Cross dipole stacking in the crystal of distyrylbenzene derivative: The approach toward high solid-state luminescence efficiency

Article abstract of DOI:10.1021/ja054661d

Through adding two substituent phenyl groups on distyrylbenzene, we have obtained the cross stacking of 2,5-diphenyl-1,4-distyrylbenzene with two trans double bonds (trans-DPDSB) in crystalline state. In such a cross-stacking mode, the solid-state emissio

Full text of DOI:10.1021/ja054661d

Published on Web 09/21/2005
Cross Dipole Stacking in the Crystal of Distyrylbenzene Derivative: The
Approach toward High Solid-State Luminescence Efficiency
Zengqi Xie,† Bing Yang,† Feng Li,† Gang Cheng,†,‡ Linlin Liu,† Guangdi Yang,† Hai Xu,† Ling Ye,†
Muddasir Hanif,† Shiyong Liu,‡ Dongge Ma,§ and Yuguang Ma*,†
Key Lab for Supramolecular Structure and Materials of Ministry of Education, and State Key Laboratory of
Integrated Optoelectronics, Jilin UniVersity, Changchun130012, P.R. China, and State Key Laboratory of
Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences,
Changchun 130022, P.R. China
Received July 13, 2005; E-mail: ygma@jlu.edu.cn
Poly(p-phenylene vinylene) (PPV)-type polymers and oligomers
have attracted more attention as luminescent materials due to their
applications in organic light-emitting diodes (OLEDs)1-4 and
lasers.5-7 However, H-aggregation that exists widely in such
π-conjugated compounds dramatically decreases the luminescence
quantum yield upon going from dilute blends or solutions to
films.8-11 The most efficient configuration to prevent any decrease
in luminescence efficiency, prefigured by theoretical investigations
in the clusters of conjugated molecules, is to adjust the long axes
of adjacent chains to be perpendicular in condensed media.12-14 In
such a cross stacking, the luminescence emission will be close to
Figure 1. Different conformational structures of trans-DPDSB in crystal.
the single molecular emission for the strong oscillator strength in
the transition between the ground state and the lowest excited state.
This type of cross-packing mode has been expected in some liquid
crystal molecules, but the accurate crystal structure has not been
determined.15,16 In our continued effort to inspect the packing mode
of distyrylbenzene (DSB) derivatives,17,18 we were surprised to find
that 2,5-diphenyl-1,4-distyrylbenzene with two trans double bonds
(trans-DPDSB) tends to adopt a stable cross-stacking mode in
crystalline state. The following experiments demonstrate that this
material exhibits excellent thermal stability, charge-transfer ability,
attractive OLEDs performance, and the amplified spontaneous
emission (ASE) in the needlelike crystalline state. Herein we report
the cross-stacking structure and the attractive luminescence proper-
ties of trans-DPDSB.
trans-DPDSB was synthesized by Wittig reaction and was fully
characterized by NMR, FT-IR, elemental analysis, and differential
scanning calorimeter (DSC) methods. Single crystals of trans-
DPDSB were prepared by vaporizing a mixtureof chloroform and
methanol (1:2) slowly. The crystal data reveal that there are two
crystallographically independent conformations of trans-DPDSB
(Figure 1).
Figure 2. (a) Crystals of trans-DPDSB under UV light (365 nm). (b)
Stacking of 1D molecular columns. (c) Cross-stacking molecules in 1D
molecular column. (d) Schematic of the aromatic CH/π hydrogen bonds
between two adjacent molecules. Interaction distances for I and II are 3.08
and 2.67 Å respectively. Conformation 1 is drawn in blue and 2 in red.
As shown in Figure 2, the most attractive structural feature of
the crystal packing is that the molecules are packed into one-
dimensional molecular columns along the b axis. They overlap upon
the central phenyl rings, and one molecule is rotated relative to the
other by an angle of 70° about an axis that passes through the
centers of both molecules. For the excited transition dipoles of PPV-
type conjugated molecules along their long axis, the observed
molecular packing in trans-DPDSB single crystal is the first
example of the cross dipole stacking of conjugated molecules in
solid states, which may enhance the solid-state fluorescence
prefigured by theoretical analysis.12,13 As shown in Figure 3, in
Figure 3. Splitting of the optically allowed transitions of a single chain
(sketched in the middle) in a cross-stacking configuration (trans-DPDSB)
and a parallel-stacking configuration (H-aggregate).
the parallel-stacking dimer, the lowest excited state corresponds to
the destructive combination of the transition dipole moment of the
individual chains, and it is optically forbidden. However, in the
cross-stacking dimer, the energy splitting between the lowest two
excited states of the dimer is reduced, and a progressive transfer
† Key Lab for Supramolecular Structure and Materials of Ministry of Education,
Jilin University.
‡ State Key Laboratory of Integrated Optoelectronics, Jilin University.
§ State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute
of Applied Chemistry, Chinese Academy of Sciences.
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C O M M U N I C A T I O N S
of intensity from the second excited state to the lowest excited state
occurs. This promotes a finite transition dipole moment between
the ground state and the lowest excited state of the clusters, which
is highly beneficial for the light-emission properties. Additionally,
the overlapped central phenyl rings along one-dimensional molec-
ular columns (b axis) can act as a channel for electron transport,
producing high carrier mobility. Actually, the single crystal of trans-
DPDSB exhibits very strong pure blue emission, when excited under
an ultraviolet lamp (365 nm) (shown in Figure 2a). OLED studies
have demonstrated good electron-transporting property of trans-
DPDSB films (see Supporting Information).
The distance between the central rings of adjacent molecules is
4.1 Å (b/2), which means there is no face-to-face π-π interaction.
As shown in Figure 2d, there are two different types of aromatic
CH/π hydrogen bonds between the adjacent two molecules in each
column, where two carbon atoms of conformation 2 act as proton
donors and the two phenyl substituents of conformation 1 act as
acceptors. The interaction distance and the angle of C-H-π center
for interaction I are 3.08 Å and 154°, and for interaction II are
2.67 Å and 157°, respectively. Considering that the enthalpy of
aromatic CH/π hydrogen bonds 6.5-10.3 kJ/mol for one unit19-22
but 10.3 kJ/mol seems to be the best value,22 we believe that such
CH/π interactions are the key driving force for the cross-stacking
mode. At the same time the attractive force of such CH/π
intermolecular interactions induces the different molecular confor-
mations in the crystal. The distyrylbenzene segment of conformation
1 is planar, and that of conformation 2 is torsional (Figure 1). In
detail, the torsional angle θ1 in conformation 1 is 0.5°, while in
conformation 2 the aromatic CH/π hydrogen bond I raises the
torsional angle to 24.1°. A similar influence by the aromatic CH/π
hydrogen bond II on the torsional angle of p-terphenyl segment
can be observed.
trans-DPDSB shows excellent thermal stability (melting point
of 251-254 °C determined by DSC), good electron-transporting
property, and strong pure blue emission in solids, rendering it as
good OLED material. The achieved maximum luminescence and
luminous efficiency of trans-DPDSB-based OLEDs with a simple
device structure are 2100 cd m-2 and 1.53 cd A-1, respectively,
which are much higher than that of the device using p-bis(p-
styrylstyryl)benzene as the emitting layer.23 The big difference in
performance can be attributed to the different stacking modes
between trans-DPDSB (cross stacking) and p-bis(p-styrylstyryl)
benzene (H-aggregate). Interestingly, trans-DPDSB tends to form
transparent and needlelike crystals, and from Figure 2a, the tip of
needlelike single crystal shows stronger fluorescence than the body
surface of the crystal, indicating a natural self-wave-guided structure
of this crystal.24,25 Therefore, obtained needlelike trans-DPDSB
crystals with strong fluorescence emission and self-wave-guided
structure are expected as ideal gain media for optically pumped
solid-state lasers.26,27 The ASE with threshold power of about 0.68
mJ/pulse and the initial broad blue emission collapsing into the
intense and narrow blue band from needlelike trans-DPDSB single
crystals with the increase of pumping intensity have been observed
in primary experiments for optically pumped lasers.
To summarize, we are the first to obtain the cross stacking of
trans-DPDSB, where the aromatic CH/π hydrogen bonds act as
the driving force. The crystal of trans-DPDSB has excellent thermal
stability, good electron-transporting property, and strong pure blue
emission, which make this material, a promising candidate for
OLED and laser. The primary experiments on OLED and laser
imply the potential applications of this material, and further
investigation is presently underway.
Acknowledgment. We are grateful for financial support from
National Science Foundation of China (grant numbers 20474024,
20125421, 90101026, 50473001), Ministry of Science and Technol-
ogy of China (grant number 2002CB6134003) and PCSIRT.
Supporting Information Available: Complete ref 27; synthesis
and characterization, crystal structure determination, photoluminescence
properties, OLED fabrication, ASE experiment of trans-DPDSB; and
X-ray crystallographic file (CIF). This material is available free of
charge via the Internet at http://pubs.acs.org.
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