Synthesis and two-photon absorption properties of unsymmetrical metallosalophen complexes

Article abstract of DOI:10.1016/j.poly.2012.09.050

Several new unsymmetrical metallosalphen complexes derived from the condensation of a monoimine and salicylaldehyde in the presence of a transition metal ion as a template have been synthesized and characterized. Their linear and third-order non-linear optical (NLO) properties have been investigated in detail. With different substituted donor or acceptor groups at the 5-position of the salicylidene fragment, these metallosalophen complexes exhibit slightly different two-photon absorption (TPA) cross-section σ⁽²⁾ values. Theoretical calculations based on density functional theory were also carried out to help with the interpretations of the experimental results.

Full text of DOI:10.1016/j.poly.2012.09.050

Polyhedron 49 (2013) 121–128
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Polyhedron
journal homepage: www.elsevier.com/locate/poly
Synthesis and two-photon absorption properties of unsymmetrical
metallosalophen complexes
d
d
d
Jie Zhang a,b, Cheng Zhong c, XunJin Zhu b, , Hoi-Lam Tam , King-Fai Li , Kok-Wai Cheah ,
⇑
Wai-Yeung Wong b, , Wai-Kwok Wong b, , Richard A. Jones e,
⇑
⇑
⇑
a School of Pharmaceutical and Chemical Engineering, Taizhou University, 318000 Taizhou, PR China
b Institute of Molecular Functional Materials (Areas of Excellence Scheme, University Grants Committee, Hong Kong), Department of Chemistry and Institute of Advanced
Materials, Hong Kong Baptist University, Waterloo Road, Hong Kong, PR China
c College of Chemistry and Molecular Sciences, Wuhan University, 430072 Wuhan, PR China
d Department of Physics, Hong Kong Baptist University, Waterloo Road, Hong Kong, PR China
e Department of Chemistry and Biochemistry, The University of Texas at Austin, 1 University Station, A5300, Austin, TX 78712-0165, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
Several new unsymmetrical metallosalphen complexes derived from the condensation of a monoimine
and salicylaldehyde in the presence of a transition metal ion as a template have been synthesized and
characterized. Their linear and third-order non-linear optical (NLO) properties have been investigated
in detail. With different substituted donor or acceptor groups at the 5-position of the salicylidene frag-
ment, these metallosalophen complexes exhibit slightly different two-photon absorption (TPA) cross-sec-
Received 14 August 2012
Accepted 22 September 2012
Available online 15 October 2012
tion
with the interpretations of the experimental results.
r
(2) values. Theoretical calculations based on density functional theory were also carried out to help
Keywords:
Metallosalophen
Two-photon absorption
Unsymmetrical
Non-linear optics
Synthesis
Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction
metal coordination complexes possess very large r(2) values of
up to 10,736 GM, as measured by a femtosecond open aperture
Z-scan technique [7]. Transition metal N2O2 Schiff base complexes,
in which the ligands are derived from salicylaldehyde and dia-
mines (generically coined as salen or salophen), have also been
studied as a class of efficient chromophores exhibiting potentially
large NLO responses [6–9]. They are particularly interesting be-
cause of their preparative accessibility and their high thermal sta-
bility. In such compounds, metal complexation leads to the
formation of geometrically constrained acentric, generally planar
structures that always involve an enhancement of optical non-
linearity of the Schiff base complexes compared to that of the related
free ligands [7,10]. Furthermore, the electron-donating (D) and -
accepting (A) capabilities of organometallic fragments have been
successfully applied for the design and development of new highly
efficient dipolar chromophores for achieving high second-order
NLO responses [9g,10a,11]. To the best of our knowledge, only a
few examples of the TPA properties of N2O2 Schiff base metal com-
plexes have been reported in the literature, and these are rarely de-
rived from unsymmetric metallosalophens [7,12]. We present here
several new metal-containing chromophores based on unsymmet-
ric donor-acceptor Schiff base frameworks. The TPA properties
of these unsymmetric metallosalophens were investigated by
fluorescence emission spectroscopy as well as by the femtosecond
Z-scan technique to obtain their r(2) values. Besides, theoretical
In the past decade, there has been growing interest in the de-
sign and construction of new molecular materials displaying a
two-photon absorption (TPA) because of their potential applica-
tions in two-photon up-conversion lasing [1], three-dimensional
optical data storage [2], photodynamic therapy [3], non-linear pho-
tonics [4] and so on. The TPA process is a third-order non-linear
optical (NLO) phenomenon, and the efficiency of this process is
determined by measuring the values of the TPA cross-section
r
(2). The most important feature of large TPA molecules is the pres-
ence of a long conjugated system consisting of substituents of
strong donor and/or acceptor groups, leading to potential intramo-
lecular charge transfer capabilities [5]. Various organic molecules
containing
p-conjugated electrons with appropriate donor and
acceptor groups exhibit moderate to strong TPA properties. In re-
cent years, the development of metal complexes containing TPA
chromophores has received a great deal of interest because of their
structural diversity and multifunctional nature [5a,6]. Bharadwaj
and co-workers observed that bis-cinnamaldiminato Schiff base
⇑
Corresponding authors. Tel.: +852 3411 5157 (X. Zhu); fax: +852 3411 7348 (X.
Zhu, W.-Y. Wong, W.-K. Wong), +1 512 471 8648 (R.A. Jones).
E-mail addresses: xjzhu@hkbu.edu.hk (X. Zhu), rwywong@hkbu.edu.hk (W.-Y.
Wong), wkwong@hkbu.edu.hk (W.-K. Wong), rajones@mail.utexas.edu (R.A. Jones).
0277-5387/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.poly.2012.09.050

122
J. Zhang et al. / Polyhedron 49 (2013) 121–128
calculations employing linear and quadratic response functions
were carried out at the density functional theory (DFT) level to
aid the interpretations of the experimental results.
2.1.3. Two-photon absorption measurements
The two-photon-absorption spectra (i.e. Z-scan traces) were
measured at 800 nm by the open-aperture Z-scan method using
100 fs laser pulses with a peak power of 276 GW cmꢁ2 from an
optical parametric amplifier operating at a repetition rate of
1 kHz, generated from a Ti:sapphire regenerative amplifier system
[15]. The laser beam was split into two parts by a beam splitter.
One was monitored by a photodiode (D1) as the incident intensity
reference, I0, and the other was detected as the transmitted inten-
sity by another photodiode (D2). After passing through a lens with
f = 20 cm, the laser beam was focused and passed through a quartz
cell. The position of the sample cell, z, was moved along the direc-
tion of the laser beam (z axis) by a computer-controlled translat-
able table so that the local power density within the sample cell
could be changed under the constant incident intensity laser
power level. Finally, the transmitted intensity from the sample cell
was detected by the photodiode D2. The photodiode D2 was inter-
faced to a computer for signal acquisition and averaging. Each
transmitted intensity datum represents the average of over 100
measurements. Assuming a Gaussian beam profile, the non-linear
absorption coefficient, b, can be obtained by curve-fitting to the ob-
2. Experimental
2.1. Chemicals and measurements
2.1.1. General
The commercially available chemicals were purchased and used
as received. Silica gel 60 (0.04–0.063 mm) for column chromatog-
raphy was purchased from Merck. NMR spectra were recorded on a
Bruker Ultrashield 400 Plus NMR spectrometer. Low-resolution
FAB mass spectra were obtained with a Finnigan TSQ710 mass
spectrometer. High-resolution matrix-assisted laser desorption/
ionization time-of-flight (MALDI-TOF) mass spectra were recorded
with a Bruker Autoflex MALDI-TOF mass spectrometer. All solution
spectroscopic measurements were prepared in spectrophotometric
grade N,N0-dimethylformamide (DMF) (Aldrich) and degassed with
freeze-pump-thaw cycles (three times). Electronic absorption
spectra in the UV–Vis region were recorded with a Varian Cary
100 UV/Vis spectrophotometer, and elemental analyses were per-
formed on a Carlo Erba 1106 instrument. The IR spectra (KBr pel-
lets) were recorded with a Nicolet Magna 550 FTIR spectrometer.
Steady-state visible fluorescence and PL excitation spectra were re-
corded on a Photon Technology International (PTI) Alphascan spec-
trofluorimeter and visible decay spectra on a pico-N2 laser system
(PTI Time Master) with kexc = 337 nm. Quantum yields of visible
emissions were calculated according to the literature method using
quinine sulfate in 1.0 N H2SO4 as the reference standard (Uf = 0.55
in air-equilibrated water) [13,14].
served open-aperture traces, T(z), with Eq. (1) [16], where a0 is the
linear absorption coefficient, l is the sample length (1 mm quartz
cell) and z0 is the diffraction length of the incident beam. After
obtaining the non-linear absorption coefficient, b, the TPA cross-
section,
r -
(2), of the sample molecule (in units of 1 GM = 10ꢁ50
cm4 s photonꢁ1) can be determined using Eq. (2), where NA is
Avogadro’s constant, d is the concentration of the sample compound
in solution, h is Planck’s constant and v is the frequency of the
incident laser beam.
a
0l
bI0ð1 ꢁ eꢁ
Þ
2
TðzÞ ¼ 1 ꢁ
ð1Þ
ð2Þ
2
a0½1 þ ðz=zoÞꢂ
2.1.2. Two-photon induced fluorescence (TPIF) measurements
The 750 nm excitation femto-second laser pulses were generated
by a mode-locked Ti:Sapphire laser (Tsunami, Spectra-Physics)
and Ti:Sapphire regenerative amplifier (Spitfire, Spectra-Physics)
with a pulse energy of 1 mJ per pulse, duration of ꢀ120 fs and a
repetition rate of 1 kHz. Two polarizers were used to alter the
excitation laser intensity before it reached the sample. The
emission from the sample was monitored by a spectrometer
system with a monochromator (SpectraPro 2300i, ACTON research
corporation) and photomultiplier tube (R636-10, Hamamatsu).
1000bh
¼
NAd
m
r2
2.2. Preparation of the unsymmetric metallosalophen complexes
The salicylaldehydes 1a [17], 1b–1c, 1d [18], 1e [19], 1f [20],
and 1g [21] were prepared from the commercially available chem-
ical 2-hydroxy-5-iodo-3-methoxybenzaldehyde in moderate yields
CHO
OH
1a R1 = 4-cyanophenyl
1b
CHO
Pd(PPh3)4 /Na2CO3
R1 = 4-N,N-dimethylaminophenyl
1c R1 = 4-tert-butylphenyl
1d
R1
B(OH)2
R1
+
I
OH
Toluene/N2
R1 = 4-methylphenyl
1e R1 = phenyl
OCH3
OCH3
C4H9
Sn
CHO
CHO
OH
Pd (PPh3)4
1f
R2 = 3-methylimidazolyl
1g R2 = 3-pyridyl
R2
C4H9
+
Br
OH
R2
Toluene/N2
C4H9
OCH3
OCH3
A
N
NH2
CH3CH2OH
Reflux
NH2
2a
R1 = 4-cyanophenyl
R1
+
OH
OCH3
R1
OH
2b R1 = 4-N,N-dimethylaminophenyl
H2N
OCH3
Scheme 1. Synthesis of electron-donating and electron-accepting functionalized salicylaldehydes 1a–1g and monoimines 2a and 2b.

J. Zhang et al. / Polyhedron 49 (2013) 121–128
123
CHO
N
O
N
N
OH
NH2
M
M(OAc)2
+
OH
R1
R2
R2
O
R1
CH3CH2OH/Reflux
OCH3
OCH3
H3CO
OCH3
2a 2b
-
1a 1g
-
3a
3b
3c
3d
3e
3f
R1 = 4-cyanophenyl, R2 = 4-N,N-dimethylaminophenyl, M = Zn(II)
R1 = 4-cyanophenyl, R2 = 4-tert-butylphenyl, M = Zn(II)
R1 = 4-cyanophenyl, R2 = 4-methylphenyl, M = Zn(II)
R1 = 4-cyanophenyl, R2 = phenyl, M = Zn(II)
R1 = 4-cyanophenyl, R2 = 1-methylimidazolyl, M = Zn(II)
R1 = 4-N,N-dimethylaminephenyl, R2 = 3-pyridyl, M = Zn(II)
3g R1 = 4-cyanophenyl, R2 = 4-N,N-dimethylaminophenyl, M = Ni(II)
3h R1 = 4-cyanophenyl, R2 = 4-methylphenyl, M = Ni(II)
3i R1 = 4-cyanophenyl, R2 = phenyl, M = Ni(II)
3j R1 = 4-cyanophenyl, R2 = 1-methylimidazolyl, M = Ni(II)
4 R1 = R2 = H, M = Zn(II)
Scheme 2. Synthesis of D–p–A unsymmetric salophens 3a–3j.
Table 1
Linear and non-linear optical data for the metallo-salophen complexes.
Table 2
Photophysical data of the zinc salophen complexes in DMSO.
Emissiona (kem/nm)
293 K
Lifetime (
293 K
s
/ns)
Ufb
Complex Absorption kmax/nm ([log(
e
/
r(2)
/
Td/
°C
Complex Excitation
dm3 molꢁ1 cmꢁ1)])a
GMb
(kexc/nm)
3a
3b
3c
3d
3e
3f
3g
3h
3i
342 (4.73), 442 (4.13)
1424
1176
1490
1754
1447
1626
1275
1547
1403
1650
254
298
237
238
370
323
416
424
369
352
3a
3b
3c
3d
3e
3f
314, 381
313, 377
313, 376
315, 378
311, 369
304, 374, 422
581
538
543
543
543
582
2.39
2.89
3.67
3.19
2.58
1.75
0.011
0.050
0.062
0.048
0.065
0.042
312 (4.55), 354 (4.62), 436 (4.13)
314 (4.56), 356 (4.64), 437 (4.16)
356 (4.64), 436 (4.14)
256 (4.58), 311 (4.87), 431 (4.21)
258 (4.55), 313 (4.88), 437 (4.13)
313 (4.79), 335 (4.80), 503 (3.79)
257 (4.63), 300 (4.71), 361 (4.70), 495 (3.92)
258 (4.71), 296 (4.74), 363 (4.70), 493 (3.94)
288 (4.58), 362 (4.60), 495 (3.88)
Measurements were performed on 1 ꢃ 10ꢁ6 M solutions in DMSO.
a
b
The quantum yield was measured relative to quinine sulfate in 1.0 N H2SO4
(
Uem = 0.55).
3j
a
Measurements of absorption were performed on 1 ꢃ 10ꢁ6 M solutions in DMSO.
b
TPA cross-sections are expressed in units of GM, where 1 GM = 10ꢁ50 cm4
-
separated as a red solid. The product was collected by filtration and
dried, in a yield of 82–91%.
s photonꢁ1, and the measurements were performed on 1 ꢃ 10ꢁ4 M solutions in
DMSO.
2a: 1H NMR (400 MHz, CDCl3) d (ppm): 13.85 (s, 1H, OH), 8.72
(s, 1H, C@NH), 7.67–7.75 (m, 4H, Ar–H), 7.29 (d, J = 2.0 Hz, 1H,
Ar–H), 7.20 (d, J = 2.0 Hz, 1H, Ar–H), 7.10–7.20 (m, 2H, Ar–H),
6.80–6.84 (m, 2H, Ar–H), 4.05 (s, 2H, NH2), 4.03 (s, 3H, OCH3).
HRMS (MALDI-TOF, positive mode, DMSO) m/z: 344.1386 [M+H]+
(C21H17N3O2+H: calcd. 344.1394).
(58–84%) via Suzuki-Miyaura or Stille coupling reactions, and fur-
ther purified on a silica gel column to afford the building blocks for
the unsymmetric metallosalophens (see Supporting information).
General procedure for the preparation of 2a and 2b: A mixture
of o-phenylenediamine (427 mg, 3.95 mmol) and 1a or 1b
(1.98 mmol) in EtOH (200 mL) was refluxed for 12 h. Then, the
solution was slowly concentrated, upon which the title compound
2b: 1H NMR (400 MHz, DMSO-d6) d (ppm): 12.84 (s, 1H, OH),
8.89 (s, 1H, C@NH), 7.53 (d, J = 8.4 Hz, 2H, Ar–H), 7.49 (d,
J = 2.0 Hz, 1H, Ar–H), 7.29 (d, J = 2.0 Hz, 1H, Ar–H), 7.14 (d,
Fig. 1. Two-photon (a) and one-photon (b) induced fluorescence spectra excited at 750 and 375 nm in DMSO, respectively, (1.0 ꢃ 10ꢁ4 M).

124
J. Zhang et al. / Polyhedron 49 (2013) 121–128
J = 8.4 Hz, 1H, Ar–H), 7.01 (t, J = 8.0 Hz, 1H, Ar–H), 6.77–6.81 (m,
3H, Ar–H), 6.63 (t, J = 8.0 Hz, 1H, Ar–H), 5.08 (s, 2H, NH2), 3.90 (s,
3H, OCH3), 2.49 (s, 6H, N(CH3)2). FAB-MS (positive mode) m/z:
362 [M+H]+.
3f: 1H NMR (400 MHz, DMSO-d6) d (ppm): 9.16 (s, 1H, C@NH),
9.14 (s, 1H, C@NH), 8.92 (d, J = 2.0 Hz, 1H, Ar–H), 8.44 (d,
J = 4.7 Hz, 1H, Ar–H), 8.05–8.07 (m, 1H, Ar–H), 7.90–7.96 (m, 2H,
Ar–H), 7.49–7.52 (m, 3H, Ar–H), 7.39–7.42 (m, 3H, Ar–H), 7.31 (d,
J = 2.0 Hz, 1H, Ar–H), 7.25 (d, J = 2.0 Hz, 1H, Ar–H), 7.13 (d,
J = 2.0 Hz, 1H, Ar–H), 6.79 (d, J = 8.8 Hz, 2H, Ar–H), 3.89 (s, 3H,
General procedure for the preparation of 3a–3j: A mixture of the
monoimine 2a or 2b (0.15 mmol), salicylaldehyde (1a–1g)
(0.17 mmol) and Zn(OAc)2ꢄ2H2O or Ni(OAc)2ꢄ4H2O (0.17 mmol) in
EtOH (50 mL) was refluxed for 12 h. Then, the product was col-
lected by filtration and dried to furnish a deep red solid (3a–3j)
or brown solid (3j) in a yield of 81–92%.
OCH3), 3.86(s, 3H, OCH3), 2.92 (s, 6H, N(CH3)2). IR (KBr)
m
(cmꢁ1):
3435, 1615, 1584, 1523, 1465, 1396, 1363, 1277, 1224, 1196,
981, 806, 750. HRMS (MALDI-TOF, positive mode, DMSO) m/z:
634.1595 [M]+ (C35H30N4O4Zn: calcd. 634.1553). UV/Vis (DMSO,
3a: 1H NMR (400 MHz, DMSO-d6) d (ppm): 9.17 (s, 1H, C@NH),
9.13 (s, 1H, C@NH), 7.84–7.90 (m, 6H, Ar–H), 7.62 (d, J = 2.0 Hz, 1H,
Ar–H), 7.49 (d, J = 8.8 Hz, 2H, Ar–H), 7.39–7.42 (m, 2H, Ar–H), 7.31
(d, J = 2.4 Hz, 1H, Ar–H), 7.28 (d, J = 2.4 Hz, 1H, Ar–H), 7.13 (d,
J = 2.0 Hz, 1H, Ar–H), 6.79 (d, J = 8.8 Hz, 2H, Ar–H), 3.88 (s, 3H,
20 °C) kmax (nm) [log(e
/dm3 molꢁ1 cmꢁ1)]: 258 (4.55), 313 (4.88),
437 (4.13). C35H30N4O4Zn (636.03): calcd. C 66.09, H 4.75, N
8.81; found C 66.21, H 4.63, N 8.99%.
3g: 1H NMR (400 MHz, DMSO-d6) d (ppm): 9.27 (s, 1H, C@NH),
9.21 (s, 1H, C@NH), 8.08–8.12 (m, 2H, Ar–H), 7.87 (m, 4H, Ar–H),
7.75 (m, J = 2.0 Hz, 1H, Ar–H), 7.49 (d, J = 8.8 Hz, 2H, Ar–H), 7.46
(d, J = 2.0 Hz, 1H, Ar–H), 7.35–7.37 (m, 2H, Ar–H), 7.24 (d,
J = 2.0 Hz, 1H, Ar–H), 7.12 (d, J = 2.0 Hz, 1H, Ar–H), 6.77 (d,
J = 8.8 Hz, 2H, Ar–H), 3.86 (s, 3H, OCH3), 3.84 (s, 3H, OCH3), 2.92
OCH3), 3.85 (s, 3H, OCH3), 2.91 (s, 6H, N(CH3)2). IR (KBr)
m
(cmꢁ1): 3291, 2907, 2220, 1617, 1582, 1519, 1463, 1446,
1360,1274, 1193, 980, 812. HRMS (MALDI-TOF, positive mode,
DMSO) m/z: 658.1570 [M]+ (C37H30N4O4Zn: calcd. 658.1553). UV/
Vis (DMSO, 20 °C) kmax (nm) [log(
e
/dm3 molꢁ1 cmꢁ1)]: 342 (4.73),
(s, 6H, N(CH3)2). IR (KBr) m
(cmꢁ1): 3522, 2933, 2221, 1612, 1600,
442 (4.13). C37H30N4O4Zn (660.05): calcd. C 67.33, H 4.58, N
1578, 1522, 1460, 1441, 1362, 1281, 1201, 989, 815. HRMS (MAL-
DI-TOF, positive mode, DMSO) m/z: 675.1572 [M+Na]+ (C37H30N4-
NiO4+Na: calcd. 675.1523). UV/Vis (DMSO, 20 °C) kmax (nm)
8.49; found C 67.49, H 4.44, N 8.31%.
3b: 1H NMR (400 MHz, DMSO-d6) d (ppm): 9.17 (s, 1H, C@NH),
9.14 (s, 1H, C@NH), 7.84–7.93 (m, 6H, Ar–H), 7.57–7.63 (m, 3H,
Ar–H), 7.40–7.44 (m, 5H, Ar–H), 7.28 (d, J = 2.0 Hz, 1H, Ar–H), 7.18
(d, J = 2.0 Hz, 1H, Ar–H), 3.89 (s, 3H, OCH3), 3.86 (s, 3H, OCH3), 1.13
[log(e
/dm3 molꢁ1 cmꢁ1)]: 313 (4.79), 335 (4.80), 503 (3.79). C37-
H30N4NiO4 (653.35): calcd. C 68.02, H 4.63, N 8.58; found C
68.05, H 4.53, N 8.89%.
(s, 9H, C(CH3)3). IR (KBr)
m
(cmꢁ1): 3292, 2957, 2221, 1618, 1582,
3h: 1H NMR (400 MHz, DMSO-d6) d (ppm): 9.54 (m, 2H, C@NH),
8.11–8.17 (m, 2H, Ar–H), 7.89 (m, 4H, Ar–H), 7.75–7.76 (s, 1H, Ar–
H), 7.56–7.58 (m, 3H, Ar–H), 7.38–7.41 (m, 2H, Ar–H), 7.21–7.30
(m, 4H, Ar–H), 3.88 (s, 3H, OCH3), 3.87 (s, 3H, OCH3), 2.34 (s, 3H,
1538, 1464, 1395, 1362, 1275, 1193, 980, 829. HRMS (MALDI-TOF,
positive mode, DMSO) m/z: 694.1656 [M+Na]+ (C39H33N3O4Zn+Na:
calcd. 694.1666). UV/Vis (DMSO, 20 °C) kmax (nm) [log(e
/dm3 molꢁ1
cmꢁ1)]: 312 (4.55), 354 (4.62), 436 (4.13). C39H33N3O4Zn (673.09):
calcd. C 69.59, H 4.94, N 6.24; found C 69.62, H 4.82, N 6.14%.
3c: 1H NMR (400 MHz, DMSO-d6) d (ppm): 9.18 (s, 1H, C@NH),
9.16 (s, 1H, C@NH), 7.86–7.94 (m, 6H, Ar–H), 7.63 (d, J = 2.0 Hz,
1H, Ar–H), 7.57 (d, J = 8.0 Hz, 2H, Ar–H), 7.41–7.43 (m, 3H, Ar–H),
7.29 (d, J = 2.0 Hz, 1H, Ar–H), 7.24 (d, J = 8.0 Hz, 2H, Ar–H), 7.19
(d, J = 2.0 Hz, 1H, Ar–H), 3.90 (s, 3H, OCH3), 3.88 (s, 3H, OCH3),
CH3). IR (KBr) m
(cmꢁ1): 3487, 2916, 2222, 1601, 1579, 1545,
1464, 1365, 1282, 1202, 989, 815. HRMS (MALDI-TOF, positive
mode, DMSO) m/z: 646.1225 [M+Na]+ (C36H27N3NiO4+Na: calcd.
646.1258). UV/Vis (DMSO, 20 °C) kmax (nm) [log(e
/dm3 molꢁ1
cmꢁ1)]: 257 (4.63), 300 (4.71), 361 (4.70), 495 (3.92). C36H27N3-
NiO4 (624.31): calcd. C 69.26, H 4.36, N 6.73; found C 69.33, H
4.11, N 6.54%.
2.33 (s, 3H, CH3). IR (KBr)
m
(cmꢁ1): 3391, 2911, 2220, 1616,
3i: 1H NMR (400 MHz, DMSO-d6) d (ppm): 9.54 (m, 2H, C@NH),
8.09–8.12 (m, 2H, Ar–H), 7.85–7.86 (m, 4H, Ar–H), 7.77 (d,
J = 2.0 Hz, 1H, Ar–H), 7.66 (d, J = 7.6 Hz, 2H, Ar–H), 7.58 (d,
J = 2.0 Hz, 1H, Ar–H), 7.42–7.46 (m, 2H, Ar–H), 7.31–7.39 (m, 2H,
Ar–H), 7.28–7.31 (m, 1H, Ar–H), 7.25 (d, J = 2.0 Hz, 1H, Ar–H),
1581, 1512, 1463, 1393, 1364, 1275, 1191, 979, 812. HRMS (MAL-
DI-TOF, positive mode, DMSO) m/z: 652.1158 [M+Na]+ (C36H27N3-
O4Zn+Na: calcd. 652.1196). UV/Vis (DMSO, 20 °C) kmax (nm)
[log(e
/dm3 molꢁ1 cmꢁ1)]: 314 (4.56), 356 (4.64), 437 (4.16). C36-
H27N3O4Zn (631.01): calcd. C 68.52, H 4.31, N 6.66; found C
7.22 (d, J = 2.0 Hz, 1H, Ar–H), 3.86 (m, 6H, OCH3). IR (KBr) m
68.61, H 4.21, N 6.79%.
(cmꢁ1): 3468, 3034, 2219, 1614, 1599, 1543, 1460, 1369, 1283,
1201, 987, 828. HRMS (MALDI-TOF, positive mode, DMSO) m/z:
632.1134 [M+Na]+ (C35H25N3NiO4+Na: calcd. 632.1101). UV/Vis
3d: 1H NMR (400 MHz, DMSO-d6) d (ppm): 9.18 (s, 1H, C@NH),
9.17 (s, 1H, C@NH), 7.85–7.91 (m, 6H, Ar–H), 7.67 (d, J = 7.3 Hz, 2H,
Ar–H), 7.63 (d, J = 2.3 Hz, 1H, Ar–H), 7.40–7.45 (m, 5H, Ar–H), 7.29
(d, J = 2.3 Hz, 1H, Ar–H), 7.25 (t, J = 7.3 Hz, 1H, Ar–H), 7.21 (d,
J = 2.3 Hz, 1H, Ar–H), 3.89 (s, 3H, OCH3), 3.88 (s, 3H, OCH3). IR
(DMSO, 20 °C) kmax (nm) [log(e
/dm3 molꢁ1 cmꢁ1)]: 258 (4.71),
296 (4.74), 363 (4.70), 493 (3.94). C35H25N3NiO4 (610.28): calcd.
C 68.88, H 4.13, N 6.89; found C 68.71, H 4.10, N 7.03%.
(KBr)
m
(cmꢁ1): 3298, 2927, 2221, 1615, 1584, 1538, 1463, 1394,
3j: 1H NMR (400 MHz, DMSO-d6) d (ppm): 9.75–10.08 (m, 2H,
C@NH), 8.14 (m, 2H, Ar–H), 7.86–7.99 (m, 4H, Ar–H), 7.75 (s, 1H,
Ar–H), 7.68 (s, 1H, Ar–H), 7.34–7.44 (m, 4H, Ar–H), 7.03–7.05 (m,
2H, Ar–H), 3.90 (s, 3H, OCH3), 3.84 (s, 3H, OCH3), 3.71 (s, 3H,
1365, 1275, 1191, 979, 829. HRMS (MALDI-TOF, positive mode,
DMSO) m/z: 638.1063 [M+Na]+ (C35H25N3O4Zn+Na: calcd.
638.1040). UV/Vis (DMSO, 20 °C) kmax (nm) [log(e -
/dm3 molꢁ1
cmꢁ1)]: 356 (4.64), 436 (4.14). C35H25N3O4Zn (616.98): calcd. C
68.13, H 4.08, N 6.81; found C 67.92, H 4.37, N 6.92%.
NCH3). IR (KBr) m
(cmꢁ1): 3400, 2218, 1600, 1580, 1544, 1464,
1369, 1281, 1199, 987, 829. HRMS (MALDI-TOF, positive mode,
3e: 1H NMR (400 MHz, DMSO-d6) d (pm): 9.18 (s, 1H, C@NH), 9.10
(s, 1H, C@NH), 7.86–7.92 (m, 6H, Ar–H), 7.73 (s, 1H, Ar–H), 7.64 (d,
J = 2.4 Hz, 1H, Ar–H), 7.40–7.44 (m, 2H, Ar–H), 7.29 (d, J = 2.4 Hz,
1H, Ar–H), 7.16 (d, J = 2.0 Hz, 1H, Ar–H), 6.91 (d, J = 2.0 Hz, 2H, Ar–
H), 3.89 (s, 3H, OCH3), 3.80 (s, 3H, OCH3), 3.70 (s, 3H, NCH3). IR
DMSO) m/z: 614.1373 [M+H]+ (C33H25N5NiO4: calcd. 614.1332).
UV/Vis (DMSO, 20 °C) kmax (nm) [log(e
/dm3 molꢁ1 cmꢁ1)]: 288
(4.58), 362 (4.60), 495 (3.88). C33H25N5NiO4 (614.28): calcd. C
64.52, H 4.10, N 11.40; found C 64.44, H 4.28, N 11.56.
(KBr)
m
(cmꢁ1): 3435, 2932, 2220, 1614, 1584, 1537, 1466, 1397,
3. Results and discussion
1336, 1274, 1193, 981, 832. HRMS (MALDI-TOF, positive mode,
DMSO) m/z: 620.1298 [M+H]+ (C33H25N5O4Zn: calcd. 620.1270).
3.1. Synthesis and characterization
UV/Vis (DMSO, 20 °C) kmax (nm) [log(e
/dm3 molꢁ1 cmꢁ1)]: 256
(4.58), 311 (4.87), 431 (4.21). C33H25N5O4Zn (620.97): calcd. C
63.83, H 4.06, N 11.28; found C 63.67, H 3.97, N 11.45%.
The condensation of a diamine and 2 equiv of a salicylaldehyde
has been generally practiced as the standard method for the

J. Zhang et al. / Polyhedron 49 (2013) 121–128
125
LUMO+1
LUMO+2
LUMO+1
LUMO
HOMO
HOMO-1
HOMO-2
LUMO
HOMO
HOMO-1
HOMO-3
HOMO-4
HOMO-2
HOMO-3
Fig. 2. Contour plots of the frontier molecular orbitals of 4 (left) and 3a (right).
preparation of symmetric salophen or salen ligands in high yields
[22]. However, the synthesis of unsymmetrical salens has proven
to be challenging [23]. Recently, salen-type Schiff base ligands
incorporating two different benzylidene moieties and a diamine
backbone were synthesized in high yields (80–90%) under mild
conditions via a stepwise approach [23a,24]. However, the method
could not be applied to prepare unsymmetric salophen-type Schiff
base ligands. Here, a well-known template procedure starting from
a monoimine, an equimolar amount of salicylaldehyde and a tran-
sition metal ion has been used to synthesize unsymmetrical salo-
phen Schiff base metal complexes directly [25]. Firstly, several
salicylaldehydes (1a–1g) functionalized with electron-donating
or -accepting capabilities were prepared from the commercially
available chemical 2-hydroxy-5-iodo-3-methoxybenzaldehyde in
moderate yields (58–84%) via Suzuki-Miyaura or Stille coupling
reactions, and further purified on a silica gel column to afford the
building blocks for unsymmetric metallosalophens (Scheme 1)
[26]. Then, monoimine intermediates (2a–2b) were synthesized
by the condensation of 1a or 1b with excess o-phenylenediamine
[27]. Lastly, the neutral unsymmetric Schiff base complexes 3a–
3f were readily synthesized from the monoimines 2a–2b, an equi-
molar amount of the salicylaldehydes 1a–1f and zinc(II) acetate
tetrahydrate, as outlined in Scheme 2. The product was precipi-
tated directly from the reaction mixture and simply isolated by

126
J. Zhang et al. / Polyhedron 49 (2013) 121–128
Table 3
Theoretical results obtained by means of the response function formalism within the DFT framework [B3LYP/6-31+G(d)] for 4 and 3a in vacuum. Major transition (percentage),
transition energies (E), oscillator strengths (f) and TPA transition probabilities (D) are shown.
Singlet excited state
Major transition
E (nm)
f
D (a.u.)
4
S1
S2
S3
S4
S5
S6
S7
S8
S9
S10
HOMO ꢁ 1 ? LUMO (98%)
HOMO ? LUMO (93%)
459.61
456.8
424.9
412.27
361.53
342.25
333.19
318.25
307.64
302.85
0.0666
0.0350
0.0010
0.1429
0.4159
0.0386
0.0
0.0
0.6356
0.0522
1.36E+02
2.56E+03
7.08E-01
2.19E+03
9.44E+02
3.96E+03
3.20E-01
1.05E-02
7.99E+02
1.57E+02
HOMO ? LUMO + 1 (98%)
HOMO ꢁ 1 ? LUMO + 1 (91%)
HOMO ꢁ 2 ? LUMO (98%)
HOMO ꢁ 2 ? LUMO + 1 (74%)
HOMO ꢁ 4 ? LUMO (91%)
HOMO ꢁ 4 ? LUMO + 1 (65%)
HOMO ꢁ 3 ? LUMO (71%)
HOMO ꢁ 3 ? LUMO + 1 (97%)
3a
S1
S2
S3
S4
S5
S6
S7
S8
S9
S10
HOMO ? LUMO (98%)
548.26
495.43
452.88
415.10
403.93
399.90
375.73
364.81
353.01
339.86
0.0555
0.0411
0.1754
0.0310
0.0203
0.0085
0.0384
0.7070
0.3861
0.1856
0.185E+05
0.147E+04
0.194E+04
0.155E+04
0.156E+05
0.419E+04
0.459E+04
0.553E+04
0.328E+05
0.592E+04
HOMO ? LUMO + 1 (95%)
HOMO ꢁ 1 ? LUMO (93%)
HOMO ꢁ 1 ? LUMO + 1 (92%)
HOMO ꢁ 2 ? LUMO (84%)
HOMO ? LUMO + 2 (96%)
HOMO ꢁ 2 ? LUMO + 1 (91%)
HOMO ꢁ 3 ? LUMO (93%)
HOMO ꢁ 1 ? LUMO + 2 (94%)
HOMO ꢁ 3 ? LUMO + 1 (73%)
one filtration step [25b,28]. In order to understand the optical re-
sponse upon the electronic properties of the metal center, some
nickel(II) salophen complexes (3g–3j) were also obtained by a sim-
ilar method. It should be noted that appropriate metal(II) ions,
such as Zn(II) and Ni(II), are selected as templates for simplifying
the synthesis and purification. The symmetric compound 4 was
also prepared for comparison (Scheme 2).
These unsymmetrical salophen complexes (3a–3j) were charac-
terized by 1H NMR, IR, elemental analysis and UV–Visible spectros-
copies. NMR analysis was performed in DMSO-d6, and for all
complexes, typical spectral patterns were observed that can be
associated with the formation of a Zn(II)-centered unsymmetrical
salophen complex. From the IR data, one can find the typical vibra-
tions of C@N at about 1594 cmꢁ1 and CAH of the methoxyl group
at about 1369 cmꢁ1 for 3a. All of these complexes are thermally
stable, as judged by thermal gravimetric analysis (TGA), with onset
decomposition temperatures (Td) within the range of 237–298 °C
(Table 1).
in this region of the absorption spectra. The two-photon induced
fluorescence (TPIF) spectra of 3a–3g were measured in DMSO solu-
tions. Upon excitation at 750 nm at room temperature with a
mode-locked femtosecond Ti:Sapphire laser, the compounds
3a–3d strongly emitted broadband fluorescence with a peak wave-
length of 576, 528, 530 and 527 nm, respectively (Fig. 1). Since no
linear absorption in the wavelength range of 750 nm was observed
for these zinc complexes, this indicates that the emission induced
by 750 nm excitation could not be attributed to a linear process,
but rather to a non-linear process. The upconverted emissions of
these complexes are nearly identical to those observed in their cor-
responding single-photon excited emission, indicating that the
molecules are promoted from the ground state to the same excited
state, either by 1PA or 2PA (Fig. 1, Table 2).
To further confirm the non-linear processes of the complexes,
the dependence of the upconverted luminescence intensities on
the incident laser power was measured at 750 nm. The result
shows the plots of log(emission intensity at 540 nm) versus log(la-
ser power) of 3b and 3d, which gave straight lines with slopes close
to 2.0 (Fig. S1). The slope of 2 in the log(emission intensity) versus
log(laser power) supports the two-photon nature of the process
[29].
3.2. Photophysical properties
The linear and non-linear photophysical properties of com-
plexes 3a–3j have been investigated and the results are summa-
rized in Tables 1 and 2, respectively. The room temperature
solution electronic absorption, excitation, and emission spectra of
3a–3f are very similar and characteristic of intraligand transitions
of Schiff base metal complexes. The absorption between 300 and
The TPA cross-sections r(2) of these unsymmetrical salophen
complexes were measured at 800 nm using 100 fs laser pulses
with the open-aperture Z-scan method (Table 1). With different
substituted donor groups, from N,N-dimethylaminophenyl to a
phenyl unit, 3a–3j exhibited slightly different r(2) values, be-
tween 1176 and 1754 GM. Despite the different electronic config-
urations for Zn(II) and Ni(II), there is no obvious trend for the
observed TPA cross-sections r(2) of these salophen complexes.
Although the exceptionally large r(2) values of 10,736 GM on a
Schiff base through metal complexation has been reported by
Bharadwaj et al. [7], the r(2) values of up to 1754 GM are still
among the highest known values for any mono-salophen metal
complexes to date. For comparison, we also synthesized the sym-
metric salophen complex 4 and measured its absolute r(2) value
to be 1465 GM. This indicates that the attachment of appropriate
D and A fragments would have certain effects on the electronic
properties of some of unsymmetric metallosalophen complexes,
although the exact structure-property relationship is yet to be
established.
500 nm can be assigned to the
ature emission of 3a–3f, with lifetimes (
p
–
p⁄ transition. The room temper-
) ranging from 2.39 to
s
3.67 ns and quantum yields (Uf) of 0.01 to 0.06, can be assigned
to the intraligand singlet state emission. It should be noted that
the emission spectra of both complexes 3a and 3f are red-shifted
due to the electron-donating property of the substituted 4-N,N0-
dimethylaminophenyl functional group. As for the nickel(II) salo-
phens, the low energy absorption band is significantly red-shifted
as compared to the zinc(II) congeners, and this band has been as-
signed to metal-to-ligand charge transfer transitions [8b]. Addi-
tionally, the nickel(II) salophen complexes exhibited very weak
luminescence.
Non-linear optical measurements were performed in the near-
infrared region since all the metal complexes 3a–3j are transparent

J. Zhang et al. / Polyhedron 49 (2013) 121–128
127
3.3. Theoretical calculations
metallosalophen complexes can show slightly better r(2) values
than the symmetric salophen complexes. Theoretical calculations
were also carried out at the density functional theory level to aid
the interpretations of the experimental results. In general, these
salophen complexes possess moderate fluorescence quantum
In the last decade, efforts have been made to establish a rela-
tionship between the TPA properties of compounds and their
molecular structures [30–33]. To aid the interpretation of our
experimental results, we have carried out theoretical calculations
on 3a and 4 to predict their electronic transitions, either related
to one- (1PA) or two-photon (TPA) absorption. All the theoretical
computations were carried out within the DFT framework. Their
ground state geometries were optimized using B3LYP/6-31G(d)
[34] on light atoms and LANL2DZ [35] on the zinc atom. Then, their
electronic transitions were calculated using TD-B3LYP with the
same basis set on the ground state geometry. All calculations were
performed using the Dalton2011 Program [36]. The resulting fron-
tier molecular orbitals are shown in Fig. 2. The theoretical results
for one- and two-photon absorptions of compounds 4 and 3a are
displayed in Table 3, respectively, which includes transition ener-
gies (E), oscillator strengths (f) and TPA transition probabilities (D).
Generally, the frontier molecular orbitals play a key role in the
electronic transition during the one- or two-photon processes [37].
From Table 3, we observed that the first strongest transition, S8 via
1PA, for 3a occurs at E = 364 nm (f = 0.7070), which is in close
agreement with the experimental one (342 nm). Analysis of the
yields and large
r
(2) values, and show two-photon induced fluores-
cence (TPIF). Modification of the salophen core to achieve aqueous
solubility for these complexes and their potential application for
tissue image work are currently underway.
Acknowledgments
We thank Hong Kong Baptist University (FRG/07-08/II-58,
FRG2/11-12/007 and IRACE/11-12/04) and the Hong Kong Re-
search Grants Council (HKBU 202307) for the financial support.
W.-K.W. and W.-Y.W. are also thankful for a grant from the Areas
of Excellence Scheme, University Grants Committee of HKSAR,
China (Project No. [AoE/P-03/08]). R.A.J. thanks the Welch
Foundation (Grant F-816) for financial support.
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.poly.2012.09.050.
molecular orbitals of 3a indicates that S8 has
p ?
p⁄ character, cor-
responding to HOMO ꢁ 3 ? LUMO, which is independent of the
donor or acceptor group anchored on the salophen core. The S3
transition state, corresponding to HOMO ꢁ 1 ? LUMO, has an
intramolecular charge transfer (ICT) character. However, the oscil-
lator strength of this state is very weak due to the small spatial
overlap between the HOMO and LUMO. Other states related to
the donor or accepter orbitals also have weak oscillator strength.
When comparing the calculated electronic transitions of symmet-
ric and unsymmetrical metallosalophen complexes 4 and 3a, we
find many more allowed transitions for the latter (with consider-
able oscillator strength and 2PA transition probability). According
to the aforementioned, the molecules are promoted from the
ground state to the same excited state, either by 1PA or 2PA, indi-
cating in turn that the two-photon absorption band matches the
linear absorption one. The major transitions S9 for 4 and S9 for
3a with considerable oscillator strength and TPA transition proba-
bility can be considered as the major intermediate states in the TPA
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