Rhodamine-based probes for metal ion-induced chromo-/fluorogenic dual signaling and their selectivity towards Hg(ii) ion

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Rhodamine-based probes for metal ion-induced chromo-/ﬂuorogenic dual

signaling and their selectivity towards Hg(^II) ion†

Bamaprasad Bag* and Ajoy Pal

Received 15th December 2010, Accepted 7th March 2011

DOI: 10.1039/c0ob01179g

The new signaling probes 2–6, rhodamine-B derivatives of various receptors which contain different

donor atoms for effective metal ion coordination, were synthesized and their absorption as well as

ﬂuorescence spectral responses were evaluated in the presence of various metal ions. All these probes

along with the reference probe 1 have exhibited optimal metal ion-induced absorption and ﬂuorescence

enhancement with Hg(II) ion in the longer wavelength region (>500 nm) in MeCN, exploiting the

spectral characteristics of metal ion-induced structural transformation of rhodamine. The selectivity

and sensitivity towards Hg(II) ion were better pronounced in MeCN–H₂O (1 : 1 v/v) medium, implying

the role of the solvent molecules, water in particular, in the preferential Hg(II) coordination

environment. Complexation of Hg(II) to 1–6 not only enhanced the absorption at ~560 nm, which

turned the colourless solution into pink to facilitate a naked eye detection, but also ampliﬁed the

ﬂuorescence intensity simultaneously to offer high sensitivity of detection at lower concentration. The

Hg(II)-induced photo-physical spectral responses of 1–6 in presence of other competitive metal ions

rendered their high selectivity towards Hg(II). Further, their reversible dual channel signaling pattern

under the action of counter anions, exploiting coordination tendency of anions towards Hg(II), which

compete with probe-metal interaction, implied the reversibility in their Hg(II) coordination. The

selectivity, sensitivity and reversibility, in principle, establishes the potential of these probes as

chemosensors for Hg(II) ion detection.

as chemical sensors in biomedical and environmental research

and as molecular logics in molecular information processing.

Introduction

Transition and heavy metal ions play a predominant role¹in many

The design and synthetic perspective of such probes involve

biological processes in living systems, apart from their extensive

various methodological designs, perturbation of involved photo-

domestic and industrial utility. Despite being essential for bio-

physical processes and manifestation of output signal. Designed

systems, their absence or presence in excess of the requisite amount

through various combinations of receptor, spacer and signaling

can lead to various physiological disorders. Further, their presence

subunits, these miniaturized supramolecular architectures respond

in the environment as pollutants and, thus, their toxic effects

to chemical information changes occurring in the receptor unit

that pose severe health hazards are matters of concern.²Thus,

upon analyte binding and communicate the information through

selective detection of such important yet hazardous metal ions at

perturbation of operative photophysical processes to be measured

the sub-micromolar level for biological, environmental and clinical

at the signaling sub-unit. Chromogenic signaling probes offer a

purposes is highly desirable and indispensable. Although various

naked eye detection of analytes as the output signal modulation

analytical methods, such as atomic absorption and emission

appears as a mode of color change or variation in absorption.

spectrometry, inductively coupled plasma mass spectroscopy, neu-

Fluorescent-signaling probes have proven to be essential and

tron activation analysis, X-ray ﬂuorescence spectrophotometry,

etc., have been employed to detect these metal ions, synthetic

probes which exhibit signal transduction upon analyte binding,

powerful tools to monitor in vitro and/or in vivo biologically

relevant species such as metal ions due to their advantages over the

other analytical methods in terms of sensitivity, high selectivity,

metal ions in particular, ﬁnd wide-domain utility and impact

shorter response time, non-complicated sampling, non-destructive

and non-invasive properties, and offer real-time monitoring of

the processes occurring at different time scales. Although heavy

Colloids and Materials Chemistry Department, Institute of Minerals and

and transition metal ions quench ﬂuorescence via the spin-

orbit coupling effect, a large number of ﬂuorescent probes³have

been reported for these metal ions that overcome their inherent

quenching nature through a suitable design of the probe. Most of

these reported ﬂuorescent turn-on probes for metal ions exhibit

Materials Technology (CSIR), P.O.: R.R.L., Bhubaneswar, 751013, India.

E-mail: bpbag@immt.res.in

1

† Electronic supplementary information (ESI) available: H-, ¹³C-NMR,

ESI-MS spectra of the probes 1–6, their absorption and emission spectral

data in various solvents, in presence of various metal ions at different

operational conditions. See DOI: 10.1039/c0ob01179g

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Scheme 1 Schematic representation of the probes 1–6.

chelation-enhanced ﬂuorescence enhancement in organic media,

responsive towards several metal ions or even exhibit a restricted

detection limit. Hence, the quest for development of new signaling

probes for metal ions still pertains to the selectivity and sensitivity

issue. In this context, it should be advantageous to design probes

that facilitate both chromogenic as well as ﬂuorogenic signaling

upon selective analyte binding.

is responsible for metal ion binding plays a crucial role. As the

receptor provides a number of donor atoms for effective metal ion

coordination, the nature and selectivity of a probe can be tailored

by the selection of appropriate donor atoms at strategic positions

with a varied ligand design topology to accommodate a particular

ion. Thus, in rhodamine-based signaling probes for metal ions,

receptor subunits with varied spatial disposition of donor atoms

in a covalent framework may be envisaged to exhibit different

selectivity towards different metal ions.

With this strategic design, we have synthesized rhodamine-

based probes (1–6, Scheme 1) with different receptor units which

impart different stereo-electronic environments for effective metal-

probe interaction. Further, their dual channel ﬂuorogenic and

chromogenic signaling in response to various metal ions have been

the focus of investigation in order to develop real-time monitoring

protocols for selective detection of these ions. These new probe

molecules (2–6) exhibit highly Hg(II)-selective chromogenic and

ﬂuorogenic signaling among all the metal ions investigated. The

probe 1, for which the synthesis has been reported^6i,8learlier, also

shows maximum absorption and ﬂuorescence enhancement in the

presence of Hg(II) ion. In the context of mercury contamination,¹⁵

its toxicity, subsequent biological and environmental impacts¹⁶

and the recent increasing interest to develop Hg(II)-selective

rhodamine based probes^5–7etc. has instigated a protocol to design

such probes with high selectivity, sensitivity and reversibility

in a simple synthetic route to contribute signiﬁcantly towards

development of an easy to handle method for real-time moni-

toring of mercury concentration in biological and environmental

samples.

The excellent spectroscopic properties of rhodamine dye, such

as high absorption coefﬁcient, high ﬂuorescence quantum yield,

absorption and emission at longer wavelength, etc., have re-

oriented the focus on designing rhodamine-based probes⁴for

various analytes, which also offer the advantageous virtue of

their ability to exhibit optical signal modulation in aqueous

media. Various rhodamine-based chromogenic and ﬂuorogenic

chemosensors have recently been developed for selective detection

of many transition and heavy metal ions such as Hg(II),^5–7

Cu(II),⁸Fe(III),^6j,9Cr(III),^9h,10Pb(II),^6j,11Ag(I),^5fAu(III),¹²Pd(II),¹³

Eu(III)¹⁴etc.. The signaling mechanism of these probes follows a

straight forward protocol, i.e. the colourless and non-ﬂuorescent

spirocyclic rhodamine structurally equilibrates to the colored and

highly ﬂuorescent ring-opened amide form in the presence of

a metal ion. The rhodamine-based probes exert simultaneous

chromogenic and ﬂuorogenic signaling as they not only exhibit

excellent enhancement in absorption and ﬂuorescence intensity in

response to metal ion binding, but also develop a strong colour as

a signature of the sensing event to facilitate naked eye detection.

The selectivity, sensitivity, operational conditions and reversibility

are the vital parameters to be manifested for any metal ion

selective probe; hence, the modular design of the receptor which

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taken after averaging ten measurements, were found to be rather

complicated with three emission peaks at ~540, ~560 and ~580 nm

attributed to those of the lactonized rhodamine. The observed

Results and discussion

The rhodamine derivatives 1–3 were synthesized by derivatization

of rhodamine-B hydrochloride with ethylene diamine, triethylene

tetramine and tetraethylene pentamine, respectively, in ethanol.

Although 1 has been reported earlier as a precursor to various

rhodamine-based probes,^6i,8lto the best of our knowledge the

detailed photophysical behavior and their signal modulation

with various metal ions have not been elaborately studied. The

probes 4–6 were synthesized through Schiff-base condensation of 1

with 2-hydroxy benzaldehyde, 4-(diethylamino)benzaldehyde and

4-(N,N-bis(2-hydroxyethyl)amino)benzaldehyde, respectively, in

ethanol followed by reduction with NaBH₄. The compounds were

quenched emissions of 1–6 (ﬂuorescence quantum yield f_FT

<

0.001) in almost all the solvents were contributed to by combined

inﬂuential factors: (a) rhodamines existence in spirolactam form

which facilitates an efﬁcient intersystem crossing and (b) operative

photo-induced electron transfer processes from distal donor

groups present in their receptor moiety. Though quenched, the

emission quantum yields (f_F) of all these compounds expectedly

varied with solvent polarity function. The ﬂuorescence emissions

of 1–6 were more pronounced in protic solvents such as MeOH,

EtOH and 1-PrOH in comparison to aprotic solvents, which may

be attributed to the solvent-assisted ring-opening of the spiro-

structure in traces of the probes in protic environment, as also

observed in other rhodamine-based probes.⁹ⁱ

1

characterized by H-NMR, ¹³C-NMR and ESI-MS. The char-

acteristic peak near ~66 ppm (9-carbon) in ¹³C-NMR spectrum

of the compounds in CDCl₃conﬁrms that the rhodamine exists

in its spirolactam conformation. The absorption and emission

spectral properties of 1–6 in various solvents with varying polarity

such as n-hexane, 1-propanol, chloroform, dichloromethane,

tetrahydrofuran, ethyl acetate, acetone, N,N-dimethylformamide,

dimethylsulfoxide, methanol, ethanol and acetonitrile were inves-

tigated. As the reversible lactonization-delactonization process of

rhodamine highly depends on the organic solvent media or pH,

the signaling responses of these probes in different pH and in

organic aqueous media were also carried out. The solutions of 1–

6 in acetonitrile/acetonitrile–water binary mixtures are colourless

and non-ﬂuorescent, which supports rhodamine’s predominant

existence in spirolactam form.

Absorption spectral pattern of 1–6 in the presence of various metal

ions

As the probes 1–6 contain different receptor units with varying

spatial disposition of donor atoms attached to the rhodamine

moiety and are expected to have a different binding mode with

various metal ions exploiting spatial compatibility for optimal

overlap and effective metal–ligand interaction; various metal ions

such as Na(I), K(I), Mn(II), Fe(II), Co(II), Ni(II), Cu(II), Zn(II),

Pb(II), Ag(I), Cd(II) and Hg(II) were added to the solution of

1–6 in MeCN to investigate the effect of metal ion addition on

absorption in visible wavelength region (400–650 nm window).

As observed, the UV-Vis spectra of 1–6 in MeCN exhibited no

absorption in the 400–700 nm region due to rhodamine spiro-

conformation in solutions, however, addition of metal ions lead

to appearance of a strong absorption transition at ~560 nm with

a shoulder at ~515 nm (Fig. 2a & b) in all the probes, which

is characteristic of rhodamine-based dyes. Metal ion addition

also resulted in a colour change in the solution of the probes

in MeCN, from colourless to pink, which implies a metal-induced

delactonization of rhodamines initial spirolactam form to its ring-

opened amide conformation that facilitates the complexation.

Among all the metal ions investigated, all the probes exhibited

maximum absorption enhancement (e_M.L/e_L) and subsequent

colourless→pink colour transition in presence of Hg(II) ions

yielding a signiﬁcantly high absorption (e = 2.3 ¥ 10⁴–4.2 ¥

10⁴dm³mol^-1cm^-1) at ~560 nm in MeCN. A few of the metal

ions such as Pb(II) and Fe(II) also have induced an appreciable

enhancement in absorption at ~560 nm, though to a signiﬁcantly

lower extent in comparison to that of Hg(II), whereas the rest of

the metal ions induced a negligible absorption change. The extent

of absorption enhancement at ~560 nm in the presence of metal

ions varies with nature of the probe, type of the metal ion and

their binding interactions, because the spatial disposition of the

donor atoms of receptor units and the structural rigidity of probes

facilitate different coordination environment towards different

metal ions. The absorption enhancement factor (De _~560) at ~560 nm

of 1–6 upon Hg(II) coordination were found to be as high as 3588,

2311, 2126, 3683, 2503 and 2330-fold respectively (Fig. 3). Next to

Hg(II), Pb(II) exhibited relatively higher absorption at 550 nm upon

complexation among other metal ions with 1, 2, 3, 4 and 6 resulting

in respective 941, 618, 1068, 1731 and 705-fold enhancement. The

Absorption and emission pattern of 1–6

The absorption of the probes (1–6) observed at ~315 nm and

~270 nm in all the solvents under investigation were due to ligand

localized p–p* transitions and found to be slightly solvatochromic

(Dl = 10 nm) in nature. The molar extinction coefﬁcient (e) of these

transitions varied with the nature of the substituent attached to

the rhodamine moiety. The absence of any absorption transition

at 400–600 nm region (Fig. 1) and appearance of the colorless

solution ascertained rhodamines lactonized conformation in these

compounds. On the other hand, their emission spectral patterns

(Fig. 1) upon excitation at 350–500 nm region, which were

Fig. 1 Absorption (1 ¥ 10^-4M) and emission (1 ¥ 10^-6M, l_ex= 500 nm)

spectral pattern of 1–6 in MeCN.

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Fig. 2 Absorption spectra of (a) 2 (1 ¥ 10^-4M) and (b) 5 (1 ¥ 10^-4M) in MeCN in the absence and presence of various metal ions.

Fig. 3 Absorption enhancement factor of 1–6 (10 mM) in the presence of various metal ions (50 mM) in MeCN.

absorption (A_~560) of 5 did not enhance with Pb(II), implying a non-

to be more pronounced with Hg(II) in comparison to other

metal ions, as observed in the case of MeCN. In order to verify

the inﬂuence of water molecules on Hg(II)-selective chromogenic

signaling in 2–6, the metal ion-induced changes in absorption

in these probes were investigated in different acetonitrile–water

binary mixtures of varied proportions (9 : 1, 7 : 3 and 1 : 1 v/v).

Interestingly, the chromogenic signaling behavior of all these

probes implies that their selectivity towards Hg(II) ion enhanced

gradually with increase in water proportion in acetonitrile–water

binary mixture. Other metal ions such as Pb(II) and Fe(II), which

induced a small enhancement in absorption in all the probes

(except in 5, where Pb(II) did not exhibit any change) in MeCN,

failed to exhibit such absorption enhancement in MeCN–H₂O

coordinating environment of the probe towards that particular

metal ion, while Fe(II) enhanced its absorption up to 853-fold. The

De₅₆₀due to complexation with other metal ions in comparison to

that with Hg(II) indicates that all these probes preferably facilitate

an efﬁcient coordination environment for Hg(II) ion.

When investigated in acetonitrile : water (1 : 1 v/v) medium

rather than in acetonitrile, the absorption (e_~560) at ~560 nm of 2–6

increased selectively in presence of Hg(II) ion among all the metal

ions investigated and subsequent pink colour development was

also observed (Fig. 4c & d). However, similar Hg(II)-selectivity

could not be observed for the probe 1 in MeCN–H₂O (1 : 1 v/v)

mixture although the absorption enhancement(De_~560) was found

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Fig. 4 Absorption spectra of 3 (1.8 ¥ 10^-5M) in (a) MeCN and (b) MeCN–H₂O (1 : 1 v/v) in absence and presence of various metal ions. The colour

changes of 6 (10 mM) in response to various metal ions (100 mM) in (c) MeCN and (d) MeCN–H₂O (1 : 1 v/v). The colour change due to added metal

ions to the solution are in the order of blank, Na(I), K(I), Mn(II), Fe(II), Co(II), Ni(II), Cu(II), Zn(II), Ag(I), Cd(II), Hg(II) and Pb(II) (from left to right).

(1 : 1 v/v) medium. The suppression of chromogenic signaling

pattern of Pb(II) and Fe(II) in organic–aqueous medium may

be attributed to a restricted metal ion–probe interaction in the

presence of water molecules.

concentrations of metal ions in all the titration plots of different

probe–metal combination, the increase in absorption peak at

354 nm, which is attributed to the perturbation of associated

p→p* transitions, and a distinct isosbestic point at ~330 nm in

the absorption spectra indicate formation of a single component

upon complexation, i.e. coordinated delactonized rhodamine. The

absorption spectral pattern of 2, 3 and 6 on titration with different

equivalents of Hg(II), Pb(II) and Fe(II) in MeCN revealed that

absorption attains a maximum upon addition of 2 equiv. of metal

ions (Fig. 5) and remains constant up to addition of 10 equiv.

of the cation. Further, it was observed from the titration plots

that the absorption maximum (A_~560) of the probes remains almost

the same up to addition of 1 equiv. of metal ion and subsequent

addition up to 2 equiv. leads to its gradual enhancement to

The time-course studies for each of the probes with Hg(II)

revealed that the complexation induced colour change attained

optimal absorbance in almost 1 min and remained constant for at

least 24 h. As the high selectivity of any probe towards a particular

metal ions is conferred by reduced or negligible interference of

other competitive metal ions, the change in absorption of 1–6

due to Hg(II) were also investigated in presence of other metal

ions in both MeCN and MeCN–H₂O media. It was observed that

Hg(II) ion still produces a change in absorption in the presence of

other cations in probes 2–6 almost similar to that in their absence,

which implies the high selectivity of the probes towards Hg(II) ion.

Thus, the unique feature of selective Hg(II)-induced absorption

ampliﬁcation and subsequent colour change enables the probes

2–6 to potentially be employed for naked-eye detection of Hg(II)

within a concentration threshold.

The plot of absorbance at 560 nm of 1–6 as a function of

mole fraction of added metal ions (Jobs plot) in MeCN as well

as MeCN–aqueous medium reveals that these probes bind to the

metal ion in different stoichiometry depending upon their covalent

architecture. The probe–metal stoichiometries were found to be

1 : 1 for probes 1 and 4, 1 : 2 for probes 2, 3 and 6, while a peculiar

ratio of 1 : 1.5 was observed for probe 5, which implies a different

binding mode of each probe towards the metal ions added, Hg(II)

in particular. The 1 : 1.5 probe–metal complexation stoichiometry

arises when at least two probe molecules (5) bind to three

metal ions (Hg(II) and Fe(II)) during complexation. The UV-vis

absorption titration of 1–6 (1.0 ¥ 10^-4M–1.0 ¥ 10^-5M) with various

metal ions (1.0 ¥ 10^-6M–1.0 ¥ 10^-3M) were carried out in MeCN as

well as in MeCN–H₂O (1 : 1 v/v), which have shown an appreciable

change in absorption and subsequent colour transition. Apart

from metal ion-induced increase in absorption (A_~560) at different

Fig. 5 Absorption spectra of the titration of 2 (1 ¥ 10^-5M) with different

amount of Hg(II) added in MeCN–H₂O (1 : 1 v/v). Inset: Jobs plot,

absorption as a function of Hg(II) added and change in absorption of

2 at 557 nm as a function of Hg(II) concentration.

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attain the maximum, which indicates that these probes ﬁrst bind

to the metal ion through distal donor atoms of their respective

receptor units yielding almost no change in absorption response,

then followed by coordination at delactonized rhodamine unit

which results in increased A_~560and brings about the colour change.

A similar observation in the absorption titration plots for 2, 3 and

6 with Hg(II) in MeCN–H₂O(1 : 1 v/v), where these probes exhibit

selectivity towards Hg(II) among all the metal ions investigated,

have indicated towards their 1 : 2 binding mode which were well

supported by respective Jobs plots. The complex stability con-

stants (K_s) for Hg(II) coordination, determined through Benesi–

Hildebrand method⁷for 1 : n stoichiometry as a reciprocal of the

slope of the linear regression of the plot of 1/DA vs. 1/[Hg(II)]ⁿ

were found to be 0.99 ¥ 10⁷, 3.02 ¥ 10⁸and 1.01 ¥ 10⁷M^-2with 2, 3

and 6, respectively (n = 2, r²> 0.99). The higher K_svalues for these

probes correlate to their covalent architecture, i.e. the number of

available coordination sites in the receptor. When titrated with

Hg(II) or Fe(II), the absorption of 5 (10 mM) attained maximum

with addition of 1.5 equiv. of metal ion and remained constant

up to 20 equiv. further addition. The linear regression of the

plot for determination of K_sdid not ﬁt with a good correlation

(correlation coefﬁcient < 0.7) with 1 : 1 or 1 : 2 stoichiometry of

5ÃHg(II) complex (either n = 1 or n = 2), rather exhibited a good

correlation with 1 : 1.5 complex stoichiometry and its K_swith

Hg(II) was estimated to be 1.06 ¥ 10⁵M^-1.5(n = 1.5, r²> 0.99),

which supports the complex stoichiometry pattern as deduced by

the Jobs plot. Similarly, absorption titration of 4 (10 mM) with

Hg(II)/Pb(II)/Fe(II) in MeCN and that of 1 (10 mM) with Hg(II)

indicated towards a 1 : 1 complexation stoichiometry, as observed

in respective Jobs Plots. The K_sof Hg(II) coordination in MeCN–

H₂O (1 : 1 v/v) was determined to be 1.20 ¥ 10⁴M^-1for 4 while

it was 5.73 ¥ 10⁴M^-1for 1. The determined K_svalues for Hg(II)

coordination with these probes in MeCN–H₂O (1 : 1 v/v) revealed

that 3, which incorporates a higher number of amino donor sites in

its architecture, has a higher binding afﬁnity towards Hg(II). The

observable naked eye detection of the pink colour development in

these probes (mM) upon Hg(II) addition have a detection limit of

0.3–0.5 ¥ 10^-6M of Hg(II).

The absorption titration proﬁles suggested that addition of

excess of metal ions (>100 equivalents of that of probe) to the

solution of probes (>1 ¥ 10^-5M) lead to decrease in the metal-

induced enhanced absorption. The absorption titration spectra of

5 (3 ¥ 10^-4M) with Fe(II) in MeCN revealed that the absorption at

558 nm enhanced to attain maximum, though small in comparison

to that with Hg(II), remained constant up to addition of 50 equiv.

of Fe(II) and gradually decreased upon further addition (>100

equiv.). Apart from that, a new absorption peak simultaneously

appeared at 470 nm and gradually enhanced with further increase

in Fe(II) concentration exhibiting a clear isosbestic point at 500 nm.

The absorption transition of 5 at 470 nm in presence of excess of

Fe(II) may be attributed to a ligand–metal charge transfer band,

which arises due to a different, but preferential, binding mode of

5 with Fe(II) when present in excess. Except Fe(II) addition to 5,

similar absorption peak appearance could not be observed for any

other metal ions when added in excess to any of the probes.

MeCN revealed that their non-ﬂuorescent solution becomes highly

ﬂuorescent with enhancement of ﬂuorescence intensity at ~580 nm

emission maximum (l_em^max) upon metal addition (Fig. 6). This

implies a delactonization process, induced by metal ion coordi-

nation, of the non-ﬂuorescent spirocyclic form of rhodamine to

its highly ﬂuorescent ring opened form, a primitive characteristic

of rhodamine dye. The extent of chelation-enhanced ﬂuorescence

effects were found to depend highly upon nature of the probes

and interacting metal ions. Among all the metal ions investigated,

max

these probes exhibited high ﬂuorescence enhancement at l_em

=

~580 nm (ﬂuorescence quantum yield f_FT= 0.464–0.659) in the

presence of Hg(II) ion in MeCN with 970, 579, 725, 1315, 1773

and 510-fold ﬂuorescence intensity enhancement factors (F/F₀)

respectively in 1–6. Among all the probes, 5 exhibited maximum

Hg(II)-induced ﬂuorescence enhancement (F/F₀> 1700-fold),

showing its higher afﬁnity towards Hg(II) in comparison to other

probes. Barring Hg(II), only addition of Pb(II) and Fe(II) ions

promote a small increase in F/F₀in these probes (except for

5 where Pb(II) did not change the ﬂuorescence intensity) in

MeCN, while other metal ions failed to respond with a similar

appreciable change in ﬂuorescence under identical conditions.

The ﬂuorescence enhancement factor and corresponding quantum

yields of 1–6 in the presence of various metal ions are collected

in Table 1. When investigated in MeCN–H₂O binary mixtures

(9 : 1, 7 : 3, 1 : 1 v/v), the selectivity of 2–6 towards Hg(II) ion

enhances with an increase in the water proportion in the solvent

mixture, as also observed with absorption change. Thus in

MeCN–H₂O (1 : 1 v/v) medium, all these probes exhibited high

ﬂuorescence enhancement selectively in presence of Hg(II) while

their ﬂuorogenic signaling behavior with Pb(II) and Fe(II) observed

in MeCN were suppressed, and other metal ions exhibited no

or negligible change in ﬂuorescence emission. In order to verify

the selectivity of these probes towards Hg(II) ions over other

competitive metal ions, the ﬂuorescence intensity change (F/F₀)

of 1–6 (1 mM) upon addition of Hg(II) (10 mM), other metal ions

(10 mM), and Hg(II) along with different metal ions in both MeCN

and MeCN–H₂O (1 : 1 v/v) were evaluated. The Hg(II) addition

to the solution of 1–6 enhanced the ﬂuorescence intensity up to

same extent (error <5%) in either presence or absence of other

interfering cations in both the solvent conditions, while other

Emission spectral pattern in presence of metal ions

The ﬂuorescence spectral pattern of 1–6 when excited in the

500–550 nm region in the presence of various metal ions in

Fig. 6 Fluorescence spectra (l_ex= 500 nm) of 6 (1 mM) in presence of

various metal ions (5 mM) in (a) MeCN and (b) MeCN–H₂O (1 : 1 v/v).

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Table 1 Absorbance, emission enhancement factor and corresponding quantum yields of the probes in presence of various metal ions^a

0

Absorbance (log e)

Emission EF (I_F/I_F

)

Quantum yield (f_FT)

Probe

Ionic Input

MeCN

MeCN–H₂O (1 : 1 v/v)

MeCN

MeCN–H₂O (1 : 1 v/v)

MeCN

MeCN–H₂O (1 : 1 v/v)

1

Blank

Na(I)

K(I)

1

~1

1

<0.001

0.015

~0.002

0.003

0.035

0.026

~0.003

~0.002

0.485

<0.001

0.009

0.001

0.002

0.021

0.026

~0.003

~0.002

0.392

1.146

1.114

2.512

3.331

1.973

2.170

3.757

3.637

1.176

1.447

4.555

3.974

1.556

1.112

1.085

2.501

3.176

1.932

2.101

3.725

3.619

1.119

1.389

4.132

3.883

3.155

1.2

1.4

30.9

3.7

6.6

70

Mn(II)

Fe(II)

Co(II)

Ni(II)

Cu(II)

Zn(II)

Ag(I)

Cd(II)

Hg(II)

Pb(II)

H⁺

21.3

1.7

2.9

62.2

47.7

2.8

1.9

897.6

207.2

71.5

52.5

5.2

4.4

970.4

255

~1

0.127

<0.001

0.102

0.023

2

3

4

5

Blank

Na(I)

K(I)

1

1.024

1.062

1.044

1.098

1.505

1.995

1.267

1.229

1.004

1.217

1.516

3.979

1.893

3.669

1

2.1

1.2

93.1

22.7

31.1

32.5

39.4

1.4

1

1.2

1.6

0.9

3.22

1.86

2.5

3.7

0.9

1.3

0.9

339.5

12.5

67.8

0.001

<0.001

~0.002

<0.001

0.002

0.003

0.001

<0.001

0.258

1.342

1.079

2.706

3.741

3.215

3.505

3.067

3.574

2.640

3.144

4.364

3.791

1.708

<0.002

~0.001

0.077

0.019

0.029

0.030

0.033

~0.001

~0.002

0.475

Mn(II)

Fe(II)

Co(II)

Ni(II)

Cu(II)

Zn(II)

Ag(I)

Cd(II)

Hg(II)

Pb(II)

H⁺

2.2

579.3

100.6

1

0.083

~0.001

0.007

0.021

Blank

Na(I)

K(I)

1.041

1.532

1.114

1.568

3.694

2.526

2.725

3.229

3.099

2.438

2.210

4.346

4.044

1.380

1

1.1

1.2

4.2

103.3

1.6

1.3

16.1

10.5

9.5

1

0.001

~0.001

0.004

0.094

~0.002

~0.001

0.015

0.010

0.003

0.659

0.135

~0.001

<0.001

0.002

<0.001

~0.002

<0.001

0.366

1.041

1.011

1.009

2.677

1.132

1.213

1.699

1.994

1.126

1.227

4.347

2.963

3.615

1.01

1.02

0.97

2.39

1.03

0.95

2.69

1.34

1.01

0.98

448.6

9.37

61.3

Mn(II)

Fe(II)

Co(II)

Ni(II)

Cu(II)

Zn(II)

Ag(I)

Cd(II)

Hg(II)

Pb(II)

H⁺

3.5

725.3

148.6

~1

0.008

0.020

Blank

Na(I)

K(I)

1.041

0.903

1.176

1.799

3.865

2.114

2.013

3.196

3.118

1.176

1.756

4.608

4.280

1.204

1

<0.001

0.107

0.004

0.002

0.028

0.025

<0.001

0.658

<0.001

0.010

<0.001

0.001

0.998

1.051

1.034

2.214

1.997

1.198

2.221

2.179

1.319

1.551

4.499

3.276

4.063

~1

1.07

1.05

1.02

23.59

1.04

0.91

2.24

1.35

0.94

0.91

1038.6

15.9

98.8

~1

Mn(II)

Fe(II)

Co(II)

Ni(II)

Cu(II)

Zn(II)

Ag(I)

Cd(II)

Hg(II)

Pb(II)

H⁺

1.6

214

8.4

4.4

55.1

49.7

2.8

0.001

<0.001

0.479

0.008

0.042

2.9

1315.5

685.9

~1

0.342

<0.001

Blank

Na(I)

K(I)

Mn(II)

Fe(II)

Co(II)

Ni(II)

Cu(II)

Zn(II)

1.079

1.380

1.568

2.354

4.011

2.260

3.256

2.553

3.332

1.003

1.104

1.226

2.004

3.009

1.018

2.176

3.029

2.011

1

1.4

3.7

22.2

361.9

20.3

73.5

36.7

72.6

1

<0.001

0.001

0.007

0.109

0.006

0.022

0.011

0.022

<0.001

0.006

<0.001

0.002

1.02

1.1

1.2

22.6

1.2

2.6

10.4

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Table 1 (Contd.)

0

Absorbance (log e)

Emission EF (I_F/I_F

)

Quantum yield (f_FT)

Probe

Ionic Input

MeCN

MeCN–H₂O (1 : 1 v/v)

MeCN

MeCN–H₂O (1 : 1 v/v)

MeCN

MeCN–H₂O (1 : 1 v/v)

Ag(I)

Cd(II)

Hg(II)

Pb(II)

H⁺

1.505

1.833

4.478

1.732

1.389

1.339

1.523

4.402

3.024

3.747

2.5

1.9

1773.3

1.3

1

1.4

1.2

1360.6

1.5

114.7

<0.001

0.532

<0.001

0.364

<0.001

0.049

6

Blank

Na(I)

K(I)

1

<0.001

~0.001

0.089

~0.001

0.028

<0.001

0.007

<0.001

0.002

<0.001

0.366

1.079

1.146

2.013

3.822

1.681

2.072

3.364

3.198

1.602

1.699

4.624

4.108

1.690

1.014

1.037

1.882

2.673

1.463

1.839

2.322

2.047

1.274

1.359

4.058

2.833

3.348

1.4

1.1

98.4

1

1.3

30.7

25.4

1.1

1.2

510.8

159.9

1

1.2

1.6

9.6

1.4

2.1

7.9

1.3

1

1.3

443.2

22.1

32.6

Mn(II)

Fe(II)

Co(II)

Ni(II)

Cu(II)

Zn(II)

Ag(I)

Cd(II)

Hg(II)

Pb(II)

H⁺

0.023

~0.001

0.464

0.145

~0.001

0.016

0.019

^aExperimental conditions: Absorption: [probes] = 1 ¥ 10^-6–1 ¥ 10^-5M; [metal ion] = 1 ¥ 10^-4M; MeCN. Emission experimental conditions: concentration

of free probe: 1 ¥ 10^-6–1 ¥ 10^-8M; added metal ion = 1 ¥ 10^-6M; l_ex= 500 nm; excitation and emission band-pass: 5 nm; T = 298 K; MeCN. The error in

f_Fis within 5%. EF: The enhancement factors are in comparison to that of corresponding metal free probes.

metal ions did not induce any signiﬁcant change. This establishes

the signiﬁcant feature of high selectivity of these probes towards

Hg(II) over other competitive metal ions.

The ﬂuorescence titration proﬁle of 1–6 (1.0 ¥ 10^-7M) versus

various concentration of Hg(II) in MeCN–H₂O (1 : 1 v/v) revealed

max

that ﬂuorescence intensity of these probes with l_emat 580 nm

enhanced signiﬁcantly with gradual increase in Hg(II) concentra-

tion and remained constant thereafter saturation of complexation

within a concentration threshold. Fluorescence spectral pattern

of 2 (1 mM) in MeCN–H₂O(1 : 1 v/v) upon addition of various

concentrations of Hg(II) is given in Fig. 7. The other probes

were also exhibited similar titration proﬁle with Hg(II). The

ﬂuorescence enhancement factor (F/F₀) were proportional to the

concentration of Hg(II) added ([Hg(II)] = 1.0 ¥ 10^-8M–1.0 ¥ 10^-6M)

to each of the probes. The complex stability constants for Hg(II)

coordination with 1–6 were estimated to be 2.62 ¥ 10⁶M^-1, 3.83 ¥

10¹²M^-2, 6.60 ¥ 10¹³M^-2, 8.99 ¥ 10⁸M^-1, 4.18 ¥ 10¹⁰M^-1.5and 1.08 ¥

10¹²M^-2, respectively, (error < 5%) on the basis of linear ﬁtting

Fig. 7 Fluorescence spectral pattern of 2 (1 mM) in MeCN–H₂O

(1 : 1 v/v) upon addition of various concentrations of Hg(II). Each

(inset, Fig. 7) of the double reciprocal plot of ﬂuorescence intensity

[1/{(F/F₀) - 1}] as a function of concentration of Hg(II) added

spectrum was taken after 1 min of Hg(II) addition. Inset (top): Fluorescence

{1/[Hg(II)]ⁿ}. The good coefﬁcient (r²> 0.99) of linear regression

enhancement change (normalized with the emission of free probe 2) as a

function of equivalents of Hg(II) added. Inset (bottom): Linear regression

for the assigned complex stoichiometry was also well supported

by their corresponding binding mode as obtained through Jobs

plot of ﬂuorescence intensity change 1/{(F/F₀) - 1} as a function of

concentration 1/[Hg(II)]²M^-2.

plots and absorption titration pattern. From the titration data of

the probe 5 with Hg(II), it was observed that linear regression

of the plot for determination of K_sdid not ﬁt with a good

correlation (correlation coefﬁcient <0.6–0.8) with 1 : 1 or 1 : 2

stoichiometry of 5Hg(II) complex (either n = 1 or n = 2), but rather

exhibited a good correlation with 1 : 1.5 complex stoichiometry,

similar to that observed in the case of absorption titration. The

ﬂuorescence enhancement factors and subsequent higher K_svalues

(>10⁷) for 1–6 with Hg(II) corresponding to their higher binding

ability suggested that these probes have the potential to detect

Hg(II) ion at a very low concentration level. Based on their

ﬂuorescence signal modulation, the detection limit (ﬂuorescence

signal S/N = 3) for Hg(II) were measured to be very low (3.0 ¥

10^-8M–5 ¥ 10^-9M) and comparable to those of Hg(II)-selective

rhodamine-based probes,^5t,6l,8land even with other receptor-based

probes.¹⁷The ﬂuorescence signal responses, in comparison to that

of absorption, induce higher sensitivity of these probes towards

Hg(II) coordination. Moreover, the essential factor for a probe in

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chemo-sensing application is the real-time monitoring of analyte

concentration. The time course studies for each of the probes with

Hg(II) revealed that the probe–metal complexation occurred in

almost 1 min, as reﬂected through maximum ﬂuorescence intensity

change, and remained constant for at least 12 h.

coordination to the probe also decreases substantially upon

addition of chelating agents such as ethylenediamine-tetraacetic

acid (EDTA) with simultaneous change in colour from pink

to colourless. When the probe itself is added to the saturated

solution containing Hg(II)–probe complex, a negligible decrease

in absorption and emission spectral intensity of the Hg(II)–probe

complex was observed due to a concentration variation of the

probe in the solution.

Reversibility in signaling responses

The reversibility is an important aspect of any probe to be

employed as a chemical sensor for detection of speciﬁc metal

ions. To investigate the reversibility in chromo- and ﬂuorogenic

signaling of 1–6 rendered by Hg(II) complexation, ammonium salts

The photophysical spectral behaviours of 1–6 monitored in

different pH have shown that no observable change in absorption

and ﬂuorescence intensity occur in pH 7.0 to 10.0, which indicates

that these probes can potentially be used in biological systems.

However, their enhanced absorption and ﬂuorescence intensity

proportional to the decreased pH (<7.0) revealed that delactoniza-

tion of the rhodamine spiro-ring occurred under acidic conditions

even in absence of any metal ion. When monitored at pH 4.0 in

comparison to that in basic or neutral pH (10.0–7.0), appearance

of the absorption peak at 557 nm (e₅₅₇= 5637, 4678, 1563, 2580,

245 and 6624 dm³mol^-1cm^-1respectively for 1–6) and pink colour

of the solution was observed for all the probes. Simultaneously, a

~40 nm red-shift in ﬂuorescence maxima along with an enhance-

ment in intensity (F_pH4.0/F_pH7.0> 10-fold) was also observed. In

this context, a negligible change in absorption or ﬂuorescence

intensity of these probes was observed in presence of H⁺(HClO₄)

in MeCN. However, when H⁺was added to their solution in EtOH,

MeOH and even in acetonitrile–aqueous medium (9 : 1, 7 : 3 and

1 : 1 v/v), the absorption (A_~560) and ﬂuorescence (F/F₀) were

substantially enhanced. This implies a hindrance of protonation-

induced delactonization of the rhodamine spiro-ring in these

probes in aprotic solvents like MeCN, which gets facilitated in

protic solvents. Both absorption and ﬂuorescence changes in the

presence of metal ions in MeCN revealed that 2–6 promoted

signiﬁcant signal ampliﬁcation in the presence of Hg(II) while a

few other metal ions such as Fe(II) and Pb(II) also induced smaller

changes. It may be argued that the hydrated metal perchlorate

salts are known to generate protons in organic solvents, and hence

might be contributing to the turn-on signaling responses of these

probes in the presence of metal ions through proton-induced

delactonization of the spirolactam ring. However, if the proton

generation in the medium by the metal salts were responsible, then

the extent of enhancement should have been almost the same in all

the cases. The signaling responses of these probes in MeCN–H₂O

(1 : 1 v/v) indicated their Hg(II) selectivity where other metal ions

impart no or negligible spectroscopic changes (Fig. 9 & Table 1).

Similar water-mediated tuning of selectivity towards Hg(II) could

not be observed in 1, although it has exhibited optimal ﬂuorogenic

and chromogenic signal ampliﬁcation in the presence of Hg(II)

among all the metal ions investigated. The reason for enhanced

Hg(II) selectivity of 2–6 may be rather complicated, however

the possibility of solvent molecules playing the crucial role in

metal–probe complexation may not be overlooked. MeCN being a

coordinating solvent, it actively indulges in complexation of Hg(II)

as well as with a few other metal ions with these probes through

solvent-assisted coordination apart from the probes binding to the

metal ion. The coordination of other metal ions to these probes

in the presence of water molecules is restricted due to their strong

hydration ability, whereas only Hg(II) coordination to 2–6 gets

facilitated in acetonitrile–aqueous medium to exhibit a Hg(II)-

selective absorption signal ampliﬁcation with change in colour

of different anions such as SO₄^2-, NO₃, PO₄^3-, CO₃^2-, SCN^-, Cl^-,

-

F^-, I^-, OAc^-and HCO₃were added to the solution containing

-

the probe and Hg(II), and their absorption and ﬂuorescence

intensity change were monitored after 1 min. Among all the

anions, the enhanced absorption due to Hg(II) coordination to

the probe immediately reduced upon addition of acetate and

iodide with subsequent disappearance of pink colour of the

solution to colourless. Simultaneously, the enhanced ﬂuorescence

was also quenched to exhibit a signal with ﬂuorescence intensity

almost equal to that of a metal-free probe. This implies that

these anions have a higher coordinating tendency towards Hg(II)

and thus replace the probes to bind with the metal ion. Similar

diminishing signaling patterns of the Hg(II)-complexed probes

were also observed with Cl^-after 12 h of addition of the anion.

The absorption and emission spectral pattern of the solution

containing 3 and Hg(II) upon addition of various anions is given

in Fig. 8. Other probes also responded similarly to establish

reversible chromo- and ﬂuorogenic signaling of Hg(II) ion as a

function of counter anions. The lowered absorption and emission

intensity due to anion addition was observed to enhance again

upon further addition of Hg(II), turning the colour of the solution

from colourless to pink, which suggests the possibility of reuse of

these probes as chemosensors for selective Hg(II) detection. Apart

from anions, a few chelating agents and the probes themselves were

also added to the Hg(II)–probe complex in MeCN–H₂O (1 : 1 v/v)

medium in order to verify the reversibility in Hg(II)-induced

absorption and emission signaling responses. It was observed that

the ampliﬁed absorption and emission due to selective Hg(II)

Fig. 8 Change in (a) absorption and (b) ﬂuorescence intensity upon

addition of various anions (10 equiv.) to the solution containing 3 (1

equiv.) and Hg^II(5 equiv.). Conc. of 3 = 1.0 ¥ 10^-4M (abs) and 1.0 ¥ 10^-8

(em). l_ex= 500 nm.

M

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Fig. 9 Fluorescence enhancement of (a) 3 and (b) 5 in the presence of various metal ions in various compositions of MeCN–H₂O binary mixture as

medium. (Conc. of ligand = 1 ¥ 10^-6M, conc. of metal = 1 ¥ 10^-5M, RT, l_ex= 500 nm.)

from colourless to pink (increased A_~560) and simultaneous ﬂuo-

rescence enhancement. Further, in order to determine the better

probe in terms of selective Hg(II) sensing among 2–6, a controlled

experiment was carried out with addition of Hg(II) to a solution

containing all these competitive probes (equimolar proportion) in

MeCN–H₂O (1 : 1 v/v) medium. Both the absorption and emission

intensity of the mixture solution was enhanced (F/F₀ª 1300-fold)

to the extent of that of just 5 upon Hg(II) addition under similar

experimental conditions, which implied that the selective Hg(II)

sensing was better observed in 5 among all these probes.

Structure–function correlation

Although the new probes 2–6 were expected to exhibit chro-

mogenic and ﬂuorogenic signaling selectively in the presence

of different metal ions, due to their structural variation in the

rhodamine-appended receptor, which provides different binding

sites for complexation, rather interestingly 1–6 have shown optimal

spectroscopic response preferentially with Hg(II). The Hg(II)

selectivity of these probes might be due to their common structural

feature, i.e. the substituted amino-ethyl-amido-phenyl segment of

the ring-opened rhodamine, which coordinates selectively to Hg(II)

and is responsible for metal ion-induced enhanced absorption and

ﬂuorescence signals modulation. Hence, the substituted amino-

ethyl moiety in all these probes, when appended to rhodamine,

provided an optimal stereo-electronic situation for effective Hg(II)

coordination irrespective of the nature of the substituent and ren-

dered subsequent spectral responses for selective Hg(II) detection.

In this regard, the absorption spectral pattern of 5 in the presence

of Cu(II) in various compositions of MeCN–H₂O medium has

provided vital evidence (Fig. 10a). The Cu(II) induced absorption

of the ring-opened rhodamine at 557 nm (e = 3950 dm³mol^-1cm^-1),

though much lower than that in the presence of Hg(II), was

reduced signiﬁcantly while the absorption transition at 478 nm

(e = 18 306 dm³mol^-1cm^-1) along with its vibronic structures

observed in MeCN subsequently increased with the increase in

aqueous composition in organic–aqueous binary mixtures with

an isosbestic point at 508 nm. Excitation of the (0,0) transition at

478 nm and its vibronic structures did not exhibit any emission,

whereas excitation of that due to ring-opened rhodamine exhibited

Fig. 10 (a) Absorption spectral responses of 5 (2.5 ¥ 10^-5M) in presence

of Cu(II) ion (1 ¥ 10^-4M) in various composition of MeCN–H₂O mixture

as medium. (b) Absorption spectra of 5 (2.5 ¥ 10^-5M) in the presence

of various metal ions in MeCN–H₂O (1 : 1 v/v). Inset: Metal ion-induced

absorption modulation as a function of composition of medium.

a ﬂuorescence emission at ~580 nm. The Cu(II)-induced absorption

at 478 nm thus may be assignable to a ligand-to-metal charge

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transfer transition which arises due to binding of Cu(II) to the

distal N atom of the phenyl-amino-ethyl unit of 5. The increase

in Cu(II)-induced LMCT band and subsequent reduction of ring-

opened rhodamine absorption with an increase in the amount of

the aqueous component in the binary composition of the medium

implied that the presence of water gradually restricts the ability

of Cu(II) to delactonize the spiro-ring of rhodamine, and rather

promoted the metal ion to bind preferentially at the distal amino

end. It has been observed here that the probe 5 exhibited an altered

chromogenic signaling selectively with two different metal ions

when monitored at two different output wavelengths (Fig. 10b).

Its absorption spectral pattern in the presence of various metal

ions in MeCN–H₂O (1 : 1 v/v) revealed that the absorption peak at

557 nm enhanced (e = 25 569 dm³mol^-1cm^-1) selectively in presence

of Hg(II) with a colour change from colourless to pink, while that

at 478 nm enhanced (e = 18 671 dm³mol^-1cm^-1) selectively in the

presence of Cu(II) with a colour change from colourless to yellow.

The altered signaling responses of a probe selective towards two

different inputs when monitored at two different output channels

contribute signiﬁcantly towards development of transducers that

perform complex chemical logic action.

To verify the coordination geometry around the substituted

amido-ethyl-amino unit of these probes upon complexation and

their preferential binding to Hg(II) ion, geometrical optimization

calculations of a model system containing the same binding unit

and its complexes with various metal ions were performed at DFT

level with the Gaussian 09 program¹⁸using B3LYP/6-31G* basis

set for ligand and B3LYP/LANL2DZ basis set for metal ions,

which includes relativistic small-core effects with effective core

potential. The non-bonded N_amino–O_amidodistances of the model

system and its corresponding metal complexes revealed that Hg(II)

ion favorably ﬁts to the chelation cavity of amido-ethyl-amino

receptor unit in comparison to other metal ions. Its correlation

with these probes, which contain the same donor sites for effective

metal ion binding, may arguably support that a preferential Hg(II)

coordination by these probes leads to subsequent metal ion-

induced delactonization of the rhodamine spiro-ring resulting

in optimal Hg(II)-mediated chromo- and ﬂuorogenic signaling

among all the metal ions investigated. The other binding sites

on the receptor attached to amino-ethyl-amido-rhodamine unit

might not be contributing to the selective Hg(II)-induced spiro-

ring opening, and subsequent coordination for effective signaling,

however, contributes signiﬁcantly towards the sensitivity of the

Hg(II) detection by these probes providing increased complex

stability. In this context, it has been found that for the 4-[(N,N-

bis(2-hydroxyethyl)amino)phenyl receptor, when attached to the

rhodamine-ethyl-amino-methyl unit, the photo-physical spectral

pattern of the probe 6 has shown selectivity towards Hg(II),

however a 4,4-diﬂuoro-4-bora-3a,4a-diaza-s-indacene (BODIPY)

derivative of the same receptor unit has been reported¹⁹to exhibit

ﬂuorogenic signaling selective with Pb(II). Similarly, a rhodamine

B derivative of triethyl tetramine (probe 2) exhibited Hg(II)-

selective spectral responses, while a rhodamine 6G derivative of

the same receptor unit has been reported^9hto exhibit Cr(III)-

selective signaling. Emphasis on the structural pattern would

have beneﬁcially contributed to understand the Hg(II)-selective

signaling behaviours of these probes through structure–function

correlation before a conclusion can be put forward regarding

its coordination environment; however, all attempts to obtain

X-ray quality crystals of 1–6 with Hg(II) remained unsuccessful.

Nevertheless, based on the photo-physical signaling responses,

the new probes 2–6 can potentially be employed for qualitative

and quantitative detection of Hg(II) in real-time sensing in

environmental aspects.

Conclusion

In summary, the new rhodamine-based signaling probes 2–6,

designed and synthesized through a simple synthetic route by

condensation of different receptors containing amino/oxo donor

atoms in their architecture with rhodamine-B, have been shown

to exhibit both ﬂuorogenic and chromogenic signalling induced

by metal ions, exploiting the structure–function correlation of

rhodamine. Among all the metal ions under investigation, Hg(II)-

induced optimal signal ampliﬁcation. All these probes not only

exhibited maximum absorption enhancement in the presence of

Hg(II) and turned the colour of the solution from colourless

to pink with an observable naked eye detection limit of 0.3–

0.5 ¥ 10^-6M of Hg(II) concentration, but also simultaneously

exhibited optimal Hg(II)-induced ampliﬁcation of ﬂuorescence

intensity to two orders in magnitude with lower detection limit

(<3 ¥ 10^-8M). A few other metal ions such as Pb(II) and

Fe(II) have also induced appreciable absorption and emission

enhancement though much lower in comparison to that due

to Hg(II). The selectivity of 2–6 enhanced towards Hg(II) in

organic–aqueous binary mixture medium, implying a vital role

of water molecules in regulating the coordination environment

of complexation which modulates the signalling pattern, whereas

the reference probe 1 failed to exhibit Hg(II) selectivity under

similar conditions. As other competitive metal ions promote no

or negligible change in absorption and ﬂuorescence intensity and

induced negligible interference in Hg(II)-induced observable and

detectable spectral changes, these probes facilitate high selectivity

towards Hg(II). The spectral response of each probe towards Hg(II)

varied with their structural design, which incorporates different

binding sites on their receptor unit, exploiting their binding afﬁnity

and different binding modes of complexation. Among all the

probes, 5 exhibited optimal signaling responses in terms of Hg(II)

sensing. The spectral response of 1–6 towards Hg(II) was found

to be reversible with addition of counter anions such as iodide

or acetate, which establishes these probes to be chemosensors for

Hg(II). Considering the advantages of high sensitivity, selectivity,

reversibility and real-time response of 2–6 towards Hg(II), these

probes might potentially be used for environmental analysis or

in vitro Hg(II) detection in living cells. Our subsequent work will

be focused along this direction.

Materials and methods

All the reagent grade chemicals were used without puriﬁca-

tion unless otherwise speciﬁed. Rhodamine-B hydrochloride,

ethylenediamine, triethylene tetramine, tetraethyl pentamine,

2-hydroxybenzaldehyde, 4-(diethylamino)benzaldehyde, 4-(N,N-

bis(2-hydroxyethyl)amino)benzaldehyde and the metal perchlo-

rate salts were obtained from Aldrich (USA) and used as received.

Anhydrous sodium sulfate, acids, buffers and the solvents were

received from S. D. Fine Chemicals (India). All the solvents were

freshly distilled prior to use for ﬂuorescence measurements by

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following the literature procedures²⁰and all the reactions were

carried out under N₂atmosphere. Chromatographic separations

were done by column chromatography using 100–200 mesh silica

gel.

removed under reduced pressure. Water (30 mL) was added to

the pale yellow solid and extracted with CHCl₃(330 mL). The

combined organic layers, after drying over anhydrous Na₂SO₄,

were evaporated under reduced pressure to obtain 2 as a pale

brown solid. It was further puriﬁed by passing through a column

(100–200 mesh silica gel) with chloroform and methanol mixture

(98 : 2 v/v). Yield: 0.43 g (76%); mixed mp: 112–114 ^◦C; ESI-MS,

m/z⁺(%): 571 [2]⁺(45%); ¹H-NMR (400 MHz, CDCl₃, 25 C, TMS,

d): 7.80 (br s, 1H), 7.35 (t, J = 3.99 Hz, 1H), 7.22 (br s, 1H), 7.0

(br s, 1H), 6.34 (d, J = 7.99 Hz, 2H), 6.30 (s, 2H), 6.20 (d, J =

3.99 Hz, 2H), 3.25 (d, J = 3.99 Hz, 8H), 2.75 (br s, 2H), 2.61 (br s,

2H), 2.51 (br s, 8H), 2.30 (br s, 4H), 1.08 (t, J = 7.99 Hz, 12H);

¹³C-NMR (100 MHz, CDCl₃, 25 C, TMS, d):168.51, 153.13(d),

148.64, 132.27(d), 130.98(t), 128.55(d), 127.94, 123.65, 122.57,

107.95, 105.36(t), 97.56, 64.77(d), 50.94, 48.47, 48.05, 47.47, 44.20,

40.73, 39.85, 12.44; anal. calcd for C₃₄H₄₆N₆O₂: C, 71.55, H, 8.12,

N, 14.72%; found: C, 71.46, H, 8.21, N 14.63%.

The compounds were characterized by elemental analyses, ¹H-

1

NMR, ¹³C-NMR and mass (ESI) spectroscopy. H-NMR and

¹³C-NMR spectra were recorded on a JEOL JNM-AL400 FT

V4.0 AL 400 (400 MHz and 100 MHz, respectively) instrument

in CDCl₃with Me₄Si as the internal standard. Electrospray mass

(ESI) spectral data were recorded on a MICROMASS QUATTRO

II triple quadruple mass spectrometer. The dissolved samples

of the compounds in suitable solvents were introduced into the

ESI source through a syringe pump at the rate of 5 mL min^-1,

ESI capillary was set at 3.5 kV with 40 V cone voltage and

the spectra were recorded at 6 s scans. The DART-MS were

recorded on a JEOL-AccuTOF JMS-T100LC mass spectrometer

having a DART source. Melting points were determined with a

melting point apparatus by PERFIT, India and were uncorrected.

Elemental analyses were done in an Elementar Vario EL III Carlo

Erba 1108 elemental analyzer. UV-visible spectra were recorded

on a Perkin Elmer Lambda 650 UV/VIS spectrophotometer at

298 K in 10^-4–10^-6M concentration. Steady-state ﬂuorescence

spectra were obtained with a Fluoromax 4P spectroﬂuorometer at

298 K. Fluorescence quantum yields were determined as per the

procedure reported earlier from our laboratory.²¹All the samples

were excited at 500 nm only to compare their uncorrected spectra

with that of rhodamine-G in ethanol²²as standard (f_F= 0.95)

for determination of ﬂuorescence quantum yields. The detection

limit of the metal ion was determined by a procedure reported in

literature.²³

Compound 3. To a stirring solution of rhodamine B hydrochlo-

ride (0.478 g, 1 mmol) in EtOH (50 mL), tetraethylene pentamine

(1 mL, 5.25 mmol, excess) was added and the reaction mixture

was heated to reﬂux for 12 h with constant stirring until the colour

of the solution changes from pink to orange. The solution was

then cooled to room temperature, ﬁltered and the solvent was

removed under reduced pressure. CHCl₃(30 mL) was added to

the light brown semi-solid and washed thoroughly with water

(330 mL). The organic layer, after drying over anhydrous Na₂SO₄,

was evaporated under reduced pressure to obtain a pale brown

semisolid as desired product. It was further puriﬁed by passing

through a column (100–200 mesh silica gel) with chloroform and

methanol (97 : 3 v/v) as eluent. Yield: 0.44 g (72%); ESI-MS, m/z⁺

(%): 614 [3]⁺(53%); ¹H-NMR (400 MHz, CDCl₃, 25 C, TMS, d):

7.79 (br s, 1H), 7.38 (br s, 1H), 7.00 (br s, 1H), 6.33 (s, 2H), 6.30

(br s, 1H), 6.21 (br s, 2H), 6.03 (br s, 2H), 3.26 (br s, 13H), 3.02

(br s, 2H), 2.72 (br s, 2H), 2.64 (br s, 2H), 2.53 (br s, 10H), 1.08

(br s, 12H); ¹³C-NMR (100 MHz, CDCl₃, 25 C, TMS, d):168.49,

153.56, 153.31, 148.75 (d), 132.44, 131.13, 129.01, 128.76, 128.05,

123.81, 122.75, 108.08, 105.53, 97.71(d), 65.01, 60.96, 57.80, 53.15

(d), 50.57, 47.65, 45.81, 44.37, 40.02, 38.64, 12.60; anal. calcd for

C₃₆H₅₁N₇O₂: C, 70.44, H, 8.37, N, 15.97%; found: C, 70.29, H,

8.58, N 15.82%.

The complex stability constants (K_s) were determined from the

change in absorbance or ﬂuorescence resulting from the titration

of dilute solutions (~10^-4–10^-6M) of the probes against metal

ion solution following the Benesi–Hildebrand method⁷for a

complexation of 1 : n probe : metal stoichiometry as depicted in

the eqn(1)

1

=

+

(1)

2

X −X ₀K_S(X _max−X ₀)[M(II)]₀

X _max−X ₀

where X₀is the absorbance or ﬂuorescence of the probes at

a particular wavelength, X is the absorbance or ﬂuorescence

intensity obtained with added [M(II)], A_maxis the absorbance or

ﬂuorescence obtained with excess amount of metal ion added

and [M(II)]_ois the concentration of metal ion added. The double

reciprocal plot of absorption or ﬂuorescence spectral change

(1/X - X₀) as a function of added metal ion concentration (1/[M]ⁿ)

results in a linear regression and its slope determines K_s(M^-n).

Compound 4. To a completely dissolved solution of 1 (0.5 g,

1.03 mmol) in EtOH (40 mL), 2-hydroxy benzaldehyde (0.2 mL,

1.9 mmol) was added and allowed to react at room temperature for

24 h. with constant stirring. NaBH₄was the added to the reaction

mixture to reduce the Schiff base thus formed. It was further

allowed to react for 2 h at room temperature with constant stirring

to ensure the complete reduction. The solvent was evaporated to

dryness under reduced pressure. Water (30 mL) was added to

the pale yellow solid and extracted with CHCl₃(330 mL). The

combined organic layers, after drying over anhydrous Na₂SO₄,

were evaporated under reduced pressure to obtain the crude

product, which was further puriﬁed by column chromatography

(CHCl₃–MeOH = 95 : 5 v/v) to obtain 4 as a pale yellow solid.

Yield: 0.48 g (79%); mixed mp: 82–85 C; DART-MS, m/z⁺(%):

Synthesis

Compound 1, aminoethyl rhodamine. This compound was

synthesized following the procedure reported^6i,8k,21earlier.

Compound 2. To a stirring solution of rhodamine B hydrochlo-

ride (0.478 g, 1 mmol) in EtOH (50 mL), triethylene tetramine

(0.7 mL, 5 mmol, excess) in EtOH (20 mL) was added dropwise

and allowed to react under reﬂux condition for 15 h until the

colour of the solution changes from pink to orange. The solution

was then cooled to room temperature, ﬁltered and the solvent was

1

591.3 [4 + 1]⁺(100%); H-NMR (400 MHz, CDCl₃, 25 C, TMS,

d): 7.91 (d, J = 7.99 Hz, 1H), 7.44 (t, J = 3.99 Hz, 1H), 7.10

(m, 2H), 6.86 (d, J = 3.99 Hz, 1H), 6.79 (d, J = 7.99 Hz, 1H),

6.69 (t, J = 7.99 Hz, 2H), 6.40 (d, J = 7.99 Hz, 2H), 6.36

4478 | Org. Biomol. Chem., 2011, 9, 4467–4480

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(d, J = 3.99 Hz, 2H), 6.22 (d, J = 7.99 Hz, 2H), 3.80 (s, 2H),

3.28–3.34 (br s, 10H), 2.51 (s, 2H), 1.15 (br s, 12H); ¹³C-NMR

(100 MHz, CDCl₃, 25 C, TMS, d): 169.92, 159.23, 153.42, 149.58,

134.24, 132.29, 129.13, 129.01, 124.84, 124.26, 119.62, 116.73,

108.09, 106.11, 97.54, 65.62, 52.48, 47.71, 44.48, 39.18, 12.53; anal.

calcd for C₃₇H₄₂N₄O₃: C, 75.23, H, 7.17, N, 9.48%; found: C, 73.09,

H, 7.27, N 9.32% [C₃₇H₄₂N₄O₃·H₂O].

52.38, 47.43, 44.57, 39.79, 29.89, 12.80; anal. calcd for C₄₁H₅₁N₅O₄:

C, 72.64, H, 7.58, N, 10.33%; found: C, 72.55, H, 7.69, N 10.26%.

Acknowledgements

BPB wishes to thank the Department of Science and Technology,

New Delhi, for the ﬁnancial support (SR/FTP/CS-138/2006) for

this work. The authors sincerely wish to thank the Director, IMMT

for a CSIR-diamond jubilee research internship to AP.

Compound 5. To a stirring ethanolic (40 mL) solution of

1 (0.5 g, 1.03 mmol), 4-(diethylamino)benzaldehyde (0.2 g,

1.13 mmol) was added and heated to reﬂux for 10 h. The

reaction mixture was cooled to room temperature and the Schiff

base thus formed was reduced with NaBH₄. After completion

of the reduction, the solvent was evaporated to dryness under

reduced pressure. Water (30 mL) was added and extracted with

CHCl₃(330 mL). The combined organic layers, after drying

over anhydrous Na₂SO₄, were evaporated under reduced pressure

to obtain a pale yellow semi-solid product, which was further

puriﬁed by column chromatography (CHCl₃–MeOH = 97 : 3 v/v)

to obtain 5 as a pale yellow solid. Yield: 0.49 g (74%); mp: 76

C; HR-MS(ESI⁺), m/z⁺(%): 646.5386 [5+1]⁺(59%), calculated

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for C₄₁H₅₁N₅O₂= 645.4043; H-NMR (300 MHz, CDCl₃, 25 C,

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Rhodamine-based probes for metal ion-induced chromo-/fluorogenic dual signaling and their selectivity towards Hg(ii) ion

DOI: 10.1039/c0ob01179g

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