Bimodal fluorescence signaling based on control of the excited-state conformational twisting and the ground-state protonation processes

Article abstract of DOI:10.1016/j.tetlet.2007.02.120

Amino-based fluoroionophores 1 and 2 can selectively sense alkaline earth metal ions in MeCN under both neutral and acidic conditions by different signaling mechanisms. The fluoroionophoric behavior for the neutral probes is characterized by an 'off-on' photoinduced electron transfer (PET)-like fluorescence intensity response due to a switching from a twisted internal charge transfer (TICT) to a planar internal charge transfer (PICT) state. For the protonated probes (i.e., 1/H⁺ and 2/H⁺), the fluorescing species is the localized stilbene fluorophores, but dual fluorescence is induced upon metal-ion recognition through a deprotonation process.

Full text of DOI:10.1016/j.tetlet.2007.02.120

Tetrahedron Letters 48 (2007) 3097–3102
Bimodal fluorescence signaling based on control
of the excited-state conformational twisting and
the ground-state protonation processes
Jye-Shane Yang,a,* Chung-Yu Hwangb and Mon-Yao Chenb
aDepartment of Chemistry, National Taiwan University, Taipei 10617, Taiwan
bDepartment of Chemistry, National Central University, Chung-Li 32054, Taiwan
Received 25 December 2006; revised 16 February 2007; accepted 16 February 2007
Available online 1 March 2007
Abstract—Amino-based fluoroionophores 1 and 2 can selectively sense alkaline earth metal ions in MeCN under both neutral and
acidic conditions by different signaling mechanisms. The fluoroionophoric behavior for the neutral probes is characterized by an
‘off–on’ photoinduced electron transfer (PET)-like fluorescence intensity response due to a switching from a twisted internal charge
transfer (TICT) to a planar internal charge transfer (PICT) state. For the protonated probes (i.e., 1/H+ and 2/H+), the fluorescing
species is the localized stilbene fluorophores, but dual fluorescence is induced upon metal-ion recognition through a deprotonation
process.
ꢀ 2007 Elsevier Ltd. All rights reserved.
Development of fluorescent probes or chemosensors for
biological and/or environmental applications continues
to play a key role in the field of probe design, because
fluorescence measurement is an inherently sensitive
and practically simple technique.1 Among the various
fluorescence signaling mechanisms, the utilities of photo-
induced electron transfer (PET)2 and internal charge
transfer (ICT)3 have been well demonstrated. For
PET-based probes, dramatic changes in fluorescence
intensity lead to either an ‘off–on’ (i.e., fluorescence
enhancement) or an ‘on–off’ (i.e., fluorescence quench-
ing) response.2 In contrast, the sensing behavior of
ICT-based probes is characterized by spectral shifts in
both the absorption and fluorescence spectra, which
allows ratiometric measurement.3 A particular type of
ICT probes relies on the presence of a twisted ICT
(TICT) state in addition to the normal planar ICT
(PICT) state.4,5 When both the PICT and TICT states
are fluorescent, the free probes show dual fluorescence.
When the TICT state is predominant but nonfluo-
rescent, the free probes would be weakly fluorescent,
resembling the off state of PET-based probes. As a
result, the fluorescence signaling behavior for TICT-
based probes might be an off–on or ratiometric type,
depending on the nature of fluorophore.
The vast majority of the reported (T)ICT and PET
probes adopt one or more amino group(s) as the elec-
tron donor (D).1–4 However, the operation of these ami-
no-based probes generally requires careful pH control
for success, and in most cases the signal function is
invalidated under acidic conditions due to the proton-
ation of the amino group. An additional problem
associated with (T)ICT fluoroinophores of simple
donor–acceptor (D–A) constitution is the phenomenon
of excited-state ion decoordination, which reduces the
probe sensitivity.6 We report herein new amino-based
D–A typed fluoroionophores 1 and 2, which can operate
under both neutral and acidic conditions with high
sensitivity to alkaline earth metal ions, particularly
Mg2+, through different signaling mechanisms. Whereas
the fluorescence response is mainly an off–on behavior
under neutral conditions, dual fluorescence with ratio-
metric behavior is observed in the presence of 10 mM
HClO4.
The design of fluoroionophores 1 and 2 was inspired by
our recent works on the fluorescence behavior of N-aryl
substituted aminostilbenes 3 and 4.7 When the substitu-
ent in the N-aryl group of 3 or 4 (i.e., the R0 group) is
Keywords: Fluoroionophores; Signaling; PET; TICT; PICT; Dual
fluorescence.
*
Corresponding author. Fax: +886 2 23636359; e-mail: jsyang@
ntu.edu.tw
0040-4039/$ - see front matter ꢀ 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2007.02.120

3098
J.-S. Yang et al. / Tetrahedron Letters 48 (2007) 3097–3102
methoxy (i.e., 3OM and 4OM), the fluorescence quantum
yield is rather low in acetonitrile (Ufl ꢀ 0.006) due to the
efficient formation of a weakly fluorescent TICT state.
However, TICT-state formation is negligible for those
with a less electron-donating or weak electron-withdraw-
ing substituent such as methyl (3Me and 4Me), hydrogen
(3H and 4H), or chloro (3Cl), leading to much higher
fluorescence quantum yields (Ufl = 0.13–0.43). Because
of such a unique substituent effect, we consider that
trans-4-(N-arylamino)stilbenes are potential TICT-based
fluorescent probes upon introducing appropriate
receptors at the N-aryl group. To test this hypothesis,
benzocrown derivatives 1 and 2 were investigated. We
reasoned that 1 and 2 might retain the weak fluorescence
nature of 3OM and 4OM, and a small amount of
electronic perturbation on the oxygen donor might be
sufficient to ‘turn on’ the fluorescence of 1 and 2 through
the inhibition of the TICT process. In addition, unlike the
conventional PET and (T)ICT probes, the amino
nitrogen in 1 and 2 is not a part of the receptor (i.e.,
ionophore), which might minimize the potential influence
of excited-state cation decoordination.
Table 1 shows the maxima of absorption and fluores-
cence spectra and fluorescence quantum yield (Ufl) for
1 and 2 along with the data of 3 and 4 in acetonitrile.
The photophysical data for 1 and 2 are all similar to
those for 3OM and 4OM, respectively, except for the
fluorescence maxima for 2 versus 4OM. This indicates
that the presence of an additional alkoxy group at the
meta-position of the N-(4-anisole) group does not affect
the TICT process. Since the fluorescence spectra for 1, 2,
3OM, and 4OM result from an unresolved overlapping
of the PICT and the TICT fluorescence,7 the fluores-
cence maxima would depend on the relative intensity
of the PICT and TICT fluorescence. Thus, the difference
in the fluorescence maxima between 2 (547 nm) and
4OM (583 nm) can be attributed to a relatively stronger
fluorescence for the PICT than the TICT for 2 versus
4OM, because the PICT fluorescence is blue shifted rel-
ative to the TICT fluorescence.
Figure 1 shows the absorption and fluorescence titration
spectra of 1 and 2 in MeCN with Mg2+. Whereas the
absorption spectra are little affected, the fluorescence
spectra are blue shifted by 25–52 nm and the fluores-
cence intensity is increased by as large as 54–70 times
(i.e., switching ‘on’ the fluorescence) when the concen-
tration of Mg2+ is 150 lM. Apparently, complexation
between the benzocrown and Mg2+ (vide infra) perturbs
the electron-donating ability of the crown oxygens and
in turn the overall electron-donating ability of the aryl-
amino group. Such a perturbation on the electron donor
is sufficient to inhibit the TICT process, and thus the flu-
orescing state becomes the highly fluorescent PICT state
due to the amino-conjugation effect.11 It should be noted
that the significant fluoroionophoric behavior for 1 and
The synthesis of 1 and 2 is shown in Scheme 1. Inter-
mediates 5–9 are all known compounds.8 Their prepara-
tions were carried out by following the literature
procedures and their characterization data conform to
the reported values. Palladium-catalyzed amination
reactions9 between 8 or 9 and 7 provided the desired
compounds 1 and 2.10
O
O
R'
O
h
O
g
O
e
f
a
b
i
d
NH
NH
Table 1. Absorption maxima (kabs), fluorescence maxima (kfl), fluo-
rescence quantum yield (Ufl), and binding constants (logK) for
compounds 1–4 and for the complexes of 1 and 2 in MeCN
b
R
R
c
OM R' = OMe
Me R' = Me
1
2
R = H
3
4
R = H
Compounda
kabs (nm)
kfl (nm)
Ufl
logK
H
Cl
R' = H
R' = Cl
R = CN
R = CN
1
355
497
445
447
455
456
358
547
522
525
531
536
363, 522
502
457
442
437
583
531
504
0.007
1/Mg2+
1/Ca2+
1/Sr2+
1/Ba2+
1/H+
2
350
351
351
351
298, 310
386
378
377
377
379
318, 383
356
354
351
351
384
382
379
0.38 (54)
0.29 (41)
0.22 (31)
0.13 (19)
4.2
4.1
3.9
3.8
3.1
O
O
O
NaOH
O
O
+
Cl
O
O
O
Cl
Butanol
52%
HO
OH
0.002
5
2/Mg2+
2/Ca2+
2/Sr2+
2/Ba2+
2/H+
3OMc
3Mec
3Hc
0.14 (70)
0.07 (35)
0.04 (20)
0.02 (10)
5.2
5.0
4.8
4.7
3.0
N2H4 nH2O
10% Pd/C
65% HNO3
CH3COOH
O
O
O
H2N
O2N
O
O
O
O
1,4-Dioxane
100%
CHCl3
98 %
O
O
O
0.007
0.41
0.34
0.43
<0.005
0.13
6
7
1
Br
Br
3Clc
NaOtBu,
racemic- BINAP,
7
4OMd
4Med
4Hd
1. P(OEt)3, ¡µ
2. NaH, DMF
or
2
0.35
Pd2(dba)3, Toluene
80oC 12hr
Br
R
a The counter anion for the cation is perchlorate (ClO4ꢁ).
b Value in the parentheses is the fluorescence enhancement factor rel-
O
R
27-40%
85-90%
ative to the free probe (i.e., Ufl/U0, where U0 is the Ufl for the free
8: R = H
9: R = CN
probe).
c Data from Ref. 7a.
d Data from Ref. 7b.
Scheme 1.

J.-S. Yang et al. / Tetrahedron Letters 48 (2007) 3097–3102
3099
0.6
a
2
Ha,Hb
445 nm
355 nm
1
Hf
0.5
0.4
0.3
0.2
0.1
0.0
Hi
He
Hc
Hd Hh
350 nm
H
g
*
b
c
2/Mg2+
*
2/H+
He
Hd
Hf
Hh,Hi
Hg
522 nm
386 nm
2
0.4
2/Mg2+/H+
378 nm
d
0.3
0.2
0.1
0.0
7.8 7.6 7.4 7.2 7.0 6.8
ppm
4.4 4.2 4.0 3.8 3.6 3.4
Figure 2. 1H NMR spectra (400 MHz) of (a) 2; (b) 2/Mg2+; (c) 2/H+;
and (d) 2/Mg2+/H+ in acetonitrile-d3 (counter ion ClO4ꢁ). The con-
centration for 2, Mg2+, and H+ are 1, 10, and 10 mM, respectively.
250 300 350 400 450 350 400 450 500 550 600 650
The marks
and d denote the peaks of NH and the excess H+,
ꢂ
Wavelength (nm)
respectively, and the dash lines are to correlate the shifting of specific
proton signals. See the molecular structure for the designation of
protons.
Figure 1. Absorption and fluorescence spectra of (a) 1 (1 · 10ꢁ5 M
excitation at 325 nm) and (b) 2 (1 · 10ꢁ5 M, excitation at 340 nm) in
MeCN upon addition of 0, 1, 3, 5, 10, 15, 30 equiv of Mg(ClO4)2. For
clarity, the absorption spectra are shown at 0, 5, and 30 equiv of
Mg(ClO4)2.
formed between the fluoroionophores and the alkaline
earth metal ions at low concentrations (the total concen-
tration is at 1 · 10ꢁ5 M). Their binding constants,
expressed as logK, determined by absorption titration
spectra13 along with the spectral data for 1 and 2 are
reported in Table 1.
2 does not necessarily indicate the absence of any ex-
cited-state cation decoordination behavior. It is more
likely that the cation decoordination effect is smaller
for 1 and 2 than those where the amino nitrogen partici-
pates in the cation binding. Provided that the electron-
donating ability for the donors in the complex forms
of 1 and 2 is far off the threshold for TICT formation,
a small degree of cation decoordination would not
resume the TICT process. As shown by the case of 2,
recognition of Mg2+ by the benzocrown moiety is
Interestingly, the function of 1 and 2 is not interfered
even under highly acidic conditions, but the signaling
mechanism is distinct (Fig. 4). As exemplified by the case
of 1, the fluorescence spectra undergo a large blue shift
(Dkfl = 139 nm) in the presence of 10 mM HClO4, and
the fluorescence spectrum resembles that for a simple
stilbene fluorophore. Thus, a full protonation of the
amino group in the ground as well as the excited state
is indicated. Upon addition of Mg2+, the PICT fluores-
cence band is generated, leading to dual fluorescence.
Apparently, the coordination of Mg2+ expels the
ammonium proton, presumably due to charge–charge
repulsions. The phenomenon of metal-ion induced
deprotonation of the ammonium ion that triggers a
PET process and thus quenches the fluorescence has
recently been reported.14 However, the deprotonation
behavior that triggers an ICT process and leads to dual
fluorescence is unprecedented. The corresponding ratio-
metric plot against the concentration of metal ions for 1
is shown in Figure 5. It should also be noted that at low-
er concentration of HClO4 such as 1 mM the fluores-
cence signaling behavior resembles that under neutral
conditions. In other words, fluoroionophore 1 has little
responses to or affected by protons at low concentra-
tions, as demonstrated by the case of 1 to Mg2+
(Fig. 3a). This phenomenon can be attributed to the
1
supported by the H NMR spectra (spectra a and b in
Fig. 2), where the benzocrown protons display a much
larger downfield shifting (Dd = 0.2–0.4 ppm) than the
rest of the protons (Dd < 0.1 ppm).
Compounds 1 and 2 also display fluorescence responses
to the other alkaline earth metal ions Ca2+, Sr2+, and
Ba2+, but the size of the spectral shifts and intensity
enhancement is relatively lower than the case of Mg2+
.
In contrast, little or no fluorescence responses were
found for alkali metal ions such as Li+, Na+, and K+.
The larger fluorescence response of 1 and 2 to Mg2+ ver-
sus the other metal ions could be attributed to its higher
charge density (e.g., Mg2+ (0.75) > Ca2+ (0.24)).12 As
exemplified by the case of 1, the ion selectivity based
on the intensity-ratio plot of the fluorescence quantum
yields against the concentration of these metal ions is de-
picted in Figure 3a. According to the Job plots based on
the changes in fluorescence (Fig. 3b), a 1:1 complex is

3100
J.-S. Yang et al. / Tetrahedron Letters 48 (2007) 3097–3102
60
50
2+
0.5
Mg
a 298 nm
445 nm
1
2
310 nm
2+
+
Mg + H
0.4
0.3
0.2
0.1
0.0
40
30
2+
Ca
2+
Sr
350 nm
358
nm
20
2+
Ba
10
0
+
325 nm
H
0
5
10
15
20
25
30
b
522 nm
305 nm
318 nm
[cation]/[1]
0.4
0.3
378 nm
Mg2+
Ca2+
Sr2+
0.2
0.1
0.0
363 nm
Ba2+
340 nm
250
300 350
400
350 400 450 500 550 600 650
Wavelength (nm)
0
0.2
0.4
0.6
0.8
1.0
Figure 4. Absorption and fluorescence spectra of (a) 1 (1 · 10ꢁ5 M,
excitation at the isosbestic point, 325 nm) and (b) 2 (1 · 10ꢁ5 M,
excitation at the isosbestic point, 340 nm) in MeCN with 0.01 M of
HClO4 upon addition of 0, 1, 3, 5, 10, 15, 30 equiv of Mg(ClO4)2.
molar fraction of 1
Figure 3. (a) The plot of changes in the ratio of the fluorescence
quantum efficiency (Ufl/U0, where U0 is the Ufl for the free probe)
against the ratio of the concentration of alkaline earth metal ions
Mg2+, Ca2+, Sr2+, Ba2+, and proton (counter ion ClO4ꢁ) versus 1
(1 · 10ꢁ5 M, excitation at 325 nm) in MeCN, and (b) the Job plot of 1
with metal ions in MeCN (the total concentration of 1 and metal ions
is 1 · 10ꢁ5 M).
Mg2+
60
50
weak basicity of diarylamines and thus the low proton
binding constants (Table 1). Similar fluoroionophoric
behavior was also observed for 2 (Fig. 4b). The above-
proposed fluorescence sensing mechanisms are sup-
Ca2+
40
30
Sr2+
1
20
ported by the H NMR studies based on 2. As shown
in Figure 2, H+ induces a much larger downfield shifting
for the protons in the diphenylamino group than
the other protons (spectrum c in Fig. 2), which is
explicitly different from the effect of Mg2+ and consis-
tent with the protonation of the amino nitrogen. When
Mg2+ was added at such an acidic condition, the signals
of protons Hd–Hf shifts upfield to a significant extent
(Dd = 0.1–0.2 ppm), but the benzocrown protons under-
go downfield shifting with a size similar to that in
neutral conditions (spectrum d in Fig. 2), a behavior
consistent with the phenomenon of Mg2+-binding-
induced deprotonation reaction.
Ba2+
10
0
0
50
100
150
200
[M(II)]/[1]
Figure 5. The plot of changes in the ratio of the intensity of
fluorescence maxima (Imax/I358, where Imax is the fluorescence maxima
in the presence of metal ions) against the ratio of the concentration of
alkaline earth metal ions Mg2+, Ca2+, Sr2+, and Ba2+ and proton
versus 1 (1 · 10ꢁ5 M, excitation at 325 nm) in MeCN with 0.01 M of
HClO4.
The difference in fluorescence properties between 1 and 2
deserves our attention. When compared to 1, the pres-
ence of an electron-withdrawing cyano group in the stil-
bene moiety for 2 enhances the ICT character and thus
results in a red shift in the spectra and reduces the fluo-
rescence quantum efficiency. Regarding the fluoroiono-
phoric behavior, the selectivity and sensitivity toward
Mg2+ appears to be slightly better for 2 versus 1 (Table
1) under neutral conditions. The higher sensitivity is
illustrated by the higher fluorescence enhancement
factor (70 vs 54), and the higher selectivity is evidenced
by the larger reduction in the fluorescence enhancement

J.-S. Yang et al. / Tetrahedron Letters 48 (2007) 3097–3102
3101
O
O
O
O
O
ratiometric
mode II
off-on
NH
R
+
+
mode I
PICT state
O
O
= metal ions
O
O
O
O
O
O
O
O
+ H+
NH
NH2
R
R
_
H+
TICT state
LE state
Figure 6. Schematic presentation of the bimodal fluoroinophoric behavior of 1 and 2.
factor on going from Mg2+ to Ca2+ and to Ba2+. Under
the acidic conditions, the absorption band for the free
probe (386 nm) is not completely disappeared (Fig. 3),
which can be accounted for by the lower basicity of 2
due to the cyano group.
3. For recent examples of ICT-based probes, see: (a) Yang,
J.-S.; Hwang, C.-Y.; Hsieh, C.-C.; Chiou, S.-Y. J. Org.
Chem. 2004, 69, 719–726; (b) Yang, J.-S.; Lin, Y.-D.;
Chang, Y.-H.; Wang, S.-S. J. Org. Chem. 2005, 70, 6066–
6073; (c) Caballero, A.; Marti’nez, R.; Lloveras, V.;
´
Ratera, I.; Vidal-Gancedo, J.; Wurst, K.; Tarraga, A.;
Molina, P.; Veciana, J. J. Am. Chem. Soc. 2005, 127,
15666–15667; (d) Cheung, S.-M.; Chan, W.-H. Tetra-
hedron 2006, 62, 8379–8383; (e) Liu, L.-H.; Zhang, H.; Li,
A.-F.; Xie, J.-W.; Jiang, Y.-B. Tetrahedron 2006, 62,
10441–10449.
In conclusion, we have demonstrated that trans-4-(N-
arylamino)stilbenes are potential fluorophores for sen-
sor design. In particular, as shown by the benzocrown
derivatives 1 and 2, the resulting fluorescent probes
can function under acidic as well as neutral conditions
through different signaling mechanisms. The summa-
rized fluoroionophoric behavior for 1 and 2 is depicted
in Figure 6. Whereas the fluorescence response of 1
and 2 is a combination of an off–on intensity changes
and wavelength shifts under neutral conditions, fluores-
cence signaling under acidic conditions results in metal
ion-induced deprotonation of the ammonium ion and
LE-PICT dual fluorescence. Since the selectivity of a
probe and the complex stability mainly depend on the
receptor, the use of more selective and powerful recep-
tors should improve the performance (e.g., function in
aqueous solutions) of N-(arylamino)stilbene-derived
fluorescent probes. Further investigation toward this
issue is in progress.
´
4. (a) Letard, J.-F.; Delmond, S.; Lapouyade, R.; Braun, D.;
Rettig, W.; Kreissler, M. Recl. Trav. Chim. Pays-Bas 1995,
114, 517–527; (b) Collins, G. E.; Choi, L.-S.; Callahan, J.
H. J. Am. Chem. Soc. 1998, 120, 1474–1478; (c) Choi,
L.-S.; Collins, G. E. Chem. Commun. 1998, 893–894; (d)
Morozumi, T.; Anada, T.; Nakamura, H. J. Phys. Chem.
B 2001, 105, 2923–2931; (e) Sibert, J. W.; Forshee, P. B.
Inorg. Chem. 2002, 41, 5928–5930; (f) Aoki, S.; Kagata,
D.; Shiro, M.; Takeda, K.; kimura, E. J. Am. Chem. Soc.
2004, 126, 13377–13390; (g) Liu, B.; Chen, J.; Yang, G.;
Li, Y. Res. Chem. Intermed. 2004, 30, 345–353; (h) Li, Y.
Q.; Bricks, J. L.; Resch-Genger, U.; Spieles, M.; Rettig,
W. J. Fluoresc. 2006, 16, 337–348.
5. The PICT state often refers to the locally excited (LE)
state.
6. (a) Crochet, P.; Malval, J.-P.; Lapouyade, R. Chem.
Commun. 2000, 289–290; (b) Rurack, K.; Rettig, W.;
Resch-Genger, U. Chem. Commun. 2000, 407–408; (c)
Malval, J.-P.; Gosse, I.; Morand, J.-P.; Lapouyade, R. J.
Am. Chem. Soc. 2002, 124, 904–905.
Acknowledgment
7. (a) Yang, J.-S.; Liau, K.-L.; Wang, C.-M.; Hwang, C.-Y.
J. Am. Chem. Soc. 2004, 126, 12325–12335; (b) Yang,
J.-S.; Liau, K.-L.; Hwang, C.-Y.; Wang, C.-M. J. Phys.
Chem. A 2006, 110, 8003–8010.
Financial support for this research was provided by the
National Science Council of Taiwan, ROC.
8. (a) Bance, S.; Barber, H. J.; Woolman, A. M. J. Chem.
Soc. 1943, 1–4; (b) Kumari, N.; Kendurkar, P. S.; Tewari,
R. S. J. Organomet. Chem. 1975, 96, 237–241; (c) Berezina,
R. N.; Kobrin, V. S.; Kusov, S. Z.; Lubenets, E. G. Russ.
J. Org. Chem. 1998, 34, 1517–1518; (d) Bogaschenko, T.;
Basok, S.; Kulygina, C.; Lyapunov, A.; Lukyanenko, N.
Synthesis 2002, 2266–2270.
9. (a) Wolfe, J. P.; Wagaw, S.; Marcoux, J.-F.; Buchwald, S.
L. Acc. Chem. Res. 1998, 31, 805–818; (b) Hartwig, J. F.
Angew. Chem., Int. Ed. 1998, 37, 2046–2067.
10. Typical synthetic procedures for aminostilbenes have been
reported.11 Characterization data for compounds 1 and 2
are shown in the following: Compound 1: yield 27%; mp
68–69 ꢁC; 1H NMR (400 MHz CDCl3) 3.80–3.82 (m, 8H),
3.93–3.95 (m, 4H), 4.11–4.17 (m, 4H), 6.69 (dd, J = 2.2 and
8.4 Hz, 1H), 6.74 (d, J = 2.2 Hz, 1H), 6.86 (d, J = 8.4 Hz,
1H), 6.95 (d, J = 8.4 Hz, 2H), 6.98 (d, J = 16.3 Hz, 1H),
7.08 (d, J = 16.3 Hz, 1H), 7.25 (t, J = 7.2 Hz, 1H), 7.37 (t,
J = 7.5 Hz, 2H), 7.42 (d, J = 8.4 Hz, 2H), 7.51 (d,
References and notes
1. (a) de Silva, A. P.; Gunaratne, H. Q. N.; Gunnlaugsson,
T.; Huxley, A. J. M.; McCoy, C. P.; Rademacher, J. T.;
Rice, T. E. Chem. Rev. 1997, 97, 1515–1566; (b) Valeur, B.;
Leray, I. Coord. Chem. Rev. 2000, 205, 3–40; (c) de Silva,
A. P.; Fox, D. B.; Huxley, A. J. M.; Moody, T. S. Coord.
Chem. Rev. 2000, 205, 41–57; (d) GoKel, G. W.; Leevy, W.
M.; Weber, M. E. Chem. Rev. 2004, 104, 2723–2750.
2. For recent examples of PET-based probes, see: (a) Yang,
J.-S.; Lin, Y.-D.; Lin, Y.-H.; Liao, F.-L. J. Org. Chem.
2004, 69, 3517–3525; (b) Chen, G.; Yee, D. J.; Guberna-
tor, N. G.; Sames, D. J. Am. Chem. Soc. 2005, 127, 4544–
4545; (c) Ghosh, K.; Masanta, G. Tetrahedron Lett. 2006,
47, 2365–2369; (d) Kim, H. J.; Kim, J. S. Tetrahedron Lett.
2006, 47, 7051–7055; (e) Ueno, T.; Urano, Y.; Kojima, H.;
Nagano, T. J. Am. Chem. Soc. 2006, 128, 10640–10641.

3102
J.-S. Yang et al. / Tetrahedron Letters 48 (2007) 3097–3102
J = 7.8 Hz, 2H) ppm; 13C NMR (100 MHz CDCl3): 68.85,
70.63, 71.02, 71.06, 108.01, 109.97, 113.55, 115.63, 115.70,
119.43, 123.45, 126.65, 127.97, 128.54, 132.42, 132.72,
136.04, 142.82, 145.41, 145.89, 150.32 ppm; IR (KBr) 3230,
69.55, 69.80, 69.97, 70.49, 70.67, 70.99, 71.03, 107.32,
112.92, 115.83, 116.07, 125.66, 126.17, 127.03, 127.70,
128.45, 128.64, 129.14, 136.52, 137.86, 144.32, 144.72,
150.07 ppm; IR (KBr) 3244, 1599, 1513, 939 cmꢁ1; FAB-
HRMS calcd for C28H31NO5: 461.2202. Found: 461.2198.
Compound 2: yield 40%; mp 74–75 ꢁC; 1H NMR
(400 MHz, CD2Cl2) 3.66–3.70 (m, 8H), 3.83–3.84 (m,
4H), 4.04–4.07 (m, 4H), 5.83 (bs, 1H), 6.69 (d, J = 8.5 Hz,
1H), 6.72 (d, J = 2.3 Hz, 1H), 6.84 (d, J = 8.5 Hz, 1H),
6.93 (d, J = 8.6 Hz, 2H), 6.94 (d, J = 16.3 Hz, 1H), 7.16 (d,
J = 16.3 Hz, 1H), 7.40 (d, J = 8.6 Hz, 2H), 7.55 (d,
J = 8.4 Hz, 2H), 7.60 (d, J = 8.4 Hz, 2H) ppm; 13C NMR
(100 MHz, CD2Cl2): 69.02, 69.60, 69.84, 69.93, 70.48,
2217, 1590, 1511, 938 cmꢁ1
; FAB-HRMS calcd for
C29H30N2O5: 486.2155. Found: 486.2156.
11. Yang, J.-S.; Chiou, S.-Y.; Liau, K.-L. J. Am. Chem. Soc.
2002, 124, 2518–2527.
12. Watanabe, S.; Ikishima, S.; Matsuo, T.; Yoshida, K. J.
Am. Chem. Soc. 2001, 123, 8402–8403.
13. Connors, K. A. Binding Constants—The measurement of
Molecular Complex Stability; John Wiley & Sons: New
York, 1987; Chapter 4.
14. Bu, J.-H.; Zheng, Q. U.; Chen, C.-F.; Huang, Z. T. Org.
Lett. 2004, 6, 3301–3303.