Synthesis, structure and spectral properties of O,N,N coordinating ligands and their neutral Zn(ii) complexes: A combined experimental and theoretical study

Article abstract of DOI:10.1039/c3dt51571k

Mononuclear Zn(ii) complexes with the general formula [Zn(L)₂] have been synthesized in good yields by reacting Zn(OAc)₂ with HL in a ratio of 1 : 2 in methanol solvent. Here L is the deprotonated form of 6-[(quinolin-8-ylamino)methy

Full text of DOI:10.1039/c3dt51571k

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Dalton Trans. (Revised)
Manuscript ID: DT-ART-06-2013-051571.R1
July, 2013
Synthesis, structure and spectral properties of O, N, N coordinating
ligands and their neutral Zn(II) complexes: a combined
experimental and theoretical study†
Amar Hens, Pallab Mondal, and Kajal Krishna Rajak*
1

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Abstract
The mononuclear Zn(II) complexes of general formula [Zn(L)2] have been
synthesized in good yields by reacting Zn(OAc)2 with HL in ratio of 1:2 in methanol
solvent.
Here
L
is
the
deprotonated
form
of
6ꢀ[(quinolinꢀ8ꢀ
ylamino)methylene]cyclohexaꢀ2,4ꢀdienone
ylamino)methylene]cyclohexaꢀ2,4ꢀdienone
(HL1),
(HL2),
4ꢀchloroꢀ6ꢀ[(quinolinꢀ8ꢀ
4ꢀmethylꢀ6ꢀ[(quinolinꢀ8ꢀ
ylamino)methylene]cyclohexaꢀ2,4ꢀdienone (HL3), 2,4ꢀdimethylꢀ6ꢀ[(quinolinꢀ8ꢀylamino)
methylene]cyclohexaꢀ2,4ꢀdienone 2ꢀmethoxyꢀ6ꢀ[(quinolinꢀ8ꢀylamino)
(HL4),
methylene]cyclohexaꢀ2,4ꢀdienone (HL5). The electronic structure and photophysical
properties of the ligands were calculated by DFT and TDDFT method. The Xꢀray
structure of one complex has been reported. The ligands have strong binding ability
[(0.75ꢀ15.37) x 104] and ratiometric response to Zn2+ ions. Addition of Zn2+ ions to the
ligands in THF solution a sharp color change is observed visually as well as there occurs
a significant enhancement of the fluorescence intensity and the quantum yield for this
series. Introduction of other metal ions having biological and environmental effect either
keeps unaltered or quenches emission intensity. However we observed the sensing
property of the ligands strongly depends on the substituents at the ortho position of the
phenol group. The DFT calculation reveals that ICT process take place from
salicyldehyde (donor moiety) to quinoline (acceptor moiety) which is responsible for
enhancement of fluorescence intensity of ligands after complexation.
2

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Introduction
The research on electroluminescent materials has received a great deal of
attention for decades because of their potential application in the fabrication of organic
light emitting diodes (OLED).1 A variety of fluorescentꢀsensing approaches have been
extensively investigated pertaining to the signaling mechanisms of internal charge
transfer2 (ICT), photoinduced electron transfer/energy transfer3 (PET), metalꢀligand
charge transfer4 (MLCT), excimer/exciplex formation as well as the excited state
intramolecular/intermolecular proton transfer5 (ESIPT).
Zn(II) ion is spectroscopically as well as magnetically silent due to its 3d10
electronic configuration. However it can modulate the ligand luminescence properties by
means of chelation enhanced fluorescence’s (CHEF).6ꢀ18 The generated CHEF effect is
regulated by photoinduced electron transfer (PET) mechanism. In this context the Schiff
base complexes of Zn(II) ion are particularly interesting due to their potential
photochromic applications.19ꢀ22
In the free Schiff base ligands the nitrogen lone pair electrons at the
α position of
the fluorophore are induced to the system of the fluorophores resulting the quenching
π
of their fluorescence. However in the complex the donation of the −C=N− nitrogen lone
pair to the metal center enhances the fluorescence intensity through CHEF mechanism.
In the present work we report the successful synthesis of mononuclear Zn(II)
complexes containing O, N, N coordinating ligands derived from 8ꢀaminoquinoline and
salicylaldehyde and its derivatives. The Xꢀray structure of a complex is reported as a
representative case. Here we describe the spectral properties of the synthesized ligands
as well as their neutral Zn(II) complexes. The sensing properties of such compounds are
governed by the close environment about the metal ion and the small change in the ligand
architecture can strongly influence such properties, as in Scheme 1. These demands
further investigation of coordination chemistry of zinc(II) incorporating Schiff base
ligands having O, N, N coordinating sites to understand how a ligand environment
modulate their spectroscopic properties.
3

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Fluorophore
(Quinoline moiety)
N
t
]
s
)
e
I
Controlling
Part
(Salicylaldehyde
moiety)
I
u
(
G
n
Z
[
Scheme 1
Here, we also present a full density functional theory (DFT) and timeꢀdependent
density functional theory (TDDFT) investigation to get better insight into the geometry,
electronic structure, and optical properties of these systems. Geometry optimization of
the singlet groundꢀstate was carried out by means of DFT calculations. TDDFT
calculations of several singlet states have been performed for better understanding of the
electronic origin of the absorption and emission spectra.
Experimental section
Materials
All the starting chemicals were analytically pure and used without further
purification.
Caution! Perchlorate salts are highly explosive, and should be handled with care and in
small amounts.
Physical Measurements
UV–Vis spectra were recorded on
a
Perkin–Elmer LAMBDA 25
Perkin–Elmer Lꢀ0100
spectrophotometer. IR spectra were obtained with
a
spectrophotometer. 1H spectra were measured on Bruker FT 300 MHz spectrometer. The
1
atomꢀnumbering scheme used for H is same as that used in the crystallography.
Elemental analyses (C, H, N) were performed on Perkin–Elmer 2400 series II analyzer.
The emission data were collected on a PerkinꢀElmer LS 55 fluorescence spectrometer.
Astd
Ar
Ir
ηr2
(1)
Φr = Φstd
2
Istd
ηstd
4

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For all luminescence measurements excitation and emission slit width of 5 nm was used.
Quantum yields of the complexes were determined in freezeꢀpumpꢀthawꢀdegassed
solutions of the complexes by a relative method using quinine sulfate in the same solvent
as the standard.23 The quantum yields were calculated using eq 1,24 where Φr and Φstd are
the quantum yields of unknown and standard samples [Φstd = 0.54 (at 298 K) in THF at
λex = 460 nm], Ar and Astd (<0.1) are the solution absorbance at the excitation wavelength
(λex), Ir and Istd are the integrated emission intensities, and ηr and ηstd are the refractive
indices of the solvent. Experimental errors in the reported luminescence quantum yields
were about 10%.
Computational Details
Among the five ligands (HL1 to HL5), DFT calculation were carried out for two
cases (HL1 and HL2) only
. The geometrical structures in their singlet ground state (S0)
and excited state (S1) were optimized by the DFT25 and timeꢀdependent DFT (TDDFT)26
method with B3LYP exchange correlation functional27 approach associated with the
conductorꢀlike polarizable continuum model (CPCM).28
To calculate the stability of the keto tautomeric form over the enol form for the
ligands (HL) in ground S0 state we perform the potential energy scan according to the
“distinguished coordinate approach”29 i.e. by specifying a reaction coordinate (in the
present case it is the coordinate for translocation of the proton from Ndonor to Oacceptor, i.e.
elongation of the Ndonor−H bond axis) along which energy change is observed. For the
ground S0 state all of the other degrees of freedom are relaxed without imposing any
symmetry constraints.
On the basis of the optimized ground (S0) and excited state (S1) geometry
structures of the ligands, the absorption and emission spectral properties in
tetrahydrofuran (THF) media were calculated by timeꢀdependent density functional
theory (TDDFT) approach associated with the conductorꢀlike polarizable continuum
model (CPCM). We computed the lowest 20 singlet – singlet transition both in ground
state (S0) and excited state (S1) and results of the TDDFT calculations. Due to the
presence of electronic correlation in the TDDFT (B3LYP) method it can yield more
accurate electronic excitation energies.
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DFT calculations were done for two complexes (2 and 4). The geometry of the
complexes in their ground S0 state was optimized in solution phase. But we are unable to
optimize their singlet excited state (S1) geometry. For the complexes we also calculated
60 singlet – singlet transition using their ground S0 state geometry by the same procedure
as in ligands.
The effective core potential (ECP) approximation of Hay and Wadt was used for
describing the (1s22s22p6) core electron for zinc whereas the associated “doubleꢀξ”
quality basis set LANL2DZ was used for the valence shell.30 For H atoms we used 6ꢀ
31(g) basis set and for C, N and O atoms we employed 6−31+g and for Cl atoms we
employed 6ꢀ31+g(d,p) as basis set for the optimization of both the ground state and the
excited state geometries. The calculated electronic density plots for frontier molecular
orbitals were prepared by using the GaussView 5.0 software. All the calculations were
performed with the Gaussian 09W software package.31 GaussSum 2.1 program32 was
used to calculate the molecular orbital contributions from groups or atoms.
Crystalographic Studies
The single crystal suitable for Xꢀray crystallographic analysis of the complex
[Zn(L4)2].CH3OH,
4. The Xꢀray intensity data were collected on Bruker AXS SMART APEX CCD
diffractometer (Mo Kα, = 0.71073 Å) at 293 K. The detector was placed at a distance
6.03 cm from the crystal. Total 606 frames were collected with a scan width of 0.3ο in
different settings of
. The data were reduced in SAINTPLUS33 and empirical absorption
4.CH3OH was obtained by slow evaporation of methanolic solution of
λ
φ
correction was applied using the SADABS package.33 Metal atom was located by
Patterson Method and the rest of the nonꢀhydrogen atoms were emerged from successive
Fourier synthesis. The structure was refined by full matrix leastꢀsquare procedure on F2
.
All nonꢀhydrogen atoms were refined anisotropically. All calculations were performed
using the SHELXTL V 6.14 program package.34 Molecular structure plots were drawn
using the Oak Ridge thermal ellipsoid plot (ORTEP).35 Relevant crystal data is given in
Table 1.
6

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Table 1 Crystal Data and Structure Refinement Parameters for 4.CH3OH
4
.CH3OH
Formula
C37H34N4O3Zn
648.05
Mr
Crystal system
Space group
Triclinic
Pꢀ1
a
b
c
/ Å
/ Å
/ Å
/°
11.806(5)
12.117(5)
12.312(5)
85.883(5)
62.824(5)
87.652(5)
1562.7(11)
2
α
β
γ
/°
/°
/Å3
V
Z
D
ꢀ
θ
T
calcd /mg m−3
1.371
/mm−1
0.830
/°
1.69 − 27.78
293(2)
/K
Reflns collected
R1,a wR2b [I > 2
GOF on F2
7281
0.0500, 0.1346
σ(I)]
1.048
aR1 = ΣFoꢀ Fc/ Σ Fo.
b wR2 = [Σ[ Fo2 ꢀ Fc2)2] / Σ[ Fo2)2]]1/2
w
(
w(
Synthesis of ligands
The ligands were prepared by the same general method. Details are given here for
a representative case (HL1).
HL1. To a methanolic solution (20 mL) of 2ꢀHydroxyꢀbenzaldehyde (245 mg, 2
mmol) and 8ꢀaminoquinoline (288 mg, 2 mmol) in presence of 3ꢀ4 drops of acetic acid
was refluxed in water bath for 2 h. After cooling to room temperature, the solvent was
removed under reduced pressure. The crude mass was subjected to column
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chromatography on a silica gel column (60ꢀ120 mesh). A red band was eluted using 5%
ethylacetate in hexane solution. A red colored solid was obtained after removal of solvent
under reduced pressure to afford the desired ligand (HL1). Yield: 406 mg (82%).
Elemental Anal. Calcd. for C16H12N2O: C, 77.04; H, 4.87; N, 11.28. Found: C, 77.12; H,
4.92; N, 11.36. 1H NMR {300 MHz, CDCl3, δ (ppm),
J (Hz)}: 8.93 (H1, d, J = 5.9), 8.08
(H10, s), 7.51−6.7 (9H, ArH), 3.51 (NH, bs).
HL2. Yield: 435 mg (77%). Elemental Anal. Calcd. for C16H11ClN2O: C, 67.90;
1
H, 3.92; N, 9.91. Found: C, 68.18; H, 4.06; N, 10.06. H NMR {300 MHz, CDCl3, δ
(ppm), J (Hz)}: 8.99 (H1, d, J = 5.7), 8.19 (H10, s) 7.74−7.26 (8H, ArH), 3.51 (NH, bs).
HL3. Yield: 424 mg (81%). Elemental Anal. Calcd. for C17H14N2O: C, 77.84; H,
5.38; N, 10.16. Found: C, 78.12; H, 5.44; N, 10.91. 1H NMR {300 MHz, CDCl3 δ (ppm),
(Hz)}: 8.77 (H1, d, = 5.8), 8.09 (H10, s) 7.40−6.04 (8H, ArH), 3.49 (NH, bs) 2.27
(CH3, s).
J
J
HL4. Yield: 474 mg (86%). Elemental Anal. Calcd. for C18H16N2O: C, 78.24; H,
5.84; N, 10.14. Found: C, 78.67; H, 5.97; N, 10.28. 1H NMR {300 MHz, CDCl3 δ (ppm),
J
(Hz)}: 8.99 (H1, d,
J = 5.7), 8.16 (H10, s) 7.69−6.91 (7H, ArH), 4.98 (NH, s) 2.33
(CH3, s), 2.30 (CH3, s).
HL5. Yield: 444 mg (80%). Elemental Anal. Calcd. for C17H14N2O2: C, 73.37; H,
5.07; N, 10.07. Found: C, 73.71; H, 5.15; N, 10.29. 1H NMR {300 MHz, CDCl3, δ (ppm),
J
(Hz)}: 8.95 (H1, d,
J = 5.8), 8.10 (H10, s) 7.53−6.85 (8H, ArH), 3.49 (NH, s) 3.95
(OCH3, s).
Synthesis of complexes
The complexes were synthesized by the same general procedure. Details are given
here for a representative case [Zn(L1)2],
1
.
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[Zn(L1)2], 1. To a solution of Zinc(II) acetate dihydrate (34 mg, 0.15 mmol) in
methanol (20 mL) was added HL1 (76 mg, 0.30 mmol), stirred for 2 h. The resulting dark
orange colored solution on slow evaporation yielded dark orange crude product. Yield:
74 mg (88%). Elemental Anal. Calcd. for C32H22N4O2Zn: C, 68.64; H, 3.96; N, 10.01.
Found: C, 68.71; H, 4.01; N, 10.07. IR (KBr, cm−1): ν(C=N) 1641; ν(C−Ophenoxo) 1249.
1H NMR {300 MHz, DMSO−d6, δ (ppm),
J (Hz)}: 8.97 (H1, d, J = 5.9), 8.012 (H10, s),
7.53−6.65 (9H, ArH).
[Zn(L2)2], 2. Yield: 84 mg (89%). Elemental Anal. Calcd. for C32H20Cl2N4O2Zn:
C, 64.77; H, 3.40; N, 9.44. Found: C, 64.92; H, 3.57; N, 9.68. IR (KBr, cm−1): ν(C=N)
1
1630; ν(C−Ophenoxo) 1217. H NMR {300 MHz, DMSO−d6, δ (ppm),
J (Hz)}: 9.09 (H1,
d,
J
= 5.8), 8.23 (H10, s), 7.78−6.30 (8H, ArH).
[Zn(L3)2], 3. Yield: 90 mg (86%). Elemental Anal. Calcd. for C34H26N4O2Zn: C,
69.45; H, 4,46; N, 9.53. Found: C, 69.52; H, 4.48; N, 9.60. IR (KBr, cmꢀ1): ν(C=N) 1627;
1
ν(C−Ophenoxo) 1213. H NMR {300 MHz, DMSO−d6, δ (ppm),
5.7), 8.11 (H10, s), 7.44−6.09 (8H, ArH), 2.31 (CH3, s).
J (Hz)}: 8.79 (H1, d, J =
[Zn(L4)2], 4. Yield: 75 mg (81%). Elemental Anal. Calcd. for C36H30N4O2Zn: C,
70.19; H, 4.91; N, 9.04. Found: C, 70.23; H, 4.92; N, 9.10. IR (KBr, cm−1): ν(C=N)
1
1623; ν(C−Ophenoxo) 1220. H NMR {300 MHz, DMSO−d6, δ (ppm),
J (Hz)}: 9.04 (H1,
d,
J
= 5.7), 8.21 (H10, s), 7.71−6.97 (7H, ArH), 2.35 (CH3, s), 2.32 (CH3, s).
[Zn(L5)2], 5. Yield: 72 mg (78%). Elemental Anal. Calcd. for C34H26N4O4Zn: C,
65.86; H, 4.23; N, 9,04. Found: C, 65.91; H, 4.25; N, 9.08. IR (KBr, cm−1): ν(C=N)
1
1616; ν(C−Ophenoxo) 1225. H NMR {300 MHz, DMSO−d6, δ (ppm),
J (Hz)}: 8.97 (H1,
d, J = 5.9), 8.15 (H10, s), 7.57−6.89 (8H, ArH), 3.99 (OCH3, s).
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Results and discussion
Synthesis
Five tridentate ligands HL1ꢀ HL5 (general abbreviation, HL) used in this work
have been synthesized by reacting 8ꢀaminoquinoline with the appropriate aldehyde in
methanol are given in Chart 1. The ligands derived from 8ꢀaminoquinilne and
salicylaldehyde derivatives exist in the keto form in the solid state36 however they exist
both in the keto and enol form in solution.37
R2
R1
O
R2
R1
Chart 1.
HL1
HL2
R2 = H
R2 = Cl
R1 = H
R1 = H
OH
HL3
R1 = H
R2 = CH3
R1 = CH3 R2 = CH3 HL4
HL5
NH
N
R1 = OMe
R2 = H
N
N
.
The reaction of Zn(OAc)2 2H2O with HL in a ratio of 1:2 in methanol at room
temperature in air affords orange colored complexes of composition [Zn(L)2] in excellent
yields. In the complex the ligands behave as a monoanionic O, N, N coordinating
tridentate ligand. A schematic representation for the synthesis of complexes is given in
Scheme 2.
N
N
Zn(OAc)2.2H2O
HL
N
Zn
O
O
CH3OH
M:HL = 1:2
N
Scheme 2 Schematic representation for the synthesis of the complexes
IR spectra of the complexes were recorded as KBr disks. In the complexes
C−Ophenoxo stretches observed near 1213−1249 cm−1 whereas the C=N stretch occurs as a
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sharp peak in the frequency range 1616−1641 cmꢀ1. The complexes are diamagnetic and
1
display well resolved H NMR spectra in DMSOꢀd6 solution. In complexes the azo
methine proton was observed as a singlet near 8.20 ppm, and chemical shift at about 9.00
ppm is associated with the H atom adjacent to the pyridyl N atom of the coordinated
ligands. The aromatic protons span in the range 7.80−6.30 ppm. The CH3 groups are
observed in the range 2.31−2.35 ppm whereas the OCH3 signal occurs at 3.99 ppm. The
details data for IR and NMR spectra are given in Experimental section.
Crystal Structure
The molecular structure of [Zn(L4)2],
diffractometry. The complex crystallizes in Pꢀ1 space group. The selected bond distances
and angles for are listed in Table 2, and the molecular view is shown in Fig.1.
4 was determined by singleꢀcrystal Xꢀray
4
Fig. 1 ORTEP plot and atom labeling scheme of 4. Hydrogen atoms have been omitted
for clarity.
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In complex
4
the ligand (HL4) acts as monoanionic N, N, O coordinating
tridentate ligand and binds meridionally. Here the metal centre occupies a distorted
octahedral geometry where the axial positions are occupied by one pyridyl nitrogen atom
and phenoxo group and the equatorial positions are occupied by two imine and one
pyridyl nitrogen atoms and one phenoxo group. In the complex, the Zn−Ophenoxo bond
length is found to be 2.03 Å, while the Zn−N bond distances fall in the range 2.12ꢀ2.23
Å.
Table 2 Selected Bond Lengths (Å) and Angles (o) for
4
Bond Lengths (Å)
Zn1–N1
Zn1–N2
Zn1–N3
2.234(2)
2.134(2)
2.216(3)
Zn1–N4
Zn1–O1
Zn1–O2
2.125(2)
2.047(2)
2.034(2)
Bond Angles (o)
N1–Zn1–N2
N1–Zn1–N3
N1–Zn1–N4
75.16(9)
85.77(9)
98.47(9)
N2–Zn1–O2
97.79(9)
N3–Zn1–N4 76.02(10)
N3–Zn1–O1 91.93(9)
N1–Zn1–O1 158.38(9)
N1–Zn1–O2 94.18(9)
N3–Zn1–O2 163.17(9)
N4–Zn1–O1 101.82(9)
N2–Zn1–N3 98.47(10)
N2–Zn1–N4 172.02(9)
N4–Zn1–O2
O1–Zn1–O2
87.36(9)
94.11(9)
N2–Zn1–O1
83.96(8)
Geometry Optimization and Electronic Structure
The geometry optimizations of the ligands were performed in solution (THF) both
in singlet ground state (S0) and excited state (S1) by DFT and TDDFT method
respectively. The potential energy scan (Fig. 2) of HL1 at S0 state reveals that the keto
(diheadral angle between two planes made by salicylaldehyde moiety and quinoline
moiety is 0.4o) tautomeric form is more stable by an amount of energy 2.175 kcal mol−1
than the corresponding nonplanar enol form (diheadral angle between two planes made
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by salicylaldehyde moiety and quinoline moiety is 26.6o), which is consistent with the Xꢀ
ray structure of this type of Schiff bases.36
-502915
-502916
-502917
-502918
-502919
-502920
-502921
1.0
1.2
1.4
1.6
1.8
2.0
R
Ndonor−H
Fig. 2 Potential energy curve at ground S0 state for HL1 calculated at DFT/B3LYP level.
For the complexes we are able to optimize only the ground S0 state geometries.
Moreover we are unable to grow the single crystals suitable for structure determination
for other systems. So the significant bond distances and angles of the optimized geometry
of compound
compared with the crystal structure parameter of
complex and are also shown in Fig. 3 as representative case.
Here the Zn(II) centre occupies a distorted octahedral geometry. The crystal
structure data of (Table 3) is in good agreement with optimized parameters and the
4
along with one representative case
2
in their singlet ground S0 state are
4
(Table 3) The optimized structure for
2
4
4
slight discrepancy (maximum deviation 0.068 Å for Zn1−N4 bond distance) comes from
the crystal lattice distortion existing in real molecules. The optimized geometries of all
the complexes do not show significant differences in the coordinate sphere around the
metal center. It reveals that the ligands bind in a similar fashion in the complexes.
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Table 3 Selected Optimized Geometrical Parameters of complexes
2
and
4
in the Ground
(S0) State at B3LYP Levels and Experimental Bond length (Å) and Angle (o) for complex
4
Solution phase
Experimental
2
4
4
2.226
2.200
2.226
2.200
2.071
2.069
2.234
2.193
2.234
2.193
2.067
2.067
2.234
2.134
2.216
2.125
2.047
2.034
Zn1−N1
Zn1−N2
Zn1−N3
Zn1−N4
Zn1−O1
Zn1−O2
161.490
172.170
161.526
161.043 158.38
171.463 172.02
161.024 163.17
N1−Zn1−O1
N2−Zn1−N4
N3−Zn1−O2
4
2
Fig. 3 Optimized molecular structures of
2 and 4. (Zn: Pale violet, Cl: Green, N: Blue, O:
Red, C: Grey. Hydrogen atoms are omitted for clarity)
The electron density at both HOMO and HOMO − 1 orbitals mainly reside on the
salicylaldehyde moiety while the electron density at LUMO, LUMO + 1, LUMO + 2 and
LUMO + 3, LUMO + 4 orbitals mainly reside on quinoline moiety. Similar pattern is
observed for other complexes. The compositions of HOMO and LUMO are useful in
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understanding the nature of transition as well as the absorption spectra of the complexes
vide infra).
(
Ligand Photophysical Study
The UVꢀvis absorption spectra of the ligands used in this present work were
studied at room temperature in THF. All the ligands show two well resolved peaks in the
region (450−473) and (337−360) nm. Fig. 4 shows experimental absorption spectra of all
the ligands. The detailed spectral data are collected in Table 4.
The absorption energies associated with their oscillator strengths, the main
configurations and their assignments calculated using TDDFT method using the S0
geometry for HL2 is discussed here and the related data are given in Table 5 whereas that
for HL1 is given in ESI (Table S1,†).
2.5
2.0
HL1
HL2
1.5
HL3
HL4
HL5
1.0
0.5
0.0
300
350
400
450
500
λ/nm
Fig. 4 Experimental UVꢀvis absorption spectra of all the ligands at room temperature in
THF.
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Table 4 Photophysical Parameters of the Ligands
ꢁνa
Ligand
ΦF
(nm) (x 10−3)
λ
max, nm
λemi,
nm
533
(ε, M−1 cm−1)
468 (63)
HL1
HL2
HL3
HL4
HL5
65
81
1.42
2.30
1.60
2.80
3.80
344 (10066)
455 (149)
360 (8690)
450 (28)
360 (2550)
473 (247)
351 (6162)
454 (29)
536
523
539
557
73
66
103
337 (6539)
a Stokes shifts.
Table 5 Selected Parameters for the Vertical Excitation (UVꢀvis Absorptions) and the
Emission of HL2; Electronic Excitation Energies (eV) and Oscillator Strengths (
),
f
Configurations of the LowꢀLying Excited States of HL2; Calculation of the S0−S1 Energy
Gaps Based on Optimized GroundꢀState Geometries (UVꢀvis Absoption) and the
Optimized ExcitedꢀState Geometries (Fluorescence) (THF used as solvent)
Electronic
Transitions
Composition
Excitation Oscillator
energy strength
CI
λexp
(nm)
Process
(f)
2.6388 eV 0.4180
(470 nm)
3.4097 eV 0.2564
(363 nm)
0.70565 455
S0 → S1
S0 → S3
HOMO → LUMO
Absorption
Emission
0.59290 360
0.37802
HOMO – 1 → LUMO
HOMO → LUMO + 1
HOMO → LUMO
2.3087 eV 0.3229
(537 nm)
0.70710 536
S1 → S0
In case of the ligand HL2 at ground state (S0) HOMO is composed of 76 % of
salicylaldehyde moiety and 24 % of the quinoline moiety while HOMO − 1 is composed
of 36 % of salicylaldehyde moiety and 64 % of the quinoline moiety and HOMO − 1 is
stabilized by 0.946 eV compared to HOMO. Similarly in LUMO the contribution of
salicylaldehyde moiety (62 %) is greater than that of quinoline moiety whereas in LUMO
+ 1 the electron density is mainly residing at quinoline moiety (82 %). The energy
difference between HOMO and LUMO is 3.504 eV. The calculated absorption bands are
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located at 470 and 363 nm for HL2 (Fig. 6), which are in good agreement with
experimental results of 455 and 360 nm (Table 5). These two absorption bands can be
assigned to the S0 → S1 and S0 → S3 transitions, respectively with ILCT character (Table
5). Similar calculations were carried out also for HL1 and given in ESI (Fig. S1 and Table
S1,†).
The fluorescence behavior of all the ligands discussed in this present work was
studied at room temperature in THF solution (Fig. 5). All the ligands display red shifted
emission spectra and their maxima occur in the range 530−560 nm with large Stokes shift
(65−103 nm). In solution, at room temperature, all the ligands were weak emitters having
low quantum yields (ΦF) in the range (1.02−3.8) x 10−3. The luminescence parameters for
all the ligands are listed in Table 4. The fluorescence intensity of HL1 is much greater
compared to other ligand system and this may be due to the incorporation of bulky group
in case of other ligands the probability of intersystem crossing increases which reduces
the fluorescence intensity for other ligand system compared to HL1.
175
150
HL1
HL2
125
HL3
100
HL4
HL5
75
50
25
0
500
550
600
650
700
λ
(nm)
Fig. 5 Experimental Emission spectra of all the ligands at room temperature in THF.
In order to study the emission property, the excited state (S1) geometry was
optimized38ꢀ39 in THF solution by timeꢀdependent DFT (TDDFT) method using CPCM
for all the ligands. The energy gap between the S0 and S1 state, calculated with the
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LUMO+1
−1.523 eV
IC
CT
LUMO
−2.503 eV
LUMO
−2.668 eV
S0
f = 0.2564
ex = 363 nm
S3
S1
S0
λ
S0
S1
f = 0.3229
f = 0.4180
ex = 470 nm
λ
em = 537 nm
λ
CT
HOMO
−5.374 eV
HOMO
−5.557 eV
HOMO−1
−6.503 eV
Fig. 6 Frontier molecular orbitals involved in the UVꢀvis absorption and emission of
HL2. IC stands for internal conversion and CT stands for conformation transformation.
Excitation and radiative decay process are marked as solid lines and the nonꢀradiative
processes are marked by dotted lines.
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optimized S1 state geometry, is the fluorescence emission wavelength.38ꢀ39 DFT study
shows more coplanar structure at S1 excited state compared to S0 ground state for all the
ligands. This geometry relaxation upon photoexcitation imparts remarkable effect on the
energy level of the molecular orbitals. In case of HL2 the LUMO is stabilized by 0.1657
eV at the S1 state geometry compared to that at S0 state geometry while the HOMO is
destabilized by 0.1834 eV for S1 state geometry compared to that at S0 state geometry. As
a result, the energy difference between the HOMO and LUMO is greatly decreased at the
S1 state compared to that at S0 state geometry and this geometry relaxation is the main
origin of large Stoke shift. The similar type of situation is observed for other ligands. For
HL2 the fluorescence wavelength was calculated as 537 nm (in THF) which is in very
good agreement with the experimental value of 536 nm (Table 5). The calculated
fluorescence spectral data for HL1 is given in ESI as a representative case (Table S1,†).
Absorption Study of the Complexes
The mode of coordination of ligands (HL1 − HL5) with Zn2+ was investigated by
absorption spectrophotometric titration at 25oC in THF solution. Fig. 7 describes a
representative UVꢀvis titration curve of HL2 with various concentration of Zn2+ ion. From
Fig. 7, it has been observed that with increasing concentration of Zn2+ ion the absorption
intensity of free ligand HL2 at 360 nm gradually decreases with small blue shift and the
absorption intensity at 455 nm gradually increases with an isosbestic point at 407 nm. It
is to be noted that there is no changes of the absorption intensity both at 360 and 455 nm
after the addition of excess of 0.5 equivalent of Zn2+ ion with respect to 1.0 equivalent of
HL2. This result corroborates with the formation of 1:2 complex in solution. The same
spectral behaviors are also documented for other ligands and the spectral changes for
UVꢀvis titrations of other systems are given in ESI (Fig. S2 to S5,†).
Comparative experiments on Zn2+ꢀsensing characteristics with the ligands series
were carried out with 0.1 mM HL solutions with the addition of 0.5 equivalent of Zn2+.
The experimental absorption spectra and the spectral parameters for all the complexes are
given in Fig. 8 and Table 6. The isolated complexes exhibit similar spectral behaviour.
To get better insight to experimental absorption values TDDFT calculations were
done for two complexes (
2
and
4
) on the basis of the optimized geometry. The calculated
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absorption energies associated with their oscillator strengths, the main configurations and
their assignments of
(Table S2,†).
2 is given in Table 7 whereas that for complexes 4 is given in ESI
2.0
1.5
1.0
0.5
0.0
300
350
400
450
500
Wavelength (nm)
Fig. 7 Spectrophotometric titrations of HL2 (0.1 mM) with various numbers of equivalent
.
of Zn(ClO4)2 6H2O in THF at room temperature ([Zn2+] = 0, 0.01, 0.02, 0.03, 0.04, 0.05
mM).
Table 6 Photophysical Parameters of the complexes
ꢁνa
Complex
ΦF
λ
max, nm
λemi,
(ε, M−1 cm−1)
466 (781)
nm
(nm)
540 74
546 92
546 96
571 98
572 118
0.3923
0.4409
0.241
1
350 (10150)
454 (2590)
374 (5455)
450 (558)
350 (2472)
473 (533)
2
3
0.0049
0.0104
4
351 (6140)
454 (990)
5
346 (4235)
a Stoke shift.
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2.0
1.5
1.0
0.5
0.0
Zn(L1)2
Zn(L2)2
Zn(L3)2
Zn(L4)2
Zn(L5)2
300
350
400
450
500
550
λ/nm
Fig. 8 Experimental UVꢀvis absorption spectra of all the complexes at room temperature
in THF.
Table 7 Main calculated optical transition for complex 2 with composition in terms of
molecular orbital contribution of the transition, Vertical excitation energies and Oscillator
strength in THF
Excit Composition
ation
Excitation Oscillator CI
energy strength
Assign
λexp
(nm)
(f)
1
2
3
4
2.6836 eV 0.3393
(462 nm)
2.7100 eV 0.1852
(457 nm)
3.2934 eV 0.0144
(376 nm)
0.6253
−0.3189
0.6932
−0.1171
0.6759
0.1760
ILCT
ILCT
ILCT
ILCT
ILCT
ILCT
454
HOMO − 1 → LUMO
HOMO → LUMO + 1
HOMO − 1 → LUMO + 1
HOMO → LUMO
HOMO − 1 → LUMO + 2
HOMO → LUMO + 3
HOMO − 5 → LUMO
HOMO − 4 → LUMO + 1
HOMO − 3 → LUMO
HOMO − 1 → LUMO + 2
HOMO → LUMO + 3
374
3.3726 eV 0.1286
(368 nm)
ILCT
ILCT
ILCT
ILCT
ILCT
−0.1428
−0.1574
−0.1578
−0.1931
0.6117
Here lowest lying distinguishable singlet → singlet absorption band at 450 nm
can be attributed to the HOMO − 1 → LUMO, HOMO → LUMO + 1, HOMO − 1 →
LUMO + 1 and HOMO → LUMO transitions and these can be assigned to π(L) → π∗(L)
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−
1.631 eV LUMO+3
−1.803 eV
LUMO+2
−2.471 eV LUMO+1
LUMO
−2.524 eV
f = 0.0144
λex = 376 nm
f = 0.1286
λex = 368 nm
f = 0.1852
λex = 457 nm
f = 0.3393
λex = 462 nm
−5.498 eV HOMO
HOMO-1
−5.601 eV
Fig. 9 Frontier molecular orbitals involved in the UVꢀvis absorption of complex 2.
transitions with ILCT character. The absorption band at 374 nm consists of more than
one excitation. The excitations observed at energies are 3.2934 eV ( = 376 nm and f =
λ
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0.0144) and 3.3726 eV (λ = 368 nm and f = 0.1286) and these are due to HOMO →
LUMO + 3, HOMO − 1 → LUMO + 2, HOMO − 3 → LUMO, HOMO − 4 → LUMO +
1, HOMO − 5 → LUMO transitions with ILCT character. Frontier molecular orbitals
involved in the UVꢀvis absorption for complex
2 are given in Fig. 9. In summary, the
TDDFT calculations reveal that the ICT process is occurring from salicylaldehyde moiety
(donor) to quinoline moiety (acceptor).
Fluorescent Properties of Complexes
The coordination mode as well as binding constant of HL to Zn2+ ion was
investigated by fluorescence titration in THF at room temperature. The ligands emit very
weak fluorescence near 530 nm (λex = 450 nm) with quantum yields span in the range
(1.02−3.8) x 10−3 (Table 4). However the fluorescence intensity enhances remarkably
with small bathochromic shift when excited at 450 nm upon addition of Zn2+ ions. The
details luminescence parameters are listed in Table 6. The enhancement of quantum
yields are 275, 190, 150, 1.75 and 2.75 fold for HL1−HL5, respectively (Table 6). Fig. 10
illustrates the enhancement of emission intensity of HL2 with the increase concentration
of Zn2+ ions from 0 to 1.0 equivalent. Inset of Fig. 10 describes a plot of emission
intensity at 546 nm against the titration of Zn2+ from 0 to 1.0 equivalent. It is clear from
the plot that the fluorescence intensity reaches a plateau after addition of 0.5 equivalents
of Zn2+ ions and there is no significant enhancement of the fluorescence intensity on
further addition of Zn2+. This result strongly corroborates with the formation of 1:2
complex between Zn2+ and HL2. The titration curves of other ligands are given in ESI
(Fig. S6 to S9,†).
In complex, the ligands bind as monoanionic form to the Zn2+ forming a sixꢀ
membered and fiveꢀmembered chelating ring with the lignads. TDDFT study reveals that
the transition near 450 nm is mainly associated with the HOMO and LUMO. The
electron density in HOMO is mainly localized on phenol ring while that in LUMO is
localized on the quinoline ring, which results in complete ICT from the phenol ring
(donor, HOMO) to quinoline ring (acceptor, LUMO) with the enhancement of
fluorescence intensity via CHEF mechanism.
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1000
800
600
400
200
0
1000
750
500
250
0
0.00
0.02
0.04
0.06
0.08
0.10
Zn(II) in mM
450
500
550
600
650
700
750
Weavlenth (nm)
Fig. 10 Fluorescence emission spectra of HL2 upon addition of Zn(II) in THF, λex= 454
nm at room temperature ( [HL2]= 0.1mM, [Zn(II)] = 0.01, 0.02, 0.03, 0.04 and 0.05 mM).
Inset: Fluorescence intensity vs different concentration of Zn(II) ion.
Table 8 Binding constant values for all complexes
Binding Constant (× 104)
Complex
2.80
1.25
0.75
15.37
3.00
1
2
3
4
5
The binding constant values have been determined from the emission intensity
data38 using the modified BenesiꢀHildebrand equation:
1 / ꢁ
to establish the binding abilities of the ligands with Zn2+. Here, ꢁ
F∞ 0, where F0 x and
of Zn2+, at an intermediate Zn2+ concentration, and at a concentration of complete
F
= 1 / ꢁ
F
max + (1 /
K
[C])(1 / ꢁFmax
)
F
=
Fx−F0 and ꢁFmax =
−
F
,
F
F∞ are the emission intensities of the ligand used in the absence
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interaction, respectively, and where
K
is the binding constant and [C] the Zn2+
concentration. The binding constant values for the complexes are given in Table 8. The
binding constants values reveal that the ligands have strong binding abilities to Zn2+.
Comparative experiments on Zn2+ꢀsensing characteristics with the ligands series
were carried out with 0.1 mM HL solutions with the addition of 0.5 equivalents Zn2+ and
the corresponding emission spectra are shown in Fig. 11. The isolated complexes exhibit
similar spectral behaviour. We found that the ligands HL4 and HL5 is a very weak and a
moderate emitter, respectively in the presence of Zn2+, where the other three ligands
(HL1, HL2, HL3) exhibit remarkable fluorescence enhancement after metalꢀion
complexation (Table 6 ).
1000
HL1+Zn2+
800
HL2+Zn2+
HL3+Zn2+
600
HL4+Zn2+
HL5+Zn2+
400
200
0
500
550
600
650
700
λ/nm
Fig. 11 Experimental Emission spectra of all the complexes at room temperature in THF.
It is interesting to note that the complexes which are associated with the ligands
having bulky group (HL4 and HL5, Chart 1) at the ortho position of the phenol group of
salicyladehyde moiety obviously caused the low quantum yields which may be attributed
to the vibrational effects. Therefore it is believed that the effect of substitution on
reducing the emission intensity may be attributed to the loss of conjugation or vibrational
quenching due to steric effect originated from the bulky group at ortho position.
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Metal ion competition Studies
We have studied the fluorescence intensity behavior upon addition of various
metal ions. All the ligands in the present study behave similarly. The metal ion binding
selectivity was assayed in 0.1 mM THF with excitation at 454 nm. Fig. 12 shows the
fluorescence intensity of HL2 in the presence of different metal ions. We observed that
among the metal ions studied only Zn2+, Cd2+ metal ions change the emission behavior of
HL2 (Fig. 13). However, the enhancement of the fluorescence intensity in case of Zn(II)
is much higher than that of Cd(II) ion.15,42 Other biogenic cations, such as Na+ and K+ in
living cells, neither enhanced nor quenched the emission intensity, as shown in Fig. 13
and this can be attributed to the very weak complexing ability of alkali metal ions with
the ligand. Other metal ions such that Ca2+, Ni2+, Co2+, Fe3+, Mn2+, and Cu2+ were
quenched the emission intensity as there is a probability of an electron and energy
transfer between the metal ion and ligands.
HL2
HL2+ Na+
1000
HL2+ K+
HL2+ Ca++
HL2+ Cu++
800
HL2+ Ni++
HL2+ Co++
600
HL2+ Fe+++
HL2+ Zn++
HL2+ Cd++
400
200
0
500
550
600
650
700
750
Weavlength (nm)
Fig. 12 Relative fluorescence intensity change of HL2 in the presence of various metal
ions at room temperature in THF, λex = 454 nm.
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1000
800
600
400
200
0
2+Cu2+
K+ Ca
Fe3+
2+ Mn2+
Ni
2+ Cd2+
Zn
Na+
HL2
Fig. 13 Histogram plot of fluorescence intensity change of HL2 in the presence of various
metal ions at room temperature in THF, λex = 454 nm.
Conclusion
In summary we have characterized five tridentate ligands having quinoline moiety
and their binding behavior with Zn(II) metal ion. The geometry optimization of the
ligands reveals that ligand stable in their keto tautomeric form in ground state. The
photophysical properties of the ligands were calculated with TDDFT approach. Emission
intensity of all the ligands are enhanced significantly through the complexation with
Zn(II) metal ion and form 1:2 (M:L) complex which was confirmed by fluorescence
titration as well as Xꢀray structure. Our study reveals that in complexes the quantum yield
(ΦF) decreases significantly when bulky group incorporated at the orthoꢀposition with
respect to the phenolic group of the ligand. The geometry of the complexes as well as
their spectral behaviors was studied by DFT and TDDFT method.
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Acknowledgements
Financial assistance received from the Council of Scientific and Industrial
Research, New Delhi, India, Science & Engineering Research Board, New Delhi, India is
gratefully acknowledged. We are also thankful to Department of Science and
Technology, New Delhi, India for the data collection on the CCD facility setup (Jadvpur
University) under DSTꢀFIST program. We also acknowledge CAS, Department of
Chemistry, Jadavpur University and DSTꢀPURSE program for other facilities. Pallab
Mondal and Amar Hens are also thankful to CSIR, New Delhi and UGC, New Delhi for
the research fellowship. Authors are also thankful to Karan Goswami for preliminary
work.
Notes
Inorganic Chemistry Section, Department of Chemistry, Jadavpur University,
Kolkata,
kkrajak@chemistry.jdvu.ac.in
† Electronic Supplementary Information (ESI) available: Xꢀray crystallographic
700
032,
India.
Eꢀmail:
kajalrajak@hotmail.com
//
file in CIF format for
944740.
4.CH3OH; Figure S1ꢀS9 and Table S1ꢀS2. CCDC reference number
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