Two-photon absorption dyes with thiophene as π electron bridge: Synthesis, photophysical properties and optical data storage

Article abstract of DOI:10.1016/j.dyepig.2011.06.028

Four novel dyes are prepared by thiophene as π bridge between carbazole central core and other terminal groups by Suzuki and Heck coupling reactions. These dyes are fully characterized by IR, ¹H NMR, ¹³C NMR, MS and elemental analysis. Linear absorption, single- and two-photon excited fluorescence in various solvents are experimentally investigated. The calculated two-photon absorption cross sections of 9-Hexyl-3,6-di((5-phenyl)-2-thienyl) carbazole (1), 9-Hexyl-3,6-di((5-thienyl)-2-thienyl)carbazole (2), 9-Hexyl-3,6-di((5-p-vinylpyridyl)-2-thienyl)-carbazole (3) and 9-Hexyl-3,6-di-((5-o-vinylpyridyl)-2-thienyl)carbazole (4) for the lowest excited state are 537.84, 550.76, 1292.95 and 1340.40 × 10^-50 cm⁴ s photon^-1, respectively. Calculated and experimental data have shown that thiophene as π electron bridge improves the two-photon absorption cross sections greatly. Two-photon optical data recording experiments have been carried out at 820 nm laser radiation.

Full text of DOI:10.1016/j.dyepig.2011.06.028

Dyes and Pigments 92 (2011) 633e641
Contents lists available at ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Two-photon absorption dyes with thiophene as
p
electron bridge:
Synthesis, photophysical properties and optical data storage
,
a
a
a
a
Hongping Zhou a , Feixia Zhou , Shiya Tang , Peng Wu , Yixin Chen ,
*
Yulong Tu 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
Article history:
Four novel dyes are prepared by thiophene as
p bridge between carbazole central core and other terminal
groups by Suzuki and Heck coupling reactions. These dyes are fully characterized by IR, 1H NMR, 13C NMR,
MS and elemental analysis. Linear absorption, single- and two-photon excited fluorescence in various
solvents are experimentally investigated. The calculated two-photon absorption cross sections of 9-Hexyl-
3,6-di((5-phenyl)-2-thienyl)carbazole (1), 9-Hexyl-3,6-di((5-thienyl)-2-thienyl)carbazole (2), 9-Hexyl-
3,6-di((5-p-vinylpyridyl)-2-thienyl)-carbazole (3) and 9-Hexyl-3,6-di-((5-o-vinylpyridyl)-2-thienyl)
Received 28 April 2011
Received in revised form
19 June 2011
Accepted 23 June 2011
Available online 6 July 2011
carbazole (4) for the lowest excited state are 537.84, 550.76, 1292.95 and 1340.40 ꢀ 10ꢁ50 cm4 s photonꢁ1
,
Keywords:
Thiophene
Linear absorption
Single-photon excited fluorescence
Two-photon excited fluorescence
Calculation
respectively. Calculated and experimental data have shown that thiophene as
p electron bridge improves
the two-photon absorption cross sections greatly. Two-photon optical data recording experiments have
been carried out at 820 nm laser radiation.
ꢀ 2011 Elsevier Ltd. All rights reserved.
TPA data storage
1. Introduction
electron could interact through pseudo-conjugated system
(
s
system) between two
In order to obtain more information, we have designed a series
of dyes with systematic molecular structure of types Ae eDe eA
and , where D is carbazole, A is pyridine, and represents
-conjugated vinyl or thiophene ring. The carbazolyl group is
p systems [6].
Organic molecules that can simultaneously absorb two or more
photons to be promoted to their excited states have recently been
the subject of much research due to the growing interest in
advanced photonic applications, such as optical limiting [1], three-
dimensional optical storage [2], microfabrication [3], and up con-
verted lasing [4]. To fully realize these applications, an intense
worldwide effort has been focused on the design of organic
p
p
pe
s
ep
p
a
p
chosen due to its rigid plane, long conjugation length, good hole
transporting properties and their charge transporting compounds
creating free carriers in the visible region through the photo-carrier
materials with a large TPA cross section (
lengths. Up to now, several efficient design strategies have been put
forward to enhancing dTPA, such as the -conjugated was employed
to connect donor (D) and acceptor (A) symmetrically or asym-
metrically to form DeAeD, De eA, De eD, De eA and
Ae eDe eA structures, macrocycles, dendrimers, polymers and
multibranched molecules. Furthermore, several other studies on
structureeproperty relationship reveal that the dTPA increase with
the D/A strength, chain length, and planarity of the p-center [5]. In
recent years, some researchers get the information that the
d
) at desirable wave-
generation process, and thiophene-based p-conjugating spacers
have been proved effective in photovoltaic cells because of their
chemical and environmental stability as well as their electronic
tunability [7].
In this paper, four novel dyes (1, 2, 3 and 4) were synthesized via
Suzuki and Heck coupling reactions as illustrated in Scheme 1.
Structural confirmation data for the four new dyes are presented.
Linear absorption, steady-state fluorescence and two-photon
absorption were measured. Our experimental and ability study
results demonstrate that not only do the TPA cross sections of
p
p
p
p
p
p
AepeDepeA (3 and 4) increase, but also those of pesep type
molecules (1 and 2) with introduction of thiophene ring increase.
Furthermore, we attempted to use the new dyes as optical data
storage materials by the method of void creation in polymer
* Corresponding author. Tel.: þ86 551 5108151; fax: þ86 551 5107342.
E-mail addresses: zhpzhp@263.net, zhoufxfx@126.com (H. Zhou).
0143-7208/$ e see front matter ꢀ 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.dyepig.2011.06.028

634
H. Zhou et al. / Dyes and Pigments 92 (2011) 633e641
Scheme 1. Synthetic routes to four dyes 1e4.
according to references [8]. The optical system that we used for
data recording and reading with the new material was similar to
the instrumental setup used in the polymerization [9]. Experi-
mental results show the novel dyes exhibit good optical memory.
31 mmol)/ethanol (20 mL) was dropped to the mixture at 80 ꢂC. The
reaction mixture was refluxed for 2 h, cooled to room temperature
and filtered. The product was crystallized with ethanol and
produced pale blue crystals. Yield: 89.02%. IR (KBr, cmꢁ1) selected
bands: 2947 (m), 2919 (s), 2848 (m), 1855 (w), 1718 (w), 1582 (w),
1467 (vs), 1424 (s), 1374 (m), 1342 (m), 1281 (s), 1227 (m), 1189 (w),
1145 (m), 1052 (w), 1003 (w), 856 (m), 795 (vs), 725 (m), 626 (m),
2. Experimental
2.1. Characterization
561 (m). 1H NMR: (CDCl3, 400 MHz),
d (ppm): 0.88 (s, 3H), 1.31
(s, 6H), 1.83 (t, J ¼ 6.40 Hz, 2H), 4.23 (t, J ¼ 7.20 Hz, 2H), 7.18
Elemental analyses were performed with a Perkins-Elmer 240 B
instrument. IR spectra were recorded with a Nicolet FT-IR NEXUS
870 instrument (KBr discs) in the 4000e400 cmꢁ1 region. 1H NMR,
13C NMR spectra were obtained on Bruker Avance 400 MHz spec-
trometer in CDCl3 solution (with TMS as internal standard). Mass
spectrum was determined with a Micro mass GTC-MS (EI source).
The solvents were purified by conventional methods before use.
The excitation source for the TPEF experiments was a mode-locked
femtosecond Ti:Sapphire laser (Spectra-physics, 100 fs, 82 MHz)
tuned to 720 nm. The maximum average laser power available for
these experiments was about 100 mW. An optical streak camera
was used as a recorder. All the samples are done by the concen-
tration of 10ꢁ3 mol/L. The compounds and the methyl methacrylate
resin were dissolved in CH2Cl2. Upon evaporation of the solvent at
room temperature, the resin formed a film on a glass disk.
(d, J ¼ 8.00 Hz, 2H), 7.73 (d, J ¼ 8.40 Hz, 2H), 8.34 (s, 2H). 13C NMR
(CDCl3, 100 MHz),
d (ppm): 13.985, 22.502, 26.876, 28.795, 31.500,
43.262, 81.637, 110.898, 123.984, 129.359, 134.498, 139.506. Anal.
Calc. For C18H19I2N: C, 42.97; H, 3.81; N, 2.78. Found: C, 42.58; H,
3.64; N, 2.36%. MS, m/z (%): 502.96 (Mþ,100), 431.87 ([M-n-C5H11]þ).
2.2.2. Intermediate B
A (3.00 g, 6 mmol) and DMF (10 mL) under nitrogen were added
to a three-necked round-bottomed flask (80 mL) equipped with
a magnetic stirrer, a reflux condenser and a nitrogen input tube.
10 mL of Et3N was added until all of the precipitate dissolved at
70 ꢂC. Thiophen-2-ylboronic acid (2.55 g, 20 mmol) and Pd(OAc)2
were successively added, then the reaction mixture was refluxed in
an oil bath at 130 ꢂC under nitrogen. The reaction mixture was
refluxed for 6 h and cooled to room temperature. The solution was
dissolved in methylene chloride (200 mL), washed three times with
distilled water, and dried with anhydrous magnesium sulfate. Then
it was filtered and concentrated. The product was crystallized with
ethyl acetate and gave dark yellow columnar crystals. Yield: 80.11%.
IR (KBr, cmꢁ1) selected bands: 3417 (m), 3102 (m), 2963 (m), 2928
(m), 2849 (m), 1732 (s), 1684 (s), 1601 (s), 1484 (vs), 1424 (s), 1294
(s), 1234 (m), 1214 (m), 1160 (m), 1077 (w), 1047 (w), 865 (w), 826
2.2. Synthesis
Considering thiophene is a better donor and can be readily
functionalized in the 2- and/or 5-position, we prepared four new
dyes by thiophene as
p electron bridge between carbazole central
core and other terminal groups by Suzuki and Heck coupling
reaction. The synthetic route toward the four compounds is
described in Scheme 1.
(m), 800 (s), 694 (s). 1H NMR: (CDCl3, 400 MHz),
d (ppm): 0.92
(t, J ¼ 7.20 Hz, 3H), 1.42e1.34 (m, 6H), 1.92e1.86 (m, 2H), 4.28
(t, J ¼ 7.20 Hz, 2H), 7.17 (d, J ¼ 4.00 Hz, 2H), 7.31 (d, J ¼ 5.20 Hz, 2H),
2.2.1. Intermediate A
7.39 (t, J ¼ 4.40 Hz, 4H), 7.77 (d, J ¼ 8.00 Hz, 2H), 8.38 (s, 2H). 13
C
Hexylcarbazole (2.51 g, 10 mmol) and ethanol (10 mL) were
added to a three-necked flask equipped with a magnetic stirrer,
a reflux condenser, an isobaric dropping funnel, ICl (5.00 g,
NMR (CDCl3, 100 MHz), d (ppm): 14.043, 22.563, 26.969, 29.013,
31.588, 43.330, 109.196,118.021, 122.138, 123.238, 123.727, 124.625,
125.939, 128.031, 140.449,145.690. Calcd for C26H25NS2: C, 75.14; H,

H. Zhou et al. / Dyes and Pigments 92 (2011) 633e641
635
6.06; N, 3.37%. Found: C, 75.50; H, 6.39; N, 3.60%. MS, m/z (%):
415.14 (Mþ, 100), 344.05 ([M-n-C5H11]þ), 262.07 ([M-n-
C5H11eC4H3S]þ).
was charged with C (1.32 g, 2.00 mmol), DMF (30 mL) and water
(10 mL) under a nitrogen atmosphere. Et3N (10 mL) was added until
all of the precipitate dissolved at 70 ꢂC. 4-vinylpyridine (5 mL) was
added and refluxed for 2 h, then PPh3 (0.10 g, 0.38 mmol) and
Pd(OAc)2 (0.033 g, 0.15 mmol) were successively added. The
mixture was allowed to warm to 90 ꢂC. The mixture was refluxed
for 6 h and cooled to room temperature. The solution is dissolved in
methylene chloride (200 mL), washed three times with distilled
water, and dried with anhydrous magnesium sulfate. Then it was
filtered and concentrated. After removal the solvent, the residue
was purified by column chromatography on a silica gel with
petroleum ether/ethyl acetate (1:1) as eluant to yield pure red
powder. Yield: 40.23%. IR (KBr, cmꢁ1) selected bands: 3551 (w),
3471 (m), 3412 (m), 2950 (m), 2925 (m), 2947 (m), 1017 (s), 1619
(m), 1593 (s), 1549 (w), 1439 (s), 1237 (w), 1206 (w), 1158 (w), 1136
(w), 1051 (w), 985 (w), 875 (w), 853 (w), 802 (s), 541 (m). 1H NMR:
2.2.3. Intermediate C
A round-bottomed flask (50 mL) equipped with a magnetic
stirrer, was charged with B (0.13 g, 0.31 mmol) in acetone (9 mL). The
reaction mixture was stirred at room temperature until all the
precipitate was dissolved, and then NIS (0.42 g, 1.87 mmol) in
acetone (4 mL) was added in portions. After 4 h of reflux, the reac-
tion was checked by TLC (petroleum ether:ethyl acetate ¼ 4:1)
indicates that it was completed, and then dissolved in methylene
chloride (200 mL), washed three times with distilled water and
dried with anhydrous magnesium sulfate. Then it was filtered and
concentrated. The solid was crystallized with ethanol and yellow
acicular crystals were obtained. Yield: 54.21%. IR (KBr, cmꢁ1
)
selected bands: 3415 (s), 3234 (m), 2924 (w), 2849 (m), 1638 (s),
(DMSO-d6, 400 MHz),
d
(ppm): 0.79 (t, J ¼ 6.80 Hz, 3H), 1.25e1.15
1616 (m), 1487 (w), 1384 (m), 802 (w), 623 (m). 1H NMR: (CDCl3,
(m, 6H), 1.78 (d, J ¼ 6.00 Hz, 2H), 4.42 (t, J ¼ 5.60 Hz, 2H), 6.92
(d, J ¼ 16.00 Hz, 2H), 7.37 (d, J ¼ 3.20 Hz, 2H), 7.55 (d, J ¼ 5.20 Hz,
6H), 7.67 (d, J ¼ 8.80 Hz, 2H), 7.83e7.74 (m, 4H), 8.53 (d, J ¼ 4.80 Hz,
400 MHz),
d
(ppm): 0.90 (d, J ¼ 7.20 Hz, 3H),1.33 (t, J ¼ 4.00 Hz, 6H),
1.87 (d, J ¼ 7.20 Hz, 2H), 4.29 (t, J ¼ 7.20 Hz, 2H), 7.04 (d, J ¼ 4.00 Hz,
2H), 7.27 (d, J ¼ 4.00 Hz, 2H), 7.39 (d, J ¼ 8.40 Hz, 2H), 7.65
4H), 8.64 (s, 2H). 13C NMR (DMSO-d6, 100 MHz),
d (ppm): 14.278,
(q, J ¼ 1.60 Hz, 2H), 8.23 (s, 2H). 13C NMR (CDCl3, 100 MHz),
d
(ppm):
22.438, 26.514, 28.983, 31.391, 43.027, 110.794, 118.245, 119.950,
121.037, 121.229, 123.064, 123.754, 124.566, 124.653, 125.333,
127.134, 130.948, 139.889, 140.830, 144.562, 145.888, 150.411. Calcd
for C40H35N3S2: C, 77.26; H, 5.67; N, 6.76%. Found: C, 77.62; H, 5.29;
N, 6.45%. MS, m/z (%): 621.23 (Mþ, 100), 550.10 ([M-n-C5H11]þ).
14.014, 22.534, 26.939, 28.984, 31.546, 43.377, 70.751, 109.351,
117.871,123.133,123.604,124.452,125.237,137.911,140.610,151.608.
Anal. Calc. For C26H23I2NS2: C, 46.79; H, 3.47; N, 2.10. Found: C,
46.43; H, 3.61; N, 2.32%. MS, m/z (%): 666.9 (Mþ,100), 595.85 ([M-n-
C5H11]þ), 541.04 ([M ꢁ I]þ), 469.95 ([M ꢁ I-n-C5H11]þ).
2.2.7. 9-Hexyl-3, 6-di((5-o-vinylpyridyl)-2-thienyl)carbazole 4
Yellow powder was obtained by a similar method to 3 using 2-
vinylpyridine instead of 4-vinylpyridine. Compound 4 was purified
by column chromatography on silica gel using petroleum ether:-
ethyl acetate ¼ 5:1 as eluant. Yield: 56.45%. IR (KBr, cmꢁ1) selected
bands: 3466 (m), 3414 (m), 3061 (w), 2958 (m), 2925 (m), 2850 (m),
1622 (s), 1581 (s), 1559 (m), 1445 (m), 1423 (s), 1256 (s), 1199 (m),
1092 (s), 869 (m), 793 (m), 758 (w), 736 (m), 575 (w), 534 (m). 1H
2.2.4. 9-Hexyl-3, 6-di((5-phenyl)-2-thienyl)carbazole 1
Yellow powder was obtained by a similar method to B using C
instead of A and benzene-2-ylboronic acid instead of thiophen-2-
ylboronic acid. Compound 1 was purified by column chromatog-
raphy on silica gel using petroleum ether as eluant. Yield: 53.23%. IR
(KBr, cmꢁ1) selected bands: 3414 (m), 2957 (m), 2922 (m), 2850 (m),
1599 (s), 1482 (s), 1451 (s), 1385 (s), 1291 (m), 1240 (m), 1217 (w),
1156 (w), 799 (s), 748 (s), 687 (m). 1H NMR: (CDCl3, 400 MHz),
NMR: (CDCl3, 400 MHz),
d
(ppm): 0.81 (t, J ¼ 6.20 Hz, 3H),1.40e1.20
d
(ppm): 0.91 (t, J ¼ 6.00 Hz, 3H),1.38 (m, 6H),1.93 (t, J ¼ 7.20 Hz, 2H),
(m, 6H), 1.84 (t, J ¼ 6.80 Hz, 2H), 4.25 (t, J ¼ 7.00 Hz, 2H), 7.14e6.92
(m, 6H), 7.25 (d, J ¼ 3.60 Hz, 2H), 7.29 (d, J ¼ 7.60 Hz, 2H), 7.35 (d,
J ¼ 8.40 Hz, 2H), 7.70 (d, J ¼ 6.00 Hz, 2H), 7.71 (m, 4H), 8.31 (s, 2H),
4.34 (t, J ¼ 7.20 Hz, 2H), 7.37e7.29 (m, 6H), 7.45e7.41 (m, 6H), 7.71 (q,
J ¼ 7.20 Hz, 4H), 7.80 (q, J ¼ 1.60 Hz, 2H), 8.41 (s, 2H).13C NMR (CDCl3,
100 MHz),
d
(ppm): 14.009, 22.541, 26.965, 29.020, 31.571, 43.396,
8.54 (d, J ¼ 4.80 Hz, 2H). 13C NMR (CDCl3, 100 MHz),
d (ppm):
109.272, 117.729, 122.976, 123.276, 124.046, 124.316, 125.522,
125.901, 127.242, 128.906, 134.593, 140.512, 142.433, 144.982. Calcd
for C38H33NS2: C, 80.38; H, 5.86; N, 2.47%. Found: C, 80.72; H, 6.24; N,
2.81%. MS, m/z (%): 567.2 (Mþ, 100), 496.1 ([M-n-C5H11]þ).
14.012, 22.540, 26.951, 29.007, 31.558, 43.404, 109.355, 117.896,
121.789, 122.271, 122.753, 123.260, 124.436, 125.712, 126.739,
129.782, 136.347, 137.042, 140.303, 140.678, 146.052, 149.103,
155.143. Calcd for C40H35N3S2: C, 77.26; H, 5.67; N, 6.76%. Found: C,
77.64; H, 5.95; N, 6.37%. MS, m/z (%): 621.20 (Mþ, 100), 550.20 ([M-
n-C5H11]þ).
2.2.5. 9-Hexyl-3, 6-di((5-thienyl)-2-thienyl)carbazole 2
Yellow powder was obtained by a similar method to B using C
instead of A. Compound 2 was purified by column chromatography
on silica gel using petroleum ether:ethyl acetate ¼ 20:1 as eluant.
Yield: 45.12%. IR (KBr, cmꢁ1) selected bands: 3422 (m), 2955 (m),
2922 (m), 2854 (w),1602 (w),1480 (w),1454 (w),1385 (s),1237 (w),
3. Results and discussion
3.1. Theoretical calculation
794 (m), 761 (w), 692 (w). 1H NMR: (CDCl3, 400 MHz),
d (ppm): 0.90
(t, J ¼ 6.80 Hz, 3H), 1.36 (m, 6H), 1.91 (t, J ¼ 6.80 Hz, 2H), 4.31
(t, J ¼ 6.80 Hz, 2H), 7.08 (d, J ¼ 4.00 Hz, 2H), 7.25e7.21 (m, 6H), 7.30
(d, J ¼ 4.00 Hz, 2H), 7.41 (d, J ¼ 8.80 Hz, 2H), 7.75 (d, J ¼ 8.80 Hz, 2H),
The electronic properties of dyes 1e4 have been investigated by
means of theoretical calculations. The transition probability of OPA
is given by oscillator strength
8.36 (s, 2H). 13C NMR (CDCl3, 100 MHz),
d (ppm): 14.024, 22.549,
X
26.960, 29.015, 31.569, 43.387, 109.308, 117.706, 122.663, 123.239,
123.282, 124.026, 124.285, 124.695, 125.631, 127.853, 135.554,
137.800, 140.509, 144.486. Calcd for C34H29NS4: C, 70.42; H, 5.04; N,
2.42%. Found: C, 70.71; H, 5.34; N, 2.77%. MS, m/z (%): 579.1
(Mþ, 100), 508.05 ([M-n-C5H11]þ).
2
uf
2
dop
¼
jh0jma
f
(1)
j ij
3
a
where j0> denotes the ground state, jf > the final state, uf the
corresponding excitation frequency, and ma is the Cartesian
component of the electronic dipole moment operator. The
summation is performed over the molecular x, y and z axes.
The TPA cross section which can be directly comparable with
experimental measurement is defined as
2.2.6. 9-Hexyl-3, 6-di((5-p-vinylpyridyl)-2-thienyl)carbazole 3
A three-necked round-bottomed flask (80 mL) equipped with
a magnetic stirrer, a reflux condenser and a nitrogen input tube,

636
H. Zhou et al. / Dyes and Pigments 92 (2011) 633e641
4
2a50
p a
u
15c0
2gð
u
Þ
dtp
stp
¼
(2)
Gf
Here a0 is the Bohr radius, c0 is the speed of light,
a is the fine
structure constant, is the photon energy of the incident light, g(u)
u
denotes the spectral line profile, and Gf is the lifetime broadening of
the final state, which is assumed to be 0.1 eV here [10]. dtp is written
as follows [11]
h
i
X
dtp
¼
F ꢀ SaaS*bb þ G ꢀ SabS*ab þ H ꢀ SabS*ba
(3)
ab
where F, G and H are coefficients dependent on polarization of the
light. Sab is the TPA transition matrix element for the two-photon
resonant absorption of identical energy, and can be written as [12]
"
#
X
m
j
j
u
m
f
0jmb
j j f
m
h0j aj ih j bj i
h
j ih j aj i
Sab
¼
þ
(4)
uj
ꢁ
f =2
uj
ꢁ
u
f =2
j
where
a,
b
˛ðx; y; zÞ, uj and uf are the excitation frequency for the
intermediate state jj> and the final state jf > , respectively.
The most straightforward approach to analyze optical properties
of molecules is the response theory [13], which provides an
analytical solution for the TPA cross section. The equilibrium
geometries of molecules in gas phase are optimized by Gaussian
package [14] at the hybrid density functional theory (DFT/B3LYP)
level with 6-31G* basis set. The OPA and TPA properties are calcu-
lated by use of the response theory at DFT level implemented in
DALTON [15].
In order to further investigate the electronic properties of 1e4
by means of theoretical calculations, we have plotted the HOMO
and LUMO for dyes 1e4 in the gas phase, which is visualized by use
of MOLEKEL program (pictures of HOMOeLUMO are available in
Fig. 1). The HOMO is mainly located on the central carbazole ring in
1e4, while the LUMO is localized on the central nodes and the
linkages between those heterocycle and the carbazole ring for all
dyes. Distributions of HOMO and LUMO levels are separated in all
dyes indicating that the HOMOeLUMO transition can lead to
charge-transfer.
Fig. 1. HOMO and LUMO of 1e4 in the ground state (gas phase).
(see Fig. 3), which is visualized by the use of MOLEKEL program
[17]. The blue pare represents where the electrons go and the gray
pare represents where the electrons come from. For 1e4, the CT
plots are very similar. It can be seen that upon excitation, charges
are mainly transferred from the two branches to the
p-center. In the
CT state, there is more electron density at the -center on average,
p
indicating that the molecule could be ready to give away its elec-
tron to its surrounding.
The linear and nonlinear calculated optical properties for 1e4 in
gas phase are listed in Table 1. The absorption maxima of the four
dyes located from 343.25 to 401.08 nm, which correspond to pep*
3.2. Linear absorption and single-photon excited
transition for 1e2 but main CT transition for 3e4. The energy gaps
of the two-photon absorption band are almost equate to the energy
gaps of the single-photon absorption band, which is tally with the
theories of the nonlinear optics. From Table 1, one can observe that
the calculated optical properties depend on the nature of the
peripheral substituting groups due to their same principal frame-
work part (9-hexyl-3, 6-bis(2-thienyl) carbazole). The responding
field model is used to study the two-photon absorption cross
section of the molecules. The largest two-photon absorption cross
sections of molecules are 537.84 for 1, 550.76 for 2, 1292.95 for 3,
1340.40 GM (1 GM ¼ 10ꢁ50 cm4 s photonꢁ1) for 4, respectively,
which is obviously increased in comparison with that of the
intermediate B and the reported 3, 6-bis[2-(4-pyridyl)ethenyl]-9-
fluorescence (SPEF)
The photophysical properties (absorption and fluorescence) of
the new series of dyes in five different solvents (1.0 ꢀ 10ꢁ5 mol/L)
are collected in Table 2 (including fluorescence quantum yields and
lifetimes). Linear absorption spectra of 3 in toluene, THF, acetone,
DMF and benzyl alcohol with
a solution concentration of
c ¼ 1.0 ꢀ 10ꢁ5 mol/L are shown in Fig. 4. The linear absorption
Table 1
Calculated single- and two-photon-related photophysical properties of dyes 1e4
and intermediate B in gas phase.
Compd
D
E1
lðm1aaxÞ /nmb
DE2
lðm2afÞx/nmd
de
a
c
ethylcarbazole
(L1),
3,
6-bis[2-(2-pyridyl)ethenyl]-9-
ethylcarbazole (L2) (Fig. 2) by our team performed based on the
same TPA calculation [16]. Compounds 2, 3 and 4 are different from
B, L1 and L2 by one thiophene, and the TPA cross sections are
increased equally by about 400 GM [16]. The results revealed that
B
1
2
3
4
4.41
3.61
3.40
3.09
3.10
280.81
343.25
364.35
401.08
399.64
4.35
3.87
3.66
3.26
3.28
568.53
639.05
675.72
785.63
754.00
177.51
537.84
550.76
1292.95
1340.40
the thiophene as
, and Ae
For the better understanding of the two-photon progress, we
p
electron bridge could increase dTPA, for both the
a
The energy gap of the single-photon absorption band.
Peak position of the longest absorption band.
p
es
ep
p
eDe eA type molecules.
p
b
c
The energy gap of the two-photon absorption band.
Peak position of the two-photon absorption band.
have plotted the charge density difference between the ground
state and the first excited state (CT state) for 1e4 in the gas phase
d
e
Two-photon absorption cross section in GM (1 GM ¼ 10ꢁ50 cm4 s photonꢁ1).

H. Zhou et al. / Dyes and Pigments 92 (2011) 633e641
637
Table 2
Photophysical properties of dyes 1e4 in several of different polar solvents.
Compd
Solvents
lðm1aaxÞ /nma
lðm1afÞx/nmb
Fc
Δn
/cmꢁ1d
1
Toluene
THF
Acetone
DMF
Benzyl alcohol
Toluene
THF
Acetone
DMF
Benzyl alcohol
Toluene
THF
Acetone
DMF
Benzyl alcohol
Toluene
THF
Acetone
DMF
344, 364
344, 364
344, 364
344, 369
344, 364
356, 379
359, 376
357, 374
358, 379
361, 376
401
401
401
405
411
401
401
401
403
420, 435
425, 440
434
442
436
426, 450
431, 449
446
450
438, 453
467
500
507
517
523
460
482
493
501
0.068
1.000
0.20
3663
3831
4431
4475
4536
4163
4274
4416
4163
4520
3524
4937
5213
5348
5210
3198
4190
4653
4853
4746
0.15
0.054
0.069
0.778
0.069
0.931
0.839
0.152
0.044
0.045
0.038
0.024
0.094
0.090
0.083
0.080
0.045
2
3
4
Fig. 2. Schematic drawing of L1 and L2.
Benzyl alcohol
408
506
a
Peak position of the longest absorption band.
b
c
Peak position of SPEF, exited at the absorption maximum.
Quantum yields determined by using RhB and coumarin as standard.
Stokes’ shift in cmꢁ1
spectra of 3 are slightly red-shifted with the increasing solvent
polarity and show little solvatochromic behavior. Table 2 shows the
order of absorption maxima is 3 ꢃ 4 > 2 > 1 in the same solvent.
This confirms the fact that it flows of electrons through the thio-
phene ring than through a benzene ring [7,18,19]. The greater
d
.
which exhibits red shift to longer wavelength [21]. Protons from
benzyl alcohol molecules can form hydrogen bond with the
nitrogen heteroatom of pyridine rings and such hydrogen bond
usually enhances charge separation.
extent of
p-electron delocalization makes an extraordinary bath-
ochromic shift of the absorption band. Here, protonation of the
pyridine group in 3 and 4 results in a much stronger electron-
accepting group, thereby enhancing charge separation of
p-elec-
However, with increasing polarity of the solvent, single-photon
excited fluorescence (SPEF) spectra of all the dyes show remarkable
bathochromic shifts. As shown in Fig. 5 and Table 2, for example,
lmax (SPEF) of 3 is located at 467 nm in toluene and red-shifted to
517 and 523 nm in DMF and benzyl alcohol, respectively. These
results suggest that the molecule of the fluorescent exited state
“assumed to be the first exited stated” S1 must be larger than that of
the ground state, as the enhanced dipoleedipolar interactions
caused by the increasing polarity of solute and/or solvent will lead
to a more significant energy level decrease for the exited state. In
benzyl alcohol solution, it is the hydrogen bonds that stabilize the
separated charge states.
trons in the conjugated donoreacceptor structure [20]. The net
effect of the protonation enhances ICT in benzyl alcohol comparing
to the aprotic organic solvents such as acetone, THF and toluene,
The SPEF spectra of the dyes (1e4) at equimolar concentration
were recorded in DMF solution (Fig. 6). Similar to absorption
maxima, it can be seen that excited maxima is 3 ꢃ 4 > 2 > 1 in DMF.
0.6
Toluene
Benyl alcohol
THF
DMF
Acetone
0.5
0.4
0.3
0.2
0.1
0.0
350
400
450
500
550
600
Wavelength(nm)
Fig. 3. Density difference between the charge-transfer and ground states of 1e4 in the
gas phase.
Fig. 4. Linear absorption spectra of 3 in five solvents.

638
H. Zhou et al. / Dyes and Pigments 92 (2011) 633e641
1000
1000
600
500
400
300
200
100
0
800
Toluene
Benyl alcohol
THF
DMF
600
800
400
200
600
Acetone
0
0
10000
20000
30000
40000
2
2
400
200
0
I
(mW)
in
Experimental data
Fitted result
0
20
40
60
80 100 120 140 160 180 200 220
400
500
600
I (mW)
in
Wavelength(nm)
Fig. 7. Dependence of the output intensity (Iout) of compound 3 in DMF on the input
laser power (Iin). The inset shows the linear dependence of Iout on Ii2n
.
Fig. 5. Single-photon fluorescence spectra of 3 in five solvents with differing polarity.
shifts of 3 and 4 are increased very rapidly with the increasing
polarity. This is the typical characterization of CT correlating with
abilities study [23].
The quantum yields (F) of four compounds in different solvents
were determined by using RhB for 1 and 2, Coumarin for 3 and 4 as
standard (Table 2). In order to further demonstrate the influence of
solvent on fluorescence. Table 2 lists the Stokes’ shift of the four
dyes in solvents with different polarities. The Stokes’ shift is defined
as the loss of energy between absorption and reemission of light,
which is a result of several dynamic processes. These processes
include losses due to dissipation of vibrational energy, redistribu-
tion of electrons in the surrounding solvent molecules induced by
the altered dipole moment of the excited dye, reorientation of the
solvent molecules around the excited state dipole, and specific
interactions between the fluorophore and the solvent or solutes.
The Lippert equation is widely used to describe the effects of the
physical properties of the solvent on the emission spectra of fluo-
rophore [22].
3.3. TPA properties
As shown in Fig. 4, there is no linear absorption in the wave-
length range 475e900 nm for 3, which indicates that there are no
energy levels corresponding to an electron transition in this spec-
tral range. The linear dependence on the square of input laser
power suggests a two-photon excitation mechanism at 750 nm for
all the four molecules (Fig. 7). Fig. 8 shows TPEF of 1e4 in DMF at
750 nm excited wavelength. The peak wavelength of the TPEF is
red-shifted relative to the SPEF for all the compounds, which is due
to reabsorption in the high-concentrated solution. Fig. 9 shows that
ꢀ
ꢁ
ꢀ
ꢁ
the TPEF of
3 in different solvents. It shows positive sol-
Dn
¼
nabs
ꢁ
nem
¼
2=cha3
D
f
me
ꢁ
mg 2þconst
vatochromism effect, the same trend as the one photon fluores-
cence. By tuning the pump wavelengths increment at 10 nm from
720 to 900 nm while keeping the input power fixed and then
recording TPEF intensity, TPEF spectra are obtained.
TPA cross sections have been measured using fs TPEF in DMF.
The obtained dTPA for 1 and 2 at 770 nm are 150 GM and 100 GM,
the largest dTPA for 3 and 4 at 770 nm are 703 GM and 565 GM,
In this equation, h is Planck’s constant, c is the speed of light, and
a is the radius of the cavity in which the fluorophore resides. The
wavenumbers of the absorption and emission are na and nf (in
cmꢁ1), respectively. In aprotic solvents, from Table 2, the Stokes’
shifts are roughly approximately proportional to the orientation
polarizability for all the four compounds, especially the Stokes’
1.0
1
2
3
4
2000
0.8
1
2
3
4
1500
1000
500
0
0.6
0.4
0.2
0.0
400
500
600
700
400
450
500
550
600
650
700
Wavelength (nm)
Wavelength (nm)
Fig. 8. Two-photon fluorescence spectra of 1e4 in DMF (c ¼ 1.0 ꢀ 10ꢁ3 mol/L) at
Fig. 6. Single-photon fluorescence spectra of 1e4 in DMF (c ¼ 1.0 ꢀ 10ꢁ5 mol/L).
750 nm excited wavelength.

H. Zhou et al. / Dyes and Pigments 92 (2011) 633e641
639
Toluene
Benyl alcohol
THF
DMF
Acetone
1.0
0.8
0.6
0.4
0.2
0.0
400
450
500
550
600
650
Wavelength(nm)
Fig. 9. The two-photon fluorescence spectra of 3 in five solvents with differing polarity
(c ¼ 1.0 ꢀ 10ꢁ3 mol/L).
respectively (Fig. 10). Considering the ideal molecules used in the
calculation, it is easily to understand that the experimental values
are smaller than the calculation results. Compared with the mole-
cule we prepared before (L1), the calculation outcomes are
acceptable. The experimental dTPA for 3 is larger than the one for the
reported L1, by about 500 GM in experiment (703 GM for 3 vs
123 GM for L1) [16]. L1 is different from 3 by one thiophene, the
calculated value of 3 is also larger than that of L1 (1292.95 GM for 3
vs 947 GM for L1). In conclusion, system with thiophene ring can
greatly enhance the TPA cross sections. Previous reports show the
contribution to TPA cross sections per double bonds is 155 GM at
most. Here, we get the information that donor group thiophene is
an effective ‘electron bridge’ that increases TPA cross sections [24].
Fig. 11. Example of one-layer data storage using 3 as initiator. The distance between
two adjacent bits in the layer is 5 mm.
formation of voids in an undoped PMMA film by the use of single-
shot ultrashort pulses with a wavelength of 400 nm. It has been
shown that the voids in the PMMA can be read out under two-
photon excitation. Moreover, Day and Gu [8] reported on the
formation of submicrometer voids within dye-doped PMMA under
multiphoton absorption (MPA) excited by an infrared laser beam.
In the experiment, the used storage medium was PMMA doped
with new compounds. For example, compound 3 (99.5%) and
PMMA were mixed together (with a suitable weight ratio about
1:50) in a chloroform solution, and the film was coated onto a glass
3.4. TPA data storage
slide with thickness of about 150
mm in ambient air at room
For the sake of long time storage, the materials must be doped
into polymer. A polymer material has a low threshold of optical
damage and provides the possibility of doping with absorbing dyes,
which would allow the use of a long excitation wavelength to
reduce the scattering effect and the manipulation of the refractive
index. Poly(methyl methacrylate) (PMMA) is an interesting mate-
rial because of its high chemical resistance, advantageous optical
properties, and low cost. Recently, PMMA and dye-doped PMMA
are extensively studied with respect to fs laser machining [8].
A recent publication by Yamasaki et al. [8] demonstrates the
temperature. There is almost no absorption above 500 nm by the
chromophore; therefore, it undergoes a typical two-photon exci-
tation on using an 820 nm femtosecond laser as the writing laser.
Thus, as described in the literature [8], bits were recorded succes-
sively by a single pulse. A computer-controlled two-axis translation
stage was used to move the storage medium between pulses. The
recorded bits were retrieved through parallel reading with
a reflection-type confocal microscope, as shown in Fig. 11. The
shadow and blank can be detected and classified as stored 1 and
800
1
2
3 in DMF
4 in DMF
200
700
600
500
400
300
200
100
0
150
100
50
0
700
750
800
850
900
720
740
760
780
800
820
840
Wavelength(nm)
Wavelength(nm)
Fig. 10. Two-photon (from a 200 fs, 76 MHz Ti:sapphire laser) absorption cross sections of 1, 2, 3 and 4 in DMF vs excitation wavelengths of identical energy of 0.100 W
(experimental uncertainties:10%).

640
H. Zhou et al. / Dyes and Pigments 92 (2011) 633e641
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about 5 m. The laser irradiation duration was 50 ms for each bit
[25]. The difference in the bit size may be caused by the inhomo-
geneity of the doped chromophore in PMMA.
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The synthetic methodology employing Heck and Suzuki reac-
tion conditions facilitated a systematic variation of functional
groups of differing electronic characters. The dyes show good SPEF
and TPEF behavior and exhibit relatively large TPA cross section,
and can be used for optical data storage when initiated at 820 nm.
In this paper, the TPA properties were explored by extending the
conjugation in dipolar type compounds: benzene, thiophene and
vinylpyridine groups attached to a 9-hexyl-3, 6-bis(2-thienyl)
carbazole unit. The results demonstrate that introduction of donor
group thiophene as
p electron bridge improves the two photons
absorption cross sections for both AepeDepeA and pesep type
molecules greatly.
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(i) Morales AR, Belfield KD, Hales JM, Stryland EWV, Hagan DJ. Synthesis of two-
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2006;20:4972e80;
This work was supported by a grant for the National Natural
Science Foundation of China (21071001, 50873001), Education
committee of Anhui Province (KJ2010A030), The team for Scientific
Innovation Foundation of Anhui Province (2006KJ007TD), Science
and Technological Fund of Anhui Province for Outstanding Youth
(10040606Y22), The 211 Project of Anhui University, Ministry of
education funded projects focus on retured overseas scholar.
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