Thermally Induced trans-to-cis Isomerization and Its Photoinduced Reversal Monitored using Absorption and Luminescence: Cooperative Effect of Metal Coordination and Steric Substituent

Article abstract of DOI:10.1002/chem.201900204

For ethene derivatives with large groups the cis-isomer is often quite unstable and unavailable. Herein, we report an exception of two stable coordination complexes, (cis-L)ZnCl₂, starting from trans-1,2-bis(1-R-benzo[d]imidazol-2-yl)ethene (R=H, L1; R=CH₃, L2) ligands under solvothermal condition (T ≥140 °C). Using the intensity of the absorption and luminescence spectra as probes we proposed its progressive cis-to-trans reversal upon irradiation with UV light, which was confirmed by powder X-ray diffraction (PXRD). Similar results observed in the series of (cis-L2)M^IICl₂ [M=Fe (4), Co (5), Ni (6)] demonstrate the universal strategy. The results of PXRD, NMR spectroscopy, ESI-MS and DFT calculations support the above conclusion. NMR spectroscopy indicates that irradiation of 1 converts an optimized 71 % of the cis-isomer to trans, whereas the free trans-L1 ligand transforms to only 15 % cis-isomer under similar conditions.

Full text of DOI:10.1002/chem.201900204

A Journal of
Accepted Article
Title: Thermally-induced trans-to-cis isomerization and its photo-
induced reversal monitored using absorption and luminescence:
Cooperative effect of metal coordination and steric substituent
Authors: Minghua Zeng, Jun-Quan Zhang, De-Shan Zhang, Qiu-Jie
Chen, Hai-Bing Xu, Mohamedally Kurmoo, and Ming-Hua
Zeng
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To be cited as: Chem. Eur. J. 10.1002/chem.201900204
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Thermally-induced
trans-to-cis
isomerization
and
its
photo-induced reversal monitored using absorption and
luminescence: Cooperative effect of metal coordination and steric
substituent
Jun-Quan Zhang,[a] De-Shan Zhang,[a] Qiu-Jie Chen,[b] Hai-Bing Xu,*[b] Mohamedally Kurmoo,[c]
Ming-Hua Zeng* [a,b]
Abstract: For ethene derivatives with large groups the cis-isomer is
often quite unstable and unavailable. Herein, we report an exception
of two stable coordination complexes, (cis-L)ZnCl2, starting from
trans-1,2-bis(1-R-benzo[d]imidazol-2-yl)ethene (R = H, L1; R = CH3,
L2) ligands under solvothermal condition (T ≥.140 ºC). Using the
intensity of the absorption and luminescence spectra as probes we
proposed its progressive cis to trans-reversal upon irradiation with
UV light, which was confirmed by PXRD. Similar results observed in
the series of (cis-L2)MIICl2 (M = Fe (4), Co (5), Ni (6)) demonstrate
the universal strategy. The results of PXRD, NMR, ESI-MS and DFT
support the above conclusion. NMR indicates irradiation of 1
converts an optimized 71% of the cis-isomer to trans-one, whereas
the free trans-L1 ligand transforms to only 15% cis-isomer under
similar conditions.
have been revealed. Recently, novel structure and performance
of photo-responsive metal compounds (Table S1) have been
developed and established.[7] For instance, McGimpsey used
2,2’-dipyridylethylene ligands to modulate surface wettability by
photo-induced
cis-
/
trans-isomerization.[8]
Employing
photo-switchable ligand into a MOF, that undergoes a reversible
conformational change upon light exposure, as soft materials are
also promising.[9] Yet the applications especially for the cis- /
trans- isomerization about C=C bonds are limited somewhat by
two intrinsic drawbacks: 1) information on the cis-isomer remains
[2,6]
insufficient in most cases, due to their instability,
and 2)
although the spectra could be modulated by substitution at
corresponding position, the spectral change for cis- and
trans-isomers is subtle, [10] severely limiting their developments.
It has been established by
a combination of X-ray
crystallography and mass spectrometry (MS), the mechanisms of
the formation of polyoxometalate clusters, [11] coordination cages,
Introduction
[12]
[13,14]
and 3d-metal clusters
could be successfully unveiled.
Recently, combination magnetic resonance spectroscopy (NMR),
followed by the former two techniques, a plausible reaction
mechanism of an Fe2+-catalyzed domino in situ N-alkylation
reaction of 2-(Hydroxymethylhydroxymethyl) quinoline-8-ol
(HL-OH) with NEt3 / NH3 for a trinuclear iron cluster [Fe3(L3-N)2]
were proposed.[15] Furthermore, utilizing the former two
techniques and isotope labeling, a longest domino reaction
involves 14 steps and the formation of 12 covalent bonds and 3
heterocycles of a new type of triheteroarylmethyl radical had
been clearly unraveled.[16] Encouraged by these successes, we
want to deduce the possible structures of cis- / trans-isomers of
photo-responsive metal compounds under photoexcitation by
single crystal and powder crystallography, ESI-MS, and NMR.
To the best of our knowledge, stabilizing cis-isomer by
chelating metal ion with photo-switching trans-molecules, and
improving photo-isomerization efficiency has yet to be reported.
Herein, stable (cis-L)ZnCl2 (L1, 1; L2, 2) were afforded by
Photo-switchable molecules are considered as promising
elements in future devices, because their physical and chemical
properties, such as absorption spectra, fluorescence,
conjugation, coordination, geometrical structure and so on, could
be tuned by light during photo-excitation.[1,2] Till now, there are
mainly two homologous photo-switching derivatives with cis- /
trans- isomerization about C=C (stilbene) and N=N (azobenzene)
Scheme 1).[3] Due to the significantly higher barrier for thermal
isomerization about C=C investigations of stilbene derivatives
are less common compared to those of azobenzene ones.
Regardless, the progress on photo-isomerization dynamics[4,5]
and the strategy on modulation spectra[6] of stilbenes derivatives
[a]
[b]
[c]
J.-Q. Zhang, D.-S. Zhang, Prof. Dr. M.-H. Zeng
Key Laboratory for the Chemistry and Molecular Engineering of
Medicinal Resources, School of Chemistry and Pharmaceutical
Sciences, Guangxi Normal University, Guilin, 541004, P. R. China.
E-mail: zmh@mailbox.gxnu.edu.cn
Q.-J. Chen, Prof. Dr. H.-B. Xu, Prof. Dr. M.-H. Zeng
Key Laboratory for the Synthesis and Application of Organic
Functional Molecules, College of Chemistry and Chemical
Engineering, Hubei University, Wuhan 430062, P. R. China.
E-mail: xhb@hubu.edu.cn
chelating
ZnCl2
starting
with
trans-1,2-bis(1-R-benzo
[d]imidazol-2-yl)ethene (R = H, L1; R = CH3, L2), and it was
found that 71% cis-configuration could convert into trans-one for
1, while only 15% of pure trans-L1 transforms into cis-isomer
during irradiation. Although, subtle changes of spectra present in
both L1 and L2, markedly intensity enhancement in both
absorption and emission of 1 and 2 are observed during
irradiation within second scale. Similar results of a comparative
study on a series of (cis-L2)MIICl2 (M = Fe (4), Co (5), Ni (6))
verify the strategy is universal. Through the coincident results of
crystallography, ESI-MS, NMR, PXRD, and DFT, we deduced
the configurational transition of 1 during photo-isomerization.
Dr. M. Kurmoo
Université de Strasbourg, Institut de Chimie de Strasbourg, CNRS-
UMR7177, 4 rue Blaise Pascal, Strasbourg Cedex 67070, France.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201xxxxxx
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Scheme 1. (a) Photo-isomerization of photo-switches through double bonds (the functional group on the phenyl rings may be the same or different); (b) Synthetic
procedures of L1 / L2 and metal compounds 1–2; (c) Synthetic procedure of reference compound 3; (H atoms of carbon are omitted for clarity).
To verify whether photo-isomerization and photo-cycloaddition
reactions may occur during the syntheses of compounds 1 and 2
by irradiation, an analog ligand with C-C single bond bridging two
benzo[d]imidazole, 1,2-bis(1H-benzo[d]imidazol-2-yl)ethane (L’)
Results and Discussion
was used to make the corresponding L’ZnCl2 (3) as reference.
Although L1 and 1 had been characterized previously (Table
S1),[17] the reported spectral properties were preliminary and of
low resolution. The temporal effect of irradiation was not studied
and photo-induced isomerization for both L1 and 1 not reported.
Given the principle of orbital symmetry conservation, the
photo-cycloaddition[18] in relatively concentrated solutions were
also not discussed.
Crystal Structures
The compounds were all characterized by elemental analyses,
IR spectroscopy, TG, PXRD, ESI-MS, 1H-NMR, and X–ray
crystallography. The crystallographic data of L1, L2, 1, 2, L’ and
3-6 are summarized in Table S2. Selected atomic distances and
angles for 1–3 are presented in Table 1. The crystal structures
are depicted in Figure 1.
Syntheses
The crystal structures of L1 and L2 are similar to that of L’
(Figure 1). They can be viewed as two benzo[d]imidazole
separated by C=C for L1 and L2 and C‒C for L’ in a symmetric
trans-configuration. The C‒C bond multiplicities were confirmed
by the bond lengths of C8‒C8a (1.338 Å) of L1 and L2 being
shorter than that of L’ (1.501 Å). Due to the steric hindrance from
–CH3 of imidazole groups, the angle of C7‒C8‒C8a in L2 (123.8°)
is slightly smaller than that in L1 (125.2°), but all much larger
than that in L’ (113.0°).
In-situ solvothermal reaction at 140 ºC for 24 hours led to the
dehydration of 1,2-bis(1-R-benzo[d]imidazol-2-yl)ethanol in
methanol to exclusively trans-1,2-bis(1-R-benzo[d]imidazol-2-yl)
ethene [R = H, L1; R = CH3, L2] as pale yellow block crystals
(Scheme 1). While their reactions with ZnCl2 led to the
(cis-L)ZnCl2 compounds (L1, 1; L2, 2). The metal complexes
were also obtained in
a one-pot reaction of ZnCl2 with
1,2-bis(R-benzo[d] imidazol-2-yl)ethanol. It appears that through
the slow in-situ dehydration the target crystals suitable for X–ray
diffraction were formed. To get a deeper insight into the reaction
procedure, we synthesize trans-L to react with ZnCl2 under the
same condition, also achieving (cis-L)ZnCl2, the productivity
based on L1 and L2 is higher than that of the former. In parallel,
a series of (cis-L2)MIICl2 (M = Fe (4), Co (5), Ni (6)) were
synthesized under the same conditions.
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Photophysical properties
UV-Vis absorption spectra of L1, L2, L’ and 1–3 in DMSO are
shown in Figure 2, and their molar absorption coefficients are
listed in Table S3. Due to electron-donating effect of the methyl in
L2, the maximal absorptions show slight bathochromic shifts to
378 nm extending to 420 nm from 364 nm reaching to 400 nm for
L1. As for inferior conjugacy of L’, significant hypsochromic shifts
to 284 nm trailing to 300 nm is observed.
From the DFT calculations (Table S4), the absorption bands
from 200 to 400 nm are assigned as π‒π and n-π transitions.
Since different symmetries of the cis- and trans-isomers for L1
and L2, superior π‒π stacking among trans-isomer to that of
cis-one induces redshift of absorption bands, which is also
confirmed by the steric hindrance from two benzo[d]imidazo
groups in the structure of cis-isomer. Additionally, the more π‒π
stacking interactions the higher transition probability induce
enhancement on absorption intensity, thus the intensity of
trans-isomer is higher than that of the cis-one.
After coordination with ZnCl2, similar absorption bands
ascribed to the intraligand transitions appear, since the
corresponding bands are found for the free ligands. However, the
molar absorption coefficients of 1 and 2 are much lower than
those of the free ligands L1 and L2 (Table S3), because of their
different configurations in trans-L and (cis-L)ZnCl2.
Figure 1. Crystal structures of L1, L2, L’ and compounds 1–3. (H atoms are
omitted for clarity).
Similarly, the crystal structures of 1 and 2 resemble that of 3, in
which the Zn2+ exhibits distorted tetrahedral coordination with two
N atoms from a chelating cis-configured ligand and two terminal
Cl ions. The C7–C8 bond lengths of 1.452 Å in 1, and 1.441 Å in
2 and C9–C10 of 1.450 Å in 1, and 1.441 Å in 2 are shorter than
those in 3 (1.502 Å and 1.503 Å). The bond lengths of Zn1-Cl1
(2.281 Å in 1, and 2.305 Å in 2) and Zn1-Cl2 (2.229 Å in 1, and
2.231 Å in 2) are different where Zn1-Cl1 is longer than Zn1-Cl2,
but in 3 they are similar (2.250 Å and 2.253 Å, respectively). Due
to double bond (C=C) between two benzo[d]imidazole in both 1
and 2, the bond lengths of C8=C9 (1.336 Å in 1, and 1.323 Å in 2)
are much shorter than that in 3 with single bond (1.539 Å).
However, the angles of C7–C8=C9 (134.0° in 1, and 135.5° in 2),
C8=C9–C10 (134.8° in 1, and 135.1° in 2) are much larger than
those in 3 ((115.2° and 115.1°, respectively). It is interesting to
note that the angles of N1–Zn1–N3 in 1 (105.1°) and 2 (103.2°)
are smaller than that in 3 (106.4°), indicating potential energy
within the structure of 1 and 2.
Table 1. Selected Bond Lengths (Å) and Angles (°) for L, 1, 2, L’, and 3.
Bonds and Angles
C7–C8
1
2
3
Figure 2. UV-Vis absorption spectra of ligands and corresponding metal
compounds in DMSO.
1.452(1)
1.336(1)
1.450(1)
2.281(2)
2.229(2)
105.1(3)
105.1(5)
134.0(8)
134.8(8)
L1 (R = H)
1.338(3)
125.2(4)
1.441(7)
1.323(7)
1.441(6)
2.305(4)
2.231(2)
103.2(2)
113.5(9)
135.5(4)
135.5(4)
L2 (R = CH3)
1.321(4)
123.8(2)
1.502(7)
1.539(7)
1.503(7)
2.250(4)
2.253(6)
106.4(0)
111.0(9)
115.2(4)
115.2(4)
L’
C8–C9
C9–C10
The absorption spectra of the ligands and metal complexes are
characterized by a band centered at ca. 370 nm which exhibits a
vibrational progression with an average separation of 1300 cm-1.
While the resolution appears to increase for 1 compared to L1, it
is reduced from L2 to 2. The intensities of the band for the metal
complexes are less intense compared to the corresponding
ligands. Interestingly, upon irradiation with two 18-Watt 365 nm
UV lamps, the absorption band intensities of the UV-Vis spectra
are marginally lowered for both L1 (0.98 times, Figure S5a) and
L2 (0.91 times, Figure S5c). In contrast, the absorption
Zn1–Cl1
Zn1–Cl2
N1–Zn1–N3
Cl1–Zn1–Cl2
C7–C8–C9
C8–C9–C10
C8–C8a
1.501(4)
113.0(2)
C8a–C8–C7
Symmetry code: (a) −x+1, −y, −z+1.
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intensities of (cis-L)ZnCl2 increase markedly during irradiation
within timescale of seconds (Figure 3). For 2, the absorption
bands gradually become clear and discernable, finally like the
profile of L2, and 2.60 times of absorption intensity enhancement
could be induced within 15 seconds (Figure S5d). In parallel, only
1.22 times of absorption intensity enhancement could be attained
within 27 seconds for 1 under the same condition (Figure S5b).
All the facts, on the one hand, suggest large enhancement on
absorption intensity could be afforded in (cis-L)ZnCl2 by chelation
metallic ion, thus the absorption intensity enhancement for 2
(2.60 times) and 1 (1.22 times) is more obvious than those
attentions of L2 (0.91 times) and L1 (0.98 times). On the other
hand, steric substitution in the ortho-position also benefit for
improving spectral intensity, inducing larger enhancement for 2
than that for 1. Finally, as discussed in the crystal structure
section, the potential energy within the structure contributes for
photo-isomerization, inducing the faster rate for 2 (15 seconds)
than for
1 (27 seconds). Consequently, due to lacking
photo-isomerization, negligible changes of absorption intensity in
both L’ (Figure S5e) and 3 (Figure S5f) could be observed.
Figure 3. Absorption intensity enhancement of L1, 1 and 2 in DMSO during irradiation with 365 nm.
Upon excitation, bright blue luminescence centered at ca. 420
nm for L1 (Figure S6a), 442 nm for L2 (Figure S6c), and 406 nm
for L′ in DMSO (Figure S6e) occurs. Although negligible emission
changes during irradiation is observed for all of them, significant
emission enhancement is found by chelating them with metallic
ion to afford (cis-L)ZnCl2 under the same condition. Since the
substituents on the imidazole nitrogen for 2 (–CH3) is bulkier than
that for 1 (–H), emission increment for 2 (5.10 times, Figure S6d)
is larger than that of 1 (2.75 times, Figure S6b) during irradiation.
Noteworthy, the emission enhancement of 2 (2.3 times) is much
faster than that of 1 (1.6 times) within the first three seconds
(Figure 4). Moreover, after 1 in DMSO was excited at 365 nm for
30 seconds, the absorption and emission intensities almost
remain unchanged under dark environment for 12 and 24 hours
(Figure S7), indicating the photo-isomerization procedure is
irreversible.
Figure 4. Emission intensity enhancement of L1, 1 and 2 in DMSO during irradiation.
In order to verify the universal strategy, we synthesize and
performed a comparative study of a series of (cis-L2)MIICl2 (M =
Zn (2), Fe (4), Co (5), Ni (6)). From their crystal structures (Figure
S1), the bond lengths of C8=C9 vary from 1.330 Å in
(cis-L2)NiCl2 to 1.338 Å in (cis-L2)FeCl2, and the angles of N1–
M–N3 are from 98.1° in (cis-L2)FeCl2 to 103.2° in (cis-L2)CoCl2
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(Table S5). Correspondingly, the absorption intensity
enhancements are from 1.60 times for (cis-L2)FeCl2 (Figure S8a)
to 2.30 times for (cis-L2)NiCl2 (Figure S8c), and the ones of
luminescence are from 2.29 times for (cis-L2)NiCl2 (Figure S8c)
to 6.54 times for (cis-L2)FeCl2 (Figure S8a) during irradiation
(Table S6). The take away information photoinduced cis /
trans-isomerization through C=C bonds can be regulated
incorporating steric substituents and coordination to metallic ions
(Figures 3-4).
of the conjugation, the latter can have little difference, we studied
the reference compound 3 which has a C-C single bond center to
obtain the reference point in spectral energy (Figure S13). The
small energy shifts in the spectra of 1 and 2 (Figures S5b-S5d)
therefore suggests there is no photocycloaddition (Figures 2 and
3). Thus, the change in spectral intensity is associated with
photoinduced isomerization.
Additionally, after irradiation of the metal compounds 1 and 2
for a long time (24 hours), their absorption spectra could not
overlap completely with those of free L1 and L2 (Figure S9),
respectively, indicating cis-configurational metal compounds
could not completely convert into trans-isomers. With reference
to the photo-induced cis-/trans- isomerization structures of
2,2’-dipyridylethylene CuII compound,[8] we may deduce that the
Zn2+ chelating with cis-L converts into Zn2+ bridging with trans-L
during photo-isomerization as Scheme 2. Further to confirm it,
we react ZnCl2 with L1 and 1,2-bis(1-R-benzo[d]imidazol-2-yl)
ethanol by solvothermal reaction in methanol at 130 °C instead of
140 °C for 24 hours, it was found that 1 could not be obtained as
verified by the PXRD (Figure S11). This sets the barrier energy
required to convert trans to cis for chelation with the Zn2+ to kT (T
= 140 °C. On the other hand, mixture of ZnCl2 with L1 / L2 in
1
DMSO, there are some downfield shifts in the H-NMR spectra
compared to that of free L1 / L2 (Figures S12a and S12b),
suggesting the ZnCl2 bridges with L1 / L2 as shown in Scheme
2.
Figure 5. The PXRD patterns of the reaction products of ZnCl2 with L1 in
methanol at 25 °C (black), solvothermal at 130 °C (red) and 140 °C (blue) for
24 hours, and the one of the irradiated 1 in DMSO for 6 hours followed by
evaporation to dryness in air (dark cyan).
Configurational transitions of photo-isomerization by
ESI-MS
Due to higher solubility of 1 than 2, we dissolve the crystals of
1 in DMSO and diluted with CH3OH and collected MS under
different in-source energies (Figures 6a and S14), it was found
that three single series of peaks in the positive mode spectra at
m/z 261.11 [L1 + H]+, 359.00 [1-Cl]+, and 437.02 [1 + DMSO -
Cl]+ appear until the in-source energy reached 60 eV, suggesting
the integrity of the cores of 1 is stable.
The photo-isomerization process of 1 monitored by ESI-MS
under irradiation (Figures 6b and S15) with two 18-Watt 365 nm
UV lamps within various times were performed. Three peaks in
the positive mode spectra of [1+(Solv)2-Cl]+ (Solv = DMSO + H2O,
455.03; DMSO + CH3OH, 469.04; 2DMSO, 515.03), beside three
peaks at 261.11, 359.00 and 437.02 as previous experiment, are
observed. The results together with the abundance of fragment
[1+(Solv)2-Cl]+ increasing with the irradiation time (Figures 6c),
indicate the (cis-L1)ZnCl2 activated by irradiation transfers into
trans-one, favoring for the more solvent molecules (including
DMSO / H2O / CH3OH) coordination with Zn2+ (as suggest in
Scheme 2).
Scheme 2. Possible structures of (cis-L)ZnCl2 under photo-isomerization in
DMSO.
On the question of reversibility, we examine the crystal phases
using powder diffraction due to the lack of single crystals from
reactions of ZnCl2 with the trans-L1 at temperature less than
140 °C. For reactions at 25 °C and those treated solvothermally
up to 130 °C (Figure 5), the strikingly similar patterns with
different intensity suggest the same compound is formed. While
the pattern changed when the temperature is set at 140 °C and it
corresponds to that simulated from the crystal data of
(cis-L1)ZnCl2. The pattern for the irradiated (cis-L1)ZnCl2 in
DMSO for 6 hours followed by evaporation to dryness in air was
the same as those of the powders obtained up to 130 °C without
any Bragg reflection originating from the (cis-L1)ZnCl2. All these
facts further confirm the transformation of (cis-L1)ZnCl2 by
irradiation and reversal of the ligand conformation from cis to
trans as suggested in Scheme 2.
As for L1 under the same condition, no new peaks especially
of dimer signals of photocycloaddition product in the positive
[18]
Given the possibility of photo-cycloaddition
instead of
[12]
photo-isomerization,
where the former effect is expected to
blue shits the absorption and emission energies due to removal
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mode spectra (Figure S15a) could be observed during irradiation, Density Functional Theory Calculations (DFT)
indicating photocycloaddition reaction does not happen in L1.
According to the typical composition of peaks of the MS
spectra, the possible structures of fragments are fully optimized
and their corresponding Gibbs free energies (Figure S16) were
calculated. As revealed from the calculated coordination
energies, changing from trans-L1 to cis-L1 is easier than to
intermolecular [2 + 2] photocycloaddition product, because two
C=C bonds need to be broken into two C‒C bonds with bonding
energy as high as 162 kcal/mol, beside the energy to surmount
the steric hindrance. In parallel, transforming cis-1 into trans-1
with Zn‒N broken is more feasible than intermolecular [2 + 2]
photocycloaddition of the metal compound. As for 2, the former
transformation instead of the latter one is more workable than
those of 1, as the bulky substituent contributes to isomerization,
but impedes dimerization by steric hindrance. Thus, DFT
calculations indicate Zn2+ coordination to trans-L is more
favorable than with cis-L. With irradiation the cis-L is more easily
transformed into trans-L. Consequently, the energetics favor Zn2+
bridging with trans-L (Scheme 2). All the results are coincident
with the suggestion of the ESI-MS.
Conversion efficiency of photo-isomerization by 1H-NMR
In order to verify the conversion efficiency during
photo-isomerization reaction, the 1H-NMR measurements were
carried out to monitor the configuration changes during
irradiation for 0.5 h with 365 nm. Before irradiation, L1 in
DMSO-d6 at 298 K exhibits four distinguishable proton signals at
12.93, 7.63, 7.60, and 7.23 ppm (Figure 7a), ascribed
respectively to (a) two H on the nitrogen atoms in imidazole, (b)
two H on the C=C, (c) four H on the benzene near the imidazole,
and (d) four H on the benzene opposite the imidazole. After
coordination with ZnCl2 to form (cis-L1)ZnCl2, we can clearly
detect the proton signals of two H on the C=C in cis-isomer of 1
up-field to 7.18 from 7.63 in trans-L1 (Figure 7c). Upon irradiation
of trans-L1 for 0.5 h with two 18-Watt 365 nm UV lamps, three
sets of proton signals at 7.85, 7.35 and 7.05 ppm ascribed to the
typical cis-isomer appear besides those of the trans-L1 (Figure
7b), manifesting the formation of cis-L1 in NMR time scale.
Similarly, irradiation of 1 for 0.5 h, three sets of proton signals at
7.24, 7.62 and 7.68 ppm ascribed to the typical trans-isomer like
trans-L1 appear besides those of the (cis-L1)ZnCl2 (Figure 7d).
From the changes of the proton peak area, we can clearly detect
ca. 15% trans-L1 transformation into cis-L1, and 71% conversion
of coordinated cis-L of 1 to trans-L during photo-isomerization
reaction.
Although the solubility of 2 and L2 is poorer than that 1 and L1
in DMSO, the 1H-NMR measurements of L2 and 2 (Figure S17)
with weak signals under the same condition were also carried out.
It was found that there are almost no changes for trans-L2 before
and after irradiation, while five sets of proton signals at 7.95,
7.69-7.67, 7.63-7.62, 7.31-7.25 and 4.03 ppm ascribed to the
typical trans-configuration, like those of trans-L2, appeared
besides those of cis-one for 2. These facts indicate that trans-L2
is too stable to transform into cis-L2, and it is feasible to stabilize
(cis-L2)ZnCl2 (2) by chelating Zn2+ with trans-L2, the quantum
yields for cis → trans is ca. 34%. Due to the steric hindrance from
Figure 6. Positive mode ESI-MS spectra of crystal of
1 dissolved in
DMSO/CH3OH under various in-source energies (a) and after irradiation for
various times (b), the peak intensity in the region of m/z 455.03 and 469.04
have been amplified for clarity, and the abundance of fragment [1+(Solv)2-Cl]+
increasing with the irradiation time (c).
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the two -CH3 of L2, it is necessary to overcome larger energy for
L2 than that for L1, confirmed by the DFT calculations, to transfer
into cis-isomer, thus the photo-isomerization of L1 is more
obvious than that of L2. On the other hand, as discussed in
crystal structure, although the potential energy within the
structure, which would benefit for improving the cis → trans
conversion by energy releasing, the steric hindrance from the two
-CH3 of 2 inhibits the cis → trans conversion, thus the conversion
efficiency of 2 (34%) is lower than that of 1 (71%).
Figure 7. 1H-NMR spectra of L1 and 1 before and after irradiation for 0.5 hour.
developing new member of photo-responsive coordination
compounds.
Conclusions
Experimental Section
While trans-1,2-bis(1-R-benzo[d]imidazol-2-yl)ethene (R = H, L1;
R = CH3, L2) are sterically more stable than their cis-isomers,
thermal treatments at T ≥ 140 ºC (isomerization barrier energy) of
the trans-isomers in the presence of divalent metal halides (M =
Fe, Co, Ni, Zn) crystallize (cis-L)MCl2 in preference. The
photo-excitation at 365 nm of the complexes progressively
transform the chelated cis-isomer ligand to its trans-isomer in a
time scale of less than one minute. Due to the difference of the
intensities of the UV-Vis absorption and luminescence of the two
isomers (trans > cis), it was used to monitor the progress of the
transformation. PXRD and NMR suggest the ligand remains
coordinated after photoinduced isomerization and irradiation
reverses the ligand configuration from cis back to trans. Steric
substituent on the ligand also affects the intensities and rate.
Combining the results of ESI-MS, NMR, PXRD, and DFT, we
infer that the metallic ions are possibly bridged by trans-L instead
of chelated by the cis-L following irradiation. This strategy of
trapping the unstable isomer by coordination may be used in
Trans-1,2-di(1H-benzo[d]imidazol-2-yl)ethene
(L1).
Benzene-1,2-diamine (15.14 g, 140 mmol) and fumaric acid (8.12 g, 70
mmol) were dissolved in 100 mL 4 M hydrochloric acid. The solution was
heated to reflux for 24 hours where it changed to green-yellow. After
cooling, it was neutralized with ammonia to give a yellow precipitate which
was filtered. The powder was recrystallized from 250 mL 4 : 1 ethanol–
water followed by filtering, washing and drying. Yield 9.50 g (52%).
Selected IR (KBr): 2746 (m), 1590 (w), 1529 (w), 1426 (s), 1322 (m), 1235
(m), 1148 (m), 1034 (m), 973 (m), 870 (m), 745 (s) cm-1. 1H-NMR (400
MHz, DMSO-d6): δ 12.88 (s, 2H), 7.63 (s, 2H), 7.66 (d, J = 7.8 Hz, 2H),
7.52 (d, J = 7.8 Hz, 2H), 7.25 (t, J = 7.3 Hz, 2H), 7.20 (t, J = 7.6 Hz, 2H).
ESI-MS: m/z = 261 [C16H12N4+H]+.
Trans-1,2-di(1-methyl-1H-benzo[d]imidazol-2-yl)ethene
(L2).
N-Methyl-1,2-phenylene diamine (27.3 g, 140 mmol) and fumaric acid
(8.12 g, 70 mmol) were dissolved in 150 ml 4 M hydrochloric acid. The
solution was heated to reflux for 24 hours. After cooling, a green
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compound crystallized from the brown solution. The powder was filtered
and dissolved in 500 mL 4:1 ethanol–water. After adding active carbon,
the solution was heated to reflux for 10 minutes. The green solution was
neutralized with ammonia giving a yellow precipitate which was filtered,
washed and dried. Yield 5.25 g (25%). Selected IR (KBr): 3067 (w), 2956
(w), 1 602(w), 1471 (s), 1346 (s), 1405(m), 1306 (m), 1238 (m), 923 (m),
854 (m), 751 (s), 563 (m) cm-1. ESI-MS: m/z = 289 [C18H16N4+H]+.
were performed using Vario Micro Cube. Thermogravimetric analyses
(TGA) were performed in a flow of nitrogen at a heating rate of 5 °C min-1
using a NETZSCH TG 209 F3. Infrared spectra of compounds were
obtained by transmission through KBr pellets containing ca. 0.5% of the
compound using a PE Spectrum FTIR spectrometer (400–4000 cm-1).
Powder X-ray diffraction (PXRD) intensities were measured at 296 K
using a Rigaku D/max-IIIA diffractometer (Cu Kα). The crystalline powder
sample was prepared by crushing the crystals, and scans of 5–60° were
conducted at a rate of 5° min-1. The calculated diffraction patterns of the
compounds were generated using the Mercury 3.0 software. A THERMO
Finnigan LCQ Advantage Max ion trap mass spectrometer was used to
collect ESI-MS spectra. A Perkin-Elmer Lambda 35 spectrometer was
used to measure the UV-Vis absorption spectra of ligands and
compounds. The emission luminescence and lifetime properties were
recorded with an Edinburgh FLS 920 fluorescence spectrometer.
1,2-di(1H-benzo[d]imidazol-2-yl)ethane (L’). Benzene-1,2-diamine
(15.14 g, 140 mmol) and succinic acid (8.26 g, 70 mmol) were dissolved
in 100 ml 4 M hydrochloric acid. The solution was heated to reflux for 24
hours to a faint yellow color. After cooling, it was neutralized with
ammonia and the yellow precipitate was filtered. The powder was
recrystallized in 300 mL 4 : 1 ethanol–water. After cooling, the white
precipitate was filtered, washed and dried. The yield of L’ was 11.25 g
(61%). Selected IR (KBr): 2744 (s), 1625 (w), 1541 (m), 1435 (s), 1319
(w), 1268 (m), 1221 (m), 1032 (m), 911 (m), 870 (m), 740 (s) cm-1. 1H
NMR (400 MHz, DMSO-d6): δ = 7.85 (m, 4H, ArH), 7.65 (m, 4H, ArH) and
3.82 (s, 4H, CH2). ESI-MS: m/z = 263 [C18H14N4+H]+.
X-ray Crystallography. Single-crystal X-ray diffraction data for L1, L2
and L’ and 1‒6 were collected on a Rigaku R-AXIS SPIDER IP
diffractometer employing graphite-monochromated Cu-Kα radiation (λ =
1.54184 Å) using the θ-ω scan technique at 150 K. Their structures were
solved by direct methods using ShelXS and refined using full matrix
Synthesis of metal compounds (1-3). A mixture of ZnCl2 (1.0 mmol,
0.14 g), L1 (1.0 mmol, 0.28 g) / L2 (1 mmol, 0.31 g) / L’ (1 mmol, 0.26 g)
and methanol (15 mL) was sealed in a 25 mL Teflon-lined steel autoclave
and heated at 140 °C for 24 h. After the autoclave was cooled to room
temperature at a rate of 10 °C min-1, light yellow block crystals of the
metal compounds (1-3) were obtained. The absence of solvent was
confirmed by thermogravimetric and elemental analyses (EA). Phase
purity was confirmed by PXRD (Fig. S4).
[19]
least-squares technique within ShelXL2015 and OLEX.2.
All
non-hydrogen atoms were refined with anisotropic thermal factors.
Because of the noncentrosymmetric space group and a Flack parameter
about 0.5 of 1 and 3 an absolute structure refinement was performed
using the SHELX-2015 TWIN instruction. [20] Crystallographic data have
been deposited at the Cambridge Crystallographic Data Center. The
crystallographic data (CCDC-1887730, 1887731, 1887732, 1887733,
1887734, 1887735, 1887746, 1887747, 1887748 for 1, L1, L2, 2, L3, 3, 4,
5, 6, respectively) can be found in the Supporting Information or can be
obtained free of charge from the Cambridge Crystallographic Data Centre
via https://summary.ccdc.cam.ac.uk/structure-summary-form.
1．Anal. Calcd for Zn(C16H12N4)Cl2: C 48.46, H 3.56 and N 14.13. Found
(%): C 48.32, H 3.69, and N 14.09. Selected IR (KBr): 3221 (s), 1590 (w),
1457 (m), 1349 (m), 1230 (w), 1146 (w), 1042 (m), 805 (m), 749 (s), 587
(m). Yield, 55 mg, 14% based on L1.
Electrospray Ionization Mass Spectrometry (ESI-MS). ESI-MS
measurements were conducted at a capillary temperature of 275 °C.
Aliquots of the solution were injected into the device at 0.3 mL/h. The
mass spectrometer used for the measurements was a Thermo Exactive
Plus, and the data were collected in positive and negative ion modes. The
spectrometer was calibrated with the standard tune mix to give a precision
of ca. 2 ppm in the region of 200−2000 m/z. The capillary voltage was 50
V, the tube lens voltage was 150 V, and the skimmer voltage was 25 V.
The in-source energy was set to the range of 0−100 eV with a gas flow
rate at 15% of the maximum. Preliminary ESI-MS has been used to probe
the integrity and behaviour of the cluster in solution, and detection of trace
intermediates of time dependent under photoexcitation. Due to the
solubility limitation in CH3CN, CH3OH and H2O, the crystals of them were
dissolved in DMSO and diluted with CH3OH for ESI-MS at 275 °C with
positive mode.
2．Anal. Calcd for Zn(C18H16N4)Cl2: C 50.91, H 3.80, and N 13.19. Found
(%): C 50.85, H 3.77 and N 13.25. Selected IR (KBr): 3068 (w), 2957 (w),
1593 (w), 1470 (s), 1339 (m), 1240 (m), 1146 (w), 1018(w), 918 (w), 857
(w), 748 (s), 559 (w). Yield, 80 mg, 19% based on L2.
3．Anal. Calcd for Zn(C16H14N4)Cl2: C 48.21, H 3.54 and N 14.06. Found
(%): C 48.32, H 3.63, and N 14.17. Selected IR (KBr): 3289 (s), 3196 (s),
1620 (w), 1542 (m), 1462 (s), 1340 (m), 1279 (m), 1208 (m), 1060 (s), 552
(m). Yield: 45 mg, 23% based on L’.
4. Anal. Calcd for Fe(C16H14N4)Cl2: C 52.08, H 3.89 and N 13.50. Found
(%): C 51.92, H 4.01, and N 13.27. Selected IR (KBr): 3078 (w), 2954 (w),
1602 (w), 1474 (s), 1366 (m), 1341 (m), 1307 (m), 1239 (m), 1151 (m),
858(m), 753 (s), 558 (w). Yield: 115 mg, 28% based on L2.
Density Functional Theory Calculations. Density functional theory
calculations were carried out using B3LYP/6-311g*, functional adding the
D3 version of Grimme’s dispersion with Becke-Johnson damping, SDD for
Zn and 6-31+g basis sets for other elements using Gaussian 09 software.
5. Anal. Calcd for Co(C16H14N4)Cl2: C 51.70, H 3.86 and N 13.40. Found
(%): C 51.12, H 3.99, and N 13.68. Selected IR (KBr): 3058 (w), 2945 (w),
1602 (w), 1466 (s), 1342 (s), 1307 (m), 1243 (m), 1154 (w), 851(m), 756
(s), 537 (w). Yield: 178 mg, 43% based on L2.
[21]
[22]
Bonding order were analyzed using Multiwfn software.
The
geometry of LZnCl2 were fully optimized assuming C4 symmetry, Mayer
bond order was then calculated based on the optimized geometry.
Attempt to calculate complexation energies of different possible
fragments based on single-crystal geometry failed due to scf convergence
problem.
6. Anal. Calcd for Ni(C16H14N4)Cl2: C 51.73, H 3.86 and N 13.41. Found
(%): C 51.27, H 4.11, and N 13.13. Selected IR (KBr): 3068 (w), 2944 (w),
1593 (w), 1470 (s), 1336 (m), 1341 (m), 1303 (w), 1236 (m), 1156 (m),
858(m), 753 (s), 564 (w). Yield: 205 mg, 49% based on L2.
Materials and methods
Optical Properties. Emission and excitation spectra and emission
lifetimes were measured using the same slit and iris. The solid-state
quantum yields of powder samples in sealed quartz cuvettes and as films
spin-coated on quartz substrates were measured using the integrating
All chemicals were obtained from commercial sources and used as
received without further purification. Elemental analyses for C, H, and N
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sphere (142 mm in diameter) of Edinburgh FLS980 Spectro
fluorophotometer (the ratio between signal to noise ca. 6000 : 1 by using
the Raman peak of water) with the same slit (1.9980 mm) and iris (10, the
largest one is 100 in our experiments).
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This work was supported by the National Science Foundation for
Distinguished Young Scholars of China (No. 21525101), NSF
(No. 21571165), the NSF of Hubei Province innovation group
project (2017CFA006), and the NSF of China and Guangxi
Province (2014GXNSFFA118003, 2017GXNSFDA198040), the
BAGUI talent and scholar program (2014A001), the Project of
Talents Highland of Guangxi Province. MK is funded by the
CNRS-France.
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Keywords: Trans / cis conversion• Thermally or photo induced
isomerization • Absorption and luminescence • Metal
coordination • Steric substituent
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Entry for the Table of Contents
FULL PAPER
Thermal treatment at 140 ºC of trans-L
in the presence of MIICl2 (M = Fe, Co,
Ni, Zn) led to stable (cis-L)MCl2 and
photoexcitation of the latter transform
it back to (trans-L)MCl2(solvent). The
kinetics can be followed using UV-Vis
absorption and luminescence. The
results are founded by further ESI-MS,
Jun-Quan Zhang, De-Shan Zhang,
Qiu-Jie Chen, Hai-Bing Xu,*
Mohamedally Kurmoo, Ming-Hua Zeng*
365 nm
Page No.1 – Page No.9
Uv-Vis
irradiation
PL
irradiation
Thermally-induced trans-to-cis
isomerization and its photo-induced
reversal monitored using absorption
and luminescence: Cooperative effect
of metal coordination and steric
substituent
NMR,
and
PXRD
and
DFT
calculations.
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