Synthesis, structures, and optical properties of two novel two-photon initiators derived from 2,2′:6′,2′-terpyridine

Article abstract of DOI:10.1246/bcsj.80.986

Efficient aqueous-phase aldol condensation, Michael addition, and solvent-free Wittig reactions were successfully employed to synthesize two two-photon initiators 9-ethyl-3-[4-(2,2′:6′,2″-terpyridinyl- 4′-yl)styryl]carbazole (T1) and 9-{4-[4-(2,2′:6′,2″-

Full text of DOI:10.1246/bcsj.80.986

986 Bull. Chem. Soc. Jpn. Vol. 80, No. 5, 986–993 (2007)
ꢀ 2007 The Chemical Society of Japan
Synthesis, Structures, and Optical Properties of Two Novel
Two-Photon Initiators Derived from 2,20:60,200-Terpyridine
ꢀ1;2
ꢀ1;2
Zhang-Jun Hu,1 Jia-Xiang Yang,
Hong-Ping Zhou,1 Gui-Bao Xu,1 Wen-Tao Yu,1 Yun-Xing Yan,2 Yuan-Hong Sun,3
Chuan-Kui Wang,3 Xiao-Qiang Yu,2 and Min-Hua Jiang2
Yu-Peng Tian,
Xu-Tang Tao,2 Lei Tian,1
1Department of Chemistry, Anhui University, Hefei 230039, P. R. China
2State Key Laboratory of Crystal Materials, Shandong University, Jinan 250100, P. R. China
3Department of Physics, Shandong Normal University, Jinan 250014, P. R. China
Received August 22, 2006; E-mail: jxyang@ahu.edu.cn
Efficient aqueous-phase aldol condensation, Michael addition, and solvent-free Wittig reactions were successfully
employed to synthesize two two-photon initiators 9-ethyl-3-[4-(2,20:60,200-terpyridinyl-40-yl)styryl]carbazole (T1) and
9-{4-[4-(2,20:60,200-terpyridinyl-40-yl)styryl]phenyl}carbazole (T2). The two initiators with carbazolyl moiety attached
to 2,20:60,200-terpyridine present D–ꢀ–A-type framework, where A is a ꢀ-deficient terpyridine ring. The crystal struc-
tures were determined by single-crystal X-ray diffraction determination. The experimental results confirmed that the
two initiators have sensitive single-photon-excited fluorescence (SPEF) and two-photon-excited fluorescence (TPEF)
properties. The experimental and theoretical two-photon absorption (TPA) cross-sections were investigated. Two-photon
initiation polymerization (TPIP) microfabrication experiments were carried out, and possible polymerization mecha-
nisms are discussed based on the theoretical evaluation.
Materials with a large TPA cross-section have attracted
considerable attention due to their various applications in the
areas of optical limiting,1 two-photon photodynamic therapy,2
two-photon-pumped up-conversion lasing,3 three-dimensional
(3D) fluorescence imaging,4 optical data storage,5 and the
two-photon micro-fabrication (TPMF).6,7 As far as the TPMF
technique is concerned, it has paved a way to direct laser fab-
rication of 3D microstructures in one step with high spatial res-
olution, making possible many applications, such as photonic
devices, micromachines, zero-threshold lasers, and integrated
optical waveguides.8,9
TPIP is the most important approach to realize TPMF,
which utilizes high-efficient two-photon initiators and excites
them by laser pulses in the near IR region. However, efficient
two-photon initiators are still not abundant. Therefore, a con-
trol of the molecular geometry as well as the electronic struc-
ture and optimization of the physical properties of the initiators
to develop the new initiators is of crucial importance to pre-
pare more efficient TPIP initiators. Recently, many D–ꢀ–D-
type initiators have been developed.6 Our group also have
developed a series of two-photon initiators with symmetric
and asymmetric molecular structure of types: A–ꢀ–D–ꢀ–A,
D–ꢀ–A, and D–ꢀ–D. These initiators show efficient TPA
and were applied in TPIP successfully.6c,7 Although these
TPA dyes, based on conjugated aromatic rings, are now avail-
able, some complicated hetero-aromatic compounds, which are
often used for special purposes in the two-photon area due to
their favorable coordinate abilities, such as 2,20:60,200-terpyri-
dine with a rigid framework plane and a superb ability to co-
ordinate with various metal ions, are still not well explored.10
Its derivatives are often used for many physical and chemical
applications.11 Recently, there have been a few reports about
TPA response of some D–ꢀ–A-type terpyridine derivatives.12
The recent success of the research on green and facile prepa-
rations of the key precursor 1,5-di(2-pyridinyl)-3-p-tolylpen-
tane-1,5-dione (30)13 has aroused our interests in studying these
derivatives from a two-photon-initiator point of view. By in-
corporating carbazolyl unit into the terpyridine backbone, D–
ꢀ–A-type initiators, in which the terpyridyl ring is the acceptor
part and a carbazolyl group the donor one, were prepared.
In this work, a facile synthesis route for precursor com-
pound 2,20:60,200-terpyridine involving aqueous-phase aldol
condensation and Michael addition reactions, is presented.
Furthermore, solvent-free Wittig reactions were used in build-
ing the ꢀ-conjugated system. The single-crystal structures and
the linear/nonlinear optical properties of two compounds as
well as a brief discussion about the relationships of struc-
ture/properties are described. TPIP microfabrication experi-
ments demonstrated that they could act as efficient two-photon
photopolymerization initiators. A possible mechanism of the
photopolymerization is discussed.
Experimental
Instruments and Measurements. The 1H NMR and 13C NMR
spectra were recorded at 25 ꢁC on a Bruker Avance 500 or 600
spectrometer, and the chemical shift are reported as parts per
million (ppm) from TMS (ꢁ). Coupling constants J are given in
Hertz. Mass spectra were acquired on a Micromass GCT-MS (EI
source). Elemental analysis was performed on a Perkin-Elmer 240
instrument. The melting points were measured on a METTLER-
Published on the web May 14, 2007; doi:10.1246/bcsj.80.986

Z.-J. Hu et al.
Bull. Chem. Soc. Jpn. Vol. 80, No. 5 (2007)
987
CHO
O
O
2-acetylpyridine
2-acetylpyridine
20% NaOH aq.
NH4OAc
N
N
EtOH
2% NaOH aq.
N
N
N
O
N
1'
2'
3'
4'
Br
PPh3Br
RCHO, t-BuOK
grind
PPh3
NBS, BPO
T1 or T2
CCl4
benzene
N
N
N
N
N
N
5'
6'
N
N
N
N
N
N
N
N
T1
T2
Scheme 1. Synthesis strategy for the preparation of two initiators (RCHO = 9-ethylcarbazole-3-carbaldehyde (a) and 4-(9-carbazo-
lyl)benzaldehyde (b)).
TOLEDO DSC822 differential scanning calorimeter at a heating
rate of 20 ꢁC minꢂ1 under a nitrogen atmosphere. IR spectra were
recorded on a NEXUS 870 (Nicolet) spectrophotometer in the
400–4000 cmꢂ1 region using a powder sample on a KBr plate.
X-ray diffraction data of single crystals were collected on a
Siemens Smart 1000 CCD diffractometer and a Bruker P4 four-
circle diffractometer. The determination of unit cell parameters
and data collections were performed with Mo Kꢂ radiation (ꢃ ¼
air at room temperature. TPA cross-sections were measured using
flouorescein as reference.
Materials and Synthesis. All chemicals were available com-
mercially, and the solvents were purified as conventional methods
before use. 9-Ethylcarbazole-3-carbaldehyde (a), 4-(9-carbazolyl)-
benzaldehyde (b) were synthesized according to the reported
methods.15 As shown in Scheme 1, we synthesized the title com-
pounds by a series of modified reactions. The trans-isomer prod-
ucts were recrystallized from different solvents. These compounds
˚
0:71073 A). Unit cell dimensions were obtained with least-squares
1
refinements, and all structures were solved by direct methods
using SHELXS-97. 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.
UV–vis 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
SPEF quantum yields ꢀ were measured by using a standard meth-
od under the same experimental conditions for all compounds.
Coumarin 307 dissolved in ethanol (ꢀ ¼ 0:56)14 at the same con-
centration as the other sample was used as the standard. Lifetime
values were obtained by reconvolution fit analysis of the decay
profiles with the aid of F900 analysis software. The fitting results
were judged by their values of ‘‘reduced chi-squared.’’ TPEF spec-
tra and TPIP microfabrication 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
were characterized by H NMR, 13C NMR, FT-IR, and MS spec-
trometry.
40-(p-Tolyl)-2,20:60,200-terpyridine (40): A flask charged with
2-acetylpyridine (3.0 g, 24.8 mmol), freshly distilled 4-methyl-
benzaldehyde (10) (3.0 g, 24.8 mmol) and 2% aqueous sodium hy-
droxide (100 mL) was vigorously stirred for about 8 h at room
temperature, during which time a bright yellow solid product 20,
that was not isolated precipitated. Another molar amount of 2-ace-
tylpyridine (3.0 g, 25.0 mmol) was added into the preceding reac-
tion system and the concentration of aqueous sodium hydroxide
was changed from 2 to 20%. The reaction was monitored by
TLC. After another 8 h of constant stirring at 80 ꢁC, a dark-brown
ropy mixture containing 1,5-di(2-pyridyl)-3-p-tolylpentane-1,5-
dione (30) were gained after removing the aqueous phase. The
ropy mixture was added to a stirred solution of ammonium acetate
(18.0 g, excess) in ethanol (250 mL). The reaction was heated at
reflux for 5 h, affording a dark red solution. Cooled to the room
temperature, bright yellow product was crystallized from the fil-
tered solution. The white acicular product was recrystallized from
ethanol and dried in vacuum to give 40 (6.0 g, 75%). Mp: 167.8 ꢁC.
1H NMR (CDCl3, 600 MHz): ꢁ 2.44 (s, 3H), 7.33 (d, J ¼ 7:87 Hz,
2H), 7.39 (t, J ¼ 6:05 Hz, 2H), 7.85 (d, J ¼ 7:87 Hz, 2H), 7.92 (t,

988 Bull. Chem. Soc. Jpn. Vol. 80, No. 5 (2007)
Two-Photon Initiators
J ¼ 7:38 Hz, 2H), 8.70 (d, J ¼ 7:87 Hz, 2H), 8.76 (m, 4H).
13C NMR (CDCl3, 600 MHz): ꢁ 156.27, 155.77, 150.18, 149.04,
139.09, 136.91, 135.41, 129.63, 127.14, 123.77, 121.38, 118.65,
and 21.23. MS: m=z (%) = 324.15 (100). FT-IR (KBr, cmꢂ1):
ꢄ ¼ 3774, 3702, 3049, 1581, 1468, 1391, 1262, 1036, 882, 821,
790, 728, 687, 615, 492.
120.47, 120.20, 118.94, and 110.03. MS: m=z (%) = 578.23
(100), 288.12 (2.91). FT-IR (KBr, cmꢂ1): ꢄ ¼ 3415, 3046, 1583,
1514, 1446, 1388, 1226, 1113, 1036, 968, 897, 833, 791, 745, 631,
582, 536. Calcd for C41H28N4 (576.7): C, 85.39; H, 4.89; N,
9.72%. Found: C, 84.40; H, 4.78; N, 9.70%.
Crystallographic Data. Crystallographic data have been de-
posited with Cambridge Crystallographic Data Centre: Deposition
number CCDC-228394 for T1 and CCDC-228395 for T2. Copies
of the data can be obtained free of charge via www: http://www.
ccdc.cam.ac.uk/conts/retrieving.html (or from The Cambridge
Crystallographic Data Centre, 12, Union Road, Cambridge, CB2
1EZ, UK; fax: +44 1223 336033; e-mail: deposite@ccdc.cam.ac.
uk). Structural factors are available on request from the authors.
40-[4-(Bromomethyl)phenyl]-2,20:60,200-terpyridine (50): Com-
pound 40 (5.0 g, 15.5 mmol), NBS (2.9 g, 16.1 mmol) and a catalyt-
ic amount of benzoyl peroxide (BPO) were dissolved in anhydrous
tetrachloromethane (200 mL). The mixture was refluxed for 8 h.
After the mixture cooled to room temperature, the solid residue
was filtered. The filtrate was washed with water and brine and then
dried over anhydrous magnesium sulfate. After filtration and sol-
vent evaporation, the residual solid was recrystallized from anhy-
drous ethanol, to give 5.0 g white crystals 50 in an 80% yield. Mp:
176 ꢁC. 1H NMR (CDCl3, 600 MHz): ꢁ 4.58 (s, 2H), 7.38 (m, 2H),
7.56 (d, J ¼ 8:00 Hz, 2H), 7.90 (m, 4H), 8.69 (d, J ¼ 8:00 Hz, 2H),
8.45–8.77 (m, 4H). 13C NMR (CDCl3, 400 MHz): ꢁ 156.07,
155.95, 149.50, 149.09, 138.62, 138.57, 136.94, 129.63, 127.76,
123.89, 121.38, 118.82, and 32.96. MS: m=z (%) = 324.15 (100),
404.05, 401.05, 405.06. FT-IR (KBr, cmꢂ1): ꢄ ¼ 3779, 3697,
3635, 3049, 1807, 1719, 1658, 1581, 1483, 1386, 1221, 1031,
893, 831, 785, 738, 672, 590, 492.
Results and Discussion
Structure Features. Single crystals of T1, suitable for the
X-ray analysis, were obtained from slow evaporation of
CH2Cl2 covered with 2-propanol at room temperature several
days later. A structure is shown in Fig. 1. Crystal data are list-
ed in Table 1. In the electron-acceptor moiety (terpyridinyl
end), the center pyridinyl plane is labeled P0. The other three
ring planes (labeled P2, P3, and P4) were arranged with dihe-
4-(2,20:60,200-Terpyridin-40-yl)-benzyltriphenylphosphonium
Bromide (60): A mixture of 50 (5.0 g, 12.4 mmol) and triphenyl-
phosphine (TPP) (4.9 g, 18.6 mmol) in anhydrous benzene (150
mL) was refluxed for 18 h. The product was precipitated from
the resulting solution. The off-white precipitates were collected
via filtration and dried under vacuum to give 60 (7.4 g, yield 90%).
It was used in the next reactions without characterization.
9-Ethyl-3-[4-(2,20:60,200-terpyridinyl-40-yl)styryl]carbazole
(T1): t-BuOK (1.3 g, 12.3 mmol) was placed into a dry mortar
and milled to very small, then 60 (2.0 g, 3.0 mmol) and a (0.7 g,
3.14 mmol) were added and mixed. The mixture was milled vigo-
rously 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 (moni-
tored by TLC), the mixture was dispersed in 100 mL ethanol. The
residual solid was filtered and recrystallized from anhydrous
dichloromethane/ethanol, to give yellow crystals T1 (1.3 g, yield
80%). Mp: 310 ꢁC. 1H NMR (CDCl3, 500 MHz): ꢁ 1.49 (t, J ¼
7:23 Hz, 3H), 4.42 (q, J ¼ 7:23 Hz, 2H), 7.24 (d, J ¼ 16:3 Hz,
2H), 7.25–7.30 (m, 2H), 7.40–7.53, (m, 6H), 7.73 (t, J ¼ 6:75
Hz, 3H), 7.95 (t, J ¼ 7:57 Hz, 2H), 8.00 (d, J ¼ 8:13 Hz, 2H),
8.17 (d, J ¼ 7:67 Hz, 1H), 8.30 (s, 1H), 8.74 (d, J ¼ 7:92 Hz, 2H),
8.80 (d, J ¼ 4:45 Hz, 2H), 8.85 (s, 2H). 13C NMR (CDCl3, 600
MHz): ꢁ 157.57, 156.17, 155.76, 148.95, 140.43, 139.92, 138.97,
137.13, 130.65, 128.43, 127.61, 126.74, 125.88, 125.29, 124.59,
123.87, 123.40, 123.03, 121.53, 120.54, 119.12, 118.98, 118.68,
108.71, 108.66, 98.13, 25.37, and 13.86. MS: m=z (%) = 528.23
(100), 513.19 (84). FT-IR (KBr, cmꢂ1): ꢄ ¼ 3047, 2972, 1584,
1564, 1468, 1441, 1387, 1344, 1328, 1230, 1151, 1126, 1081,
1037, 962, 821, 792, 738, 685, 661, 623, 514. Calcd for C37H28N4
(528.7): C, 84.06; H, 5.34; N, 10.60%. Found: C, 84.13; H, 5.20;
N, 10.12%.
Fig. 1. Crystal structure of T1, arranged perpendicularity in
a crystal. Hydrogen atoms are omitted for clarity.
Table 1. Crystal Data Collection and Structure Refinement
of the Two Initiators
T1
T2
Formula
C75H58Cl2N8
1142.17
Triclinic
C41H28N4
576.67
Orthorhombic
Pbcn
293(2)
Formula weight
Crystal system
Space group
Temperature/K
ꢁ
P1
298(2)
0.71073
˚
ꢃ of Mo Kꢂ/A
0.71073
0.075
Absorption coefficient/mmꢂ1 0.071
˚
a/A
11.853(5)
13.683(6)
19.596(9)
108.393(8)
94.379(8)
94.826(8)
2987.0(2)
2
13.286(17)
22.946(3)
9.936(11)
˚
b/A
˚
c/A
ꢂ/ꢁ
ꢅ/ꢁ
ꢆ/ꢁ
3
˚
V/A
Z
3028.9(6)
4
3445
2673
0.0464
9-{4-[4-(2,20:60,200-Terpyridinyl-40-yl)styryl]phenyl}carbazole
(T2): Compound T2 was also prepared similarly to T1 via b and
60 to afford solid product (yield 83%). 1H NMR (DMSO-d6,
600 MHz): ꢁ 7.30–7.70 (m, 12H), 7.8–8.30 (m, 10H), 8.60–8.80
(m, 6H). 13C NMR (CDCl3, 600 MHz): ꢁ 156.40, 156.08, 149.91,
149.15, 141.09, 138.20, 137.95, 137.37, 137.19, 136.65, 128.78,
128.18, 127.90, 127.47, 127.36, 126.14, 124.00, 123.72, 121.67,
Reflections collected
Reflections unique
Rint
15895
10400
0.0570
R1 ¼ 0:0725, R1 ¼ 0:0606,
wR2 ¼ 0:1683 wR2 ¼ 0:1326
Final R indices[I > 2ꢇðIÞ]

Z.-J. Hu et al.
Bull. Chem. Soc. Jpn. Vol. 80, No. 5 (2007)
989
ꢁ
˚
Table 2. Selected Bond Lengths (A) and Angles ( ) of the
Two Initiators
Table 3. Photophysical Data of the Two Initiators
SPEF
max
SPEF
max
aÞ
bÞ
eÞ
Compound Solvent ꢃmax
ꢃ
ꢀcÞ ꢈdÞ
ꢃ
T1
T1
Toluene
CHCl3
THF
Acetone
DMF
EtOH
360
445
459
462
499
509
518
0.68 1.47
0.52 1.94
0.67 2.11
0.53 2.14
0.69 2.53
0.56 2.06
491
496
497
502
516
521
361
360
361
364
362
C(19)–C(22)
C(23)–C(24)
N(4)–C(35)
C(3)–C(16)
C(1)–C(6)
1.459(6) C(22)–C(23)
1.455(6) N(4)–C(27)
1.388(6) N(4)–C(36)
1.478(6) C(5)–C(11)
1.301(6)
1.377(5)
1.462(6)
1.469(7)
1.490(7) C(27)–N(4)–C(36) 126.4(5)
C(27)–N(4)–C(35) 108.2(4) C(35)–N(4)–C(36) 125.4(4)
T2
Toluene
CHCl3
THF
Acetone
DMF
EtOH
349
350
348
347
351
347
430
446
446
459
464
460
0.93 1.53
0.87 1.91
0.88 2.06
0.86 2.32
0.79 2.63
0.45 2.14
473
487
490
497
502
499
T2
C(10)–C(11)
C(13)–C(12)
C(7)–N(1)
1.493(5) C(11)–C(12)
1.484(5) C(6)–N(1)
1.429(7) C(19)–C(20)
1.338(5)
1.392(6)
1.480(6)
C(16)–C(17) 1.487(8) C(6)#1–N(1)–C(7) 125.9(3)
C(6)–N(1)–C(6)#1 108.2(6) C(6)–N(1)–C(7) 125.9(3)
a) ꢃmax of the one-photon absorption spectra in nm. b) ꢃmax of
the one-photon fluorescence spectra in nm. c) Fluorescence
quantum yield. d) The lifetime of one-photon fluorescence
in ns. e) ꢃmax of the two-photon fluorescence spectra in nm.
between P0 and P3 (21.5ꢁ) and that of P2 and P3 (15.7ꢁ),
which shows that the ꢀ-conjugated system has become inter-
laced and slightly skewed.
Two structural features suggest that all nonhydrogen atoms
are highly conjugative, leading to eximious ꢀ-bridges for the
charge transfer from carbazolyl to pyridyl. As shown in Figs. 1
and 2, it was found that the C=C exists in trans-isomer, which
provides further evidences to support that this facile synthetic
technique is efficient in constructing practical ꢀ-conjugated or
ꢀ-delocalized systems of optical dye molecules.6c,13
Fig. 2. Crystal structure of T2. Hydrogen atoms are omitted
for clarity.
dral angles of 9.6ꢁ between P0 and P2, 11.6ꢁ between P0 and
P3 and 10.6ꢁ between P0 and P4. At the N-based donor end,
carbazolyl was basically co-planar (labeled P1), which may
cause the delocalization of the lone pair of electrons into the
ꢀ-system. The dihedral angle between P1 and the phenyl plane
P2 was only 5.0ꢁ, and the linkage between these two planes
was conjugated with bond lengths of 1.459(6) (C19–C22),
Linear Absorption Spectra and Single-Photon-Excited
Fluorescence (SPEF). The photophysical properties of the
two compounds are summarized in Table 3. The linear absorp-
tion spectra of two compounds were measured in solvents with
different polarities at a concentration of 1 ꢃ 10ꢂ5 mol Lꢂ1. The
SPEF spectra were measured with the same concentration as
that of the linear absorption spectra. As shown in Table 3,
there were visible blue-shifts from the peak positions (absorp-
tion and SPEF spectra) of T1 to those of T2 in the same sol-
vent polarity. The linear absorption and SPEF spectra of the
two compounds in DMF are shown in Fig. 3, as well as these
visible shifts. As shown in the molecular structures, the struc-
ture modification occurs only in terminal electron-donor
group. The sequence in the blue-shifts was explained by the
electron-donating strength of donor groups. It is well known
that a strong electron donor can stabilize the charge-separated
excited state of a molecule. It indicates that T1 contains the
stronger donor. Investigating the structure of T2, we found that
the carbazolyl plane had a big dihedral angle (68.2ꢁ) to the
next phenyl plane (serious twist of the carbazolyl units to
the ꢀ-bridges). Thus, the lone electron pair may be delocalized
onto the carbazolyl group mainly, reducing charge transfer and
molecular polarity, especially in the excited state. However,
the carbazolyl plane in T1 was coplanar with plane of the ꢀ-
bridges. Thus, the carbazolyl ligand can donate more electrons
than that in T2.
˚
1.302(6) (C22=C23), and 1.455(6) A (C23–C24) (Table 2),
which suggests that all non-hydrogen atoms between carbazole
donor and terpyridine acceptor are highly conjugated.
Single crystals of T2 (Fig. 2 and Table 1), suitable for the
X-ray analysis, were obtained from slow evaporation of CHCl3
covered with ethanol at room temperature after several days.
There was disorder in crystal structure, which affects C11 and
C12 in ꢀ-bridge. The disorder had two orientations, and the
major part (C11A–C12A) had an occupancy factor of 0.50.
As shown in Fig. 2, in the electron-acceptor moiety, the central
pyridinyl (labeled P0) was also arranged with dihedral angles
of 21.5ꢁ between P0 and P3, 2.2ꢁ between P0 and P4, and 2.2ꢁ
between P0 and P5, indicating that the terpyridinyl is more co-
planar than that in T1, which may be good for its electron-
accepting ability. The dihedral angle between the carbazolyl
plane (labeled P1) and its neighboring phenyl plane (labeled
P2) was 68.2ꢁ, while the dihedral angle between planes P2
and P3 was 15.7ꢁ. The linkage between the two planes was
conjugated with the bond lengths are: 1.493(5) (C10–C11),
˚
1.338(6) (C11=C12), and 1.484(6) A (C12–C13) (Table 2).
In further survey of the ꢀ-conjugated plane, extending to the
terpyridinyl group, it was revealed that the dihedral angle
between P0 and P2 (5.8ꢁ) was almost the difference of that
From Table 3 and Fig. 4, the maximum absorption peaks
showed slight solvatochromism (less than 4 nm), the maximum
fluorescence peaks clearly were red-shifted, and the fluores-

990 Bull. Chem. Soc. Jpn. Vol. 80, No. 5 (2007)
Two-Photon Initiators
Fig. 3. Linear absorption spectra (c ¼ 1 ꢃ 10ꢂ5 mol Lꢂ1),
SPEF spectra (c ¼ 1:0 ꢃ 10ꢂ5 mol Lꢂ1), and TPEF (c ¼
1 ꢃ 10ꢂ3 mol Lꢂ1) spectra of two initiators in DMF.
Fig. 5. The linear dependence of Log Iout on Log Iin of two
initiators. Excitation carried out at 760 nm, with c ¼ 1:0 ꢃ
10ꢂ3 mol Lꢂ1 in DMF.
Fig. 4. Linear absorption spectra (c ¼ 1 ꢃ 10ꢂ5 mol Lꢂ1
)
and SPEF spectra (c ¼ 1:0 ꢃ 10ꢂ5 mol Lꢂ1) of T1 in var-
Fig. 6. TPEF spectra (c ¼ 1:0 ꢃ 10ꢂ3 mol Lꢂ1) of T1 in
ious solvents.
various solvents.
cent lifetime were longer, except for ethanol, with an increase
in the polarity of the solvent for each compound. This can be
explained by the fact that the excited state of two compounds
may possess higher polarity than that of the ground state, since
solvatochromism is associated with the energy level lowering.
Increased dipole–dipole interaction between the solute and sol-
vent leads to lowering of the energy level.16 In the case of
ethanol, hydrogen bonding between the solvent and solute
molecules will push the excitation energy lower.17
the chromophores were excited by using a femtosecond laser
beam with a wavelength of 720 nm, strong fluorescence with
colors changing from blue to green were observed and detect-
ed (Table 3). As shown in Fig. 1 and Table 3, the TPEF spec-
tra of two dyes were quite similar to the SPEF spectra and had
similar blue-shifts from T1 to T2 in the same solvent. The
TPEF characteristics of the compounds were also affected by
their molecular environments. We found that there are similar
vibronic structures and visible solvatochromism in the TPEF
and SPEF spectra (Fig. 6). This implies that they come from
the same fluorescent excited state. In addition, we also ob-
served greater red-shifts in the peak positions of in the TPEF
spectra relative to their corresponding SPEF with a decrease
in the polarity of the solvent for each compound (Table 3).
This is due to re-absorption of the fluorescence in the concen-
trated solution. However, when the Stokes’ shifts are large
enough that the emission and absorption bands hardly over-
lap, this re-absorption is weakened. It was concluded that
the smaller the solvent polarity, the smaller the Stokes’ shifts,
Two-Photon-Excited Fluorescence (TPEF) and Absorp-
tion (TPA). As shown in Fig. 3, there was no linear absorp-
tion in the range of 600–800 nm for two initiators, which sug-
gests that there are no molecular energy levels corresponding
to this spectral range. Therefore, upon excitation from 600 to
800 nm, it was impossible to produce single-photon-excited
up-converted fluorescence. If frequency up-converted fluores-
cence, which is induced with a laser in this spectral range,
appears, it should be mainly attributed to two-photon-excited
fluorescence (TPEF). Figure 5 shows a log–log plot of the
excited fluorescence signal vs excited light power. It provided
direct evidence for the squared dependence of excited fluores-
cence power and input laser intensity.
TPEF
max
SPEF
.
max
and more visible the red-shifts from ꢃ
to ꢃ
The TPA cross-sections were measured using the two-
photon-induced fluorescence technique with the following
equation:18
When the concentrated solutions (1:0 ꢃ 10ꢂ3 mol Lꢂ1) of

Z.-J. Hu et al.
Bull. Chem. Soc. Jpn. Vol. 80, No. 5 (2007)
991
fine structure constant, h! is the photon energy of the incident
ꢁ
ꢀref cref nref
F
ꢇ ¼ ꢇref
;
ð1Þ
light, gð!Þ denotes the spectral line profile, which is assumed
to be a ꢁ-function., and ꢊf is the lifetime broadening of the
final state, which is assumed to be 0.1 eV.23
ꢀ
c
n Fref
where ref stands for the reference molecule, ꢇ 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 in-
tensities of the solution emitted at the exciting wavelength, and
ꢀ is the fluorescence quantum yield. The ꢇref value was taken
from the literature. By referencing the TPA cross-section of
flouorescein to be 19 GM (1 GM = 10ꢂ50 cmꢂ4 s photonꢂ1),19
the obtained TPA cross-sections of the two initiators were
128 and 68 GM, respectively. According to Albota et al.,20
the terminal substitution with strong electron donors causes
an increase in electron delocalization in the excited state, re-
sulting in a substantial increase in the transition dipole mo-
ment, which is the major contributor to the enhanced TPA
cross-section. The experimental results basically satisfied this
principle.
We utilized the Gaussian-98 program package to optimize
the molecular geometrical structure hybrid density functional
theory (DFT/B3LYP) and a basis set 6-31G. The few-state
model is used to study the TPA cross-section of the molecules.
The calculated largest TPA cross-sections of both molecules
were 2060 and 1510 GM, respectively. The calculation results
were bigger than the experimental results and reasonably pre-
dicted the dependence of TPA ꢇ value on the structure varia-
tion. Considering the sensitivity of TPA ꢇ on measurement
conditions, such as solvent, intensity level, and the pulse dura-
tion of the laser beam, we think that the calculated ꢇ values are
acceptable. Therefore, the theoretical and experimental results
provide powerful evidence that these dyes can be used as sen-
sitive two-photon polymerization initiators.
Theoretical Calculation of Two-Photon Cross Sections.
The transition probability of one-photon absorption (OPA) is
given by an oscillator strength, where ꢉꢂ is the electric dipole
moment operator, !f denotes the excitation
Two-Photon-Initiating Polymerization Microfabrication.
In our TPIP experiments, three-dimensional cross-linked lat-
tice periodic microstructures were fabricated with negative
resins system that contained oligomer (methyl methacrylate)
and 0.5% of T1 or T2 as initiator and a little 1,2-dichloro-
ethane, which increased the compatibility and helped to con-
trol the viscosity. The same mode-locked Ti:sapphire laser that
was used in the TPF measurements was utilized for TPMF at
720 nm. The laser was tightly focused via an objective lens
(ꢃ50, NA ¼ 0:45), and the focal point was modulated in the
gel resin, which was on a xyz-step monitored stage, with a
speed of 80 mm sꢂ1. The pulse energy after being focused by
the objective lens was ꢄ30 mW. Polymerization was observed
with a polarization microscope (Opton, Germany), and the
photographs of the microstructures were shown in Fig. 7a.
The polymerization solid skeletons were uniform and orderly,
and their width was about 3 mm. The distance between neigh-
boring streaks was 10 mm. The polymerization reaction did not
take place in the part out of the laser focus, which is easily
washed out by tetrahydrofuran (Fig. 7b). In order to further
explore the fabrication of practical devices, we designed and
introduced artificial defects into the microstructures. Figure 7b
shows that simple defects were introduced successfully, which
is necessary for applied micro devices sometimes. The micro-
structures were stable for a long time in air without volume
shrinkage and distortion. The polymerization reaction in the
skeletons went to completion, and the inner structure was com-
pact and rigid.
X
2!f
2
ꢁop
¼
jh0jꢉꢂjfij
ð2Þ
3
ꢂ
energy of the excited state jfi, j0i denotes the ground state,
and the summation is performed over the x, y, and z axes of
the molecule.
In terms of sum-over-state formula, the two-photon matrix
element for the two-photon resonant absorption of identical en-
ergy21 is written as
ꢀ
ꢁ
X
h0jꢉꢂjjihjjꢉꢅjf i h0jꢉꢅjjihjjꢉꢂjfi
S
ꢂꢅ
¼
þ
;
ð3Þ
!j ꢂ !f =2
!j ꢂ !f =2
j
where j0i and jfi denote the ground state and the final state,
respectively, jji stands for all the intermediate states, including
the ground state, !j is the excitation energy of the excited
states, and ꢉ is the electronic dipole moment. In general, the
total TPA probability requires the information for all of the ex-
cited states, which is difficult to obtain. On the other hand, a
few-state model often provides reasonable results for TPA
cross-section in a visible region.20 The TPA cross-section is
given by orientational averaging over the two-photon absorp-
tion probability22
h
i
X
ꢀ
ꢀ
ꢀ
S
ꢂꢅ
ꢅꢂ
ꢁtpa
¼
F ꢃ S
S
þ G ꢃ S
S
þ H ꢃ S
;
ð4Þ
ꢂꢂ
ꢂꢅ
ꢅꢅ
ꢂꢅ
According to Cumpston et al.,6a strong donor substituents
ꢂꢅ
where the coefficients F, G, and H are related to the incident
radiation. For linearly polarized light, F, G, and H are 2, 2,
and 2, but for ircular polarized light, they are ꢂ2, 3, and 3, re-
spectively. In the present work, we only consider the results
with the linearly polarized laser beam. The summation goes
over the molecular axes ꢂ, ꢅ ¼ fx; y; zg.
The TPA cross-section directly comparable with experimen-
tal measurements is defined as
4ꢀ2a50ꢂ !2gð!Þ
ꢇtpa
¼
ꢁtpa
;
ð5Þ
15c0
ꢊf
Fig. 7. Optical micrographs of the lattice fabricated via
two-photon polymerization using T1 as an initiator.
where a0 is the Bohr radius, c0 is the speed of light, ꢂ is the

992 Bull. Chem. Soc. Jpn. Vol. 80, No. 5 (2007)
Two-Photon Initiators
(Nos. 2006KJ032A and 2006KJ135B), Key Foundation of
the Ministry of Education of China (No. 06060347), Team
for Scientific Innovation Foundation of Anhui Province
(No. 2006KJ007TD) and Person with Ability Foundation of
Anhui Province (No. 2002Z021). We also wish to thank Prof.
D. Q. Wang of Liao Cheng University for his assistance with
the X-ray structure determinations.
using T1 as an initiator
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In conclusion, we reported two D–ꢀ–A-type functional ter-
pyridine derivatives which are efficient TPA initiators. The
structure/properties relationships were discussed. The experi-
mental and theoretical results revealed that the compounds
with excellent SPEF and TPEF properties can be obtained
by conjugating different terminals with terpyridyl groups.
The photopolymerization reactions were successfully initiated
by the two initiators. It was verified that the present synthetic
approaches are very practical to prepare efficient two-photon
initiators. It is noteworthy that facile syntheses were employed
in building these terpyridine derivatives, demonstrating that it
is easy to construct efficient complicated hetero-aromatic TPA
initiators for realizing cheaper TPIP technology using safer
and greener methods.
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