Fluorescent zinc(II) complex exhibiting "onoffon" switching toward Cu2+ and Ag+ ions

Article abstract of DOI:10.1021/ic1018086

Binuclear zinc(II) and copper(II) complexes based on a new Schiff base ligand N,N0-bis(2-hydroxybenzilidene)- 2,4,6-trimethylbenzene-1,3-diamine (H₂L) have been synthesized. The ligand H₂L and complexes under investigation have been

Full text of DOI:10.1021/ic1018086

ARTICLE
pubs.acs.org/IC
Fluorescent Zinc(II) Complex Exhibiting “On-Off-On” Switching
Toward Cu2þ and Agþ Ions
Rampal Pandey,† Prashant Kumar,† Ashish Kumar Singh,† Mohammad Shahid,† Pei-zhou Li,‡
Sanjay Kumar Singh,‡ Qiang Xu,‡ Arvind Misra,† and Daya Shankar Pandey*,†
†Department of Chemistry, Faculty of Science, Banaras Hindu University, Varanasi -221 005 (U.P.), India
‡National Institute of Advanced Industrial Science and Technology (AIST), 1-8-31 Midorigaoka, Ikeda, Osaka 563-8577, Japan
S Supporting Information
b
ABSTRACT: Binuclear zinc(II) and copper(II) complexes
based on a new Schiff base ligand N,N0-bis(2-hydroxybenzili-
dene)-2,4,6-trimethylbenzene-1,3-diamine (H2L) have been
synthesized. The ligand H2L and complexes under investigation
have been characterized by elemental analyses, spectral (FT-IR,
1H, 13C NMR, ESI-MS, electronic absorption, emission), and
electrochemical studies. The structures of H2L and complexes
[{Zn(C23H18N2O2)}2] (1) and [{Cu(C23H18N2O2)}2] H2O
3
(2) have been determined crystallographically. Selective “On-Off-
On” switching behavior of the fluorescent complex 1 has been
studied. The fluorescence intensity of 1 quenches (turns-off)
uponadditionofCu2þ, while enhances (turns-on) inthe presence
of Agþ ions. The mechanisms of “On-Off-On” signaling have
been supported by 1H NMR, ESI-MS, electronic absorption, and
emission spectral studies. Job’s plot analysis supported 1:1 and
1:2 stoichiometries for Cu2þ and Agþ ions, respectively. Association and quenching constants have been estimated by the
BenesiꢀHildebrand method and SternꢀVolmer plot. Moreover, 1 mimics a molecular keypad lock that follows correct chemical
input order to give maximum output signal.
’ INTRODUCTION
containing salicylaldiminato Schiff bases, which act as blue emitters
and find potential applications in various areas.7 However, binuclear
metallacycles based on salen framework exhibiting “On-Off-On”
switchability toward HTM ions have scarcely been investigated.8
Molecular devices based on chemical systems may be em-
ployed in the construction of molecular logic networks within a
single molecule in vivo or in nano spaces.9 Generally, in a keypad
lock system the output signal depends on the correct order of
input signals; therefore, a molecular device that is capable of
distinguishing different chemical sequences might be advanta-
geous over simple molecular logic gates.10 To date, only a few
systems have been devised that can selectively and reversibly
recognize HTM ions by emitting distinct signals leading to their
application as molecular logic gates.10 With the objective of devel-
oping an HTM ion sensor applicable under aqueous conditions, we
have synthesized a binuclear zinc(II) complex [{Zn(C23H18
N2O2)}2] (1) based on N,N0-bis(2-hydroxybenzilidene)-2,4,6-tri-
methyl-benzene-1,3-diamine. It has been demonstrated that the
present system serves as a fluorescent probe under aqueous
conditions (H2O/MeCN 7:3, v/v and in Tris-HCl buffer system,
Fluorescent sensing for selective detection of heavy and
transition metal (HTM) ions has drawn considerable current
attention owing to its high sensitivity and simplicity.1 The
synthesis of biologically and environmentally important HTM
ion sensors is interesting, since it can easily undergo chelation in
organic media. However, because of a strong hydration factor,
most of these have limitations to aqueous environment.2 In this
context, metal ion sensors based on metalꢀligand coordination
or chemical reactions have been developed.3 The syntheses of
sensors capable of efficient sensing and detection of multiple
analytes is challenging.3 It is well documented that copper acts as
a catalytic cofactor for a variety of metalloenzymes, and at the
same time, because of widespread applications, it is an important
metal pollutant.4 Likewise, complexes based on silver find appli-
cations in medicine and agriculture; however, its prolonged use
leads to irreversible darkening of the skin and mucous mem-
brane.5 Therefore, highly selective and sensitive fluorescent
sensors for Cu2þ and Agþ ions have been developed.6 Although
most of the HTM ions act as fluorescence quenchers,3 the emission
performance of their complexes strongly depends on the nature of
ligand and the structure of resulting compounds. In search of HTM
ion sensors, reseachers have synthesized numerous compounds
Received: May 25, 2010
Published: March 11, 2011
r
2011 American Chemical Society
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dx.doi.org/10.1021/ic1018086 Inorg. Chem. 2011, 50, 3189–3197
|

Inorganic Chemistry
ARTICLE
Table 1. Crystal Data and Refinement Parameters for H2L, 1, and 2
H2L
1
2
empirical formula
FW
C
23H42N2O2
C46H40N4O4Zn2
843.61
C46H40N4O4Cu2.H2O
857.96
358.43
cryst. syst.
space group
orthorhombic
P212121
monoclinic
P21/n
monoclinic
P21/n
ꢀ
a (Å)
7.0939(14)
12.542(3)
21.535(4)
90
10.536(2)
15.095(3)
13.134(3)
111.13(3)
1948.4(7)
white, block
4
8.2097(16)
17.960(4)
14.016(3)
103.09(3)
2013.0(7)
black, block
4
ꢀ
b (Å)
ꢀ
c (Å)
β (deg)
volume (Å3)
color and habit
Z
1916.1(7)
yellow, block
4
density calcd. (gcmꢀ3
)
1.242
1.438
1.445
abs. coeff. (mmꢀ1
)
0.080
1.281
1.112
F(000)
760
872
908
cryst. size (mm3)
θ range (deg)
reflns collected
ind. reflns
0.60 ꢁ 0.32 ꢁ 0.30
3.02 to 27.48
18783
0.09 ꢁ 0.08 ꢁ 0.07
3.17 to 27.43
17659
0.36 ꢁ 0.32 ꢁ 0.30
2.98 to 27.47
18464
4381
4410
4598
[Rint = 0.0614]
4381/0/250
2435
[Rint = 0.112]
4410/0/257
2180
[Rint = 0.134]
4598/3/273
2225
reflns/restraint/params
reflns obs.[I > 2σ(I)]
goodness-of-fit on F2
final R ind [I > 2σ(I)]
1.025
1.070
1.050
R1 = 0.0478
wR2 = 0.0875
R1 = 0.1102
wR2 = 0.1094
R1 = 0.0624
wR2 = 0.0993
R1 = 0.1602
wR2 = 0.1432
R1 = 0.1005
wR2 = 0.1364
R1 = 0.2012
wR2 = 0.1686
R indices (all data)
pH ∼7.0) for selective detection of Cu2þ/Agþ ions. Through this
work, we present the first report dealing with a dimeric zinc complex
[{Zn(C23H18N2O2)}2] acting as a fluorescent probe for selective
detection of Cu2þ/Agþ ions under aqueous conditions and a
molecular keypad lock through reversible “On-Off-On” switching
with distinct signals in the presence of these ions.
cell using platinum wire as the counter electrode, a glassy carbon working
electrode, and Ag/Agþ reference electrode.
Preparation of N,N0-bis(2-hydroxybenzilidene)-2,4,6-tri-
methylbenzene-1,3-diamine (H2L).
A methanolic solution
(10 mL) of 2,4,6-trimethylbenzene-1,3-diamine (0.751 g, 5.0 mmol)
was added to a solution of salicylaldehyde (1.06 mL, 10.0 mmol) in methanol
(5 mL), and contents of the flask were heated under reflux for 10 h. After
cooling to room temperature it gave yellowish thick oil which upon addition
of methanol afforded yellow solid. Recrystallization from dichloromethane/
methanol (1:4) gave the desired product as yellow crystals. Yield (1.250 g;
70%). Anal. Calcd. [C23H22N2O2]: C 77.51, H 5.66, N 7.86%; Found: C
77.43, H 5.62, N 7.82%. 1H NMR (CDCl3, δH ppm): 13.02 (s, 2H, OH),
8.32 (s, 2H), 7.33ꢀ7.40 (m, 4H), 6.95ꢀ7.06 (m, 4H), 6.92 (s, 1H), 2.18 (s,
6H), 2.04 (s, 3H). 13CNMR (CDCl3, δC ppm): 167.1, 161.17, 146.8, 133.2,
132.4, 132.2, 130.0, 124.7, 119.2, 119.0, 118.7, 117.3, 18.2, 14.00. ESI-MS
(m/z): 359.20 [(Mþ1)þ, 100%], 360.20 [(Mþ2)þ, 60%]. IR (KBr
pellets, cmꢀ1): 460 (w), 639 (w), 753 (s), 814 (m), 902 (w), 984 (w),
1082 (m), 1147 (w), 1189 (m), 1275 (s), 1403 (m), 1461 (s), 1572 (s),
1621 (vs), 2926 (w), 2976 (w), 3053 (w), 3450 (w). UVꢀvis. (CH2Cl2:
MeCN, λmax, nm, ε Mꢀ1 cmꢀ1): 332 (3.83 ꢁ 104), 266 (3.04 ꢁ 104).
Preparation of [{Zn(C23H18N2O2)}2] (1). To an alkaline solu-
tion of L2ꢀ [prepared by dissolving H2L (0.358 g, 1.0 mmol) in MeOH
(15 mL) and adding KOH (0.112 g, 2.0 mmol) under stirring conditions
’ EXPERIMENTAL SECTION
General Information and Materials. The reagents and solvents
were procured from commercial sources. Solvents were dried and
distilled following standard literature procedures.11 Elemental analyses
for C, H, and N were performed on a CE-440 Elemental Analyzer.
Infrared and electronic absorption spectra were acquired on a Varian
3300 FT-IR and Shimadzu UV-1601 spectrophotometers, respectively.
1H (300 MHz) and 13C (75.45 MHz) NMR spectra were obtained on a
JEOL AL300 FT-NMR spectrometer using tetramethylsilane (TMS) as
an internal reference. Fluorescence spectra were recorded on a Varian
Cary Eclipse Fluorescence spectrophotometer using Tris-HCl buffer
[pH 7ꢀ8; water, acetonitrile (7:3)] at room temperature. Electrospray
ionization mass spectrometry (ESI-MS) were obtained on a THERMO
Finningan LCQ Advantage Max ion trap mass spectrometer. The
samples (10 μL) were dissolved in dichloromethane/acetonitrile (3:7,
v/v) and introduced into the ESI source through a Finningan surveyor
auto sampler. The mobile phase MeOH/MeCN (90:10): H2O flowed at
a rate of 250 μL/min. The ion spray voltage was set at 5.3 KV and
capillary voltage at 34 V. The MS scan run up to 2.5 min, and spectra
print outs were averaged of over 10 scans. Cyclic voltammetric measure-
ments were performed on a CHI 620c electrochemical analyzer at room
temperature. The experiments were made in an airtight single compartment
over half an hour], Zn(NO3)2 6H2O (0.594 g, 2.0 mmol) dissolved in
3
MeOH (10 mL) was added dropwise and stirred at room temperature
for 1 h. Slowly a white precipitate separated, which was collected by
filtration washed with methanol and diethyl ether. Slow diffusion of
diethyl ether over a dichloromethane solution of the complex afforded
white block shaped crystals in a couple of days. Yield (0.638 g; 76%).
Anal. Calcd. [C46H40Zn2N4O4]: C 65.49, H 4.78, N 6.64%; Found:
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Inorganic Chemistry
ARTICLE
C 65.50, H 4.80, N 6.66%. 1H NMR (CDCl3, δH ppm): 7.89 (s, 4H),
7.36 (t, J1 = 6.9 Hz, J2 = 7.2 Hz, 4H), 7.04 (d, J = 7.5 Hz, 4H), 6.93 (d, J =
8.4 Hz, 4H), 6.85 (s, 2H), 6.60 (t, J1 = 7.2 Hz, J2 = 7.5 Hz, 4H), 2.30 (s,
6H), 1.5 (s, 12H). 13C NMR (CDCl3, δC ppm): 174.4, 171.8, 146.5,
136.3, 129.9, 129.5, 124.5, 123.7, 117.8, 114.9, 18.2, 13.00. ESI-MS
(m/z): 843.3 [(M)þ, 12%], 847.2 [(Mþ4)þ, 10%], 359.3 [(Lþ1)þ,
100%]. IR (KBr pellets, cmꢀ1): 482 (w), 648 (vw), 753 (s), 860 (vw),
922 (w), 1024 (w), 1084 (m), 1143 (m), 1192 (m), 1326 (s), 1352 (m),
1442 (s), 1532 (s), 1609 (vs), 2924 (w), 3423 (w). UVꢀvis. (CH2Cl2:
MeCN, λmax nm, ε Mꢀ1 cmꢀ1): 398 (0.773 ꢁ 105), 328 (0.951 ꢁ 105),
278 (2.01 ꢁ 105).
Figure 1. Formation of the complexes 1 and 2.
Preparation of [{Cu(C23H18N2O2)}2] H2O (2). To an alkaline
Supporting Information, Table S1, respectively. Preliminary data on
space group and unit cell dimensions as well as intensity data were
collected on a R-AXIS RAPID II diffractometer using graphite mono-
chromatized MoꢀKR radiation (λ = 0.71073 Å) at room temperature.
The structures were solved by direct methods (SHELXS 97) and refined
by full-matrix least-squares on F2 (SHELX 97).12 Non-hydrogen atoms
were refined anisotropically. All the hydrogen atoms were geometrically
fixed and refined using a riding model. The PLATON computer
program was used for analyzing the interaction and stacking distances.13
Cambridge Crystallographic Data Centre (CCDC) No.'s CCDC-743409
(H2L), -743411 (1), -743410 (2) provide access to the supplementary
crystallographic data for this paper.
3
solution of L2ꢀ [prepared by dissolving H2L (0.358 g, 1.0 mmol) and
adding KOH (0.112 g, 2.0 mmol) under stirring over half an hour] a
methanolic solution (10 mL) of Cu(NO3)2 3H2O (0.465 g, 2.0 mmol)
3
was added dropwise and stirred at room temperature for 1 h. Slowly a
black precipitate separated, which was collected by filtration, washed
with methanol and diethyl ether. Black block shaped crystals were obtained
by slow diffusion of diethyl ether to a dichloromethane solution of the
complex over a couple of days. Yield: (0.699 g; 82%). Anal. Calcd. for
C46H40Cu2N4O4 H2O: C 64.40, H 4.93, N 6.53; Found: C 64.28, H 4.84,
3
N, 6.49%. ESI-MS (m/z): 841 [(Mþ1)þ, 18%], 359.3 [(Lþ1)þ, 100%].
IR (KBr pellets, cmꢀ1): 480 (w), 592 (vw), 753 (s), 831 (vw), 916 (w),
1031 (w), 1090 (m), 1148 (m), 1193 (m), 1320 (s), 1387 (m), 1441 (s),
1531 (s), 1606 (vs), 2915 (w), 3408 (w). UVꢀvis. (CH2Cl2, λmax nm, ε
Electrochemical Studies. Redox properties of H2L, 1, and 2 in the
potential range þ2.0 to ꢀ2.0 V vs Ag/AgCl were followed by cyclic
voltammetry in CH2Cl2 containing 0.1 M tetrabutylammonium per-
chlorate as the supporting electrolyte.
M
ꢀ1 cmꢀ1): 390 (0.871 ꢁ 105), 326 (1.01 ꢁ 105), 277 (2.08 ꢁ 105).
Preparation of [{Zn(C23H18N2O2)}2]2 [Cu]2 (3). To a solution
3
of 1 (0.843 g, 1.0 mmol) in acetonitrile (10 mL), Cu(NO3)2 3H2O
UVꢀvis and Fluorescence Studies. The stock solution of 1 (1.0 ꢁ
10ꢀ5 M) for electronic absorption and emission spectral studies was
prepared using spectroscopic grade MeCN-H2O (3:7, v/v) in 0.01 M
Tris-HCl buffer. The solutions of alkali- and alkaline earth metal ions, first
3
(1.161 g, 5.0 mmol) dissolved in water (10 mL) was added dropwise and
stirred at room temperature for 2 h, whereupon the yellowish green
solution turned brown. It was stirred for an additional 4ꢀ5 h to ensure
that its brown color remained unchanged. The resulting solution was
completely evaporated and product dried under vacuum. Product yield
and elemental analyses for 3 could not be estimated because of the
row transition metals, and other cations, namely, Naþ, Kþ, Ca2þ, Mg2þ
,
Mn2þ, Fe2þ, Fe3þ, Co2þ, Ni2þ, Cu2þ, Zn2þ, Agþ, Cd2þ, and Hg2þ were
prepared by dissolving nitrate salts in triple distilled water (1 ꢁ 10ꢀ2 M).
In the titration studies 3.0 mL solution of 1 (1.0 ꢁ 10ꢀ5 M) was taken in a
quartz cell of 1 cm path length, and solutions of the metal ions were added
gradually to the cell.
1
presence of an excess of paramagnetic Cu(NO3)2 3H2O. H NMR
3
(CDCl3, δH ppm): 11.01 (s, 8H), 9.90 (s, 8H), 8.32 (s, 4H), 7.57ꢀ7.53
(m, 8H), 7.32ꢀ7.23 (m, 8H), 7.02ꢀ6.98 (m, 8H), 2.18 (s, 12H), 2.02 (s,
24H). ESI-MS (m/z): 847 [(1þ4)þ, 16%], 359.3 [(Lþ1)þ, 16%]. IR
(KBr pellets, cmꢀ1): 482 (w), 648 (vw), 759 (s), 860 (vw), 922 (w),
1024 (w), 1084 (m), 1153 (m), 1203 (m), 1283 (s), 1383 (m), 1469 (s),
1546 (s), 1606 (vs), 2929 (w), 3423 (w). UVꢀvis. (CH2Cl2: MeCN,
λmax nm, ε Mꢀ1 cmꢀ1): 357 (1.73 ꢁ 105), 269 (3.01 ꢁ 105).
’ RESULTS AND DISCUSSION
Synthesis and Characterization. The ligand N,N0-bis(2-
hydroxybenzilidene)-2,4,6-trimethylbenzene-1,3-diamine
Preparation of [{Zn(C23H18N2O2)}2] [Ag]2 (4). To a solution
3
(H2L) was prepared by condensation of 2,4,6-trimethyl-ben-
zene-1,3-diamine and salicylaldehyde (1:2) in methanol under
refluxing conditions. Initially an oily product was obtained, which
upon addition of dry methanol afforded yellow crystalline
compound. Deprotonated H2L reacted with the hydrated metal
nitrates Zn(NO3)2 6H2O and Cu(NO3)2 3H2O (1:2) to afford
of 1 (0.841 g, 1.0 mmol) in acetonitrile (10 mL) an aqueous solution
(10 mL) of AgNO3 (0.849 g, 5.0 mmol) was added dropwise and stirred
at room temperature for 2 h. It was stirred for an additional 4ꢀ5 h to
ensure that its yellowish green color remains unchanged. The resulting
solution was completely evaporated, and the product air-dried. Product
yield and elemental analyses for 4 could not be estimated because of the
presence of an excess of AgNO3. 1H NMR (CDCl3, δH ppm): 11.43 (s,
4H), 8.33 (s, 4H), 7.43ꢀ7.33 (m, 4H), 7.06ꢀ7.01 (m, 2H), 6.98ꢀ6.92
(m, 2H), 6.85 (s, 2H), 6.62ꢀ6.57 (t, J1 = 7.5 Hz, J2 = 7.2 Hz, 4H), 2.30
(s, 6H), 2.18 (s, 12H). ESI-MS (m/z): 845 [(1þ2)þ, 15%], 847
[(1þ4)þ, 12%], 360.6 [(Lþ2)þ, 8%], 188 [{Ag(NO3) (H2O)}þ,
100%]. IR (KBr pellets, cmꢀ1): 482 (w), 648 (vw), 756 (s), 860 (vw),
920 (w), 1028 (w), 1081 (m), 1146 (m), 1190 (m), 1328 (s), 1382 (s),
1451 (s), 1537 (s), 1607 (vs), 2924 (w), 3423 (w). UVꢀvis. (CH2Cl2:
MeCN, λmax nm, ε Mꢀ1 cmꢀ1): 398 (0.773 ꢁ 105), 328 (0.951 ꢁ 105),
278 (2.01 ꢁ 105).
3
3
binuclear complexes 1 and 2 in reasonably good yields. A simple
scheme showing the synthesis of complexes is depicted in
Figure 1. The complexes 1 and 2 were characterized by analytical,
spectral (IR, NMR, UVꢀvis, emission, ESI-MS), and electro-
chemical studies. Crystal structures of 1 and 2 have been authen-
ticated by single crystal X-ray diffraction analyses.
1H NMR spectrum of H2L is expected to exhibit singlets
associated with phenolic and aldimine protons in the downfield
side, multiplets corresponding to phenolic ring protons in aro-
matic region, and resonances due to methyl protons in the
highfield side. Expectedly, in CDCl3 it displayed singlets at δ
13.02 and 8.32 ppm assignable to -OH and -CH=N- protons,
respectively. Multiplets at δ 7.33ꢀ7.40 and 6.95ꢀ7.06 ppm have
been assigned to phenolic ring protons while, singlets at δ 2.18
and 2.04 ppm to the methyl protons of 1,3,5-trimethylphenyl ring
Crystallography. The structures of H2L and binuclear complexes
1 and 2 have been determined crystallographically. The Oak Ridge
thermal-ellipsoid plot (ORTEP) views along with the atom numbering
scheme are depicted in Figure 2(aꢀc). Important crystallographic data
and selected geometrical parameters are summarized in Table 1 and
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Inorganic Chemistry
ARTICLE
Crystal structure determination revealed that in cetrosymmetric
complexes 1 and 2 deprotonated ligand L2ꢀ bridges and chelates
two metal(II) centers through phenolato oxygen and imine
nitrogen from two different ligands (Figure 2b,c). The zinc(II)
centers in 1 are coordinated by phenolato oxygen and imine
nitrogen from two ligands leading to a distorted tetrahedral
geometry (Td) about the metal centers with Zn Zn separation
3 3 3
of 7.141 Å.14 The bond distances ZnꢀO and ZnꢀN range from
1.905(4) to 2.016(4) Å [Zn1ꢀN1, 2.016(4); Zn1ꢀN2,
1.999(4); Zn1ꢀO1, 1.907(4); Zn1ꢀO2, 1.905(4)]. Angles
between the planes containing O1ꢀZnꢀN1 and O2ꢀZnꢀN2
are in the range 92.40 to 132.70° [O2ꢀZn1ꢀN2, 96.02(2);
O1ꢀZn1ꢀN1, 96.07(2)°; O1ꢀZn1ꢀN2, 122.55(2)°
O2ꢀZn1ꢀN1, 124.29(2)° ] while OꢀZnꢀO and NꢀZnꢀN
[N2ꢀZn1ꢀN1, 111.73(2)°; O2ꢀZn1ꢀO1, 108.44(2)°] are
consistent with distorted tetrahedral geometry about the zinc(II)
centers.
The CuꢀO and CuꢀN bond distances in 2 are normal and
ranges from 1.88(4) to 1.96(5) Å [Cu1ꢀO2, 1.88(4); Cu1ꢀO1,
1.89(4); Cu1ꢀN2, 1.95(5); Cu1ꢀN1, 1.96(5)].14 It is evident
from the O(N)ꢀCuꢀN(O) angles [O2ꢀCu1ꢀO1, 90.70(2)°;
N1ꢀCu1ꢀN2, 104.00(2)°, O1ꢀCu1ꢀN1, 94.30(2)°;
O1ꢀCu1ꢀN2, 141.90(2)°; O2ꢀCu1ꢀN2, 94.62 (2)°, and
O2ꢀCu1ꢀN1, 141.90(2)°] that the coordination geometry
about copper(II) centers in 2 are almost halfway between square
planar and tetrahedral (Figure 2c). An angle of 51.66° between
the planes containing O1ꢀCuꢀN1 and O2ꢀCuꢀN2 further
supported intermediate coordination geometry. Similar observa-
tions have been made in other copper(II) complexes.14 The
intermediate geometry may arise from enhancement of geo-
metric ligand strain by the JahnꢀTeller effect of Td copper(II)
ions. The metal centers in this complex are separated through an
m-mesitylene spacer by 7.314 Å that is slightly longer in
comparison to other copper(II) complexes.14 The inter layer
centroid-centroid distance between two mesitylene spacers is
3.906 Å. Crystal structure of 2 revealed the presence of a water
molecule in its crystal lattice which is involved in strong hydrogen
bonding interactions. It leads to hydrogen-bonded array between
O3ꢀH O1 and O3ꢀH O2 on both the sides of dimeric
3 3 3
3 3 3
unit (Supporting Information, Figure S4).
Electrochemical Studies. The cyclic volammogram of H2L
displayed oxidative and reductive responses at þ1.0 V and ꢀ1.60
V, respectively which may be ascribed to the oxidation of
phenolic and reduction of 1,3-diimine moieties.15 Additional
waves in the cyclic voltammogram of 1 and 2 are absent in the
positive or negative region. As the zinc(II) center is redox
inactive, waves at Eox = þ1.092 V and Ered = ꢀ1.564 V in its
cyclic voltammogram (Supporting Information, Figure S5) may
be assigned to ligand based oxidation and reduction,
respectively.15 The complex 2 also exhibited an analogous
pattern wherein, oxidative and reductive waves appeared at
þ1.072 and ꢀ1.476 V, corresponding to ligand-centered oxida-
tion and reduction, respectively. As compared to H2L the
potentials for 1 and 2 anodically shifted, suggesting electron
withdrawing effect in the ligand through coordination to Zn(II)/
Cu(II) metal centers upon complexation.
Figure 2. ORTEP views of (a) H2L, (b) complex 1, and (c) 2 at 50%
thermal probability.
(Supporting Information, Figure S1). The position and integrated
intensity of various signals strongly supported the formulation of H2L
wherein, N,O-donor sites are trans- disposed (Figure 2a). Upon
complexation with the metal centers (Zn/Cu) in binuclear metalla-
cycles 1 and 2, it adopted cis- position which has been confirmed by
spectral and structural studies. The ligand H2L crystallizes in
orthorhombic space group P212121. The crystal structure of H2L
exhibited the linkage of 1,3,5-trimethylphenyl ring through pheno-
lato imine group and planarity of phenolato imine and mesitylene
rings. Donor sites presented by C17ꢀN2 and C7ꢀN1 are arranged
in such a way that they can maintain a minimum distance for
intramolecular OꢀH N hydrogen bonding between N1
Absorption Spectral Studies. The electronic absorption
spectrum of the H2L displays intense bands at 314 and 258 nm
corresponding to nꢀπ* and πꢀπ* transitions. In the complex 1
these bands exhibited a red shift and appear at 398 and 278 nm
(Figure 3a). The red shift in the position of the absorption bands
in comparison to H2L may be attributed to the formation of a
3 3 3
3 3 3
H1ꢀO1 and N2 H2ꢀO2. The bond distances C7ꢀN1 and
3 3 3
C17ꢀN2 are 1.280 Å and 1.279 Å, respectively.
Crystal Structure of the Complexes. The complexes 1 and 2
crystallize in the monoclinic system and P21/n space group.
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Figure 3. (a) Relative absorption spectra of H2L, 1, 2, 1 þ Cu2þ (3),
and 1 þ Agþ (4) in 0.01 M Tris-HCl buffer (H2O/acetonitrile, 7:3, v/v;
pH 7.0, c = 1 ꢁ 10ꢀ5 M). (b) Relative fluorescence spectra for H2L, 1, 2,
1 þ Cu2þ (3), and 1 þ Agþ (4) in Tris- HCl buffer (0.01 M, H2O/
MeCN, 7:3, v/v; pH 7.0, c = 1 ꢁ 10ꢀ5 M). λex = 350 nm, λem= 458 nm,
slit width (5 nm/5 nm, 500 V PMT).
Figure 5. (a) Absorption spectra of 1, 1 þ EDTA (10.0 equiv), and 1 þ
EDTA þ Cu2þ (50.0 equiv) upon addition of 20.0 equiv of various metal
ions. λex = 350 nm, λem= 458 nm, slit width (5 nm/5 nm, 500 V PMT).
(b) Fluorescence titration spectrum of 1 in the presence of Cu2þ ions
(2.0ꢀ30.0 equiv) in Tris-HCl buffer (0.01M, H2O/MeCN, 7:3, v/v; pH
7.0, c = 1.0 ꢁ 10ꢀ5 M).
owing to the higher affinity of Cu2þ ion for EDTA. On the other
hand, addition of EDTA (10 equiv) to a solution of 1 exhibited
insignificant changes in the position of absorption bands indicating
that the Zn2þ is not being liberated from 1 to form a Zn-EDTA
complex (Figure 5a). Again, addition of a large excess of Cu2þ (60
equiv) to the same solution leads to the emergence of the band at
∼357 nm, suggesting interaction between 1 and Cu2þ present in the
solution. Further, to ensure that the band at 357 nm is not due to
binuclear copper complex formed by substitution of Zn2þ, absorp-
tion spectra of 2was acqiured in the same solvent system. It displayed
transitions at 390, 326, and 277 nm. The presence of a low energy
band at 390 nm instead of the one at 357 nm indicated that the
observed spectral changes are not due to substitution of Zn2þ by
Cu2þ. It suggested that some sort of interaction is certainly taking
place between 1 and Cu2þ ions, but is not arising from the
substitution of Zn2þ by Cu2þ ions. A blue shift of ∼30 nm may
be attributed to interaction of Cu2þ ions probably through imine
units of the complex 1. Job’s plot analysis (Supporting Information,
Figure S9) further supported 1:1 stoichiometry between 1 and
Cu2þ ions.
Fluorescence Spectral Studies. The ligand H2L and complex
2 do not show any noticeable fluorescence upon excitation at
350 nm. Further, H2L do not show any substantial fluorescence
changes in the presence of various metal ions (20.0 equiv) except
Zn2þ (Figure 3b, Supporting Information, Figure S8; Top). The
complex 1 displays weak fluorescence at 458 nm with a Stokes
shift of ≈108 nm upon excitation at 350 nm. High fluorescent
behavior of 1 may be attributed to the complex formation leading
to reduction of PET process.16 Six-membered chelate ring
present in the zinc complex 1 increases rigidity in comparison
to H2L, which in turn, reduces the loss of energy by vibrational
decay and enhances the fluorescence intensity by a factor of
∼11.17 The fluorescence behavior of 1 has also been examined in
dichloromethane (DCM), and we found that it produces mea-
surable signal even in nanomolar (nM) range (Supporting
Information, Figure S8; bottom). The quantum yield (ΦF) for
H2L (0.05), 1 (0.65), and 2 (0.06) has been determined with
respect to the Zn(salen) (ΦF = 0.10 in dichloromethane).18 The
quantum yield of the complex 1 is comparable to that of other
fluorescent salen based Zn-complexes.18aꢀc
Figure 4. (a) Fluorescence spectra of 1, upon addition 20.0 equiv. of
various metals ions. λex = 350 nm, λem= 458 nm, slit width (5 nm/5 nm,
500 V PMT). (b) Absorption titration spectrum of 1 in the presence of
Cu2þ ions (2.0ꢀ30.0 equiv) in Tris- HCl buffer (0.01 M, H2O/MeCN, 7:3,
v/v; pH 7.0, c = 1 ꢁ 10-5 M).
binuclear metallacycle. The stability of 1 was followed in the pH
range 6ꢀ12 and it was observed that the most stable species
exists between pH 7.0ꢀ7.4 (Supporting Information, Figure S6).
The metal ion interaction studies were performed by addition of
nitrate salts of the alkali- and alkaline-earth metal ions, first-row
transition metal, and other cations, namely, Naþ, Kþ, Ca2þ
Mg2þ, Mn2þ, Fe2þ, Fe3þ, Co2þ, Ni2þ, Cu2þ, Zn2þ, Agþ, Cd2þ
,
,
and Hg2þ to a solution of 1. Interstingly, absorption spectra exhi-
bited a significant change in the presence of Cu2þ, while other
metal ions displayed insignificant changes (Supporting Informa-
tion, Figure S7; Top).
To have better understanding of the affinity of 1 for Cu2þ ions,
absorption titration studies were performed by the addition of
Cu(NO3)2 3H2O (2.0 to 30.0 equiv, Tris-HCl buffer, MeCN/
3
H2O, 3:7, v/v, pH ∼7.0) to a solution of 1 (Figure 4b). The
absorption bands corresponding to 1 disappeared, and an intense
band emerged at 357 nm with an isosbestic point at 281 nm. It is
quite possible that this band may arise by replacement of the
coordinated Zn2þ by Cu2þ ion. To ascertain this possibility, a
strong chelating agent like ethylenediaminetetraacetic acid (EDTA,
10.0 equiv) was added to the solution. Absorption bands associated
with 1 restored (Supporting Information, Figure S7; bottom)
suggested that the Zn2þ has not been replaced by Cu2þ ion. It is
reasonable to consider that the substitution of Zn2þ by Cu2þ ion
may lead to formation of [Cu-L]2, which in turn upon addition of
EDTA may dissociate to form a more stable complex Cu-EDTA
The complex 1 may serve as a potential fluorescent sensor. To
demonstrate this behavior nitrate salts of the tested metal ions
(H2O; 20.0 equiv, c = 1 ꢁ 10ꢀ2 M) were added to a solution of
1 and emission spectral changes monitored. The fluorescence
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1H NMR Studies. To have clear idea about the mechanism of
quenching/enhancement taking place in the presence of Cu2þ
/
Agþ ions, 1H NMR studies were performed on H2L, 1, 2, 1 þ
Cu2þ (3), and 1 þ Agþ (4) (Figure 7). For the sake of clarity
shifts in the position of aldimine proton -CH=N- (denoted as
1
H1) have been followed by H NMR spectral studies. The -
CH=N- proton of H2L resonates at δ 8.32 ppm. Upon com-
plexation with the metal center Zn, it exhibited an upfield shift
and resonated at δ 7.89 ppm (Figure 7 and Supporting Informa-
tion, Figure S2; Top). The H1 and H2 protons of 3 displayed a
downfield shift and resonated at 11.01 and 9.90 ppm, respectively
(Figure 7 and Supporting Information, Figure S3; Top). The
downfield shift in the position of H1 and H2 suggested that Cu2þ
is interacting with the -CH=N- moiety (Scheme) in 1:1 stoichio-
metric ratio.19a,b From the crystal structures of 1 and 2 it is clear
that the only difference between these complexes lies in their
metal center. Therefore, such a large shift in the position of H1
Figure 6. (a) Fluorescence titration spectra upon addition of Cu2þ ion
(0.0ꢀ40.0 equiv) to a solution of 1; (b) Change in the fluorescence spectra
upon addition of Agþ ion (0.0ꢀ24.0 equiv) to a solution of 1. λex = 350 nm,
λem = 458 nm, slit width (5 nm/5 nm, 500 V PMT).
quenching/enhancements were observed only upon addition of
Cu2þ/Agþ ions (Figure 4a and Supporting Information, Figure
S9; top). Addition of the Cu2þ and Agþ ions (20.0 equiv) displayed
∼60% fluorescence quenching (Figure 4a and 6a), and ∼56%
enhancement, respectively (Figure 4a and 6b). Quenching by Cu2þ
has been investigated by electronic absorption and emission titra-
tions. Addition of Cu2þ ion (2.0ꢀ40.0 equiv; interval time, 5 min.)
leads to the fluorescence quenching (“turn-off” ∼89%, Figure 6a),
and the quantum yield decreases to 0.005.18e To gain deep insight
into the mode of quenching by Cu2þ, an axcess of EDTA (10.0
equiv) was added to a solution of 1. The fluorescence due to 1
completely quenched and could not be restored further by additions
of Cu2þ (40.0 equiv). On the other hand, addition of Cu2þ (40.0
equiv) to a solution of 1 leads to the fluorescence quenching, “turn-
off” (∼90%) which could not be restored by subsequent addition of
10.0 equiv of EDTA (Figure 5b). It suggested that the observed
fluorescence quenching is not due to the substitution of Zn2þ from 1.
Detailed information about the mode of quenching in the
presence of Cu2þ has been deduced from SternꢀVolmer plot (Ksv =
8,500 Mꢀ1). A straight line obtained in the SꢀV plot suggested
dynamic quenching (Supporting Information, Figure S10). The
association constants (Ka) deduced22 for 1:1 stoichiometry, based
on emission and absorption titration studies are 3.29 and 3.54 log Ka,
respectively (Supporting Information, Figure S11). It was further
observed that an increase in the time interval for addition of Cu2þ
leads to insignificant changes in the absorption as well as fluorescence
spectral patterns.
and H2 protons is not expected if Zn2þ is substituted by Cu2þ
.
1
Further, as expected a well resolved H NMR spectrum for 2
could not be obtained because of paramagnetic Cu2þ (d9) ions
and ferromagnetic coupling.14 Interaction of Cu2þ with aldimine
moieties may subsequently be responsible for metal to ligand
charge transfer (MLCT) transitions and quenching.
In comparison to 3, the -CH=N- protons in 4 exhibited a
downfield shift and resonated at δ 11.43 ppm, while the H2
proton resonated in the high field side at δ 8.34 ppm (Figure 7
and Supporting Information, Figure S3; Bottom). It suggested
that Agþ is interacting more strongly than Cu2þ with the
-CH=N- moiety, which has further been supported by a higher
association constant value (8.05 log Ka) for Agþ ion.23 It also
supported the results from fluorescence titration studies that the
addition of 24.0 equiv of Agþ leads to saturation of fluorescence
enhancement plot, while 40.0 equiv is required for Cu2þ
.
Electrospray Mass Spectrometry. The ESI-MS of H2L
displayed a molecular ion peak [Mþ1]þ (100%) at m/z 359.2
(Supporting Information, Figure S12; Top). In its mass spectrum
1 displayed peaks associated with [M]þ, [Mþ4]þ, and [Lþ1]þ
at m/z 843.3 (12%), 847.2 (10%), and 359.3 (100%), respec-
tively. The presence of various peaks strongly supported for-
mulation of 1 (Supporting Information, Figure S12; middle).
Similarly, 2 displayed molecular ion peak at m/z 841.2 (18%)
alongwith a peak associated with [Lþ1]þ at m/z 359.3 (100%)
(Supporting Information, Figure S12; bottom). To have an idea
about the quenching/enhancement in the presence of Cu2þ
/
Similarly, the fluorescence titration experiments employing
Agþ ions (2.0 to 24.0 equiv) display gradual fluorescence enhance-
ment “turn-on” (Figure 6b) with an improved quantum yield
(0.76).18f To ensure that displacement of Zn2þ by Agþ ion is not
responsible for “turn-on”behavior, an unsuccessful attempt was made
to isolate the silver complex by direct reaction of AgNO3 with H2L.14
As direct reaction of AgNO3 with H2L does not give a Ag-complex,
the possibility of displacement mode can be ruled out. It suggested
that the fluorescence enhancement possibly arises from charge
transfer through weak interaction between 1 and Agþ ion.19,23
The Job’s plot analysis (Supporting Information, Figure S9) shows
1:2 stoichiometry between 1 and Agþ ion and the association
constant (Ka) from a BꢀH plot based on emission titration studies
(Supporting Information, Figure S10) is 8.05 log Ka.22 Although the
color of solutions containing H2L, 1, 3, and 4 under UV light are
not significantly different, however, the fluorescence intensity of 1
diminishes and enhances in the presence of Cu2þ and Agþ ions,
respectively (Figure 8a, inset).
Agþ ions, mass spectra of 3 and 4 were acquired. In its MS
spectrum 3 exhibited prominent peak at m/z 847.6 (16%)
assignable to [1þ4]þ (Supporting Information, Figure S13;
top); on the other hand, 4 displayed peaks at m/z 845 (15%)
and 847 (12%) corresponding to [1þ2]þ and [1þ4]þ
(Supporting Information, Figure S13; bottom). The observed
ESI-MS spectral patterns for 3 and 4 are quite different from that
of 1. It may be concluded that Cu2þ and Agþ ions are interacting
with 1 (Supporting Information, Figure S12 and S13). The
spectral data are consistent with the conclusions drawn from
1H NMR spectral studies.
1
On the basis of H NMR and ESI-MS spectral studies the
probability of displacement of Zn2þ from 1 by Cu2þ/Agþ has
been ruled out. The observed dynamic fluorescence quenching
can be accounted by considering two-way charge transfer be-
tween aldimine (-CdN-) moiety of 1 and paramagnetic Cu2þ
ions.19 Interaction of 1 with Cu2þ may lead to the formation of
nonfluorescent species by bringing paramagnetic Cu2þ close to
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Figure 7. 1H NMR spectrum of H2L, 1, 3, and 4.
Figure 8. (a) Relative changes in the fluorescence spectra of 1, upon addition of various equiv of Agþ ion (0.0ꢀ24.0 equiv) and Cu2þ ion (0.0ꢀ40.0
equiv); Inset shows fluorescent distinction visible by the naked eye under UV-light irradiation at 254 nm. (b) Output for 1, corresponding to six possible
input combinations at 458 nm. Inset shows a molecular keypad lock generating emission at 458 nm when a correct password, namely, CZA, is entered.
Keys C, Z, and A hold the relevant inputs Cu2þ, Zn2þ, and Agþ, respectively.
the chromophore with enhanced intersystem crossing.20 Simi-
larly, observed enhanced fluorescence signal in the presence
of Agþ may be attributed to one way electron transfer from
aldimine (-CH=N-) to diamagnetic Agþ ion (d10).21aꢀc Further,
interaction of Agþ ion with both aldimine and phenolato oxygen
atom in 1:2 stoichiometry may result in restricted PET that
enhances the fluorescence.21d,e
system as a sequence dependent molecular keypad lock using the
Cu2þ, Zn2þ, and Agþ metal ions as three different chemical
inputs. It was observed that 1 displayed distinct fluorescence
signals corresponding to given chemical input. Inputs in the
order Cu2þfZn2þfAgþ provided maximum output whereas
AgþfZn2þfCu2þ afforded minimum output (the Zn2þ ion
was used as an intermediate chemical input because the addition
of Zn2þ leads to restoration of the fluorescence of 1). These
chemical inputs may be executed in the form of Cu2þ, Zn2þ, and
Agþ, designated as ‘C’, ‘Z’, and ‘A’, respectively. Among the
possible six input combinations, CZA, CAZ, ZAC, ZCA, ACZ,
and AZC, the combination CZA gave maximum fluorogenic
output, whereas minimum output signaling was displayed by
AZC (Figure 8b). The keypad shown in figure 8b (inset)
contains various keys, (i.e., A, B, C, D, E, F, U, V, W, X, Y, Z)
like that in the electronic keypad (containing A to Z) and follows
correct password order (CZA). The mechanism behind the
maximum or minimum fluorogenic outputs have been ascribed
As Molecular Keypad Lock. Recently, Kumar et al.24c re-
ported a fluorogenic chemosensor based on a calix[4]arene
which acts as an ‘‘On-Off’’ reversible switch with chemical inputs
of Hg2þ and Kþ ions and works as a molecular keypad lock in the
presence of Fꢀ ions. On the basis of the findings of Kumar et al
and other research groups, it was realized that the present system
may also serve as a potential molecular keypad lock system.10,24
Complex 1 under investigation exhibits reversible fluorescence
quenching (turn-off)/enhancement (turn-on) in the presence of
Cu2þ/Agþ ions, respectively. The reversibility and selectivity of
1 toward Cu2þ/Agþ ions prompted us to consider the present
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to the weak interaction {M-π (-CH=N-)} between 1 and Cu2þ
/
Technology (AIST), Osaka, Japan for extending use of the
single-crystal X-ray diffraction facility.
Agþ ions. Since, Cu2þ and Agþ ions do not displace Zn2þ from 1,
the added metal ions (Cu2þ/Zn2þ/Agþ) weakly interact with 1.
The addition of a particular metal ion (Cu2þ/Zn2þ/Agþ) domi-
nates over the metal ion present in solution and weakly interacts
with 1 thus exhibiting the fluorescence owing to added metal ion.
As none of the added metal ions interact with 1 through
covalent/coordinate bond, it is possible that the incoming metal
ion may interact with 1 replacing the other, as a result complex 1
mimic between three distinct fluorescence states. Such types of
chemical systems may be applicable to protect the information at
molecular level as it requires correct password entries.10
Although, a few molecular keypad lock systems are available in
literature,10,24 all these are based on organic molecules whereas
the system described in this work is based on a binuclear Zn
complex.25
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In summary, through this work we have described the synth-
esis, spectral and structural characterization of binuclear zinc(II)
and copper(II) complexes based on a new salen type ligand N,N0-
bis(2-hydroxybenzilidene)-2,4,6-trimethylbenzene-1,3-diamine.
The zinc complex displays strong fluorescence, which is measur-
able even at nanomolar concentrations. It exhibits “turn-off” and
“turn-on” in the presence of Cu2þ and Agþ ions in aqueous
environment at neutral pH. The “Off-On” processes are rever-
sible, and it has been established that they arises from weak
interaction between Cu2þ/Agþ and -CH=N- unit. The observed
fluorescence quenching of 1 in the presence of paramagnetic
Cu2þ has been attributed to interaction of Cu2þ with the -CH=N-
unit, subsequently metal to ligand charge transfer (MLCT) to the
chromofore of 1. On the other hand, fluorescence enhancement in
the presence of Agþ possibly arises from ligand to metal charge
transfer (LMCT) between 1 and Agþ (d10). Moreover, 1 can be
applied as a molecular keypad lock system which follows correct
chemical input order CZA (Cu2þfZn2þfAgþ) because of rever-
sible “switch-off” and “switch-on” signaling in the presence of
Cu2þ and Agþ ions, respectively. The present approach can be
further extended to yield a new class of chemosensors with potential
applications.
’ ASSOCIATED CONTENT
S
Supporting Information. Contains characterization
b
data, UVꢀvis and emission spectra related to this work, and
crystal data in CIF format (CCDC-743409, 743411, 743410) for
H2L, complex 1 and 2. This material is available free of charge via
the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: dspbhu@bhu.ac.in. Phone: þ 91 542 6702480. Fax:
þ 91 542 2368174.
(11) Perrin, D. D.; Armango, W. L. F.; Perrin, D. R.; Purification of
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(12) (a) Sheldrick, G. M. SHELXL-97, Program for X-ray Crystal
Structure Refinement; G€ottingen University: G€ottingen, Germany, 1997.
(b) Sheldrick, G. M. SHELXS-97, Program for X-ray Crystal Structure
Solution; G€ottingen University: G€ottingen, Germany, 1997.
’ ACKNOWLEDGMENT
Thanks are due to the Department of Science and Technol-
ogy, New Delhi, India for financial assistance through the
Scheme SR/S1/IC-15/2006. The authors are also grateful
to the National Institute of Advanced Industrial Science and
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