A highly sensitive ESIPT-based ratiometric fluorescence sensor for selective detection of Al3+

Article abstract of DOI:10.1021/acs.inorgchem.6b01170

An excited-state intramolecular proton transfer (ESIPT)-based highly sensitive ratiometric fluorescence sensor, 1H was developed for selective detection of aluminum (AI³⁺) in acetonitrile as well as in 90% aqueous system. Single-crystal X-ray d

Full text of DOI:10.1021/acs.inorgchem.6b01170

Article
pubs.acs.org/IC
A Highly Sensitive ESIPT-Based Ratiometric Fluorescence Sensor for
Selective Detection of Al3+
Sanghamitra Sinha, Bijit Chowdhury, and Pradyut Ghosh*,†
†Department of Inorganic Chemistry, Indian Association for the Cultivation of Science, 2A & 2B Raja S. C. Mullick Road, Kolkata
700 032, India
S
* Supporting Information
ABSTRACT: An excited-state intramolecular proton transfer (ESIPT)-based highly sensitive ratiometric fluorescence sensor,
1H was developed for selective detection of aluminum (Al3+) in acetonitrile as well as in 90% aqueous system. Single-crystal X-
ray diffraction analysis reveals almost planar and conjugated structure of 1H. Photophysical properties of the sensor as well as its
selectivity toward Al3+ are explored using UV−visible, steady-state, and time-resolved fluorescence spectroscopic studies. The
bright cyan (λem = 445 nm) fluorescence of 1H in acetonitrile turns into deep blue (λem = 412 nm) with ∼2.3-fold enhancement
in emission intensity, in the presence of parts per billion level Al3+ (detection limit = 0.5 nM). Interestingly, the probe 1H
exhibits increased selectivity toward Al3+ in H2O/acetonitrile (9:1 v/v) solvent system with a change in fluorescence color from
pale green to deep blue associated with ca. sixfold enhancement in emission intensity. Density functional theoretical (DFT)
calculations provide the ground- and excited-state energy optimized structures and properties of the proposed aluminum
complex [Al(1) (OH)]22+, which is in harmony with the solution-state experimental findings and also supports the occurrence of
ESIPT process in 1H. The ESIPT mechanism was also ascertained by comparing the basic photophysical properties of 1H with a
similar O-methylated analogue, 1′Me.
Exploiting this idea a number of oxygen- and nitrogen-rich
chelating ligands like Schiff bases,6a−g imidazoline,2i 8-hydrox-
yquinoline4d-based fluorescence turn-on Al3+ sensors have been
reported, where the mechanisms utilized are mostly based on
chelation-enhanced fluorescence,7 photoinduced electron trans-
fer (PET),2i,4b,8a,b metal−ligand charge transfer,5e etc. Most of
these show enhanced emission intensity with little or no spectral
shift,4a−d and Schiff-base derivatives are generally prone to
hydrolysis in aqueous system.9 In recent years, excited-state
intramolecular proton transfer (ESIPT) process and ratiometric
fluorescence sensing have come into the limelight due to their
theoretical and practical importance.3a−k,10 ESIPT is a fast
(picosecond) intramolecular proton-transfer process, occurs in
excited state of suitable molecules containing −OH/−NH/−SH,
CN/CO, etc. functionalities, which results large Stokes
shift.10 A very few fluorescence turn-on ESIPT-based sensors for
Al3+ have been reported; however, those are based on Schiff-base
INTRODUCTION
■
During the past few years, a huge amount of research interest has
been devoted to the area of sensing of trace metal ions because of
their numerous effects on the biosphere.1a−k,2a−h,3a−l However,
the number of reports on effective aluminum sensing is still
limited.2a−i Aluminum is the most abundant (∼8% of the earth
surface) metal in the planet and is mostly present in its ionic form
Al3+, in atmosphere.4a−c,5a−m Increased concentration of
aluminum is poisonous for aquatic species like fish, algae,
bacteria, plants, etc. and also hazardous to human
health.4a−dAccording to World Health Organization, the
bearable aluminum intake for a human body is estimated to be
7 mg week−1 kg−1 body weight.4a−d,5a−m In this context,
development of fluorescent probes for Al3+ is becoming
increasingly more common to address the sensibility
issue.4a−d,5a−o But achieving this goal has been found to be
comparatively more difficult than that for the other metal ions
because of the poor coordination ability, strong hydration
aptitude, and the lack of spectroscopic characteristics of Al3+.
Being a hard acid, Al3+ prefers to bind with hard base.4c,d
Received: May 12, 2016
© XXXX American Chemical Society
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generated skeleton.5l,11a,b Thus, development of aqueous stable
ESIPT-based ratiometric sensor for Al3+ remains a challenging
task. In this work, we developed a simple ratiometric ESIPT-
based non-Schiff-base fluorescent sensor 1H, which can
selectively detect Al3+ in parts per billion level both in organic
and in 90% aqueous system even in the presence of excess
amount of other metal ions.
site toward aluminum. Further to ascertain the role of −OH
group in Al3+ binding via tuning the ESIPT mechanism, a similar
O-methylated system (1′Me) was developed.
Synthesis and Characterization. Sensor 1H is synthesized
by refluxing 9,10-phenanthrenequinone, 4-(diethylamino)-2-
hydroxybenzaldehyde, and ammonium acetate in glacial acetic
1
1
acid for 4 h. It is fully characterized by H, H−1H COSY, 13C
and 1H DEPT-135-HSQC NMR, (Scheme 1 and Figures S1−S4,
Supporting Information), electrospray ionization mass spec-
trometry (ESI-MS; Figure S5, Supporting Information), and
elemental analysis techniques. Finally, the solid-state structure of
1H is confirmed by single-crystal X-ray diffraction analysis. In the
NMR spectrum of 1H, all the protons and carbons resonate at
their expected frequency ranges, while the protons of −OH and
−NH groups resonate downfield near 13 ppm. This could be due
to the intramolecular hydrogen bond formation between
phenolic −OH proton with nitrogen of imidazole CN and
intermolecular hydrogen bond between −NH proton and the
oxygen in deuterated dimethyl sulfoxide (DMSO-d6). In ESI-MS
of 1H a single peak for molecular ion ([C25H23N3O][H+] is
observed at 382.14 (calculated 382.18). Synthesis and character-
ization of 1′Me are given in Scheme S1 and Figures S6 and S7,
Supporting Information.
RESULTS AND DISCUSSION
■
Designing Aspect of 1H. Among varieties of ESIPT-based
ligands, 2-(2-hydroxyphenyl)benzoxazole (HBO), 2-(2-
hydroxyphenyl)benzothiazole (HBT), and (2-(2-
hydroxyphenyl)benzimidazole (HBI) derivatives have turned
out to be the most promising sensors due to their preferential
photophysical properties like large Stokes shift and distinctly
different coloration upon guest binding with very high selectivity
and sensitivity in the recent years.3e Besides these, they can also
provide hard basic chelating sites containing oxygen and
nitrogen, which would be favorable for aluminum binding.
However, HBI derivatives possess higher quantum yields than
those of HBO and HBT.3e Keeping these things in mind an HBI-
based ligand (1H) was designed, where a diethylamino phenolic
unit is coupled with 9,10-phenanthrene imidazole moiety
resulting in a highly conjugated intramolecularly proton
transferable structure (Chart 1). The phenanthrene unit was
Single Crystal X-ray Diffraction Analysis. Diffractable
quality single crystals of 1H are obtained by slow evaporation
from its ethyl acetate−petroleum ether (1:5) solution at room
temperature. 1H crystallizes in orthorhombic crystal system with
Pbca space group. Asymmetric unit of the crystal contains one
ligand molecule and one ethyl acetate (Figure 1 and Figure S8,
Chart 1. Chemical Structure of the Ligand 1H
attached to extend the delocalization as well as to improve the
quantum efficiency by reducing nonradiative deactivation via
rigidification of the structure. It is expected that the ESIPT unit
itself, due to its hard basic nature, would act as the coordinating
Figure 1. Single-crystal X-ray structure of the ligand 1H (thermal
ellipsoids are drawn at 50% probability level). Color code: O, red; N,
blue; C, gray; H, dark blue.
Scheme 1. Synthetic Scheme of the Ligand 1H
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Figure 2. (a) UV−vis and fluorescence spectra of 1H, measured in acetonitrile at room temperature (concentration: 20 and 2.5 μM, respectively). (b)
Fluorescence (2.5 μM) spectral changes of 1H in the presence of various metal ions (10 equiv) as their perchlorate salts, in acetonitrile at room
temperature.
Supporting Information). Detailed crystallographic parameters
are listed in Table S1, Supporting Information. Structural analysis
reveals that the molecule is almost planar and highly conjugated.
Al3+ Sensing Studies in Acetonitrile. Photophysical
properties of the ligand are thoroughly studied using absorbance
(UV−visible), photoluminescence (PL), and time-resolved
spectroscopic methods. UV−vis spectrum of 1H has a prominent
signature of the chromophoric phenanthrene unit. It shows
bands at ∼243 (ε = 41 350 M−1 cm−1), ∼260 (ε = 30 235 M−1
cm−1), ∼284 (ε = 15 572 M−1 cm−1), ∼310 (ε = 18 150 M−1
cm−1), ∼355 (ε = 38 600 M−1 cm−1), and ∼375 nm (ε = 34 100
M−1 cm−1) in acetonitrile at 25 °C (Figure 2a), among which the
first three bands at 243, 260, and 284 nm can be considered as
intraligand (IL) π−π* transitions. The bands at 310, 355, and
375 nm may be due to the electronic transitions from
nonbonding orbitals on the heteroatoms to ligand π* orbitals,
that is, n−π* transition.12a Excitation at either 355 or 375 nm
wavelengths results in a large Stokes shift giving the same
emission profile consisting of an intense emission band at 445
nm (Figure 2a). It indicates that whatever the excitation energy
is, emission is occurring from the same excited singlet state,
making the molecule highly fluorescent with bright cyan color
under UV irradiation. Such a large Stokes shift can be explained
by comparatively low energy gap between keto excited state and
the keto ground state (than that in the enol form), which like the
other ESIPT systems should be responsible for the emission
spectra of the molecule (Chart S1, Supporting Informatio-
n).10,11a,b This may be visualized from the computational energy
level diagram that will be described later. The ESIPT process in
1H is further confirmed by UV−vis and PL studies of the control
compound 1′Me, where ESIPT cannot occur (Figure S9,
Supporting Information). It shows only 45 nm (∼3169 cm−1)
Stokes shift, which can be originated from nonradiative
vibrational decays, the reason for Stokes shifts in normal
fluorescent molecules.
Absorption spectrum does not change significantly in the
presence of other metal ions except in case of Cr3+, where a
similar spectral shift is observed without any prominent change
in absorbance.
A large change in the emission spectra of 1H (2.5 μM in
acetonitrile) is observed upon addition of a very small amount of
Al3+ (Figure 2b). The peak at 445 nm diminishes, while two new
peaks appear at 412 and 430 nm with ∼2.3-fold higher intensity.
Cr3+ and In3+ show same shift at higher concentration, where the
increase in intensity is much less (Figure 2b and Figure S11,
Supporting Information). However, Cu2+, because of its
paramagnetic quenching property, reduces the fluorescence
intensity of 1H, while the other metal ions remain silent to this
system. The change in absorption and emission spectra of 1H in
the presence of Al3+ can easily be correlated with the theoretically
computed energy-level diagram of the frontier molecular orbitals
of the free receptor and its Al3+ complex, which will be discussed
later in the theoretical part. On the one hand, the energy
difference between highest occupied molecular orbital (HOMO)
and lowest unoccupied molecular orbital (LUMO) in the Al3+
complex is less compared to that of 1H(enol), which is
responsible for the ligand absorption profile, and thus it shows
a bathochromic shift in the UV−visible spectrum upon metal
coordination. On the other hand, the same difference is greater
than that of the keto form, which may be responsible for the
ligand emission profile, thus leading to hypsochromic shift of
fluorescence peaks. However, the fluorescence spectrum of 1′Me
shows 10 nm bathochromic shift that can be rationalized by the
lowering of the HOMO−LUMO energy gap upon Al3+
coordination (Figure S9, Supporting Information).
Considering quinine sulfate as standard,11c the quantum yield
(ϕf) value of 1H comes out to be 0.424 (Figure S12, Supporting
Information). This high ϕf value in solution state may be the
outcome of the intramolecular hydrogen bonding between
phenolic −OH and the imidazole N, which makes the molecule
rigid, by restricting rotation about the single bond connecting
diethylaminophenolic and the phenanthrene−imidazole unit,
and lowers the nonradiative decays via rotational and vibrational
relaxation pathways. Complexation with Al3+ further increases
the quantum yield value to 0.5474, which may be attributed to
more rigidification by metal coordination via deprotonation and
Metal binding properties of 1H are investigated to check its
selectivity toward metal ions. Addition of various metal ions (10
equiv) as their corresponding perchlorate salts to 20 μM solution
of 1H in acetonitrile results red shift of the absorption bands
from 355 and 375 nm to 375 and 390 nm, respectively, with an
increase in absorbance in case of Al3+ (Figure S10, Supporting
Information). A clear isosbestic point is observed at 363 nm.
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Figure 3. (a) UV−vis (20 μM) and (b) fluorescence (2.5 μM) titration profiles of 1H with Al3+ in acetonitrile at room temperature. (inset) Fluorescence
colors of 1H (left) and its Al3+ complex (right) in acetonitrile.
Figure 4. (a) An equivalent plot from PL titration data of 1H (2.5 μM) with Al3+ and (b) Benesi−Hildebrand plot for calculation of association constant
from PL titration of ligand 1H with Al3+ in acetonitrile at room temperature.
hard−hard interaction than the rigidification by comparatively
weaker H-bond formation.
and no further increase in the fluorescence intensity is observed.
Enhancement of intensity is ∼2.3-fold as compared to the free
ligand. The Job’s plot analysis reveals 1:1 host−guest
stoichiometry between 1H and Al3+ (Figure S14, Supporting
Information). Binding constant is calculated as 3.36 × 105 M−1,
from Benesi−Hildebrand plot analysis (Figure 4b).
For better understanding the Al3+ binding properties of 1H,
absorption and fluorescence titration experiments are performed.
Upon incremental addition of Al3+ into acetonitrile solution of
1H (20 μM) a red shift in the absorption bands is observed. The
absorbance at 355 nm is gradually decreased with concomitant
increase in the absorbance at 375 nm and simultaneous
development of a new peak at 390 nm (Figure 3a). After
addition of 1 equiv of Al3+, saturation point is reached, and no
more change is observed in the absorption spectrum (Figure
S13a, Supporting Information). A single isosbestic point at 363
nm indicates the presence of two species: the free ligand and the
ligand−Al3+ complex. Job’s plot analysis shows inflection point at
0.5, suggesting 1:1 host−guest stoichiometric binding event
(Figure S13b, Supporting Information). PL titration experiment
is performed by gradually adding Al3+ into the acetonitrile
solution of 1H (2.5 μM). With increasing concentration of Al3+,
the strong emission band at 445 nm is gradually blue-shifted, and
two new emission maxima at 412 and 430 nm are observed
(Figure 3b). Intensities of these peaks increase up to addition of 1
equiv of Al3+ (Figure 4a), after which saturation point is achieved,
To check the selectivity of 1H toward Al3+, the change in
fluorescence intensity of the ligand upon addition of 1 equiv of
Al3+ is monitored in the presence of excess amount of other
competitive metal ions (e.g., Cr3+, In3+, Cu2+, Mn2+, Mg2+, Ag2+,
Hg2+, Zn2+, Pb2+, Fe3+, Ni2+, and Co2+; 10 equiv each). As shown
in Figure S15a, Supporting Information, the selectivity along
with the emission enhancement remains almost unperturbed in
the presence of all metal ions. A plot of fluorescence intensity
change (I − I0) versus concentration of Al3+ yields the detection
limit as low as 0.5 nM, which is much lower than the maximum
tolerable limit of Al3+ (Figure S15b, Supporting Information).
Such a low detection limit as well as the selective emission
enhancement and large blue shift with Al3+ reach to a conclusion
that 1H is an effective sensor for selective detection of Al3+ even
in the presence of a large excess of other competitive metal ions
in acetonitrile.
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Figure 5. Time-resolved luminescence decays of 1H (2.5 μM; λem = 445 nm) (a) in the presence of various metal ions as their perchlorate salts and (b)
upon addition of increasing amount of Al3+ in acetonitrile at room temperature.
Time-Resolved Spectroscopic Study. The conclusion
made by steady-state spectroscopic measurements is again
supported by fluorescence lifetime study. Lifetimes (τ) of free
ligand (1H) and the ligand in the presence of various metal ions
are determined using time-correlated single-photon count
(TCSPC) experiment (Table S2, Supporting Information).
Lifetime of 1H (5.37 ns) is decreased in the presence of Al3+
(1.14 ns), while no remarkable change is observed in the
presence of other metal ions. Decay pattern of 1H also remains
unchanged in the presence of all metal ions except Al3+ (Figure
5a). These observations indicate that the nature of the free ligand
(1H) is retained in cases of all other metal ions. Moderate
lifetime value as well as high quantum yield of 1H suggests that
the rate of nonradiative decay (κnr) is not large in 1H. This can
also be explained by the formation of intramolecular hydrogen
bond between phenolic −OH and imidazole N, which rigidifies
the system and hence suppresses nonradiative decay pathways.
And this rigidification is strengthened by Al3+ coordination. So,
κnr does not differ largely between 1H and its Al3+ adduct, and
unlike most of the cases, here it is not the governing factor for
lifetime determination. The radiative decay rate (Γ), which is an
intrinsic property of a fluorophore and depends on its absorption
and emission spectra and refractive index of the medium, plays a
major role in this case.12b Coordination with Al3+ ion shifts the
excitation and emission maxima as well as increases the
absorbance and fluorescence intensity, resulting in an increase
in radiative decay rate of Al3+ complex as compared to that of 1H,
which is reflected in their corresponding lifetime values. No
change in decay pattern of 1H with other metal ions indicates
that no adduct is formed with these metal ions, because of which
ligand is the only contributor to the overall lifetime. Titration
with Al3+ shows gradual decrease in lifetime with change in the
decay pattern from single exponential to biexponential and finally
again to single exponential at 1 equiv of Al3+, which remains
unchanged upon further addition of Al3+ (Figure 5b; Table S3
and Figure S16a, Supporting Information). This observation
suggests that initially free ligand is the only species present in
solution and is the solitary contributor to lifetime value.
Incremental addition of Al3+ increases the concentration of the
Al3+ adduct, which starts to contribute in overall lifetime,
resulting in biexponential decay with shorter lifetime. After
addition of 1 equiv of Al3+, all the ligand molecules undergo
adduct formation with Al3+ resulting in Al3+ complex, having
shorter lifetime, as the solitary contributor to the new single-
exponential decay. The similar decay pattern as well as the
lifetime values of 1H (5.35 ns) and 1H in the presence of Al3+
(1.27 ns) at longer wavelength (525 nm) again support the
ESIPT mechanism (Figure S16b, Supporting Information).
Al3+ Sensing by 1H in Aqueous Medium. In aqueous
environment metal ions generally remain surrounded by solvent
media, which makes it difficult to sense metal ions in water. To
check whether the selective sensing property of 1H toward Al3+ is
retained in aqueous environment, UV−visible and PL studies are
performed with various metal ions (e.g., Cr3+, In3+, Cu2+, Mn2+,
Mg2+, Ag2+, Hg2+, Zn2+, Pb2+, Fe3+, Ni2+, Co2+, and Al3+) in H2O/
acetonitrile (9:1 v/v) solvent system, where the metal ions are
taken in 100% water. Absorption spectrum of 1H shows two
strong bands at 246 nm (ε = 26 283 M−1 cm−1) and 366 nm (ε =
22 461 M−1 cm−1), whereas excitation at 366 nm results emission
at ∼463 nm. Fluorescence intensity of 1H is less in case of H2O/
acetonitrile (9:1 v/v) solvent as compared to that in acetonitrile
(Figure S17, Supporting Information). Ligand 1H shows greater
selectivity toward Al3+ in H2O/acetonitrile (9:1 v/v) solvent with
∼sixfold enhancement in fluorescence intensity, associated with a
change in fluorescence color from pale green to blue (Figure 6a).
However, a large excess of Cr3+ and In3+ causes same color
change as Al3+ with very small increase in intensity. The
improvement in selectivity toward Al3+ in aqueous solvent can be
explained by the hard−hard interaction between N- and O-
containing chelating site of 1H and strong Lewis acidic Al3+ that
allows 1H to overcome the high solvation energy barrier in case
of Al3+, which is not possible for rest of the metal ions.
Absorbance and fluorescence titration experiments are
performed by gradually adding Al3+ (dissolved in water) into a
H2O/acetonitrile (9:1 v/v) solution of 1H, to understand the
binding properties of 1H with Al3+. Incremental addition of Al3+
results in a steady decrease in the absorbance at 366 nm with
simultaneous formation of a new band at 430 nm associated with
an isosbestic point at 405 nm (Figures S18 and S19, Supporting
Information). In fluorescence titration, upon addition of Al3+ the
emission band at 463 nm shifts toward higher energy region
resulting in a new emission band containing two peaks at 412 and
430 nm (Figure 6b). Intensities of the peaks are increased
gradually to ∼sixfold as that of the free ligand. Saturation point is
reached at 1 equiv of Al3+, which, along with the corresponding
Jobs plot analysis, proves 1:1 stoichiometric binding between 1H
and Al3+ (Figure 6c and Figure S20, Supporting Information).
Association constant between 1H and Al3+ is determined from
Benesi−Hildebrand plot as 8 × 104 M−1, which is less as
compared to that in acetonitrile (Figure S21a, Supporting
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performed in DMSO-d6 solvent. Incremental addition of Al3+
into a DMSO-d6 solution of 1H broadens the peaks of −OH and
−NH protons at 13.26 and 12.99 ppm, respectively. Intensity of
the −OH peak decreases slowly, and it disappears at 1 equiv of
Al3+ concentration (Figure 7). All the spectral changes continue
Figure 6. (a) Fluorescence (2.5 μM) spectral changes of 1H in the
presence of various metal ions (10 equiv). (b) Fluorescence titration
profile of 1H (2.5 μM) with Al3+. (inset) Fluorescence colors of 1H
(left) and its Al3+ complex (right) in H2O/acetonitrile (9:1 v/v). (c)
Corresponding Job’s plot experiment of 1H (7.5 μM) with Al3+ (7.5
μM). (d) Selectivity graph of 1H with Al3+ in the presence of other metal
Figure 7. Changes in 1H NMR spectra of 1H (7.9 mM) upon addition of
increasing amounts of Al3+ in DMSO-d6 at room temperature.
ions in H2O/acetonitrile (9:1 v/v) solvent at room temperature (λem
=
412 nm for Al3+, Cr3+, and In3+ and 463 nm for other metal ions). (pink)
Fluorescence intensities of 1H in the presence of all metal ions (10
equiv). (green) Fluorescence intensities in the presence of all metal ions
and Al3+. Codes used: (1) only 1H, (2) Ag+, (3) Zn2+, (4) Cd2+, (5)
Pb2+, (6) Mn2+, (7) Hg2+, (8) Mg2+, (9) Cu+, (10) Fe3+, (11) Co2+, (12)
Ni2+, (13) In3+, (14) Cr3+, (15) Al3+.
to that point only, after which saturation is reached. The results
indicate breaking of the intramolecular H bond between phenolic
−OH and imidazole nitrogen (CN), followed by deprotona-
tion of the phenolic proton in the presence of Al3+.
The two isolated doublets at 8.545 and 8.445 ppm in free
ligand NMR spectrum, corresponding to Hd and Hd′ protons,
merge into a single doublet in the presence of Al3+. This result
may be explained by the deprotonation of phenolic proton,
which along with the metal coordination indirectly equalizes the
electronic environment around these two apparently electroni-
Information). Detection limit (12 nM) of 1H for Al3+ is higher in
H2O/acetonitrile (9:1 v/v) binary solvent than in acetonitrile
(Figure S21b, Supporting Information).
To check the selectivity of 1H toward Al3+ in the presence of
other metal ions in H2O/acetonitrile (9:1 v/v), change in
fluorescence intensity of 1H upon addition of 1 equiv of Al3+ (in
100% water) is monitored in the presence of excess amount of
other competitive metal ions (10 equiv each). The result shows
same fluorescence enhancement with Al3+ as in absence of
competitive metal ions (Figure 6d). The time-resolved
fluorescence titration data in H2O/acetonitrile (9:1 v/v) solvent
(Figure S22, Supporting Information) shows gradual decrease in
lifetime of 1H upon incremental addition of Al3+, which stops
beyond 1 equiv of Al3+. The small lifetime of 1H (36 ps) suggests
that ESIPT process becomes weaker in aqueous system. Further
it is observed that the pH of the solution before and after the
addition of Al3+ remains the same, which in turn justifies the fact
that the observed change in emission profiles is solely due to the
binding of Al3+ and not to the effect of pH. On the one hand, the
sensing efficiency of 1H is decreased in acidic pH, which may be
due to the protonation of imidazole nitrogen atoms as well as
prohibition of the deprotonation of phenolic −OH group, which
is necessary for Al3+ coordination. On the other hand, at alkaline
pH, OH− ions compete for Al3+ with the ligand, which reduces
the sensitivity of 1H toward Al3+.11a Thus, the overall result
suggests that 1H can be used as an even better fluorescent sensor
for Al3+ in H2O/acetonitrile (9:1 v/v) system than in pure
acetonitrile.
1
cally nonequivalent protons. H NMR titration experiment is
also performed in 1:1 D2O/CD3CN solvent, which shows
downfield shifts of the Ha, Hb, and Hc peaks (Figure S23,
Supporting Information). Effect of Al3+ on −OH and −NH
peaks could not be monitored, as these two peaks get broadened
in polar protic solvent. The overall outcome suggests that
deprotonation of the phenolic −OH group as well as the hard−
hard interaction between Al3+ and the O− (of phenolate anion)
and N (of imidazole CN) atom of the ligand are the key factors
in binding toward Al3+ (Scheme 2 and Figure S24, Supporting
Information).
Scheme 2. Proposed Binding Mechanism of 1H with Al3+
1
Mechanistic Investigation using H Nuclear Magnetic
Resonance Spectroscopy and Electrospray Ionization
Mass Spectrometry. To understand the mechanism of Al3+
1
binding by 1H, H NMR spectroscopic titration experiment is
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The ESI-MS of the isolated Al3+complex of 1H shows peaks at
424.34 and 887.76 m/z, which could be assigned for [Al(1)
(OH)]22+ and [Al(1)(O)]2K+, respectively. Distribution pattern
of both 1+ and 2+ charged species match well with their
corresponding theoretically calculated patterns (Figures S25 and
S26, Supporting Information). This approximated structure
agrees with the experimental findings as well as the previous
reports3l and the three-centered two-electron bond formation
property of aluminum.2h,5l Several trials to get crystals of the Al3+
complex failed. So DFT calculations are made to ascertain the
proposed structure of the Al3+ complex as well as the ESIPT
mechanism.
restricted rotation around C−C single bond between phenyl and
imidazole ring, is responsible for high quantum efficiency of the
ligand 1H. The energy difference between HOMO and LUMO is
2+
in the order of 1H (enol) > [Al(1) (OH)]2 > 1H (keto).
2+
Ground-state optimized structure of [Al1(OH)]2 shows the
presence of Al−O−Al three-centered two-electron bonds, where
the Al−O bond lengths are ∼1.83 and 1.84 Å (Figure S27 and
Tables S5 and S6, Supporting Information). Phenolic O and
imidazole N (CN) act as the chelating site and form
coordinate bonds with Al center with a bite angle of ∼102°.
The Al−O−Al bond angle is observed as ∼101°, while < O−Al−
O is ∼80°. The bond angles around Al vary from ∼80° to ∼127°,
which suggests a distorted tetrahedral geometry around both the
Al centers. This is in accord with the literature reports on 3c−2e
bond formation by Al.2h,3l,5l Frontier molecular orbitals in [Al(1)
(OH)]22+ are mainly laid on the ligand rather than on Al centers,
where the HOMO is mostly contributed by phenolic imidazole
part of the ligand, and LUMO spreads over total ligand moiety.
In other words, it can be said that the absorption bands are only
due to intraligand n−π* and π−π* electronic transitions not for
charge transfer from metal to ligand or vice versa. Excitation
energies of 1H and [Al(1) (OH)]22+ are calculated, which show
that the absorption bands at 355 and 375 nm for 1H correspond
to HOMO-to-LUMO+1 and to HOMO-to-LUMO electronic
transitions, respectively, while the 375 and 390 nm bands in
Theoretical Calculations and Molecular Orbital Anal-
ysis of 1H and Its Al3+ Complex. To get insight into the
sensing mechanism and the structure−property relationship of
2+
the [Al(1) (OH)]2 dimer, DFT calculations on ground (S0)
and excited states (S1) are performed with hybrid B3LYP
functional12c as incorporated in Gaussian 09 package,12d mixing
the exact Hartree−Fock-type exchange with Becke’s exchange
functional12e and that proposed by Lee−Yang−Parr for the
correlation contribution.12f The 6-311G(d)12g basis set is used
for all the atoms with conductor-like polarizable continuum
model (CPCM)12h for acetonitrile solvent (Figure 8 and Figures
2+
[Al(1) (OH)]2 are mainly due to the electron transfer from
HOMO−1 to LUMO and from HOMO to LUMO, respectively.
CONCLUSIONS
■
In conclusion, an ESIPT-based ratiometric fluorescent sensor,
1H, is developed, which can selectively detect Al3+ in parts per
billion level, both in acetonitrile and in 90% H2O−acetonitrile
system. Ligand 1H exhibits two distinctly different colors in free
form as well as in the presence of Al3+. Higher selectivity of 1H
toward Al3+ in 90% H2O−acetonitrile solvent makes it suitable
for real-life application. DFT calculations suggest the presence of
3c−2e bonds (Al−O−Al) with distorted tetrahedral geometry
around aluminum in the resulting Al3+ complex. ESIPT-based
sensing mechanism is also studied in molecular level using DFT.
In summary, the work provides a new and effective fluorescence
sensor for selective detection of Al3+ with an insight about its
binding mechanism.
Figure 8. Frontier molecular orbitals of 1H and [Al(1) (OH)]22+ with
their corresponding energy gap as calculated from DFT B3LYP/6-
311G(d) computational level using CPCM model for acetonitrile
[isovalue = 0.02].
EXPERIMENTAL SECTION
■
Materials. The reaction and workup procedures were performed at
ambient condition. Perchlorate salts of Al3+, Cr3+, Cu2+, Mn2+, Mg2+,
Ag2+, Hg2+, Zn2+, Pb2+, Fe3+, Ni2+, and Co2+ were purchased from
Aldrich, and nitrate salts were purchased from Alfa Aesar; both of them
were used as received. Glacial acetic acid was purchased from
Spectrochem Pvt. Ltd., India. HPLC-grade solvents and doubly distilled
water were used in spectral measurements. NMR solvent DMSO-d6 was
purchased from Aldrich.
S27 and S28, Supporting Information). Theoretical outcome
shows that in ground state, 1H is present in its enol form, as it
possesses lower energy than the corresponding keto ground
state. But after excitation, tautomerization becomes favored
toward the excited keto form. This is because of the lower energy
of keto S1 state as compared to that of enol S1. Thus, the
absorption spectrum of ligand 1H nicely matches with the
theoretically calculated UV−visible spectrum of the enol form
(Figure S29, Supporting Information). Details about the
electronic transitions are given in Table S4, Supporting
Information. The ground-state optimized structure of 1H is
consistent with its single-crystal structure. As depicted in Figure 8
and Figure S28, Supporting Information, both the HOMO and
LUMO of 1H (in enol as well as keto form) are almost evenly
distributed on overall ligand molecule, which reduces the chance
of PET process from diethylamino group of the molecule to the
phenanthrene moiety, which could result in fluorescence
quenching in this type of molecule. This, along with the
Methods. Fourier transform infrared (FTIR) spectra were recorded
on Shimadzu FTIR-8400S infrared spectrophotometer with KBr pellets.
High-resolution mass spectrometry (HRMS) analysis was performed on
1
QTOF−Micro YA 263 mass spectrometer in positive ESI mode. H,
13C, 1H−1H COSY, and 1H-DEPT-135-HSQC NMR experiments were
performed on FT-NMR Bruker DPX 300/400/500 MHz NMR
spectrometer, and chemical shifts for 1H and 13C NMR were reported
in parts per million (ppm), calibrated to the residual solvent peak set.
Absorption spectra were recorded in a PerkinElmer Lambda 900 UV/
vis/NIR spectrometer (using quartz cuvette of 1 cm path length), and
emission spectra were recorded in a FluoroMax-3 spectrophotometer,
from Horiba Jobin Yvon. Elemental analysis was performed on
PerkinElmer 2500 series II elemental analyzer, PerkinElmer, USA.
G
DOI: 10.1021/acs.inorgchem.6b01170
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Picoseconds diode laser (IBH Nanoled-07) was used to excite samples
in an IBH Fluorocube apparatus for TCSPC measurements. The
luminescence decays were recorded on a Hamamatsu MCP photo-
multiplier (R3809), and IBH DAS6 software was used to analyze the
data. All geometry optimizations were performed with the Gaussian
0912d program package using DFT. The B3LYP12c functional and 6-
311G(d) basis set12g were used for all atoms. All geometries were fully
optimized in the ground states (So). Time-dependent DFT calculations
using CPCM12h for acetonitrile solvent were performed to examine low-
energy excitations at the ground-state geometry at the same level of
calculation as employed for geometry optimizations. Gaussview5.0 was
used for visualizations of the optimized structures and the MOs.
Caution! Metal perchlorate salts are extremely explosive in the presence of
open flames and sparks or heat. Aluminum nitrate and perchlorate both are
skin irritants. All due precautions should be taken while handling these.
Calculation of Association Constants. Association constant
between 1H and Al3+ was calculated using UV and PL titration data.
Association constants were calculated using Benesi−Hildebrand eq 1.
78.71; H, 6.08; N, 11.02; O, 4.19. Found: C, 78.25; H, 6.34; N, 11.56.
FTIR in KBr disc (ν/cm−1): 3315, 3057, 2974, 2924, 2897, 2868, 1699,
1637, 1568, 1527, 1489, 1456, 1416, 1375, 1354, 1302, 1277, 1225,
1198, 1148, 1080, 1043, 1014, 962, 854, 787, 756, 723, 656. ESI-MS
1
[C25H23N3O][H+] calcd: m/z 382.18. Found: m/z 382.14. H NMR
(300 MHz, DMSO-d6, Si(CH3)4): δ 13.26 (s, 1H, OH), 12.99 (s, 1H,
NH), 8.88 (t, J = 9 Hz, 2H, Hg,g′), 8.54 (d, J = 9 Hz, 1H, Hd), 8.44 (d, J =
9 Hz, 1H, Hd′), 8.00 (d, J = 9 Hz, 1H, Hc), 7.76 (m, 2H, He,e′), 7.65 (t, J =
9 Hz, 2H, Hf,f′), 6.44 (d, J = 9 Hz, 1H, Hb), 6.25 (s, 1H, Ha), 3.41 (q, J = 6
Hz, 4H, Hh), 1.15 (m, 6H, Hi). 13C NMR (100 MHz, DMSO-d6,
Si(CH3)4): δ 159.2 (1C, Ck), 150.8 (1C, Cm), 149.9 (1C, Cj), 127.4−
127.1(4C, Cp,q,r,s), 126.8 (1C, Cc), 125.5−125.2 (6C, Ce,e′,f,f′,o,n), 124.1−
123.8 (2C, Cd,d′), 121.8−121.6 (2C, Cg,g′), 103.5 (1C, Cl), 100.9 (1C,
Cb), 98.0 (1C, Ca), 43.7 (2C, Ch), 12.59 (2C, Ci).
Synthesis of 2-(2-Methoxyphenyl)-1H-phenanthro[9,10-d]-
imidazole (1′Me). A solution of 9,10-phenanthrenequinone (0.250 g,
1.2 mmol), 2-methoxybenzaldehyde (0.163 g, 1.2 mmol), and
ammonium acetate (1.5 g, 19.5 mmol) in glacial acetic acid (8 mL)
was stirred at 90 °C for 4 h. The reaction mixture was poured into ice-
cold water. Resulting precipitate was filtered, washed properly with
water, and dried. Pure product was obtained as faded white powder.
(0.32 g, 83%). Elemental analysis: Calcd (%) for C22H16N2O: C, 81.46;
H, 4.97; N, 8.64; O, 4.93. Found: C, 81.22; H, 5.13; N, 8.31; O, 4.65.
ESI-MS [C22H16N2O][H+] calcd: m/z 325.13. Found: m/z 325.03. 1H
NMR (500 MHz, DMSO-d6, Si(CH3)4): 12.73 (s, 1H), 8.86 (d, J = 8 Hz,
2H), 8.64 (s, 2H), 8.23 (d, J = 8 Hz, 1H), 7.72 (t, J = 8 Hz, 2H), 7.63 (t, J
= 8 Hz, 2H), 7.51 (t, J = 8 Hz, 1H), 7.28 (d, J = 8 Hz, 1H), 7.17 (t, J = 8
Hz, 1H), 4.04 (s, 3H).
1/(I − I0) = 1/(I − Is) + 1/{k(I − I0)[Al3+]}
(1)
where I0 is the PL intensity of free ligand (1H), and I is the PL intensity
upon each addition of Al3+. Is is the PL intensity at saturation point. The
errors were calculated within 10%.
Calculation of Detection Limit. Detection limit (DL) was
calculated using the following eq 2:
DL = (3 × SD)/slope
(2)
where SD corresponds to standard deviation of the blank sample,
measured by 15 consecutive scans of the black sample. Slope is obtained
from the linear fit plot of PL intensity changes versus concentration of
Al3+ added.
Calculation of Excited-State Lifetimes. Equation (3) was used to
analyze the time-resolved emission decays:
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.6b01170. CCDC No. 1475692 (L1) contains the
supplementary crystallographic data for this paper. These data
can be obtained free of charge from the Cambridge Crystallo-
graphic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
■
S
P(t) = B +
a e−t/ti
∑
i
(3)
i
where P(t) is decay, i is the number of discrete emissive species, B is a
baseline correction, αi is the pre-exponential factor, and τi is the excited-
state lifetime associated with the ith component, respectively. In case of
multiexponential decays eq 4 was used to calculate average lifetime:
All NMR, ESI-MS, UV−vis, and PL titration data for 1H
and Al3+ (PDF)
Crystallographic information file for 1H (CIF)
⟨τ⟩ =
a τ
∑
i i
(4)
i
AUTHOR INFORMATION
Corresponding Author
*E-mail: icpg@iacs.res.in.
Notes
■
where ai is the contribution of the ith decay component, and ai = αi/Σαi.
X-ray Crystallographic Refinement Details. The crystallo-
graphic details of ligand 1H is given in Table S1, Supporting
Information. A diffractable-size crystal was collected from the mother
liquor, dipped in paratone oil, and then cemented on the tip of a glass
fiber using epoxy resin. Intensity data of the crystal were collected using
Mo Kα (λ = 0.7107 Å) radiation on a Bruker SMART APEX
diffractometer, equipped with a CCD area detector at 150 K. Data
integration and reduction were processed by SAINT13a software.
Empirical absorption correction to the collected reflections was done by
applying SADABS.13b Structure was solved using SHELXTL14 and was
refined on F2 by the full-matrix least-squares technique using the
SHELXL-9715 program package. PLATON-9716 and MERCURY 3.117
were used to generate graphics. Additional crystallographic information
is available in the Supporting Information.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
P.G. gratefully acknowledges Science and Engineering Research
Board, India, for financial support and Alexander von Humboldt
Foundation for donating a fluorimeter. S.S. and B.C. would like
to thank CSIR, India, for research fellowship. The authors
acknowledge S. Mondal, IACS, for his help in theoretical
calculations.
Synthesis of 5-(Diethylamino)-2-(1H-phenanthro[9,10-d]-
imidazole-2-yl)phenol (1H). A solution of 9,10-phenanthrenequinone
(0.250 g, 1.2 mmol), 4-(diethylamino)-2-hydroxybenzaldehyde (0.232
g, 1.2 mmol), and ammonium acetate (1.5 g, 19.5 mmol) in glacial acetic
acid (10 mL) was stirred at 90 °C for 4 h. The reaction mixture was
poured into ice-cold water. Resulting precipitate was filtered, washed
properly with water, and dried in air. The brownish-white residue was
purified by medium-pressure column chromatography with silica gel of
60−120 mesh size using petroleum ether/ethyl acetate as eluent. Pure
product was isolated at 20% ethyl acetate concentration as faded white
powder (0.35 g, 77%). Elemental analysis: Calcd (%) for C25H23N3O: C,
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