Tuning of the optical properties of chiral Schiff base Zn(II) complexes by substituents

Article abstract of DOI:10.1016/j.poly.2013.04.051

Three new chiral Schiff base zinc(II) complexes, bis(N-R-1-phenylethyl-3- methoxysalicydenaminato) zinc(II), bis(N-R-1-naphthylethyl-3- methoxysalicydenaminato) zinc(II) and bis(N-R-1-naphthylethyl-5- methoxysalicydenaminato) zinc(II), have been prepared and characterized structurally. They are found to have a distorted square planar or compressed tetrahedral trans-[ZnN₂O₂] coordination geometry. Their UV-Vis and CD spectra exhibit intense intraligand transition bands, which are affected by the positions of the substituted methoxy groups of the aldehyde moieties rather than the extension of the π-conjugated ring (phenyl or naphthyl) groups of the amine moieties. A similar dependence of the molecular structures can also be found in the case of the fluorescence spectra, which is in agreement with a comparison with analogous zinc(II) complexes. The results of computational chemistry, which was performed to simulate UV-Vis and CD spectra for these complexes, suggest that the correlation between the effects of substitution and HOMO-LUMO gaps associated with the most intense intraligand transitions can be reasonably explained.

Full text of DOI:10.1016/j.poly.2013.04.051

Polyhedron 59 (2013) 107–114
Contents lists available at SciVerse ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Tuning of the optical properties of chiral Schiff base Zn(II) complexes
by substituents
⇑
Takashi Onodera, Takashiro Akitsu
Department of Chemistry, Faculty of Science, Tokyo University of Science, 1-3 Kagurazaka, Shinjuku-ku, Tokyo 162-8601, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Three new chiral Schiff base zinc(II) complexes, bis(N-R-1-phenylethyl-3-methoxysalicydenaminato) zin-
c(II), bis(N-R-1-naphthylethyl-3-methoxysalicydenaminato) zinc(II) and bis(N-R-1-naphthylethyl-5-
methoxysalicydenaminato) zinc(II), have been prepared and characterized structurally. They are found
to have a distorted square planar or compressed tetrahedral trans-[ZnN2O2] coordination geometry. Their
UV–Vis and CD spectra exhibit intense intraligand transition bands, which are affected by the positions of
Received 10 March 2013
Accepted 24 April 2013
Available online 8 May 2013
Keywords:
Schiff base
Zinc(II)
Chirality
Fluoresence spectra
Computational chemistry
the substituted methoxy groups of the aldehyde moieties rather than the extension of the p-conjugated
ring (phenyl or naphthyl) groups of the amine moieties. A similar dependence of the molecular structures
can also be found in the case of the fluorescence spectra, which is in agreement with a comparison with
analogous zinc(II) complexes. The results of computational chemistry, which was performed to simulate
UV–Vis and CD spectra for these complexes, suggest that the correlation between the effects of substitu-
tion and HOMO–LUMO gaps associated with the most intense intraligand transitions can be reasonably
explained.
Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction
Herein, we have prepared three new chiral Schiff base Zn(II)
complexes (3, 4 and 6) and have undertaken a systematic compar-
In the past decade, molecular logic gates (circuits) have received
much attention as alternative devices to silicon [1]. Following the
first paper on the original concept, a lot of optically-switchable
multifunctional materials or hybrid systems have been reported
for exhibiting molecular logic gates by photo-chemical external
stimuli [2–8]. In the course of work on organic/inorganic hybrid
materials composed of organic photochromic dyes and inorganic
magnetic materials or chiral Schiff base metal complexes [9–15],
we have also designed a molecular logic gate containing spiropy-
ran (SP) (or its photoisomerized form, merocyanine (MC)) and a
chiral Schiff base compound [16] or its Zn(II) complexes (see later
section). In contrast to the organic compounds synthesized, one of
the advantages of organic/inorganic hybrid systems is the inclusion
of photo-switching or molecular aligning functions into non-pho-
toactive inorganic metal complexes without changing the elec-
tronic states or the molecular structures. However, some
problems such as overlap of excitation or emission wavelengths
and overlap of absorption (and the corresponding circular dichro-
ism (CD)) band components and undesirable quenching of the
emission intensity by energy transfer arose in the preliminary
systems.
ison with related known Zn(II) complexes (1, 2 and 5) [17,18],
looking at optical properties and computational studies (Fig. 1).
The working hypothesis of the molecular design is that introducing
electron-donating methoxy substituent groups on the aldehyde
moiety is more effective than introducing
the amine moiety. As for organic/inorganic hybrid materials, the
overlap of intense bands of Zn(II) complexes and dyes at
p-conjugate groups on
p–p
around 390 nm makes it difficult to interpret the results. Not only
fluorescence spectra, but also UV–Vis absorption and the corre-
sponding CD spectra are employed for a comparison with the com-
putational results, which provide the spatial distribution of the
ground state (HOMO in the
p-conjugate moiety) and excited state
(LUMO in the -conjugate moiety) orbitals and their energy gaps.
p
The main purpose of this study is chemical tuning of the optical
properties of chiral Schiff base Zn(II) complexes, aimed at con-
structing molecular logic gates, and the testing as molecular logic
gates is only mentioned as Supplementary data from the viewpoint
of inorganic coordination chemistry.
2. Experimental
2.1. Synthesis
⇑
Chemicals of the highest commercial grade available, purchased
from Aldrich, Wako, TCI and Kanto were used as received without
Corresponding author. Tel.: +81 3 5228 8271; fax: +81 3 5261 4631.
E-mail address: akitsu@rs.kagu.tus.ac.jp (T. Akitsu).
0277-5387/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.poly.2013.04.051

108
T. Onodera, T. Akitsu / Polyhedron 59 (2013) 107–114
831m, 914w, 975w, 1036m, 1076m, 1111m, 1159s, 1220s, 1297m,
1396s, 1437s, 1535w, 1605s (C@N),1624s, 2358w, 2832w, 2976w.
2.2. Physical measurements
Elemental analyses (C, H, N) were carried out with a Perkin–El-
mer 2400II CHNS/O analyzer at the Tokyo University of Science.
Infrared spectra were recorded as Nujol mulls on a JASCO FT-IR
4200 plus spectrophotometer in the range 4000–400 cmÀ1 at
298 K. Absorption spectra were measured on a JASCO V-570 UV/
Vis/NIR spectrophotometer in the range 800–200 nm at 298 K.
Circular dichroism (CD) spectra were measured on a JASCO
J-820 spectropolarimeter in the range 800–200 nm at 298 K. Fluo-
rescence spectra were recorded on a JASCO FP-6200 spectropho-
tometer at 298 K. Photo-illuminations were carried out using a
lamp (1.0 mW/cm2) with optical filters (UV k = 200–400 nm) lead-
ing to a sample using optical fibers and a polarizer through optical
filters.
Fig. 1. Structures of new chiral Schiff base zinc (II) complexes (3, 4, and 6) and
known complexes (1, 5 and 2) for comparison.
2.3. X-ray crystallography
further purification. The known compounds 1 and 5 [17], and 2
[18] were prepared according to the literature procedures.
Pale yellow prismatic single crystals of 3, 4 and 6 were glued on
top of a glass fiber and coated with a thin layer of epoxy resin to
measure the diffraction data. Intensity data were collected on a
Bruker APEX2 CCD diffractometer with graphite monochromated
2.1.1. Preparation of bis(N-R-1-phenylethyl-3-
methoxysalicydenaminato)zinc(II) (3)
Mo Ka radiation (k = 0.71073 Å). Data analysis was carried out
To a solution of o-vanillin (0.62 g, 4.0 mmol) dissolved in meth-
anol (30 mL), (R)-(+)-1-phenylethylamine (0.49 g, 4.0 mmol) was
added dropwise and stirred at 313 K for 2 h to give a yellow solu-
tion of the ligand. Zinc(II) acetate dehydrate (0.44 g, 2.0 mmol) and
triethylamine (0.40 g, 4.0 mmol) were added and stirred for 2 h.
After filtration, pale yellow single crystals suitable for X-ray analy-
sis were obtained. Yield 0.55 g (48.0%). Anal. Calc. for C32H32N2O4Zn:
C, 66.96; H, 5.62; N, 4.88. Found: C, 66.76; H, 5.61; N, 4.86%. IR (KBr,
cmÀ1): 541m, 703s, 743s, 773m, 858m, 922w, 975s, 1039w, 1084s,
1211s, 1239s, 1361s, 1444s, 1548s, 1607s (C@N), 2365w, 2834w,
2924w, 2974, 3040w.
with the SAINT program package. The structures were solved by di-
rect methods with SHELXS-97 [19] and expanded by Fourier tech-
niques and refined by full-matrix least-squares methods based
on F2 using the program SHELXL-97 [19]. An empirical absorption
correction was applied by the program SADABS. All non-hydrogen
atoms were readily located and refined by anisotropic thermal
parameters. All hydrogen atoms were located at geometrically cal-
culated positions and refined using riding models.
Unfortunately, for 6, seriously thermal displacement was ob-
served for methanol molecules of the crystalline solvent, despite
low-temperature measurements. Although the quality of the data
became quite poor, there was sufficient precision for the molecules
of 6 to enable its characterization and starting structure for optimi-
zation. As is often the case with crystal structure analysis, even at
low temperature measurement, crystalline solvent molecules fixed
weakly with intermolecular van der Waals forces or hydrogen
bonds indicate serious thermal motion in the space made from
the main crystal framework of complex molecules. The reason for
high R values is the quite high thermal displacement of the crystal-
line solvent molecules, whilst the important part of complex mol-
ecules are finely analyzed without any problems for the present
purpose and precision, namely confirming the chemical structure
prepared. As for structural characterization, although 1H NMR
spectra were measured for 3, 4 and 6, integration of unclearly re-
solved peaks for the aromatic protons could not be carried out rea-
sonably to the best of our own skills. For this reason, we did not
include 1H NMR data as a part of the characterization data. How-
ever, we have reliably confirmed the preparation of three new
complexes (3, 4 and also 6) just by means of X-ray crystallography,
without any supporting 1H NMR data.
2.1.2. Preparation of bis(N-R-1-naphthylethyl-3-
methoxysalicydenaminato)zinc(II) (4)
To a solution of o-vanillin (0.62 g, 4.0 mmol) dissolved in
methanol (30 mL), (R)-(+)-1-(1-naphthyl)ethylamine (0.68 g,
4.0 mmol) was added dropwise and stirred at 313 K for 2 h to
give a yellow solution of the ligand. Zinc(II) acetate dehydrate
(0.44 g, 2.0 mmol) and triethylamine (0.40 g, 4.0 mmol) were
added and stirred for 2 h. After filtration, pale yellow single crys-
tals suitable for X-ray analysis were obtained. Yield 0.964 g
(71.7%). Anal. Calc. for C40.5H38N2O4.5Zn: C, 70.48; H, 5.55; N,
4.06. Found: C, 70.58; H, 5.62; N, 3.94% (containing 0.5 methanol
as crystalline solvent). IR (KBr, cmÀ1): 508m, 745s, 780s, 809m,
857w, 915w, 982m, 1051m, 1080m, 1109w, 1213s, 1245s,
1335m, 1441s, 1471s, 1510w, 1545m, 1611s (C@N), 2831m,
2902w, 2947w, 2988w, 3048w.
2.1.3. Preparation of bis(N-R-1-naphthylethyl-5-
methoxysalicydenaminato)zinc(II) (6)
To a solution of 2-hydroxy-5-methoxybenzaldehyde (0.62 g,
4.0 mmol) dissolved in methanol (30 mL), (R)-(+)-1-(1-naph-
thyl)ethylamine (0.68 g, 4.0 mmol) was added dropwise and stir-
red at 313 K for 2 h to give a yellow solution of the ligand.
Zinc(II) acetate dehydrate (0.44 g, 2.0 mmol) and triethylamine
(0.40 g, 4.0 mmol) were added and stirred for 2 h. After filtration,
pale yellow single crystals suitable for X-ray analysis were ob-
tained. Yield 1.13 g (64.8%). Anal. Calc. for C40.5H38N2O4.5Zn: C,
70.48; H, 5.55; N, 4.06. Found: C, 70.51; H, 5.50; N, 3.95% (contain-
ing 0.5 methanol as crystalline solvent). IR (KBr, cmÀ1): 775s, 804s,
Crystallographic data for 3. C32H32N2O4Zn, crystal size 0.23 Â
0.14 Â 0.11 mm, Mw = 573.97, tetragonal, space group P41 (#76),
a = 11.4565(12) Å, c = 21.518(2) Å, V = 2824.2(5) Å3, Z = 4, Dcalc
=
1.350 Mg/m3, F(000) = 1200, R1 = 0.0271, wR2 = 0.0745 (4626 reflec-
P
tions), S = 0.625, Flack parameter = 0.006(7) (where R1 = ||Fo| À |Fc||/
P
P
P
|Fo|. Rw = ( w(|Fo| À |Fc|)2/ w|Fo|2)1/2, w = 1/(
r
2(Fo) + (0.1P)2), P =
(Fo2 + 2Fc2)/3).
Crystallographic data for 4. C40H36N2O4Zn, crystal size 0.22 Â
0.18 Â 0.16 mm, Mw = 674.08, monoclinic, space group C2 (#9),
a = 20.216(3) Å, b = 6.7493(9) Å, c = 14.4831(19) Å, b = 124.7220(10)°,

T. Onodera, T. Akitsu / Polyhedron 59 (2013) 107–114
109
V = 1624.2(4) Å3, Z = 2, Dcalc = 1.378 Mg/m3, F(000) = 704, R1 = 0.0225,
wR2 = 0.0623 (3016 reflections), S = 0.488, Flack parameter =
Table 1
Selected geometric parameters for 3.
P
P
P
0.029(7) (where R1 = ||Fo| À |Fc||/ |Fo|. Rw = ( w(|Fo| À |Fc|)2/
Bond distances (Å)
Bond angles (°)
P
w|Fo|2)1/2, w = 1/(
r
2(Fo) + (0.1P)2), P = (Fo2 + 2Fc2)/3).
Zn1–O2
Zn1–O1
Zn1–N1
Zn1–N2
O1–C7
O2–C23
N1–C1
N1–C10
N2–C17
N2–C26
C1–C2
C9–C10
C10–C11
C17–C18
C25–C26
C26–C27
1.9184(14)
1.9300(15)
1.9919(18)
1.9970(16)
1.311(3)
1.306(2)
1.298(3)
1.494(3)
1.291(3)
1.495(2)
1.450(3)
1.527(3)
1.523(3)
1.452(3)
1.518(3)
1.508(3)
O2–Zn1–O1
O2–Zn1–N1
O1–Zn1–N1
O2–Zn1–N2
O1–Zn1–N2
N1–Zn1–N2
C7–O1–Zn1
C23–O2–Zn1
C1–N1–C10
C1–N1–Zn1
C10–N1–Zn1
C17–N2–C26
C17–N2–Zn1
C26–N2–Zn1
N1–C1–C2
114.73(7)
115.03(7)
96.64(7)
96.38(6)
110.19(7)
124.91(8)
123.65(15)
125.16(13)
116.12(19)
120.45(16)
123.43(13)
115.90(16)
120.97(13)
123.09(12)
127.3(2)
Crystallographic data for 6. C40H36N2O4Zn, crystal size 0.18 Â
0.12 Â 0.08 mm, Mw = 674.08, monoclinic, space group P21 (#4),
a = 8.5407(13) Å, b = 30.464(5) Å, c = 13.509(2) Å, b = 96.757(2)°,
V = 3490.5(9) Å3, Z = 4, Dcalc = 1.283 Mg/m3, F(000) = 1408, R1 =
0.1120, wR2 = 0.1514 (3016 reflections), S = 0.862, Flack parame-
P
P
P
ter = 0.024(11) (where R1 = ||Fo| À |Fc||/ |Fo|. Rw = ( w(|Fo| À
P
|Fc|)2/ w|Fo|2)1/2, w = 1/(
r
2(Fo) + (0.1P)2), P = (Fo2 + 2Fc2)/3).
2.4. Computational methods
All calculations were performed using the Gaussian 09 W soft-
ware Revision A.02 (GAUSSIAN, Inc.) [20]. The gas phase geometry
optimizations were carried out using the PM6 functional. The ver-
tical excitation energy was calculated with the ZINDO method
based on the singlet ground state geometry.
O1–C7–C2
125.2(2)
N1–C10–C11
N1–C10–C9
N2–C17–C18
O2–C23–C18
N2–C26–C27
N2–C26–C25
108.69(17)
109.08(19)
127.52(18)
124.71(18)
111.02(17)
108.92(16)
3. Results and discussion
3.1. Crystal structure of 3
3.2. Crystal structure of 4
The molecular structure of 3 is depicted in Fig. 2. Complex 3 af-
fords a distorted tetrahedral trans-[ZnN2O2] coordination geometry
in which the central Zn atom is not located on the center of sym-
The molecular structure of 4 is depicted in Fig. 3. Complex 4 af-
fords a distorted tetrahedral trans-[ZnN2O2] coordination geometry
in which the central Zn atom is not located on the center of sym-
metry. The molecule in an examined crystal adopts the
D(R,R) con-
formation, with an umbrella conformation having stacked benzene
rings. Selected bond distances (Å) and bond or torsion angles (°) are
as follows: Zn1–O2 = 1.9184(14), Zn1–O1 = 1.9300(15), Zn1–
N1 = 1.9919(18), Zn1–N2 = 1.9970(16), O2–Zn1–O1 = 114.73(7),
O2–Zn1–N1 = 115.03(7), O1–Zn1–N1 = 96.64(7), O2–Zn1–N2 =
96.38(6), O1–Zn1–N2 = 110.19(7), N1–Zn1–N2 = 124.91(8). As
listed in Table 1, most of these values are within the normal ranges
for Schiff base zinc(II) complexes [11,17,18,21–68]. The molecular
packing in the solid state is essentially formed by weak van der
Waals forces.
metry. The molecule in an examined crystal adopts the
D(R,R) con-
formation, with an umbrella conformation having stacked benzene
rings. Selected bond distances (Å) and bond or torsion angles (°) are
as follows: Zn1–O1 = 1.9147(12), Zn1–N1 = 1.9944(13), O1–Zn1–
O1 = 113.85(8), O1–Zn1–N1 = 95.95(5), O1–Zn1–N1 = 114.45(5),
N1–Zn1–N1 = 123.37(8). As listed in Table 2, most of these values
are within normal ranges for Schiff base zinc(II) complexes
[11,17,18,21–68]. The molecular packing in the solid state is essen-
tially formed by weak van der Waals forces.
3.3. Crystal structure of 6
The molecular structure of 6 is depicted in Fig. 4. There are two
crystallographically independent molecules (which belong to Zn1
Fig. 2. Molecular structure of 3 showing the selected atom labelling scheme.
Hydrogen atoms are omitted for clarity.
Fig. 3. Molecular structure of 4 showing the selected atom labelling scheme.
Hydrogen atoms are omitted for clarity. Symmetry operation (i) (1 À x, y, 1 À z).

110
T. Onodera, T. Akitsu / Polyhedron 59 (2013) 107–114
Table 2
1.992(4), Zn1–N2 = 1.999(4), Zn2–O4 = 1.902(4), Zn2–O3 =
1.930(4), Zn2–N4 = 2.000(4), Zn2–N3 = 2.019(4), O2–Zn1–O1 =
115.50(15), O2–Zn1–N1 = 120.13(17), O1–Zn1–N1 = 94.87(14),
O2–Zn1–N2 = 98.77(16), O1–Zn1–N2 = 113.51(15), N1–Zn1–N2 =
115.14(16), O4–Zn2–O3 = 117.02(17), O4–Zn2–N4 = 118.39(16),
O3–Zn2–N4 = 95.45(17), O4–Zn2–N3 = 96.44(16), O3–Zn2–N3 =
112.46(16), N4–Zn2–N3 = 118.48(17). As listed in Table 3, most
of these values are within normal ranges for Schiff base zinc(II)
complexes [11,17,18,21–68]. Although the crystals contain metha-
nol molecules as crystalline solvent, the molecular packing in the
solid state is also mainly formed by weak van der Waals forces.
Selected geometric parameters for 4. Symmetry operation (i) (1 À x, y, 1 À z).
Bond distances (Å)
Bond angles (°)
Zn1–O1
Zn1–N1
O1–C1
N1–C7
N1–C9
C1–C6
C6–C7
C8–C9
C9–C10
1.9147(12)
1.9944(13)
1.302(2)
1.290(2)
1.499(2)
1.416(2)
1.443(2)
1.527(3)
1.530(2)
O1–Zn1–O1i
O1–Zn1–N1
O1–Zn1–N1i
N1–Zn1–N1i
C1–O1–Zn1
C7–N1–C9
C7–N1–Zn1
C9–N1–Zn1
O1–C1–C6
O1–C1–C2
O2–C2–C3
O2–C2–C1
N1–C7–C6
N1–C9–C8
N1–C9–C10
113.85(8)
95.95(5)
114.45(5)
123.37(8)
125.47(11)
117.30(14)
121.33(11)
121.29(10)
124.99(15)
117.69(15)
125.45(17)
113.40(15)
127.69(15)
107.67(14)
112.92(13)
3.4. UV–Vis and CD spectra
Fig. 5 depicts the UV–Vis and CD spectra for 1–6, in which the
known complexes (1, 2 and 5) are shown for systematic compari-
son and discussion. As shown in Fig. 1, complexes 1, 3 and 5 have a
phenyl group at R3, while complexes 2, 4 and 6 have a 1-naphthyl
and Zn2) in the asymmetric unit. Complex 6 affords a considerably
distorted tetrahedral trans-[ZnN2O2] coordination geometry in
which the central Zn atom is not located on the center of symme-
Table 3
Selected geometric parameters for 6.
try. The molecule in an examined crystal adopts the
K(R,R) confor-
mation, with an umbrella conformation having stacked benzene
rings. Selected bond distances (Å) and bond or torsion angles (°)
are as follows: Zn1–O2 = 1.898(4), Zn1–O1 = 1.932(3), Zn1–N1 =
Bond distances (Å)
Bond angles (°)
Zn1–O2
Zn1–O1
Zn1–N1
Zn1–N2
Zn2–O4
Zn2–O3
Zn2–N4
Zn2–N3
N1–C15
N1–C30
O1–C21
O2–C22
O3–C3
O4–C13
N3–C8
N3–C54
N4–C1
N4–C64
C3–C2
C1–C2
1.898(4)
1.932(3)
1.992(4)
1.999(4)
1.902(4)
1.930(4)
2.000(4)
2.019(4)
1.293(6)
1.496(5)
1.333(6)
1.329(6)
1.310(6)
1.313(7)
1.292(7)
1.496(6)
1.285(7)
1.481(6)
1.424(7)
1.452(7)
1.419(7)
1.432(7)
1.393(7)
1.404(7)
1.449(6)
1.409(8)
1.460(7)
1.274(6)
1.483(6)
1.516(6)
1.533(8)
1.530(7)
1.546(6)
1.508(7)
1.498(8)
1.526(7)
1.531(8)
O2–Zn1–O1
O2–Zn1–N1
O–Zn1–N1
115.50(15)
120.13(17)
94.87(14)
98.77(16)
113.51(15)
115.14(16)
117.02(17)
118.39(16)
95.45(17)
96.44(16)
112.46(16)
118.48(17)
118.5(4)
121.9(3)
119.6(3)
124.2(3)
123.0(3)
120.1(3)
124.4(3)
116.7(4)
120.4(3)
122.5(3)
121.7(4)
119.7(3)
118.5(3)
125.1(6)
115.5(5)
119.1(5)
124.0(5)
126.0(5)
117
118.2(5)
125.4(5)
127.4(5)
124.6(4)
118.3(4)
127.1(4)
117.4(5)
125.1(5)
121.3(4)
119.9(4)
118.7(3)
127.4(5)
107.6(4)
107.7(4)
116.7(4)
108.6(4)
108.0(4)
111.4(4)
108.4(4)
115.4(5)
O2–Zn1–N2
O1–Zn1–N2
N1–Zn1–N2
O4–Zn2–O3
O4–Zn2–N4
O3–Zn2–N4
O4–Zn2–N3
O3–Zn2–N3
N4–Zn2–N3
C15–N1–C30
C15–N1–Zn1
C30–N1–Zn1
C21–O1–Zn1
C22–O2–Zn1
C3–O3–Zn2
C13–O4–Zn2
C8–N3–C54
C8 N3 Zn2
C54–N3–Zn2
C1–N4–C64
C1–N4–Zn2
C64–N4–Zn2
C7–C6–O6
O6–C6–C5
O3–C3–C4
O3–C3–C2
N4–C1–C2
C13–C9
C8–C9
C16–C17
C16–C21
C16–C15
C23–C22
C23–C28
N2–C28
N2–C42
C31–C30
C29–C30
C41–C42
C42–C44
C56–C54
C54–C53
C65–C64
C64–C63
N4–C1–H1
O4–C13–C14
O4–C13–C9
N3–C8–C9
O1–C21–C16
O1–C21–C20
N1–C15–C16
O2–C22–C26
O2–C22–C23
C28–N2–C42
C28–N2–Zn1
C42–N2–Zn1
N2–C28–C23
N1–C30–C31
N1–C30–C29
N2–C42–C41
N2–C42–C44
N3–C54–C53
N3–C54–C56
N4–C64–C65
N4–C64–C63
Fig. 4. Molecular structure of 6 showing the selected atom labelling scheme.
Hydrogen atoms and crystalline solvent (methanol molecule) are omitted for
clarity. The asymmetric unit consists of two crystallographically independent
molecules ((a) shows Zn1 and (b) shows Zn2).

T. Onodera, T. Akitsu / Polyhedron 59 (2013) 107–114
111
227 nm for 4) and the R2-methoxy pair (at 393 and 225 nm for
5; 395 and 226 nm for 6).
The tendency of the corresponding CD spectral peaks is also in
agreement with the peaks of the UV–Vis spectra. Obvious CD spec-
tral peaks are observed at 442 (positive), 375 (negative), 344 (po-
sitive), 316 (negative), 274 (positive) and 255 (negative) nm for
1, 453 (positive), 376 (negative), 344 (positive), 311 (negative)
and 277 (positive) nm for 2, whose R3 groups of the amine moiety
are different. This tendency is also supported by both the R1-meth-
oxy pair (at 466 (positive), 393 (negative), 353 (positive), 334 (neg-
ative), 280 (positive) and 258 (negative) for 3; 487 (positive), 395
(negative), 354 (positive), 330 (negative), 316 (positive), 301 (neg-
ative), 283 (positive) and 255 (negative) for 4) and the R2-methoxy
pair (at 473 (positive), 403 (negative), 350 (negative shoulder), 278
(positive) and 252 (positive) nm for 5; 461 (positive), 411 (nega-
tive), 374 (positive), 330 (negative), 296 (negative) and 279 (posi-
tive) nm for 6).
3.5. Fluorescence spectra
Figs. 6 and 7 depict the florescence spectra of 1–6, whose exci-
tation wavelengths kex, 360 and 270 nm respectively, are deter-
mined according to the wavelengths of the intense peaks of the
UV–Vis absorption spectra mentioned above. Each pair having
identical R1 and R2 groups exhibit similar emission bands, namely
1 and 2, 3 and 4, and 5 and 6. For excitation at kex = 360 nm, emis-
sion peaks appeared at kem = 451, 458, 486, 490, 489 and 496 nm
for 1–6, respectively. For excitation at kex = 280 nm, emission peaks
appeared at kem = 451, 458, 486, 489, 499 and 502 nm for 1–6,
respectively. It should be noted that only one emission band could
be predominantly observed for 1–6, regardless of two excitation
wavelengths. Thus, the molecular electronic structures associated
with the R1 and R2 groups of the aldehyde moiety rather than
the R3 groups of the amine moiety predominantly dominate the
optical properties of these complexes.
The effect of the substituent groups can be summarized as fol-
lows: the magnitude of the short wavelength shift of the absorp-
tion band (and the corresponding CD band) at 390 nm is
methoxy (R2) and H (R1) > methoxy (R1) and H (R2) > H (R1 and
R2). Of course, the excitation and emission wavelengths of the
Zn(II) complexes are also shifted according to this tendency.
Strictly speaking, the differences in the excitation wavelengths re-
Fig. 5. UV–Vis and CD spectra in 0.05 mM methanol solutions for 1–6.
group at R3, regardless of the R1 and R2 groups. On the other hand,
complexes 1 and 2, complexes 3 and 4, and complexes 5 and 6 have
no methoxy groups at R1 and R2, a methoxy group at R1, and a
methoxy group at R2, respectively. Because of the d10 electronic
configuration of zinc(II) complexes, the electronic and CD spectra
contained bands around 220–380 nm due to just intraligand (p–
p⁄ and n–p⁄) transitions for 1–6. Assignment of the bands of the
experimental spectra [11] is in agreement with and supported by
the present computational results, qualitatively (see later section).
In contrast to R3, substitution of the R1 or R2 groups of the alde-
hyde moiety plays an important role in shifting the peaks. Obvious
UV–Vis spectral peaks are observed at 363, 273, 238 and 223 nm
for 1, and 364, 273 and 225 nm for 2, which indicates that substi-
tuting the R3 group of the amine moiety play a small role in shift-
ing the peaks. This tendency is also supported by both the
R1-methoxy pair (at 380, 283 and 236 nm for 3; 381, 284 and
Fig. 6. Fluorescence spectra in 0.005 mM methanol solutions (kex = ca. 360 nm) for 1–6.

112
T. Onodera, T. Akitsu / Polyhedron 59 (2013) 107–114
Fig. 7. Fluorescence spectra in 0.005 mM methanol solutions (kex = ca. 280 nm) for 1–6.
Fig. 8. (a) Simulated UV–Vis spectra, (b) simulated CD spectra and (c) molecular
orbitals of the ground state (HOMO) and excited state (LUMO) for 3.
Fig. 9. (a) Simulated UV–Vis spectra, (b) simulated CD spectra and (c) molecular
orbitals of the ground state (HOMO) and excited state (LUMO) for 4.
sulted in small differences in the emission wavelengths between
related Zn(II) complexes. The tendency is appropriate for the
reduction of overlapping bands of Zn(II) complexes and MC dyes
[69], resulting in quenching emission intensity due to energy
transfer (hybrid systems for logic gates are mentioned in a later
section). The computational results (see next section) elucidated
that the methoxy groups at the R2 position are effective in making
a large HOMO–LUMO gap in six-membered aldehyde moieties.
Unfortunately, a direct correlation between the substituent groups

T. Onodera, T. Akitsu / Polyhedron 59 (2013) 107–114
113
Table 4
Selected results of calculations for 3–6, having R1 and R2 groups.
3
4
5
6
2
CHOMO2–CLUMO
eHOMO
eLUMO
0.0610
À0.2838
À0.0279
0.2559
0.0610
0.0619
À0.2842
À0.0281
0.2560
0.0619
0.1105
À0.2789
À0.0267
0.2522
0.1105
0.1097
À0.2786
À0.0265
0.2521
0.1097
HOMO–LUMO gap
levels, energy gap and partial electron density of the HOMO and
LUMO for 3–6. These calculated values of the HOMO–LUMO gap
(evaluated from the differences of the squares of the ‘coefficients’
of carbon atoms connected to the methoxy groups, in perturbation
approximation treatment and the assumption of a one-electron ex-
cited configuration) also support the similarity of complexes 3 and
4 and complexes 5 and 6, in other words the importance of the R1
and R2 groups for optical properties. Moreover, both pairs indi-
cated different HOMO–LUMO gap values, which is also in agree-
ment with the longer wavelength shift of the experimental UV–
Vis spec tra for a methoxy group on the R2 site (5 or 6) rather than
the R1 site (3 or 4).
4. Conclusions
In summary, we have newly prepared and characterized three
chiral Schiff base zinc(II) complexes and compared the UV–Vis,
CD and florescence spectra for six compounds, changing the substi-
tuent groups on the amine and aldehyde moieties systematically.
Experimental results of these spectra elucidated that the substitu-
ent position of the methoxy groups on the aldehyde moieties was
more effective for spectral shifts than the substituent groups on
the amine moieties for these six compounds. The calculation of
the HOMO–LUMO gaps with the simulated spectra also theoreti-
cally supports the importance of the substituent position of meth-
oxy groups on the aldehyde moieties.
The present results suggest a reasonable molecular design
methodology for chiral Schiff base zinc(II) complexes as flores-
cence (or chiral) probes employed in logic circuits composed of
photochromic SP (MC) dyes by using intermolecular quenching
of emission (fluorescence spectra for the hybrid systems 3–6 and
SP (MC) are shown in Figs. S4–S7, respectively). Both the molecular
design of each component (corresponding to the present report)
and the material design as a hybrid system are important in order
to construct such a logic circuit, whose concept and whole system
are depicted in Fig. S8.
Fig. 10. (a) Simulated UV–Vis spectra, (b) simulated CD spectra and (c) molecular
orbitals of the ground state (HOMO) and excited state (LUMO) for 6.
Appendix A. Supplementary data
and the intensity of fluorescence spectra could not be elucidated in
this treatment.
CCDC 866099, 866100 and 866101 contain the supplementary
crystallographic data for 6, 4 and 3. These data can be obtained free
of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html, or
from the Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336 033; or e-mail: depos-
it@ccdc.cam.ac.uk. Supplementary data associated with this article
can be found, in the online version, at http://dx.doi.org/10.1016/
j.poly.2013.04.051.
3.6. Computational results
In order to rationalize the importance of the molecular elec-
tronic structure associated with the R1 and R2 groups for 1–6,
(a) simulated UV–Vis spectra, (b) simulated CD spectra and (c)
molecular orbitals of ground and excited states were calculated
and are shown in Figs. 8–10(a)–(c) for 3, 4, and 6, respectively
(for comparison, Figs. S1–S3 are also shown for the known com-
plexes 1, 2 and 5, respectively). For all the compounds, predomi-
nant absorption bands can be assigned to be a transition from
the LUMO to the HOMO, which are distributed on the six-mem-
bered aldehyde moieties and not on the amine moieties. It is a rea-
sonable explanation that the spectral shifts are attributed to the
substitution of R1 and R2 groups. Table 4 summarizes the energy
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