Translocation of copper within the cavity of cryptands: Reversible fluorescence signaling

Article abstract of DOI:10.1039/b806426a

The movement of a copper ion inside the cavity of cryptands having two distinct metal binding sites at each end is linked to a fluorescence ON-OFF process in a reversible manner, providing a new way of fluorescence switching. The Royal Society of Chemistry.

Full text of DOI:10.1039/b806426a

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Translocation of copper within the cavity of cryptands: reversible
fluorescence signalingw
Kalyan K. Sadhu and Parimal K. Bharadwaj*
Received (in Cambridge, UK) 15th April 2008, Accepted 10th June 2008
First published as an Advance Article on the web 23rd July 2008
DOI: 10.1039/b806426a
The movement of a copper ion inside the cavity of cryptands
having two distinct metal binding sites at each end is linked to a
fluorescence ON–OFF process in a reversible manner, providing
a new way of fluorescence switching.
Transition metal ions used for signal transduction in fluorescence
signaling systems are attractive due to the availability of various
redox states for the metal ion. We have been using aza cryptands1
as receptors in the configuration ‘‘fluorophore–spacer–receptor’’
for emission enhancement by transition/heavy metal ions. Herein,
we describe two new cryptand receptors with NS3 and N4 donor
sets at each end of the cavity, segregated by aromatic spacers. Our
design is based on the fact that Cu(I) has a preference to bind soft
S donors, whereas Cu(II) prefers N donors. Thus, a copper ion,
depending upon its oxidation state, can translocate between the
ends of the cryptand receptor, leading to fluorescence switch-
ability (Scheme 1). This appears to be the first example of the
Scheme 2 The synthetic route to investigated compounds L1 and L2.
translocation of a metal ion inside a cryptand cavity. Transloca-
tion and fluorescence modulation2,3 by metal ions in cyclic and
Freshly purified THF was used as the solvent in all UV-vis
acyclic systems has been achieved through a change of pH2d and
spectral measurements. The unsubstituted cryptands L1e and L2e
redox state of the metal ion. Besides this, the use of a triple-
exhibited a strong absorption band at around 290 nm, attributable
stranded helical complex as molecular redox switch,3a a calixarene
as a molecular syringe3b for metal ions and metal tunneling3c are
some of the other important results in this field.
to a combination of transitions involving amines4 and aromatic
thioether5 moieties. Upon addition of a metal ion, such as Cu(II),
Co(II) or Zn(II), no noticeable change in the UV region was
observed. This indicates that these metal ions occupy the N4 end
of the cavity, because thioether S - M(II) ligand to metal charge
transfer (LMCT) transitions usually appear6 around 360 nm. The
N - M(II) LMCT band for secondary amines usually appears7
around 280 nm, merging with the band at 290 nm in the present
cases. The ligand field transitions for Cu(II) in our two cases
appear as broad low-intensity bands centered around B790 nm.
For the anthryl derivatives L1 and L2, three strong bands
characteristic8 of 9-alkyl-substituted anthracene appear at 389,
369 and 349 nm, in addition to the 290 nm band. Both L1 and L2
show a small (B2 nm) bathochromic shift of the anthryl bands
upon addition of a metal ion, such as a first row transition metal,
Cd(II) or Ag(I), with no significant change in molar absorptivity.
Upon addition of Hg(II) or Pb(II), or upon protonation, no change
in band positions could be observed.
Syntheses of the cryptands were achieved in several steps, as
illustrated in Scheme 2. The key intermediate in each case is a
tripodal trialdehyde, incorporating thioether groups, which
undergoes Schiff base condensation with tris-(2-aminoethyl)
amine to afford the desired cryptand.
Scheme 1 A schematic view of the translocation of a redox-active
metal ion within a cryptand cavity.
All emission measurements were undertaken using 10À5
M
solutions of freshly purified THF. The parent cryptands L1e
and L2e were non-emissive in nature. However, cation-free L1
and L2 in THF exhibited an emission that consisted of a well-
resolved anthracene monomer emission, corresponding1a to a
locally excited (LE) (p–p*) state, along with a red-shifted,
broad, structureless emission centered at 525 nm.w This struc-
tureless emission, visible at high concentration, is due to
Department of Chemistry, Indian Institute of Technology Kanpur,
Kanpur 208016, India. E-mail: pkb@iitk.ac.in
w Electronic supplementary information (ESI) available: Character-
ization and synthesis of the ligands L1 and L2: FAB-MS, 13C-NMR,
1H-NMR, UV-vis and fluorescence spectral data. See DOI: 10.1039/
b806426a
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Fig. 2 Emission spectra of L1CCu(II) under different conditions.
Fig. 1 (a) Emission spectra of L1, L1 + Cu(II), L1 + Ag(I), L1+
Zn(II), L1 + H+ (concentration of L1 = 1.1 Â 10À6 M) in THF.
(b) Emission spectra of L2, L2 + Cu(II), L2 + Ag(I), L2 + H+
(concentration of L2 = 1.0 Â 10À6 M) in THF.
Hydrated metal perchlorate and nitrate salts can generate
protons in organic solvents. To verify that the fluorescence
enhancement was due to the metal ion and not to protonation,
several controlled experiments were carried out.1a,e We also
recorded emission with Cu(II) as an input in the presence of
2,6-di-tertbutylpyridine (10À2 M), which is a poorly coordinat-
ing base. No change in emission enhancement was observed.
Fluorescence titrations of L1 and L2 were carried out using
metal ions and protons as titrants. The plotw of fluorescence
quantum yield as a function of cation concentration indicates
1 : 1 complex formation with metals and 1 : 2 complex
formation with protons. The complex stability constants (Ks)
were determined from linear regression plots obtained from
the changes in fluorescence quantum yield by following a
reported procedure.10 The values obtained were found to be
consistent with those available in the literature,11 with good
correlation coefficients ( Z 0.98). Ks was found to be in the
order of B106 MÀ1 for Cu(II) and Zn(II) with either L1 or L2.
Since Cu(I) prefers soft thioether S rather than hard amino
N centres, it was decided to utilize site preference for the two
oxidation states of the metal ion, with the ultimate goal of
having reversible fluorescence signaling. The green Cu(II)
inclusion complexes of both L1 and L2 dissolved in freshly
purified THF, giving emission spectra very similar to those
obtained by adding Cu(BF4)2ÁxH2O to either L1 or L2, with
comparable quantum yields. Upon addition of one equivalent
of NaBH(OAc)3 to the THF solution of the L1CCu(II)
complex, the metal ion was reduced to Cu(I) instantaneously,
accompanied by a quenching of the fluorescence (Fig. 2) as the
Cu(I) ion moved away from the ‘N4’ end, making PET
operational once again. Upon exposure to air, Cu(I) was
oxidized to Cu(II) and moved to the ‘N4’ end. This blocked
PET, and fluorescence was recovered to the original fFM value
in 12 h. Cu(II) could then be reduced again to Cu(I), and so the
exciplex formation, as observed earlier both by us1a and by
others,9 and is formed between the N lone pair and the anthryl
moiety. In case of metal-free L1 and L2, observed fFT (fFT
=
fFM + fFE, where fFT is the total quantum yield, fFM is the
quantum yield of the monomer emission and fFE is the
quantum yield of the exciplex emission) values are very low
due to efficient photoinduced electron transfer (PET) from the
HOMO of the donor N-atom to the excited anthracene
fluorophore.
Upon addition of a metal ion as input to either L1 or L2,
fFE vanished as expected, while fFM increased greatly (Fig. 1)
due to the N lone pairs at the N4 end being coordinated to the
metal ion, blocking PET. The extent of enhancement de-
pended on the nature of the metal ion, as well as on the
receptor (Table 1). Noticeable enhancements could be ob-
served in presence of protons in both systems.
Table 1 Fluorescence quantum yields of L1 in THF with different
ionic inputsa
Quantum yield f (enhancement factor) (lex= 370 nm)
L1
L2
fFM fFE
fFT
fFM
0.001
fFE
fFT
Input
None
Mn(II) 0.109
0.001 0.001 0.002
0.001 0.002
—
—
—
n.d.
—
—
—
—
—
—
—
—
0.109 (55) 0.194
0.063 (34) 0.038
0.112 (56) 0.015
—
—
—
n.d.
—
—
—
—
—
—
—
—
0.194 (97)
Co(II)
Ni(II)
0.063
0.112
0.038 (19)
0.015 (7)
n.d.
0.218 (109)
0.240 (120)
0.243 (122)
0.226 (113)
0.220 (110)
0.008 (4)
Cu(I)b n.d.
n.d.
n.d.
Cu(II)
Zn(II)
Cd(II)
Ag(I)
Tl(I)
0.222
0.222 (111) 0.218
0.113 (57) 0.240
0.134 (67) 0.243
0.121 (62) 0.226
0.082 (41) 0.220
0.009 (5)
0.012 (6)
0.155 (73) 0.204
0.113
0.134
0.121
0.082
0.009
Pb(II)
0.008
0.012
Hg(II) 0.012
H+
0.012 (6)
0.204 (102)
0.155
a
Experimental conditions: medium = THF, concentration of both L1
and L2 = 1.1 Â 10À6 M in THF and concentration of ionic input =
10À5 M. Excitation at 350 nm with an excitation band-pass = 5 nm,
emission band-pass = 5 nm and temperature = 298 K. f calculated
by comparison of the corrected spectrum with that of anthracene
(f = 0.297), taking the area under the total emission. The error in f is
within 10% in each case, except for the free ligands, where the error in
Fig. 3 Quantum yield plot of the L1CCu(II) complex after reduction
by NaBH(OAc)3 and subsequent aerial oxidation.
b
f is within 15%. n.d. = not determined.
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spectra gave a 100% peak for the metal-free L3 and L4; no
peak due to L3CCu(I) or L4CCu(I) was observed. This shows
that upon reduction, the metal ion came out of the cavity in L3
and L4 but did not in L1 and L2.
In conclusion, we have shown for the first time that a copper
ion can translocate inside the cavity of thio-aza cryptands,
depending upon its oxidation state. This property of the metal
ion is utilized here to have a reversible ON–OFF fluorescence,
which represents the first of a new generation of fluorescence
signaling systems. We are currently working on enhancing the
rate of translocation with different cryptand receptors.
P. K. B. acknowledges the financial support of the Depart-
ment of Science and Technology, New Delhi, India (grant no.
SR/S5/NM-38/2003). K. K. S. thanks the CSIR, India for a
Senior Research Fellowship.
Scheme 3 Aza-oxa cryptand-based fluorescence signaling systems.
cycle continued. The receptor in L2 is more rigid,12 and this
helped in the movement of the metal ion from one end to the
other (Fig. 3). Accordingly, in L2, the complete recovery of
fluorescence upon exposure of the Cu(I) complex to air took
5 h. There is a possibility that the metal ion actually went
outside of the cavity when it was reduced to Cu(I) and then
came back in again upon re-oxidation. To investigate this, we
probed two aza-oxa cryptands1a (L3 and L4, Scheme 3) as
receptors. Both L3CCu(II) and L4CCu(II) complexes showed
high fluorescence enhancement in THF. Upon addition of one
equivalent of NaBH(OAc)3 to THF solutions of the L3CCu(II)
or L4CCu(II) complexes, the fluorescence was quenched.
However, upon exposure to air, no fluorescence recovery
could be observed. It is believed that in these two cases, the
metal ion came out of the cavity because it did not prefer the
NO3 binding site. When the metal ion came out of the cavity
and went into the bulk medium, it did not return to cavity
upon oxidation.
Notes and references
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Upon undertaking a mass spectral investigation, it was
found that both L1CCu(II) and L2CCu(II) showed B40%
peak intensity.w Upon reduction of L1 and L2 with one
equivalent of NaBH(OAc)3, the corresponding species showed
a 100% peak intensity due to L1CCu(I) and L2CCu(I),
respectively, showing that the metal was still inside the cavity.
The possibility of the binding of Cu(I) in the N4 region was
overruled from the fluorescence spectra, as no enhancement
was observed when Cu(I) was generated (Fig. 2). Therefore,
the only possible binding site of Cu(I) was the NS3 region,
whereas Cu(II) was in the N4 region, blocking PET. The
different binding zones for Cu(I) and Cu(II) supports the
movement of the ion within the cavity. Cyclic voltamograms
for the L1CCu(II) and L2CCu(II) complexesw show well-
defined cyclic responses for the Cu(II)/Cu(I) couple, although
DEp (the difference of the oxidation and reduction peak
potentials) is more than 60 mV. Thus, it can be concluded
that this couple is chemically reversible but electrochemically
quasi-reversible. In comparison, with receptors L3 and L4, the
ESI mass spectra of the Cu(II) inclusion complexes gave a 50%
peak, both for L3CCu(II) and L4CCu(II). However, on reduc-
tion with one equivalent of NaBH(OAc)3, the ESI mass
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