Synthesis, structure and photophysical properties of 2-benzhydryl-4-methyl- 6-(aryliminomethyl)phenol ligands and the zinc complexes thereof

Article abstract of DOI:10.1016/j.ica.2012.03.046

The Schiff-base ligands 2-benzhydryl-4-methyl-6-((phenylimino)methyl)phenol (L1-L5), and their corresponding zinc complexes (C1-C5), were prepared and fully characterized. The UV-Vis absorption and fluorescence spectra of the ligands and their Zn(II) comp

Full text of DOI:10.1016/j.ica.2012.03.046

Inorganica Chimica Acta 392 (2012) 292–299
Contents lists available at SciVerse ScienceDirect
Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
Synthesis, structure and photophysical properties of 2-benzhydryl-4-methyl-6-
(aryliminomethyl)phenol ligands and the zinc complexes thereof
b
d,
b,
Zihong Zhou a,b, Wen Li c, Xiaohua Hou b, Langqiu Chen a, , Xiang Hao , Carl Redshaw , Wen-Hua Sun
⇑
⇑
⇑
a Key Laboratory of Environmentally Friendly Chemistry and Application of Ministry of Education, College of Chemistry, Xiangtan University, Xiangtan 411105, 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 Key Laboratory of Photochemical Conversion and Optoelectronic Materials, Technical Institute of Physics and Chemistry, The Chinese Academy of Sciences, Beijing 100190, China
d School of Chemistry, University of East Anglia, Norwich NR47TJ, UK
a r t i c l e i n f o
a b s t r a c t
Article history:
The Schiff-base ligands 2-benzhydryl-4-methyl-6-((phenylimino)methyl)phenol (L1–L5), and their corre-
sponding zinc complexes (C1–C5), were prepared and fully characterized. The UV–Vis absorption and
fluorescence spectra of the ligands and their Zn(II) complexes were measured in solvents such as meth-
anol, dichloromethane, THF, or toluene, respectively. The results showed that the fluorescence intensity
of the ligands was very weak, but upon coordination with Zn2+, the fluorescence intensity was greatly
enhanced, and the fluorescent quantum yield ratio of C5 (0.46) to L5 (0.0004) in methanol increased
by around 1150 times. The enhanced fluorescence of the zinc complexes is due to the intramolecular
charge transfer (ICT) and the stronger geometrical rigidity of the delocalized bonding planes in these mol-
ecules. Transient absorption spectrum bands with peaks at about 310 and 460 nm were attributed to trip-
let state absorptions and absorption dynamic curve fitting showed that lifetimes were of microsecond
level scale. There was no time-resolved property observed for the bleaching spectral bands at 350 nm
and at ca. 460 nm. This spectral property, like the Stokes shift, can be attributed to the typical behavior
of nanosecond transient absorption of the compounds with the excited-state intramolecular proton
transfer (ESIPT).
Received 20 February 2012
Received in revised form 23 March 2012
Accepted 26 March 2012
Available online 3 April 2012
Keywords:
Zinc complexes
UV–Vis absorption
Fluorescence
ESIPT
Transient absorption spectrum
X-ray diffraction
Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction
no damage to the human body, and ease of preparation of the com-
plexes [7–9]. The coordination modes of salicylaldehyde Schiff
Fluorescent sensors are an indispensable tool for visualizing or
monitoring metal ions in real-time and real-space at a molecular
level, and are of use in many fields such as medical diagnostics,
environmental control, living cells, and electronics [1]. Zinc is an
abundant component of most living cells [2], and plays important
roles in various biological processes such as gene expression [3],
regulation of metalloenzymes, neural signal transmission [4], and
apoptosis [5]. Moreover, the zinc ion is also a contributory factor
in neurological disorders such as epilepsy, Alzheimer’s and Parkin-
son’s diseases [1,6]. Consequently, research efforts on zinc-based
organic materials with luminescent properties have been the focus
of great interest for a number of decades.
As evidenced in the literature and by our previous studies, zinc
complexes bearing bi- or tetra-dentate (N,O-donor atoms) Schiff
base ligands have been extensively investigated. Zinc complexes
have many advantages such as the combination of emitting and
electron transferring roles, high environmental stability, little or
bases are similar to those of the 8-hydroxy-quinolines: such
ligands have at least one hydroxyl group, a potentially coordinat-
ing nitrogen atom and a delocalized p-system. Metal-chelate com-
plexes of salicylaldehyde Schiff base ligands tend to also exhibit
good luminescent properties [10]. In addition, it has also been
reported that a molecule possessing a C@N imine moiety generally
has a high affinity for coordinating with metal ions such as zinc or
cadmium ions [11]. Given this, we have designed a new type of
Schiff base complex in which the ligands were synthesized from
the reaction between salicylaldehyde derivatives possessing a
bulky ortho substituent and an aniline (Scheme 1), and subse-
quently have investigated some photophysical properties of their
neutral Zn(II) complexes. It was found in some cases that in the
absence of Zn(II), the ligands exhibited almost no fluorescence
emission, but the fluorescence intensity was largely enhanced after
coordination with Zn(II) due to intramolecular charge transfer
(ICT). ICT processes are, of course, highly desirable for this kind
of molecular structure, and give large spectral shifts. Coordination
with Zn(II) changes the electronic structure of the ligand, leading
⇑
Corresponding authors. Tel.: +86 10 62557955; fax: +86 10 62618239.
E-mail addresses: chengood2003@263.net (L. Chen), Carl.Redshaw@uea.ac.uk
to the formation of a new extended
ligand and Zn(II) [12–18].
p-conjugation system between
(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.2012.03.046

Z. Zhou et al. / Inorganica Chimica Acta 392 (2012) 292–299
293
Scheme 1. Synthesis of ligands L1–L5 and complexes C1–C5.
2. Results and discussion
2.1. Molecular structures
The ligands were deprotonated with potassium hydride (KH) in
THF at ꢀ78 °C, and the resultant potassium salts were reacted fur-
ther with 0.5 equivalents of anhydrous ZnCl2 in THF to afford
glassy yellow products (Scheme 1). All complexes were character-
ized by elemental analysis and IR spectroscopy. Ligand L4 and
complexes C2 and C5 were further characterized by single-crystal
X-ray crystallography (Figs. 1–3). The O(1)–C(1)–C(6)–C(7)–N(1)
atoms of L4 (Fig. 1) are almost coplanar, with the largest deviation
Fig. 2. Ortep plot of C5 with thermal ellipsoids drawn at the 30% probability level.
Hydrogen atoms have been omitted for clarity.
0
0
from planarity being 0.0548 ÅA for O(1) and 0.0350 ÅA for N(1). In
addition, the N atom deviates from the basal plane (C(1)–C(2)–
0
C(3)) by 0.2074 ÅA (Fig. 1). The other important bond lengths and
angles are listed in Table 1.
The selected bond lengths and angles of C2 and C5 are also
listed in Table 1. The molecular structure of C5 is shown in
Fig. 2. Atoms N(1), O(1) and C(7) form the equatorial coordination
plane, and the zinc center slightly deviates from the equatorial
0
plane by 0.2060 ÅA. The two nitrogen atoms with the zinc core form
an angle of 115.3(2)°, larger than the O(1)–Zn(1)–O(1A) bond angle
0
109.66(18)°. The Zn(1)–O(1) bond length is 1.926(3) ÅA, shorter
0
than the Zn(1)–N(1) bond length 2.001(3) ÅA. Moreover, one coordi-
nation plane (O(1)–Zn(1)–N(1)) is approximately perpendicular to
the other (O(1a)–Zn(1)–N(1a)) with a dihedral angle of 83.5°, and
smaller than the corresponding value for C2 (96.7°) (Fig. 3). In
the solid-state structure of C2, the geometry at each zinc center
can be best described as a distorted tetrahedron in which two che-
lating ligands are placed in a similar disposition about the metal
center [7], but slight differences still occur. In particular, a longer
Fig. 3. Ortep plot of C2 with thermal ellipsoids drawn at the 30% probability level.
Hydrogen atoms have been omitted for clarity.
0
Zn(1)–O(1) bond length of 1.915(2) ÅA and a smaller dihedral angle
(N(1)–C(7)–O(1) and N(1)–Zn(1)–O(1)) of 8.9° were observed for
one ligand, while the respective values for the second ligand are
0
1.925(2) ÅA for Zn(1)–O(1a) and 9.7° for N(1a)–C(7a)–O(1a) and
N(1a)–Zn(1)–O(1a). The Zn(1)–N(1)–O(1) plane and phenolate ring
are almost coplanar with each other, forming a dihedral angle of
9.6°, which is smaller than the corresponding value for C5
(10.7°). Moreover, the distance of the N atom (C2 and C50 , Figs. 3
and 2) from the C(1)–C(2)–C(3) ring is 0.0265 and 0.0054 ÅA, respec-
0
tively, which is notably smaller than that of L4 (0.2074 ÅA). The
dihedral angles between the phenol ring (O(1)–C(1)–C(6)) and
the aniline plane are notably different: 52.9° in L4, 75.9° in C2,
and 104.3° in C5, these significant differences may be due to the
steric effects of the other ligand present.
2.2. UV–Vis absorption spectra in the organic solvents
The UV–Vis spectral data of the Zn complexes in different
solvents are shown in Table 2. The absorption of the complexes
Fig. 1. Ortep plot of L4 with thermal ellipsoids drawn at the 30% probability level.
Hydrogen atoms have been omitted for clarity.

294
Z. Zhou et al. / Inorganica Chimica Acta 392 (2012) 292–299
p ?
p⁄ tran-
Table 1
broad band at about 347 nm could be assigned to the
0
Selected bond lengths (AÅ) and angles (°) for complexes L4, C2 and C5.
sition [20,21]. Similar red shifts were also observed in other sol-
vents, however, there were no observed significant changes due
to the influence of the substituents.
L4 C2
C5
Bond lengths
Zn(1)–O(1)
Zn(1)–O(1a)
Zn(1)–N(1)
Zn(1)–N(1a)
N(1)–C(7)
N(1)–C(9)
O(1)–C(1)
C(6)–C(7)
1.915(2)
1.925(2)
2.003(3)
2.014(3)
1.301(4)
1.450(4)
1.313(4)
1.445(6)
1.422(5)
1.433(5)
1.926(3)
1.926(3)
2.001(3)
2.001(3)
1.293(5)
1.451(5)
1.317(5)
1.442(6)
1.425(6)
1.431(6)
2.3. Fluorescence properties of ligands and zinc complexes
The fluorescence properties of the Schiff base ligand and their
zinc complexes were studied at room temperature (298 K) in
methanol, THF, dichloromethane and toluene solution. The zinc
complexes exhibited a number of interesting photophysical prop-
erties such as large Stokes shifts and enhanced luminescence
intensity. As expected, L1–L5 have very small fluorescent quantum
yields, so much that they are almost too difficult to measure in our
experiments. The large Stokes shifts, such as the emission bands
for L1 exhibited values of 198 nm in dichloromethane, 141 nm in
methanol, 105 nm in THF, 204 nm in toluene, which belong to
the ESIPT phenomenon (Table 3). A potential factor due to this phe-
nomenon is an excited-state energy reduction of the keto tautomer
after proton transfer, compared to that of excited enol configura-
tion (Scheme 2).
Following coordination with Zn(II), the fluorescence emission
spectrum was significantly blue shifted compared to correspond-
ing ligands. Rate constants for radiative (kr) and non-radiative
(knr) decays of all complexes were calculated and are presented
in Table 3. It is interesting to observe that all complexes (C1–C5)
in methanol solution exhibited larger radiative decay rate con-
stants (Table 3; kr; C1, C2, C3, C4, and C5 in methanol solution).
Furthermore, complexes in methanol showed longer fluorescence
lifetime than that observed when employing other solvents (see
Table 3). This might be caused by the possible presence of dis-
solved oxygen in the methanol, which results in hydrogen bonds
between the protic solvent and the zinc complexes, thereby
strengthening the delocalization and the planarity/rigidity
(Scheme 3) [22–24]. Regarding the observations in dichlorometh-
ane, which can often contain amounts of acid, the nitrogen atoms
within the complex could be protonated and thereby be capable
of forming further hydrogen bonding [25]. In general, all the zinc
complexes showed stronger fluorescence than did the correspond-
ing ligands and this can be understood in terms of the ring formed
between zinc and the bi-dentate ligand, leading to a more rigid
molecular skeleton.
1.286(6)
1.419(5)
1.359(5)
1.466(6)
1.411(6)
1.399(6)
C(1)–C(6)
C(1)–C(2)
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)
O(1)–C(1)–C(6)
C(6)–C(7)–N(1)
C(7)–N(1)–C(9)
C(1)–C(6)–C(7)
O(1)–C(1)–C(2)
C(2)–C(1)–C(6)
110.40(10)
95.06(10)
120.80(10)
121.39(10)
95.49(10)
115.58(10)
123.8(3)
127.7(3)
117.3(3)
124.6(4)
118.5(3)
109.66(18)
96.36(13)
120.36(13)
120.36(13)
96.36(13)
115.3(2)
123.9(4)
128.7(4)
117.4(3)
124.5(4)
118.6(3)
117.5(3)
120.8(4)
122.6(4)
118.9(4)
121.1(4)
119.5(4)
119.7(4)
117.5(3)
Table 2
The spectral properties of ligands and Zn complexes in different solvents.
Solvent
Ligands
kabs-max (nm)
Complexes
kabs-max (nm)
Methanol
THF
Dichloromethane
Toluene
Methanol
THF
Dichloromethane
Toluene
Methanol
THF
Dichloromethane
Toluene
Methanol
THF
Dichloromethane
Toluene
L1
342
346
344
345
344
347
344
346
343
345
346
345
345
346
343
349
344
346
343
347
C1
232
237
294
294
216
239
282
293
238
265
279
297
215
242
280
294
217
263
281
294
L2
L3
L4
L5
C2
C3
C4
C5
The fluorescence intensity for the ligands and their zinc com-
plexes in THF and toluene solutions are shown in Fig. 5, and it is
evident that all complexes again exhibit strong fluorescence in
comparison to the corresponding ligands. In the presence of the
lone pairs of electrons of the donor atoms in the ligand, the fluores-
cence of the ligand was probably quenched by the occurrence of an
ICT process or ESIPT [26–32]. A conceptually related system (L1–
L5) contains the N^O salicylaldimine, which deprotonates upon
Zn(II) entry at the ligand to form a six-membered ring can signifi-
Methanol
THF
Dichloromethane
Toluene
was very sensitive to the solvent. For example, in strongly polar
solvents, the maximum absorption wavelengths of the complexes
were blue-shifted compared to weakly polar solvents, and this
indicated that the energy gap between the ground state and the ex-
cited state gradually became larger with the increase in solvent
polarity. The absorption wavelengths of the Zn complexes (C1–
C5) and the ligands (L1–L5) in CH2Cl2 are shown in Fig. 4. Accord-
ing to Fig. 4(a), all complexes have two new absorption bands at
about 410 and 236 nm, which can be assigned to ligand to metal
charge transfer (LMCT) and a low-energy metal to ligand charge-
transfer (MLCT, d ? p⁄ transition) [19], respectively. The two
absorption bands around 347 and 280 nm associated with the cor-
responding ligands are shown in Fig. 4(b), and these absorption
bands were shifted between 2 and 10 nm, consistent with internal
charge transfer (ICT) within the ligand [16]. Shown in Fig. 4(b) for
the ligands (L1–L5) in CH2Cl2, the narrow band at about 280 nm
could be assigned to the n ? r⁄ transition, whilst the intense
cantly reduce the energy gaps between the
p
and p⁄ orbital, the
HOMO and LUMO, as well as the orbitals with energies close to
the HOMO and LUMO [33,34]. In addition, coordination with the
Zn atom can enhance the rigidity of the ligand [12,35], which can
diminish the loss of energy via vibrational motions and increase
the emission efficiency, thereby resulting in further fluorescence
enhancement [23,36].
2.4. Transient absorption spectra and possible excited species
Fig. 6(a) and (b) shows the transient absorption spectra of L5 in
toluene solvent, conducted with and without de-oxygenation,
respectively. Clearly, there are two prominent transient absorption
bands with peaks at about 312 and 470 nm for the both deoxygen-
ating and oxygenated system. The only spectral difference was that

Z. Zhou et al. / Inorganica Chimica Acta 392 (2012) 292–299
295
Fig. 4. (a) UV–Vis absorption spectra of zinc complexes (C1–C5) in THF. Concentration: 2 ꢁ 10ꢀ4 M. (b) UV–Vis absorption spectra of ligands (L1–L5) in THF. Concentration:
2 ꢁ 10ꢀ4 M.
Table 3
The excitation wavelengths and fluorescence lifetime of ligands and Zn complexes in different solvent.
Solvent
Ligands
Complexes
kmaxEm (nm)
489
kmaxEm (nm)
kEx (nm)
D
ka (nm)
kEx (nm)
D
ka (nm)
s
(ns)
UF
kr
knr
b
c
Methanol
THF
Dichloromethane
Toluene
Methanol
THF
Dichloromethane
Toluene
Methanol
THF
Dichloromethane
Toluene
Methanol
THF
Dichloromethane
Toluene
L1
486
455
538
549
436
536
547
555
484
446
542
546
437
480
543
548
528
433
538
547
345
350
340
345
345
345
340
345
345
345
340
345
345
350
340
350
345
345
340
350
141
105
198
204
91
191
207
210
139
101
202
201
92
C1
C2
C3
C4
C5
340
405
330
410
340
405
340
410
340
405
350
410
340
405
347
415
340
405
340
415
146
84
160
85
145
83
149
80
143
79
141
81
137
81
140
72
5.748
6.430
6.616
6.253
6.236
6.277
6.686
6.358
6.072
7.103
7.166
6.974
4.934
5.530
4.915
5.188
5.100
7.386
5.107
5.452
0.08
0.01
0.03
0.03
0.34
0.01
0.06
0.02
0.26
0.01
0.02
0.03
0.29
0.01
0.05
0.06
0.46
0.01
0.12
0.05
13.92
1.555
4.534
4.798
54.52
1.593
8.974
3.146
42.82
1.408
2.791
4.302
58.78
1.808
10.17
11.56
90.20
1.354
23.49
9.171
1.601
1.540
1.466
1.551
1.058
1.577
1.406
1.541
1.219
1.394
1.368
1.391
1.439
1.790
1.933
1.812
1.059
1.340
1.723
1.742
489
490
495
488
488
489
490
486
484
491
491
480
486
487
487
485
485
488
488
L2
L3
L4
L5
130
203
198
183
88
Methanol
THF
Dichloromethane
Toluene
142
80
148
73
198
197
a
b
c
D
k = Stokes shift.
kr = UF
(106 sꢀ1).
knr = (1 ꢀ kr ꢂ )/
(108 sꢀ1).
/
s
s
s
Scheme 2. The excited-state energy reduction comparison between the keto tautomer and enol configuration.

296
Z. Zhou et al. / Inorganica Chimica Acta 392 (2012) 292–299
addition, the same measurements were done for C5 and the
absorption spectra showed similar results, when compared to L5.
3. Conclusions
Scheme 3. The plausible interaction between zinc complex and solvent methanol.
A series of 2-benzhydryl-4-methyl-6-((phenylimino)methyl)-
phenol ligands L1–L5, and their Zn(II) complexes (C1–C5) were
prepared and characterized. Molecular structures of L5, C2 and
C5 were confirmed by single-crystal X-ray diffraction. The UV–
Vis absorption and fluorescence spectral properties of ligands
and their Zn(II) complexes were investigated. In strong polar sol-
vents, the UV absorption wavelengths of the complexes were
blue-shifted, whereas the fluorescence emission wavelengths of
the complexes were red-shifted in lower polarity solvents. The
fluorescence emission peaks of the ligands were very weak due
to ICT or isomerization of the ligands, but when the zinc ion was
bound by the chelating ligands (L1–L5) an ICT resulted in fluores-
cence enhancement.
the spectral intensity of the deoxygenated sample was about 10%
higher than that of the sample without de-oxygenation. Also there
was a bleaching band with a peak at 350 nm. On recalling previous
transient absorption spectra of some Schiff bases [37,38], the tran-
sient absorption spectra clearly were from their excited triplet
state species due to similar spectrum bands. The experiments with
and without deoxygenation also supported this assignment. That
is, absorption decay curves of all samples with deoxygenation have
longer exponential function fitting lifetimes due to the removal of
oxygen as triple state quenching agent and some values are listed
in Table 4. Here, one important point we should pay attention to is
that the bleaching band was almost stable over time. In other
words, the bleaching band differs from the positive transient
absorption band by having a time-resolved feature. This seems to
imply that the excited state species does not go back to the ground
state (Scheme 2). However, the series of decay curves have shown
that the excited state giving rise to the transient absorption signals
indeed went back to the ground state. In regard to the bleaching
bands without the time-resolution along with the absorption at
about 460 nm, these may be attributed to the compounds with
ESIPT phenomenon. In fact, so long as we recall that fluorescence
properties with large Stoke shift are due to ESIPT, the fluorescence
lifetimes of the compounds with the light-induced keto state are
only about 0.6–1.0 ns (see Table 4) on average by employing the
excited source of the 355 nm pulse-width laser. When the absorp-
tion excited state decays to zero, this means that the excited state
can directly go back to the ground state (enol) and does not go back
to the keto form. Therefore, this bleaching band has almost lost the
time-resolved feature within the nanosecond (ns) scale. Up to now,
experimental characterization of the ESIPT phenomenon has been
made by their time-resolved fluorescence with a large Stokes shift
as a feature. As far as we are aware, there is no evidence reported
from corresponding transient absorption experiments in the liter-
ature. In the present work, we not only gave fluorescence experi-
mental evidence, but also present the first transient absorption
experimental evidence for the ESIPT phenomenon at ns scale. In
4. Experimental
4.1. General considerations and materials
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. ZnCl2 was dried and re-
fluxed over thionyl chloride. 3-Methyl-5-benzhydryl-6-hydroxy
bentaldehyde was prepared according to the literature method
[39]. All aniline derivatives were purchased from Aldrich or Acros
Chemicals. Absorption spectra were determined on SHIMADZU
UV-1601PC UV–Vis Spectrophotometer. 1H and 13C NMR spectra
were recorded on a Bruker DMX400 MHz instrument at ambient
temperature using TMS as an internal standard. IR spectra were re-
corded on a Perkin-Elmer System 2000 FT-IR spectrometer using
KBr disc in the range of 400–4000 cmꢀ1. Elemental analyses were
performed on a Flash EA 1112 microanalyzer. The steady-state
fluorescent spectra were measured on an F4500-FL fluorescence
spectrophotometer; fluorescence lifetimes were obtained using
the time-correlated single-photon counting technique (Edinburgh
Analytical Instruments F900 fluorescence spectrofluorimeter). Thin
films of the samples were prepared on quartz slides (1 cm) through
spin-coating. Fluorescence quantum yields (UF) were calculated
Fig. 5. (a) Fluorescence spectra of the complexes C1–C5 in THF, the inset shows the fluorescence spectra of the ligands L1–L5 in THF. Concentration: 7 ꢁ 10ꢀ5 M. Excitation
wavelength = 350 nm for L1 and L4, 345 nm for L2, L3 and L5, 405 nm for C1, C2, C3 and C4, 400 nm for C5. (b) Fluorescence spectra of the complexes C1–C5 in toluene, the
inset shows the fluorescence spectra of ligands L1–L5. Concentration: 7 ꢁ 10ꢀ5 M. Excitation wavelength = 410 nm for C1, C2 and C3, 415 nm for C4 and C5, 345 nm for L1, L2
and L3, 350 nm for L4 and L5.

Z. Zhou et al. / Inorganica Chimica Acta 392 (2012) 292–299
297
Fig. 6. Time-resolved transient absorption spectra of L5 and C5 in toluene solvent: (a) L5 were deoxygenated for about 30 min by bubbling through the solution with high
purity nitrogen gas (99.999%); (b) was the same sample (L5) without deoxygenation; (c) C5 were deoxygenated for about 30 min by bubbling through the solution with high
purity nitrogen gas (99.999%); and (d) was the same sample (C5) without deoxygenation. The inset shows the corresponding transient decays by exponential function fitting
at 464 nm for (a), (b) and 448 nm for (c), (d). Concentration: 5 ꢁ 10ꢀ5 M.
Table 4
Edinburgh Instruments Ltd. (England, UK). A nanosecond Nd:YAG
laser system was used for sample excitation (Surelite Continuum la-
Exponential fitting vales of some decay curves.
ser, pulse width 5–8 ns) and Xe-lamp with 450 W was used as
probe light. The conversion of light to electricity was completed
with the HAMAMATSU PMT R955. Laser light was inducted into a
sample quartz cell (1 cm ꢁ 1 cm ꢁ 4 cm) by right angle direction
with probe light on the plane of the optical stable, and finally the
signals were recorded with a Tektronix digital 100 MHz TDS3012B
oscilloscope. Here, 355 nm laser was used as exciting source for
all flash photolysis measurements.
Decay of sample
Fitting lifetimes (relative concentration)
Single: 1027 s; double:
101.8 s (2.7), 972.0 s (97.3)
Single: 703.6 s; double:
L5 (deoxygenated) 464 nm
l
l
l
L5 (air) 464 nm
l
18.91
934.4
845.1
l
l
l
s (2.1), 683.2 ls (97.9)
s
s
C5 (deoxygenated) 448 nm
C5 (air) 448 nm
4.2. Syntheses and characterization
according to the comparative method [7,40,41], using anthracene
in THF (UF = 0.297) as a standard [42].
4.2.1. 2-((2,6-Dimethylphenylimino)methyl)-6-benzhydryl-4-
methylphenol (L1)
R
2
ꢂ ð1 ꢀ 10ꢀA Þ ꢂ ðnxÞ
s
iF;x
iF;s
ð
m
Þd
m
m
UF;x
¼
UF;s
R
2
x
2,6-Dimethylbenzenamine (0.40 g, 3.30 mmol) and bentalde-
hyde (1.00 g, 3.30 mmol) were refluxed in absolute ethyl alcohol
(40 mL) in the presence of p-toluenesulfonic acid (0.15 g,
0.87 mmol) for 4 h. The reaction mixture was cooled to room tem-
perature. After solvent evaporation, the crude product was purified
by column chromatography on alumina with petroleum ether/
ethyl acetate (100/1 v/v) as eluent to afford the product as a yellow
powder in 25.3% (0.34 g, 0.84 mmol) yield. Mp 148–149 °C, IR (KBr;
ð
mÞd
ꢂ ð1 ꢀ 10ꢀA Þ ꢂ ðnsÞ
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
IF,x
(v
(v
) are fluorescence intensities at wavelength for the
sample and the standard, respectively. Ax and As are absorptions
of the sample and standard, nx and ns are refractive indices of the
solvents. The transient absorption spectra were obtained by the
nanosecond laser flash photolysis technique, and the used instru-
ment is a conventional LFP instrument set-up named as LP920 by
cmꢀ1):
982, 752. 1H NMR (CDCl3, 400 MHz, TMS): d 13.14 (s, 1H), 8.27
m 3025, 2916, 1622, 1585, 1493, 1448, 1361, 1265, 1192,

298
Z. Zhou et al. / Inorganica Chimica Acta 392 (2012) 292–299
Table 5
129.6, 130.8, 132.1, 135.0, 139.0, 143.7, 146.5, 159.9, 166.8. Anal. Calc.
for C33H35NO: C, 85.86; H, 7.64; N, 3.03. Found: C, 85.63; H, 7.34; N,
3.19%.
Summary of crystallographic data for L4, C2 and C5.
Complex
L4
C2
C5
Empirical formula
Formula weight
Crystal system
Space group
C
30H29NO
C62H60N2O2Zn
930.49
C64H64N2O2Zn
958.54
orthorhombic
Pbcn
4.2.4. 2-Benzhydryl-6-((mesitylimino)methyl)-4-methylphenol (L4)
Using the above procedure with 2,4,6-trimethylbenzenamine
(0.78 g, 5.80 mmol), 3-methyl-5-benzhydryl-6-hydroxy bentalde-
hyde (1.75 g, 5.80 mmol) and p-toluenesulfonic acid (0.26 g,
1.53 mmol) was added and refluxed for 3 h. L4 was obtained as a
yellow powder in 37.0% (0.90 g, 2.15 mmol) yield. Mp 152–
419.54
monoclinic
P2(1)/n
triclinic
ꢀ
P1
Unit cell dimensions
0
8.6521(17)
26.552(5)
10.833(2)
12.433(3)
15.346(3)
15.863(3)
21.925(4)
16.424(3)
16.866(3)
a (AÅ)
0
b (AÅ)
0
153 °C, IR (KBr; cmꢀ1):
m 3027, 2916, 1620, 1591, 1493, 1446,
c (AÅ)
1361, 1262, 1204, 984, 764. 1H NMR (CDCl3, 400 MHz, TMS): d
13.26 (s, 1H), 8.26 (s, 1H), 7.31–7.00 (m, 10H), 6.89 (s, 1H), 6.84
(s, 1H), 6.83 (s, 1H), 6.06 (s, 1H), 2.28 (s, 3H), 2.22 (s, 3H), 2.14
(s, 6H). 13C NMR (CDCl3, 100 MHz, TMS): d 18.1, 20.2, 20.3, 48.5,
117.7, 125.7, 126.9, 127.8, 128.5, 129.0, 130.1, 131.5, 133.8,
134.3, 143.2, 156.4, 166.3. Anal. Calc. for C30H29NO: C, 85.88; H,
6.97; N, 3.34. Found: C, 85.61; H, 6.93; N, 3.31%.
a
(°)
90.00
111.14(3)
90.00
70.36(3)
70.72(3)
72.60(3)
2629.7(9)
90.00
90.00
90.00
6073(2)
b (°)
c
(°)
0
2321.0(8)
V (AÅ3)
Z
4
2
4
Dcalc (g cmꢀ3
l
)
1.201
0.072
173(2)
896
1.175
0.511
173(2)
984
1.048
0.444
173(2)
2032
(mmꢀ1
)
T (K)
F(000)
h range (°)
Number of
reflections
collected
Number of unique
reflections
1.53–25.36
14868
1.41–27.49
25893
1.55–25.00
39198
4.2.5. 2-((2,6-Diethyl-4-methylphenylimino)methyl)-6-benzhydryl-4-
methylphenol (L5)
Similarly, using 2,6-diethyl-4-methylbenzenamine (2.41 g,
14.80 mmol) in the reaction (3-methyl-5-benzhydryl-6-hydroxy
bentaldehyde (4.46 g, 14.80 mmol), p-toluenesulfonic acid
(0.67 g, 3.89 mmol)) gave the L5 in 18.2% (1.19 g, 2.66 mmol) yield.
4244
11993
1.108
5338
Goodness-of-fit
1.238
1.179
(GOF) on F2
Mp 121–122 °C, IR (KBr; cmꢀ1):
m 3024, 2970, 1620, 1594, 1494,
R indices (all data)
R1 = 0.1338
wR2 = 0.2764
R1 = 0.0852
wR2 = 0.1918
R1 = 0.0899
wR2 = 0.2111
1396, 1263, 1203, 983, 758. 1H NMR (CDCl3, 400 MHz, TMS): d
13.20 (s, 1H), 8.26 (s, 1H), 7.32–7.17 (m, 10H), 7.01 (s, 1H), 6.92
(s, 2H), 6.85 (s, 1H), 6.07 (s, 1H), 2.48 (q, J = 7.51 Hz, 4H), 2.32 (s,
3H), 2.23 (s, 3H), 1.11 (t, J = 7.52 Hz, 6H). 13C NMR (CDCl3,
100 MHz, TMS): d 14.5, 20.2, 20.6, 24.3, 48.4, 117.6, 125.7, 126.7,
126.9, 127.8, 129.0, 130.2, 131.5, 133.8, 134.0, 134.3, 143.2,
156.4, 166.3. Anal. Calc. for C32H33NO: C, 85.87; H, 7.43; N, 3.13.
Found: C, 85.12; H, 7.47; N, 3.06%.
(s, 1H), 7.32–6.84 (m, 15H), 6.06 (s, 1H), 2.23 (s, 3H), 2.16 (s, 6H).
13C NMR (CDCl3, 100 MHz, TMS): d 18.7, 20.7, 49.0, 118.2, 124.9,
126.3, 127.5, 128.4, 128.5, 129.6, 130.8, 132.1, 135.0, 143.7,
148.4, 156.9, 166.9. Anal. Calc. for C29H27NO: C, 85.89; H, 6.71; N,
3.45. Found: C, 85.88; H, 6.95; N, 3.32%.
4.2.2. 2-((2,6-Diethylphenylimino)methyl)-6-benzhydryl-4-
methylphenol (L2)
4.2.6. Bis((2-((2,6-dimethylphenylimino)methyl)-6-benzhydryl-4-
methyl)phenoxy)zinc (C1)
Using the same procedure as for the synthesis of L1, 2,6-
diethylbenzenamine (1.02 g, 6.86 mmol) was used instead of 2,
6-dimethylbenzenamine, and reacted with 3-methyl-5-benzhy-
dryl-6-hydroxy bentaldehyde (2.07 g, 6.86 mmol) and p-toluene-
sulfonic acid (0.31 g, 1.81 mmol) in absolute ethyl alcohol
(100 mL). The yellow powder L2 was isolated in 31.4% (0.92 g,
To a THF solution (20 mL) of L1 (0.19 g, 0.50 mmol) was added
KH (0.020 g, 0.50 mmol) at ꢀ78 °C. The mixture was allowed to
warm to room temperature and stirred for additional 2 h, then
ZnCl2 (0.034 g, 0.25 mmol) was added. The resultant mixture was
stirred for 12 h at room temperature. The residue, obtained by
removing the solvent under vacuum, was dissolved in 20 mL tolu-
ene. The mixture was then filtered, and the filtrate was evaporated
to dryness under reduced pressure to give complex C1, which was
obtained as a glassy yellow solid in 61.3% (0.16 g, 0.18 mmol) yield.
2.12 mmol) yield. Mp 158 °C–159 °C, IR (KBr; cmꢀ1):
m 3026,
2963, 1602, 1583, 1494, 1447, 1365, 1262, 1158, 988, 797. 1H
NMR (CDCl3, 400 MHz, TMS): d 13.09 (s, 1H), 8.27 (s, 1H), 7.32–
7.02 (m, 15H), 7.01 (s, 1H), 6.85 (s, 1H), 6.07 (s, 1H), 2.47 (q,
J = 7.50 Hz, 4H), 1.12 (t, J = 7.52 Hz, 6H). 13C NMR (CDCl3,
100 MHz, TMS): d 14.4, 20.2, 24.3, 48.4, 124.7, 125.7, 125.9,
127.0, 127.8, 129.0, 130.2, 131.5, 133.9, 134.4, 143.2, 156.3,
166.3. Anal. Calc. for C31H31NO: C, 85.87; H, 7.21; N, 3.23. Found:
C, 85.62; H, 7.19; N, 3.27%.
IR (KBr; cmꢀ1):
m 3025, 1597, 1588, 1536, 1433, 1395, 1259, 1193,
984, 765. Anal. Calc. for C58H52N2O2Zn: C, 79.67; H, 5.99; N, 3.20.
Found: C, 79.54; H, 6.31; N, 3.23%.
4.2.7. Bis((2-((2,6-diethylphenylimino)methyl)-6-benzhydryl-4-
methyl)phenoxy)zinc (C2)
Using the same procedure for the synthesis of C1, 2-((2,6-dieth-
ylphenylimino)methyl)-6-benzhydryl-4-methylphenol (L2, 0.43 g,
1.00 mmol) and KH (0.040 g, 1.00 mmol) were dissolved in 20 mL
THF solution and stirred for additional 2 h at room temperature,
and ZnCl2 (0.068 g, 0.50 mmol) was added and stirred for further
12 h. C2 was obtained as a glassy yellow solid in 55.2% (0.26 g,
m 3026, 1596, 1588, 1535,
1426, 1388, 1259, 1183, 983, 730. Anal. Calc. for C62H60N2O2Zn:
C, 80.02; H, 6.50; N, 3.01. Found: C, 79.89; H, 6.72; N, 3.04%.
4.2.3. 2-((2,6-Diisopropylphenylimino)methyl)-6-benzhydryl-4-
methylphenol (L3)
Using the above procedure as described for L2, 2,6-diisopropyl-
benzenamine (0.89 g, 5.03 mmol), 3-methyl-5-benzhydryl-6-hydro-
xy bentaldehyde (1.52 g, 5.03 mmol) and p-toluenesulfonic acid
(0.23 g, 1.33 mmol) were refluxed in absolute ethyl alcohol
(100 mL) for 3 h. L3 was obtained as a yellow powder in 31.2%
0.28 mmol) yield. IR (KBr; cmꢀ1):
(0.72 g, 1.56 mmol) yield. Mp 178–179 °C, IR (KBr; cmꢀ1):
m
3023,
2962, 1601, 1586, 1494, 1457, 1360, 1260, 1182, 989, 795. 1H NMR
(CDCl3, 400 MHz, TMS): d 13.08 (s, 1H), 8.24 (s, 1H), 7.32–6.82 (m,
13H), 6.80 (s, 1H), 6.08 (s, 1H), 3.01–2.91 (m, 2H), 2.24 (s, 3H), 1.16
(d, J = 6.84 Hz, 12H). 13C NMR (CDCl3, 100 MHz, TMS): d 20.7, 22.6,
23.8, 28.1, 48.9, 118.1, 122.9, 123.3, 125.4, 126.3, 127.6, 128.4,
4.2.8. Bis((2-((2,6-diisopropylphenylimino)methyl)-6-benzhydryl-4-
methyl)phenoxy)zinc (C3)
Using the same procedure, 2-((2,6-diisopropylphenylimino)
methyl)-6-benzhydryl-4-methylphenol (L3, 0.46 g, 1.00 mmol)

Z. Zhou et al. / Inorganica Chimica Acta 392 (2012) 292–299
299
was reacted with KH (0.040 g, 1.00 mmol), and after 2 h, ZnCl2
via www.ccdc.cam.ac.uk/data_request/cif. Supplementary data
associated with this article can be found, in the online version, at
http://dx.doi.org/10.1016/j.ica.2012.03.046.
(0.068 g, 0.50 mmol) was added. C3 was obtained as a glassy yel-
low solid in 61.1% (0.30 g, 0.30 mmol) yield. IR (KBr; cmꢀ1):
m
3022, 1622, 1578, 1537, 1462, 1393, 1258, 1167, 988, 728. Anal.
Calc. for C66H68N2O2Zn: C, 80.33; H, 6.95; N, 2.84. Found: C,
80.12; H, 7.19; N, 2.73%.
References
[1] M. Hosseini, Z. Vaezi, M.R. Ganjali, F. Faridbod, S.D. Abkenar, K. Alizadeh, M.
Salavati-Niasari, Spectrochim. Acta, Part A 75 (2010) 978.
[2] C.J. Fahrni, T.V. O’Halloran, J. Am. Chem. Soc. 121 (1999) 11448.
[3] A.E. Majzoub, C. Cadiou, I. Déchamps-Olivier, B. Tinant, F. Chuburu, Inorg.
Chem. 50 (2011) 4029.
[4] M.-M. Yu, Z.-X. Li, L.-H. Wei, D.-H. Wei, M.-S. Tang, Org. Lett. 10 (2008) 5115.
[5] M. Rouffet, C.A.F.D. Oliveira, Y. Udi, A. Agrawal, I. Sagi, J.A. McCammon, M.C.
Seth, J. Am. Chem. Soc. 132 (2010) 8232.
[6] Z. Dai, G. Proni, D. Mancheno, S. Karimi, N. Berova, J.W. Canary, J. Am. Chem.
Soc. 126 (2004) 11760.
[7] 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.
4.2.9. Bis((2-benzhydryl-6-((mesitylimino)methyl)-4-
methyl)phenoxy)zinc (C4)
Using the same procedure, 2-benzhydryl-6-((mesitylimi-
no)methyl)-4-methylphenol (L4, 0.21 g, 0.50 mmol) was reacted
with KH (0.034 g, 0.25 mmol) to form the phenolate, following
which ZnCl2 (0.034 g, 0.25 mmol) was added. The yellow solid of
C4 was obtained in 50.2% (0.11 g, 0.13 mmol) yield. IR
(KBr; cmꢀ1):
m 3023, 1617, 1589, 1536, 1438, 1390, 1260, 1167,
[8] A. Majumder, G.M. Rosair, A. Mallick, N. Chattopadhyay, S. Mitra, Polyhedron
25 (2006) 1753.
[9] G. Consiglio, S. Failla, I.P. Oliveri, R. Purrello, S.D. Bella, Dalton Trans. (2009)
10426.
1021, 743. Anal. Calc. for C60H56N2O2Zn: C, 79.85; H, 6.25; N,
3.10. Found: C, 79.73; H, 6.48; N, 2.89%.
[10] T. Yu, W. Su, W. Li, Z. Hong, R. Hua, M. Li, B. Chu, B. Li, Z. Zhang, Z. Hu, Inorg.
Chim. Acta 359 (2006) 2246.
4.2.10. Bis((2-((2,6-diethyl-4-methylphenylimino)methyl)-6-
benzhydryl-4-methyl)phenoxy)zinc (C5)
[11] L. Li, Y.-Q. Dang, H.-W. Li, B. Wang, Y. Wu, Tetrahedron Lett. 51 (2010) 618.
[12] L. Wang, W. Qin, X. Tang, W. Dou, W. Liu, J. Phys. Chem. A 115 (2011) 1609.
[13] Z. Xu, Y. Xiao, X. Qian, J. Cui, D. Cui, Org. Lett. 7 (2005) 889.
[14] J. Wang, X. Qian, J. Cui, J. Org. Chem. 71 (2006) 4308.
[15] L. Wang, W. Qin, W. Liu, Inorg. Chem. Commun. 13 (2010) 1122.
[16] F. Qian, C. Zhang, Y. Zhang, W. He, X. Gao, P. Hu, Z. Guo, J. Am. Chem. Soc. 131
(2009) 1460.
[17] J. Kabatc, B. Je˛drzejewska, A. Bajorek, J. Pa˛czkowski, J. Fluoresc. 16 (2006) 525.
[18] O.A. Bozdemir, R. Guliyev, O. Buyukcakir, S. Selcuk, S. Kolemen, G. Gulseren, T.
Nalbantoglu, H. Boyaci, E.U. Akkaya, J. Am. Chem. Soc. 132 (2010) 8029.
[19] X.-L. Tong, D.-Z. Wang, T.-L. Hu, W.-C. Song, Y. Tao, X.-H. Bu, Cryst. Growth Des.
9 (2009) 2280.
Similarly, after 2-((2,6-diethyl-4-methylphenylimino)methyl)-
6-benzhydryl-4-methylphenol (L5, 0.54 g, 1.20 mmol) was depro-
tonated by KH (0.048 g, 1.20 mmol) in toluene, ZnCl2 (0.078 g,
0.58 mmol) was added; and the yellow solid of C5 (0.42 g,
0.44 mmol, 73.4% yield) was obtained. IR (KBr; cmꢀ1):
m 3025,
1616, 1590, 1537, 1430, 1390, 1216, 1166, 986, 745. Anal. Calc.
for C64H64N2O2Zn: C, 80.19; H, 6.73; N, 2.92. Found: C, 80.12; H,
7.07; N, 2.81%.
[20] K. Artur, W. Maret, J. Am. Chem. Soc. 129 (2007) 10911.
[21] B.R. François, X.B. Li, M.J. Katz, A.R. Geisheimer, D.B. Leznoff, H. Patterson,
Inorg. Chem. 50 (2011) 231.
4.3. X-ray crystallographic studies
[22] A.P.D. Silva, H.Q.N. Gunaratne, T. Gunnlaugsson, A.J.M. Huxley, C.P. McCoy, J.T.
Rademacher, T.E. Rice, Chem. Rev. 97 (1997) 1515.
[23] J. Grzegorzek, A. Filarowski, Z. Mielke, Phys. Chem. Chem. Phys. 13 (2011)
16596.
[24] K. Sakai, S. Takahashi, A. Kobayashi, T. Akutagawa, T. Nakamura, M. Dosen, M.
Kato, U. Nagashima, Dalton Trans. 39 (2010) 1989.
[25] D. Mahmut, T. Nyokong, Polyhedron 26 (2007) 2767.
[26] S. Basak, S. Sen, S. Banerjee, S. Mitra, G. Rosair, M.T.G. Rodriguez, Polyhedron
26 (2007) 5104.
[27] Y. Zhang, R. Yang, F. Liu, K. Li, Anal. Chem. 76 (2004) 7336.
[28] K.-H. Chen, J.-S. Yang, C.-Y. Hwang, J.-M. Fang, Org. Lett. 10 (2008) 4401.
[29] E.J. Shin, D. Kim, J. Photochem. Photobiol. 152 (2002) 25.
[30] H. Görner, S. Khanra, T. WeyhermÜller, P. Chaudhuri, J. Phys. Chem. A 110
(2006) 2587.
[31] Y. Ito, K. Amimoto, T. Kawato, Dyes Pigments 89 (2011) 319.
[32] O. Kotova, K. Lyssenko, A. Rogachev, S. Eliseeva, I. Fedyanin, L. Lepnev, L.
Pandey, A. Burlov, A. Garnovskii, A. Vitukhnovsky, M.V.D. Auweraer, N.
Kuzmina, J. Photochem. Photobiol. 218 (2011) 117.
[33] S.-L. Zheng, J.-H. Yang, X.-L. Yu, X.-M. Chen, W.-T. Wong, Inorg. Chem. 43
(2004) 830.
[34] Y.-P. Tong, S.-L. Zheng, X.-M. Chen, Inorg. Chem. 44 (2005) 4270.
[35] L. Zhang, L. Zhu, J. Org. Chem. 73 (2008) 8321.
Single crystals of complexes C2 and C5 suitable for X-ray dif-
fraction were grown by the slow diffusion of n-heptane/ethyl ether
into each THF solution, respectively. Single crystals of ligand L4
were obtained by slow evaporation of THF. Crystallographic data
of compounds L4, C2, and C5 are summarized in Table 5. X-ray
studies were carried out on a Rigaku Saturn724 + CCD with graph-
0
ite-monochromatic Mo K
a radiation (k = 0.71073 ÅA) at 173(2) K,
cell parameters were obtained by global refinement of the posi-
tions of all collected reflections. Intensities were corrected for Lor-
entz 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 [43].
Acknowledgments
[36] D.-D. Qin, Z.-Y. Yang, G.-F. Qi, Spectrochim. Acta, Part A 74 (2009) 415.
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.
ˇ
[37] J. Kollár, P. Hrdlovic, S. Chmela, M. Sarakha, G. Guyot, J. Photochem. Photobiol.
170 (2005) 151.
[38] F. Wang, L. Qin, M. Fan, Res. Chem. Intermed. 4 (1993) 299.
[39] Ø.W. Akselsen, L. SkattebØl, T.V. Hansen, Tetrahedron Lett. 50 (2009) 6339.
[40] W.R. Dawson, M.W. Windsor, J. Phys. Chem. 72 (1968) 3251.
Appendix A. Supplementary material
ˇ
[41] P. Hrdlovic, J. Kollár, S. Chmela, J. Photochem. Photobiol. 163 (2004) 289.
[42] S. Banthia, A. Samanta, J. Phys. Chem. B 110 (2006) 6437.
[43] G.M. Sheldrick, SHELXTL-97, Program for the Refinement of Crystal Structures,
University of Göttingen, Germany, 1997.
CCDC 851615, 851616, and 851617 contain the supplementary
crystallographic data for L4, C2 and C5. These data can be obtained
free of charge from The Cambridge Crystallographic Data Centre