Crystal structure, spectroscopic studies, and nonlinear optical (NLO) properties of Schiff base metal complexes containing an N, S donor ligand

Article abstract of DOI:10.1139/v04-142

A new ligand, 4-dipropylaminobenzaldehyde thiosemicarbazone (abbreviated as HL), and its complexes ML₂ (M = Ni, Pd, and Cu) have been synthesized and fully characterized by elemental analyses, EI-MS, IR and UV-vis spectroscopy, and photoluminescence measurements. All of the compounds exhibit a blue-green color with dual fluorescence in DMF solution. The quantum yield ratio (Φ(CT)/Φ(LE)) decreases in the order NiL₂ > HL > PdL₂ > CuL₂. The ligand HL crystallizes in the monoclinic system, space group P2₁/n, with a = 15.1680, b = 8.5573, and c = 25.3920_A, β = 100.3457°, and V = 3242.2 A³. The nickel complex (NiL₂) crystallizes in the triclinic system, space group P1, with a = 8.3093, b = 9.7540, and c = 10.2802 A, α = 101.383°, β = 107.693°, γ = 96.703°, and V = 764.12 A³. In these complexes, HL has lost a proton from its tautometric thiol form and acts as a single negatively charged bidentate ligand coordinating to the nickel ion via the mercapto sulfur and β-nitrogen atoms. The geometry around Ni(II) is almost square-planar with two equivalent Ni-N and two equivalent Ni-S bonds. The monomeric structure of HL or NiL₂ is extended into an infinite two-dimensional network via hydrogen bonds. The two-photon absorption of HL and ML₂ solutions (in DMF) was measured at 532 nm by the open-aperture Z-scan technique.

Full text of DOI:10.1139/v04-142

1700
Crystal structure, spectroscopic studies, and
nonlinear optical (NLO) properties of Schiff base
metal complexes containing an N, S donor ligand
Zhao–Ming Xue, Ying-Wen Tang, Jie–Ying Wu, Yu–Peng Tian, Min–Hua Jiang,
H.-K. Fun, and A. Usman
Abstract: A new ligand, 4-dipropylaminobenzaldehyde thiosemicarbazone (abbreviated as HL), and its complexes ML2
(M = Ni, Pd, and Cu) have been synthesized and fully characterized by elemental analyses, EI-MS, IR and UV–vis
spectroscopy, and photoluminescence measurements. All of the compounds exhibit a blue–green color with dual fluo-
rescence in DMF solution. The quantum yield ratio (Φ′(CT)/Φ(LE)) decreases in the order NiL2 > HL > PdL2 > CuL2.
The ligand HL crystallizes in the monoclinic system, space group P21/n, with a = 15.1680, b = 8.5573, and c =
25.3920 Å, β = 100.3457°, and V = 3242.2 Å3. The nickel complex (NiL2) crystallizes in the triclinic system, space
group P1, with a = 8.3093, b = 9.7540, and c = 10.2802 Å, α = 101.383°, β = 107.693°, γ = 96.703°, and V = 764.12
Å3. In these complexes, HL has lost a proton from its tautometric thiol form and acts as a single negatively charged
bidentate ligand coordinating to the nickel ion via the mercapto sulfur and β-nitrogen atoms. The geometry around
Ni(II) is almost square-planar with two equivalent Ni—N and two equivalent Ni—S bonds. The monomeric structure of
HL or NiL2 is extended into an infinite two-dimensional network via hydrogen bonds. The two-photon absorption of
HL and ML2 solutions (in DMF) was measured at 532 nm by the open-aperture Z-scan technique.
Key words: crystal structure, metal complex, dual fluorescence, NLO.
Résumé : On a réalisé la synthèse d’un nouveau ligand, la thiosemicarbazone du 4-dipropylaminobenzaldéhyde (abrév.:
HL), et de ses complexes ML2 (M = Ni, Pd et Cu) qu’on a caractérisés par analyse élémentaire, par spectroscopie
« EI-MS », IR et UV–visible ainsi que par des mesures de photoluminescence. En solution dans le DMF, tous les com-
posés présentent une couleur bleu-vert avec double fluorescence. L’ordre des rapports des rendements quantiques
(Φ′(CT)/Φ(LE)) est NiL2 > HL > PdL2 > CuL2. Le ligand HL cristallise dans le système monoclinique, groupe d’espace
P21/n avec a = 15,1680, b = 8,5573 et c = 25,3920 Å, β = 100,3457° et V = 3242,2 Å3. Le complexe de nickel (NiL2)
cristallise dans le système triclinique, groupe d’espace P1, avec a = 8,3093, b = 9,7540 et c = 10,2802 Å, α =
101,383°, β = 107,693° et γ = 96,703° et V = 764,12 Å3. Le ligand HL a perdu un proton à partir de sa forme tauto-
mère thiol et il agit vis-à-vis du nickel comme un ligand bidentate portant une seule charge négative qui conduit à une
coordination avec l’ion nickel par le biais des atomes de soufre du mercapto et de l’azote en β. La géométrie autour du
Ni(II) est pratiquement plan-carré avec deux liaisons Ni—N et Ni—S équivalentes, respectivement. La structure mono-
mère de HL et de NiL2 se répercute dans un réseau bidimensionnel infini par le biais de liaisons hydrogènes.
L’absorption de deux photons à partir de solutions de HL et de NiL2 (dans le DMF) a été mesurée à une longueur
d’onde d’environ 532 nm par la technique du balayage Z à grande ouverture.
Mots clés : structure cristalline, complexe métallique, double fluorescence, NLO.
[Traduit par la Rédaction] Xue et al. 1706
Introduction
Received 5 January 2003. Published on the NRC Research
Press Web site at http://canjchem.nrc.ca on 3 February 2005.
There have been extensive studies of π-conjugated organic
compounds concerning their large two-photon absorption
(TPA) (1–4) and dual fluorescence phenomena (5–8). In
contrast, TPA of transition metal complexes has received lit-
tle attention (9, 10) and there has been virtually no report on
dual fluorescence. In recent years, π-conjugated metal com-
plexes have found applications in many areas of chemistry
and materials science. For example, luminescent metal com-
pounds can function as emitters in light-emitting devices
(11). They are known to be very stable to intense irradiation
in the near-infrared region and have been investigated as
promising optical limiting materials (12, 13). To our knowl-
edge, a metal complex with both efficient dual fluorescence
and TPA has not been reported. The organic chromophore 4-
Z.-M. Xue,1 Y.-W. Tang, J.-Y. Wu, and Y.-P. Tian.2,3
Department of Chemistry, Anhui University, Hefei, 230039,
China.
M.-H. Jiang. State Key Laboratory of Crystal Materials,
Shandong University, Jinan, 250100, China.
H.-K. Fun and A. Usman. X-ray Crystallography Laboratory,
School of Physics, Universiti Sains Malaysia, 11800, Penang,
Malaysia.
1Corresponding author (e-mail: zmxue@ahu.edu.cn).
2Corresponding author (e-mail: yptian@ahu.edu.ca).
3Also at State Key Laboratory of Crystal Materials, Shandong
University, Jinan, 250100, China, and State Key Laboratory
of Coordination Chemistry, Nanjing University, Nanjing,
210093, China.
Can. J. Chem. 82: 1700–1706 (2004)
doi: 10.1139/V04-142
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Xue et al.
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dipropylaminobenzaldehyde thiosemicarbazone, which is a
π-electron delocalized system containing mixed sulfur and
nitrogen donors, was considered as a potential new efficient
compound for obtaining one-photon- and two-photon-
excited dual fluorescence (14) and prepared in our labora-
tory. Following the success of our initial work (15–18), we
are particularly interested in further using it as a ligand to
coordinate to transition metal ions to form metal complexes
that show efficient dual fluorescence and TPA. We have se-
lected Ni2+, Pd2+, and Cu2+ as the central metal ions to form
metal complexes with it. In this paper, we present the dual
fluorescence and two-photon absorption properties of the
complexes.
Syntheses of ML2
M(OAc)2@nH2O (M = Ni(II), Cu(II)) or PdCl2(CH3CN)2
(1 mmol) and HL (2 mmol) were disso1ved in ethanol
(50 mL). After refluxing for 4 h, the mixture was cooled to
room temperature. The precipitate was thoroughly washed
with ethanol three times and dried in vacuo over P2O5. Yield
90%. Anal. calcd. for C28H42N8S2Ni (%): C 54.63, H 6.83,
N 18.21; found: C 54.64, H 6.82, N 18.24. mp 249-250 °C.
Black crystals of NiL2 were isolated by slow evaporation of
a CH2Cl2 solution covered with ethanol. Anal. calcd. for
C28H42N8S2Pd (%): C 50.11, H 6.23, N 17.31; found: C
50.23, H 6.34, N 17.41. Anal. calcd. for C28H42N8S2Cu (%):
C 53.66, H 6.67, N 18.54; found: C 53.82, H 6.74, N 18.32.
X-ray crystal structure determinations
Theory
A crystal of HL with approximate dimensions 0.50 ×
0.28 × 0.20 mm3 and a crystal of complex NiL2 with ap-
proximate dimensions 0.44 × 0.38 × 0.22 mm3 were used for
the X-ray structure determinations. Data were collected on a
Siemens SMART-CCD area detector diffractometer using
Mo Kα radiation in ω-scan mode at 293 K. The data collec-
tion covered a hemisphere of reciprocal space by a combina-
tion of three sets of exposures; each set had a different φ
angle (0.88° and 180°) for the crystal and each exposure of
10 s. The crystal-to-detector distance was 4 cm, and the de-
tector swing angle was –35°. Coverage of the unique set is
over 99% complete. Crystal decay was monitored by repeat-
ing 30 initial frames at the end of data collection and analyz-
ing the duplicate reflections and was found to be negligible.
The unit cell parameters were determined using SMART
(21). The three sets of data collected were reduced using the
program SAINT (22). The structure was solved with direct
methods using the program SHELXTL (23). All the non-
hydrogen atoms were located from the trial structure and
then refined anisotropically with SHELXTL using a full-
matrix least-squares procedure. Weighting schemes of w =
Dual fluorescence
4-(N,N-Dimethylamino)benzonitrile (DMABN) is the pro-
totype of a group of organic donor–acceptor compounds that
can undergo intramolecular charge transfer (ICT) in the ex-
cited singlet state. In polar solvents, it exhibits dual fluores-
cence. Apart from normal emission from the locally excited
(LE) state, which is also present in the gas phase and in
nonpolar solvents, the spectrum observed in polar solvents
exhibits a second, anomalous strongly red-shifted band,
which is attributed to a charge-transfer (CT) state (19). This
phenomenon has been mostly explained by the twisted intra-
molecular charge transfer (TICT) mechanism (20). In the
TICT model, a 90° twist of the dimethylamino group with
respect to the phenyl ring was considered to have occurred
in the CT state of DMABN. The dimethylamino group was
thereby assumed to be electronically decoupled from the rest
of the molecule — the principle of minimum overlap. The
CT/LE fluorescence quantum yield ratio Φ′(CT)/Φ(LE) in-
creases with the efficiency of the ICT reaction.
2
1/[σ2(Fo ) + (0.0777P)2], with P = [(Fo)2 + 2(Fc)2]/3, and
2
w = 1/[σ2(Fo ) + (0.1207P)2], with P = [(Fo)2 + 2(Fc)2]/3,
Experimental
were applied for HL and NiL2, respectively. The hydrogen
atoms were geometrically fixed at calculated positions at-
tached to their parent atoms and were treated as riding on
them.
General procedures
All chemicals and solvents were dried and purified by
usual methods. Elemental analyses were performed with a
Perkin-Elmer 240 instrument. IR spectra were recorded with
a Nicolet FT-IR 170SX instrument (KBr discs) in the 4000–
400 cm–1 region. UV–vis spectra were recorded with a
Shimadzu UV-265 spectrophotometer. The photoluminescence
measurements were carried out in DMF solutions, and the
spectra were collected with a Perkin-Elmer LS55 spectro-
fluorimeter. Mass spectra were obtained with a ZAB-HS
mass spectrometer (FAB source).
Results and discussion
Description of the structures of HL and NiL2
Selected crystallographic experimental data for HL and
NiL2 are given in Table 1. The ORTEP diagrams of HL and
NiL2 with the atom numbering schemes are shown in Fig.1.
Selected bond lengths and angles are listed in Tables 2 and
3. A view of the molecular packing of HL in the crystal is
given in Fig. 2. Additional crystallographic data are avail-
able as Supplementary material.4
Strategy of syntheses of HL and ML2
4-Dipropylaminobenzaldehyde and 4-dipropylaminoben-
zaldehyde thiosemicarbazone (HL) were synthesized as de-
scribed previously (14).
The ORTEP drawing of HL shows that there are two dif-
ferent structures of HL, cis and trans (refer to the structures
of dipropyl). The corresponding bond lengths and angles of
4 Supplementary data may be purchased from the Directory of Unpublished Data, Document Delivery, CISTI, National Research Council
Canada, Ottawa, ON K1A 0S2, Canada (http://www.nrc.ca/cisti/irm/unpub_e.shtml for information on ordering electronically). CCDC
200310 and 200311 contain the crystallographic data for this manuscript. These data can be obtained, free of charge, via
www.ccdc.cam.ac.uk/conts/retrieving.html (or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ,
U.K.; fax +44 1223 336033; or deposit@ccdc.cam.ac.uk).
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Can. J. Chem. Vol. 82, 2004
Table 2. Selected bond lengths (Å) for HL and NiL2.
Table 1. Selected crystallographic experimental data for HL and
NiL2.
HL
NiL2
HL
NiL2
Ni(1)—N(2)
Ni(1)—N(2A)
Ni(1)—S(1)
Ni(1)—S(1A)
S(1)—C(8)
N(1)—C(3)
N(1)—C(12)
N(1)—C(9)
N(2)—C(7)
N(2)—N(3)
N(3)—C(8)
N(4)—C(8)
C(1)—C(2)
C(6)—C(7)
1.937(3)
1.937(3)
2.1629(14)
2.1629(14)
1.722(4)
1.358(6)
1.448(8)
1.628(10)
1.299(5)
1.401(5)
1.279(6)
1.356(6)
1.383(6)
1.450(6)
Formula
C28H44N8S2
C28H44N8S2Ni
Formula weight
Crystal system
Space group
556.85
Monoclinic
P21/n
0.50 × 0.28 ×
0.20
15.1680(8)
8.5573(5)
25.3920(10)
—
100.3457(5)
—
3242.2(3)
4
615.54
Triclinic
P1
0.44 × 0.38 ×
0.22
8.3093(1)
9.7540(3)
10.2802(2)
101.383(1)
107.693(1)
96.703(1)
764.12(3)
1
1.683(4)
1.369(4)
1.457(5)
1.480(5)
1.255(4)
1.372(4)
1.334(4)
1.327(4)
1.371(4)
1.452(5)
Crystal dimensions (mm)
a (Å)
b (Å)
c (Å)
α (°)
β (°)
γ (°)
V (Å3)
Z
µ (mm–1)
0.194
1.145
1208
8221
22 330
0.0769
0.1598
0.331, –0.337
0.804
1.338
328
3565
5423
0.0927
0.2282
0.929, –1.179
Table 3. Selected bond angles (<) for HL and NiL2.
d (calcd.) (g cm–3)
F(000)
HL
NiL2
Unique reflections
Reflections observed
R
N(2)-Ni(1)-N(2A)
N(2)-Ni(1)-S(1)
N(2A)-Ni(1)-S(1)
N(2)-Ni(1)-S(1A)
N(2A)-Ni(1)-S(1A)
S(1)-Ni(1)-S(1A)
C(8)-S(1)-Ni(1)
C(7)-N(2)-Ni(1)
N(3)-N(2)-Ni(1)
180.00(2)
85.09(10)
94.91(10)
94.91(10)
85.09(10)
180.00(6)
97.50(15)
124.60(3)
120.3(3)
123.0(5)
121.1(5)
115.4(5)
115.1(3)
113.5(3)
Rw
∆(ρ)max,min (e Å–3)
the two structures are not distinctly different. The bond
lengths of N(2)—N(3) (1.372 Å) and C(8)—N(3) (1.334 Å)
suggest that these bonds have some double bond character
but are nearly single bonds. The lengths of S(1)—C(8)
(1.683 Å) and C(7)—N(2) (1.255 Å) suggest that these
bonds have some double bond character. All the carbon at-
oms in the benzene ring are almost coplanar (mean deviation
form the plane 0.0071 Å), and C(7), N(2), N(3), C(8), N(4),
and S(1) are also nearly coplanar (mean deviation from the
plane 0.0091 Å). The dihedral angle of the two planes
is 3.31° and shows that they are nearly coplanar. All
these crystallographic data suggest that there is a highly π-
conjugated and electron-delocalized system in the molecule.
The nickel atom is coordinated in a square-planar configu-
ration with two equivalent Ni—N bonds (l.937 Å) and two
equivalent Ni—S bonds (2.l63 Å). The HL has lost a proton
from its tautomeric thiol form and acts as a single negatively
charged bidentate ligand coordinating to the nickel ion via
the mercapto sulfur and β-nitrogen atoms. S(1), N(2), S(1a),
N(2a), and Ni(II) are almost coplanar, as are S(1), C(8),
N(3), N(2), and Ni(II) (mean deviation from the plane
0.0048 Å), and these two planes form a dihedral angle of
9.6°. The very small dihedral angle indicates the high extent
of delocalization of electrons in the π-conjugated system of
the two Schiff base ligands and the coordination system.
The monomeric structure of HL is extended into an infi-
nite two-dimensional network via hydrogen-bond contacts
between two neighboring molecules (Fig. 3). There are no
intramolecular donor hydrogen bonds. A similar case exists
for NiL2. For HL, four hydrogen atoms attached to amino
nitrogen atoms N(3A), N(3B), N(4A), and N(4B) have the
potential to form donor hydrogen bonds (Fig. 3). Two hydrogen
atoms attached to N(4A) and N(3B) formed intermolecular
C(3)-N(1)-C(12)
C(3)-N(1)-C(9)
C(12)-N(1)-C(9)
C(7)-N(2)-N(3)
C(8)-N(3)-N(2)
122.5(3)
120.0(3)
117.5(3)
115.1(3)
122.5(3)
donor hydrogen bonds of the types 4AB (N(4A)-HþS(1B)
(1 – x, 1 – y, 1 – z)) and 3BA (N(3B)-HþS(1A) (1 – x, 1 – y,
1 – z)). The NþS separations are 3.478(3) Å for
N(4A)þS(1B) and 3.472(6) Å for N(3B)þS(1A). The N-HþS
angles are 161.7(4)° for N(4A)-HþS(1B) and 157.6(6)° for
N(3B)-HþS(1A). Two hydrogen atoms attached to N(3A)
and N(4B) also formed intermolecular donor hydrogen
bonds of the types 3AB (N(3A)-HþS(1B) (1 – x, –y, 1 – z))
and 4BA (N(4B)-HþS(1A) (1 – x, –y, 1 – z)). The NþS
separations are 3.430(4) Å for N(3A)þS(1B) and 3.614(0) Å
for N(4B)þS(1A). The NþS angles are 156.1(0)° for N(3A)-
HþS(1B) and 163.6(2)° for N(4B)-HþS(1A).
For NiL2, one hydrogen atom attached to amino nitrogen
atom N(4) has the potential to form a donor hydrogen bond.
The hydrogen atom attached to N(4) formed an intermole-
cular donor hydrogen bond of the type N(4)-HþN(3) (–x,
–1 – y, –z). The N(4)þN(3) separation is 3.307(4) Å, and the
N(4)-HþN(3) angle is 149.8(1)°.
It is interesting to compare the structure of HL in the free
state with that in the nickel complex. The largest difference
is found in the bond lengths around C(8). In the free ligand,
the bond distances of C(8)—N(3) (1.334 Å) and C(8)—S(1)
(1.683 Å) suggest that C(8)—N(3) has nearly single bond
character and C(8)—S(1) has nearly double bond character,
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Xue et al.
1703
Fig. 1. ORTEP drawings of HL and NiL2 with the atom numbering schemes.
are almost the same length (1.452(5) and 1.450(6) Å,
respectively). These results, together with the spectral char-
acterization of the nickel complex, indicated the presence of
the C–S–M group formed by enolization of the -NH-C=S
group in the free ligand to -N=C-SH and coordination to the
metal ion through the sulfur after deprotonation (24, 25).
The deprotonation and formation of the metal complexes
made the configuration of the ligand quite different. The
ligand undergoes some conformational changes during the
complex formation. Figure 4 shows the superposition of the
ligand in the complex and in the free state. The major
conformational changes are in the NH2NHC(S)NH2 moiety.
The trans → cis and cis → trans (referring to the structure of
NC(S)N) transitions around certain bonds facilitate the coor-
dination of HL to the metal ion as a bidentate ligand via the
β-nitrogen atom and the thio S atom.
Fig. 2. A view of the molecular packing of HL in the crystal.
while in the complex the bond distances of C(8)—N(3)
(1.279 Å) and C(8))—S(1) (1.722 Å) show that C(8)—N(3)
is a double bond and C(8)—S(1) is a single bond. Com-
paring the bond lengths for C(7)—N(2) and C(6)—C(7) in
HL with those in NiL2, C(7)—N(2) is a shorter bond in HL
(1.225 Å) than in NiL2 (1.299 Å) and the C(6)—C(7) bonds
FT-IR spectroscopy of HL and ML2
FT-IR data for HL and ML2 (M = Ni, Pd, Cu) are listed in
Table 4. The spectrum of HL exhibits a strong band at
3152 cm–1, which can be assigned to
The peak disappears in the IR spectra of the metal com-
νN-H
of the free ligand.
plexes, which suggests that the proton on the α-nitrogen
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Can. J. Chem. Vol. 82, 2004
Fig. 3. H-bonds between two neighboring HL molecules.
Fig. 4. Superposition of the ligand molecule in the complex
(solid lines) and the free state (dashed lines).
Table 4. FT-IR data of HL and ML2.
νN-H
νC=S
νC=N
νC-N
νN-N
νPh
HL
NiL2
3152
—
1046
—
1589
1571
1291
1312
955
965
1518
1516
PdL2
CuL2
—
—
—
—
1578
1576
1299
1298
964
965
1514
1516
These FT-IR data for HL and ML2 (M = Ni, Pd, Cu) as
well as the results of elemental analyses and MS, together
with the crystal structures of HL and NiL2, show that the
Schiff base ligand formed stable metal complexes with the
three metal ions. The ligand HL was deprotonated on
complexation to the divalent M(II), forming the stable neu-
tral complex ML2 (as shown in Scheme 1).
atom is lost upon complex formation with the metal ion. A
strong band at 1291 cm–1 in the IR spectrum of HL is as-
signed to νC-N. This band is shifted to higher frequencies by
ca. 11–19 cm–1 in the IR spectra of the metal complexes.
This increase of νC-N on complexation is attributed to an in-
crease of the C(8)—N(3) bond order as a result of depro-
tonation of the α-nitrogen atom. This tendency of HL to
deprotonate may be attributed to the stabilization of the
deprotonated form by conjugation of the -C=N-N=C– group.
A strong band at 1046 cm–1 in the IR spectrum of HL is ten-
tatively assigned to νC=S. This band is absent in the spectra
of the metal complexes. This observation can be explained
by the change in the nature of the C(8)—S(3) bond on coor-
dination of the ligand through the sulfur atom. This is
proven by the crystal structure determinations for HL and
NiL2, which yielded C(8)—S(3) bond lengths of 1.683(4)
and 1.722(4) Å, respectively. The peak at ca. 1589 cm–1 may
be safely assigned to the C(7)=N(2) stretching vibration of
the azonal group, since the frequency of this band is lowered
by ca. 14–18 cm–1 in the spectra of the metal complexes.
This decrease in νC=N on complexation is attributed to a de-
crease in the C=N bond order as a result of M–N bond for-
mation. The very strong peak at ca. 1518 cm–1 in the IR
spectra of HL is attributed to the aromatic C=C vibration.
The band is very little affected upon complex formation.
Absorption spectra
The electronic absorption spectra of HL, NiL2, PdL2, and
CuL2, measured in DMF solution (d0 = 1.0 × 10–5 mol/L),
are shown in Fig. 5. HL, NiL2, PdL2, and CuL2 all exhibit an
intense and low-lying (near-UV region) absorption band and
at least two peaks between 250 and 380 nm, with tailing at
long wavelengths. The large molar absorption coefficients
(log ε = 3.8–4.8) of the near-UV band are indicative of
highly π-onjugated systems. The two peaks at shorter wave-
lengths are at 264 and 366 nm for HL, 267 and 377 nm for
NiL2, 266 and 374 nm for PdL2, and 266 and 376 nm
for CuL2. The shorter-wavelength peak in each couple is the
π→π* absorption of larger π-conjugated systems, and the
second peak is the n→π* absorption associated with p–π
conjugation of nitrogen lone-pair electrons with benzene. An
additional peak due to a metal-to-ligand charge transfer
(MLCT) absorption is observed at 425, 393, and 394 nm in
the spectra of NiL2, PdL2, and CuL2, respectively. There is
no linear absorption in the wavelength range from ~500 to
800 nm.
Comparing the two peaks between 250 and 380 nm in the
spectra of HL with those in the spectra of ML2, the shorter-
wavelength peak for ML2 is red-shifted by about 2–3 nm
compared to that of HL, and the longer-wavelength peak is
red-shifted by about 8~11 nm. Such red shifts are consistent
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Xue et al.
1705
Fig. 6. Open-aperture Z-scan data. Normalized transmittance of
Scheme 1. Strategy of syntheses of HL and ML2.
ML2.
Table 5. TPA coefficients β and cross sections σ (in DMF solu-
tion).
β (cm/GW)
σ × 1046 (cm4 s/photon)
Fig. 5. Room-temperature UV–vis absorption and emission spec-
tra of HL and ML2 in DMF solution: (a) HL, one-photon excita-
tion (OPE) at 400 nm; (b) NiL2, OPE at 400 nm; (c) PdL2, OPE
at 434 nm; (d) CuL2, OPE at 337 nm. Dotted curves represent
“anomalous” emission from the CT state.
HL
NiL2
—
0.47
—
0.84
PdL2
CuL2
0.48
0.51
0.98
0.99
electronic density on the larger π-conjugated system and the
p–π conjugation.
One-photon-excited dual fluorescence
The one-photon steady-state emission spectra of HL,
NiL2, PdL2, and CuL2, measured in DMF solutions (d0 =
1.0 × 10–7 mol/L), are shown in Fig. 5. HL, NiL2, PdL2, and
CuL2 emit a blue–green color with dual peaks and a large
Stokes shift (LE peak, ~30–180 nm) when irradiated (one-
photon processes) by UV at 400, 320, 320, and 337 nm,
respectively. The characteristic benzaldehyde moiety bands
had peaks at 523 and 430 nm for HL, 507 and 488 nm for
NiL2, 493 and 444 nm for PdL2, and 444 and 434 nm for
CuL2. The quantum yield ratio Φ′(CT)/Φ(LE) (d0 = 1.0 ×
10–7 mol/L) decreased in the order NiL2 (2.12) > HL
(0.79) > PdL2 (0.45) > CuL2 (0.43).
Two-photon absorption
The two-photon absorptions of HL and ML2 solutions (in
DMF) were measured by use of the open-aperture Z-scan
technique (26) at a wavelength of 532 nm and are shown in
Fig. 6. The normalized transmittance T(z) in Fig. 6 was ana-
lyzed theoretically with the following equation:
∞
[1 − q0(z)]m
T(z, s = 1) =
for q0 < 1
∑
(m +1)3/ 2
m=0
where s = 1 indicates an open aperture, and q0(z) =
2
βI0(t)Leff/(1 + z2/z0 ),where β is the TPA coefficient, I0(t) is
the intensity of the laser beam at the focus (z = 0), Leff = [1 –
exp(–α0L)]/α0 is the effective thickness with α0 the linear
absorption coefficient and L the sample thickness, z0 is the
diffraction length of the beam, and z is the sample position.
Nonlinear absorption of ML2 in an open-aperture Z-scan,
which is used to observe third-order phenomena that all mol-
ecules exhibit, can be explained mainly by the TPA mecha-
with the fact that when the N and S atoms of HL are coordi-
nated to M(II), the new rings enhance the π-electron de-
localization along the unsaturated system and reinforce the
© 2004 NRC Canada

1706
Can. J. Chem. Vol. 82, 2004
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Conclusions
We have synthesized a new class of novel blue lumines-
cent complexes based on a Schiff base. All compounds emit
a blue-green color with dual fluorescence in DMF solution
and show a large Stokes shift (LE peak, ~30–180 nm). The
quantum yield ratio Φ′(CT)/Φ(LE) (d0 = 1.0 × 10–7mol/L) de-
creases in the order NiL2 (2.12) > HL (0.79) > PdL2 (0.45) >
CuL2 (0.43). All of the compounds exhibit good one-photon
fluorescent properties. The complexes have more highly π-
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Chin. J. Inorg. Chem. 10, 1009 (2002).
This work was supported by the National Natural Science
Foundation of China (50272001, 50335050, 50325311) and
the Natural Science and Person with Ability Foundation of
Anhui Province (03044701, 2002Z021). H.K.F. thanks the
Malaysian Government and Universiti Sains Malaysia for
the research grant R&D No. 305/PFIZIK/610961.
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© 2004 NRC Canada