Photophysical property vs. medium: Mononuclear, dinuclear and trinuclear Zn(II) complexes

Article abstract of DOI:10.1039/c4ra13606c

Three complexes [Zn₂(L)₂(μ^1,2-OAc)(μ^1,1-OAc)]·C₉H₈N₂1·C₉H₈N₂, [Zn₃(L)₄]·(ClO₄)₂2 and [Zn(L)₂

Full text of DOI:10.1039/c4ra13606c

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Photophysical property vs. medium: mononuclear,
dinuclear and trinuclear Zn(II) complexes†
Cite this: RSC Adv., 2015, 5, 4219
Amar Hens and Kajal Krishna Rajak*
1,2
1,1
2 2 9 8 2 9 8 2 3 4 4 2 2 3
Three complexes [Zn (L) (m -OAc)(m -OAc)]$C H N 1$C H N , [Zn (L) ]$(ClO ) 2 and [Zn(L) ]$CH OH 3
were synthesized using a common Schiff base 2,4-dimethyl-6-((quinolin-8-ylimino)methyl)phenol (HL)
1
and characterized by various methods such as elemental analysis, H NMR, FT-IR, UV-Vis and mass
spectroscopy. The single crystal XRD analysis revealed that complex 1 is dinuclear, complex 2 trinuclear
and complex 3 mononuclear in nature. The photophysical properties of the ligand and complexes were
also investigated by different techniques such as UV-Vis spectroscopy, fluorescence spectroscopy,
fluorescence lifetime measurement, DFT, and TDDFT calculations in solvents of different polarity. The
complexes exhibited higher quantum yields than the ligand in solution. The trinuclear complex had
higher emission intensity than the mono and dinuclear complexes in solution as well as in the solid state.
In addition, complex 1 exhibited a unique dissymmetric coordination mode of the oxygen atom in the
acetate bridging moiety.
Received 1st November 2014
Accepted 28th November 2014
DOI: 10.1039/c4ra13606c
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concentrations or in aggregated states, and thus cannot serve as
ideal uorescent compounds.
Introduction
Schiff bases play a vital role in modern coordination chem-
A diversity of uorescent-sensing approaches have been
1–3
istry, where tridentate salen-type ligands are one of the most extensively investigated pertaining to the signaling mechanisms
4–6
21a
popular classes of Schiff-base ligands amongst the coordi- of metal–ligand charge transfer (MLCT),
internal charge
21b
nation chemists. These systems possess an extra dimension to transfer (ICT), photo induced electron transfer/energy trans-
their optical properties as metal coordination signicantly fer (PET), metal–ligand charge transfer (MLCT), excimer/
21c
21d
alters the photophysical processes they are capable of. These exciplex formation as well as the excited state intramolecular/
21d
intriguing metal coordination-driven photophysical properties intermolecular proton transfer (ESIPT). The uorescence of
6
have enabled various applications including the development of the ligand (HL) appears due to ESIPT mechanism but the
7
–9
10,11
nonlinear optical materials, sensitizers for solar cells,
metal ion sensors.
and emission intensity is very weak in contrast with the complexes,
probably due to quenching by the lone pair of electrons of the
12–14
21c
Salen type complexes have an important aspect of their donor atoms in the ligands through the PET mechanism. For
emission behavior. In most cases, metal coordination enhances Zn(II) complexes, no emission originating from metal-centered
the charge-transfer character in the excited state of the chro- MLCT/LMCT excited states are expected, since the Zn(II) ion is
mospheres. The uorescence of the salen type complexes
with several elements is very strong, particularly the salen type- uration. Thus, the emission observed in the complexes is
1
0
15–17
difficult to be oxidized or reduced due to its stable d cong-
22
zinc(II) complexes which exhibit electroluminescence as well as tentatively assigned to the p / p* intraligand uorescence.
18,19
21c
photoluminescence.
attracted intensive interest because of their potential applica- complexation of HL with metal ions, as well as CHEF
tions in organic light emitting diodes (OLED), organic eld mechanism which reduces the loss of energy by thermal
The organic uorescent materials have Moreover, in complex the PET process is prevented by the
23,24
20
21
effect transistors (OFET), uorescent sensors, etc. Conven- vibrational decay
enhances emission intensity during
tional uorescent dyes of large delocalized p-conjugated complexation. The lattice structure of dinuclear complex
moieties typically suffer from uorescence quenching at high contains non coordinated free amine in solid state and it
undergoes PET process to quench its emission intensity.
Our strategy is to change the concentration ratio of metal
and ligand and then synthesize different types of polynuclear
complexes to nd the genesis behind their different photo-
Inorganic Chemistry Section, Department of Chemistry, Jadavpur University, Kolkata,
7
00 032, India. E-mail: kajalrajak@hotmail.com
physical phenomena. Recently, we reported the synthesis and
in CIF format for 1 and 2; Fig. S1–S17 and Tables S1–S5. CCDC 1015063 and the structure of a Zn(II)-mononuclear complex [ZnL
]. This
2
†
Electronic supplementary information (ESI) available: X-ray crystallographic le
6
1
015064. For ESI and crystallographic data in CIF or other electronic format see
bischelate Zn(II) complex displayed a moderately strong
DOI: 10.1039/c4ra13606c
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
uorescence in THF solution. We synthesized HL in different LS 55 uorescence spectrometer. For the luminescence
ways besides the two unique coordination modes of dinuclear measurements, excitation and emission slit width of 5 nm was
and trinuclear Zn(II) complexes, synthesized to achieve our goal. used for the complex and 10 nm slit width for the ligand.
Our main focus was to compare the change in excited state Quantum yields of the complexes were determined in freeze–
emission behavior of the above stated complexes in solution pump–thaw-degassed solutions of the complexes by a relative
state and in solid state. Interestingly, the complexes show method using quinine sulfate in the same solvent as the stan-
2
6b
signicant differences but maintain a regular trade in their dard [Fstd ¼ 0.54 (at 298 K) in 0.1 M H
2
SO
4
at lex ¼ 350 nm] by
photoluminescence behaviors.
usual method. The uorescence life time was determined by
26a
Here we report an unexpected and exclusive formation of usual method.
bisacetato bridged dinuclear complex, which contain different
mode of coordination with the oxygen atom of acetate group.
We also focus on how different modes of coordination of acetate
group can modulate the geometrical environment of central
atom from trigonal bipyramide to square pyramidal shape.
Some aspects of these unforeseen ndings were rationalized
by DFT and TDDFT calculations. We also calculated and
analyzed the singlet ground state natural transition orbitals
Computational details
0
The geometrical structures of the singlet ground state (S ) were
optimized in solution by the DFT method with B3LYP
2
7
2
8
exchange correlation functional approach. The vibrational
frequency calculation was also performed for the complexes to
ensure that the optimized geometries represent the local
minima and there are only positive eigen values. On the basis of
these optimized ground state geometry structure, the absorp-
tion spectral properties in solvents with different polarity were
(NTOs) derived from TDDFT results and compared them with
the ground state molecular orbitals (MOs) obtained from DFT
calculations. The computational modeling of the NMR param-
eter is also of abiding interest, and such DFT calculations have
emerged as a promising approach for the prediction of nuclear
29
calculated by TDDFT associated with the conductor-like
3
0
polarizable continuum model (CPCM). TDDFT calculations
were performed for complex 1 only as both the complexes show
similar nature. We computed the lowest 40 singlet–singlet
transition.
The effective core potential (ECP) approximation of Hay and
Wadt was used for describing the (1s 2s 2p ) core electron for
25
shielding and coupling constants of NMR active nuclei. Thus,
we computed the proton NMR chemical shis and also the
1
1
H– H spin–spin coupling constant using the gauge-
independent atomic orbital (GIAO)-DFT method, which was
aimed at providing the denitive characterization of the
complexes. The solid-state structure of the complexes stays in
solution, as revealed by the combined experimental and theo-
retical NMR spectral analyses.
2
2
6
zinc whereas the associated “double-x” quality basis set
LANL2DZ was used for the valence shell.
3
1,32
6-311+G and 6-
311+G* basis sets were used for H atom and for C, N and O
atoms respectively. The calculated electronic density plots for
frontier molecular orbitals were prepared by using the Gauss
View 5.0 soware. All the calculations were performed with the
Experimental section
Materials
3
3
34
Gaussian 09W soware package. Gauss Sum 2.1 program
was used to calculate the molecular orbital contributions from
groups or atoms.
3,5-Dimethyl phenol was purchased from sigma Aldrich.
Zn(OAc) $2H O. All the solvents were spectroscopic grade and
2
2
1
In addition, the H NMR properties of the complexes were
used aer proper distillation. The purity was also veried by
recording the emission spectra in the studied spectral region.
Zinc perchlorate hexahydrate was prepared by the standard
procedure.
calculated with the magnetic eld perturbation method with the
35
GIAO algorithm with the NMR ¼ spin–spin keyword incorpo-
rated in the Gaussian 09W program. In calculation, the 6-311+G
(2d,p) basis set was employed for non-metal. The relative
Caution! Perchlorate salts are potentially explosive and
should be handled with care and in small amounts, although we
encountered no problems.
chemical shi of a given nucleus X in the molecule was dened
calc
ref
calc
as d
molecule optimized at the same level of theory.
account for the solvent effect, we used the integral equation-
X
[ppm] ¼ s
X
ꢀ s
X
where TMS was used as a reference
35b,36a
In order to
36b,c
formalism polarizable continuum model (IEFPCM) method.
Physical measurements
UV-Vis spectra were recorded on a Perkin-Elmer LAMBDA 25
Crystallographic studies
spectrophotometer. IR spectra were recorded on a Perkin-Elmer
1
L-0100 spectrophotometer. H NMR spectra were measured on The X-ray intensity data of single crystals of compound 1 and 2
Bruker FT 300 MHz spectrometer with TMS as the internal were collected on Bruker AXS SMART APEX CCD diffractometer
˚
standard. Elemental analyses (C, H, N) were performed on (Mo Ka, l ¼ 0.71073 A) at 293 K. The data were reduced in
37
Perkin-Elmer 2400 series II analyzer. The molar conductivity SAINTPLUS and empirical absorption correction was applied
37
was determined using Systronics Conductivity Meter 304 in using the SADABS package. Metal atom was located by Pat-
acetonitrile solution at room temperature. Electrospray ioniza- terson Method and the rest of the non-hydrogen atoms were
tion mass spectrometry (ESI-MS) measurements were done on a generated from successive Fourier synthesis. The structures
2
Micromass Qtof YA 263 mass spectrometer using acetonitrile as were rened by full matrix least-square procedure on F . All
a solvent. The emission data were collected on a Perkin-Elmer non-hydrogen atoms were rened anisotropically. Calculations
4220 | RSC Adv., 2015, 5, 4219–4232
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3
8
were performed using the SHELXTL V 6.14 program package.
16 2
Yield 4.6 g (83%). Elemental anal. calc. for C18H N O: C,
1
For complex 2, a solvent void remained in which no satisfactory 78.24; H, 5.84; N, 10.14. Found: C, 78.85; H, 5.69; N, 10.22%. H
solvent model could be constructed and thus the structure was NMR {300 MHz, CDCl d (ppm), J (Hz)}: 8.16 (CH]N, s), 4.98
3
rened using the SQUEEZE option of PLATON program. The (OH, bs) 7.69–7.16 (8H, ArH), 2.33 (CH , s), 2.30 (CH , s).
3
3
ꢀ
1
squeeze output was also appended in the CIF le of complex 2. Selected FT-IR bands (KBr, cm ): 3543 (b), 3454 (s), 3348 (s),
3
9a
39b
Molecular graphics were drawn using ORTEP and Mercury
3033 (w), 1962 (w), 1616 (s), 1565 (m), 1498 (s), 1467 (s), 1363 (s),
1332 (s), 1270 (w), 1130 (m), 1080 (m), 793 (s). ESI-MS (CH CN):
m/z calc. 277.1341, found: 277.1502 (100%) (HL + H) , 300.2122
sowares. The relevant crystal data are given in Table 1.
3
+
+
(
52%) (HL + Na) .
Synthesis of 2-hydroxy-3,5-dimethylbenzaldehyde (A)
A mixture of 3.5 g (25 mmol) of 2,4-dimethylphenol, 1.89 g (13.5
mmol) of hexamethylenetetramine and 1.89 g (21 mmol) of
paraformaldehyde was heated to 100 C, with stirring. 7.5 mL of
Synthesis of complexes
ꢁ
1,2
1,1
[Zn
CN solution (20 mL) of HL (0.276 g, 1.00 mmol), methanolic
$2H O (0.219 g, 1.00 mmol) was
(140 mL, 0.102 g, 2.00 mmol)
2 2 9 8 2 9 8 2
(L) (m -OAc)(m -OAc)]$C H N , 1$C H N . To the
acetic acid was then added drop wise. The whole system was CH
3
ꢁ
brought to 110 C. over a period of 15 minutes, with stirring. 1.7 solution (10 mL) of Zn(OAC)
2
2
mL of sulphuric acid and 0.6 mL of water were then added drop added drop wise followed by NEt
3
wise over a period of 15 minutes. Stirring was continued at 110 and stirred for 4 h at room temperature. The color of the solu-
ꢁ
C. for 30 minutes, and then the mixture was poured into 50 mL tion was changed from colorless to intense orange. The solution
of hot water. The preparation was extracted 3 times with was kept for slow evaporation. The orange single crystals suit-
methylene chloride, and then the combined organic phases able for single-crystal X-ray analysis were obtained from CH
were washed until neutral with an aqueous sodium hydrogen solution aer one week. Yield: 375 mg (80%). Elemental anal.
carbonate solution. Drying over sodium sulphate, evaporation calc. for C49 Zn : C, 62.36; H, 4.70; N, 8.91. Found: C,
and removal of the solvent by distillation yielded 3.04 g (80%) of 62.48; H, 4.61; N, 8.88%. H NMRexpt {300 MHz, CDCl
yellow oil. Elemental anal. calc. for C
: C, 71.98; H, 6.71. J (Hz)}: 8.74 (H10, s), 8.34 (H1, d, J ¼ 4.8), 7.77–6.88 (8H, ArH),
3
CN
H
44
N
6
O
6
2
1
3
, d (ppm),
9
10 2
H O
1
1
Found: C, 72.09; H, 6.68%. H NMR {300 MHz, CDCl , d (ppm), J 2.40 (H17, s), 2.02 (H40, s). H NMR
{d (ppm) aq ¼ amino-
3
theor
(
(
Hz)}: 11.07 (CHO, s), 9.80 (OH, s), 7.25–7.15 (2H, ArH), 2.29, quinoline, Me ¼ methyl, ace ¼ acetate and al ¼ aldehyde }:
CH , s), 2.18 (CH , s). ESI-MS (CH CN): m/z calc. 150.0681, 8.68 (H10), 8.54 (H1), 7.93 (H3), 7.66 (H2), 7.47 (H7), 7.30 (H5),
3
3
3
+
found: 151.0772 (100%) (A + H) .
Preparation of ligand [2,4-dimethyl-6-((quinolin-8-ylimino)- CH
methyl)phenol] (HL). (20 mmol) of 2-hydroxy-3,5- (w), 2358 (s), 2330 (s), 1561 (s), 1363 (s), 1219 (s), 1046 (m),
dimethylbenzaldehyde (A) and 2.88 g (20 mmol) of 8-amino- 825(m). IRtheor (n cm ): ( C–Hasym) 3218, 3220, 3229 and 3236
7.10 (H14), 6.58 (H12), 2.31 (H35, CH
3
[ace]) and 1.93 (H38,
ꢀ
1
3
[al]). Selected FT-IRexptl bands (KBr, cm ): 3452 (b), 2918
3
g
ꢀ
1
aq
aq
Me
quinoline were taken in 50 mL methanol solution. Then the ( C–Hsym) 3193, 3196, 3204, 3206 and 3209; ( C–Hsym) 3005
ace
Me
mixture was reuxed in water bath for 2 hours. The solvent was and 2987; (C]NHsym) ( C]O
1605, 1601; C–Hvib) 1537
asym
al
al
ace
removed under reduced pressure to obtain the desired ligand (HL). and 1541; n( C–H ) 1360 ( C–Osym) 1365 and ( C–Osym) 1363.
vib
Table 1 Crystal data and structure refinement parameters for complex 1$C
9
H
8
N
2
and 2
, 1$C
1,2
1,1
[Zn
2
(L)
2
(m -OAc)(m -OAc)]$C
9
H
8
N
2
9
H
8
N
2
[Zn
3
(L)
4
]$(ClO
4
)
2
, 2
Formula
C
49
H
44
N
6
O
6
Zn
2
C
72
H58Cl
2
N
8
O
12Zn
3
M
r
943.68
1494.33
Crystal system
Space group
Triclinic
P1ꢀ
8.262(2)
14.563(3)
18.991(4)
75.372(10)
77.580(10)
84.010(10)
2156.53(8)
2
1.453
1.171
1.1–27.5
293(2)
9814
Monoclinic
C2/c
24.966(9)
20.629(7)
16.929(6)
90
111.647(2)
90
8104.1(5)
4
˚
a/A
˚
b A
˚
c/A
ꢁ
ꢁ
a/
b/
ꢁ
g/
3
˚
V/A
Z
D
ꢀ3
calcd/mg m
1.225
1.004
ꢀ
1
m/mm
q/
ꢁ
1.8–27.6
293(2)
9438
0.062, 0.182
1.01
T/K
Rens collected
a
b
R
1
, wR
2
2[I > 2s(I)]
0.044, 0.137
1.07
GOF on F
a
b
2
o
2 2
2 2 1/2
R
1
¼ SrrF
o
r ꢀ rF
c
rr/SrF
o
r. wR
2
¼ [S[w(F
ꢀ F
c
) ]/S[w(F
o
) ]]
.
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Scheme 1 Schematic representation for the synthesis of the ligand.
ESI-MS (CH CN): m/z calcd 740.4173, found: 740.3229 (100%).
3
ꢀ
1
2
ꢀ1
Molar conductance, L
Zn (L) ]$(ClO , 2. To the CH
0.276 g, 1.00 mmol), methanolic solution (10 mL) of
Zn(ClO $6H O (0.372 g, 1.00 mmol) was added dropwise fol-
lowed by NEt (140 mL, 0.102 g, 2.00 mmol). The reaction
M
: (CH
3
CN solution) 8 U cm mol
.
[
3
4
4
)
2
3
CN solution (20 mL) of HL
(
4
)
2
2
3
mixture was reuxed under water bath for 2 h, then cooled and
stirred for 6 h at RT. The color of the solution changed from
colorless to intense orange. The solution was kept for slow
evaporation. The orange single crystals suitable for single-
crystal X-ray analysis were obtained from solution aer 2 days.
2 8 12
Yield: 4.25 g (84%). Elemental anal. calc. for C72H58Cl N O -
Zn : C, 57.87; H, 3.91; N, 7.50. Found: C, 57.95; H, 3,87; N,
3
1
7
.57%. H NMR {300 MHz, CDCl
3
, d (ppm), J (Hz)}: 8.804 (H1, s),
8
.75 (H10, d, J ¼ 5.9), 8.09–7.76 (8H, ArH), 2.18 (H27, s), 2.14
1
(H26, s). H NMR
{d (ppm)}: 8.77 (H1), 8.68 (H10), 8.07 (H3),
theor
7.86 (H2), 7.71 (H7), 7.59 (H5), 7.74 (H14), 7.52 (H12), 2.11 (H17,
–
CH
3
1
[al]) and 2.19 (H18, –CH
3
[al]). Selected FT-IR bands (KBr,
ꢀ
cm ): 3488 (b), 2918 (w), 2361 (s), 1843 (s), 1770 (s), 1653 (s),
ꢀ
1
1
(
418 (s), 1269 (m), 1219(b), 1048 (w), 810 (w). IRtheor (n cm ):
C–Hasym) 3324, 3308, 3289 and 3265; ( C–Hsym) 3213, 3209,
195 and 3190; ( C–Hsym) 3017 and 3001; ((C]NHsym) 1614;
C–Hvib) 1542 and 1536; ( C–H ) 1524 and 1518; ( C–Hvib)
aq
aq
Me
3
Me
al
al
vib
al
1
362 and ( C–O ) 1365. ESI-MS (CH CN): m/z calcd 646.8229,
sym
3
Fig. 1 Asymmetric unit of complex 1$C
ellipsoids drawn at the 25% probability level. Hydrogen atoms have
been omitted for clarity.
9
H
8
N
2
with displacement
found: 646.9227 (100%). Molar conductance, L : (CH CN
M
3
ꢀ1
2
ꢀ1
solution) 252 U cm mol
.
Scheme 2 Schematic representation for the synthesis of the complexes.
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Table 2 Selected bond distance ( A˚ ) for complex 1 and 2
Bond length
1,2
1,1
[
Zn
2
(L)
2
(m -OAc)(m -OAc)]$C
9
H
8
N
2
, 1
3 4 4 2
[Zn (L) ]$(ClO ) , 2
Zn1–O1
Zn1–O2
Zn1–O4
Zn1–N1
Zn1–N2
1.988(2)
2.098(2)
2.066(19)
2.148(3)
2.077(2)
Zn2–O3
Zn2–O4
Zn2–O6
Zn2–N3
Zn2–N4
2.000(3)
2.029(18)
1.999(2)
2.166(3)
2.086(2)
Zn1–O1
Zn1–O2
Zn1–N1
Zn1–N2
Zn1–N3
2.133(3)
2.140(3)
2.150(3)
2.095(3)
2.122(3)
Zn1–N4
Zn2–O1
Zn2–O2
Zn2–O1
Zn2–O2
2.150(3)
1.954(3)
1.940(3)
1.954(3)
1.940(3)
i
i
A schematic representation for the synthesis of complexes is
given in Scheme 2. The elemental analysis and Electrospray
ionization mass spectroscopic measurements conrmed the
Result and discussion
Synthesis of the probe
The preparation of Schiff base 2,4-dimethyl-6-((quinolin-8- formation of the synthesized complexes (see Experimental
ylimino)methyl)phenol (HL) was presented in Scheme 1. The section).
stoichiometric reaction of Zn(OAC) $2H O with the ligand HL
2
2
in acetonitrile produced the orange colored acetate bridge Crystal structures
dinuclear complex 1$C in good yields. However, when the
same reaction was carried out in presence of stoichiometric
amounts of Zn(ClO $6H O it gave a dark orange colored
9
H
8
N
2
1,2
1,1
[
Zn (L) (m -OAc)(m -OAc)]$C H N , 1$C H N . The asym-
2 2 9 8 2 9 8 2
ꢀ
metric units of complex 1 contain two L , two Zn(II) ion, two
acetate anion and one free amine (Fig. 1). The geometry around
the Zn1 atom is intermediate between square based pyramid
and distorted trigonal bipyramid (s ¼ 0.48) whereas that of Zn2
4
)
2
2
phenoxo bridge trinuclear complex, 2. In these complexes, the
ligand HL bound as a monoanionic N, N, O coordinating tri-
dentate ligand and formed neutral complex 1$C H N , whereas
9
8
2
41
is distorted trigonal bipyramidal (s ¼ 0.63). The equatorial
complex 2 showed cationic nature. The molar conductivity
values of both the complexes were determined in acetonitrile
solution at room temperature. The values of the molar
positions at Zn1 are occupied by O2, N2 and O4 atoms and the
axial position are engaged by N1 and O1 respectively, while for
Zn2 center the equatorial positions are occupied by O3, O4 and
N4 atoms and axial positions are engaged by O6 and N3
respectively. The Zn(II) ions in complex 1 have the same coor-
ꢀ1
2
ꢀ1
conductivity were 8 and 252 U cm mol for 1$C
9 8 2
H N and 2,
respectively. The value for complex 2 corresponded to a 2 : 1
40
9 8 2
type of electrolyte behavior, whereas the complex 1$C H N
dination environment as that of ZnN
2
O
3
in which the Zn–O and
was nonconductance.
˚
Zn–N bond distances vary between 1.988(2)–2.098(2) A and
Fig. 2 Asymmetric unit of complex 2, with displacement ellipsoids drawn at the 30% probability level. Hydrogen atoms have been omitted for
clarity.
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ꢁ
a
Table 3 Selected bond angle ( ) for complex 1 and 2
ꢁ
Bond angle ( )
1,2
1,1
[
Zn
2
(L)
2
(m -OAc)(m -OAc)]$C
9
H
8
N
2
, 1
3 4 4 2
[Zn (L) ]$(ClO ) , 2
O1–Zn1–O2
O1–Zn1–O4
O1–Zn1–N1
O1–Zn1–N2
O2–Zn1–O4
O2–Zn1–N1
O2–Zn1–N2
O4–Zn1–N1
O4–Zn1–N2
N1–Zn1–N2
92.53(9)
98.03(8)
165.85(9)
89.58(9)
92.35(8)
90.45(9)
129.90(9)
95.67(9)
136.82(9)
77.94(9)
O3–Zn2–O4
103.40(8)
96.81(10)
93.16(11)
128.34(9)
94.86(9)
O1–Zn1–O2
O1–Zn1–N1
O1–Zn1–N2
O1–Zn1–N3
O1–Zn1–N4
O2–Zn1–N1
O2–Zn1–N2
O2–Zn1–N3
O2–Zn1–N4
N1–Zn1–N2
77.05(10)
160.33(11)
85.09(11)
101.14(11)
96.69(11)
96.52(12)
103.91(11)
82.70(11)
158.41(11)
78.38(13)
86.25(11)
97.63(10)
N1–Zn1–N3
N1–Zn1–N4
N2–Zn1–N3
N2–Zn1–N4
N3–Zn1–N4
O1–Zn2–O2
O1–Zn2–O1
96.37(12)
95.58(13)
171.89(14)
96.00(12)
78.24(12)
86.25(11)
119.59(11)
126.28(10)
126.28(10)
116.93(11)
97.42(11)
O3–Zn2–O6
O3–Zn2–N3
O3–Zn2–N4
O4–Zn2–O6
O4–Zn2–N3
O4–Zn2–N4
O6–Zn2–N3
O6–Zn2–N4
N3–Zn2–N4
92.57(9)
i
i
127.30(9)
165.84(10)
88.92(9)
O1–Zn2–O2
i
O1 –Zn2–O2
i
76.99(10)
O2–Zn2–O2
i
i
O1 –Zn2–O2
Zn1–O1–Zn2
Zn1–O2–Zn2
a i
Symmetry code: ꢀx, y, 1/2 ꢀ z.
1
,2
˚
2
.077(2)–2.166(3) A, respectively (Table 2). The ligand acts as a Zn2 ions by two different bridging fashions syn–syn m
1
,1
mono anionic tridentate chelated ligand with N, N, O donors in bidentate and m monodentate resulting in diacetato bridged
each metal centre. The two acetate groups bridged the Zn1 and dinuclear Zn(II) complex.
[
Zn
3 4 4 2
(L) ]$(ClO ) , 2. The crystal structure of complex 2
2
+
consists of discrete tri nuclear [Zn
3
(L)
4
]
cation and two iso-
lated prechlorate anions in the ratio of 1 : 2. The asymmetric
ꢀ
units of complex 2 contain two Zn(II) ions, two L and two ClO
4
anion (Fig. 2). Two different types of coordination environment
are observed around the Zn(II) ions. The terminal zinc atoms
i
Zn1 and Zn1 are symmetrically equivalent and hexacoordi-
nated. The bond lengths and angles are given in the Tables 2
and 3. The central Zinc atom Zn2 adopts tetrahedral coordi-
nation environment with four oxygen donor atoms. The
˚
distance between Zn1–Zn2 is 3.074 A. The bond lengths of Zn–N
and Zn–O in Zn1 vary in the range 2.129 and 2.136 respectively.
The bond lengths of Zn–O in Zn2 vary in the range 1.947. In
i
ꢁ
complex 2, angular Zn1–Zn2–Zn1 disposition is nearly 170 .
The Zn1 is distorted in octahedral geometry which is coordi-
nated by two phenoxido oxygen atoms (O1 and O2), two pyridyl
nitrogen of the quinoline moiety (N1 and N4) and two imine
nitrogen atoms (N2 and N3) of the ligands. In octahedral
geometry, cis angles ranging from 77 to 103 and the trans angle
1
Fig. 3 Linear correlation between the experimental and calculated H
NMR chemical shifts of 1 in aliphatic and aromatic regions.
Table 4 Frontier molecular orbital composition (%) in the ground state for 1
Contribution (%)
Orbital
Energy (eV)
Zn (M)
Acetate (OAc)
Quinoline (Qu)
Aldehyde (al)
Main bond type
L + 4
L + 3
L + 2
L + 1
LUMO
HOMO
H ꢀ 1
H ꢀ 2
H ꢀ 3
H ꢀ 4
H ꢀ 5
ꢀ0.98
ꢀ1.31
ꢀ1.56
ꢀ2.13
ꢀ2.37
ꢀ5
0
0
0
0
0
0
1
1
7
6
7
0
0
0
0
0
1
1
1
5
99
56
61
73
68
15
15
46
24
40
16
1
43
38
27
32
83
83
51
64
51
42
p* (Qu)
p* (Qu) + p* (al)
p* (Qu) + p* (al)
p* (Qu) + p* (al)
p* (Qu) + p* (al)
p (Qu) + p (al)
p (Qu) + p (al)
p (Qu) + p (al)
ꢀ5.3
ꢀ6.01
ꢀ6.19
ꢀ6.28
ꢀ6.43
M(d) + p (OAc) + p (Qu) + p (al)
M(d) + (OAc) + p (Qu) + p (al)
M(d) + (OAc) + p (Qu) + p (al)
3
35
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Fig. 4 Optimized energy diagram with optimized molecular structures of 1, 1a and 1b. (Zn: pale violet, N: blue, O: red, C: grey. Hydrogen atoms
are omitted for clarity) DEDIS means the destabilization energy gap.
ꢁ
from 158 to 178 clearly indicates the amount of distortion. In Table 6 Photophysical parameters of the ligands in different solvent at
room temperature
contrast, Zn2 is tetrahedral coordinated by four phenoxo oxygen
atom from the four individual ligands.
a
2
7
9
Solvent
Hexane
l
abs
l
em
(F
f
)
s (ns)
c
k
r
ꢂ 10
k
nr ꢂ 10
323
325
329
332
342
348
419
445
457
464
476
480
2.98
2.86
2.67
1.02
0.78
0.73
0.91
0.94
1.01
1.53
2.27
3.01
1.13
1.21
0.91
0.89
1.01
1.19
3.2
3.0
2.6
0.67
0.42
0.32
1.06
1.06
0.96
0.64
0.51
0.44
Mass spectra
CCl
4
The ligand (HL) and the desired aldehyde (A) were diluted with DCM
acetonitrile for mass spectrometry. Mass spectral analysis in the
Acetone
CH CN
3
positive ion mode showed a major peak at m/z (%) ¼ 277.1502
DMF
(
100), 300.2122 (60) and 151.0772 (100), which were assigned to
a
ꢀ2
+
+
+
¼
10 .
the mono cationic form of [HL + H] , [HL + Na] and [A + H]
respectively. The mass spectrum of the HL and A are given in
ESI Fig. S1 and S2.† The mass spectra of complexes 1 and 2 were
carried out in acetonitrile solution. A major peak appeared at
m/z (%) ¼ 740.3229 (100) and 646.9227 (100), which was allo-
observed at strongly deshielding eld near at 11.07 ppm and
the peak appeared at 9.80 ppm mainly responsible for
phenolic proton. Two doublet aromatic protons appeared at
aromatic span 7.25–7.15 ppm. Two sharp singlet peaks
appeared at a higher shielding eld near 2.29 and 2.18 ppm
due to the two methyl proton in ESI Fig. S5.† The peak that
appeared at high deshielding eld for aldehyde proton in (A)
cated for the monocationic monoacetate bridging of dinuclear
+
complex [Zn
2
(L)
2
(m-OAc)] and bicationic trinuclear complex
2
+
[Zn
3
(L)
4
]
respectively. The mass spectrums of the complexes
are given in ESI Fig. S3 and S4.† It also suggested that in solu-
tions the major species existing are as same as the molecular
formula in their solid state.
1
complete disappeared in H NMR spectra of HL, whereas a
new sharp singlet azo methine peak appeared at 8.16 ppm and
the aromatic protons were observed near 7.69 to 7.16 ppm in
ESI Fig. S6.† A broad singlet peak that appeared at 4.98 ppm
was attributed for phenolic proton. In HL two singlet methyl
protons showed slightly higher eld compared to the methyl
protons of (A). In both the complexes the phenolic proton of
HL completely disappeared, the azo methine proton
NMR spectra
The ligand (HL), the desired aldehyde (A) and complexes are
diamagnetic and display well resolved H NMR spectra. For
complex 1, (A) and HL, H NMR spectra were done in CD Cl
solution, whereas, complex 2 was dissolved in CD CN solution
1
1
3
3
respectively. A sharp singlet peak of aldehyde proton of (A) was
Table 5 Selected optimized geometrical parameters of complexes 1, 1a and 1b in the ground (S
length ( A˚ ) and angle ( )
0
) state at B3LYP levels and experimental bond
ꢁ
˚
Zn–Zn ( A˚ )
ꢁ
s (ref. 41) (Zn1)
s (ref. 41) (Zn2)
Zn–Obrig (A)
O(m)–Zn–O(m) ( )
Energy (a.u.)
1
1
1
0.15
0.63
0.068
0.045
0.62
0.025
2.0405
2.0022
2.0615
3.5918
4.1693
3.2035
97.18 & 98.54
108.11 & 129.75
79.42 & 76.68
ꢀ5775.17223673
ꢀ5775.16628225
ꢀ5775.15511220
a
b
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IR spectra
The IR spectra of the complexes were recorded in a KBr disk.
The calculated IR spectra of all the complexes were reported.
The characteristic IR data were given in the Experimental
section. In both complexes C–Ophenoxo stretches were observed
ꢀ
1
ꢀ1
at 1220 cm whereas C]N stretches at 1610 and 1605 cm for
and 2, respectively. The characteristic stretching frequency of
1
ꢀ
ꢀ
n (COO ) and n (COO ) for 1 were observed at about 1561 and
as
s
ꢀ
1
1
363 cm . Both the complexes exhibited broad band at the
ꢀ1
region of 3500 cm due to aromatic C–H stretching vibrations
whereas the band at 2925 cm exhibited asymmetric Csp3
ꢀ1
ꢀ
H
stretching vibrations for methyl group respectively. The bands
ꢀ1
in the region of 1600–1430 cm
were consistent with the
skeletal vibration of the aromatic system. The detailed data for
IR and NMR spectra are given in Experimental section. The IR
spectra of the complexes are given in ESI Fig. S9 and S10.†
Scheme 3 Jablonski diagram for fluorescence with solvent relaxation.
underwent deshielding eld at 8.74 ppm in 1 and 9.07 ppm in
Geometry optimization, structure analysis and different
bridging mode of complex 1
2
, whereas in ligand it appeared at 8.16 ppm. Both these events
The complexes are diamagnetic at room temperature indicating
6
4
reveal the indication of complex formation. The methyl proton
also shied in upeld region during complexsation near about
2
their singlet ground state t2g
all the complexes was performed in solution phases in their
ground singlet (S ) spin state. The optimized structures of the
g
e . The geometry optimization for
1
ppm. The H NMR spectra of 1 and 2 are depicted in ESI
0
Fig. S7 and S8.†
complexes 1 and 2 at singlet ground state are shown in ESI
Fig. S11.† Calculated structures are in excellent agreement with
experimental data for the complexes 1 and 2, for which X-ray
data are available. The signicant bond distances and angles
of the optimized geometry of complex 1 in their singlet ground
Complex 1 showed two methyl spectra, one at 2.39 ppm
and the other at 2.02 ppm, with the last one showing
comparatively lower eld region. However, complex 2 depic-
ted only one sharp singlet methyl proton at 2.40 ppm. This
observation clearly says that two types of methyl proton exist
in complex 1 whereas complex 2 occupied only one type of
methyl proton. The peak at 2.40 is attributed for the methyl
group present at ligand system and the unique peak at 2.02
ppm in 1 for the methyl proton present at acetate bridge
system. This is clear evidence that the di-nuclear and tri-
nuclear form of the complex 1 and 2 are present at solid
state as well as in solution state also. The correlation between
S
0
state compared with their crystal structure parameter is given
in ESI Table S1.† In this complex, the calculated Zn–N and Zn–O
˚
bond distances occur near 2.10 and 2.00 A, respectively which
are in very good agreement with the experimental values and
the slight discrepancy comes from the crystal lattice distortion
existing in real molecules.
The partial frontier molecular orbital compositions and
0
energy levels of 1 in singlet ground state (S ) are listed in Table 4.
1
the experimental and calculated H NMR chemical shi of 1
is shown in Fig. 3 as a representative case.
The partial molecular orbital diagram with some isodensity
frontier molecular orbital which are mainly involved in the
ꢀ
5
Fig. 5 (a) The absorbance spectra of 1 (ꢃ1 ꢂ 10 M) in different solvents at room temperature. (b) A representative diagram of emission intensity
vs. solvent polarity of the complex 1.
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Table 7 Photophysical parameters of complex 1 in different solvent at room temperature
2
7
8
Solvent
labs (nm)
lem (nm)
Quantum yield (F
f
)
s
1
(ns)
s
2
(ns)
c
k
r
ꢂ 10
k
nr ꢂ 10
CCl
DCM
CH CN
DMF
4
449
453
458
463
577
582
592
597
0.28
0.21
0.10
0.05
0.17
0.41
1.25
1.20
3.96
4.41
5.01
5.15
1.10
1.22
1.40
1.04
7.1
4.8
2.0
0.9
1.81
1.79
1.79
1.84
3
electronic transitions for mononuclear complexes 1 and 2 are (Fig. 4). In 1b both the ligand moieties surrounding metal
shown in ESI Fig. S12.† The energy difference between HOMO atoms prefer SP geometry, while in case of 1a it prefers TBP
41
(
H) and LUMO (L) occurs in the 2.63 and 2.56 eV for complex 1 shapes. Hence two metal atoms maintain nearly symmetrical
and 2 respectively. The L and L+1 are almost degenerate (energy environments towards ligand moiety when the bridging acetate
difference ꢃ0.24 eV in 1, ꢃ0.06 eV in 2). But L+2 is destabilized groups used their oxygen atoms to coordinate with the metal in
by an amount ꢃ0.6 eV compared to L+1. In dinuclear complexes, same fashion. On the other hand in 1 the geometry surrounding
the electron density in H and Hꢀ1 mainly reside on aldehyde the metal centre is non symmetric as the coordination modes of
moiety (83%) while in case of Hꢀ3 and Hꢀ4 the little contribu- bridging atoms are dissymmetric in nature (Table 5).
tion of electron density arises from acetate bridging group and
metal d orbital and the major contribution comes from aldehyde
moiety (ꢃ60%). L, L+1, L+2 and L+3 of the polynuclear complexes
Photophysical study of HL in different solvent
originates from ligand p* orbital localized on quinoline moiety
and aromatic system attached to C]N bond contribution.
Here in we described that the nature of geometry of the
complex is inuenced by the mode of coordination of oxygen
atom in bridging acetate group. We compared optimized
geometry between two hypothetical dinuclear complexes
The UV-Vis spectrum of the sensor HL was recorded in presence
of various types of solvents at room temperature, which dis-
played a well resolved peak in the range 325 to 350 nm
(Fig. S13a†). The uorophore emits a weak emission band in the
region of 420 to 480 nm when excited at 330 nm in presence of
different polarity solvent (Fig. S13b†). Both the absorption
maxima (labs) and emission maxima (lem) of this particular
1
,2
1,2
1,1
1,1
[Zn
2 2 2 2
(L) (m -OAc)(m -OAc)], 1a and [Zn (L) (m -OAc)(m -OAc)],
1
,2
1,1
1
b with the crystalline form of [Zn (L) (m -OAc)(m -OAc)], 1. It
2 2
was observed that the naturally selected crystalline complex 1 is

uorophore shied towards longer wavelength as the polarity of
more stabilized than the two hypothetical proposed forms
the solvent increases (Table 6).
Table 8 Main calculated optical transition for complex 1 with composition in terms of molecular orbital contribution of the transition, vertical
excitation energies and oscillator strength in acetonitrile
Oscillator
Electronic transitions
Composition
Excitation energy
strength (f)
CI
0.17021
0.74314
0.11988
Assign
l
exp (nm)
1
S
0
/ S
/ S
/ S
1
2
3
H ꢀ 1 / L
H / L
H / L + 1
2.6396 eV (463.70 nm)
0.3965
ILCT
ILCT
ILCT
ILCT
ILCT
ILCT
ILCT
ILCT
466
1
1
1
S
0
H ꢀ 1 / L
2.6741 eV (458.65 nm)
0.0665
0.16688
1
H / L
ꢀ0.68227
1
H / L + 1
0.19988
1
S
S
0
0
H ꢀ 1 / L
2.6852 eV (457.72 nm)
3.5506 eV (349.19 nm)
0.0372
0.2545
0.58303
1
H / L + 1
ꢀ0.31162
ꢀ0.15908
ꢀ0.23882
0.48128
ꢀ0.33256
ꢀ0.13673
ꢀ0.13467
0.31146
1
1
/ S11
H ꢀ 5 / L
H ꢀ 4 / L
H ꢀ 3 / L
H ꢀ 2 / L
H ꢀ 2 / L + 1
H ꢀ 5 / L + 1
H ꢀ 4 / L + 1
H ꢀ 2 / L
H ꢀ 2 / L + 1
H ꢀ 5 / L
H ꢀ 4 / L + 1
H ꢀ 3 / L
H ꢀ 2 / L
H ꢀ 2 / L + 1
MLCT/ ILCT
1
1
1
1
1
1
1
1
1
1
1
1
1
MLCT/ ILCT
MLCT/ ILCT
ILCT
ILCT
MLCT/ ILCT
MLCT/ ILCT
ILCT
ILCT
MLCT/ ILCT
MLCT/ ILCT
1
345
1
S
S
0
0
/ S12
/ S13
3.5736 eV (346.94 nm)
3.5909 eV (345.28 nm)
0.2092
0.2790
1
0.44082
ꢀ0.35880
ꢀ0.18707
ꢀ0.20622
0.28261
0.41547
0.46091
1
1
1
MLCT/ ILCT
ILCT
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the S
1
state which leads bathochromic shi towards more polar
solvent. This typical uorophore contains the more polar enol
6
form in S state rather than the keto tautomeric form. As a
1
result a larger dipole moment in the excited state (m ¼ 5.2085
E
D) was observed than in the ground S
0
state (m
G
¼ 4.9389 D).
Following excitation the solvent dipoles can reorient or relax
around the excited state, which lowers the energy of the S state
Scheme 3).
More structured and intense emission spectra were
1
(
observed for the uorophore in the least polar solvent like
hexane (Fig. S13b†). As the polarity of the solvent increase the
intensity of outcome uorescence spectra sharply diminishes
r
and broadens (Fig. S13b†). The radiative rate constant (k ) and
non radiative rate constant (knr) value were also determined in
presence of different polarity solvent (Table 6). It was
observed that with increase in dielectric constant decrease in
r
k values are more than knr (Table 6). The uorescence life
time decay of the ligands was measured in different polarity
solvent at 280 nm and the nature of the decay was tted with
mono exponential decay curve (Fig. 10a). The life time (s) of
excited state was more under high polar solvent. The mono
exponential emission nature of the uorophore revealed that
the excited state contains only one tautomeric form (enol) in
Fig. 6 Natural transition orbitals (NTOs) for the complexes 1 illus-
trating the nature of optically active singlet excited states in the
absorption bands 460 and 345 nm in acetonitrile as a solvent.
Table
9
Main calculated optical transition for complex 1 with excess.
composition in terms of molecular orbital contribution of the transi-
tion, vertical excitation energies and oscillator strength in solvents of
different polarity
Photophysical studies of complexes in different solvent
The absorption spectra of the complexes 1, 2 and 3 were
recorded in different polarity solvent at room temperature. The
representative UV-Vis spectra of complex 1 was presented in
Fig. 5a, while for the complexes 2 and 3 they were depicted in
ESI Fig. S14a and b.† A new band appeared longer wavelength at
Solvent
CCl
4
DCM
CH
3
CN
DMF
1
1
1
1
Transition (S
Oscillator strength (f)
0
/ S
1
)
ILCT
0.4356
446.01
447
ILCT
ILCT
0.3965
463.75
466
ILCT
0.2879
456.91
454
0.5013
480.61
477
l
theo (nm)
exp (nm)
l
450 nm in addition to the ligand characteristic peak around 345
Excitation energy (eV)
2.7173
2.6648
2.6390
2.5841
nm. This lower energy characteristic band indicated the
complex formation. The absorbance maxima underwent bath-
ochromic shi with increasing dielectric constant of the
solvent. The Photophysical data of the complex 1 is given in
Table 7, and for complexes 2 and 3 in ESI Tables S2 and S3.†
This indicates that the excited state S
1
of the uorophore is
state. The excess vibrational energy
is rapidly lost by the polar solvent molecules and stabilized by
42
more polar than ground S
0
Fig. 7 (a) The black, red and green lines are represented complex 1, 2 and 3 respectively; from top to bottom the lines are represented the
ꢀ1
absorption maxima, fluorescence maxima and stokes shifts ( ꢀn abs max, ꢀn fl max and D ꢀn Stokes shifts respectively, in cm ) of 1, 2 and 3 against the solvent
polarity function, Df. (b) A bar diagram of fluorescence intensity vs. different medium of the respective complexes was plotted.
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Table 10 The C–H/p interactions parameters for 1, 2 and 3
H/C(g) ( A˚ )
Y–H/C(g) ( )
ꢁ
Y/C(g) ( A˚ )
Symmetry
Y–H(I)/Cg(J)
Complex 1
N(6)–H(6B)/Cg(1)
C(14)–H(14)/Cg(3)
C(43)–H(43)/Cg(4)
2.71
2.96
2.98
159
174
156
3.525(6)
3.888(4)
3.850(9)
ꢀ1 + x, 1 + y, z
x, ꢀ1 + y, z
x, y, z
Complex 2
C(27)–H(27A)/Cg(1)
C(31)–H(31)/Cg(2)
2.86
2.54
162
170
3.790(6)
3.456(5)
ꢀx, ꢀy, ꢀz
1/2 ꢀ x, 1/2 ꢀ y, 1 ꢀ z
Complex 3
C(7)–H(6)/Cg(8)
C(19)–H(19)/Cg(1)
2.69
2.90
135
105
3.389(4)
3.284
ꢀx, 1 ꢀ y, 1 ꢀ z
x, y, z
To get better insight on experimental absorption values
TDDFT calculations were done for complex 1 on the basis of the
optimized geometry. The calculated absorption energies asso-
ciated with their oscillator strengths, the main congurations
and their assignments of 1 is given in Table 8. In order to
analyze the nature of absorption, we performed an NTO analysis
44
based on the calculated transition density matrices. Here we
referred to the unoccupied and occupied NTOs as “electron”
and “hole” transition orbitals, respectively (Fig. 6). Based on our
TDDFT NTOs analysis the bands in the region 300–470 nm for
this complex can be characterized as an ILCT states. A very little
amount of orbital contribution (less than 5%) of metal d-orbital
in higher energy transition (<S10) was observed. As illustrated in
Fig. 6, optical excitations occur from the occupied (hole) tran-
sition orbitals to the unoccupied (electron) transition orbitals.
Hole NTOs contributing to the bands were localized on the p
orbital of ligand while the electron NTOs were mainly delo-
calized over the p* orbital of the ligand moiety. The bath-
ochromic shi of the complex 1 in UV-Vis spectrum with
increasing solvent polarity was veried by theoretical calcula-
tion. We calculated TDDFT and Gaussum2.0 analysis on the
basis of the optimized geometry of complex 1 using different
polarity solvents. In every case it was observed that the lowest
lying distinguishable singlet / singlet absorption band at
Fig. 8 Packing diagram of both the complexes 1, 2 and 3 with C–H/p
and p–p stacking interactions.
a
Table 11 The p–p stacking interaction parameters for 1, 2 and 3
Cg–Cg ( A˚ )
CgI_Perp ( A˚ )
CgJ_Perp ( A˚ )
ꢀ3.4440(13)
a ( )
ꢁ
Symmetry
Rings I–J
Complex 1
Cg(1)/Cg(6)
3.6817(18)
ꢀ3.4298(13)
3.25(15)
2 ꢀ x, 1 ꢀ y, 1 ꢀ z
Complex 2
Cg(5)/Cg(5)
Cg(1)/Cg(1)
3.764(2)
3.851(3)
ꢀ3.3797(17)
ꢀ3.6487(18)
ꢀ3.3797(17)
ꢀ3.6485(18)
0
14
1/2 ꢀ x, 1/2 ꢀ y, 1 ꢀ z
1/2 ꢀ x, 1/2 ꢀ y, 1 ꢀ z
Complex 3
Cg(5)/Cg(5)
Cg(7)/Cg(5)
3.595(2)
3.624(2)
3.3967(15)
3.3989(14)
3.3967(15)
3.3961(14)
0
ꢀx, ꢀy, 1 ꢀ z
ꢀx, ꢀy, 1 ꢀ z
2.29(17)
a
Cg(1) ¼ C11/C16, Cg(2) ¼ N1/C9, Cg(3) ¼ N5/C49, Cg(4) ¼ C29/C34, Cg(5) ¼ N4/C36, Cg(6) ¼ C4/C9. Cg(n) ¼ ring number; a ¼ dihedral angle
between planes I and J; Cg–Cg ¼ distance between ring centroids; Cg(I)_Perp ¼ perpendicular distance of Cg(I) on ring J; Cg(J)_Perp ¼
perpendicular distance of Cg(J) on ring I.
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ꢀ1
(
ꢀn em max) band maxima (in cm ) for 1, 2 and 3 in different
solvents were plotted vs. Df. The higher slope for the ꢀn em max vs.
Df plot as compared to that for the ꢀn abs max vs. Df plot (Fig. 7a)
suggested that the excited state of the complexes were more
45
polar than the ground state. To substantiate these results, we
also tried to correlate the Stokes' shis (i.e., D ꢀn ¼ ꢀn abs ꢀ ꢀn em
)
with the Df values of the solvents.
The solid state emission was performed for all the complexes.
All of them showed a bathochromic shi during excitation at 450
nm. The emission maxima are 550 nm, 554 nm and 572 nm for
complex 1, 2 and 3 respectively, (in ESI Fig. S16†). In the crystal
packing, several noncovalent interactions such as hydrogen
bonding (ESI Tables S4–S6†), C–H/p, p/p were found. The
Fig. 9 An extended 2D network perspective view of complexes.
9 8 2
C–H/p interactions in 1$C H N were generated by interaction of
4
50 nm could be attributed to p (L) / p* (L) transitions with
ILCT character. It was further observed that the energy gap
between HOMO and LUMO in S / S transition gradually
H6B, H14 and H43 with centroid of the rings Cg(1), Cg(3) and
Cg(4) whereas in 2 they are generated by interaction of H27A and
H31 with centroid of the rings Cg(1) and Cg(2) and in 3 by
interaction of H6 and H19 with centroid of the rings Cg(8) and
Cg(1) respectively (Table 10). The p–p stacking interaction was
found to be involved in aromatic rings with centroid-to-centroid
0
1
decreased with increasing solvent polarity (Table 9).
Emission spectral properties
˚
distance minimum for mono nuclear complex (3.595(2) A, Table
The emission spectral behaviors of the complexes were studied
at room temperature in solvent of different polarity. As shown in
Fig. 5b it was observed that the emission intensity gradually
decreased with increasing solvent polarity for 1, while for
complexes 2 and 3 they are given in ESI Fig. S15a and b.† We
measured the absorption (labs max) and uorescence (lem max
band maxima, quantum yield (F ) and lifetime (s) in different
solvents along with the solvent polarity function, Df (eqn (1))
11 and Fig. 8). The face angle (a) of zero with minimum distance
between two aromatic ring clouds led to larger bathochromic
shi rather than for di- and tri- nuclear system. The emission
intensity increased gradually from mononuclear to trinuclear
(Fig. 7b). A strong p–p was involved between two quinoline
)
moieties (Fig. 8) in complex 2 led to more red shied emission
band besides incorporation of non coordinating ammine group
which quenched the complex 1 through the PET mechanism
f
45
21
2
2
taking place in the lone pair of nitrogen atom of free amine.
Df ¼ (3 ꢀ 1/23 + 1) ꢀ (h ꢀ 1/2h + 1)
(1)
Besides the smaller inter nuclear distance between two Zn atom
˚
where 3 and h are the static dielectric constant and refractive (3.074 A), complex 2 underwent spin orbital coupling. On the
index of the solvent, respectively. With increase in solvent other hand, similar type interaction did not take place in
˚
polarity, bathochromic shis in absorbance and emission complex 1 as zinc atoms were far enough (3.599 A) by acetate
spectra were observed (Tables 7, S3 and S4†), which revealed bridge than their van der Waal's radius of isolated zinc atom
that the more polar excited state was stabilized more effectively (2.25 A˚ ). Such type of spin orbital interaction somehow
43
42
in solvents of higher polarity. A reasonably good linear ts enhanced the uorescence intensity in solid state. An extended
were obtained when the absorption ( ꢀn abs max) or emission 2D network of relevant complexes is given in Fig. 9.
Fig. 10 Changes in the time-resolved photoluminescence decay of HL (a) and 1 (b) in different polarity solvent at room temperature.
4230 | RSC Adv., 2015, 5, 4219–4232
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Time resolved luminescence spectra proved to be an
important tool to understand the decay process and the emis-
sive nature of the complexes in different polarity solvent. All the
complexes display a bi-exponential decay nature irrespective of
solvent polarity and the decay plot for the complex 1 is shown in
Fig. 10b, whereas complexes 2 and 3 are given in ESI Fig. 17a
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1
and b.† The value of s life time of the complexes was of similar
order with the lifetime with ligand which revealed that in
excited state the biexponential decay nature of the complexes
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complex itself. The smaller knr value (nearly ten times) for the
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Conclusion
1
1
1
1
1
So as a whole we have synthesized three zinc complexes con-
taining Schiff base with different stoichiometric ratio and its
photo physics has been characterized by absorption, emission,
time-resolved emission spectroscopic techniques as well as by
theoretical study. The highest emission intensity was observed
in trinuclear complex. In summary we conclude that the
incorporation of more zinc atom during complexation leads to
stronger uorescence intensity with gradually bathochromic
shi in solution of different polarity. The solvent polarity
function plot shows that excited state of the complexes are more
polar than the ground state. Moreover, it shows a regular
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2
increasing trade of life time (s ) as the polarity of investigated
solvent is increased. The quantum yield as well as radiative rate
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2
constant (k ) value is signicantly monitored by solvent polarity
r
while the non radiative rate constant values (k ) remain almost
nr
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Acknowledgements
Amar Hens acknowledge UGC, New Delhi for the research
fellowship. We are also thankful to Department of Science and
Technology (DST), New Delhi, India for the data collection on
the CCD facility setup (Jadavpur University) under DST-FIST
program. We also acknowledge CAS, Department of Chem-
istry, Jadavpur University and DST-PURSE program for other
facilities. The author acknowledges Ashok Sasmal of Jadavpur
University, for help in weak interaction calculation.
2
22 G.-X. Liu, Y.-Y. Xu, Y. Wang, S. Nishihara and X.-M. Ren,
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