6-Benzhydryl-4-methyl-2-(1H-benzoimidazol-2-yl)phenol ligands and their zinc complexes: Syntheses, characterization and photoluminescence behavior

Article abstract of DOI:10.1016/j.ica.2012.03.057

The series of 6-benzhydryl-4-methyl-2-(1H-benzoimidazol-2-yl)phenol derivatives (HL) and their zinc complexes (ZnL₂) were synthesized and fully characterized. Single-crystal X-ray diffraction studies were conducted on representative complexes (C1 and C4), which revealed a distorted tetrahedral geometry at zinc. By comparison with these organic compounds (HL), the fluorescent quantum yields of the corresponding zinc complexes were increased. The fluorescence intensities of zinc complexes were affected by the solvents used, and enhanced intensities in methanol were observed compared with those observed in other solvents such as dichloromethane and toluene. The fluorescence decay mainly followed a single exponential in toluene, whereas a double exponential decay was observed in the presence of two active species in methanol and dichloromethane.

Full text of DOI:10.1016/j.ica.2012.03.057

Inorganica Chimica Acta 392 (2012) 345–353
Contents lists available at SciVerse ScienceDirect
Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
6-Benzhydryl-4-methyl-2-(1H-benzoimidazol-2-yl)phenol ligands and their zinc
complexes: Syntheses, characterization and photoluminescence behavior
d
b,
Zihong Zhou a,d, Wen Li a, Xiang Hao b, Carl Redshaw c, , Langqiu Chen , Wen-Hua Sun
⇑
⇑
a Key Laboratory of Photochemical Conversion and Optoelectronic Materials, Technical Institute of Physics and Chemistry, The Chinese Academy of Sciences, Beijing 100190, China
b Key Laboratory of Engineering Plastics and Beijing National Laboratory for Molecular Science, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China
c School of Chemistry, University of East Anglia, Norwich NR4 7TJ, UK
d Key Laboratory of Environmentally Friendly Chemistry and Application of Ministry of Education, College of Chemistry, Xiangtan University, Xiangtan 411105, China
a r t i c l e i n f o
a b s t r a c t
Article history:
The series of 6-benzhydryl-4-methyl-2-(1H-benzoimidazol-2-yl)phenol derivatives (HL) and their zinc
complexes (ZnL2) were synthesized and fully characterized. Single-crystal X-ray diffraction studies were
conducted on representative complexes (C1 and C4), which revealed a distorted tetrahedral geometry at
zinc. By comparison with these organic compounds (HL), the fluorescent quantum yields of the corre-
sponding zinc complexes were increased. The fluorescence intensities of zinc complexes were affected
by the solvents used, and enhanced intensities in methanol were observed compared with those observed
in other solvents such as dichloromethane and toluene. The fluorescence decay mainly followed a single
exponential in toluene, whereas a double exponential decay was observed in the presence of two active
species in methanol and dichloromethane.
Received 18 January 2012
Received in revised form 26 March 2012
Accepted 30 March 2012
Available online 6 April 2012
Keywords:
6-Benzhydryl-4-methyl-2-(1H-
benzoimidazol-2-yl)phenol
Zinc complexes
Ó 2012 Elsevier B.V. All rights reserved.
Fluorescence
UV-absorption spectra
ESIPT
1. Introduction
2. Results and discussion
The photophysical properties of various benzimidazole deriva-
tives have been intensively studied in recent years, and their po-
tential application in molecular electronics and biological
fluorescence probing has been recognized [1–6]. In addition, a
number of imidazole derivatives have been considered as inhibi-
tors of copper corrosion [7–9]. With the potential for use as fluo-
rescence probes in mind, the influence of the surrounding
environment (the solvent) on the fluorescent properties has been
considered [10–12]. In general, both pH values and the various
substituents present impacted on the fluorescent properties of
benzimidazole derivatives [13–17]. In view of this, herein new
benzimidazole ligands containing a phenoxy group in combination
with a bulky benzhydryl moiety have been prepared, and their zinc
complexes were synthesized and characterized. Furthermore, the
photophysical properties of both the organic and zinc compounds
were investigated, which revealed that the intensities and size of
the observed shifts relied on the coordination environment and
the solvent.
2.1. Synthesis and characterization
A
series of 6-benzhydryl-4-methyl-2-(1H-benzoimidazol-2-
yl)phenol derivatives was synthesized by the condensation reaction
of 6-benzhydryl-4-methylsalicylaldehyde with the corresponding
2-aminoaniline or N-alkyl-2-nitroaniline (Scheme 1), and these li-
gands were fully characterized by FT-IR and NMR spectroscopy as
well as by elemental analysis. Further reaction with 0.5 equivalents
of zinc acetate in tetrahydrofuran (THF) afforded the zinc com-
plexes bis(6-benzhydryl-4-methyl-2-(1H-benzoimidazol-2-yl)phe-
nolate) in good yields (C1–C6, Scheme 1). The zinc complexes were
characterized by FT-IR spectroscopy, elemental analysis and by sin-
gle-crystal X-ray diffraction for the representative complexes C1
and C4.
2.2. X-ray crystallographic study
Single crystals of complexes C1 and C4 suitable for X-ray struc-
tural determinations were grown by the slow diffusion of n-hep-
tane into THF solutions. Both molecular structures (Figs. 1 and 2)
revealed a distorted tetrahedral geometry at zinc, comprising of
two monoanionic bidentate ligands; selected bond lengths and
angles are tabulated in Table 1.
⇑
Corresponding authors. Tel.: +86 10 62557955; fax: +86 10 62618239.
E-mail addresses: Carl.Redshaw@uea.ac.uk (C. Redshaw), whsun@iccas.ac.cn
(W.-H. Sun).
0020-1693/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.ica.2012.03.057

346
Z. Zhou et al. / Inorganica Chimica Acta 392 (2012) 345–353
NO2
NH
Ph
Ph
R1
Ph
OH
R2
Ph
N
N
R1
R2
DMF/H2O, 50°C
OH
CHO
L1-L6
Zn(OAc)2
THF
R2
N
R1
Ph
R1 R2
No.
N
Ph
1
2
3
4
5
6
H
H
O
Zn
N
O
H Me
Ph
Ph
H
H
Et
i-Pr
Bz
H
R1
N
R2
H
Cl
C1-C6
Scheme 1. Synthesis of organic compounds and their zinc complexes.
Fig. 2. ORTEP plot of C4 with thermal ellipsoids drawn at the 30% probability level.
Hydrogen atoms linked on carbon atoms have been omitted for clarity.
In the solid state of complex C1, there are two independent
molecules, which interact via an intermolecular hydrogen bond be-
Table 1
tween the benzimidazole N–H group and the phenol oxygen atom
0
0
Selected bond lengths (AÅ) and angles (°) for complexes C1 and C4.
(N(4)ꢀ ꢀ ꢀO(1a) = 2.842 ÅA). There are slight differences for the bond
lengths and angles at zinc for the two ligands: for one ligand,
C1
C4
0
0
Zn(1)–O(1) (1.918(3) ÅA), Zn(1)–N(1) (1.982(4) ÅA), and O(1)–
0
Bond lengths (ÅA)
Zn(1)–N(1) (94.52(15)°), whilst for the other ligand, Zn(1)–O(1a)
0
0
Zn(1)–O(1)
Zn(1)–O(1a)
Zn(1)–N(1)
Zn(1)–N(1a)
N(1)–C(7)
N(1a)–C(7a)
N(1)–C(9)
N(1a)–C(9a)
O(1)–C(1)
O(1a)–C(1a)
N(2)–C(7)
N(2a)–C(7a)
C(6)–C(7)
1.918(3)
1.939(3)
1.982(4)
1.986(4)
1.319(6)
1.327(6)
1.383(6)
1.386(6)
1.326(6)
1.329(5)
1.358(6)
1.355(6)
1.476(7)
1.466(7)
1.924(3)
1.924(3)
1.990(4)
1.990(4)
1.341(6)
1.341(6)
1.387(6)
1.387(6)
1.321(5)
1.321(5)
1.377(6)
1.377(6)
1.470(6)
1.470(6)
(1.939(3) ÅA), Zn(1)–N(1a) (1.986(4) ÅA), and O(1a)–Zn(1)–N(1a)
(94.41(16)°) (Table 1). In the molecular structure, the atoms O(1),
Zn(1), and N(1) are almost coplanar with the phenolate and imid-
azolyl planes with individual dihedral angles of 166.6° and 169.3°,
respectively, whereas the dihedral angles of the plane of atoms
O(1a), Zn(1), and N(1a) with those of the other ligand are 149.5°
and 162.3°, respectively. The planes formed by atoms O(1), Zn(1),
and N(1) and O(1a), Zn(1), and N(1a) are almost vertical to each
other with a dihedral angle of 88.1° (Fig. 1).
Similar to complex C1, the molecular structure of complex C4
(Fig. 2) revealed a distorted tetrahedral geometry at zinc, how-
ever the two ligands exhibited equivalent bonding at zinc. The
dihedral angles between the plane of atoms O(1), Zn(1), and
N(1) with the planes of the imidazolyl and phenolate in complex
C4 are 150.3° and 156.6°, respectively, which are slightly smaller
those observed in the complex C1. Similar coordination features
around zinc were observed in the analogous complexes bearing
C(6a)–C(7a)
Bond angles (°)
O(1)–Zn(1)–O(1a)
O(1)–Zn(1)–N(1)
O(1a)–Zn(1)–N(1)
O(1)–Zn(1)–N(1a)
O(1a)–Zn(1)–N(1a)
N(1)–Zn(1)–N(1a)
117.32(14)
94.52(15)
114.72(15)
117.86(16)
94.41(16)
119.87(17)
121.4(2)
93.32(14)
119.36(15)
119.36(15)
93.32(14)
111.8(2)
2-(1H-benzoimidazol-2-yl)phenolate, which though they lacked
the bulky 6-benzhydryl group [18], did possess stacking
interactions. The bulky substituents present for the ligands here-
p–p
in, prevented the formation of
p–p stacking interactions for C1
and C4.
2.3. Absorption spectra
The UV–Vis spectra of the 6-benzhydryl-4-methyl-2-(1H-ben-
zoimidazol-2-yl)phenol ligands (HL, L1–L6) and their zinc com-
plexes (ZnL2) were each measured in anhydrous methanol,
dichloromethane and toluene solutions; the spectral characteris-
tics are tabulated in Table 2. The absorptions of the compounds
were significantly affected by the solvent employed. For example,
all organic compounds (L1–L6) exhibited absorption peaks at
around 225 nm in methanol, 230 nm in dichloromethane, and
330 nm in toluene solutions, indicating that their absorptions were
blue-shifted on increasing the polarity of the solvent. These could
Fig. 1. ORTEP drawing of complex C1 with thermal ellipsoids drawn at the 30%
probability level. Hydrogen atoms are omitted for clarity (two independent
molecules are included, only one structure is listed).
be assigned to
p–
p⁄ transitions on the basis of the extinction

Z. Zhou et al. / Inorganica Chimica Acta 392 (2012) 345–353
347
Table 2
UV–Vis absorption properties of L1–L6 and C1–C6.
Media
Ligands
kabs–max (nm)
e(kmax) (Mꢁ1 cmꢁ1
)
Complexes
kabs–max (nm)
e(kmax) (Mꢁ1 cmꢁ1
)
Methanol
CH2Cl2
Toluene
Solid
L1
224
231
332
415
33980
22888
17400
–
C1
224
234
301
393
32454
40380
25080
–
Methanol
CH2Cl2
Toluene
Solid
L2
L3
L4
L5
L6
223
231
332
408
30646
26224
16106
–
C2
C3
C4
C5
C6
229
234
299
391
37048
39946
34416
–
Methanol
CH2Cl2
Toluene
Solid
222
231
333
398
32396
21382
16782
–
221
233
298
393
26590
38494
18826
–
Methanol
CH2Cl2
Toluene
Solid
221
232
327
384
33408
29522
15488
–
221
235
297
377
8186
42080
29258
–
Methanol
CH2Cl2
Toluene
Solid
220
233
333
418
31714
33958
19292
–
234
234
300
389
39480
38574
22636
–
Methanol
CH2Cl2
Toluene
Solid
225
231
336
407
32010
24480
14526
–
223
236
306
384
10258
44282
21552
–
coefficients (e(kmax), Table 2). The dipolar interactions and the
protonation at the basic sp2-N centers led to red-shifts; whereas
the protonation of the hydroxyl group of the phenol derivatives
resulted in blue-shifts. Such phenomena were consistent with
observations reported in the literature [19–21].
By comparison to the organic compounds, the absorption bands
of all the zinc complexes were strongly enhanced and shifted with
the same trends according to the polarity of the solvent used. As
shown in Fig. 3, the UV-absorption spectra (A) of all compounds
in toluene were shifted to the red in comparison to those of the
corresponding compounds in dichloromethane (B); the red-shifts
were about 100 nm for the organic compounds (L1–L6), and
70 nm for the zinc complexes (C1–C6).
Δ
Ep
Δ
En
polar solvent
nonpolar solvent
The solvent polarity affected the energy gap and the electronic
transitions due to the excited-state species possessing a large di-
pole moment and facile electronic reorganization. The energy gap
Scheme 2. The effect of solvent on the electronic transition energies.
(D
E) between the ground state and the excited state was gradually
enlarged on increasing the polarity of the solvent used ( Ep > En)
(Scheme 2) [22,23]. In the solid thin films, all the zinc complexes
D
D
(C1–C6) showed significantly blue-shifted absorptions compared
to those of the corresponding ligands (L1–L6) [24–26].
Fig. 3. UV-absorption spectra of complexes ZnL2 and corresponding ligands HL in solution (5 ꢂ 10ꢁ5 M) of toluene (A) and dichloromethane (B).

348
Z. Zhou et al. / Inorganica Chimica Acta 392 (2012) 345–353
2.4. Fluorescence spectra and quantum yields
using dichloromethane or toluene as solvent (Fig. 4D and F), com-
plexes C1, C4, and C6 showed larger fluorescence quantum yield
Fluorescence spectra (Fig. 4) of all compounds were obtained in
different solvents, which were selected for their different polarities
and potential for hydrogen bonding, data are collected in Table 3.
On comparing the absorptions for the ligands versus the corre-
sponding zinc complexes, it was evident that coordination to zinc
of the monoanionic ligand sets resulted in significant blue-shifts.
(
UF) than their analogs C2, C3, and C5.
In methanol, the intensities of the large Stokes-shifted fluores-
cence for the ligands were reduced because of the intermolecular
hydrogen bonding interactions inhibiting the ESIPT process in
HL; exceptions were L1 and L6 (Fig. 5, Stokes shifts 153 and
159 nm, respectively, Table 3). The influence of methanol on L1
and L6, both of which possess an –NH group at the benzoimidazole,
was different due to the differing protonation/hydroxyl interac-
tions of the methanol with these ligands (Scheme 3).
The emissions of L2, L3, L4 and L5 for the Stokes shift showed
bands at 146, 152, 157 and 150 nm belonging to ESIPT in MeOH.
There were lower Stokes shift bands alongside the ESIPT bands,
The blue-shifted absorptions were the result of increased
p-elec-
tron donation through the coordination of the Zn(II) to the anionic
ligands, i.e. the electron was transferred to the vacant orbitals of
Zn(II). In addition, on increasing the polarity of solvent used, the
fluorescence emission and fluorescence excitation spectra of all
the ligands (L1–L6) were in general blue-shifted. Interestingly,
Fig. 4. Fluorescence spectra of ligands HL and zinc complexes ZnL2 in solution (5 ꢂ 10ꢁ5 M) of anhydrous methanol (A and B), dichloromethane (C and D) and toluene
(E and F).

Z. Zhou et al. / Inorganica Chimica Acta 392 (2012) 345–353
349
Table 3
Emission data for compounds L1–L6 and C1–C6.
Media
Ligands
Complexes
kmaxEm (nm)
kEx (nm)
D
ka (nm)
kmaxEm (nm)
kEx (nm)
D
ka (nm)
Methanol
CH2Cl2
Toluene
Solid
L1
L2
L3
L4
L5
L6
481
489
496
498
328
330
345
415
153
159
151
83
C1
C2
C3
C4
C5
C6
431
439
437
462
365
380
380
393
66
59
57
69
Methanol
CH2Cl2
Toluene
Solid
442, 365
500
504
296
330
335
408
146, 69
170
169
443
442
444
467
353
300
300
391
90
142
144
76
487
79
Methanol
CH2Cl2
Toluene
Solid
441, 361
503
507
289
330
335
398
152, 72
173
172
440
444
444
431
355
300
300
393
85
144
144
38
493
95
Methanol
CH2Cl2
Toluene
Solid
443, 363
509
518
286
330
330
384
157, 77
179
188
442
445
445
437
350
360
370
397
92
85
75
40
496
112
Methanol
CH2Cl2
Toluene
Solid
447, 370
506
510
297
330
335
418
150, 73
176
175
446
445
448
435
355
300
300
389
91
145
148
46
492
74
Methanol
CH2Cl2
Toluene
Solid
490
497
501
499
331
345
350
407
159
152
151
92
437
444
443
430
372
380
390
384
65
64
53
46
a
D
k = stokes shift.
Fig. 5. Normalized excitation and emission spectra of L1 (A) and L6 (B) in different solvents.
OCH3
which corresponded to significant primary photo-relaxation for L2,
L3, L4 and L5 in methanol (Table 3). Such unusual phenomena
were caused by the synergic influence of the excited enol-configu-
ration and the excited keto-configuration [27,28]. For all ligands
(L1–L6), the rapid excited-state intramolecular proton-transfer
(ESIPT) occurred through the cis-enol structure, with intramolecu-
lar hydrogen-bonding between the phenol and the Nimidazole atom
(Scheme 4). Compared with the ligands, less blue-shifts were
observed within the spectra of the corresponding zinc complexes,
consistent with literature observations [29–31]. In all cases, the
photo-excitation of the closed cis-enol form resulted in ESIPT to
form the excited state keto form, illustrating the large Stokes
shifted emission [29,32,18,33]. The emission spectra of all ligands
in the solid state were measured at room temperature (Table 3),
and showed photoluminescence peaks at about 500 nm; which
were generally similar to the values observed in solution.
H
R
N
N
R=H
solvent: CH3OH
Ph
N
H
N
O
H
Ph
HO
Ph
Ph
R=Me, Et, iPr, Bz
solvent: CH3OH
R
N
R
N
N
×
N
H
H O
Ph
H
HO
Ph
H3CO
Ph
OCH3
Ph
Scheme 3. The proposed protonation of methanol with the ligands.

350
Z. Zhou et al. / Inorganica Chimica Acta 392 (2012) 345–353
Sn (n>1)
Sn (n>1)
S1 (Enol-excited state)
ESIPT
S1
Keto-excited
state
Absorption
Fluorescence
R1
R1
Absorption
Stokes-shifted
Fluorescence
Ph OH N
Ph O HN
N
N
Ph
Ph
R2
R2
Keto-ground
state
S0
S
0 (Enol-ground state)
HL (L1-L6)
ZnL2 (C1-C6)
B
A
Scheme 4. Electronic transitions in the photo-luminescent process for the zinc complexes (A) and ligands (B).
Table 4
Excited singlet state lifetimes (ns) and the values of radiative decay rate (kr) of ligands (L1–L6) and zinc complexes (C1–C6).
Media
Ligands
Complexes
UF
s1 (ns)
kr1 (Sꢁ1
a
)
s2 (ns)
kr2 (Sꢁ1
a
)
UF
s1 (ns)
kr1 (Sꢁ1
a
)
s2 (ns)
kr2 (Sꢁ1
a
)
Methanol
CH2Cl2
Toluene
L1
L2
L3
L4
L5
L6
0.06
0.07
0.07
3.2
3.8
4.1
1.88
1.84
1.71
–
–
–
–
–
–
C1
C2
C3
C4
C5
C6
0.10
0.09
0.09
3.2
3.3
3.4
3.13
2.73
2.65
–
–
–
–
–
–
Methanol
CH2Cl2
Toluene
0.016
0.005
0.01
4.8(37.7)
4.3(5.2)
0.8
0.33
0.12
1.25
2.8(62.3)
0.6(94.8)
–
0.57
0.83
–
0.13
0.01
0.01
3.8
1.6(17.6)
4.1(71.5)
3.42
0.63
0.24
–
–
0.23
0.63
4.4(82.4)
1.6(28.5)
Methanol
CH2Cl2
Toluene
0.007
0.005
0.01
3.8(83.5)
0.6(83.5)
0.8
0.18
0.83
1.25
1.1(16.5)
2.7(16.5)
–
0.64
0.19
–
0.16
0.02
0.02
4.5(34.8)
4.3(74.0)
3.9(76.3)
3.56
0.47
0.51
2.8(65.2)
1.2(26.0)
1.3(23.7)
5.71
1.67
1.54
Methanol
CH2Cl2
Toluene
0.008
0.002
0.003
1.9(23.1)
3.2
2.6(10.2)
0.42
0.06
0.12
4.0(76.9)
–
0.5(89.8)
0.20
–
0.60
0.17
0.12
0.16
4.1(83.5)
4.2(82.8)
3.7
4.15
2.86
4.32
2.0(16.5)
1.6(17.2)
–
8.50
7.60
–
Methanol
CH2Cl2
Toluene
0.009
0.004
0.01
3.7
0.6
0.9
0.24
0.67
1.11
–
–
–
–
–
–
0.13
0.02
0.02
4.0
1.6(21.2)
1.2(20.7)
3.25
1.25
1.67
–
–
0.43
0.50
4.6(78.8)
4.0(79.3)
Methanol
CH2Cl2
Toluene
0.06
0.05
0.08
3.6
7.1(8.4)
3.7
1.67
0.70
2.16
–
–
1.67
–
0.19
0.08
0.08
1.9(11.4)
2.0(14.0)
3.5
10.0
4.00
2.29
3.8(88.6)
3.9(86.0)
–
6.00
2.05
–
3.0(91.6)
–
Concentration: 5 ꢂ 10ꢁ5 M.
a
kr = UF/s
(Sꢁ1 = 107 sꢁ1).
According to Table 3, all ligands (L1–L6) showed small Stokes shift
in the solid state in comparison to their respective solutions, indi-
cating that the ESIPT does not occur in the solid state.
consistent with previous observations [36,37]. Meanwhile, the
intermolecular hydrogen bonds (with methanol) resulted in in-
creased planarity of the atoms C(6), C(7), O(1), Zn(1), N(I), and C(1).
The fluorescence decay was observed by using excitation at 330
or 350 nm (laser), and the results are listed in Table 4. The fluores-
cence decay followed a single exponential in toluene for all com-
pounds, with the exception of compounds L4, C2, C3, and C5.
Using methanol or dichloromethane as solvent, double exponential
decay was mostly observed. The two different lifetimes were
caused by the presence of different excited state species due to
the conversion rate of the two species being slower than the emis-
sion rate, indicating that these species were not in equilibrium
[34]. In another word, there was one kind of species present in tol-
uene, but two kinds of species present in the other solvents used
herein [21,35].
The fluorescence intensities of the zinc complexes ZnL2 were
significantly enhanced on comparison to those of the correspond-
ing ligands (HL). Compared to L2–L5, compounds L1 and L6 having
a N–H group, exhibited stronger fluorescent intensity due to intra-
molecular hydrogen bonding. In methanol solution, the quantum
yields of the zinc complexes were increased because the bulky sub-
stituents present slowed down the rate of radiationless decay,
3. Conclusion
The series of 6-benzhydryl-4-methyl-2-(1H-benzoimidazol-2-
yl)phenol derivatives (HL) and their zinc complexes (ZnL2) were
synthesized and fully characterized. The molecular structures of
the complexes C1 and C4 exhibited distorted tetrahedral geometry
at zinc. The maximum UV-absorption bands of both the organic
(HL) were blue-shifted on increasing the polarity of the solvent
used. By comparison with the organic compounds (HL), the fluo-
rescent quantum yields of the corresponding zinc complexes were
increased. The fluorescence intensities of the zinc complexes were
heavily affected by the solvents used, with better intensities ob-
served in methanol than in other solvents such as dichloromethane
and toluene. The fluorescence decay mainly followed a single expo-
nential in toluene, whereas in methanol and dichloromethane, the
double exponential decay was observed indication the presence of
two active species.

Z. Zhou et al. / Inorganica Chimica Acta 392 (2012) 345–353
351
2.13 mmol) yield. mp 200–201 °C, IR (KBr; cmꢁ1):
m
3362, 3019,
4. Experimental
1601, 1583, 1519, 1469, 1397, 1375, 1247, 1078, 741, 621. 1H
NMR (CDCl3, 400 MHz, TMS):
d 7.67 (d, J = 7.46 Hz, 1H),
4.1. General consideration
7.38ꢁ7.17 (m, 14H), 6.79 (s, 1H), 6.09 (s, 1H), 4.02 (s, 3H), 2.28
(s, 3H). 13C NMR (CDCl3, 100 MHz, TMS): d 21.27, 33.27, 50.10,
109.60, 112.63, 118.84, 123.06, 123.28, 125.84, 126.27, 126.91,
128.37, 129.62, 132.86, 133.12, 135.86, 140.52, 143.77, 152.19,
154.62. Anal. Calc. for C28H24N2O: C, 83.14; H, 5.98; N, 6.93. Found:
C, 83.05; H, 6.04; N, 6.79%.
All manipulations of air and/or moisture-sensitive compounds
were carried out under an atmosphere of nitrogen using standard
Schlenk techniques. THF was refluxed over sodium-benzophenone
and distilled under nitrogen prior to use. 3-Benzhydryl-2-hydroxy-
5-methylbenzaldehyde was prepared according to the literature
method [38]. All aniline derivatives were purchased and used as
the obtained. 1H and 13C NMR spectra were recorded on a Bruker
DMX400 MHz instrument at ambient temperature using TMS as
an internal standard. Absorption spectra were determined on a
SHIMADZU UV-1601PC UV–Vis Spectrophotometer. IR spectra
were recorded on a Perkin–Elmer System 2000 FT-IR spectrometer
using a KBr disc in the range of 400–4000 cmꢁ1. Elemental analyses
were performed on a Flash EA 1112 microanalyzer. The steady-
state fluorescent spectra were measured on an F4500-FL fluores-
cence spectrophotometer; fluorescence lifetimes were obtained
using the time-correlated single-photon counting technique (Edin-
burgh Analytical Instruments F900 fluorescence spectrofluorime-
ter). Thin films of the samples were prepared on quartz slides
4.2.3. 2-Benzhydryl-6-(1-ethyl-benzoimidazol-2-yl)-4-methylphenol
(L3)
Using the above procedure as for the synthesis of L2, but using
N-ethyl-2-nitrobenzenamine (1.31 g, 7.92 mmol), 3-benzhydryl-2-
hydroxy-5-methylbenzaldehyde (2.39 g, 7.92 mmol), and sodium
hydrosulfite (7.18 g, 41.2 mmol) was added, respectively. L3 was
as a yellow powder in 30.2% (0.85 g, 2.03 mmol) yield. mp 155–
156 °C, IR (KBr; cmꢁ1):
m 3375, 3022, 1599, 1579, 1493, 1468,
1398, 1378, 1246, 1076, 741, 696. 1H NMR (CDCl3, 400 MHz,
TMS): d 7.66 (d, J = 7.38 Hz, 1H), 7.42 (d, J = 7.50 Hz, 1H), 7.32–
7.09 (m, 11H), 6.80 (s, 1H), 6.50 (s, 1H), 6.09 (s, 1H), 5.67 (s, 1H),
4.48-4.43 (q, 2H), 2.28 (s, 3H), 1.64 (t, J = 7.2 Hz, 3H). 13C NMR
(CDCl3, 100 MHz, TMS): d 15.34, 21.14, 21.35, 40.72, 50.15, 51.24,
109.74, 112.64, 118.95, 123.06, 123.27, 125.20, 126.26, 126.99,
128.37, 128.63, 129.48, 129.51, 129.63, 130.97, 132.99, 133.12,
135.00, 140.69, 142.89, 143.81, 151.60, 154.65. Anal. Calc. for
(1 cm) through spin-coating. Fluorescence quantum yields (UF
)
were calculated according to the comparative method, using
anthracene in methanol (UF = 0.29) as a standard [27,39–41].
R
2
IF;x
IF;s
ð
v
v
Þd
v
v
ꢀ ð1 ꢁ 10ꢁAsÞ ꢀ ðnxÞ
UF;x
¼
UF;s
R
2
ð
Þd
ꢀ ð1 ꢁ 10ꢁAxÞ ꢀ ðnsÞ
C29H26N2O: C, 83.22; H, 6.26; N, 6.69. Found: C, 83.15; H, 6.33;
N, 6.35%.
R
where UF,s is the quantum yield of standard, integrals IF,x
(v
)dv and
R
IF,s
(v
)dv are the areas under curves of the sample and standard,
) and IF,s
sample and the standard, respectively. Ax and As are absorptions
of the sample and standard, nx and ns are refractive indices of the
solvents.
IF,x
(v
(v
) are fluorescence intensities at wavelength for the
4.2.4. 2-Benzhydryl-6-(1-isopropyl-benzoimidazol-2-yl)-4-
methylphenol (L4)
Using the above procedure, but using N-isopropyl-2-nitroben-
zenamine (2.33 g, 12.9 mmol) was used instead of N-ethyl-2-
nitrobenzenamine, reaction with 3-benzhydryl-2-hydroxy-5-
methylbenzaldehyde (3.90 g, 12.9 mmol) and sodium hydrosulfite
(11.7 g, 67.2 mmol). The product was obtained as a yellow powder
in 13.6% (0.76 g, 1.76 mmol) yield. mp 158–159 °C, IR (KBr; cmꢁ1):
4.2. Preparation of ligands
4.2.1. 6-Benzhydryl-4-methyl-2-(benzoimidazol-2-yl)phenol (L1)
A modified synthetic procedure for the ortho-nitroaniline deriv-
atives (1.21 g, 11.2 mmol) was employed: DMF/H2O = 4:1
(80 mL:20 mL), sodium hydrosulfite (10.1 g, 58.3 mmol), 3-benz-
hydryl-2-hyd roxy-5-methylbenzaldehyde (3.38 g, 11.2 mmol)
were combined under nitrogen. The reaction mixture was refluxed
for 5 h. The solvent was then removed in vacuo, and the resultant
residue was dissolved with dichloromethane. The combined organ-
ic extracts were dried and purified on an alumina column using
petroleum ether/ethyl acetate (v/v = 25:1) as the eluent to obtain
m
3341, 3054, 2910, 1599, 1493, 1441, 1391, 1367, 1244, 1069, 740,
699. 1H NMR (CDCl3, 400 MHz, TMS): d 7.72 (d, J = 7.50 Hz, 1H),
7.66 (d, J = 7.52 Hz, 1H), 7.34–7.13 (m, 11H), 6.83 (s, 1H), 6.53 (s,
1H), 6.12 (s, 1H), 5.70 (s, 1H), 5.22 (m, 1H), 2.30 (s, 3H), 1.75 (d,
J = 6.88 Hz, 6H). 13C NMR (CDCl3, 100 MHz, TMS): d 21.14, 21.28,
21.60, 50.10, 51.24, 112.86, 113.32, 119.52, 122.65, 126.22,
126.29, 126.72, 127.24, 128.37, 128.62, 129.51, 129.64, 130.97,
133.01, 133.15, 142.13, 142.89, 143.72, 151.78, 153.86. Anal. Calc.
for C30H28N2O: C, 83.30; H, 6.52; N, 6.48. Found: C, 83.24; H,
6.67; N, 6.30%.
the target compound as
a white powder in 16.7% (0.76 g,
1.87 mmol) yield. mp 241ꢁ242 °C, IR (KBr; cmꢁ1):
m 3382, 3025,
2921, 1628, 1596, 1525, 1448, 1383, 1252, 1073, 739, 690. 1H
NMR (CDCl3, 400 MHz, TMS): d 7.53 (m, 2H), 7.31–7.16 (m, 13H),
6.77 (s, 1H), 6.08 (s, 1H), 2.24 (s, 3H). 13C NMR (CDCl3, 100 MHz,
4.2.5. 2-Benzhydryl-6-(1-benzyl-benzoimidazol-2-yl)-4-methylphenol
(L5)
TMS):
d 14.34, 20.94, 49.89, 60.65, 111.45, 123.35, 123.44,
Following the same procedure described for the formation of L4,
treatment of N-benzyl-2-nitrobenzenamine (1.70 g, 7.47 mmol),
3-benzhydryl-2-hydroxy-5-methylbenzaldehyde (2.26 g, 7.47
mmol), and sodium hydrosulfite (6.78 g, 39.0 mmol) gave L5
126.37, 128.42, 129.62, 132.94, 133.67, 143.59, 151.61, 154.60.
Anal. Calc. for C27H22N2O: C, 83.05; H, 5.68; N, 7.17. Found: C,
82.97; H, 5.72; N, 7.02%.
(0.52 g, 2.28 mmol, 14.5%). mp 196–197 °C, IR (KBr; cmꢁ1):
m
4.2.2. 2-Benzhydryl-4-methyl-6-(1-methyl-benzoimidazol-2-
yl)phenol (L2)
3026, 2913, 1605, 1496, 1444, 1397, 1250, 1155, 740, 691. 1H
NMR (CDCl3, 400 MHz, TMS): d 7.71 (d, J = 7.71 Hz, 1H), 7.38–
7.14 (m, 18H), 7.10 (s, 1H), 6.74 (s, 1H), 6.08 (s, 1H), 5.60 (s, 2H),
2.04 (s, 3H). 13C NMR (CDCl3, 100 MHz, TMS): d 20.97, 35.16,
50.10, 110.16, 112.19, 119.04, 123.39, 123.66, 125.73, 125.99,
126.27, 127.08, 128.10, 128.36, ‘129.40, 132.84, 133.27, 135.94,
136.39, 140.82, 143.76, 152.64, 154.53. Anal. Calc. for C34H28N2O:
C, 84.97; H, 5.87; N, 5.83. Found: C, 84.68; H, 5.94; N, 5.79%.
Using the same procedure as for the synthesis of L1, but using
a solution of N-methyl-2-nitrobenzenamine (1.46 g, 9.63 mmol)
and 3-benzhydryl-2-hydroxy-5-methylbenzaldehyde (2.91 g, 9.63
mmol) in DMF/H2O = 4:1 (80 mL:20 mL) was treated with about
5.2 equivalents of sodium hydrosulfite (8.73 g, 50.2 mmol) under
nitrogen. L2 was obtained as a yellow powder in 22.1% (0.86 g,

352
Z. Zhou et al. / Inorganica Chimica Acta 392 (2012) 345–353
Table 5
4.2.6. 2-Benzhydryl-6-(5-chloro-benzoimidazol-2-yl)-4-methylphenol
(L6)
Summary of crystallographic data for C1 and C4.
Using the same procedure as for the synthesis of L1, the reaction
of 5-chloro-2-nitrobenzenamine (2.02 g, 11.7 mmol), 3-benzhy-
dryl-2-hydroxy-5-methylbenzaldehyde (3.53 g, 11.7 mmol) and
sodium hydrosulfite (10.6 mmol, 60.9 mol) gave L6 (0.67 g,
C1
C4
Empirical formula
Formula weight
Crystal system
C
54H42N4O2Zn
C60H54N4O2Zn
928.46
monoclinic
C2/C
844.29
monoclinic
P2(1)/c
3.90 mmol, 13.5%). mp 232–233 °C, IR (KBr; cmꢁ1):
m 3384, 3027,
Space group
0
13.942(3)
16.626(3)
a (ÅA)
2920, 1582, 1519, 1494, 1441, 1374, 1247, 1102, 1059, 738, 701.
1H NMR (CDCl3, 400 MHz, TMS): d 8.05 (s, 1H), 7.31–7.17 (m,
13H), 6.78 (s, 1H), 6.07 (s, 1H), 2.24 (s, 3H). 13C NMR (CDCl3,
100 MHz, TMS): d 20.97, 50.10, 110.16, 112.19, 119.04, 123.39,
123.66, 125.73, 125.99, 126.27, 127.08, 128.10, 128.36, 129.40,
129.61, 132.84, 133.27, 135.94, 136.39, 140.82, 143.76, 152.64,
154.53. Anal. Calc. for C27H21ClN2O: C, 76.32; H, 4.98; N, 6.59.
Found: C, 76.19; H, 5.11; N, 6.39%.
0
21.575(4)
36.686(7)
10.260(2)
28.932(6)
b (AÅ)
0
c (AÅ)
a
(°)
90.00
94.56(3)
90.00
90.00
99.36(3)
90.00
b (°)
c
(°)
0
11000(4)
4869.6(16)
V (AÅ3)
Z
8
4
Dcalcd (g cmꢁ3
l
)
1.020
0.484
173(2)
3520
1.266
0.553
293(2)
1952
(mmꢁ1
)
T (K)
F(000)
4.3. Preparation of zinc complexes
h Range (°)
Number of reflections
collected
1.10–25.34
70616
1.43–27.47
18011
4.3.1. Zinc di(6-benzhydryl-4-methyl-2-(benzoimidazol-2-
yl)phenolate) (C1)
Number of unique
reflections
20091
5501
To a stirred solution of L1 (0.19 g, 0.5 mmol) in THF (15 mL) at
room temperature, zinc acetate dihydrate 0.056 g (0.25 mmol) was
added. The reaction mixture was stirred at room temperature for
12 h, and petroleum ether was added to precipitate the complex.
The resulting precipitate was filtered, washed with petroleum
ether, and dried under vacuum to furnish the product C1 as a white
Goodness-of-fit (GOF) on F2 1.053
1.322
R1 = 0.1135,
wR2 = 0.2544
R indices (all data)
R1 = 0.1255,
wR2 = 0.2484
powder in 81% (0.17 g, 0.21 mmol) yield. IR (KBr; cmꢁ1):
m 3612,
4.3.6. Zinc di(6-benzhydryl-4-methyl-2-(5-chloro-benzoimidazol-2-
yl)phenolate) (C6)
3057, 2967, 1625, 1599, 1531, 1451, 1335, 1248, 1081, 739, 670.
Anal. Calc. for C54H42N4O2Zn: C, 76.82; H, 5.01; N, 6.64. Found: C,
76.73; H, 4.89; N, 6.57%.
The procedure was similar to that used for C5, but using L6
(0.19 g, 0.45 mmol), affording C6 in 78.4% (0.16 g, 0.18 mmol). IR
(KBr; cmꢁ1):
m 3496, 3025, 1658, 1624, 1529, 1445, 1381, 1248,
4.3.2. Zinc di(6-benzhydryl-4-methyl-2-(1-methyl-benzoimidazol-2-
yl)phenolate) (C2)
1098, 1027, 754, 695. Anal. Calc. for C54H40Cl2N4O2Zn: C, 71.02;
H, 4.41; N, 6.14. Found: C, 71.14; H, 4.35; N, 6.09%.
Using the same procedure as for the synthesis of C1, but using
L2 (0.20 g, 0.5 mmol), to which zinc acetate dihydrate 0.056 g
(0.25 mmol) was added at room temperature, thereby affording
4.4. X-ray crystallographic studies
C2 in 95% (0.21 g, 0.25 mmol) yield. IR (KBr; cmꢁ1):
m 3727, 3023,
1553, 1470, 1444, 1398, 1325, 1247, 1077, 741, 696. Anal. Calc.
for C56H46N4O2Zn: C, 77.10; H, 5.31; N, 6.42. Found: C, 77.01; H,
5.36; N, 6.39%.
Single crystals of complexes C1 and C4 suitable for an X-ray
structural determination were grown by the slow diffusion of n-
heptane into THF solution. X-ray studies were carried out on a
Rigaku Saturn724+ CCD with graphite-monochromatic Mo K
a
0
radiation (k = 0.71073 ÅA) at 173(2) K, cell parameters were ob-
tained by global refinement of the positions of all collected reflec-
tions. Intensities were corrected for Lorentz and polarization
effects and empirical absorption. The structures were solved by
direct methods and refined by full-matrix least squares on F2. All
hydrogen atoms were placed in calculated positions. Structure
solution and refinement were performed by using the SHELXL-97
package (Table 5) [42].
4.3.3. Zinc di(6-benzhydryl-4-methyl-2-(1-ethyl-benzoimidazol-2-
yl)phenolate) (C3)
As above, but using a solution of ligand L3 (0.21 g, 0.5 mmol) in
THF (15 mL), which was added zinc acetate dihydrate 0.056 g
(0.25 mmol) at room temperature, to afford C3 in 72.7% (0.16 g,
0.18 mmol) yield. IR (KBr; cmꢁ1):
m 3550, 3026, 1590, 1552, 1443,
1399, 1334, 1239, 1076, 747, 695. Anal. Calc. for C58H50N4O2Zn: C,
77.37; H, 5.60; N, 6.22. Found: C, 77.40; H, 5.44; N, 6.17%.
4.3.4. Zinc di(6-benzhydryl-4-methyl-2-(1-isopropyl-benzoimidazol-
2-yl)phenolate) (C4)
Acknowledgments
The procedures are similar to that for C3, but using L4 (0.22 g,
0.5 mmol), resulting in C4 in 78.3% (0.18 g, 0.20 mmol) yield. IR
m 3785, 3026, 2975, 1604, 1549, 1491, 1437, 1413,
1373, 1246, 1073, 748, 696. Anal. Calc. for C60H54N4O2Zn: C,
The authors thank the National Natural Science Foundation of
China for support of this research (Grant No. 20773149). C.R.
thanks the EPSRC for an Overseas Travel Grant.
(KBr; cmꢁ1):
77.61; H, 5.86; N, 6.03. Found: C, 77.35; H, 5.79; N, 5.97%.
Appendix A. Supplementary material
4.3.5. Zinc di(6-benzhydryl-4-methyl-2-(1-benzyl-benzoimidazol-2-
yl)phenolate) (C5)
CCDC 863307 and 863308 contains the supplementary crystal-
lographic data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif. Supplementary data associ-
ated with this article can be found, in the online version, at http://
dx.doi.org/10.1016/j.ica.2012.03.057.
The procedure was similar to that for C4, but using L5 (0.23 g,
0.48 mmol), leading to C5 in 83.3% (0.20 g, 0.20 mmol) yield. IR
(KBr; cmꢁ1):
m 3938, 3027, 2918, 1553, 1496, 1443, 1398, 1252,
1155, 733, 697. Anal. Calc. for C68H54N4O2Zn: C, 79.71; H, 5.31; N,
5.47. Found: C, 79.69; H, 5.11; N, 5.37%.

Z. Zhou et al. / Inorganica Chimica Acta 392 (2012) 345–353
[20] S.K. Dogra, J. Lumin. 118 (2006) 45.
353
References
[21] S. Banthia, A. Samanta, J. Phys. Chem. B. 110 (2006) 6437.
[22] P.C. Tway, L.J.C. Love, J. Phys. Chem. 86 (1982) 5223.
[23] T. Qiu, X. Xu, X. Qian, J. Photochem. Photobio. A. 214 (2010) 86.
[24] M.M. Oliva, J. Casado, J.T.L. Navarrete, J. Phys. Chem. C 111 (2007) 18778.
[25] M. Kurt, P.C. Babu, N. Sundaraganesan, M. Cinar, M. Karabacak, Spectrochim.
Acta Part A Mol. Biomol. Spectrosc. 79 (2011) 1162.
[26] Ì. Sıdır, Y.G. Sıdır, Ì. Kayagil, Spectrochim. Acta Part A Mol. Biomol. Spectrosc.
81 (2011) 339.
[27] A.O. Eseola, W. Li, R. Gao, M. Zhang, X. Hao, T. Liang, N.O. Obi-Egbedi, W.-H.
Sun, Inorg. Chem. 48 (2009) 9133.
[28] M. Forés, M. Duran, M. Solà, J. Phys. Chem. A 103 (1999) 4525.
[29] M. Mosquera, J.C. Penedo, M.C.R. Rodríguez, F. Rodríguez-Prieto, J. Phys. Chem.
100 (1996) 5398.
[30] M.M. Henary, C.J. Fahrni, J. Phys. Chem. A 106 (2002) 5210.
[31] Y.-P. Tong, S.-L. Zheng, X.-M. Chen, Inorg. Chem. 44 (2005) 4270.
[32] R.M. Tarkka, X. Zhang, S.A. Jenekhe, J. Am. Chem. Soc. 118 (1996) 9438.
[33] S. Park, J. Seo, S.H. Kim, S.Y. Park, Adv. Funct. Mater. 18 (2008) 726.
[34] S. Santra, G. Krishnamoorthy, S.K. Dogra, J. Mol. Struct. 559 (2011) 25.
[35] S. Santra, G. Krishnamoorthy, S.K. Dogra, Chem. Phys. Lett. 311 (1999) 55.
[36] S. Santra, S.K. Dogra, Chem. Phys. 226 (1998) 285.
[1] M.L. McKelvy, T.R. Britt, B.L. Davis, J.K. Gillie, F.B. Graves, L.A. Lentz, Anal. Chem.
70 (1998) 119R.
[2] S.K. Dogra, J. Mol. Struct. 734 (2005) 51.
[3] J.-G. Guo, Y.-M. Cui, H.-X. Lin, X.-Z. Xie, H.-F. Chen, J. Photochem. Photobio. A.
219 (2011) 42.
[4] M.N. Manjunatha, A.G. Dikundwar, K.R. Nagasundara, Polyhedron 30 (2011)
1299.
[5] S. Demirayak, I. Kayagil, L. Yurttas, Eur. J. Med. Chem. 46 (2011) 411.
[6] Z. Tian, W. Wu, W. Wan, A.D.Q. Li, J. Am. Chem. Soc. 133 (2011) 16092.
[7] G. Verdasco, M.A. Martín, B.de1. Castillo, P. López-Alvarado, J.C. Menéndez,
Anal. Chim. Acta 303 (1995) 73.
[8] A.W. White, R. Almassy, A.H. Calvert, N.J. Curtin, R.J. Griffin, Z. Hostomsky, K.
Maegley, D.R. Newell, S. Srinivasan, B.T. Golding, J. Med. Chem. 43 (2000) 4084.
[9] J.L. Morales, J. Krzeminski, S. Amin, G.H. Perdew, Chem. Res. Toxicol. 21 (2008)
472.
[10] L. Fabbrizzi, G. Francese, M. Licchelli, A. Perotti, A. Taglietti, Chem. Commun.
(1997) 581.
[11] J. Kabatc, B. Je˛drzejewska, A. Bajorek, J. Pa˛czkowski, J. Fluoresc. 16 (2006) 525.
[12] R.N. Dsouza, U. Pischel, W.M. Nau, Chem. Rev. 111 (2011) 7941.
[13] S. Santra, G. Krishnamoorthy, S.K. Dogra, J. Mol. Struct. 559 (2001) 25.
[14] R. Yang, K. Li, K. Wang, F. Zhao, N. Li, F. Liu, Anal. Chem. 75 (2003) 612.
[15] Y. Zhang, R.H. Yang, F. Liu, K.A. Li, Anal. Chem. 76 (2004) 7336.
[16] M. Huang, P. Liu, Y. Chen, J. Wang, Z. Liu, J. Mol. Struct. 788 (2006) 211.
[17] H.J. Jung, N. Singh, D.O. Jang, Tetrahedron Lett. 49 (2008) 2960.
[18] Y.-P. Tong, S.-L. Zheng, X.-M. Chen, Eur. J. Inorg. Chem. (2005) 3734.
[37] C.J. Fahrni, M.M. Henary, D.G. VanDerveer, J. Phys. Chem. A 106 (2002) 7655.
[38] W.R. Dawson, M.W. Windsor, J. Phys. Chem. 72 (1968) 3251.
ˇ
[39] P. Hrdlovic, J. Kollár, S. Chmela, J. Photochem. Photobio. A. 163 (2004) 289.
[40] Ø.W. Akselsen, L. Skattebøl, T.V. Hansen, Tetrahedron Lett. 50 (2009) 6339.
[41] A.O. Eseola, W. Li, W.-H. Sun, M. Zhang, L. Xiao, J.A.O. Woods, Dyes Pigm. 88
(2011) 262.
[42] G.M. Sheldrick, SHELXTL-97, Program for the Refinement of Crystal Structures,
University of Göttingen, Germany, 1997.
[19] G. Krishnamoorthy, S.K. Dogra, Spectrochim. Acta Part
Spectrosc. 55 (1999) 2647.
A Mol. Biomol.