Molecular Interactions Control Quantum Chain Reactions toward Distinct Photoresponsive Properties of Molecular Crystals

Article abstract of DOI:10.1021/jacs.7b06261

In this work, we fabricated four diphenylcyclopropenone (DPCP) crystals, which involved various molecular interactions encoded in individual molecular structures 1-4. On the basis of crystalline structural analysis and photoresponsive characterization of

Full text of DOI:10.1021/jacs.7b06261

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Communication
Molecular Interactions Control Quantum Chain Reactions toward
Distinct Photoresponsive Properties of Molecular Crystals
Yanjun Gong, Yifan Zhang, Wei Xiong, Ke Zhang, Yan-Ke Che, and Jincai Zhao
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b06261 • Publication Date (Web): 27 Jul 2017
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Molecular Interactions Control Quantum Chain Reactions toward Dis-
tinct Photoresponsive Properties of Molecular Crystals
Yanjun Gong,†,§,‡ Yifan Zhang,∥,‡ Wei Xiong,†,§ Ke Zhang,*,#,§ Yanke Che,* ,†,§ and Jincai Zhao†,§
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†
Key Laboratory of Photochemistry, CAS Research/Education Center for Excellence in Molecular Sciences, Institute of
Chemistry, Chinese Academy of Sciences, Beijing 100190, China
#State Key Laboratory of Polymer Physics and Chemistry, Institute of Chemistry, Chinese Academy of Sciences, Beijing
100190, China
§University of Chinese Academy of Sciences, Beijing 100049, China
∥IBS Center for Soft and Living Matter, Ulsan National Institute of Science and Technology, Uljuꢀgun, Ulsan 689ꢀ789,
South Korea
Supporting Information Placeholder
bons 1-4 were fabricated through engineering various molecular
ABSTRACT: In this work, we fabricated four diphenylcycloꢀ
interactions encoded in individual molecular structures 1-4 (Figꢀ
ure 1). Specifically, when photoꢀtriggering the DPCP microribbon
terminal with a length of 10 µm, microribbon 1 with a length of ca.
100 µm quickly and totally melted while microribbon 2 achieved
a photodecarbonylation migration of ca. 40 µm. This directly
revealed the quantum chain amplification factors of 9 and 4 for
microribbons 1 and 2, respectively. By contrast, microribbons 3
and 4 with strong Hꢀbonding interactions exhibited no quantum
chain reactions even at elongated irradiation time and only unꢀ
derwent the photodecarbonylation at the photoirradiated terminals.
These results provided valuable information to control quantum
chain reactions toward the enhanced mechanical photoresponses
of molecular crystals.
propenone (DPCP) crystals, which involved various molecular
interactions encoded in individual molecular structures 1-4. Based
on crystalline structural analysis and photoresponsive characteriꢀ
zation of the resultant singleꢀcrystal microribbons 1-4, we demonꢀ
strated that the magnitude of molecular interactions could effecꢀ
tively control the quantum chain reaction and the photoresponsive
property of the DPCP crystals. The microribbons 1 and 2 having
weak molecular interactions exhibited an efficient chain reaction
and large mechanical photoresponses (i.e., photomelting and phoꢀ
todeforming), while the microribbons 3 and 4 with strong molecuꢀ
lar interactions exhibited no chain reaction and mechanical morꢀ
phology change. Our work presented a new way to achieve moꢀ
lecular crystals with enhanced mechanical photoresponses.
Photoresponsive molecular crystals have attracted intensive atꢀ
tention over the last decades because of the potential application
as actuators.1ꢀ23 Many impressive advances have been made in
this area, particularly the development of materials based on the
photoisomers (e.g., azobenzene11ꢀ14 and diarylethene15ꢀ19) and
photoreactions20ꢀ23. It is still a great challenge, however, to use the
photochemical reactions to fully transform the molecular conforꢀ
mational changes into the macroscopic morphology changes of a
molecular crystal. This is because photoirradiation of a molecular
crystal only gave rise to partially reacted regions within the crysꢀ
tal and some unreacted regions always remained.1ꢀ23 Garciaꢀ
Garibay and coworkers made the pioneering work on recognizing
a quantum chain reaction of solidꢀstate photoꢀdecarbonylation that
resulted from efficient energy transfer in the solid state.24,25 This
may offer the opportunity to achieve a complete transformation of
molecular conformation changes into enhanced mechanical phoꢀ
toresponses. Nonetheless, the information of control over quanꢀ
tum chain reactions toward mechanical photoresponses of moꢀ
lecular crystals is very limited.
In this work, we report that the magnitude of molecular interacꢀ
tions could effectively control the quantum chain reaction toward
distinct photoreactionꢀinduced responsibility of diphenylcycloꢀ
propenone (DPCP) crystals. Four DPCP singleꢀcrystal microribꢀ
Figure 1. (aꢀd) Molecular structures 1-4 and singleꢀcrystal XRD
structures of the resultant microribbons 1-4.
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The synthesis of molecules 1-4 bearing different substituents on
pixel, 200 × 200 pixels per frame) for 15 s to initiate the reaction.
1
the same molecular backbone and the preparation of correspondꢀ
ing singleꢀcrystal microribbons 1-4 were described in the supportꢀ
ing information. Singleꢀcrystal Xꢀray structures and crystal paꢀ
rameters of DPCP microribbons 1-4 are shown in Figure 1 and
Table S1. Microribbon 1 belonged to a triclinic system, space
group of P n a 21, in which molecules 1 stacked along c axis with
a dꢀspacing value of 4.03 Å (Figure 1a). This large intermolecular
distance suggested the relatively weak interactions between
neighbor molecules. Microribbon 2 belonged to a monoclinic
system, space group of P21/n, in which molecules 2 stacked along
b axis with the dꢀspacing value of 4.04 Å, as shown in Figure 1b.
Although the intermolecular distance of microribbon 2 along b
axis was similar to that of microribbon 1 along c axis, the oxygen
atom in the methoxy group was very close to the phenyl in microꢀ
ribbon 2 (i.e., 3.56 Å), indicating the presence of a loneꢀpair elecꢀ
tronsꢀπ interaction in microribbon 2. Compared to microribbon 1,
such an accessorial interaction in microribbon 2 could offer better
resistance for the single crystal to the phase transition or chemical
reaction. In the microribbons 3 and 4, a more molecular interacꢀ
tion of Hꢀbonding (the bond length of ca. 1.8 Å) was introduced
(Figures 1cꢀd). Particularly in the microribbon 4, an Hꢀbonding
network was formed (Figure 1d), which could greatly augment the
magnitude of the molecular interactions. The differential scanning
calorimetry (DSC) measurements verified the magnitude trends of
the molecular interactions in microribbons 1-4. As shown in Figꢀ
ure S1, the endothermic peaks were observed at 83 °C, 168 °C,
238 °C, and 253 °C for microribbons 1-4, assignable to their reꢀ
spective phase transition temperature. This indicated the magniꢀ
tude of molecular interactions followed the order of microribbon 1
< microribbon 2 < microribbon 3 < microribbon 4.
Surprisingly, microribbon 1 with a length of 100 µm rapidly meltꢀ
ed in 15 s after photoꢀtriggering, indicating a highly efficient
propagation of the chain reaction where the excited products
(DPA*) transferred energy to neighboring molecules of unreacted
areas. This result also meant the quantum chain amplification
factor of microribbon 1 was more than 9. Here the weak molecuꢀ
lar interactions in microribbon 1 should facilitate both the photoꢀ
reaction and the breakdown of the crystal structure. The melting
behavior could even be transferred between the microribbons that
stacked each other (Figure S2), again indicating the high propagaꢀ
tion efficiency of the chain reaction. The photoꢀtriggered chain
reaction was also observed on microribbon 2 after the photoirradiꢀ
ation of its terminal (Figures 2c and d). However, the migration
length of the chain reaction in microribbon 2 was much shorter
(ca. 40 µm) and the reacted area didn’t melt but became darker.
Scanning electron microscopy (SEM) imaging of the darker area
revealed that the smooth microribbon granulated after the photoꢀ
decarbonylation (Figure S3). The distinct propagation ability of
the chain reactions over microribbons 1 and 2 should be correlatꢀ
ed with the different molecular interaction magnitude. In microꢀ
ribbon 2, the stronger molecular interactions yielded a higher
energy barrier for the changes of molecular volume needed in the
photodecarbonylation and thereby decreased the propagation disꢀ
tance of the chain reaction.
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Figure 3. (a) Brightꢀfield and (b) fluorescenceꢀmode optical miꢀ
croscopic images of the photodecarbonylation of microribbon 3
after UV irradiating the microribbon terminal (10 µm). (c) Brightꢀ
field and (d) fluorescenceꢀmode optical microscopic images of the
photodecarbonylation of microribbon 4 after the UV irradiating
microribbon terminal (10 µm).
Figure 2. (a) Schematic representation and (b) brightꢀfield optical
microscopic images of the photomelting of microribbon 1 after
UV irradiating the microribbon terminal (10 µm). (c) Schematic
representation and (d) brightꢀfield optical microscopic images of
the photoꢀdeforming of microribbon 2 after UV irradiating the
microribbon terminal (10 µm).
The chain reaction could be completely blocked when the
magnitude of the molecular interactions in molecular crystals was
largely increased. As shown in Figure 3, microribbons 3 and 4
with strong molecular interactions (e.g., Hꢀbonding) exhibited no
propagation of the chain reaction when the microribbon terminal
was irradiated even for elongated time (e.g., 2h). Interestingly, no
apparent changes were observed over microribbons 3 and 4 under
the brightꢀfield CLSM. However, the photodecarbonylation of
DPCP into DPA at the terminal with irradiation could be revealed
by fluorescenceꢀmode CLSM, where the emission of the phoꢀ
toirradiated area became much weaker than that of the rest area
Given that the molecular interactions could hinder the molecuꢀ
lar volume changes of DPCP to diphenylacetylene (DPA) in moꢀ
lecular crystals and thereby yield an energy barrier for the photoꢀ
reactions, we envisioned that distinct chain reaction amplification
should be observed over microribbons 1-4. To confirm this, we
utilized a confocal laser scanning microscopy (CLSM) to directly
compare the chain reaction amplification. As illustrated in Figꢀ
ures 2a and 2b, the microribbon terminal with 10 µm in length
was irradiated by a 375 nm scanning laser (18 W mm−2, 4 µs per
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(Figures 3b and 3d). Notably, compared to microribbon 3, microꢀ
showed that the blue fluorescence gradually turned into yellow
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ribbon 4 exhibited a much slower photochemical reaction that
took ca. 2 h to finish the emission changes (Figure 3d). To further
compare the photodecarbonylation rate in microribbons 1-4, we
used Fourierꢀtransform infrared (FTꢀIR) spectroscopy to realꢀtime
monitor the vibration intensity changes of O=Cꢀ group under the
UV irradiation (5 mW/cm2). As shown in Figure S4, for microribꢀ
bons 1 and 2, the adsorption peak of O=Cꢀ group at 1840 cmꢀ1
quickly disappeared after 9 s and 16 s, respectively. For microribꢀ
bons 3 and 4, however, this characteristic peak slowly disappeared
after 10 min and 120 min, respectively. These observations conꢀ
sisted well with the aforementioned photoresponses observed
under CSLM.
with increasing the UV irradiation time (Figure 4cꢀd and Movie
S3), indicative of the production of DPA from DPCP via the phoꢀ
tochemical decarbonylation. Moreover, the rate of the fluoresꢀ
cence change of microribbon 3 was much faster than that of miꢀ
croribbon 4 (Figure 4cꢀd), similar to those in Figure 3. Interestingꢀ
ly, XRD characterization of microribbons 3 and 4 before and after
the UV irradiation indicated that their crystal structures did not
change despite the large changes in molecular volume caused by
the photochemical decarbonylation (Figure S7). Furthermore,
SEM imaging displayed that the surface of microribbons 3 and 4
remained smooth after UV irradiation (Figure S8). These results
indicated that the molecular interactions in 3 and 4 microribbons
were strong enough to endure the impact of large molecular volꢀ
ume changes by UV irradiation and maintain their total original
ordered structure.
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To further demonstrate that the distinctly different photoreꢀ
sponsive properties of singleꢀcrystal microribbons 1-4 were atꢀ
tributed by the different intermolecular interactions engineered in
crystals but not by the slight difference of molecular structures 1-
4 at the molecular level, the photodecarbonylation reaction of
molecules 1-4 was investigated in solution phase. Figure S9
shows the timeꢀdependent fluorescence changes of molecules 1-4
acetonitrile solution (5 mM) under UV irradiation (365 ± 20 nm,
9.6 mW/cm2). Apparently, the solution photodecarbonylation
rates of molecules 1-4 were very similar and the photodecarꢀ
bonylation could be efficiently finished in ca. 11 min for all cases.
This strongly indicated that the difference of molecular structures
1-4 with different groups on phenyl groups (ortho versus para,
methoxy versus hydroxyl) had no effect on their photodecarꢀ
bonylation reactions at the molecular level. Resultantly, the comꢀ
parable photodecarbonylation of molecules 1-4 at the molecular
level confirmed that the different photoresponsive properties of
singleꢀcrystal microribbons 1-4 were attributed by the different
intermolecular interactions engineered in crystals. In addition, no
apparent fluorescence was observed in Figure S9 for molecules 1-
4 acetonitrile solution with UV irradiation less than ca. 8 min.
This is because the fluorescence spectra of the photodecarbonylaꢀ
tion products partly overlapped with the UVꢀVis adsorption specꢀ
tra of the reactants 1-4 (Figure S10). In this case, the fluorescence
of the small amounts of photodecarbonylation products were abꢀ
sorbed by the large amounts of unreacted reactants at the initial
stage of the reaction.
Figure 4. (a, b) Brightꢀfield optical microscopy image of the phoꢀ
tomelting of microribbon 1 (a) and the photodeforming of microꢀ
ribbon 2 (b) upon the UV light irradiation. (c, d) Fluorescenceꢀ
mode optical microscopy image of the photoresponse of microꢀ
ribbons 3 (c) and 4 (d). Scale bar represented 10 ꢁm.
We next investigated the photodecarbonylation and the resultꢀ
ing photoresponses of microribbons 1-4 with entire UV irradiation
(365±5 nm) under optical microscopy (Figure 4). As shown in
Figure 4a and Movie S1, microribbon 1 deposited on a glass slide
rapidly collapsed and melted in ca. 15 s under UV irradiation,
indicating that the rapid photochemical decarbonylation in microꢀ
ribbon 1 destroyed the ordered crystal structure. In addition, Xꢀray
diffraction (XRD) characterization of microribbon 1 before and
after 15 s UV irradiation confirmed that the single crystal strucꢀ
ture of microribbon 1 completely disappeared and the amorphous
DPA products were simultaneously formed (Figure S5a). Unlike
microribbon 1, microribbon 2 didn’t collapse and melt but deꢀ
formed from the photochemical decarbonylation upon UV irradiaꢀ
tion (Figure 4b and Movie S2). XRD characterization of the miꢀ
croribbon 2 before and after the UV irradiation indicated that the
volume changes from DPCP to DCA and CO only partially deꢀ
stroyed the crystal structure and part of the ordered structure still
remained (Figure S5b). SEM imaging revealed that the smooth
surface of microribbon 2 granulated, similar to that in Figure 2d.
Apparently, the weak molecular interactions in microribbon 1 was
not enough to maintain the ordered structure when large molecuꢀ
lar volume changes took place upon UV irradiation, while the
relatively strong molecular interactions in microribbon 2 could
keep part of original crystal structure under similar conditions.
In conclusion, we demonstrated that the quantum chain reacꢀ
tion and the photoresponsive property could be effectively conꢀ
trolled by the magnitude of molecular interactions involved in
DPCP crystals. Specifically, the weak molecular interactions
yielded a relatively low energy barrier for the molecular volume
changes in the photoreaction, which facilitated the propagation of
the chain reaction and the creation of large mechanically photoreꢀ
sponsive properties, e.g., photomelting for microribbon 1 and
photodeforming for microribbon 2. In contrast, the strong molecuꢀ
lar interactions in microribbons 3 and 4 hindered the chain reacꢀ
tion and mechanical morphology changes. These findings providꢀ
ed valuable information for crystal engineering of molecular maꢀ
terials with enhanced mechanical photoresponses.
AUTHOR INFORMATION
Corresponding Author
ykche@iccas.ac.cn; kzhang@iccas.ac.cn
This hypothesis was again supported by the photoresponsive
behavior of microribbons 3 and 4. As shown in Figure S6, no
photoresponse was observed under brightꢀfield optical microscoꢀ
py even upon elongated UV irradiation for both cases. However,
fluorescence optical microscopy imaging of microribbons 3 and 4
Author Contributions
‡These authors contributed equally.
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Notes
The authors declare no competing financial interests.
ACKNOWLEDGMENT
This work was supported by 973 project (No. 2013CB632405),
NSFC (Nos. 21577147, 21590811, 21521062, 21622406, and
21622406), the “Strategic Priority Research Program” of the CAS
(No. XDA09030200).
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Supporting Information Available: Experimental section,
Table S1, Figure S1ꢀS10 and Movie S1ꢀS3. This material is availꢀ
able free of charge via the internet at http://pubs.acs.org.
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