Three new five-coordinated mercury (II) dyes: Structure and enhanced two-photon absorption

Article abstract of DOI:10.1016/j.dyepig.2011.03.024

A new donor-bridge-acceptor (D-π-A) type ligand (L: 4′-(4-[4- (imidazole)styryl]phenyl)-2,2′:6′,2″-terpyridine) with two-photon absorption and coordination ability was designed and synthesized. Self-assembly of the ligand with HgX₂ (X = Cl, Br,

Full text of DOI:10.1016/j.dyepig.2011.03.024

Dyes and Pigments 91 (2011) 237e247
Contents lists available at ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Three new five-coordinated mercury (II) dyes: Structure
and enhanced two-photon absorption
,
a
a
a
a
a
Hongping Zhou a , Feixia Zhou , Peng Wu , Zheng Zheng , Zhipeng Yu , Yixin Chen ,
*
Yulong Tu a, Lin Kong a, Jieying Wu a, Yupeng Tian a
,b,c
a Department of Chemistry, 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 new donorebridgeeacceptor (Dep
eA) type ligand (L: 40-(4-[4-(imidazole)styryl]phenyl)-2,20:60,200-
Article history:
Received 6 November 2010
Received in revised form
17 March 2011
Accepted 17 March 2011
Available online 23 March 2011
terpyridine) with two-photon absorption and coordination ability was designed and synthesized. Self-
assembly of the ligand with HgX2 (X ¼ Cl, Br, I) yielded a series of new coordination complexes (Dyes 1e3)
with five-coordinated mercury (ΙΙ), which were characterized by single crystal X-ray diffraction determi-
nation. Solvent molecules and various weak interactions, including hydrogen bonds (CeH$$$N, CeH$$$X)
and
pep interactions played significant roles in the final topological structures. Linear and nonlinear
optical properties of the ligand and three dyes were described. Experimental results revealed that two-
photon absorption cross sections of three dyes are extraordinary stronger than that of ligand.
Ó 2011 Elsevier Ltd. All rights reserved.
Keywords:
Single crystal
Hydrogen bonds
pep interactions
Linear absorption
Fluorescence
Two-photon absorption
1. Introduction
exhibits enhanced fluorescence and TPA activity by aggregation.
The elegant work of Prasad [4] demonstrated that two-photon
Two-photon absorption (TPA) is a process in which two photons
are simultaneously absorbed to an excited state in a medium via
a virtual state under intense laser pulses when the sum of the
energies of two absorbed photons matches well with the energy
gap between an allowed excited state and the ground state. Since
the theory was first proposed by Göppert-Mayer in 1931 [1], many
researchers have been interested in seeking novel TPA materials
due to their promising applications in optical limiting, two-photon
laser scanning fluorescence imaging, microfabrication, 3D optical
data storage, photodynamic therapy and so on [2]. They have done
substantial research on structureeproperty relationships. Although
it is still under exploration, thanks to the effort of scientists, some
efficient molecular design strategies were put forward to provide
guidelines for the development of materials with large TPA cross
sections [3].
active dyes could be incorporated inside nanometer-sized silica
nano-bubbles via a simple reverse micelle approach. The encap-
sulation of hydrophobic dyes within silica not only improves their
water dispersibility but also leads to an increase in emission life-
time and efficiency as well as photostability. The studies of organic
chromophores near the surface of silver nanoparticles with random
fractal geometry or self-assembled ones on the nanoparticle
surface exhibit large enhancement in two-photon activities relative
to isolated chromophores [5,6].
Our group has developed a series of two-photon absorption
materials [7] and modified nanoparticles [8]. Recently, a new series
of organiceinorganic hybrid dyes had been reported to enhance the
TPA and two-photon polymerization (TPP) properties via changing
anions [9]. These results revealed that the packing and environment
of the chromophores have profound influence on the TPA proper-
ties. Many TPA dyes reported based on conjugated aromatic rings
are now available. Some complicated hetero-aromatic compounds
are often used for special purposes in the two-photon area due to
their favorable coordinate abilities. For example, 2, 20:60, 200-ter-
pyridine with a rigid framework plane and a superb ability to
coordinate with various metal ions, is still not well explored [10,11].
Scaling down these materials has recently afforded an exciting
new class of highly tailorable organiceinorganic hybrids, which
* Corresponding author. Tel.: þ86 551 5108151; fax: þ86 551 5107342.
E-mail addresses: zhoufxfx@126.com, zhpzhp@263.net (H. Zhou).
0143-7208/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.dyepig.2011.03.024

238
H. Zhou et al. / Dyes and Pigments 91 (2011) 237e247
Its derivatives are often used for many physical and chemical
applications [12]. The recent success of the research on green and
facile preparations of the key precursor 1,5-di(2-pyridinyl)-3-p-
tolylpentane-1,5-dione (30) [13] has aroused our interest in studying
these derivatives from the two-photon materials point of view. By
incorporating imidazolyl unit into the terpyridine backbone, a new
were characterized by 1H NMR, FT-IR, and MS spectrometry. Starting
materials (20e60) were prepared according to literature procedures
[15] (Scheme 1) and 4-(1H-Imidazol-1-yl)benzaldehyde was
prepared according to literature procedures [16].
2.2.1. Preparation of the ligand: 40-(4-[4-(imidazolyl)styryl]
phenyl)-2,20:60,20’-terpyridine
DepeA type compound, in which the terpyridyl ring is the acceptor
part and an imidazolyl group as the donor one, was prepared (Fig.1).
In this work, solvent-free Wittig reactions were used in building
t-BuOK (1.38 g, 12.3 mmol) was placed into a dry mortar and
well milled into powder, then 60 (1.99 g, 3.0 mmol) and 4-(1H-
Imidazol-1-yl)benzaldehyde (0.55 g, 3.2 mmol) were added and
mixed. The mixture was milled vigorously for about 20 min. The
mixture became sticky, and then tetrahydrofuran (5 mL) was
added. The mixture was continuously milled for another 10 min.
After completion of the reaction (monitored by Thin Layer chro-
matography (TLC)), the mixture was dispersed in 100 mL ethanol.
The residual solid was filtered and recrystallized from anhydrous
dichloromethane/ethanol, to give yellow crystals L (1.10 g, yield
the
p-conjugated system (Scheme 1). Our synthetic strategy is
based on the molecular self-assembly of organic ligand with TPA
properties and different inorganic salts. A series of new coordina-
tion complexes (Dyes 1e3) with five-coordinated mercury (ΙΙ) were
yielded by self-assembling the ligand with HgX2 (X ¼ Cl, Br, I),
which were characterized by single crystal X-ray diffraction
determination. Linear and nonlinear optical properties of the ligand
and three dyes were investigated. Results showed these three new
dyes exhibit enhanced TPA cross-section values, exceeding corre-
sponding ligand, which offered a reference in designing and
synthesizing new TPA materials with high performance.
76%). 1H NMR (CDCl3, 400 MHz):
d 7.17 (s, 1H), 7.45 (s, 2H),
7.53e7.56(m, 2H), 7.70e7.73 (d, J ¼ 8.50 Hz, 2H), 7.81e7.84 (m, 4H),
7.86 (s, 1H), 7.98e8.00 (d, J ¼ 8.30 Hz, 2H), 8.06e8.07 (m, 2H), 8.34
(s, 1H), 8.68e8.70 (d, J ¼ 8.00 Hz, 2H), 8.76 (s, 2H), 8.78e8.79 (d,
J ¼ 4.51 Hz, 2H). MS: m/z (%) ¼ 321.12 (100). FT-IR (KBr, cmꢁ1): 3408
(w), 3051 (w), 1584 (s), 1565 (s), 1466 (m), 1387 (s), 1242 (m), 1140
(m), 989 (m), 788 (s), 740 (s), 634 (m), 614 (m). Anal. Calcd for
C32H23N5: C, 80.48; H, 4.85; N, 14.66%. Found: C, 80.89; H, 4.50; N,
15.04%.
2. Experimental section
2.1. Materials and physical measurements
Elemental analyses were performed with a PerkineElmer 240C
elemental analyzer. The 1H NMR spectra were recorded at 25 ꢀC on
a Bruker Avance 400 spectrometer, and the chemical shift are
2.2.2. Preparation of dyes 1e3
reported as parts per million (ppm) from TMS (d). Coupling
constants J are given in Hertz. Mass spectra were acquired on
a Micromass GCT-MS (EI source). IR spectra were recorded on
a NEXUS 870 (Nicolet) spectrophotometer in the 400e4000 cmꢁ1
region using a powder sample on a KBr plate. UVevis absorption
spectra were recorded on a UV-3100 spectrophotometer. Fluores-
cence measurements were carried out using an Edinburgh FLS920
fluorescence spectrometer equipped with a 450W Xe lamp and
a time-correlated single-photon counting (TCSPC) card. All the
fluorescence spectra were collected. The single-photon excited
HgLCl2$CHCl3(Dye 1): HgCl2 (10.88 mg, 0.04 mmol) in 15 mL of
MeOH was layered onto a solution of L (19.08 mg, 0.04 mmol) in
5 mL of CHCl3 and stood for ten days to give yellow cube single
crystals. Yield: 22 mg (73%). FT-IR (KBr, cmꢁ1): 3050 (w), 1595 (s),
1574 (s), 1521 (w), 1473 (m), 1401 (m), 1248 (m), 807 (s), 789 (s).
Anal. Calcd for C33H24Cl5HgN5: C, 45.64; H, 2.79; N, 8.06%. Found: C,
45.19; H, 2.68; N, 7.65%.
HgLBr2$CHCl3(Dye 2): HgBr2 (14.48 mg, 0.04 mmol) in 15 mL of
MeOH was layered onto a solution of L (19.08 mg, 0.04 mmol) in
5 mL of CHCl3 and stood for ten days to give yellow cube single
crystals. Yield: 24 mg (72%). FT-IR (KBr, cmꢁ1): 3057 (w), 1596 (s),
1573 (s), 1522 (w), 1475 (m), 1400 (m), 1247 (m), 807 (s), 789 (s).
Anal. Calcd for C33H24Br2Cl3HgN5: C, 41.40; H, 2.53; N, 7.32%. Found:
C, 41.06; H, 2.44; N, 7.08%.
HgLI2$CHCl3(Dye 3): HgI2 (18.24 mg, 0.04 mmol) in 15 mL of
MeOH was layered onto a solution of L (19.08 mg, 0.04 mmol) in
5 mL of CHCl3 and stood for ten days to give yellow cube single
crystals. Yield: 29 mg (78%). FT-IR (KBr, cmꢁ1): 3055 (w), 1595 (s),
1541 (s), 1521 (w), 1475 (m), 1399 (m), 1246 (m), 807 (s), 788 (s).
Anal. Calcd for C33H24Cl3HgI2N5: C, 37.70; H, 2.30; N, 6.66%. Found:
C, 37.33; H, 2.27; N, 6.31%.
fluorescence (SPEF) quantum yields (
a standard method under the same experimental conditions for all
compounds. Rhodamine B dissolved in ethanol (
F) were measured by using
F
¼ 0.56) [14] at
the same concentration as the other samples was used as the
standard. The two-photon excited fluorescence (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.
2.2. Materials and synthesis
All chemicals were available commercially, and the solvents were
purified as conventional methods before use. These compounds
2.3. X-ray crystallography
X-ray diffraction data of single crystals were collected on
a Siemens Smart 1000 CCD diffractometer. The determination of
unit cell parameters and data collections were performed with Mo
N
Ka
radiation (
l
¼ 0.71073 Å). Unit cell dimensions were obtained
with least-squares refinements, and all structures were solved by
direct methods using SHELXS-97 [17]. 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
riding on the concerned atoms. Crystallographic crystal data and
N
N
N
N
Fig. 1. Schematic drawing of the ligand (L).

H. Zhou et al. / Dyes and Pigments 91 (2011) 237e247
239
CHO
O
O
2-acetylpyridine
2-acetylpyridine
20% NaOH aq.
NH4OAc
EtOH
N
N
2% NaOH aq.
N
N
N
O
N
3'
1'
2'
4'
Br
PPh3Br
N
N
CHO
PPh3
toluene
NBS, BPO
CCl4
L
grind
t-BuOK
N
N
N
N
N
N
5'
6'
Scheme 1. Synthesis strategy for the preparation of L.
processing parameters for dyes 1e3 are shown in Table 1. Selected
bond lengths and bond angles for dyes 1e3 are listed in Table 2.
provided Cl, d(H34$$$Cl2) ¼ 2.659 Å and :(C34eH34$$$Cl1) ¼
153.24ꢀ).
As shown in Fig. 2b, the imidazole ring does not directly
involve in coordination, but the lone pair electrons from the N
atom still have a strong Pro-e-competence, which generates a 1-D
chain along c-axis through CeH$$$N bond interactions (rose
dashed line, d(C4eH4$$$N4) ¼ 2.689 Å, yellow dashed line,
d(C17eH17$$$N4) ¼ 2.694 Å, the angle of C4eH4eN4 is 155.15ꢀ and
the angle of C17eH17eN4 is 166.27ꢀ). Also it can be seen that there
3. Results and discussion
3.1. Structure of dyes 1e3
3.1.1. Structure of HgLCl2$CHCl3 (dye 1)
Dye 1 crystallizes in the monoclinic form with space group C2/c
as shown in Fig. 2a. The unit consists of one mercury ion connecting
three N from one L and two chlorine atoms, and a free solvent
chloroform molecule. The bond lengths Hg1eN1, Hg1eN2,
Hg1eN3 are 2.400(8), 2.403(7), 2.441(9) Å, respectively. Each bond
angle around the mercury atom is in the range 68.2(3)e133.4(3)ꢀ,
indicating a quite distorted tetrahedral geometry. Selected bond
lengths and angles are listed in Table 2. Besides, the chloroform
molecule connects to mercury salt through CeH$$$Cl hydrogen
bonds (the chloroform molecule provided H and mercury salt
are
pep stacking contacts (red dashed line) with the shortest
distance to be 3.470 Å between two neighboring dimers. Extraor-
dinarily, the chloroform molecules play the vital role in the dimmer
formation (pink dashed line, d(C34eH34$$$Cl2) ¼ 2.659 Å and the
angle of C34eH34$$$Cl2 is 153.23ꢀ, sky blue dashed line,
d(Cl1$$$Cl5) ¼ 3.236 Å, and the van der waals radii of H and Cl atom
are 1.20 and 1.80 Å,).
Table 2
Selected bond lengths (Å) and angles (ꢀ) for dyes 1e3.
Dye 1
Hg1eN1
Hg1eN2
Table 1
2.400(8)
2.403(7)
2.441(9)
2.428(3)
2.425(3)
68.2(3)
Cl1eHg1eCl2
Cl1e Hg1eN1
Cl1eHg1eN2
Cl1eHg1eN3
Cl2eHg1eN1
Cl2eHg1eN2
Cl2eHg1eN3
118.27(12)
105.0(2)
109.89(19)
99.5(2)
104.1(2)
131.3(2)
98.1(2)
Crystallographic data for dyes 1e3.
Compound
Dye 1
Dye 2
Dye 3
Hg1eN3
Hg1eCl1
Hg1eCl2
N1eHg1eN2
N1eHg1eN3
Empirical formula
Formula weight
Crystal system
Space group
a [Å]
C32H23Cl2HgN5
868.41
Monoclinic
C2/c
21.220(5)
17.455(5)
17.578(5)
90.000(5)
94.353(5)
90.000(5)
6492(3)
8
298(2)
1.777
5.186
1.51e25.00
5684
C32H23Br2HgN5
957.33
Monoclinic
C2/c
21.280(5)
17.438(5)
17.597(5)
90.000(5)
92.899(5)
90.000(5)
6522(3)
8
298(2)
1.950
7.449
1.51e25.00
5757
C32H23I2HgN5
1051.31
Monoclinic
C2/c
21.476(5)
17.520(5)
17.983(5)
90.000(5)
92.016(5)
90.000(5)
6762(3)
8
298(2)
2.065
6.647
1.50e24.99
5970
133.4(3)
Dye 2
Hg1eN1
Hg1eN2
Hg1eN3
Hg1eBr1
Hg1eBr2
N1eHg1eN2
N1eHg1eN3
b [Å]
c [Å]
2.402(9)
2.391(8)
2.447(10)
2.5431(15)
2.5496(17)
67.4(3)
Br1eHg1eBr2
Br1eHg1eN1
Br1eHg1eN2
Br1eHg1eN3
Br2eHg1eN1
Br2eHg1eN2
Br2eHg1eN3
117.97(5)
103.2(2)
131.6(2)
98.5(2)
107.0(2)
109.9(2)
99.6(3)
a
[ꢀ]
[ꢀ]
b
g
[ꢀ]
V [Å3]
Z
131.8(3)
T [K]
Dcalcd[g cmꢁ3
]
m
[mmꢁ1
]
Dye 3
q
range [ꢀ]
Hg1eN1
Hg1eN2
Hg1eN3
Hg1eI1
Hg1eI2
N1eHg1eN2
N1eHg1eN3
2.483(9)
2.434(8)
2.429(8)
2.7128(11)
2.6993(11)
66.1(3)
I1eHg1eI2
I1eHg1eN1
I1eHg1eN2
I1eHg1eN3
I2eHg1eN1
I2eHg1eN2
I2eHg1eN3
120.95(3)
97.7(2)
108.2(2)
108.9(2)
98.8(2)
Total no. data
No. unique data
No. params refined
R1
wR2
GOF
3029
398
0.0523
0.1258
3072
397
0.0522
0.1333
3729
397
0.0458
0.1252
130.3(2)
101.15(19)
1.034
0.958
0.930
131.2(3)

240
H. Zhou et al. / Dyes and Pigments 91 (2011) 237e247
Fig. 2. (a) Coordination environments of Hg with the atom numbering scheme. (b) The 1-D framework of dye 1 showing the CeH$$$N (rose and yellow) hydrogen bond and
stacking (red) along the c-axis. (c) The 2-D framework of dye 1 showing stacking along the a-axis. (d) The 3-D architecture of dye 1 showing the CeH$$$Cl (red and rose)
hydrogen bond along the b-axis. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
pep
pep
From Fig. 2c, dye 1 formed a 2-D framework through
p
e
p
from L, two bromine atoms and one chloroform molecule. The X-
ray single crystal reveals that dye 2 also crystallizes in the mono-
clinic form with space group C2/c as shown in Fig. 3a. The bond
lengths Hg1eN1, Hg1eN2, Hg1eN3 are 2.402, 2.391, 2.447 Å,
respectively. Each bond angle around the mercury atom is in the
range 65.9(3)e131.8(3)ꢀ. Selected bond lengths and angles are lis-
ted in Table 2. However, different from dye 1, the chloroform
molecule connects to the ligand through CeH$$$Cl hydrogen bonds
(the chloroform molecule provided Cl and the ligand provided H, d(
H25$$$Cl2) ¼ 2.671 Å and :(C25eH25$$$Cl2) ¼ 160.21ꢀ).
stacking interactions (red dashed line, the shortest distance is
3.543 Å, plum dashed line, the shortest distance is 3.470 Å)
between the neighboring dimers along a-axis.
Finally, the extended 3-D topological structures are formed
along the b-axis via CeH$$$Cl interactions (red dashed line,
d(C1eH1$$$Cl1) ¼ 2.939 Å, d(C12eH12$$$Cl1) ¼ 2.683 Å, the angle
of C1eH1eCl1 is 131.87ꢀ and the angle of C12eH12eCl1 is 154.75ꢀ)
(Fig. 2d).
3.1.2. Structure of HgLBr2$CHCl3 (Dye 2)
Similar to dye 1, dye 2 also indicates a quite distorted tetrahedral
geometry, which consists of one mercury (ΙΙ) connecting three N
Fig. 3b shows that the weak force of CeH$$$Br is widely spread
in the complex to construct this super molecular structure. Also we
can find that the chloroform molecules play a very important role

H. Zhou et al. / Dyes and Pigments 91 (2011) 237e247
241
Fig. 3. (a) Coordination environments of Hg with the atom numbering scheme. (b) Weak interactions form solvent in dye 2. (c) The 1-D framework of dye 2 showing the CeH$$$N
(green and light orange) hydrogen bond and stacking (red) along the c-axis. (d) The 2-D framework of dye 2 showing stacking along the a-axis. (e) The 3-D architecture of
pep
pep
dye 2 showing the CeH$$$Br (red and pink) hydrogen bond along the b-axis. (For interpretation of the references to color in this figure legend, the reader is referred to the web
version of this article.)
as follows: (1)Cl2 and C22, H22, C25, H25 of dye form of CeH$$$Cl
bonds (green dashed line, d(C22eH22$$$Cl2) ¼ 2.932 Å, light
CeH$$$Br bond with the distance of C33eH33$$$Br1 is 2.971 Å
(rose dashed line) and the angle of C33eH33eBr1 is 150.94ꢀ.
Similar to dye 1, dye 2 generates a 1-D chain along c-axis
through CeH$$$N bond interactions (bright green dashed line,
orange dashed line, d(H25$$$Cl2)
¼
2.671 Å, the angle of
C22eH22eCl2 is 174.62ꢀ and the angle of C25eH25eCl2 is
160.21ꢀ). (2) Cl1 and C3, H3 of dye form of CeH$$$Cl bond with the
distance of C3eH3$$$Cl1 is 2.798 Å (red dashed line) and the angle
of C3eH3eCl1 is 160.67ꢀ. (3) Br1 and C33, H33 of dye form of
d(C12eH12$$$N5)
¼
2.672 Å, light orange dashed line,
d(C18eH18$$$N5) ¼ 2.665 Å, the angle of C12eH12eN5 is 155.72ꢀ
and the angle of C18eH18eN5 is 167.40ꢀ). Also it can be seen that

242
H. Zhou et al. / Dyes and Pigments 91 (2011) 237e247
there are
p
e
p
stacking interactions (red dashed line, the shortest
C30eH30eBr1 is 142.78ꢀ and the angle of C33eH33eBr1 is
150.93ꢀ) along b-axis (Fig. 3e).
distance is 3.473 Å) between two neighboring dimmers (Fig. 3c).
From Fig. 3d, dye 2 forms a 2-D framework through pep
stacking (red dashed line, the shortest distance is 3.579 Å, plum
dashed line, the shortest distance is 3.473 Å) between the neigh-
boring dimers along a-axis.
3.1.3. Structure of HgLI2$CHCl3 (Dye 3)
Dye 3 adopts the same coordination mode as dye 2. The single
crystal analysis of dye 3 reveals that the mercury (ΙΙ) is five-coor-
dinated via three N atoms from L and two I atoms. The bond lengths
Hg1eN1, Hg1eN2, Hg1eN3 are 2.483, 2.434, 2.429 Å, respectively
(Fig. 4a). Each bond angle around the mercury atom is in the range
Finally, dye 2 forms a stable 3-D architecture through CeH$$$Br
interactions (red dashed line, d(C30eH30$$$Br1) ¼ 3.048 Å, pink
dashed line, d(C33eH33$$$Br1)
¼
2.972 Å, the angle of
Fig. 4. (a) Coordination environments of Hg with the atom numbering scheme. (b) Weak interactions form solvent in dye 3. (c) The 1-D framework of dye 3 showing the CeH$$$N
(turquoise and light orange) hydrogen bond, the CeH$$$I (dark yellow and sea green) hydrogen bond and stacking (red) along the c-axis. (d) The 2-D framework of dye 3
showing stacking along the a-axis. (e) The 3-D architecture of dye 3 showing the CeH$$$I (gold and orange) hydrogen bond along the b-axis. (For interpretation of the
references to color in this figure legend, the reader is referred to the web version of this article.)
pep
pep

H. Zhou et al. / Dyes and Pigments 91 (2011) 237e247
243
Table 3
Photophysical properties of L, Dye 1, Dye 2 and Dye 3 in several of different polar solvents.
Cmpd
Solvents
Benzene
Ethyl acetate acetate
THF
Ethanol
Acetonitrile
DMF
L
l
l
/nma
282, 333
390, 410
283, 331
409
283, 331
414
283, 332
422
281, 333
424
284, 334
434
/nmb
Fc
0.11
5640
0.07
5762
0.09
6057
0.06
6424
0.07
6445
0.11
6899
Dn/cmꢁ1d
Dye 1
Dye 2
l
l
/nma
/nmb
281, 328
391, 410
282, 326
409
282, 331
412
283, 327
422
282, 328
424
281, 334
432
Fc
0.12
6098
0.07
6225
0.07
5940
0.05
6884
0.08
6903
0.10
6792
Dn/cmꢁ1d
l
l
/nma
/nmb
282, 327
391, 410
275, 323
408
278, 331
413
285, 326
424
275, 327
424
279, 335
434
Fc
0.19
6191
0.08
6450
0.09
5999
0.08
7090
0.10
6996
0.11
6809
Dn/cmꢁ1d
Dye 3
l
l
/nma
/nmb
279, 328
391, 410
275, 324
407
278, 331
414
280, 325
423
275, 322
425
283, 331
433
Fc
0.18
6098
0.09
6294
0.09
6057
0.07
7129
0.08
7526
0.11
7117
Dn/cmꢁ1d
a
Peak position of the longest absorption band.
b
c
Peak position of SPEF, exited at the absorption maximum.
Quantum yields determined by using coumarin as standard.
d
Stokes’ shift in cmꢁ1
.
66.1(3)e131.2(3)ꢀ, indicating a quite distorted tetrahedral geom-
etry. Selected bond lengths and angles are listed in Table 2. Similar
to dye 2, both the chloroform molecules and pep stacking between
two neighboring dimmers play a significant role in 1-D and 2-D
framework, as well as CeH$$$I interactions.
interesting, which is different from the most common coordination
framework of mercury atoms that is four-coordination and six-
coordination [18]. In our dyes, the Hg atoms are all five-coordina-
tion, which is probably due to the specific structure of the ligand for
its good planarity. Meanwhile, the nitrogen atoms distribute evenly
on the rings, which is advantageous in forming the spatial structure
of the distorted tetrahedron.
From the crystal structures of three dyes, the five-coordination
mode of Hg (ΙΙ) with a quite distorted tetrahedral geometry is very
Fig. 5. Linear absorption spectra of L, dye 1, dye 2 and dye 3 in six solvents.

244
H. Zhou et al. / Dyes and Pigments 91 (2011) 237e247
Fig. 6. The one-photon fluorescence spectra of L, dye 1, dye 2 and dye 3.
4. Linear absorption and single-photon excited
fluorescence (SPEF)
shift are observed for L and three dyes in six solvents due to strong
solvent-solute dipoleedipole interactions, a manifestation of the
large dipole moment and orientational polarizability. Because L and
three dyes contain an electron-donating group and an electron-
withdrawing group, they exhibit a significant dipole moment in the
ground state, which depends on charge separation in the fluo-
rophore. In the excited state, it is likely that this charge separation
increases, resulting in a larger dipole moment than that in the
ground state [19]. Thus, such an increase in dipole moment would
explain the sensitivity of the emission spectra of these dipolar
compounds to solvent polarity. Besides, the fluorescence intensity
of L decreases gradually with increasing solvent polarity. From
Fig. 6, the two peaks phenomenon in the minimum polar solvent of
benzene, has been observed, which may be attributes to the twisted
intramolecular charge transfer (TICT) [20]. The quantum yields (V)
The photophysical properties (absorption and fluorescence) of
the new series of dyes in six different solvents are collected in
Table 3 including fluorescence quantum yields.
4.1. Absorption properties
Linear absorption spectra of L, dye 1, dye 2 and dye 3 in benzene,
ethyl acetate, THF, ethanol, acetonitrile and DMF with a solution
concentration of c ¼ 1.0 ꢂ 10ꢁ5 mol/L are shown in Fig. 5. From
Table 3, one can observe that each of their absorption spectra
exhibits two peaks between 250 nm and 400 nm and the absorp-
tion maxima of L and three dyes are located at 334, 334, 335 and
331 nm in DMF, respectively, which corresponds to the
pep
*
transition of the main chain. The absorption spectrum is not very
sensitive to the solvent polarity. The linear absorption spectra of L
and three dyes in six different solvents are shown in Fig. 5 and one
can observe that there is no linear absorption in the entire spectral
range from 450 to 900 nm.
4.2. Fluorescence properties
The SPEF spectra were measured at the same concentration as
that of the linear absorption spectra. As shown in Fig. 6 and Table 3,
the SPEF spectra of L, dye 1, dye 2 and dye 3 display pronounced
positive solvatochromism (i.e., bathochromic shift with increasing
solvent polarity) in their emission spectra. The emission maxima of
L and dyes are about 410 nm in benzene. In high polar solvent DMF,
the peak is located at 434 nm, which is red-shifted by 24 nm due to
the different molecular environment. Large values of the Stokes
Fig. 7. The Solid-State fluorescence spectra of L, dye 1, dye 2 and dye 3.

H. Zhou et al. / Dyes and Pigments 91 (2011) 237e247
245
that there are no molecular energy levels corresponding to an elec-
tron transition in the spectral range. Therefore, upon excitation from
450 to 850 nm, it is impossible to produce single-photon excited up-
converted fluorescence. If frequency up-converted fluorescence
induced with a laser in this spectral range appears, it can be ascribed
to two-photon excited fluorescence (TPEF). Fig. 8 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.
As shown in Fig. 9, 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 10ꢁ3 mol/
L. The TPA spectra of dye 2 and dye 3 are similar to that of dye 1. The
absorption maxima locate at 448 and 487 nm for L, 526 nm for dye
1, dye 2 and dye 3 in DMF, respectively.
The TPA cross sections (d) are determined by comparing their
TPEF to that of fluorescein in DMF, according to the following
Fig. 8. Output fluorescence (Iout) vs. the square of input laser power (Iin) for L. Exci-
tation carried out at 720 nm, with c ¼ 1.0 ꢂ 10ꢁ3 mol/L in DMF.
equation [22]:
of the ligand and the three complexes in different solvents are
determined by using RhB as a standard. Upon increasing the solvent
polarity, the fluorescence quantum yields exhibit no obvious
decrease. This behavior is different from previous reports [21].
As shown in Fig. 7, the Solid-State fluorescence maxima of L
locates at 460 nm and three coordination complexes dye 1, 2 and 3
locate at 526, 503 and 511 nm, respectively. The Solid-State fluo-
rescence spectrum shows a marked red shift of the maximum
corresponding to the metal to ligand charge transfer (MLCT) tran-
sition upon coordination L with Hg (II) salts.
Fref cref nref
F
Fref
d
¼
dref
F
c
n
Here, the subscripts ref. stands for the reference molecule.
the TPA cross-section value, c is the concentration of solution, n is
the refractive index of the solution, F is the TPEF integral intensities
of the solution emitted at the exciting wavelength, and
fluorescence quantum yield. The dref value of reference was taken
from the literature [23].
Detailed experiments reveal that from 715 to 830 nm, the peak
position in the TPEF spectra of these complexes is independent of
the excitation wavelength, but the TPA cross sections are depen-
dent over this range. By tuning the pump wavelength incrementally
from 715 to 830 nm while keeping the input power fixed and then
recording the TPEF intensity, the two-photon absorption spectra
d is
F is the
4.3. Two-photon excited fluorescence (TPEF)
There is no linear absorption in the wavelength range
450e850 nm for the ligand and the three complexes, which indicates
Fig. 9. The TPA spectra of L, dye 1, dye 2 and dye 3 in DMF under different excitation wavelengths.

246
H. Zhou et al. / Dyes and Pigments 91 (2011) 237e247
Appendix. Supplementary material
Supplementary material associated with the article can be found
in online version, at doi:10.1016/j.dyepig.2011.03.024.
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for scientific Innovation Foundation of Anhui Province
(2006KJ007TD), Science and Technological Fund of Anhui Province
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