A light triggered optical and chiroptical switch based on a homochiral Eu2L3helicate

Article abstract of DOI:10.1039/d0tc01044h

Optical and chiroptical photoswitches have found potential applications in advanced information technologies. Herein, a pair of chiral triple-stranded helicates [Eu₂(o-L^RR)₃](TOf)₆and [Eu₂(o-L^SS)₃](TOf)₆, which have chirality, luminescence and light stimuli-responsive properties, have been designed and synthesized. The ligands o-L^RR/SSare composed of photochromic diarylethene moieties as spacers and two chiral 2,6-dipicolinic amides as coordination units. Through the comprehensive spectral characteristics in combination with semiempirical geometry optimization using the Sparkle/RM1 model, it was confirmed that (R/S)-1-phenylethanamines decorated at the terminals of ligands successfully controlled the stereoselective self-assembly of the helicates. Upon alternating UV and visible light irradiation, the diarylethene experienced a reversible change between a colorless open-ring state and a pink closed-ring state. This photochromic process caused variations in the electronic structure and ligand conformation, and led to the presence of light-responsive optical (UV and PL) and chiroptical (ECD and CPL) switching properties. The helicate may, therefore, be considered as a completely reversible +/- ECD and on/off CPL switch. Furthermore, on the basis of these light-switchable optical properties, the INHIBIT and IMPLICATION logic gates were fabricated.

Full text of DOI:10.1039/d0tc01044h

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DOI: 10.1039/D0TC01044H
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A light triggered optical and chiroptical switch based on a
homochiral Eu2L3 Helicate
Jianpeng Zhang, Yanyan Zhou, Yuan Yao, Zhenyu Cheng, Ting Gao, Hongfeng Li* and Pengfei Yan
Received 00th January 20xx,
Accepted 00th January 20xx
Optical and chiroptical photoswitches have found potential applications in advanced information technologies. Herein, a
pair of chiral triple-stranded helicates [Eu2(o-LRR)3](TOf)6 and [Eu2(o-LSS)3](TOf)6, which have chirality, luminescence and light
stimuli-responsive properties, have been designed and synthesized. The ligands o-LRR/SS are composed of a photochromic
diarylethene moieties as spacer and two chiral 2,6-dipicolinic amides as coordination units. Through the comprehensive
spectral characteristics in combination with semiempirical geometry optimization using Sparkle/RM1 model, it was
confirmed that (R/S)-1-phenylethanamines decorated at the terminals of ligand successfully controlled the stereoselective
self-assembly of the helicates. Upon alternating UV and visible light irradiation, the diarylethene experienced a reversible
change between a colorless open-ring state and a pink closed-ring state. This photochromic process caused the variations
of electronic structure and ligand conformation, and led to the presence of light-responsive optical (UV, PL) and chiroptical
(ECD, CPL) switching properties. The helicate may therefore be considered as a completely reversible +/- ECD and on/off CPL
switch. Furthermore, on the basis of these light-switchable optical properties, the INHIBIT and IMPLICATION logic gates were
fabricated.
DOI: 10.1039/x0xx00000x
www.rsc.org/
IL and IR represent the left and right polarized emission
intensities, respectively.
To facilitate the detection of CPL signal, the switches should
Introduction
Optical photoswitches have attracted much interest because of
their promising applications in optical storage,1 logic gate2 and
sensors.3 In recent years, following with the rapidly increasing
demands for massive information storage and/or encryption, it
might be a feasible strategy to extend the dimension of output
signals, which can add extra storage density and increase the
difficulty to decode data. Chiroptical photoswitches could
demonstrate the changes of chiroptical signals besides the usual
optical outputs upon alternating light irradiation.4 Circular
dichroism (CD) and circularly polarized luminescence (CPL)
spectroscopies can reflect the ground state and excited state
chirality of molecules,5 which measure the differences of
absorption and emission of left circularly polarized light versus
right circularly polarized light, respectively. In comparison with
the CD−based photoswitches,6 the examples exhibiting dynamic
photoswitching of CPL are rare due to the relatively low
luminescence dissymmetry factor (glum) of the most chiral
luminophores,7 which increase the difficulties on the detection
of CPL signals. The glum value represents the degree of
preference of one type of circularly polarized light over the other,
which is defined as glum = 2(IL-IR) / (IL+IR), where |glum|≤2, and
have the high luminescence quantum yields (QYs) and/or large
glum values. Generally, the high glum values could be obtained
from the emission systems with electric dipole forbidden and/or
magnetic dipole-allowed transitions.8 For most of the CPL
materials comprising of organic fluorophores, such as those of
small organic molecules,9 polymers10 and supramolecular
assemblies,11 their magnetic transition dipole moments (m) are
much smaller than the electric transition dipole moments (μ), and
thus, the glum values are typically within the range of 10–5–10–3.
In contrast, some of the f‒f transitions in lanthanide ions are
magnetic dipole-allowed transitions, which render the lanthanide
CPL materials the larger glum values, typically in the range of 10–
2–0.5.12 For instance, the Cs[Eu((+)-hfbc)4] represents the
highest reported glum value (1.38) of CPL materials in solution.13
As a consequence, the chiral lanthanide complexes might be the
more valuable CPL photoswitch candidate due to their large glum
values.
Diarylethenes (DAE) and their derivatives are known as the
most widely used photo-responsive components in chiroptical
switches besides the spiropyran and azobenzenes.14,15 However,
the examples that could realize the reversible photoswitching of
chiroptical signals in lanthanide assemblies are much more
limited.16 As we known, there was only one example of a
lanthanide assembly for simultaneously realizing CD and CPL
photoswitching.16a
In chiroptical switches, the introduction of chiral elements into
the chromophore/luminophore or photochromic moieties is the
general strategy for realizing the modulation of chiroptical
a. Key Laboratory of Functional Inorganic Material Chemistry, Ministry of
Education; School of Chemistry and Materials Science, Heilongjiang University,
Harbin 150080, China. E-mail: lihongfeng@hlju.edu.cn; Yanpf@vip.sina.com.
b. †Electronic Supplementary Information (ESI) available: [1H NMR, 13C NMR, ESI-
TOF-MS, thermal stability, fatigue resistant property spectra, emission spectra and
the methord for Φo-c and Φc-o,]. For ESI or other electronic format See
DOI:10.1039/x0xx00000x
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signals.17 Helical structure is one of the most widespread and which were composed of the photochromic diarylethene
View Article Online
DOI: 10.1039/D0TC01044H
fascinating structures in nature, which is intrinsically chiral, moieties as spacer and two chiral 2,6-dipicolinic amide as
possessing P or M helical senses. Although the reports are coordination units. As expected, a pair of homochiral triple-
limited, a few chiral lanthanide dinuclear and multinuclear stranded helicates [Eu2(o-LRR)3](TOf)6 and [Eu2(o-LSS)3](TOf)6
helical architectures have been developed by Gunnlaugsson,18 with CPL were successfully isolated, which showed light-
Law,19 Sun,20 and us.21 As previously reported, the switchable optical and chiroptical properties upon alternating
photoisomerization of diarylethene (DAE) units in helicate UV and visible light irradiation. Finally, the helicate as a logic
resulted in the dramatic changes of CD spectra in the visible gate material was investigated and the INHIBIT and
region.16b On the basis of above points, herein, we designed and IMPLICATION logic gates were successfully fabricated
synthesized a pair of chiral ditopic ligands o-LRR and o-LSS,
.
Scheme 1 Synthetic routes of o-LSS and the complex [Eu2(o-LSS)3](TOf)6.
single species in solution. The Eu(III) complexes show a single
set of broadened but distinguishing signals (Figures S12 and
S13). In comparison with the free ligands, most of the resonance
peaks undergo high-field shifts. To more clearly confirm the
formation of the single species and their diastereopurities in
solution, the corresponding Y(III) complexes were isolated as
Results and discussion
Synthesis and characterization of the ligands and complexes
The synthetic procedures for the enantiopure DAE-based
bis(tridentate) ligands, o-LRR/o-LSS and their corresponding
complexes are outlined in Scheme 1. The ligands are composed
of three subunits, namely two coordinated 2,6-dipicolinic amides
and one photochromic diarylethene. The former was prepared
according to previously reported procedures,22 where the
chirality was introduced through a condensation reaction
between 7a and chiral (R/S)-1-phenylethanamine. The 3a was
synthesized using the known methods.23 Regarding the nitration
of 3a, HNO3/SnCl4 was selected as nitration agent to substitute
the general HNO3/acetic anhydride partner, which effectively
avoided the ring-opening of benzofurans. Finally, the two
functional groups were connected via the amide-coupling
reaction to give the final ligands. The ligands and some
intermediates were characterized by 1H, 13C NMR spectroscopy
and mass spectrometry (Figures S1‒S11).
1
substitute for H NMR experiments. As shown Figure 1a, a
single set of sharp resonance peaks are obtained, which displays
about the same full-width half-maximum compared with the free
ligand. Taking the Hi as representative, only one sharp single
peak is observed, which indicates the complex matching the C2
symmetric feature of the ligand and the time-averaged C3
symmetry of this triple-stranded helicate.
High-resolution electrospray ionization mass spectrometry
(ESI-TOF-MS) analyses further confirm the formation of a
triple-stranded helicate with a formula of [Eu2(o-LRR)3](TOf)6
and [Eu2(o-LSS)3](TOf)6 (Figure 1b and S14). The spectra show
three cluster peaks with the correct isotopic distributions
corresponding to the given charge state containing differing
ratios of TOf‒ counter anion ions. For example, the cluster of
peaks with m/z at 878.6821, 1221.2135 and 1906.2639 could be
assigned to [[Eu2(o-LRR)3](TOf)2]4+, [[Eu2(o-LRR)3](TOf)3]3+,
and [[Eu2(o-LRR)3](TOf)4]2+ charge states (Figure 1b). The
assignments are further verified by carefully comparing the
The Ln(III) triple-stranded helicates were obtained by reaction
of the ligands (o-LR/S) with the corresponding Ln(TOf)3 (Ln =
Eu, Y and Gd) in a 3:2 ratio in CH3CN at room temperature for
1
1 h. The H NMR characterizations suggest the formation of a
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corresponding simulated isotopic patterns with high-resolution from three ligands, giving rise to a tricapped trigonal prismatic
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experimental data. The corresponding Y and Gd complexes also geometry. The Eu–Eu distance is determDinOeId: 1t0o.10b3e9/1D70.T4CÅ01.0I4t4iHs
show the similar spectral patterns and the correct isotopic noted that the DAE units in spacer all adopt the antiparallel
distributions (Figures S15‒S18).
conformations, which is the essential for realizing light
cyclization reaction.
Fig. 2 Sparkle/RM1 ground state geometries of the [Eu2(o-LRR)3](TOf)6 before
(upper graph) and after (lower graph) irradiation with UV light.
Optical switch by UV‒Vis and CD absorption
The optical and chiroptical photoswitching properties of the
pair of helicates were firstly investigated by UV–Vis and CD
spectra. Figure 3 shows the spectral changes of two ligands
before and after UV irradiation. As shown in Figure 3a, the
colorless o-LRR/o-LSS solution shows a broad band in range of
270‒390 nm with the maximum absorbance at 303 nm before
UV irradiation. Following UV light irradiation at 275 nm, the
absorbance at 303 nm gradually decreases and a new band
appeares at 508 nm (Figures S20 and S21), leading to an obvious
color change of the solution from colorless to pink (Figure 3a,
inset). This is the result of the photocyclization of the
dithienylethene from open-ring to close-ring forms. Upon visible
light irradiation at 526 nm for 3 min, the spectra reversibly
returned to that of the original states (Figures S22 and S23), and
these photochromic conversions could be repeated for several
cycles without appreciable light fatigue (Figures S24 and S25).
Moreover, the ligands also show excellent thermal stabilities in
photostationary state (PSS). There are almost no changes of
spectral shapes and intensities after heating for 2 h at 55 °C
(Figures S26 and S27). On the basis of the plots of irradiation
time dependence of absorption intensity at 508 nm and
conversion ratios of photoisomerization (estimated by 1H NMR
in PSS, Figures S28 and S29), the photocyclization (Φo-c) and
photocycloreversion (Φc-o) quantum yields were calculated to be
0.039, 9.7 × 10–3 for LRR and 0.044, 8.7 × 10–3 for LSS,
respectively. To avoid the influence of high concentration of
NMR testing solution on the accuracy of the conversion ratio of
photoisomerization, the mole fraction of o-LRR/c-LRR were also
Fig. 1 Self-assembly of o-LRR with Ln(TOf)3. (a) 1H NMR assignment of ligand and 1H
NMR (400 MHz) spectra of free ligand LRR and the self−assembled Y complex [Y2(o-
LRR)3](TOf)6 in CD3CN. (b) ESI-TOF-MS spectrum of [Eu2(o-LRR)3](TOf)6 with inset
showing the observed (Obs.) and simulated (Sim.) isotopic pattern of [[Eu2(o-
LRR)3](TOf)3]3+ cation peaks.
To obtain a better structural insight into this helical structure,
a molecular mechanical modeling was built by using the
MOPAC 2016 program implemented in the LUMPAC 3.0
software with a Sparkle/RM1 model.24 It is known that the chiral
terminal of ligand could effectively control the stereoselectivity
during the assembly process. Herein, two possible
conformations, P or M in one helicate were both considered in
calculated process (Figure S19). In [Eu2(o-LRR)3]6+, a M helical
sense with the metal centers the ɅɅ configuration give a lower
system energy than that with a P configuration; conversely, a ΔΔ
configuration and a P helical sense is favored energetically in
[Eu2(o-LSS)3]6+. As shown in Figure 2, in one helicate, each
Eu(III) ion is nine-coordinated by six O atoms and three N atoms
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determined by high performance liquid chromatography (HPLC) lower open/close-ring conversion ratios are also reflected in
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method (Figure S42). The result shows that the photoconversion changes of solution color, which change fDroOmI: 1p0i.1n0k39in/Df0rTeCe0l1ig04a4nHd
is 21.3%, which is consistence with the result calculated from the to a light pink in complex (Figure 3a and 3b, insets). Upon
1H NMR test (21%).
alternating UV and visible light irradiation, the helicates show
reversible photoswitching absorption and thermal stability
(Figures S32‒S39).
The antiparallel conformations have two kinds of helical
senses, P and M. Since the chiral centers localize in terminal of
the ligands, such chirality cannot be effectively transferred to the
chromophore and/or cause the diastereomer excess between P
and M conformations. As a result, the CD signals of both ligands
are rather weak in their open-ring and close-ring states (Figure
3a). However, this point chirality could be effectively transferred
to the chromophores as helical chirality after self-assembly with
Ln(III) ions. As shown in Figure 3b, the open-ring helicates show
the mirror-image and strong CD signals in the corresponding UV
region. Moreover, two pairs of positive exciton couplets with
Cotton effect at 260, 276 nm and 312, 364 nm are observed in
[Eu2(o-LRR)3](TOf)6, while the mirror-imaged CD signals
exhibit in [Eu2(o-LSS)3](TOf)6. Generally, the pattern of exciton
couplet correlates with the absolute configuration of the metal
center,25 wherein a positive exciton couplet correlating to a Λ
configuration; on the contrary, correlating to a Δ configuration.
On the basis of this empirical rule, the Eu(III) ions are suggested
to adopt ΛΛ configurations in [Eu2(o-LRR)3](TOf)6. This result
is consistent with the optimized molecular structures by
molecular mechanical modeling.
Under exposure to UV light, the spectra of the helicates
display strong CD signals with bisignated exciton couplets in
visible region. This result implies that the chirality of (R/S)-1-
phenylethanamine has been successfully transferred into close-
ring DAE units, wherein the helical chirality is formed. In
comparison with open-ring form, the CD signals show the
decrease of intensities and slight red-shift in UV region. Upon
alternative irradiation with UV and visible light, the chiroptical
switching by CD signals are realized.
Fig. 3 (a) UV-Vis absorption (lower curves) and CD spectra (upper curves) of the ligands
(o-LRR, blue dash line; LRR-PSS, orange dash line and o-LSS, blue solid line; LSS-PSS, orange
solid line) (1.0 × 10–5 M, CH3CN, 293 K). (b) UV-Vis absorption (lower curves) and CD
spectra (upper curves) of [Eu2(LRR/SS)3](TOf)6 (3.3 × 10–6 M, CH3CN, 293 K).
In comparison with the absorptions of free ligands, the
helicates show some differences in open-ring and
photostationary state (PSS). First, the absorption maximum of
helicate is red-shifted by about 30 nm compared with the ligand
(Figure 3b). We suggest that the red shift should originates from
the electron-withdrawing effect of metal ion, which facilitates
the intramolecular charge transfer transition, leading to the
decrease of transition energy. Second, the helicate shows the
lower absorbance than free ligand in visible light region after UV
irradiation, which indicates that the ligand in complex should has
the lower photocyclization quantum yield. Unfortunately, the 1H
NMR spectrum of [Y2(o-LRR/SS)3](TOf)6 did not offer the
information to estimate the conversion ratio between o-LRR/SS
and c-LRR/SS forms, which only displayed a slight decrease of
intensity and no new signals attributed to close-ring isomers or
free ligands were found upon UV irradiation for 12 h (Figures
S30 and S31). We infer that the protons in closed forms might
have very similar magnetic environments, whose resonance
signals are probably overlapped by those in open-ring isomers.
However, the photoconversion ratio of close-ring isomers could
be estimated by high performance liquid chromatography
(HPLC) method (Figure S43). In PSS, a peak attributed to close-
ring form was observed at retention time of 14.67 min, by which
the conversion ratio of photoisomerization was calculated to be
2% and the photocyclization (Φo-c) and photocycloreversion (Φc-
o) quantum yields were calculated to be 7.6 × 10–3 and 0.057. The
Optical switch by PL and CPL
To investigate the photomodulate luminescence properties of
[Eu2(o-LRR/SS)3](TOf)6, the emission from center Eu(III) ions
was determined by excited at λ = 400 nm (Figures S40, with a
low intensity at this wavelength). In open-ring state, the helicates
show four characteristic emission bands of Eu(III) ion at 594,
5
615, 650, and 695 nm, corresponding to D0 → 7FJ (J = 1–4)
transitions (Figure 4). After reaching closed photostationary
state (PSS), the emission intensities decrease to 24% of its initial
value. This is the result of the photocyclization of the
5
dithienylethene unit, which deactivate the D0 excited state of
Eu(III) ions. In photochromic fluorescent switches, the
luminescent quenching generally depends on Förster resonance
energy transfer (FRET) mechanism.26 The absorption of close-
ring helicate covers the visible spectral region of 430‒680 nm
(Figure 3b), thus it seems that the FRET quenching mechanism
is reasonable. However, in lanthanide systems, the Dexter
electron exchange between the ligand triplet state and excited
state of Ln(III) as sensitization mechanism should not be
excluded.27 Thus, the triplet state energy level (T, 3ππ*) of ligand
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Table 1. glum values for 5D0 → 7FJ of Eu3+ ion. View Article Online
in open-ring and PSS are estimated by the phosphorescence
spectra of [Gd2(LSS)3](TOf)6 (Figure S41). The [Gd2(o-
LSS)3](TOf)6 shows a characteristic phosphorescence band of
lanthanide complex with three vibrational progressions. From
the highest energy band, the open-ring To energy level was
5
7
DOI: 10.1039/D0TC01044H
glum D0 → FJ (J = 1, 2, 3, 4)
Complexes
J = 1
J = 2
J = 3
J = 4
(589 nm)
(613 nm)
(651 nm)
(694 nm)
Eu2(o-LRR)3(TOf)6
Eu2(o-LSS)3(TOf)6
Eu2(c-LRR)3(TOf)6
Eu2(c-LSS)3(TOf)6
–0.100
0.101
0.050
–0.051
0.027
–0.056
0.055
0.030
–0.030
0.025
5
calculated to be 21 231 cm−1 (471 nm, higher than D0 energy
level, 17 500 cm−1), which implied that an effective energy
transfer from ligand to metal center could occur and turn on the
emission of Eu(III) ion. However, in close-ring PSS state,
[Gd2(LSS)3](TOf)6 show a new band at 613 nm besides a
hypsochromic shift high energy band at 434 nm. This lower
energy band could be attributed to the close-ring ligand
phosphorescence. By this band, the close-ring Tc was estimated
–0.040
0.040
–0.025
0.025
–0.028
–0.024
Application in logic circuit
It is clear from the above results that the optical (UV, PL)
chiroptical (CD, CPL) signals could be effectively modulated by
UV and visible light stimuli. As a result, the helicates can be used
as the molecular logic gate (MLG) when the UV and visible light
are selected as input and the intensity variations of absorption
(UV and CD) and emission (PL and CPL) as output. To construct
the logic switch, the following conditions are defined: the open-
ring form of helicate is used as an initial state of MLG, UV light
(λ = 365 nm) is employed as input (In 1) and visible light (λ =
526 nm) as input (In 2), and the logical inputs 0 and 1 are
assigned to the absence and presence of stimulus, respectively.
Regarding to the output, the absorption at λ = 500 nm in UV and
CD spectra and the emission at λ = 615 nm in PL and CPL
spectra are considered as the detectable signals, and named as
OCD, OUV, OPL and OCPL, respectively. OUV and OCD are defined
as 1, when the absorbances λ = 500 nm are large than zero,
otherwise OUV and OCD are 0. As shown in Figure 5, on the basis
of these definitions, two INHIBIT logic gates were successfully
fabricated by UV and CD signals. It implies only when In1 = 1
and In2 = 0, the output signal (OUV and OCD) are 1. In PL and
CPL, OPL and OCPL are defined as 1, when the emission
intensities at λ = 615 nm are large than 50% of the maxima values,
otherwise they are 0. Thus, the IMPLICATION logic gates were
fabricated by determining the variations of luminescent
intensities. In a word, two kinds of logic gates based on optical
and chiroptical switching properties were successfully fabricated.
5
to be 16 313 cm−1, which is lower than the D0 energy level,
which indicates that the sensitization to Eu(III) ion would be
invalid. In our opinion, the FRET and Dexter energy transfer
mechanisms might coexist in the photomodulate luminescent
process.
Fig. 4 Total luminescence (lower curve, right axis) and CPL spectra (upper curves,
left axis) of the helicates in open-ring [Eu2(o-LRR/SS)3](TOf)6 and PSS states.
The photomodulate luminescence properties are also
reflected in the spectral variations of CPL. In CPL spectra, the
mirror-image CPL signals corresponding to above four
characteristic transitions of Eu(III) ion are observed before and
5
after UV irradiation. Among these bands, the D0→7F2 and
5D0→7F1 transitions display the larger emission intensities than
the other bands, which are consistent with those observed in PL
spectra. In PSS, in view of the decrease of total emission
intensities, the ΔI also show the significant decreases in CPL
intensities. On the other hand, the luminescence dissymmetry
factor, glum is the essential parameter for estimating the degree of
CPL, where glum = 2(IL – IR)/(IL + IR), (–2 ≤ glum ≤ 2). The higher
glum indicates the more intense polarized emission relative to
total emission, and the easier to be detected. Generally, the chiral
luminophores with large magnetic dipole transition often exhibit
the large glum. Herein, the 5D0→7F1 transition is magnetic dipole
transition, which shows the largest |glum| values of 0.1 and 0.04
in open-ring and PSS states, respectively. The corresponding
|glum| values for other transitions are listed in Table 1. Under
irradiation of visible light, the CPL intensities and glum values
can be recovered, which indicate the chiroptical switches by CPL
are obtained.
Fig. 5 The truth table and symbolic representation of the logic circuit triggered by In1
(irradiation with 365 nm light) and In2 (irradiation with526 nm light); the open [Eu2(o-
LRR)3] isomer as the initial state. The output is defined as OCD (positive CD signal at 500
nm), OCPL (CPL signal at 615 nm), OUV (UV-Vis absorbance at 500 nm), OPL (PL signal at 615
nm).
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GEO-OK, SCFCRT = 1.D-10 (to increase the SCFVcieownAvrteicrlegOennlicnee
DOI: 10.1039/D0TC01044H
criterion) and XYZ (for Cartesian coordinates).
Conclusions
In summary, a pair of chiral triple-stranded helicates [Eu2(o-
LRR)3](TOf)6 and [Eu2(o-LSS)3](TOf)6 that have light-responsive
optical and chiroptical switching properties were successfully
designed and synthesized. The helicate integrates the molecular
chirality, helical chirality, light stimuli-response, and lanthanide
luminescence in one architecture. Upon alternatively UV and
visible light irradiation, the helicate realized a reversible change
between a colorless open-ring state and a pink closed-ring state.
In close-ring photostationary state, the new absorption bands in
UV and CD spectra were observed in the visible light region,
meanwhile the emissions of Eu(III) ions were partly quenched,
leading to the decrease of intensities of PL and CPL. Notably,
the luminescence dissymmetry factor, glum was found to have a
decrease from 0.1 to 0.04 (by monitoring the 5D0→7F1 transition),
which indicated that the photochromic transformation causing
conformation variation of helical structure. After irradiation with
visible light, the spectra reversibly returned to those of the
original states. Thus, the helicate could be considered as a
completely reversible optical and chiroptical switch. Finally, on
the basis of above photoswitching properties, two INHIBIT and
IMPLICATION logic gates were described.
Synthetic procedures
2-Methylbenzofuran (1a). To a solution of benzofuran (1.2 g,
10 mmol) in THF (20 mL), n-BuLi (5.2 mL, 13.0 mmol, 2.5 M
in hexane,) was added at –78 °C and kept stirring for 2 hours.
After the solution become a pale suspension, CH3I (2.5 mL, 40
mmol) was added and stirred for 8 h at room temperature. The
solution was then quenched with water and extracted three times
with CH2Cl2 and dried over anhydrous MgSO4. Purification by
flash chromatography (pentane) afforded 2-methylbenzofuran
1
(1a) as a colourless liquid in 85 % yield (1.2 g). H NMR (400
MHz, CDCl3) δ = 7.51 (d, J = 7.5 Hz, 1H), 7.44 (d, J = 7.5Hz,
1H), 7.22 (m, J = 7.5, 6.6 Hz, 2H), 6.40 (s, 1H), 2.49 (s, 3H). EI-
MS m/z = 132.16 M+.
3-Bromo-2-methylbenzofuran (2a). To a solution of 1a (1.5 g,
10.3 mmol) in THF (100 mL), N-bromosullirimide (2.0 g, 11.4
mmol) was added at 0 °C, then stirred for 24 h at room
temperature. The reaction mixture was poured into sodium
thiosulfate solution and extracted with ethyl acetate. The product
was purified by column chromatography (petroleum ether as the
eluent) and give 1.2 g of 2a as a light yellow liquid in 48% yield.
1H NMR (400 MHz, CDCl3, TMS): δ 2.49 (s, 3H), 7.25–7.34 (m,
3H), 7.40–7.47 (m, 1H). EI-MS m/z = 212.06 M+.
Experimental
1,2-Bis(2-methyl-1-benzofuran-3-yl)perfluorocyclopentene
(3a). To a solution of 2a (1.2 g, 5.7 mmol) in THF (40 mL), n-
Materials and instruments
BuLi (4.0 mL, 6.4 mmol, 1.6 M in hexane,) was added at –78 °C
After stirring for 30 min, the octafluorocyclopentene (0.6 g, 2.8 mmol)
was added slowly to the reaction mixture at 78 °C, then stirred for
.
All commercially available reagents and chemicals involved
in this experiment were purchased from commercial companies
and were used without further purification if not otherwise
–
12 h. The reaction mixture was poured into concentrated sodium
chloride solution and extracted with ethyl acetate. The organic phase
was dried over anhydrous MgSO4 and evaporated in vacuo. The crude
product was purified by column chromatography (petroleum ether as
1
stated. The H NMR and 13C NMR spectra are recorded on a
Bruker Avance III 400 MHz spectrometer. HPLC analysis were
recorded on a SHIMASZU LC 20AT HPLC instrument with
InertSustain C18 column (Acetonitrile; flow rate: 1.0 mL/min).
Elemental analyses were carried out by using an Elementar Vario
EL cube analyzer. Electrospray time-of-flight (ESI-TOF) and
Electron ionization (EI) mass spectra were measured on Bruker
maXis mass spectrometers and Agilent 5975N, respectively. CD
and CPL spectra were performed on an Olis DM245
spectrofluorimeter. UV‒Vis spectra were obtained from a
Perkin-Elmer Lambda 25 spectrometer. Emission and excitation
spectra were recorded using an Edinburgh FLS 1000
fluorescence spectrophotometer. Luminescence lifetimes were
measured by using a single-photon-counting spectrometer from
Edinburgh Instrument (FLS 1000) with a microsecond pulse
lamp as the excitation sources. The data were analyzed by
software supplied by Edinburgh Instruments. The values
reported were the average of three independent determinations
for each sample.
1
the eluent) to give 0.6 g of 3a as colorless crystals in 42% yield. H
NMR (200 MHz) δ 2.07 (s, 3H), 2.16 (s, 3H), 7.14-7.30 (m, 4H),
7.36–7.48 (m, 2H), 7.59–
7.77 (m, 2H). EI-MS m/z = 436.35 M+.
1,2-bis(2-methyl-6-nitro-1-benzofuran-3-yl)perfluorocyc-
lopentene (4a). To a dried CH2Cl2 (5 mL) cooled to 0 °C, the
fuming nitrate (0.2 mL) and SnCl4 (0.6 mL, 3.0 mmol) was
slowly added and stirred for 15 min. Then it was added slowly to
a solution of 3a (0.1 g, 0.2 mol) in CH2Cl2 at 0 °C. The reaction
was monitored via TLC. After the reaction completion, the
reaction mixture was poured into the water (40 mL) and
neutralized to pH = 7. The mixture was then extracted with ethyl
acetate (3 × 30 mL) and dried over MgSO4. The product was
purified by column chromatography (hexane/ethyl acetate, v/v,
5/1) and give 0.1 g of 4a as a yellow solid in 83% yield. 1H NMR
(400 MHz, Chloroform-d) δ 8.32–8.37 (d, J = 2.0 Hz, 2H), 8.10–
8.21 (dd, J = 8.7, 2.0 Hz, 2H), 7.50–7.57 (d, J = 8.7 Hz, 2H),
2.23 (s, 6H). ESI-TOF-MS m/z = 549.0772 [M + Na]+.
Computational Details
1,2-bis(2-methyl-6-amino-1-benzofuran-3-yl)perfluoroc-
yclopentene (5a).To a solution of 4a (0.2 g, 0.4 mmol) in EtOH,
NiCl2·6H2O (0.21 g, 0.9 mmol) was added at 0 °C and kept
stirring for 15 min. Then NaBH4 (0.2 mg, 5.6 mmol) was added
slowly and stirred for 2 h at room temperature. The reaction
mixture was poured into the dichloromethane (20 mL). The
The ground state geometries of [Eu2(LRR)3](TOf)6 and
[Eu2(LSS)3](TOf)6 were optimized by calculations using
LUMPAC with a Sparkle/RM1 model implemented in the
MOPAC 2016 software. The keywords used in the calculation
reported here were RM1, PRECISE, BFGS, GNORM = 0.25,
6 | J. Name., 2012, 00, 1−3
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Page 7 of 10
Journal of Materials Chemistry C
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Journal Name
ARTICLE
mixture then was washed with brine and dried over Na2SO4. The the reaction was taken up in EtOAc (10 mL) and extracted with
View Article Online
product was purified by column chromatography copious amounts of saturated NaHCO3 (1D0OmI:L10×.1033)9./TDh0eTCo0r1g0a4n4iHc
(CH2Cl2/acetone, v/v, 10/1), and give 180 mg of 5a as a gray layer was evaporated under vacuum and purified by column
solid in 56% yield. 1H NMR (400 MHz, CD3CN) δ 7.07–7.13 (d, chromatography (hexane/ethyl acetate, v/v, 1/3), and give 0.13g
1
J = 8.4 Hz, 2H), 6.67-6.72 (d, J = 2.0 Hz, 2H), 6.50–6.57 (dd, J of LRR/SS as a light pink solid in 64.5% yield. H NMR (400
= 8.4, 2.1 Hz, 2H), 4.29 (s, 4H), 2.08 (s, 6H). ESI-TOF-MS m/z MHz, Acetonitrile-d3) δ 10.13 (s, 2H), 8.65–8.72 (d, J = 8.3 Hz,
= 467.1380 [M + H]+.
2H), 8.31–8.37 (dd, J = 7.7, 1.2 Hz, 2H), 8.25–8.31 (dd, J = 7.8,
Dimethyl Pyridine-2,6-dicarboxylate (6a). To a solution of 1.2 Hz, 2H), 8.05–8.14 (m, 6H), 7.34–7.42 (m, 6H), 7.23–7.31
dipicolinic acid (5.0 g, 30.0 mmol) in MeOH (100 mL), (dd, J = 8.3, 6.9 Hz, 6H), 7.14–7.23 (m, 2H), 5.27 (p, J = 7.2 Hz,
concentrated H2SO4 (2.5 mL) was added dropwise. Then, the 2H), 2.18 (s, 6H), 1.56 (d, J = 7.1 Hz, 6H). ESI-TOF-MS m/z =
mixture was heated to reflux for 24 h. The reaction was 993.3150 [M + Na]+.
monitored via TLC. After the reaction completion, the solvent Synthesis [Ln2(o-L)3](TOf)6. To a solution of the (o-L) (0.1 mg,
was removed and gave 5.9 g of 6a as a white solid in 99% yield. 0.1 mmol) in CH3CN (10 mL) was added dropwise a solution of
No further purification was needed. 1H NMR (400 MHz, DMSO- Ln(TOf)3 (Ln = Eu, Y, Gd; 0.067 mmol) in 10 mL CH3CN at
d6): δ 8.26–8.13 (m, 3H), 3.90 (s, 6H). EI-MS m/z = 195.19 M+. room temperature. After stirring for 2 h, the resulting solution
6-(Methoxycarbonyl)picolinic Acid (7a). To a solution of 6a was dropped slowly into dry diethyl ether and washed with
(1.9 g, 9.9 mmol) in MeOH (75 mL), the KOH pellets (557 mg, 9.9 CHCl3 (3 × 10 mL), then dried under vacuum to give the desired
mmol) was added at 0 °C and kept stirring for 4 h. The reaction was products.
monitored via TLC. After the reaction completion, the MeOH was [Eu2(o-LRR)3](TOf)6. Yield: 82% Anal. calc. for
removed by evaporation in vacuo. The white salt was washed with C165H120Eu2F36N18O36S6 (4111.0955): C, 48.21; H, 2.94; N, 6.13.
EtOAc (3 × 30 mL) then dissolved in water (25 mL), and the solution Found: C, 48.16; H, 2.97; N, 6.19. ESI-TOF-MS: m/z =
was acidified to pH = 2. The solution was extracted with chloroform 1221.2135 [[Eu2(o-LRR)3](TOf)3]3+.
(3 × 25 mL), then dried over MgSO4. The organic layer was [Eu2(o-LSS)3](TOf)6.
evaporated and give 1.5 g of 7a as a white powder in 81% yield H C165H120Eu2F36N18O36S6 (4111.10): C, 48.21; H, 2.94; N, 6.13;
Yield:
79%
Anal.
calc.
for
1
NMR (400 MHz, DMSO-d6): δ 8.23
MS m/z = 180.09 [M
H]–.
–
8.14 (m, 3H), 3.90 (s, 3H). ESI- O, 14.01. Found: C, 48.24; H, 3.02; N, 6.11. ESI-TOF-MS: m/z
= 1221.2243 [[Eu2(o-LSS)3](TOf)3]3+.
–
Methyl (R/S)-6-((1-phenylethyl)carbamoyl)picolinate (8a) [Gd2(o-LRR)3](TOf)6. Yield: 81% Anal. calc. for
and (R/S)‐6‐(2‐phenylpropylcarbamoyl)-picolinic acid (9a). C165H120Gd2F36N18O36S6 (4121.6675): C, 48.08; H, 2.93; N,
To a solution of 7a (0.4 g, 2.2 mmol), 1-ethyl-3-(3- 6.12. Found: C, 47.98; H, 2.96; N, 6.16. ESI-TOF-MS: m/z =
dimethylaminopropyl)carbodiimide (EDC, 0.5 g, 2.6 mmol, 1.2 1225.5792 [[Gd2(o-LRR)3](TOf)3]3+.
equiv), and hydroxybenzotriazole (HOBT, 0.4 g, 2.6 mmol, 1.2 [Gd2(o-LSS)3](TOf)6.
Yield:
86%
Anal.
calc.
for
equiv) in dry DMF (10 mL), then stirred for 30 min at room C165H120Gd2F36N18O36S6 (4121.6675): C, 48.08; H, 2.93; N,
temperature. The (R/S)-1-methylphenethylamine (0.3 g, 2.6 6.12. Found: C, 48.02; H, 2.98; N, 6.19. ESI-TOF-MS: m/z =
mmol, 1.2 equiv) was then added to the reaction mixture and 1225.2454 [[Gd2(o-LSS)3](TOf)3]3+.
reacted overnight at room temperature. DMF was evaporated [Y2(o-LRR)3](TOf)6.
Yield:
76%
Anal.
calc.
for
under vacuum, and the resulted solid was taken up in EtOAc (10 C165H120Y2F36N18O36S6 (3984.9792): C, 49.73; H, 3.04; N, 6.33.
mL) and extracted with copious amounts of saturated NaHCO3 Found: C, 49.75; H, 3.12; N, 6.37. ESI-TOF-MS: m/z =
(10 mL × 3). The organic layer was evaporated under vacuum 1179.1815 [[Y2(o-LRR)3](TOf)3]3+.
and purified by EtOAc and hexane column chromatography. [Y2(o-LSS)3](TOf)6.
Yield:
78%
Anal.
calc.
for
Hydrolysis was then performed by stirring the product in a C165H120Y2F36N18O36S6 (3984.9792): C, 49.73; H, 3.04; N, 6.33.
solution of 1 M NaOH (8 mL) and THF (2 mL) at room Found: C, 49.64; H, 3.18; N, 6.25. ESI-TOF-MS: m/z =
temperature. Once the hydrolysis was complete by TLC, the 1179.1823 [[Y2(o-LSS)3](TOf)3]3+.
solution was acidified with 4 M HCl to pH 4 and the precipitate
was collected via vacuum filtration and give 3.9 g of 9a as white
solid in 46% yield. 1H NMR (400 MHz, DMSO-d6, δ): 13.13 (s,
1H), 9.42–9.48 (d, J = 8.5 Hz, 1H), 8.21–8.29 (dq, J = 8.0, 1.7
Hz, 2H), 8.16–8.24 (m, 1H), 7.37–7.43 (d, J = 7.2 Hz, 2H), 7.29–
7.37 (dd, J = 8.5, 6.7 Hz, 2H), 7.19–7.28 (m, 1H), 5.24 (p, J =
7.2 Hz, 1H), 1.52–1.59 (d, J = 7.0 Hz, 3H) ESI-MS m/z = 270.30
M+.
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
This work is financially supported by the National Natural
Science Foundation of China (Nos. 51773054 & 51872077). We
also thank the Key Laboratory of Functional Inorganic Material
Chemistry, Ministry of Education, P. R. China for supporting
this work.
Ligand LRR and LSS. A solution containing 9a (0.272g, 1.0
mmol), 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide (EDC,
0.05 g, 0.26 mmol), and hydroxybenzotriazole (HOBT, 0.035 g,
0.26 mmol, 1.2 equiv) in dry DMF (10 mL) was firstly stirred for
30 min at room temperature. Then 5a (0.1 g, 0.21 mmol, 1
equiv), was added to above reaction mixture and react overnight
at room temperature. DMF was evaporated under vacuum, and
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1−3 | 7
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Journal of Materials Chemistry C
Page 8 of 10
ARTICLE
Journal Name
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Journal of Materials Chemistry C
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DOI: 10.1039/D0TC01044H
A light triggered optical and chiroptical switch based on a homochiral
Eu2L3 Helicate
Jianpeng Zhang, Yanyan Zhou, Yuan Yao, Zhenyu Cheng, Ting Gao, Hongfeng Li* and Pengfei Yan
A pair of homochiral triple-stranded helicates [Eu2(o-LRR)3](TOf)6 and [Eu2(o-LSS)3](TOf)6 show light-responsive
optical (UV, PL) and chiroptical (ECD, CPL) switching properties upon alternating UV and visible light irradiation.