Mechanofluorochromism based on BOPIM complexes: The effect of substituents and regulation of the direction of the emission color changes

Article abstract of DOI:10.1016/j.dyepig.2019.04.001

Novel boron 2-(2′-pyridyl)imidazole (BOPIM) complexes T1, T2 and T3 with different substituents (including bromo, tert-butyl and methoxyl) on the benzene ring of BOPIM dyes have been designed and synthesized, and their optical properties in both solution and the solid state were investigated and compared. The three compounds exhibited typical intramolecular charge transfer (ICT) characteristics. Solvent-dependent UV-vis absorption, fluorescence emission spectra and quantum chemical calculation indicated a molecular push-pull electronic structure. Their ICT degrees increased with the sequence of T1 < T2 < T3. The analysis of the X-ray crystal structure revealed the twisted molecular conformation of BOPIM dyes. Furthermore, they showed remarkable reversible mechanofluorochromic (MFC) features under mechanical force. It was found that the MFC activities could be tuned easily by changing the substituents on the BOPIM dyes. T1 exhibited emission color change from bright green to yellowish green with a spectral red-shift of only 22 nm under mechanical stimuli, whereas T2 and T3 gave the large spectral red-shifts of 36 and 30 nm. Electronic and steric effects of the substitutes were proved playing significant roles in regulating the ICT effect and intermolecular interactions. More importantly, the remarkable effect of substituents on the MFC behaviors of BOPIM dyes will provide an effective way to obtain novel high-contrast MFC dyes.

Full text of DOI:10.1016/j.dyepig.2019.04.001

DyesandPigments167(2019)1–9
Contents lists available at ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Mechanofluorochromism based on BOPIM complexes: The effect of
substituents and regulation of the direction of the emission color changes
Yong Zhana,∗, Xin Wangb, Yunhan Wangb, Yongnan Xub
a Department of Pharmacy, College of Life Sciences, China Jiliang University, Hangzhou, 310018, PR China
b Key Laboratory of Structure-Based Drug Design and Discovery, Ministry of Education, Shenyang Pharmaceutical University, Shenyang, 110016, PR China
A B S T R A C T
Novel boron 2-(2′-pyridyl)imidazole (BOPIM) complexes T1, T2 and T3 with different substituents (including bromo, tert-butyl and methoxyl) on the benzene ring of
BOPIM dyes have been designed and synthesized, and their optical properties in both solution and the solid state were investigated and compared. The three
compounds exhibited typical intramolecular charge transfer (ICT) characteristics. Solvent-dependent UV-vis absorption, fluorescence emission spectra and quantum
chemical calculation indicated a molecular push-pull electronic structure. Their ICT degrees increased with the sequence of T1 < T2 < T3. The analysis of the X-ray
crystal structure revealed the twisted molecular conformation of BOPIM dyes. Furthermore, they showed remarkable reversible mechanofluorochromic (MFC)
features under mechanical force. It was found that the MFC activities could be tuned easily by changing the substituents on the BOPIM dyes. T1 exhibited emission
color change from bright green to yellowish green with a spectral red-shift of only 22 nm under mechanical stimuli, whereas T2 and T3 gave the large spectral red-
shifts of 36 and 30 nm. Electronic and steric effects of the substitutes were proved playing significant roles in regulating the ICT effect and intermolecular inter-
actions. More importantly, the remarkable effect of substituents on the MFC behaviors of BOPIM dyes will provide an effective way to obtain novel high-contrast MFC
dyes.
1. Introduction
boron-difluoride molecule (BODIPY) feature high degree of planarity,
resulting in fluorescent quenching in the aggregated state [12]. Gen-
erally, the strong emission in the solid state is the prerequisite for ex-
cellent MFC materials. Thus, a series of boron 2-(2′-pyridyl)-imidazole
(BOPIM) complexes with strong fluorescent emission in the solid state
might be candidates for MFC materials [13]. These molecules not only
have high fluorescence quantum yield, large Stokes shift, and good
thermal stability, but also can interact with adjacent molecules through
multiple intermolecular non-covalent bondings to significantly alleviate
π-π stacking-induced fluorescence quenching [14]. To date, BOPIM
dyes are rarely explored as MFC materials. In addition, structural tai-
loring of BOPIM dyes for fine-tuning of their photophysical properties
has offered great chances toward organic fluorescent materials. With
these in mind, herein, we reported the synthesis, photophysical prop-
erties and MFC behaviors of three BOPIM based fluorescent dyes (T1,
T2 and T3, Fig. 1), in which different substituents (including bromo,
tert-butyl and methoxyl) are selected to attach to the benzene ring of
BOPIM dyes. These compounds exhibited high fluorescence quantum
yield in both solution and the solid state. The optical properties of
BOPIM dyes were studied and compared. The emission color could be
tuned via the introduction of different substituents on the BOPIM dyes.
The single crystal structure of T1 reflected the non-planar orientation
between BOPIM core and benzene rings, which leaded to close π-π
stacking. It is interesting that three dyes revealed reversible
In recent years, the exploitation of solid-state fluorescent organic
materials has attracted a great deal of attention due to their potential
application in photoelectronic devices and sensors [1]. Among these
materials, mechanofluorochromic (MFC) materials exhibiting variable
colors in response to external forces are attracting intensive interest due
to their potential applications in sensors, memory chips and security
systems [2]. Up to now, various kinds of π-conjugated organic molecule
such as tetraphenylethene [3], triphenylacrylonitrile [4], cyanoethy-
lene [5], 9,10-divinylanthracene [6] and difluoroboron β-diketone
(BF2dbk) boron complexes [7] have been found to show obvious me-
chanochromic luminescence change under mechanical force stimuli.
The molecular structure, conformation, packing modes and inter-
molecular interactions play an important role in modulating fluores-
cence under mechanical forces [8]. A short synthetic route and simple
molecular structure are preferred. However, there are extremely rare of
clear design strategy of already reported MFC materials [9]. In-
vestigations on the groups of structural analogues are usually an ef-
fective way to understand the MFC mechanisms and explain the cor-
responding structural feature [10].
Recently, organoboron fluorescent dyes have attracted considerable
attention because of their high fluorescence yield, large absorption
coefficient and excellent photochemical stability [11]. However, typical
∗ Corresponding author.
E-mail address: zhanyong@cjlu.edu.cn (Y. Zhan).
https://doi.org/10.1016/j.dyepig.2019.04.001
Received 11 January 2019; Received in revised form 1 March 2019; Accepted 1 April 2019
Availableonline08April2019
0143-7208/©2019PublishedbyElsevierLtd.

Y. Zhan, et al.
DyesandPigments167(2019)1–9
Fig. 1. Molecular structures of BOPIM dyes T1-T3.
mechanofluorochromism through grinding and solvent fuming treat-
ment. The MFC activities of three compounds are increased in a se-
quence of T1 < T3 < T2. For T1, it showed minimum spectral red-
shift upon external force due to its least ICT degree in the excited state
in the three luminogens. In the case of T2 and T3, although T2 ex-
hibited the less ICT degree than T3, T2 exhibited the maximum MFC
behaviors. We proposed that the introduction of tert-butyl owing to the
steric hindrance effect would lead to more loose π-π stacking in ag-
gregated state, which was favorable for yielding efficient MFC [15]. The
above results demonstrated that the MFC behaviors of BOPIM dyes
were influenced by electronic and steric effects of the substitutes. Ra-
tional introduction of substituents allowed convenient modulation of
photophysical properties and MFC behaviors of BOPIM dyes.
2.2. Synthesis process
2.2.1. 2-(4,5-Bis(4-bromophenyl)-1H-imidazol-2-yl)pyridine (L1)
2-Cyanopyridine (0.53 g, 5 mmol), 4-bromobenzaldehyde (1.85 g,
10 mmol) and NH4OAc (3.85 g, 50 mmol) were dissolved in 6 mL of
acetic acid and the mixture was heated at 150 °C overnight to reflux in a
round-bottom flask. After cooling to room temperature, the mixture was
poured into saturated solution of NaHCO3 and acidified with 0.1 M
hydrochloric acid. A pale yellow solid was collected by filtration and
dried under vacuum. The crude product was purified by column chro-
matography (silica gel) using petroleum ether/ethyl acetate (v/v = 5:1)
to give L1 as a white solid (1.20 g) in a yield of 53%. 1H NMR
(600 MHz, TMS, DMSO‑d6) δ = 13.31 (s, 1H), 8.65 (d, J = 4.2 Hz, 1H),
8.14 (d, J = 7.8 Hz, 1H), 7.94–7.91 (m, 1H), 7.61 (d, J = 8.0 Hz, 2H),
7.53 (d, J = 8.4 Hz, 2H), 7.48–7.44 (m, 4H), 7.43–7.41 (m, 1H) (Fig.
S5, ESI); HRMS (ESI): m/z = 455.1453, found: 455.9536 [M+H]+
(Fig. S6, ESI).
2. Experimental section
2.1. Materials and measurements
2.2.2. 2,3-Bis(4-bromophenyl)-5,5-difluoro-5H-5l4,6l4-imidazo[1′,2':3,4]
[1–3]di-azaborolo [1,5-a]pyridine (T1)
1H and 13C NMR spectra were measured on a Bruker AMX-400 NMR
spectrometer at 600 MHz and 150 MHz in DMSO as the solvent at room
temperature. FT-IR spectra were measured with a Nicolet-360 FT-IR
spectrometer by the incorporation of samples in KBr disks. High re-
solution mass spectrometry analysis was carried out on an Agilent 6500
Q-TOF mass spectrometer (Version Q-TOF B.05.01). The UV-vis ab-
sorption spectra were determined on a Beijing purkinje TU-1810
Spectrophotometer. Fluorescence emission spectra were obtained on a
Shimadzu RF-5301 PC Spectrofluorophotometer. The fluorescence
quantum yields in solvents were estimated by comparing to a standard
(9,10-diphenyl anthracene in benzene, ΦF = 0.85). The absolute
fluorescent quantum yields were estimated on an Edinburgh FLS920
steady state spectrometer using an integrating sphere. Powder X-ray
diffraction (XRD) measurements were carried out with a Bruker
Advances D8 X-ray diffractometer. DSC measurements were carried out
using a Mettler Toledo 3 thermos system at a heating rate of 10 °C/min.
The frontier orbital plots of the HOMO and LUMO of T1, T2, and T3
were obtained by density functional theory (DFT) calculations at the
B3LYP/6-31G level with the Gaussian 09W program package. Single
crystal of T1 was obtained by slowing solvent evaporation in mixture of
CH2Cl2 and petroleum ether, and selected for X-ray diffraction analysis
To a solution of compound L1 (1.0 g, 2.2 mmol) in dry CH2Cl2
(30 mL), then Et3N (4.4 mL, 31.7 mmol) was added, and the mixture
was heated under N2 atmosphere to reflux. After that, BF3·Et2O (4.4 mL,
34.9 mmol) was added dropwise. The mixture was refluxed with stir-
ring for 8 h under an atmosphere of nitrogen. After cooling to room
temperature, the reaction was quenched by adding water (100 mL), and
the mixture was extracted with CH2Cl2 (3 × 50 mL). The organic phase
was dried over anhydrous MgSO4. After removal of solvent, the residue
was purified by column chromatography on silica gel using petroleum
ether/ethyl acetate (v/v = 3:1), then recrystallized from CH2Cl2 and
petroleum ether to afford T1 as a green solid (0.30 g) in a yield of 27%.
Mp: 228 °C (obtained from DSC); 1H NMR (600 MHz, TMS, DMSO‑d6)
δ = 8.92 (d, J = 5.4 Hz, 1H), 8.53 (td, J = 7.8 Hz, 1.2 Hz, 1H), 8.24 (d,
J = 7.8 Hz, 1H), 7.85–7.83 (m, 1H), 7.70–7.67 (m, 2H), 7.57–7.55 (m,
2H), 7.46–7.44 (m, 4H) (Fig. S7, ESI); 13C NMR (150 MHz, DMSO‑d6) δ
(ppm) = 149.49, 147.20, 146.37, 145.93, 143.97, 143.39, 143.14,
137.94, 133.78, 132.42, 131.89, 130.68, 130.50, 130.02, 129.89,
125.67, 124.26, 122.25, 121.29, 120.94, 120.72, 118.60 (Fig. S8, ESI);
IR (KBr, cm−1): 3439, 2923, 1634, 1548, 1477, 1400, 1164, 1127,
1011, 833, 715, 591; HRMS (ESI): m/z = 502.9452, found: 503.9516
[M+H]+ (Fig. S9, ESI).
in
a Rigaku RAXIS-RAPID diffractometer using graphite-mono-
chromated Mo-Kα radiation (λ = 0.71073 Å). The crystal was kept at
room temperature during data collection. The structures were solved by
the direct methods and refined on F2 by full-matrix least-square using
the SHELXTL-97 program. The C, N, O and H atoms were easily placed
from subsequent Fourier-difference maps and refined anisotropically.
CCDC number for T1: 1858002. DCM was distilled over calcium hy-
dride under nitrogen immediately prior to use. The other chemicals
were used as received without further purification. The ground powders
were obtained by grinding the as-prepared powders with a pestle in the
mortar. The fumed samples were prepared by fuming the ground
powder with DCM for 1 min.
2.2.3. 2-(4,5-Bis(4-tert-butylphenyl)-1H-imidazol-2-yl)pyridine (L2)
By following the synthetic procedure for L1, L2 was synthesized by
using 2-cyanopyridine (0.53 g, 5 mmol), 4-tert-butylbenzaldehyde
(1.62 g, 10 mmol) and NH4OAc (3.85 g, 50 mmol) as the reagents. The
crude product was purified by column chromatography (silica gel)
using petroleum ether/ethyl acetate (v/v = 6:1) to give L2 as a white
solid (1.10 g) in a yield of 54%. 1H NMR (600 MHz, TMS, DMSO‑d6)
δ = 12.96 (s, 1H), 8.63–8.62 (m, 1H), 8.11 (d, J = 7.8 Hz, 1H), 7.91
(td, J = 7.8 Hz, 1.8 Hz, 1H), 7.51 (s, 1H), 7.49–7.47 (m, 2H), 7.47 (s,
1H), 7.43–7.41 (m, 2H), 7.39–7.37 (m, 1H), 7.35 (s, 1H), 7.33 (s, 1H),
1.32 (s, 9H), 1.29 (s, 9H) (Fig. S10, ESI); HRMS (ESI): m/z = 409.2518,
found: 410.2586 [M+H]+ (Fig. S11, ESI).
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Y. Zhan, et al.
DyesandPigments167(2019)1–9
2.2.4. 2,3-Bis(4-(tert-butyl)phenyl)-5,5-difluoro-5H-5l4,6l4-imidazo
[1′,2':3,4] [1–3]diazaborolo [1,5-a]pyridine (T2)
were characterized by 1H and 13C NMR, FT-IR and high resolution mass
spectroscopy analyses. T1, T2 and T3 are soluble in common organic
solvents, such as CH2Cl2, THF, toluene and DMF, but show poor solu-
bility in alcohols and aliphatic hydrocarbon solvents.
This compound was synthesized following the same procedure de-
scribed for the synthesis of compound T1 from L2 (1.0 g, 2.4 mmol),
Et3N (4.9 mL, 35.2 mmol), and BF3·Et2O (4.9 mL, 38.6 mmol). The
crude product was purified by column chromatography on silica gel
using petroleum ether/ethyl acetate (v/v = 3:1), then recrystallized
from CH2Cl2 and petroleum ether to afford T2 as a yellow-green solid
(0.37 g) in a yield of 33%. Mp: 170 °C (obtained from DSC); 1H NMR
(600 MHz, TMS, DMSO‑d6) δ = 8.86 (d, J = 6.0 Hz, 1H), 8.50 (td,
J = 7.8 Hz, 1.2 Hz, 1H), 8.18 (d, J = 8.4 Hz, 1H), 7.79 (t, J = 6.6 Hz,
1H), 7.49–7.46 (m, 6H), 7.36 (s, 1H), 7.35 (s, 1H), 1.32 (s, 9H), 1.29 (s,
9H) (Fig. S12, ESI); 13C NMR (150 MHz, DMSO‑d6) δ (ppm) = 151.06,
149.93, 146.97, 145.64, 144.27, 144.14, 142.88, 132.83, 132.22,
128.31, 127.49, 125.93, 125.42, 125.12, 118.21, 34.85, 34.69, 31.50,
31.45 (Fig. S13, ESI); IR (KBr, cm−1): 3439, 2962, 1632, 1477, 1402,
1267, 1132, 1002, 837, 707, 562; HRMS (ESI): m/z = 457.2501, found:
458.2587 [M+H]+ (Fig. S14, ESI).
3.2. Crystal structure of T1
In order to understanding the MFC behaviors of three compounds,
we intended to obtain their single crystals, but only single crystal of T1
was prepared by slow evaporation from the mixture of ethyl acetate and
petroleum ether. The collected crystal was good enough for single
crystal X-ray diffraction analysis, and the data are summarized in Table
S1(ESI). Crystal information revealed that T1 crystal belonged to a
triclinic space group P-1. The single crystal structure and crystal
packing diagram of T1 are shown in Fig. 2. It was clear that the
fluorine-boron(Ⅲ) chelation moiety was almost a plane, however, 4-
and 5-substituted benzene rings adopted a non-planar orientation with
dihedral angle (30.9° for 4-position and 55.23° for 5-position sub-
stituted benzene ring) between BOPIM core and side benzene ring (Fig.
S1, ESI). Therefore, T1 adopted a non-planar conformation. As shown
in Fig. 2b, the BOPIM core was arranged in an anti-parallel arrange-
ment, with partial overlap between neighboring BOPIM core, indicating
it form dimer by π-π stacking interactions [17]. The crystal packing
mode showed that the unit cell of T1 contained four molecules with the
presence of weak intermolecular interactions, such as B-F…H-C
(d = 3.15 Å), B-F…C (distance in the range of 2.34–2.66 Å), Br…Br
(d = 3.61 Å), Br…H-C (d = 3.00 Å), C…C (d = 3.34 Å), C…H-C
(d = 2.80 Å) and C-H…H-C (d = 2.28 Å) (Fig. S2, ESI). The single
crystal data revealed the twisted geometry and existence of weak in-
termolecular interactions, resulting loose packing in the crystal state,
which is a key factor influencing its MFC behavior [18].
2.2.5. 2-(4,5-Bis(4-methoxyphenyl)-1H-imidazol-2-yl)pyridine (L3)
By following the synthetic procedure for L1, L3 was synthesized by
using 2-cyanopyridine (0.53 g, 5 mmol), 4-methoxybenzaldehyde
(1.39 g, 10 mmol) and NH4OAc (3.85 g, 50 mmol) as the reagents. The
crude product was purified by column chromatography (silica gel)
using petroleum ether/ethyl acetate (v/v = 6:1) to give L3 as a white
solid (1.20 g) in a yield of 67%. 1H NMR (600 MHz, TMS, DMSO‑d6)
δ = 12.95 (s, 1H), 8.63–8.62 (m, 1H), 8.11 (d, J = 7.8 Hz, 1H), 7.90 (t,
J = 7.8 Hz, 1H), 7.43 (d, J = 7.2 Hz, 4H), 7.37 (s, 1H), 6.93 (s, 4H),
3.77 (s, 6H) (Fig. S15, ESI); HRMS (ESI): m/z = 357.1477, found:
358.1549 [M+H]+ (Fig. S16, ESI).
2.2.6. 2,3-Bis(4-methoxyphenyl)-5,5-difluoro–5H-5l4,6l4-imidazo
[1′,2':3,4] [1–3]diazaborolo [1,5-a]pyridine (T3)
3.3. UV-vis absorption and fluorescence emission spectra in solutions
This compound was synthesized following the same procedure de-
scribed for the synthesis of compound T1 from L3 (1.0 g, 2.8 mmol),
Et3N (5.6 mL, 40.3 mmol), and BF3·Et2O (5.6 mL, 39.6 mmol). The
crude product was purified by column chromatography on silica gel
using petroleum ether/ethyl acetate (v/v = 3:1), then recrystallized
from CH2Cl2 and petroleum ether to afford T3 as a yellow solid (0.38 g)
in a yield of 34%. Mp: 205 °C (obtained from DSC); 1H NMR (600 MHz,
TMS, DMSO‑d6) δ = 8.85 (d, J = 5.4 Hz, 1H), 8.48 (t, J = 7.8 Hz, 1H),
8.16 (d, J = 7.8 Hz, 1H), 7.77 (t, J = 6.6 Hz, 1H), 7.43–7.41 (m, 4H),
7.00 (d, J = 8.4 Hz, 2H), 6.91 (d, J = 8.4 Hz, 2H), 3.80 (s, 3H), 3.76 (s,
3H) (Fig. S17, ESI); 13C NMR (150 MHz, DMSO‑d6) δ (ppm) = 159.50,
158.79, 149.71, 146.92, 145.23, 144.34, 143.79, 142.86, 137.97,
132.30, 130.01, 129.94, 129.92, 129.08, 127.43, 124.92, 123.40,
118.10, 114.65, 114.37, 114.15, 55.55, 55.44 (Fig. S18, ESI); IR (KBr,
cm−1): 3436, 2926, 1633, 1520, 1476, 1406, 1248, 1179, 1031, 1008,
834, 787, 704, 612, 545; HRMS (ESI): m/z = 405.1460, found:
406.1550 [M+H]+ (Fig. S19, ESI).
To investigate the substituent effect on the optical properties, the
normalized absorption and fluorescence emission spectra of T1, T2 and
T3 in different solvents (1.0 × 10−5 M) are shown in Figs. 3 and 4 and
Fig. S3 (ESI), and the corresponding photophysical data are summar-
ized in Table S2 (ESI). As shown in Fig. 3a, it was found that the three
compounds showed two major absorption bands. For example, the ab-
sorption peaks of T1 were located at 306 nm and 398 nm in THF. The
formed band might be attributed to the π-π* electronic transition,
whereas the latter one could be derived from intramolecular charge-
transfer (ICT) transition. In order to confirm the occurrence of CT
transition, the solvent-dependent fluorescence emission spectra of
BOPIM dyes are shown in the Fig. 4c and d and Fig. S3b (ESI). It was
clear that the maximum emission peaks of the three compounds red-
shifted with increasing polarity of the solvents. For instance, in hexane,
T1 showed a strong emission peak at 505 nm, but with the increasing
solvent polarity, its fluorescence band red-shifted to 532 nm in DMF
accompanied by emission bands broaden. Meanwhile, the Stokes shifts
of T1 increased from 3783 cm−1 in hexane to 6778 cm−1 in DMF
(Table S2, ESI). Besides, T1 was highly emissive in hexane with a
fluorescence quantum yield (ΦF) of 0.72 using 9,10-diphenylanthracene
(ΦF = 0.85 in benzene, λex = 390 nm) as the standard (Table S2, ESI),
but ΦF was only 0.21 in DMF. Combined with the broadening and red-
shift of the emission band, accompanied by an obvious decreasing of ΦF
and the increasing Stokes shifts in polar solvents, we deemed that ICT
transition of T1 from electron-donating groups to electron-accepting
BOPIM skeleton taking place in polar solvents and the longer wave-
length emissions might be assigned to ICT emissions [19]. The ICT
transition peak of T2 and T3 red-shifted to 414 nm and 424 nm, re-
spectively, which might be due to the increased electron-donating
ability and thereby reduced the energy gaps between the excited and
ground states [20]. Indeed, compared with compounds T1 and T2, T3
with the strong electron-donating methoxyl group showed the most
3. Results and discussion
3.1. Synthesis of T1, T2 and T3
The pivotal roles of boron (Ⅲ) chelation are to render the π system
planar, thereby enhancing conjugation and charge transfer along the
main molecular axis. The synthetic routes of BOPIM dyes are shown in
Scheme 1. Firstly, starting from the corresponding aromatic aldehydes
1–3 and 2-cyanopyridine, modification of BOPIMs relied on the for-
mation of their 2-(1H-imidazol-2-yl)pyridine ligands L1-L3 through
multi-component one-pot reaction with the yields of 53%–67%. Then,
the produced L1-L3 were then directly reacted with boron difluoride
diethyl ether to give T1, T2 and T3 with the yields of 27%–34% [16].
All the intermediates and target molecules were purified by column
chromatography on silica gel, and the target molecules T1, T2 and T3
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Y. Zhan, et al.
DyesandPigments167(2019)1–9
Scheme 1. Synthetic routes of T1, T2 and T3.
Fig. 2. (a) Single crystal structure of T1; (b) crystal packing diagram of T1 with multiple intermolecular interactions. For clarity, the hydrogen atoms are not shown;
(c) single crystal under natural light.
obvious variation of absorption maximum (Δλabs = 42 nm) upon in-
creasing the solvent polarity from hexane to DMF. In addition, solva-
tochromic effect was obtained for BOPIM dyes, that was, the lowest-
energy absorption-bands were marked blue-shift with increasing sol-
vent polarity, corresponding to classic ICT characteristics (Fig. 4a and b
and Fig. S3a, ESI). Owing to their ICT feature, the PL spectra of BOPIM
dyes are dependent on solvent polarity as well. As shown in Fig. 3b, it
was found that the maximum emission of T1, T2 and T3 in THF located
at 523 nm, 543 nm and 562 nm, respectively. The obvious bath-
ochromic shift of T3 should be ascribed to the increasing electron-
Fig. 3. (a) Normalized UV-vis absorption of T1, T2 and T3 in THF; (b) fluorescence spectra of T1, T2 and T3 (1.0 × 10−5 M, excited at 410 nm for T1, 420 nm for T2
and 435 nm for T3) in THF.
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Y. Zhan, et al.
DyesandPigments167(2019)1–9
Fig. 4. Normalized UV-vis absorption spectra of T1 (a), T2 (b), and fluorescence emission spectra of T1 (c, λex = 410 nm), T2 (d, λex = 420 nm) in different solvents
(1.0 × 10−5 M).
Fig. 5. Electron density distributions in the HOMO and LUMO states and optimized conformation of T1, T2 and T3 calculated by DFT in Gaussian 09W at the B3LYP/
6-31G(d) level.
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DyesandPigments167(2019)1–9
distributed on the 5-substituted benzene moieties, whereas the LUMO
was mostly located at the boron-fluorine chromophore unit. This result
demonstrated that the ICT occurred for the excited state. Compared
with T1, the HOMO of T2 and T3 with the electron-donating groups
showed upshift more seriously than their LUMO, respectively. Thus, the
energy gaps of compound T2 and T3 were smaller than that of com-
pound T1, which well explained that the absorption and emission bands
of compounds T2 and T3 were longer than those of compound T1
(Table S3, ESI). Moreover, T3 possessed larger ICT degree in the excited
state than that of T2. The above inference was consistent with the ob-
served optical properties. In addition, the three compounds adopted a
twisted conformation in their optimized lowest energy state, which
disfavored close molecular packing and π-π interactions in the solid
state [21].
Fig. 6. Photographs of T1 (a), T2 (b), and T3 (c) color changes under the
treatment of grinding and fuming stimuli. (For interpretation of the references
to color in this figure legend, the reader is referred to the Web version of this
article.)
3.5. Mechanofluorochromic behaviors
donating ability, which strengthens D-A effect. In addition, concerning
the structure features, it is not surprised that T1, T2 and T3 showed
solid state fluorescent emission with high quantum efficiency of 59.2%,
55.1%, and 46.5%, respectively.
As discussed above, T1, T2 and T3 adopted a twisted spatial con-
formation in their optimized structures, thus, assembly of the molecules
into closely packed structures was difficult. The crystals were easily
destroyed by external force because of weak intermolecular interactions
[22]. Herein, three compounds were greatly anticipated to be MFC-
active materials. To investigate whether the three compounds T1, T2
and T3 exhibit mechanochromic luminescence, their emission color
change in the solid state were investigated. The naked-eye-visible
changes in fluorescence color were recorded and the images are shown
in Fig. 6. The pristine T1, T2 and T3 solids could emit bright green,
bright green and yellowish-green light under UV irradiation, their
emitting colors changed into bright yellowish-green, bright yellow and
yellow light, respectively, after grinding by using a spatula or a pestle,
suggesting the significant MFC behaviors of the three molecules. The
3.4. Theoretical calculation
Density functional theory (DFT) calculations were performed on the
three compounds with the Gaussian 09W package to better clarify the
influence of the geometric and electronic structures of BOPIM dyes on
their photophysical properties. The geometrical structures are opti-
mized at the B3LYP/6-31G(d) level. The frontier orbital plots of the
HOMO and LUMO are shown in Fig. 5, and the corresponding data are
listed in Table S3 (ESI). It was found that the HOMO was mainly
Fig. 7. Normalized fluorescence spectra of T1 (a), T2 (b), and T3 (c) in different solid states: pristine, grinding and fuming (excited at 380 nm for T1, 420 nm for T2,
435 nm for T3); (d) Maximum fluorescence emission of T1 upon treated by grinding and fuming with DCM repeatedly.
6

Y. Zhan, et al.
DyesandPigments167(2019)1–9
Fig. 8. PXRD patterns of (a) T1, (b) T2, and (c) T3 in different solid states: pristine, grinding and fuming.
normalized fluorescence emission spectra of samples in different solid
states (pristine, grinding and fuming) are shown in Fig. 7. It was clear
that the emission peaks of T1, T2 and T3 in the pristine crystals ap-
peared at 504, 510 and 530 nm and red-shifted to 526, 546 and 560 nm
in the ground powders, respectively, indicating that the grinding
treatment induced the spectral bathochromic shifts of 22, 36 and 30 nm
for T1, T2 and T3 (Table S4, ESI). In addition, after fuming with DCM
for several seconds, the emission colors were restored to the original
states, similar to their pristine crystals, and the corresponding max-
imum fluorescence wavelengths could blue-shift to the pristine crystals.
If the fuming powders were reground, their emission colors were again
changed as the first grinding. This MFC conversion could be repeated
many times without fatigue (Fig. 7d and Fig. S4, ESI), indicating ex-
cellent reversibility in the switching processes. In contrast, when the
ground powders of the three compounds were heated at certain tem-
perature, the emissions could not be converted back to their original
states. Compared with T1, T2 and T3 with electron-donating groups
exhibited the larger spectral shifts (Δλ = 36 nm for T2 and Δλ = 30 nm
for T3) after grinding the pristine crystal (Table S4, ESI), which could
be ascribable to the key role of the ICT transition, which was well-
known to contribute greatly to MFC behavior with higher sensitivity
toward mechanical force [23]. Besides, T2 exhibited a high-contrast
MFC relative to T3. The reason might be that the tert-butyl group could
made the molecule more twisted and loosely packed through its steric
hindrance effect [24].
intensity but increased peak widths, indicating that the crystalline
phase was damaged into amorphous state during the grinding process,
which resulted in the different fluorescence emission spectra of the
ground powders [25]. However, when the ground powders were treated
by DCM fuming, sharp and intense diffraction peaks could be appeared
into an ordered crystalline phase. Therefore, the mechanochromism of
the three compounds was attributed to the transformations between the
ordered crystalline and amorphous states.
Differential scanning calorimetry (DSC) measurements were per-
formed to further study their thermal properties. The DSC curves of T1,
T2 and T3 in different solid states are shown in Fig. 9. It was found that
the DSC curves of the pristine crystals of T1, T2 and T3 exhibited only a
single endothermal peak at 228 °C, 170 °C and 205 °C, respectively,
corresponding to their melting points. However, the ground powders of
T1 and T3 showed an additional one exothermic peak at 113 °C and
95 °C, respectively, and the ground powder of T2 exhibited two weak
exothermic transition peaks at 106 °C and 147 °C before melting point,
which could be assigned to their cold-crystallization temperature, and
also demonstrated that the amorphous state was a metastable state
[26]. T1, T2 and T3 possessed an additional metastable state, which
could be converted to a crystalline state upon thermal annealing, in-
dicating the phase transition form a poorly organized phase to a well-
ordered phase.
When a filter paper containing the pristine solids of T1, T2 and T3
were pressed by streaking a stainless steel pen, a color trails were ob-
served. As shown in Fig. 10a, three English letters, “BZY”, with yel-
lowish green fluorescence under UV irradiation were written using a
ball pen. The three letters were hidden by DCM vapor fuming for 10 s,
and the entire solid film emitted green fluorescence under 365 nm light.
Similarly, T2 and T3 solid film might be used as recording material
(Fig. 10b and c). Bright yellow letters “BZY” could be written in green
(T2 solid) and yellowish green (T3 solid) background, and could be
erased after fuming DCM vapor. Thus, these solid film based on BOPIM
To understand the mechanism of the MFC behaviors of T1, T2 and
T3, the powder wide-angle X-ray diffraction (PXRD) measurements
were conducted. According to the PXRD measurements, T1, T2 and T3
formed different molecular aggregates before and after grinding treat-
ment. As shown in Fig. 8, it was clear that the PXRD patterns of the
pristine crystals exhibited many strong and sharp diffraction peaks,
indicating
a well-ordered microcrystalline-like structures. After
grinding, the PXRD patterns displayed significant decreased peak
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Y. Zhan, et al.
DyesandPigments167(2019)1–9
Fig. 9. DSC curves of compounds T1 (a), T2 (b) and T3 (c) for the pristine crystals and ground powders under nitrogen atmosphere at a heating rate of 10 °C/min.
Fig. 10. Photographic images of (a) T1, (b) T2, and (c) T3 films on pieces of filter paper in response to grinding, fuming under the irradiation of 365 nm UV light.
dyes as a smart materials can be used to record information.
three synthesized complexes. The solid emission studies of T1, T2 and
T3 illustrated that their emission color could change reversibly upon
the treatment of grinding and fuming with DCM. The MFC behaviors of
these three compounds increased with the same order of
T1 < T3 < T2, which depended on electronic and steric effects of the
peripheral substituents on the BOPIM dyes. Moreover, information
written by force can be hidden using DCM vapor. Thus, BOPIM dyes
could act as smart materials for data security protection. The current
studies provided valuable information for designing new MFC materials
that have potential application in optical-recording materials.
4. Conclusions
In summary, a series of BOPIM complexes was obtained by in-
troducing different substituents (including bromo, tert-butyl and
methoxyl) on the benzene ring of BOPIM dyes. Their molecular struc-
ture, photophysical properties and MFC behaviors have been in-
vestigated and compared. Single crystal structure illustrated that the
non-planar orientation of BOPIM dyes. Spectral measurements and
quantum chemical calculation indicated a push-pull electronic structure
and non-planar geometry, and they gave unique ICT emission in dilute
solutions. Reversible mechanofluorochromism were detected for the
8

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DyesandPigments167(2019)1–9
Acknowledgements
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