Metal cation-induced multifluorescence of azacrown-substituted (tetrafluorophenyl)imidazo[1,2-a]pyridine

Article abstract of DOI:10.1080/10610270903173987

ortho-Azacrown-substituted (tetrafluorophenyl)imidazo[1,2-a]pyridine (2) in an acetonitrile solution emits 380 nm light in the presence of Li⁺ cation and emits 460 nm light in the presence of Zn²⁺, Mg²⁺ or H⁺ cation. In contrast, para-azacrown-substituted analogue (1) emits three different fluorescent lights responding to Li⁺, Zn²⁺ or H⁺ cation, respectively; 388 nm light to Li+ cation, 433 nm light to Zn²⁺ cation or 469 nm light to H⁺ cation.

Full text of DOI:10.1080/10610270903173987

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Metal cation-induced multifluorescence of azacrown-
substituted (tetrafluorophenyl)imidazo[1,2-a]pyridine
Kiyoshi Tanaka a , Takashi Takeyama a , Ryousuke Yukawa a , Satoru Iwata a & Tohru
Kurushima a
a Laboratory of Molecular Control , Faculty of Science and Technology, Seikei University ,
Musashino-shi, Tokyo, 180-8633, Japan
Published online: 20 Aug 2009.
To cite this article: Kiyoshi Tanaka , Takashi Takeyama , Ryousuke Yukawa , Satoru Iwata & Tohru Kurushima (2010) Metal
cation-induced multifluorescence of azacrown-substituted (tetrafluorophenyl)imidazo[1,2-a]pyridine, Supramolecular
Chemistry, 22:3, 186-193, DOI: 10.1080/10610270903173987
To link to this article: http://dx.doi.org/10.1080/10610270903173987
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Supramolecular Chemistry
Vol. 22, No. 3, March 2010, 186–193
Metal cation-induced multifluorescence of azacrown-substituted
(tetrafluorophenyl)imidazo[1,2-a]pyridine
Kiyoshi Tanaka*, Takashi Takeyama, Ryousuke Yukawa, Satoru Iwata and Tohru Kurushima
Laboratory of Molecular Control, Faculty of Science and Technology, Seikei University, Musashino-shi, Tokyo 180-8633, Japan
(Received 18 March 2009; final version received 5 July 2009)
ortho-Azacrown-substituted (tetrafluorophenyl)imidazo[1,2-a]pyridine (2) in an acetonitrile solution emits 380 nm light in
the presence of Liþ cation and emits 460 nm light in the presence of Zn2þ, Mg2þ or Hþ cation. In contrast, para-azacrown-
substituted analogue (1) emits three different fluorescent lights responding to Liþ, Zn2þ or Hþ cation, respectively; 388 nm
light to Liþ cation, 433 nm light to Zn2þ cation or 469 nm light to Hþ cation.
Keywords: multifluorescence; cation recognition; azacrown; imidazo[1,2-a]pyridine
Introduction
a similar manner. In this case, no formation of ortho-
substituted products was detected (Scheme 1).
Selective recognition of chemical species by a fluorophore
modified with a recognition site has been designed for a
molecular device such as a logic gate, where a couple of
chemical species play a role as input signals and the
difference in the fluorescent intensity as an output signal
(1–5). In most cases, a sole wavelength fluorescence is
utilised as an output signal. However, multiple emissions
of a single molecule responding to several chemical
species have recently been developed as multifluorescent
probes (6–11). The single-molecular multifluorescence is
also of interest from the viewpoint of an optoelectronic
device such as a photoconverter.
Absorption spectra of 1 in an acetonitrile solution
were slightly red-shifted and its absorbance was
substantially strong, compared with those of 2-(penta-
fluorophenyl)imidazo[1,2-a]pyridine (12). Its fluores-
cence around 380 nm wavelength was considerably
weak and the intensity was affected by the polarity of
the solvent; the intensity in nonpolar solvents such as
toluene and dioxane was stronger than that in a polar
solvent such as acetonitrile (Figure 1). A similar trend
was observed with the piperidine analogue 3. In
contrast, fluorescence intensity of 2 was quite small but
its wavelength was affected by the polarity of the
solvents, around 380 nm in toluene and 460 nm in
acetonitrile (Figure 2). The longer wavelength fluores-
cence in acetonitrile would be ascribed to a charge-
transferred species induced by the electron transfer from
the azacrown-nitrogen atom into the imidazopyridine
ring (14).
In a continuing research on the application of
fluorinated 2-(2-hydroxyphenyl)benzazoles to fluorescent
probes sensing metal cations, we reported that fluor-
escence of 2-(pentafluorophenyl)imidazo[1,2-a]pyridine
is strongly enhanced by the protonation with acids (12,
13). In this paper, we wish to describe that azacrown-
modified (tetrafluorophenyl)imidazo[1,2-a]pyridine (1)
emits three different fluorescent lights responding to Liþ,
Zn2þ or Hþ cation, respectively; 388 nm light to Liþ
cation, 433 nm light to Zn2þ cation or 469 nm light to
Hþ cation.
Cation-induced changes in absorption and fluor-
escence of the ortho-substituted imidazo[1,2-a]pyridine
2 were first evaluated in acetonitrile. Although the
absorptions of 2 are definitely changed by cations, its
fluorescence changes are small, but its trend is clear. Thus,
Hþ, Zn2þ and Mg2þ cations induced blueshifts in the
absorptions and increased the fluorescence around 460 nm
wavelength, whereas Liþ cation caused blueshifts in both
absorption and fluorescence (Figure 3). It should be noted
that Naþ and Kþ cations do not cause any meaningful
changes in absorption and fluorescence. Reliable job plots
of 2 could not be taken with Liþ cation but can be obtained
with Zn2þ or Mg2þ cation using the absorbance changes.
These results supported the existence of a 1:1 complex
Results and discussion
Compound 1 was obtained by the substitution of 2-
(pentafluorophenyl)imidazo[1,2-a]pyridine with 1,4,7-
trioxa-10-azacyclododecane in DMSO, together with the
ortho-substituted isomer 2. The substituted positions were
determined by the 19F NMR analysis. For the comparison
of 1, the piperidine-substituted analogue 3 was prepared in
*Corresponding author. Email: tanaka@st.seikei.ac.jp
ISSN 1061-0278 print/ISSN 1029-0478 online
q 2010 Taylor & Francis
DOI: 10.1080/10610270903173987
http://www.informaworld.com

Supramolecular Chemistry
187
Scheme 1. Synthesis of 1–3.
Figure 1. Absorption and fluorescence spectra of 1 in toluene, dioxane and acetonitrile. [1] ¼ 1 £ 1025 M, lex ¼ 306 nm.
with Zn2þ cation and a 2:1 complex with Mg2þ cation
(Figure 4). Binding constants were determined by the
curve fittings using the absorbance changes and were
estimated to be K ¼ (3.5 ^ 0.7) £ 104 M21 for Zn2þ
cation and K ¼ (1.9 ^ 0.4) £ 109 M22 for Mg2þ cation
(Figure 5).
Fairly deshielded shift of the 3-proton of the
imidazopyridine ring, compared with that of 1, is
characteristic of 2. In the presence of one equivalent of
cation, its proton shifted upfield and the other aromatic
protons and the azacrown protons are all shifted downfield.
It is noticed that the downfield shift of the aromatic protons
Figure 2. Absorption and fluorescence spectra of 2 in toluene, dioxane and acetonitrile. [2] ¼ 1 £ 1024 M for absorption spectra and
1 £ 1025 M for fluorescence spectra, lex ¼ 285 nm.

188
K. Tanaka et al.
Figure 3. Caþtion-induced spectral changes of 1–3 in acetonitrile. [1] ¼ [2] ¼ [3] ¼ 1 £ 1025 M, lex ¼ 308 nm, metal cation;
thiocyanate, H ; trifluoroacetic acid.
induced by Liþ cation is relatively small (Figure 6). The
deshielded shift of the 3-proton of 2 itself would be
ascribed to an effect of the ortho-azacrown ring and the
upfield shift in the presence of Hþ, Zn2þ or Mg2þ cation
would be caused by the ligation of the cation at the
1-nitrogen atom of 2, which draws the azacrown ring in the
direction of the 1-nitrogen atom, in the counter direction of
the 3-proton. The azacrown ring associated with Liþ
cation similarly goes in the direction of the 1-nitrogen
atom. Assumed structures of the complexes are depicted in
Figure 7. It is assumed that, in the excited states, the
coordination of Hþ, Zn2þ or Mg2þ cation with
the azacrown-nitrogen atom is weakened to result in the
formation of a charge-transferred species, which emits the
460 nm light and, reversely, the association of Liþ cation
with the azacrown-nitrogen atom is strong to retard the
electron transfer from its nitrogen atom, giving rise to the
enhancement of the 380 nm fluorescence (15).
The effects of cation on absorption and fluorescence of
the para-substituted imidazo[1,2-a]pyridine 1 were next
evaluated in an acetonitrile solution. The absorption and
the fluorescence were both red-shifted in the presence of
trifluoroacetic acid, which is in sharp contrast to the
blueshift of the absorption in the case of 2, and the
fluorescence intensity around 469 nm was substantially
enhanced with the increase of trifluoroacetic acid (Figures
3 and 8). Addition of excess amount of trifluoroacetic acid
caused the slight increase in the fluorescence around
1
the 360 nm wavelength. The H NMR spectra of 1 in the
presence of an equivalent of trifluoroacetic acid show the
deshielded aromatic protons, along with the slightly
deshielded protons adjacent to the azacrown-nitrogen
atom (Figure 9). These spectral changes support the
contribution of the protonation at the 1-nitrogen atom
causing the delocalisation of the positive charge to the
azacrown-nitrogen atom in the ground state, as depicted in
Figure 10. In the case of excess trifluoroacetic acid, the
delocalisation would be retarded by the second protona-
tion at the azacrown-nitrogen atom, increasing the
fluorescence around the 360 nm wavelength. As antici-
pated with the piperidine analogue 3, the similar redshifts
of the absorption and the fluorescence were also noticed in
the presence of trifluoroacetic acid (Figure 3).
Figure 4. Job plots for the complexations of 2 with (a)
Zn(SCN)2 and (b) Mg(SCN)2 in acetonitrile. Total concentration
of 2 plus Zn(SCN)2 or Mg(SCN)2 is maintained at 1 £ 1024 M.

Supramolecular Chemistry
189
Figure 5. Titration of 2 with (a) Zn(SCN)2 and (b) Mg(SCN)2 in acetonitrile. [2] ¼ 1 £ 1024 M, [Zn(SCN)2] ¼ 0–10 £ 1024 M,
[Mg(SCN)2] ¼ 0–5 £ 1024 M.
Addition of Liþ cation to an acetonitrile solution of 1
did not cause the appreciable change in the absorption but
enhanced the fluorescence around the 388 nm wavelength
(Figure 3). Although Liþ cation did not affect the 1H NMR
spectra of 1 distinctly, this fluorescence enhancement
would be ascribed to the association of Liþ cation with the
azacrown site from the fact that a similar fluorescence
enhancement was not observed in the case of 3 and from
the consideration of the size-matching of Liþ cation with
the 12-azacrown ring (16). The curve fitting of the
fluorescence intensity of 1 supported the formation of a 1:1
complex of 1 and Liþ cation, its binding constant being
estimated to be K ¼ (4.7 ^ 0.7) £ 102 M21 (Figure 11).
The effects of Zn2þ cation on the spectral properties of
1 are very complicated. Thus, the absorption was slightly
blue-shifted in the presence of Zn2þ cation but its
fluorescence was reversely red-shifted and the emission
around the 433 nm wavelength was enhanced with the
1
increase of Zn2þ cation. The H NMR spectra of 1 in the
presence of an equivalent of Zn2þ cation show a similar
tendency to those with trifluoroacetic acid, and also show
the deshielded aromatic protons, along with the slightly
deshielded protons adjacent to the azacrown-nitrogen
atom (Figure 9). In this case, it is not so likely that
the ligation of Zn2þ cation at the 1-nitrogen atom
causes the delocalisation of the positive charge to the
Figure 6. 1H NMR spectra of 2 (a) itself, (b) with trifluoroacetic acid, (c) with Mg(SCN)2, (d) with Zn(SCN)2 and (e) with LiSCN.
Solvent; CD3CN, [2] ¼ [CF3COOH] ¼ [Mg(SCN)2] ¼ [Zn(SCN)2] ¼ [LiSCN] ¼ 1 £ 1023 M.

190
K. Tanaka et al.
Figure 7. Assumed structures of the complexes of 2 with Zn2þ, Mg2þ or Liþ.
Figure 8. Spectral changes of 1 with the increase of trifluoroacetic acid in acetonitrile. [1] ¼ 1 £ 1025 M, lex ¼ 308 nm,
[trifluoroacetic acid] ¼ 0–1 equiv. of 1.
azacrown-nitrogen atom in the ground state, because the
absorption of 1 is not red-shifted but slightly blue-shifted.
(Figure 12), titration was found to be difficult because of
the non-existence of reliable isosbestic points in the
absorption spectra. A similar trend in absorption and
fluorescence was observed with 3 (Figure 3), suggesting
1
Although the Job plot using H NMR analysis supports
the existence of a 1:1 complex of 1 with Zn2þ cation
Figure 9. 1H NMR spectra of 1 (a) itself, (b) with LiSCN, (c) with Zn(SCN)2 and (d) with trifluoroacetic acid. Solvent; CD3CN,
[1] ¼ [CF3COOH] ¼ [Mg(SCN)2] ¼ [Zn(SCN)2] ¼ [LiSCN] ¼ 1 £ 1023 M.

Supramolecular Chemistry
191
Figure 10. Assumed structure of the protonated 1.
the formation of a similar species to that with 1. From
these observations, we assume that the 1–Zn2þ complex
ligated at the 1-nitrogen atom by Zn2þ cation forms a
dimer or an oligomer where the Zn2þ cation is weakly
bridged with the azacrown-nitrogen atom of the other
1–Zn2þ complex in the ground state, this weak bridging is
released in the excited state (15), followed by the electron
transfer from the azacrown-nitrogen atom into the
imidazopyridine ring, and then the formed charge-
transferred species emits the 433 nm light (14). This
consideration would be supported by the difference
between the absorption spectra and the excitation spectra
of 1 in the presence of 10 equiv. of Zn2þ cation as shown in
Figure 13, where the similar spectra of 3 with Zn2þ cation
is depicted.
Figure 12. Job plot for the complexation of 1 with Zn2 þ in
acetonitrile. Total concentration of 1 plus Zn(SCN)2 is
maintained at 1 £ 1022 M.
and Ca2þ do not cause any meaningful changes in the
absorption and fluorescence.
Conclusions
para-Azacrown-substituted (tetrafluorophenyl)imidazo-
[1,2-a]pyridine (1) and its ortho-analogue (2) were simply
prepared by the substitution of 2-(pentafluorophenyl)imi-
dazo[1,2-a]pyridine with 1,4,7-trioxa-10-azacyclodode-
cane. Multiple fluorescence was found to be enhanced by
several kinds of cations in an acetonitrile solution. Thus, 2
emits the 380 nm light in the presence of Liþ cation and
emits the 460 nm light in the presence of Zn2þ, Mg2þ or
Hþ cation. In contrast, 1 emits three different fluorescent
lights responding to Liþ, Zn2þ or Hþ cation, respectively;
388 nm light to Liþ cation, 433 nm light to Zn2þ cation or
469 nm light to Hþ cation.
It was confirmed that fluorescence and absorption of 1
were not affected by Mg2þ cation, which is in contrast to
those of 2. It is assumed that ligation of Mg2þ cation at the
1-nitrogen atom of the imidazopyridine ring is weaker than
that of Zn2þ cation, from the considerations of the rule of
hard and soft acids and bases. Therefore, ligation at the 1-
nitrogen atom of the imidazopyridine ring and cooperative
association with the ortho-substituted azacrown ring in 2
would be necessary for the complexation of Mg2þ cation.
It is also noted that other metal cations such as Naþ, Kþ
Experimental
The FT/IR spectra were recorded on a JASCO FTIR-
460 plus spectrophotometer and samples were run as
potassium bromide pellets. The UV–vis and fluor-
escence spectra were recorded with JASCO V-530 and
JASCO FP-6500 spectrometers, respectively, and
measured in solvents of the highest quality for
spectroscopy (KOKUSAN Chemical Co., Tokyo,
Japan) without further purification. The 1H and 19F
NMR spectra were recorded with a JEOL JNM-LA400
spectrometer and the chemical shifts are given in d
(ppm) downfield using tetramethylsilane as an internal
1
standard for H NMR spectra and trifluoroacetic acid as
an external standard for 19F NMR spectra; J values are
given in Hz. The MALDI-TOF MS spectra were taken
with a SHIMAZU AXIMA-CFR Plus mass spec-
trometer. The elemental analyses were measured with
a Perkin-Elmer 2400 II CHN analyser.
Figure 11. Titration of 1 with LiSCN in acetonitrile.
[1] ¼ 1 £ 1025 M, [LiSCN] ¼ 0–20 £ 1025 M.

192
K. Tanaka et al.
Figure 13. Absorption and excitation spectra of (a) 1 and (b) 3 with Zn(SCN)2 in acetonitrile. [1] ¼ [3] ¼ 1 £ 1025 M,
[Zn(SCN)2] ¼ 1 £ 1024 M, lem ¼ 434 nm.
2-(Pentafluorophenyl)imidazo[1,2-a]pyridine was pre-
pared by the reaction of bromomethyl pentafluorophenyl
ketone, according to the previously reported paper (12).
mixture was washed with water and brine, dried over
magnesium sulphate and evaporated. The resulting solid
was chromatographed on silica gel (hexane–ethyl acetate,
1:1 and then 1:2) to give 110 mg (18% yield) of 1 and
90 mg (14% yield) of 2, which were recrystallised from
hexane–ethyl acetate.
2-(2,3,5,6-Tetrafluorophenyl-4-piperidino)imidazo[1,2-
a]pyridine (3)
Compound 1
A mixture of 2-(pentafluorophenyl)imidazo[1,2-a]pyri-
dine (220 mg, 0.77 mmol), piperidine (2 ml) and triethyl-
amine (3 ml) in 10 ml of dried DMSO was stirred at 658C
for 2 h. Ethyl acetate was added to the reaction mixture and
the mixture was washed with water and brine, dried over
magnesium sulphate and evaporated. The resulting solid
was chromatographed on silica gel (hexane–ethyl acetate,
1:1) to give 170 mg (63% yield) of 3, which was
recrystallised from hexane–ethyl acetate. Yellow crystals;
mp 153–1548C; FT/IR (KBr): n 3179 (CZH), 3045
Yellow crystals; mp 110–1118C; FT/IR (KBr): n 3157
(CZH), 3042 (CZH), 2942 (CZH), 2858 (CZH), 1650
(CZF), 1130 (CZOZC) cm21; UV–vis (acetonitrile):
l
max [log 1 (litres mol21 cm21)] 276.5 (4.36), 306.5 (4.41),
1
313 (4.41) nm; H NMR (400 MHz, CDCl3): d 3.55 (t,
J ¼ 4.9 Hz, 4H), 3.71 (m, 8H), 3.81 (t, J ¼ 4.9 Hz, 4H),
6.83 (ddd, J ¼ 6.8, 6.8, 0.98 Hz, 1H), 7.21 (ddd, J ¼ 9.2,
6.8, 0.98 Hz, 1H), 7.69 (d, J ¼ 9.2 Hz, 1H), 7.91 (s, 1H),
8.16 (d, J ¼ 6.6 Hz, 1H); 19F NMR (376 MHz, CDCl3): d
274.2 (AA0BB00, 2F), 267.5 (AA0BB00, 2F); MS (MALDI-
TOF) m/z: calcd for C21H21F4N3O3 [MþH]þ 440.15;
found [MþH]þ 440.09. Anal. Calcd for C21H21F4N3O3:
C, 57.40; H, 4.82; N, 9.56. Found: C, 57.26; H, 4.73; N,
9.29.
(CZH), 2930 (CZH), 2845 (CZH), 1648 (CZF) cm21
;
UV–vis (acetonitrile): lmax [log 1 (litres mol21 cm21)]
275 (4.31), 301 (4.29), 312 (4.27) nm; 1H NMR (CDCl3): d
1.66 (m, 6H), 3.26 (t, J ¼ 4.3 Hz, 4H), 6.83 (dd, J ¼ 6.8,
6.5 Hz, 1H), 7.21 (ddd, J ¼ 9.2, 6.5, 0.97 Hz, 1H), 7.70 (d,
J ¼ 9.2 Hz, 1H), 7.91 (s, 1H), 8.15 (dd, J ¼ 6.8, 0.97 Hz,
19
1H); F NMR (CDCl3): d 276.2 (AA0BB00, 2F), 268.0
Compound 2
(AA0BB00, 2F); MS (MALDI-TOF) m/z: calcd for
C18H15F4N3 [MþH]þ 350.12; found [MþH]þ 350.10.
Anal. Calcd for C18H15F4N3: C, 61.89; H, 4.33; N, 12.03.
Found: C, 61.95; H, 4.31; N, 11.97.
Yellow crystals; mp 180–1828C; FT/IR (KBr): n 3165
(CZH), 2906 (CZH), 2858 (CZH), 1631 (CZF), 1132
(CZOZC) cm21; UV–vis (acetonitrile): lmax [log 1
(litres mol21 cm21)] 279 (3.83), 293 (3.81), 317 (3.82)
1
nm; H NMR (400 MHz, CDCl3): d 3.23 (t, J ¼ 4.2 Hz,
4H), 3.38 (m, 8H), 3.71 (t, J ¼ 4.1 Hz, 4H), 6.76 (dd,
J ¼ 6.8, 6.6 Hz, 1H), 7.14 (dd, J ¼ 9.0, 6.6 Hz, 1H), 7.61
(d, J ¼ 9.0 Hz, 1H), 8.48 (d, J ¼ 6.8 Hz, 1H), 9.12 (s,
1H); 19F NMR (376 MHz, CDCl3): d 283.6 (dd,
J ¼ 21.0, 21.0 Hz, 1F), 281.7 (dd, J ¼ 21.0, 20.0 Hz,
1F), 269.6 (dd, J ¼ 20.0, 9.0 Hz, 1F), 263.8 (m, 1F);
MS (MALDI-TOF) m/z: calcd for C21H21F4N3O3
[MþH]þ 440.15; found [MþH]þ 440.13. Anal. Calcd
for C21H21F4N3O3: C, 57.40; H, 4.82; N, 9.56. Found: C,
57.47; H, 4.94; N, 9.51.
10-(2,3,5,6-Tetrafluoro-4-(imidazo[1,2-a]pyridin-2-
yl)phenyl)-1,4,7-trioxa-10-azacyclododecane (1) and 10-
(3,4,5,6-tetrafluoro-2-(imidazo[1,2-a]pyridin-2-yl)
phenyl)-1,4,7-trioxa-10-azacyclododecane (2)
A mixture of 2-(pentafluorophenyl)imidazo[1,2-a]pyri-
dine (420 mg, 1.48 mmol), 1,4,7-trioxa-10-azacyclodode-
cane (250 mg, 1.43 mmol) and triethylamine (3 ml) in
25 ml of dried DMSO was stirred at 1008C for 3 days.
Ethyl acetate was added to the reaction mixture and the

Supramolecular Chemistry
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Acknowledgements
This work was financially supported by the Promotion and
Mutual Aid Corporation for Private Schools of Japan through the
High-Tech Research Center Project.
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