Phenanthroimidazole derivatives with minor structural differences: crystalline polymorphisms, different molecular packing, and totally different mechanoluminescence

Article abstract of DOI:10.1039/c9tc05218f

Two crystalline polymorphs of tPTI-Bpin displayed opposite mechanoluminescence (ML) performance, and had different space groups of P2₁/c (ML-active tPTI-B1) and P1 (ML-inactive tPTI-B2), respectively. For comparison, another analogue molecule o

Full text of DOI:10.1039/c9tc05218f

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Phenanthroimidazole derivatives with minor
structural differences: crystalline polymorphisms,
different molecular packing, and totally different
Cite this: J. Mater. Chem. C, 2019,
7, 13759
Received 23rd September 2019,
Accepted 22nd October 2019
mechanoluminescence†
a
Yun Yu,a Yuanyuan Fan,a Can Wang,a Yao Wei,a Qiuyan Liao,a Qianqian Li
and
DOI: 10.1039/c9tc05218f
ab
Zhen Li
*
rsc.li/materials-c
Two crystalline polymorphs of tPTI-Bpin displayed opposite
Thanks to the efforts of scientists, many luminogens with
mechanoluminescence (ML) performance, and had different space strong ML emissions have been reported, and the inherent
groups of P21/c (ML-active tPTI-B1) and P1 (ML-inactive tPTI-B2), mechanism has been explored step by step.5 And actually, the
%
respectively. For comparison, another analogue molecule of PTI- results from the crystalline polymorphism contribute much to
Bpin was designed. Careful analysis of their crystal structures the deep understanding of the interesting ML phenomena.6
demonstrated that the special triangle-like molecular packing Also, some isomers gave important information on the influence of
mode of tPTI-B1 mainly accounted for its distinctive ML behavior. the molecular packing on the ML property, in which the only
Regardless of its minor difference in chemical structure from that of difference is the linkage position of the aromatic blocks. For
tPTI-Bpin, PTI-B without the ML property has similar parallelogram- example, two isomers of a phenanthro[9,10-d]imidazole derivative
like molecular packing to tPTI-B2, further confirming that the ML exhibited opposite ML properties.7 Thus, all the cases of poly-
property is highly related to the molecular packing rather than the morphisms and isomers confirmed the importance of the molecular
chemical structure.
packing in addition to the molecular structure, demonstrating the
urgent requirement for exploring the structure-packing-property
Organic luminogens have attracted increasing interest due to relationship. Accordingly, more ML luminogens should be obtained
their wide applications in OLEDs (organic light-emitting to provide more information, especially those polymorphisms.
diodes), organic lasers, stimuli-responses, bioimaging and so
Analyzing the reported ML polymorphisms carefully, in many
on.1 Besides the electronic nature of molecular structures, the cases, the information of molecular packing was collected from
molecular packing plays a key role in realizing high functional just the two polymorphs of one ML luminogen, and there were no
performance, exhibiting the Molecular Uniting Set Identified other analogues with minor structural differences for comparison,
Characteristic (MUSIC).2 As typical examples, the crystalline although some important information could be expected from the
polymorphisms of organic molecules demonstrated different analogues. From this point, we would like to design polymorphs
and even opposite properties, regardless of their same chemical with ML properties and comparable analogues. Accordingly,
structures.3 This case also occurred for purely organic lumino- based on our previous work, 1,2-diphenylphenamimidazole was
gens with mechanoluminescence (ML), which is a unique light selected as the basic aromatic ring, and the boronic ester group
emission induced by external mechanical stimulation, but not was introduced to construct tPTI-Bpin, since the aryl boronic ester
normal photo excitation (Chart S1, ESI†).4,6 Thus, to realize group could benefit the ML property for its enhancement of the
good ML performance, the molecular packing of luminogens efficient intermolecular interactions.8 For comparison, PTI-Bpin
should be paid much attention, in addition to their chemical was also synthesized, which had almost the same chemical
structures.
structure to tPTI-Bpin, but only without the tert-butyl moiety
(Fig. 1a). Excitingly, through simple slow evaporation in different
solutions, two crystalline polymorphs (tPTI-B1 and tPTI-B2) of
tPTI-Bpin were cultured with different space groups of P21/c and
a Sauvage Center for Molecular Sciences, Department of Chemistry,
Wuhan University, Wuhan 430072, China. E-mail: lizhen@whu.edu.cn,
lichemlab@163.com; Fax: +86-27-68755767
%
P1, and one crystalline (PTI-B) of PTI-Bpin was obtained with the
b Institute of Molecular Aggregation Science, Tianjin University, Tianjin 300072,
space group of C2/c. The tPTI-B1 crystal displayed the ML
emission, while others are ML inactive. Interestingly, PTI-B had
a similar molecular packing like tPTI-B2, and well explained their
ML inactivity, further confirming the importance of molecular
packing. Herein, we present the syntheses, ML characterizations,
China
† Electronic supplementary information (ESI) available: Synthetic procedures,
NMR spectral data, photophysical properties, and single crystal X-ray analysis
data. CCDC 1948484, 1948485 and 1948492. For ESI and crystallographic data in
CIF or other electronic format see DOI: 10.1039/c9tc05218f
This journal is ©The Royal Society of Chemistry 2019
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Fig. 1 (a) Chemical structures of tPTI-Bpin and PTI-Bpin. (b) ML spectrum of tPTI-Bpin, and PL spectra of tPTI-Bpin and PTI-Bpin in different states
(inset: the images of crystal PTI-B by optical microscope Leica M123). (c) ML photo of tPTI-Bpin/B1. (d) The images of crystals tPTI-B1 and tPTI-B2 by
optical microscope Leica M123, respectively.
crystal analyses and theoretical calculations of tPTI-Bpin and group on the outer phenyl ring compared to tPTI-Bpin. The
PTI-Bpin in detail, to understand the structure-packing-property morphology images of these crystals are shown in Fig. 1, which
relationship.
could be easily distinguished by the naked eye. All the three
The synthetic routes of tPTI-Bpin and PTI-Bpin are illustrated crystals emitted intense deep blue fluorescence under UV
in Scheme S1 (ESI†). The key intermediate compounds of tPTI-Br illumination at room temperature (Fig. 1b and Table 1). Crystal
and PTI-Br were synthesized according to the literature.9 The tPTI-B1 showed a similar emission spectrum to the powder of
target compounds were synthesized through the typical Miyaura tPTI-Bpin, with the emission peaks locating at 414 and 411 nm,
Borylation reaction,10 and purified by silica gel chromatography respectively. Crystal tPTI-B2 shows a different emission band
followed by recrystallization. All the target compounds were shape with the main peak at 419 nm and a shoulder at 401 nm,
characterized by 1H and 13C NMR, mass spectrometry and due to its different molecular packing mode from that of tPTI-
elemental analysis. The thermogravimetric analysis (TGA) indi- B1.3,11 Similar emission spectra were observed for crystal PTI-B
cated the good thermal stability for tPTI-Bpin and PTI-Bpin, of and the powder of PTI-Bpin with the main emission peaks at
which the decomposition temperatures (Td) are 322 1C and 410 and 411 nm, respectively and the shoulders at 430 nm.
302 1C respectively (Fig. S2, ESI†). The UV-visible spectra of Moreover, their PL quantum yields (fF) and fluorescence life-
tPTI-Bpin and PTI-Bpin were measured in dilute tetrahydro- times (t) were measured in solid states (Table 1 and Fig. S1,
furan (THF) solution. As shown in Fig. S1 (ESI†), they have ESI†). The PL quantum yields of tPTI-Bpin in different solid
similar absorption spectra with two absorption bands centred states are generally higher than that of PTI-Bpin, and all of
at about 261 and 330 nm, which are attributed to the p–p* local them have very short lifetimes ranging from 1.37 to 2.71 ns.
electron transition of the conjugated system. Similar photo- When scraping the tPTI-B1 crystal or the crystalline powder of
luminescence (PL) spectra of tPTI-Bpin and PTI-Bpin in tPTI-Bpin at room temperature, bright blue emission centred at
solution are also observed in Fig. S1 (ESI†) with the maximum 427 nm could be observed without UV irradiation (Fig. 1b and c).
emission wavelength at 397 and 386 nm, respectively.
However, no emission was observed for the tPTI-B2 or PTI-B
The single crystals of tPTI-Bpin and PTI-Bpin were cultured crystal. This extremely opposite ML performance in different
by solvent evaporation at room temperature, with their struc- polymorphs of tPTI-Bpin should be related to the different
tural data summarized in Table 1. Two polymorphs of tPTI- molecular packing modes in the crystals, which should also
Bpin were obtained by changing the solvent mixture: prism-like account for the different ML properties of PTI-Bpin and tPTI-
crystal tPTI-B1 was grown from a solvent mixture of methanol Bpin. To further explore the structure-ML property relationship,
and dichloromethane (DCM), another block-like crystal tPTI-B2 more investigations and analyses should be conducted based on
was from a solvent mixture of n-hexane and DCM. However, single crystals and other experimental results.
there was only stick-like crystal PTI-B obtained from different
The powder X-ray diffraction (PXRD) of the samples in different
solvent mixtures for PTI-Bpin, which only lacks a tert-butyl states was carried out. As readily seen, the as-prepared crystalline
Table 1 Optical properties and single crystal information of tPTI-Bpin and PTI-Bpin
Emission (nm)
PLQY (%)
Lifetime (ns)
Crystal system
Space group
Symmetry
ML activity
tPTI-Bpina
tPTI-B1b
tPTI-B2b
PTI-Bpina
PTI-Bb
411
414
419
411
410
29.14
36.62
58.04
14.94
10.72
1.84
1.38
2.71
1.84
1.37
—
—
—
Inactive
Active
Inactive
Inactive
Inactive
Monoclinic
Triclinic
—
P21/c
Centrosymmetric
Centrosymmetric
—
%
P1
—
C2/c
monoclinic
Centrosymmetric
a
b
tPTI-Bpin and PTI-Bpin as prepared powder;. tPTI-Bpin and PTI-Bpin in the crystalline state.
13760 | J. Mater. Chem. C, 2019, 7, 13759--13763
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single crystal information of PTI-Bpin (PTI-B) and two poly-
morphs of tPTI-Bpin (tPTI-B1 and tPTI-B2). The space group of
%
PTI-B is C2/c, while P21/c for tPTI-B1 and P1 for tPTI-B2,
respectively. All of them are centrosymmetric, indicating that
there are no piezoelectric effects in these crystals.4c,12 The
intermolecular and intramolecular interactions in the unit cells
of all three crystals were analysed carefully, with the results
listed in Tables S1–S4 (ESI†). The single molecules in the three
crystals all possess twisted structures, and the outer phenyl
rings both twist towards the planar phenanthroimidazole (PTI)
core with different torsion angles (Fig. S4, ESI†). Among them,
tPTI-B1 possesses the most twisted structure with two torsion
angles of 108.0071 and 29.0971, respectively. Due to the different
intramolecular interactions of the three crystals, the side tert-
butylbenzene group in tPTI-B1 is closer to the PTI core, while the
side phenyl rings in tPTI-B2 and PTI-B are closer to the aryl boronic
ester moiety. Thus, tPTI-B1 demonstrated a totally different twisted
conformation of a single molecule compared to tPTI-B2 and PTI-B,
leading to different molecular packing modes of tPTI-B1 and
tPTI-B2, and PTI-B (Fig. S3, ESI†).
Fig. 2 The PXRD patterns of (a) tPTI-B1, (b) tPTI-B2, and (c) PTI-B in
crystalline form, after grinding for 1 min and 5 min. (d) The DSC curves of
tPTI-Bpin and PTI-Bpin in different states.
As illustrated in Fig. 3a, there are three types of molecular
powders of tPTI-B1, tPTI-B2 and PTI-B all exhibited non-negligible dimers with efficient C–HÁ Á ÁO, C–HÁ Á ÁN and C–HÁ Á Áp inter-
peaks due to their good crystallinity (Fig. 2). Then, the as-prepared actions existing in the unit cell of the ML active crystal tPTI-B1.
crystalline powders were ground for different durations by using a Dimer-1 possesses an antiparallel conformation with 4 C–HÁÁÁp
mortar and pestle to give two different degrees of grinding (3.593–3.633 Å) and 8 C–HÁÁÁO (2.663–3.987 Å) interactions.
powders. For tPTI-B1, some diffraction peaks at around 241 Dimer-2 displays a parallel conformation with 7 C–HÁÁÁp (2.693–
disappeared after grinding for 1 minute. When 5 minutes of 3.971 Å) and 7 C–HÁÁÁN (2.746–3.975 Å) interactions. And mole-
continuous grinding was conducted, almost all of the diffraction cules in dimer-3 stack in a nonparallel conformation with a
peaks disappeared. Accordingly, we could speculate that the lattice certain angle and 11 C–HÁÁÁp (3.136–3.932 Å) and 4 C–HÁÁ ÁN
of the tPTI-B1 crystal is easily destroyed under mechanical (3.201–3.900 Å) interactions. With efficient intermolecular inter-
stimulus, and the crystal is breakable to some extent. On the actions, the three types of dimers form a triangle-like conforma-
contrary, the sharp and high diffraction peaks of tPTI-B2 and tion in the unit cell of tPTI-B1, which is stable to some extent and
PTI-B still remained even after grinding for 5 minutes, indicating
their relatively high lattice stability and high hardness. The PXRD
results could well demonstrate the different hardness and lattice
stabilities of these crystals, which may affect their ML perfor-
mances, since the ML process is often accompanied by crystal
breakage. As depicted in the differential scanning calorimetry
(DSC) curves, no obvious Tg was found for the tPTI-B2 crystal. The
tPTI-B1 crystal showed a lower glass transition temperature (Tg) of
139 1C than PTI-B (170 1C), also indicating the higher lattice
stability of PTI-B than that of tPTI-B1. The two polymorphs of
tPTI-Bpin exhibited extremely opposite ML properties along with
their different crystal lattice stabilities: the ML-active tPTI-B1
showed a relatively low lattice stability, while tPTI-B2 had a high
lattice stability without the ML property. For another isomer of
PTI-Bpin, its crystal PTI-B showed a similar high lattice stability
and ML inactivity just like tPTI-B2. Thus, the ML properties were
affected by the lattice stability of the crystals, which is highly
related to the molecular packing and intermolecular interactions
in the crystal.
The single crystal X-ray diffractions were carried out for
PTI-B1, PTI-B2 and PTI-B, and the single crystal structure and
molecular packing mode were carefully analyzed to investigate
Fig. 3 The molecular packing and molecular dimers including inter-
the deeper relationship between the crystal packing modes and
ML performances. Table 1 and Table S5 (ESI†) show the basic
molecular interactions in the unit cell of crystals (a) tPTI-B1, (b) tPTI-B2,
and (c) PTI-B, respectively.
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liable to breaking rather than slipping under a mechanical force.
The ML inactive tPTI-B2 crystal, another polymorph of tPTI-Bpin,
displays an extremely different molecular packing from tPTI-B1.
In tPTI-B2, there are two types of molecular dimers in the unit cell,
including dimer-1 with 16 C–HÁÁÁp (2.686–3.996 Å) and 10
C–HÁÁÁN (3.326–3.692 Å) interactions, and dimer-2 with 18
C–HÁÁÁp (2.952–3.953 Å) and 2 C–HÁÁÁN (3.359 Å) interactions.
Dimer-1 and dimer-2 both possess similar antiparallel stacking to
the dimer-1 in tPTI-B1, and also stack parallelly to each other,
resulting in a parallelogram-like packing mode in the unit cell.
When mechanical force is applied, tPTI-B2 with a parallelogram-
like packing tends to generate a molecular slippage with the
possible energy loss through a non-radiative relaxation route,
Fig. 4 The HOMO and LUMO of single molecules in tPTI-B1, tPTI-B2, and
resulting in its ML inactivity. However, since triangles are hard PTI-B calculated at the B3LYP/6-31G(d,p) level.
to slip rather than parallelograms, tPTI-B1 with the triangle-like
packing is liable to breaking instead of slippage, effectively
LUMO (the lowest unoccupied molecular orbital) delocalized
suppressing the non-radiative energy loss. Thus, the triangle-like
packing mode of crystal tPTI-B1 should be partially responsible
for its ML activity. A similar molecular packing mode to tPTI-B2 is
found in the ML inactive PTI-B crystal, in which, the molecules
also pack parallell to each other and form a parallelogram-like
packing in the unit cell. And there are two molecular dimers
including dimer-1 with 12 C–HÁÁÁp (3.206–3.817 Å), 4 C–HÁÁ ÁN
(2.556–3.722 Å), 4 C–HÁÁÁO (3.190–3.253 Å), and 2 pÁÁÁp (3.793 Å)
interactions, and dimer-2 with 6 C–HÁÁÁp (3.285–3.704 Å), 8
C–HÁÁÁN (2.750–3.615 Å), and 4 C–HÁÁÁO (3.012–3.709 Å) inter-
actions. Combined with the PXRD results, the different molecular
packing modes and intermolecular interactions of these PTI
derivatives could really affect the lattice stability and hardness
of the crystals, resulting in different ML performances under
mechanical stimulation.
As discussed above, in the two polymorphs of tPTI-Bpin
(tPTI-B1 and tPTI-B2), the conformations of single molecules are
very different, directly leading to their different molecular packing
modes. As for the isomer of PTI-Bpin, it only lacks a tert-butyl
group in the chemical structure compared to tPTI-Bpin, but the
single crystal of PTI-Bpin displays a similar molecular packing
mode as tPTI-B2. Thus, considering their chemical structures,
molecular packing in the aggregated state, and emissive proper-
ties together, the molecular packing rather than the chemical
structure mainly accounts for their different ML behaviors. The
triangle-like molecular packing gives tPTI-B1 higher rigidity to
realize the ML property, while the parallelogram-like molecular
packing mode of tPTI-B2 and PTI-B is easy to slip under a
mechanical force, leading to the increase of possible energy loss
through non-radiative channels.
on the PTI core and the phenyl ring linked to the carbon of PTI,
indicating the weak intramolecular charge transfer in these
luminogens.
In summary, two luminogens of tPTI-Bpin and PTI-Bpin
with ignorable difference in the chemical structure have been
designed and synthesized, to investigate the structure-packing-
ML property relationship. The two crystalline polymorphs of
tPTI-Bpin show opposite ML activities, derived from their
different conformations of packing modes in the crystals. The
crystal (PTI-B) of another isomer PTI-Bpin shows a similar
molecular packing mode and ML inactivity to tPTI-B2, further
confirming that the molecular packing rather than the chemical
structure mainly accounts for their diverse ML properties. There-
fore, the molecular packing is particularly important for the
modulation of organic ML properties, which is of great signifi-
cance for future research on organic ML materials.
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
This work was supported by the National Natural Science
Foundation of China (No. 51573140 and 21734007), Hubei
Province (No. 2017CFA002) through financial support and the
Special funds for basic scientific research services in central
colleges and Universities (No. 2042017kf0247).
Density functional theory (DFT) calculations (B3LYP/
6-31g(d,p)) were carried out on the isolated molecules (derived
from their ground state geometries in single crystals). The
molecules were in different conformations with different
torsion angles between the peripheral phenyl rings and the
central PTI core, which is well consistent with the molecular
conformations in single crystal structures (Fig. 4). No distinct
orbital delocalization was found on the isolated molecules,
since there is no apparent D–A structure in these luminogens.
Both the HOMO (the highest occupied molecular orbital) and
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