Design, crystal structures and enhanced frequency-upconverted lasing efficiencies of a new series of dyes from hybrid of inorganic polymers and organic chromophores

Article abstract of DOI:10.1039/b914656c

A new series of hybrid dyes, trans-4-(4′-N,N-dialkylaminostyryl)-N- methyl pyridinium [Cd(SCN)₃^-]_n [alkyl = Me(1), Et(2), Pr(3) and n-Bu(4)] have been successfully synthesized from hybrids of inorganic polymers and organic

Full text of DOI:10.1039/b914656c

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PAPER
www.rsc.org/materials | Journal of Materials Chemistry
Design, crystal structures and enhanced frequency-upconverted lasing
efficiencies of a new series of dyes from hybrid of inorganic
polymers and organic chromophores†
a
Fuying Hao, Xuanjun Zhang, Yupeng Tian,
c
abc
a
Hongping Zhou, Lin Li,a Jieying Wu,a Shengyi Zhang,a
*
*
Jiaxiang Yang,a Baokang Jin,a Xutang Tao,b Guangyong Zhoub and Minhua Jiangb
Received 20th July 2009, Accepted 25th September 2009
First published as an Advance Article on the web 23rd October 2009
DOI: 10.1039/b914656c
ꢀ
A new series of hybrid dyes, trans-4-(40-N,N-dialkylaminostyryl)-N-methyl pyridinium [Cd(SCN)3
]
n
[alkyl ¼ Me(1), Et(2), Pr(3) and n–Bu(4)] have been successfully synthesized from hybrids of inorganic
polymers and organic chromophores. The structures of four new hybrid dyes were characterized by
single-crystal X-ray diffraction first. The particular features of the hybrid dyes are (1) the inorganic
anions form isolated polymeric one-dimensional zig-zag chains of [Cd(SCN)3ꢀ]n with an extended p-
conjugation system within the polymeric –[M–S–ChN]–M. chain and high polarizabilities; and (2)
the organic cationic chromophores determine their optical properties. Experimental results revealed
that the TPA cross-section values and overall energy conversion efficiencies of hybrid dyes with
different alkyl pendant groups have obviously increased from 1 to 4, and in particular, have been
substantially enhanced compared to their corresponding iodide analogues 10–40. Therefore, we
speculate that such increases are attributable to arrangements of infinite anionic [Cd(SCN)3ꢀ]n chains
and synergic effects with chromophore cations.
chromophores with various electron-donor and electron-
Introduction
acceptor moieties, which are symmetrically or asymmetrically
attached to a p-conjugated linker. The effects of varying the
strength of the end groups as electron-donating or electron-
accepting and introducing additional groups into the middle to
vary the charge redistribution associated with excitation, and
varying the conjugation length have been studied to understand
design criteria for producing structures with enhanced two-
photon activity. Meanwhile, other attempts to enhance the TPA
activity have been also performed, such as changing the
arrangements or environment of the TPA molecules.9 The
elegant work of Prasad10 demonstrated that two-photon active
dyes could be incorporated inside nanometre-sized silica nano-
bubbles via a simple reverse micelle approach. This encapsula-
tion of hydrophobic dyes within silica not only improves their
water dispersability but also leads to an increase in emission
lifetime and efficiency as well as photostability. The studies11 of
organic chromophores near the surface of silver nanoparticles
with a random fractal geometry or self-assembled on the nano-
particle surface12 exhibit large enhancement in two-photon
activities relative to isolated chromophores. These results
revealed that the packing and environment of the chromophores
have profound influence on the TPA properties.
The emergence of molecular electronics and photonics as
multidisciplinary frontiers of science and technology has led to
great interest in searching for new optical materials. Two-photon
absorption (TPA), a third-order nonlinear optical process in
which two photons are simultaneously absorbed to an excited
state in a medium via a virtual state,1 has recently attracted
considerable attention due to its interesting frequency up-
conversion mechanism and exciting applications ranging from
optical limiting2 to three dimensional (3D) fluorescence micros-
copy,3 3D microfabrication either through direct electron
transfer from a chromophore to a monomer4 or through two-
photon photoacid generation,5 optical data storage,4 and
photodynamic therapy,6 bioimaging.7 To realize these applica-
tions, several design approaches have been reported for the
synthesis of organic molecules with large TPA cross sections.
Recent reports8 from different groups show design strategies for
efficient TPA molecules by a systematic investigation of
aDepartment of Chemistry, Anhui University and Key laboratory of
Inorganic Materials Chemistry of Anhui Province, Hefei, 230039, China.
E-mail: yptian@ahu.edu.cn
bThe State Key Laboratory of Crystal Materials, Shandong University,
Although many organic molecules with large two-photon
activities have recently been reported, there are only a limited
number of molecules that have been demonstrated to show two-
photon pumped (TPP) lasing properties.13 It should be noted that
the major requirements for TPP lasing dyes are different from
those for chromophores designed for other applications, where
a larger TPA cross section is the important consideration. On the
other hand the most essential requirements are the ease of
establishing population inversion and a higher lasing efficiency.
Jinan, 250100, P. R. China
cThe State Key Laboratory of Coordination Chemistry, Nanjing
University, Nanjing, 210093, P. R. China
† Electronic supplementary information (ESI) available: General
methods, crystal data for all compounds, crystal Structures for 1, 2 and
3, two-photon fluorescence lifetime, some photophysical properties and
thermal properties, UV-vis absorption spectra and photograph of dye
doped poly-methyl methacrylate. CCDC reference numbers 245757,
246081–246084, 726094. For ESI and crystallographic data in CIF or
other electronic format see DOI: 10.1039/b914656c
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This may be the very reason why a small number of two photon
active chromophores employed for TPP lasing have been
reported. So far, we do not know exactly what specific parame-
ters, molecular structures, and dynamic processes are the criteria
to judge the TPP lasing capability of a dye. However, as a rule of
thumb, most TPP lasing molecules are salt-type asymmetric, and
have an excellent planar configuration consisting of a large polar
p-conjugated system with donor–acceptor substituents.12,14
Considering that trans-4-(40-N,N-dialkylaminostyryl)-N-methyl
pyridinium and its derivatives are typical non-linear optical
chromophores with a D-p-A structure,15 we have synthesized
a short series of dyes (10–40) to investigate their two-photon
properties.
Scheme 1
Experimental results revealed that the TPA and TPP lasing
were enhanced with the gradual elongation of the R groups,
which may result from the stronger donating abilities of –NR2
groups with the elongation of alkyl chains. However, the
enhancement of the TPA and TPP is not so remarkable when
the attached alkyl chain contains 4 carbon atoms or more (see
Table 1). It is reasoned that excessive carbon atoms of alkyl
chains will destroy the planarity of dye molecules. Therefore, we
searched for another approach to enhance the TPA and TPP
properties via changing anions by hybrid.
Scheme 2
(R ¼ Et, Pr and n-Bu) are pale yellow oil. For 4-(N, N-diethyl-
amino)benzaldehyde, IR (KBr, cmꢀ1) 2974 (m), 1665 (s), 1598 (s),
1566 (m), 1528 (s), 1470 (m), 1409 (s), 1357 (s), 1330 (m), 1179 (s)
1
1002 (w), 839 (m), 591 (w); H NMR (400 MHz, d6-acetone),
Herein, we report the novel self-assembly of organic–inorganic
hybrids, which exhibit enhanced TPA cross-section values, TPP
up-conversion lasing efficiency, as well as thermal stability
exceeding those of corresponding iodine analogues. Our
synthetic strategy is based on the molecular self-assembly of
d (ppm): 1.19 (6H, t, J ¼ 7.0 Hz), 3.51(4H, m), 6.80 (2H, d,
J ¼ 8.8 Hz), 7.69 (2H, d, J ¼ 9.2 Hz), 9.68 (1H, s). For 4-(N,N-
dipropylamino)benzaldehyde, IR (KBr, cmꢀ1) selected bands:
2963 (s), 2874 (m), 1667 (s), 1595 (s), 1526 (s), 1405 (s), 1365 (s),
1
1239 (s), 1170 (s), 998 (w), 815 (s), 607 (m), 510 (m); H NMR
organic chromophore cations with TPA properties and large
ꢀ
(400 MHz, d6–acetone), d (ppm): 0.95 (6H, t, J ¼ 7.2 Hz), 1.65
(4H, m), 3.40 (4H, t, J ¼ 7.8 Hz), 6.78 (2H, d, J ¼ 8.8 Hz), 7.67,
(2H, d, J ¼ 9.2 Hz), 9.69 (1H, s). For 4-(N,N-dibutylamino)-
benzaldehyde, IR (KBr, cmꢀ1) selected bands: 2958 (s), 2931 (m),
2871 (m), 1669 (s), 1596 (s), 1525 (s), 1464 (w), 1405 (s), 1367 (s),
1313 (w), 1224 (m), 1168 (s), 814 (s), 607 (m), 510 (m); 1H NMR
(400 MHz, d6-acetone), d (ppm): 0.94 (6H, t, J ¼ 7.4 Hz), 1.38
(4H, m), 3.40 (4H, m), 3.42 (4H, t, J ¼ 7.8 Hz), 6.76 (2H, d,
J ¼ 9.2 Hz), 7.65 (2H, d, J ¼ 8.8 Hz), 9.67 (1H, s).
inorganic anions. Here, [Cd(SCN)3
] was selected, which can
n
form a 1D chain structure in the presence of some ions such as
K+, R4N+ (R ¼ H, Me, Et, etc.) and has been successfully applied
in nonlinear optics.16 (Scheme 1)
Experimental section
Synthesis: The synthetic route is described in Scheme 2.
4-(N,N-Dialkylamino)benzaldehyde was synthesized according
to the literature method.17 At room temperature, the compounds
1,4-Dimethylpyridinium iodide was obtained as white powder.17
IR (KBr, cmꢀ1) selected bands: 3451 (m), 3023 (m), 1644 (s), 1517
Table 1 Photophysical propertiesa of all dyes
Single-photon-related properties
Two-photon-related properties at 1064nm
lmax (abs)/nm
lmax (SPEF)/nm
TPF lmax/nm
s/GMb
Laser/nm
Efficiency (%)
10
1
471
471
482
482
486
485
486
487
603
597
601
598
604
605
605
602
621
621
627
627
627
631
625
625
134
781
122
870
136
940
123
1130
623
628
627
630
626
632
628
632
< 0.1
1.68
5.44
5.65
5.99
7.89
5.26
9.39
20
2
30
3
40
4
a
b
In methanol solution. The TPA cross section; 1 GM ¼ 10ꢀ50 cm4 s photonꢀ1
.
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(m), 1477 (m), 1289 (s), 1182 (s), 1043 (m), 809 (s), 697 (s), 485 (s);
1H NMR (400 MHz, d6-DMSO), d (ppm): 2.61 (3H, s), 4.29 (3H,
s), 7.97 (2H, d, J ¼ 6.4 Hz), 8.84 (2H, d, J ¼ 6.4 Hz); M+
(MS/ESI), 108.48.
11.89%; M+ (MS/ESI), 295.5. C21H27N3S requires C, 71.59; H,
7.52; N, 11.59%; M+, 295.4. IR (KBr, cmꢀ1): 2956 (w), 2051 (s),
1644 (w), 1574 (s), 1521 (s), 1470 (w), 1432 (w), 1343 (w), 1175 (s),
1151 (s), 1100 (w), 979 (m), 834 (m). For 400, found: C, 72.59; H,
8.52; N, 11.59%; M+ (MS/ESI), 323.2. C23H31N3S requires C,
72.39; H, 8.19; N, 11.01%; M+, 323.5. IR (KBr, cmꢀ1): 2957 (w),
2051 (s), 1575 (s), 1526 (s), 1472 (w), 1436 (w), 1345 (m), 1177 (s),
1152 (s), 1043 (w), 982 (m), 832 (m).
Cd(SCN)2 was synthesized using the method reported by
Singh and Gupta.18
Trans-4-[40-(N, N-dimethylamino)styryl]-N-methyl pyridinium
iodides (10)
Trans-4-[40-(N, N-dimethylamino)styryl]-N-methyl pyridinium
cadmium thiocyanate (1)
10 was synthesized according to a literature method.17 Yield, 3.54
g, 84%. Found: C, 52.55; H, 5.32; N, 7.48%; M+ (MS/ESI), 239.3.
C16H19N2I requires C, 52.46; H, 5.19; N, 7.65%; M+, 239.2. IR
(KBr, cmꢀ1) selected bands: 2917 (w), 1644 (m), 1577 (s), 1527
(w), 1507 (w), 1432 (w), 1371 (m), 1164 (s), 982 (m), 824 (m).
The methods for the synthesis of 20, 30 and 40 were similar to
that of 10, except that 4-(N,N-dialkylamino)benzaldehyde (R ¼
Et, Pr and n–Bu) was used instead of 4-(N,N-dimethylamino)-
benzaldehyde. Yields for 20, 30 and 40 are 3.83 g, 89%; 4.14 g, 86%
and 3.75 g, 86%, respectively. For 20, Found: C, 54.75; H, 5.72;
N, 7.18%; M+ (MS/ESI), 267.4. C18H23N2I (20) requires C, 54.82;
H, 5.84; N, 7.11%; M+, 267.3. IR (KBr, cmꢀ1): 2967 (w), 1644
(m), 1585 (s), 1521 (s), 1471 (w), 1434 (w), 1344 (m), 1180 (s),
1152 (m), 1078 (w), 974 (m), 825 (m). For 30, Found: C, 54.61; H,
6.68; N, 6.14%; M+ (MS/ESI), 295.2. C20H27N2I$H2O(30$H2O)
requires C, 54.55; H, 6.59; N, 6.36%; M+, 295.4. IR (KBr, cmꢀ1):
3486.3(m), 3429.7 (m), 2955 (w), 1645 (m), 1581 (s), 1547 (w),
1521 (s), 1469 (w), 1434 (w), 1339 (m), 1178 (s), 1155 (m), 1098
(w), 976 (m), 828 (m). For 40, Found: C, 54.45; H, 7.33; N, 5.69%;
M+ (MS/ESI), 323.4. C22H31N2I$2H2O (40$2H2O) requires C,
54.32; H, 7.20; N, 5.76%; M+, 323.5. IR (KBr, cmꢀ1): 3419.7 (m),
2957 (w), 1646 (m), 1586 (s), 1548 (w), 1528 (s), 1472 (w),
1434 (w), 1341 (w), 1174 (s), 1151 (m), 1043 (w), 992 (m), 832 (m).
100 (0.89 g, 3.01 mmol) and Cd(SCN)2 (0.68 g, 3.01 mmol) were
refluxed in 25 mL methanol. Red product was obtained by
filtration. Yield, 1.45 g, 92%. Found: C, 44.04; H, 3.97; N,
12.60%; M+ (MS/ESI), 239.0. C19H19CdN5S3 requires C, 43.38;
H, 3.64; N, 13.32%; M+, 239.2. IR (KBr, cmꢀ1) selected bands:
2096 (s), 1641 (m), 1584 (s), 1526 (m), 1373 (m), 1333 (w),
1180 (s), 1167 (s), 973 (m), 826 (m).
The methods for the synthesis of 2–4 were similar to that of 1
with the yields of 1.50 g, 90%; 1.56 g, 89% and 1.72 g, 91%,
respectively. For 2, found: C, 46.52; H, 4.44; N, 14.08%; M+
(MS/ESI), 267.5. C23H26CdN6S3 (2$CH3CN) requires C, 46.43;
H, 4.37; N, 14.13%; M+, 267.4. IR (KBr, cmꢀ1) selected bands:
2103 (s), 1641 (m), 1582 (s), 1524 (s), 1355 (m), 1336 (m), 1180 (s),
1155 (s), 975 (w), 828 (m). For 3, found: C, 47.51; H, 4.45; N,
12.13%; M+ (MS/ESI), 295.3. C23H27CdN5S3 requires C, 47.45;
H, 4.68; N, 12.03%; M+, 295.4. IR (KBr, cmꢀ1): 2104 (s), 1641
(w), 1586 (s), 1523 (m), 1364 (w), 1336 (w), 1179 (s), 1155 (s), 971
(w), 827 (w). For 4, found: C, 49.51; H, 5.45; N, 11.13%; M+
(MS/ESI), 323.7. C25H31CdN5S3 requires C, 49.21; H, 5.12; N,
11.48%; M+, 323.5. IR (KBr, cmꢀ1): 2090 (s), 1640 (w), 1566 (s),
1521 (s), 1337 (m), 1165 (s), 1148 (m), 980 (w), 831 (w).
Single crystals suitable for X-ray analysis for 1–4 were
obtained by slow evaporation of acetonitrile solutions at room
temperature.
Trans-4-[40-(N,N-dimethylamino)styryl]-N-methyl pyridinium
thiocyanates (100)
AgNO3 (0.92 g, 5.41 mmol) in 20 mL methanol was added to
30 mL methanol solution of 10 (1.99 g, 5.41 mmol). The mixture
was refluxed for 3 h, and then filtered to remove silver iodide
precipitation. The filtrate was added into 20 mL methanol
solution of KSCN (0.27 g, 5.41 mmol), followed by refluxing for
about 3 h and then filtered to remove KNO3. During evaporation
of the clear red solution, shining microcrystals were formed,
filtered, washed with water and methanol, pure product was
obtained. Yield, 1.32 g, 89%. Found: C, 68.26; H, 6.52; N,
14.59%; M+ (MS/ESI), 239.4; C17H19N3S requires C, 68.65; H,
6.43; N, 14.13%; M+, 239.2. IR (KBr, cmꢀ1) selected bands:
2055 (s), 1646 (m), 1574 (s), 1527 (m), 1372 (m), 1317 (w),
1160 (s), 941 (w), 833 (m).
The methods for the synthesis of 200–400 were similar to that of
100 except that corresponding trans-4-[40-(N,N-dialkyl amino)-
styryl]-N-methyl pyridinium iodides were used. Yields for 200,
300and 400 are 1.49 g, 90%; 1.46 g, 83% and 1.52 g, 80%, respec-
tively. For 200, found: C, 69.56; H, 7.06; N, 12.17%; M+ (MS/
ESI), 267.5. C19H23N3S requires C, 70.11; H, 7.12; N, 12.91%;
M+, 267.3. IR (KBr, cmꢀ1): 2973 (w), 2051 (s), 1647 (w), 1576 (s),
1528 (s), 1473 (w), 1438 (w), 1343 (m), 1178 (s), 1155 (s), 1079
(m), 980 (w), 831 (w). For 300, found: C, 71.34; H, 7.70; N,
Crystal structure determination
The crystal structures were determined by single crystal X-ray
diffraction analyses. Data collections for 30, 40, 2 and 4 were
performed using a Bruker P4 four-circle diffractometer by the q/
ꢀ
2q scan method [l ¼ 0.71073 A]. Data for 1 and 3 were collected
at 293 K on a Siemens SMART CCD area detector diffrac-
tometer with Mo Ka radiation with an u-scan mode [l ¼ 0.71073
ꢀ
A]. The structures were solved with direct methods using the
SHELXTL program19 and refined anisotropically with
SHELXTL using full-matrix least squares procedure. The
structural plots were drawn using SHELXTL. The hydrogen
atom positions were geometrically idealized and allowed to ride
on their parent atoms and fixed displacement parameters (see
Table S1 in the ESI†). For 40, H atoms of the water molecules
were located in a difference Fourier map and refined isotropi-
cally, with the O/H and H/H distances restrained to 0.84(1)
ꢀ
and 1.37(2) A, respectively. The C19–C20 and C20–C21
ꢀ
ꢀ
distances were restrained to 1.45(2) A and C20–C21 to 1.54(2) A.
For 30, the C15–C16 and C16–C17 distances were restrained to
ꢀ
1.48(2) A.
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Fig. 1 ORTEP drawing of 30 with 50% probability ellipsoids. Hydrogen
Fig. 3 ORTEP drawing of 40 with 50% probability ellipsoids. Hydrogen
atoms have been omitted for clarity.
atoms have been omitted for clarity.
Results and discussion
40. In the four structures, their anionic [Cd(SCN)3ꢀ]n chains are
very alike. The Cd atoms are octahedrally coordinated with three
nitrogen and three sulfur atoms from six SCNꢀ ligands, as
illustrated in Fig. 5 for 4 (dyes 1–3 see Fig. S1–S3 in the ESI†).
The trans influence dictates that the sulfurs are trans to the
nitrogens. The three N–Cd–S angles average to 174.0ꢁ in 1,
172.8ꢁ in 2, 172.9ꢁ in 3 and 172.2ꢁ in 4, respectively, which are
very close to linearity. The Cd atoms form an infinite zig-zag
Structural features
The crystal structures were divided into two groups with different
anions: iodine anion (30 and 40) and cadmium thiocyanate anion
(1–4), respectively.
The crystal structures of 30 and 40 are shown in Fig. 1 and 3,
respectively. Selected bond lengths are listed in Table S2 (ESI†).
Molecular structures of 30 and 40 include one and two co-crys-
tallized water molecules, respectively, which can be seen in Fig. 2
and 4. Various hydrogen bonds can also be seen in Fig. 2, such as
the C–H/O bonding between the hydrogen of the benzene ring
and the oxygen of the water, the O–H/I bonding between the
hydrogen of water and the iodine ion as well as C–H/I between
the hydrogen of the benzene ring and the iodine ion. From Fig. 4,
one can see another kind of hydrogen bond, O–H/O–H
between water molecules in 40. By comparison, hydrogen
bonding is more abundant in 40 than in 30. Compared to crystal
30, crystal 40 possesses superior crystalline habits, such as ease of
crystallization from solution and growing into a large three-
dimensional crystal. This reveals that water molecules and iodine
ions related hydrogen bonds play an important part in the
crystalline process and the crystalline morphology.
ꢀ
chain with the Cd/Cd distances of 5.45, 5.51, 5.44 and 5.45 A
and the Cd/Cd/Cd angles of 161.4ꢁ, 162.3ꢁ, 162.3ꢁ and 161.1ꢁ
in 1–4, respectively. The zig-zag chains are ‘‘parallel’’ to one
another and run along the crystallographic b axis. If the bridging
thiocyanate ligands are considered, there are three infinite zig-zag
chains of –[S–ChN–Cd]N–. As exemplified by 4, one of the three
–[S–ChN–Cd]N– chains is –[S1–C23hN3–Cd]N– with Cd–S1–
C23, S1–C23–N3, C23–N3–Cd and N3–Cd–S1 angles of 95.1ꢁ,
179.5ꢁ, 157.1ꢁ and 172.4ꢁ, respectively. The remaining two chains
are –[S3–C25hN5–Cd]N– with the corresponding Cd–S3–C25,
S3–C25–N5, C25–N5–Cd, N5–Cd–S3 angles of 101.4ꢁ, 178.6ꢁ,
138.6ꢁ and 171.5ꢁ; and the –[S2–C24hN4–Cd]N– chain with the
Cd–S2–C24, S2–C24–N4, C24–N4–Cd and N4–Cd–S2 angles of
98.2ꢁ, 178.4ꢁ, 148.3ꢁ and 172.6ꢁ, respectively (see Table S3 of the
ESI†). The net result is three zig-zag –[S–ChN–Cd]N– chains
bundled around the central zig-zag Cd/Cd chain, with Cd
atoms as knots (see Fig. 5). These metal-thiocyanate chains
make ꢂ100ꢁ kinks at the sulfur atoms and ꢂ30ꢁ bents at the
nitrogen atoms, resulting in nearly colinear five-atom fragments
S–C–N–Cd–S (see Table S3 in the ESI†).16
From Fig. 1 and 3, one can see that the pyridinum cations in 30
and 40 are almost perfectly planar. Their mean deviations from
0
0
ꢀ
ꢀ
the plane (N1, C2–C14) are 0.029 A for 3 and 0.0702 A for 4 ,
respectively. In fact, the dihedral angles of the benzene and
pyridinium rings are 4.1ꢁ in 30 and 8.5ꢁ in 40 and their bond
lengths are all of aromatic character. These structural features
suggest that two cations have highly delocalized p-electron
systems, leading to the charge transfer from donor to acceptor,
which is a necessary condition for two photon absorption and
up-conversion emission.20
The structures of 1–4 mainly consist of trans-4-[40-(N,N-dia-
ꢀ
lkylamino)styryl]-N-methyl pyridinium cation and [Cd(SCN)3
]
n
anion, where the structures of cation are similar to those in 30 and
Fig. 2 Packing diagram of 30.
Fig. 4 Packing diagram for 40.
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the backbone of styryl pyridinium that determine the wave-
lengths of linear absorption peaks. Then the absorption maxima
of the dyes with the same anions show larger red shift with
enhanced electron-donating abilities of the R groups.
Single-photon excited fluorescence (SPEF) and quantum yield
One-photon fluorescence spectra and quantum yields of all the
dyes in methanol solvent were measured on a Perkin Elmer
LS-55B fluorospectrometer. The Rh6G ethanol solution was
used as the quantum yield standard (F ¼ 0.97), and a RhB
ethanol solution was used to calibrate the system. 2 ꢃ 10ꢀ5
M
dilute solutions of samples were prepared. In order to avoid
a significant error of the measurements due to the sensitivity of
the fluorescence spectrophotometer and other environmental
conditions, every sample at least was carried out three times
under identical experimental conditions within an error of 3.0%
and before taking the average. Fig. 6 shows the one-photon-
excited fluorescence spectra of 4. The emission spectra of all the
dyes exhibit mirror images to the absorption spectra with large
Stokes shifts ranging from 115 to 132 nm and low fluorescence
quantum yields (all of them < 0.6%), which are consistent with
those reported by Prasad and coworkers21 All data lie in Table S5
in the ESI.† In addition, compared to corresponding iodide
analogues, the relative fluorescence quantum yields of all hybrid
dyes (with [Cd(SCN)3ꢀ]n anion) increase considerably by factors
of ꢂ14.2, ꢂ3.8, ꢂ8.1 and ꢂ4.9, respectively.
Fig. 5 ORTEP drawing of the coordination environment of Cd atoms
and the cation in 4 with atomic numbering at 50% probability ellipsoids
(a) the packing diagram (b) hydrogen atoms and butyl groups have been
omitted for clarity.
ꢀ
The average distances of S–C and C–N in 4: 1.646 and 1.147 A,
indicate partial p-delocalization along the metal-thiocyanate
chains for all three chains. This observation is consistent with the
fact that the SCNꢀ ligand is easily polarizable [–S–ChN 4
S]C]N–], while the three –[S–ChN–Cd]N– chains are not
identical. Interestingly, one chain, –[S1–C23hN3–Cd]N–, has
Two-photon excited luminescence spectra and TPA cross-section
Fig. 6 presents two-photon-excited fluorescence spectra of 4. The
emission peaks of all the other samples are summarized in
Table 1, from which one can see that two-photon-excited fluo-
rescence peaks are about 20 nm red-shifted as compared to
those of their single-photon counterparts. This is due to the
re-absorption effect20 of the dye solution at high concentration.
In addition, with the increase of solvent polarity, the peaks were
red-shifted gradually. This may enable them to serve as good
candidates for TPA sensing of polarity. However, different R
groups (from methyl to butyl) apparently do not influence the
ꢀ
shorter Cd–S1 and N3–Cd distances of 2.707 and 2.267 A,
respectively (in comparison with the corresponding average
ꢀ
distances of 2.741 and 2.351 A for the other two chains) and
a smaller angle Cd–S1–C23 of 95.1ꢁ (in comparison with the
corresponding average value of 100.3ꢁ for the other two chains)
and a larger C23–N3–Cd angle of 157.1ꢁ (in comparison with
the corresponding average value of 143.5ꢁ for the other two
chains) respectively. Therefore, the–[S1–C23hN3–Cd]N– chain
in 4 exhibits a larger partial p-delocalization in comparison
with the other two chains –[S3–C25hN5–Cd]N– and –[S2–
C24hN4–Cd] N– in 4.
In short, the partial p-delocalization (in terms of both
p-donation and p-back-donation) along the infinite –[S–ChN–
Cd]N– chains, coupled with the highly polarizable ‘‘soft’’ metal
such as Cd2+ ions, by synergic effect with chromophores cation,
makes these cadmium thiocyanate polymers highly promising
candidates for two-photon up-conversion materials.
Linear absorption spectra
The methanol solutions of all the studied dyes show similar linear
absorption properties which feature one intense absorption band
between 470 and 490 nm (see Table 1 and Fig. S5 in the ESI†).
There is no significant linear absorption in the spectral range
from 600 to 1200 nm (See Fig. S4 in the ESI†). From Table 1, one
can observe that the dyes with the same cation exhibit the same
linear absorption maxima, which indicates that it is cations with
Fig. 6 (a) One-photon excited fluorescence spectrum of 4 in methanol
(2 ꢃ 10ꢀ7 M), (b) two-photon excited fluorescence spectrum and (c) the
superradiance spectrum of 4 in methanol (0.01 M).
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J. Mater. Chem., 2009, 19, 9163–9169 | 9167

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emission peaks by the spectra data summarized in Table 1. As
mentioned above, no linear absorption occurs in the spectra
range from 600 nm to 1200 nm, so upon irradiation with the laser
pulse at 1064 nm, sequential stepwise one-photon absorption
followed by excited-state emission can be excluded and the
strong frequency up-converted fluorescence can be assigned to
a simultaneous two-photon absorption mechanism.
abilities of R groups. Especially, hybrid dyes display a consider-
able increase (1.68, 5.65, 7.89 and 9.39%, respectively). Secondly,
for the same chromophore cation, all hybrid dyes show
remarkable up-conversion lasing efficiencies compared with
their corresponding iodide analogues which indicates that
ꢀ
[Cd(SCN)3
]
anions have a profound influence on the upcon-
n
ꢀ
vertion efficiencies. Therefore, we speculated that [Cd(SCN)3
]
n
The effective TPA cross-section values of the dye solutions
were measured by using two-photon induced fluorescence
method. From Table 1, we can see that for the same cation, all
hybrid dyes exhibit a significant increase in the TPA cross-section
values compared to corresponding iodide analogues by a factor
of ꢂ5.8, 7.1, 6.9 and 9.1, respectively (R ¼ Me, Et, Pr, n-Bu). In
addition, TPA cross-section values of the dyes are rapidly
enlarged with the stronger electron-donating abilities and better
anions take part in the frontier molecular orbitals to some extent,
lower pump threshold and favor intramolecular charge transfers.
In our study, the observed up-conversion efficiency is highest for
4 in a methanol solution. Such high up-conversion lasing
efficiency is closely related to arrangements of infinite anionic
[Cd(SCN)3ꢀ]n chains. (see mentioned structural feature).
Maybe one suspects that infinite [Cd(SCN)3ꢀ]n chains will be
destroyed in solution at pump energy, but previous reports22
exhibit that oligomers bridged by different bidentate ligands
show themselves NLO effects in solution because they tend to
preserve their original linear structural characters in themselves
to some degree. Our previous work using NCSꢀ as a bridge also
solubility of the R groups from 1 to 4, namely 781, 870, 940 and
ꢀ
1130 GM, respectively. This reveals that the [Cd(SCN)3
anions have a profound influence on the TPA.
]
n
ꢀ
revealed that Cd(SCN)3 formed linear coordination polymers,
The TPP lasing spectra and up-conversion efficiency
which were used as a template for the synthesis of nanowire
bundles.23 Similarly, the molecules of 1–4 in this work may
arrange in this linear matrix as arranged in the crystals, although
aggregation chains may not be infinite as those in the solid
crystals. The enhancement of the TPA activity and up-conver-
sion efficiency most probably resulted from the arrangements of
the dye molecules in the [Cd(SCN)3ꢀ]n matrix. Furthermore, the
dyes as practical TPP lasing materials are used in high concen-
tration solutions or in a polymer matrix. Thereby, we are not
concerned by their actions in dilute solutions.
The TPP lasing experiments: the 1064 nm, 40 ps pulse from
a passively mode-locked Nd: YAG laser was adopted as pump
source. Through a f ¼ 12 cm lens, the pump beam was focused
vertically onto the center of a 1 cm path quartz cell that was full
of solution sample. The lasing output beam was dispersed by
a polychromator and the spectra were recorded by a streak
camera (Hamamatsu Model C1587).
The TPP lasing spectrum of 4, with the peak located at 632 nm,
is shown in Fig. 6 and peaks of the other dyes are summarized in
Table 1. Most of the lasing emission peaks are slightly red-shifted
compared to the two-photon excited fluorescence, which is due to
losses caused by fluorescence re-absorption.20 The super-
radiances of 40 and 4 (in methanol) at different pump energies are
shown in Fig. 7, from which one can find two facts: (1) the
upconverted efficiencies increase with the increase of pump
intensity until the relationship between upconverted intensities
and pump intensities deviates from the square law. After that,
upconverted efficiencies turn to decline due to the saturation
effect; and (2) under identical experimental conditions, the
overall energy conversion efficiency is 5.28% for 40 (output/input
energy is 0.235/4.444 mJ) and 9.39% for 4 (output/input energy is
0.348/3.705 mJ) respectively. In addition, on the basis of Table 1,
we can find that the up-convertion efficiencies of dyes with the
same anions enlarge with the increase of electron-donating
Thermogravimetric analysis (TGA) measurements of these
dyes revealed that all hybrid dyes presented slightly enhanced
thermal stability compared with those of the iodide analogues
(see Table S5, ESI†). Interestingly, the hybrid dyes of the organic
chromophores with inorganic coordination polymers did not
influence the doping in the organic polymer matrix. Preliminary
experimental results displayed that hybrid dyes could be well
doped in poly(methyl methacrylate) via both heat polymerization
and illumination polymerization using sun-light (see Fig. S6,
ESI†). This enables them potential applications as laser
materials. Further work is still in progress.
Conclusions
In brief, a series of hybrid dyes have been synthesized and
characterized by single crystal X-ray diffraction analysis. Struc-
tures indicated hybrid dye’s own novel features of (1) the partial
p-delocalization along the infinite –[S–ChN–Cd]N– chains and
high polarizabilities; and (2) highly delocalized cationic chro-
mophore with optical properties. Thus, by synergic effects hybrid
dyes may make highly promising candidates for two-photon up-
conversion materials. Experimental results show that the TPA
cross-section values and overall energy conversion efficiencies
have apparent enhancement compared to their corresponding
iodide analogues 10–40 and rapid increment with increase of R
groups from 1 to 4. In our study, the observed overall energy
conversion efficiency is highest for 4, which consists of
a [Cd(SCN)3ꢀ]n chain and butyl group moiety. Therefore, these
results support our assumption that high up-conversion lasing
Fig. 7 Up-conversion efficiencies of 4 and 40 in methanol (0.01 M).
9168 | J. Mater. Chem., 2009, 19, 9163–9169
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View Article Online
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efficiency is closely related to the arrangement of an infinite
ꢀ
anionic [Cd(SCN)3
] chain and R group in the cationic chro-
n
mophore. We expect that our experimental results can offer
a reference in designing and synthesizing new TPP materials with
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Acknowledgements
This work was supported by a grant for the National Natural
Science Foundation of China (50532030, 20771001, 50871001).
Scientific foundation of Anhui Province for Innovation Team
2006K007TD. Education Committee of Anhui Province
(2006KJ032A,
2006KJ135B,
KJ2007B095,
2007jq1144,
KJ2009A52, 2007jq1019). We thank Prof. Hoongkun Fun, Dr
Zhaodi Liu and Dr Stephen Barlow for single crystal determi-
nation, crystal structure analysis, and helpful discussions,
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