Novel heterocycle-based organic molecules with two-photon induced blue fluorescent emission

Article abstract of DOI:10.1039/b300924f

Two-photon absorption and two-photon induced blue emission characteristics of a series of heterocycle-based organic molecules are investigated experimentally and by quantum-chemical computations. The molecules consist of a typical A-π-A′ structure, where heterocycle, styryl and formyl groups are employed as A, π-conjugated and A′ moieties, respectively. Experimental results indicate that significant enhancements in the blue emission efficiency and two-photon absorption cross-sections can be achieved by replacing S and O atoms with an N atom in the heterocycle acceptor moiety, which is also supported by the quantum-chemical computations. Additionally, larger two-photon absorption cross-sections can be obtained by choosing appropriate solvents, as indicated by the computations.

Full text of DOI:10.1039/b300924f

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Novel heterocycle-based organic molecules with two-photon induced
blue fluorescent emission
Zhen-Li Huang,a Hong Lei,a,d Na Li,a Zhi-Ren Qiu,a He-Zhou Wang,*a
Jing-Dong Guo,b Yi Luo,b Zeng-Pei Zhong,c Xu-Feng Liuc and Zhi-Hong Zhouc
aState Key Laboratory of Optoelectronic Materials and Technologies, Zhongshan University,
Guangzhou 510275, P. R. China. E-mail: stswhz@zsu.edu.cn; Fax: 186-20-84037423
bTheoretical Chemistry, Royal Institute of Technology, SCFAB, S-106 91 Stockholm, Sweden
cSchool of Chemistry and Chemical Engineering, Zhongshan University, Guangzhou 510275,
P. R. China
dSchool of Information Science and Engineering, Shandong University, Jinan 250100,
P. R. China
Received 19th September 2002, Accepted 23rd January 2003
First published as an Advance Article on the web 20th February 2003
Two-photon absorption and two-photon induced blue emission characteristics of a series of heterocycle-based
organic molecules are investigated experimentally and by quantum-chemical computations. The molecules
consist of a typical A–p–A’ structure, where heterocycle, styryl and formyl groups are employed as A,
p-conjugated and A’ moieties, respectively. Experimental results indicate that significant enhancements in the
blue emission efficiency and two-photon absorption cross-sections can be achieved by replacing S and O atoms
with an N atom in the heterocycle acceptor moiety, which is also supported by the quantum-chemical
computations. Additionally, larger two-photon absorption cross-sections can be obtained by choosing
appropriate solvents, as indicated by the computations.
Among the design strategies for TPIF materials, it is pointed
1. Introduction
out that introducing a heteroatom into a molecular structure is
an efficient way to obtain excellent TPIF molecules because
p-deficient and p-excessive heterocycles may act as efficient
acceptor and donor moieties, respectively.7 Thus the inves-
tigation of the influence of different heteroatoms on the
photophysical properties of molecules is beneficial to the
development of TPIF molecules, especially to the synthesis of
organic TPIF molecules with efficient blue emission.
In this paper, an investigation of the TPIF and TPA
characteristics of three heterocycle-based organic molecules
using experimental measurements and quantum-chemical
computations is reported. By means of a special structural
modification of the acceptor moiety, the influence of hetero-
atoms on the photophysical properties of these kinds of
molecules are discussed. The solvent effects have also been
studied by means of quantum-chemical calculations.
Organic two-photon induced fluorescent (TPIF) materials have
been widely studied recently due to their various applications,
especially in TPIF microscopy.1–3 It has become clear that the
development of TPIF microscopy would greatly benefit from
the design and synthesis of efficient organic TPIF materials
with large two-photon absorption (TPA) cross-sections and
desired emitting characteristics. A large number of papers
have been devoted to these kinds of materials4–7 and report
a wide variety of applications of TPIF microscopy in biolo-
gical systems including single molecule studies,8–10 immunoas-
say development,11,12 DNA fragment sizing,13 chemical and
biosensing,14–16 microchip17 and capillary electrophoresis sepa-
ration detection.18 However, the emitting wavelengths of the
reported molecules are mostly in the 500–700 nm region and
the study of TPIF molecules with efficient two-photon induced
blue emission is still lacking, which is seriously restricting the
development of multi-channel TPIF microscopy. As we know,
a diverse range of autofluorescent proteins with different colors
and intrinsic chromophores, which exist in biological cells and
organisms, possess TPIF characteristics and thus produce an
autofluorescence background that encompasses most of the
visible spectral range, which is undesirable in biological
imaging as it limits the detection sensitivity of fluorescent
probes.19–21 Therefore, probe natural molecules, efficient TPIF
molecules with different emission wavelengths are helpful to
reduce background light scattering and autofluorescence. For
example, efficient two-photon induced blue emitting molecules
could be used to reduce the autofluorescence background from
green and red proteins. Unfortunately, little work has been
focused on the study of efficient TPIF molecules with blue
emission. Furthermore, efficient two-photon induced blue
emission molecules are absolutely necessary for tagging some
specific cellular components.22 In a word, the study of organic
molecules with efficient two-photon induced blue emission
appears to be an urgent and attractive area of research.
2. Experimental
2.1 Preparation
The organic molecules discussed in this paper are trans-
2-[p-formylstyryl]benzothiazole (abbreviated as ASBT),
trans-2-[p-formylstyryl] benzoxazole (ASBO) and trans-2-[p-
formylstyryl]benzimidazole (ASBM), respectively. These three
organic molecules were obtained by similar synthetic proce-
dures, and the procedure for the preparation of ASBM is
shown below as an example. All chemicals are reagent grade
and were used without further purification.
trans-2-[p-Formylstyryl]benzimidazole (ASBM). A mixture
of 2-methylbenzimidazole (1.33 g, 0.01 mol), p-phthaldialde-
hyde (1.34 g, 0.01 mol), acetic anhydride (3 ml) and acetic acid
(1.5 ml) was stirred at 120 uC for 6 h, then cooled to room
temperature. After adding concentrated hydrochloric acid
(15 ml), the mixture was left for an hour, filtered and washed
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This journal is # The Royal Society of Chemistry 2003
DOI: 10.1039/b300924f

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with water. The filtrate was treated with 30% aqueous sodium
hydroxide solution (30 ml) and gave yellowish precipitate that
was filtered and dried. The crude product was purified by
column chromatography on silica gel using petroleum ether–
ethyl acetate (10 : 1) as eluent. Bright yellowish green crystals
were thus obtained in 76% yield and mp 178–180 uC. 1H NMR
(300 MHz, CDCl3, TMS, ppm): 7.293 (d, J ~ 6.99 Hz, 2H,
CHLCH), 7.312 (d, J ~ 3.28 Hz, 1H), 7.333 (d, J ~ 3.20 Hz,
1H), 7.655 (d, J ~ 5.9 Hz, 1H), 7.694 (d, J ~ 10.94 Hz, 1H),
7.619 (d, J ~ 8.21 Hz, 2H), 7.869 (d, J ~ 8.27 Hz, 2H), 7.657
(s, NH), 10.015 (s, CHO). Elemental analysis (%): calcd.: C
77.40, H 4.87, N 11.28; found: C 77.20, H 4.74, N 10.97.
yields were measured on a Hitachi model F-4500 fluorescence
spectrophotometer. The quantum yields, Wf, were measured by
the optically dilute measurement method29 using Coumarin120
(whose Wf is assumed to be 77% in methanol30) as the reference.
The excitation source for the TPIF experiments was a mode-
locked femtosecond Ti : Sapphire laser (Spectra-physics, 100 fs,
82 MHz) tuned to 750 nm. The maximum average laser power
available for these experiments was about 300 mW. An optical
multi-channel analyzer was used as a recorder.
The experimental set-up for the TPIF excitation cross-
section measurements was similar to that used by Xu et al.23,24
and has been described in detail in our previous work.25 In
brief, the femtosecond laser was split into two beams. The
weaker one was used as a reference beam and monitored by the
detector of a power meter (Coherent, FieldMaster NO.33-
0506). The intense beam was focused on the sample by a lens.
Using a telescopic system the up-converted fluorescence was
firstly collected at a direction perpendicular to the pump beam,
and then passed through a liquid barrier filter (1 cm pathlength,
1 mol l21 CuSO4 solution) to exclude excitation illumination.25
To minimize the re-absorption effect, the excitation beam was
focused as closely as possible on the front wall of the quartz
cell. A photomultiplier tube (Hamamatsu R928) was used for
the TPIF detector and the signal was read using a Lock-in
Amplifier (EG&G 5210).
trans-2-[p-Formylstyryl]benzothiazole (ASBT). ASBT is
obtained with a yield of 84% and mp 159.2–159.7 uC. 1H
NMR (300 MHz, CDCl3, TMS, ppm): 7.431 (t, J ~ 7.55 Hz,
1H), 7.512 (t, J ~ 7.63 Hz, 1H), 7.576 (d, J ~ 3.38 Hz, 2H,
CHLCH), 7.755 (d, J ~ 8.19 Hz, 2H), 7.944 (d, J ~ 8.22 Hz,
2H), 7.910 (d, J ~ 8.41 Hz, 1H), 8.054 (d, J ~ 8.11 Hz, 1H),
10.051 (s, CHO). 13C NMR (75 MHz, CDCl3, ppm): 121.628,
123.278, 125.114, 125.808, 126.582, 127.802, 130.30, 135.754,
136.488, 141.193, 191.464. Elemental analysis (%): calcd.: C
72.43, H 4.18, N 5.28, S 12.08; found: C 72.55, H 4.51, N 4.95, S
11.92.
trans-2-[p-Formylstyryl]benzoxazole (ASBO). ASBO is
obtained with a yield of 80% and mp 178.4–179.6 uC. 1H
NMR (300 MHz, CDCl3, TMS, ppm): 7.264 (d, J ~ 16.5 Hz,
1H), 7.908 (d, J ~ 16.0 Hz, 1H), 7.334 (d, J ~ 7.05 Hz, 2H,
CHLCH), 7.580 (t, J ~ 4.5 Hz, 1H), 7.783 (t, J ~ 4.5 Hz, 1H),
7.775 (d, J ~ 8.5 Hz, 2H), 7.947 (d, J ~ 8.0 Hz, 2H), 10.141 (s,
CHO). Elemental analysis (%): calcd.: C 77.40, H 4.75, N 6.00;
found: C 77.10, H 4.81, N 5.61.
2.3 Computation
The TPA cross-section s2 is defined as26
Cf2
s2~s2, max
Dv2zC2f
(1)
4p2a50a v2
s2, max
~
dtp
The molecular structures of the heterocycle-based organic
molecules are shown in Fig. 1. These molecules consist of a
typical A–p–A’ structure, where the heterocycle, styryl and
formyl groups are employed as A, p-conjugated and A’
moieties, respectively. To our knowledge, this kind of
heterocycle-based molecule has never been studied as a TPA
material by other researchers. Moreover, such molecules with
short conjugation lengths are expected to possess blue emis-
sions. In the heterocyclic moieties, the benzothiazole group is a
frequently used acceptor,7 while benzoxazole and benzimida-
zole acceptors have been seldom studied. Thus the three
heterocycle-based molecules have been used in this work to
investigate the influences of heteroatoms (S, O and N) on the
TPIF and TPA properties of molecules. Additionally, such a
structural modification leads to negligible changes in the
p-conjugated length. As we know, lengthening a p-conjugated
system can lead to a red-shift in the emission wavelength of a
molecule, so that this kind of structural modification can be
expected to give organic molecules with different TPIF per-
formances, but with similar emission wavelengths. Chloroform
is used as the solvent for these molecules in our experiments.
15c0 Cf
where a0 is the Bohr radius, c0 is the speed of light, a is the fine
structure constant and hv is the photon energy of fundamental
light. The level broadening Cf of the final state is set to the
commonly used value Cf ~ 0.1 eV, which corresponds to a
lifetime of a few fs. Dv ~ v 2 vof, where vof is the excitation
energy of the final state f.
It is known that the two-photon absorption process of a
donor–acceptor-conjugated system is dominated by the con-
tributions from two states, namely the ground and the charge-
transfer (CT) states.26 We have adopted here the generalized
two-states model27 for computing the two-photon cross-
sections, in which the TPA probability dtp can be written as
–
(Dꢀ)2(ꢀ0f )2
dtp~8
(2 cos2 (hD0fꢀ)z1):
(2)
(DE)2
Here, Dm is the module of vector D~ꢀ~~ꢀff{~ꢀ00, m0f is the module
0f
Dm
of vector ~ꢀ0f , h is the angle between D~ꢀ and ~ꢀ0f , and DE ~ vf/2.
From eqn. (2), it is easily seen that in order to maximize the TPA
cross-section, one needs not only to increase the magnitude of the
ꢀ
ꢀ
ꢀ
ꢀ
0f
two vectors, jD~ꢀj and ~ꢀ , but also to line them up.
2.2 Apparatus
The excitation energy and transition dipole moment were
calculated by using the time-dependent DFT/B3LYP method
implemented in the GAUSSIAN package.28 A finite-field
approach was used to obtain the dipole moment change between
the ground and the CT states. The solvent effects were also
studied using the polarizable continuum model (PCM) of
GAUSSIAN. The geometries of all the molecules under
investigation were optimized at the B3LYP level. A 6-3111G*
basis set was used to investigate the different properties.
Thelinearabsorptionandtransmittancespectrawererecordedona
Shimadzu UV-3101PC UV-Vis-NIR scanning spectrophotometer.
The single-photon fluorescent (SPF) spectra and quantum
3. Results and discussion
The linear absorption spectra of these heterocycle-based
organic molecules at a concentration of 1 6 1025 mol l21
Fig. 1 Molecular structures of the heterocycle-based molecules.
J. Mater. Chem., 2003, 13, 708–711
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Fig. 3 The dependence of the up-converted fluorescence intensity on
the incident intensity of ASBT (5 6 1024 mol l21).
Fig. 2 Linear absorption and normalized fluorescent spectra of the
heterocycle-based molecules at a concentration of 1 6 1025 mol l21
.
The TPA cross-sections s2 of the organic molecules were
determined by comparing their TPIF with the two-photon
fluorescence excitation cross-sections sTPE of Rhodamine 6G
(at a concentration of 561024 mol l21 for all of the solutions)
according to23,24
are presented in Fig. 2. The linear absorption peaks of the
molecules are located around 350nm and 270 nm. As shown in
Fig. 1, structural modification occurs only in the heterocycle-
based acceptor moiety, where the S atom was replaced by an O
or N atom. Such a modification leads to negligible changes in
the p-conjugated length and so results in linear absorption
spectra with similar profiles.
The linear transmittance spectra of these three organic
molecules in thick chloroform solution (5 6 1024 mol l21) were
also measured. No linear absorption in the wavelength range
from 500 nm to 1100 nm was observed, which implies that the
emission induced by 750 nm excitation can be attributed to the
nonlinear two-photon process.
The SPF spectra of the three heterocycle-based molecules at
a concentration of 1 6 1025 mol l21 are also shown in Fig. 2. It
can be observed from Fig. 1 and 2 that, although structural
modification of the acceptor moiety leads to similar linear
absorption spectra, the emission profile of the ASBT solution is
very different from those of ASBO and ASBM, which may
result from the presence of empty d orbitals that can contribute
to the molecular p orbitals. Additionally, the quantum yields,
Wf, of the organic molecules vary considerably (see Table 1).
Of the three heterocycle-based organic molecules, ASBM
possesses the highest quantum yield value.
When excited by a mode-locked femtosecond laser tuned to
750 nm, all three organic molecules at a concentration of 5 6
1024 mol l21 emit blue up-converted fluorescence. The profiles
of the fluorescent spectra of the organic molecules are similar to
their corresponding SPF spectra, and their peak values are
shown in Table 1. Additionally, the dependence of the up-
converted fluorescence on the incident intensity for all the
organic molecules at a concentration of 5 6 1024 mol l21 were
measured, and a representative dependence curve for the ASBT
solution is shown in Fig. 3. The nearly perfect quadratic
dependence indicates that the up-converted fluorescence is
induced by TPA.
TPE Ccal ncal
sTPE~scal
F
C nFcal
(3)
sTPE
WF
s2~
where C is the concentration, n is the refractive index and F is
the time-averaged fluorescence. The subscript cal refers to the
standard reference solution. In the experiment, methanol was
used as the solvent for Rhodamine 6G and the sTPE of
Rhodamine 6G was assumed to be 25 GM, which was taken
from the literature.24 The time-averaged laser power at 750 nm
was kept constant for the unknown and the standard.
The TPA cross-sections of ASBT, ASBO and ASBM were
measured and calculated and the values are 24, 52 and 180 GM
(1 GM ~ 10250 cm4 s photon21), respectively. The TPA cross-
sections at a given wavelength (750 nm) are enhanced
remarkably by replacing the frequently used benzothiazole
acceptor with the benzoxazole and benzimidazole acceptors.
From Table 1, one notices that ASBM possesses not only the
highest quantum yield (Wf ~ 38%) but also the largest TPA
cross-section (s2 ~ 180 GM), which can probably be attributed
to the structure of the benzimidazole acceptor. Work is
currently underway to reveal the origins of such enhancements.
However, it should be pointed out that the measurement
wavelength of the TPA cross-sections is far away from the
wavelength of the TPA peaks, because as shown in the
literature,19,31 twice the frequency of the TPA peak should be
on the blue side of the corresponding peak of the linear
absorption spectrum. A significant enhancement of the s2 value
could be expected at the TPA peak.
The quantum-chemical computation results are summarized
in Table 2. The computational linear absorption energies of the
three organic molecules in CHCl3 solution exhibit 0.2–0.4 eV
red-shifts relative to those of the experimental results. As the
calculated energy is the vertical excitation energy, an error of
0.5 eV is considered to be good for theoretical results for
molecules of such size. The maximum TPA cross-sections of
ASBT, ASBO and ASBM have been computed to be 506, 647
and 779 GM, respectively, in chloroform solution, and 598, 803
and 974 GM, respectively, in methanol solution. The funda-
mental wavelength for the TPA excitation is half the frequency
of the computed linear absorption peak. To evaluate the
discrepancy between the calculated and experimental results,
we calculated the off-resonant TPA cross-section s2,maxoff using
eqn. (1). The energy detuning value was determined from the
Table 1 Experimental photophysical characteristics of the three
heterocycle-based molecules in chloroform solutiona
Molecule labsmax/nm lspfmax/nm ltpfmax/nm Wf (%)b s2c/GM
ASBT
ASBO
ASBM
357
343
359
431
428
454
436
450
470
6.8
2.4
38
24
52
180
alabsmax, lspfmax, ltpfmax: peak wavelengths in the linear absorption,
SPF and TPIF spectra. bCoumarin120 was used as the reference
c
(Wf ~ 77% in methanol30). TPA cross-section values given in GM at
750 nm, Rhodamine 6G was used as the reference (sTPE is assumed
to be 25 GM in methanol at 750 nm24); 1 GM (Go¨ppert-Mayer) ~
10250 cm4 s photon21
.
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Table 2 Quantum-chemical computational results of three heterocycle-based molecules in the gas phase, chloroform and methanol solutionsa
Molecule
E/eVb
l
max/nmb
f
s2,max/GM
s2,maxoff/GMc
In the gas phase
In CHCl3
ASBT
ASBO
ASBM
ASBT
3.229
3.233
3.124
3.207
(3.47)
3.223
(3.62)
3.102
(3.45)
3.201
3.212
3.113
384
383
397
387
(357)
385
(343)
400
1.146
1.098
0.999
1.139
306
422
509
506
131d, 47e
(0.17 eV)d
62d
ASBO
ASBM
1.099
0.995
647
779
(0.31 eV)d
240d
(359)
387
386
(0.15 eV)d
In CH3OH
ASBT
ASBO
ASBM
1.149
1.095
1.012
598
803
974
398
aExcitation energy (E), one-photon absorption wavelength (lmax), one-photon absorption oscillator strength (f) and maximum TPA cross-sec-
tion (s2,max). bExperimental results are given in parentheses. cThe off-resonant TPA cross-section s2,max has been estimated using eqn. (1).
off
e
dThe energy detuning value is estimated from the experimental spectra. The energy detuning value is determined relative to the calculated exci-
tation energy of ASBO.
6
7
8
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experimental results: (one-photon absorption maximum) 2
750/2. Due to the vibronic contributions, the maximum of the
absorption peak does not necessarily correspond to the vertical
excitation energy. Such an uncertainty can introduce error into
the energy detuning value, therefore, resulting in the dis-
crepancy between the calculated and experimental results,
especially for the ASBT compound. It is noted that the
calculated vertical transition energies of ASBO and ASBT are
almost identical. It would thus be reasonable to believe that
both compounds could have the same energy detuning value,
i.e. 0.31 eV. With this value, the off-resonant TPA cross-section
for ASBT is about 47 GM, in much better agreement with the
experimental result. Furthermore, from Table 2, the TPA
cross-section is seen to be sensitive to the polarity of the
solution, thus by choosing the appropriate solvents, the TPA
cross-section can be enhanced.
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Through experimental measurements and quantum-chemical
computations, the TPIF and TPA characteristics of a series of
heterocycle-based organic blue emission molecules have been
investigated. Experimental results indicate that remarkable
enhancements of the fluorescence quantum yields and TPA
cross-sections can be obtained by varying the heteroatoms in the
acceptor moiety. Quantum-chemical computations show that the
maximumtwo-photonabsorptioncross-sectionscanbeenhanced
by varying both the heteroatoms in the acceptor moiety and the
solvents used. Additionally, such structural modifications results
intheir emissionwavelengths staying in the same regionas there is
no significant change in the p-conjugated length.
23 C. Xu and W. W. Webb, J. Opt. Soc. Am. B, 1996, 13, 481.
24 M. A. Albota, C. Xu and W. W. Webb, Appl. Opt., 1998, 37, 7352.
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Acknowledgements
28 M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria,
M. A. Robb, J. R. Cheeseman, V. G. Zakrzewski, J. A. Montgomery,
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C. Y. Peng, A. Nanayakkara, C. Gonzalez, M. Challacombe,
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Project supported by the National Natural Science Foundation
of China, the National Key Basic Research Special Foundation,
the Natural Science Foundation of Guangdong Province and the
Natural Science Foundation of the Education Ministry of China.
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