Solid-state photochromic molecular switches based on axially chiral and helical spiropyrans

Article abstract of DOI:10.1016/j.dyepig.2020.108597

Solid-state photochromic switches with reversible luminescence modulation property have wide applications in optical data storage, anticounterfeiting, sensing, and bioimaging, however, it is a significant challenge to design and prepare such materials. In the present work, a novel class of chiral and helical binaphthyl-substituted spiropyrans has been synthesized and characterized. The introduction of binaphthyl can not only promote the solid-state luminescence but also achieve the interesting chiroptical switching properties for the potential applications in chiral recognition and circularly polarized luminescence. The single-crystal structures and density functional theory calculations reveal that the presence of strong electron-withdrawing nitro groups can promote photoswitch efficiency by stabilizing the electron-rich conjugated merocyanine with the lower electron cloud density and LUMO/HOMO levels. For charge transfer in excited states, π → π* transition would enhance emission, but n → π* transition and intramolecular charge transfer show the opposite effect. Therefore, these photochromic switches provide a novel paradigm in the design of multiple-responsive, chiral, and luminescent materials.

Full text of DOI:10.1016/j.dyepig.2020.108597

Dyes and Pigments 181 (2020) 108597
Contents lists available at ScienceDirect
Dyes and Pigments
journal homepage: http://www.elsevier.com/locate/dyepig
Solid-state photochromic molecular switches based on axially chiral and
helical spiropyrans
Lang Qu, Xuemei Xu, Jintong Song, Dehua Wu, Lei Wang, Weilan Zhou, Xiangge Zhou,
Haifeng Xiang*
College of Chemistry, Sichuan University, Chengdu, 610041, China
A R T I C L E I N F O
A B S T R A C T
Keywords:
Solid-state photochromic switches with reversible luminescence modulation property have wide applications in
optical data storage, anticounterfeiting, sensing, and bioimaging, however, it is a significant challenge to design
and prepare such materials. In the present work, a novel class of chiral and helical binaphthyl-substituted spi-
ropyrans has been synthesized and characterized. The introduction of binaphthyl can not only promote the solid-
state luminescence but also achieve the interesting chiroptical switching properties for the potential applications
in chiral recognition and circularly polarized luminescence. The single-crystal structures and density functional
theory calculations reveal that the presence of strong electron-withdrawing nitro groups can promote photo-
switch efficiency by stabilizing the electron-rich conjugated merocyanine with the lower electron cloud density
Spiropyran
Chiral BINOL
Molecular switch
Chiropital
Anticouterfeting
and LUMO/HOMO levels. For charge transfer in excited states,
π → π* transition would enhance emission, but n
→ π* transition and intramolecular charge transfer show the opposite effect. Therefore, these photochromic
switches provide a novel paradigm in the design of multiple-responsive, chiral, and luminescent materials.
1. Introduction
SP switches with a high emission quantum yield (Φ).
Organic
π-conjugated luminescent materials have attracted great
Spiropyran (SP) [1], as one important class of photochromic switches
[2], has been extensively investigated in the past two decades due to its
versatile applications in optical data storage, anticounterfeiting,
sensing, and bioimaging. As shown in Fig. 1, under external stimulus of
light, heat, and chemicals, SP switches usually undergo three different
isomers including colorless and non-emissive closed-ring SP, purple and
red-emissive open-ring merocyanine (MC), and yellow and weakly
emissive protonated merocyanine (MCH) [1b]. Previous literature re-
ports have demonstrated that electron-withdrawing groups, such as
nitro and methyl pyridinium, can enhance the UV response of photo-
switches, because electron-withdrawing groups are known to stabilize
the electron-rich conjugated open-ring MC [3]. On the other hand, these
electron-withdrawing groups might have tendency to quench the
interests in a variety of scientific communities owing to their wide and
useful applications [7]. In order to improve Φ of SP switches, many
strategies, such as immobilization of the MC moieties into polymer
nanoparticles [8] and micelles [9], are adopted to restrict their confor-
mational flexibility and protect the fluorophores from quenching by
nonradiative processes. For a chemist, however, a simpler and more
efficient and challenging way is to design and synthesize luminescent SP
switches through the modification of molecular structures. Recently, to
solve the ACQ problem to achieve solid-state luminescence, some pop-
ular aggregation-induced emission [5] (AIE)-active moieties, e.g., tet-
raphenylethene (TPE) [10] and 9,10-distyrylanthrance [11] are
combined into SP switches. Furthermore, ACQ-active-pyrene-based SP is
demonstrated to exhibit blue-red emission both in solution and in the
solid state [12].
emission [4]. In fact,
π-conjugated open-ring MC is planar and conse-
quently far more likely to encounter the problem of emission “aggre-
1,10-binaphthyls, 1,10-binaphthalene-2,20-diol (BINOL) and 1,10-
binaphthalene-2,20-diamine (BINAM), are an important class of axially
chiral molecules for many applications including supramolecular
chemistry [13], asymmetric catalysis [14], enantioselective fluorescent
recognition [15], and circularly polarized luminescence (CPL) [16]. To
gation-caused quenching” (ACQ) through intermolecular π‒π stacking
interactions [5]. Moreover, except the existence of SP-MC photo-
isomerization, the trans-cis photoisomerization of MC is also reported
[6], which would further increase the challenge to design and prepare
* Corresponding author.
E-mail address: xianghaifeng@scu.edu.cn (H. Xiang).
https://doi.org/10.1016/j.dyepig.2020.108597
Received 23 December 2019; Received in revised form 22 April 2020; Accepted 29 May 2020
Available online 10 June 2020
0143-7208/© 2020 Elsevier Ltd. All rights reserved.

L. Qu et al.
Dyes and Pigments 181 (2020) 108597
2. Results and discussion
2.1. Synthesis and characterization
(S/R)1-MCH [20], (S/R)2-SP [21], and (S/R)3-SP [20] were pre-
pared according to three different ways, as shown in Fig. 3. Compound 4
[20], 5 [22], 6 [23], 7 [24], 8 [24] and 9 [25] were prepared according
to the previous reports. The best stable form of 1 and 2/3 is MCH [10a]
and SP [17,18], respectively. The purpose of the presence of sulfonate
group to (S/R)1 is to improve the solubility in water for sensing appli-
cations [10a,26]. We failed to prepare pure closed-ring (S/R)1-SP by
adding base to (S/R)1-MCH solution, because (S/R)1-SP is salt and thus
could not be purified by running chromatography. (S/R)2-SP could be
synthesized by Suzuki coupling reaction [21] with a moderate yield of
42%, but (S/R)3-SP could not obtained by the same way because the
electron-withdrawing nitro group would significantly diminish the yield
of Suzuki coupling reaction. Therefore, (S/R)3-SP was prepared by a
modified way (Fig. 3c). (S/R)1-MCH is soluble in MeOH, DMSO, and
DMF rather than CH2Cl2, MeCN, ethyl acetate, and toluene. (S/R)2-SP
and (S/R)3-SP are soluble in CH2Cl2, ethyl acetate, THF, DMSO, DMF
and toluene rather than MeOH, EtOH, and hexane. We successfully
obtained some good-quality single crystals (yellow particles) of (S)
1-MCH (CCDC 1944289) by the vapor diffusion of CH2Cl2/MeOH. Un-
fortunately, for (S/R)2-SP and (S/R)3-SP, oil-like thin films were al-
ways obtained by various methods. We would like to point out that
(S/R)2-SP and (S/R)3-SP with a large molecular weight (924 and 1014)
are highly difficult to crystallize.
Fig. 1. Photochromism and acidochromism of spiropyran.
The photophysical properties, including UV/visible absorption and
fluorescence data, of all synthesized (S/R)1–3 at room temperature are
listed in Table S1 (in Supporting Information). The photophysical
properties, except circular dichroism (CD), of (S) and (R)1–3 are similar,
which are consistent with our previous reports [4d,4e,12,16c,16d,27].
2.2. Solution tautomerization properties of (S)1 under base/acid
treatments
We first studied the tautomerization properties of (S)1 in solution
under base/acid treatments. Density functional theory (DFT) and time-
dependent-DFT (TD-DFT) were also performed by Gaussian 09 pro-
gram package (B3LYP 6-31G(d,p)) to investigate the UV/visible ab-
sorption spectra and charge transfer in excited states. NO solid-state
tautomerization is found for (S)1. (S)1-SP, (S)1-MC, and (S)1-MCH are
non-emissive or weakly emissive in solution or solid.
Fig. 2. BINOL-based SP molecules: (a) the previous works; (b) this work.
In dilute DMSO solution, (S)1-MCH exhibits a dominating absorp-
tion peak (λabs ¼ 425 nm), which is predicted well by theoretical
calculation (λabs ¼ 416 nm) (Fig. 4e). The optimized molecular struc-
ture, energy level diagram, and frontier molecular orbitals of (S)1-MCH
are also displayed in Fig. 4a. Unlike our previous report [26a], which
demonstrates that the contribution of sulfonate group to the electron
cloud density of the highest occupied molecular orbital (HOMO),
HOMO-1, the lowest unoccupied molecular orbital (LUMO), and
LUMOþ1 is relatively small and thus the sulfonate group has little effect
on the photophysical properties, the sulfonate group (lone pair electrons
of O atoms) of (S)1-MCH has obvious contributions to the electron cloud
density of HOMO, HOMO-1, and HOMO-3 (Fig. 4a) due to its
electron-rich nature. Moreover, the dihedral angle between the two
naphthyl rings of optimized (S)1-MCH is 85.5� (Fig. S1), which indicates
that two naphthyl rings are close to vertical and lead to the broken
date, only a limited number of conjugated [17] and un-conjugated [18]
BINOL-based SP molecules (Fig. 2a) have been reported for the recog-
nition of amino acids, cations, acid/base. However, owing to the absence
of electron-withdrawing groups, there is no report on photochromic
transformation from SP to MC under the UV irradiation. Moreover, for
these conjugated BINOL-based SP molecules, no luminescent property is
demonstrated, which might be caused by the fact that their SP unites are
covalently bonded at 2,3-postion of BINOL and thus the ring opening of
SP might change the dihedral angle between two naphthyl rings and
quench the emission consequently. In the present work, combining the
axial chirality and AIE property [19] of highly sterically bulky BINOL,
we covalently linked SP moieties to BINOL at 3-postion to achieve novel
photochromic chiral switches, (S/R)1–3 (Fig. 2b), which exhibit highly
efficient and reversible solid-state luminescence switching under alter-
nating UV light and heat/visible light treatment.
π
-conjugation system and the unsymmetrical distribution of electron
cloud density [28]. It should be noticed that the lower energy absorption
of (S)1-MCH is mainly originated from Excited State 5 (416 nm,
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L. Qu et al.
Dyes and Pigments 181 (2020) 108597
Fig. 3. Synthesis of (S/R)1–3.
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L. Qu et al.
Dyes and Pigments 181 (2020) 108597
Fig. 4. Computational optimized molecular
structures and frontier molecular orbitals of
(S)1-MCH (a) and (S)1-SP (b). Computa-
tional optimized molecular structure of (S)1-
MC (c). Relative enthalpy and free energy
levels of (S)1-SP and (S)1-MC (d). Compu-
tational and experimental absorption spectra
(10
μ
mol dmÀ 3) of (S)1-MCH (black line)
and (S)1-SP (red line) in DMSO (e). (For
interpretation of the references to color in
this figure legend, the reader is referred to
the Web version of this article.)
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L. Qu et al.
Dyes and Pigments 181 (2020) 108597
2.3. Solution tautomerization properties of (S)2 under base/acid
treatments
The dihedral angle between the two naphthyl rings and the pyrrole
ring and the pyran ring of optimized (S)2-SP is 87.9� and 88.0� (Fig. 6a
and Fig. S1), respectively. Compared with (S)1-SP, (S)2-SP gives a little
blue-shifted absorption with a shoulder at ~350 nm (Fig. S2), which can
be classified into Excited State 1 (358 nm, fosc ¼ 0.3624, HOMO →
LUMO (89.2%), HOMO → LUMOþ2 (4.5%), HOMO-1 → LUMO (2.1%))
with π→ π* character in the naphthyl pyran unit (Fig. 6). Therefore, (S)
2-SP emits blue emission (0.6% in MeCN) from naphthyl pyran unit
(Table S1 and Fig. S2 and S3) [13e,15]. Moreover, the solid powders (Φ
¼ 1.1%) and aggregated nanoparticles in THF/H2O (Φ ¼ 0.5%) and
MeCN/H2O (Φ ¼ 1.0%) of (S)2-SP show AIE characters (Fig. 7 and
Fig. S4). The central rigid BINOL still exhibits intramolecular motions
(IMs) [30] through some small-angle rotations around the chiral axis
[16c,d]. These IMs would not destroy their chiral configurations but
result in emission quenching in a dilute solution. For aggregated nano-
particles and solid powders, however, the IMs would be restricted by the
strong intermolecular interactions and thus the emission is enhanced.
Upon adding HCl, the colorless MeOH solution of (S)2-SP will
change into yellow (Fig. 6 and Fig. S5) immediately, revealing the for-
mation of (S)2-MCH (λabs ¼ 430 nm) (Fig. 1). This reaction is also
reversible if NaOH is added further. The dihedral angle between the two
naphthyl rings of optimized (S)2-MCH is 120.0� (Fig. 6b and Fig. S1).
The absorption band of (S)2-MCH can be mainly contributed to Excited
State 3 (499 nm, fosc ¼ 0.3592, HOMO-1 → LUMO (92.3%), HOMO-2 →
LUMO (5.9%)) and 4 (484 nm, fosc ¼ 0.5961, HOMO-2 → LUMOþ1
(94.6%), HOMO-1 → LUMOþ1 (3.3%)) with ICT character from the bi-
naphthyl unit to the MCH unit. This ICT character would lead to the
absence of emission for (S)2-MCH (Fig. 7). Owing to the absence of
electron-withdrawing groups, no photochromic tautomerization from
(S)2-SP into (S)2-MC is observed under the UV irradiation.
Fig. 5. Absorption spectra and photographs of (S)1-MCH, (S)1-MC ((S)1-MCH
upon adding 500 equivalents of NEt3), and (S)1-SP ((S)1-MCH upon adding
500 equivalents of NaOH) in DMSO (0.1 mmol dmÀ 3).
oscillator strength fosc ¼ 0.2203, HOMO-4 → LUMO (92.4%) and
HOMO-2 → LUMO (6.1%)) rather than common Excited State 1 (HOMO
→ LUMO). This might be caused by the facts that (1) HOMO-4 → LUMO
and HOMOÀ 2 → LUMO are the allowable and symmetrical
π→π*
transition in the same central πÀ conjugation of naphthyl-based MCH
unit; (2) The level of LUMOþ1 (À 1.16 eV) is too high for transitions; (3)
HOMO → LUMO, HOMO-1 → LUMO, and HOMO-3 → LUMO are
involved in asymmetrical n →π* and intramolecular charge transfer
(ICT).
Upon adding 500 equivalents of NaOH, the yellow DMSO solution of
(S)1-MCH will change into colorless (Fig. 4e) immediately, revealing
the formation of (S)1-SP (λabs ¼ 390 nm) (Fig. 1). This reaction is
reversible if HCl acid is added further ((S)1-SP → (S)1-MCH). The
dihedral angle between the two naphthyl rings of optimized (S)1-SP is
94.9� (Fig. 4b and Fig. S1). It’s well-known that SP is non-planar. The
dihedral angle between the five-membered pyrrole ring and the six-
membered pyran ring is 84.1� for (S)1-SP. By contrast, these units in
(S)1-MCH is almost planar. Therefore, the absorption band of (S)1-SP
blue shifts into 390 nm, which is mainly contributed to Excited State 6
(398 nm, fosc ¼ 0.0858, HOMO-2 → LUMO (97.1%)) with ICT character
from the benzopyrrole unit to the naphthyl pyran unit. Owing to the
absence of electron-withdrawing groups, no photochromic tautomeri-
zation from (S)1-SP into (S)1-MC is observed under the UV irradiation.
In order to trigger to isomerize from (S)1-MCH to (S)1-MC (Fig. 1),
NaOH is replaced by a weak base of NEt3. When adding 500 equivalents
of NEt3 to a high concentration solution of (S)1-MCH (0.1 mmol dmÀ 3),
the orange-yellow solution will change into yellow-green immediately,
indicating the possible formation of (S)1-MC (Fig. 5). However, the
resultant yellow-green solution is not stable and would change into
colorless (S)1-SP in several minutes. The dihedral angle between the
two naphthyl rings of optimized (S)1-MC is 75.7� (Fig. 4c and Fig. S1).
Indeed, our calculations reveal that the enthalpy (sum of electronic and
thermal enthalpies) and free energy (sum of electronic and thermal free
energies) between (S)1-SP and (S)1-MC is 6.55 and 4.46 kcal/mol
(Fig. 4d), respectively. This indicates that the ring-opening reaction is
endothermic and (S)1-SP is thermodynamically more stable than (S)1-
MC [29].
If 500 equivalents of NEt3 is added to a high concentration MeOH
solution of (S)2-MCH (0.1 mmol dmÀ 3), the orange-yellow solution will
change into blue-purple immediately, indicating the possible formation
of (S)2-MC. However, the resultant blue-purple solution is highly un-
stable and will become into colorless (S)2-SP very quickly, and thus we
had no time to take the photo and measure the absorption spectrum. The
dihedral angle between the two naphthyl rings of optimized (S)2-MC is
119.6� (Fig. 6c and Fig. S1). The enthalpy and free energy between (S)2-
SP and (S)2-MC is 14.6 and 9.56 kcal/mol, respectively (Fig. 6d).
2.4. Solution and solid tautomerization properties of (S)3 under base/
acid and UV light treatments
The dihedral angle between the two naphthyl rings and the pyrrole
ring and the pyran ring of optimized (S)3-SP is 82.4� and 87.5� (Fig. 8a
and Fig. S1), respectively. Like (S)2-SP, (S)3-SP shows a similar ab-
sorption spectrum with a shoulder at ~350 nm (Fig. S6). In contrast to
that of (S)2-SP, the electron cloud density in LUMO and LUMOþ1 of (S)
3-SP is manly located at the nitro groups. The low-energy absorption
band of (S)3-SP can be contributed to Excited State 29 (365 nm, fosc
¼
0.2425, HOMO → LUMOþ5 (93.5%)) and 30 (351 nm, fosc ¼ 0.1218,
HOMO-1 → LUMOþ5 (96.0%)) with ICT character from the benzo-
pyrrole unit to the naphthyl pyran unit. Therefore, (S)3-SP is non-
emissive in solution and solid.
Even upon adding 1000 equivalents of HCl, the colorless MeOH so-
lution of (S)3-SP shows little changes (Fig. 9), revealing the absence of
(S)3-MCH (Fig. 1) and the inhibition of chemical switching from the
nitro groups. However, if further irradiated by 365 nm UV light for 5
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L. Qu et al.
Dyes and Pigments 181 (2020) 108597
Fig. 6. Computational optimized molecular
structures and frontier molecular orbitals of
(S)2-SP (a) and (S)2-MCH (b). Computa-
tional optimized molecular structure of (S)2-
MC (c). Relative enthalpy and free energy
levels of (S)2-SP and (S)2-MC (d). Energy
level diagram of (S)2-MC and (S)3-MC (e).
Computational and experimental absorption
spectra (10
μ
mol dmÀ 3) of (S)2-MCH (red
line) and (S)2-SP (black line) in MeOH (f).
(For interpretation of the references to color
in this figure legend, the reader is referred to
the Web version of this article.)
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L. Qu et al.
Dyes and Pigments 181 (2020) 108597
thermodynamically stable (S)3-MC (ΔG ¼ 16.1 kcal/mol, Fig. 8c). ΔG
between (S)3-SP and (S)3-MC is bigger than that between (S)2-SP and
(S)2-MC, which is contrary to the fact that the reaction of (S)3-SP → (S)
3-MC is more efficient than the reaction of (S)2-SP → (S)2-MC. The
possible reason is that the reaction of SP → MC is very complicated and
there are some high-energy intermediate compounds [29]. The dihedral
angle between the two naphthyl rings of optimized (S)3-MC is 104.1�
(Fig. 8b and Fig. S1). The absorption band (λabs ¼ 530 nm, Fig. 8e) of (S)
3-MC can be mainly contributed to Excited State 1 (531 nm, fosc
1.4867, HOMO → LUMO (91.6%), HOMO-1 → LUMO (5.4%), HOMO-1
→ LUMOþ1 (2.2%)) with * character in the MC unit and nitro
¼
π
→ π
group and high fosc. It is unusual that the strong electron-withdrawing
nitro groups have contribution to electron cloud density of both
HOMO/HOMO-1 and LUMO/LUMOþ1 and consequently stabilize the
electron-rich conjugated (S)3-MC (Fig. 8b) by reducing the electron
cloud density. Moreover, (S)3-MC with nitro groups has a much lower
LUMO level of 0.61 eV and HOMO level of 0.71 eV than (S)2-MC
(Fig. 6e). Therefore, the above two factors may lead to the efficient
photoswitching in (S)3-MC. (S)3-MC is non-emissive in dilute solution
and shows weak red AIE in solid powders and aggregated nanoparticles
(Table S1 and Fig. S7).
In order to extend the application field in solid state, we doped (S)3-
SP into a polymer host of polymethyl methacrylate (PMMA). (S)3-SP-
doped PMMA film exhibits excellent photochromic-switching properties
as well (Figs. 10 and 11). The transparent and colorless (S)3-SP-doped
PMMA film (wt % ¼ 1.0%, thickness ¼ 0.5 mm) will change into purple
when irradiated by 365 nm UV light. It needs about 10 min UV irradi-
ation to achieve the saturation of purple and the recovery time for
colorless film is about 120 min and 3 min under sunlight and heating at
80 �C, respectively. This color change is easily distinguishable by a
naked eye. Furthermore, (S)3-SP-doped PMMA film shows good color
reversibility and stability against photobleaching.
It is unexpected that, under the UV irradiation, red emission (λabs
¼
~640 nm, Fig. S8) from (S)3-MC of PMMA film (Φ ¼ 2.2%) is much
stronger than that of solid powders. Doping technology could reduce not
only the IMs but also intermolecular π‒π stacking interactions of (S)3-
MC. The corresponding fluorescence changes provide a high contrast
between turn off and turn on (Fig. 11).
Fig. 7. Top: emission spectra of (S)2-SP in THF/H2O with different volume
fractions of water (fwater) (10
μ
mol dmÀ 3) and solid powder (excited at 310 nm).
In order to evaluate the conversion efficiency of (S)3-SP → (S)3-MC,
thin-layer chromatography (TLC) analysis is studied. As shown in
Fig. 12, after the elution with ethyl acetate: petroleum ether ¼ 1 : 5, the
spot of purple (S)3-MC solution on SiO2 TLC appears only one colored
ribbon under room light (purple), 254 nm UV light (dark), and 365 nm
UV light (red), which illustrates that the conversion efficiency is nearly
quantitative. The strong absorption at ~590 nm in MeCN solution and
PMMA film further verifies the high conversion efficiency (Fig. 10).
Bottom: emission spectra of (S)2-SP in MeOH (5
μ
mol dmÀ 3) with adding 0 to
2000 equivalents of HCl (excited at 310 nm).
min, the above (S)3-SP solution will change into yellow (S)3-MCH so-
lution (λabs ¼ 430 nm). The dihedral angle between the two naphthyl
rings of optimized (S)3-MCH is 106.7� (Fig. 8d and Fig. S1). The ab-
sorption band of (S)3-MCH can be mainly contributed to Excited State 5
(484 nm, fosc ¼ 0.7539, HOMO-2 → LUMO (56.0%), HOMO-3 → LUMO
(31.8%), HOMO-2 → LUMOþ1 (9.5%)) with ICT character from the bi-
naphthyl benzopyrrole unit to the MCH unit. This ICT character would
lead to the absence of emission for (S)3-MCH as well. Moreover, the
nitro groups have no contrition to the electron transitions.
2.5. X-ray single crystal structures
The molecule chemical structures and X-ray single-crystal arrange-
ments of organic materials act an important role in their photophysical
properties [4]. In order to investigate the effect of molecule arrange-
ments, the X-ray single-crystal structures of (S)1-MCH are depicted in
Fig. 13 and Fig. S9. There is no (R)1-MCH in the X-ray single-crystal
structures, which confirms the fact that the (R)/(S)-chiral configura-
tions of BINOL are stable enough for chemical modification. Except one
propane-1-sulfonate unit and two methyl groups, other the atoms of (S)
1-MCH are located in two planes: one is peripheral naphthyl ring, the
other is naphthyl-based MCH. The dihedral angle between the two
naphthyl rings in X-ray single-crystal structure (75.2�) is smaller than
that in optimized structure (85.5�), revealing that the binaphthyl ex-
hibits the IMs through a rotation around the chiral axis in gas state or
With the help of nitro groups, (S)3-SP in solution exhibits excellent
photochromic-switching properties (Fig. 10). When irradiated by 365
nm UV light, the colorless (S)3-SP solution will change into purple,
indicating the formation of (S)3-MC. It needs about 10 min UV irradi-
ation to achieve the saturation of purple and the recovery time for
colorless solution under sunlight is about 5 min. If heated at 80 �C, the
purple color will disappear immediately.
Although the ring-opening reaction of (S)3-SP → (S)3-MC is endo-
thermic (ΔH ¼ 19.2 kcal/mol, Fig. 8c) and heating may help to form (S)
3-MC, heating would promote the equilibrium of reaction to yield
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Dyes and Pigments 181 (2020) 108597
Fig. 8. Computational molecular structures and frontier molecular orbitals of (S)3-SP (a), (S)3-MC (b), and (S)3-MCH (d). Relative enthalpy and free energy levels
of (S)3-SP and (S)3-MC (c). Computational and experimental absorption spectra (10
μ
mol dmÀ 3) of (S)3-SP (back line), (S)3-MC (red line), and (S)3-MCH (green
line) in MeOH (e). (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
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L. Qu et al.
Dyes and Pigments 181 (2020) 108597
Fig. 9. Absorption spectra of (S)3-SP (5
μ
mol dmÀ 3) in MeOH with adding 0 to
1000 equivalents of HCl. Absorption spectrum of (S)3-SP (10
μ
mol dmÀ 3) in
MeOH with adding 1000 equivalents of HCl and 5 min UV irradiation. Insert:
photographs of (S)3.
dilute solution [16c,d]. Although (S)1-MCH is propeller-type, some
intermolecular π‒π stacking interactions are still observed in its X-ray
single-crystal structures (Fig. S9) [31]. Furthermore, many strong
intermolecular interactions including hydrogen bond O⋅⋅⋅H (1.895 Å),
S⋅⋅⋅H (2.323 Å), and H⋅⋅⋅H (2.197 Å) are found between the neighboring
(S)1-MCH molecules. These strong intermolecular interactions could
eliminate the IMs and help to self-assemble (S)1-MCH molecules to
build 1-D hierarchical helices of helices (helical pitch ¼ 22.3 Å) in one
crystal cell, which indicate that these materials might be superior in the
CPL switching application [16].
2.6. Chiroptical properties
The chirality properties of (R/S)2-SP and (R/S)3-SP in dilute MeCN
solution and PMMA are investigated by CD spectra (Fig. 14). Even in
dilute MeCN, the CD signals of (R/S)2-SP and (R/S)3-SP are still strong.
(S)2-SP, the (S) enantiomer, shows positive Cotton peak at 270 nm and
negative Cotton peaks at 236, 289, and 340 nm. (R)2-SP, the (R)
enantiomer, exhibits exactly the mirrored CD spectrum indicating that
(S)2-SP and (R)2-SP are a pair of enantiomers. The similar phenomena
are observed for (S)3-SP and (R)3-SP enantiomers. Combining with
previous work [13e,16c,d,32], the CD signals are mainly originated
from chiral binaphthyl itself.
Since (R/S)3-SP exhibits excellent photochromic-switching proper-
ties, its chiroptical-switching properties are examined in both dilute
solution and PMMA film (Fig. 14). It is unexpected that, when irradiated
(R/S)3-SP solution by 365 nm UV light for 10 min, the resultant purple
(R/S)3-MC solution do not show any Cotton peak in longer wavelength
regions (>400 nm). This indicates that (R/S)3-MC has strong absorption
at longer wavelength regions, but the corresponsive CD signals are much
lower compared to those in shorter wavelength regions [17b,33] and
might be caused by the fact that the absorption of (R/S)3-MC at longer
Fig. 10. Top: absorption spectra changes of (S)3-SP (10
μ
mol dmÀ 3) in MeCN
upon irradiation with 365 nm UV light. Insert: photographs of (S)3. Middle:
absorption spectra changes of (S)3-SP-doped (wt % ¼ 1.0%) PMMA film upon
irradiation with 365 nm UV light and the recovery under heating at 80 �C.
Bottom: absorption dynamics at the peak (590 nm) of the (S)3-SP-doped (wt %
¼ 1.0%) PMMA film when exposed to continuous 365 nm UV irradiation. Insert:
its absorbance contrast profiles under 365 nm UV irradiation and heating (80
�C) treatment with switching cycles.
wavelength regions is mainly originated from π→ π* character in the MC
unit and nitro group and the central chiral binaphthyl unit has little
contribution. For PMMA film, however, the Cotton peak around 590 nm
is still weak but detectable.
2.7. Thermochromism
Thermochromism denotes the phenomenon of a color change with
dependence on temperature [34]. As shown in Fig. 15, when (S)3-MC in
MeCN solution is cooled to 77 K in liquid nitrogen, the purple color will
9

L. Qu et al.
Dyes and Pigments 181 (2020) 108597
Fig. 11. Absorption spectra changes of (S)3-SP-doped (wt % ¼ 1.0%) PMMA film (thickness ¼ 0.5 mm) upon irradiation with 365 nm UV light and the recovery
under heating at 80 �C.
anticounterfeit mask with both visual color (colorless ↔ continuously
increased purple) and fluorescence (dark ↔ continuously increased red)
changes under room light and UV light, respectively.
3. Conclusions
In summary, we have prepared and reported a unique serial of
binaphthyl-substituted spiropyrans, which show not only the interesting
chiroptical switching properties but also the solid-state luminescence.
The introduction of strong electron-withdrawing nitro groups would
help to promote photochromic switching but hinder chemical switching.
The nitro groups can stabilize the electron-rich conjugated merocyanine
with the lower LUMO and HOMO levels. Therefore, these multi-stimuli-
responsive molecular switches have potential applications in not only
optical data storage, anticounterfeiting, sensing, and bioimaging but
also chiral recognition and circularly polarized luminescence.
Fig. 12. Photographs of (S)3-SP (MeCN solution upon irradiation with 10 min
365 nm UV light) on SiO2 TLC using ethyl acetate: petroleum ether ¼ 1:5 as
eluent (a: under room light; b: under 254 nm UV light; c: under 365 nm
UV light).
4. Experimental section
4.1. Materials and instrumentation
All reagents were purchased from commercial suppliers and used
without further purification. 1H NMR (400 MHz) spectra were recorded
in DMSO‑d6. Chemical shifts are reported in ppm using tetramethylsi-
lane as internal standard. UV/vis absorption spectra were recorded
using a U5100 (Hitachi) spectrophotometer with quartz cuvettes of 1
and 0.1 cm pathlength for solution and PMMA film, respectively. Fluo-
rescence spectra were obtained using F-7000 Fluorescence spectropho-
tometer (Hitachi) at room temperature. The slit width was 5 nm and 2.5
nm for excitation and emission. The photon multiplier voltage was 400
V. CD spectra were recorded using a Chirascan plus qCD (Applied
Photophysics) at room temperature. HR-MS were obtained with a
Waters-Q-TOF-Premier (electrospray ionization mass spectrometry,
ESI).
change into orange-red color. When temperature is increased to room
temperature, the color will recover to purple color and finally become
colorless. This color change is reversible and visible to the naked eyes.
As previously mentioned, the ring-opening reaction of (S)3-SP → (S)
3-MC is endothermic, and thus cooling would help to form colorless (S)
3-SP from purple (S)3-MC. Consequently, the color of frozen mixture
will change from purple into orange-red color.
2.8. Anticounterfeiting mark
The counterfeiting of good and money has important economic im-
plications and is also a threat to health and security [35]. Turn-on
fluorescence is widely adopted for a normal anticounterfeiting mark,
when it was irradiated by UV light. If (S)3-SP-doped PMMA film (wt %
¼ 1.0%) is used as anticounterfeiting mark, it is possible to provide more
usefully signal changes for counterfeiting. As shown in Figs. 11 and 16,
combining photoetching technology, (S)3-SP can be used as a promising
4.2. Measurement of fluorescence quantum yield (Φ)
The quantum yield of a solution sample was measured by the optical
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L. Qu et al.
Dyes and Pigments 181 (2020) 108597
Fig. 13. X-ray single crystal structures and packing of (S)1-MCH molecules: packing in a unit cell from a view down the crystallographic a (a), b (b and e), and c (c)
axes; intermolecular interactions (d). H atoms are omitted in (e).
dilute method of Demas and Crosby with a standard of quinine sulfate
(Φr ¼ 0.55, quinine in 0.05 mol dmÀ 3 sulfuric acid) calculated by: Φs ¼
Φr(Br/Bs)(ns/nr)2(Ds/Dr), where the subscripts s and r refer to the sample
and reference standard solution respectively; n is the refractive index of
the solvents; D is the integrated intensity. The excitation intensity B is
calculated by: B ¼ 1–10–AL, where A is the absorbance at the excitation
wavelength and L is the optical path length (L ¼ 1 cm in all cases). The
refractive indices of the solvents at room temperature are taken from
standard source. The quantum yield of a solid sample was measured by
an integrating sphere.
4.4.1. Synthesis of (R/S)1-MC
7 (1.41 g, 5.0 mmol) and 8 (0.63 g, 2.0 mmol) were added into
anhydrous ethanol (30 mL), and then the mixture was allowed to reflux
overnight. The organic solid was filtered, and then the precipitate was
washed with EtOH afford to a dark yellowish brown solid (0.76 g, 66%).
1H NMR (400 MHz, DMSO‑d6) δ 9.52 (s, 1H), 9.26 (d, J ¼ 11.1 Hz, 2H),
8.82 (d, J ¼ 16.3 Hz, 1H), 8.26 (d, J ¼ 16.3 Hz, 1H), 8.08 (d, J ¼ 7.9 Hz,
2H), 7.97–7.85 (m, 3H), 7.69–7.59 (m, 2H), 7.37 (dd, J ¼ 7.8, 5.7 Hz,
2H), 7.29 (ddd, J ¼ 14.8, 8.2, 1.1 Hz, 2H), 7.25–7.19 (m, 1H), 6.96 (d, J
¼ 8.3 Hz, 1H), 6.87 (d, J ¼ 8.3 Hz, 1H), 5.01–4.87 (m, 2H), 2.78–2.69
(m, 2H), 2.28 (s, 2H), 1.82 (s, 6H). 13C NMR (101 MHz, DMSO‑d6) δ
181.75, 154.04, 151.83, 149.64, 143.73, 140.99, 136.83, 134.22,
131.64, 130.06, 129.96, 129.87, 129.48, 129.44, 129.28, 129.09,
128.71, 128.42, 128.22, 126.42, 124.63, 123.93, 123.12, 122.58,
118.86, 118.03, 115.28, 113.52, 112.98, 52.09, 47.44, 45.73, 39.52,
26.29, 24.81. HR-MS (ESI): m/z: calcd: 577.1923; found: 578.1998
[MþH]þ, 600.1840 [MþNa]þ.
4.3. X-ray crystallographic analysis
The determination of the unit cell and data collection for four single
crystal samples were performed on a Xcalibur E X-ray single crystal
diffractometer equipped with graphite monochromator Mo K
α (λ ¼
0.71073 Å) radiation. The data collection was executed using CrysA-
lisPro program. Structures were solved by direct method and successive
Fourier difference syntheses (SHELXS-97), and were refined by full
matrix least-squares procedure on F2 with anisotropic thermal param-
eters for all nonhydrogen atoms (SHELXL-97).
4.4.2. Synthesis of (R/S)2-SP
(R/S)-4 (1.88 g, 3.0 mmol), 3 (2.14 g, 6.0 mmol), and Cs2CO3 (4.90
g, 15.0 mmol) were transferred into a round-bottom flask, which was
full of nitrogen gas. 30 mL THF and 6 mL distilled water were injected
into the flask. When the solution was stirred uniformly and freezed with
liquid nitrogen, Pd(PPh3)4 (0.16 g) was added. The contents of the flask
were evaporated, and nitrogen was pumped for three cycles to replace
the residual oxygen. After the solution melted and the temperature
returned to room temperature, the reaction liquid was heated to reflux
for 16 h. Then, 60 mL water was added into the flask, and the solution
was extracted three times with ethyl ether. The organic layer was
combined and dried with anhydrous sodium sulfate. Then, the solvent
was evaporated, and crude product was obtained. The crude product
4.4. Computational details
Calculations were carried out using the Gaussian 09 software pack-
age [B3LYP 6-31G(d,p)] based on the X-ray single-crystal structure or its
modified structures. The ground state geometry was optimized using
DFT. The excited states were predicted using the ground state geometry
using TD-DFT, from where UV/Vis absorption (pcm method for solvent
effect; 100 excited states) were predicted.
11

L. Qu et al.
Dyes and Pigments 181 (2020) 108597
Fig. 15. Photographs of (S)3-SP in MeCN (5
μ
mol dmÀ 3) at room temperature
(I and II) or 77 K (III: whole was cooled by liquid nitrogen; IV: half bottom was
cooled by liquid nitrogen).
the residual oxygen. After the solution melted and the temperature
returned to room temperature, the reaction liquid was heated to reflux
for 18 h. Then, 100 mL water was added into the flask, and the solution
was extracted three times with ethyl ether. The organic layer was
combined and dried with anhydrous sodium sulfate. Then, the solvent
was evaporated, and crude product was obtained. The crude product
was purified by flash column chromatography (ethyl acetate: petroleum
ether ¼ 1 : 1) to give target compound 5 a reddish brown oil. The reddish
brown oily liquid was taken out of the solvent in vacuo to give a light
purple solid (1.69 g, 52%). This product was used directly in the next
step without further purification. The crude organic 5 (1.69 g, 2.36
mmol), 5-nitrosalicylaldehyde (1.00 g, 6.14 mmol) and anhydrous EtOH
(150 mL) were added into a flask. The reaction solution under a nitrogen
atmosphere was heated to reflux for 48 h. Then, the solvent was evap-
orated to obtain a red powder as a crude product. The crude compound
was purified by column chromatography (ethyl acetate: Petroleum
ether ¼ 1 : 10) to afford a pale green powder solid (1.41 g, 58%). 1H
NMR (400 MHz, DMSO‑d6) δ 8.25–8.23 (m, 2H), 8.07 (d, J ¼ 2.0 Hz,
2H), 8.02 (ddd, J ¼ 7.8, 6.7, 2.9 Hz, 4H), 7.49 (d, J ¼ 9.6 Hz, 4H), 7.44
(d, J ¼ 7.9 Hz, 2H), 7.30 (t, J ¼ 7.6 Hz, 2H), 7.26 (d, J ¼ 10.3 Hz, 2H),
7.12–7.05 (m, 2H), 6.92–6.87 (m, 2H), 6.77 (d, J ¼ 7.9 Hz, 2H), 6.03 (d,
J ¼ 10.5 Hz, 2H), 4.47 (t, J ¼ 5.0 Hz, 1H), 4.42 (dd, J ¼ 5.2, 2.4 Hz, 1H),
4.35–4.30 (m, 2H), 2.74 (s, 6H), 2.39 (dd, J ¼ 6.2, 2.0 Hz, 6H), 1.27 (d, J
¼ 12.5 Hz, 6H), 1.18 (d, J ¼ 7.1 Hz, 6H). 13C NMR (101 MHz, DMSO‑d6)
δ 159.31, 150.62, 146.87, 140.53, 135.98, 135.49, 132.63, 130.61,
129.61, 128.98, 128.36, 127.94, 126.06, 125.88, 125.75, 125.57,
125.06, 122.83, 122.63, 121.24, 118.90, 115.32, 106.24, 97.50, 55.45,
51.93, 30.95, 28.54, 25.58, 22.05, 19.58. HR-MS (ESI): m/z: calcd:
1014.3840; found: 1015.3921 [MþH]þ, 1037.3720 [MþNa]þ.
Fig. 14. Top: CD spectra of (R/S)2-SP and (R/S)3-SP (without and with 5 min
and 10 min UV irradiation) in MeCN (5
μ
mol dmÀ 3). Bottom: CD spectra of (S)
3-SP-doped PMMA film (wt % ¼ 1.0%) without and with 10 min UV
irradiation.
was purified by flash column chromatography (ethyl acetate: petroleum
ether ¼ 1 : 100~1 : 25) to give target compound (R/S)2-SP (1.16 g,
42%) as a white amorphous solid. 1H NMR (400 MHz, DMSO‑d6) δ
8.12–7.99 (m, 4H), 7.52–7.42 (m, 6H), 7.29 (t, J ¼ 7.6 Hz, 2H), 7.18 (d,
J ¼ 7.4 Hz, 2H), 7.14–7.06 (m, 4H), 7.03 (d, J ¼ 10.2 Hz, 2H), 6.84 (td, J
¼ 7.4, 2.7 Hz, 2H), 6.68 (dd, J ¼ 14.8, 7.6 Hz, 4H), 5.79 (d, J ¼ 10.2 Hz,
2H), 4.50–4.41 (m, 2H), 4.34 (td, J ¼ 5.4, 2.7 Hz, 2H), 2.71 (s, 6H), 2.39
(s, 6H), 1.29–1.23 (m, 6H), 1.15 (s, 6H). 13C NMR (101 MHz, DMSO‑d6)
δ 153.89, 150.65, 147.31, 136.52, 135.59, 132.59, 130.63, 129.78,
129.38, 129.09, 128.79, 127.90, 126.92, 125.91, 125.59, 125.01,
122.51, 120.23, 119.02, 118.58, 114.30, 106.69, 103.98, 97.46, 55.43,
51.39, 30.95, 28.58, 25.54, 22.06, 19.85. HR-MS (ESI): m/z: calcd:
924.4138; found: 925.4216 [MþH]þ, 947.3998 [MþNa]þ.
Declaration of competing interest
There are no conflicts to declare.
CRediT authorship contribution statement
Lang Qu: Investigation, Formal analysis, Visualization, Writing -
original draft. Xuemei Xu: Writing - original draft. Jintong Song:
Conceptualization, Methodology. Dehua Wu: Conceptualization,
Methodology. Lei Wang: Investigation. Weilan Zhou: Validation.
Xiangge Zhou: Methodology. Haifeng Xiang: Resources, Supervision,
Writing - original draft.
4.4.3. Synthesis of (R/S)3-SP
(R/S)-4 (3.76 g, 6.0 mmol), 2 (4.40 g, 15.4 mmol), and Cs2CO3 (10.0
g, 30.7 mmol) were transferred into a round-bottom flask, which was
full of nitrogen. 60 mL THF and 12 mL distilled water were injected into
the flask. When the solution was stirred uniformly and freezed with
liquid nitrogen, Pd(PPh3)4 (0.33 g) was added. The contents of the flask
were evaporated, and nitrogen was pumped for three cycles to replace
12

L. Qu et al.
Dyes and Pigments 181 (2020) 108597
Fig. 16. Photographs of (S)3-SP-doped (wt % ¼ 1.0%) PMMA film (thickness ¼ 0.5 mm) on 1 yuan RMB without (a) and with 0.5 min (c) and 10 min (b,d) UV
irradiation under room light or UV light. The PMMA film with panda shielding mask under room light or UV light (e).
[2] (a) Feringa BL, van Delden RA, Koumura N, Geertsema EM. Chiroptical molecular
Acknowledgements
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from molecules to materials. Adv Mater 2010;22:3348–60.(c) Ko CC, Yam VWW.
This work was supported by the National Natural Science Foundation
of China (no. 21871192) and Sichuan Science and Technology Program
(no. 2018JY0559). We acknowledge the comprehensive training plat-
form of the specialized laboratory of the College of Chemistry, Sichuan
University, for material analysis. We would like to thank the Analytical
& Testing Center of Sichuan University for CCD X-ray single crystal
diffractometer work and circular dichroism spectrometer work. We are
grateful to Daibing Luo and Yani Xie for help with the single crystal and
circular dichroism measurements.
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