Self-assembly of twisted tetrachloroperylenediimide chromophores into two dimensional brick-stone aggregates: Exciton dynamics and photoconductivity

Article abstract of DOI:10.1039/c2cc32112b

2D self-assembly has been demonstrated in perylenediimides with twisted chromophores, in which the π-π stacked units are interconnected via hydrogen bonding interactions. Spectroscopic measurements and theoretical calculations suggest a weak J-type exciton coupling in the assembly. High photoconductivity of the 2D crystal makes it a promising candidate for further opto-electronic applications.

Full text of DOI:10.1039/c2cc32112b

View Article Online / Journal Homepage / Table of Contents for this issue
Dynamic Article Links
ChemComm
Cite this: Chem. Commun., 2012, 48, 6402–6404
www.rsc.org/chemcomm
COMMUNICATION
Self-assembly of twisted tetrachloroperylenediimide chromophores into
two dimensional brick-stone aggregates: exciton dynamics and
photoconductivityw
ab
a
a
a
a
a
Xinqiang Cao, Shuming Bai, Yishi Wu, Qing Liao, Qiang Shi, Hongbing Fu* and
Jiannian Yao*a
Received 23rd March 2012, Accepted 3rd May 2012
DOI: 10.1039/c2cc32112b
2
D self-assembly has been demonstrated in perylenediimides
with twisted chromophores, in which the p–p stacked units are
interconnected via hydrogen bonding interactions. Spectroscopic
measurements and theoretical calculations suggest a weak
J-type exciton coupling in the assembly. High photoconductivity
of the 2D crystal makes it a promising candidate for further
opto-electronic applications.
Perylenediimides (PDIs) are particularly attractive building
1
blocks in supramolecular self-assembly, because of their
exceptional stability and excellent optical properties, and
promising applications in opto-electronic devices such as field
2
3
4
effect transistors, waveguides, and solar cells. Chemical
modifications of PDIs at either the imide or the bay positions
allow versatile tunability of the supramolecular self-assembled
architectures. The strong unidirectional p–p interactions in
planar PDI cores generally lead to the formation of one-
dimensional (1D) assemblies with a face-to-face molecular
Fig. 1 SEM (a) and TEM (b) images of the TCPDI microsheets (inset
shows the corresponding larger magnification view and SAED pattern
of microsheets). (c) Predicted morphology based on attachment
energies principle, viewing perpendicular to the (002) plane. (d)
XRD patterns of TCPDI microsheets and single crystal data.
5
arrangement, such as nanowires, nanobelts, and nanoribbons.
¨
Recently, Wurthner et al. reported that core twisting of
the perylene backbone by introducing substituents in the
bay regions drives the self-assembly of slipped alignment,
6
however, still gives rise to 1D morphology. On the other
PDI into 2D structures still remains seldom explored, in spite
of its crucial importance in comprehensive understanding of
the relationship among molecular packing, exciton dynamics
hand, two-dimensional (2D) crystal assemblies, which main-
tain a thickness comparable to their 1D counterparts and
meanwhile have a side-length suitable for the device fabri-
cation, might suggest peculiar excitonic properties and be
1
1
and opto-electronic properties.
Herein, we synthesized twisted perylene chromophores
(TCPDI) with four chlorine substituents at the bay regions
(Fig. 1a, inset) and employed a colloid chemical method to
7
superior for opto-electronic applications, such as carrier
8
transport,
9
electrophotography, and photoconductivity
1
2
1
studies. To the best of our knowledge, self assembly of
0
induce the 2D self assembly. Spectroscopic measurements and
theoretical calculations have revealed distinct exciton dynamics
in the assemblies, compared with those commonly observed
H-type couplings in the 1D structures for those N-substituted
a
Beijing National Laboratory for Molecular Science (BNLMS),
Institute of Chemistry, Chinese Academy of Sciences,
Beijing 100190, P. R. China. E-mail: jnyao@iccas.ac.cn,
hongbing.fu@iccas.ac.cn; Fax: +86-10-62522039;
Tel: +86-10-62522039
Graduate University of Chinese Academy of Sciences (GUCAS),
Beijing 100049, P. R. China
13
PDIs. Moreover, these highly fluorescent single crystalline 2D
microsheets demonstrate high photoconductivity, suggesting
potential applications in opto-electronic devices.
b
TCPDI was synthesized via direct amination of tetra-
chloroperylene tetracarboxylic acid dianhydride with excess
14
ammonium acetate in propionic acid (Scheme S1, ESIw).
w Electronic supplementary information (ESI) available: Experimental
details, theoretical calculations, detailed experimental setup and more
optical–electric measurements. CCDC 858806. For ESI and crystallo-
graphic data in CIF or other electronic format see DOI: 10.1039/
c2cc32112b
Because of the poor solubility of TCPDI in common organic
solvents, di-anions that have relatively better solubility were
6
402 Chem. Commun., 2012, 48, 6402–6404
This journal is c The Royal Society of Chemistry 2012

View Article Online
used to facilitate the self-assembly of TCPDI in solution via
1
a two-step reaction:
2
2 2 4
TCPDI + Na S O + 4NaOH -
+
Na
2ꢀ
2
TCPDI + 2Na
2
SO
3
+ 2H
2
O
(1)
In the first step, di-anions were prepared according to reaction
(
1) through reduction of TCPDI using dithionite as the reducing
agent (Fig. S1, ESIw). Second, di-anions were gradually oxidized
by oxygen molecules according to reaction (2) to release neutral
TCPDI molecules which will undergo 2D assembly:
Fig. 2 (a) Diffuse reflectance absorption spectrum (black line) and
fluorescence spectrum (red line) of obtained TCPDI microsheets.
Dashed lines show the absorption (black) and emission spectra (red)
of monomer solution in THF. (b) Decay and fitted curves of TCPDI
monomeric THF solution and microsheets on quartz plate.
2
ꢀ
ꢀ
2
TCPDI + O + 2H O - 2TCPDI(s)k + 4OH (2)
2 2
Table 1 Fluorescence quantum yields and fitting results of the
fluorescence decay of TCPDI monomer solution and microsheets
Fig. 1 shows the scanning (SEM) and transmission (TEM)
electron microscopy images of the obtained microstructures. 2D
microsheets have been observed with tens of micrometers of
the edge length (Fig. 1a) and a thickness of around 300 nm
a
b
8
Sample
labs,max/nm lem,max/nm t/ns Ff (%) k (ꢁ10 )
f
Monomer 508
Microsheets 603
540
650
5.44 62
0.16
1.14
3.12
(Fig. S2, AFM image, ESIw). Moreover, the typical selected
area electron diffraction (ED) pattern (Fig. 1b, inset) has clearly
5
a
The fluorescence quantum yield (F) measurements were performed
under ambient condition using a relative method for monomer solution
validated the single crystalline structure. On the basis of the
1
5
orthorhombic single crystal data, the square microsheets
exhibit a 2D in-plane growth along (110) and (1–10) crystal
faces, as pointed out by circled and triangle sets of spots with
and an absolute method for microsheets using an integration sphere
b
(see ESIw for details).
k
f
f f
was calculated via equation: k = F /t.
8
ꢀ1
˚
d-spacing values of 6.2 A. Additionally, X-ray diffraction
kf,sheet = 3.12 ꢁ 10 s is demonstrated for the 2D crystal,
8
ꢀ1
.
(
Fig. 1d) has confirmed the formation of single crystalline
microsheets with only a sequence of peaks corresponding to
002) crystal planes. Theoretical calculation (Fig. 1c) using
compared to that of monomer: k
= 1.14 ꢁ 10 s
f, m
In order to gain a better insight into the excited states in the
TCPDI 2D aggregate, the coupling of molecules is calculated
using the Transition–Density–Cube (TDC) Method developed
(
growth morphology algorithm also anticipates a sheet-like
thermodynamic stable shape. The lowest attachment energy
obtained for the (002) face (Table S1, ESIw) suggests that it will
be prominent in the crystal morphology, since the growth rate
of the crystal face is assumed to be proportional to its magni-
tude of attachment energy. 2D p–p stacking along (110) and
16
by Fleming et al. Single crystal data (Fig. 3, inset) were used in
the calculations. Interestingly, results (Table S2, ESIw) show
that molecular pairs (1, 1 ) and (3, 3 ) along the direction of
0
0
hydrogen bonding within the same p-stacked layer give out the
ꢀ1
largest excitonic coupling strength (ꢀ336.76 cm ), much larger
than that of other molecular pairs, such as (1, 2) in the p–p
stacking direction and (2, 3) with Van der Waals forces. As we
pointed out before, the p–p stacks are interconnected by
hydrogen bonding interactions along a-axis, thus the largest
coupling strength in this direction may indicate the long ‘‘head-
(
1–10) faces with larger attachment energies (i.e., larger growth
rates) has also been clearly revealed in this calculated morpho-
logy, in consistence with the ED result in Fig. 1b. Besides,
Fig. 1c (red arrow) shows the direction of closet intermolecular
contact (hydrogen bonding) between imide hydrogen and car-
17
˚
bonyl oxygen with a separation of 2.86 A (Fig. S3a, ESIw).
to-tail’’ J-type aggregation in the crystal, compared to those
commonly observed H-type aggregates in N-substituted PDIs.
Moreover, as shown in Fig. 3, the ratio of 0–0 to 0–1 emission
intensities of the microsheets increases from 1.72 to 2.87 when the
temperature decreases from 300 K to 77 K. F. C. Spano demon-
strated that in typical J-aggregates, the ratio of the 0–0 to 0–1
These hydrogen bonding interactions advance the growth along
a-axis and would result in different excitonic coupling behavior
compared to those N-substituted PDIs, as we will discuss
below. Therefore, the growth of the TCPDI microsheets
demonstrates a hydrogen bonding assisted p–p stacking of the
twisted perylene chromophores into a 2D polymer chain like
structure, which obviously will provide us with good opportu-
nities to explore its distinctive exciton properties.
Fig. 2a shows the absorption and emission spectra of
TCPDI microsheets as well as monomer tetrahydrofuran
(
THF) solution. The microsheets exhibit a red-shifted absorp-
tion peak centered at 603 nm, compared with that of the
monomer (508 nm, Table 1). Concomitantly, they also display
a strong fluorescence (Quantum yield: 5%) with small stokes
shift (maximum at 650 nm) as shown in Fig. 2a. In addition,
as shown in Fig. 2b and Table 1, the monomer emission
can be fitted single-exponentially with a lifetime of tm
=
=
5
.44 ns and so can be the microsheets emission with tsheet
Fig. 3 Experimental emission spectra of obtained microsheets measured
0
.16 ns. Therefore, an enhanced radiative decay rate with
at 77 K and 300 K. Inset: unit cell of TCPDI single crystal.
This journal is c The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 6402–6404 6403

View Article Online
efficiency of 12%. Our results suggest a molecular design route
to the crystal engineering, thus to modulate its exciton dynamic
properties in the solid state to meet the specific demands of
device optimization in photon-to-electrical energy conversion.
This work was supported by the National Natural Science
Foundation of China (Nos 20903108, 20873163), the Chinese
Academy of Sciences (‘‘100 Talents’’ program), and the
National Research Fund for Fundamental Key Project 973
Fig. 4 (a) I–V curves measured in dark and under different light
intensities, inset shows the photograph of microsheet on two silver
electrodes during the test. (b) On–off switching behavior of microsheet
(
2006CB806200, 2006CB932101).
ꢀ
2
based device under a light density of 1.74 mw cm , and an applied
bias voltage 15 V.
Notes and references
1
¨
(a) F. Wurthner, Chem. Commun., 2004, 1564–1579; (b) W. Wang,
L.-S. Li, G. Helms, H.-H. Zhou and A. D. Q. Li, J. Am. Chem.
Soc., 2003, 125, 1120–1121.
1
8a
emission intensities decreases with increasing temperature.
Thus, in combination with the enhanced radiative decay rate
and red-shifted absorption, these may suggest a weak
2
A. L. Briseno, S. C. B. Mannsfeld, C. Reese, J. M. Hancock, Y. Xiong,
S. A. Jenekhe, Z. Bao and Y. Xia, Nano Lett., 2007, 7, 2847–2853.
1
8
3 (a) Q. Bao, B. M. Goh, B. Yan, T. Yu, Z. Shen and K. P. Loh, Adv.
Mater., 2010, 22, 3661–3666; (b) Y. Che, X. Yang, K. Balakrishnan,
J. Zuo and L. Zang, Chem. Mater., 2009, 21, 2930–2934.
4 I. A. Howard, F. Laquai, P. E. Keivanidis, R. H. Friend and
N. C. Greenham, J. Phys. Chem. C, 2009, 113, 21225–21232.
J-type coupling in the crystal.
The unique photophysical properties of TCPDI microsheet
have implied its promising application in photo-to-electric
energy conversion. A device configuration shown in Fig. S7a
5
L. Zang, Y. K. Che and J. S. Moore, Acc. Chem. Res., 2008, 41,
596–1608.
T. E. Kaiser, V. Stepanenko and F. Wu
2009, 131, 6719–6732.
(
ESIw) was used to monitor the current change upon light
illumination for the 2D microsheets. When the light intensity
1
6
¨
rthner, J. Am. Chem. Soc.,
ꢀ2
ꢀ2
(
Pin) increases from 0.09 mw cm to 2.34 mw cm , the current
within the voltage sweep interval increases accordingly
Fig. 4a), which could be attributed to more photons absorbed
7
8
M. Osada and T. Sasaki, Adv. Mater., 2012, 24, 210–228.
A. L. Briseno, S. C. B. Mannsfeld, M. M. Ling, S. H. Liu,
R. J. Tseng, C. Reese, M. E. Roberts, Y. Yang, F. Wudl and
Z. N. Bao, Nature, 2006, 444, 913–917.
(
by the photoconductor. Additionally, Fig. 4b displays the time
response of single TCPDI microsheet, which exhibits a function
9 D. S. Weiss and M. Abkowitz, Chem. Rev., 2009, 110, 479–526.
10 Y. Chen, K. Li, W. Lu, S. S.-Y. Chui, C.-W. Ma and C.-M. Che,
Angew. Chem., Int. Ed., 2009, 48, 9909–9913.
ꢀ2
of switcher when light (Pin = 1.74 mw cm ) was switched on
and off at a constant applied voltage of Vsd = 15 V, and no
fatigue is observed during the whole cycle. The photocurrent
1
1
1
1 Z. C. Wang, Z. Y. Li, C. J. Medforth and J. A. Shelnutt, J. Am.
Chem. Soc., 2007, 129, 2440–2441.
2 X. Q. Cao, Y. S. Wu, H. B. Fu and J. N. Yao, J. Phys. Chem. Lett.,
2011, 2, 2163–2167.
3 D. Chaudhuri, D. B. Li, Y. K. Che, E. Shafran, J. M. Gerton,
L. Zang and J. M. Lupton, Nano Lett., 2011, 11, 488–492.
14 F. Wurthner, C. Thalacker and A. Sautter, Adv. Mater., 1999, 11,
¨
(
I ) is found to switch promptly with light on and off. By using
ph
ꢀ
d
t/t 19
the equation: Iph p e
,
we could obtain a decay time
constant t E 423 ms (Fig. S8, ESIw) after the light was turned
d
ꢀ
11
off. Moreover, the dark current (I ) was only 5.8 ꢁ 10
A.
dark
ꢀ2
754–758.
15 Crystal data for TCPDI: C24
Upon 1.74 mw cm light illumination, the Iph could approach
B5.7 ꢁ 10ꢀ A, indicating an on/off ratio of B100, relatively
higher than that reported for a single crystalline inorganic
9
6 4 2 4
H Cl N O , M = 528.11, ortho-
˚
rhombic, a = 14.324(3) A, b = 7.1135(14) A, c = 19.269(4) A,
˚
˚
3
˚
a = 90.001, b = 90.001, g = 90.001, V = 1963.4(7) A
space group Pbcn, Z = 4, 5341 reflections measured, 2234
independent reflections. The final R values were 0.0619 (I > 2s(I)).
The final wR(F ) values were 0.1288 (I > 2s(I)). The final R values
, T = 173(2) K,
20
nanowire. Given that the average irradiation light is 550 nm,
1
we can estimate the external responsivity (Rex) of the TCPDI
2
ꢀ1
1
microsheet photoconductor to be 0.06 A W at an electric
1
2
were 0.0682 (all data). The final wR(F ) values were 0.1330 (all data).
ꢀ
21
2
The goodness of fit on F was 1.166. CCDC 858806.
16 B. P. Krueger, G. D. Scholes and G. R. Fleming, J. Phys. Chem. B,
field of E = 2 V mm via equation Rex = Iph/Pin
,
better than
2
2
the single polymer-nanowire based photodetector. Accord-
ingly, the corresponding external quantum efficiency was
determined to be 12% under the same electric field strength
1
998, 102, 5378–5386.
7 L. van Dijk, P. A. Bobbert and F. C. Spano, J. Phys. Chem. B,
009, 113, 9708–9717.
1
2
(
see ESIw for details). This value is larger than those reported
for 1D porphyrin rods and PDI based organic nanofibril
18 (a) F. C. Spano, Acc. Chem. Res., 2010, 43, 429–439; (b) S. H. Lim,
T. G. Bjorklund, F. C. Spano and C. J. Bardeen, Phys. Rev. Lett.,
2
3
2004, 92, 107402.
9 J.-J. Wang, J.-S. Hu, Y.-G. Guo and L.-J. Wan, J. Mater. Chem.,
2011, 21, 17582–17589.
0 J. J. Wang, F. F. Cao, L. Jiang, Y. G. Guo, W. P. Hu and
L. J. Wan, J. Am. Chem. Soc., 2009, 131, 15602–15603.
1 T. Zhai, L. Li, Y. Ma, M. Liao, X. Wang, X. Fang, J. Yao,
Y. Bando and D. Golberg, Chem. Soc. Rev., 2011, 40, 2986–3004.
2
4
heterojunctions. The obtained high photoconductivity gain
may originate from the split of excitons into free charge
carriers in pristine organic crystals by bulk traps and bending
1
2
2
2
5
of excitonic bands near the crystal surface.
To conclude, 2D single crystalline TCPDI microsheets have
been self assembled via a colloid chemical reaction method
from its dianion. Spectral analysis and theoretical calculations
have shown that synergistic 2D p–p stacking of the twisted
perylene cores and orientation by the hydrogen bonding in the
imide positions together advance the weak J-type coupling in
the crystal. A high performance photodetector based on
TCPDI single microsheet has been fabricated with a fast
response, large on/off ratio, and an external quantum
22 G. A. O’Brien, A. J. Quinn, D. A. Tanner and G. Redmond,
Adv. Mater., 2006, 18, 2379–2383.
2
3 A. D. Schwab, D. E. Smith, B. Bond-Watts, D. E. Johnston,
J. Hone, A. T. Johnson, J. C. de Paula and W. F. Smith, Nano
Lett., 2004, 4, 1261–1265.
24 Y. K. Che, H. L. Huang, M. A. Xu, C. Y. Zhang, B. R. Bunes,
X. M. Yang and L. Zang, J. Am. Chem. Soc., 2011, 133, 1087–1091.
2
5 (a) T. M. Clarke and J. R. Durrant, Chem. Rev., 2010, 110,
736–6767; (b) Y. Wakabayashi, J. Takeya and T. Kimura, Phys.
Rev. Lett., 2010, 104, 066103.
6
6
404 Chem. Commun., 2012, 48, 6402–6404
This journal is c The Royal Society of Chemistry 2012