Studies of the isomerization and photophysical properties of a novel 2,2′:6′,2′′-terpyridine-based ligand and its complexes

Article abstract of DOI:10.1039/c1dt10752f

A novel 2,2′:6′,2′′-terpyridine-based ligand L and its complexes [ML₂](ClO₄)₂·CH ₂Cl₂ (M = Cd 1, Zn 2, Cu 4, Mn 5), [CoL ₂](ClO₄)₂3, CdLI₂6 and CdL(SCN)

Full text of DOI:10.1039/c1dt10752f

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PAPER
Studies of the isomerization and photophysical properties of a novel
2,2¢:6¢,2¢¢-terpyridine-based ligand and its complexes†
Dongmei Li,a,b Qiong Zhang,a Peng Wang,a Jieying Wu,a Yuhe Kan,c Yupeng Tian,*a,d,e Hongping Zhou,*a
Jiaxiang Yang,a Xutang Taoc and Minhua Jiangd
Received 25th April 2011, Accepted 20th May 2011
DOI: 10.1039/c1dt10752f
A novel 2,2¢:6¢,2¢¢-terpyridine-based ligand L and its complexes [ML2](ClO4)2·CH2Cl2 (M = Cd 1, Zn 2,
Cu 4, Mn 5), [CoL2](ClO4)2 3, CdLI2 6 and CdL(SCN)2 7 were synthesized and fully characterized. The
crystal structures of 1–6 were solved by single crystal X-ray diffraction analysis. The linear absorption
and emission properties, and third-order nonlinear optical (NLO) properties of all the complexes were
systematically investigated. The equilibrium of the trans- and cis- isomers of L was studied both
experimentally and theoretically. The configurations and photophysical properties of the complexes
display a large dependence on the choice of metal ions and anions.
number density of two-photon active components (i.e., ligands),
or as an important structural control in the intramolecular charge-
transfer process, leading to an enhanced s value.
Introduction
Since the 1990s, increasing attention has been devoted to two-
photon absorption (2PA) materials because of their applications
The control of the molecular geometry as well as the electronic
in photonic and biophotonic areas, such as three-dimensional
structure, which are interconnected, is of crucial importance to
optical data storage and microfabrication,1 power limiting,2 up-
optimize physical properties of the coordination compounds. The
conversion lasing,3 three-dimensional fluorescence imaging,4 and
architectures of them rest on several factors, but herein, we mainly
photodynamic therapy.5 For this reason, extensive efforts have
discuss the influences from the ligand, metal centers and anions.
been concentrated on the synthesis of various materials with
Firstly, the design of ligands is a useful way of manipulating the
large 2PA cross-sections (s), such as polymers,6 small organic
structures. For example, 2, 2¢:6¢,2¢¢-terpyridine (tpy) derivatives
molecules,7 semiconductors or metal nanoparticles8 and coordina-
are important tridentate ligands, which can be incorporated into
tion compounds.9 Recently, material design directed towards 2PA
various supramolecular structures, such as for the preparation
optimization has had extensive research for certain applications.10
of wire-type components, helicates, molecular cycles, wheels and
scorpionates.11 The three conformations observed in the tpy ligand
are well documented (Scheme 1a).12 Although a large number
of tpys and their metal complexes have been synthesized,11,13 the
studies of the third-order nonlinear optical (NLO) properties and
structure–property relationships of them are scarcely reported.
Based on our interest in searching for the optimized 2PA molecules
having large s value, we selected the tpy unit as a strong acceptor
(A) group with three pyridine rings preferring a planar geometry
to achieve maximum conjugation, which is helpful for the p-
electron delocalization. Phenothiazine (PTZ) was chosen as the
donor group (D) as PTZs are well-known dyes, antioxidants, and
pharmaceuticals.14 Compared with the coplanar molecules, the
nonplanar PTZ ring is a stronger electron donor and shows better
2PA properties.7d,9c Here we use a styryl moiety as the p-bridge
to build a novel ligand 10-ethyl-3-[4-(2, 2¢:6¢,2¢¢-terpyridinyl-4¢-
yl) styryl] phenolthiazine (L), in which the vinyl double bond
can adopt cis or trans configurations (Scheme 1b); the tpy unit,
tridentate ligand, can easily coordinate to various metal ions.
The metal-ion binds to the acceptor, forming a D–p–A motif
ligand, rather than a donor site, it should yield an increase
rather than decrease of intraligand charge transfer (ICT) upon
Additionally, good physical and chemical stabilities are important
to a novel material as an applicable material and must be addressed
in parallel to continuing efforts to enhance the effective s values.
In contrast with organic molecules, coordination compounds can
be easily synthesized in high yield and are expected to show better
physical and chemical stability, in which the metal ions can serve
either as a multidimensional template for increasing the molecular
aDepartment of Chemistry, Anhui Province Key Laboratory of Func-
tional Inorganic Material Chemistry, Anhui University, Hefei, 230039,
P. R. China. E-mail: yptian@ahu.edu.cn, zhpzhp@263.net; Fax: +86-551-
5107304; Tel: +86-551-5108151
bHenan Electric Power Research Institute, Zhengzhou, 450052, P. R., China
cDepartment of Chemistry, Jiangsu Province Key Laboratory for Chemistry
of Low-Dimensional Materials, Huaiyin Teachers College, Huaian, 223001,
P. R. China. E-mail: yhkan@yahoo.cn
dState Key Laboratory of Crystal Materials, Shandong University, Jinan,
250100, P. R. China
eState Key Laboratory of Coordination Chemistry, Nanjing University,
Nanjing, 250100, P. R. China
† Electronic supplementary information (ESI) available: DFT frontier
orbital components available. CCDC reference numbers 660614, 673758,
700924–700926 and 700929. For ESI and crystallographic data in CIF or
other electronic format see DOI: 10.1039/c1dt10752f
8170 | Dalton Trans., 2011, 40, 8170–8178
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All of the reactions were monitored by TLC (silica gel plates,
GF254). Silica gel 60 (100–200 mesh) was used for column
chromatography. Elemental analyses were carried out on a Perkin–
Elmer 240 analyzer. IR spectra were recorded on a Nicolet FT-
1
IR-instrument with KBr discs in the 4000 ~ 400 cm-1 range. H-
NMR spectra were performed on a Bruker 400 Hz Ultrashield
spectrometer and were reported as parts per million (ppm) from
TMS (d). The linear absorption spectra were measured on a UV-
265 spectrophotometer. The linear emission spectra and fluores-
cence quantum yields were performed using F-2500 fluorescence
spectrophotometer. The concentration of the sample solution was
1.0 ¥ 10-5 M. NLO measurements were investigated with 532 nm
laser pulses of 18 ns duration in 1.5 ¥ 10-4 M DMF solution.
2
X-ray crystallography and structure solution
Single crystal X-ray diffraction measurements were carried out
on a Siemens Smart 1000 CCD diffractmeter equipped with a
graphite crystal monochromator situated in the incident beam
for data collection at room temperature. The determinations of
unit cell parameters and data collections were performed with
Mo-Ka radiation. Unit cell dimensions were obtained with least-
squares refinements, and all structures were solved by direct
methods with SHELXS–97 and refined with SHEXLX–97. All
the non-hydrogen atoms were located in successive difference
Fourier syntheses. The final refinement was performed by full-
matrix least-squares methods with anisotropic thermal parameters
for non-hydrogen atoms on F2. The hydrogen atoms were added
theoretically and riding on the concerned atoms.
Scheme 1 (a) Three conformations of the terpyridine unit, (b) trans and
cis tautomerization of the ligand L.
excitation. Consequently, metal-ion binding is expected to result
in an increased s and enhanced fluorescence brightness.15,16
Secondly, different metal ions can influence the structures and
photophysical properties of resulting complexes to a large extent.
Therefore, in the present work, a series of metal complexes were
synthesized based on the designed ligand above. The isomerization
and photophysical properties of L and its complexes were investi-
gated, along with time-dependent density functional theory (TD-
DFT) computational studies to elucidate the electronic structures
of the ground and excited states of the two isomers of L.
3
Synthesis of 10-ethyl-3-[4-(2,2¢:6¢,2¢¢-terpyridinyl-4¢-yl)
styryl]phenothiazine L
t-BuOK (1.3 g, 12.3 mmol), 4-(2,2¢:6¢,2¢¢-terpyridinyl-4¢-yl)-
benzyltriphenyl-phosphonium bromide a17 (2.0 g, 3.0 mmol) and
10-ethylphenothiazine-3-carbaldehyde b18 (0.8 g, 3.1 mmol) were
added to a dry mortar and milled vigorously for about 35 min.
And then the mixture was washed with water and extracted
with 3 ¥ 100 mL CH2Cl2. The crude product was purified by
chromatography on a silica gel with CH2Cl2/methanol mixture
(20 : 1 v/v) as the eluent. Yellow solid was collected, yield: 0.71 g
Experimental section
1
General procedures
All chemicals were purchased as reagent grade and used without
further purification. The solvents were dried and distilled accord-
ing to standard procedures. The synthetic routes towards L and
its complexes are described in Scheme 2.
◦
(42%). Mp: 386 C. Anal. Calcd for C37H28N4S (560.7): C 79.26,
H 5.04, N 9.99, S 5.71; found: C 79.04, H 5.20, N 9.88, S 5.88. IR
(KBr): n = 1583 (vs), 1564 (s), 1467 (vs), 1443 (m), 1385 (s), 1250
(m), 791 (m), 749 (m), 620 (w). 1H NMR (400 MHz, d6-DMSO): d
1.29 (t, 3H, J = 6.78, 6.78 Hz), 3.90 (q, 2H, J = 6.27, 6.78, 6.27 Hz),
6.93 (t, 1H, J = 7.53, 7.53 Hz), 6.99 (d, 2H, J = 8.78 Hz), 7.14 (d,
1H, J = 7.53 Hz), 7.17–7.26 (m, 3H), 7.42 (t, 2H, J = 5.27, 5.27 Hz),
7.51 (t, 2H, J = 5.78, 5.53 Hz), 7.72 (d, 2H, J = 8.03 Hz), 7.89 (d,
2H, J = 8.03 Hz), 8.02 (t, 2H, J = 7.53, 7.53 Hz), 8.65 (d, 2H,
J = 7.78 Hz), 8.71 (s, 2H), 8.76 (d, 2H, J = 4.02 Hz). MS: m/z =
560. 13C NMR (100 MHz, d6-DMSO): d 13.06, 41.69, 115.90,
118.02, 118.19, 121.43, 122.79, 122.97, 123.56, 124.99, 125.14,
126.40, 126.92, 127.50, 127.64, 128.16, 128.82, 131.84, 136.39,
137.95, 139.02, 144.32, 149.40, 149.80, 155.45, 156.15, 162.775.
4
Synthesis of the complexes
L (56.0 mg, 0.1 mmol) in 10 mL of CH2Cl2 was added into a
50 mL colorimeter tube, layered with 18 mL of methanol, and then
M(ClO4)2·nH2O (M = Cd 1, Zn 2, Co 3, Cu 4, Mn 5) (0.05 mmol)
Scheme 2 Synthetic routes to L and its complexes.
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or CdX2·nH2O (X = I for 6, SCN for 7) (0.1 mmol) in 10 mL of
methanol was added. The solution was left for slow evaporation
and a lot of red crystals were collected for each case in two
weeks.
CdLI2 6. Yield: 93%. Anal. Calcd for C37H28N4I2SCd (926.93):
C 47.94, H 3.04, N 6.04, S 3.46; found: C 47.86, H 3.15, N 5.95,
S 3.63. IR (KBr): n = 1592 (vs), 1572 (s), 1467 (vs), 1401 (s), 1360
1
(s), 1248 (s), 1012 (m), 787 (m), 732 (m). H NMR (400 MHz,
d6-DMSO): d 1.33 (t, 3H, J = 6.58, 6.88 Hz), 3.95 (q, 2H, J = 6.88,
7.18, 6.58 Hz), 6.96 (t, 1H, J = 7.48, 7.48 Hz), 7.04 (d, 2H, J =
8.98 Hz), 7.16 (d, 1H, J = 7.78 Hz), 7.22 (t, 1H, J = 7.78, 7.78 Hz),
7.29 (d, 1H, J = 16.16 Hz), 7.39 (d, 1H, J = 16.16 Hz), 7.49 (m, 2H),
7.84 (d, 2H, J = 8.08 Hz), 7.92 (s, 2H), 8.33 (d, 4H, J = 25.13 Hz),
8.83 (d, 2H, J = 4.49 Hz), 9.05 (d, 4H, J = 18.55 Hz). TGA: The
main structure decomposed at 470 ◦C.
[CdL2](ClO4)2·CH2Cl2 1. Yield: 91%. Anal. Calcd for
C75H58Cl4N8O8S2Cd (1517.65): C 59.35, H 3.85, O 8.43, N 7.38, S
4.23; found: C 59.23, H 3.90, O 8.33, N 7.48, S 4.31. IR (KBr):
n = 1597 (vs), 1572 (s), 1465 (s), 1385 (m), 1249 (m), 1090 (vs),
1016 (m), 791 (m), 729 (m), 620 (m).1H NMR (400 MHz, d6-
DMSO): d 1.34 (t, 6H, J = 6.78, 7.03 Hz), 3.96 (q, 4H, J = 7.03,
7.03, 7.03 Hz), 6.96 (t, 2H, J = 7.53, 7.28 Hz), 7.04 (d, 4H, J =
8.53 Hz), 7.16 (d, 2H, J = 7.78 Hz), 7.22 (t, 2H, J = 7.78, 7.53 Hz),
7.28 (d, 2H, J = 16.31 Hz), 7.36 (d, 2H, J = 16.31 Hz), 7.48 (m,
4H), 7.61 (m, 4H), 7.83 (d, 4H, J = 7.53 Hz), 8.17 (m, 8H), 8.79
(m, 12H). TGA: The first stage of weight loss occurred at 220 ◦C
corresponding to the lost of CH2Cl2 molecules. The second stage
CdL(SCN)2 7. Yield: 90%. Anal. Calcd for C39H28N6S3Cd
(789.29): C 59.35, H 3.58, N 10.65, S 12.19; found: C 59.47, H
3.65, N 10.76, S 12.08. IR (KBr): n = 1594 (vs), 1570 (vs), 1461
(vs), 1401 (vs), 1360 (s), 1249 (s), 1013 (m), 785 (s), 751 (s). TGA:
The first stage of weight loss occurred at 390 ◦C attributed to the
lost of two SCN-, respectively. The main structure decomposed at
600 ◦C.
◦
-
started at 410 C is attributed to the lost of anions ClO4 . The main
structure decomposed at 500 ◦C exhibiting high thermal stability.
[ZnL2](ClO4)2·CH2Cl2 2. Yield: 92%. Anal. Calcd for
C75H58Cl4N8O8S2Zn (1470.64): C 61.25, H 3.98, O 8.70, N 7.62, S
4.36; found: C 61.38, H 3.82, O 8.71, N 7.50, S 4.47. IR (KBr): n =
1598 (vs), 1572 (vs), 1547 (m), 1469 (vs), 1404 (m), 1252 (m), 1091
Results and discussion
1
Synthesis and characterization
1
Through the solvent-free Wittig reaction, L was afforded as a
yellow solid in moderate yield (42%) (Scheme 2 and Fig. S1†). 1H
NMR spectra of L in d6-DMSO and CDCl3 show that the signals
of the vinyl and aromatic protons are overlapped, consequently,
the course of cis-trans isomerization of the free ligand can not
(vs), 1017 (m), 793 (m), 732 (m), 620 (m). H NMR (400 MHz,
d6-DMSO): d 1.35 (t, 6H, J = 7.03, 6.78 Hz), 3.97 (q, 4H, J = 7.03,
7.03, 6.78 Hz), 6.98 (t, 2H, J = 7.28, 7.28 Hz), 7.07 (t, 4H, J =
8.53, 7.53 Hz), 7.17 (d, 2H, J = 7.78 Hz), 7.23 (t, 2H, J = 8.03,
7.53 Hz), 7.35 (d, 2H, J = 16.06 Hz), 7.50 (m, 10H), 7.97 (m, 8H),
8.30 (t, 4H, J = 7.28, 7.53 Hz), 8.50 (d, 4H, J = 8.03 Hz), 9.18 (d,
4H, J = 8.03 Hz), 9.42 (s, 4H). TGA: The first stage of weight loss
occurred at 220 ◦C attributed to the lost of CH2Cl2 molecules. The
1
be followed through this way. H NMR spectrum of 6 measured
immediately after its dissolution in d6-DMSO shows that the vinyl
protons of the trans- isomer of L in 6 exhibit doublets at d 7.39
(J = 16 Hz) and 7.29 (J = 16 Hz) (Fig. S1†), respectively. After 6 h
◦
second stage started at 395 C is attributed to◦the lost of anions
1
-
irradiation, H NMR spectrum of 6 shows that vinyl protons of
ClO4 . The main structure decomposed at 480 C.
cis- isomer was shifted to d 7.34 (J = 7 Hz) and 7.31 (J = 7 Hz)
(Fig. S2†). It is also distinguished that the downfield shifts of the
CH2 and CH3 protons in trans vs. cis d = 3.96 and 1.33 vs. 4.47
and 1.47 ppm were observed, respectively. The dissolution of 6 in
d6-DMSO for 6 h results in the formation of a 7 : 8 cis/trans ratio.
The methyl proton regions of the 1H NMR spectra of 2 are shown
in Fig. S3.† The chemical shift at d 1.35 was observed for the CH3
protons of the Trans–Trans isomers of 2. After 6 h (Fig. S3,† inset),
a new triplet at d 1.49 shall be assigned to the methyl protons of
the Trans–Cis or Cis–Cis, or both of them (shown in Scheme 3).
[CoL2](ClO4)2
3. Yield:
95%.
Anal.
Calcd
for
C74H56Cl2N8O8S2Co (1379.23): C 64.44, H 4.09, O 9.28, N
7.63, S 4.37; found: C 64.48, H 4.15, O 9.10, N 7.50, S 4.28. IR
(KBr): n = 1615 (vs), 1570 (m), 1470 (s), 1401 (vs), 1249 (m), 1085
(s), 793 (m), 751◦(m), 620 (m). TGA: The first stage of weight loss
-
occurred at 390 C is attributed to the lost of anions ClO4 . The
main structure decomposed at 480 ◦C.
[CuL2](ClO4)2·CH2Cl2 4. Yield: 87%. Anal. Calcd for
C75H58Cl4N8O8S2Cu (1468.75): C 61.33, H 3.98, O 8.71, N 7.63, S
4.37; found: C 61.45, H 4.05, O 8.84, N 7.78, S 4.46. IR (KBr): n =
1618 (vs), 1597 (s), 1471 (m), 1383 (s), 1249 (m), 1085 (s), 791 (w),
745 (w), 620 (m). TGA: The first stage of weight loss occurred at
190 ◦C corresponding to the lost of CH2Cl2 molecules. The second
◦
-
stage started at 360 C is attributed to the lost of anions ClO4 .
The main structure decomposed at 450 ◦C.
[MnL2](ClO4)2·CH2Cl2 5. Yield: 85%. Anal. Calcd for
C75H58Cl4N8O8S2 Mn (1460.16): C 61.69, H 4.00, O 8.77, N 7.67,
S 4.39; found: C 61.65, H 3.92, O 8.69, N 7.83, S 4.50. IR (KBr):
n = 1598 (vs), 1571 (vs), 1472 (vs), 1401 (s), 1249 (m), 1087 (vs),
790 (m), 733 (m), 620 (m). TGA: The first stage of weight loss
occurred at 220 ◦C corresponding to the lost of CH2Cl2 molecules.
The second stage started at 390 ◦C is attributed to ◦the lost of
-
anions ClO4 . The main structure decomposed at 500 C.
Scheme 3 Representations of three configurations of the complexes 1–5.
8172 | Dalton Trans., 2011, 40, 8170–8178
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Table 1 Conformational Parameters for complexes 1–6
1
2
3
4
5
6
a
P3,1
P3,2
P3,4
P5,4
P5,6
P9,7
P9,8
7.95
2.67
7.92
2.93
21.50
20.37
53.88
8.91
7.77
5.23
8.79
24.48
13.69
8.03
11.52
33.10
26.14
13.43
7.24
2.97
8.73
2.62
20.58
18.89
54.03
8.96
4.93
3.25
9.88
16.73
27.56
—
—
—
—
—
18.56
17.58
53.56
7.04
10.22
29.28
10.30
26.62
19.82
20.98
53.84
9.08
12.83
29.09
9.61
9.51
9.90
P9,10
P10,11
P11,12
28.37
10.20
25.49
28.75
10.80
26.88
26.76
a P3,1 represent the dihedral angles between P3 and P1, and so on.
No evidence was available for the co-existence of the three
configurations of 2, but at least two of them are presented. The
constitution of the cis- isomer of L in 6 is much larger than
that in 2 after they are kept in the same solvent prolonging
the same time. The study as to whether this isomerization is
induced photochemically or not is in progress. For the IR spectra
-
of 1–5, n of ClO4 at ca. 1090 cm-1 shows strong signals. The
thermogravimetric analysis (TGA) shows that the decompositions
of the main structures of all complexes occurred above 450 ◦C. It is
obvious that the thermal stability of the complexes is higher than
that of the free ligand L. (Mp: 386 ◦C).
Fig. 1 (a) ORTEP plot of l showing the conformational parameters
P1–P12 with the 50% probability ellipsoids. Inset (right bottom corner):
Coordination geometry around the metal ion. (b) Packing diagram of l
shows the 3D structure, view down b-axis. Solvent molecules and anions
were omitted for clarity. (c) C–H ◊ ◊ ◊ p interactions in l indicated by dashed
lines, as an example.
2
Crystallographic results
Crystallographic data, selected bond lengths and angles for 1–6 are
summarized in Table S1 and S2.† The non-covalent interactions in
the complexes 1–5 are shown in Table S3.† In order to illustrate the
conformation of the ligand, the parameters P1–P12 are presented
in Table 1. The crystal structures of 1–5 are very similar, and the
molecular structure of 1 is shown in Fig. 1 as an example.
complex, molecules interacted with each other to form a 2D layer
extending along the b-axis (Fig. 1b). The 2D layers were further
organized to form a 3D supramolecular structure through non-
covalent interactions, such as H–bonds and C–H ◊ ◊ ◊ p interactions
(Fig. 1c).
Comparison of crystal structures of 1–5.
Differences between the crystal structures of 1–5. (i) The ligands
show different conformations in 1–5. The dihedral angles of the
investigated planes P1–P12 were summarized in Table 1. When
the dihedral angles are close to 0 or 180◦, it means that the planes
in one ligand are close to being coplanar. (ii) The number and
type of the H–bonds are very different in 1–5 (Table S3†). (iii) The
separations of the C–H ◊ ◊ ◊ p interactions for 1–5 are different, and
the shortest one was found in the crystal structure of 2.
Similarities of their structures. (i) Each M(ClO4)2·nH2O reacted
with two equiv of L; (ii) all the complexes 1–5 crystallize in the
triclinic P¯ı space group (Table S1†); (iii) the M(II) atoms are all
coordinated by six N atoms from two tpy groups of the two
different L with the slightly distorted octahedral coordination
2+
environments and the tpy units in {M(tpy)2} adopted the cis,cis
conformation (Fig. 1a); (iv) the ligands in the complex molecules
are all in the trans configuration, consequently, the complexes
exist in a Trans–Trans configuration in the solid state; (v) the
conformations of the two ligand units in each complex molecule
are different. For example, in the molecular structure of 1, the PTZ
unit on the left side is folded about the S–N axis with a dihedral
angle of 126.44◦ (Fig. 1a and Table 1). The dihedral angles formed
by P3 with the two pyridine rings P1 and P2 are 7.95◦ and 2.67◦,
respectively. The dihedral angles formed by P4 with P3 and P5 are
18.56◦ and 17.58◦, respectively. On the right side, the PTZ unit is
folded about the S–N axis with a dihedral angle of 143.38◦. The
dihedral angles formed by P9 with the two pyridine rings P8 and P7
are 7.04◦ and 10.22◦, respectively. The dihedral angles formed by
P10 with P9 and P11 are 29.28◦ and 10.30◦, respectively. (vi) There
Comparison of crystal structures of 1 and 6.
Differences between the crystal structures of 1 and 6. (i) Each
Cd(II) ion in 6, shown in Fig. 2, is five-coordinate with three N
atoms from one tpy unit and two I atoms, while Cd(II) ion is six-
coordinate in 1. (ii) The conformations of L in the two complexes
are different (Fig. 2a and Table 1). It is clear that the dihedral
angles in 6 are smaller than that in 1. The planarity of L in 6 is
coplanar, which will facilitate the p-electron delocalization in the
complex. (iii) In the crystal structure of 6, the complex molecules
interacted with each other to form a 2D layer extending along
the b-axis, which were further linked to give a 3D supramolecular
structure through C–H ◊ ◊ ◊ I and p ◊ ◊ ◊ p interactions (Fig. 2b and
c). I ◊ ◊ ◊ H(C) distances of the two types of H–bonds a and b are
exists not only the electrical attraction between [ML2]2+ and ClO4 ,
but also the strong H–bonds, where the H–bonding acceptor is
from one oxygen atom of the perchlorate anion. (vii) In each
-
◦
˚
3.083 and 3.096 A, while C–H ◊ ◊ ◊ I angles are 154.79 and 157.36 ,
respectively. The separation of the p ◊ ◊ ◊ p interaction between one
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Fig. 3 Linear absorption (a) and emission (b) spectra of L in six solvents.
The linear emission spectra of L exhibit dual emissions,
which show strong solvatochromism. With increasing polarity
of the solvent, their emission maxima (lmaxem) show remarkable
bathochromic shifts. As shown in Fig. 3b, the dual emission
wavelengths in hexane are located at 449 and 503 nm, whereas
in DMF red-shifted to 496 and 561 nm, respectively. These results
suggest that the molecular polarity of the excited state of L must
be larger than that of the ground state, as the enhanced dipole–
dipole interactions caused by increasing the polarity of solute
and/or solvent will lead to decreasing energy level significantly
for the excited state.
The ratio of the intensities of the dual emission bands varies
with different excitation wavelengths, L in hexane as an example
shown in Fig. 4a (Fig. S4† shows those of L in THF and DMF). In
such cases, it is generally taken as an evidence for a heterogeneous
sample with more than one absorbing and emitting species. The
bands at 449–496 nm and 503–561 nm were tentatively assigned to
the emissions of trans and cis originating from the ICT transitions,
respectively. It was supported by the fact that the both emission
bands show strong solvatochromism. The hypothesis of different
absorbers can be checked readily by recording fluorescence
excitation spectra with observation wavelengths of the fluorescence
bands relevant to the cis and trans configuration of the molecule,
respectively (Fig. 4b). The similarity in the overall appearance
of the excitation spectra is in accordance with the assumption
that the two absorbing species are different configurations of the
same molecular species, which exhibit slightly different absorption
characteristics. When the detection wavelength was chosen at
450 nm rather than 505 nm, all the maxima in the excitation
spectra shifted toward longer wavelengths, indicating that the trans
absorbs at a lower energy region than the cis. The assignments
above were further corroborated by theoretical calculations.
Fig. 2 (a) ORTEP plot of 6 showing the conformational parameters
P1–P6 with the 50% probability ellipsoids. Inset: Coordination geometries
around the metal ions. (b) Representation of hydrogen bonds. (c) Packing
diagram shows the 3D structure, view down b-axis.
pyridine ring and phenyl ring of PTZ unit from another molecule
˚
is 3.362 A.
To summarize, the different metal centers in 1–5 with similar co-
ordination geometries have large influences on the conformations
of the ligands and the non-covalent interactions in the complex
molecules in solid state. While the coordination geometries of the
Cd(II) centers vary a lot with different anions in 1 and 6. In the
solid state, L in all the complexes adopts the trans- configuration.
3
Linear absorption and emission spectra
The photophysical data of all the compounds were summarized
in Table 2. The linear absorption and emission spectra of L in
six organic solvents are shown in Fig. 3, while those of all the
compounds in DMF are depicted in Figs. 5 and 6, respectively.
Solvatochromism. Generally, the linear absorption spectra of
L feature an intense absorption band at 293–306 nm and a
moderately intense absorption in the 378–394 nm range in all
the solvents (Fig. 3a). The low-energy band originating from a
HOMO (H)→LUMO (L) transition was assigned to intraligand
charge transfer (ICT) transitions, while the high-energy band was
assigned to a H-1→L transition (ICT) and mixed with H-1→L+5
transition due to the p–p* transitions of the tpy. It was further
corroborated by calculations. As shown in Fig. 3a and Table 2,
weak solvatochromism was observed in the ICT absorption bands,
indicating the L molecule with fairly small dipole moments and
the difference in dipoles between ground and excited state.19
8174 | Dalton Trans., 2011, 40, 8170–8178
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Table 2 Photophysical data of all the compounds.
a
b
c
d
e
f
L
Solvent
lmax
lmax
f
b
t
s
Hexane
Benzene
CH2Cl2
THF
CHCl3
DMF
Solid
293, 378
294, 386
300, 388
302, 390
~, 391
449, 503
451, 528
454, 535
457, 539
461, 553
491, 561
530
306, 394
0.62/0.44
0.67/0.42
0.47/0.28
0.27/0.18
0.18/0.24
0.16/0.23
0.33/0.31
0.42/0.45
14
49
51
45
38
34
47
43
1.4/1.2
1.6/1.2
1.7/1.5
1.6/1.2
1.6/1.4
1.5/1.2
1.8/1.4
1.6/1.2
5.8
20
21
19
15
14
19
18
1
2
3
4
5
6
7
DMF
Solid
DMF
Solid
DMF
Solid
DMF
Solid
DMF
Solid
DMF
Solid
DMF
Solid
310, 396
315, 397
326, 401
308, 396
311, 397
320, 301
320, 400
492, 564
602
494, 562
623
496, 564
604
499, 572
595
495, 572
583
491, 571
648
491, 570
626
a Linear absorption maxima (nm). b Linear emission maxima (in nm, under the excitation wavelength of 310 nm). c x/y represent quantum yields obtained
by comparison of emission of RHB ethanol solution with that of trans/cis under identical experimental conditions. d Nonlinear absorption coefficient
(10-3 cm GW-1). e Fluorescence lifetime (ns). f 2PA cross-section in 103 GM (1 GM = 10-50 cm4 s·photon-1).
Fig. 5 Linear absorption spectra of all the compounds in DMF.
1, 6 and 7 with the same metal ions, the red shifts of 6 and 7 are
slightly larger than that of 1.
The linear emission spectra of all the compounds exhibit similar
emission characteristics (Fig. 6), which give the evidence again that
at least two configurations of each complex (1–5) are co-existing in
DMF. As shown in Fig. 6a, under the same excitation wavelength
ex
(l ) of 287 nm, the ratio of the intensities of the dual emission
bands varies from different compounds with different metal ions
or anions.
To summarize, in the solvent, the configuration of ligand
Fig. 4 Fluorescence emission (a) and excitation (b) spectra of L in hexane
at room temperature. Inset: the excitation wavelengths.
in the complexes varies with either different metal centers or
anions, leading to the changes in photophysical properties of the
complexes.
Comparison of the complexes with the ligand. The observation
of similar absorption bands for the complexes and L suggested
their assignments as the metal-perturbed ICT (Table 2 and Fig.
5). The absorption maxima (lmaxab) of the complexes show a small
red shift compared to that of the ligand, indicating a little increase
Linear emission spectra of all the compounds in solid state. All
the compounds exhibit one emission band at 530–648 nm in the
solid state at ambient temperature (Table 2), corresponding to the
results of single crystal X-ray diffraction analysis that the ligands
in the complexes exist in trans in solid state. In 1–5, the emission
peaks (lmax) show the order of 5 < 4 < 3 < 1 < 2. While lmax
of the complexes 1, 6 and 7 exhibit the sequence of 1 < 7 < 6.
ab
of the ICT in the complex. In 1–5 with the same anions, lmax of
3 exhibits the largest red shift of all the complexes. In complexes
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Fig. 6 Linear emission spectra of all the compounds (L, 1–7) in DMF (a)
ex
ex
l
= 287 nm, (b) l = 368 nm.
The stronger non-covalent interactions between molecules in 2
and 6 may be in responsible for the larger red shifts of the solid-
state emission bands compared to their respective counterparts in
solution. Table 2 also shows that the fluorescence lifetimes (t) of
the complexes are a little longer than that of the ligand when they
are all under the excitation wavelength of 370 nm. The quantum
yields of trans in each compound show the sequence of 1 > L >
2 > 7 > 6 > 3 > 4 > 5, while those of cis follow the order of 7
> L > 1 > 6 > 2 > 4 > 5 > 3. Obviously, 3, 4 and 5 show much
lower quantum yields of both trans and cis compared to those of
the other complexes.
Fig. 7 Molecular orbital plots of (a) the ground state, (b) the excited state,
left: cis¢; right: trans¢. The molecular structures of the species studied were
calculated at TDDFT level of theory.
isomers is localized on the D and p-bridge groups. The unoccupied
molecular obitals L and L+2 are localized on the A and p-bridge
groups. From the calculation results, two transitions in the lower
energy region resemble those observed in the experimental linear
absorption spectra. The transitions at ca. 420 nm (f = 0.27) and 330
nm (f = 0.53) originating from H→L and H-1→L transitions are
assigned as the ICT transitions of cis¢, respectively. The transitions
at 427 nm (f = 0.73) and 341 nm (f = 1.01) originating from H→L
and H-1→L transitions are all assigned as the ICT transitions of
trans¢, respectively. The transitions at 318 nm (f = 0.16 and 0.33)
of cis¢ and trans¢ both originating from H-1→L+5 transitions
are assigned as the p–p* transitions of the terpyridine. The most
difference in the absorption spectra of the two isomers is that the
oscillator strengths of the two ICT absorption bands of trans¢ are
2-fold larger than those of cis¢. This suggests that there is a factor of
2 higher probabilities for the two ICT transitions in trans¢, which
is a result of better electronic coupling between the PTZ donor
and tpy acceptor moieties in trans¢ compared to that in cis¢. A
better coplanarity between the D/A units can result in superior
D/A coupling in the ground state of trans¢. It is reaffirmed that
the ICT transitions of trans¢ are red-shifted compared to those of
cis¢. Basically, the calculated singlet-singlet transitions in L are in
reasonable agreement with the experimental lmaxab in the two major
absorption bands observed in the experimental spectra, except
the two closely overlapping transitions, H-1→L and H-1→L+5
transitions, that may only exhibit one high energy band in the
experimental result.
4
Computational studies
The initialized geometry of cis and trans pre-optimized by AM1
was obtained by further geometryoptimization in vacuum (B3LYP
(6-31G(d)) for S0, CIS (6-31G(d)) for S1). Then TDDFT (B3LYP
(6-31G(d,p)) calculations were performed on the optimized struc-
ture. All the calculations including optimizations and TDDFT
were conducted using G03 software. In the calculation of the
optical absorption spectrum, the 25 lowest spin-allowed singlet-
singlet transitions, up to an energy of ~5 eV, were taken into
account. Moreover, the lowest 25 singlet and singlet excited states
were calculated for each calculated optimized geometry.
For the sake of reducing the computational cost, trans¢ and cis¢
were used instead of trans and cis, in which the ethyl groups on
the PTZ moieties were replaced by a methyl group.20
The selected singlet-singlet excitations of the two isomers of L
in the range detected in the absorption spectra are listed in Table 3.
The nature of the orbitals involved in the excitation is also shown
in Fig. 7a. The frontier filled orbitals H of trans¢ and cis¢ are
mainly localized on the PTZ (D) and vinyl units. H-1 of the two
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Table 3 Selected Low-Lying Singlet (Sn) Excited States Computed by TDDFT method, with the Orbitals Involved (OI) in the Excitations. Transition
coefficient (Tc), Excitation eneries (eV and nm), and Oscillator Strengths (f ) in cis¢ and trans¢
c
Isomer
Sn
Ca
OI
Tcb
Eex
f d
cis¢
S1
S4
S5
S15
S1
S4
S6
S12
ICT
H→Lf
0.68
0.66
0.54
0.44
0.67
0.61
0.57
0.49
2.95/420.04
3.76/329.50
3.90/317.92
4.35/285.15
2.91/426.53
3.63/341.37
3.89/318.34
4.26/292.01
0.27
0.53
0.16
0.21
0.73
1.01
0.33
0.12
ICT
H-1→L
H-1→L+5
H-1→L+2
H→L
p–p*(D)e
ICT
trans¢
ICT
ICT
p–p*(D)
ICT
H-1→L
H-1→L+5
H-1→L+2
a C: Character of the transitions. b The excitations with transition coefficients less than 0.4 were not shown. c Excitation energy in eV nm-1. d Only the
singlet excited stated with f > 0.1 were listed. e p–p* transition of PTZ(D) unit. f H(HOMO) and L(LUMO).
In order to study the nature of the emitting singlet states and the
structural changes from the corresponding ground states, CIS (6-
31G(d)) method was used to optimize the low-lying singlet states
in trans¢ and cis¢ (Fig. 7b). The calculated emission maxima of
the ICT transitions, corresponding to 540 and 474 nm for cis¢ and
trans¢, are correlated well with the experimental emissions in the
range of 503–561 nm and 449–491 nm for cis and trans.
5
Nonlinear optical properties (NLO)
The third-order NLO properties of all the compounds in DMF
were studied using the Z-scan method. Fig. 8 shows the typical
Z-scan measurement of 2 in DMF. The nonlinear absorption
component was evaluated under an open aperture. The filled
squares represent the experimental data measured under this
condition. It is clearly illustrated that the absorption increases
as the incident light irradiance rises. The solid line in Fig. 8 is the
theoretical curve from the eq 1.21
Fig. 8 Z-scan data for 2 in DMF, obtained under an open aperture
configuration. The black dots are the experimental data and the solid
curve is the theoretical fit.
where hn is the energy of the incident photon. NA is the Avogadro
constant. d is the concentration of the 2PA compound in mol cm-3
units.23
+∞
1
T (z ) =
ln 1+q(z ) exp(−t 2 )dt
[ ]
(1)
The nonlinear absorption coefficient b and the 2PA cross-
section s of all the compounds were listed in Table 2. s of L
was 5.8 ¥ 103 GM. Considering the molecular weight difference
between the complexes and free ligand, the 2PA cross-section
values for all the complexes are approximately 2.4- to 3.6-fold
larger than that for L. Generally, the different metal centers are
responsible for the different 2PA properties of the complexes. The
detailed experimental information is given in Table 2, and further
explanations are listed as follows. (i) The s values of the complexes
1–5 exhibit the order of 5 < 3 < 4 < 1 < 2. (ii) The configurations
of L in 1–5 vary with the different metal centers, e.g. the ratio of
trans of L in 1, 2 and 3 are much larger than that in 4 and 5, which
may be a potential reason for the enhanced NLO properties of 1,
2 and 3. (iii) Another factor for the larger s of 1 and 2 compared
to those of 3, 4 and 5 is attributed to the closed-shell d10-metal
centers being non-detrimental to fluorescence. (iv) Considering
each complex molecule of 1 contains two ligand molecules, it is
slightly surprising that 1 (with two ligands L) has such similar
values to 6 and 7 (one ligand L).
∫
q(z ) p
−∞
q(z) = bI0Leff/(1 +z2/z0 )
(2)
(3)
2
Leff = (1 - e-aL)/a
where I0 is the input intensity at the focus z = 0, L is the sample
length, a is the linear absorption coefficient and b is the 2PA
2
coefficient. z0 = pw0 /l is the Rayleigh diffraction length, w0 is
the radius of beam at focus. Thus, once an open-aperture Z-
scan is performed, the nonlinear absorption coefficient b can
be unambiguously deduced.22 The open-aperture transmittance
is symmetric with respect to the focus (z = 0), where it has a
minimum transmittance. The intensity dependent transmittance
indicates the nonlinear absorption for 2. No linear absorption
above 500 nm was observed in the linear absorption spectrum of 2
in DMF, we attribute the nonlinear absorption to the 2PA effect.
Using the above equation, we can get the nonlinear absorption
coefficient b (in units of cm GW-1). In addition, the molecular 2PA
cross-section s (in units of cm4 s·photon-1) can be determined by
the following expression:
Although bis(tpy) complexes 1–5 are bound to be linear 1D,
different from metallo-octupolar D3 ter(bipyridine) complexes
that showing enhanced 2PA response. Functionalized TPY ligands
give rise to octahedral and pyramidal metal complexes, showing
high 2PA response.24 Large enhanced 2PA response in 6 and
s = hnb/NAd ¥ 10-3
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7 may arise from an extended p-electron delocalization after
complexation and an increase of intramolecular charge transfer
(ICT) upon excitation. The s values of the complexes are larger
than those of most transition metal complexes.25 In conclusion,
the results indicate that the novel type ligand combined terpy with
PTZ offers a new opportunity to enhance the nonlinear optical
properties in different aspects.
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Conclusions
A novel 2,2¢:6¢,2¢¢-terpyridine-based ligand L and its complexes 1–
7 have been synthesized and fully characterized. The equilibrium
of the trans- and cis- isomers of L was investigated both experimen-
tally and theoretically. All the complexes were easily obtained in
high yield and show better thermal stability than that of L, which
exhibit good one- and two-photon excited fluorescence properties.
Compared with those of L, the larger 2PA cross-section values
and longer fluorescence lifetimes of the complexes were obtained.
The molecular structures and photophysical properties of the
complexes display large dependence on the choice of the metal
ions and anions. The joint theoretical and experimental study
gives reasonable interpretation for their photophysical properties
based on sufficient crystallographic data from the series of the
compounds. The results show that terpy group and PTZ are
linked by a p-conjugated system to form a novel two-photon
absorbing ligand. Its metal complexes (M = Zn, Cd) with the
closed-shell d10 configuration and anions (X = SCN, I) possess the
larger 2PA cross-section values. Consequently, simple and efficient
preparation of these complexes for enhanced 2PA properties may
be a better alternative to those organic compounds obtained by
significant skill.
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This work was supported by a grant for the National Natural Sci-
ence Foundation of China (21071001, 50873001, and 20875001),
Department of Education of Anhui Province (KJ2010A030), the
Team for Scientific Innovation Foundation of Anhui Province
(2006KJ007TD), and Ministry of Education Funded Projects
Focus on returned overseas scholar. Science and Technological
Fund of Anhui Province for Outstanding Youth (10040606Y22)
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8178 | Dalton Trans., 2011, 40, 8170–8178
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