Isomers of Pyrene-Imidazole Compounds: Synthesis and Configuration Effect on Optical Properties

Article abstract of DOI:10.1021/acs.orglett.5b02879

Two pyrene-imidazole-based structural isomers, axisymmetric syn-PyDTI and centrosymmetric anti-PyDTI, have been easily produced and successfully separated in a one-pot reaction. Crystalline syn-PyDTI exhibited a 2-fold higher quantum efficiency than anti-PyDTI and displayed a much better performance in OLEDs with a η_c.max of 11.4 cd A^-1 (8.8 cd A^-1 for anti-PyDTI), although they possess the same apparent electronic structure. Observations revealed that the molecular excited-state properties are derived from distinct structural symmetries.

Full text of DOI:10.1021/acs.orglett.5b02879

Letter
pubs.acs.org/OrgLett
Isomers of Pyrene−Imidazole Compounds: Synthesis and
Configuration Effect on Optical Properties
Yulong Liu,† Tong Shan,† Liang Yao,† Qing Bai,† Yachen Guo,† Jinyu Li,† Xiao Han,† Weijun Li,†
Zhiming Wang,§ Bing Yang,† Ping Lu,*,† and Yuguang Ma‡
†State Key Laboratory of Supramolecular Structure and Materials, Jilin University, 2699 Qianjin Avenue, Changchun 130012, China
‡Institute of Polymer Optoelectronic Materials and Devices, State Key Laboratory of Luminescent Materials and Devices, South
China University of Technology, Guangzhou 510640, China
§School of Petrochemical Engineering, Shenyang University of Technology, Liaoyang 111003, China
S
* Supporting Information
ABSTRACT: Two pyrene-imidazole-based structural isomers,
axisymmetric syn-PyDTI and centrosymmetric anti-PyDTI, have
been easily produced and successfully separated in a one-pot
reaction. Crystalline syn-PyDTI exhibited a 2-fold higher quantum
efficiency than anti-PyDTI and displayed a much better perform-
ance in OLEDs with a ηc.max of 11.4 cd A−1 (8.8 cd A−1 for anti-
PyDTI), although they possess the same apparent electronic
structure. Observations revealed that the molecular excited-state
properties are derived from distinct structural symmetries.
somers are one of the central issues in chemistry, especially
for applications in medicinal1,2 and materials sciences.3,4 In
most important and thoroughly investigated organic chromo-
phores in previous years.17−19 Its 4, 5, 9, and 10 positions can
be functionalized to yield periphery-modified pyrene-4,5,9,10-
tetraones,20 providing the opportunity to further create a
polycyclic skeleton.12,21 One of the insufficiently resolved issues
involving the pyrene π-system is that it suffers heavily from
aggregation-caused quenching that originates from the strong
π−π interaction between molecules.22 To address this concern,
the covalent bonding of an aggregation-induced emission
(AIE)-active unit, such as tetraphenylethylene (TPE), has
proved to be a powerful way to achieve high solid-state
photoluminescence.23 Based on this consideration, here, novel
syn-/anti-structural isomers consisting of multiple functional
groups, including pyrene (rigid π-structure), imidazole (non-
symmetrical units), and TPE (AIE block), are obtained in a
one-pot reaction. Axisymmetric syn-PyDTI and centrosymmet-
ric anti-PyDTI are readily formed during the synthesis process.
These isomers are successfully separated with high purity. It is
interesting to note that the syn-PyDTI exhibits 2-fold higher
fluorescent quantum efficiency in the crystal and 1.5-fold higher
quantum efficiency in evaporated amorphous film than that of
anti-PyDTI, although they possess the same apparent electronic
structure. The present findings can pave the way for the
development of a new type of structural isomer and promotes
the development of efficient organic luminescent materials that
are regulated by molecular design.
I
the optoelectronics field, isomers have also attracted tremen-
dous attention.5 For instance, structural isomers, which contain
the same bond structure but differ in the geometrical positions
of their atoms in space, do not necessarily share similar
properties, even if they have the same functional groups; thus,
they can be utilized as valuable and model compounds to
investigate the relationship between structure and properties.6,7
A typical example involves the intensive study of the syn-/anti-
isomeric effects on the critical changes of physical and optical
properties of thiophene derivatives.8−11 Pure anti-isomers
exhibit much higher charge mobilities than their corresponding
syn-counterparts in organic field effect transistors, and this
phenomenon has been described by various groups for
anthradithiophene, dicyanomethylenedithiophene, anthrabis-
benzothiophene, and other systems.9−11 One possible reason
for this phenomenon may be attributable to the distinct dipole
moments of syn-/anti-isomers. These valuable studies have
significantly promoted the development of functional materials
in optoelectronics and have contributed to the basic knowledge
of isomers in organic chemistry.
In prior studies, syn-/anti-isomers have generally been
synthesized independently through various synthetic
routes.8−12 It would be beneficial if these materials could be
obtained via a one-step route and conveniently purified. In this
paper, we demonstrate a strategy for achieving this goal and
found a dramatic configuration effect on the optical property.
1,3-Imidazole is a commonly used, five-membered-ring func-
tional group in efficient luminescent materials because of its
amphoteric characteristics, which originate from the two types
of sp2 hybrid nitrogen atoms.13−16 Pyrene has been one of the
Scheme 1 depicts the synthesis of the syn-isomer and the
anti-isomer of PyDTI. These two compounds could be formed
Received: October 31, 2015
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.5b02879
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Scheme 1. Synthetic Approach to PyDTI Isomers
Figure 2. (A, C) Molecular structure side views of syn- and anti-PyDTI
crystals. (B, D) Unit cell structure top views of crystalline syn- and
anti-PyDTI.
configuration and a slip-stacked packing mode along the long
molecular axis (Figure 2D and Figure S12-(2), Supporting
Information). In contrast, syn-PyDTI manifested a clear twisted
configuration (Figure 2A). Because the axisymmetric structure
of syn-PyDTI would induce a dipole along the short axis
direction (Figure S7 and Table S1, Supporting Information), it
is proposed that the dipole−dipole interactions drove the
formation of a flat dimer in one unit cell by alternating the
antiparallel alignment26 of the pyrene−imidazole unit with an
interfacial distance of ∼4.0 Å (Figure 2B and Figure S12-(1),
Supporting Information). The successful separation and
confirmation of isomers is significant for achieving a deep
understanding of the resulting optical properties at the
molecular level.
simultaneously via a “click-like” one-pot reaction.21,24 The
observed Rf values for these two isomers on a silica gel plate
varied significantly when eluted with dichloromethane. This
was a consequence of the various symmetries of the syn- and
anti-isomers, which produced rigid polycyclic aromatic hydro-
carbons with enhanced differences in molecular shape and
polarity. Thus, the syn- and anti-PyDTI could be separated
successfully by column chromatography in a nearly 1:1 ratio. In
1
the H NMR spectra (Figure 1), it can be seen that the
Both isomers showed AIE activity. They were nonemissive in
THF but exhibited intense emission in their aggregate states
(Figure S10, Supporting Information). Comparing the crystals’
emissions revealed a red-shift for the anti-isomer; that is, the
syn-PyDTI crystal exhibited an emission peak at 465 nm,
whereas the anti-isomer crystal displayed an emission peak at
507 nm (Figure 3A). The high-quality single crystals of syn- and
1
Figure 1. H NMR spectra of syn-PyDTI (A) and anti-PyDTI (B) in
C4D8O (500 M Hz, 298 K).
resonance peaks of protons H1 and H3 in syn-PyDTI occurred
at 7.32 and 7.25 ppm, respectively, while those on the other
side of the pyrene units, such as H2 and H4, displayed relatively
low-field chemical shifts of 9.02 and 8.16 ppm, respectively.
This difference could be a result of the fact that the ring-current
effect,25 which originated from two axisymmetric phenyl rings
attached to the N atom, gave rise to a relatively strong shielding
effect of protons H1 and H3, leading to a concomitant upfield
chemical shift. In contrast, anti-PyDTI possessed a centrosym-
metric structure, and the corresponding signals of the aromatic
protons H1 and H3 presented a downfield shift at approximately
7.42 and 7.69 ppm. The characteristic 1H NMR signals for both
protons remained unchanged after UV irradiation. This
indicated that photochemical isomerization could not occur
in syn/anti-PyDTIs suggesting the good stability of these
isomers.
Figure 3. Emission spectra (A, B) and time-resolved peak fluorescence
by time-correlated single-photon counting measurements (C, D) of
syn-PyDTI and anti-PyDTI as crystals and evaporated films.
anti-PyDTI were blue and green under UV light, respectfully,
which directly confirmed their different luminescent properties.
As shown in their packing diagrams (Figure S12, Supporting
Information), the intermolecular interactions were relatively
small because of the large distance between the slip-packed
adjacent molecules in anti-PyDTI, which would not signifi-
cantly increase the effective conjugation length. The blue-shift
observed in the syn-PyDTI crystal’s emission spectrum was
The single-crystal measurement further confirmed the
configuration of syn/anti-PyDTI. As depicted in Figure 2,
both crystals belonged to the triclinic space group P-1. The
anti-PyDTI crystal showed a centrosymmetric, coplanar
B
DOI: 10.1021/acs.orglett.5b02879
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Organic Letters
Letter
probably associated with the formation of dimers, which
resulted in a flexible geometry and a decreased conjugation
length. It can also be confirmed by their absorption spectra,
where the initial band of anti-PyDTI was found at 438 nm and
that of syn-PyDTI was located at 403 nm (Figure S9,
Supporting Information). More importantly, the isomers
manifested significantly different quantum efficiencies, as
revealed by measurements obtained by an integrating sphere
system. The efficiency of the anti-PyDTI crystal was calculated
as 32%, while that for the syn-crystal was even higher to be 65%.
In evaporated amorphous films, both species presented the
same broad red-shifted emission spectra that peaked at 495 nm,
indicating strengthened intermolecular interactions (Figure
3B). However, syn-PyDTI exhibited a quantum efficiency (φf)
of 53%, which was higher than the anti-PyDTI efficiency of
35%.
Figure 4. Current efficiency versus brightness curves for syn-PyDTI
and anti-PyDTI. Inset: Electroluminescence (EL) spectra for syn-
PyDTI and anti-PyDTI measured at 9 V.
In the excited state dynamics study, the nonradiative decay
rate constant for the evaporated film of the syn-isomer was
calculated to be 1.9 × 108 s−1, which was much smaller than
that of anti-PyDTI (3.1 × 108 s−1). This indicates that the
molecular vibrational modes of excited state of syn-PyDTI
might effectively suppress the energy loss through nonradiative
decay processes and afford a higher efficiency (Table 1). This
In summary, two structural isomers with a pyrene−imidazole
structure, syn-PyDTI and anti-PyDTI, were conveniently
synthesized and successfully separated. When compared with
anti-PyDTI, syn-PyDTI exhibits a 2-fold higher quantum
efficiency as a crystal and 1.5-fold higher quantum efficiency
as an evaporated amorphous film, although the two isomers
possess the same apparent electronic structure. This observa-
tion reveals the dependence of molecular excited state
properties derived from distinct structural symmetries. Syn-
PyDTI also displays much better performance in OLEDs with a
ηc.max of 11.42 cd A−1 (8.12 cd A−1 for anti-PyDTI). Such
findings are very relevant to OLED materials because precise
separation is essential for enhanced device performance. The
current work reports the establishment of stable imidazole-
based isomers with very convenient accessibility and multi-
functionality. We expect that the present strategy can provide a
generic route toward novel and advanced structural isomers and
that these materials may find practical applications in
optoelectronics.
Table 1. Parameters for Exciton Dynamics in Crystals and
Evaporated Films
a
b
b
Φf
τ (ns)
kr/108 (s−1
)
knr/108 (s−1
)
syn-crystal
anti-crystal
syn-film
0.65
0.32
0.53
0.35
2.12
1.66
2.45
2.10
3.1
1.9
2.2
1.7
1.6
4.1
1.9
3.1
anti-film
a
b
Measured by an integrating sphere system. The radiative and
nonradiative exciton decay rate constants kr and knr can be calculated
using equations S1−S4 in the Supporting Information.
type of situation was more obvious in the crystal because the
time-resolved fluorescence spectroscopy of the syn-PyDTI
crystal showed monoexponential decays from the excited
singlet state. This behavior suggested that the fluorescence
originated from only one excited state and that no competing
radiative deactivation process existed (Figure 3C). Thus, the
various symmetries characteristic of syn- and anti-isomers
induced crucial changes in the photophysical propeties.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.5b02879.
■
S
Experimental procedures, DFT calculations, and charac-
terization data (PDF)
OLEDs were fabricated with the classical structure of ITO/
PEDOT:PSS (40 nm)/NPB (50 nm)/syn-PyDTI or anti-
PyDTI (20 nm)/TPBI (40 nm)/LiF (0.5 nm)/Al (120 nm).
The electroluminescent properties are summarized in Figure 4
and Table S4. The turn-on voltages were measured as 2.8 V for
both devices, suggesting that the barriers for carrier injection
were quite low. They both exhibited green emissions, with
peaks centered at 500 nm for syn-PyDTI and 508 nm for anti-
PyDTI, that were very similar to their emission spectra in the
film state. The maximum current efficiency (ηc.max) and power
efficiency (ηp.max) of syn-PyDTI were as high as 11.42 cd A−1
and 10.39 lm W−1, respectively, which were rarely obtained in
pyrene-based OLEDs.27 Even when the brightness reached
10000 cd m−2, the ηc.max remained nearly unchanged and
displayed a very low roll-off in efficiency. Comparatively, the
device using anti-PyDTI as the active layer showed a moderate
performance with a ηc.max of 8.12 cd A−1 and a ηp.max of 6.71 lm
W−1. The higher efficiency and better device performance of
syn-PyDTI make it the better candidate for OLED applications.
X-ray crystal details for syn-PyDTI (CIF)
X-ray crystal details for anti-PyDTI (CIF)
AUTHOR INFORMATION
Corresponding Author
■
*E-mail: lup@jlu.edu.cn.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported financially by the Ministry of Science
and Technology of China (2013CB834801) and the National
Science Foundation of China (21374038 and 91233113).
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