A new window towards multidimensional sensing of transition metal cations through dual mode sensing ability of N-benzyl-(3-hydoxy-2-naphthalene): Emission enhancement coupled remarkable spectral shift

Article abstract of DOI:10.1016/j.saa.2011.02.035

A structurally simple Schiff base N-benzyl-(3-hydroxy-2-naphthalene) (NBHN32) has been synthesized and characterized by ¹H NMR, ¹³C NMR, and DEPT spectroscopy. The photophysical behaviour of NBHN32 in response to the presence of vari

Full text of DOI:10.1016/j.saa.2011.02.035

Spectrochimica Acta Part A 79 (2011) 197–205
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
A new window towards multidimensional sensing of transition metal cations
through dual mode sensing ability of N-benzyl-(3-hydoxy-2-naphthalene):
Emission enhancement coupled remarkable spectral shift
Bijan Kumar Paul, Subrata Mahanta1, Rupashree Balia Singh1, Nikhil Guchhait∗
Department of Chemistry, University of Calcutta, 92 A.P.C. Road, Calcutta 700009, India
a r t i c l e i n f o
a b s t r a c t
Article history:
A structurally simple Schiff base N-benzyl-(3-hydroxy-2-naphthalene) (NBHN32) has been synthesized
and characterized by 1H NMR, 13C NMR, and DEPT spectroscopy. The photophysical behaviour of NBHN32
in response to the presence of various transition metal cations has been explored by means of steady-
state absorption, emission and time-resolved emission spectroscopy techniques. Efficient through space
intramolecular photoinduced electron transfer (PET) between the naphthalene fluorophore and the imine
group has been argued for extremely low fluorescence yield of NBHN32 compared to the parent molecule
3-hydroxy-2-naphthaldehyde (HN32) containing the same fluorophore but lacking the receptor moiety.
Transition metal ion-induced emission enhancement is thus addressed on the lexicon of perturbation of
the PET by the metal ions. Apart from fluorescence enhancement, transition metal ion imparts remarkable
shift of the emission maxima of NBHN32, which is another unique aspect on the proposed ability of
NBHN32 to function as a fluorescence chemosensor.
Received 28 October 2010
Received in revised form 4 February 2011
Accepted 16 February 2011
Keywords:
N-benzyl-(3-hydroxy-2-naphthalene)
PET
Transition metal ions
Fluorescence enhancement
Dual mode sensitivity
© 2011 Elsevier B.V. All rights reserved.
1. Introduction
chemosensors consist of a signaling part which is usually a fluo-
rophore, and a recognition part in close proximity which is usually
Fluorescence signaling systems can be designed for signal
transduction upon analyte binding [1–13] and this opens up
new windows to use them as chemical sensors in a variety of
immensely significant areas such as biomedical research, prepa-
ration of chemical logics for molecular information processing
and so forth [14–27]. Also the high degree of sensitivity, non-
invasive nature, specificity and real-time monitoring with fast
response time are the advantages that add additional edge to flu-
orescent chemosensors over other techniques. Development of
fluorescent chemosensors for the sensing of metal ions thus quite
naturally seems an important goal in various research fields. The
concept of fluorescence enhancement via chelation is one of the
design techniques that has been exploited over the past few years
[28–34].
a Lewis base site (that can coordinate to a metal ion, such as
amines/imines). In the absence of the guest metal ion, the fluo-
rescence intensity of the fluorophore is reduced to a considerable
extent (photoinduced electron transfer (PET) is the most com-
monly exploited mechanism). However, as the presence of metal
ion engages the lone pair of the donor amine group by chelation,
the oxidation potential of the amine is increased and this leads to a
significant enhancement in the fluorescence intensity of the fluo-
rophore by turning off (or hampering) the PET process and thereby
constitutes the basis for actuating mechanism of functioning as a
sensor. The selection of transition metal ions for these studies is
obviously important given their vital roles in clinical, biological,
environmental studies [35–42].
The present programme is designed to focus on examining the
possibility of exploring the photophysics of a new synthetic Schiff
base N-benzyl-(3-hydroxy-2-napthalene) (NBHN32) towards sens-
ing of transition metal ions. Modulation of steady-state absorption
and emission spectral properties of NBHN32 upon interaction with
metal ions has been exploited for the purpose.
In fact, systems capable of sensing guest molecules, ions, pro-
tons, etc. have received a great deal of current research interest
because of their potential caliber to form a promising avenue
for applicative research in an extensive arena. In general, such
Also NBHN32 belongs to the family of Schiff base which
are normally known to be adamant towards tuning of spectro-
scopic properties with simple governing factors like variation of
medium polarity/pH, co-ordination towards suitable metal ions,
etc. [43–48]. Despite such difficulties the present work documents
∗
Corresponding author. Tel.: +91 33 2350 8386; fax: +91 33 2351 9755.
E-mail address: nguchhait@yahoo.com (N. Guchhait).
Present address: BioMolecular Optics, Department of Physics, Ludwig-
1
Maximilians-Universität, Oettingenstr 67, D-80538 Munich, Germany.
1386-1425/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2011.02.035

198
B.K. Paul et al. / Spectrochimica Acta Part A 79 (2011) 197–205
for NBHN32 that it can form a potential avenue for applicability,
particularly in the field of sensory research.
1.5
1.0
0.5
0.0
CYC
CCl
4
0.12
0.06
0.00
A major effort on sensory research has focused on tuning the
sensory capability by rational designing and synthesis of suitable
probes. For example, the emission wavelength and intensity can be
tuned by changing the chemical structure of the sensor molecule
[49–51]. On this perspective of tuning sensory capability, NBHN32
seems to be an efficient candidate since fluorescence enhancement
is found to be coupled with shifting of emission maxima as well,
revealing its unique dual mode sensing ability for transition metal
ions. Simultaneously, NBHN32 exhibits an appreciable diversity on
the issue of responding to the presence of metal cations as it shows
the effect for a series of the same with commendable efficiency.
Whereas differential mode and extent of modification of its pho-
tophysics with different metal ions can be taken as the test of its
selectivity.
THF
ACN
EtOH
Water
320
360
400
440
Wavelength (nm)
300
350
400
450
Wavelength (nm)
2. Experimental
Fig. 1. Absorption spectra of NBHN32 in various solvents as indicated in the figure
legend. Inset shows the magnified view of the absorption spectra in the lower energy
region.
2.1. Materials
The detailed synthetic procedure and purification of the Schiff
base NBHN32 is described in the Supplementary Information.
Spectroscopic grade acetonitrile (ACN) was purchased from
Spectrochem, India and used after proper purification according
to standard procedures. Further, from ET (30) measurements it was
confirmed that the ACN used was almost free from moisture [52].
The solvent appeared transparent and the purity was also verified
by recording the emission spectra in the studied spectral region.
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 and Co(ClO4)2(H2O)6. The metal salts were pro-
cured locally and used as received. Perchlorate salts have been
preferred because of the low coordinating ability of the counter
anion.
All other solvents and reagents such as methylcyclohexane
(MCH), methanol (MeOH), heptane (HEP), cyclohexane (CYC),
tetrahydrofuran (THF) were of spectroscopic grade (Spectrochem,
India) and were used after proper distillation. Ethanol (EtOH), sul-
phuric acid (H2SO4), sodium hydroxide (NaOH) and triethylamine
(TEA) from E-Merck were used as received. Triple distilled water
was used for preparing aqueous solutions.
The mean (average) fluorescence life-times for the decay curves
were calculated from the decay times (ꢀi) and the relative con-
tribution of the components (ai) using the following equation
[1–4,33,34,53]:
ꢀ
iaiꢀi2
ꢀ
ꢀꢀꢁ = ꢀavg
=
(1)
iaiꢀi
The quality of the fits has been assessed over the entire decay range
and tested with a visual inspection of the residuals of the fitted
functions to the actual data and other statistical parameters, e.g. ꢁ2
criteria and Durbin–Watson (DW) parameter. A value of ꢁ2 close to
unity and value of DW > 1.8 ascertained quality fit obtained in each
case.
Cyclic voltametry: The voltametric measurements were per-
formed 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 as reference electrode. The cyclic
voltagrams were acquired in a N2-bubbled acetonitrile solution
containing ∼0.2 M tetraethylammonium perchlorate (TEAP) as sup-
porting electrolyte. The scan speed was 50 mV/s.
2.2. Instrumentations and methods
Steady-state spectral measurements: The absorption and emis-
sion spectra were acquired on Hitachi UV–vis U-3501 spectropho-
tometer and PerkinElmer LS-50B fluorimeter, respectively. For all
spectroscopic measurements, the sample concentration was main-
tained at ca. 7 × 10−6 mol/dm3 in order to avoid reabsorption and
aggregation effects. Only freshly prepared solutions were used for
spectroscopic measurements. All experiments have been carried
out at ambient temperature (300 K) unless otherwise specified.
The effect of metal ions on the spectral characteristics of
NBHN32 was monitored by adding few microlitre amounts (addi-
tion restricted to 100.00 ␮L so as to eliminate the possibility of any
significant volume change) of stock solution of the metal salts to a
known volume of NBHN32 solution in ACN (2.50 ml).
Time-resolved fluorescence decay: Fluorescence lifetimes were
measured by Time Correlated Single Photon Counting (TCSPC) tech-
nique using nanoLED-07 (IBH, U.K.) as the light source at 340, 375
and 440 nm to trigger the fluorescence of NBHN32 or NBHN32:Mn+
complex, and the signals were collected at the magic angle of
54.7◦ to eliminate contributions from time-resolved fluorescence
anisotropy decay [3,53]. The decays were deconvoluted on data
station v2.3 IBH DAS6 data analysis software.
3. Results and discussions
Before moving on to the spectral behaviour of NBHN32 in pres-
ence of transition metal ions, it is necessary to present an overview
of the photophysics of the compound in bulk homogeneous sol-
vent, so that spectral modifications induced by the metal ions can
be looked over the milieu of the photophysics of NBHN32 in homo-
geneous solvents.
The absorption spectra of NBHN32 recorded in different solvents
are presented in Fig. 1 with the corresponding spectroscopic data
being summarized in Table 1. Fig. 1 shows the appearance of two
absorption bands of NBHN32 in all solvents, viz., at ∼373 nm and
∼290 nm. Now the possibilities are that the two bands originate
from excitation of one species to two different excited states or due
to absorption of two different entities. Comparing the absorption
spectra of NBHN32 with the parent molecule HN32 [54] and 2-
hydroxy-1-naphaldehyde [55] the higher energy absorption band
(∼290 nm) is assigned to the open conformer while the lower
energy band (∼373 nm) being due to the closed conformer of
NBHN32 (Scheme 1), in conformity with extra degree of stabiliza-
tion by virtue of the intramolecular hydrogen bonding in the latter.

B.K. Paul et al. / Spectrochimica Acta Part A 79 (2011) 197–205
199
Table 1
4
Spectroscopic parameters for NBHN32 in different solvents obtained from absorp-
tion and emission spectroscopy.
max
abs
Medium
ꢂ
(nm)
ꢂmemax (nm)(ꢂex = 370 nm)
3
2
1
0
MCH
ACN
EtOH
MeOH
293
293
293
293
322
322
378
370
371
373
425
410
425
423
425
425
425
419
–
–
498
498
470
490
1
MeOH+OH−
MeOH+H+
16
H
3
6
1
8
O
2
O
4
5
H
N
10
H
H
H
N
315
360
405
450
495
C
C
C
H
C
H
9
7
H
Wavelength (nm)
C6H11
C6H11
Fig. 3. Absorption spectra of NBHN32 in ACN in the presence Zn2+. Curves 1 → 16
correspond to [Zn2+] = 0, 4.44, 8.89, 13.33, 17.78, 22.23, 26.67, 31.12, 35.56, 40.01,
44.46, 48.90, 53.35, 57.79, 62.24, 66.69 ␮M).
Closed Conformer
λabs ~ 375 nm
λem ~ 425 nm
Open Conformer
λabs ~ 293 nm
Scheme 1. Schematic of structures of the open and closed conformers of NBHN32.
individual Gaussian curves, which reveal the presence of two peaks
at ∼425 nm and ∼498 nm). Based on a comparison with literature,
the ∼425 nm band is assigned to the local emission of NBHN32
closed conformer, while the ∼498 nm band is presumed to be due to
a zwitterionic structure. An unequivocal assignment for the emis-
sion spectral properties of NBHN32 is a quite difficult and tedious
job. Nevertheless, an elaborate assessment has been endeavoured
in Supplementary Information.
Furthermore, the absorption maximum of 2-naphthol is found to
be located at ∼318 nm [54] with no absorption peak at 370 nm
wavelength region. This supports our spectral assignments, since
an analogous intramolecularly hydrogen bonded closed conformer
is not possible in 2-naphthol. The lower energy absorption band
of the intramolecular hydrogen bonded species is seen to be lit-
tle blue shifted with increasing solvent polarity, presumably due
to weakening of the intramolecular hydrogen bond as a result of
solvent–solute dipolar interaction.
3.1. Transition metal ion-induced modulation of spectral
properties of NBHN32
Fig. 2 depicts the emission spectra of NBHN32 in various sol-
vents and the corresponding spectral parameters are summarized
in Table 1. NBHN32 exhibits a single emission band in non-polar sol-
vents at ∼425 nm, but in case of polar protic solvents an additional
hump is observed at around 498 nm (the inset of Fig. 2 shows a more
conspicuous view by resolving the spectral profile in methanol into
3.1.1. Absorption spectra
Theabsorption spectraofNBHN32 in presence ofincreasing con-
centration of transition metal ions have been monitored and the
representative spectral profile with Zn2+ is presented in Fig. 3 with
the spectroscopic parameters being summarized in Table 2. The
∼373 nm absorption band, i.e. the band due to ␲ → ␲* transition of
the lowest energy closed conformer of NBHN32 (Scheme 1) is found
to undergo little red-shift with concomitant decrease in absorbance
with increasing metal ion concentration. Simultaneously, two new
bands are developed at ∼320 nm and ∼430 nm. That the addi-
tion of strong base (NaOH) to a methanolic solution of NBHN32
accompanied a red-shift of the absorption band from ꢂabs ∼ 373 nm
to ∼425 nm due to anion formation (Fig. S2 in the Supplemen-
tary Information), prompted us to ascribe the ∼430 nm band to
NBHN32:Mn+ (Mn+ stands for transition metal ions) complex. This
is consistent with the idea of deprotonation of the –OH group dur-
ing complexation with Mn+, since deprotonated –OH moiety can
function as a better coordinating agent than neutral –OH group.
The higher energy band (∼320 nm) may be due to transition to some
higher excited state of the NBHN32:Mn+ complex (S2 ← S0 or higher
transition). However, in analogy to the photophysical behaviour of
the parent molecule HN32, it is not unlikely that deprotonation of
the open conformer of NBHN32 in presence of metal ions results in
its characteristics absorption band in the higher energy region.
The absorption spectra also underline the fact that with only
Ni2+, Co2+, Cr3+ and Zn2+ ions two distinct isobestic points are
obtained (at ∼341 nm and ∼385 nm for Zn2+; ∼350 nm and
∼380 nm for Co2+; ∼344 nm and ∼383 nm for Ni2+; ∼352 nm and
∼387 nm for Cr3+; Figs. S3–S8 in the Supplementary Information)
whereas isobestic point is lacking with other metal ions. Such dif-
ferential response of NBHN32 to different metal ions (lack of a
400 450 500 550 600
Wavelength (nm)
CYC
ACN
DOX
MeOH
400
450
500
550
600
Wavelength (nm)
Fig. 2. Emission spectra (ꢂex = 370 nm) of NBHN32 in various solvents as indicated
in the figure legend. Inset shows the deconvoluted emission profile of the probe in
methanolic medium into individual Gaussian curves. Blue lines denote the resolved
bands and the red line denotes the simulated profile based on the resolved bands.
The deconvolution provides a clarified view for the presence of two species under
the broad experimental spectrum.

200
B.K. Paul et al. / Spectrochimica Acta Part A 79 (2011) 197–205
Table 2
Spectroscopic parameters for NBHN32 in the presence of Mn+ in ACN.
Sample
Absorption ꢂ (nm)
Emission ꢂ (nm)
ꢂex ∼ 325 nm
ꢂ1
ꢂ2
ꢂ3
ꢂex ∼ 370 nm
ꢂex ∼ 430 nm
NBHN32
–
∼293
∼370
∼370
∼430
–
∼422
–
NBHN32 + Mn+
∼325
∼525
∼422, ∼525
∼525
common pattern) seems quite natural and is argued on the grounds
of varying tendency of NBHN32 to different metal ions towards
complex formation and also varied stability of the complexes so-
formed [9,30–34,56]. The presence of an isobestic points indicates
equilibrium between NBHN32 and NBHN32:Mn+ complex in course
of complex formation in respective cases.
of other transition metal ions; Figs. S9–S13). It is seen that the
normal emission of NBHN32 (ꢂem ∼ 425 nm) undergoes quenching
with increasing metal ion concentration with simultaneous appear-
ance of a new band at ∼525 nm. However, the emergence of the
new band (∼525 nm) is not equally prominent for all metal ions
and becomes quite distinct in case of Ni2+, Co2+ (Figs. S9 and S11 of
Supplementary Information) and Cu2+ (Fig. 4b). Most interestingly
for Zn2+ ion (Fig. 4a), the ∼525 nm band undergoes considerable
intensity enhancement as a function of Zn2+ ion concentration.
The emission band at ∼425 nm has been ascribed to the nor-
mal emission of NBHN32 coming from the locally excited state of
its closed conformer and its quenching due to addition of metal
ions may be the result of NBHN32:Mn+ complex formation which
consequently accompanies decrease of population of free NBHN32
molecules. At the same time, the ∼525 nm emission band seems
attributable to the NBHN32:Mn+ complex. With increase in metal
ion concentration, the population of the NBHN32:Mn+ complex
should increase relative to that of free NBHN32 molecules, a man-
ifestation of which is found in the relative intensity modulations
of the ∼525 nm (intensity enhancement) and ∼425 nm (intensity
reduction) bands as a function of metal ion concentration (Fig. 4).
Fig. 5 reveals that for ꢂex ∼ 320 nm (for Zn2+ and Cu2+ ions)
the intensity of the ∼525 nm emission band is appreciably higher
in comparison to that of the ∼425 nm emission band (see
Supplementary Information for the effect of other transition metal
ions; Figs. S14–S18).
3.1.2. Emission spectra
The presence of three prominent bands in the absorption profile
of the metal complex naturally guided us to monitor the emission
spectral behaviour of NBHN32 individually after exciting at the cor-
responding absorption maxima. Fig. 4 displays the effect of addition
of Cu2+ and Zn2+ metal ions on the emission profile of NBHN32
when ꢂex ∼ 370 nm (see Supplementary Information for the effect
(a)
12
We encounter similar observations for ꢂex = 430 nm. The
∼430 nm absorption band was said to be originating from the nor-
mal S1 ← S0 transition of the NBHN32:Mn+ complex, whence quite
naturally appropriate excitation of which led to discernible inten-
sity enhancement coupled with a marked shift of the emission
wavelength to ∼525 nm (Fig. 6 for Zn2+ and Cu2+ ions and see
Supplementary Information for the effect of other transition metal
ions; Figs. S19–S23). Indeed, the relative magnitude of emission
enhancement with ꢂex = 430 nm is seen to be unusually high since
for the specified excitation wavelength of 430 nm the free com-
pound NBHN32 exhibited barely any fluorescence at all, while the
presence of transition metal ions imparts dramatic fluorescence
enhancement.
1
420
455
490
525
560
595
630
Wavelength (nm)
(b)
525 nm
425 nm
The difference in the pattern of spectral modifications of
NBHN32 in course of interaction with various metal ions is
a manifestation of the varied coordinating abilities to differ-
ent transition metal ions and the stabilities of the resultant
complexes [9,31–34,57–59]. The contribution of the inner-filter
effect resulting in reduction of fluorescence intensity (emanating
from absorption of the exciting photon by metal salts) has been
negated in −v1iew of considerably low molar extinction co-efficient
(∼30–40 M cm−1) of the metal salts compared to NBHN32
(1.423 × 104 M−1 cm−1) at ꢂex = 320 nm [33,34].
3.1.3. Chemosensory response
It is thus established that interaction with transition metal ions
brings about profound modification to the ground and excited state
photophysics of NBHN32. Table 3 illustrates the magnitude of flu-
orescence intensity enhancement (IE) of NBHN32 as a function
of different transition metal ions. Table 3 reveals that maximum
intensity enhancement can be achieved at a strikingly low concen-
tration of the metal ion, i.e. NBHN32 has an excellent efficiency
450
500
550
600
Wavelength (nm)
Fig. 4. Emission spectra of NBHN32 in ACN in the presence of (a) Zn2+ (curves 1 → 12
correspond to [Zn2+] = 0, 5.33, 10.67, 16.00, 21.34, 26.68, 32.01, 37.35, 42.68, 48.02,
53.36, 58.69 ␮M) and (b) Cu2+ (curves 1 → 10 correspond to [Cu2+] = 0, 5.33, 10.67,
16.00, 21.34, 26.68, 32.01, 37.35, 42.68, 48.02 ␮M) when ꢂex ∼ 370 nm.

B.K. Paul et al. / Spectrochimica Acta Part A 79 (2011) 197–205
201
(a)
(a)
6
1
8
1
405
450
495
540
585
480
510
540
570
600
630
Wavelength (nm)
Wavelength (nm)
(b)
(b)
4
10
1
1
400
450
500
550
600
Wavelength (nm)
Fig. 5. Emission spectra of NBHN32 in ACN in the presence of (a) Zn2+ (curves 1 → 6
correspond to [Zn2+] = 0, 5.33, 10.67, 16.00, 21.34, 26.68 ␮M) and (b) Cu2+ (curves
1→10 correspond to [Cu2+] = 0, 5.33, 10.67, 16.00, 21.34, 26.68, 32.01, 37.35, 42.68,
48.02 ␮M) when ꢂex ∼ 430 nm.
480
510
540
570
600
630
Wavelength (nm)
Fig. 6. Emission spectra of NBHN32 in ACN in the presence of (a) Zn2+ (curves 1 → 6
correspond to [Zn2+] = 0, 5.33, 10.67, 16.00, 21.34, 26.68 ␮M) and (b) Cu2+ when
ꢂex ∼ 430 nm (curves 1 → 10 correspond to [Cu2+] = 0, 5.33, 10.67, 16.00, 21.34, 26.68,
32.01, 37.35, 42.68, 48.02 ␮M) when ꢂex ∼ 430 nm.
in sensing metal ions down to the level of 10−6 to 10−7 M order.
At the same time, apart from IE, the marked shifting of the emis-
sion maxima in presence of transition metal ions can also afford
a promising ground in serving as a signaling mode for sens-
ing of metal ions, i.e. dual mode sensing ability of NBHN32 is
revealed.
Another salient and important aspect of the present study is, of
course, the dual mode sensing ability.
Differential ratiometric chemosensory response of NBHN32
towards a given concentration of various transition metal ions
marks the sensing selectivity of NBHN32 (Fig. 7). Also a comparison
of the figures (Fig. 7a–c) reveals a possibility of viable expansion to
tune the selectivity of NBHN32 depending on the mode of obser-
vation. The exploration of the chemosensory ability of the probe
on a lexicon of ratiometric sensing scheme is noteworthy since
it indicates the use of NBHN32 as a multidimensional sensor. A
ratiometric sensing scheme is independent of the possible contam-
inations coming from experimental and instrumental artifacts such
as fluctuations in probe concentration, excitation source intensity,
light scattering, stability under illumination, environment around
the sensor molecule (medium pH/polarity, temperature, pressure,
etc.) and so forth. Thus the strategy of ratiometric chemosensory
response ensures a remarkable enhancement of the sensitivity of
the technique.
Despite the large volume of reports on the use of various com-
pounds as chemosensor for metal ions, the ability of a sensor
molecule to respond to the presence of such low concentrations is
relatively sporadically addressed in the literature [9,31–34,60–71].
Table 3
Fluorescence response of NBHN32 as a function of different transition metal ions in
ACNa.
Metal ions
[Mn+] × 10−7 (M)b
IEc (ꢂex ∼ 320 nm)
IEc (␭ex ∼ 430 nm)
Zn2+
Cu2+
Mn2+
Ni2+
Co2+
Cr3+
Fe3+
4.00
7.20
4.80
4.00
3.20
4.00
4.00
143
49
27
38
30
20
22
183
22
9
10
7
6
4
a
Experimental condition: ∼7.0 ␮M of the compound was used in ACN at 300 K.
b
Represents the concentration of metal ion for which maximum intensity
3.1.4. Excitation spectra
enhancement (IE) is observed, any further addition of metal ion leads to usual
fluorescence quenching.
The excitation spectra have been recorded at the correspond-
ing emission maxima under similar experimental conditions and
the representative spectra for increasing concentration of Zn2+
c
With respect to fluorescence intensity of the compound in the absence of metal
ion.

202
B.K. Paul et al. / Spectrochimica Acta Part A 79 (2011) 197–205
5
1
330
360
390
420
450
480
510
Wavelength (nm)
Fig. 8. Excitation spectra (ꢂmonitored = ꢂmemax) of NBHN32 in the presence of increasing
concentration of Zn2+ in ACN. Curves 1 → 5 correspond to [Zn2+] = 5.33, 21.34, 32.01,
42.68, 58.69 ␮M.
is retardation of nonradiative deactivation channels with conse-
quent opening up of radiative decay routes leading to emission
intensity enhancement. However, this explanation, in analogy to
literature reports [1–4,8–12,30–34,60–71], does not seem to suf-
fice for the magnitude of emission intensity enhancement observed
in the present study. This eventually guided us to account for
the findings on the basis of photoinduced electron transfer (PET)
mechanism, which is a commonly exploited strategy for fluo-
rosensors for metal ions employing an amine as the receptor
[9,31–34,72,73]. This mechanism commonly states that in the
absence of a guest, the interaction (PET) between the receptor
(imine group of NBHN32) and the fluorophore (naphthalene chro-
mophore) results in quenching of fluorescence. In the presence of
the guest the fluorophore–receptor communication is inoperating
due to binding of the guest at the receptor site and this leads to
recovery of the original fluorescence. Thus “switching ON” of the
fluorescence or fluorescence enhancement indicates the presence
of a guest. With a view to the commonly observed fluorescence
quenching action of transition metal ions to various fluorophores,
[74,75] it is obvious that in order to serve the purpose of fluores-
cence enhancement induced by transition metal ions, specifically
designed molecular systems are required in which the receptor
when bound to the metal ions will be inaccessible to the fluo-
rophore for quenching. Few such successes have been reported
where a cryptand receptor has been employed to trap the metal
ion in its cavity so as to suppress the quenching activity [76–79].
In the present study, our target molecule, NBHN32 has a relatively
simpler molecular structure but is found to undergo notable flu-
orescence enhancement in the presence of transition metal ions.
Most efficient enhancement is observed with poor quencher Zn2+
ion, as expected, but some good quenchers also reveal considerable
enhancement (Table 3). To the best of our knowledge, it is the first
documentation of a fluorescent chemosensor which throws light
into the utilization of an imine group as a possible receptor unit in
a molecular system working on the concept of PET mechanism. Also
our interpretation of results in terms of PET mechanism seems veri-
fied on the milieu of excitation spectral profile (vide Section 3.1 and
Fig. 8). However, reinforcing support is deduced from fluorescence
lifetime measurements as discussed in the forthcoming section.
Fig. 7. Differential ratiometric chemosensory response of NBHN32 towards vari-
ous transition metal ions based on emission and absorption spectral changes and
expressed as the ratio of (a) I525nm/I425nm (ꢂex ∼ 320 nm), (b) A320nm/A367nm and
(c) A430nm/A367nm. Here 1 → Zn2+, 2 → Cu2+, 3 → Ni2+, 4 → Mn2+, 5 → Co2+, 6 → Cr3+
,
7 → Fe2+
.
are given in Fig. 8. The excitation spectral profile does not bear
reasonable similarity with the absorption profile under similar
experimental conditions. This observation probably signals the
operation of some sort of excited state affair as the actuating
mechanism behind the transition metal ion-induced modulation
of emission spectral properties of the probe [54,57–59].
3.2. Mechanism of intensity enhancement
Metal ions are normally known to quench emission intensity
of a fluorophore so that a reverse observation obviously captures
attention and demands a suitable explanation. In this section we
endeavour to present an explanation for the experimental findings.
The enhancement of emission intensity as a result of complex-
ation may be a cumulative outcome of several factors operating
simultaneously. At the first glance, complexation with a transition
metal ion is necessarily to impart rigidity to the entire molecular
architecture. This will result in reduction of rotational/vibrational
degrees of freedom of the system, a direct consequence of which
3.3. Time-resolved fluorescence decay measurements
Fluorescence lifetime measurements often serve as a sensitive
indicator of the local microenvironment of a fluorophore, and it is

B.K. Paul et al. / Spectrochimica Acta Part A 79 (2011) 197–205
203
Fig. 9. Time-resolved fluorescence decay profile of NBHN32 in the absence and presence of Zn2+ and Cu2+ ([NBHN32] ∼2 ␮M and [Mn+] = 16.00 ␮M), for (a) ꢂex = 340 nm, (b)
ꢂex = 375 nm and (c) ꢂex = 440 nm.
responsive to various excited state affairs [1–4,9,31–34,57–59,80].
Thus in order to achieve a deeper insight into the photophysics
of NBHN32 in the absence and presence of transition metal ions,
fluorescence lifetimes have been monitored under differential cir-
cumstances. Fig. 9 depicts the time-resolved fluorescence decay
profile of NBHN32 in the absence and presence of Zn2+ and Cu2+ ions
and the relevant data are compiled in Table 4. The decay behaviour
of either the fluorophore or its metal complex is found to be com-
plicated and is best fitted to bi- and triexponential functions.
It is interesting to note that the complicated triexponential
decay pattern of NBHN32 reflects the presence of a short-lived
major component (∼0.021 ns, 92.41% in ACN), which seems to
be a representative of the lifetime of PET-quenched fluorophore
[9,31–34,57–59,81–83]. The long-lived minor components may in
principle arise from the products of electron transfer reactions such
as an exciplex. However, the absence of any fluorescence peak of
NBHN32 at longer wavelengths and the high polarity of the medium
(ACN, ε = 37.5; where exciplexes are unstable) leads to negate the
possibility stated above. Further reinforcing support in favour of
negation of the possibility of exciplex comes from a similar fluores-
cence decay pattern observed in non-polar solvent MCH (Table 4a
and Fig. S24 of Supplementary Information). 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 ␲-orbitals of the
ring. The long-lived components arise from species in which the
donor and acceptor groups are well separated, while the predomi-
nant short-lived component represents the quenched fluorophore
having the donor imine in close proximity [31–34,57–59,82,83].
However, as we move on to the decay behaviour of metal
complexes of NBHN32 it gets enormously modified though the
complicacy persists and it becomes cumbersome to explain the
observations by placing equal emphasis on each decay compo-
nent. Thus, instead of placing too much importance on individual
decay components the average (mean) fluorescence lifetimes (ꢀavg
)
have been calculated (Table 4) and the observations are found to
stride along the steady-state spectral findings. In order to obtain a
deeper insight into the decay behaviour of NBHN32:Zn2+ complex,
the decay pattern was monitored individually after exciting at dif-
ferent absorption maxima. The considerable increment of average
fluorescence lifetime of the complex (NBHN32:Zn2+) with respect
to that of the bare fluorophore (NBHN32) is in line with the rela-
tion: ꢀavg = (1/kr)˚f [57–59]. At the same time, it is noteworthy
(Table 4b) that in the presence of metal ions the relative contribu-
tion from the component representing PET-quenched fluorophore
is lowered compared to that in the absence of the same. This obser-
vation appears to support the postulate that the presence of metal
ion resists the operation of PET process. Also maximum enhance-
ment of ꢀavg observed in case of ꢂex = 440 nm (ꢂmax∼430 nm for
S1 ← S0 transition of the metal complex) seems atbos be an excel-
lent corroboration of the steady-state observations (as discussed in
Section 3.1). In case of NBHN32:Cu2+ complex, recording of fluores-

204
B.K. Paul et al. / Spectrochimica Acta Part A 79 (2011) 197–205
Table 4a
Time-resolved fluorescence decay parameters of NBHN32 in different solvents.
Medium
ꢀ1 (ns)
ꢀ2 (ns)
ꢀ3 (ns)
a1
a2
a3
ꢀavg (ns)
ꢁ2
DWa
ACN
MCH
MeOH
0.021
0.107
0.147
1.057
2.20
1.763
4.461
5.61
6.089
0.924
0.95
0.97
0.05
0.04
0.008
0.026
0.01
0.02
3.052
2.113
2.827
1.10
1.05
1.20
1.82
1.90
1.81
a
Durbin–Watson parameter.
Table 4b
Time-resolved fluorescence decay parameters of NBHN32 in the presence of Zn2+ and Cu2+ ions in ACN.
Sample (ꢂex/nm)
ꢀ1 (ns)
ꢀ2 (ns)
ꢀ3 (ns)
a1
a2
a3
ꢀavg (ns)
ꢁ2
DWa
NBHN32 (375)
0.021
1.884
2.176
3.244
1.644
0.019
1.057
8.094
8.565
8.869
0.185
0.00585
4.461
–
–
0.924
0.809
0.411
0.104
0.306
0.791
0.050
0.191
0.589
0.896
0.564
0.188
0.026
–
–
3.052
5.015
7.601
8.639
4.906
3.549
1.10
1.82
1.88
1.90
1.88
1.81
1.81
NBHN32 + Zn2+ (340)
NBHN32 + Zn2+ (375)
NBHN32 + Zn2+ (440)
NBHN32 + Cu2+ (375)
NBHN32 + Cu2+ (440)
1.09
1.03
1.00
1.17
1.02
–
–
7.176
0.975
0.131
0.021
a
Durbin–Watson parameter.
cence decay at ꢂex = 340 nm was not possible due to instrumental
limitations.
ating sensory scheme from experimental and/or instrumental
artifacts.
Also, the wide flexibility in generating sensing responses
depending on the mode of observation hints at the possibility of
viable expansion of the technique to further consequences.
3.4. Redox behaviour
In an attempt to assess the electron deficiency of the
fluorophore unit in NBHN32 we have carried out cyclic volta-
metric measurements with NBHN32. The Ered(fluoro) appears at
−0.971 V (irreversible) while the oxidation potential is observed
at Eox(fluoro) = 0.912 V (irreversible). Now, with the measured
reduction potential coupled with the spectral data, a quantita-
tive estimate of the thermodynamic driving force for PET process
in NBHN32 can be achieved in terms of the free energy change
(ꢃG) of the process [57–59]. The ꢃG value has been calculated
using the equation: ꢃG = 23.06[Eox(recep) − Ered(fluoro)] − E0,0, in
which Eox(recep) is the oxidation potential of the receptor moiety,
Ered(fluoro) is the reduction potential of the fluorophore and E0,0
represents the energy of the fluorescent state [57–59]. The value
of E0,0 used in the calculation has been estimated from location of
the first peak position in the fluorescence spectrum (ꢂem ∼ 425 nm)
[57–59]. Using Eox(recep) = 0.49 V [9,31,33,34,57–59] (for triethy-
lamine, literature SCE value corrected for Ag/AgCl electrode by
subtracting 0.27 V), ꢃG value comes out to be −33.60 kcal/mol,
which dictates the thermodynamic feasibility of the process of PET
in the studied system and also advocates for addition of an extra
dimension to our previous arguments and interpretations.
Acknowledgements
B.K.P. gratefully acknowledges C.S.I.R., New Delhi, India for
research fellowship. The authors gratefully acknowledge the
instrumental facility of Indian Association for the Cultivation of
Science, Jadavpur, Calcutta 700032, India for fluorescence lifetime
measurements. N.G. acknowledges D.S.T., New Delhi (Project no.
SR/S1/PC/26/2008), Government of India for financial support.
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.saa.2011.02.035.
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