Modulation of piezochromic fluorescence behavior by subtle structural change

Article abstract of DOI:10.1016/j.dyepig.2019.03.039

Two piezochromic molecules, (2Z,4E)-2-(4-(Diphenylamino)phenyl)-5-phenylpenta-2,4-dienenitrile (1a) and (2Z,4E)-2-(4-(acridin-10(9H)-yl)phenyl)-5-phenylpenta-2,4-dienenitrile (1b), were synthesized by using Knoevenagel and Buchwald-Hartwig cross-coupling

Full text of DOI:10.1016/j.dyepig.2019.03.039

Accepted Manuscript
Modulation of piezochromic fluorescence behavior by subtle structural change
Tao Man, Nian-Sheng Xu, Zhen-Hai Yu, Yong Huo, Ye Yuan, Zi-Fa Shi, Xiao-Ping
Cao, Lin Wang, Hao-Li Zhang
PII:
S0143-7208(18)32532-4
https://doi.org/10.1016/j.dyepig.2019.03.039
DYPI 7428
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 26 November 2018
Revised Date: 14 March 2019
Accepted Date: 15 March 2019
Please cite this article as: Man T, Xu N-S, Yu Z-H, Huo Y, Yuan Y, Shi Z-F, Cao X-P, Wang L, Zhang H-
L, Modulation of piezochromic fluorescence behavior by subtle structural change, Dyes and Pigments
(2019), doi: https://doi.org/10.1016/j.dyepig.2019.03.039.
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to
our customers we are providing this early version of the manuscript. The manuscript will undergo
copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please
note that during the production process errors may be discovered which could affect the content, and all
legal disclaimers that apply to the journal pertain.

ACCEPTED MANUSCRIPT
N
N
CN
CN

ACCEPTED MANUSCRIPT
Modulation of Piezochromic Fluorescence Behavior by Subtle
Structural Change
Tao Man[a]†, Nian-Sheng Xu[a]†, Zhen-Hai Yu[b], Yong Huo[a], Ye Yuan[b], Zi-Fa Shi*[a], Xiao-Ping Cao[a],
Lin Wang*[b], and Hao-Li Zhang*[a]
aState Key Laboratory of Applied Organic Chemistry & College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou,
730000 (P. R. China)
bCenter for High Pressure Science & Technology Advanced Research (Shanghai), Shanghai, 201203, P.R. China
Abstract: Two piezochromic molecules, (2Z,4E)-2-(4-(Diphenylamino)phenyl)-5-phenylpenta-2,4-dienenitrile (1a) and (2Z,4E)-2-(4-(acridin-
10(9H)-yl)phenyl)-5-phenylpenta-2,4-dienenitrile (1b), were synthesized by using Knoevenagel and Buchwald-Hartwig cross-coupling
reactions. Their piezochromic fluorescence behavior was tuned by subtle structural change. The system indicates typical intramolecular
charge-transfer properties. The luminescence intensity of 1a decreased with increasing pressure with the redshift of the emission wavelength
when the hydrostatic pressure in a diamond anvil cell (DAC) was applied on its crystal. However, the emission wavelength of 1b which has a
methylene unit more than 1a had no obvious shift and the intensity increased gradually at the pressure range from 0.66 GPa to 5.72 GPa.
Keywords: piezochromic material • modulation • triphenylamine • donor-acceptor structure• intramolecular charge transfer
1. Introduction
Piezochromic materials whose emission colors or intensity change upon mechanical grinding/shearing or hydrostatic
compression are of great interest from a scientific viewpoint over the past decade owing to their potential applications in pressure
sensors, optical storage, rewritable media, and security ink [1-6]. Various novel organic molecules showing piezochromic modulation
have been developed [2-16]. Most of the piezochromic materials contain the donor-acceptor (D-A) structure [5, 7-16].
Triphenylamine (TPA), as one of the important structural units for functional organic materials,[17-20] was widely used to
construct D-A stuctures for piezochromic organic crystals because TPA units may undergo conformational change under high-
pressure conditions. TPA or it’s derivatives could be riveted with different acceptors, such as cyanostibene [15, 21-25],
dicyanovinylbenzene [26-29], quinoline [30], alkynyl gold complexes [31, 32], and other groups [33-39], to form
mechanofluorochromic materials. The luminescence quantum yield of most of these organic compounds decreases with increasing
external pressure.
Effective structural modification of traditional fluorophores is an important method to achieve organic mechanofluorochromic
*Corresponding author
E-mail adress: shizf@lzu.edu.cn (Zi-Fa Shi), wanglin@hpstar.ac.cn (Lin Wang), haoli.zhang@lzu.edu.cn (Hao-Li Zhang)
†These authors are contributed equally.

ACCEPTED MANUSCRIPT
molecules. Herein, we report two luminescence analogues, (2Z,4E)-2-(4-(Diphenylamino)phenyl)-5-phenylpenta-2,4-dienenitrile (1a)
and (2Z,4E)-2-(4-(acridin-10(9H)-yl)phenyl)-5-phenylpenta-2,4-dienenitrile (1b) (Figure 1). Their piezochromic fluorescence behavior
was tuned by subtle structural change. By linking the terminal benzene on the nitrogen with a methylene, 1a could be simply modified
to get the compound 1b. As most of piezochromic materials, the luminescence intensity of 1a decreased with increasing pressure
under hydrostatic pressure in a diamond anvil cell (DAC) [40]with the redshift of the emission wavelength. While the emission
wavelength of 1b had no obvious shift and the intensity increased gradually in the pressure range from 0.66 GPa to 5.72 GPa.
Figure 1. Structures of compounds 1a and 1b.
2. Experimental Section
2.1 General Information
Chemicals were purchased from commercial suppliers and used without further purification unless otherwise stated. Solvents
were dried and purified according to the standard procedures before use. Reactions were monitored by TLC. Flash column
chromatography was performed on silica gels (200-300 mesh). 1H NMR and 13C NMR (400 and 100 MHz, respectively) spectra were
recorded on a Bruker 400 MHz NMR spectrometer in CDCl3. 1H NMR chemical shifts are reported in ppm (δ) relative to
tetramethylsilane (TMS) with the solvent resonance employed as the internal standard (CDCl3, δ 7.26 ppm). Data are reported as
follows: chemical shift, multiplicity (s = singlet, d = doublet, t = triplet, m = multiplet), coupling constants (Hz) and integration. 13C NMR
chemical shifts are reported in ppm from tetramethylsilane (TMS) with the solvent resonance as the internal standard (CDCl3, δ 77.0
ppm). HRMS data were obtained on a Thermo Orbitrap Elite mass spectrometer (ESI). Melting points were determined by use of a
microscope apparatus which was uncorrected. Single crystal X-ray diffraction measurements were made on an Agilent SuprtNova
diffractometer working with mirror monochromated Cu-Kα or MoKα radiation.
2.2 Synthesis of 1b
Compound 1a was synthesized according to the procedure reported by us previously [41]. The analogue 1b was prepared by
Knoevenagel reaction and Buchwald-Hartwig cross-coupling reaction according to the route given in Scheme 1.

ACCEPTED MANUSCRIPT
Scheme 1. Synthesis of compound 1b.
2.2.1 Synthesis of 4
To a stirred solution of 2 (784 mg, 4.0 mmol) and cinnamaldehyde 3 (529 mg, 4.0 mmol) in EtOH (20 mL) was added freshly
prepared 1 M NaOCH3 solution in methanol (4.2 mL, 4.2 mmol). Then the mixture was stirred at room temperature for 10 h, and the
precipitate formed. The reaction mixture was filtered, and the solid was washed with cold EtOH to provide 4 as a yellow solid (744 mg,
1
60%). m.p. 124–125 °C; H NMR (400 MHz, CDCl3): δ = 7.58–7.52 (m, 4H), 7.51–7.46 (m, 2H), 7.43–7.33 (m, 5H), 7.09–7.00 ppm
(m, 1H); 13C NMR (100 MHz, CDCl3): δ = 148.2 (C), 137.8 (CH), 136.4 (C), 136.0 (CH), 128.8 (CH), 128.7 (CH), 127.1 (CH), 126.9
(CH), 125.8 (CH), 120.0 (C), 117.4 (C), 113.4 (C), 111.5 (CH), 44.4 (CH2), 12.6 ppm (CH3), one overlapping Ar-C resonance; HRMS
(ESI): m/z calcd for C17H13NBr+: 310.0226 [M+H]+; found: 310.0232.
2.2.2 Synthesis of 1b
To a mixture of 9,10-dihydroacridine (312 mg, 1.76 mmol), Pd(OAc)2 (9.0 mg, 0.04 mmol), P(t-Bu)3 (1.94 mg, 0.01 mmol), and
Cs2CO3 (1.043 g, 3.20 mmol) in dry toluene (60 mL) was added 4 (496 mg, 1.60 mmol). The mixture was refluxed under nitrogen until
the reaction was completed (24 h). Then, the reaction mixture was cooled to room temperature and filtered. The solvent was
removed under vacuum. The residue was purified by silica-gel flash chromatography (hexane/dichloromethane, 3/1) to give 1b as a
light yellow solid (426 mg, 65%). m.p. 250–251 °C; 1H NMR (400 MHz, CDCl3): δ = 7.88 (d, J = 8.4 Hz, 2H), 7.59 (d, J = 7.0 Hz, 2H),
7.55–7.35 (m, 7H), 7.16 (d, J = 7.0 Hz, 2H), 7.09 (d, J = 14.6 Hz, 1H), 7.00–6.93 (m, 2H), 6.91–6.85 (m, 2H), 6.22 (d, J = 8.1 Hz, 2H),
4.23 ppm (s, 2H); 13C NMR (100 MHz, CDCl3): δ = 142.4 (CH), 142.0 (CH), 135.7 (C), 133.2 (C), 132.1 (CH), 129.8 (CH), 129.0 (CH),
128.5 (CH), 128.0 (CH), 127.7 (CH), 126.7 (CH), 125.1 (CH), 121.0 (C), 120.8 (C), 116.8 (CH), 113.9 (CH), 112.3 (C), 31.9 ppm
(CH2), two overlapping Ar-C resonance; HRMS (ESI): m/z calcd for C30H21N2 : 409.1699 [M-H] +; found: 409.1694.
+
3. Results and Discussion
With the target compounds in hand, UV-vis absorption and emission spectra of 1a and 1b were first investigated (Figure S1). In
THF solution, compound 1a showed absorption band with the maximum (λmax) at 420 nm and 320 nm. In contrast, 1b exhibited an
absorption band with the λmax of 348 nm and a weak shoulder at 430 nm.
The fluorescence of 1a and 1b showed vast difference in different polar solvents (Figure 2). The fluorescence peak of 1a had
remarkably redshift (509 nm in toluene, 530 nm in THF, 552 nm in EtOAc, 583 nm in EtOH, 599 nm in acetone, and 622 nm in

ACCEPTED MANUSCRIPT
DMSO) with the increase of the solution polarity, which clearly illustrated that 1a was a typical D–A dipolar molecule with clear
intramolecular charge transfer (ICT) properties [15]. The fluorescence peaks of 1b in toluene, THF, and EtOAc were redshift about 50
nm successively (565 nm in toluene, 618 nm in THF, 664 nm in EtOAc). However, when compound 1b was dissolved in the more
polar solvents, the fluorescence peaks had an obvious bathochromic shift (546 nm in acetone, and 416 nm in DMSO). There were
two peaks at 520 nm and 422 nm in EtOH, which could be attributed to the ICT and locally excited state (LE) luminescence
respectively. In DMSO, there was only the LE emision at 416 nm.
Figure 2. Normalized fluorescence spectra of compounds 1a (left) and 1b (right) in different solvents (10–5 M).
To further understand the electronic transitions, density functional theory (DFT) calculations were performed with the geometry
optimized at the B3LYP/6-31G (d) level (Table S1 and S2). It’s obvious that the configuration of the diphenyl-diethylene pigment of 1a
and 1b both inclined to form a planar configuration. However, the configuration of the two terminal phenyl rings were inconsistent.
The diphenylamine pigment of 1a was a propeller-like shape, and the torsion angles were small which facilitated the electronic
transfer between the two moieties. In contrast, the acridine pigment of 1b was approximately perpendicular to the diphenyl-diethylene
pigment, which baffled the electronic transfer. Based on the optimized configuration, the TD-DFT calculation were performed at the
B3LYP/6-31G level. According to the calculation results, the absorption at 420 nm and 320 nm of 1a were attributed to the transition
from HOMO to LUOM (S1) and from HOMO-1 to LUMO (S2) respectively. But the intensities (f) were greatly different. Both the
transitions from S0 to S1 (f = 0.8884, 487.20 nm) and from S0 to S2 (f = 0.7641, 356.93 nm) of 1a were strong, so the absorption at
420 nm and 320 nm both were strong. While, the f (S0-S1, 0.0002, 579.97 nm) of 1b was extremely small and the f (S0-S2, 1.5381,
380.24 nm) was large enough, so the absorption band only located at 348 nm. Correspondingly, the transition intensities also
reflected in the fluorescence spectrum. The solution emission of 1a located at 548 nm, which could be ascribed to the transition from
S1 state according to the Kasha’s rule [42]. The emission of 1b in DMSO located at 416 nm (LE emission), which could be ascribed to
the transition from high excited state [43]. While the emission from THF solution (618 nm) could be ascribed to a twisted ICT [44-46].
In an effort to understand the effect of the molecular conformations and packing arrangements on the emission properties, the
crystals of 1a (the CCDC numbler is 1879514) and 1b (the CCDC number is 1879515) were formed by the slow evaporation of
solvents in DMSO and DCM solution respectively. In the crystal state, 1a exhibited a strong orange emission at λmax = 570 nm with a
41.2% fluorecence quantum yeild when excited by 423 nm light (Figure 3). By contrast, 1b showed a yellow fluorescence emission at
λmax = 523 nm with a lower fluorecence quantum yield at 18.1%.
To explore the modulation of piezochromic fluorescence behavior by molecular structure and packing in crystal, we applied
hydrostatic pressure on the crystals in a diamond anvil cell (DAC) and detected the changes of the fluorescence, excited with 523 nm
laser [47]. The two compounds show completely different piezochromic fluorescence behavior. When the external pressure was

ACCEPTED MANUSCRIPT
gradually increased from 1.37 GPa to 15.23 GPa, the emission wavelength of 1a crystal underwent a remarkable variation from 581
nm to 686 nm and its intensity gradually decreased (Figure 3). As a contrast, the emission wavelength of 1b had no obvious shift and
its intensity increased gradually at the pressure range from 0.66 GPa to 5.72 GPa (Figure 3). As compression went beyond 5.72 GPa,
the fluorescence began to be redshift (688 nm at 19.05 GPa) and weak (Figure 3).
Figure 3. a) Fluorescence spectra of 1a and 1b crystals excited by 423 nm. b) Fluorescence spectra of 1a crystal excited by 532 nm laser under external
pressure. c) and d). Fluorescence spectra of 1b crystal excited by 532 nm laser under external pressure.
To understand the effects of the molecular conformations and packing arrangements on ICT piezochromic fluorescence behavior,
the structural analysis of the single crystals of 1a and 1b and the analog calculation based on the conformations in the crystals under
different pressures were performed. Single crystal X-ray diffraction measurements were used to the structural analysis of the crystals
of compounds 1a and 1b. On the basis of crystal structures, the packing geometries of 1a and 1b at 0, 2, and 4 GPa were optimized
by the density functional theory (DFT) simulation by the CASTEP code [12, 13, 48, 49].
a)
b)
I
c)
II
Figure 4. a) The packing pattern of 1a crystal. b) the packing and structure of conformation I. c) the π-π interactions of the conformation II.

ACCEPTED MANUSCRIPT
As was shown in Figure 4, two conformations were observed in the crystal of 1a. There was no π-π interaction in conformation I,
and the twist angle between the A, B, and C benzene rings and the central plane were 45.32° (θA), 41.16° (θB), and 26.45° (θC)
respectively. However, the conformation II adopted head-to-tail stacking, in which the intermolecular π-π interaction was detected
and the distance is 3.335 Å. The dihedral angle between the A, B, and C benzene rings and the central plane were changed to
36.48° (θA), 45.02° (θB), and 33.52° (θC) respectively. The distance of secondary bond interactions (C–H…π and C–H…N) existing in
two neighboring molecules, were range from 2.709 Å and 2.869 Å. The strong and multiple secondary bond interactions played a
significance role in crystal stability. The molecule was locked by the secondary bond interactions, which largely reduced the energy
loss via non-radiative relaxation channels and made 1a highly emissive [50].
The simulation calculations on the basis of the structure data of single crystal were employed to elucidate the piezochromic
mechanism. The TD-DTF calculation results indicated that the transition from HOMO to LUMO (ICT) was permitted (f kept high level,
0.6495 to 0.9054) for both conformation I and II of 1a at 0, 2, and 4 GPa (Figure 5). When external pressure was applied, the twist
angle between the A, B, and C benzene rings and the central plane decreased gradually in two conformation. When 2 GPa pressure
was applied, The bandgap between HOMO and LUMO obviously reduced (∆EI from 2.54 eV to 2.38 eV, ∆EII from 2.65 eV to 2.45 eV)
compared with it at 0 GPa. However, when the external pressure increased to 4 GPa from 2 GPa, the calculation results of
conformation I and II were different. The bandgap between HOMO and LUMO was almost same for conformation I, but it decreased
for conformation II. Thus, the conformation change from external pressure resulted in the red-shift emission. Also, when the high
pressure compressed the crystal, the intermolecular distance drastically shortened which could enhance the polarization effect of
adjacent molecules and would reduce the energy of the lowest excited state and the radiative transition rate, leading to a red-shift in
the fluorescent wavelength and a decrease in PL intensity [14, 51].
Figure 5. Energy diagrams and the frontier orbitals contribution of 1a in fixed conformation based on the gradually decreased torsion angel (θ) between
dihydroacridine and central benzene ring, and their lowest- and second-lowest-energy transitions estimated by TD-DFT calculations at the B3LYP/6-31G(d)*level.
As was shown in Figure 6, the molecules of 1b in crystal was displayed a highly twisted molecular conformation. The dihedral
angle between the central phenyl and the acridine pigment was 83.85° (θ). The π-π interaction distance between the adjacent
molecules was 3.311 Å, but the overlap between the adjacent molecules was small. Similar with 1a, multiple weak intermolecular C–
H…π and C–H…N interactions were also detected.

ACCEPTED MANUSCRIPT
a)
c)
b)
Figure 6. a) The packing pattern of 1b. b) weak interactions in the crystal. c) the dihedral angle between the middle benzene ring and the acridin pigment.
Based on the highly twisted conformation of 1b in the initial crystal, the oscillator strength transition from HOMO to LUMO (ICT)
was very small (f = 0.0097, Figure 7) according to the calculation results. When 2 GPa pressure was applied, the torsion angle
decreased to 82.36°, the transition from HOMO to LUMO enormously improved (f = 0.0224), and the bandgap significantly reduced.
With the pressure increased (4 GPa), the torsion angle further decreased (79.60°). However, the bandgap enlarged with the intensity
increased continuously. Thus, under the external pressure, the ICT emission was turned on and its intensity increase with the
pressure increase without waveleghth redshift, which matched the experiment data (0.55 GPa to 5.72 GPa). When the pressure
continuously increased (over 5.75 GPa), the ICT emission wavelength was red shift and the intensity decreased, which was simillar
with that of 1a.
Figure 7. Energy diagrams and the frontier orbitals contribution of 1b in fixed conformation based on the gradually decreased torsion angel (θ) between
dihydroacridine and central benzene ring, and their lowest- and second-lowest-energy transitions estimated by TD-DFT calculations at the B3LYP/6-31G(d)*level.

ACCEPTED MANUSCRIPT
4. Conclusions
Two D-A fluorophores (1a and 1b) had been synthesized. Their fluorescence properties were modulated by the little difference of
the molecular structures. The ICT piezochromic fluorescence behaviors were completely different when their crystals were excited
with 532 nm laser. Under the crescent pressure, the emission peaks of compound 1a in crystal exhibited gradual red-shift and the
emission intensity became weak, which was attributed to the intermolecular interaction enhancement. But for compound 1b, the
emission peaks kept at 590 nm under the increased pressure from 0.55 GPa to 5.72 GPa and the intensities were increased
graudally because the ICT emission was turned on by the external pressure. Both the color-switchable feature of 1a and the intensity-
switchable feature of 1b may have potential applications for the pressure-sensing materials.
Acknowledgements
This work was supported by the National Natural Science Foundation of China (21572086, 21572085, and 51733004); the
Program for Changjiang Scholars and Innovative Research Team in University (PCSIRT: IRT_15R28); the Fundamental Research
Funds for the Central University (FRFCU: lzujbky-2016-ct02); and the 111 Project of MOE (111-2-17) for financial supports.
Appendix A. Supplementary data
Supplementary is related to this article can be found at:
References
[1] Harun-ur-Rashid M, Seki T, Takeoka Y. Structural colored gels for tunable soft photonic crystals. Chem Rec 2009;9:87−105. https://doi.org/10.1002/tcr.20169.
[2] Sagara Y, Kato T. Mechanically induced luminescence changes in molecular assemblies. Nat Chem 2009;1:605−10. https://doi.org/10.1038/nchem.411.
[3] Teng M-J, Jia X-R, Yang S, Chen X-F, Wei Y. Reversible tuning luminescent color and emission intensity: a dipeptide-based light-emitting material. Adv Mater
2012;24:1255−61. https://doi.org/10.1002/adma.201104592.
[4] Mei J, Hong Y, Lam JWY, Qin A, Tang Y, Tang BZ. Aggregation-induced emission: the whole is more brilliant than the parts. Adv Mater 2014;26:5429−79.
https://doi.org/10.1002/adma.201401356.
[5] Wang L, Wang K, Zou B, Ye K, Zhang H, Wang Y. Luminescent chromism of boron diketonate crystals: distinct responses to different stresses. Adv Mater
2015;27:2918−22. https://doi.org/10.1002/adma.201500589.
[6] Sagara Y, Yamane S, Mitani M, Weder C, Kato T. Mechanoresponsive luminescent molecular assemblies: an emerging class of materials. Adv Mater
2016;28:1073−95. https://doi.org/10.1002/adma.201502589.
[7] Sagara Y, Mutai T, Yoshikawa I, Araki K. Material design for piezochromic luminescence:ꢀ hydrogen-bond-directed assemblies of a pyrene derivative. J Am
Chem Soc 2007;129:1520−1. https://doi.org/10.1021/ja0677362.
[8] Zhang G, Lu J, Sabat M, Fraser CL. Polymorphism and reversible mechanochromic luminescence for solid-state difluoroboron avobenzone. J Am Chem Soc
2010;132:2160−2. https://doi.org/10.1021/ja9097719.
[9] Varghese S, Das S. Role of molecular packing in determining solid-state optical properties of π-conjugated materials. J Phys Chem Lett 2011;2:863−73.
https://doi.org/10.1021/jz200099p.

ACCEPTED MANUSCRIPT
[10] Chi Z, Zhang X, Xu B, Zhou X, Ma C, Zhang Y, et al. Recent advances in organic mechanofluorochromic materials. Chem Soc Rev 2012;41:3878−96.
https://doi.org/10.1039/C2CS35016E.
[11] Dong Y, Xu B, Zhang J, Tan X, Wang L, Chen J, et al. Piezochromic luminescence based on the molecular aggregation of 9,10-bis((E)-2-(pyrid-2-
yl)vinyl)anthracene. Angew Chem 2012;124:10940−3. https://doi.org/10.1002/ange.201204660.
[12] Nagura K, Saito S, Yusa H, Yamawaki H, Fujihisa H, Sato H, et al. Distinct responses to mechanical grinding and hydrostatic pressure in luminescent
chromism of tetrathiazolylthiophene. J Am Chem Soc 2013;135:10322−5. https://doi.org/10.1021/ja4055228.
[13] Qi Q, Qian J, Tan X, Zhang J, Wang L, Xu B, et al. Remarkable turn-on and color-tuned piezochromic luminescence: mechanically switching intramolecular
charge transfer in molecular crystals. Adv Funct Mater 2015;25:4005−10. https://doi.org/10.1002/adfm.201501224.
[14] Wang X, Liu Q, Yan H, Liu Z, Yao M, Zhang Q, et al. Piezochromic luminescence behaviors of two new benzothiazole-enamido boron difluoride complexes:
intra- and inter-molecular effects induced by hydrostatic compression. Chem Commun 2015;51:7497−500. https://doi.org/10.1039/C5CC01902H.
[15] Zhang Y, Wang K, Zhuang G, Xie Z, Zhang C, Cao F, et al. Multicolored-fluorescence switching of ICT-type organic solids with clear color difference:
mechanically controlled excited state. Chem Eur J 2015;21:2474−9. https://doi.org/10.1002/chem.201405348.
[16] Wu J, Cheng Y, Lan J, Wu D, Qian S, Yan L, et al. Molecular engineering of mechanochromic materials by programmed C–H arylation: making a
counterpoint in the chromism trend. J Am Chem Soc 2016;138:12803−12. https://doi.org/10.1021/jacs.6b03890.
[17] Zhang Y, Feng Y-Q, Wang J-H, Han G, Li M-Y, Xiao Y, et al. Moiety effect on the luminescent property of starshaped triphenylamine (TPA) derivatives as
mechanochromic materials. RSC Adv 2017;7:35672−80. https://doi.org/10.1039/c7ra03123h.
[18] Wu J, Wang H, Xu S, Xu W. Comparison of shearing force and hydrostatic pressure on molecular structures of triphenylamine by fluorescence and Raman
spectroscopies. J Phys Chem A 2015;119:1303−8. https://doi.org/10.1021/jp511380a.
[19] Liang Y, Peng B, Liang J, Tao Z, Chen J. Triphenylamine-based dyes bearing functionalized 3,4-propylenedioxythiophene linkers with enhanced performance
for dye-sensitized solar cells. Org Lett 2010;12:1204−7. https://doi.org/10.1021/ol902973r.
[20] Patra A, Anthony SP, Radhakrishnan TP. Tris(4-cyanophenyl)amine: simple synthesis via self-assembly; strong fluorescence in solution, nano/microcrystals,
and solid. Adv Funct Mater 2007;17:2077−84. https://doi.org/ 10.1002/adfm.200600931.
[21] Kundu A, Karthikeyan S, Moon D, Anthony SP. Self-reversible thermofluorochromism of D-A-D triphenylamine derivatives and the effect of molecular
conformation and packing. Crystengcomm 2017;19:6979−85. https://doi.org/10.1039/c7ce01538k.
[22] Ouyang M, Zhan L, Lv X, Cao F, Li W, Zhang Y, et al. Clear piezochromic behaviors of AIE-active organic powders under hydrostatic pressure. RSC Adv
2016;6:1188−93. https://doi.org/10.1039/C5RA21218A.
[23] Sun J, Dai Y, Ouyang M, Zhang Y, Zhan L, Zhang C. Unique torsional cruciform π-architectures composed of donor and acceptor axes exhibiting
mechanochromic and electrochromic properties. J Mater Chem C 2015;3:3356−63. https://doi.org/10.1039/C4TC02046D.
[24] Sun J, Lv X, Wang P, Zhang Y, Dai Y, Wu Q, et al. A donor–acceptor cruciform π-system: high contrast mechanochromic properties and multicolour
electrochromic behavior. J Mater Chem C 2014;2:5365−71. https://doi.org/10.1039/C4TC00516C.
[25] Zhang Y, Zhuang G, Ouyang M, Hu B, Song Q, Sun J, et al. Mechanochromic and thermochromic fluorescent properties of cyanostilbene derivatives. Dyes
and Pigments 2013;98:486−92. https://doi.org/10.1016/j.dyepig.2013.03.017.
[26] Chen W, Wang S, Yang G, Chen S, Ye K, Hu Z, et al. Dicyanomethylenated acridone based crystals: torsional vibration confinement induced emission with
supramolecular structure dependent and stimuli responsive characteristics. J Phys Chem C 2016;120:587−97. https://doi.org/10.1021/acs.jpcc.5b10954.
[27] Hariharan PS, Moon D, Anthony SP. Reversible fluorescence switching and topochemical conversion in an organic AEE material: polymorphism, defection
and nanofabrication mediated fluorescence tuning. J Mater Chem C 2015;3:8381−8. https://doi.org/10.1039/C5TC01369K.
[28] Wei J, Liang B, Cheng X, Zhang Z, Zhang H, Wang Y. High-contrast and reversible mechanochromic luminescence of a D–π–A compound with a twisted
molecular conformation. RSC Adv 2015;5:71903−10. https://doi.org/10.1039/C5RA12050K.
[29] Cao Y, Xi W, Wang L, Wang H, Kong L, Zhou H, et al. Reversible piezofluorochromic nature and mechanism of aggregation-induced emission-active
compounds based on simple modification. RSC Adv 2014;4:24649−52. https://doi.org/10.1039/C4RA03042G.
[30] Hariharan PS, Mothi EM, Moon D, Anthony SP. Halochromic isoquinoline with mechanochromic triphenylamine: smart fluorescent material for rewritable and
self-erasable fluorescent platform. Acs Appl Mater Interfaces 2016;8:33034−42. https://doi.org/10.1021/acsami.6b11939.

ACCEPTED MANUSCRIPT
[31] Chen Z, Liu G, Pu S, Liu SH. Triphenylamine, carbazole or tetraphenylethylene-based gold(I) complexes: tunable solid-state room-temperature
phosphorescence and various mechanochromic luminescence characteristics. Dyes and Pigments 2018;159:499−505.
https://doi.org/10.1016/j.dyepig.2018.07.016.
[32] Wong BY-W, Wong H-L, Wong Y-C, Au VK-M, Chan M-Y, Yam VW-W. Multi-functional bis(alkynyl) gold(III) N^C complexes with distinct mechanochromic
luminescence and electroluminescence properties. Chem Sci 2017;8:6936−46. https://doi.org/10.1039/c7sc02410j.
[33] Hariharan PS, Parthasarathy G, Kundu A, Karthikeyan S, Sagara Y, Moon D, et al. Drastic modulation of stimuli-responsive fluorescence by a subtle
structural change of organic fluorophore and polymorphism controlled mechanofluorochromism. Cryst Growth Des 2018;18:3971−9.
https://doi.org/10.1021/acs.cgd.8b00310.
[34] Zhang Y, Miao Y, Song X, Gao Y, Zhang Z, Ye K, et al. Single-molecule-based white-light emissive organic solids with molecular-packing-dependent
thermally activated delayed fluorescence. J Phys Chem Lett 2017;8:4808−13. https://doi.org/10.1021/acs.jpclett.7b02213.
[35] Qi C, Ma H, Fan H, Yang Z, Cao H, Wei Q, et al. Study of red-emission piezochromic materials based on triphenylamine. ChemPlusChem 2016;81:637−45.
https://doi.org/10.1002/cplu.201600104.
[36] Lei Y, Liu Y, Guo Y, Chen J, Huang X, Gao W, et al. Multi-stimulus-responsive fluorescent properties of donor-π-acceptor indene-1,3-dionemethylene-1,4-
dihydropyridine derivatives. J Phys Chem C 2015;119:23138−48. https://doi.org/10.1021/acs.jpcc.5b06432.
[37] Song Q, Chen K, Sun J, Wang Y, Ouyang M, Zhang C. Mechanical force induced reversible fluorescence switching of two 3-aryl-2-cyano acrylamide
derivatives. Tetrahedron Lett 2014;55:3200−5. https://doi.org/10.1016/j.tetlet.2014.04.024.
[38] Sun H, Zhang Y, Yan W, Chen W, Lan Q, Liu S, et al. A novel ultrasound-sensitive mechanofluorochromic AIE-compound with remarkable blue-shifting and
enhanced emission. J Mater Chem C 2014;2:5812−7. https://doi.org/10.1039/C4TC00553H.
[39] Zhang Z, Wu Z, Sun J, Yao B, Xue P, Lu R. β-Iminoenolate boron complex with terminal triphenylamine exhibiting polymorphism and
mechanofluorochromism. J Mater Chem C 2016;4:2854−61. https://doi.org/10.1039/C5TC02386F.
[40] Mao HK, Xu J, Bell PM. Calibration of the ruby pressure gauge to 800 kbar under quasi-hydrostatic conditions. J Geophys Res B 1986;91:4673−6.
https://doi.org/10.1029/JB091iB05p04673.
[41] An P, Xu N-S, Zhang H-L, Cao X-P, Shi Z-F, Wen W. Facile preparation of α-cyano-α,ω-diaryloligovinylenes: a new class of color-tunable solid emitters.
Chem Asian J 2015;10:1959−66. https://doi.org/10.1002/asia.201500473.
[42] Kasha M. Characterization of electronic transitions in complex molecules. Discuss Faraday Soc 1950;9:14−9. https://doi.org/10.1039/DF9500900014.
[43] Qian H, Cousins ME, Horak EH, Wakefield A, Liptak MD, Aprahamian I. Suppression of Kasha’s rule as a mechanism for fluorescent molecular rotors and
aggregation-induced emission. Nat Chem 2016;9:83−7. https://doi.org/10.1038/nchem.2612.
[44] Rettig W. Charge separation in excited states of decoupled systems: twisted intramolecular charge transfer (TICT) compounds and implications for the
development of new laser dyes and for the primary process of vision and photosynthesis. Angew Chem 1986;98:969−86.
https://doi.org/10.1002/ange.19860981104.
[45] Wu J, Liu W, Ge J, Zhang H, Wang P. New sensing mechanisms for design of fluorescent chemosensors emerging in recent years. Chem Soc Rev
2011;40:3483−95. https://doi.org/10.1039/c0cs00224k.
[46] Jamorski Jödicke C, Lüthi HP. Time-dependent density functional theory (TDDFT) study of the excited charge-transfer state formation of a series of aromatic
donor−acceptor systems. J Am Chem Soc 2003;125:252−64. https://doi.org/10.1021/ja020361+.
[47] Sui Q, Ren X-T, Dai Y-X, Wang K, Li W-T, Gong T, et al. Piezochromism and hydrochromism through electron transfer: new stories for viologen materials.
Chem Sci 2017;8:2758−68. https://doi.org/10.1039/c6sc04579k.
[48] Perdew JP, Burke K, Ernzerhof M. Generalized gradient approximation made simple. Phys Rev Lett 1996;77:3865−8.
https://doi.org/10.1103/PhysRevLett.77.3865.
[49] , Clark SJ, Segall MD, Pickard CJ, Hasnip PJ, Probert MIJ, et al. First principles methods using CASTEP. Z Kristallogr 2005;220:567−70.
https://doi.org/10.1524/zkri.220.5.567.65075.
[50] Yuan WZ, Tan Y, Gong Y, Lu P, Lam JWY, Shen XY, et al. Synergy between twisted conformation and effective intermolecular interactions: strategy for
efficient mechanochromic luminogens with high contrast. Adv Mater 2013;25:2837−43. https://doi.org/10.1002/adma.201205043.
[51] Feng C, Wang K, Xu Y, Liu L, Zou B, Lu P. Unique piezochromic fluorescence behavior of organic crystal of carbazole-substituted CNDSB. Chem Commun
2016;52:3836−9. https://doi.org/10.1039/C5CC09152G.

ACCEPTED MANUSCRIPT

ACCEPTED MANUSCRIPT
Highlights
ò
ò
Molecules 1a and 1b with donor-acceptor structure were synthesized.
Their piezochromic fluorescence behavior was modulated by a small difference in their
structures.
ò
ò
The luminescence intensity of 1a decreased with increasing hydrostatic pressure with
the redshift of the emission wavelength.
The emission wavelength of 1b which differs from 1a by a methylene unit had no
obvious shift and the intensity increased gradually at the pressure range from 0.66 GPa
to 5.72 GPa.