Alteration of selectivity in rhodamine based probes for Fe(iii) and Hg(ii) ion induced dual mode signalling responses

Article abstract of DOI:10.1039/c2ob07182g

The probes for metal ion induced chromo- and fluorogenic signalling responses alter their selectivity depending upon the nature of substituent as well as a function of solvent medium. 2 has shown selectivity towards Fe(iii) ion, 4 towards Hg(ii) ion while 3 is responsive towards both Fe(iii) and Hg(ii) ions.

Full text of DOI:10.1039/c2ob07182g

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COMMUNICATION
Alteration of selectivity in rhodamine based probes for Fe(III) and Hg(II) ion
induced dual mode signalling responses†
Bamaprasad Bag* and Biswonath Biswal
Received 27th December 2011, Accepted 10th February 2012
DOI: 10.1039/c2ob07182g
selectivity towards different metal ions. We report herein the
substituted aminoethyl-rhodamine based probe 2, where a p-
cyanobenzyl group is substituent at the donor amino end,
to exhibit simultaneous enhanced absorption and emission sig-
nalling responses selectively with Fe(III) ion in MeCN. 3, which
The probes for metal ion induced chromo- and fluorogenic
signalling responses alter their selectivity depending upon
the nature of substituent as well as a function of solvent
medium. 2 has shown selectivity towards Fe(III) ion, 4
towards Hg(II) ion while 3 is responsive towards both Fe(III)
and Hg(II) ions.
incorporates
a p-bromobenzyl substituent exhibits similar
responses with both Hg(II) and Fe(III) ions, while 4 with a p-
nitrophenyl substituent responds selectively in the presence of
Hg(II) ion in MeCN. The physiological importance2,3,6 and the
lethal toxic effect7,8 of Fe(III) and Hg(II) ions on human health
and environment beyond requisite concentration threshold is
the matter of concern, thus, promotes for development of new
rhodamine based signalling probes which are capable of detect-
ing these metal ions selectively under physiological conditions.
Although many Hg(II)9 and Fe(III)10-selective probes based on
complexation induced delactonization of rhodamine are known,
these new probes (2–4) not only exhibit dual mode signalling for
highly selective and sensitive detection of these metal ions, but
also provide a structural basis to alter selectivity as a function of
attached substituent to rhodamine’s ‘amino-ethyl-amido’ frame-
work. Further, 2–4 exhibits both chromogenic and fluorogenic
signalling responses selectively with Hg(II) ion in MeCN–H2O
medium. Hence, probe 2 which signals in both chromogenic and
fluorogenic pathway switches its Fe(III) selectivity to Hg(II) ion
in a dual order manner: (a) upon structural transformation to the
core and (b) in a mixed organic–aqueous medium. In rhodamine
based signalling systems, examples of switching in the metal
ion selectivity by the same probe under different structural or
functional manifestation are not much known, baring few, such
as an acetylacetone coupled rhodamine-hydrazide derivative11
switches selectivity between Fe(III) and Cu(II) ions as a function
of pH, a rhodamine-phenyl urea conjugate12 changes the Pb(II)
selectivity to Hg(II) ion depending upon the medium, and signal-
ling of rhodamine-hydrazide derivatives alter selectivity between
Cu(II)–VO(II)13 or Cu(II)–Hg(II)14 ion pairs as a function of the
mode of detection. To the best of our knowledge, no method-
ologies involving rhodamine based probes that impart dual
mode signalling responses have been reported so far to exhibit a
two-fold switching in selectivity between Fe(III) and Hg(II) ions
as in 2.
Metal ion responsive signalling probes1 find their wide-domain
utility and impact in on-site, real time, selective analysis of
various metal ions for biological, environmental, clinical moni-
toring and assessment.2,3 The synthetic and operational aspects
of such probes follow various methodological designs; most
involve either metal ion mediated perturbation of photo-physical
processes or exploitation of structure–function correlation. A
probe that facilitates complexation-induced chromogenic as well
as fluorogenic dual mode of signalling is advantageous in selec-
tive metal ion detection. In this regard, rhodamine based
probes1a,4 have attracted immense interest recently because of
rhodamine’s excellent spectroscopic properties, where the signal
transduction pathway mostly exploits its contrast structure–func-
tion correlation in lactonized–delactonized conformations. As
selectivity, sensitivity, reproducibility and reversibility are vital
parameters to be manifested for any metal ion selective probe,
the structural motif of its receptor which facilitates disposition of
donor atoms in space for effective metal ion coordination plays a
crucial role. An appropriate selection of donor atoms at strategic
positions of the receptor to accommodate a particular ion brings
about selectivity5 in a probe. The selectivity may also be tailored
by modulation of electron densities over those donor atoms, in
turn, perturbation of stereo-electronic situation to tune the
binding preferences. Thus, in rhodamine based signalling probes
for highly selective and sensitive detection of metal ions, re-
ceptor subunits with varied substituents at donor atoms in its
covalent framework may be envisaged to exhibit different
Colloids and Material Chemistry Department, Institute of Minerals and
Materials Technology (CSIR-IMMT), P.O. R.R.L., Bhubaneswar,
751013, India. E-mail: bpbag@immt.res.in; Fax: +91 674 258 1637;
Tel: +91 674 237 9254
†Electronic supplementary information (ESI) available: Detailed syn-
thetic procedure, characterization (1H-, 13C-NMR, ESI-MS, ESI:
Fig. S26–S37) absorption and emission spectroscopic and structural data
of 2–4. CCDC 855291 (for 4). For ESI and crystallographic data in CIF
or other electronic format see DOI: 10.1039/c2ob07182g.
The first step of the synthesis of these probes (Scheme 1) was
involved in condensation of ethylenediamine with rhodamine-B
hydrochloride in EtOH to obtain 1.15 Its subsequent reaction
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Scheme 1 Pictorial representation of the probes (1–5).
with 4-cyanobenzaldehyde and 4-bromobenzaldehyde in EtOH
followed by reduction with NaBH4 afforded the reduced Schiff
bases 2 and 3 respectively as desired probes. The aromatic
nucleophilic substitution reaction of 1 with 4-fluoro-1-nitroben-
zene resulted in 4, which crystallizes‡ from acetonitrile through
a slow evaporation technique. The diethylenetriamine appended
rhodamine16 (5) was synthesized and its metal ion induced sig-
nalling responses in organic–aqueous medium were compared
with those of 2–4. The characteristic peak near ∼66 ppm (Cq) in
13C-NMR spectrum of 1–5 in CDCl3 ascertains the predominant
existence of rhodamine’s spirolactam conformation.
The absorption and emission spectra of 2–5 in various sol-
vents (ESI: Fig. S1–S2†) revealed that these probes do not
absorb in 500–600 nm region and exhibit a weak fluorescence
(ϕFT < 0.001) in MeCN or MeCN–H2O (1 : 1 v/v), a character-
istic behaviour attributed to that of lactonized rhodamine. Apart
from the high energy ligand localized absorption transitions for
Fig. 1 Absorption spectral responses of 2 alone and in the presence of
various metal ions in (a) MeCN and (b) MeCN–H2O (1 : 1 v/v) medium.
Absorption spectra of (c) 3 and (d) 4 in presence of various metal ions
in MeCN. [probe] = 1 × 10−5 M, [M(I/II)] = 2 × 10−4 M in all the cases.
all probes, 4 also exhibits a solvatochromic absorption (λmax
=
360 nm in hexane, 40 nm red-shifted in DMSO) due to the
intramolecular charge transfer17 occurring from distal donorAmino
to acceptorNitro moiety through a π-spacer. The quenched
emission in each probe is attributed to an efficient intersystem
crossing occurring in the lactonized rhodamine, apart from the
operative photo-induced electron transfer processes from distal
Amino−donor groups present in their receptor moiety.
In order to investigate the metal ion induced signalling
responses in these probes, various metal ions such as Na(I), K(I),
Mn(II), Fe(II), Fe(III), Co(II), Ni(II), Cu(II), Zn(II), Pb(II), Ag(I), Cd
(II) and Hg(II) were added to the probe solution in MeCN or
MeCN–H2O (1 : 1 v/v) [Fig. 1, ESI: Fig. S3†]. The colourless
solution of 2 in MeCN leads to the appearance of an absorption
peak at 557 nm (Fig. 1a) with a subsequent change in colour to
pink selectively in presence of Fe(III) ion (ε2+Fe(III) = 11 448 dm3
mol−1 cm−1), while the presence of other metal ions imparts a
smaller or no change (ε2+M(I/II)/ε2 < 90 fold) in comparison to
that with Fe(III) ion (ε2+Fe(III)/ε2 = 1047 fold). Its metal ion
induced change in emission (I580) spectral pattern follows its
absorption spectral behaviour in MeCN, fluoresces (ϕFT = 0.632)
selectively in presence of Fe(III) ion (Fig. 2a). The absorption
and emission amplifications as well as the colourless → pink
colour transition of 2 in MeCN selectively in the presence of Fe
(III) ions is attributed to the complexation induced delactoniza-
tion of the rhodamine spiro-ring to its ring-opened amide
conformation.
Fig. 2 Fluorescence spectral pattern of (a) 2 and (b) 4 alone and in the
presence of various metal ions in MeCN. [2 and 4] = 0.5 × 10−6 M, λex
= 500 nm, RT, ex. and em. b.p. = 5 nm.
presence of Hg(II) ions (Fig. 1b). The Fe(III) ion, which resulted
in enhancement in absorption and emission spectral responses of
2 in MeCN (ESI: Fig. S4†), even fails to exhibit such A557 and
I580 amplification in MeCN–H2O medium, although it exhibits
an enhanced absorption at 356 nm (Fig. 1b, ε356 = 71 333 dm3
mol−1 cm−1). Thus, 2 exhibits altered signalling responses by
switching the selectivity from Fe(III) to Hg(II) ions as a function
of medium where the MeCN–H2O binary mixture promotes
selective Hg(II) coordination to 2 and restricts the coordination
of other metal ions. A controlled experiment (ESI: Fig. S5†) was
carried out to demonstrate the switching of metal ion selectivity
between Fe(III) and Hg(II) ions in 2 as a function of medium.
When water was added to the solution containing 2 and Fe(III)
When measured in MeCN–H2O (1 : 1 v/v) rather than
MeCN, 2 renders such signal amplification [A557(ε = 15 556
dm3 mol−1 cm−1) and I580 (ϕFT = 0.603)] selectively in the
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ions in MeCN (50% v/v, maintaining the concentration of 2
same), the Fe(III)-ion induced enhanced absorption and emission
decreases almost to the extent of the metal free probe with pink
colour of the solution disappearing. Further addition of Hg(II) to
the same solution results in enhancement in A557/I581 peaks and
the reappearance of the pink colour of the solution. This infers
that 2, which exhibits preferential Fe(III) ion induced chromo-
and fluorogenic signal amplification in MeCN through delactoni-
zation of the rhodamine unit, gets decomplexed with Fe(III) ions
in the presence of water due to a preferential hydration of Fe(III)
ions over probe-metal coordination and restores its spirolactam
configuration. Addition of Hg(II) to the same solution leads
to aqueous promoted selective Hg(II)-complexation to render
the colourless → pink transition with enhanced A557 and I581
peaks through delactonization of the rhodamine again. Addition
of other metal ions to the aqueous mediated Fe(III) decomplexed
colourless solution of 2 fails to induce delactonization of the
spiro ring to induce a similar effect. This establishes that 2
switches its selectivity from Fe(III) ions in MeCN to Hg(II) ions
in MeCN–H2O (1 : 1 v/v) medium.
Fig. 3 Extent of absorption enhancement of 2 in the presence of other
metal ions prior and after the addition of Fe(III) ions in MeCN. [2] = 1 ×
10−5 M, [M] = 2 × 10−4 M.
here similar responses selectively in the presence of Hg(II)
ions in organic–aqueous mixed medium. In comparison, the p-
nitrophenyl substitution as in 4 selectively promotes Hg(II)-
coordination to delactonize rhodamine’s spiro-ring only, to
exhibit such dual mode signalling process in both solvent
conditions.
The absorption and emission spectral pattern of 3 in MeCN in
the presence of all these metal ions revealed that it does not
exhibit any selectivity; it’s rhodamine spiro-ring delactonizes in
the presence of either Hg(II) or Fe(III) ions to exhibit enhance-
ment in absorption (Fig. 1c) with high molar extinction coeffi-
cients (ε3+Hg(II) = 9096 and ε3+Fe(III) = 12 292 dm3 mol−1 cm−1
)
As the cross-sensitivity of any probe towards different metal
ions limits its utility in selective detection of a metal ion, it has
also been investigated with these probes (ESI: Fig. S8–S11†).
Addition of other competitive metal ions to the solution contain-
ing 4 with Hg(II) ions in MeCN or MeCN–H2O (v/v, varied pro-
portions) does not perturb its absorption and emission spectral
pattern. On the contrary, the lowered absorption and quenched
emission of the individual solutions containing 4 and metal ions
other than Hg(II) immediately results in A557 and I580 spectral
enhancement along with the colourless solutions turning to pink
upon addition of Hg(II) ions to the same extent of that of 4 alone
in the presence of Hg(II) ions. This infers its high selectivity
towards Hg(II) ions and the non-interference of other competitive
metal ions. A similar experiment with 2 in MeCN ascertains its
Fe(III) ion selective dual mode signalling responses (Fig. 3).
Further, the time course studies as a function of spectral changes
in absorption [(A/A0)557] and emission [(IF/I0)580] of these
probes with corresponding metal ions which selectively delacto-
nize their rhodamine unit suggest that the probe–metal com-
plexation occurs in almost 1 min and remains constant for at
least 12 h in each case.
Further, the absorption/fluorescence spectral pattern of 2–4 in
the 400–650 nm window does not exhibit any appreciable
change with pH variation in 4.0–10.0 range. The spectral
response of these probes in the absence and presence of Hg(II) in
varying pH reveals that they yield stable complexation induced
amplified signals in the same region without any interference
from protons. However, the spectroscopic signal intensity of the
metal free probes increase with a decrease in pH values (ESI:
Fig. S12†) under more acidic conditions (pH < 4.0), yet it was
found to be responsive towards corresponding specific metal ion
coordination. Nevertheless, the signal stability of these probes
and their complexes over a wide pH range indicate their potential
utility in complicated/biological systems.
and emission (ϕFT(3+Hg(II)) = 0.598, ϕFT(3+Fe( = 0.613) while
III))
other metal ions fails to initiate the ring-opening process. Probe
4 absorbs (ε557 = 16 970 dm3 mol−1 cm−1, Fig. 1d) and fluor-
esces (ϕFT(581) = 0.689, Fig. 2b) in MeCN selectively in the pres-
ence of Hg(II) ions. Thus, the substituents attached to the distal
donor amino group of the ‘amino-ethyl-amido’ rhodamine core
alter the metal ion selective binding preferences as observed
through chromogenic as well as simultaneous fluorogenic ‘turn-
on’ signalling responses. It is worth mentioning that 3 and 4 also
exhibit dual channel signalling responses selectively in the pres-
ence of Hg(II) ions in MeCN–H2O (v/v, varied proportions)
medium (ESI: Fig. S6†).
Addition of various metal ions to the colourless solution of 5
in MeCN leads19 to a pink colour as well as amplification in its
absorption and emission spectral pattern to an extent varying
with the nature of the metal ions added. Although it attains
maximum absorption (A557, ε5+Hg(II)/ε5 = 1720 fold) and emis-
sion (ϕFT = 0.608) enhancement in the presence of Hg(II) ions,
other metal ions also induce an appreciable spectral change
(ε5+Pb(II)/ε5 = 1045 fold, ϕ5+Pb(II) = 0.329). However, 5 exhibits
spectral enhancements in A557 (ε = 20 261 dm3 mol−1 cm−1
)
and I580 (ϕFT = 0.643) selectively in the presence of Hg(II) when
measured in MeCN–H2O (1 : 1 v/v) rather than in MeCN (ESI:
Fig. S7†). The observed aqueous promoted Hg(II)-selectivity
of 5 over other metal ions is consistent with other amino-[(ethyl-
amino)1,3,4]-derivatized rhodamine probes18 under similar
conditions. Although, the signalling responses of a diethylene-
triamine attached rhodamine-G based probe20 has been reported
to exhibit Fe(III) selectivity, the signal perturbation in the pres-
ence of Hg(II) ions has not been evaluated in that case. There-
fore, another unit of aminoethyl-substitution to the aminoethyl-
rhodamine core as in 5 exhibits a preferential Hg(II) ion
induced dual mode ‘turn-on’ signalling in MeCN and exhibits
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The plot of absorption spectral responses of 2 as a function of
molar fraction of added Fe(III) ion in MeCN (continuous vari-
ation method) establishes its 1 : 1 (L : M) complexation stoichi-
ometry. Similar plots (ESI: Fig. S13†) for absorption spectral
responses of 3 with either Hg(II) or Fe(III), and that of 4 with Hg
(II) also ascertain their stoichiometry of complexation to be 1 : 1
(L : M). The fluorescence spectral pattern of the titration of 4
(0.6 μM) with Hg(II) in MeCN–H2O (1 : 1 v/v, pH = 7.0)
revealed that the emission I580 increases with increasing Hg(II)
concentration gradually up to 5 equiv. of Hg(II) and remains con-
stant even up to addition of 25 equiv. of Hg(II). A further
increase in Hg(II) concentration leads to an expected decrease in
emission (I580). The absorption and emission titration studies for
other probes also follow similar spectroscopic trends (ESI:
Fig. S14–S19†). From the fluorescence spectral changes (IF/I0)
of these probes as a function of concentrations of added metal
ions (Fig. 4), the association constants (ka) of complexation are
determined21 to be 5.81 × 105 M−1 for 2 ⊂ Fe(III), 3.829 × 105
M−1 for 3 ⊂ Fe(III), 1.226 × 105 M−1 for 3 ⊂ Hg(II), 8.746 × 104
M−1 for 4 ⊂ Hg(II) and 8.527 × 104 M−1 for 5 ⊂ Hg(II) com-
plexes respectively. The ka were also determined through absorp-
tion spectral changes of similar titrations of these complexes,
which are consistent (ESI: Table ST1†) with those obtained
through fluorescence spectral changes (the correlation factor log
[ka(fluorescence)/ka(absorption)] are found to be in 0.08–1.60
range).
of other counter anions exhibits a negligible decrease in spectral
responses and the colour of the solution remained pink for at
least 12 h, except for I− which also reduces the spectral pattern
substantially though not to the same extent of AcO− ions. The
optical spectral responses of Hg(II) ⊂ 3 and Hg(II) ⊂ 5 com-
plexes, however, get perturbed by many anions to an extent
depending upon the probe–metal and metal–anion interactions.
Apart from anions, 2–4 also regenerate their initial spirolactam
state upon subsequent addition of chelating agents such as
EDTA and ethylenediamine to these complexes in solution (ESI:
Fig. S23†). Further addition of Hg(II) ions to a colourless sol-
ution of anion (AcO−) mediated Hg(II)-decomplexed 4 (ϕFT
≤
0.002) results in the reappearance of the pink colour and exhibits
enhanced A557 and I580 peaks almost to the extent of those upon
initial Hg(II) addition to 4. This establishes its reusability as a
probe for selective Hg(II) ion detection.
The X-ray diffracted crystallographic structure of 4 (Fig. 5)
revealed that attachment of the electron-withdrawing 4-
nitrophenyl group has lowered the degree of pyramidalization of
the donor Namino-atom (N4) almost to a planar conformation.
Further, the N4–C31 bond (1.357(3) Å) distance is much shorter
than the normal C–N bond (average N–C bond length: 1.450 Å)
due to delocalization of electron density from N4 to the attached
aromatic π-system inducing a partial double bond character and
imposing a structural rigidity in the electron-withdrawing substi-
tuent, which is consistent with the ICT transition17 of the D-π-A
unit. The structural aspect of a signalling probe provides
vital information toward the thermodynamics of metal binding
and their coordination module. The variation in the size
and spatial orientation of the chelation cavity of ‘amino-ethyl-
amido’-rhodamine core upon attachment of different substituents
might have contributed significantly towards the stereo-electronic
situation of the probe–metal coordination and hence, altered its
metal ion selectivity preferences. Unfortunately, all attempts to
get the crystals of other probes remained unsuccessful. More-
over, the crystal structures are favoured by the packing forces
rather than by the intrinsic differences in free energies, thus,
the probes might have different geometries in solution and may
not be helpful in comparing their structures in solid states to
understand the spatial orientation of the molecules. Hence, the
geometrically optimized structures of 2–4 obtained by density
functional theory (DFT) calculations22 at B3LYP level were
compared in order to verify the effect of substituent induced
The reversibility in metal ion induced absorption and emission
signal responses of these probes were evaluated with subsequent
addition of ammonium salts of various counter anions in MeCN :
H2O (1 : 1 v/v) medium (ESI: Fig. S20–S22†). The Hg(II)-
induced enhancement in absorption (A557) and emission (I580) of
4 significantly decreases almost to that of the metal free probe 4
and the pink coloured solution turns colourless within 1 min in
the presence of AcO− anions. In the case of 4 ⊂ Hg(II), addition
Fig. 4 Fluorescence spectral pattern of 2 as a function of added Fe(III)
ions in MeCN. [2] = 1 × 10−6 M, λex = 500 nm, RT, ex. and em. b.p. =
5 nm. (Inset, top) Change in fluorescence intensity (IF/I0) of 2 as a
equivalents of Fe(III) ion added and (bottom) corresponding linear
Fig. 5 Perspective view of the X-ray crystallographic structure of 4
(the hetero-atoms numbering scheme, H-atoms are omitted for clarity).
regression plot of I0/(IF − I0) vs. 1/[Fe(III)] M−1
.
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structural modification to the chelation cavity comprising donor
atoms on altered complexation preferences of these substituted
‘amino-ethyl-amido’-rhodamine framework (ESI: Fig. S24–S25,
Table ST2–ST3†). The sum of angles around Ndonor–atom (N4)
to which the substituent is attached infers to its non planar geo-
metry in 2 (346.56°) and 3 (346.54°), while to a lower degree of
pyramidalization in 4 (359.99°) and is consistent with that of the
X-ray diffracted crystallographic structure of 4. The shorter N4–
C31 bond distance in 4 (1.378 Å), in comparison to that in 2
(1.464 Å) and 3 (1.462 Å), suggests that attachment of 4-nitro-
phenyl induces a partial double bond character and restricts the
rotation around it to impose a structural rigidity on 4. Apart from
spatial orientation, the non-bonded distances among the donor
atoms, which constitute the chelation cavity, provide vital infor-
mation towards effective and preferential metal ion coordination
through modulation of the extent of orbital overlap. The non-
bonded Oamido(O2)⋯Namino(N4) and Namido(N3)⋯Namino(N4)
distances, which vary depending upon the nature of the sub-
stituent, are found to be 4.462 Å and 3.822 Å respectively
in 2, 4.491 Å and 3.820 Å respectively in 3, while a larger dis-
tance of 4.557 Å and 4.533 Å respectively is found in 4.
Notes and references
‡Crystal data for 4: C36H39N5O4; Mw = 605.72; needle-shaped; pale-
yellow crystals, triclinic, space group P1, a = 9.225(2) Å, b = 12.144(3)
Å, c = 14.886(4) Å, α = 74.73(1), β = 78.29(2), γ = 83.99(4), U =
1572.9(6)Å3, T = 296(2) K, Z = 2, μ(Mo Kα) = 0.085 mm−1, F (000) =
644, ρcalc = 1.279 mg m−3, 7814 reflection data with 414 parameters,
5517 [I ≥ 2 σ(I)] unique reflections used in calculations. The final R1 =
0.0771, wR2 = 0.2389, S = 1.040.
ˉ
1 For selected reviews, see: (a) J. S. Kim and D. T. Quang, Chem. Rev.,
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Elements: The Inorganic Chemistry of Life. Oxford University Press,
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B. Sarkar, Marcel Dekker Inc., New York, 2002; (c) D. Beyersmann,
in Metals and Their Compounds in the Environment, ed. E. Merian,
Wiley-VCH, Weinheim, 1990.
3 (a) M. A. Lynes, Y. J. Kang, S. L. Sensi, G. A. Pedrizet and
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A
smaller cavity dimension could accommodate only a
smaller cation, hence, 2 preferentially binds to Fe(III) ions,
which has a comparatively smaller ionic radius. The larger non-
bonded distances, the electron and charge densities over
donor atoms, and the structural rigidity of 4 suggests that Hg(II)
ions favorably fits into its chelation-cavity and therefore it
exhibits Hg(II)-selective signalling responses. The preliminary
optimized geometries of these probes, which are presumed to
follow a pre-organizational approach prior to complexation,
establish their preferential coordination pattern among various
metal ions.
In summary, the substituted aminoethyl-rhodamine based
probe has been demonstrated here to switch its selectivity from
Fe(III) to Hg(II) ion in a dual order. First, the p-cyanobenzyl-
attached probe 2, which exhibits Fe(III) selective fluorogenic and
chromogenic signal amplification in MeCN through complexa-
tion-induced rhodamine delactonization, alters its selectivity to
Hg(II) ions in MeCN–H2O medium. Second, the Fe(III) selective
dual signalling responses of 2 in MeCN switches its selectivity
to Hg(II) ions when the electron density over the distal Namino
donor drifts away upon attachment of a p-nitrophenyl-group
as in 4. Similar methodological investigations of structural
modifications might lead, in principle, to the design of dual
mode signalling probes for selective detection of other metal
ions. Further, the mixed aqueous–organic medium has shown
here to promote Hg(II) selectivity in these probes as reflected
by their signalling responses and other metal ions failing to
bind under similar conditions; observations infer a competition
between metal–ligand interaction and hydration of those
metal ions, however, the mechanistic approach for understanding
this process properly in rhodamine based signalling probes
still remains elusive. We are presently working along this
direction.
5 (a) M. Kruppa and B. Konig, Chem. Rev., 2006, 106, 3520;
(b) P. K. Bharadwaj, Prog. Inorg. Chem., 2003, 51, 251.
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Biol. Met., 1991, 4, 1.
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The authors wish to thank the Council of Scientific and
Industrial Research, New Delhi for the financial support under
CSIR-EMPOWER scheme (CSIR-IMMT-OLP-021) for this
work and UGC, New Delhi for a senior research fellowship to
B. Biswal.
This journal is © The Royal Society of Chemistry 2012
Org. Biomol. Chem., 2012, 10, 2733–2738 | 2737

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This journal is © The Royal Society of Chemistry 2012