Opposite mechanoluminescence behavior of two isomers with different linkage positions

Article abstract of DOI:10.1039/c8cc03083a

Two isomers of mm-TPE(PI)₂ and pp-TPE(PI)₂, constructed by the two same aromatic blocks of tetraphenylethene and phenanthro[9,10-d]imidazole, exhibit totally different mechanoluminescence, as a result of the ignorable different linkage positions on the tetraphenylethene moiety. Detailed analysis and theoretical calculations demonstrate the structure-packing-property relationship of organic mechanoluminescent luminogens, with the emphasis on the important role of molecular packing in the solid state.

Full text of DOI:10.1039/c8cc03083a

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Opposite mechanoluminescent behavior of two isomers with the
different linkage position
Received 00th January 20xx,
Accepted 00th January 20xx
Fan liu,a Jin Tu,a Xiaorui Wang,a Jiaqiang Wang,a Yanbing Gong,a Mengmeng Han,a Xianxi Dang,a
Qiuyan Liao,a Qian Pengb, Qianqian Li,a and Zhen Li*a
DOI: 10.1039/x0xx00000x
www.rsc.org/
Two isomers of mm-TPE(PI)2 and pp-TPE(PI)2, constructed by the molecular electronic structure. And our recent research exhibited
two same aromatic blocks of tetraphenylethene and that the molecular electronic structure had a heavy influence on the
phenanthro[9,10-d]imidazole,
exhibit
totally
different molecular packing in solid state, partially due to the different
mechnoluminescence, as a result of the ignorable different linkage intramolecular charge transfer.6 Actually, as presented in the
position on the tetraphenylethene moiety. Detailed analysis and textbook of “Basic Organic Chemistry”, different linkage positions on
theoretical calculations demonstrate the structure-packing- the aromatic ring could result in the different conjugation degree
property relationship of organic mechanoluminescent luminogens, and the accompanied intramolecular charge transfer, which had also
with the emphasis on the important role of molecular packing in been confirmed by a series of blue AIEgens designed by us
solid state.
previously,7 with a para-linkage enjoying a good conjugation effect
and an meta-linkage possessing a poor conjugation effect. Thus, to
explore ML luminogens constructed by TPE and other big aromatic
blocks, the linkage mode should be investigated, to obtain more
information for the rational molecular design of ML materials.
According to the above considerations, coupled with the
cognition of its widely utilization,8 phenanthro[9,10-d]imidazole (PI)
was chosen to construct two isomers of mm-TPE(PI)2 and pp-
TPE(PI)2 with TPE (Fig. 1), in which, the linkage modes were different
between these two blocks. Really, due to the minor difference,
these two isomers demonstrated totally different ML performance:
mm-TPE(PI)2 is ML active, while not for pp-TPE(PI)2. Herein, we
would like to present the synthesis, characterization, theoretical
calculations, crystal structure, and mechanolumonescence of these
two isomers of mm-TPE(PI)2 and pp-TPE(PI)2.
For their huge potential applications in the field of displays, lighting
and stress sensing,1 and the importance in the deep understanding
of the emission mechanism, organic mechanoluminescent (ML)
luminogens, first found by Francis Bacon in 1605,2 have re-attracted
the increasing attention in recent three years upon its marriage with
the characteristic of aggregation induced emission (AIE), which
tackled the weak emission of previous organic ML molecules.3
However, so far, purely organic ML molecules with strong emission
were still very scarce, and many reported organic
mechanoluminescent AIEgens were the derivatives of
tetraphenylethene (TPE), with only modifications by small groups of
methoxy, aldehyde, or ethynyl moieties (these molecular weight are
less than 460, Chart S1). Actually, in the research filed of AIE, TPE
was a star molecule and has been utilized to successfully construct
lots of excellent AIEgens with different aromatic rings,4 especially
those bearing big π systems. Thus, if the present TPE-based ML
molecules with the modification of small substituents could be
expanded to those bearing other big aromatic constructing blocks,
the ML family should be enlarged rapidly.
The target compounds were prepared according to the
synthetic routes shown in Scheme S1. Briefly, dialdehyde-based
tetraphenylethylenes of 4 and 5 were prepared as key intermediates
through Suzuki coupling reactions between (2,2-dibromoethene-
1,1-diyl)dibenzene and monoaldehyde phenylboronic acids of 2 or 3,
respectively. Subsequently, coupled with aniline and phenanthrene-
9,10-dione, Compounds 4 and 5 were converted to mm-TPE(PI)2 and
pp-TPE(PI)2 correspondingly, which were well characterized by
nuclear magnetic resonance (NMR) spectroscopy (Fig. S1-S4), mass
spectrum, and elemental analysis.
On the other hand, as demonstrated in the previous ML
luminogens of ours and other scientists,5 the molecular packing
plays an important role for the ML property, in addition to the
UV-visible absorption spectra of mm-TPE(PI)2 and pp-TPE(PI)2
were measured in dichloromethane (Fig. S5). There were two sets of
absorption bands in the 310-370 nm region, which were attributed
to the π-π* transitions and the intramolecular charge transfer (ICT)
respectively. In comparison with pp-TPE(PI)2, the intramolecular π-
a. Department of Chemistry, Wuhan University, Wuhan, 430072, China. E-mail:
lizhen@whu.edu.cn, lichemlab@163.com.
b. Key Laboratory of Organic Solids, Beijing National Laboratory for Molecular Science,
Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China.
† Electronic Supplementary Information (ESI) available: Synthetic procedures, NMR
spectral data, photophysical properties, (TD)DFT results, and single crystal X-ray
analysis data. See DOI: 10.1039/x0xx00000x
conjugation
in
mm-TPE(PI)2
between
imidazole
and
tetraphenylethene moiety was interrupted by the meta linkage in a
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non-radiation rates of 4.11 × 107 S-1 and 19.99 × 107 S-1 for pp-
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DOI: 10.1039/C8CC03083A
TPE(PI)2 and mm-TPE(PI)2, Fig. S9 and Table S1-S3).
When scraping the solid samples of mm-TPE(PI)2 heavily at
room temperature, obvious blue light could be observed, even
under daylight (Fig. 2a). In comparison with other ML-active TPE-
based luminogens (Chart S1), the -system was much bigger due to
the presence of a big planar conjugated ring of phenanthro[9,10-
d]imidazole. As shown in Figure 2b, its ML spectrum exhibited red-
shifted emission peak at 445 nm versus those of the as-prepared
and crystalline ones (405 and 417 nm), in a good accordance to the
fluorescence emission of mm-TPE(PI)2 crystal. Interestingly, the
emissive peak at about 390 nm for the as-prepared mm-TPE(PI)2,
disappeared in its crystal, possibly due to the orderly molecular
arrangement, which suppressed the emission of individual PI
moieties (at 390 nm). On the contrary, pp-TPE(PI)2 was ML-inactive,
regardless of its similar molecular structure to that of mm-TPE(PI)2.
As confirmed by UV-visible absorption spectra and the corr
esponding DFT calculations, the conjugation degree of these two
isomers was much different, as expected in the molecular design.
Then, this difference was fully responsible to the totally different ML
property, or the molecular packing was also important? Accordingly,
single crystals of pp-TPE(PI)2 and mm-TPE(PI)2 were cultured by
Fig. 1 Chemical structures of the two isomers.
large degree, which led to its blue-shifted emission, agreeing well
with the ΔEg values calculated by the density functional theory (DFT)
(Fig. S6). We also calculated the rotational barriers in the ground
state for isolated mm-TPE(PI)2 and pp-TPE(PI)2 (Figure S7), and mm-
TPE(PI)2 is more twisted than pp-TPE(PI)2. As the same to many TPE
derivatives, mm-TPE(PI)2 and pp-TPE(PI)2 possessed the AEE or AIE
feature (Fig. S8):9 both of them emitted faint light in dilute THF
slowly evaporating their solutions in
a mixture solvent of
dichloromethane and ethanol. The unit cell of pp-TPE(PI)2 was non-
centrosymmetric with a space group of P-1, while that of mm-
TPE(PI)2 was centrosymmetric with the space group of P21/n (Table
S4). As reported in literatures,3a, 5c the non-centrosymmetric crystals
were favorable to obtaining piezoelectric properties, which could
lead to the electronic discharge at the crack surface of crystals upon
mechanical stimulation, directly resulting in the dye excitation and
generation of ML for the crystals. However, the non-
centrosymmetric pp-TPE(PI)2 was ML inactive, opposite to the
previous cases.
solution with the quantum yields ( Φ ) of 4.86% and 0.72%
F
respectively, however, their PL intensity started to increase when
the water fractions (fw) of the THF-water mixture solvent increased
to higher than 80% and 70%, respectively. At the fw of 99%, mm-
TPE(PI)2 emitted intense blue lights while the cyan one for pp-
TPE(PI)2. When the solvent changed from pure THF solution to
aggregate states in 99% aqueous mixtures, the PL quantum yield
value of pp-TPE(PI)2 reached 88.15%, 118-fold higher than that in
pure THF, demonstrating the typical AIE effect. For mm-TPE(PI)2, the
PL quantum yield increased only to a limited extent (14.72%) from
the original one of 4.86% in THF solution, exhibiting an AEE
characteristic. Obviously, the small structural change resulted in the
big difference in their luminescence characteristics, with the
different behavior of AIE and AEE. In general, for AIE luminogens,
the restriction of intramolecular motion (RIM) is widely accepted as
their internal AIE mechanism. Thus, the different fluorescent
behaviors of these two isomers should be ascribed to the different
rotational barriers between the TPE core and the PI peripheries. As
mm-TPE(PI)2 is more twisted than pp-TPE(PI)2, its intramolecular
rotation might be more restricted in THF solution. Moreover, at the
fw value of 99%, the emission peak is at 460 nm for mm-TPE(PI)2,
which is blue shifted relative to that at fw of 80% (468 nm), while
the peak of pp-TPE(PI)2 almost remains the same (494 to 493 nm)
with the fw value changing from 80% to 99%. This should be
ascribed to the morphological change, that is to say, mm-TPE(PI)2 is
more easy to crystallize when the water fraction increases. And, in
solid states, mm-TPE(PI)2 (405 nm) demonstrates much blue-shifted
emissions, in comparison with that of pp-TPE(PI)2 (458 nm), due to Fig. 2 (a) ML images of mm-TPE(PI)2 under daylight (left) and in dark
the decreased conjugation and crystallization. Interestingly, the (right) at room temperature. (b) PL spectra of as prepared (top),
same fluorescence lifetimes of about 2.88 ns were measured for crystal (middle) for mm-TPE(PI)2 and ML spectra (bottom) of mm-
these two compounds at fw of 99%, as the results of the conflicting TPE(PI)2. (c) Fluorescence decay curves of mm-TPE(PI)2 (excited at
properties of their different quantum yields and non-radiation rates 418 nm) and pp-TPE(PI)2 (excited at 480 nm) powder under
(QYs of 88.15% and 14.72% for pp-TPE(PI)2 and mm-TPE(PI)2, the different states. (d) XRD patterns of mm-TPE(PI)2.
2 | J. Name., 2012, 00, 1-3
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could not impede the intramolecular rotation anVdiewreAdrtiuclceeOntlihnee
DOI: 10.1039/C8CC03083A
energy loss via non-radiative relaxation channels under the stimulus
of a force, causing its inactive mechanoluminescence.
The powder X-ray diffraction (PXRD) spectra were also
measured to evaluate the patterns of crystals for these two
luminogens. As shown in Fig. 2d and S11, the mm-TPE(PI)2 crystal
had much better compressive resistance rather than the pp-TPE(PI)2
one. After grinding lightly (pressure was between 0.5 N and 0.7 N),
mm-TPE(PI)2 still possessed the same and even more diffraction
peaks, however, pp-TPE(PI)2 gave much weakened and less PXRD
peaks. When vigorous grinding (pressure was between 1.7 N and
1.9 N) was conducted, mm-TPE(PI)2 showed the diffraction peaks
partly and pp-TPE(PI)2 did not have any obvious peaks. Thus, the
PXRD spectra could well demonstrate the different packing degree
of their crystals and interpret the information of their different ML
properties to some extent.
Fig. 3 (a) Unit cell (left) and relevent molecular configurations (right)
of pp-TPE(PI)2. (b) Unit cell (left) and relevent molecular
configurations (right) of mm-TPE(PI)2.
In order to further investigate the influence of the molecular
configurations on their ML effect, density functional theory (DFT)
calculations were then used to obtain the dipole moments and the
ΔEg for PP-1, PP-2 and MM-1, MM-2, at the 6-31G(d, p) level (Fig. 4),
based on their ground state geometries in single crystals. In
comparison with PP-1 and PP-2, the HOMO and LOMO levels of
MM-1 and MM-2 almost totally located on the different single
molecule, whereas those of PP-1 and PP-2 mainly delocalized over
the two molecules. Consequently, the ΔEg values of MM-1 and MM-
2 were larger than those of PP-1 and PP-2. Correspondingly, the
dipole moments of MM-1 and MM-2 were 4.90 and 13.80 debye,
respectively, larger than those of PP-1 (0.01 debay) and PP-2 (0.01
debay). The larger dipole moments of the configurations and the
vertical arrangement might result in a larger net-dipole moment of
the crystalline structures, and subsequently lead to a stronger
piezoelectric effect in the mm-TPE(PI)2 crystal upon the crystal
breaking, thus contributing much to its mechanoluminescence. On
the other hand, although the pp-TPE(PI)2 crystal was non-
centrosymmetric, an anti-parallel packing of PP-1 and PP-2 might
cause the molecular slippage easily under grinding or scraping for
pp-TPE(PI)2, opening the non-radiative channel when the large
crystals were fractured into smaller ones.
As shown in Fig. 3, there were two main different molecular
configurations in the crystals of pp-TPE(PI)2 and mm-TPE(PI)2,
named as PP-1, PP-2 and MM-1, MM-2, respectively. The most
notable difference between these two crystals is the θ between the
PI rings of adjacent molecules in each configuration (Fig. S10), which
represented the packing mode of the crystal. The conformation of
pp-TPE(PI)2 was nearly anti-parallel (θ = 0.014°, 0.018°) in the PP-1,
PP-2 forms, in contrast, the conformation of mm-TPE(PI)2 was
vertical (θ = 85.728°) in the MM-1, MM-2 forms, showing that
adjacent molecules were packed in a vertical arrangement.
Interestingly, there were no π–π interactions in these two
crystals, due to the introduction of the the highly twisted TPE
moieties, which could enlarge the intermolecular distance to restrict
the π–π stacking. However, phenanthro[9,10-d]imidazole contained
two different N heteroatoms, which were typically electron rich
with lone pair electrons, leading to the increased molecular electron
density for the enhanced intermolecular interactions. As shown in
Figure 3, many C–H…N and C-H…π interactions could be observed
in crystals of these two isomers. To simplify the packing analyses,
only the strong intermolecular interactions (with distances shorter
than 3.5 Å) were summarized in Fig. 3 and Table S5-7. In the MM-1
form, strong C-H…N bonds, as short as 2.748 Å, and eight types of
C-H…π bonds with distances ranging from 2.995 to 3.445 Å, were
observed, while only one type of C-H…N bond (3.405 Å) was found
in the MM-2 form. Thus, in the mm-TPE(PI)2 crystal, most of the
strong intermolecular interactions existed in the MM-1 form.
Similarly, in the pp-TPE(PI)2 crystal, strong intermolecular
interactions mainly existed in the PP-1 form, including C-H … N
bonds as short as 2.754 Å and five types of C-H … π bonds with
distances ranging from 2.804 to 3.332 Å. Furthermore, two types of
C-H…π bonds (3.386 Å, 3.281 Å) were found in the PP-2 form. In
addition, there were two MM-1 forms in the unit cell of mm-
TPE(PI)2 crystal, but, only one PP-1 form for the unit cell of pp-
TPE(PI)2 crystal. These results demonstrated that the stronger
interactions in the mm-TPE(PI)2 crystal made it pack in a more tight Fig. 4 The HOMO (lower images) and LUMO (upper images) of the
molecular conformation, that is to say, bad conjugation but tight four conformations of pp-TPE(PI)2 and mm-TPE(PI)2 calculated at
packing. On the contrary, the looser packing of pp-TPE(PI)2 crystal the B3LYP/6-31G(d, p) level.
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(d) H. Nakayama, J. I. Nishida, N. Takada, H. Sato and Y.
Additionally, in consideration of the twist molecular
conformations, their mechanochromic effect was tested. Really, as
other members of the TPE family, they showed similar
mechanofluorochromic properties, as the results of the phase
transition from crystalline to amorphous.10 Accordingly, the
fluorescence of mm-TPE(PI)2 changed from blue to cyan by grinding,
and went back to the blue emission immediately upon fuming
treatment with dichloromethane. Similarly, pp-TPE(PI)2 could
change from cyan to green upon grinding, and back to the cyan one
again upon fuming with dichloromethane (Fig. S12). Interestingly,
after grinding, the Φ F values improved from 15.57 to 21.16% for
mm-TPE(PI)2 and from 42.34 to 44.97% for pp-TPE(PI)2,
accompanying with the fluorescence lifetimes changing from 2.41 to
4.25 ns for mm-TPE(PI)2 and 2.06 to 2.80 ns for pp-TPE(PI)2, owing
to the non-radiation rates decreased from 35.03 × 107 S-1 to 18.55 ×
107 S-1 for mm-TPE(PI)2 and from 27.99 × 107 S-1 to 16.95 × 107 S-1 for
pp-TPE(PI)2, as well as the radiation rates (from 6.46 × 107 S-1 to 4.98
× 107 S-1 for mm-TPE(PI)2 and from20.55 × 107 S-1 to 16.06 × 107 S-1
for pp-TPE(PI)2), as demonstrated in Table S1-S2.
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3
4
5
In summary, for the first time, very big planar conjugative block
of phenanthro[9,10-d]imidazole was utilized to construct the ML-
active mm-TPE(PI)2 with the star molecule of TPE. Oppositely, its
isomer of pp-TPE(PI)2 with the subtly different linkage mode
between the PI and TPE groups is ML-inactive, regardless of its
wonderful AIE characteristic and non-centrosymmetric crystal
favorable for the ML property. Careful analysis of their crystals,
coupled with theoretical calculations, demonstrated that the
vertical arrangement, larger molecular dipolar moment and tight
packing account for the outstanding ML performance, no matter the
centrosymmetric crystal of mm-TPE(PI)2. Thus, ML property is
heavily related to the intramolecular linkage mode never reported
previously, in addition to the molecular configuration and packing
mode. In other words, the regulation of intramolecular conjugation
is a good choice for expanding the ML family. Thus, the clear
relationship between the conjugation of molecules, packing in the
crystals and the ML properties might provide some guidance for the
design of efficient organic ML materials, especially, how to design
and achieve the organic “high molecular weight” ML luminogens.
This work was supported by the National Science Foundation
of China (no. 51573140, 21734007), Hubei Province (2017CFA002),
and Special funds for basic scientific research services in central
colleges and Universities (2042017kf0247).
6
7
Conflicts of interest
8
9
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There are no conflicts to declare.
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4 | J. Name., 2012, 00, 1-3
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