Mononuclear aluminum complex derived from 1,1,1,1-tetrakis[(2- salicylaldiminomethyl)]methane acting as a zinc sensor: Crystal structure, emission and lifetime studies

Article abstract of DOI:10.1016/j.poly.2012.03.041

The mononuclear complex [AlHL] (1) has been prepared using AlCl ₃, H₄L and triethylamine in a 1:1:3 ratio (H₄L = 1,1,1,1-tetrakis[(2-salicylaldiminomethyl)]methane). [Al(HL)] crystallizes in the monoclinic space group P2(1)/c. Although H₄L does not emit at room temperature, a remarkable CHEF (Chelation Enhancement of Fluorescence Emission) effect is observed upon the formation of the complex [Al(HL)] (1). When excited at a wavelength of 350 nm, complex 1 emits at 433 nm. Blue emitting aluminum complexes find special importance as far as OLEDs are concerned and complex 1 is one of the best blue emitting compounds to the best of our knowledge. Emission titration of a solution of complex 1 in acetonitrile with Zn²⁺, Cd²⁺, Hg²⁺ and Pb²⁺ shows selectivity towards the Zn²⁺ ion. Unlike Cd²⁺, Hg ²⁺ and Pb²⁺, which quench the emission, Zn²⁺ enhances the fluorescence intensity by 1.64-folds. The quenching process follows the Stern-Volmer equation and in order to get an insight of whether the quenching is static or dynamic in nature, lifetime measurements were carried out. Interestingly, mercury was found to follow static quenching, while cadmium, lead and nickel adopted dynamic quenching pathways.

Full text of DOI:10.1016/j.poly.2012.03.041

Polyhedron 40 (2012) 72–80
Contents lists available at SciVerse ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Mononuclear aluminum complex derived from
,1,1,1-tetrakis[(2-salicylaldiminomethyl)]methane acting as a zinc sensor:
Crystal structure, emission and lifetime studies
1
a,b,
⇑
, Papu Biswas c,
⇑
Supriya Dutta
a
Department of Inorganic Chemistry, Indian Association for the Cultivation of Science, Jadavpur, Kolkata 700 032, India
Department of Chemistry, Budge Budge Institute of Technology, Nischintapur, Budge Budge, Kolkata 700 137, India
Department of Chemistry, Bengal Engineering and Science University, Shibpur, Howrah 711 103, India
b
c
a r t i c l e i n f o
a b s t r a c t
Article history:
The mononuclear complex [AlHL] (1) has been prepared using AlCl , H L and triethylamine in a 1:1:3
3
4
Received 25 January 2012
Accepted 16 March 2012
Available online 12 April 2012
ratio (H
space group P2(1)/c. Although H
Enhancement of Fluorescence Emission) effect is observed upon the formation of the complex [Al(HL)]
1). When excited at a wavelength of 350 nm, complex 1 emits at 433 nm. Blue emitting aluminum
complexes find special importance as far as OLEDs are concerned and complex 1 is one of the best blue
emitting compounds to the best of our knowledge. Emission titration of a solution of complex 1 in ace-
4
L = 1,1,1,1-tetrakis[(2-salicylaldiminomethyl)]methane). [Al(HL)] crystallizes in the monoclinic
4
L does not emit at room temperature, a remarkable CHEF (Chelation
(
Keywords:
Aluminum
Crystal structure
Absorption
Emission
2
+
2+
2+
2+
2+
2+
2+
2+
tonitrile with Zn , Cd , Hg and Pb shows selectivity towards the Zn ion. Unlike Cd , Hg and Pb
,
2
+
which quench the emission, Zn enhances the fluorescence intensity by 1.64-folds. The quenching
process follows the Stern–Volmer equation and in order to get an insight of whether the quenching is sta-
tic or dynamic in nature, lifetime measurements were carried out. Interestingly, mercury was found to
follow static quenching, while cadmium, lead and nickel adopted dynamic quenching pathways.
Ó 2012 Elsevier Ltd. All rights reserved.
Lifetime
1
. Introduction
Apart from their biological importance, aluminum complexes,
3
mainly aluminum tris(quinolin-8-olate) (AlQ ), also find a special
The coordination chemistry of the group 13 (IIIA) metal ions is of
place in the field of organic light emitting diodes (OLEDs) by virtue
of their excellent emission properties, high quantum yields and good
color purity. So far as flat full-colour displays are concerned, lumino-
phores with color tunability encompassing the entire visible range
(red, green and blue) are desired. Although efficient green and red
emitting OLEDs have been designed [16–22], the scarcity of aluminum
complexes emitting in the indigo-blue (430–500 nm) region restricts
their extensive use. Designed fabrications of ligands have enabled a
few aluminum complexes to emit at around 475 nm [23], while only
one complex, to the best of our knowledge, emits at 438 nm [24].
In our efforts to explore the metal complex chemistry of tetrapo-
dal ligands with a carbon bridge head, we have prepared the ligand
1,1,1,1-tetrakis[(2-salicylaldiminomethyl)]methane. Herein we re-
port the synthesis, structure and photophysical properties of an
aluminum complex that emits in the blue region at 433 nm, and
serves as an excellent sensor for zinc.
biomedical interest because of the involvement of aluminum with
neurological dysfunctions, being suspected [1,2] of inducing neuro-
logical dysfunctions such as Alzheimer’s disease. In addition to
Al(III), Zn(II) is also thought to play a vital role in certain neurolog-
ical disorders [3]. Disruption of the uptake of zinc may lead to the
generation of amyloid plaques in Alzheimer’s disease [4,5]. Thus,
due to the presence of zinc as an essential component of many bio-
logical enzymes and substrates such as carbonic anhydrase [6] as
well as its use in the area of cancer, neuroscience [7,8], etc., sensing
of zinc in solution has become a prime area of research during the
last decade. Macrocyclic zinc sensing fluorophores developed by
Kimura et al. showed an 8-fold increase of fluorescence intensity
[
1
9,10], while Nagano et al. enhanced the fluorescence intensity to
4-fold by irradiating with visible light [11]. The Zinpyr [12–15]
family developed by Lippard and co-workers still serves as one of
the best models for fluorescence based detection of zinc.
2
. Experimental
⇑
Corresponding authors. Addresses: Department of Chemistry, Budge Budge
Institute of Technology, Budge Budge, Kolkata 700 137, India (S. Dutta), Department
of Chemistry, Bengal Engineering and Science University, Shibpur, Howrah 711 103,
India. Fax: +91 33 2668 4564 (P. Biswas).
2.1. Materials
Reagent grade chemicals obtained from commercial sources
were used as received. Solvents were purified and dried according
E-mail addresses: supriyo78@rediffmail.com (S. Dutta), biswaspapu@rediffmail.
com (P. Biswas).
0
277-5387/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.poly.2012.03.041

S. Dutta, P. Biswas / Polyhedron 40 (2012) 72–80
73
to standard methods [25]. The tetrapodal ligand H
has been synthesized according to the procedure previously re-
ported by us [26,27].
4
L (see Chart 1)
Table 1
Crystallographic data of [Al(HL)] (1).
Complex
1
Formula
C
33 29 4 4
H N O Al
2.2. Physical measurements
Formula weight
T (K)
572.58
120(2)
Crystal system, space group
Unit cell dimensions
Monoclinic, P2(1)/c
Elemental (C, H and N) analyses were performed on a Perkin-
Elmer 2400 II elemental analyzer. IR spectra were recorded using
KBr disks on a Shimadzu FTIR 8400S spectrometer. The electronic
spectra were recorded using a Perkin-Elmer 950 UV–Vis/NIR spec-
trophotometer. The luminescence spectra were recorded on a
Perkin-Elmer LS 55 luminescence spectrophotometer at room tem-
perature. Lifetime measurement for photo-emission was carried
out with a Horiba Jobin YVON steady state spectrometer. Emission
and lifetime titrations of [Al(HL)] with various metal ions were car-
ried out at room temperature in acetonitrile medium; the concen-
a (Å)
b (Å)
c (Å)
12.5964(6)
14.7675(7)
16.8111(8)
90
90.144(2)
90
3127.1(3)
4, 1.216
a
(°)
b (°)
(°)
c
3
V (Å )
3
Z, qcalc (Mg/m )
ꢁ
1
l
(mm
)
0.107
1200
F(000)
Crystal size (mm )
No. of data/restraints/parameters
No. of refections [I > 2
Goodness-of-fit (GOF)on F
Limiting indices
Final R indices[I > 2
R indices (all data)
3
0.44 ꢀ 0.32 ꢀ 0.28
ꢁ5
tration of [Al(HL)] (1) was 1 ꢀ 10 (M) in every case. Acetonitrile
solutions of Zn(ClO O, Cd(ClO ꢂ6H O, Hg(ClO ꢂ6H O,
ꢂ4H
Pb(ClO O, Cu(ClO ꢂ6H O, Ni(ClO ꢂ6H O and Co(ClO
ꢂ6H
O were used to perform the titrations. The concentrations of
5720/0/379
4
)
2
2
4
)
2
2
4
)
2
2
r
(I)]
5720
1.092
2
4
)
2
2
4
)
2
2
)
4 2
2
4
)
2
ꢂ
ꢁ15 6 h 6 15ꢁ17 6 k 6 17ꢁ19 6 l 6 20
6H
2
a
a
b
b
2
+
2+
2+
2+
2+
2+
2+
r
(I)]
R
R
1
= 0.0595,wR
= 0.0756,wR
2
= 0.1216
= 0.1268
Zn , Cd , Hg , Pb , Cu , Ni and Co used during the absorp-
ꢁ4
ꢁ1
1
2
tion, emission and lifetime titrations were 1 ꢀ 10 , 1 ꢀ 10 , 1 ꢀ
P
P
ꢁ2
ꢁ3
ꢁ3
ꢁ1
a
1
R (F) = ||F
o
| ꢁ |F
c
||/ |F
o
|.
1
0
, 1 ꢀ 10 , 1 ꢀ 10 , 1 and 1 ꢀ 10 (M), respectively. ESI-MS
P
P
b
ðF Þ ¼ ½ wðF ꢁ F Þ = wðF Þ ꢃ1=2.
2
2
2
2
2
o
2
wR
2
o
c
in acetonitrile were obtained on a Micromass Qtof YA 263 mass
spectrometer. H NMR spectra were obtained on a Bruker Avance
1
DPX spectrometer at 500 MHz.
0
0
Ar-H(a)); 6.99 (t, 2H, J
1
2
= 6.46 Hz, J = 7.13 Hz, Ar-H(c + d )); 6.71
2
.3. Preparation of the metal complex
1 2
(d, 3H, J = 8.15 Hz, Ar-H(d)); 6.68 (t, 3H, J = 8.00 Hz, J = 7.50 Hz,
0
Ar-H(c)); 4.13 (d, 3H, J = 13.25 Hz, CH
2
(g/g )); 3.71 (dd, 2H,
0
[
Al(HL)]ꢂCH
containing the ligand H
0.14 g, 1.04 mmol) was added dropwise a solution of triethyl-
3
CN (1): To a boiling acetonitrile solution (25 mL)
J = J = J = 12.91 Hz, CH (h)); 3.50 (d, 3H, J = 12.91 Hz, CH (g /g)).
1
2
3
2
2
1
4
L (0.55 g, 1.00 mmol) and anhydrous AlCl
3
H NMR (500 MHz, CD CN) d: 12.79 (s, 1H, Ar-OH); 8.48 (s, 1H,
3
0
(
CH(e )@N); 8.13 (s, 3H, CH(e)@N); 7.37 (d, 1H, J = 8.50 Hz, Ar-
H(a )); 7.30 (t, 1H, J = 8.00 Hz, J = 7.50 Hz, Ar-H(b )); 7.20–7.16
1 2
0
0
amine (0.30 g, 2.97 mmol) in acetonitrile. During the addition of
the base the color of the solution changed from yellow to almost
colorless. The solution was heated under reflux for 1 h, after which
it was evaporated to dryness on a rotary evaporator. The pale yel-
low solid thus obtained was recrystallized from 1:1 acetonitrile-
ethanol. Single crystals for X-ray analysis were obtained by slow
diffusion of diethyl ether into a concentrated acetonitrile solution
of the complex.
(m, 6H, Ar-H(a + b)); 6.89 (t, 1H, J = 7.50 Hz, J = 7.50 Hz, Ar-
1
0
2
0
H(c )); 6.85 (d, 1H, J = 8.50 Hz, Ar-H(d )); 6.55 (t, 3H, J = 7.50 Hz,
1
0
J = 7.00 Hz, Ar-H(c)); 6.38 (d, 3H, J = 8.00 Hz, Ar-H(d )); 3.98 (d,
3H, J = 13.50 Hz, CH (g/g )); 3.61 (dd, 2H, J = J = 13.00 Hz,
J = 17.00 Hz, CH (h)); 3.49 (d, 3H, J = 13.50 Hz, CH (g /g)).
2
0
2
1
3
0
2
2
2
2.4. X-ray crystallography
Yield: 0.46 g (75%). Anal. Calc. for C35
32 5 4
H N O Al: C, 68.51; H,
5
.22; N, 11.42. Found: C, 68.45; H, 5.17; N, 11.48%. ESI-MS (positive)
Crystals suitable for structure determination of [Al(HL)] (1)
1
+
1
+
3
in CH CN: m/z = 572.86 [Al(HL )+H] (100%), 594.83 [Al(HL )+Na]
were obtained by slow diffusion of diethyl ether into a concen-
trated acetonitrile solution of the complex. A crystal was mounted
on a glass fiber using perfluoropolyether oil. Intensity data were
collected on a Bruker-AXS SMART APEX II diffractometer at
ꢁ1
(
(
(
95%). FT-IR (KBr, m/cm ): 3433 (w, br), 2914 (w), 1629 (s), 1602
m, sh), 1545 (s), 1473 (s), 1454 (s), 1402 (m), 1342 (m), 1323
m), 1276 (w), 1209 (m), 1149 (m), 1031 (w), 904 (w), 808 (w),
1
7
61 (s), 740 (w), 626 (m), 507 (w), 460 (w), 426 (w). H NMR
1
0
20(2) K using graphite-monochromated Mo K
a radiation (k =
0
(
500 MHz, CD
2
Cl
2
) d: 12.64 (s, 1H, Ar-OH); 8.52 (s, 1H, CH(e )@N);
.71073 Å). The data were processed with SAINT [28] and absorption
0
0
8
.15 (s, 3H, CH(e)@N); 7.40 (d, 2H, J = 7.50 Hz, Ar-H(a + b )); 7.33
corrections were made with SADABS [28]. The structure was solved
by direct and Fourier methods and refined by full-matrix least-
(
t, 3H, J
1
= 8.06 Hz, J
2
= 7.78 Hz, Ar-H(b)); 7.23 (d, 3H, J = 7.78 Hz,
2
squares methods based on F using SHELX-97 [29]. For the structure
N
solution and refinement, the SHELXTL software package [30] was
used. A highly disorder solvent molecule (acetonitrile) in com-
pound 1 was deleted using the SQUEEZE facility of PLATON [31].
The non-hydrogen atoms were refined anisotropically, while the
hydrogen atoms were placed at geometrically calculated positions
with fixed thermal parameters. Crystal data and details of data col-
lection are listed in Table 1.
HO
N
N
N
OH HO
OH
3
. Results and discussion
3.1. Syntheses and characterization
H L
4
The ligand H
4
L was obtained in 75–80% yield by reacting the
Chart 1. Schematic diagram of the ligand 1,1,1,1-tetrakis[(2-salicylaldiminometh-
yl)]methane (H L).
4
hydrochloride salt of 1,1,1,1-tetrakis(2-aminomethyl)methane,

7
4
S. Dutta, P. Biswas / Polyhedron 40 (2012) 72–80
Fig. 1. ORTEP representation of [Al(HL)] (1) at 50% probability ellipsoids. Hydrogen atoms have been removed for clarity.
triethylamine and salicylaldehyde in a 1:4:4 molar ratio in metha-
Table 2
Selected bond distances (Å) and angles (°) of [Al(HL)] (1).
nol [13]. Complex 1 was obtained in a reaction involving 1 equiv-
3 4
alent of AlCl , 1 equivalent H L and 3 equivalents of triethylamine
1
in boiling acetonitrile. Crystals suitable for X-ray diffraction were
obtained by diffusing slowly diethyl ether in a concentrated solu-
tion of 1 in acetonitrile.
Al–N(1)
Al–N(2)
Al–N(3)
Al–O(1)
Al–O(2)
Al–O(3)
2.037(2)
2.050(2)
2.042(2)
1.837(2)
1.831(2)
1.826(2)
O(1)–Al–O(2)
O(1)–Al–O(3)
O(2)–Al–O(3)
N(1)–Al–O(1)
N(1)–Al–O(2)
N(1)–Al–O(3)
N(2)–Al–O(1)
N(2)–Al–O(2)
N(2)–Al–O(3)
N(3)–Al–O(1)
N(3)–Al–O(2)
N(3)–Al–O(3)
N(1)–Al–N(2)
N(1)–Al–N(3)
N(2)–Al–N(3)
92.62(9)
95.14(10)
91.53(9)
88.37(10)
94.40(10)
172.97(10)
172.18(10)
89.25(9)
92.39(10)
93.52(9)
173.78(10)
88.94(9)
83.91(9)
84.76(9)
84.53(9)
The significant IR spectral bands observed for complex 1 are a
broad band due to hydrogen bonded phenolic O–H at about
ꢁ1
ꢁ1
3
440 cm , and two strong bands between 1630 and 1600 cm
due to metal coordinated C@N and uncoordinated
m
C@N
m .
The electrospray ionization mass spectrum (ESI-MS positive) of
complex 1 has been measured in acetonitrile. The peaks (shown in
III
+
III
+
Fig. S1) observed for 1 are [Al (HL)+H] and [Al (HL)+Na] at m/
z = 572.86 (100%) and m/z = 594.84 (95%), respectively. As seen
from Fig. S1, the observed and the simulated peaks are in good
agreement with each other.
3.2. Description of the crystal structure
Table 3
Metrical parameters for C–Hꢂ ꢂ ꢂ
p
interactions in [Al(HL)] (1).
The ORTEP representation of [Al(HL)] (1) is shown in Fig. 1. Se-
a
Compound
C–Hꢂ ꢂ ꢂ
p
atom
Hꢂ ꢂ ꢂ
p
C–Hꢂ ꢂ ꢂ
p
lected bond distances and bond angles for 1 are given in Table 2.
Compound 1 crystallizes in the monoclinic P2(1)/c space group.
The coordination environment around the central metal ion is
slightly distorted octahedral, in which the Al(III) center is coordi-
nated by three imine nitrogens and three phenolate oxygens
distance (Å)
angle (°)
[
Al(HL)] (1)
C(2)–H(2B)ꢂ ꢂ ꢂ
C(3)–H(3B)ꢂ ꢂ ꢂ
C(5)–H(5B)ꢂ ꢂ ꢂ
p
p
p
p
p
C(27)ꢂ ꢂ ꢂC(32)
C(13)ꢂ ꢂ ꢂC(18)
C(6)ꢂ ꢂ ꢂC(11)
2.564
3.294
2.931
3.184
145.14
150.90
149.01
156.04
141.24
C(24)–H(24)ꢂ ꢂ ꢂ
C(31)–H(31)ꢂ ꢂ ꢂ
C(6)ꢂ ꢂ ꢂC(11)
provided by three pendant salicylaldimines. Each N
3
and O
3
donor
C(13)ꢂ ꢂ ꢂC(18) 3.100
set coordinates to the metal ion in a facial way to form a distorted
octahedral geometry. Fig. 1 shows that in compound 1 one of the
salicylaldimine moieties remains uncoordinated and the phenolic
OH and the imine nitrogen are intramolecularly hydrogen-bonded.
The average Al(III)–N(imine) and Al(III)–O(phenolate) distances are
a
p
designates the centroid of the aromatic ring.
albeit the majority of them lie between 2.6 and 3.1 Å, indicating
strong intermolecular C–Hꢂ ꢂ ꢂ
p interactions.
2
.043(2) and 1.831(2) Å, respectively. The transoid angles in 1 vary
between 172.18(10)° and 173.78(10)°, while the cisoid angles vary
between 83.91(9)° and 95.14(10)°. The packing diagram of com-
pound 1 reveals several short contacts between either a methylene
hydrogen atom or an aromatic hydrogen atom and the centroid of
3.3. Proton NMR spectra
1H NMR spectroscopic studies were carried out for complex 1 in
Cl as well as in CD CN. The observed chemical shifts for com-
the
p
cloud of another aromatic ring. These C–Hꢂ ꢂ ꢂ
p
interactions
CD
2
2
3
are intermolecular in nature. Generally, the upper acceptable limit
pound 1 (Fig. 2) along with the spectral assignments are given in
the experimental section, based on the proton numbering scheme
as shown in Chart 2. The hydrogen-bonded phenolate O–Hꢂ ꢂ ꢂN is
for the distance of a C–H hydrogen atom from the centroid of an
aromatic ring in a C–Hꢂ ꢂ ꢂ
parameters for the C–Hꢂ ꢂ ꢂ
indicate that the C–Hꢂ ꢂ ꢂ
p
interaction is 3.5 Å [32,33]. The metrical
interactions in compound 1 (Table 3)
distances range from 2.564 to 3.335 Å,
p
2 2
observed as a sharp singlet at 12.64 and 12.79 ppm in CD Cl and
p
3
CD CN, respectively. Compound 1 shows 12 signals, including a

S. Dutta, P. Biswas / Polyhedron 40 (2012) 72–80
75
e'
d'
N
h
c'
c
f
HO
g'
b'
d
g
a'
g'
g
e
N
O
N
N
h
Al
O
O
a
b
[
Al(HL)]
4
.2
4.1
4.0
3.9
3.8
3.7
3.6
3.5
3.4
d
b
a
c
e
e'
a'+ b'
c' + d'
8
.5
8.0
7.5
7.0
6.6
*
*
f
1
2
11
10
9
8
7
6
5
4
3
2
1
0
ppm
Fig. 2. 1H NMR spectrum of [Al(HL)] (1) in CD
2
2
Cl . Solvent peaks are marked with an asterisk. Assignments of the signal are done on the basis of the proton numbering as
shown in Chart 2.
1
:1:3:3. Further, the eight methylene protons appear as two dou-
blet and one doublet of doublets in the range 4.13–3.50 ppm (in
CD Cl ) and 3.99–3.47 ppm (in CD CN), with an intensity ratio of
:1:2 in either case. It seems that the methylene protons of the
e'
d'
N
2
2
3
h
c'
c
2
f
HO
three metal chelated pendant salicylaldiminomethyl arms are
anisochronous and are observed as two doublets, but they are
isochronous for the metal free fourth arm.
b'
d
g
a'
g'
1
e
H NMR titration of [Al(HL)] (1) with incremental addition of
N
O
N
N
Zn(ClO
aromatic as well as the aliphatic protons of the uncoordinated pen-
dant arm. With the addition of 0.31 equivalents of Zn(ClO O,
the originally sharp, well defined aromatic/aliphatic peaks, mostly
due to the aromatic protons bound to the metal ion, broadened,
accompanied by a small shift towards the low field region, as
shown in Fig. S2 (Supporting Information). The relatively less per-
turbed aromatic signals (as shown in Fig. S2, Supporting Informa-
tion), corresponding to the metal free pendant arm, signifies a
4
)
2 2 3
ꢂ4H O in CD CN showed a slight perturbation for the
Al
O
O
4
)
2
ꢂ 4H
2
a
b
[
Al(HL)]
Chart 2. Proton labelling for the ligand [Al(HL)] (1).
2
+
weak interaction of Zn with the uncoordinated salicylaldimine
moiety.
2 2
pair of singlets due to CH@N at 8.52 and 8.15 ppm (in CD Cl ) and
at 8.48 and 8.13 ppm (in CD
:3, in coherence with a structure with one pendant arm lying
uncoordinated to the metal center. The 16 aromatic protons of
each ligand occur between 7.40 and 6.68 ppm (in CD Cl ), with
the intensity ratio of the signals being 2:3:3:2:3:3, and between
.38 and 6.37 ppm (in CD CN) having an intensity ratio of 1:1:6:
3
CN), with a proton intensity ratio of
1
3.4. Absorption spectra
2
2
The absorption spectrum of complex 1 recorded in acetonitrile
ꢁ1
ꢁ1
7
3
show two peaks at 350 nm (e = 20000 M cm ) and 263 nm

7
6
S. Dutta, P. Biswas / Polyhedron 40 (2012) 72–80
2
+
3
+
Fig. 4. Change of absorption profile of the titration of complex 1 with the Hg ion
Fig. 3. Change of absorption profile of the titration of the ligand H
4
L with the Al
2+
3
+
in acetonitrile at room temperature. Concentration of complex 1 and Hg ions are
ion in acetonitrile at room temperature. Concentration of H L and Al ion are
4
1 ꢀ 10ꢁ and 0.01 (M), respectively.
5
ꢁ
5
ꢁ3
1
ꢀ 10 and 2 ꢀ 10 (M), respectively.
ꢁ
1
ꢁ1
(e
= 45000 M cm ), accompanied by a shoulder at 275 nm. Both
⁄
the peaks are likely to be ligand based
p
?
p
transitions [24,34].
The formation of complex 1 in solution was monitored spectro-
photometrically as well as spectrofluorometrically by titrating a
ꢁ
5
1
ꢀ 10 (M) solution of the ligand H
4
L by the gradual addition
solution (2 ꢀ 10 (M)) in acetonitrile. Fig. 3 shows
L with the
. Among the three peaks originally observed in
the spectrum of H L at 319, 258 and 215 nm, the peaks at 319
ꢁ3
of an AlCl
3
the change in the absorption profile of the ligand H
addition of AlCl
4
3
4
3+
and 258 nm diminish with the addition of the Al ion, and new
absorption peaks emerge at 350 and 263 nm, along with a shoulder
at 275 nm, at the expense of the original peaks. However, the
intensity of the peak at 215 nm increases without any shift of
wavelength. Addition of 0.92 equivalents of Al3 ion with respect
+
4
to the ligand H L led to saturation, and the spectrum thus obtained
is identical to that of complex 1, implying the formation of the
mononuclear complex [Al(HL)] only, in solution as well as in solid
state as depicted by the crystal structure.
1
0
2+
2+
2+
Titration of complex 1 with d metal ions (Zn , Cd , Hg and
2
+
2+
2+
2+
Pb ) yields almost unchanged spectra for Cd and Pb , while Zn
and Hg show a small but distinct change in the ground state
absorption spectra. Fig. 4 shows the change in the absorption spec-
tra of complex 1 on gradual addition of the Hg ion, while the ini-
tial and final spectra obtained by titrating complex 1 with Zn ,
2
+
Fig. 5. Change of emission profile of the titration of the ligand H
4
L with the Al3+ ion
in acetonitrile at room temperature. Concentration of H
4
L and Al3 ions are 1 ꢀ 10
+
ꢁ5
and 2 ꢀ 10ꢁ (M), respectively.
3
2+
2
+
2
+
2+
2+
Cd , Hg and Pb are shown in Fig. S3(a)–(d). In the case of the
2
+
titration of complex 1 with Zn , the small absorption spectral
3.5. Emission spectra
change observed (Fig. S2, Supporting Information) is in coherence
1
with the H NMR titration, which also shows a small perturbation
The emission spectrum of complex 1 is shown in Fig. S4 (Sup-
porting Information). The ligand H L is non-emissive at room tem-
of the signals due to a weak interaction of Zn2+ with the aromatic
protons of the uncoordinated salicylaldimine moiety. Upon titra-
4
perature. Coordination of Al(III) with the subsequent formation of
[Al(HL)] (1) leads to a CHEF effect [35,36] (Chelation Enhancement
of Fluorescence Emission) with a remarkable increase of emission
intensity at kem = 433 nm when complex 1 is excited (kex) at
350 nm in acetonitrile at room temperature. To the best of our
knowledge, blue emitting aluminum complexes have been re-
ported with kem as low as 438 nm [11], while complex 1 is a better
blue emitter by 5 nm.
2
+
tion of 1 with the Hg ion, the absorption maximum at 350 nm
undergoes a red shift of 5 nm, while in the UV region the intensity
of the shoulder at 275 nm is increased at the expense of the dimin-
ishing peak at 263 nm. Cd2 , somehow, does not alter the absorp-
tion spectra of 1, although it has a small quenching effect upon
the emission intensity of 1, as discussed later. The unavailability
of sufficient space to accommodate the larger Pb in the uncoordi-
nated pendant arm is likely to be the reason that the metal ion can-
not interact with the CH@N as well as the phenolic-OH groups of
the free salicylaldimine moiety, thereby keeping the ground state
absorption spectra unperturbed.
+
2+
As discussed in the previous section, the fluorometric titration of
ꢁ
5
ꢁ3
a 1 ꢀ 10 (M) solution of H
4
L with a solution of AlCl
3
(2 ꢀ 10 (M))
in acetonitrile gave rise to photoluminescence at 433 nm when ex-
cited at kmax = 350 nm, as shown in Fig. 5. Saturation was attained

S. Dutta, P. Biswas / Polyhedron 40 (2012) 72–80
77
Table 4
Absorption, emission and lifetime data of [Al(HL)] (1) with Zn2+, Cd2+, Hg2+, Pb2+, Cu2+
Ni and Co metal ions.
,
2+
2+
Compound abs (nm)
k
k
em
U
s
0
(ns)
s
(ns)
0
F /
c
F
0
s /s
a
c
(
nm)
[
Al(HL)]
1)
350
(20000)
433
0.002
0.570
–
–
–
–
(
2
63
45000)
345
16700)
350
17200)
64
39500)
354
33400)
75
70000)
350
19000)
64
47500)
(
(
1) + Zn2+
1) + Cd2+
438
433
0.003
0.552b 0.685 0.61
(
(
0.0018 0.567b 0.434 1.35
1.31
(
2
(
(
1) + Hg2+
1) + Pb2+
440
433
433
434
432
0.0012 0.537b 0.565 2.20 1 ± 0.2
(
2
(
(
0.0008 0.573b 0.257 2.50
2.23
(
2
(
(1) + Cu2+
354
(21870)
270
(44815)
360
0.0013 0.590b
–
4.10
–
Fig. 6. Increase of emission intensity of [Al(HL)] (1) with the addition of
2
+
+
approximately 0.3 equivalents of Zn
at room temperature in acetonitrile.
2
ꢁ5
ꢁ4
Concentration of complex 1 and Zn ions are 1 ꢀ 10
and 1.5 ꢀ 10
(M),
respectively.
(
1) + Ni2+
1) + Co2+
0.0015 0.751b 0.292 2.44
0.0019 0.547b 0.570 1.15
2.57
0.96
(
32200)
64
54200)
350
23400)
64
53350)
2
(
(
(
2
(
a
b
c
k
ex = 350 nm.
Lifetime of (1) without addition of the corresponding M2+.
Value of the ratio of the emission intensity without addition of metal ion to the
emission intensity at saturation.
Fig. 7. Quenching of the emission intensity of complex 1 with the gradual addition
2+
of the Ni ion at room temperature in acetonitrile. Concentration of complex 1 and
2
+
ꢁ5
Ni ions are 1 ꢀ 10 and 1 (M), respectively. Inset showing emission intensity vs.
2+
equivalents of Ni plot.
for a ligand (H
mation of the mononuclear Al(III) complex.
Interestingly, complex 1 when titrated with d metal ions
L):Al3+ mole ratio of 1:0.92, again confirming the for-
4
10
+
2+
2+
2+
(
Zn2 , Cd , Hg and Pb ), as well as other first row transition me-
2
+
2+
2+
2+
tal ions like Cu , Ni and Co , shows the ability to sense Zn . As
2+
2+
2+
shown in Fig. S5(a)–(c), Cd , Hg and Pb simply quench the
emission intensity practically without altering the emission wave-
length (kem = 433 nm). The extent of quenching increases with hea-
Fig. 8a. Extent of the quenching of emission intensity of [Al(HL)] (1) with various
2+
M
(M = Zn, Cd, Hg, Pb, Cu, Ni, Co) ions in acetonitrile at room temperature.
2
+
2+
2+
vier ions such as Cd , Hg and Pb , reducing the emission
intensity by 1.35-, 2.2- and 2.5-fold, respectively. This quenching
of emission intensity may be ascribed to the heavy atom effect
approximately 5 nm, as shown in Fig. 6. The increase of emission
37], especially in case of Hg2 and Pb . On the other hand, when
+
2+
2+
[
intensity up to the addition of 0.31 equivalents of Zn may prob-
2+
2
+
complex 1 is titrated with Zn , the emission intensity is enhanced
by 1.64-fold on addition of 0.31 equivalents of Zn2 ion with
respect to complex 1. This increase of fluorescence intensity is
accompanied by a red-shift of the emission wavelength (kem) by
ably indicate a Zn :[AlHL] = 1:3 weak interaction, where three
uncoordinated salicylaldimine moieties from three molecules of
+
2
+
complex 1 weakly coordinate with Zn to enhance the emission
intensity.

7
8
S. Dutta, P. Biswas / Polyhedron 40 (2012) 72–80
2
r
2
std
A
A
std
I
r
g
g
U
r
¼
U
std
ð1Þ
r
I
std
where
U
r
and
U
std are the quantum yields of unknown and stan-
std = 0.042 at 298 K in CH CN, A and Astd
<0.1) are the solution absorbance at the excitation wavelength
ex), I and Istd are the integrated emission intensities, and and
std are the refractive indices of the solvent.
dard samples [38] (
U
3
r
(
(
k
r
g
r
g
Thus, complex 1 itself acts as a zinc sensor, which can sense
Zn2+ to a concentration as low as 1.0 ꢀ 10 (M). A comparative
ꢁ7
2
+
study is shown in Fig. 8a where the sensing of Zn ions by means
of fluorescence enhancement is clearly depicted against the
quenching effect of the other metal ions. Fig. 8b shows the same
observation quantitatively in the form of a bar diagram in terms
of (F ꢁ F
0 0
) [F = emission peak intensity for AlHL (1) in the absence
of quencher; F = emission peak intensity of 1 saturated with the
quencher] for the metal ions in a more perceptible manner. In
quest of having an insight into the nature of the quenching,
Fig. 8b. Bar diagram showing the selectivity of complex 1 towards the Zn2+ ion by
enhancement of the emission intensity as compared to other metal ion quenchers.
0
Stern–Volmer plots [39] (F /F versus [Q] where [Q] = quencher
When complex 1 is titrated with 3d-transition metal ions like
concentration) were drawn. In all cases, F /F versus [Q] gave linear
plots (Fig. S6(a)–(c), Supporting Information). KSV values [23] ob-
0
2
+
2+
2+
Cu , Ni and Co , all led to the quenching of the emission to
varying extents. Fig. 7 shows the representative emission quench-
tained from the slopes of F /F versus [Q] plots for the quenching
0
ing profile of complex 1 by the Ni2 ion. It is to be noted that Cu is
+
2+
2+
2+
2+
of emission intensity of [Al(HL)] (1) by Cd , Hg and Pb are
found to be the most efficient quencher among the transition metal
20.90, 9834.22 and 9612.49, respectively. The K values clearly
SV
2
+
2+
2+
2+
ions, followed by Ni and Co ions. Cu quenches the emission of
indicate that Cd has very little effect upon the quenching of fluo-
complex 1 by 4.10-fold, while Ni and Co2+ quench the emission
by 2.44- and 1.15-fold, respectively. The quantum yields of
complex 1 saturated with M2 ions (M = Zn, Cd, Hg, Pb, Cu, Ni
2+
2+
2+
rescence of complex 1, while Hg and Pb strongly quench the
emission of complex 1 due to the heavy atom effect [37]. Excellent
+
2+
2+
linear fits were also obtained for (F /F) versus [Q] (Q = Cu , Ni
0
2
+
and Co), measured against [Ru(bpy)
Table 4.
3
]Cl
2
using Eq. (1), is given in
and Co ) plots, as shown in Fig. S7(a)–(c) (Supporting Information)
with KSV values of Cu , Ni and Co being 31760, 21.5 and 53,
2
+
2+
2+
Fig. 9a. Lifetime profile of [Al(HL)] (1) at room temperature in acetonitrile. Red points represent the experimental data, green line shows the best fit line and the blue points
represent the data of the promt.

S. Dutta, P. Biswas / Polyhedron 40 (2012) 72–80
79
AlNi1
AlNi2
AlNi3
AlNi4
AlNi5
AlNi6
AlNi7
AlNi8
AlNi9
AlNi10
AlNi11
AlNi12
Prompt
1000
100
10
-9
-9
-8
-8
-8
4
.0x10
8.0x10
1.2x10
1.6x10
2.0x10
2
+
τ / sec
Fig. 9b. Comarison of the lifetime decay profiles of complex 1 without Zn (green
2
+
squares) and complex 1 with Zn ion (red squares). The solid green and red lines
represent the best fits obtained for the decays. The black and white squares
represent the decay profile of the promt.
Fig. 10. Lifetime decay profiles of complex 1 without and with incremental
2
O in acetonitrile. Concentration of complex 1 and Ni ions
are 1 ꢀ 10 and 1 (M), respectively. The purple spheres represent the decay profile
2+
amounts of Ni(ClO
4
2
) ꢂ6H
ꢁ
5
of the promt.
0
respectively. Although linear plots of (F /F) versus [Q] imply that
the quenching process follows the Stern–Volmer equation, we can-
not say for sure whether the quenching process follows a static or
dynamic route. Hence, lifetime measurements were carried out for
shift of the kmax value (Fig. S3(c)) indicating a strong ligand–metal
coordination, which is a pre-requisite for static quenching. On the
other hand, the absorption spectral profile of complex 1 saturated
the d1 metal ions (Cd , Hg , and Pb ) to ascertain whether the
0
2+
2+
2+
2
+
2+
with Cd and Pb ions showed no change (Fig. S3(b) and (d))
indicating no substantial ligand–metal coordination. Thus the
quenching of emission intensity of complex 1 by these ions was
static or dynamic in nature.
2
+
2+
quenching effect of Cd and Pb is expected to follow a dynamic
quenching pathway, which is also corroborated by the observa-
tions made through lifetime measurement experiments. Table 4
3.6. Lifetime measurements
0 0
shows the corresponding (F /F) and (s /s) values for the titration
2
+
2+
2+
2+
Fig. 9a shows the lifetime profile of AlHL (1) at room tempera-
of 1 with Zn , Cd , Hg and Pb . The peak intensities without
2
+
2+
2+
ture in acetonitrile. Lifetime titrations of complex 1 with Zn , Cd ,
the addition of M (M = Zn, Cd, Hg, Pb, Cu, Ni and Co) ions and sat-
urated with the said metal ions along with the KSV values are given
in Table S1.
2
+
2+
Hg and Pb were carried out in same way as the emission titra-
tions using acetonitrile solutions at room temperature. A compar-
ison of the decay profiles of the lifetimes of complex 1 (green
The lifetime titrations of complex 1 with gradual addition of
2
+
squares) and complex 1 containing 0.31 equivalents of Zn ion
red squares) are shown in Fig. 9b. The solid green and red lines
represent the best fits obtained for the above-mentioned decays.
4
Ni(ClO )
2
ꢂ6H
2
O in acetonitrile were carried out in a similar fashion
(
as that of the fluorometric titration described in the previous sec-
tion. The lifetime of complex 1 without addition of the Ni2+ ion was
found to be 0.75 ns, which gradually decreased with incremental
Due to strong quenching by Cu2 (F
+
/F = 4.10) ion, lifetime data of
0
2
+
2+
complex 1 saturated with the Cu ion could not be recorded.
addition of the Ni ion, as shown in Fig. 10. Upon saturation with
2
+
Moreover, due to a negligible change in emission intensity of com-
the Ni ion, the lifetime was recorded as 0.29 ns. The F
0
/F (= 2.44)
(= 2.57) values obtained by titrating complex 1 with Ni
O, as given in Table S1, clearly indicate that quenching
plex 1 when quenched by Co2 (F
+
0
and s /s
0
/F = 1.15), lifetime titrations
could not be performed. Hence, we were unable to determine the
4
(ClO )
2
ꢂ6H
2
nature of quenching, i.e. whether quenching by Cu2 and Co
+
2+
2+
of complex 1 by Ni ion follows a dynamic quenching pathway.
followed a static or dynamic quenching pathway. It is to be noted
2
+
that addition of Zn ions does not quench the emission intensity,
rather it enhances the fluorescence intensity of complex 1, hence
the (s /s) value for complex 1 with 0.31 equivalents of Zn does
0
4. Conclusions
2
+
not provide us with any significant information and hence it has
In view of making a blue emitting aluminum complex, the mono-
not been mentioned in Table 4.
nuclear complex [Al(HL)] (1) has been prepared using the ligand
Table 4 lists the room temperature lifetimes of AlHL (1) in the
absence of quencher and saturated with quencher. Fig. S8(a)–(d)
1,1,1,1-tetrakis[(2-salicylaldiminoaminomethyl)]methane,
AlCl
3
and triethylamine in a ratio of 1:1:3. Although the ligand is non-
emissive at room temperature, 1 shows a remarkable CHEF effect
with kem = 433 nm, one of the most blue emitting aluminum com-
plexes known to us. Moreover, an acetonitrile solution of complex
1 when titrated with zinc perchlorate (0.31 equivalents with respect
to complex 1) enhanced the emission intensity of 1 by 1.64-fold. All
(
1
Supporting Information) shows the lifetime profiles of complex
2
+
2+
2+
2+
with Zn , Cd , Hg and Pb , respectively. While the lifetime
2+
titration of complex 1 with Hg ions gave a (
ing static quenching [39], the lifetime titration of complex 1 with
Cd and Pb showed dynamic quenching [39], with (F
s
s
0
/s
ꢄ 1) value imply-
2
+
2+
0
/F) ꢄ (
0
s /
1
0
2+
2+
2+
2+
2+
2+
), in coherence with the ground state absorption spectral profile,
other d metal ions (M = Cd , Hg and Pb ) as well as Cu , Ni
2
+
as shown in Fig. S3(b)–(d) (Supporting Information). It may be
mentioned in this context that the ground state absorption spectra
of complex 1 saturated with the Hg2 ion showed a substantial blue
and Co ions only quench the emission of [AlHL] (1) to varying ex-
tents. Thus [AlHL] (1) acts as an excellent sensor for zinc that can
+
2+
sense the presence of Zn ions at concentrations as low as

8
0
S. Dutta, P. Biswas / Polyhedron 40 (2012) 72–80
ꢁ
7
1
ꢀ 10 (M), and shows completely opposite emission behavior as
[4] C.C. Curtain, F. Ali, I. Volitakis, R.A. Cherny, R.S. Norton, K. Beyreuther, C.J.
Barrow, C.L. Masters, A.I. Bush, K.J. Barnham, J. Biol. Chem. 276 (2001) 20466.
compared to other d-transition metal ions. All the metal ions that
quench the emission of complex 1, however, followed the Stern–
[
5] S.W. Suh, K.B. Jensen, M.S. Jensen, D.S. Silva, P.J. Kesslak, G. Danscher, C.J.
Frederickson, Brain Res. 852 (2000) 274.
Volmer equation, as is evident from the linear (F
0
/F) versus quench-
[6] J.M. Berg, Acc. Chem. Res. 28 (1995) 14.
[
7] E. Kimura, S. Aoki, E. Kikuta, T. Koike, Proc. Natl. Acad. Sci. USA 100 (2003)
731.
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er concentration plots. In order to understand the nature of the
quenching, lifetime measurements were carried out by adding
3
[
2
+
2+
2+
2+
2+
2+
Cd , Hg , Pb , Cu , Ni and Co ions to a solution of complex
[9] T. Koike, T. Abe, M. Takahashi, K. Ohtani, E. Kimura, M. Shiro, J. Chem. Soc.,
Dalton Trans. (2002) 1764.
10] S. Aoki, S. Kaido, H. Fujioka, E. Kimura, Inorg. Chem. 42 (2003) 1023.
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(2000) 1052.
10
2+
1
. It is to be noted that among the d metal ions, only Zn enhances
the fluorescence of complex 1, while Cd and Pb quench the emis-
[
[
2+
2+
sion by 1.35- and 2.53-fold, respectively, through dynamic quench-
2
+
[12] S.C. Burdette, C.J. Frederickson, W. Bu, S.J. Lippard, J. Am. Chem. Soc. 125 (2003)
778.
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644.
ing pathway. Hg , on the other hand, follows a static quenching
pathway to quench the emission intensity by 2.20-fold. However,
1
[
2
+
Cu emerges out to be the most efficient quencher, decreasing
2+
1
the emission intensity by 4.10-fold, while Ni quenched the emis-
sion intensity by 2.44-fold following a dynamic quenching pathway.
[
5
2
+
Co stands out as the weakest of the quenchers, reducing the fluo-
rescence intensity by just 1.15-fold.
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[
[
We are thankful to Prof. K. Nag, Department of Inorganic Chem-
istry, Indian Association for the Cultivation of Science for helpful
suggestions during work and for preparation of the manuscript.
We are also thankful to Dr. Sujoy Kumar Baitalik and Debasish Saha,
Department of Chemistry, Inorganic Chemistry Section, Jadavpur
University, Kolkata 700 032 for their suggestions and help during
the preparation of the manuscript. Thanks are due to the Depart-
ment of Science and Technology, Government of India for establish-
ing the National X-ray Diffractometer facility at the Department of
Inorganic Chemistry, Indian Association for the Cultivation of
Science.
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Appendix A. Supplementary data
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