Selective fluorescence sensing of Cu(ii) and Zn(ii) using a new Schiff base-derived model compound:...

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http://dx.doi.org/10.1039/c3an00155e

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Selective fluorescence sensing of Cu(II) and Zn(II) using a Schiff base-derived new

model compound: Naked eye detection and spectral deciphering of the mechanism

of sensory action

Aniruddha Ganguly†, Bijan Kumar Paul

†,§, Soumen Ghosh, Samiran Kar

‡, Nikhil

Guchhait*

Department of Chemistry, University of Calcutta, 92, A.P.C. Road, Kolkata-700009, India

*Corresponding author. Tel.: +913323508386; fax: +913323519755.

E-mail address: [email protected] (N. Guchhait).

† Equal contribution.

§Present address: Department of Chemistry and Biochemistry, University of Colorado,

Boulder, Colorado 80309-0215.

‡Present address: Research and Development section, Navin Fluorine International

Limited, Dewas Area, Madhya Pradesh, India

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View Article OnlineDOI: 10.1039/C3AN00155E


2

Abstract

A new Schiff base compound 2-((benzylimino)-methyl)-naphthalen-1-ol (2BIMN1O) has

been synthesized and characterized by 1H NMR,

13C NMR, DEPT, FT-IR and Mass

spectroscopic techniques. A significantly low fluorescence yield of the compound has

been rationalized in connection with photo-induced electron transfer (PET) from the

imine receptor moiety to the naphthalene fluorophore unit. Subsequently, an evaluation

for the transition metal ion-induced modification of the fluorophore-receptor

communication reveals the promising prospect of the title compound to function as a

fluorosensor for Cu2+

and Zn2+

ions selectively, through remarkable fluorescence

enhancement. While perturbation of PET process in 2BIMN1O has been argued to be the

responsible mechanism behind the fluorescence enhancement, the selectivity for these

two metal ions has been interpreted on the grounds of an appreciably strong binding

interaction. Particularly notable aspects regarding the chemosensory activity of the

compound is its ability to detect the aforesaid transition metal ions down to the level of

micromolar concentration (detection limit being 0.82 and 0.35 µM respectively), along

with a simple and efficient synthetic procedure. Also the spectral modulation of

2BIMN1O in the presence of the transition metal ions paves way for construction of a

calibration curve in the context of its fluorescence signaling potential.

Keywords: Schiff base, PET, Fluorophore-receptor communication, Fluorosensor,

Transition metal ions.

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Introduction

Molecules capable of performing light-induced logic operations have drawn

extensive attention since years owing to their viable application in the development of

molecular switches and information storage devices at the molecular level [1-7].

Moreover, light induced fluorescence switching in such systems paves way for their

potential use as signaling systems capable of responding in the presence of a foreign

analyte [8,9]. In fact, systems capable of sensing guest molecules, ions, protons, etc. have

received a great deal of current research interest because of their potential caliber to form

a promising avenue for applicative research. In general, these types of chemosensors

consist of a signaling part which is typically a fluorophore, and a recognition part in close

proximity which is usually a Lewis base site (that can coordinate to a metal ion, such as

amines/imines). In the absence of the guest metal ion, the fluorescence intensity of the

fluorophore is generally very low (photoinduced electron transfer (PET) is the most

commonly encountered mechanism). However, in the presence of metal ion or proton the

lone pair of the donor amine group is engaged, which leads to a significant enhancement

in the fluorescence intensity of the fluorophore by turning off (or hampering) the PET

process and thereby constitutes the basis for actuating mechanism of functioning as a

sensor [10-15]. Further, an architecture based on fluorescence signaling promises to have

additional edges such as specificity, sensitivity, real time monitoring with fast response

time and cost-effectiveness [16–19].

On the other hand, sensing of transition metal ions, especially Cu(II) and Zn(II)

has long been realized as an important goal in chemistry and biology with a view to their

important biological and environmental roles [1,9,20–22]. The close proximity of the


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values of ionic radius and charge density as well as similar coordinating ability of both

the aforesaid ions, often leads to simultaneous sensing of both [23-26]. Keeping these in

mind, the present work has been framed which describes the synthesis and photophysical

characterization of a Schiff base-derived new model compound viz., 2-((benzylimino)-

methyl)-naphthalen-1-ol (2BIMN1O, Scheme 1). Though Schiff base compounds are

often found to be adamant towards tuning of spectroscopic properties with simple

governing factors like variation of medium polarity/pH, co-ordination towards suitable

metal ions, etc. [27-30], the title compound displays remarkable sensitivity towards a few

transition metal ions. Transition metal ion-induced modification of its photophysics has

subsequently been argued to be a promising possibility for the development of a new

fluorescence chemosensor for Cu(II) and Zn(II). A detailed assessment over the range of

first row transition metal ions and alkaline earth cations reveals the selectivity of

2BIMN1O for Cu(II) and Zn(II). The mechanism of the sensory action has been

rationalized in connection with PET mechanism. At the same time, the structural

simplicity as well as the remarkable simplicity of its synthetic procedure is noteworthy.

Experimental

Synthesis of 2BIMN1O and materials

The detailed synthetic procedure and purification of the Schiff base 2BIMN1O is

described in the supplementary material.

Spectroscopic grade acetonitrile (ACN) was purchased from Spectrochem, India

and used after proper purification according to the standard procedures. The solvent was

found free from impurities and appeared transparent in the spectral region of interest. The

purity was also verified by recording the emission spectra in the studied spectral region.

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The metal salts used in the present investigation are as follows: Zn(ClO4)2(H2O)6,

CrCl3(H2O)6, Cu(ClO4)2(H2O)6, Fe(ClO4)2(H2O)6, Ni(ClO4)2(H2O)6, Co(ClO4)2(H2O)6.

The metal salts were procured locally and used as received. Perchlorate salts have been

preferred because of the low coordinating ability of the anionic counterpart.

All other solvents and reagents such as dioxane (DOX), methanol (MeOH), hexane

(HEX), n-butanol (BuOH), tetrahydrofuran (THF) were of spectroscopic grade

(Spectrochem, India) and were used after proper distillation. Trifluoroacetic acid (TFA),

and sodium hydroxide (NaOH) from E-Merck were used as received. Triple distilled

water was used for preparing aqueous solutions. CDCl3 and d6-DMSO for NMR

experiments were used as received from Sigma-Aldrich, USA.

Instrumental details and methods

The absorption and emission spectra were recorded on Hitachi UV–Vis U-3501

spectrophotometer and Perkin-Elmer LS55 fluorimeter, respectively. In all measurements

the sample concentration was maintained at 2× 10–6

M in order to avoid aggregation and

reabsorption effects. All absorption and emission spectral measurements were performed

with proper background corrections and with freshly prepared solutions only.

Fluorescence lifetimes were measured by the method of Time Correlated Single-

Photon Counting (TCSPC) using a HORIBA Jobin Yvon Fluorocube-01-NL fluorescence

lifetime spectrometer. The sample was excited using a laser diode at 375 nm and the

signals were collected at the magic angle of 54.7° to eliminate any considerable

contribution from fluorescence anisotropy decay [9,16]. The typical time resolution of

our experimental setup is ~ 100 ps. The decays were deconvoluted using DAS-6 decay

analysis software. The acceptability of the fits was judged by χ2 criteria and visual


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6

inspection of the residuals of the fitted function to the data. Mean (average) fluorescence

lifetimes were calculated using the following equation [18,19]:

∑

∑=

ii

ii

avτα

τατ

2 (1)

in which αi is the pre-exponential factor corresponding to the ith

decay time constant, τi.

Fluorescence quantum yield (Фf) was determined using recrystallized β-naphthol

(Фf = 0.23 in MCH) as the reference [9] using the following equation [9,30]:

2

2

)(

)(

R

S

S

R

R

S

RSAbs

Abs

A

A

η

η××Φ=Φ (2)

where “A” terms denote the integrated area under the fluorescence curve, ‘‘Abs’’ denote

absorbance, η the refractive index of the medium and Ф, the fluorescence quantum yield.

Subscripts ‘‘S’’ and ‘‘R’’ stand for denoting respective parameters for the studied sample

and reference, respectively.

The cyclic voltammetric measurements were performed with a Sycopel model

AEW21820F/L instrument. The experiments were carried out in a standard three-

component cell equipped with a glassy carbon working electrode, a Pt wire as an

auxiliary electrode and Ag/AgCl reference electrode. The cyclic voltagrams were

acquired in a N2-bubbled DMSO solution containing ~ 0.2 M tetraethylammonium

perchlorate (TEAP) as supporting electrolyte. The scan speed was 50 mV/s.

All NMR spectra were recorded with TMS as internal standard on Bruker, AV

300 Supercon Digital NMR system. The solvents used in NMR spectral study are

mentioned in respective parts of the discussions.

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Mass spectroscopic analysis of the ligand was performed in a QTOF Micro

YA263 ESI-TOF mass spectrometer using dichloromethane as solvent.

Results and discussions

Absorption study

The absorption spectra of 2BIMN1O recorded in various solvents are displayed in

Figure 1a, with the corresponding spectroscopic parameters being summarized in Table

1. The compound exhibits three absorption bands viz. at ~ 270 nm, ~ 360 nm and ~ 420

nm wavelength regions in non-polar solvents, whereas in polar solvents the ~ 360 nm

band is obscured. For assignments of the spectral bands of 2BIMN1O we consider the

anticipation with the spectral properties of the parent compound 1-hydroxy-2-

naphthaldehyde (HN12) [31] and another structurally related compound 2-[(Naphthalen-

1-ylmethylimino)-methyl]-naphthalen-1-ol (2N1YMN1O) [9]. HN12 exhibits two

prominent absorption bands at ~ 300 nm and ~ 370 nm which are assigned to the open

form and intramolecularly hydrogen bonded closed form of HN12, respectively [31]. As

obvious, the closed form absorbs at lower energy region (~ 370 nm) owing to its greater

degree of stability due to the presence of the intramolecular hydrogen bond. In direct

correspondence to the spectral features of the parent molecule (HN12) [31] and other

structurally similar systems [9,32–35], the ~ 360 nm absorption band for 2BIMN1O

(Figure 1a) has been ascribed to the intramolecularly hydrogen bonded lowest energy

ground state conformer, i.e. conformer I in Scheme 1, while the higher energy band at ~

270 nm in turn appears to reflect the signature of the non-hydrogen bonded open

conformer (conformer II in Scheme 1). However, as mentioned earlier, the band at ~ 360

nm is seen to be obscured on moving from non-polar to polar solvents (Figure 1a) which


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may result from increasing degree of solute-solvent interaction in polar/polar protic

media [9,31].

Though the assignment of the lowest energy absorption band at ~ 420 nm (Figure

1a) remains a matter of debate, a direct comparison with spectroscopic properties of

structurally related compounds [9,32–35] tends to attribute the band to the zwitterionic

species of 2BIMN1O (Scheme 1). This observation also seems satisfactory on the ground

of the fact that the zwitterionic species is capable of undergoing substantial stabilization

through delocalization of its formal charges and thus absorbs in the higher wavelength

region. Additionally, it is interesting to note that the absorption band for the zwitterionic

species gains a greater intensity compared to that for the closed form (i.e. conformer I in

Scheme 1) in polar solvents, while the reverse pattern is noted in non-polar solvent (cf.

Figure 1a). Such an observation is not unlikely since the charge separated zwitterionic

species is expected to be more stabilized and hence the corresponding conformer is

expected to be more populated in polar solvents than in non-polar solvents. Also, it is

pertinent to mention that the absorption bands of 2BIMN1O corresponding to the open

(λabs ~ 270 nm) and closed (λabs ~ 360 nm) conformers are relatively blue shifted

compared to those of the parent compound HN12 (λabs ~ 300 nm for the open form and

λabs ~ 370 nm for the closed form [31]). This observation is reasonable since the

intramolecular hydrogen bonding in 2BIMN1O involving weaker acceptor nitrogen is

expected to be weaker than that in HN12 involving comparatively stronger acceptor

oxygen as is consistent with earlier reports also [9].

Emission study

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The emission spectra of 2BIMN1O following photoexcitation at both λex = 360

nm and 420 nm have been recorded in various solvents and the representative profiles

obtained with λex = 420 nm are illustrated in Figure 1b, while the corresponding

spectroscopic parameters are collected in Table 1. Excitation at either of the two bands is

found to produce identical spectral profile with the emission maxima at λem ~ 480 nm,

which is thus subsequently ascribed to the emission from the locally excited state of the

zwitterionic species (Scheme 1). Such observation also suggests that the intramolecular

hydrogen bonded closed form of 2BIMN1O undergoes a transformation to the

zwitterionic form on the excited state potential energy surface prior to emission [9].

Within the range of solvents assayed the emission maxima position is found to exhibit no

noteworthy dependence on solvent polarity (Figure 1b and Table 1). However, it is likely

to consider the presence of more than one species (e.g., the closed conformer I, the open

form (conformer II), the zwitterionic species, etc.) under the broad emission spectral

profile [32–35]. The excitation spectra (Figure S3 in the supplementary information) are

found to exhibit three bands at ~ 300 nm, ~ 380 nm and at ~ 430 nm, i.e. the excitation

spectra corresponds well with the absorption spectra.

The calculated values of quantum yields in different solvents are summarized in

Table 1 and are found to be considerably lower as compared to those of its parent

molecule HN12 [31], which at a glance suggests that the title compound is an inefficient

fluorophore compared to its constituent compound.

Effect of acid and base on the spectral properties of 2BIMN1O

Modification of spectral properties of a fluorophore in the presence of external

agents often serves as a sensitive indicator for a more critical assessment of its


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photophysics. Here, we have exploited this technique for the compound under

investigation using acid and base-induced modulation of its spectral properties. Figure

S4a (see supplementary information) exhibits the effect of addition of base NaOH to an

acetonitrile solution of 2BIMN1O on the absorption profile resulting in an appreciable

blue shift of the absorption maxima from 420 nm to ~ 400 nm. In analogy to the effect of

base on the absorption spectral features of the parent molecule HN12 [31], the blue

shifted absorption band (~ 400 nm) seems attributable to the corresponding anion of

conformer I (Scheme 1).

On the emission profile, addition of base results in sharpening of the emission

band with a little blue shift of the emission maxima (~ 5 nm) (Figure S4b in the

supplementary information), which is said to be due to the anion of the closed form

(conformer I). Similar observation was also noted for the parent molecule HN12 [31].

The corresponding excitation spectra (not shown) juxtapose well with the absorption

spectra, which thus substantiates our previous interpretation.

On the other hand, Figure 2a exhibits the effect of addition of trifluoroacetic acid

(TFA) to an acetonitrile solution of the compound on the absorption profile, where a

slight blue shift from 420 nm to ~ 410 nm is observed, which might be the outcome of

the formation of a protonated species. However, on the emission profile, addition of acid

results in a remarkable enhancement of the intensity of the emission band with no

noteworthy shift of the emission maxima (Figure 2b) whereas the corresponding

excitation spectra (Inset of Figure 2b) do not corroborate well with the corresponding

absorption spectra which infers the occurrence of some excited state affair. The

underlying interpretation has been argued in detail in the forthcoming sections.

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Transition metal ion-induced modulation of photophysics of 2BIMN1O

Steady-state spectral properties

In course of monitoring the effect of transition metal ions on the steady-state

spectral properties of 2BIMN1O, absorption spectra are found to remain almost silent to

the presence of the transition metal ions (vide Figure S5 in the supplementary

information). Only with Cu2+

, an exception could be noted (Figure 3a). Gradual addition

of Cu2+

ion to an ACN solution of the Schiff base (2BIMN1O) is found to accompany a

decrease in absorbance at ~ 420 nm with a simultaneous emergence of a new band at ~

570 nm which is supposed to be the signature of a ligand to metal charge transfer

(LMCT). This observation leads to an apparent formulation of a ratiometric sensing

scheme for Cu2+

based on its response on the absorption profile in terms of plotting the

ratio of absorbance at 570 nm and 420 nm (A570/A420) as a function of Cu2+

concentration, as depicted in the inset of Figure 3a. Such ratiometric sensing via simple

absorption measurement gives an additional impetus to this study, as such a scheme is

claimed to be more fruitful as an actuating tool due to its independence from a number of

common experimental and instrumental artifacts e.g., fluctuations in probe concentration

and excitation source intensity, light scattering and so forth, and hence the method does

not call for frequent calibration during estimation of the analyte [9,16,36].

Interestingly, on the emission profile, transition metal ion-induced changes are

more fascinating to note. Only Zn2+

and Cu2+

ions are found to impart a significant

modulation to the emission characteristics of 2BIMN1O, while others fail to impart any

(vide Figure 3c. For a clearer detail see Figure S6 in the supplementary information). It is

interesting to note that Zn2+

failed to convey any significant modification in the


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absorption profile of 2BIMN1O, as evident from Figure 3b, and this significant difference

of absorption spectral response of the sensor (2BIMN1O) to the presence of the two

transition metal ions Cu2+

and Zn2+

can serve as an actuating index to discriminate

between these two transition metal ions spectroscopically. The remarkable emission

intensity enhancement of the studied compound in presence of increasing concentration

of Cu2+

and Zn2+

is presented in Figure 4 (a and b respectively). The excitation spectra

presented in the insets of Figure 4 exhibit a notable disagreement with the corresponding

absorption spectra in presence of the two metal ions (vide Figure 3a and 3b), indicating

that the transition metal ion-induced modification of the emission characteristics of

2BIMN1O corresponds to an excited state event.

Figure 4c depicts the photograph showing visual colour changes of 2BIMN1O in

the presence of Zn2+

and Cu2+

ions. The solution of 2BIMN1O in acetonitrile is found to

show no significant change of colour in the presence of Zn2+

ion, whereas the presence of

Cu2+

ion (even in the presence of other metal ions) induces a prominent colour change

(from yellow to purple). This result explores the potential of the studied compound

2BIMN1O to discriminate between the presence of Zn2+

and Cu2+

ions in a solution via

visual inspection only.

Intentiometric chemosensory response of 2BIMN1O

The spectacular modification of the emission profile of the title compound with

Zn2+

and Cu2+

subsequently leads to the concept of utilizing such modulations of spectral

response of 2BIMN1O as a credible tool for exploiting the sensory action of the

compound. The chemosensory emission spectral response of 2BIMN1O to the series of

first row transition metal ions is constructed on the basis of variation of relative


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fluorescence intensity of the compound (i.e. I/I0 where I0 and I are fluorescence intensities

in the absence and presence of a defined amount of the transition metal ion, respectively)

as a function of concentration of the metal ions and is illustrated in Figure 5a. The figure

clearly advocates for the selective chemosensory response of the Schiff base to Cu2+

and

Zn2+

ions only, to different extent though.

Apart from such chemosensory response, the utilization of the intensity of the

emission spectral response has been further extended to the construction of calibration

curves for sensing of Cu2+

and Zn2+

ions. The variation of relative fluorescence intensity

(I/I0) of the title compound over a range of metal ion concentration exhibits a linear

regression (Figure 5b and c) which could be used to estimate the aforesaid metal ions in

an unknown sample utilizing the chemosensory property of the compound. It is also

worth noting at this stage that the proposed sensor is able to sense transition metal ions

(selectively Cu2+

and Zn2+

) up to a sufficiently low concentration, in our case in the

micromolar order, the corresponding detection limit being 0.82 and 0.35 µM respectively

for Cu2+

and Zn2+

ions (limit of quantification being 2.5 and 1.06 µM respectively for

Cu2+

and Zn2+

) as calculated using the reported procedure [36].

Mechanism of sensory action

As previously stated, the mechanism of sensory action of the studied compound,

as well as the effect of protic acid has been attempted to be rationalized on the basis of

PET mechanism, which is a commonly exploited strategy for metal sensing fluorosensors

showing intensity enhancement in the presence of protic acid and metal ion(s) [16,18,19].

In order to substantiate the postulate, the following series of experiments and arguments


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have been undertaken. Firstly, we focus on establishing the efficiency of the studied

molecular architecture to function as a suitable PET system.

A search for the extent of electron deficiency of the fluorophore unit in 2BIMN1O has

been employed as the actuating index for this purpose [16,18,19]. A direct evidence of

the electron deficient nature of the fluorophore unit comes from a scrutiny of the quantum

yield values which are considerably less than that of the parent compound HN12 [31]

(see Table 1).

Cyclic voltammetric measurements have been performed to assess the electron deficiency

of the fluorophore unit in 2BIMN1O. The Ered(fluoro) appears at – 0.649 V (irreversible)

while the oxidation potential is observed at Eox(fluoro) = 0.711 V (irreversible). Now,

with the measured reduction potential coupled with the spectral data, a quantitative

estimate of the thermodynamic driving force for PET process in 2BIMN1O can be

achieved in terms of the free energy change (∆G) of the process [9,16,19]. The ∆G value

has been calculated using the equation [9]:

[ ] 0,0)()(06.23 EfluoroErecepEG redox −−=∆ (3)

where, Eox(recep) and Ered(fluoro) represent the oxidation potential of the receptor moiety

and the reduction potential of the fluorophore, respectively whereas E0,0 represents the

energy of the fluorescent state [9,16]. The value of E0,0 used in the calculation has been

estimated from location of the first peak position in the fluorescence spectrum (λem ~ 480

nm). Using Eox(recep) = 0.49 V [9,16] (for triethylamine, literature SCE value corrected

for Ag/AgCl electrode by substracting 0.27 V), ∆G value is estimated to be – 33.32

kcal/mol, which dictates the thermodynamic feasibility of the PET process in the studied

molecular system.


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Initially it is surprising to note a contrasting behaviour of the transition metal ions

(Zn2+

and Cu2+

) resulting in fluorescence enhancement, which are usually known for their

quenching actions. Herein, we make an effort to answer this riddle by taking into account

the individual interactions operative in the presence and absence of the guest and by

comparing the effects for 2BIMN1O and its parent compound, HN12.

The constituent fluorophore HN12 is found to undergo usual quenching as induced by the

transition metal ions Zn2+

and Cu2+

. The extent of quenching has been quantitatively

estimated on the Stern–Volmer equation [9,16]:

][1][10

0 QkQKI

IqSVτ+=+=

(4)

in which KSV is the Stern–Volmer quenching constant and kq is the bimolecular

quenching constant. I0 is the original fluorescence intensity, I is the quenched intensity, τ0

is the excited state fluorescence lifetime of the unquenched fluorophore and [Q] is the

molar concentration of the quencher [16,18,19]. Cu2+

ion (with one unpaired electron) is

found to be a more efficient quencher for HN12 than Zn2+

ion, with KSV = 7.75 × 104

M−1

for Cu2+

and 1.13 × 103

M−1

for Zn2+

(correspondingly the bimolecular quenching rate

constant values are kq = 7.61 × 1013

M−1

s−1

for Cu2+

and 1.11 × 1012

M−1

s−1

for Zn2+

[9]

and τ0 (= 1.019 ns) is the fluorescence lifetime of HN12 in the absence of quencher [31]).

Thus interaction between the fluorophore (naphthalene unit of 2BIMN1O) and transition

metal ions (Cu2+

and Zn2+

) results in fluorescence quenching, whereas that between the

receptor (imine moiety of 2BIMN1O) and the metal ions leads to fluorescence

enhancement. Hence it is presumed that it is the net result of these two opposing

interactions that plays the governing role behind the resultant consequence (quenching or


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enhancement) of interaction of 2BIMN1O with the transition metal ions (Cu2+

and Zn2+

).

Since the fluorescence yield of 2BIMN1O is very low (vide Table 1) in the absence of the

guest which in turn reflects the presence of a communication (PET) between the

flurophore and the receptor resulting in decreasing the fluorescence yield, it is not

unlikely that the interaction with the guest ion (H+ or transition metal ions) will lead to

modulation of the fluorophore–receptor communication in a manner that will facilitate

fluorescence enhancement by impairing the aforesaid communication. Such a proposition

is also in consensus with available literature reports [16,18,19].

Time-resolved fluorescence decay behaviour

Fluorescence lifetime measurements often serve as a sensitive indicator of the

local environment of a fluorophore, and it is responsive to excited state affairs

[16,18,37,38]. Thus in order to attain a deeper insight into the photophysics of 2BIMN1O

and its modulation in the presence of protic acid as well as transition metal ions,

fluorescence lifetimes has been monitored under differential circumstances. Figure 6

represents the time resolved fluorescence decay profile of 2BIMN1O under different

experimental conditions and the relevant data are compiled in Table 2. The decay

behaviour of the bare fluorophore, its metal complex or the protonated fluorophore is

found to be complicated and is best fitted to triexponential functions. The decay

characteristics have been monitored at λex = 375 nm in all solvents studied. It is

interesting to note that the complicated triexponential decay pattern of 2BIMN1O reflects

the presence of a short-lived major component (~ 44 ps, ~ 96% in ACN), which seems to

be a representative of the lifetime of PET-quenched fluorophore [16,18,19,37]. The long-

lived minor components may in principle originate from the products of electron transfer


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reactions such as an exciplex. However, the absence of any fluorescence peak of

2BIMN1O at longer wavelengths and the high dielectric constant of the medium (ACN, ε

= 37.5; where exciplexes are unstable) discards this possibility [16,18]. Further

reinforcing support in favour of negation of the possibility of exciplex formation comes

from a similar fluorescence decay pattern observed in non-polar solvent n-hexane (Figure

8 and Table 2). The triexponential nature of the fluorescence decay can be explained by a

through space electron transfer mechanism involving the overlap of the lone pair orbital

of the distal nitrogen and the π-orbitals of the ring. The long-lived components arise from

species in which the donor and acceptor groups are well separated, while the predominant

short-lived component represents the quenched fluorophore having the donor imine in

close proximity of the acceptor. [16,18,19,37].

The overall decay behaviour of 2BIMN1O in the presence of the two transition

metal ions and protic acid undergoes a noteworthy modification (Figure 7 and Table 2)

though the complicacy of the decay persists and it becomes even more cumbersome to

assign specific mechanistic model to each decay component. Thus without paying too

much importance on individual decay components, we concentrated on the average

(mean) fluorescence lifetimes (τav) (vide Table 2) and the observations are found to agree

with the steady-state findings. As stated earlier, in the presence of both metal ions (Cu2+

and Zn2+

) and H+ ion, the basic complicacy of the decay profile is found to persist,

however an overall increase of the average fluorescence lifetime in the presence of the

aforesaid foreign ions is distinctly detectable (Table 2). A more critical analysis of the

data has been undertaken with a view to mark out the contributions from radiative (kr)

and nonradiative (knr) decay rate constants according to the following equations [16]:


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f

f

rkΦ

⟩⟨=τ

(5)

1−⟩⟨=+ fnrr kk τ (6)

The calculated results are summarized in Table 3. A careful examination of the data

(Table 3) reveals that interaction of 2BIMN1O with the ions results in only a nominal

depletion of nonradiative decay rates, while a marked enhancement of the radiative decay

rates, which appears to play the essential role behind the observed fluorescence

enhancement. Though it might be argued that a sort of binding interaction of 2BIMN1O

with the transition metal ion would impart rigidity to the entire molecular framework

whereby reducing the rotation/vibrational degrees of freedom and hence would contribute

towards the observed fluorescence enhancement, this explanation does not seem to be

adequate for explaining the high magnitude of the observed enhancement.

At this stage it is pertinent to compare between the efficiency of Cu2+

and Zn2+

to

perturb the PET mechanism resulting in observed emission enhancement of the studied

compound. From the time-resolved fluorescence data for 2BIMN1O in various

environments it is evident that both the metal ions (Cu2+

and Zn2+

) as well as the protic

acid (H+ ion) induce perturbation of PET in the molecule which is evident from the

increment of lifetime of the short-lived major component of fluorescence decay along

with the noticeable decrement of the associated amplitude which is the representative of

the PET-quenched fluorophore. However, the presence of Zn2+

is found to impart a much

greater perturbation of the PET mechanism (as can be described by greater extent of

lowering of the amplitude for the short-lived decay component along with increase of the

corresponding lifetime to a greater extent (Table 2)). This comparison is further


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substantiated from the observation reported in Figure 5 (b and c respectively) which

shows that in the presence of the same concentration (25.0 µM) of the metal ions (Cu2+

or

Zn2+

) fluorescence enhancement in 2BIMN1O occurs to a significantly greater extent for

Zn2+

. This could be linked to the greater binding efficiency of the Schiff base with Zn2+

than Cu2+

, as is elaborated in the forthcoming section.

Strength of 2BIMN1O – metal ion interaction and the probable functional groups

involved in the interaction

Herein, we endeavor to estimate the strength of the 2BIMN1O–metal ion

interaction assuming a complexation reaction between the two moieties involved. An

assessment of the emission intensity data on the modified Benesi-Hildebrand equation

has been exploited for the said purpose. A detailed discussion on Benesi-Hildebrand

equation is avoided since it is routine and abundantly available in the literature [38]. We

thus start with the equation [16,38,39]:

( ) ( ) ( ) [ ]+∞∞−

+−

=− nMKIIIIII

000

111 (7)

in which I0, I and I∞ are the emission intensities, respectively, in the absence of, at an

intermediate and infinite concentration of the metal ion (Mn+

).

A linear regression for the plot of [I-I0]-1

vs. [Mn+

]-1

for both Cu2+

and Zn2+

ions (Figure 8)

dictates a 1:1 association between 2BIMN1O and each of the two metal ions. Assuming a

binding interaction between the two partners, the strength of binding can be

quantitatively evaluated in terms of binding constant as K = intercept to slope ratio of the

Benesi-Hildebrand plot = (4.32 ± 0.8) × 104

M-1

for Zn2+

and (6.55 ± 0.8) × 103

M-1

for

Cu2+

. Consequently the free energy change for the process of complexation reaction is


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calculated to be KRTG ln−=∆ = – 26.44 kJ mol–1

for Zn2+

and – 21.77 kJ/mol–1

for

Cu2+

, i.e. a negative free energy change which dictates the thermodynamic feasibility for

the occurrence of the binding interaction. Also a relatively stronger binding of 2BIMN1O

with Zn2+

ion is manifested through a higher binding constant value as compared with

Cu2+

ion.

To unveil the functional groups of 2BIMN1O that are involved in the interaction

with the transition metal ions, the 1H NMR spectroscopic technique has been exploited.

Figure 9 displays the 1

H NMR spectral profiles of 2BIMN1O in the absence and presence

of Zn2+

ion (for reasons of compatible solubility of both the compound 2BIMN1O and

the perchlorate salt of the metal ion the spectra in Figure 9 are recorded in d6-DMSO).

The 1H NMR spectra of 2BIMN1O is found to exhibit some distinct modifications of

chemical shift upon interaction with Zn2+

ion. In presence of Zn2+

ion, the peak

corresponding to the -OH proton is found to disappear indicating deprotonation at that

site which is quite likely in case of such complex formation involving the oxygen atom.

On the other hand, such complexation of the title compound with Zn2+

ion is expected to

reduce the electron density of the coordination sites and induce a down-field shift of the

nearby proton signals [40]. In our case, prominent down-field shifts of the N–CH2–

protons and –CH=N– proton (δ 4.67→4.75 and δ 7.93→8.36 respectively, vide Figure 9)

are observed accounting for the electron deficiency of the corresponding sites. Such an

observation at once suggests for the involvement of the imine nitrogen in complexation.

Surprisingly, the proton at C8 shows an up-field shift (δ 8.35→8.2, vide Figure 9) and

possibly owing to this fact, the protons at C6 and C7 shows a significantly higher degree

of separation between their peaks (∆δ 0.14→0.39). The protons in the remote part of the


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molecular frame work from the site of interaction do not experience notable change in

chemical shift. However, in order to deduce a more indicative inference on the issue the

X-ray crystallographic structure elucidation of the 2BIMN1O–Mn+

complex system

should have been a more appropriate candidate. Unfortunately we refrain from the issue

since even after our repeated attempts the quality of the crystal of the 2BIMN1O–Mn+

complex system was not suitable for X-ray crystallographic analysis.

Conclusion

A Schiff base-derived new model compound, 2BIMN1O has been synthesized

and its photophysics has been characterized by steady-state absorption, emission and

time-resolved emission spectroscopic techniques. The fluorescence yield of the

compound is found to be considerably low compared to its constituent compound, HN12,

which has been rationalized in connection with the operation of PET mechanism from the

imine moiety to the fluorophore unit in the multicomponent system of interest. The

specific structural architecture of the title compound in fluorophore-spacer-receptor

(FSR) format paves way for a perusal of the perturbation of its fluorescence properties

with the addition of transition metal ions. The compound 2BIMN1O reveal significant

fluorescence enhancement in the presence of Cu2+

and Zn2+

ions selectively. Hence, this

compound has subsequently been argued to be a promising candidate for the development

of Cu2+

and Zn2+

ion selective fluorescence chemosensor. A calibration curve has been

constructed for the estimation of the transition metal ions in an unknown sample down to

the limit of micromolar concentration. The prominent difference on the absorption profile

of 2BIMN1O in the presence of Cu2+

and Zn2+

ions also paves way for demarcating the

response of the sensor compound to the two transition metal ions. Additional edges of


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2BIMN1O to act as a fluorescence chemosensor are its relatively simple structure and

facile synthetic route. Also such commendable selectivity in generating sensing responses

indicates the possibility of viable expansion of the technique to further consequences in

sensory research.

Acknowledgements

AG and SG gratefully acknowledge Junior Research Fellowships respectively

from CSIR and UGC, New Delhi, Govt. of India. The authors are thankful to their

colleague Dr. Sasanka Dalapati for stimulating discussions. NG likes to acknowledge

UPE and CRNN, CU and DST, India (Project No. SR/S1/PC/26/2008) from funding.

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Figure captions:

Figure 1: (a) Absorption spectra and (b) emission spectra (λex = 420 nm) of 2BIMN1O in

different solvents as specified in the figure legend. The sample concentration was

maintained at 2×10–6

M.

Figure 2: Effect of increasing concentration of acid (TFA) on the (a) absorption profile

and (b) emission profile of 2BIMN1O in ACN solvent. Inset shows the corresponding

excitation profile (sample concentration was maintained at 2×10–6

M).

Figure 3: Absorption spectra of 2BIMN1O (a) in the presence of increasing

concentration of Cu2+

ion. Direction of arrow indicates increasing concentration of Cu2+

(0.0, 6.67, 16.12 and 30.01 µM) and (b) in the presence of 30.0 µM concentration of Zn2+

ion. (c) Emission spectra (λex = 420 nm) of 2BIMN1O in ACN solvent and in the

presence of 50.0 µM of different transition metal ions as indicated in the figure legends

(sample concentration was kept at 2×10–6

M) . The inset of figure (a) shows the

ratiometric sensing scheme for Cu2+

based on its response on absorption profile in terms

of plotting the ratio of absorbance at 570 and 420 nm (A570/A420) as a function of Cu2+

concentration.

Figure 4: Emission spectra of 2BIMN1O in ACN solvent in the presence of increasing

concentration of transition metal ions, (a) Cu2+

and (b) Zn2+

(λex = 420 nm). Inset shows

the excitation spectra in the presence of increasing concentration of respective transition

metal ions. Curves (i) → (vii) correspond to [Mn+

] = 0, 6.67, 12.24, 20.01, 26.68, 33.35,

40.02 µM, (c) The photograph of (left to right) 2BIMN1O alone (R) in ACN solvent and

in the presence of Zn2+

and Cu2+

and Mn+

(mixture of Zn2+

,Cu2+

,Fe2+

,Ni2+

,Co2+

,Cr3+

and

Mn2+

) as indicated in the figure (sample concentration was maintained at 2×10–6

M).


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Figure 5: (a) Intentiometric chemosensory emission spectral response of 2BIMN1O in

the presence of different transition metal ions (concentration of each of transition metal

cation = 25.0 µM). The construction of calibration curve for detection of (b) Cu2+

and (c)

Zn2+

based on emission spectral data (sample concentration was kept at 2×10–6

M).

Figure 6: Typical time-resolved fluorescence decay profile of 2BIMN1O (a) in various

solvents as indicated in the figure legend (sample concentration 2× 10–6

M). The lower

panel shows the residual plot for the respective fitted functions to the actual data.

Figure 7: Typical time-resolved fluorescence decay profile of 2BIMN1O in the presence

of 30.0 µM of Cu2+

and Zn2+

ions in ACN solvent (sample concentration 2×10–6

M). The

lower panel shows the residual plot for the respective fitted functions to the actual data.

Figure 8: Benesi-Hildebrand plot ([I-I0]-1

vs. [Mn+

]-1

) for complexation between

2BIMN1O and the transition metal ions ((a) Cu2+

and (b) Zn2+

), derived from emission

spectral data.

Figure 9: 1H NMR spectra of 2BIMN1O in the (i) absence and (ii) presence of Zn

2+ ion.


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28

Table 1: Spectroscopic parameters for 2BIMN1O in various solvents as obtained

from absorption and emission measurements

a: λex = 420 nm for all solvents.

b: Quantum yields are × 10-2

order (error ± 5%).

c: f is the factor by which the fluorescence yield of 2BIMN1O is lower than its parent

compound, HN12.

Solvent λabs

(nm)

λema (nm) Quantum yield

b

(ΦΦΦΦf)

fc

λ1 λ2 λ3

HEX 268 360 422 480 0.23 20.43

THF 268 360 419 475 0.13 48.46

DOX 269 358 420 479 0.18 43.07

ACN 270 357 416 480 0.10 51.48

MeOH 270 356 420 484 0.13 48.46

BuOH 274 358 420 482 0.15 21.33


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29

Table 2: Time-resolved fluorescence decay parameters of 2BIMN1O in different

environments

Environment τ1b

(ns)

τ2b

(ns)

τ3b

(ps)

α1 α2 α3 τav

(ns)

χ2

HEX 1.69 7.86 501 0.358 0.028 0.614 2.56 1.03

MeOH 1.22 6.02 51 0.003 0.009 0.988 3.11 1.05

ACN 1.54 5.47 44 0.035 0.007 0.958 2.20 1.09

aH

+ 1.28 10.65 59 0.607 0.025 0.368 3.60 0.99

aCu

2+ 1.31 14.30 144 0.209 0.078 0.713 10.91 1.08

aZn

2+ 3.51 10.93 644 0.702 0.053 0.243 4.70 1.01

a: in the presence of 30.0 µM TFA or metal ion in ACN.

b: error in measurement ± 3%.


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30

Table 3: Fluorescence quantum yield (ΦΦΦΦf) and kinetic fluorometric and nonradiative

parameters for 2BIMN1O in the presence and absence of transition metal ions in

ACN solvent

Environment ΦΦΦΦf a τav (ns) kr (s

-1) knr

(s

-1)

ACN 0.10 × 10-2

2.20 4.59 × 105 4.54 × 10

8

H+ 0.03 3.60 8.33 × 10

6 2.69 × 10

8

Zn2+

0.24 4.70 5.06 × 107 1.62 × 10

8

Cu2+

0.16 10.91 1.44 × 107 7.72 × 10

7

a: error in measurement ± 5%.


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31

Figure 1

250 300 350 400 4500.0

0.1

0.2

0.3

MeOH

DOX

THF

HEX

ACN

Ab

sorb

an

ce

Wavelength (nm)

(a)

450 500 550 600E

m.

inte

nsi

ty (

a.u

.)

Wavelength(nm)

ACN

BuOH

HEX

DOX

THF

MeOH

(b)


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32

Figure 2

300 350 400 4500.0

0.2

0.4

0.6

0.8

Increasing [H+]

Ab

sorb

an

ce

Wavelength (nm)

(a)

300 360 420

440 480 520 560 600

In

ten

sity

(a

.u.)

Wavelength (nm)

Increasing [H+]Em

. in

ten

sity

(a

.u.)

Wavelength (nm)

(b)


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33

Figure 3

300 400 500 6000.0

0.1

0.2 Sensor only

Sensor + 30 µµµµM Zn2+

(b)

Ab

sorb

an

ce

Wavelength (nm)

440 480 520 560

Sensor

Sensor + Cu2+

Sensor + Zn2+

Sensor + Ni2+

Sensor + Co2+

Sensor + Fe2+

Sensor + Mn2+

Sensor + Cr3+

Em

. In

ten

sity

(a

.u.)

Wavelength (nm)

(c)

300 400 500 600

0.0

0.1

0.2

0 5 10 15 20 25 300.0

0.5

1.0

1.5

2.0

Ab

sorb

an

ce

Wavelength (nm)

[Cu2+]

(a)A

57

0/A

42

0

[Cu2+] in µµµµM


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34

Figure 4

450 500 550 600

240 300 360 420

Em

. in

ten

sity

(a

.u.)

Wavelength (nm)

[Cu2+]

(a)

i

vii

In

ten

sity

(a.

u.)

Wavelength (nm)

440 480 520 560

240 300 360 420

i

vii

Em

. in

ten

sity

(a

.u.)

Wavelength (nm)

[Zn2+]

(b)

Inte

nsi

ty (

a.

u.)

Wavelength (nm)


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35

Figure 5

0

20

40

60

Zn2+Cu2+Ni2+Co2+Fe2+Mn2+Cr3+

Rel

ati

ve

inte

nsi

ty,

I/I 0

(a)

0 5 10 15 20 25 300

20

40

60

80

I/I 0

[Zn2+] (µµµµM)

(c)

0 5 10 15 20 25 300

10

20

30

40

50

I/I 0

[Cu2+] (µµµµM)

(b)


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36

Figure 6


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37

Figure 7


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38

Figure 8

0.04 0.08 0.12 0.16

0.002

0.004

0.006

0.008

[I-I

0]-1

1/ [Mn+] (µµµµM-1)

Cu2+

Zn2+


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39

Figure 9


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40

Scheme 1: Schematic structures of different conformers of 2BIMN1O

O N

H

NO

H

Conformer I

H

H

Zwitterionic species

O N

H

H

Conformer II


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Graphical abstract

Ratiometric chemosensor of fluorophore-spacer-receptor type new synthetic Schiff base

selective for Cu2+ and Zn

2+ ion with sufficiently low detection limit.

Fluorophore Receptor

e-

e-

PET ON

PET OFF

Fluorescence ON

Fluorescence OFF

hνa

hνf

hνf

Mn+/H+

Fluorophore Receptor

e-

e-

PET ON

PET OFF

Fluorescence ON

Fluorescence OFF

hνa

hνf

hνf

Mn+/H+


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Date post:	11-Dec-2016
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Selective fluorescence sensing of Cu(ii) and Zn(ii) using a new Schiff base-derived model compound:...

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