Synthesis and two-photon-excited fluorescence of benzothiazole-based compounds with various π-electron donors

Article abstract of DOI:10.1002/ejoc.200300272

We have synthesized a series of new D-π-A compounds that feature various electron donors and a fixed benzothiazolyl unit as an electron acceptor. The crystal structure of compound 3 [trans,trans-2-{4-[(4-N-carbazolyl)styryl]styryl}-1,3-benzothiazole, CSSB

Full text of DOI:10.1002/ejoc.200300272

FULL PAPER
Synthesis and Two-Photon-Excited Fluorescence of Benzothiazole-Based
Compounds with Various π-Electron Donors
Du-Xia Cao,[a] Qi Fang,*[a] Dong Wang,[a] Zhi-Qiang Liu,[a] Gang Xue,[a] Gui-Bao Xu,[a] and
Wen-Tao Yu[a]
Keywords: Fluorescence / Sulfur heterocycles / Wittig reactions
We have synthesized a series of new D−π−A compounds that
feature various electron donors and a fixed benzothiazolyl
unit as an electron acceptor. The crystal structure of com-
to-orange region. The measured TPEF cross-section of com-
pound [trans,trans-2-{4-[4-(N,N-diphenylamino)styryl]-
styryl}-1,3-benzothiazole, DPSSB], for example, is about 6.1
2
pound 3 [trans,trans-2-{4-[(4-N-carbazolyl)styryl]styryl}-1,3- times that of Coumarin 307. Photophysical data indicate that
benzothiazole, CSSB] was determined. All these compounds
show high fluorescence quantum yields and 3 in toluene
gives the most intense blue emission around 450 nm with a
quantum yield of Φ = 0.69. When excited at 800 nm by a
Ti:sapphire femtosecond laser, these compounds exhibit
strong two-photon-excited fluorescence (TPEF) in the blue-
these compounds are polar in the ground state and have an
enhanced polarity in the excited state, and that the electron
donating ability of a dialkylamino group is much stronger
than that of a diarylamino group.
( Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2003)
Introduction
TPA cross-section (σ), high SPEF and TPEF quantum
yields (Φ and ΦЈ, respectively), and large TPEF cross-sec-
tions (which can be formulated as σΦЈ). Photostability and
proper response wavelengths also have to be taken into con-
sideration. From the viewpoint of molecular engineering,
structural features such as the π-conjugation style, the mo-
lecular planarity, and the donating and withdrawing abili-
ties of the electron donor (D) and acceptor (A) all play an
important role in increasing σ, Φ, and σΦЈ. Most of the
newly synthesized TPEF-active molecules can be classified
as either asymmetrical DϪπϪA or symmetrical DϪπϪD/
AϪπϪA types.[2,10Ϫ17]
Two-photon-excited fluorescence (TPEF) is characterized
by the unusual excitation manner of simultaneous absorp-
tion of two photons, each of which individually matches
only half of the molecular energy level. Typically, as for the
examples in this paper, blue TPEF can be excited by red
light, i.e., the frequency of TPEF has been up-converted
relative to that of the excitation light. Increasingly, two-
photon absorption (TPA) and TPEF show several advan-
tages over common linear absorption and single-photon ex-
cited fluorescence (SPEF) techniques, such as increased
depth of penetration, reduced photodamage and greatly en-
hanced three-dimensional spatial resolution, which intrin-
sically originate from the quadratic dependence of TPA and
TPEF on the intensity of the incident laser.[1,2] In recent
years, new compounds with large TPA cross-sections or
TPEF cross-sections have received much research interest
and have shown promising applications in, for example,
three-dimensional optical data storage,[3] optical power lim-
iting,[4,5] two-photon-pumped frequency up-conversion las-
ing,[6,7] TPEF-based microscopy,[8] and photodynamic ther-
apy.[9]
By fixing a definite donor and π-bridge and then chang-
ing a series of different acceptors, Kannan et al. recognized
that benzothiazolyl is an excellent acceptor that gives rise
to a DϪπϪA type compound (AF-240, Scheme 1) that
shows an optimized TPA cross-section when compared with
the AF-X series of TPA compounds.[10] Kannan et al. also
reported that the chemical, thermal, and photochemical
stabilities of benzothiazolyl derivatives are much better than
structurally similar compounds.[10] By fixing the benzothia-
zolyl unit as an optimized acceptor and then changing a
series of different donors or π-bridges, the molecular struc-
tures may be further optimized for TPA and TPEF.
In the design of novel and useful TPA-based compounds
for various applications, some figures of merit must be
taken into consideration. Of crucial importance are a large
Based on these ideas, we recently synthesized a series of
new DϪπϪA-type compounds, where D is a number of ter-
tiary amino or thiophene groups, A is fixed as a benzothia-
zolyl unit and the π-bridge is extended to two conjugated
styryl units. We believed that the skeleton of this π-bridge
maintained basically planar, since planarity is commonly
[a]
State Key Laboratory of Crystal Materials,
Shangdong University,
Shanda South Road 27, Jinan, 250100, China
Fax: (internat.) ϩ86-531-8565403
E-mail: fangqi@icm.sdu.edu.cn
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DOI: 10.1002/ejoc.200300272
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Two-Photon-Excited Fluorescence of Benzothiazole-Based Compounds
FULL PAPER
modified Wittig reaction. By a similar method described
by Rose,[18] 2-aminothiophenol and 4-methylcinnamic acid
were cyclized in phosphorus oxychloride to afford 5Ј with
high yield. To prepare the phosphonium salt for Wittig re-
action, bromination is necessary, but the yield of bromin-
ation of 5Ј to the product 6Ј by using N-bromosuccinimide
(NBS) is quite low (ca. 35%). This low yield may be because
of the presence of an alkaline N atom on the benzothiazolyl
unit. The phosphonium salt 7Ј can be prepared con-
veniently from 6Ј and one equivalent triphenylphosphane.
Finally, the title compounds were obtained by Wittig reac-
tions using 7Ј and the aldehydes 1ЈϪ4Ј. We stress that the
Wittig reactions would produce mixtures of the trans- and
cis-alkenes. The trans-configured object compounds possess
higher chemical-, themal- and photo-stability and better
fluorescence properties. To obtain pure trans-object com-
pounds, all the crude products were treated with a trace
amount of iodine in toluene, and the trans-configured ones
were separated out by column chromatography on silica gel
and purified by recrystallization from chloroform. All these
1
new compounds were characterized by H NMR, IR, and
UV spectroscopy, MS spectrometry, and elemental analysis
(the details are shown in the Exp. Sect.).
Scheme 1. Molecular structures of the compounds studied in this
work and of AF-240[10]
2. Structural Features of 3
There are two types of disorder: one that affects the C19
and C20 atoms in the π-bridge and the other that affects
the C2 and C11 atoms in the carbazolyl unit (shown in
Figure 1). The disorder of the π-bridge (C19, C20) is over
two orientations and the major part (C19Ј, C20Ј) has an
occupancy factor of 0.63. Other disorder pairs (C2 and C2Ј,
C11 and C11Ј) have similar occupancy.
The molecular structure of 3 has been clearly charac-
terized by inspecting the planarity of the carbazolyl and
benzothiazolyl groups and several dihedral angles in the
molecule. As shown in Figure 1, the dihedral angle between
the carbazolyl plane (P1) and its neighboring phenyl plane
regarded as a positive structural factor in enhancing the
molecular TPA, SPEF, and TPEF properties.
As expected, these compounds possess high fluorescence
quantum yields in the blue-to-orange region. Particularly,
when excited by a tunable femtosecond laser at an exci-
tation wavelength of 800 nm, they showed strong TPEF. In
this paper, we report the detailed synthesis, structure and
photophysical properties of these new compounds.
Results and Discussion
1. Synthesis
As outlined in Scheme 2, we synthesized the title com- (P2: C13 to C18) is 53.95°, while the dihedral angle between
pounds by cyclization, bromination, and subsequently a P2 and the central phenyl plane (P3: C21 to C26) is only
Scheme 2. Synthetic strategy for the preparation of the compounds (D ϭ diethylamino, diphenylamino, carbozolyl)
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D.-X. Cao, Q. Fang, D. Wang, Z.-Q. Liu, G. Xue, G.-B. Xu, W.-T. Yu
FULL PAPER
Figure 1. Molecular structure of 3. Hydrogen atoms have been omitted for clarity
14.28°, and the dihedral angle between P3 and the terminal form sheets along the c axis, and that all the main molecular
phenyl plane (P4: C30 to C35) is 7.91°. These data indicate planes are parallel to each other with neighboring mol-
that the benzothiazolyl acceptor is so highly conjugated ecules in the a direction being arranged in a head-to-tail
with the π-bridge that it has become part of the whole π- manner.
delocalization system; the donor, however, which is seri-
ously twisted with respect to the main molecular plane,
does not conjugate well with the π-bridge. Thus, the co-
planar benzothiazolyl unit can make good use of its elec-
3. Linear Absorption and Single-Photon-Excited
Fluorescence
The photo-physical properties of the title compounds in
tron accepting ability. A further survey of the π-bridge,
four kinds of solvent are listed in Table 1. Figure 3 displays
which extends to the benzothiazolyl group, reveals that the
the typical linear absorption and SPEF spectra of these
compounds in THF. The positions of the fluorescence
peaks of these compounds are independent of the excitation
wavelength in the absorption region and the quantum yields
are also basically independent of the excitation wavelength
from 370 nm to 430 nm. Some notable spectral features can
be summarized as follows.
dihedral angle between P2 and P4 (22.16°) is almost equal
to the sum of that between P2 and P3 (14.28°) and that
between P3 and P4 (7.91°), showing that the planar π-
bridge has become gradually and slightly skewed. Although
detailed X-ray structures of 1, 2, and 4 are not available,
we believe that these benzothiazole-based title compounds
probably have similar structures to that of 3 in that their
conjugated π-bridges and benzothiazolyl groups are basi-
cally coplanar.
3.1. The Spectral Sequence and the Ground-State Polarity
As shown in Table 1 and Figure 3, the main absorption
and fluorescence peaks of 3 appear at relatively shorter
wavelengths, those of 1 appear at much longer wavelengths,
and those of 2 are located in between. Thus, the peak posi-
tions (with respect to wavelength) occur in a sequence 1 Ͼ
2 ϾϾ 3 (red-shift sequence).
This spectral sequence is maintained invariably in all of
the solvents used. It may correlate with the sequence of
electron-donating strengths of corresponding donors and,
therefore, be in accord with their sequence of molecular po-
larities.
It is interesting to survey whether the lone electron pair
of the amino group is delocalized and, if it is, how it is
delocalized. As shown in the structure of 3, the NC3 core
in the carbazolyl unit basically is planar, with the distance
˚
of the N atom to the C3 plane being only 0.0181 A (this
distance can be defined as the pyramidalization of the NC3
core). The NC3 plane is co-planar with the carbazolyl plane
rather than co-planar with the next phenyl plane (P2). The
Figure 2. Packing diagram for 3 viewed along the c axis
Figure 2 shows the view along the c axis of the packing NϪC bond lengths in the carbazolyl unit [N(1)ϪC(5): 1.405
˚
˚
diagram of compound 3 in the crystal. The structures can (7) A, N(1)ϪC(8): 1.386 (7) A] are considerable shorter
˚
be regarded as consisting of alternating (010) layers. The than the N(1)ϪC(13) length [1.410 (7) A]. These structural
molecules in two adjacent layers are nearly perpendicular features are quite common in all of the carbazolyl deriva-
to each other, forming a herring-bone-like structure, and all tives collected by the Cambridge Structural Database
of the carbazolyl groups are located between the adjacent (CSD). Thus, the lone electron pair is highly delocalized
layers. By viewing a certain molecular (010) layer, we ob- and is mainly delocalized within the carbazolyl group (P1
serve that molecules of 3 form columns along the a axis and plane).
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Two-Photon-Excited Fluorescence of Benzothiazole-Based Compounds
Table 1. Photophysical data of the title compounds
FULL PAPER
Solvents λmax(abs.) λmax(SPEF) ∆ν˜ [cmϪ1
]
103 ϫ m[c] ∆µge [D] Φ[a] [e] t [ns] λmax(TPEF) σ[g] [GM][h] σe
[d]
[i]
[nm][a]
[nm][a] [b]
[nm][f]
1 (DESSB) toluene 428
THF 425
528
590
644
656
514
552
606
614
444, 460
486
516
526
438, 460
473
4638
7018
8306
8738
4601
6578
8070
8501
3468, 4242
5880
7162
7589
3046, 4360
5611
0.54
0.35
0.13
0.11
0.61
0.49
0.22
0.17
0.69
0.61
0.54
0.49
0.45
0.41
0.33
0.29
1.6
2.3
1.9
2.0
1.3
2.0
2.2
2.3
1.0
1.4
2.0
2.2
0.9
1.0
1.0
1.0
528
590
116
206
3.3
3.8
acetone 423
CH3CN 421
13.78
13.04
12.48
10.58
24.5
27.2
25.7
2 (DPSSB) toluene 417
515
551
190
221
6.1
5.7
THF
412
acetone 408
CH3CN 406
toluene 387
3 (CSSB)
4 (TESB)
449, 472
487
99
118
3.6
3.8
THF
384
acetone 381
CH3CN 380
toluene 388
443, 467
474
42
65
1.0
1.4
THF
380
acetone 377
CH3CN 377
494
502
6525
6840
19.4
[a]
[b]
Linear absorption and SPEF properties were measured at c ϭ 5.0·10Ϫ6 . For 1 and 2, the excitation wavelength was 420 nm; for
[c]
[d]
3 and 4, it was 390 nm.
The slope of plots of ∆ν˜ versus ∆f.
The change in dipole moment, derived from the LippertϪMataga
equation. Determined using Coumarin 307 as a standard (Φ ϭ 0.56)[19] with the excitation at 400 nm. TPEF maxima excited with
[e]
[f]
an 800-nm laser at c ϭ 1.0·10Ϫ4 . The TPA cross-sections were measured by comparing their TPEF with that of Coumarin 307.
[g]
[h]
1 GM ϭ 10Ϫ50 (cm4·s)/photon. The relative TPEF cross-sections assuming that of Coumarin 307 is equal to 1.
[i]
ation of the NC3 core in them is larger than that in carbazo-
lyl derivatives. For example, the pyramidalization of NC3 in
˚
(E)-4,4Ј-bis(diphenylamino) stilbene is 0.0436 A and its
three NϪC bond lengths are very similar.[20] Thus, the lone
electron pairs in triphenylamino compounds may be delo-
calized averagely over the three phenyl units. The NC3 core
in alkylamino compounds also tends to be planar and so
the electron pair is also highly delocalized.[21] In contrast to
the case of 3, it must be delocalized mainly toward the ac-
ceptor in 1.
Because of the delocalization of the lone pair of electrons
to the diphenyl or carbazolyl (more significantly) units and
the serious twist of the carbazolyl unit relative to the π-
conjugated bridge, the electron-donating strength should be
in the order diethylamino Ͼ diphenylamino ϾϾ carbazolyl
(donating sequence). This sequence is supported by the cor-
1
responding H NMR chemical shift data of the aldehyde
protons in the precursor compounds: δ ϭ 9.69, 9.78, and
10.01 ppm for 1Ј, 2Ј, and 3Ј, respectively.
3.2. The Stokes Shifts and the Increased Excited-State
Polarity
The solvatochromic effect for these compounds is enor-
mous. For any one of these compounds, upon increasing
the solvent polarity the absorption λmax shows a small blue
shift, the emission λmax is remarkably red-shifted, and the
quantum yields decrease. Thus, as shown in Table 1, the
Stokes shifts ∆ν˜ of this series of compounds are all signifi-
cantly large in various solvents and occur in the sequence 1
Ͼ 2 ϾϾ 3 (sequence of Stokes shift).
Figure 3. Linear absorption and single-photon-excited fluorescence
in THF at c ϭ 5.0·10Ϫ6

For all triphenylamino compounds collected in the CSD,
the three phenyl rings are arranged in a propeller-like, non-
For polar compounds, the solvating interactions can be
coplanar fashion. As a statistical result, the pyramidaliz- mainly attributed to the dipoleϪdipole interactions between
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D.-X. Cao, Q. Fang, D. Wang, Z.-Q. Liu, G. Xue, G.-B. Xu, W.-T. Yu
Benzothiazolyl seems not to be so well known as are
FULL PAPER
the solute and the solvent molecules. Increased polarity of
a solvent and/or increased polarity of a solute will lead to other electron acceptors. Our photophysical data indicate
greater lowering of the energy level. The bathochromic ef- that the benzothiazolyl unit behaves as an acceptor in the
fect of SPEF above indicates that the dipole moment of excited state. The benzothiazolyl group is also a conjugated
these compounds should be much larger in the excited state delocalization system, as can be deduced by the X-ray
than that in the ground state. In another words, the dipole structure of 3. The polarity, π-delocalization, and photoind-
moment in the ground state may be greatly enhanced in the uced charge-transfer characteristics of a molecular system
excited state by the photoinduced charge transfer.
are basic structural factors in various branches of molecular
According to the LippertϪMataga equation,[22] the nonlinear optics, including TPA- and TPEF-related photo-
˜
Stokes shifts ∆ν can be related to the difference in dipole physics.
moments (∆µge ϭ µe Ϫ µg) between the ground and the
Among these compounds, 3 stands out as having the best
excited states:
SPEF qualities: high quantum yields, a short blue-green
emission wavelength, high chemical and photochemical sta-
bility, and easily crystallized, i.e., easy to purify.
(1)
4. Two-Photon-Excited Fluorescence Properties
In this equation, a is the cavity radius of the molecule, ∆f
is the so-called orientation polarizability with ε and n being
the dielectric constant and the refractive index, respectively,
of the solvent.
The setup for TPEF measurement is depicted in Figure 5.
A laser beam from a Ti:sapphire mode-locked femtosecond
laser (Coherent, Mira 900-D) was used as a pump source
and a streak camera (Hamamatsu, C5680) in conjunction
with an imaging spectrograph (Hamamatsu, C5094) was
used as a recorder. TPEF was detected at a direction per-
pendicular to the pump beam. To minimize the effects of
re-absorption, the excitation beam was focused as close as
possible to the wall of the quartz cell, which faced the slit
of the imaging spectrograph.
By varying different solvents, the Stokes shifts ∆ν˜ show
a fairly good linear relation with ∆f (shown in Figure 4).
Thus, the slope m of the fitted line of the ∆ν˜ vs. ∆f data will
give the term 2∆µge2/hca3, which can serve as a convenient
experimental measure of the dipole moment increase be-
tween the ground and excited states. The AM1 geometry
optimization procedure gives the total surface area of a
molecule and its equivalent a value was obtained by as-
suming the molecule to be a sphere having this surface area.
In this way, the slope m and the resulting value of ∆µge were
obtained and are listed in Table 1. Compound 1 has the
largest m value and 2 has the largest value of ∆µge because
of its largest a value. All ∆µge values are positive, suggesting
a considerably increased dipole moment for the excited
state.
Figure 5. Experimental setup for TPEF measurement
As shown in Figure 3, there is no detectable linear ab-
sorption from 550 to 1000 nm and, therefore, there should
be no such molecular energy level corresponding to this
wavelength range. Thus, upon excitation from 710 nm to
900 nm, there should be no possibility of producing single-
photon-induced up-conversion fluorescence and no such in-
termediate step allowing a cascading excitation, such as is
the case for some inorganic, frequency up-conversion mate-
rials. The only reasonable excitation mechanism is a two-
photon mechanism, i.e., simultaneous absorption of two
photons by each molecule.
Figure 4. Stokes shift ∆ν˜ vs. orientation polarizability ∆f of the
solvents. (The scatter dots are the experimental data and the lines
are the linearly fitted results.)
As shown in Figure 6, the TPEF intensity of compound
2 shows a quadratic dependence on the input power when
the input laser power is below 0.13 W, which suggests the
The structure type of 4 is quite different from that of the two-photon excitation mechanism. Above 0.13 W, however,
other three compounds. Because of the shorter conjugated TPEF intensity increases slowly and the quadratic rule is
chain, the absorption and fluorescence peaks in various sol- obviously not in effect, implying some uncertain photo-
vents are blue-shifted relative to the other compounds, physical processes, which cause the fluorescence saturation,
while the solvent-induced red-shift behavior is the same.
are in effect.
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Two-Photon-Excited Fluorescence of Benzothiazole-Based Compounds
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Figure 6. The output fluorescence intensity vs. the input power for
2 in THF with c ϭ 1.0·10Ϫ3
Figure 8. Two-photon-excited fluorescence spectra of the com-
pounds in THF and of Coumarin 307 in MeOH at c ϭ 1.0·10Ϫ4


pounds show much stronger frequency up-converted fluor-
escence than that of Coumarin 307.
The TPEF characteristics of the compounds are also af-
fected by their molecular environments. Just like the effect
of solvent on SPEF, the peak position of TPEF is red-
shifted with increasing the solvent polarity from toluene to
THF. The TPEF peak positions of 3 and 4 in toluene show
a small red-shift relative to their corresponding SPEF
peaks. This red-shift might originate from the re-absorption
of the fluorescence within the concentrated solution. Taking
3 in toluene as an example, the shorter-wavelength side of
the SPEF peak and the longer-wavelength side of the linear
absorption band are overlapped to some extent (see Fig-
ure 9). For all the compounds in THF, and 1 and 2 in tolu-
ene, the Stokes shifts are large enough that the emission
and absorption bands hardly overlap and, thus, their TPEF
peaks have no obvious shift. For the Coumarin 307 solution
in MeOH, the position of the TPEF peak (490 nm) is the
same as that of the SPEF peak.
Figure 7. Two-photon excitation (TPE) spectra of 1 and 2 in THF
with c ϭ 1.0·10Ϫ3

Based on the similarities of the SPEF and TPEF of these
benzothiazole-based compounds, we recognize that the pri-
Detailed experiments reveal that from 710 to 900 nm the
peak positions in the TPEF spectra of these compounds are
independent of the excitation wavelengths, but the emission
intensities of TPEF are dependent over that range. By tun-
ing the pump wavelengths incrementally from 710 to
900 nm while keeping the input power fixed and then re-
cording TPEF intensity, two-photon excitation (TPE) spec-
tra were obtained; the TPE spectra of 1 and 2 are shown in
Figure 7. The spectra display two excitation peaks, and this
feature is quite similar to that observed in the linear absorp-
tion spectra, except that the wavelengths are roughly
doubled. The optimal excitation wavelengths for 1 and 2
are ca. 820 nm. The TPE peak of 2 is essentially twice that
of linear absorption maximum (412 nm), but the TPE peak
for 1 is at an energy slightly higher than twice that of the
corresponding linear absorption maximum (425 nm).
The TPEF spectra of all these compounds, shown in Fig-
ure 8, were taken when they were excited at 800 nm in THF.
The TPEF of Coumarin 307 was measured as a standard
Figure 9. The UV absorption and single-photon-excited fluor-
escence (SPEF) spectra (c ϭ 5.0·10Ϫ6 ⁾ and the two-photon-ex-
under the same experimental conditions. All of our com- cited fluorescence (TPEF) spectrum (c ϭ 1.0·10Ϫ4 ⁾ of 3 in toluene
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D.-X. Cao, Q. Fang, D. Wang, Z.-Q. Liu, G. Xue, G.-B. Xu, W.-T. Yu
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mary excited states obtained by single-photon absorption The photophysical data of these compounds indicate that
and two-photon absorption may be different because of the benzothiazolyl is a true electron acceptor in its excited state.
different parity selection rules, but they are likely to relax All these compounds show very strong two-photon-excited,
quickly to the same fluorescence emission states.
frequency up-converted fluorescence ranging from blue to
orange, with the TPA cross-sections on the order of 102
GM. By fixing the benzothiazolyl acceptor and then testing
a series of different donors, we found that although the di-
ethylamino group provides the strongest donating proper-
ties, the most suitable donor for TPA and TPEF may be the
diphenylamino group.
5. TPA and TPEF Cross-Sections
The TPA and TPEF cross-sections (σ and σe, respec-
tively) are basic parameters to evaluate a material’s TPA
and TPEF properties. From TPEF intensity data, σe and σ
can be evaluated by using Equations (2) and (3),[23,24] where
r stands for the reference compound, n for the refractive
index, and F for the integral fluorescence intensity.
Experimental Section
(2)
(3)
General Remarks: Nuclear magnetic resonance spectra were re-
corded on a Bruker AV 600 spectrometer. Infrared spectra were
recorded on a Nicolet 20SX FT-IR spectrophotometer. Mass spec-
tra were obtained on an Agilent 5973N MSD spectrometer. The
melting points were measured on a MettlerϪToledo DSC822e
Differential Scanning Calorimeter at a heating rate of 20 °C·minϪ1
under a nitrogen atmosphere. Elemental analyses were carried out
on a PE 2400 autoanalyzer. 2-Aminothiophenol, 4-methylcinnamic
acid, and 2-thiophenecarbaldehyde (4Ј) were obtained from Acros
Ltd. 4-(Diethylamino)benzaldehyde (1Ј) and 4-(N-carbazolyl)ben-
zaldehyde (3Ј) were synthesized according to a literature pro-
cedure.[25] 4-(Diphenylamino)benzaldehyde (2Ј) was prepared by a
standard Vilsmeier reaction. All the solvents used for absorption
and fluorescence measurements were HPLC grade. Linear absorp-
tion spectra of dilute solutions (c ϭ 5.0·10Ϫ6 ) were recorded on
a Hitachi U-3500 UV/Vis-IR spectrophotometer using a quartz cu-
vette having 1-cm path length. Steady-state fluorescence spectra
and fluorescence decay curves were measured on 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 corrected. The SPEF quantum yields
Φ were measured by using a standard method with excitation at
400 nm under the same experimental conditions for all com-
pounds.[26] Coumarin 307 dissolved in ethanol, at the same concen-
tration as the other samples, was used as the standard.[19] Lifetime
values were obtained by reconvolution fit analysis of the decay pro-
files with the aid of F900 analysis software. The fitting results were
judged by their values of ‘‘reduced chi-squared’’. Two-photon-exci-
tation (TPE) and TPEF spectra (excited by 800 nm laser) were
measured using a Mira 900-D Ti:sapphire femtosecond laser with
a pulse width of 200 fs and a repetition rate of 76 MHz.
We obtained the relative TPEF cross-sections σe of these
compounds by comparing their TPEF to that of Coumarin
307 under exactly the same experimental conditions, and by
specifying that the value for Coumarin 307 be equal to 1.
As listed in Table 1, compound 2 shows the largest TPEF
cross-section, which is 6.1 times that of Coumarin 307, at
the 800 nm excitation.
The TPEF cross-section σe is supposed to be linearly de-
pendent on the TPA cross-section (σ) with the TPEF quan-
tum yield ΦЈ as the coefficient.[24] In most reports, the
SPEF quantum yield Φ has been adopted instead of ΦЈ,
because ΦЈ is difficult to measure. By referencing the TPEF
cross-section of Coumarin 307 to be 19 GM (1 GM ϭ
10Ϫ50 cm4·s/photon),[23] the TPA cross-sections of the title
compounds were obtained (Table 1); they are a few times
larger than that of the Coumarin 307 reference. Compound
2 in THF shows the largest TPA cross-section (221 GM).
Compound 4 shows the smallest TPA and TPEF cross-sec-
tions, which may be because of its shorter conjugation sys-
tem. Although compound 3 possesses the best SPEF qualit-
ies, compound 2 exhibits the best TPEF properties by hav-
ing the largest TPA and TPEF cross-sections at the two-
photon excitation wavelength (800 nm). By comparison
with compound 1, the much better SPEF and TPEF
properties of 2 might result from its propeller-shaped di-
phenylamino group, which (1) can properly partition the
lone electron pair and stabilize the reduced positive charge
and (2) can help to form and stabilize the fluorescent state
2-[2-(4-Methylphenyl)ethenyl]-1,3-benzothiazole (5Ј): 4-Methylcin-
namic acid (8.1 g, 0.05 mol) and 2-aminothiophenol (5.4 mL, 6.3 g,
having charge-transfer character. By comparison with com- 0.05 mol) were suspended in phosphorus oxychloride (ca. 60 mL)
and then the mixture was heated at 100 °C under nitrogen for 5 h.
After the mixture had cooled to room temperature, it was carefully
and slowly poured into a beaker of stirring distilled water (200 mL).
The pH was adjusted to 7.0 by addition of sodium hydroxide pel-
lets. The crude product was extracted into dichloromethane and
washed twice with distilled water. The organic solvent was removed
using a rotary evaporator. The separated precipitate was repeatedly
recrystallized from ethanol to give colorless platelet crystals (10.0 g,
pound 3, which also possesses an aromatic arylamino-sub-
stituted donor, the superior TPEF properties of 2 might be
attributable to (1) the greater electron donating ability of a
diphenylamino group relative to that of a carbazolyl group
and (2) the TPEF excitation wavelength (800 nm) for 3 not
being short enough, considering the λmax (384 nm in THF)
of the absorption spectra of 3.
1
80%). H NMR (CDCl3): δ ϭ 2.38 (s, 3 H), 7.10Ϫ7.61 (m, 8 H),
7.75Ϫ8.06 (m, 2 H) ppm.
Conclusion
2-{2-[4-(Bromomethyl)phenyl]ethenyl}-1,3-benzothiazole (6Ј): Com-
pound 5Ј (6.3 g, 0.025 mol) was dissolved in freshly distilled tetra-
We have synthesized a series of new benzothiazole-based
compounds incorporating various electron donor groups. chloromethane (200 mL) and then N-bromosuccinimide (4.5 g,
3634  2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim www.eurjoc.org
Eur. J. Org. Chem. 2003, 3628Ϫ3636

Two-Photon-Excited Fluorescence of Benzothiazole-Based Compounds
0.025 mol) and a small amount of benzoyl peroxide were added crystallography, were obtained by recrystallization from chloro-
FULL PAPER
together. The mixture was heated under reflux for 8 h. After cool-
ing to room temperature, the precipitate was filtered off and the
filtrate was purified by column chromatography on silica gel using
chloroform/petroleum ether (1:9) as eluent. A colorless powder was
obtained (2.9 g, 35%). 1H NMR (CDCl3): δ ϭ 4.50 (s, 2 H),
7.21Ϫ7.62 (m, 8 H), 7.76Ϫ8.06 (m, 2 H) ppm.
form. M.p. 296Ϫ297 °C. 1H NMR (600 MHz, CDCl3): δ ϭ 7.26
(d, J(E) ϭ 16.6 Hz, 1 H), 7.31Ϫ7.33 (m, 3 H), 7.41Ϫ7.51 (m, 7 H),
7.59 (d, J(E) ϭ 16.7 Hz, 1 H), 7.61Ϫ7.64 (m, 6 H), 7.78 (d, J ϭ
8.3 Hz, 2 H), 7.90 (d, J ϭ 7.8 Hz, 1 H), 8.04 (d, J ϭ 8.0 Hz, 1 H),
8.17 (d, J ϭ 7.8 Hz, 2 H) ppm. MS: m/z (%) ϭ 504 (100) [Mϩ],
252 (51) [M]2ϩ. UV/Vis (THF): λmax (ε) ϭ 384 nm (7.4·104). IR
(KBr): ν˜ ϭ 1593 (m), 1513 (s), 1477 (m), 1450 (s), 1334 (m), 1312
(m), 1231 (s), 1169 (w), 1103 (m), 955 (s), 824 (m), 750 (s), 724 (s)
cmϪ1. C35H24N2S (504.7): calcd. C 83.30, H 4.79, N 5.55, S 6.35;
found C 83.04, H 4.81, N 5.50, S 6.31.
{4-[2-(1,3-Benzothiazole-2-yl)ethenyl]benzyl}triphenylphosphonium
Bromide (7Ј): A mixture of 6Ј (4.0 g, 0.012 mol) and triphenylphos-
phane (3.2 g, 0.012 mol) in freshly distilled toluene (200 mL) was
heated under reflux for 5 h. The mixture was then placed in a re-
frigerator overnight and then the crude precipitate was filtered off
and washed with ethanol. A yellow powder (5.7 g, 80%) was ob-
tained and used directly in the next step without further purifi-
cation.
trans,trans-2-{4-[2-(Thiophen-2-yl)ethenyl]styryl}-1,3-benzothiazole
(4, TESB): By reaction of 4Ј and 7Ј, a yellow-green powder (0.53 g,
38%) was obtained. M.p. 240Ϫ241 °C, 1H NMR (600 MHz,
CDCl3): δ ϭ 6.95 (d, J(E) ϭ 16.1 Hz, 1 H), 7.04 (m, 1 H), 7.13 (d,
J ϭ 3.3 Hz, 1 H), 7.25 (d, J ϭ 5.0 Hz, 1 H), 7.32 (d, J(E) ϭ 16.0 Hz,
1 H), 7.40 (t, J ϭ 7.3 Hz, 1 H), 7.44 (d, J(E) ϭ 16.1 Hz, 1 H),
7.49Ϫ7.60 (m, 6 H), 7.89 (d, J ϭ 7.9 Hz, 1 H), 8.03 (d, J ϭ 8.1 Hz,
1 H) ppm. MS: m/z (%) ϭ 344 (100) [Mϩ]. UV/Vis (THF): λmax
(ε) ϭ 380 nm (4.4·104). IR (KBr): ν˜ ϭ 1619 (m), 1591 (m), 1477
(m), 1431 (m), 1412 (m), 1312 (m), 1231 (m), 1192 (m), 956 (s), 864
(m), 821 (s), 754 (s), 728 (s), 697 (s), 666 (m) cmϪ1. C21H15NS2
(345.5): calcd. C 73.01, H 4.38, N 4.05, S 18.56; found C 72.69, H
4.29, N 4.13, S 18.44.
trans,trans-2-{4-[4-(N,N-Diethylamino)styryl]styryl}-1,3-
benzothiazole (1, DESSB): A mixture of 7Ј (2.5 g, 0.004 mol) and
1Ј (0.71 g, 0.004 mol) was suspended in freshly distilled THF
(40 mL) and then potassium tert-butoxide (0.7 g, 0.006 mol) in tert-
butyl alcohol (60 mL) was added dropwise under nitrogen with stir-
ring in an ice-bath. The mixture was continuously stirred at room
temperature for further 20 h. The mixture was poured into a beaker
containing distilled water (200 mL) and the pH value of the mix-
ture was adjusted to 7.0 by addition of dilute hydrochloric acid.
This mixture was extracted with dichloromethane, the organic
phases were washed twice with distilled water, and then the organic
solvent was removed using a rotary evaporator. To isomerize the
crude material to the trans-configured object compound, the resi-
due was dissolved in toluene (ca. 50 mL) containing a trace amount
of iodine and then the mixture was heated under reflux for further
4 h. The solvent was evaporated, the residue was purified by col-
umn chromatography on silica gel using chloroform/petroleum
ether (1:1) as eluent, and the product was then recrystallized from
chloroform to yield a red powder (0.66 g, 40%). M.p. 227Ϫ230 °C,
1H NMR (600 MHz, CDCl3): δ ϭ 1.21 (t, J ϭ 7.0 Hz, 6 H), 3.41
X-ray Crystallographic Study of 3 (CSSB): X-Ray diffraction data
of a yellow-green single crystal of 3 (0.45 ϫ 0.42 ϫ 0.08 mm) were
collected on a Bruker P4 four-circle diffractometer using Mo-Kα
radiation. Of 5781 collected reflections (4.04 Յ 2θ Յ 49.98), 4569
reflections were independent (Rint ϭ 0.0340). The structure was
resolved using the SHELXTL-97 program by a direct method and
refined by a full-matrix least-squares method on F2. The crystal
belongs to the monoclinic system, P21/c space group, with formula
C35H24N2S and molecular weight 504.62; T ϭ 293(2) K, a ϭ
˚
8.0149(9), b ϭ 40.378(4), c ϭ 8.0607(12) A, β ϭ 97.480(10)°, V ϭ
3
2586.4(5) A , Z ϭ 4, dcalcd. ϭ 1.306 Mg/m3, µ ϭ 0.153 mmϪ1
,
˚
F(000) ϭ 1072, R1 ϭ 0.1005, wR2 ϭ 0.2586 [I Ͼ 2σ(I)].
(q, J ϭ 7.0 Hz, 4 H), 6.69 (d, J ϭ 8.6 Hz, 2 H), 6.91 (d, J(E)
ϭ
16.2 Hz, 1 H), 7.13 (d, J(E) ϭ 16.2 Hz, 1 H), 7.37Ϫ7.43 (m, 4 H),
7.47Ϫ7.57 (m, 6 H), 7.88 (d, J ϭ 8.0 Hz, 1 H), 8.01 (d, J ϭ 8.1 Hz,
1 H) ppm. MS: m/z (%) ϭ 410 (100) [Mϩ], 395 (89) [M Ϫ CH3]ϩ.
UV/Vis (THF): λmax (ε) ϭ 425 nm (5.8·104). IR (KBr): ν˜ ϭ 2969
(w), 1585 (s), 1519 (s), 1478 (m), 1402(m), 1357 (m), 1266 (s), 1186
(s), 1152 (s), 961 (s), 818 (s), 757 (s), 731 (m) cmϪ1. C27H26N2S
(410.6): calcd. C 78.99, H 6.38, N 6.82, S 7.81; found C 78.68, H
6.31, N 6.65, S 7.69.
CCDC-191317 contains the supplementary crystallographic data
for this paper. These data can be obtained free of charge at
www.ccdc.cam.ac.uk/conts/retrieving.html [or from the Cambridge
Crystallographic Data Centre, 12, Union Road, Cambridge
CB2 1EZ, UK; fax: (internat.) ϩ44Ϫ1223/336-033; E-mail:
deposit@ccdc.cam.ac.uk].
Acknowledgments
trans,trans-2-{4-[4-(N,N-Diphenylamino)styryl]styryl}-1,3-benzo-
thiazole (2, DPSSB): By reaction of 2Ј and 7Ј, DPSSB was pre-
pared as a yellow powder (0.85 g, 42%) following a method similar
to that described above. M.p. 277Ϫ278 °C. 1H NMR (600 MHz,
CDCl3): δ ϭ 7.02 (d, J(E) ϭ 16.3 Hz, 1 H), 7.05Ϫ7.08 (m, 4 H),
7.13Ϫ7.16 (m, 5 H), 7.28Ϫ7.30 (m, 4 H), 7.41Ϫ7.43 (m, 3 H), 7.45
(d, J(E) ϭ 16.1 Hz, 1 H), 7.50Ϫ7.60 (m, 6 H), 7.89 (d, J ϭ 7.9 Hz,
1 H), 8.03 (d, J ϭ 8.1 Hz, 1 H) ppm. MS: m/z (%) ϭ 506 (100)
[Mϩ], 253 (24) [M]2ϩ. UV/Vis (THF): λmax (ε) ϭ 412 nm (6.9·104).
IR (KBr): ν˜ ϭ 3026 (w), 1588 (s), 1512 (m), 1489 (s), 1331 (m),
1280 (s), 1176 (m), 1107 (w), 961 (s), 826 (m), 755 (s), 729 (m), 695
(s) cmϪ1. C35H26N2S (506.7): calcd. C 82.97, H 5.17, N 5.53, S
6.33; found C 82.39, H 5.07, N 5.31, S 6.49.
This work was supported by the National Nature Science Foun-
dation of China (No. 20172034), the Foundation for University
Key Teachers by the Ministry of Education, and by a grant from
the State Key Program of China.
[1]
J. D. Bhawalkar, G. S. He, P. N. Prasad, Rep. Prog. Phys. 1996,
59, 1041Ϫ1070.
[2]
´
M. Albota, D. Beljonne, J.-L. Bredas, J. E. Ehrlich, J.-Y. Fu,
A. A. Heikal, S. E. Hess, T. Kogej, M. D. Levin, S. R. Marder,
D. McCord-Maughon, J. W. Perry, H. Röckel, M. Rumi, G.
Subramaniam, W. W. Webb, X.-L. Wu, C. Xu, Science 1998,
281, 1653Ϫ1656.
D. A. Parthenopoulos, P. M. Rentzepis, Science 1989, 245,
843Ϫ845.
G. S. He, J. D. Bhawalkar, C. F. Zhao, P. N. Prasad, Appl. Phys.
Lett. 1995, 67, 2433Ϫ2435.
J. E. Ehrlich, X. L. Wu, I.-Y. S. Lee, Z.-Y. Hu, H. Röckel, S.
R. Marder, J. W. Perry, Opt. Lett. 1997, 22, 1843Ϫ1845.
[3]
[4]
[5]
trans,trans-2-{4-[4-(N-Carbazolyl)styryl]styryl}-1,3-benzothiazole
(3, CSSB): By reaction of 3Ј and 7Ј, yellow-green microcrystals of
3 (0.91 g, 45%) were obtained following a procedure similar to that
described above. Platelet crystals, which were suitable for X-ray
Eur. J. Org. Chem. 2003, 3628Ϫ3636
www.eurjoc.org
 2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
3635

D.-X. Cao, Q. Fang, D. Wang, Z.-Q. Liu, G. Xue, G.-B. Xu, W.-T. Yu
FULL PAPER
[6]
[15]
G. S. He, C. F. Zhao, J. D. Bhawalkar, P. N. Prasad, Appl. Phys.
L. Ventelon, S. Charier, L. Moreaux, J. Mertz, M. Blanchard-
Lett. 1995, 67, 3703Ϫ3705.
Y. Ren, Q. Fang, W. T. Yu, H. Lei, Y. P. Tian, M. H. Jiang, Q.
C. Yang, T. C. W. Mak, J. Mater. Chem. 2000, 10, 2025Ϫ2030.
W. Denk, J. H. Strickler, W. W. Webb, Science 1990, 248,
73Ϫ76.
H. Stiel, K. Teuchner, A. Paul, W. Freyer, D. Leupold, J. Photo-
chem. Photobiol. A: Chem. 1994, 80, 289Ϫ298.
R. Kannan, G. S. He, L. X. Yuan, F. Xu, P. N. Prasad, A. G.
Dombroskie, B. A. Reinhardt, J. W. Baur, R. A. Vaia, L.-S.
Tan, Chem. Mater. 2001, 13, 1896Ϫ1904.
M. Rumi, J. E. Ehrlich, A. A. Heikal, J. W. Perry, S. Barlow,
Z. Hu, D. McCord-Maughon, T. C. Parker, H. Röckel, S.
Thayumanavan, S. R. Marder, D. Beljonne, J.-L. Bredas, J. Am.
Chem. Soc. 2000, 122, 9500Ϫ9510.
O.-K. Kim, K.-S. Lee, H. Y. Woo, K.-S. Kim, G. S. He, J.
Swiatkiewicz, P. N. Prasad, Chem. Mater. 2000, 12, 284Ϫ286.
Desce, Angew. Chem. Int. Ed. 2001, 40, 2098Ϫ2101.
[16]
[7]
B. R. Cho, K. H. Son, S. H. Lee, Y.-S. Song, Y.-K. Lee, S.-J.
Jeon, J. H. Choi, H. Lee, M. Cho, J. Am. Chem. Soc. 2001,
123, 10039Ϫ10045.
Z. Q. Liu, Q. Fang, D. Wang, G. Xue, W. T. Yu, Z. S. Shao,
M. H. Jiang, Chem. Commun. 2002, 2900Ϫ2901.
U. Rose, J. Heterocyclic Chem. 1992, 29, 551Ϫ557.
G. A. Reynolds, K. H. Drexhage, Opt. Commun. 1975, 13,
222Ϫ225.
X. M. Wang, D. Wang, G. Y. Zhou, W. T. Yu, Y. F. Zhou, Q.
Fang, M. H. Jiang, J. Mater. Chem. 2001, 11, 1600Ϫ1605.
Z. Q. Liu, Q. Fang, W. T. Yu, G. Xue, D. X. Cao, D. Wang,
G. M. Xia, M. H. Jiang, Chin. Chem. Lett. 2002, 13, 997Ϫ1000.
J. R. Lakowicz, Principles of Fluorescence Spectroscopy, Plenum
[8]
[17]
[9]
[18]
[19]
[10]
[20]
[11]
[21]
´
[22]
Press, New York, 1983, p. 190.
[12]
[13]
[14]
[23]
C. X u , W. W. We bb, J. Opt. Soc. Am. B 1996, 13, 481Ϫ491.
[24]
M. A. Albota, C. Xu, W. W. Webb, Appl. Opt. 1998, 37,
L. Ventelon, L. Moreaux, J. Mertz, M. Blanchard-Desce,
Chem. Commun. 1999, 2055Ϫ2056.
B. A. Reinhardt, L. L. Brott, S. J. Clarson, A. G. Dillard, J. C.
Bhatt, R. Kannan, L. X. Yuan, G. S. He, P. N. Prasad, Chem.
Mater. 1998, 10, 1863Ϫ1874.
7352Ϫ7356.
[25]
C. F. Zhao, C.-K. Park, P. N. Prasad, Y. Zhang, S. Ghosal, R.
Burzynski, Chem. Mater. 1995, 7, 1237Ϫ1242.
[26]
J. N. Demas, G. A. Crosby, J. Phys. Chem. 1971, 75, 991Ϫ1024.
Received May 7, 2003
3636
 2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
Eur. J. Org. Chem. 2003, 3628Ϫ3636