Reversible regulating of crystal structures based on isomerization of phenylhydrazones

Article abstract of DOI:10.1039/c7cc06374a

Reversible crystal transformation for phenylhydrazones 1 between the orthorhombic microporous 1-E crystal and monoclinic herringbone 1-Z crystal induced by light and heating has been demonstrated. Other than the inherent single-molecular photoreactivity,

Full text of DOI:10.1039/c7cc06374a

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Marilyn M. Olmstead, Alan L. Balch, Josep M. Poblet, Luis Echegoyenet al.
Reactivity differences of Sc3N@C2n (2n 68 and 80). Synthesis of the
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Reversible Regulating of Crystal Structures Based on Isomerization
of Phenylhydrazones
Lulu Ma,*a Zexing Yuan,a Zhenguo Huang,a Jingyi Jin,a Duxia Cao,a Ruifang Guan,a Qifeng Chen,*a
and Xuan Sunb
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
3
rotation or inversion mechanism. The single-molecular
Reversible crystal transformation for phenylhydrazones
1
between the orthorhombic microporous 1-E crystal and photochemical events at the microscopic scale thus can be
monoclinic herringbone 1-Z crystal induced by light and heating amplified or synchronized by the regular loose arrangement,
has been demonstrated. Other than the inherent single- showing observable changes at some macroscopic scale.2
molecular photoreactivity, the weak intermolecular Therefore, the construction of the porous molecular crystal
interactions and loose stacking of porous 1-E crystal favor the with photoresponsive performance via loose stacking of
efficient solid-state photoisomerization.
photochromic molecules, is an effective way to achieve the
photoswitchable crystalline materials. The porous molecular
crystals assembled by intermolecular weak interactions may
show substantial advantages in terms of preparation,
processability, structural diversification and functional
flexibility in comparison with the porous metal–organic and
covalent frameworks4 based on covalent bonds. In addition,
The development of novel optically switchable solid materials
is of great interest for separation, catalysis, photomechanical
actuation and particularly the energy conversion and storage.
Photochromic molecules capable of undergoing reversible
structural interconversion by light radiation are promising
building blocks for the implementation of photoswithable
functions within supramolecular species and bulk solid
materials. 1 However, most photochromic compounds in solid
state show suppressed photoreactivity due to the close
stacking by hydrogen bonds, π-π coupling or other
intermolecular interactions. In addition, some molecules suffer
the photo-damage upon light stimuli, which severely limits the
photo-responsive performance in solid state.2 Construction of
photoactive solid materials with photoresponsive behavior,
especially the reversible photoswitchable properties, is still a
challenge to date. Great efforts have been made on this issue
mainly in two aspects: a) optimization of the photochromic
molecular building blocks with high photoisomerization yield,
good stability, excellent cyclic reversibility and fatigue
resistance to ensure the inherent single-molecular
photoreactivity; b) organization of these photoreactive
molecules by controlled assembly into orderly and loose
stacking superstructures, such as the porous molecular
crystals. The loose stacking endows the molecules in the solid
state enough space and freedom for photoisomerization via
the
crystalline
phase
transformation
based
on
photoisomerization converts the photon absorption into the
chemical energy and stores it in the metastable crystal then
release the stored energy as heat, therefore, the novel
photoactive molecular crystals with structural flexibility and
photoswitchable performance show potential applications in
energy conversion and storage such as the solar thermal cells
a few types of compounds with E/Z
5
(STCs). While
photoisomerization properties for STCs have been reported,
most of which are azobenzene derivatives fabricated in
polymer flims.5b,c It has been indicated that the phase change
between the crystalline solid and the amorphous solid/liquid
could be a parameter for designing high energy density photon
energy storage materials.5b Accordingly, development of novel
optically switchable crystalline materials to realize energy
storage via crystal transformation is highly interest for energy
storage and conversion materials.
Hydrazones 6 that can undergo light and thermal E/Z
isomerization around the imine C=N bond are potentially
promising photochromic molecules for their higher
photoisomerization
quantum
yield
compared
Z
with
isomers.7
azobenzenes,3a and good thermal stability of the
Most studies on hydrazones by far are focused on the pH
activated isomerization in solution,6b, 8 and a few examples
a School of Material Science and Engineering, University of Jinan, No. 336, West
Road of Nan Xinzhuang, Jinan, Shandong, P. R. China 250022
b Key laboratory of Colloid and Interface Chemistry, Ministry of Education, School of have been reported on photoisomerization in solution7,9 but
Chemistry and Chemical Engineering, Shandong University, 27 Shanda Nanlu, Jinan,
Shandong, P. R. China 250100
barely in solid state. Herein, we have synthesized
phenylhydrazones
molecular crystals. The reversible photoisomerization of
1
(
Fig. 1a) to construct photoswitchable
*Corresponding author: Dr. Lulu Ma, E-mail: mse_mall@ujn.edu.cn; Dr. Qifeng
Chen, E-mail: qfchen@126.com.
1
solution has been studied in detail. By slow liquid-phase
in
Electronic Supplementary Information (ESI) available: Experimental details,
Scheme S1, Tables S1−S2, and Figures S1−S4 (PDF), Data for 1-E and 1-Z crystals
(CIFs). See DOI: 10.1039/x0xx00000x
diffusion method, the single crystals of both 1-E and 1-Z
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isomers that are hardly achieved have been obtained 10 times, Fig. 1d), showing good reversibility and
respectively. Furthermore, the crystal of 1-E exhibits regular photochemical fatigue resistance of phenylhydrazones
quasi-hexagonal extrinsic micropores,10 which is challenging to
construct. The porous crystal structure of 1-E with the inner
free volume and weak intermolecular interactions facilitates
the photoswitching of the hydrazones in solid state. Finally,
efficient reversible isomerization in the crystalline state was
realized with considerable energy conversion between the
porous structure of 1-E and close herringbone stacking of 1-Z
isomers under light or heating. The structural flexibility,
porosity and versatility of the novel photoswitchable material
with substantial energy changed may have promising
applications in energy harvesting and storage.
1.
Fig. 2. (a) Molecular packing of 1-E crystal viewed along c axis with porous fluctuated
hexagonal pattern composed of four kinds of dimers (A, B, C, D) with each pore
diameter of about 7.07 Å; (b) Molecular packing of 1-E crystal viewed along b axis; (c)
Molecular packing of 1-E crystal viewed along c axis by the spacefill style; (d) Molecular
packing of 1-Z crystal viewed along b axis by stacking alternately with four molecules
(A, B, C, D); (e) Molecular packing of 1-Z crystal viewed along c axis with staggered
herringbone mode.
The single crystals of 1-E and 1-Z isomers were obtained
respectively by slow liquid-phase diffusion method (Table S1).
As far as we know, this is the first report on the crystal of the
Z
crystal, an intramolecular hydrogen bond is formed in each
-isomer that was finely growth from solution. In the 1-E
E
-
isomer between the pyridine nitrogen and the hydrogen on
the hydrazone, leading to the formation of a six-member ring
(
rigid that may prevent
Fig. 1a, rose-carmine circle), which is somewhat twisted but
tight stacking 13 between the
Fig. 1. (a) ORTEP drawing of phenylhydrazone 1-E (red dotted line: intramolecular
hydrogen bond, rose-carmine circle: intramolecular six-member ring) and 1-Z; (b) E/Z
isomerization and possible mechanism of phenylhydrazone 1; (c) UV/vis absorption
spectral changes upon the photoisomerization of 1 in THF (0.07 mM). Absorption
spectra of 1 before (black trace, mainly 1-E) and after (red trace, photostationary state
of mainly 1-Z) irradiation by 380 nm light for 60s; the grey traces show the evolution of
a
molecules. The crystal here has indicated that such loose
stacking can lead to a porous structure, which is believed to be
a key factor to achieve the switchable performance in solid
the spectrum with irradiation for t=1, 2, 5, 9, 20, 27, 35, 45 s and the blue trace is the state.14 On account of the twisting of the H-bonded rings, two
spectrum obtained after stored in the dark for 12 hours at room temperature. (d)
molecules packed into a dimmer in slightly slipped
arrangement with rotation angle of about 121.6° and π-π
stacking distance of 3.24 Å (Fig. 2b) by four distinctive patterns
Absorbance changes of
1 in THF solution (0.07 mM) measured at λ=376 nm by
alternating irradiation by 380 nm light for 60 s and under dark for 12 hours in repeated
switching cycles.
(Fig. 2a: A, B, C and D) to form one unit cell. The unit cell
1 with pyridine unit (Fig. 1a) were composed of four different dimmers forms in the on-top
Phenylhydrazones
synthesized with the E/Z ratio of about 3:2 (Scheme S1 and Fig.
S1 in ESI†).2a, 11 The isomerization mechanism3b of such
hydrazone molecular switches typically involves the changes of
the intramolecular hydrogen-bondings, namely the hydrazone-
azobenzene tautomerization, followed by rotation around a
C-N single bond (Fig. 1b).
The photoisomerization of the freshly purified phenyl-
hydrazones 1-E in THF were characterized by UV-vis
spectroscopy (Fig. 1c and 1d).The absorption band centered at
“Zigzag” mode at ab plane (Fig. 2a, 2c) and then crystallizes in
an orthorhombic space group Pccn. Consequently, two quasi-
hexagonal pore I and pore II (Fig. 2a) are constructed by the
zigzag packing with six dimmers of A1, B1, C1, B2, A2, D3 and
C1, D1, A4, D2, C2, B2 respectively. The pores II and pores I
stacked up and down along a axis to form the fluctuated
arrangement (Fig. 2b and 2c) with uniform minimum diameter
of about 7.07 Å.15 Overall, extrinsic micropores are formed
with fluctuated quasi-hexagonal channels along c axis (Fig. 2b
and 2c) with a porosity of about 17% calculated by Platon.16
λmax=376 nm of 1-E
transition of the aromatic units, which can weaken the C=N
bond and facilitate the
isomerization.7, 12 Irradiation of the
(Fig. 1c: black line) is assigned to the π-π*
In the 1-Z crystal, only the Z-configuration molecules are
E/Z
observed, which shows increased planarity without
intramolecular hydrogen bonds (Fig. 1a). The 1-Z isomers
crystallize in the monoclinic space group P 21/n with the unit
cell composed by four molecules A, B, C and D in the on-top
“line” mode at ac plane (Fig. 2d) with intermolecular H-bonds
1-E solution with light of 380 nm results in blue shift of the
absorption band, reaching the photostationary state (PSS) at
λmax=370 nm after 60 seconds (red line), leading to the
formation of 1-Z isomers with the estimated yield of about 95%
(Part 3 in ESI†).12 Putting this reaction mixture in dark for 12
hours at room temperature, the absorption spectra reverted
back to the original state, which indicates the reversible
isomerization from 1-Z to 1-E isomers. The isosbestic points
(λ=370 and 310 nm) demonstrate that only two species (1-E
and 1-Z isomers) are involved during the isomerization
process.2a Furthermore, no obvious photodegradation
(2.31 Å) and C-H
each two molecules (B and C, A and D‘ or D and A“ , Fig. S2 in
ESI†), which further enhance the stability of -isomers in solid.
…N interactions (2.73 Å and 2.64 Å) between
Z
The four molecules stacked alternately in staggered forward
and reverse herringbone modes at ab plane (Fig. 2e). Two
neighboring molecules are oriented in a face-to-face staking to
form columns along b axis by the π-π interactions with shorter
distance of 3.18 Å (Fig. 2e). In brief, the 1-Z isomers bearing
occurred during multiple isomerization cycles of the system (
≥
2 | J. Name., 2012, 00, 1-3
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planar structure stack densely into herringbone arrangement annealing the irradiated film at 76°C under vacuum for 10 min,
with no porosity in the crystal.
the absorption spectra converted back almost completely (Fig.
3a: Curve III, blue), which indicated the isomerization.
The above shows that 1-E is the thermodynamically stable
form in solid state and the photoreaction is a nearly absolute
process.
To get deep insight into the isomerism in solid state, the X-
ray powder diffraction (XRPD) of the drop-cast film of 1-E were
performed by light irradiation or heat treatment. The XRPD
Comparing the molecular structures of the two isomers 1-E
and 1-Z, and the corresponding crystalline structures, one may
figure out the decisive role of the intramolecular H-bonds in
controlling the crystal structure. The intramolecular H-bonds
formed in the 1-E isomers induced strong tension within the
molecule (Fig. 1a), leading to the increased rigidity and
distortion of the molecule structure, therefore weaken the π-π
interaction by enlargement of the π-π distance (3.24 Å). By
virtue of the solely weak π-interaction along the c axis, the 1-E
isomers stack loosely to form porous crystalline structure.17 In
the case of the 1-Z isomers, intramolecular H-bonding is
Z→E
pattern obtained from the films of 1-E (Fig. 3b: Curve II, red) is
almost identical to the pattern calculated from single-crystal
data (Fig. 3b: Curve I, black), indicating the film structure is
resemble to that of the crystalline particles. The diffraction
peak at 2θ=7.18° in curve II can be ascribed to the refractions
from two dimmers side by side (A and B or C and D ) in the unit
infeasible because of the Z-configuration and the far distance
between the nitrogen of cyano group and the hydrogen on
hydrazone, which leaves the planar molecular structure that
favors the close face-to-face π-π stacking 18 with shorter
distance of 3.18 Å along the b axis. In addition, other
intermolecular interactions, such as the intermolecular H-
bonds and C-H…N interactions along a and c axis, are
demonstrated (Fig. S2), which coherently construct the 1-Z
crystal with tight herringbone arrangement. Considering the
multiple intermolecular interactions and the compact
molecular stacking within the 1-Z crystal but the exclusive and
weak π-stacking in the 1-E crystal, efficient transformation
between the 1-E and 1-Z crystals are expected, which will be
verified by the UV-visible absorption spectroscopy and X-ray
powder diffraction (XRPD) as illucidated vide infra. Therefore,
appropriate increase in the rigidity and distortion of the
molecular structures by intramolecular H-bonding may weaken
and reduce the intermolecular interactions to prevent the
close stacking, and therefore is benefit for the formation of
porous material.
cell of crystal 1-E (Fig. 2a). The corresponding d spacing is 12.3
Å with the miller index (110) (Table S2), representing the
length of the two dimmers (Fig. 4) according to the calculated
crystal morphology from the BFDH model
20
(Fig. S3). The
diffraction peak at 2θ=7.89° with the miller index (200) and d
spacing of 11.2 Å (Table S2) which is a half of the axial length
of a axis in the unit cell, can be ascribed to the width of the
corresponding two dimmers (Fig. 4 and Fig. S3). After 22 h
irradiation of the film by light of 475 nm, the peaks belonging
to the 1-E crystal disappeared, accompany with new peaks
emerged at 7.50° and 15.0° as shown in Fig. 3b (Curve III, blue).
According to the mentioned
E to Z photoisomerization
occurred upon 475 nm light irradiation, this photo-induced
PXRD pattern should be corresponding to the generation of 1-Z
isomers, which may be the indication of the phase transition
from the porous orthorhombic syngony of 1-E to the
herringbone monoclinic syngony of 1-Z. The corresponding d
spacing of the emerging peaks are 11.8 Å with the miller index
(10-1) and the secondary diffraction of 5.91 Å with the miller
index (20-2) (Table S2), respectively, which are well coincide
with the length of one molecule in the unit cell of crystal 1-Z
(
Fig. 4 and Fig. S3). Notably, the XRPD signals of the 1-E
crystals are almost recovered upon annealing the irradiated
film at 76 °C under vacuum for 10 min (Fig. 3b: Curve IV,
green), suggesting the restoring porous orthorhombic
crystalline phase upon heating. The above results are clearly
evident that the crystal structure change is reversible and
correlated to the E/Z interconversion.
Fig. 3. Observation of the photoisomerization of 1-E in a drop-cast thin film by
UV-visible absorption spectroscopy and X-ray powder diffraction (XRPD): (a)
Absorption spectra of 1-E film before (I) and after (II) irradiation at 475 nm for 22
h; Curve III is the spectrum obtained when annealed at 76 °C under vacuum for
10 min. Spectra taken at different irradiation times of 2, 5, 16 and 22 h are
shown in grey lines; (b) XRPD patterns obtained before (II) and after 22 h (III)
irradiation the film of 1-E at 475 nm. Curve I and V are patterns calculated from
single-crystal data of 1-E and 1-Z. Curve IV is the pattern obtained by annealing
of the irradiated film at 76 °C under vacuum for 10 min.
The solid-state photoreactivity of
1 was checked on the 1-E
drop-cast thin films by tracking the UV-visible absorption
changes with light or heat stimuli. The maximum absorption
centered at λmax = 475 nm of 1-E (Fig. 3a: Curve I, black) film
shows substantial (~100 nm) red-shift compared with the
absorption of individual molecules in solution (Fig. 1c: black
trace). This clearly suggests the J-aggregation19 and conforms
to the slipped rotating arrangement between the two
molecules within a starting dimer of the 1-E crystal (Fig. 2a).
Irradiation of the 1-E film with the light of 475 nm brings about
an obvious blue-shift of about 22 nm (Fig. 3a: Curve II, red),
Fig. 4. Schematic illustration of the phase transition process between the porous
orthorhombic crystal of 1-E and the herringbone monoclinic crystal of 1-Z by
light or heating.
A rational phase transition mechanism is proposed as shown
in Fig. 4, including the following steps: (1) when irradiation of
the porous crystal of 1-E at 475 nm, photoisomerization
indicating the efficient E→Z photoisomerization in solid state
wtih about 91% of the 1-Z isomers12 obtained at the PSS. After
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occurred on the molecular level, where the 1-E isomer combination of porosity and novel photoactive moieties opens
transforms into the 1-Z configuration accompany with the up intriguing perspectives in the field of photoresponsive
intramolecular H-bond broken; (2) the slightly slipped materials.
molecular stacking with large rotation angle of about 121.6° in
This work was supported by National Natural Science
each dimer changes into the face-to-face π-π stacking without Foundation of China (grant no. 21477047, 21371109),
rotation; (3) four dimers in the on-top “Zigzag” mode as one Shandong Province Science Foundation (ZR2013BL002), Doctor
structure repeating unit of the 1-E crystal rearrange into an on- Foundation of University of Jinan (160070104, XB1302), and
top “line” mode serving as two building blocks to the 1-Z Natural Science Foundation of University of Jinan (XKY1502,
crystal, and (4) the on-top “line” mode building blocks arrange XKY1201).
along a and c axis to form into the herringbone structure of 1-Z
crystal. The phase transition from 1-Z to 1-E crystal promoted
by heating occurs by the reverse process.
There are no conflicts to declare.
Notes and references
The efficient photoisomerization was realized in hydrazones
crystals, relying on the solely weak intermolecular interactions
and loose packing between the molecules in the porous 1-E
‡ Details of the synthesis and structural characterization are
provided in ESI† (Scheme S1 and Figure S1). CCDC numbers of
crystal, beyond the inherent good single-molecular the single crystals: 1486572 for 1-E , 1486571 for 1-Z
.
photoreactivity of the molecules. To the best of our knowledge,
substantial E→Z photoisomerization of hydrazone between
two crystalline phases is described for the first time, and on
consideration of such crystalline structure transformation,
effective enthalpy change desired for the energy storage is
1
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structure. The micropores can also be restored upon
isomerization promoted by heating with the enthalpy (
Z→E
H) of
ꢀ
32.3 kJ/mol in melting state following the reverse process. The
structural flexibility and photoswitchable performance make
this novel photoactive material interesting in energy
harvesting, storage and conversion. It also paves the way to a
better understanding of solid-state photochemical reactions
and photoinduced crystalline phase transitions. The special
4 | J. Name., 2012, 00, 1-3
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DOI: 10.1039/C7CC06374A
Reversible crystal transformation for phenylhydrazones 1 between the orthorhombic
microporous 1-E crystal and monoclinic herringbone 1-Z crystal induced by light and heating
respectively.