynylimidazole were estimated by performing
density functional theory (DFT) calculations. Moreover, the fluores-
cence and absorption properties of the compounds synthesized in
the present study (Scheme 2) were analyzed both in the solid state
and in solution. These compounds were also studied as acid vapor
detectors in the solid state.
Results and discussion
Molecular design
DFT calculations (at the B3LYP/6-311(d) level of theory) were
performed to predict the effect of the introduction of various
aryl ethynyl substituents in imidazole. In particular, the frontier
orbital energies of 4,5-bis(phenylethynyl)-1-methylimidazole (1a),
4,5-bis(4-methoxyphenylethynyl)-1-methylimidazole (1b), 4,5-bis(4-
formylphenylethynyl)-1-methylimidazole (1c), 4-phenylethynyl-
5-(4-formylphenylethynyl)-1-methylimidazole (1d), and 4-(4-
methoxyphenylethynyl)-5-(4-formylphenylethynyl)-1-methylimidazole
(1e) were calculated (Fig. 1 and Fig. S1, ESI†). Structure optimization
indicated all compounds to be planar, with small torsion angles
existing between the imidazole and aryl rings (Tables S1–S5, ESI†).
Compound 1a was estimated to have a higher HOMO energy, lower
LUMO energy, and smaller HOMO–LUMO energy gap (3.70 eV) than
those of 4,5-diphenyl-1-methylimidazole (2) (4.65 eV), owing to the
p-conjugation extension and the planarization of the phenyl rings
resulting from the introduction of the ethynylene groups. The
introduction of the methoxy groups to produce 1b was associated
with mutually similar increases in the energies of the HOMO and
LUMO; therefore, the HOMO–LUMO energy gap calculated for 1b
(3.62 eV) was almost the same as that calculated for 1a. In contrast,
the introduction of the electron-withdrawing formyl groups (to
produce compound 1c) caused the calculated value of the HOMO–
LUMO energy gap to decrease (3.22 eV). This result was due to a
decrease in the LUMO energy (DE(1c–1a) = ꢀ0.98 eV) that was larger
than that of its counterpart for HOMO (DE(1c–1a) = ꢀ0.49 eV). The
introduction of a single formyl group to produce derivative 1d was
associated with a decrease in the HOMO and LUMO energies with
Scheme 1 Atomic orbital coefficients of the frontier orbitals of imidazole
and 1-methylimidazole calculated using the PM3 method.
substituted imidazoles is hard to achieve using the classical
synthetic strategy, which consists of the condensation between
an aldehyde, a 1,2-diketone, and ammonia.11 Recently, cross-
coupling reactions involving haloimidazoles have been
reported, which enable the organic chemists to selectively
introduce the desired substituent in the 4- and 5-positions of
the substrate.12 Several 4,5-disubstituted imidazole derivatives
could be selectively synthesized using this approach, expanding
the scope of synthetically accessible imidazole-based functional
materials.
In the present study, 4,5-bis(arylethynyl)imidazole, a family
of novel small fluorophores with V-shaped structures, was
designed with the aim of exploiting the characteristic electronic
state of the imidazole skeleton (Scheme 2). In the D–p–A
molecule, the V-shaped structure spatially separates the HOMO
and LUMO, which have localized electron density. This structure
is suitable for fluorescence solvatochromism of molecules.13 The
arylethynyl group is widely used for the functionalization of
compounds like pyrene and perylene to obtain fluorophores.14
This group expands the p-conjugation of the substrate while
Scheme 2 Molecular structures of compounds 1a–e and 2.
Fig. 1 Energy diagram of the frontier orbitals of compounds 2 and 1a–e.
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respect to 1a, although both energies were higher than those of their
counterparts for 1c. The HOMO–LUMO energy gap of 1d was
calculated to be 3.21 eV, a value almost identical to that calculated
for 1c. Furthermore, introduction of a methoxy group at the R0
position of 1d (to produce compound 1e) induced an increase in the
energy of the HOMO, whereas the LUMO energy was hardly
modified. This molecule displayed the smallest calculated HOMO–
LUMO energy gap (3.05 eV) among the compounds in the 1a–e
series. According to calculations, the electron densities of the HOMO
and LUMO of 1e mainly distribute on the methoxyphenylethynyl
group and the formylphenylethynyl group, respectively. A similar
distribution of HOMO and LUMO electron densities was observed
for 1d. Notably, such large differences between HOMO and LUMO
atomic orbital coefficients often result in an intramolecular charge
transfer (ICT).
The results of DFT calculations suggest that the introduction
of electron-donating groups in the R and R0 positions of the
imidazole derivatives described above does not affect the
HOMO–LUMO energy gap, even though the individual energies
of the HOMO and LUMO increase as a consequence of
the modification. Although the introduction of electron-
withdrawing groups in the mentioned positions cause a
decrease in both the HOMO and LUMO energies, the decrease
is more substantial for the LUMO than for the HOMO. Further-
more, the introduction of an electron-withdrawing group in the
R position is associated with a decrease in the HOMO–LUMO
energy gap. Finally, the concomitant introduction of an electron-
withdrawing group in the R position and an electron-donating
group in the R’ position results in the formation of a donor–
acceptor structure and of a compound displaying a small HOMO–
LUMO energy gap. This result is well explained by the difference in
the atomic orbital coefficient between the C4 and C5 positions of
the imidazole unit illustrated in Scheme 1, according to which the
electron density of the LUMO is low at the 4-position.
Scheme 3 Synthesis of compound 1. Reagents and conditions: (i) arylace-
tylene, Pd(PPh3)2Cl2, triethylamine (NEt3), dimethylformamide (DMF), r.t.;
(ii) iPrMgBr, tetrahydrofuran (THF), 0 1C; (iii) N-bromosuccinimide, THF,
room temperature (r.t.); (iv) 2-(4-ethynylphenyl)dioxolane, Pd(PPh3)2Cl2,
NEt3, DMF, 90 1C; and (v) 1 M HCl, acetone, r.t.
Syntheses
Species containing identical arylethynyl groups as substituents compound 1e was synthesized in just the same manner as
(compounds 1a–c) were synthesized by performing Sonogashira described for 1d.
coupling15,16 between 4,5-diodo-1-methylimidazole 3 and the
relevant arylacetylene. Compounds 1d and 1e were synthesized
Spectroscopic properties in solution
using a two-step Sonogashira coupling (Scheme 3) using The absorption spectra of compounds 1a–e were measured in
5-bromo-4-iodo-1-methylimidazole (5) and exploiting the differ- CH2Cl2 (Fig. 2 and Table 1). All compounds displayed two
ence in reactivity between the Br and I substituents.12b dominant absorptions: a relatively sharp absorption with a max-
In particular, compound 5 was prepared by the selective imum at 270–290 nm and a broad absorption with a maximum at
deiodination of 317 using a Grignard reagent followed by a 330–370 nm. Focusing on the longer wavelength absorptions,
bromination step. The Sonogashira coupling between 5 and which are due to the HOMO–LUMO transition, 1a and 1b dis-
phenylacetylene at room temperature produced 5-bromo-4- played maxima near 330 nm, whereas 1c–e, which comprise
phenylethynyl-1-methylimidazole (6d), selectively. Notably, a formyl groups, displayed maxima near 370 nm. The absorption
Sonogashira coupling between 6d and 4-ethynylbenzaldehyde edges of 1a–e are observed at 363, 368, 413, 407, and 418 nm,
was attempted at 90 1C, but the target compound 1d was respectively, and the values of their optical band gaps were
not obtained. Therefore, 2-(4-ethynylphenyl)-1,3-dioxolane, in calculated to be 3.42, 3.37, 3.00, 3.05, and 2.97 eV, respectively.
which 4-ethynylbenzaldehyde is protected by ethylene glycol, These results match the results of the DFT calculations, according
was used in the Sonogashira coupling with 6d. This reaction to which the introduction of the formyl group induces a decrease
produced 7 in moderate yield and, following deprotection, in the HOMO–LUMO energy gap, whereas the introduction of
1d in good yield. The methoxy- and formyl-substituted methoxy groups has little influence on it.
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Fig. 2 Absorption and fluorescence spectra of compounds 1a (1.24 ꢁ
10ꢀ5 M), 1b (1.6 ꢁ 10ꢀ5 M), 1c (1.3 ꢁ 10ꢀ5 M), 1d (1.4 ꢁ 10ꢀ5 M), and 1e
(1.4 ꢁ 10ꢀ5 M). The excitation wavelengths are the compounds’ labs
(solid line: absorption spectra, dashed line: fluorescence spectra).
Fig. 3 Correlation between solvent polarity and fluorescence maximum
wavenumber. Blue: 1a; green: 1b; yellow: 1c; orange: 1d; and red: 1e.
ET(30): solvent polarity parameter.
The fluorescence spectra of 1a and 1b in CH2Cl2 are similar
to each other (Fig. 2). These spectra display fluorescence
maxima at about 365 and 385 nm. Compounds 1c–e displayed
fluorescence maxima at longer wavelengths than compounds
1a and 1b, similar to what had been observed in the absorption
spectra. However, the fluorescence maxima of 1c–e are quite
different from each other, as opposed to what had been
observed in the absorption spectra. Compound 1c, which has
two formyl substituents, displayed a fluorescence maximum at
450 nm in CH2Cl2. Compound 1d, which has a single formyl
group at the R position, displayed a fluorescence maximum
at 481 nm. Compound 1e, which has a methoxy substituent and
a formyl substituent, displayed a fluorescence maximum at
514 nm, the longest wavelength in the 1a–e series. In summary,
the fluorescence spectrum of 1e is characterized by a large
Stokes shift (146 nm). The wavelength differences between the
fluorescence maxima in the 1c–e series can be explained by
the existence of an ICT. In 1d and 1e, the electron density of the
HOMO is distributed around the phenyl ethynyl or methoxy-
phenylethynyl and imidazole moiety, whereas that of the LUMO
is mainly distributed around the formylphenylethynyl moiety.
The large difference in the atomic orbital coefficient between
the HOMO and LUMO results in a difference in the dipole
moment between the ground and excited states, causing an ICT
to be observed. The electron density localization of HOMO and
LUMO progressively decreases on going from 1e to 1d and to 1c,
as a consequence of the difference in the electron-donating
properties of the substituents in the R0 position. Therefore, the
ICT character of the electronic transition is suggested to
increase in strength on going from 1e to 1d and to 1c. In many
cases, fluorophores with ICT character show remarkable
fluorescence solvatochromism.18
Notably, the fluorescence spectra of 1a–e were also recorded
in toluene, 1,4-dioxane, and CH3CN solutions (Fig. S2, ESI†). The
spectrum of compound 1e displayed an 84 nm shift in wave-
length of the emission maximum when the solvent was switched
from toluene to CH3CN. The wavelength shifts displayed by 1c
and 1d under the same conditions were 46 and 60 nm, respec-
tively. In contrast, 1a and 1b displayed only small fluorescence
solvatochromism. The correlation between the solvent polarity
parameter (ET(30)) and the fluorescence wavenumber is often
used to compare the fluorescence solvatochromism properties of
compounds (Fig. 3).19 The steeper the slope of the curve of the
fluorescence wavenumber versus ET(30), the brighter the solvato-
chromism. The slopes of the mentioned curves measured for
compounds 1a–e are 19, ꢀ25, 199, 243, and 306 cmꢀ1, respectively.
These results indicate that the fluorescence solvatochromic proper-
ties increased in the following order: 1b D 1a o 1c o 1d o 1e.
This evidence also supports the order of strength of the ICT
character in the electronic transition.
Table 1 Absorption and fluorescence features of compounds 1a–e
b
l
em,sol/nm (Fsol )
ea/104 Mꢀ1 cmꢀ1
lem,cry/nm (Fcry
)
b
labsa/nm
Toluene
1,4-Dioxane
CH2Cl2
MeCN
Compound
1a
1b
1c
1d
1e
327
332
366
363
368
2.41
2.70
2.86
3.13
2.05
385 (0.05)
369 (0.01)
421 (0.06)
437 (0.04)
454 (0.06)
375 (0.06)
371 (0.09)
431 (0.07)
448 (0.04)
467 (0.04)
384 (0.03)
368 (0.007)
450 (0.20)
481 (0.07)
514 (0.03)
384 (0.03)
366 (0.004)
467 (0.07)
497 (0.02)
538 (0.008)
405 (0.22)
396 (0.07)
551 (0.05)
498 (0.28)
523 (0.25)
Excitation wavelengths are labs for all compounds. labs: absorption maximum over 300 nm. e: molar absorption ecoefficiency. lem,sol: fluorescence
maximum in solution. Fsol: quantum yield in solution. lem,cry: fluorescence maximum in the solid state. Fcry: quantum yield in the solid state.
a
b
Values were measured in CH2Cl2. Absolute quantum yields were determined employing the integrating sphere system.
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Solid-state properties
All compounds displayed fluorescence in the solid state (Fig. 4
and Fig. S3, ESI†). The wavelengths of the fluorescence maxima
of 1a, 1b, 1d, and 1e were 405, 396, 498, and 523 nm, respec-
tively. The differences in the wavelengths of the fluorescence
maxima of 1a, 1d, and 1e between solid state and solution were
relatively small. However, the wavelengths of the fluorescence
maxima of 1b and 1c in the solid state were 30 and 100 nm
longer than that in solution, respectively.
Although the fluorescence quantum yields of 1a–e in solution
were relatively low, in the solid state, the values of these para-
meters were moderately high. This observation is the result of
the suppression of molecular motions in the crystal phase.
Notably, compounds 1a–e produced single crystals that were
suitable for X-ray structural analysis (Fig. 5 and Table S6,
ESI†).20 Four crystallographically independent molecules were
observed in the 1a crystal. The largest torsion angle between the
imidazole ring and the aryl ring was observed in compound 1b
(441). The corresponding torsion angles for the other compounds
were about 251, meaning that all these compounds had a
relatively planar structure.
Fig. 5 Crystal structures of (a) 1a, (b) 1b, (c) 1c, (d) 1d, and (e) 1e.
Three major intermolecular contacts via p-orbitals were
observed in the crystals of compounds 1a–e. The first is an
antiparallel p-stacking between the arylethynylimidazole moieties
observed in 1a, 1c, and 1d (contact A, Fig. 6a). The second is an
antiparallel imidazole–imidazole stacking observed in 1a and 1e
(contact B, Fig. 6b). The third is a p-stacking between the aryl ring
and imidazole ring observed in 1b and 1c (contact C, Fig. 6c).
Additionally, molecular contact via aryl rings was observed in the
crystal of 1c (contact D, Fig. 6d). The shortest distances between
atoms in all contacts were in the range of 3.4–3.6 Å. Contact C
observed in 1b and 1c is similar to a J-aggregate-type structure,
suggesting a small excitation energy. Excitation energies of those
dimeric structures of 1b and 1c were calculated by implementing
the time-dependent DFT method (Tables S7 and S8, ESI†). The
oscillator strength of the HOMO–LUMO excitation of contact A
and D was calculated to have a value of 0 for both 1b and 1c,
suggesting a forbidden transition. Meanwhile, those of contact C
in compounds 1b and 1c were calculated to be 0.518 and 0.0283,
respectively. In 1c, the excitation energies of the dimer of contact C
and monomer were calculated to have values of 2.92 and 3.12 eV,
Fig. 6 Molecular packing in the crystal. (a) Antiparallel p-stacking
between arylethynylimidazole moieties in 1a (contact A). (b) Antiparallel
imidazole–imidazole stacking in 1e (contact B). (c) p-Stacking between the
aryl ring and imidazole ring in 1c (contact C). (d) Molecular contact via aryl
rings in 1c (contact D).
respectively. Therefore, the excitation energy decreased by about
0.2 eV as a result of the formation of the p-stacked dimer.
Similarly, the energy change associated with 1b dimer formation
was calculated to be about 0.055 eV. Similar results were obtained
in the contact C trimer structure (Tables S9 and S10, ESI†). These
calculation results are in agreement with experimental results.
Acid responsiveness
The response of compounds 1a–e to acid vapor was examined
in the solid state, relying on the basicity of the imidazole
skeleton (Fig. 7 and Fig. S4, ESI†). Notably, the binding of a
proton to the nitrogen atom in the 3-position of the imidazole
moiety caused remarkable changes in the electronic structure
and spectroscopic properties of the compounds. After single
crystals of compounds 1a–e were exposed to hydrochloric acid
vapor, their fluorescence properties underwent substantial
modifications, even though the colors of the samples changed
Fig. 4 Fluorescence spectra of compounds 1a–e in the solid-state.
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recorded in the absence of TFA. The fluorescence maxima of 1b
(374 and 393 nm) shifted to longer wavelengths as a result of the
addition of TFA. In contrast, adding TFA to solutions of 1d and 1e
caused their fluorescence maxima to shift toward shorter wave-
lengths (440 and 452 nm, respectively). Notably, adding TFA to the
solution of the mentioned compounds triggered the formation of
imidazolium species. In the case of 1b, as a consequence of
imidazolium formation, a donor–acceptor system formed between
the anisyl moieties and the imidazolium moieties (Fig. S5, ESI†). In
the case of 1d and 1e, the length between donor and acceptor
becomes shorter due to imidazolium formation. These changes in
electronic structure caused the fluorescence spectra of these
compounds to shift toward different wavelengths. Similar spectro-
scopic changes resulting from imidazolium formation have been
reported in the study of 1,10-diaryl-2,20-biimidazole derivatives.21b
Single crystals of TFA salts of 1a, 1d, and 1e were obtained by
Fig. 7 Photographs of compounds 1a–e in the solid state under UV light
(365 nm) irradiation. The photographs above were taken before the
exposure of compounds to acid vapor, the ones below were taken after
exposure to acid vapor.
only slightly. These acid responses are irreversible. The fluores- recrystallization (Fig. 9).20,22 All these molecules were characterized
cence of compounds 1c–e, the electronic transitions of which by a planar structure with small torsion angles (o201) between the
have ICT character, was quenched after the compounds were imidazole and aryl rings. In all three molecules, the nitrogen in the
exposed to the acid vapor for about 10 s. Meanwhile, following 3-position of the imidazole ring was in contact with one oxygen of
exposure to hydrochloric acid vapor, 1a and 1b displayed weak the trifluoroacetate anion (B2.6 Å). The crystal of the TFA salt of 1d
fluorescence, with quantum yields of about 0.01 and 0.03, comprises two crystallographically independent molecules, and it
respectively. Additionally, the fluorescence maximum of 1b contains 1/2 molecule of neutral TFA per one of 1d salt; notably,
was observed at 453 nm, meaning that this peak shifted toward the neutral TFA molecule is in contact with the trifluoroacetate
longer wavelengths with respect to the neutral crystal.
anion. In contrast to the crystals of the neutral species, the crystals
The effects of adding trifluoroacetic acid (TFA) to the solutions of all TFA salt derivatives displayed stacked structures along the p
of compounds 1a–e were also examined (Fig. 8).21 The quantum direction. Molecules form a slip-stacked structure in the crystals of
yields of compounds 1a and 1b increased as a result of the addition the salts of both 1a and 1d. In the crystal of the TFA salt of 1e, the
of TFA. In contrast, the fluorescence of compounds 1c–e was
quenched or weakened. In other words, the fluorescence of
compounds, the electronic transitions of which had an ICT
character, was weakened by the addition of TFA in solution. This
tendency is similar to that observed in the acid vapor exposure
experiments. The fluorescence spectra of 1a and 1b solutions
recorded following TFA addition display two maxima. The fluores-
cence spectrum of 1a after TFA addition is similar to that of 1a
Fig. 8 Fluorescence spectra of 1a, 1b, 1d, and 1e in CH2Cl2 solution in the
presence of excess trifluoroacetic acid, 1a (F = 0.33, lex = 327 nm, 1.24 ꢁ 10ꢀ5 M);
1b (F = 0.34, lex = 332 nm, 1.6 ꢁ 10ꢀ5 M); 1d (F = 0.01, lex = 363 nm, 1.4 ꢁ
10ꢀ5 M); and 1e (F = 0.03, lex = 368 nm, 1.4 ꢁ 10ꢀ5 M). Please note that
compound 1c displayed no fluorescence. F: fluorescence quantum yield;
Fig. 9 Crystal structures and molecular arrangements of trifluoroacetate
salts of (a) 1a, (b) 1d, and (c) 1e.
lex: excitation wavelength.
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by changes in their D–p–A structure. In the solid state, the
formation of J-aggregate-type stacked structures observed in 1b,
1c, TFA salt of 1a, and TFA salt of 1d induced a long-wavelength
shift of the fluorescence maxima in both neutral and imidazolium
species.
Experimental section
UV-Vis absorption spectra were measured using a JASCO V-650
instrument. Fluorescence spectra and quantum yields were
measured using a SHIMADZU RF-6000 system with integral
sphere attachment. X-ray crystal structure analysis was per-
formed using a Bruker D8 VENTURE instrument and APEX3
application. DFT calculations were performed by Gaussian 09
revision D.01.23 All purchased reagents and solvents were used
without further purification. NMR spectra were measured using
JEOL JNM-ECS-400, JEOL JNM-ECZ-400s, or JEOL JNM-LA-300
instruments. Mass spectrometry was performed using a Waters
LCT-Premier (high-resolution, ESI) instrument. IR spectra were
recorded using a JASCO FT/IR-4100 system. All IR measurement
were performed using the KBr method. Synthetic details and
characterizations are described in the ESI.†
Fig. 10 Fluorescence spectra of the trifluoroacetate salts of 1a (F = 0.13,
l
ex = 372 nm), 1d (F = 0.03, lex = 412 nm), and 1e (F = 0.06, lex = 418 nm)
in crystalline form.
formylphenyl and imidazolium moieties overlap each other, so that
they are at a distance of about 3.45 Å.
The fluorescence maxima of the crystals of the TFA salts of
1a and 1d (395 and 486 nm, respectively) appeared at longer
wavelengths than that of their counterparts observed for salts
of 1a and 1d in solution (Fig. 10). The crystal of 1e salt displayed
a similar fluorescence maximum (465 nm) to that observed for
1e salt in solution. The shifts to longer wavelengths of the
fluorescence maxima of the TFA salts of 1a and 1d in crystalline
phase with respect to that of their counterparts for salts of 1a
and 1d in solution are due to the formation of J-aggregation-type
structures, similar to those observed for the relevant neutral
compounds in the solid state.
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
This work was partly supported by Keio Gijuku Academic
Development Funds.
Conclusions
Novel V-shaped imidazole-based small fluorophores comprising
arylethynyl groups were designed and synthesized. Introducing
arylethynyl groups in the 4- and 5-positions of the imidazole
skeleton effectively modified the values of the HOMO and LUMO
energies and caused the resulting compounds to display fluores-
cence. These compound electronic structures matched quite well
with those predicted based on DFT calculations. These com-
pounds displayed moderate quantum yields not only in solution
but also in the solid state. Compounds 1c–e, which comprise
formyl substituents, displayed ICT properties as well as remark-
able fluorescence solvatochromism. In particular, in CH2Cl2
solution, the methoxy- and formyl-substituted 1e displayed a
fluorescence maximum at a wavelength of over 500 nm and a
large Stokes shift of about 150 nm. Upon exposure of 1a and 1b
in the solid state to acid vapor, the fluorescence maxima of these
compounds shifted to longer wavelengths. In the corresponding
experiments, the fluorescence of compounds 1c–e was
quenched. The fluorescence maximum of 1b in a solution to
which acid had been added appeared at a longer wavelength
than that of its counterpart to which no acid had been added. In
contrast, the fluorescence maxima of 1d and 1e in acid-added
solution appeared at a shorter wavelength than that of the
corresponding neutral solution. These results can be explained
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