FRET-derived ratiometric fluorescence sensor for Cu2+

Article abstract of DOI:10.1016/j.tet.2007.11.063

New N-(pyrenylmethyl)naphtho-azacrown-5 (1) was synthesized as an 'On-Off' fluorescent chemosensor for Cu²⁺. Excited at 240 nm corresponding to the absorption of naphthalene unit (energy donor) of 1, emission at 380 nm from pyrene unit (energy

Full text of DOI:10.1016/j.tet.2007.11.063

Available online at www.sciencedirect.com
Tetrahedron 64 (2008) 1294e1300
www.elsevier.com/locate/tet
FRET-derived ratiometric fluorescence sensor for Cu2þ
a,
Hyun Jung Kim a, Sun Young Park a, Sangwoon Yoon b, , Jong Seung Kim
*
*
a Department of Chemistry, Korea University, Seoul 136-701, Republic of Korea
b Department of Chemistry, Dankook University, Yongin 448-701, Republic of Korea
Received 29 September 2007; received in revised form 17 November 2007; accepted 20 November 2007
Available online 22 November 2007
Abstract
New N-(pyrenylmethyl)naphtho-azacrown-5 (1) was synthesized as an ‘OneOff’ fluorescent chemosensor for Cu2þ. Excited at 240 nm
corresponding to the absorption of naphthalene unit (energy donor) of 1, emission at 380 nm from pyrene unit (energy acceptor) is observed,
indicating that intramolecular fluorescence resonance energy transfer (FRET-On) occurs in 1. When Cu2þ is added to a solution of 1, however,
the fluorescence of pyrene is strongly quenched (FRET-Off) whereas that of naphthalene group is revived. Such FRET ‘OneOff’ behavior of 1 is
observed only in the case of Cu2þ binding, but not for other metal cations. The high selectivity of 1 toward Cu2þ can be potentially applied to
a new kind of FRET-based chemosensor. The FRET OneOff behavior is supported by computational studies. The calculated molecular orbitals
of HOMO and LUMOs suggest the excited-state interactions leading to FRET from naphthalene to pyrene in 1, but no electron density changes
in 1$Cu2þ complex.
Ó 2007 Elsevier Ltd. All rights reserved.
1. Introduction
Fluorescent sensors offer several distinct advantages such
as sensitivity, selectivity, time response and spatial resolution.
Photo-physical sensing processes are diverse: photo-induced
During recent two decades, there has been a great emer-
gence of interests in the development of fluorescent probes
for various metal cations.1e3 On account of their importance
in many biological and environmental processes, especially
transition metal ions have received an increasing concern.4,5
Heavy metal cations such as Cu2þ, Pb2þ and Zn2þ ions are
widely considered as pollutants as well as the essential trace
elements in biological systems.6e10 In particular, the recogni-
tion and detection of Cu2þ is of growing interest because they
are considered as highly noxious elements.6e8 Accordingly,
the design and synthesis of fluorescent chemosensors for
the Cu2þ ion have become very active research areas.6e8
However, only a few reports are available for fluorescence
electron transfer (PET),11 photo-induced charge transfer
12
(PCT),
fluorescence resonance energy transfer (FRET),13
perturbation of optical transitions, and polarizabilities, exci-
mer/exciplex formation,14a,b modification of redox potentials
in ground or excited states, and photo-regulation of binding
properties.
The FRET is defined as an excited-state energy interaction
between two fluorophores in which excited donor (D) energy
is transferred to an acceptor (A) part without any photon-emis-
sion.13 So, the FRETrequiresa certain degree of spectral overlap
between the emission spectrum of the donor and the absorption
spectrum of the acceptor. In pursuit of the FRET-based metal
cation sensors, here we report a new receptor 1 in which naph-
thalene unit as an energy donor and pyrene group as an energy
acceptor are anchored with 15-monoazacrown-5, displaying
changes in FRET behavior upon the Cu2þ cation encapsulation.
Furthermore, density functional theory (DFT) calculations
provide the complexation mechanism of the ligandeCu2þ ion
and qualitative explanation for the observed FRET OneOff
phenomena.
enhancing chemosensors upon Cu2þ ion encapsulation be-
cause the Cu2þ ion is known as inherent quenching metal
ion.16,17c,d
6
* Corresponding authors.
E-mail addresses: sangwoon@dankook.ac.kr (S. Yoon), jongskim@korea.
ac.kr (J.S. Kim).
0040-4020/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2007.11.063

H.J. Kim et al. / Tetrahedron 64 (2008) 1294e1300
1295
2. Results and discussion
Our synthesis began with a preparation of naphthalene
derivative 514c,d as shown in Scheme 1. Under N2 atmosphere,
treatment of 5 with p-toluenesulfonamide and K2CO3 in DMF
led to 4 in 78% yield. Subsequently, 214e was obtained in 85%
yield after tosyl group was removed by the use of 6% Na(Hg)
amalgam. Finally, 1 was synthesized in 73% yield from the re-
action of 2 with 3 in the presence of the K2CO3 in CH3CN.
Their molecular structures were fully characterized by the
1H NMR, 13C NMR, MS, and elemental analyses (see
Section 4 and Supplementary data).
O
Cl
Cl
OH
OH
O
O
Cl
O
OTs
K2CO3, CH3CN
O
5
O
O
O
O
O
O
Na2HPO4
Na(Hg)
H2NTs
HN
TsN
K2CO3, DMF
O
O
2
4
O
N
O
H
N
O
O
Cl
O
O
N
H
1
3
Scheme 1. Synthetic pathway to 1.
Figure 1 displays a collection of absorption and emission
spectra of 1e3 and 1$Cu2þ complex along with excitation
wavelengths for the fluorescence measurements denoted by ar-
rows. To investigate the role of the individual constituent chro-
mophores in the photodynamics of 1, we examined absorption
and emission spectra of 2 (naphthalene) and 3 (pyrene), shown
in Figure 1a and b. Most notably, the emission spectrum of
naphthalene overlaps with the absorption spectrum of pyrene,
achieving a favorable condition for the FRET-On. Figure 1b
also shows that pyrene alone exhibits very weak fluorescence
at w380 nm upon excitation at 240 nm. When both naphtha-
lene and pyrene are incorporated in 1, the emission band of
pyrene appears upon excitation at 240 nm (naphthalene ab-
sorption band) as shown in Figure 1c, indicative of an efficient
intramolecular FRET from naphthalene to pyrene in 1. When
Cu2þ is added to 1, however, the pyrene emission is
completely quenched, then the naphthalene emission is solely
observed at w330 nm [Fig. 1d].
Figure 1. Absorption (dashed line) and emission (solid line) spectra of (a) 2
and (b) 3. Emission spectra of (c) 1 and (d) 1 (6.0 mM) with 0.72 mM of Cu-
(ClO4)2 in CH3CN. The emission spectra were acquired with an excitation at
240 nm marked by downward arrows.
fluorescence from the donor is completely extinguished in
the presence of the energy acceptor (FDA¼0) whereas f be-
comes 0 (the minimum FRET efficiency) when the energy
transfer does not occur at all (FDA¼FD). We measured
f¼0.85 for free 1, but noticed that upon addition of Cu2þ
ion it decreased to f¼0.08.
The FRET efficiency (f) of naphthalene/pyrene system is
determined by Eq. 1, where FDA and FD denote the fluores-
cence intensities of naphthalene (energy donor) in the pres-
ence and absence of pyrene (energy acceptor), respectively:15a
In addition, we also measured quantum yields (Ff) of both
1, 2 and their Cu2þ ion complexes (Table 1). For 2, the fluo-
rescence quantum yield was observed to decrease 2.3-fold in
the presence of Cu2þ ions, probably due to the heavy metal
f ¼ 1 ꢀ FDA=FD
The FRET efficiency f becomes 1 (the maximum FRET
ð1Þ
efficiency) when the energy transfer works 100% and thus

1296
H.J. Kim et al. / Tetrahedron 64 (2008) 1294e1300
Table 1
Quantum yields (Ff) of both 1e3 and their Cu2þ ion complexes15b
Compound Ligand
LigandþCu2þ
365e480 nm 310e365 nm
310e365 nm
(naphthalene) (pyrene)
365e480 nm
(naphthalene) (pyrene)
1
2
3
0.000704
0.528
d
0.380
d
0.473
0.0811
0.229
d
0.0433
d
0.0995
ion effect.16,17c,d We, however, found that in 1 the quantum
yield of the naphthalene unit increases 115.0-fold with the
Cu2þ ions, indicating the FRET-Off. Judging from the quan-
tum yield data in Table 1, we inferred that for the fluorescence
changes of the naphthalene unit, the metal ion-induced FRET
OneOff effect is predominent over the heavy metal ion factor.
These FRET OneOff phenomena can be also explained by
comparison to the quantum yield of free 3 and its complex.
The fluorescence changes of 1 also show a high selectivity
toward Cu2þ over other metal cations as seen in Figure 2.
While the FRET is turned off upon addition of Cu2þ, resulting
in the fluorescence shift to w330 nm, other metal ions such as
Figure 3. Fluorescence spectra of 1 (6.0 mM) upon addition of various concen-
trations of Cu(ClO4)2 in CH3CN with excitation at 240 nm.
To quantify the compelxation ratio between 1 and Cu2þ
ion, the Job plot measurement was executed by varying the
concentration of both 1 and Cu2þ ion (Fig. S1). The maximum
point appears at the mole fraction of 0.54, close to the typical
ligand mole fraction (0.5) for 1:1 ligand-to-metal complex.
This suggests that the Cu2þ be entrapped by the monoaza-
crown-5 to form a 1$Cu2þ complex.17a,b Mass spectrum also
confirms the formation of the 1$Cu2þ complex (Fig. S2).
There are two major peaks at m/z 651 and 750, corresponding
to [1þCu]2þ and [1þCu(ClO4)]þ, respectively.
Liþ, Naþ, Kþ, Rbþ, Csþ, Agþ, Mg2þ, Ca2þ, Sr2þ, Ba2þ
,
Co2þ, and Zn2þ ions (720 mM, 120 equiv) do not induce any
changes in the fluorescence spectrum of 1.
We measured the fluorescence spectral changes as a func-
tion of Cu2þ concentration. Figure 3 shows that the fluores-
cence band switches from w380 nm (pyrene) to w330 nm
(naphthalene) by the addition of Cu2þ ion, implicating that in-
deed the complexation of Cu2þ suppresses the FRET behavior
in 1. The spectral band switch completes with the addition of
w120 equiv of Cu2þ and the excessive amount of Cu2þ
(>120 equiv) quenches both pyrene and naphthalene emission,
mainly due to a heavy metal ion effect.16,17c,17d According to
the extent of the fluorescence emission changes, we could ob-
tain the association constants17e of 1 (Ka¼1.04ꢁ104 Mꢀ1) for
Cu2þ ion.
To investigate the structure of 1$Cu2þ complex and the or-
igin of Cu2þ-induced FRET-Off, we have performed density
function theory (DFT) calculations using Gaussian 03 suite.18
The Becke-3eLeeeYangeParr (B3LYP) exchange func-
tionals19 and LANL2MB basis sets20 were employed to com-
promise the computation time and the accuracy. Calculations
either failed to converge or took too long when we used vari-
ous other basis sets including 3-21G, 6-31G(d) and
Figure 2. Fluorescence spectra of 1þCu2þ (dark line) and 1þX (gray lines) where X¼Liþ, Naþ, Kþ, Rbþ, Csþ, Agþ, Mg2þ, Ca2þ, Sr2þ, Ba2þ, Co2þ, and Zn2þ
.
The gray spectrum also includes the fluorescence of free 1. The concentrations of metal ions used were 720 mM in CH3CN, corresponding to 120 equiv of the free
1. The spectra were obtained by excitation at 240 nm.

H.J. Kim et al. / Tetrahedron 64 (2008) 1294e1300
1297
LANL2DZ. For LANL2MB, we tried other forms of initial
geometry to confirm that the optimized geometry is a global
minimum, given the calculation method and basis set.
Figure 4 presents the optimized geometry of free 1, and
1$Cu2þ complex. The free ligand has a structure where
naphthalene and pyrene are positioned perpendicular to each
other (T-shaped structure) with center-to-center distance
˚
w6.5 A. Van der Waals interactions between the pyrene-H
and the naphthalene p electrons make the major contribution
in stabilizing the overall geometry. Previous computation stud-
ies on a similar system (naphthaleneeanthracene)21 suggest
that the parallel-displaced structure be slightly more stable
than the T-shaped structure. For 1, however, a constrained am-
ide spacer keeps pyrene and naphthalene farther apart and the
resulting longer interplanar distance makes the Hep interac-
tion preferable to the pep interaction, yielding the T-shaped
structure, as shown in Figure 4a.
(a) Ligand
Upon binding Cu2þ ion, 1 undergoes dramatic structural
changes. Strong carbonyl oxygeneCu2þ complexation pulls
in the carbonyl group and subsequently rotates the pyrene moi-
ety to form the tilted parallel structure between pyrene and
naphthalene group. The strong interaction between the car-
bonyl oxygen and Cu2þ is further evidenced by the red shift
of the C]O stretching band from 1674 to 1646 cmꢀ1 in IR
spectra upon addition of Cu2þ (Fig. S3).
With the optimized geometries of 1 and 1$Cu2þ complex,
we explored the mechanism of Cu2þ-induced FRET-Off. We
first considered the change in the distance between the energy
donor and the acceptor. FRET depends on Rꢀ6 where R is the
inter-chromophore distance.15 Upon Cu2þ binding, the dis-
tance between naphthalene and pyrene increases from
(b) Cu2+...Ligand
˚
˚
w6.5 A to w11 A as shown in Figure 4. However, since the
Forster distance at which the energy transfer efficiency falls
¨
˚
˚
to 50% is typically w20 A (e.g., 16 A for naphthaleneean-
thracene),21 Cu2þ-induced distance change is not sufficient
enough to turn off the FRET.
Another possibility is the change of electronic structure
arising from the Cu2þ complexation. Figure 5 shows the
HOMO and LUMOs of the free 1 and the 1$Cu2þ complex.
Most interestingly, the LUMO for 1 is localized on the pyrene
group while the next LUMO (LUMO-1), 23 kcal/mol higher
than LUMO, is dominantly on naphthalene. Upon irradiation
at 240 nm as in the experiment, LUMO-1 is excited and the
excited-state interaction between LUMO and LUMO-1 causes
the electronic energy transfer from naphthalene to pyrene,
producing a fluorescence being emitted from the pyrene
group, consistent with our experimental observations. Then
we also found that the molecular orbital structure of 1$Cu2þ
is markedly different from that of 1. The localized electron
density of LUMO, LUMO-1, and LUMO-2 all remain on
the pyrene moiety, leaving no possibility for energy transfer
between the chromophores. Calculated geometry and the mo-
lecular orbital picture consistently explain the experimental
results.
FRET-On
O
NH
O
O
O
N
O
Cu2+
FRET-Off
3. Conclusions
HN
O
Cu2+
A fluorogenic compound 1 bearing both naphthalene and
pyrene units has been newly synthesized. In free 1, the intra-
molecular FRET is observed from the energy donor (naphtha-
lene) to the energy acceptor (pyrene) arising from a spectral
overlap between absorption band of pyrene and emission
O
O
O
N
O
Figure 4. Optimized geometry of (a) free 1 and (b) 1$Cu2þ complex. Hydro-
gen atoms are omitted for clarity.

1298
H.J. Kim et al. / Tetrahedron 64 (2008) 1294e1300
Figure 5. Molecular orbitals of HOMO, LUMO, and the second excited LUMO (LUMO-1) of (a) free ligand 1 and (b) 1$Cu2þ complex.
band of naphthalene. However, upon addition of Cu2þ, the
FRET is suppressed and the emission of naphthalene is only
observed. Density functional theory calculations show that 1
has a T-shaped structure with pyrene and naphthalene perpen-
dicular to each other by the Hep interaction. Cu2þ ion entrap-
ped by the azacrown ether strongly binds to the carbonyl
oxygen and induces the structural changes of 1 from the T-
shape to the tilted parallel structure. The molecular orbitals
of 1 and 1$Cu2þ complex suggest that the excited-state inter-
actions between pyrene and naphthalene in 1 cause the FRET
to occur while no excited energy exchange between the chro-
mophores inhibits the FRET for 1$Cu2þ complex. Such high
selectivity of compound 1 presents itself as a potential new
material for the selective Cu2þ ion sensor.
4. Experimental section
4.1. Synthesis
4.1.1. N-(Pyrenylmethyl)naphtho-azacrown-5 (1)
To a mixture of 0.50 g (1.57 mmol) of 2,3-naphtho-aza-
crown-5 (2)14e and 0.96 g (3.14 mmol) of N-(1-pyrenylme-
thyl)chloroacetamide (3)16 in 50 mL of dry CH3CN,
anhydrous K2CO3 (0.24 g, 1.57 mmol) and a catalytic amount
of NaI were added. The reaction mixture was refluxed for 2
days. After removal of the solvent in vacuo, the resulting solid
was dissolved in CH2Cl2 (100 mL) and the organic layer
was washed three times with water. The organic layer was
dried over MgSO4 and evaporated in vacuo. Column

H.J. Kim et al. / Tetrahedron 64 (2008) 1294e1300
1299
chromatography on silica gel with EtOAcehexane (1:1) as el-
uent gave 0.82 g (73%) of 1 as a white solid. 1H NMR
(200 MHz, CDCl3): d 8.55 (s, 1H, NH, pyrene), 7.99e7.54
(m, 9H, ArH, pyrene), 7.26e7.19 (m, 2H, ArnaphH ), 7.08e
7.03 (m, 2H, ArnaphH ), 6.51 (s, 2H, ArnaphH ), 4.86e4.83 (d,
2H, CONHCH2, J¼5.79 Hz) 3.53e3.51 (d, 4H, ArOCH2e,
J¼3.79 Hz), 3.41e3.34 (m, 8H, ArOCH2CH2OCH2CH2e),
3.28 (s, 2H, eNCH2COe), 2.64e2.59 (t, 4H, OCH2CH2NH,
J¼4.69 Hz); 13C NMR (100 MHz, CDCl3): 177.8, 148.8,
131.3, 130.9, 130.6, 129.3, 128.6, 127.5, 127.4, 127.0, 126.5,
126.3, 125.9, 125.2, 125.0, 124.7, 124.4, 124.2, 122.9, 108.2,
69.0, 68.4, 56.6, 41.3 ppm. FABMS m/z (Mþ): calcd, 588.6,
found, 589.0. Anal. Calcd for C37H36N2O5: C, 75.49; H, 6.16.
Found: C, 75.51; H, 6.15.
grant (KRF-2006-331-C00141) and Seoul Fellowship for
H.J.K.
Supplementary data
Supplementary data associated with this article can be
found in the online version, at doi:10.1016/j.tet.2007.11.063.
References and notes
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in 78% yield. H NMR (200 MHz, CDCl3): d 7.70e7.58 (m,
4H, ArnaphH, ArH-tosyl), 7.31e7.22 (m, 4H, ArnaphH, ArH-to-
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Fluorescence spectra were recorded with a RF-5301PC
spectrofluorophotometer. Stock solutions (1.00 mM) of metal
perchlorate salts were prepared in CH3CN. Stock solutions
of free 1e3 (0.060 mM) were prepared in CH3CN. Excitations
were carried out at 240 nm (for 1 and 3) with all excitation and
emission slit widths at 3 nm. Titration experiments were per-
formed with 6.0 mM solutions of 1 in CH3CN and various con-
centrations of metal perchlorates in CH3CN. For the Job plot
experiment, 1 (6.0 mM) in CH3CN and Cu(ClO4)2 (6.0 mM)
in CH3CN were prepared as stock solutions. The concentra-
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2 h, the fluorescence emission at 240 nm was recorded.
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