New five-coordinated mercury (II) dyes based on a novel 2,2′:6′,2″-terpyridine ligand: Structures, photophysical properties and DFT calculations to evaluate the halogen effect on the two-photon absorption

Article abstract of DOI:10.1016/j.dyepig.2012.07.005

A simple D-π-A type ligand (L: 4′-(4-[4-(1H-[1,2,4]-triazolyl) styryl]phenyl)-2,2′:6′,2″-terpyridine) was designed and synthesized, which reacted with HgX₂ (X = Cl, Br, I, SCN) yielding a series of uncommon five-coordinated mercury (II) complexes (Dyes 1-4). The ligand and Dye 4 were characterized by single crystal X-ray diffraction determination. Linear and nonlinear optical properties of the ligand and dyes were exhibited. Experimental results revealed that the two-photon absorption (TPA) cross-sections of the four dyes are considerably larger than that of the ligand. Density function theory (DFT) calculations performed on Dyes 1-4 showed that the halogens affect the accepting capability of the complexed metal due to different electron inductive effects, which finally influence the TPA cross-section values.

Full text of DOI:10.1016/j.dyepig.2012.07.005

Dyes and Pigments 95 (2012) 723e731
Contents lists available at SciVerse ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
New five-coordinated mercury (II) dyes based on a novel 2,20:60,200-terpyridine
ligand: Structures, photophysical properties and DFT calculations to evaluate the
halogen effect on the two-photon absorption
,
a
a
a
a
a
a
Hongping Zhou a , Jianqing Wang , Feixia Zhou , Dongling Xu , Yuanle Cao , Gang Liu , Lin Kong ,
*
Jiaxiang Yang a, Jieying Wu a, Yupeng Tian a
,b,c
a College of Chemistry and Chemical Engineering, Anhui University and Key Laboratory of Functional Inorganic Materials Chemistry of Anhui Province, 230039 Hefei, PR China
b State Key Laboratory of Crystal Materials, Shandong University, 250100 Jinan, PR China
c State Key Laboratory of Coordination Chemistry, Nanjing University, 210093 Nanjing, PR China
a r t i c l e i n f o
a b s t r a c t
A simple D-p
-A type ligand (L: 40-(4-[4-(1H-[1,2,4]-triazolyl)styryl]phenyl)-2,20:60,200-terpyridine) was
Article history:
Received 30 April 2012
Received in revised form
4 July 2012
Accepted 4 July 2012
Available online 13 July 2012
designed and synthesized, which reacted with HgX2 (X ¼ Cl, Br, I, SCN) yielding a series of uncommon
five-coordinated mercury (II) complexes (Dyes 1e4). The ligand and Dye 4 were characterized by single
crystal X-ray diffraction determination. Linear and nonlinear optical properties of the ligand and dyes
were exhibited. Experimental results revealed that the two-photon absorption (TPA) cross-sections of
the four dyes are considerably larger than that of the ligand. Density function theory (DFT) calculations
performed on Dyes 1e4 showed that the halogens affect the accepting capability of the complexed metal
due to different electron inductive effects, which finally influence the TPA cross-section values.
Ó 2012 Published by Elsevier Ltd.
Keywords:
Five-coordinated mercury (II)
Single crystal
Linear optical properties
Nonlinear optical properties
DFT calculations
TPA cross-sections
1. Introduction
study results reveal that TPA cross-sections increase with the
donor/acceptor strength, conjugation length, and planarity of the
Two-photon absorption (TPA) is a three-order nonlinear optical
process and offers various advantages, including localized excita-
tion at the focal point of the objective and minimal photochemical
damage. Materials with large two-photon absorption cross-section
are in great demand for a variety of applications, including 3D
microfabrication [1], two-photon excited fluorescence microscopy
[2], optical power limiting [3], and photodynamic therapy [4].
It is well established that TPA cross-sections depend critically on
the strength of the chromophore’s two-photon absorptivity. To
date, many molecules with a larger TPA cross-section (dTPA) have
been widely investigated via two main approaches. One is to design
complex structures mainly based on the intra-molecular interac-
p-center. The other is defined as inter-molecular interaction
starting from simple molecular units organized in supramolecular
self-assembled structures [6]. Such research indicates that the
existing weak interactions may induce rearrangement of electrons
and optimized orientation of compounds to enhance TPA cross-
section.
In the current work, first, terpyridine was selected as the A
(acceptor) group, because it possesses two important aspects that
make it an ideal building block for fluorescent molecular materials
[7]: (i) a superb ability to coordinate with different metal ions (ii)
a rigid framework plane, which leads generally to form a larger
planar systems. Second, the enolic styrene acts as
p-conjugated
tion, such as donorebridgeeacceptor (De
p
eA) dipoles,
unit, which can lead to a higher degree of -electron delocalization
p
donorebridgeedonor (De eD) quadrupoles, multibranched
compounds, dendrimers, octupoles, and porphyrins [5]. These
p
[8]. Finally, electron-rich triazole works as the D (donor) group and
coordination group (Fig. 1).
Synthetic strategy was based on the molecular self-assembly of
organic ligand with TPA properties and different inorganic salts to
product materials with larger dTPA. A series of new coordination
complexes (Dyes 1e4) with five-coordinated mercury (II) were
* Corresponding author. Tel.: þ86 5515108151; fax: þ86 5515107342.
E-mail address: zhpzhp@263.net (H. Zhou).
0143-7208/$ e see front matter Ó 2012 Published by Elsevier Ltd.
http://dx.doi.org/10.1016/j.dyepig.2012.07.005

724
H. Zhou et al. / Dyes and Pigments 95 (2012) 723e731
2.2.2. Preparation of Dyes 1e4
L (23.9 mg, 0.05 mmol) in 15 mL of CHCl3 was added into
a 50 mL colorimeter tube, layered with 10 mL of ethanol (or
methanol), and then HgX2 (X ¼ Cl, Br, I for SCN) (0.05 mmol) in
15 mL of ethanol (or methanol) was added. The solution was left for
slow evaporation and crystals were collected for each case after two
weeks.
2.2.2.1. HgLCl2 (Dye 1). Yield: 26 mg (69%). FT-IR (KBr, cmꢀ1): 3442
(m), 3097 (w),1595 (vs),1572 (m),1521 (vs),1474 (s),1428 (m),1403
(m), 1278 (m), 1248 (m), 1218 (m), 1011 (m), 980 (m), 839 (m), 790
(m), 745 (m). Calcd for C31H22Cl2N6Hg: C, 49.64; H, 2.96; N, 11.20%.
Found: C, 49.81; H, 2.99; N, 11.14%. From the calculated analysis
result, we can infer the unit of Dye 1 may contain one HgLCl2.
Fig. 1. Structure of the ligand (L).
2.2.2.2. [HgLBr2]2$CHCl3 (Dye 2). Yield: 31 mg (74%). FT-IR (KBr,
cmꢀ1): 3441 (m), 3098 (w), 1594 (vs), 1571 (m), 1519 (s), 1473 (s),
1428 (m), 1402 (m), 1278 (w), 1247 (w), 1216 (w), 1009 (m), 979 (m),
838 (m), 788 (m), 742 (m). Calcd for C63H45N12Br4Cl3 Hg2: C, 42.10;
H, 2.52; N, 9.35%. Found: C, 42.31; H, 2.57; N, 9.34%. From the
calculated analysis result, we can infer the unit of Dye 2 may
contain two HgLBr2 and a chloroform molecule.
obtained by self-assembling the ligand with HgX2 (X ¼ Cl, Br, I,
SCN). L and Dye 4 were characterized by single crystal X-ray
diffraction determination. Linear and nonlinear optical properties
of the ligand and four dyes were investigated.
2. Experimental section
2.1. Materials and physical measurements
2.2.2.3. [HgLI2]2$CHCl3 (Dye 3). Yield: 35 mg (75%). FT-IR (KBr,
cmꢀ1): 3442 (m), 3104 (w), 1595 (vs), 1539 (m), 1521 (s), 1474 (s),
1426 (m), 1402 (m), 1278 (m), 1248 (m), 1216 (m), 1009 (m), 978
(m), 837 (m), 788 (m), 733 (m). Calcd for C63H45N12Cl3I4Hg2: C,
38.11; H, 2.28; N, 8.47%. Found: C, 38.29; H, 2.33; N, 8.42%. From the
calculated analysis result, we can infer the unit of Dye 3 may
contain two HgLI2 and a chloroform molecule.
UVevis absorption spectra were recorded on a UV-3100 spec-
trophotometer. Fluorescence measurements were carried out using
an Edinburgh FLS920 fluorescence spectrometer equipped with
a 450 W Xe lamp and a time-correlated single-photon counting
(TCSPC) card. All the fluorescence spectra were collected. The
single-photon excited fluorescence (SPEF) quantum yields (F) were
2.2.2.4. HgL(SCN)2 (Dye 4). Yield: 26 mg (65%). FT-IR (KBr, cmꢀ1):
3429 (m), 3106 (w), 2922(w), 2112 (s), 1594 (vs), 1572 (m), 1521 (s),
1474 (s), 1404 (s), 1278 (m), 1247 (m), 1214 (m), 1145 (m), 1050 (m),
1011 (m), 977 (m), 839 (m), 790 (m). Anal. Calcd for C33H22N8S2Hg:
C, 49.84; H, 2.79; N, 14.09%. Found: C, 49.66; H, 2.64; N, 14.15%。
measured by using a standard method under the same experi-
mental conditions for all compounds. Rhodamine B dissolved in
ethanol (
F
¼ 0.56) [9] at the same concentration as the other
samples was used as the standard. The two-photon excited fluo-
rescence (TPEF) spectra were measured using a Mira 900-D Ti:
sapphire femtosecond laser with a pulsewidth of 200 fs and
a repetition rate of 76 MHz. All measurements were carried out in
air at room temperature. TPA cross-sections were measured using
fluorescein as reference.
2.3. X-ray crystallography
Data for single crystals were collected on a Siemens Smart 1000
CCD diffractometer. The determination of unit cell parameters and
2.2. Materials and synthesis
data collections were performed with Mo
Ka
radiation
(
l
¼ 0.71073 Å). Unit cell dimensions were obtained with least-
All chemicals were available commercially from Acros Organics
company (Beijing) and Aladdin (Shanghai), and the solvents were
purified via conventional methods before use. IR spectra were
recorded with a Nicolet FT-IR NEXUS 870 spectrometer (KBr discs)
in the 4000e400 cmꢀ1 region. 1H NMR and 13C NMR spectra were
recorded on a 400 MHz NMR instrument using CDCl3 as solvents.
Mass spectra were obtained on a Micromass GCT-MS Spectrometer.
Elemental analysis was recorded on Perkin Elmer 240B instrument.
squares refinements, and all structures were solved by direct
methods using SHELXS-97 [12]. The other non-hydrogen atoms
were located in successive difference Fourier syntheses. The final
refinement was performed by using full-matrix least-squares
methods with anisotropic thermal parameters for non-hydrogen
atoms on F2. The hydrogen atoms were added theoretically and
processing parameters for Dye 4 and L are shown in Table 1 and the
selected bond lengths and bond angles are listed in Table 2.
2.2.1. Preparation of the ligand: (40-(4-[4-(1H-[1,2,4]-triazolyl)
styryl] phenyl)-2,20:60,200-terpyridine)
3. Results and discussion
The ligand was readily achieved in 78.1% yield by Solid-state
Wittig reaction with 4-(2,20:60,200-terpyridine-40-yl)-benzyl-
triphenylphosphonium bromide [10] and 4-(1H-[1,2,4]-triazol-1-
yl) benzaldehyde [11]. 1H NMR (CDCl3, 400 MHz): 7.47 (s, 2H),
7.53e7.56 (m, 2H), 7.85e7.87 (m, 4H), 7.92 (d, J ¼ 8.0 Hz, 2H), 8.00
(d, J ¼ 8.0 Hz, 2H), 8.06 (t, J ¼ 8.0 Hz, 2H), 8.26 (s, 1H), 8.69 9 (d,
J ¼ 8.0 Hz, 2H), 8.77e8.79 (m, 4H), 9.35 (s, 1H). Ms: m/z (%) ¼ 478.19
(100). FT-IR (KBr, cmꢀ1): 3427 (w), 3054 (w), 1583 (s), 1564 (m),
1522 (s), 1466 (m), 1388 (s), 1277 (w), 1147 (m), 965 (m), 839 (m),
789 (s), 671 (m), 537 (m). Calcd for C31H22N6: C, 77.79; H, 4.63; N,
17.58%. Found: C, 77.57; H, 4.92; N, 17.51%.
L crystallized in the monoclinic form with the space group P2(1)/C
as shown in Fig. 2(a). The unit contains two molecules. In each
molecule, the dihedral angel between the terpyridine ring and its
bonded benzene ring is 10.841ꢁ, showing a distorted plane.
Single crystal X-ray structural analysis shows that the asym-
metric unit of Dye 4 consists of one Hg(II), one L, two thiocyanate
anions (Fig. 3(a)). The Hg (II) center is five-coordinated by three
nitrogen atoms from one L and two sulfur atoms from two thio-
cyanate anions. The bond lengths Hg1eN1, Hg1eN2, Hg1eN3 are
2.417(9), 2.367(8), 2.39(1) Å respectively. Each bond angle around

H. Zhou et al. / Dyes and Pigments 95 (2012) 723e731
725
Table 1
Crystallographic data for L and Dye 4.
Compound
HgL(SCN)2
33H22N8S2Hg
795.30
Monoclinic
C2/c
13.806(5)
25.080(5)
18.629(5)
102.360(5)
6301(3)
8
L
Empirical formula
Formula weight
Crystal system
Space group
a [Å]
C
C31H22N6
478.55
Monoclinic
P2(1)/C
9.513(19)
46.843(9)
11.521(2)
113.441(3)
4710.3(16)
8
b [Å]
c [Å]
b
[ꢁ]
V [Å3]
Z
T [K]
298(2)
1.677
5.055
1.71e25.00
5556
298(2)
1.350
0.083
0.87e25
8292
Dcalcd [g cmꢀ3
]
m
[mmꢀ1
range [ꢁ]
]
q
Total no. data
No. unique data
3314
3111
No. parameters refined
397
667
R1
wR2
GOF
0.0564
0.1905
1.107
0.0623
0.1377
0.915
the mercury atom is in the range 67.9(3)e137.5(2)ꢁ, indicating
a quite distorted tetrahedral geometry.
The data analysis demonstrated the dihedral angel between the
terpyridyl ring and its bonded benzene ring is only 3.06ꢁ, smaller
than that of L. The alkene
each phenyl ring, because the linkage bond lengths between
benzene rings and alkene -bridge unit are quiet conjugated,
C18eC21 [1.450(15)], C21eC22 [1.304(15)], C22eC23 [1.483(14)].
On the other hand, the alkene -bridge unit exhibits enolic
p-bridge unit is basally co-planar with
p
p
structure. These structure features of Dye 4 suggested that all
non-hydrogen atoms of the unit are highly conjugated, leading to a
Table 2
Fig. 2. (a) Single crystal structure of L. (b) The 1-D framework of L showing the
Selected bond lengths (Å) and angles (ꢁ) for L and Dye 4.
CeH/N (purple and red) hydrogen bond and CeH/
the c-axis. (c) The 2-D framework of L showing CeH/
p
interaction (light green) along
interaction along the b-axis.
p
L
(For interpretation of the references to colour in this figure legend, the reader is
referred to the web version of this article.)
N1eC47
N1eC49
N2eC52
N2eC56
N3eC38
N3eC42
N4eC22
N4eC24
N5eC32
N5eC35
N6eC26
N6eC30
N7eN8
1.346(6)
1.343(6)
1.336(7)
1.346(8)
1.319(8)
1.326(7)
1.339(6)
1.338(6)
1.334(8)
1.330(7)
1.340(7)
1.335(8)
1.361(7)
1.341(7)
1.321(7)
1.341(9)
1.318(6)
N10eN11
N10eC4
N11eC1
N12eC1
1.348(6)
1.324(6)
1.310(7)
1.334(7)
1.320(7)
117.7(4)
116.2(4)
118.4(5)
117.3(4)
117.3(4)
117.3(4)
108.3(4)
102.9(4)
102.6(5)
108.2(4)
102.8(4)
101.4(4)
N12eC4
p-bridge for charge transfer, which may enlarge the molecular TPA
C47eN1eC49
C52eN2eC56
C38eN3eC42
C22eN4eC24
C32eN5eC35
C26eN6eC30
C75eN7eN8
C74eN8eN7
C74eN9eC75
C4eN10eN11
C1eN11eN10
C1eN12eC2
cross-section [5a].
In addition, L generated a 1-D chain through CeH/N (purple
dashed line 2.466 Å, red dashed line 2.543 Å) and CeH/p contacts
(light green dashed line 3.197 Å, for interpretation of the references
tocolourinthis figure, thereader isreferred tothewebversionof this
article) (Fig. 2(b)). Further more CeH/
3.526 Å) resulted in a 2-D framework (Fig. 2(c)). Dye 4 formed its 1-D
chain via CeH/S (light green dashed line 2.974 Å) and stacking
p contacts (blue dashed line
N7eC75
N8eC74
N9eC74
N9eC75
pep
contacts (red dashed line), the shortest distance is 3.352 Å (Fig. 3(b),
for interpretation of the references to colour in this figure, the reader
is referred to the web version of this article). Its 2-D structure was
obtained when the CeH/N (purple dashed line 2.538 Å) interaction
pulled chains together (Fig. 3(c)). Further more, the adjacent inter-
penetrating layers are held together by CeH/S (red dashed line
2.938 Å, blackish green dashed line 2.865 Å) interactions to afford an
extended 3-D supramolecular framework (Fig. 3(d)). Structures with
different geometries between L and Dye 4 may be vital factors
affecting dTPA. Supramolecular self-assembled structures brought
size-enhanced nonlinearities which formed an extremely concen-
trated fluorophore, cooperatively enhancing dTPA [6].
HgL(SCN)2
Hg1eN1
Hg1eN2
Hg1eN3
Hg1eS1
Hg1eS2
S1eC33
2.417(9)
2.367(8)
2.39(1)
2.540(4)
2.451(4)
1.63(2)
1.55(1)
1.16(2)
1.13(1)
67.9(3)
N1eHg1eN3
S1eHg1eS2
S1eHg1eN1
S1eHg1eN2
S1eHg1eN3
S2eHg1eN1
S2eHg1eN2
S2eHg1eN3
S1eC33eN8
S2eC30eN6
136.1(3)
114.01(14)
98.4(3)
107.9(2)
100.6(2)
98.7(2)
137.5(2)
108.9(2)
176.1(15)
175.4(16)
S2eC30
N6eC30
N8eC33
N1eHg1eN2

726
H. Zhou et al. / Dyes and Pigments 95 (2012) 723e731
Table 3
The results of DFT calculation, the values of E0f, M01, Dm.
Formula
E0f (ev)
M01 (D)
Dm (D)
3.10
13.31
21.11
3.11
3.11
3.12
13.12
12.18
13.18
21.13
21.14
20.12
3.12
3.12
13.13
13.01
20.12
20.14
section dmax at the two-photon resonance energy (2h
related to the transition dipole moment M01 and the change in
n
¼ E1) is
permanent dipole moment Dm
excited state according to Eq. [18].
¼
m01
ꢀ
m0 between ground and
L4
5s20n2c2h
ð
DmÞ2M021
2
p
dmax
¼
(1)
G
Table 4
Photophysical properties of L, Dye 1, Dye 2, Dye 3, Dye 4 in different polar solvents.
Fig. 3. (a) Coordination environments of Hg with the atom numbering scheme. (b) The
1-D framework of Dye 4 showing the CeH/S (light green) hydrogen bond and
pep
Compd
Solvents
l
/nma
l
/nmb
Fc
Dn/cmꢀ1d
stacking (red) along the c-axis. (c) The 2-D framework of Dye 4 showing CeH/N
(purple) hydrogen bond along the a-axis. (d) The 3-D architecture of Dye 4 showing
the CeH/S (blackish green) hydrogen bond along the b-axis. (For interpretation of the
references to colour in this figure legend, the reader is referred to the web version of
this article.)
L
THF
286, 336
279, 327
288, 332
284, 330
283, 333
285, 336
282, 335
279, 326
290, 336
281, 331
286, 335
285, 331
284, 335
290, 322
285, 336
283, 329
290, 339
282, 334
284, 333
282, 324
284, 332
283, 329
295, 335
282, 334
281, 335
284, 323
285, 332
281, 331
290, 335
284, 334
408
404
515
412
420
424
408
404
413
412
420
425
408
404
414
412
420
424
407
405
415
414
419
424
409
405
415
413
420
423
0.468963
0.549127
0.335037
0.334894
0.496245
0.13953
5252
5829
10703
6031
6221
6177
5341
5922
5549
5940
6041
6682
5341
6303
5607
6123
5689
6355
5460
6173
6024
6241
5984
6355
5401
6268
6024
5998
6041
6299
Ethyl acetate
Ethanol
Acetonitrile
DMF
Methanol
THF
Ethyl acetate
Ethanol
Acetonitrile
DMF
Methanol
THF
Ethyl acetate
Ethanol
Acetonitrile
DMF
Methanol
THF
Ethyl acetate
Ethanol
Acetonitrile
DMF
Methanol
THF
HgCl2
0.44791
4. Theoretical calculation
0.456052
0.716585
0.38188
0.570439
0.23397
0.411701
0.039167
0.613287
0.436395
0.489241
0.265594
0.475053
0.362004
0.564316
0.421538
0.58198
0.296531
0.456047
0.106541
0.5374
0.397672
0.519437
0.27511
4.1. Calculation details
The structures were optimized by using the density-functional
theory with the 6e31G(d) basis set for the C, H, O, S atoms and
with the LANL2DZ basis set for the Hg, Br, I atoms [13]. DFT calcu-
lations were carried out using B3LYP (Becke’s three-parameter
hybrid exchange-correlation (x-c) functional) [14]. All the geom-
etry optimizations were carried out with no symmetry constraint.
Excitation energy calculations were carried out using TDDFT [15], as
implemented in the Gaussian 03 programs [16].
The structureeproperty relationships for the various design
approaches have been extensively reviewed [17]. For noncentre-
symmetric dipolar fluorophores, the attachment of an electron-
rich donor and an electron deficient acceptor group to a linear
HgBr2
HgI2
Hg(SCN)2
Ethyl acetate
Ethanol
Acetonitrile
DMF
p-system typically results in an uneven, dipolar ground state
charge distribution. Upon photoexcitation, such dipolar fluo-
rophores undergo charge redistribution from the donor to the
acceptor group to give a further polarized excited state with an
increased dipole moment compared to the ground state. Applying
a simplified two-state model that only includes the electronic
ground state S0 and the lowest excited state S1, the peak cross-
Methanol
a
Peak position of the longest absorption band.
Peak position of SPEF, excited at the absorption maximum.
b
c
Quantum yields determined by using quinine sulfate as standard.
d
Stokes’ shift in cmꢀ1
.

H. Zhou et al. / Dyes and Pigments 95 (2012) 723e731
727
where
G
is corresponds to a damping factor that represents the
TPA cross-sections of coordination compounds from a ligand with
overall bandwidth of the S0eS1 transition.
large d are often larger than one with small d. Considering a variety
of coordination modes of HgCl2, we only calculated Brꢀ, SCNꢀ, Iꢀ.
The halogen affect on the metal acceptance strength by different
electron inductive effect, Brꢀ > SCNꢀ > Iꢀ, which is consistent with
our previous study [19].
4.2. DFT calculation
In order to understand the relationship between the maximum
TPA cross-sections (d) and the different ligands, DFT calculations
5. Linear absorption and single-photon excited
fluorescence (SPEF)
were performed for all the complexes. As is established, metal
coordination to the terpyridinyl group increases the tendency for
charge transfer under excitation. E0f is corresponding to the tran-
sition energy (E0f) between the ground state (0) to the final state (f).
The photophysical properties (absorption and fluorescence) of
the novel series of dyes in six different solvents are collected in
Table 4 including fluorescence quantum yield.
According to Eq. (1) and the maximal E0f, M01, Dm (dmax) in Table 3,
these results clearly indicate that cooperative factors from M01 and
Dm that perform the TPA effect are obtained in coordination
complexes. In the table, the absorption maximum is very similar,
indicating that the metal binding may affect on the intensity of the
absorption rather than modify the route of absorption. Further
comparison of these data implies imidazole > triazole in enhancing
5.1. Absorption properties
Linear absorption spectra of L, Dye 1, Dye 2, Dye 3 and Dye 4 in
benzene, ethyl acetate, THF, ethanol, acetonitrile and DMF with
a solution concentration of c ¼ 1.0 ꢂ 10ꢀ5 mol Lꢀ1 are shown in
Fig. 4. From Table 4, one can observe that each of their absorption
d. It may be elucidated that the imidazole moieties are more liable
to excited state polarization than triazole, which may indicate that
Fig. 4. Linear absorption spectra of L, Dye 1, Dye 2, Dye 3 and Dye 4 in six solvents.

728
H. Zhou et al. / Dyes and Pigments 95 (2012) 723e731
spectra exhibits two peaks between 250 nm and 400 nm. The
solvent polarity, except in DMF. These phenomena maybe caused
by the non-radiative transition from the excited state to triples in
different polar solvents.
absorption maxima of L and four dyes are located at 333, 335, 335
and 335 nm in DMF, respectively, which corresponds to the
pe
p*
transition of the main chain. Fig. 4 is the linear absorption spectra
of L and four dyes in six different solvents, which shows that the
peak values are not significantly different and there is no linear
absorption in the entire spectral range from 450 to 900 nm.
Comparison of the
that the dyes have higher
charge transfer process (ICT). After L was coordinated with Hg2þ
a larger planarity was formed and a stronger acceptor generated,
possibly inducing charge delocalization on a greater scale.
F
of L and dyes in the same solvent indicates
F
values, which is due to intra-molecular
,
5.2. Fluorescence properties
5.3. Two-photon excited fluorescence (TPEF)
The SPEF spectra were measured at the same concentration as
that of the linear absorption spectra. As shown in Fig. 5 and Table 4,
the SPEF spectra of L, Dye 1, Dye 2, Dye 3 and Dye 4 showed slightly
red shift with increase of the polarity of solvent. This can be
explained by the fact that the excited state of compounds may
possess higher polarity than that of the ground state, for the sol-
vatochromism is associated with the energy level lowering [20].
Increased dipoleedipole interaction between the solute and
solvent leads to lowering the energy level on some scale [21]. In
The two-photon excited fluorescence spectra (TPEF) of
compounds L, Dye 1, Dye 2, Dye 3 and Dye 4 in DMF were recorded
at different excited wavelengths. The results are shown in Fig. 5.
Because there is no linear absorption in the range from 450 nm to
850 nm, the emission excited by a laser in this spectral range can be
attributed to the TPEF mechanism. Fig. 6 shows a logelog plot of the
excited fluorescence signal versus excited light power. It provides
direct evidence for the squared dependence of excited fluorescence
power and input laser intensity.
addition, obvious trends of decrease in
F occurred with increasing
Fig. 5. The one-photon fluorescence spectra of L, Dye 1, Dye 2, Dye 3 and Dye 4.

H. Zhou et al. / Dyes and Pigments 95 (2012) 723e731
729
The TPA cross-sections (
d) are determined by comparing their
TPEF to that of fluorescein in DMF, according to the following
equation [22]:
Fref cref nref
F
Fref
d
¼
dref
F
c
n
here, the subscript ref stands for the reference molecule.
d is the
TPA cross-section value, c is the concentration of solution, n is the
refractive index of the solution, F is the TPEF integral intensity of the
solution emitted at the exciting wavelength, and
F is the fluores-
cence quantum yield. The dref value of reference was taken from
m
the literature [23]. Detailed experiments reveal that from 680 to
850 nm, the peak position in the TPEF spectra of these complexes is
independent of the excitation wavelength, but the TPA cross-
sections are dependent over this range. By tuning the pump
wavelength up from 680 to 850 nm while keeping the input
power fixed and then recording the TPEF intensity, the two-photon
absorption spectra Fig. 7 are obtained. By referencing the TPA cross-
section of fluorescein to be 19 GM (1 GM ¼ 10ꢀ50 cmꢀ4 s photonꢀ1
)
Fig. 6. Output fluorescence (Iout) vs. the square of input laser power (Iin) for L. Exci-
tation was carried out at 720 nm, with c ¼ 1.0 ꢂ 10ꢀ3 mol/L in DMF.
[24], the maximum two-photon absorption cross-sections of
complexes are 97 GM for L, 144.13 GM for Dye 1, 194.14 GM for Dye
2, 160.96 GM for Dye 3 and 212.49 GM for Dye 4 at 720 nm in DMF.
Fig. 8 shows the TPA action spectrum of L, L-Hg (II) in the
680e850 nm range. It has been found that two-photon absorption
cross-sections of L-Hg (II) from 680 to 850 nm are always larger
than that of L, which is unusual for Hg2þ complexes. The behavior
observed here in the TPA spectra, can be attributed to the enlarged
As shown in Fig. 7, the TPA spectra of L and Dye 1 are determined
in the wavelength range by investigating their two-photon excited
fluorescence (TPEF) in DMF with a concentration of 1.0 ꢂ 10ꢀ3 mol/L.
The TPA spectra of the other dyes are similar to that of Dye 1.
The absorption maxima lengths locate at 442 and 484 nm for L,
526 nm for Dye 1, Dye 2 and Dye 3, 538 nm for Dye 4 in DMF,
respectively.
conjugation length upon Hg (II) addition L, a new DepeA type
compound formed, in which the terpyridyl ring is the acceptor part
and a triadozolyl group is the donor. When L is coordinated with
HgX2, the Hg2þ becomes the acceptor, which induces a larger
delocalization of the electrons. In addition, dyes with different
anions around Hg (II) have different TPA cross-section values,
which is due to different electron inductive effect inducing further
delocalization of electrons further (Fig. 9).
In order to study the effect of anions on dTPA, we tested and
calculated dTPA of the similar ligand L0 (40-(4-[4-(imidazolyl)styryl]
phenyl)-2,20:60,200-terpyridine
) and its complexes based on
mercury salts [19]. The maximum two-photon absorption cross-
section of L0 is 149 GM [19] (Fig. 9), much larger than that of L,
which agreed with the theoretical calculation. In addition,
Fig. 8. Two-photon (from
a 200 fs, 76 MHz, Ti: sapphire laser) absorption cross-
Fig. 7. The TPA spectra of L, Dye 1, Dye 2, Dye 3 and Dye 4 in DMF under different
sections of L, Dye 1, Dye 2, Dye 3 and Dye 4 in DMF versus excitation wavelengths
excitation wavelengths.
of identical energy of 0.100 w. (experimental uncertainties: 10%).

730
H. Zhou et al. / Dyes and Pigments 95 (2012) 723e731
National Nature Science Foundation of China (21071001, 51142011,
21101001), Natural Science Foundation of Education Committee of
An Hui Province (KJ 2012A024, KJ2010A030), the 211 Project of An
Hui University.
Appendix A. Supplementary data
Crystallographic data reported in this paper has been deposited
with the Cambridge Crystallographic Data Center, CCDC No.805571
for L, CCDC No.832301 for Dye 4. Copies of these information may
be obtained free of charge from the Director, CCDC, 12 Union Road,
Cambridge CB2 1EZ, UK (fax: þ44 1223336033; E-mail: deposit@
ccdc.cam.ac.uk). http://dx.doi.org/10.1016/j.dyepig.2012.07.005.
References
[1] (a) Zhang X, Sun YM, Yu XQ, et al. Synthesis and nonlinear optical properties
of several new two-photon photopolymerization initiators about dibenzo-
thiophene derivatives. Synthetic Metals 2009;159:2491e6;
(b) Li CW, Yang K, Feng Y, et al. Investigation of two-photon absorption
induced excited state absorption in a fluorenyl-based chromophore. Journal of
Physical Chemistry B 2009;113:15730e3.
[2] (a) Wang L, Tao XT, Shi JH, et al. Conjugated chromophore near the quantum-
confined cadmium sulfide sluster: quenched photoluminescence and enhanced
two-photon absorption. Journal of Physical Chemistry B 2006;110:19711e6;
(b)WangXM, ZhouXY, YuWT, FangQ, etal. Two-photonpumped lasingstilbene-
type chromophores containing various terminal donor groups: relationship
betweenlasing efficiency and intramolecular chargetransfer. JournalofMaterials
Chemistry 2000;10:2698e703.
[3] (a) Wang XM, Wang D, Zhou GY, et al. Symmetric and asymmetric charge
transfer process of two-photon absorbing chromophores: bis-donor
substituted stilbenes, and substituted styrylquinolinium and styrylpyr-
idinium derivatives. Journal of Material Chemistry 2001;11:1600e5;
(b) Yan YL, Li B, Liu KJ, et al. Enhanced two-photon absorption and ultrafast
dynamics of a new multibranched chromophore with a dibenzothiophene
core. Journal of Physical Chemistry A 2007;111:4188e94.
[4] (a) Wang XM, Jin F, Chen ZG, et al. A new family of dendrimers with naph-
thaline core and triphenylamine branching as a two-photon polymerization
initiator. Journal of Physical Chemistry C 2011;115:776e84;
(b) Wang XM, Jin F, Zhang WZ, et al. Novel De
p3e(A)1e3 multibranched
chromophores as an efficient two-photon-induced polymerization initiator.
Dyes and Pigments 2011;88:57e64.
[5] (a) Hu ZJ, Zhang RL, Sun PP, et al. Three asymmetrical conjugated De
sulfur-containing chromophores with focus on two-photon absorption.
Australian Journal of Chemistry 2011;64:174e9;
p
eD0
a
(b) Abbotto A, Beverina L, Bradamante S, et al. Design and synthesis of
heterocyclic multi-branched dyes for two-photon absorption. Synthetic
Metals 2003;136:795e7;
Fig. 9. DepeA structures of L and its complexes.
(c) He GS, Lin T, Prasad PN. Toward highly active two-photon absorbing
liquids. Synthesis and characterization of 1,3,5-triazine-based octupolar
molecules. Chemistry of Materials 2004;16:185e94.
complexes of Hg(II) with different anions also have different two-
photon absorption cross-sections as seen from Fig. 7. Considering
experimental error, the order is Iꢀ < Brꢀ < SCNꢀ, which is basically
consistent with theoretical results.
[6] Collini E. Cooperative effects to enhance two-photon absorption efficiency:
inta- versus inter-molecular approach. Physical Chemistry Chemical Physics
2012;14:3725e36.
[7] (a) Fabbrini G, Menna E, Maggini M, Canazza A, Marcolongo G, Meneghetti M.
Zinc-induced switching of the nonlinear optical properties of a functionalized
bis(styryl)benzene. Journal of American Chemical Society 2004;126:6238e9;
(b) Kim HM, Jeong MY, Ahn HC, Jeon SJ, Cho BR. Two-photon sensor for metal ions
derived from azacrown ether. Journal of Organic Chemistry 2004;69:5749e51;
(c) Righetto S, Rondena S, Locatelli D, Roberto D, Tessore F, Ugo R, et al. An
investigation on the two-photon absorption activity of various terpyridines and
related homoleptic and heteroleptic cationic Zn(II) complexes. Journal of Material
Chemistry 2006;16:1439e44.
6. Conclusions
In summary, we have synthesized and characterized novel
2,20:60,200-terpyridine-based ligand and its Hg2þ complexes. Single
crystal structures of L and Dye 4 were obtained, and supramolec-
ular structures were investigated. Linear and nonlinear optical
properties of the ligand and four dyes were exhibited. DFT calcu-
lations were performed to evaluate different anions effect on TPA
cross-section values. All results further proved that better co-
planar, inter-molecular interactions and anions may enhance the
2PA cross-section through cooperation effect.
[8] Cheng LH, Li L, Sun PP, Zhou HP, et al. Synthesis, structure and nonlinear
optical properties of two-photon absorption chromophores. Science in China
Series B e Chemistry 2009;4:529e34.
[9] Reynolds GA, Drexhage KH. New coumarin dyes with rigidized structure for
flashlamp-pumped dye lasers. Optics Communications 1975;13:222e5.
[10] Hu ZJ, Yang JX, Tian YP, Zhou HP, et al. Synthesis, structures, and optical
properties of two novel two-photon initiators derived from 2,20:60200- ter-
pyridine. Bulletin of the Chemical Society of Japan 2007;80:986e93.
[11] Jin F, Zhou HP, Tian YP, et al. Synthesis, crystal structures, and optical prop-
erties of a novel imidazole derivative and its Zn(II) complex. Journal of
Molecular Structure 2007;829:202e7.
Acknowledgments
[12] Mongin O, Porres L, Kahn C, Pons T, Mertz J, Blanchard-Desce M. Synthesis and
two-photon absorption of highly soluble three-branched fluorenylene-vinyl-
ene derivatives. Tetrahedron Letters 2003;44:8121e5.
[13] (a) Hehre WJ, Ditchfield R, Pople JA. Molecular orbital theory of the electronic
structure of organic compounds. V. molecular theory of bond separation.
Journal of American Chemical Society 1970;92:4796e801;
This work was supported by a program for New Century
Excellent Talents in university, Doctoral Program of Ministry of
Education of China (20113401110004), Science and Technological
Fund of An Hui Province for Outstanding Youth (10040606Y22),

H. Zhou et al. / Dyes and Pigments 95 (2012) 723e731
731
(b) Hehre WJ, Ditchfield R, Pople JA. Self-consistent molecular orbital methods:
XII. Further extentions of Gaussian-Type basis sets for use in molecular orbital
studies of organic molecules. Journal of Chemical Physics 1972;56:2257e61.
[14] (a) Becke AD. Density-functional thermochemistry. III. The role of exact
exchange. Journal of Chemical Physics 1993;98:5648e52;
and the design of two-photon dyes. Angewandte Chemie International Edition
2009;48:3244e66.
[18] (a) Kogej T, Beljonne D, Meyers F, et al. Mechanisms for enhancement of two-
photon absorption in donor-acceptor conjugated chromophores. Chemical
Physics Letters 1998;298:1e6;
(b) Becke AD. Density-functional exchange-energy approximation with
correct asymptotic behavior. Physical Review A 1988;38:3098e100.
[15] (a) Bauernschmitt R, Ahlrichs R. Treatment of electronic excitations within the
adiabatic approximation of time dependent density-functions theory. Chem-
ical Physics Letters 1996;256:454e64;
(b) Wiirthner F, Archetti G, Schmidt R, Kuball HG. Solvent effect on color, band
shape, and charge-density distribution for merocyanine dyes close to the
cyanine limit. Angewandte Chemie International Edition 2008;47:4529e32.
[19] Zhou HP, Zhou FX, Wu P, et al. Three new five-coordinated mercury (II) dyes:
structure and enhanced two-photon absorption. Dyes and Pigments 2011;91:
237e47.
[20] Bublitz GU, Ortiz R, Marder SR, et al. Stark spectroscopy of donoreacceptor
polyenes:correlation with nonlinear optical measurements. Journal of Amer-
ican Chemical Society 1997;119:2311e2.
(b) Casida ME, Jamorski C, Casida KC, Salahub DR. Molecular excitation
energies to high-lying bound states from time-dependent density-functional
response theory: characterizaton and correction of the time-dependent local
density approximation ionization threshold. Journal of Chemical Physics
1998;108:4439e49.
[21] Xu C, Webb WW. Measurement of two-photon excitation cross sections of
molecular fluorophores with data from 690 to 1050 nm. Journal of the Optical
Society of America B 1997;13:481e91.
[16] Frisch MJ, Trucks GW, Schlegel HB, et al. In: Gaussian 03, A.11. 4th ed. Pitts-
burgh, PA: Gaussian, Inc.; 2003.
[17] (a) Terenziani F, Katan C, Badaeva E, Tretiak S, Blanchard-Desce M. Enhanced
two-photon absorption of organic chromophores: theoretical and experi-
mental assessments. Advanced Materials 2008;20:4641e78;
[22] Varnavski O, Goodson T, Sukhomlinova L, Twieg R. Ultrafast exciton dynamics
in a branched molecule investigated by time-resolved fluorescence, transient
absorption, and three-Pulse photon echo peak shift measurements. Journal of
Physical Chemistry B 2004;108:10,484e10,492.
[23] Albota MA, Xu C, Webb WW. Two-photon fluorescence excitation cross
sections of biomolecular probes from 690 to 960 nm. Applied Optics 1998;37:
7352e6.
[24] Li ZG, Wu WB, Qin JG, et al. Dendronlike main-chain nonlinear optical (NLO)
polyurethanes constructed from “H”-type chromophores: synthesis and NLO
properties. Applied Materials & Interfaces 2009;4:856e63.
(b) He GS, Tan LS, Zheng Q, Prasad PN. Multiphoton absorbing materials:
molecular designs, characterizations, and applications. Chemical Reviews
2008;108:1245e330;
(c) Kim HM, Cho BR. Nonblinking and nonbleaching upconverting nano-
particles as an optical imaging nanoprobe and T1 magnetic resonance imaging
contract agent. Chemical Communications 2009;2:153e64;
(d) Pawlicki M, Collins HA, Denning RG, Anderson HL. Two-photon absorption