Utilizing the heterocyclic effect towards high contrast ratios of mechanoresponsive luminescence based on aromatic aldehydes

Article abstract of DOI:10.1039/c9tc04586d

A high contrast ratio is required by mechanoresponsive luminescent (MRL) materials to enable their adoption in mechanical sensors. However, the effective design principle remains restricted by the lack of understanding of the necessary features of such lu

Full text of DOI:10.1039/c9tc04586d

Journal of
Materials Chemistry C
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PAPER
Utilizing the heterocyclic effect towards high
contrast ratios of mechanoresponsive
luminescence based on aromatic aldehydes†
Cite this: J. Mater. Chem. C, 2019,
, 12328
7
Fang Zhang, Xiaozhong Liang, Da Li, Xiangkai Yin, Xia Tian, Bin Li, Hong Xu,
Kunpeng Guo * and Jie Li
*
A high contrast ratio is required by mechanoresponsive luminescent (MRL) materials to enable their
adoption in mechanical sensors. However, the effective design principle remains restricted by the lack of
understanding of the necessary features of such luminophores. In this contribution, a series of simple
aromatic aldehydes, namely, APCz, APAd, APPo, and APPt, was designed and synthesized by combining
different heterocyclic cores with 4-benzoyl substituted at the symmetrical positions. The heterocyclic
cores N-butyl-carbazole (APCz), N-butyl-9,9-dimethyl acridine (APAd), N-butyl-phenoxazine (APPo),
and N-butyl-phenothiazine (APPt) endowed the moderately twisted compounds with the ability of
emitting luminescence in the solid state and in adopting metastable states in their pristine-crystal state.
These features enabled phase and multicolor changes under external-force stimuli. Single-crystal
analyses revealed that decreasing the type of noncovalent interactions by changing the heterocyclic
cores from five- to six-membered rings with additional heteroatoms led to the formation of compounds
sensitive to external force and to a dramatic increase in MRL contrast ratio from Dl = 10 nm to Dl =
Received 19th August 2019,
Accepted 15th September 2019
DOI: 10.1039/c9tc04586d
5
3 nm. Overall, this work provided new insights into the design of high-contrast stimuli-responsive
rsc.li/materials-c
luminescent materials.
1–3
First, a material must exhibit efficient emission in solid state.
Second, a metastable state must be present to ensure that its
Introduction
7,8
The design and synthesis of mechanoresponsive luminescent properties can be changed under force stimuli. Third, a clear
MRL) materials, which exhibit altered solid emission under color difference should be present to meet the high contrast
(
8–10,14
mechanical force stimuli, are of considerable practical and luminescence recording requirements.
academic interest for applications in optical devices, mechanical Generally, introducing aggregation-induced emission building
1
–6
sensors, security paper, and disaster warning. In studying these blocks, such as highly twisted tetraphenylethylene and triphenyl-
materials, the origin of the MRL phenomenon, including the amine, is an effective strategy in realizing efficient solid
7
,10,17–21
changes in the molecular packing and molecular conformation emission.
upon external force stimuli, has been proposed to obtain addi- porated tetraphenylethylene and triphenylamine into a variety
For example, Tang et al. successfully incor-
7–10
tional insights into improving their performance.
context, organic MRL materials have attracted considerable inten- organic light-emitting diodes, bioimaging, and MRL materials.
tion because of their flexible molecular structure tailoring and Similarly, the utility of moderately and slightly twisted molecules
In this of small molecules and polymers, which display advantages in
5,12
11–16
intra/intermolecular interaction modulating.
In principle, as building blocks in organic electronics has also made them
a material should possess the three following essential charac- extensively studied molecules due to their relative facile synthesis
22–26
teristics to ensure that it exhibits prominent MRL behavior. and functionalization.
However, the molecules with small
steric hindrance in the condensed state tend to form compact
aggregates due to p–p interactions, which increased the non-
radiative path and resulted in emission quenching. To date, only
a few moderately and slightly twisted molecules exhibiting MRL
phenomena have been reported by mechanically disturbing their
molecular packing motifs in metastable nanostructures.
However, the lack of single-crystal analyses limits insights into
their molecular features in precise designing and synthesizing
Ministry of Education Key Laboratory of Interface Science and Engineering in
Advanced Materials, Research Center of Advanced Materials Science and
Technology, Taiyuan University of Technology, Taiyuan 030024, China.
E-mail: guokunpeng@tyut.edu.cn, lijie01@tyut.edu.cn
†
Electronic supplementary information (ESI) available: Experimental section,
supplementary figures, crystallographic data. CCDC 1842477, 1847380 and
847379. For ESI and crystallographic data in CIF or other electronic format
see DOI: 10.1039/c9tc04586d
23,24,27–29
1
1
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such high contrast MRL materials. Therefore, it is important to heteroatom-free aldehyde (APBz, Scheme S1, ESI†) with a
understand the necessary molecular features of high contrast 5-octylbenzene donor exhibiting nonemission and non-MRL
MRL luminophores for increasing the dimensionality of MRL behavior in solid state, APCz with a N-containing five-membered
materials.
heterocycle in the donor moiety resulted in a minimal increase
Heterocyles, which are cyclic hydrocarbons embedded with in molecular distortion and ICT effect, thereby emitting blue
one or more heteroatoms, are well established as key building luminescence in the crystal. Although the crystal of APCz was in
blocks in biostructures and functional materials such as lumi- a metastable state, no megascopic color change was observed
1
1,30–35
nescent materials.
Heteroatoms are indispensable in under grinding with the emission band shift of only 10 nm.
achieving versatile functions because they enable variable con- However, APAd, APPo, and APPt showed significantly increased
formations of molecules, exert electron-donating or withdraw- MRL contrast ratios accompanied with an evident color change
3
1,32,35
ing effects, and even induce complicated excited states.
from yellow to green, red to orange, and yellow to blue under
Meanwhile, given the excellent sensitivity of the intramolecular grinding, respectively. Theoretical and single-crystal analyses
charge transfer (ICT) emission to alterations in the external revealed that after changing the heterocyclic cores from five-
environment, the development of acceptor–donor–acceptor membered rings to six-membered rings with additional hetero-
(
A–D–A)-type ICT luminophores has raised significant concerns atoms, the sensitivity to external force increased with decreased
3
6–38
for advanced MRL materials.
Here, we present a strategy of noncovalent interaction types. With increased molecular distortion,
imposing molecular feature screening for realizing high contrast APPt, which exhibited a 53 nm emission band shift under grinding,
ratios of mechanoresponsive luminescence. This strategy was showed the highest contrast ratio among the samples. Our study
inspired by the fact that the building blocks of a compound, such is significant in providing insights into the molecular features in
as donor or acceptor units, can largely affect the molecular designing MRL materials with high contrast ratios.
conformation and intra/intermolecular interactions. Hence, we
aimed to introduce commonly used heterocyclic rings as donors
to produce mutative A–D–A luminophores with different crystal
characteristics and stability in the solid state to obtain additional
insights into the molecular features of MRL materials with high
Results and discussion
Theoretical calculations
contrast ratios.
To explore the influence of varied heterocyclic donors on
Symmetrical aromatic aldehydes can be regarded as a type of the molecular conformations and ICT characteristics of these
simple A–D–A compound that are beneficial in determining the compounds, we first evaluated the optimized molecular geo-
regularity of realizing MRL behavior in donor unit adjustment. metries and frontier orbital distributions by DFT calculations at
In this work, we designed and synthesized four A–D–A the B3LYP/6-31G* level. As illustrated in Fig. 1, the dihedral
heterocyclic-based aromatic aldehydes with 4-formylbenzene angles between the phenyl ring and the central heterocyclic
acceptors that differed in N-butyl-carbazole (APCz), N-butyl- donors were 351/351 for APCz, 311/251 for APAd, 291/291 for
9,9-dimethyl acridine (APAd), N-butyl-phenoxazine (APPo), APPo, and 351/401 for APPt. These results demonstrated that all
and N-butyl-phenothiazine (APPt) donors (Fig. 1) on the basis four molecules adopted moderately twisted conformations.
of the anticipation that the electron donative aromatic hetero- According to our previous studies, such moderately twisted
cyclic compounds can be effective in tuning the molecular conformations may result in compounds exhibiting mechano-
2
3,24,36
conformation with diverse intra/inter molecular interactions responsive properties in the solid state.
The dihedral
and molecular packing modes. The compounds can be facilely angle between the central phenyl core and adjacent phenyl
synthesized from commercially available formyl aromatic boronic rings for the referenced APBz was 331/331 (Fig. S1, ESI†), which
acid with different bromide aromatic rings via Suzuki coupling, as was smaller than that of APCz with a N-containing carbazole in
depicted in Scheme S1 (ESI†). Compared with the referenced the donor moiety. As a result, the introduction of a heteroatom
Fig. 1 Molecular structure and geometrically optimized 3D molecular models of APCz, APAd, APPo and APPt.
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into the central core of A–D–A molecules may increase the
molecular distortion.
As shown in Fig. S2 (ESI†), the spatial separation between
the highest occupied molecular orbitals (HOMOs) and the
lowest unoccupied molecular orbitals (LUMOs) of the refer-
enced APBz was not pronounced, thereby indicating a weak ICT
effect of APBz. However, from APCz to APAd, APPo, and APPt,
the HOMOs are more concentrated on the central heterocyclic
cores and the adjacent phenyl rings, although the LUMOs are
primarily distributed on the formyl groups as well as the phenyl
rings. These results indicated that the change in the central
cores from heteroatom-free phenyl rings to heterocycles with
increased ring sizes and heteroatoms would be an effective way
to improve the ICT effect of the compounds.
Photophysical properties
To evaluate the ICT characteristics of the compounds further,
we investigated the UV-vis absorption and photoluminescence
properties of APCz, APAd, APPo, and APPt, and studied the
referenced APBz in solvents with different polarities. The weak
ICT characteristic of APBz was proved by the insensitivity of
photophysical properties to solvent polarity change. As shown
in Fig. S3 (ESI†), the absorption spectra of APBz in various
solvents exhibited one characteristic absorption band in a
narrow range of 288–298 nm, and nearly no observable photo-
luminescence was observed upon changing the solvents’
polarities. After replacing the central donor core with hetero-
cycles, the absorption maximum (labs) of APCz, APAd, APPo,
and APPt was red-shifted by approximately 18–27 nm when
Fig. 2 Fluorescence spectra of (a) APCz, (b) APAd, (c) APPo and (d) APPt
in various organic solvents (10 mM). Photographs of (e) APCz, (f) APAd,
(
g) APPo and (h) APPt in various solvents under UV light (365 nm).
increasing the solvent polarity from hexane to DMSO (Fig. S4, moment by the relocation of the polar solvent molecules, thereby
ESI†). Compared with APBz, these red-shifts indicated a large leading to lowered energy and red-shifted fluorescence and
change in dipole moment in the ground state in different showing that APCz, APAd, APPo, and APPt are highly sensitive
solvents for APCz, APAd, APPo, and APPt. All heterocycle- to solvent polarity.
based aromatic aldehydes showed significant solvatochromic
effects with respect to APBz, as generally attributed to typical
Thermal stability characterization
ICT effects. We found that Dl, i.e., the maximum peak photo- To understand the difference of the thermal stability between
luminescence (lPL) shift between emissions in the nonpolar the various molecular structures, we performed differential
solvent hexane and in the highly polar solvent DMSO, was scanning calorimetry measurements. As shown in Fig. 3, the
1
6
08 nm for APCz, 137 nm for APAd, 149 nm for APPo, and pristine powders of APCz, APAd, APPo and APPt all displayed a
3 nm for APPt, thereby leading to a distinct change in phase transition peak prior to their melting points, respectively.
fluorescence color that can be clearly distinguished by the The corresponding exothermal peaks were observed at approxi-
À1
À1
naked eye under a 365 nm UV lamp (Fig. 2 and Table 1).
mately 148 1C (DH = 52.9 J g ) for APCz, 143 1C (DH = 0.45 J g
)
À1
Studies on the Lippert–Mataga equation revealed that the solvato- for APAd, 138 1C (DH = 50.93 J g ) for APPo, and 145 1C (DH =
chromic effect is related to a change in molecular dipole moment 0.19 J g ) for APPt, thereby indicating that the four heterocyclic
À1
between the excited and ground states. Fig. S5 (ESI†) shows the plots aromatic aldehydes were present in a metastable state. By contrast,
of the Stokes shift versus orientational polarizability.
without hetero atoms in the central core, the reference APBz
The plots showed one set of linearity indicative of one pristine powder only exhibited one exothermal peak at 94.4 1C
À1
dominant ICT excited state maintained in solvents with different (DH = 74.27 J g ) corresponding to its melting point (Fig. S6, ESI†).
polarity. The slopes of the best-fit line, which was related to the These results suggested that the introduction of heterocycles was
dipole moment change between the ground state and the excited beneficial, thereby inducing the aromatic aldehydes to form meta-
state, were 13 018, 14 750, 12 886, and 6498 for APCz, APAd, APPo, stable states in their pristine powders and leading to the possibility
and APPt, respectively. These results suggested that the dipole of yielding MRL behavior under external force stimuli.
moments in the excited state increased in a polar solvent when
Mechanoresponsive luminescence properties
the charges transferred from the central heterocyclic donors to the
peripheral withdrawing formyl groups. As a result, the polarized Taking their moderately twisted molecular conformations and
excited state was stabilized to adapt to the enhanced dipole metastable states in pristine crystals together, MRL behavior
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Table 1 Photophysical properties of APBz, APCz, APAd, APPo and APPt in under mechanical grinding, and the corresponding parameters
various solvents
are collected in Table 2. The original blue fluorescence of APCz
was almost maintained after grinding (Fig. 4), although a blue
shift of 10 nm from 474 nm to 464 nm was observed, and an
Stokes
4
À1
l
Abs e (10 M
l
PL
shift
F
F
À1
À1
Compounds Solvent
Df
(nm) cm
)
(nm) (cm ) (%)
increased absolute fluorescence quantum yield (F ) from 3.4%
F
APBz
APCz
APAd
APPo
APPt
Hexane
Toluene
THF
0
288 5.689
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
to 53.6% was recorded (Fig. 5a and Table 2). These results
indicated an extremely low MRL contrast ratio of APCz. When
replacing the centered five-membered ring N-butyl-carbazole in
APCz with a six-membered ring N-butyl-9,9-dimethyl acridine,
the resulting APAd exhibited a yellow emission (lPL = 558 nm
and FF = 11.7%) in its pristine powder and an enhanced
0.012 289 4.782
0.21 294 6.411
0.28 295 5.485
Acetone
Acetonitrile 0.29 295 6.855
DMSO
0.26 298 8.215
Hexane
Toluene
THF
0
350 4.689
400 3571
413 3956
435 4944
473 6948
502 7935
0.7
0.012 355 4.489
0.21 358 4.948
0.28 356 4.737
2.9 green emission (lPL = 514 nm and F = 64.1%) after grinding
F
34.7
35.2
39.9
(Fig. 4, 5b, and Table 2). Compared with APCz, the spectral shift
Acetone
of 44 nm and distinct fluorescence color change of APAd
Acetonitrile 0.29 359 4.767
DMSO
0.26 368 4.957
508 7489 39.0 observable by naked eyes indicated an evidently improved
MRL contrast ratio. The further improvement of the MRL
contrast ratio was observed with increased heteroatoms in the
Hexane
Toluene
THF
0
383 4.425
421 3937
449 4568
34.0
29.6
0.012 393 4.287
0.21 395 4.192
0.28 396 4.499
489 6049 17.6 centered moiety. The application of mechanical grinding to the
Acetone
534 7522
556 8316
558 7503
15.8
12.3
10.4
pristine APPo powder resulted in a blue shift of 48 nm from
21 nm to 573 nm and increased FF from 9.37% to 36.2%
Fig. 5c and Table 2). This process was accompanied by a
Acetonitrile 0.29 394 4.292
DMSO
6
(
0.26 408 4.328
Hexane
Toluene
THF
0
414 2.443
461 2463
494 3067
532 4351
580 6178
600 6643
610 6282
2.8
51.1
23.9
distinct fluorescence color change from red to orange (Fig. 4).
The largest MRL contrast ratio with a blue shift of 53 nm from
0.012 429 2.43
0.21 432 2.26
0.28 427 2.297
Acetone
2.6 527 nm (F
1.4
1.0
F
= 7.43%) to 474 nm (F
F
= 68.7%) was observed upon
Acetonitrile 0.29 429 2.476
DMSO
mechanical grinding of the APPt powder, accompanied by a
remarkable fluorescence color change and enhancement from
0.26 441 2.374
Hexane
Toluene
THF
0
358 2.954
429 4623
436 4312
452 4758
470 6121
5.4 yellow to blue (Fig. 4). The diversity of the MRL contrast ratio of
20.3
42.6
44.9
0.012 367 5.532
0.21 372 6.073
0.28 365 6.458
the four compounds can be further demonstrated in the
Commission Internationale de l’Eclairage (CIE) diagram to
Acetone
Acetonitrile 0.29 365 6.259
DMSO 0.26 376 6.25
482 6650 44.3 show their different sensitivity to mechanical force. As shown
492 6271
37.0
in Fig. S7a (ESI†), the CIE color coordinates of APCz changed
from (0.18, 0.24) to (0.15, 0.20) after grinding, which still
located in the blue color zone. However, in the case of APPt,
a considerable change from the yellow-green color zone (0.34,
0.60) to the blue color zone (0.16, 0.26) was realized upon
grinding. APPt exhibited an excellent MRL contrast ratio.
However, insignificant emission in the pristine powders and
mechanical grinding did not lead to any change in the fluores-
cence of APBz, which can be attributed to its low distorted
molecular conformation, weak ICT effect, and thermal stability
in the solid state. On the basis of these results, we can conclude
that the characteristics of centered heterocyclic rings can
regulate the fluorescence color and the MRL contrast ratio of
compounds significantly.
MRL mechanism study
To investigate the mechanism of the MRL behavior, we
recorded X-ray diffraction (XRD) patterns and time-resolved
spectra. As shown in Fig. 6a–d, the pristine powders of APCz,
APAd, APPo, and APPt showed many sharp and strong diffraction
peaks, thereby suggesting typical crystalline characteristics. In the
wide angle region, the peaks were observed at approximately
Fig. 3 DSC curves of compounds (a) APCz, (b) APAd, (c) APPo and
d) APPt in the pristine state.
(
was anticipated for APCz, APAd, APPo, and APPt. To verify our 251, which were primarily caused by intermolecular interactions,
speculation, we spread the pristine powders of the four hetero- such as p–p and C–HÁÁÁp. These noncovalent interactions between
cyclic aromatic aldehydes with different patterns on filter paper adjacent aromatic molecules often result in decreased or
and recorded the changes in their fluorescence characteristics quenched emission in the solid state, thereby accounting for
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Table 2 Photophysical properties of APCz, APAd, APPo and APPt in pristine and ground solid states
Pristine sample
PL (nm)
Ground sample
PL (nm)
À1
À1
À1
À1)
Compounds
l
F
F
(%)
t (ns)
k
r
(ns
)
knr (ns
)
l
F
F
(%)
t (ns)
k
r
(ns
)
knr (ns
APCz
APAd
APPo
APPt
474
558
621
527
3.4
11.7
9.37
7.43
4.28
13.72
2.53
0.0079
0.0012
0.0071
0.0031
0.2257
0.0716
0.3881
0.2521
464
514
573
474
53.6
64.1
36.2
68.7
5.56
15.94
4.46
0.0965
0.0402
0.0812
0.1295
0.0835
0.0225
0.1431
0.0590
3.92
5.31
as much as 12-, 33-, 11-, and 41-fold with respect to those of the
pristine APCz, APAd, APPo, and APPt powders, respectively.
Such increased krs values indicated the activation of radiative
pathways in the molecular arrangements of the ground samples,
F
thereby accounting for the high F s of these samples.
Subsequently, deep insights from a molecular level to under-
stand the structure–property relationship were obtained by
a single-crystal XRD (SCXRD) study in deciphering diverse
MRL contrast ratios of the luminophores. Fortunately, the
good-quality crystals of APCz, APAd, and APPo with blue, bright
yellow, and red luminescence were successfully cultured from
the binary mixture of hexane : dichloromethane (1 : 1) at 25 1C
Fig. 4 Photographs of APCz, APAd, APPo and APPt before and after (crystallographic data in detail are provided in Tables S1–S4,
mechanical grinding performed under a 365 nm UV lamp.
ESI†), respectively. However, the single crystal of APPt with
considerable quality has not been obtained in our laboratory to
satisfy the study. Considering the similar molecular structures
between APPo and APPt, we believe that the information on the
structure–property relationship would be deduced from the
3
9,40
combination of DFT calculation and SCXRD studies.
Fig. 7a–c reveal the torsion angles between the central
heterocycles and the adjacent phenyl rings of 291/291, 301/221,
and 211/211 for APCz, APAd, and APPo, respectively. These
values were slightly smaller than those of the isolated mole-
cules in the gas phase (calculated by the DFT study), thereby
suggesting that their molecules required additional planar
conformations to fit into the crystalline lattices. APCz was
crystallized in a triclinic system with the space group P1%
(a = 10.8799[7] Å, b = 13.3532[8] Å, and c = 17.0824[10] Å). The
four types of intermolecular interactions with different quan-
tities were detected between the APCz molecules, namely,
C–HÁ Á ÁOQC, C–HÁ Á ÁC–H, and C–HÁ Á Áp, and pÁ Á Áp, as shown
in Fig. 7d. Considering the intermolecular interaction-assisted
planar structure, APCz molecules formed interlaced herring-
bone patterns (Fig. 7g). In the case of APAd, each unit cell
contained 16 molecules and belonged to a monoclinic system
with the space group Cc (a = 22.7890[12] Å, b = 11.8841[7] Å, and
Fig. 5 Normalized emission spectra of the (a) APCz, (b) APAd, (c) APPo
and (d) APPt samples before and after grinding.
F
the relatively low F s of the pristine powders of APCz, APAd, c = 38.1493[18] Å). Compared with crystal APCz, the types of
APPo, and APPt. After being ground, the sharp peaks were noncovalent interactions in the crystal APAd were reduced,
considerably diminished, indicating the amorphous charac- accompanied with the fluctuation in respective quantities
teristics of the ground samples. The principle of MRL behavior (Table S4, ESI†). Three kinds of intermolecular interactions,
of APCz, APAd, APPo, and APPt can be attributed to the phase including the largest quantity of C–HÁ Á ÁOQC, medium quantity
transformation from crystal state to amorphous state thus far. of C–HÁ Á ÁC–H, and minimum quantity of C–HÁ Á Áp were
As shown in Fig. 6e–h and Table 2, the fluorescence lifetimes observed as the dominant driving forces in forming the face-
of the ground states of APCz, APAd, APPo and APPt became to-face parallel pattern (Fig. 7e and h). The types of noncovalent
longer than those in pristine powders, suggesting that phase interactions were further reduced in the crystal APPo, as shown in
transformation should occur upon mechanical grinding. The Fig. 7f. APPo crystallized in a monoclinic system with the space
radiative rate constant (krs) of the ground samples increased by group P21/m (a = 9.3553[5] Å, b = 25.1617[13] Å, and c = 9.5973[5] Å).
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Fig. 6 XRD patterns of pristine and ground samples of (a) APCz, (b) APAd, (c) APPo and (d) APPt. Time-resolved fluorescence spectra of pristine and
ground samples of (e) APCz, (f) APAd, (g) APPo and (h) APPt.
Fig. 7 Crystal structure of (a) APCz, (b) APAd and (c) APPo; multiple non-covalent interactions in the crystals of (d) APCz, (e) APAd and (f) APPo; different
molecular packing modes of (g) APCz, (h) APAd and (i) APPo. Inset: The fluorescence microscopy images of single crystal APCz, APAd and APPo,
respectively.
Each unit cell contained four molecules, which formed face-to-face compounds to the mechanical force increased with decreased types
parallel patterns under the interaction, namely C–HÁÁÁOQC and of noncovalent interactions. APCz possessed four types of non-
C–HÁÁÁp, as shown in Fig. 7f and i. The category and quantity of covalent interactions, thereby exhibiting only a 10 nm spectral shift
noncovalent interactions were close to the compound-MRL contrast under mechanical stimuli. Meanwhile, APPo only possessed two
ratio. To provide a clear idea, we mapped the types versus the types of noncovalent interactions, thereby resulting in a 48 nm
quantity of the noncovalent interactions in APCz, APAd, and APPo spectral shift regardless of its quantities. Molecular conformation is
crystals, as well as compound-MRL contrast ratio correlation, as another considerable parameter that influences the compound–
shown in Fig. 8 and Table S4 (ESI†). The MRL contrast ratio of the MRL contrast ratio. Therefore, under the same condition, APPt
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Conclusions
9
Insights into the molecular features regarding the structure–
property relationship were obtained in designing MRL materials
with high contrast ratios by carefully investigating the MRL
behavior of the four simple heterocycle based aromatic aldehydes.
Density functional theory and single-crystal analysis revealed that
the features of the heterocycle at the core of symmetrical aromatic
aldehydes significantly influenced the compounds’ stability,
molecular conformations, molecular-packing models, and inter-
molecular interactions, thereby regulating the MRL contrast ratio.
The introduction of heterocyclic cores is responsible for the
observed metastable states of APCz, APAd, APPo, and APPt in
the pristine powders. Changing the heterocyclic cores from the
five-membered rings to six-membered rings with additional hetero-
atoms decreased the types of noncovalent interactions, which
subsequently increased the MRL contrast ratio of the compounds.
Together with the appropriately increased molecular distortion,
APPt exhibited the highest MRL contrast ratio (a spectral shift of
1
1
1
1
1
1
1
1
1
1
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53 nm), accompanied by a remarkable change in fluorescence color
and enhancement from yellow to blue. Our investigation provides a
suggestion of the necessary features that a luminophore should
possess in improving the MRL contrast ratio.
Conflicts of interest
20 T. Jadhav, B. Dhokale and R. Misra, J. Mater. Chem. C, 2015,
, 9063–9068.
3
There are no conflicts to declare.
21 Y. Qi, Y. Wang, G. Ge, Z. Liu, Y. Yu and M. Xue, J. Mater.
Chem. C, 2017, 5, 11030–11038.
2
2 P.-Z. Chen, H. Zhang, L.-Y. Niu, Y. Zhang, Y.-Z. Chen, H.-B.
Fu and Q.-Z. Yang, Adv. Funct. Mater., 2017, 27, 1700332.
Acknowledgements
This work was financially supported by the National Natural 23 K. Guo, F. Zhang, S. Guo, K. Li, X. Lu, J. Li, H. Wang,
Science Foundation of China (61605138) and the Natural
J. Cheng and Q. Zhao, Chem. Commun., 2017, 53, 1309–1312.
Science Foundation of Shanxi Province (201601D011031 and 24 F. Zhang, R. Zhang, X. Liang, K. Guo, Z. Han, X. Lu, J. Xie,
201601D202030).
J. Li, D. Li and X. Tian, Dyes Pigm., 2018, 155, 225–232.
25 Y. Guo, L. Xu, H. Liu, Y. Li, C. M. Che and Y. Li, Adv. Mater.,
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