Synthesis, structures, and two-photon absorption properties of two new heterocycle-based organic chromophores

Article abstract of DOI:10.1246/bcsj.79.1614

Two new heterocycle-based two-photon absorption chromophores, 4-[4-(4,5-diphenyl-1H-imidazol-2-yl)styryl]-pyridine (2) and (E)-4-{[2-(1H-benzimidazol-2-yl)vinyl]styryl}-N-methylpyridinium iodide (4), have been synthesized and characterized. The two molecules possess A-π-A′ structures. A π-deficient heteroaromatic ring (benzimidazole, 4,5-diphenyl-1H-imidazole) is used as an acceptor (A), and a pyridine ring is used as another acceptor (A′). One-photon-excited fluorescence, one-photon-fluorescence quantum yields, two-photon-excited fluorescence, and two-photon absorption cross-sections were investigated. Pumped with 740 and 800 nm laser excitation, compounds 2 and 4 had two-photon absorption cross-sections (41 and 38 GM) and two-photon-excited fluorescence (511 and 601 nm) in DMF, respectively. The crystal structure of compound 4 was determined by using X-ray single-crystal diffraction analysis.

Full text of DOI:10.1246/bcsj.79.1614

1
614 Bull. Chem. Soc. Jpn. Vol. 79, No. 10, 1614–1619 (2006)
Ó 2006 The Chemical Society of Japan
Synthesis, Structures, and Two-Photon Absorption Properties
of Two New Heterocycle-Based Organic Chromophores
ꢀ
1;2
1
1
1
Yun-Xing Yan,
Hai-Hua Fan, Chi-Keung Lam, Hong Huang,
Jing Wang, Sheng Hu, He-Zhou Wang, and Xiao-Ming Chen1
1
1
ꢀ1
1
State Key Laboratory of Optoelectronic Materials and Technologies, School of Chemistry
and Chemical Engineering, Sun Yat-Sen University, Guangzhou 510275, P. R. China
2
State Key Laboratory of Crystal Materials, Shandong University, Jinan 250100, P. R. China
Received March 29, 2006; E-mail: yxyan@icm.sdu.edu.cn
Two new heterocycle-based two-photon absorption chromophores, 4-[4-(4,5-diphenyl-1H-imidazol-2-yl)styryl]-
pyridine (2) and (E)-4-{[2-(1H-benzimidazol-2-yl)vinyl]styryl}-N-methylpyridinium iodide (4), have been synthesized
0
and characterized. The two molecules possess A–ꢀ–A structures. A ꢀ-deficient heteroaromatic ring (benzimidazole,
0
4
,5-diphenyl-1H-imidazole) is used as an acceptor (A), and a pyridine ring is used as another acceptor (A ). One-photon-
excited fluorescence, one-photon-fluorescence quantum yields, two-photon-excited fluorescence, and two-photon ab-
sorption cross-sections were investigated. Pumped with 740 and 800 nm laser excitation, compounds 2 and 4 had two-
photon absorption cross-sections (41 and 38 GM) and two-photon-excited fluorescence (511 and 601 nm) in DMF,
respectively. The crystal structure of compound 4 was determined by using X-ray single-crystal diffraction analysis.
Two-photon absorption (TPA) in organic molecules has
attracted growing attention due to potential applications in-
Recent studies showed that the heterocycle-based two-
photon absorbing chromophores exhibit large TPA cross-sec-
tions. However, the mechanism and the fundamental structure–
property relationships for increasing their TPA cross-sections
are not completely clear. In this paper, we present the synthe-
sis, structures, and TPA cross-sections of two new heterocycle-
based organic chromophores, 4-[4-(4,5-diphenyl-1H-imidazol-
2-yl)styryl]pyridine (2) and (E)-4-{[2-(1H-benzimidazol-2-yl)-
vinyl]styryl}-N-methylpyridinium iodide (4). These two mole-
1–4
cluding microfabrication,
5
three-dimensional optical data
9,10
–8
storage, optical power limitation, localized photodynam-
ic therapy, and two-photon laser scanning fluorescence imag-
1
1
1
2–14
ing.
These applications take advantage of the fact that the
activity of two-photon absorption scales quadratically with the
intensity of the incident laser radiation. Some of these applica-
tions are further based on the fluorescence properties of two-
photon-excited molecules. In the case of two-photon-excited
fluorescence (TPEF) microscopy, high-performance fluoro-
phores must exhibit both high fluorescence quantum yields
0
cules have a general A–ꢀ–A structure. A ꢀ-deficient benzimi-
dazole or 4,5-diphenyl-1H-imidazole is used as an acceptor
0
(A), and the pyridine ring acts as a second acceptor (A ). These
(
ꢀ) and large two-photon absorption cross-sections (ꢁ) in
two molecules have strong one-photon-excited fluorescence
(SPEF) and TPEF, and the experimental results showed that
these two molecules are good TPA chromophores. The crystal
structure of compound 4 was determined by X-ray single-crys-
tal diffraction analysis (Scheme 1).
the near infrared region, corresponding to the biological opti-
mal window for reduced photodamage. Photostability is also
an important feature to consider for TPEF-based applications.
To meet market criteria for these applications, molecules with
large TPA cross-sections are required.
Experimental
So far, the TPA activities of most dyes available are not
large enough and lead to the use of either high laser intensity
and/or high fluorophore concentration. To fully exploit the
greater potential of TPA process, it is important to establish
efficient methods for the synthesis of fluorophores that are
amenable to further chemical functionalization or modifica-
tion, which, in turn, can be expanded to obtain materials with
large TPA cross-sections. In recent years, the optimization of
molecular TPA has largely focused on one-dimensional di-
Chemicals and Instruments. 1,4-Dimethylpyridinium iodide
was synthesized according to the reported method.31 4-bromobenz-
aldehyde, benzil, 4-vinylpyridine (95%), and palladium(II) acetate
(
phine was obtained from Tokyo Kasei Kogyo Co., Ltd. The
47.5% Pd) were purchased from Acros Organics. Tri-o-tolylphos-
1
H NMR spectra were recorded at 300 MHz on a Mercury-Plus
00 spectrometer. Elemental analyses were performed by using
3
Perkin-Elmer 2400 elemental analyzer. The melting points and
decomposition temperatures were determined on a Netzsch DSC
1
5–17
18–26
polar
tures.
or quadrupolar structures, and octupolar struc-
It should be pointed out that introducing a hetero-
2
04 thermal analyzer and a Netzsch TG 209 thermogravimetric
2
7–29
ꢁ
ꢂ1
analyzer at a heating rate of 10 C min under dinitrogen atmo-
sphere, respectively. Mass spectra were measured on a LCMS-
2010A liquid chromatograph mass spectrometer. UV–visible–
near-IR spectra were measured on a UV-3150 spectrophotometer.
The one-photon fluorescence spectra and quantum yields (ꢀ) were
atom into a molecular structure is another efficient way to
obtain excellent TPEF molecules because ꢀ-deficient and ꢀ-
excessive heterocycles may act as efficient acceptor and donor
moieties, respectively.2
4,30
Published on the web October 10, 2006; doi:10.1246/bcsj.79.1614

Y.-X. Yan et al.
Bull. Chem. Soc. Jpn. Vol. 79, No. 10 (2006) 1615
N
O
O
a
N
b
Br
N
NH
NH
1
2
N
N
c
N
d
CHO
NH
N
H
NH
N+ CH3I-
3
4
Scheme 1. Synthesis of compounds 2 and 4: a) NH4OAc/HOAc, reflux; b) 4-vinylpyridine/tri-o-tolylphosphine/palladium(II)
ꢁ
ꢁ
acetate/triethylamine, reflux; c) (Ac)2O/HOAc, 120 C, 8 h; d) 1,4-dimethylpyridinium iodide/EtOH/piperidine, 80 C, 12 h.
measured on a Hitachi F-4500 fluorescence spectrophotometer.
Coumarin307 (whose ꢀ is assumed to be 56% in methanol) was
used as the reference.32 The concentration was the same as that
of the linear absorption spectra. The path-length of the quartz cuv-
ettes was 1 cm. The two-photon-induced fluorescence spectra were
observed with a single sweep streak camera equipped with a poly-
chromator as a recorder (Hamamatsu Model C1587). A mode-
locked Nd:YAG laser (PC2143, pulse duration of 25 ps) was used
to pump up the OPG (Optical Parameter Generator) system oper-
ating at 10 Hz, tuned from 420 to 10000 nm. The excitation wave-
lengths for compounds 2 and 4 were 740 and 800 nm, respectively.
ESI-MS, m=z calcd for C28H21N3 399.17, found 399.25.
(E)-4-[2-(1H-Benzimidazol-2-yl)vinyl]benzaldehyde (3). Com-
pound 3 was obtained according to the reported method,30 and
it was recrystallized from ethyl acetate to afford yellow crystals
of compound 3 in 70% yield. Anal. calcd for C16H12N2O: C,
77.40; H, 4.87; N, 11.28%. Found: C, 77.01; H, 4.79; N, 11.15%.
1
H NMR (CDCl3, 300 MHz) ꢂ 10.01 (s, 1H), 7.87 (d, J ¼ 8:4 Hz,
2H), 7.69 (d, J ¼ 10:9 Hz, 1H), 7.67 (s, 1H), 7.65 (d, J ¼ 5:9 Hz,
1H), 7.62 (d, J ¼ 8:3 Hz, 2H), 7.34 (d, J ¼ 3:3 Hz, 1H), 7.31 (d,
J ¼ 3:3 Hz, 1H), 7.29 (d, J ¼ 7:0 Hz, 2H).
(E)-4-{[2-(1H-Benzimidazol-2-yl)vinyl]styryl}-N-methylpyri-
dinium iodide (4). At room temperature, (E)-4-[2-(1H-benzimida-
zol-2-yl)vinyl]benzaldehyde (1.24 g, 0.01 mol), 1,4-dimethylpyri-
dinium iodide (1.18 g, 0.01 mol), ethanol (100 mL), and piperidine
(three drops) were added to a one-neck flask (250 mL) equipped
with a stirrer and a condenser. The mixture was refluxed for 8 h.
After the solvent was removed, a red precipitate was obtained.
The solid was recrystallized from ethanol, and orange-red crystals
of compound 4 were obtained. The yield is 75% and decomposi-
ꢂ3
ꢂ1
The concentrations are 1 ꢃ 10 mol L , respectively.
Synthesis. 2-(4-Bromophenyl)-4,5-diphenyl-1H-imidazole
(
(
(
1). A mixture of benzil (1.05 g, 5 mmol), 4-bromobenzaldehyde
0.925 g, 5 mmol), ammonium acetate (2.5 g), and acetic acid
25 mL) was refluxed for 12 h and then cooled to room tempera-
ture. After adding concentrated hydrochloric acid (10 mL), the
mixture was left for 1 h, filtered, and washed with water. The fil-
trate was treated with 30% aqueous sodium hydroxide solution
ꢁ
(
20 mL) to afford a pale-yellow precipitate. The crude product
tion temperature (Td) is 295.8 C. Anal. calcd for C23H20IN3
2.5H2O: C, 54.12; H, 4.90; N, 8.23%. Found: C, 53.75; H, 4.82;
ꢄ
was purified by column chromatography on silica gel using ethyl
acetate/petroleum ether (1:15) as eluent. Compound 1, as a pale-
yellow solid, obtained in 86% yield. Anal. calcd for C21H15BrN2:
C, 67.21; H, 4.03; N, 7.47%. Found: C, 66.94; H, 3.97; N, 7.36%.
1
N, 8.17%. H NMR (DMSO-d6, 300 MHz) ꢂ 12.64 (s, 1H), 8.84
(d, J ¼ 6:6 Hz, 2H), 8.21 (d, J ¼ 6:6 Hz, 2H), 8.02 (d, J ¼ 16:2
Hz, 1H), 7.79 (t, J ¼ 8:8 Hz, 4H), 7.68 (d, J ¼ 16:5 Hz, 1H), 7.49
(q, J ¼ 5:4 Hz, 3H), 7.32 (d, J ¼ 16:5 Hz, 1H), 7.18 (d, J ¼ 3:3
Hz, 2H), 4.25 (s, 3H).
Crystallography. The X-ray diffraction data of compound 4
were collected on a Bruker Smart Apex CCD diffractometer
1
H NMR (DMSO-d6, 300 MHz) ꢂ 8.11 (d, J ¼ 8:4 Hz, 2H), 8.05
(
s, 1H), 7.71 (d, J ¼ 8:4 Hz, 2H), 7.48 (d, J ¼ 3:3 Hz, 4H), 7.375
d, J ¼ 3:3 Hz, 6H). ESI-MS: m=z calcd for C21H15BrN2 374.04,
(
found 374.15.
˚
4-[4-(4,5-Diphenyl-1H-imidazol-2-yl)styryl]pyridine (2). Com-
pound 1 (1.87 g, 5 mmol), tri-o-tolylphosphine (0.65 g, 2.15 mmol),
(Mo Kꢃ, ꢄ ¼ 0:71073 A). An absorption correction was applied
by using SADABS.33 The structure was solved by direct methods
and refined with a full-matrix least-squares analysis using
SHELXS-97 and SHELXL-97 programs, respectively.34,35 The
ethylene groups with two-fold disorder (C7–C8, C30–C31, and
C38–C39) were subjected to geometrical restraints in refinement.
Anisotropic thermal parameters were applied to all non-hydrogen
atoms. The organic hydrogen atoms were generated geometrically
4
(
-vinylpyridine (1.16 mL, 10.75 mmol), palladium(II) acetate
0.06 g, 0.27 mmol), and redistilled triethylamine (100 mL) under
dinitrogen were added to a three-necked flask equipped with a
magnetic stirrer, a reflux condenser, and a dinitrogen input tube.
The reaction mixture was refluxed in an oil bath under dinitrogen.
An orange product was obtained after heating and stirring for 24 h.
Then, the solvent was removed under reduced pressure, and the
residue was dissolved in dichloromethane, washed with distilled
water (3 ꢃ 50 mL), and dried with anhydrous magnesium sulfate.
The organic layer was filtered and concentrated. The resulting
solution was chromatographed on silica gel using ethyl acetate/
˚
(C–H 0.96 A); the hydrogen atoms except those on the water
molecules were added geometrically and refined with isotropic
temperature factors. Crystal data for C23H20IN3 2.5H2O: Triclin-
ꢄ
ꢁ
ic, space group P1, a ¼ 9:706ð2Þ, b ¼ 13:588ð3Þ, c ¼ 18:244ð3Þ
˚
ꢁ
A, ꢃ ¼ 75:821ð3Þ, ꢅ ¼ 79:282ð3Þ, ꢆ ¼ 77:691ð4Þ , Mr 505.32,
ꢁ
˚ 3
ꢂ3
,
petroleum ether (1:1) as eluent. Compound 2 (mp 301.8 C) was
obtained as a yellow-green powder in a yield of 55%. The en-
Z ¼ 4, V ¼ 2256:5ð7Þ A , T ¼ 293ð2Þ K, Dcalcd ¼ 1:487 g cm
R1 ¼ 0:0514, wR2 ¼ 0:1059 for 2467 reflections I = 2ꢁðIÞ.
ꢂ1
thalpy is 126.2 mJ mg . Anal. calcd for C28H21N3: C, 84.18; H,
CCDC reference number 610567.
1
5
.30; N, 10.52%. Found: C, 83.87; H, 5.32; N, 10.48%. H NMR
Results and Discussion
(
8
DMSO-d6, 300 MHz) ꢂ 12.72 (s, 1H), 8.54 (d, J ¼ 5:1 Hz, 2H),
.11 (d, J ¼ 8:1 Hz, 2H), 7.76 (d, J ¼ 8:1 Hz, 2H), 7.44 (m, 14H).
Structure and Characterization. Selected bond lengths

1
616 Bull. Chem. Soc. Jpn. Vol. 79, No. 10 (2006)
Two-Photon Absorption
˚
Table 1. Selected Bond Lengths (A) and Bond Angles ( ) for Compound 4
ꢁ
0
N1–C2
N1–C6
N1–C1
1.301(12)
1.322(10)
1.482(10)
1.297(10)
1.402(11)
1.395(10)
1.406(10)
1.325(10)
1.350(11)
1.459(10)
1.347(10)
1.397(14)
1.302(15)
1.348(12)
1.359(14)
1.367(15)
1.416(14)
1.455(5)
C9–C8
C9–C8
1.453(5)
1.464(5)
1.382(12)
1.400(11)
1.376(11)
1.459(5)
1.394(11)
1.315(5)
1.452(5)
1.387(12)
1.421(12)
1.381(13)
1.345(14)
1.413(12)
1.397(11)
1.340(11)
1.379(17)
1.373(18)
1.450(5)
1.457(5)
1.362(14)
1.340(15)
C32–C37
C32–C31
C32–C31
C33–C34
C34–C35
C35–C36
C35–C38
C35–C38
C36–C37
C40–C39
C40–C39
C41–C42
C41–C46
C42–C43
C43–C44
C44–C45
C45–C46
C7–C8
1.422(15)
1.452(5)
1.456(5)
1.356(12)
1.411(14)
1.390(15)
1.452(5)
1.460(5)
1.375(13)
1.453(5)
1.459(5)
1.393(11)
1.396(12)
1.384(12)
1.396(14)
1.337(14)
1.374(14)
1.313(5)
1.311(5)
1.309(5)
1.311(5)
1.311(5)
1.308(5)
0
0
0
C10–C11
C11–C12
C12–C13
C12–C15
C13–C14
C15–C16
C16–C17
C18–C19
C18–C23
C19–C20
C20–C21
C21–C22
C22–C23
C25–C26
C26–C27
C27–C28
C27–C30
C27–C30
C28–C29
C32–C33
N2–C17
N2–C18
N3–C17
N3–C23
N4–C25
N4–C29
N4–C24
N5–C41
N5–C40
N6–C40
N6–C46
C2–C3
C3–C4
C4–C5
C4–C7
C4–C7
0
0
0
0
1.458(5)
C7 –C8
C30–C31
C5–C6
C9–C14
C9–C10
1.334(12)
1.406(13)
1.445(14)
C38–C39
0
0
0
C30 –C31
0
C38 –C39
C2–N1–C6
120.7(9)
105.7(8)
106.6(8)
121.2(11)
121.6(11)
114.3(9)
121.2(10)
110.2(9)
116.8(11)
121.0(12)
126.0(11)
112.6(10)
133.5(11)
C22–C23–C18
N3–C23–C18
N4–C25–C26
C25–C26–C27
C25–N4–C29
C28–C27–C26
C28–C27–C30
C29–C28–C27
N4–C29–C28
C33–C32–C37
C32–C33–C34
C33–C34–C35
C36–C35–C34
122.3(9)
104.1(9)
122.0(10)
124.3(12)
118.1(9)
110.8(10)
100.0(16)
126.0(13)
118.7(11)
116.3(10)
125.0(12)
119.2(11)
117.7(10)
C37–C36–C35
C36–C37–C32
N6–C40–N5
120.8(12)
120.9(11)
108.5(8)
110.0(9)
107.0(10)
102.6(9)
C17–N2–C18
C17–N3–C23
N1–C2–C3
C2–C3–C4
C3–C4–C5
C19–C18–C23
N2–C18–C23
C20–C19–C18
C21–C20–C19
C20–C21–C22
C23–C22–C21
C22–C23–N3
C41–N5–C40
C40–N6–C46
N5–C41–C46
C42–C41–C46
C43–C42–C41
C42–C43–C44
C45–C44–C43
C44–C45–C46
N6–C46–C41
C45–C46–C41
0
123.2(10)
115.2(9)
120.9(10)
123.0(11)
118.3(12)
111.8(10)
119.4(12)
3.593Å
3.405Å
Fig. 1. Perspective views of ꢀ–ꢀ stacking interactions
dash line) in compound 4. Symmetry codes: A(ꢂ1 ꢂ x;
ꢂ y; 2 ꢂ z) and B(ꢂ1 ꢂ x; 3 ꢂ y; 1 ꢂ z).
Fig. 2. Hydrogen-bonding interactions in compound 4.
(
3
compound 4 are conjugated and essentially planar. In one of
the crystallographically independent organic cations, the dihe-
ꢁ
and bond angles of compound 4 are summarized in Table 1.
The molecular structure and packing diagram are depicted in
Figs. 1 and 2, respectively.
There are two crystallographically independent organic cat-
ions in the asymmetric unit. All of their bond lengths lie in the
usual range. The organic cations in the crystal structure of
dral angle between the benzimidazole and phenyl rings is 6.3 ,
ꢁ
and that between the phenyl and pyridinium rings is 10.0 . For
the other crystallographically independent organic cation, the
dihedral angle between the benzimidazole and phenyl rings
ꢁ
is 7.5 , and that between the phenyl and pyridinium rings is
ꢁ
10.5 . Therefore, compound 4 is a highly delocalized ꢀ-elec-

Y.-X. Yan et al.
Bull. Chem. Soc. Jpn. Vol. 79, No. 10 (2006) 1617
tron system, which is a necessary structural characteristic for
two-photon absorption.
solutions of compounds 2 and 4 in DMF are completely trans-
parent above 500 nm. The one-photon fluorescence spectra
were measured with the same concentrations as those of the
linear absorption spectra, and excitation wavelengths for com-
pounds 2 and 4 are 385 and 435 nm, respectively.
As shown in Table 2, the absorption maxima are not signif-
icantly different, although the one- and two-photon fluores-
cence maxima are slightly red-shifted, and the fluorescent life-
times are lengthened upon increasing the polarity of the aprotic
solvent for compound 2. This fact can be attributed to the fact
that the excited states may have a higher polarity than that of
the ground state, since solvatochromism is associated with the
lowering of energy levels. By increasing dipole–dipole interac-
Lattice water molecules are found to be co-crystallized in
the crystal structure of compound 4, forming various hydrogen
bonds, such as the O–HꢄꢄꢄN hydrogen bond between a water
ꢂ
molecule and a benzimidazole group, the O–HꢄꢄꢄI hydrogen
bond between a water molecule and an iodide ion, the N–
HꢄꢄꢄI hydrogen bond between a benzimidazole group and an
ꢂ
iodide ion, the O–HꢄꢄꢄO hydrogen bond among the water mole-
cules. Some interesting phenomena were also observed, Fig. 3.
Hydrogen-bonded hexagons are formed by O1w, O2w, I , and
ꢂ
their centrosymmetrically related partners. Apart from strong
hydrogen-bonding interactions, other weak intermolecular in-
teractions, such as aromatic ꢀ–ꢀ interactions between conju-
gated organic cations, are also found in the crystal structure
of compound 4. The two different organic cations are orientat-
ed in different directions and stacked in a head-to-tail fashion.
tions between the solute and solvent, the energy level can be
lowered greatly.3
6,37
In the case of ethanol, the fluorescence
maximum is slightly red-shifted compared to those in other
aprotic solvents. The formation of hydrogen bonds between
the solvent and solute molecules also lowers the excitation
energy.38 The absorption and one-photon fluorescence maxima
for compound 4 show a similar trend upon increasing the
polarity of solvent except ethanol.
˚
The inter-planar distances are 3.405 and 3.593 A, respectively,
indicating strong ꢀ–ꢀ interactions between adjacent organic
cations (Fig. 1).
One- and Two-Photon Fluorescence. The photophysical
properties of the two compounds are summarised in Table 2.
The linear absorption, one-photon fluorescence, and two-pho-
ton fluorescence spectra in DMF are shown in Fig. 4. The lin-
ear absorption spectra were measured in solvents of different
0
0
0
0
0
0
0
.6
.5
.4
.3
.2
.1
.0
(f)
(
c)
(d)
(e)
ꢂ5
ꢂ1
polarity at a concentration of 1 ꢃ 10 mol L , in which the
(
b)
solvent influence has been excluded. Figure 4 shows that the
(a)
3
00
400
500
600
700
800
900
Wavelength/nm
Fig. 4. Absorption and fluorescence spectra of compounds
2 and 4 in DMF: linear absorption (a), one- (b) and two-
photon (c) fluorescence spectra of compound 2; linear
absorption (d), one- (e) and two-photon (f) fluorescence
spectra of compound 4.
Fig. 3. Hydrogen-bonding interactions between iodide ions
and water molecules in compound 4. Symmetry codes:
A(1 ꢂ x; 2 ꢂ y; 1 ꢂ z), B(2 ꢂ x; 1 ꢂ y; 1 ꢂ z), and C(1 ꢂ x;
y ꢂ 1; z).
aÞ
Table 2. The Data of Absorption, One- and Two-Photon Fluorescence with Solvent Effects of Compounds 2 and 4
ð1aÞ
res/104
ð1 fÞ
ꢂ1
ð2Þ
Compds
Solvents
ꢄ
/nm
"
ꢄ
/nm
ꢂꢇ/cm
ꢈ/ns
ꢀ/%
ꢄ
/nm
max
max
max
2
DMF
368
3.39
3.20
3.50
3.38
2.70
2.43
507
7450
7603
8170
6989
7508
7134
3.16
59
78
54
72
78
90
511
Ethyl acetate
Ethanol
THF
CHCl3
Toluene
357
360
365
355
357
490
510
490
484
479
2.53
2.03
494
487
4
DMF
408
395
407
392
410
4.75
1.84
5.02
2.41
1.62
586
581
578
571
565
7445
8104
7269
7997
6691
0.08
0.73
5.23
1.52
3.50
1.18
601
Ethyl acetate
Ethanol
THF
CHCl3
ð1aÞ
ð1 f Þ
ð2Þ
a) ꢄ , ꢄ , and ꢄ
are one-photon absorption, one-photon fluorescence, and two-photon fluorescence peak
max
max
max
32
maxima, respectively. ꢀ is one-photon fluorescence quantum yield determined using coumarin 307 as the standard.
ꢈ is two-photon fluorescence lifetime. ꢂꢇ is Stokes’ shift. "res is the corresponding molar absorption coefficient.

1
618 Bull. Chem. Soc. Jpn. Vol. 79, No. 10 (2006)
Two-Photon Absorption
By comparing Table 2 and Fig. 4, we can see that the two-
This work was supported by NSFC (Nos. 20531070,
50323006, and 50402018), Guangdong Provincial Science
and Technology Bureau (No. 04205405) and China Postdoc-
toral Science Foundation (No. 2004036515).
photon fluorescence maxima of compound 2 do not shift com-
pared to the corresponding one-photon fluorescence maxima in
the same solvent. Although the two-photon fluorescence spec-
tra are measured at concentrations, higher than those need to
measure the one-photon fluorescence spectra, the Stokes’ shift
is large enough to make re-absorption effects negligible.39 As
shown in Fig. 4, there is no obvious overlap between the blue
side of the one-photon-induced fluorescence band and the red
side of the linear absorption band. For compound 4, the two-
photon fluorescence maximum has red-shifted compared to
the one-photon fluorescence maximum in DMF. This can be
ascribed to the effect of re-absorption for the linear absorption
band which slightly overlaps the emission band and to the high
concentrations in the two-photon fluorescence experiments
that made re-absorption significant. By considering the simi-
larities between SPEF and TPEF, we can conclude that, al-
though the molecules can be excited to different excited states
by one-photon absorption or two-photon absorption due to the
different spectral selection rules, they finally relax to the same
fluorescent excited state. It is clear that structural and environ-
mental factors influence the SPEF and TPEF properties.
The TPA Cross-Section. The TPA ꢁ of the two molecules
were determined by comparing their TPEF with the two-pho-
ton fluorescence excitation ꢁ of Rhodamine 6G (at a concen-
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