Precise molecular design for BN-modified polycyclic aromatic hydrocarbons toward mechanochromic materials

Article abstract of DOI:10.1039/d0ta06191c

The development of smart materials, in particular those exhibiting highly sensitive mechanochromic luminescence (MCL) is desirable, but challenging since the MCL internal mechanism and the structure-performance relationship still remain unclear. Herein, w

Full text of DOI:10.1039/d0ta06191c

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DOI: 10.1039/D0TA06191C
Precise Molecular Design for B-N Modified Polycyclic Aromatic
Hydrocarbon Toward Mechanochromic Material
Huanan Huang*a, Ying Zhoua, Yawei Wanga, Xiaohua Caoa, Chuan Hana, Guochang Liua, Zhixiong Xua,
Received 00th January 20xx,
Accepted 00th January 20xx
Changchao Zhana, Huanan Hua, You Penga, Ping Yana, Dapeng Cao*b
DOI: 10.1039/x0xx00000x
The development of intelligent materials, in particular those exhibiting the highly sensitive mechanochromic luminescent
(MCL), is desirable but challenging since the MCL internal mechanism and the structure-performance relationship still remain
unclear. Here, we report a new MCL material (BN benzo[f]tetraphene) using a molecular-level design strategy by introducing
BN unit to a π-conjugated system. By investigating the BN benzo[f]tetraphene (5) and its analogue, it is found that the
introduction of boron-nitrogen unit is the key to tailoring the molecular dipole moment and intermolecular interactions,
which can therefore form the easily deformable molecular stacking pattern and endow 5 with wonderful MCL properties.
The theoretical calculations confirm that the inherent energies like the excited singlet (S) and highly sensitive triplet (T)
states exist in the MCL process, and the formation and fracture of ordered molecular aggregates have a significant effect on
radiative and nonradiative transitions. The material also shows the high-contrasted and self-reversible properties related to
the thermal-and force-stimulus, which makes it a promising candidate for security ink, optical recording applications. This
work possibly opens up a new way to develop the efficient organic smart materials and therefore trigger the discovery
of new functions and properties of azaborine compounds.
packing modes are the two key factors in modifying the
molecular luminescence properties, 36-37 offering vital detail for
understanding photophysical processes of different emissive
Introduction
The creative design and precise synthesis of novel
mechanochromic luminescent (MCL) materials have attracted a
great deal attention for their promising potential applications in
security systems, memory devices, health monitoring, and so
forth. 1-8 Since the pioneering work reported by Francis Bacon
in 1605, 9-11 the MCL chemistry has been well established by
numerous groups. Their endeavors have greatly impelled various
building blocks being used as the core units to construct MCL
luminogens, including tetraphenylethylene, phenothiazine and
carbazole.12-22 However, so far, due to lack of reliable guidance
and unclear intrinsic mechanism,23-26 how to control the MCL
behaviors through the rational molecular design is still an
enormous challenge. Therefore, the development of universal
molecular design principle is crucial to break through this
bottleneck, which not only provides the possibility to incessantly
explore the potential performance of these materials in device
applications, but also renders the opportunity to systematically
investigate the inherent mechanism and their structure-MCL-
property relationships. It is of great importance both in the
fundamental research and practical applications in the future.
Actually, several strategies concerning the molecular design
have been proposed to promote the mechanical stimulation
responsive performances.27-34 The appropriate crystallization
capability with deformable packing patterns in principle is
essential for excellent MCL,35 which facilitate their phase
transformation in the solid state. Stated differently, a rigid
backbone should be involved in designing target molecule to
maintain its molecular structure during the compressing process,
as well as a relatively deformable packing mode to provide
enough flexible space for molecular rearrangements.
Particularly, the intermolecular interactions and molecular
forms. Rational balance of different intermolecular interactions
and effective adjustment of the packing modes are thus tackled
as crucial issues in the design of current mechano-responsive
luminescent materials. The incorporation of main-group
elements into polycyclic aromatic hydrocarbons (PAHs) is a
promising strategy for modulation of the intermolecular
interactions.38-46 Among various possible dopants, boron and
nitrogen-containing conjugated materials have been targeted for
particular focus due to their distinct electronic and optical
characteristics.47-53 The replacement of C=C fragments with
isoelectronic B-N units has been proved to be a powerful
approach to tailor molecular packing modes and photophysical
properties.54-63 However, this strategy with applied potential in
exploring the mechanoresponsive behaviors of BN-modified
luminescent materials has long been neglected. Thus, it is
fascinating to probe and verify the MCL characteristics of B-
N/C=C isosterism at the molecule level.
Considered all the above points, herein, we synthesize the
parental BN-embedded benzo[f]tetraphene 5 as a novel building
block to discuss its possible MCL effect using precise molecular
design strategy (Figure 1A).The molecule skeleton 5 contains a
proper conjugate size, ensuring that the molecule presents
crystalline state, which is an essential condition to explore the
macroscopic performance of organic materials in the aggregates.
Meanwhile, the planar conjugated fused ring, as an issue of
interest in highly efficient emission, is the prerequisite for
excellent MCL materials. More importantly, the insertion of
boron nitrogen unit may prompt special molecular packing
patterns to form in aggregated states, enhancing the possibility
of MCL behavior. To highlight the importance of BN-
incorporation into PAHs, its hydrocarbon analogue 5 is served
as a counterpart for comparison. Results indicate that the more
polarized nature of the B-N bond in 5 than C=C bond in 5
significantly alters the polarity of 5 and intermolecular forces,
which favors the adjustment of arrangement and interactions
a. School of Chemistry and Environmental Engineering; Jiangxi Province Engineering
Research Center of Ecological Chemical Industry; Jiujiang Key Laboratory of
Organosilicon Chemistry and Application, Jiujiang University, Jiujiang 332005,
China. E-mail: huanan200890@163.com
b. State Key Laboratory of Organic-Inorganic Composites, Beijing University of
Chemical Technology, Beijing, 100029, China. E-mail: caodp@mail.buct.edu.cn
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Figure 1. (A) The design strategy and target molecule for MCL materials; (B) Synthetic route to BN-benzotetraphene
of the molecule toward force-responsive characteristics. spectroscopy, high-resolution mass spectrometry and well
corresponded with its expected structure.
Compared to 5, the boron atom-induced the exposure of N-H
bond in 5 can enhance the intermolecular interactions through
possible N-H···π interactions, which is also a favorable factor for
regulating molecular stacking pattern. In addition, the internal
mechanism and potential application of 5 as the MCL smart
materials are also investigated. These meaningful findings
provide a new platform to construct BN-based MCL materials,
which open a new window for the development of intelligent
luminescent materials.
The compound 5 exhibited superior stability under ambient
conditions. Even in the destructive tests, no change of the
absorption features was observed after stirring in dilute acidic
and basic aqueous solutions. Thermogravimetric analysis in
Figure S1 indicate that only 5% weight loss of 5 appears
ataround 330 ℃, manifesting its high thermal stability. Such
remarkable thermal stability is crucial for their applications in
materials science.
To examine detailed photophysical properties of the BN-
embedded molecule 5, UV-vis absorption and fluorescence
emission spectroscopies were carried out. The solvatochromic
effects of 5 in various polar solvents were investigated as well
given that the compound 5 may possess the charge transfer (CT)
characteristics due to the polar B-N bond. The absorption and
emission properties of 5 in CH2Cl2, n-hexane and toluene exhibit
relatively weak solvatochromism (Figure S2-S3). The results
indicated that the CT interactions in this compound was
ignorable, which may be a result of the delocalized BN double-
bond characteristics in this system. To explain this phenomenon,
density functional theory (DFT) calculations were further
performed at the B3LYP/6-31G(d) level to explore the electronic
distribution of molecule 5. As shown in Figure S4 in Supporting
Information, both HOMO and LUMO orbitals delocalized over
the whole aromatic skeleton, unlike the typical
Results and discussion
The synthetic route of BN benzo[f]tetraphene was
illustrated in Figure 1B. Commercially available 2-
nitrobenzaldehyde 1 was initially converted to 2 through a
witting reaction in 80% yield. Followed by iron powder used to
reduce the nitro-group, compound 3 in 50% yield was generated.
Afterwards, substrate 3 was efficiently cyclized with 2-
bromophenyl trifluoroborane potassium salt under slightly
modified conditions from the procedure reported by Molander,64
yielding BN-naphthalene derivative 4 in 75%. The BN-
naphthalene 4 is an ideal substrate for the tandem reactions since
it features an active CBr bond, which allowed us to perform
further reactions to extend the π conjugation for improved
molecular properties through palladium-catalyzed cross-
coupling reactions. Subsequently, the key tandem reactions of 4
with 2-bromophenyl boronic acid were performed. Excitingly,
the bromophenyl-substituted BN-naphthalene 4 was easily
converted into the stable BN-benzo[f]tetraphene 5 in good yield.
The crude products were stable enough for purification by
column chromatography on silica gel. The structure of 5 was
characterized systematically by 1H, 13C and 11B NMR
CT system exhibiting completely separated HOMO and LUMO
levels,65-66 only slight charge separation can be found in 5
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Figure 2. (A) Pictures of JJU with different treatment taken under UV irradiation (365 nm); (B) Fluorescence spectra of 5 in different
solid states, excitation wavelength: 370 nm; (C) XRD profiles of original powder and powder after grinding; (D-E) SEM images of
5 in different solid states (D: original sample, E: ground sample); (F) Solid-state 13C NMR spectra of 5 (blue: original sample, red:
ground sample).
HOMO and LUMO levels.69-70 The HOMO and LUMO levels
(Figure S4), indicating the weak charge transfer interactions in
this system, which is highly consistent with experimental results.
In addition, the spectral behaviors for compound 5 in solution
and solid states as well as thin film were also measured (Figure
S5). The thin films were deposited onto a glass substrate via spin
coating. A significant red-shift (45 nm) for compound 5 can be
observed in the emission of solid powder compared with solution.
What is more, the thin film of 5 generated significant multi-
emissions ranging from 423 to 534 nm. The interesting emission
features will impart 5 great potential in further manipulation of
multiple energy levels of solid-state organic luminophor toward
a greatly widened scope of applications. Meanwhile, the
absorption of compound 5 solved in CH2Cl2 and thin film were
also explored. As shown in Figure S5, the barely changed UV-
vis spectra between solution and thin film will undoubtedly hint
were estimated to be -5.61 eV, -1.98 eV, respectively, which
were in good agreement with the calculated values (-5.71, -2.01
eV, Figure S8).
As expected, molecule 5 displayed satisfactory MCL
properties with merits of high contrast and fast response as
shown in Video S1 and Figure 2A. The as-prepared sample
exhibited a light blue emission under a 365 nm UV light,
afterwards changed directly to cyan upon mechanical grinding.
It proves that the proper boron-nitrogen molecules indeed
possess excellent MCL features, confirming our original design
strategy.
Since the designed MCL material shows high contrast and
sensitivity luminescence behavior, its applied potential in optical
recording was further explored as well. Letters “JJU” were
written on weighing paper with 5 as ink dissolved in CH2Cl2
(Figure 2A-1).When grinding up on, the high contrast cyan
letters appear directly (Figure 2A-2). The high contrast cyan
letters appear instantly when applied in grinding (Figure 2A-2),
yet nearly faded with heating at 100°C for 1 min (Figure 2A-3),
suggesting its accelerated self-reversible process under suitable
temperature. These simple operations and color changes imply
its quite prosperous applications for security ink, optical
recording etc.
67-68
at the existence of excimers.
To gain a deeper insight into the electrochemical property，
the cyclic voltammetry (CV) of
5 was performed in
dichloromethane (Figure S6-S7). All measurements were carried
out at room temperature with a conventional three-electrode (the
working electrode: glassy carbon electrode, the reference
electrode: saturated calomel electrode (SCE), and a platinum
To further explore the mechanism of the color change under
mechanical stimulation, solid-state photoluminescence spectra
wire as the auxiliary electrode). Ferrocene/ferrocenium redox for both pristine and ground samples were measured. As shown
couple (Fc/Fc+) was used as an internal reference. The oxidation
and reduction waves of 5 were irreversible, indicating the
instability of the produced radical ion. Actually, the CV
measurements in combination with the relevant empirical
formulas can also be used to calculate the
in Figure 2B, the emission of original sample was centered at 445
nm, while after grinding two new shoulder peaks appeared at 422
nm, 472 nm, respectively. As known, the multiple emissions can
effectively regulate the luminescence performance,71-72 which
are in good agreement with the observed MCL color change
phenomenon. To confirm the relationship between mechanical
force and luminescence, the quantitative experiments were
conducted. As shown in Figure S9, with the increase of vertical
pressure, the intensity of the emission peak at 470 nm increases
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gradually. The results demonstrate that the magnitude of forces
do have a significant effect on the luminescence behavior.
X-ray diffraction (XRD) and scanning electron microscopy
(SEM) were performed to gain insight into the intrinsic
mechanism of MCL (Figure 2C, Figure 2D-2E). The original
sample 5 showed a sharp and intense diffraction peak at 2θ =11.4°
followed by multiple peaks from 2θ = 14.3°-34.3°, namely a
well-ordered microcrystalline structure. As noted, the as-
prepared sample possesses typical crystalline state. While in situ
grinding, conversely, some of the sharp peaks became evidently
weak, suggesting that the major crystalline structure was
destroyed through the mechanical force and presents a tendency
to a quasi-amorphous state transition. The weakening or
disappearance of the partial diffractions illustrate that the layered
long-range order has been destroyed, while the significantly
increasing diffractions (2θ =24.2°, 27.1°) suggest that the order
within the monolayer has partially enhanced during the grinding
process. According to Bragg equation, the distances (d) were
3.71 Å and 3.30 Å. Such a change in the diffraction angles
implies an increase in the intermolecular π-π interaction
distance,73 which has a rational relation with the fluorescence
intensity and significant impact on the fluorescence emission,
further leading to the emission maximum changes. The above
phenomena demonstrated that the interactions between layers in
the pristine material 5 is tunable, and thus the position and
packing modes between adjacent monolayers can be easily
alternated upon grinding treatment. Furthermore, the SEM
results, as shown in Figure 2D-2E also support this inference.
The crystalline to quasi-amorphous transformation for solid-
state 5 is consistent with the sample morphological changes
before and after grinding, in which the long-range loose sheet-
like morphology has splintered into tiny fragments upon force
stimulus. Therefore, it is believed that grinding-induced MCL
behavior of 5 can be attributed to the changes of stacking patterns
and intermolecular interactions. To confirm it, the solid-state
nuclear magnetic technology (NMR), which mainly focuses on
the different local environments around various nuclei and can
deliver rich and detailed information to distinguish subtle
structure and around environment differences,74 was employed
as an assistant method.The chemical shift of 13C-NMR was
hence determined as the selected key signal regarding the
possible changes of chemical environment in various states. As
observed in Figure 2F, obvious change in the carbon nuclei
appears after grinding, the type of carbon signal increases
sharply with the phenomenon of signal enhancement, It hints that
the molecules within a certain direction were in a much more
constrained environment after grinding due to the partial
enhanced intermolecular interactions, which was consistent with
the results of the redshift emission and XRD. Solid-state nuclear
magnetic technology has been recognized as a powerful and
advantageous analytical tool for supporting exploration in the
complex molecular interactions and molecular configurations.
However, the potential of this method in discussing the MCL
behaviors of luminescent materials have rarely been covered. In
this regard, we firmly believe that solid-state NMR technology
will contribute greatly in clarifying the mechanism of MCL with
new directions of thinking, eventually promoting the
development of organic smart materials.
This raises an interesting issue, namely the two molecules
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of 5 and 5, despite very similar structure, present notably
DOI: 10.1039/D0TA06191C
different MCL behaviors. The molecular packing and
intermolecular interactions play an important role in bridging the
natural properties of single molecules and the macroscopic
optoelectronic performance of organic materials23. Therefore, to
get insight into the different behaviors for 5 and 5 under
mechanical stimulation, the corresponding crystal structures
were investigated in detail. We systematically analyzed the
molecular stacking patterns and intermolecular interactions of
the two emitters.
The single crystals of 5 suitable for X-ray analysis were
obtained by recrystallization from solutions of dichloromethane
and hexane. As shown in Figure S10, the five fused six-
membered rings in 5 revealed a highly planar conformation. The
B-N bond length in this structure is 1.402(3) Å, very close to the
length of a localized B-N bond (1.403(2) Å),76-77 which means
that the π-electrons localized in B-N bond was consistent with
those reported in BN aromatic systems, and shows distinct
double-bond character. In addition, the B-C and N-C bond
lengths (B-C8=1.527(3), N-C6=1.403(2) Å) in 5 are noticeably
shorter than normal B-C (ca. 1.579(5) Å) and N-C (ca. 1.47 Å)
single bonds,78 aligned with the electron delocalization over
these atoms. The single crystal analyses reveal that molecule 5 is
constructed based on a monoclinic crystal system, crystallizing
in the space group C12/c1 (Cc-form) with eight molecules in one
cell unit. These eight molecules present a unique spatial
arrangement, forming two groups of antiparallel intersecting
stacking modes. Meanwhile, the centroid positions of the two
groups of molecules are completely overlapped (Figure 3A),
benefitting from the high degree of symmetry, these molecules
show special stacking patterns in three dimensions. For instance,
in the direction of the b axis, the unique spatial arrangement
favored to construct an ordered packing mode (Figure 3B),
shaping multi-group remarkable offset π-π stacking, in which the
average distance between layers is 3.669 Å. Along the π–π
stacking direction, the crystals of 5 exhibit a straight line-packing
pattern driven by the multiple intermolecular interactions(Figure
S11 in the SI), including π··π, N-H···π and C-H···π interactions,
with distances in the range of 3.61-4.0 Å (Figure S12 in the SI).
It is worth mentioning that those interactions play a vital role in
the single crystal cultivation. No accumulation pattern of 5 can
be obtained as straight line without those interactions. More
importantly, viewing from the c axis, the antiparallel intersecting
stacking modes lead to forming multiple intersecting
parallelograms (Figure 3C). As well known, parallelograms are
inherently unstable and easily deform under the action of
external forces, which therefore provides an opportunity to
readjust the intermolecular interactions and stacking patterns,
and endows 5 with excellent stimulus-responsive property.
In terms of crystal structure for 5,79 the molecule exhibited
asymmetric system with the space group of P21, and each cell
contained four independent molecules with markedly different
orientations, the dihedral angle between the least-squares planes
through each of the molecules was 47.8 (8)°. The crystal packing
adopted a γ-type structure, where molecules stacked in the
direction of the b axis. The molecules within each stack formed
offset face-to-face attractive contacts with a perpendicular
distance of 3.740 Å, larger than the molecule 5. Afterwards, we
thoroughly examined the molecular packing mode in the crystals
of 5. Interestingly, another set of molecules were inserted
between the layers on the b axis.
To further evaluate the luminescence mechanism, the MCL
behaviors of the corresponding hydrocarbon analogue 5 was
parallelly investigated.75 Interestingly, no force-response for 5
was observed at the same condition. Thus, we may conclude that
the boron nitrogen unit assumes an irreplaceable role in
regulating luminescence behavior in this system.
.
4 | J. Name., 2012, 00, 1-3
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Figure 3. (A) Molecular conformation of 5 in one cell unit; (B) The molecular conformations of 5; (C) The stacking patterns of 5
along the c axis; (D) The molecular conformations of 5.
which is conducive to forming offset face-to-face packing mode
rather than the face-to-face stacked geometry.84 This is highly
consistent with the crystals data. Furthermore, the dipole
moment was introduced to discuss the packing patterns. As
shown in Figure S15, the dipole moment of 5 (1.3541 Debye) is
much larger than that of 5 (0.0094 Debye), and the dipole vector
extends in accordance to the direction of the B-N bond, through
These multiple molecular conformations yield an irregular
packing mode, resulting in the absence of MCL effect. Single
crystal analysis demonstrated that the molecular packing and
intermolecular interactions play significant roles in the MCL
light-emitting process, and the MCL properties are likely to
perish after subtle modifications of the molecular structure.
Therefore, to expound the direct relation of MCL property and the C-B-N angle. Benefitting from the increase of the dipole
moment, the monolayer preferred the offset-stacked geometries
in accordance with the dipole moment vector’s position and
direction. Such offset face to face packing mode can significantly
strengthen the electrostatic attraction and avoid the electrostatic
repulsion. Meanwhile, the adjacent groups of molecules tend to
form antiparallel packing mode, and such antiparallel packing
pattern indeed contribute greatly to less the Coulomb repulsion.
Thus, as mentioned above, the parallel tetrahedral column stacks
are formed along c axes. The results demonstrated that
introducing the boron nitrogen unit strategy is valid to regulate
molecular packing modes and intermolecular interactions and
thus endow certain boron nitrogen aromatic materials with the
MCL property.
As previously mentioned, the exploration of light-emitting
process is highly desirable to reveal the mechanism of force-
stimuli response. In traditional photoluminescence (PL) and
electroluminescence (EL) processes, inherent energies like the
excited singlet (S) and highly sensitive triplet (T) states also exist
in the MCL process17,30. Therefore, the strength of intersystem
crossing (ISC) may also have a great influence on MCL
phenomena. Thus, time-dependent density functional theory
(TD-DFT) calculations were respectively performed on the
isolated molecule and dimers (selected from the single-crystal
structure) to identify the actual status of the energy level in the
singlet and triplet states.The energy level diagrams and excited
state transition configurations for singlet and triplet states are
presented in Figure 4, Figure S16 and Table S5-S8. The red
orbitals refer to the effective ISC channels. Typically,
the suitable molecular packing modes, phenylamine moiety was
introduced to eliminate the molecular packing effect, the crystal
analysis showed no remarkable π-π interactions for the
referential compound 6 (Figure S14 in the SI). As expected, no
MCL emission was observed for referential compound 6 upon
grinding. This comparative experiment further revealed the
significant effect of molecular packing on MCL properties from
an alternative perspective.
Another intriguing question is why did the very similar
structured 5 and 5 show completely different stacking patterns.
Molecular electrostatic potential surface maps (ESP) and the
dipole moment have been used to visualize and understand the
molecular packing modes.80-83 Consequently, to explore reasons
behind the changes of 5 and 5 molecular packing, we calculated
the ESP maps and the dipole moments of the two molecules,
respectively. For molecular 5, no significant separation between
positive and negative potentials was observed. However, upon
substitution of C=C fragmentwith isoelectronic B-N unit, the
ESP around the nitrogen atom in 5 turned positive (Figure 4),
indicating that it is an electron acceptor. Interestingly, we cannot
find any negative potential on the potential surface, conversely,
dispersing throughout the whole conjugate skeleton, which can
effectively disperse and diminish the electron density, leading to
a weaker intermolecular electrostatic repulsion and thus smaller
π-π stacking distances. Moreover, the electrostatic potential
presented a positive edge and negative center distribution mode,
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Figure 4. (A) ESP maps of 5 and 5 (left: 5, right: 5); (B-C) Energy level diagrams and possible ISC channels from excited singlet
state (S1) to excited triplet states (Tn) for 5 (B: Dimer; C: Isolated)
when triplet (Tn) states contain the same transition orbital demonstrated that compound 5¢ does not possess the
compositions as excited S1 as well as an energy gap within ± 0.3 phenomenon of MCL.
eV, the singlet-to-triplet transition from S1 to Tn is considered as
Herein, we proposed a possible mechanism to elucidate
the valid channel for the intersystem crossing. 17, 85-86 Concerning MCL emission process using a hybrid crystal analysis and
theoretical calculations method. The embedded boron nitrogen
unit could significantly modify the electronic distribution and
polarity of the molecule 5, and create unique intermolecular
interactions, which lead to form the special stacking patterns and
the enhanced ISC transition. Apparently, nonradiative ISC
channel and radiative transition are conflict in terms of
fluorescence emission. In the original aggregation states, the
enhanced ISC transition effectively opens the nonradiative
channels to the highly sensitive and nonemissive triplet states,
resulting in the weak fluorescence state. Conversely, upon
mechanical stimulus, the solid-state morphology and the
radiation patterns were manipulated. The non-radiation channels
were destroyed accordingly, afterwards enabling the radiative
transition dominates light-emitting process to yield the bright
fluorescence state. Thus, a conclusion can be drawn that the
formation and fracture of ordered molecular aggregates with
efficient intermolecular interactions is deemed to be the
favorable factors for the MCL behavior in the light-emitting
process.
the hydrocarbon analogue 5 dimers, there were three minor ISC
channels from S1 to T7, T8, T9 with the energy gaps of -0.1228, -
0.1497 eV, -0.2600 eV, respectively. As for the 5 dimers, the
main channel is completely opened, and one main and two minor
channels were formed as well from S1 to Tn (T5, T7, T8) with the
energy gaps ranging from 0.0904 to -0.2319 eV. All of the
matched Tn states share certain parts of the same transition
orbital compositions with the S1 state (H→L, H→L+1, H-1→L,
H-1 → L+1, H-2 → L+1). The ISC, known as a nonradiative
relaxation process from excit singlet state to excited triplet state,
is usually detrimental to the fluorescence emission. However,
upon grinding, the effective ISC channels were destroyed.
Indeed, only one minor ISC channel for the isolated 5 molecule
existed, and thereby the non-radiation transition with the energy
loss decreases dramatically, leading to the enhanced radiative
transition contributed to the fluorescence behavior, and thus
resulting in the bright emission in the radiation process. In sharp
contrast, for counterpart 5, compared the dimer and monomer,
only one minor ISC channel was destroyed, meaning that the
transition forms are almost unchanged before and after grinding.
Therefore, from the perspective of calculation, it is also
Conclusions
In summary, we proposed a molecular-level design strategy
to achieve a novel MCL material by incorporating BN unit into
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11. N. C. Eddingsaas, and K. S. Suslick, J. Am. Chem. Soc., 2007,
highly planar and rigid aromatic backbone. The isoelectronic
analogues of 5 and 5 with distinctly different MCL activities in
this work can help us to make a deep understanding of the
structure-packing-performance relationship, guiding the design
of organic molecules with MCL property. Basically, the easily
adjustable packing modes with enhanced intermolecular
interactions make it fundamentally conducive to achieve the
MCL property. In this case, the introduction of boron nitrogen
unit led to forming parallelogram molecule packing, which is
easily deformed and recovered in comparison with other
packings such as the triangle. This typical example provided a
good template to explore the new strategies for optimizing the
molecular packing in the future. Additionally, theoretical
calculations confirmed the existence of some inherent energies
like singlet (S) state and triplet (T) state in the MCL process,
which provides an additional approach to understand the
emission process. In short, the incorporation of boron nitrogen
unit into polycyclic aromatic hydrocarbons is a promising
strategy to develop intelligent luminescent materials, and the
design strategy makes a big step in expanding the scope of the
MCL family.
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Acknowledgements
This work was supported by the National Natural Science
Foundation of China (Nos. 21861022, 21864015), Project
funded by China Postdoctoral Science Foundation (No.
2019M662275), the key project of natural science foundation of
Jiangxi Province (20202ACBL213003), and the Education
Department of Jiangxi Province Foundation of China
(Nos.GJJ190896, GJJ190927), National undergraduate
innovation and entrepreneurship project (201911843012). The
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Graphical abstract：
8 | J. Name., 2012, 00, 1-3
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Journal of Materials Chemistry A
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This paper reports on a novel molecular-level design strategy to achieve a novel MCL materDiaOlI:b1y0.1i0n3c9o/Drp0ToAr0a6t1i9n1Cg
BN unit into highly planar and rigid aromatic backbone. We found that the introduction of boron nitrogen unit
is the key to design the MCL smart materials, because it can tailor the molecular dipole moment and
intermolecular interactions and therefore yield the MCL performance.
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