Photo-irradiated: E / Z isomerization reaction of star-shaped isomers containing two cyanostilbene arms with charge transfer excited states

Article abstract of DOI:10.1039/c8cp05581e

The E/Z isomerization reaction of the multi-cyanostilbene molecule is still not clear. Herein, we have designed and synthesized three star-shaped molecular isomers with a triphenylamine core linked to two cyanostilbene groups with E/Z isomerization, Z,Z-TPDCF, Z,E-TPDCF and E,E-TPDCF, possessing three different isomeric molecular configurations, to investigate the specific E/Z isomerization reaction of the cyanostilbene groups in the two molecular arms. The in situ UV, ¹H NMR and HPLC spectra under UV-irradiation clearly showed that the E/Z isomerization reactions of both E,E-TPDCF and Z,Z-TPDCF firstly turned them into Z,E-TPDCF, and the Z,E-TPDCF was almost simultaneously turned into more E,E-TPDCF and less Z,Z-TPDCF due to the calculated lowest unoccupied molecular orbitals of Z,E-TPDCF on the cyanostilbene arm with the Z-configuration. In general, Z,E-TPDCF exhibited a relatively better configurational stability than Z,Z-TPDCF or E,E-TPDCF under the photo-irradiation conditions. Further research demonstrated that all three isomers exhibited excellent aggregation-induced emission (AIE) properties.

Full text of DOI:10.1039/c8cp05581e

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Photo-irradiated E/Z isomerization reaction of
star-shaped isomers containing two cyanostilbene
arms with charge transfer excited states†
Cite this: Phys.Chem.Chem.Phys.,
2018, 20, 28279
Jin Liu, Weijun Li,
and Cheng Zhang
Minjie Liu, Yujie Dong,* Yuyu Dai, Qingbao Song, Jianli Wang
*
The E/Z isomerization reaction of the multi-cyanostilbene molecule is still not clear. Herein, we have
designed and synthesized three star-shaped molecular isomers with a triphenylamine core linked to two
cyanostilbene groups with E/Z isomerization, Z,Z-TPDCF, Z,E-TPDCF and E,E-TPDCF, possessing three
different isomeric molecular configurations, to investigate the specific E/Z isomerization reaction of the
cyanostilbene groups in the two molecular arms. The in situ UV, 1H NMR and HPLC spectra under
UV-irradiation clearly showed that the E/Z isomerization reactions of both E,E-TPDCF and Z,Z-TPDCF
firstly turned them into Z,E-TPDCF, and the Z,E-TPDCF was almost simultaneously turned into more
E,E-TPDCF and less Z,Z-TPDCF due to the calculated lowest unoccupied molecular orbitals of
Z,E-TPDCF on the cyanostilbene arm with the Z-configuration. In general, Z,E-TPDCF exhibited a relatively
better configurational stability than Z,Z-TPDCF or E,E-TPDCF under the photo-irradiation conditions.
Further research demonstrated that all three isomers exhibited excellent aggregation-induced emission
(AIE) properties.
Received 3rd September 2018,
Accepted 22nd October 2018
DOI: 10.1039/c8cp05581e
rsc.li/pccp
property,20–23 aggregation-induced emission (AIE) phenom-
Introduction
enon24–26 and other special properties27,28 of the E/Z-isomers.
As a novel class of functional materials, molecules containing Despite this work playing a very important role in the advance-
cyanostilbene (CNS) groups have been developed rapidly into a ment of the cognition of E/Z isomers, the specific E/Z isomer-
significant molecular platform in the field of organic photo- ization reactions in molecules containing multiple CNS groups
chemistry and photoelectronics. And they have been widely have been much less studied. Currently, to break the bounded-
applied in fluorescent switches,1–7 bioimaging8–10 and organic ness in the functions of organic molecules containing only one
photoelectronic devices,11–15 on account of their excellent con- CNS group, more and more molecules containing multiple CNS
figurational characteristics and photophysical properties. How- groups have been designed and successfully synthesized to
ever, due to the basic feature of these molecules containing the obtain stronger-functions or multifunctional materials.29–34
double bond unit, unavoidable E/Z isomerization reactions take But the multiple CNS groups brought relatively more compli-
place to produce E/Z isomers under photo-irradiation condi- cated configurations and E/Z isomerization processes, pro-
tions.16,17 Therefore, it is necessary to figure out the E/Z ducing a series of complex unmanageable multi-component
isomerization reactions of the E/Z isomer molecules containing E/Z isomer mixtures under UV-irradiation,16,19 which impeded
the CNS groups for more explicit knowledge about them.
further developments in the photochemical properties and
Recently, much brilliant and impressive work has been applications of these functional materials. Thus, it is essential
reported to explain the E/Z isomerization reaction in the photo- and meaningful to investigate the specific E/Z configurational
physical process of E/Z-isomers containing CNS groups,16–19 isomerization of molecules containing multiple CNS groups.
and not only that, more and more related reports have also put
Our group recently reported the pure E/Z isomers, E-TPNCF
forward very useful research investigating the piezallochromy and Z-TPNCF, consisting of a triphenylamine and a cyano-
stilbene unit,41 which could retain their stable E/Z configura-
tions under photo-irradiation conditions due to their charge
State Key Laboratory Breeding Base of Green Chemistry-Synthesis Technology
transfer excited state properties. This work offered a helpful
College of Chemical Engineering, Zhejiang University of Technology,
way to obtain stable E- and Z-configurations.35–37 Based on this
Hangzhou, 310014, P. R. China
previous work, we designed three pure star-shaped molecular
isomers with a triphenylamine core linked to two cyanostilbene
† Electronic supplementary information (ESI) available: Fig. S1–S4 and Table S1.
See DOI: 10.1039/c8cp05581e
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groups, Z,Z-TPDCF, Z,E-TPDCF and E,E-TPDCF, possessing and mass spectroscopy according to the data for E/Z-TPNCF41
three different isomeric molecular configurations, to investi- with their confirmed E/Z configurations. All the reaction details
gate the specific E/Z isomerization reaction of molecules above can be found in the experimental section.
containing two cyanostilbene groups. The pure isomers of
UV and PL spectra
Z,Z-TPDCF, Z,E-TPDCF and E,E-TPDCF were all synthesized
simultaneously by the Knoevenagel reaction, with their con-
figurations characterized by H NMR spectra according to the
The UV and PL spectra were firstly measured to investigate the
basic photophysical properties of Z,Z-TPDCF, Z,E-TPDCF and
E,E-TPDCF. As shown in Fig. 2, the UV spectra of all three iso-
mers, Z,Z-TPDCF, Z,E-TPDCF and E,E-TPDCF, exhibited wide
absorption bands with different absorption maximum peaks
located at 400 nm, 392 nm and 380 nm respectively, which were
probably due to the increased conjugation from the E- to the
Z-configuration. The PL spectra of Z,Z-TPDCF and Z,E-TPDCF
overlapped with the emission maximum peaks (lmax) located at
606 nm, which indicated that the lowest excited states (S1) of
Z,Z-TPDCF and Z,E-TPDCF were equal in energy and that the S1
state of Z,E-TPDCF was dominated by the cyanostilbene arm
with the Z- not the E-configuration. The higher energy PL spectra
of E,E-TPDCF with lmax at 601 nm in comparison to that of
Z,Z-TPDCF, further demonstrated that the Z-configuration had
lower energy than the E-configuration in the TPDCF cases.
These results showed that the UV and PL properties of the
three isomers were not identical because of their different
configurations.
1
confirmed configuration signals of E-TPNCF and Z-TPNCF.41
The in situ UV, 1H NMR and HPLC spectra under UV-irradiation
clearly showed that the E/Z isomerization reactions of both
E,E-TPDCF and Z,Z-TPDCF firstly turned them into Z,E-TPDCF,
and the Z,E-TPDCF almost simultaneously turned into more
E,E-TPDCF and less Z,Z-TPDCF. The PL spectra and theoretical
calculations indicated that the calculated lowest unoccupied
molecular orbitals of Z,E-TPDCF on the cyanostilbene arm of
Z-configuration with energy close to Z,Z-TPDCF were considered
to be the reason for the generation of more E,E-TPDCF during
the E/Z isomerization reactions of Z,E-TPDCF. In general, Z,E-
TPDCF exhibited a relatively better configurational stability than
Z,Z-TPDCF or E,E-TPDCF under photo-irradiation conditions.
Furthermore, all three isomers Z,Z-TPDCF, Z,E-TPDCF and E,E-
TPDCF exhibited excellent aggregation-induced emission (AIE)
properties. Hereinafter, we will offer our particular experimental
results and discussions.
E/Z configuration isomerization
Results and discussion
To investigate the configurational stability, the in situ UV spectra
of the E/Z isomerization process for isomers Z,Z-TPDCF,
Z,E-TPDCF and E,E-TPDCF were measured, as shown in Fig. 3.
All the UV absorption intensities were normalized for the con-
venience of analyzing the peak position shifts. With the photo-
irradiation time increasing, a rapid and obvious blueshift took
place with the peak positions of Z,Z-TPDCF, and the peak
positions came close to that of Z,E-TPDCF in 20 min (Fig. 3a).
The peak positions of E,E-TPDCF also began with an obvious
redshift to come close to that of Z,E-TPDCF under the same
conditions (Fig. 3b). It was interesting that the peaks of Z,E-
TPDCF displayed a tiny blueshift and reached equilibration
quickly within 5 min (Fig. 3c), which indicated that Z,E-TPDCF
Synthesis and characterization
As shown in Fig. 1, the isomers of Z,Z-TPDCF, Z,E-TPDCF, and
E,E-TPDCF were simultaneously prepared simply by one Knoe-
venagel reaction from 4-(trifluoromethyl)benzaldehyde and the
intermediate product 20-((phenylazanediyl)bis([1,10-biphenyl]-
40,4-diyl))diacetonitrile (M3 in Fig. 1) which was newly synthe-
sized by an improved process with a yield of 50% according to
the related ref. 38–41. During this reaction, Z,Z-TPDCF was
obtained as the main product with a yield of 65%, and less Z,E-
TPDCF and E,E-TPDCF were simultaneously produced with
yields of 20% and 5% respectively. All the obtained isomer
structures could be confirmed clearly from the 1H NMR spectra
Fig. 1 The molecular structures and synthesis route of E/Z isomers
Z,Z-TPDCF, Z,E-TPDCF and E,E-TPDCF.
Fig. 2 The normalized UV and PL superposed spectra of Z,Z-TPDCF,
Z,E-TPDCF and E,E-TPDCF in dichloromethane (DCM) at 10ꢀ5 M.
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Fig. 4 The original and in situ 1H NMR spectra of Z,Z-TPDCF, Z,E-TPDCF
and E,E-TPDCF under photo-irradiation with UV light (365 nm) for 90 min
in CDCl3, with the spectra of the chemical shifts from 7.26 ppm to 7.08 ppm
magnified on the right-hand side.
Fig. 3 The in situ UV spectra of (a) Z,Z-TPDCF, (b) Z,E-TPDCF and (c) E,E-
TPDCF in DCM and (d) the in situ UV spectra of Z,E-TPDCF in highly polar
acetonitrile under UV light (365 nm) for different photo-irradiation times.
Z-TPNCF and E-TPNCF respectively.41 And almost all the char-
probably possessed a more stable Z/E configuration. When the acteristic peaks belonging to Z,Z-TPDCF and E,E-TPDCF could
same in situ UV spectral experiments of Z,E-TPDCF were carried be found in the original 1H NMR spectra of Z,E-TPDCF with
out in highly polar acetonitrile, the resulting spectra of Z,E- nearly identical chemical shifts, which further indicated the
TPDCF were almost unchanged in the peak position in compar- independent Z and E-configurations of the two cyanostilbene
ison to the original spectra under the 365 nm UV-irradiation arms in Z,E-TPDCF.
conditions for 120 min (Fig. 3d), which displayed a relatively
After 90 min of photo-irradiation under 365 nm UV light, the
more stable molecular configuration compared to Z,Z-TPDCF 1H NMR spectra of Z,Z-TPDCF, Z,E-TPDCF and E,E-TPDCF in
and E,E-TPDCF in highly polar acetonitrile. Through further CDCl3 were measured again to monitor the E/Z isomerization
research into in situ UV spectra in different solvents, as shown process. As we can see in Fig. 4, both the resulting spectra of
in Fig. S1 (ESI†), with an increase in the polarity of the solvents, Z,Z-TPDCF and E,E-TPDCF changed obviously; however, the
the rate of the E/Z isomerization reactions of the three isomers in situ spectra of Z,E-TPDCF were almost unchanged, which
gradually slowed down, which was consistent with the results of also showed that the configuration of Z,E-TPDCF possesses
our previous work. This result was ascribed to the gradually better stability. Through an analysis of the amplifying spectra
decreasing energy of the S1 state, and the E/Z isomerization of the chemical shifts in 7.26–7.08 ppm, we found that after UV-
reaction in the higher energy S2 state was effectively suppressed irradiation for 90 min, the in situ 1H NMR spectra of Z,Z-TPDCF
by a rapid internal conversion process from the S2 to the S1 gained two new peaks located at 7.20 ppm and 7.11 ppm, which
state.41 Solvatochromic experimental results demonstrated that corresponded to the characteristic peaks of Z,E-TPDCF, and
all three isomers, Z,Z-TPDCF, Z,E-TPDCF and E,E-TPDCF, this clearly showed that the pure Z,Z-TPDCF turned into a
showed large shifts of the PL spectra in different polar solvents mixture containing just Z,Z-TPDCF and Z,E-TPDCF without
(Fig. S2, ESI†) and large slope values of the linear fitting line of any E,E-TPDCF after UV-irradiation for 90 min. The resulting
the Stokes shift versus the solvent polarity according to the spectra of E,E-TPDCF exhibited similar results, and there was
Lippert–Mataga equation (Fig. S3 and Table S1, ESI†), which just a new characteristic peak located at 7.23 ppm, which indi-
indicated large excited state dipole moments for the three isomers, cated that the only product, Z,E-TPDCF, was generated from the
further proving the charge transfer character of their excited states. E/Z isomerization reactions of E,E-TPDCF after photo-irradiation
1
These indicated that Z,Z-TPDCF, Z,E-TPDCF and E,E-TPDCF pos- for 90 min. These H NMR spectral results powerfully demon-
sessed an E/Z isomerization process of a charge transfer excited strated that the E/Z isomerization reactions of Z,Z-TPDCF
state similar to those of E-TPNCF and Z-TPNCF.41–46
and E,E-TPDCF only turned them into Z,E-TPDCF after UV-
1
In order to further investigate the configurational isomeriza- irradiation for 90 min. In addition, the in situ H NMR spectra
tion process of Z,Z-TPDCF, Z,E-TPDCF and E,E-TPDCF, in situ of Z,Z-TPDCF, Z,E-TPDCF and E,E-TPDCF in highly polar
1H NMR spectra of the three isomers were obtained under DMSO-d6 demonstrated that almost nothing changed between
different photo-irradiation times to describe the E/Z isomeriza- the original spectra and the corresponding in situ spectra after
tion process, as shown in Fig. 4. The characteristic chemical photo-irradiation for 90 min (Fig. S4, ESI†), which showed that
shifts of pure Z,Z-TPDCF, Z,E-TPDCF and E,E-TPDCF were the E/Z isomerization reactions were impeded due to the
distinctly demonstrated by their original 1H NMR spectra in formation of a pure charge transfer excited state in the highly
CDCl3. The specific single peaks of the vinyl hydrogen atom polar DMSO-d6 according to our previous report.41
of Z,Z-TPDCF and E,E-TPDCF were located at d = 7.63 and
To further explore the specific E/Z isomerization reactions
7.39 ppm respectively, which were consistent with those of of Z,Z-TPDCF, Z,E-TPDCF and E,E-TPDCF, high performance
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Continuous photo-irradiation up to 180 min showed us diminish-
ing quantities of Z,Z-TPDCF (67%) as well as incremental quan-
tities of Z,E-TPDCF (28%) and E,E-TPDCF (4%). The isomerization
reactions of E,E-TPDCF (Fig. 5c) showed similar results to Z,Z-
TPDCF, in which Z,E-TPDCF was firstly generated and then Z,Z-
TPDCF was later observed with a tiny content. The main difference
was that the original HPLC data of E,E-TPDCF showed the
existence of a tiny amount of Z,E-TPDCF, which should be
ascribed to the faster E/Z isomerization of E,E-TPDCF under the
unavoidable exposure to UV light for detection in HPLC instru-
ments. Some different and special results of E/Z isomerization
were observed in the HPLC data of Z,E-TPDCF (Fig. 5b). Through a
photo-irradiation of 60 min, pure Z,E-TPDCF was concurrently
turned into Z,Z-TPDCF and E,E-TPDCF, unlike the cases of Z,Z-
TPDCF and E,E-TPDCF. As the photo-irradiation time increased,
the content of Z,E-TPDCF decreased gradually and those of Z,Z-
TPDCF and E,E-TPDCF were enhanced at the same time. But even
when the photo-irradiation time reached 180 min, the content of
Z,E-TPDCF was calculated to be 79%, with those of Z,Z-TPDCF and
E,E-TPDCF being only 3% and 8% respectively. It was obvious that
Z,E-TPDCF displayed a relatively more stable E/Z configuration
than Z,Z-TPDCF and E,E-TPDCF. In general, the above results
obviously indicated that pure Z,E-TPDCF could simultaneously
turn into Z,Z-TPDCF and E,E-TPDCF under UV-irradiation, while
Z,Z-TPDCF and E,E-TPDCF under photo-irradiation conditions
both firstly turned into Z,E-TPDCF which then was further turned
into Z,Z-TPDCF or E,E-TPDCF. In addition, it is worth mentioning
that the E/Z isomerization of Z,E-TPDCF would generate evidently
greater quantities of E,E-TPDCF than Z,Z-TPDCF.
In order to further study the different E/Z isomerizations for
Z,Z-TPDCF, Z,E-TPDCF and E,E-TPDCF, theoretical calculations
on their electronic orbital and cloud distribution were carried
out by the B3LYP method. Firstly, their HOMO, LUMO and
LUMO+1 orbitals in the ground states were obtained, as shown in
Fig. 6. The three isomers, Z,Z-TPDCF, Z,E-TPDCF and E,E-TPDCF,
Fig. 5 The content of the three isomers in solution recorded by in situ
HPLC spectra of pure (a) Z,Z-TPDCF, (b) Z,E-TPDCF and (c) E,E-TPDCF in
acetonitrile under 365 nm UV-irradiation for different times.
liquid chromatograms (HPLC) under UV light (365 nm) for
different photo-irradiation times were measured and analyzed,
which could quantitatively determinate the specific contents
of the three isomers in solution by their peak areas. Considering
the very slow E/Z isomerization reaction rates in acetonitrile, the
photo-irradiation experiments were carried out in an acetonitrile
solution of the three isomers and acetonitrile was also used as
the mobile phase for the HPLC measurements. The HPLC results
are summarized in Fig. 5.
After 60 min of UV-irradiation (365 nm), the resulting HPLC
data of Z,Z-TPDCF (Fig. 5a) showed that the content of Z,Z-TPDCF
in solution decreased to 88%, and only Z,E-TPDCF with about 10%
content was generated, without any observed existence of E,E-
TPDCF. With the time of UV-irradiation (365 nm) increased to
120 min, the content of Z,Z-TPDCF was further reduced to 77%
with that of Z,E-TPDCF increased to 20%; meanwhile a tiny peak
Fig. 6 The electronic density distributions of the frontier molecular orbitals
(HOMO, LUMO and LUMO+1) in the ground states of Z,Z-TPDCF, Z,E-TPDCF
corresponding to E,E-TPDCF with a content of about 2% appeared. and E,E-TPDCF.
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Fig. 7 The natural transition orbital (NTO) distributions of Z,Z-TPDCF,
Z,E-TPDCF and E,E-TPDCF for the S0 - S1 and S0 - S2 excitations at the
level of TD-oB97X/6-31G (d,p) using the geometry of the S0 state.
possess similar HOMO energy levels with electron clouds loca-
lized at the central triphenylamine group. Whereas the LUMO
and LUMO+1 orbitals of Z,Z-TPDCF and E,E-TPDCF displayed
similar electron cloud distributions which were both localized at
the two cyanostilbene arms, and the energies between their
LUMO and LUMO+1 orbitals were both very close with a differ-
Fig. 8 The PL spectra of (a) Z,Z-TPDCF, (b) Z,E-TPDCF and (c) E,E-TPDCF,
and (d) the line chart of the maximum PL peak intensity of the three isomers
in the DMF/water system with different water fractions (fw, v/v).
ence of 0.03–0.04 eV, indicating the totally identical electronic molecules, the PL spectra of the three isomers in dimethyl-
transitions of their two cyanostilbene arms. However, the LUMO formamide(DMF)/water mixture systems with different water
of Z,E-TPDCF was localized at the cyanostilbene arm with the fractions ( fw) were measured. As revealed by the data and the
Z-configuration and its LUMO+1 orbital was localized at the other photographs in Fig. 8, all three isomers exhibited typical AIE
arm with the E-configuration, with a relatively larger energy gap behavior in the DMF/water mixture system. For Z,Z-TPDCF and
of 0.12 eV between them, indicating that the HOMO–LUMO Z,E-TPDCF, there is almost no emission in dilute DMF solution,
transition at the central triphenylamine and the cyanostilbene and the PL intensity remains nearly unchanged in a DMF/water
arm with the Z- configuration would be more favorable in energy. mixture with a low water fraction ( fw r 30%). Yellow light was
In order to examine the nature of the electronic transitions for observed in a mixture system with a water fraction higher than
the excited states of the three isomers, their natural transition 30%, whose PL intensity increases with the increase in water
orbitals (NTO) were evaluated, as shown in Fig. 7. For the S0 - S1 fraction. Similar behavior is also found in E,E-TPDCF with the
and S0 - S2 excitations, the holes and particles of Z,Z-TPDCF and PL intensity observable at fw = 50%. The results for the PL
E,E-TPDCF were both located at the central triphenylamine and spectra of the three isomers in dilute DMF solution and a DMF/
the two cyanostilbene arms respectively. As for Z,E-TPDCF, the water mixture system with a low water fraction could be
holes for its S0 - S1 and S0 - S2 excitations were located at the ascribed to their active intramolecular rotations, which exhaust
central triphenylamine group, while the particles for its S0 - S1 the energy of the excited states and result in fluorescence
and S0 - S2 excitations were mainly located at the cyanostilbene quenching.20,26 When the water fraction is high enough, the
arms with the Z- and E-configurations respectively, indicating molecules of the three isomers tend to aggregate and form nano-
that the S1 state in Z,E-TPDCF was mainly dominated by the particles in the mixture system, which restricts the intramolecular
central triphenylamine and the cyanostilbene arm with the rotations and makes the excited states decay to the ground state
Z-configuration, which was consistent with the previous PL spectral through normal irradiative channels with strong emission
results. Thus, when one photon was absorbed, Z,Z-TPDCF and E,E- generation.20,26 In addition, we also measured the fluorescence
TPDCF would be excited to the molecular excited state, and then quantum yield of the three isomers in solution and solid state to
either but only one of the two identical cyanostilbene arms could quantify their aggregation-induced emission property by using an
undergo the E/Z isomerization reaction to generate Z,E-TPDCF. But integrating sphere instrument. All three isomers in the solution
if Z,E-TPDCF was excited, due to the Kasha rule, it would reach the DMF are nearly non-luminescent and their fluorescence quantum
S1 state, in which large numbers of electronic transitions were yields are close to 0.1%. And in the solid state the fluorescence
located at the cyanostilbene arm with the Z-configuration and a quantum yield of Z,Z-TPDCF is 45(ꢁ3)%, of Z,E-TPDCF is
very low proportion was found at the arm with the E-configuration. 45(ꢁ3)%, and of E,E-TPDCF is 50(ꢁ3)%. These results further
Thus, the cyanostilbene arm with the Z-configuration in Z,E-TPDCF demonstrate that Z,Z-TPDCF, Z,E-TPDCF and E,E-TPDCF are
would be more excited and thus greater quantities of E,E-TPDCF AIE active molecules.
than Z,Z-TPDCF would result.
Experimental details
AIE phenomenon
Materials and instrument
In our previous work, we demonstrated that E-TPNCF and
Z-TPNCF were aggregation-induced emission (AIE) active mole- All of the reagents and solvents were purchased from com-
cules.41 To examine whether Z,Z-TPDCF, Z,E-TPDCF and E,E-TPDCF, mercial sources and used directly without further purification.
possess a similar configuration to E-/Z-TPNCF, are AIE active 1H NMR spectra of the synthesized compounds were recorded
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on a Bruker AVANCE III instrument (Bruker, Switzerland). Mass
Z,Z-TPDCF, Z,E-TPDCF, and E,E-TPDCF. M3 (0.1 g, 0.2 mmol)
spectral (HRMS) analysis was recorded by a SolaX 70FT-MS and 4-(trifluoromethyl)benzaldehyde (0.22 g, 1.2 mmol) were
instrument. Photophysical properties were investigated by a dissolved in ethanol (6 ml). The reaction mixture was stirred and
Shimadzu UV-1800 spectrophotometer (Shimadzu, Japan) and heated for 24 h at 45 1C by the catalysis of sodium hydroxide. In a
a Perkin-Elmer LS-55 luminescence spectrophotometer (the short period of time, the color of the reaction system turned to
integration times were set to 0.01 s in order to shorten the orange-yellow from colorless with numerous precipitations. After
measurement time to less than 20 s). High Performance Liquid cooling to room temperature, the reaction mixture was filtered and
Chromatograms (HPLC) of the three isomers were recorded by washed with cold ethanol. The crude product was dried in
an Agilent Technologies 1260 Infinity (Agilent, America).
vacuum, and was further purified by column chromatography
with petroleum ether/dichloromethane (1/1). Z,Z-TPDCF was
obtained as the main product with small amounts of Z,E-TPDCF
Synthesis
4-Bromo-N-(4-bromophenyl)-N-phenylaniline (M1, C18H13Br2N). and E,E-TPDCF also obtained at the same time.
1
In a round-bottom flask, triphenylamine (2 g, 8.2 mmol) was
Z,Z-TPDCF (yield 65%). H NMR (500 MHz, CDCl3) d = 8.02
dissolved in trichloromethane (30 ml). The NBS (2.9 g, 16.3 mmol) (d, J = 8.3 Hz, 4H, Ar-H), 7.79 (d, J = 8.5 Hz, 4H, Ar-H), 7.76 (d, J =
was averagely divided into three groups and was added to the 8.3 Hz, 4H, Ar-H), 7.71 (d, J = 8.5 Hz, 4H, Ar-H), 7.63 (s, 2H,
solution every half hour. The reaction mixture was stirred at room vinyl-H), 7.57 (d, J = 8.7 Hz, 4H, Ar-H), 7.35 (t, J = 8.3 Hz, 2H,
temperature overnight in the dark. The crude product M1 was Ar-H), 7.25–7.21 (m, 6H, Ar-H), 7.13 (t, J = 7.4 Hz, 1H, Ar-H).
evaporated to dryness, and was further purified by column HRMS (mass m/z): 787.24 [M+] C50H31F6N3 calc. 787.24.
1
chromatography with dichloromethane. The pure product was
Z,E-TPDCF (yield 20%). H NMR (500 MHz, CDCl3) d = 8.01
obtained as a transparent oily liquid with a 60% yield. 1H NMR (d, J = 8.2 Hz, 2H, Ar-H), 7.78 (d, J = 8.5 Hz, 2H, Ar-H), 7.75 (d,
(500 MHz, CDCl3) d = 7.33 (d, J = 8.9 Hz, 4H, Ar-H), 7.24 (d, J = J = 8.3 Hz, 2H, Ar-H), 7.70 (d, J = 8.5 Hz, 2H, Ar-H), 7.62 (d, J =
8.6 Hz, 2H, Ar-H), 7.09–7.03 (m, 3H, Ar-H), 6.93 (d, J = 8.8 Hz, 6.5 Hz, 2H, Ar-H), 7.60 (s, 1H, vinyl-H), 7.59–7.50 (m, 6H), 7.43
4H, Ar-H).
(d, J = 8.4 Hz, 2H), 7.39 (s, 1H, vinyl-H), 7.35 (m, 4H, Ar-H), 7.25–
N-Phenyl-4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-N-(4- 7.19 (m, 6H, Ar-H), 7.13 (t, J = 7.4 Hz, 1H, Ar-H). HRMS (mass
(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl)aniline (M2, m/z): 787.24 [M+] C50H31F6N3 calc. 787.24.
C30H37B2NO4). A solution of M1 (3.2 g, 8 mmol), bis(pin-
E,E-TPDCF (yield 5%). 1H NMR (500 MHz, CDCl3) d = 7.60 (d,
acolato)diboron (5 g, 19.6 mmol) and CH3COOK (4.4 g, J = 8.4 Hz, 4H, Ar-H), 7.53 (d, J = 8.6 Hz, 8H, Ar-H), 7.42 (d, J =
45 mmol) in 1,4-dioxane (100 ml) was stirred and refluxed at 8.4 Hz, 4H, Ar-H), 7.39 (s, 2H, vinyl-H), 7.38–7.34 (m, 2H, Ar-H),
100 1C under nitrogen atmosphere for 24 h by the catalysis of 7.23–7.18 (m, 6H, Ar-H), 7.12 (t, J = 7.4 Hz, 1H, Ar-H). HRMS
Pd(dppf)Cl2 (0.4 g 0.54 mmol). After cooling to room tempera- (mass m/z): 787.24 [M+] C50H31F6N3 calc. 787.24.
ture, the reaction mixture was extracted with dichloromethane
and washed with distilled water. The extracted organic phase
was dried with anhydrous MgSO4 and was filtered to obtain
the crude product M2. The crude product was further purified
Conclusions
by column chromatography with petroleum ether/dichloro- Three star-shaped molecular isomers with a triphenylamine
methane (2/1). The pure product was obtained as a white solid core linked to two cyanostilbene groups with E- or Z- configu-
powder with a 90% yield. 1H NMR (500 MHz, CDCl3) d = 7.69 (d, rations, Z,Z-TPDCF, Z,E-TPDCF and E,E-TPDCF, possessing three
J = 8.5 Hz, 4H, Ar-H), 7.29 (d, J = 8.5 Hz, 2H, Ar-H), 7.14–7.10 (m, different isomeric molecular configurations, were designed and
3H, Ar-H), 7.07 (d, J = 8.4 Hz, 4H, Ar-H), 1.35 (s, 24H, methyl-H). successfully synthesized simultaneously by the Knoevenagel
2,20-((Phenylazanediyl)bis([1,10-biphenyl]-40,4-diyl))diaceto- reaction. Their configurations were characterized and confirmed
nitrile (M3, C34H25N3). M2 (0.5 g, 1 mmol), 2-(4-bromophenyl)- by 1H NMR spectra and mass spectroscopy. Compared to the
acetonitrile (0.6 g, 3 mmol) and K2CO3 (2.21 g, 16 mmol) were molecular configurations of Z,Z-TPDCF and E,E-TPDCF, Z,E-
dissolved in toluene (12 ml) and distilled water (8 ml). TPDCF was relatively more stable, especially in highly polar
Pd(pph3)4 (0.1 g, 0.09 mmol) was added to the solution as the solvents. After 365 nm UV-irradiation for a period of time, the
catalyst. The resulting solution was stirred and refluxed at 90 1C E/Z isomerization reactions of Z,Z-TPDCF and E,E-TPDCF always
for 24 h under nitrogen atmosphere. The reaction mixture was firstly turned them into Z,E-TPDCF, which then further reached
extracted with dichloromethane and washed with distilled Z,Z-TPDCF and E,E-TPDCF respectively. And Z,E-TPDCF almost
water. The extracted organic phase was dried with anhydrous simultaneously turned into more E,E-TPDCF and less Z,Z-TPDCF
MgSO4 and was filtered to obtain the crude product M3. The due to the calculated lowest unoccupied molecular orbitals of
pure product was obtained and further purified by column Z,E-TPDCF on the cyanostilbene arm with the Z-configuration.
chromatography with petroleum ether/dichloromethane (3/1). Furthermore, Z,Z-TPDCF, Z,E-TPDCF and E,E-TPDCF all exhi-
The pure product was obtained as a faint yellow solid powder bited excellent aggregation-induced emission properties. As
1
with a 60% yield. H NMR (500 MHz, CDCl3) d = 7.62 (d, J = far as we know, our research was the first attempt to investi-
8.2 Hz, 4H, Ar-H), 7.51 (d, J = 8.6 Hz, 4H, Ar-H), 7.41 (d, J = 8.1 Hz, gate the specific E/Z isomerization reaction process of E/Z
4H, Ar-H), 7.33 (t, J = 8.2 Hz, 2H, Ar-H), 7.25–7.18 (m, 6H, Ar-H), isomers containing two or more isomeric units, which will
7.11 (t, J = 7.3 Hz, 1H), 3.82 (s, 4H, methyl-H).
make a contribution to basic photochemical research on E/Z
28284 | Phys. Chem. Chem. Phys., 2018, 20, 28279--28286
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Paper
17 S. Mishra, P. Awasthi, J. Singh, R. K. Gupta, V. Singh,
R. Kant, R. Jeet, D. Goswami and A. Goel, J. Org. Chem.,
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18 Y. Zhu, M. Zheng, Y. Tu and X.-F. Chen, Macromolecules,
2018, 51, 3487–3496.
isomerization reactions. In addition, the relatively stable
Z,E-TPDCF could also be a model molecular configuration for
stable E/Z-isomer functional materials in the fields of organic
photo-electronic applications.
´
´
´
19 M. Martınez-Abadıa, S. Varghese, R. Gimenez and M. B. Ros,
Conflicts of interest
J. Mater. Chem. C, 2016, 4, 2886–2893.
20 W. Z. Yuan, Y. Tan, Y. Gong, P. Lu, J. W. Lam, X. Y. Shen,
C. Feng, H. H. Sung, Y. Lu, I. D. Williams, J. Z. Sun, Y. Zhang
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21 Z. Ma, Y. Ji, Z. Wang, G. Kuang and X. Jia, J. Mater. Chem. C,
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22 Y. Zhang, K. Wang, G. Zhuang, Z. Xie, C. Zhang, F. Cao,
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There are no conflicts to declare.
Acknowledgements
Our works are grateful for supports from projects funded by
China Postdoctoral Science Foundation (2018M632498), the
Zhejiang Provincial Postdoctoral fellowship (Z71101009), Natural
Science Foundation of China (51603185, 51673174), and Zhejiang
Provincial Natural Science Foundation of China (LY17E030001).
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