T-shaped donor-acceptor molecules for low-loss red-emission optical waveguide

Article abstract of DOI:10.1021/ol4012025

A series of T-shaped polycyclic molecules with high fluorescence were developed as optical waveguide materials. Their emissions covered almost the whole visible range from 450 to 800 nm. Compound 3-1 showed an optical loss coefficient about 0.29 dB/μm in

Full text of DOI:10.1021/ol4012025

ORGANIC
LETTERS
XXXX
Vol. XX, No. XX
000–000
T‑Shaped DonorꢀAcceptor Molecules
for Low-Loss Red-Emission Optical
Waveguide
Zi-Hao Guo, Ting Lei, Ze-Xin Jin, Jie-Yu Wang,* and Jian Pei*
Beijing National Laboratory for Molecular Sciences, the Key Laboratory of Bioorganic
Chemistry and Molecular Engineering of Ministry of Education, College of Chemistry
and Molecular Engineering, Peking University, Beijing 100871, China
jianpei@pku.edu.cn; jieyuwang@pku.edu.cn
Received April 29, 2013
ABSTRACT
A series of T-shaped polycyclic molecules with high fluorescence were developed as optical waveguide materials. Their emissions covered
almost the whole visible range from 450 to 800 nm. Compound 3-1 showed an optical loss coefficient about 0.29 dB/μm in red-emission waveguide.
Our investigations demonstrated that these molecules held great potential for organic optical waveguide due to the high fluorescence quantum
efficiency and large Stokes’ shift.
Over the past decade, one-dimensional (1D) nano- and
microstructures have attracted great attention due to their
potential application in miniaturized optoelectronics, such
as photodetectors,1 chemical sensors,2 field-effect transis-
tors (FETs),3 as well as optical waveguides.4 Recently, 1D
organic nano- and microstructures emerged as an effective
flexible media to generate or propagate light.5 Comparedto
their inorganic counterpart, organic materials have many
advantages such as versatile modification from a molecular
level, good flexibility, high luminescence efficiency, and low
cost.6 However, up to now, most of the reported organic
materials for active waveguides emit light from 400 to
600 nm. The organic waveguide materials with red-
emission above 600 nm are seldom reported.7 Therefore,
to develop new red-emissive organic waveguide materials
with high performance is still a challenge for efficient full-
color organic optical waveguides.
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Organic materials for 1D active waveguides with high
efficiency should be highly fluorescent and have large
Stokes’ shift. However, most 1D nano- and microstructures
constructed by organic small molecules8 or conjugated
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r
10.1021/ol4012025
XXXX American Chemical Society

polymers9 exhibit low fluorescence quantum efficiency,
which is mainly caused by πꢀπ stacking of their planar
backbones.10 In order to achieve high fluorescence quan-
tum efficiency in solid state, bulky groups are usually
incorporated to decrease πꢀπ interactions.11 Recently,
Ma et al. found that triphenylamine (TPA) contained small
donorꢀacceptor (DꢀA) molecules exhibited high fluores-
cence in solid state, which was attributed not only to the
bulky TPA skeleton but also to the intercrossed excited
state between the low-lying local exciton (LE) and charge
transfer (CT) exciton resulting from the DꢀA structure.12
On the other hand, the bandgap of DꢀA conjugated
systems is usually small and can be tuned conveniently by
changing donor and acceptor groups to achieve desired
emission.13
Scheme 1. Synthetic Routes to T-Shaped DꢀA Molecules 1-1,
1-2, 1-3, 1-2, 2-2, and 3-2
Scheme 1 illustrates the synthetic approaches to the
T-shaped DꢀA polycyclic molecules, 1-1, 2-1, 3-1, 1-2,
2-2, and 3-2. 4,7-Dibromo-2,1,3-benzothiadiazole was
reduced by NaBH4 in ethanol to afford 4 in 80% yield.
Condensation reactions between 4 and different 1,2-
diketone derivatives afforded compounds 5, 6, and 7, res-
pectively. A Suzuki coupling reaction between 5, 6, 7
and 4-(diphenylamino)phenylboronic acid catalyzed by
Pd(PPh3)4 afforded T-shaped molecules 1-1, 2-1, and 3-1.
Following the same Suzuki coupling procedure, 1-2, 2-2,
and 3-2 were also obtained using 5, 6, and 7 reacting them
with p-methoxyphenylboric acid as references. The distribu-
tions of the donor arms in horizontal direction and the
acceptor core in vertical direction facilitated the modifica-
tion of the donor and the acceptor segments independently
through different methods.
Figure 1. Design strategy and structures of the T-shaped DꢀA
molecules 1-1, 2-1, and 3-1.
Herein, we developed a series of T-shaped polycyclic
DꢀA molecules containing two electron-donor arms
and an electron-acceptor core. As shown in Figure 1, the
quinoxaline unit is chosen as the acceptor core, due to its
strong electron-withdrawing properties.14 To narrow the
bandgap to achieve red emission, different aromatic cores
are fused to the quinoxaline unit to enlarge the conjugated
length.15 In addition, bulky triphenylamine (TPA) donor
arms are connected to the acceptor backbone due to their
strong electron-donating property, which can further red-
shift their emission. Furthermore, these TPA-contained
DꢀA molecules may efficiently use the excitation energy
arising from the intercrossed excited state (LE and CT).12
The absorption and emission spectra of all T-shaped
DꢀA polycyclic molecules 1-1, 2-1, 3-1, 1-2, 2-2, and 3-2
were measured in dilute CH2Cl2 solution (1 ꢁ 10ꢀ5 M)
(Figures 2 and S1 and S2 in Supporting Information [SI]).
Figure 2a shows the absorption spectra of 1-1, 2-1, and 3-1
in dilute CH2Cl2 solution. Note that these three com-
pounds with the same donor arms exhibited similar ab-
sorption characteristics which was divided into three main
parts. Taking 3-1 as an example, the first absorption band
above 450 nm is attributed to the CT absorption. The
second band, peaking at 425 nm, comes from πꢀπ*
transition of the acceptor core. This is concluded from
the absorption spectra of 3-1 and 3-2, which have the same
acceptor core as shown in Figure 2b. The short wavelength
band (280ꢀ300 nm) originates from the nꢀπ* transition
(9) Hoeben, F. J. M.; Jonkheijm, P.; Meijer, E. W.; Schenning,
A. P. H. Chem. Rev. 2005, 105, 1491.
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Mater. 2001, 13, 1053. (b) Wurthner, F. Chem. Commun. 2004, 1564.
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Int. Ed. 2005, 44, 2427.
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Shen, F.; Wang, C.; Liu, D.; Liu, X.; Hanif, M.; Tang, S.; Li, W.; Li, F.;
Shen, J.; Park, J.-W.; Ma, Y. ChemPhysChem 2008, 9, 2601. (b) Li, W.;
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Adv. Funct. Mater. 2012, 22, 2797.
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Figure 2. Absorption spectra of (a) 1-1, 2-1, and 3-1 and of (b) 3-1 and 3-2 in dilute CH2Cl2 solutions; (c) Emission spectra of all six
T-shaped molecules in dilute CH2Cl2 solutions.
from the donor arm (TPA).16 To further explore the photo-
physical properties of these molecules, we also performed
the calculations using the method of B3LYP/6-31þg(d,p)
to compute the electronic transitions of 3-1 and 3-2 (see
Figures S7 and S8 in SI for a comparison between com-
puted and observed spectra). The computed absorption
spectra showed similar features with the experimental ones,
which explained the origin of each absorption peak.
conjugate molecules are very interesting and have been
reported in other DꢀA systems,17 which lead to indepen-
dent HOMOs and LUMOs of the T-shaped molecules.
Table 1. Electrochemical Properties of 1-1, 2-1, 3-1, and 3-2
compd
HOMOa(eV)
LUMOa(eV)
bandgapa(eV)
1-1
2-1
3-1
3ꢀ2
ꢀ5.25 (ꢀ5.12)
ꢀ5.25 (ꢀ5.16)
ꢀ5.25 (ꢀ5.12)
ꢀ5.52 (ꢀ5.54)
ꢀ2.92 (ꢀ2.40)
ꢀ3.11 (ꢀ2.57)
ꢀ3.25 (ꢀ2.73)
ꢀ3.23 (ꢀ2.65)
2.33 (2.46)
2.14 (2.27)
2.00 (2.14)
2.29 (2.89)
Photoluminescence (PL) behaviors of these molecules
were explored by exciting the molecules at their absorption
maxima, λmax (Figure 2c). Compound 1-2 shows the short-
est emission peaked at 513 nm, and 3-1 exhibits the longest
emission around 669 nm, which are consistent with the
trend in the absorption features. The emission peaks of 1-1
to 3-1 which contain stronger donor arms are more red-
shifting compared to those of 1-2 to 3-2. By fine-tuning the
bandgap, a series of emission spectra from 450 to 800 nm
were obtained. Significantly, all six T-shaped DꢀA mole-
cules exhibit small overlaps between the absorption and
emission spectra, thus decreasing their self-absorptions.
The electrochemical properties of these DꢀA molecules
were investigated by cyclic voltammetry (CV) (Figures S5,
S6 in SI) in CH2Cl2, and the data of 1-1, 2-1, 3-1 and 3-2 are
summarized in Table 1. All compounds display reversible
reductionbandsandoxidationbands.Interestingly,for1-1,
1-2, and 3-1 containing the same donor arms, the same
HOMO levels (about ꢀ5.25 eV) but different LUMO levels
are achieved. In addition, 3-1 and 3-2 exhibit the same
LUMO levels (about ꢀ3.25 eV) but different HOMO
levels, since they have the same acceptor core. The same
trend is observed in all six compounds, implying that the
oxidation potential is attributed to the oxidation of donor
arms and the reversible reduction wave is decided by the
reduction of the acceptor core. Figures S9 and S10 in SI
illustrate the electron distributions in frontier molecular
orbitals (FMOs) of compounds 1-1, 2-1, and 3-1 and 1-2,
2-2, and 3-2. The HOMOs are mainly localized on donor
arms, whereas the LUMOs are mostly distributed on
acceptor cores. These results explain why the HOMO and
LUMO levels depend on the donor arms and the acceptor
cores, respectively. Such spatially separated FMOs in
a Calculated from CV, reference electrode: Ag/AgCl; computational
results by the method of B3LYP/6-311þg(d,p) are in parentheses.
The red emissive molecule3-1 grows intostraight orbent
microwires in mixed solvents of CHCl3 and ethanol, as
shown in a and b of Figure 3. The microwires are about
0.2ꢀ3 μm inwidthand 100ꢀ500μm inlength, withsomeof
bent microwires. The microwires exhibit very bright red
luminescence spots at both ends and relatively weaker
emission from the body (Figure 3b). This is a typical feature
of an optical waveguide,5 suggesting that the micro-
wires are able to absorb the excitation light and propagate
the red emission toward the tips. Transmission electron
microscopy (TEM) and selected area electron diffraction
(SAED) analyses were applied to determine the crystallinity
of the microstructures. Both straight and bent microwires
of 3-1 exhibit one set of single-crystal diffraction patterns,
revealing that these microwires are well crystallized. Com-
pared to the absorption spectrum of 3-1 in solution, the
absorption of the microwires is not appreciably red-shifted
(see Figure S3 in SI), indicating the relatively weak inter-
actions among 3-1 molecules caused by the bulky TPA
groups. The absolute fluorescence quantum efficiency of
the microwires of 3-1 is 41%, which is very high in the solid
state for red-emissive materials.11 The large Stokes’ shift
is beneficial to efficient waveguiding, which decreases the
(17) (a) Wilson, J. N.; Bunz, U. H. F. J. Am. Chem. Soc. 2005, 127,
4124. (b) Zucchero, A. J.; Wilson, J. N.; Bunz, U. H. F. J. Am. Chem.
Soc. 2006, 128, 11872. (c) Zucchero, A. J.; McGrier, P. L; Bunz, U. H. F.
Acc. Chem. Res. 2010, 43, 397. (d) Uoyama, H.; Goushi, K.; Shizu, K.;
Nomura, H.; Adachi, C. Nature 2012, 492, 234.
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C.-H. Adv. Funct. Mater. 2004, 14, 83.
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C

Figure 4. Waveguide properties of (a) straight microwires and
(b) bent microwires.
Figure 3. (a) Optical microscopy and (b) fluorescence micro-
scopy images of microwires formed by 3-1. Bright-red lumines-
cence spots are marked with white circle at the ends of the
microwires. Transmission electron microscopy (TEM) images
and selected area electron diffraction (SAED) patterns of the (c)
straight microwires and (d) bent microwires.
calculated to be 0.29 dB/μm which is very low in those of
reported red-emissive organic waveguide materials.7
In conclusion, we have developed a series of T-shaped
DꢀA molecules through facile synthesis. These T-shaped
molecules exhibit different emissions covered from 450 to
800 nm by changing the different donor arm and acceptor
core. The red emissive molecule 3-1 can grow into straight
or bent microwires in mixed solvent of CHCl3 and ethanol
with quite high fluorescence quantum efficiency up to 41%.
The large Stokes’ shift and high fluorescence quantum
efficiency of 3ꢀ1 endow the microwires with a rarely low
optical loss coefficient of 0.29 dB/μm as a red-emission
waveguide materials. Our results demonstrate that intramo-
lecular donorꢀacceptor molecules with high quantum effi-
ciency can be rationally designed and used for red-emissive
organic waveguide materials, thus presenting a new mole-
cular family for full-color organic optical waveguides.
light loss caused by self-absorption. The emission spectra of
3-1 in solid state are shown in Figure S4 in SI. The Stokes’
shift of the microwires is about 130 nm which is larger than
those of the reported organic waveguide materials.5
A near-field scanning optical microscope (NSOM) was
applied to investigate the distance-dependent fluorescence
property of the microwires. The microwires were excited
by a focused laser (325 nm) at different local positions
along the length of the microwires with an NSOM collec-
tion-tip fixed over one of their ends. Asshown in Figure 4a,
b, both straight and bent microwires exhibit good wave-
guide properties. Such bent microwires may have potential
applications in various optical or optoelectronic micro/
nanodevices where shape-flexible material are needed. To
investigate the waveguiding efficiency, the emission fea-
tures at the excited site and the out-coupling light at the
ends were measured separately. The fluorescence intensi-
ties at the excited site (Iin) and the emitting tip (Iout) were
recorded to calculate the optical loss coefficient (R) by
a single exponential fitting (Iin/Iout = A eꢀ(ꢀRX), where X is
the distance between the excited site and the emitting tip)
(Figures S11,12 in SI). The R of the microwires was thus
Acknowledgment. This work was supported by the
Major State Basic Research Development Program (Nos.
2009CB623601 and 2013CB933501) from the Ministry of
Science and Technology, and National Natural Science
Foundation of China.
Supporting Information Available. Experimental de-
tails and characterization data. This material is available
free of charge via the Internet at http://pubs.acs.org.
The authors declare no competing financial interest.
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