Side chain effects on the solid-state emission behaviours and mechano-fluorochromic activities of 10H-phenothiazinylbenzo[d]imidazoles

Article abstract of DOI:10.1039/c9ra05255k

A family of 4-cyanophenyl-substituted 10H-phenothiazinylbenzo[d]imidazoles with different side chains at the 10-position are prepared and their physical properties are studied. The detailed structure-property research demonstrates that the cold crystalliz

Full text of DOI:10.1039/c9ra05255k

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Side chain effects on the solid-state emission
behaviours and mechano-fluorochromic activities
of 10H-phenothiazinylbenzo[d]imidazoles†
Cite this: RSC Adv., 2019, 9, 30381
*
Hao Jiang, Xiao-Jing Liu, Ran-Rong Jia, Tian-Hui Xu and Min Xia
A family of 4-cyanophenyl-substituted 10H-phenothiazinylbenzo[d]imidazoles with different side chains at
the 10-position are prepared and their physical properties are studied. The detailed structure-property
research demonstrates that the cold crystallization temperature of ground samples and the emission
wavelengths of pristine samples are in good accordance with the packing density, conformation
distortion and intermolecular interactions, but emission wavelengths of ground samples are slightly
chain-dependent. For benzimidazoles with alkyl chains, longer and more branched chains can produce
looser packings, which cause pristine samples to display red-shifted emission and reduced MFC activity.
For benzimidazoles with a phenyl chain, the emission wavelengths of both the pristine and the ground
samples are remarkably red-shifted. Moreover, the degree of conformation distortion is larger, and the
cold crystallization temperature is higher. Interestingly, the homologue with the n-hexyl chain displays
an intense ML effect that is mainly attributed to the heavy discharge quantity on largely enhanced
discharge areas under force stimuli due to the great fragility of this crystal.
Received 10th July 2019
Accepted 6th September 2019
DOI: 10.1039/c9ra05255k
rsc.li/rsc-advances
a positive effect will be produced with blue-shied emission in
the pristine state and improved MFC activity. In contrast, more
Introduction
Recently, mechano-uorochromic (MFC) compounds,1 which
are smart materials that are capable of making reversible and
dynamic responses to external force stimuli by producing varied
emission wavelengths or intensities, have increasingly become
a hot topic in the eld of material chemistry. This is due to their
outstanding performances in pressure or chemical sensors,2
memory chips,3 security inks4 or papers,5 and optical data
storage devices.6 Such solid-state photophysical properties are
highly sensitive to the conformation or/and packing modes.
Since chains can effectively inuence intermolecular interac-
tions, packing modes and molecular conformations through
the control of the chain length or the degree of branching, side-
chain engineering has been a practical and convenient method
for successfully tuning the MFC performances of many systems
in recent years.7 However, the side chain effect is very compli-
cated and emission wavelength shis in response to force
stimuli are uncertain. If side chains can effectively separate
molecules from each other to generate a large enough free
volume in which molecules adopt more twisted conformations,
free volume due to side chain extension may cause the confor-
mations to be more planar, and a negative effect will be pre-
sented with red-shied emission in the as-prepared state along
with weakened MFC activity. When conformations cause slight
changes in side chain separation, the force-induced shis will
be slightly sensitive to different alkyl chain lengths. In litera-
ture, the cases with positive8 or negative9 or small side effects10
have been reported. Occasionally, emission shis before and
aer force application are rstly increased and then
decreased,11 or vice versa,12 as alkyl chains are extended. Hence,
for a given uorophore and its homologues, the side chain
effect on their MFC performances is difficult to predict and
should be investigated individually. So far, for most reports on
the side chain effect, only experimental results have been pre-
sented and detailed structure-property studies in view of
molecular conformations and intermolecular interactions are
seldom carried out. In this work, deep insight into the rela-
tionship between side chains and MFC behaviours will be
afforded for a family of homologues by means of X-ray crystal-
lography and theoretical calculations.
Recently, substituted imidazoles have been reported to be
MFC-active systems.13 Most cases are focused on polycyclic
fused 1,2-diarylimidazoles,13b while only one report on 1,2-dia-
rylbenzo[d]imidazoles can be tracked in literature. In order to
extend the research in this eld, a family of 1,2-diarylbenzo[d]
imidazoles, with 4-cyanophenyl and phenothiazine (PTZ) as the
Department of Chemistry, Zhejiang Sci-Tech University, Hangzhou 310018, P. R.
China. E-mail: xiamin@zstu.edu.cn
† Electronic supplementary information (ESI) available: 1H and 13C NMR spectra,
absorption and emission spectra, PXRD patterns, DSC curves, X-ray
crystallography and others. CCDC 1919632–1919637, 1919652. For ESI and
crystallographic data in CIF or other electronic format see DOI:
10.1039/c9ra05255k
electron-donating
and
electron-withdrawing
group,
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respectively, has been conveniently obtained by the introduc-
tion of different side chains at the 10-position on PTZ. PTZ is
a unique building block with a high HOMO level and a bowl-
shaped conformation, which is endowed with the capability of
displaying good performance in thermally activated delayed
uorescence,14 aggregation-induced emission15 and the MFC
effect.16 The vast majority of reports on the chain effect have
only considered straight-chain alkyl groups, and little attention
has been paid to branched alkyls,17 while aryl chains have never
been considered. Herein, we have conducted a systematic study
on the MFC behaviours of 1,2-diarylbenzo[d]imidazoles (BIMP)
having unbranched and branched alkyl chains along with
a phenyl chain on the PTZ ring.
Fig. 2 Absorption (solid lines) and emission spectra (dashed lines) of
BIMP2 in different solvents (A); absorption (solid lines) and emission
spectra (dashed lines) of BIMP members in dichloromethane solution (B).
Results and discussion
Synthesis
slightly dependent on the solvent polarity, while the emission
wavelength (lem) maxima are sensitive to it. As the polarity
increases, lem are gradually red-shied and a positive sol-
vatochromic effect is produced. However, the degree of such an
effect is not so large that a slight shi cannot be further
generated in a solvent whose polarity is higher than that of
dichloromethane. This result indicates that an intramolecular
charge transfer (ICT) should occur in the excited state, however,
the extent of such a transfer is not great. Additionally, it was
found that the absorption and the emission band in each
solvent were almost completely isolated with little overlap,
which implies that the excited state experiences a considerably
geometric relaxation before going back to the ground state.
Actually, the rotations of 4-cyanophenyl and PTZ around the
benzimidazole core along with the ipping of the PTZ ring and
the pendulation of the ethyl group facilitate the dissipation of
excitation energy and the emergence of a very large Stoke's shi.
The similar absorption and emission behaviours in solution
also take place on other BIMP members (Fig. S1–S6, ESI†).
The absorption and emission spectra of BIMP members in
dichloromethane were surveyed (Fig. 2B), which indicated that
the lab of all BIMP uorophores with alkyl chains were almost
identical, likewise their lem. This means that alkyl chains have
no inuence on the photophysical properties of these
compounds in the isolated state. For BIMPBZ, the chain effect of
the phenyl group is slightly different from those of alkyls, since
the lab and the lem of BIMPBZ cause a slight red and blue shi,
respectively, as compared with those of other BIMP members.
The BIMP members can be readily obtained in good yields via
a three-step process (Fig. 1). The aromatic nucleophilic substi-
tution takes place between 2-uoronitrobenzene and 4-cya-
noaniline to produce 4-(2-nitrophenyl amino)benzonitrile 1 in
77% yield as an orange solid. 4-(2-Aminophenyl amino)benzo-
nitrile 2 was obtained in 88% yield in the presence of hydrated
hydrazine under the catalysis of Pd/C. Such reduction displays
the obvious colour change from orange to greyish white. The
subsequent cyclization can be smoothly carried out in 64–87%
yield by the condensation of 2 and N-substituted 10H-
phenothiazine-3-carbaldehyde in DMSO under an air atmo-
sphere. For this cyclization, DMSO acts as the oxidant and it is
better to carry out this reaction in an open vessel in order to
accelerate the rate. All the products and the intermediates were
1
determined by H and 13C NMR spectroscopy to obtain satis-
factory data (ESI†). All the BIMP homologues are solids with
intense emission under UV light irradiation and their crystals
are easily obtained by recrystallization from n-hexane/CHCl3
mixture.
Photophysical properties in solution
BIMP2 was selected to investigate its absorption and emission
behaviors in solution, and the corresponding spectra are pre-
sented in Fig. 2A. The absorption wavelength (lab) maxima are
X-ray crystallography
Sharply different from the isolated state, the physical properties
of uorophores in the solid-state are closely related to confor-
mations and intermolecular interactions. Hence, BIMP
members with different side chains will certainly have distinct
optical features and MFC behaviors. In order to explore the
structure-property relationship between the side chain and
emission wavelength in the pristine state, we rstly surveyed the
crystalline structures of these homologues in view of packing
density, molecular conformations (Table 1) and intermolecular
interactions (Table 2).
The single crystals of BIMP members can be grown from an
n-hexane/CHCl3 binary mixture by slow evaporation in air. X-ray
Fig. 1 Synthetic route to the title compounds BIMP.
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Table 1 Dihedral angles (q), bond angles (f) and packing density (D) of
BIMP members in the crystalline phase
Compd
q1 (ꢀ)
q2 (ꢀ)
q3 (ꢀ)
FN (ꢀ)
FS (ꢀ)
D (g cmꢁ3
)
BIMP1
BIMP2
BIMP6
BIMP6t
BIMP8i
BIMP8
BIMPBZ
47.43
51.94
50.21
51.45
61.82
62.41
52.02
31.55
54.33
41.74
36.89
23.56
15.14
68.83
—
—
—
—
—
—
74.87
98.01
98.56
98.60
96.91
97.58
96.64
100.70
118.19
120.03
118.27
116.32
119.04
115.45
122.27
1.351
1.329
1.277
a
—
1.223
1.207
1.374
a
Density cannot be precisely obtained as the crystal contains solvent
molecules.
Fig. 3 Molecular packing of BIMPBZ (A) and BIMP1 (B).
crystallography demonstrates that no C–H/p interactions
emerge in each tested crystal (Fig. 3, S7, S8† and 6A). Except for
the BIMPBZ crystal, no p/p interactions can be found in other
BIMP crystals with alkyl chains, where conformations are xed
by C–H/N and C–H/S interactions along with van der Waals
forces. This indicates very good accordance between the
packing density and the conformation distortion. In BIMP
crystals with alkyl chains except BIMP1, the conformations
gradually become more twisted as the packing density goes up,
when the sum of the two dihedral angles q1 and q2 is consid-
ered. In BIMP2, BIMP6 and BIMP8 crystals, the free volumes are
drastically improved due to the extension of the unbranched
alkyl chains, which permit the molecules to produce less
twisted conformations. In the BIMP8i crystal, there is a larger
packing density and a more twisted conformation as compared
to the BIMP8 crystal, since the reduced free volume triggered by
the shortening of the chain length is unable to be fully offset by
the improved one due to branching of the side chain. In the
BIMP6t crystal, however, the highly branched 2,2-dimethylbutyl
chain leads to the denser packing and more twisted confor-
mation than in the BIMP6 crystal, owing to the considerably
increased free volume that effectively conquers the reduced one
induced by the shortening of the alkyl chain length. These
results indicate that both the length and branching degree of
a side chain should be considered when MFC-tunable systems
are designed, and it seems that the latter has a greater impact
on conformations in the crystalline phase than the former.
Although the packing density in the BIMP1 crystal is the
largest for BIMP members with alkyl chains, the conformation
is abnormally less twisted with smaller dihedral angles. Unlike
all the other BIMP crystals, where the alkyl chains on PTZ and
the C]N bonds on imidazoles are located at both sides of the s-
bond linking the PTZ moiety and the imidazole ring, the methyl
chain and C]N bond are on the same side in the BIMP1 crystal,
which results in intramolecular C–H/S interactions. These
effectively reduce the free volume and make the less twisted
conformation preferable, even in dense packing (Fig. 3B). In the
ꢀ
BIMPBZ crystal (Fig. 3A), the strong p/p interaction (3.417 A)
between the phenyl chain and 4-cyanophenyl considerably
shortens the distance between adjacent molecules. Moreover,
the H-bonds in this crystal are remarkably stronger than those
in the BIMP2 crystal where the conformation is the most twisted
among BIMP homologues with alkyl chains. Therefore, the
shorter H-bonds together with the strong p/p interactions
cause the BIMPBZ crystal to have the largest packing density and
the highest conformation distortion in the BIMP family. Addi-
tionally, only very slight changes in bond angles fN and fS occur
on the PTZ moiety in BIMP crystals with different side chains,
which indicates that the conformational differences in these
crystals should be triggered by the side chain effect instead of
the opening and closing of the PTZ ring skeleton.
Table 2 Intermolecular interactions in crystals of BIMP members
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
Compd C–H/N1 (A) C–H/N2 (A) C–H/N3 (A) C–H/S (A) p–p (A)
BIMP1
BIMP2 3.357
BIMP6
BIMP6t 3.100
—
2.674
2.778
—
2.506
2.876
2.657
2.706
—
3.242a
3.102
3.458
3.011
3.260
3.559
3.086
—
—
—
—
—
—
3.417
—
—
BIMP8i
BIMP8
—
—
2.550
2.687
—
—
2.589
BIMPBZ 2.955
a
This H-bond is intramolecular.
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effectively overcomes the blue-shied one induced by highly
conformational distortion. Under force stimuli, the dihedral
angle q3 between the phenyl chain and the PTZ ring may also be
greatly reduced, causing the p-conjugated areas on BIMPBZ
molecules to be extended and the emission wavelength in
amorphous phase to be red-shied.
Chain-dependent solid-state physical property
It is well known that the emission behaviours of crystals are
closely related to molecular conformations and intermolecular
interactions. For homologues with similar types of intermolec-
ular interactions, their emission performances are dominantly
dependent on molecular conformations. Hence, the structure-
property study in this BIMP family should be focused on
conformations in the pristine state. The emission wavelengths
of BIMP samples before and aer force stimuli are listed in
Table 3.
Powder X-ray diffraction (PXRD) demonstrates that the MFC
activity of BIMP members could be attributed to the dynami-
cally reversible morphological transformations between the
crystalline and amorphous phases (Fig. S9–S15, ESI†). The
PXRD pattern of each as-prepared sample is almost identical to
that simulated by X-ray crystallography, which means that each
pristine sample has the same crystalline structure as the cor-
responding single crystal. By solvent fuming or annealing, the
highly dispersed bands on PXRD patterns of ground samples
can be completely restored to the original sharp and intense
peaks of the as-prepared ones. At the same time, the emission
spectra of ground samples can be fully recovered to those of as-
prepared ones (Fig. 4 and S16–S18, ESI†). However, the emis-
sion behavior and PXRD pattern of the ground BIMP6t sample
can only be reproduced into those of the pristine ones by
fuming in CHCl3. X-ray crystallography indicates that equimolar
ratios of CHCl3 molecules are locked in the caves of the BIMP6t
For BIMP homologues with alkyl chains, it has been
demonstrated that there is good accordance between emission
wavelengths and molecular conformations in the crystalline
phase. As the conformational distortion is gradually reduced,
the emission wavelength of the corresponding crystal is also
progressively red-shied because smaller dihedral angles can
cause molecules to contain larger p-conjugated area. Except for
BIMP1 with the special orientation of the PTZ group, the dihe-
dral angle q2 is greatly reduced but q1 is not monotonically
varied as the length and branching degree of alkyl chains are
enhanced. Overall, compared with the considerable alteration
in q2, there was only a slight change in q1. Hence, it appears that
it is q2 that has a dominant inuence on the emission wave-
length of the corresponding crystal. For BIMP members with
alkyls, except for BIMP8i, it was observed that the emission
wavelengths of the ground samples were almost identical. We
assume that hand grinding can just destroy lattices and pla-
narize the conformations but it is not strong enough to
compress the distances between adjacent molecules, which are
sensitive to the length and branching degree of the side chains.
It has been veried that only GPa-order high pressure can
shorten such distances.18 Consequently, the ground BIMP
samples with alkyl chains present almost identical emission
wavelengths, since alkyls make very little contribution to the p-
conjugation on the planarized molecules. However, the BIMP8i
ground sample displays the shorter emission wavelength and
the reason is unknown at the present time. Although BIMPBZ
molecules have the most twisted conformations among all
BIMP members, the emission wavelength of the BIMPBZ crystal
is remarkably red-shied as compared to that of the BIMP2
crystal in which the conformation is the most twisted among
BIMP homologues with alkyl chains. This indicates that the red-
shied emission triggered by the dissipation of excited energy
due to the strong p–p interaction in the BIMPBZ crystal
crystal by strong H-bonds between Nimidazole and HCHCl
3
(Fig. S8A, ESI†). Due to the irregularity and the intensely
thermal motion of 2,2-dimethylbutyl, the BIMP6t molecules in
the ground sample where CHCl3 is released from broken lattices
can hardly be packed into the original crystalline structure by
short-time annealing without a CHCl3 atmosphere.
Moreover, the thermal properties of BIMP members are
tested by differential scanning calorimetry (DSC). On the DSC
curves of all ground samples except BIMP6t, the exothermal
peaks (cold crystallization) can clearly be observed (Fig. S19†).
This indicates that the repeat of the original emission behav-
iours by annealing treatment could be ascribed to cold
Table 3 Emission wavelengths of BIMP members under different
conditions
lcry
lam
lcry
lam
Compd (nm) (nm) Dla (nm) Compd (nm) (nm) Dla (nm)
BIMP1
BIMP2
BIMP6
475
453
467
503
503
502
501
28
50
35
30
BIMP8
BIMP8i 480
BIMPBZ 476
490
502
490
513
12
10
37
Fig. 4 Solid-state emission spectra of BIMP2 (A), BIMP8 (B), BIMP8i (C)
and BIMPBZ (D) under different conditions (inset: photos of samples
taken under 365 nm UV light).
BIMP6t 471
a
Dl ¼ lam ꢁ lcry
.
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crystallization. Conformational changes from the crystalline of the BIMP6 crystal is so intense that luminescence can be
phase to the amorphous phase will produce inner stress in readily seen even under daylight conditions (Fig. 5C). It has
ground samples, which can be mostly or fully relieved under been demonstrated that the emission wavelength (498 nm) on
heat treatment, therefore, the temperature of cold crystalliza- the ML spectrum is very close to that on the PL spectrum (502
tion indicates the strength of the inner stress. Under vigorous nm) of the ground sample (Fig. 5A). Such similarity implies that
grinding, the lattices are completely destroyed and the confor- the BIMP6 crystal is very readily broken into amorphous parti-
mations may well be entirely planarized. Hence, the tempera- cles on exposure to force stimuli while the photo- or the force-
ture of cold crystallization (TCC
) implies the extent of induced emission may well be derived from the same excited-
conformational distortion in the crystalline phase. Overall, the state species.
level of TCC is in good accordance with the degree of confor-
X-ray crystallography indicates that BIMP6 molecules are
mational distortion in this BIMP family. The TCC of BIMPBZ is packed into a crystal with a monoclinic system and P21 chiral
the highest (114.7 ꢀC) while the TCC of BIMP8 is the lowest (70.0 space group. Intriguingly, such a chiral crystal is constructed by
ꢀ
ꢀC). Moreover, the TCC of BIMP8i (85.8 C) is higher than those molecules without any chiral units. It has been demonstrated
ꢀ
ꢀ
ꢀ
of BIMP6 (89.2 C) and BIMP2 (98.7 C), as the former has the that only two types of H-bonds (C–H/Ncyano 2.657 A and
ꢀ
less twisted conformation than the latter. Although the extent of C–H/S 3.458 A) emerge in the BIMP6 crystal without p–p and
the conformational distortion of BIMP1 and BIMP8 is very close C–H/p interactions (Fig. 6A). These H-bonds arrange the
to each other, the TCC of the former (84.3 ꢀC) is remarkably molecules into one-dimensional chains on the b–c plane and
higher than that of the latter. It is presumed that the inner very slight interactions occur among chains. In each chain, the
stress from intramolecular HS-bonds in the BIMP1 sample may adjacent molecules are respectively drawn together by N–H and
consume more heat energy to be relieved. For the ground S–H bonds to form couple I and II (Fig. 6C). Moreover, little
BIMP6t sample, neither cold crystallization nor melting peaks interactions appear among molecules along the a-axis. Hence,
can be observed on the DSC curve and it is gradually upraised as the crystal is extremely fragile and readily splits along the
the temperature increases, which is characteristic of an amor- directions of packed chains and a-axis with the application of
phous sample. Additionally, it was shown that the melting external force (Fig. 6B) to produce many fresh surfaces. It has
temperatures of BIMP1, BIMP2 and BIMPBZ crystals are been reported that the analysis of net dipole moments on
considerably higher than those of BIMP6, BIMP8 and BIMP8i, as couples could properly interpret the reason for ML activity for
a larger packing density means stronger intermolecular inter- a crystal;20b we carried out the DFT calculations at the B3LYP/6-
actions and more stable lattices that have to be destroyed at 31G (d, p) level based on the conformations in the crystalline
higher temperatures.
phase. It has been demonstrated that the dipole moment of an
individual molecule is just 1.886 Debye, while the net dipole
moments of couple I and II are 3.688 and 1.548 Debye (Fig. 6D),
respectively. Therefore, the non-zero net dipole moments can
Mechanoluminescence effect on BIMP6
Mechanoluminescence (ML), also known as triboluminescence, trigger discharges between couples on those freshly cracked
is a kind of unique luminescence triggered by mechanical surfaces under external force, which excites the molecules on
excitation (motion or stress force) of solid materials, which was such surfaces either by electron impact or ionization followed
rstly discovered by Francis Bacon in 1605. Unlike a great deal by electron–hole recombination, similar to electrolumines-
of MFC-active systems, ML-active systems composed of purely cence. However, these discharges are not strong enough to
organic molecules are relatively limited.19 Although there are result in the intense ML effect as the net dipole moments of
a few cases where centrosymmetric crystals exhibit excellent ML couples are still relatively small. Owing to the particular chain
effects,20 most reported ML-active crystals still have non- packing in the BIMP6 crystal, so many fresh surfaces can be
centrosymmetric or even chiral space groups. It is generally readily generated to provide largely increased discharge areas
believed that ML activity may well originate from the piezo- when the crystal is exposed to force stimuli, on which heavy
electric effect, and non-centrosymmetric crystals are preferable discharge quantities can be offered and may well be mainly
for producing electron discharge due to the non-zero net dipole responsible for the excellent ML activity of this crystal.
moment under the force-induced separation of amphoteric
charges. Such discharge can excite molecules on and very close
to freshly cracked surfaces to produce luminescence. Certainly,
it is reasonable that both the high discharge strength caused by
large net dipole moments and heavy discharge quantities
provided by enhanced discharge surface areas are helpful for
crystals to present excellent ML performance.
It is interesting to nd that the BIMP6 crystal is highly ML-
active; a very brightly greenish-yellow ashlight can be clearly
seen when the crystal is lightly pressed with a stainless steel
spatula (Fig. 5B). Sharply different from most other purely
organic ML-active molecules whose luminescence can only be
observed in the dark by application of heavy force, the ML effect
Fig. 5 PL and ML spectra of BIMP6 samples (A); photos of the ML
phenomenon on a BIMP6 crystal taken in the dark (B) and in daylight (C).
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emission wavelengths of their pristine samples are gradually
red-shied due to the progressively diminished extent of
conformation distortion as the length or the branching degree
of alkyl chains is increased, while the MFC activity is corre-
spondingly reduced due to emission wavelengths in the amor-
phous phase that are almost the same. The intramolecular S–H
bonds in BIMP1 crystals caused by the unique orientation of
PTZ can account for the unexpectedly less twisted conforma-
tion. Although the densest packing appears in the BIMPBZ
crystal with the most twisted conformations, the strong p–p
interactions make a great contribution to the remarkably red-
shied emission wavelength. Meanwhile, the enlarged p-
conjugation area generated by the planarization of the phenyl
chain under force stimuli causes the emission wavelength of the
ground BIMPBZ sample to be more red-shied as compared to
those of other ground BIMP samples. Additionally, the MFC
activity of this family can be ascribed to morphological changes
and the cold crystallization temperature is in good accordance
with the degree of conformational distortion. X-ray crystallog-
raphy and DFT calculations reveal that the very bright lumi-
nescence from the ML effect on BIMP6 crystals can mainly be
attributed to large discharge quantity rather than high
discharge strength due to the extreme fragility of this crystal
caused by particular chain packing.
Conflicts of interest
There are no conicts to declare.
Acknowledgements
We are grateful for nancial support from the Natural Science
Fund of Zhejiang Province (LY19B020015). We also thank Prof.
Zhenguo Chi and Dr Zongliang Xie very much for their kindly
help on ML spectra measurements. We thank Dr Jiyong Liu very
much for his kindly help on X-ray crystallographic measure-
ments and analysis.
Fig. 6 Molecular packing in the BIMP6 crystal (A) and schematic
diagram of the crystal before and after force stimuli (B); couples in this
crystal (C) and calculated energy levels and electron distribution on the
HOMO and LUMO of molecules in these couples (D).
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Meanwhile, the great fragility of the crystal leads to the quite
easy transformation from the large-sized particles into the tiny
ones, which makes the ML spectrum very close to the PL one in
the amorphous phase.
Conclusion
We have obtained a family of PTZ-substituted benzimidazoles
with different side chains and carried out detailed structure-
property research regarding MFC performance and molecular
conformations in the crystalline phase. It has been demon-
strated that conformation analysis is a suitable way to reveal the
intrinsic solid-state emission behaviours of homologues with
similar types of intermolecular interactions and packing
modes. Overall, for BIMP homologues with alkyl chains, the
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