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Tuning Light-driven Motion and Bending in Macroscale Flexible Molecular Crystals Based on A Cocrystal Approach Shuzhen Li, and Dongpeng Yan ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 11 Jun 2018 Downloaded from http://pubs.acs.org on June 11, 2018

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ACS Applied Materials & Interfaces

Tuning Light-driven Motion and Bending in Macroscale Flexible Molecular Crystals based on A Cocrystal Approach Shuzhen Li and Dongpeng Yan*

Beijing Key Laboratory of Energy Conversion and Storage Materials, College of Chemistry, Beijing Normal University, Beijing 100875, P. R. China (P. R. China)

*Correspondence address: D. Y. (email: [email protected]). Abstract: Flexible molecular crystals with stimuli-responsive properties are highly desirable; however, uncovering them is still a challenging goal. Herein, we report a cocrystal approach to obtain elastic molecular crystals that exhibit light-induced fluorescence changes and dynamic mechanical responses at the macroscale level. Cocrystals of naphthylvinylpyridine and tetrafluoroterephthalic acid were fabricated in different stoichiometry ratios (2:1 and 1:1), which present different shapes (2D and 1D morphologies), photoemission and mechanical properties (rigidity and flexibility). Moreover, obviously different photo-mechanical energy conversions (light-driven cracking/popping and bending/motion) occur for the 2D and 1D cocrystals respectively. Nuclear magnetic resonance (NMR) spectra show the occurrence of photo-induced [2+2] cycloaddition in both cocrystals, which is the primary mechanism for their photo-actuating behaviors. Crystal structure analysis and theoretical calculation further reveal that protonation and the hydrogen-bonding

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network play important roles in light-stimulus bendable 1D cocrystal. Thus, the transformation from rigidity to flexibility based on cocrystallization with different stoichiometry may offer an effective means to tune the dynamic light-driven responses for smart crystalline materials. Keywords: molecular crystals, photochromic fluorescence, dynamic mechanical responses, hydrogen-bonding, smart materials 1. Introduction: Molecular systems sensitive to external photo-stimuli have received much attention due to their potential applications in molecular machines,1-5 mechanical actuators,6,7 optical sensors8 and smart switches.9 For example, light-induced transformations of molecular conformations (e.g., cis-trans photoisomerization)10 and in chemical reactions (e.g., [2+2] photodimerization)11-14 can lead to alternative optical, electronic and mechanical properties. To date, compared with the solution chemistry (i.e. single-molecule state)15 and soft materials (i.e. hydrogels and polymer networks),16-19 crystalline molecular solids (also known as molecular crystals) that can achieve macroscale photo-responsive mechanical behaviors remain a challenge. This is a result of solid-state lattice energy based on intermolecular interactions, which can restrict molecular rotation and motion in the crystalline sate.20 Inherit lattice stress may lead to the mechanical rigidity and brittleness as often observed in molecular crystals, 21 which easily break upon external stress. Relative to their rigid counterparts, soft and flexible molecular crystals are highly desirable since they can be deformed more easily and can even present dynamic responses to environmental stimuli for 2


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smart device applications.22 However, flexible molecular crystals at the macroscopic scale are still rare, 23-25 and the lack of high flexibility largely restricts dynamically mechanical properties (such as bending and motion) for practical applications. Cocrystals, which are generally referred to molecular solids composed of a target molecule and a co-assembled unit (co-former) in a certain ratio, have received recent interest in pharmaceuticals,26-28 optics,29-31 sensors,32,33 and organic electronics.34 Cocrystallization offers the advantages of tunable intermolecular interactions (e.g., hydrogen bonding, van der Waals forces, electrostatic forces and/or their combinations), as well as molecular arrangements modes and aggregation states in molecular crystals.35-40 The different stacking fashions and stoichiometry ratios of the self-assembled molecules in cocrystals could also lead to the alternated performances.35,40 These have already provided a means to adjust static physiochemical properties (such as thermostability, luminescence, electronic transport, and mechanical behaviors).41-44 However,

whether

cocrystallization

formation

could

balance

the

dynamic

stimuli-responsive behaviors of molecular crystals continues to be a long-standing scientific problem.45-47 Herein, we illustrate that the crystalline flexibility and light-induced transformation of the fluorescence and mechanical behaviors in molecular crystals can be tuned effectively based on the cocrystallization of assembled units in different ratios. These findings may uncover new insight into the relationship between self-assembled fashions and macroscopic photo-responsive performances of molecular crystalline materials.

3



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Scheme 1. The cocrystals of NVP and TFA in 2:1 and 1:1 ratios, revealing different crystalline morphologies and self-assembled patterns. 4-(1-Naphthylvinyl)pyridine (NVP, Scheme 1), was selected as the photoactive building block, due to its excellent optical and electronic properties in logic gate, field effect transistors, and photonics.48 The potential [2+2] photodimerization of the vinyl unit

in

NVP

also

Tetrafluoroterephthalic

furnish acid

it

with

smart

(TFA)

was

selected

light-responsive as the

behavior.49

co-former,

since

fluorine-containing molecules can facilitate modification of the intermolecular interactions and optical performances within the cocrystals.50 Moreover, the potential for proton transfer and the formation of a hydrogen-bonding network between TFA and NVP may further accelerate the photodimerization process by tailoring the reaction pathway and reducing the activation energy.51 2. Results and Discussion: Upon cocrystallization of NVP and TFA in different stoichiometry ratios (2:1 and 1:1) in the methanol/water solution, two types of transparent and crystalline products can 4


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be obtained, which present regularly large-sized 2D sheet and 1D needle-like morphologies, respectively (Scheme 1). Single-crystal XRD measurements (CCDC: 1584274 and 1826887, Table S1 in SI) enable the comparison of the different molecular arrangements and self-assembly patterns, which show that the NVP-TFA crystalline solids assemble in the expected 2:1 and 1:1 ratios. It is also observed that solvent molecules (CH3OH and H2O) exist in two cocrystals to form solvates 2NVP.TFA.CH3OH (abbreviated to 2NVP.TFA) and NVP.TFA.H2O (abbreviated to NVP.TFA). The appearance of solvents in the crystal lattice may influence their mechanical properties, which could play the role of efficient stress dissipation.26 For the 2NVP.TFA cocrystal (Figure S1 in SI), two protons transferred from the carboxylic groups in TFA to N atoms in the pyridine unit of NVP; the resulting 2NVP.TFA motif involves two types of hydrogen bonds between one TFA and two NVP molecules, which further extends to the 2D plane organized by weak C‒H…F and C‒H…O hydrogen bonding between the adjacent motifs. Moreover, compared with those in the pristine NVP solid (torsion angles: 149.8o-151.8o, Table S1 in SI), the conformation of the NVP molecules demonstrate relative planarity, with the torsion angle of 174.6o between the naphthyl and pyridine planes. Two neighboring NVP molecules present an anti-parallel arrangement (distance: 3.47 Å) based on π…π and cation…π interactions between the naphthyl group and the protonated pyridine (Figure S1 in SI). For the cocrystal in the 1:1 ratio (Figure S2 in SI), only one proton transfer took place between TFA and NVP. The NVP aggregation presents an interval arrangement, in which the TFA molecules are intercalated into the NVP spaced units. 5



Moreover, the anti-parallel NVP molecules present a twisted conformation with the torsion angle of 154.6o. Powder XRD patterns of the crystal ensembles are consistent with the simulated results from the single crystal structures (Figure S3 in SI), confirming the purity of the products during cocrystallization. XRD also indicates the typical crystals of 2NVP.TFA and NVP.TFA present preferred (001) and (010) orientations (Figure S4 in SI). The pristine NVP crystal exhibits an irregular 2D sheet-like morphology and blue fluorescence (ØPLQY=11.95%, τ404nm=1.57 ns), with the major and shoulder peaks located at 404 and 419 nm (Figure S5 in SI), corresponding to the 0-0 and 0-1 vibrational energy levels respectively. Under UV irradiation (365 nm), there is no change in the morphology and sharp (Figure S5C), and the fluorescence intensity and position remain nearly the same during the irradiation time range of 0‒8 minutes (Figure S5AB). Upon formation of the sheet-like 2NVP.TFA cocrystal, the 2D crystal emits a bluish green fluorescence (λem=492 nm, ØPLQY=3.49%, τ427nm=5.06 ns, Figure S6 and Table S2 in SI) with clear emission at the edge of the crystal under UV light (Figure 1A (b)), suggesting a conventional 2D photo-waveguide effect in the sheet-like morphology. Immediately, the 2NVP.TFA crystal starts to shear off in the short-axis direction. As the UV irradiation time increases, the macro-sized 2NVP.TFA undergoes rapid and obvious cracking and breaks down into individual micro/nanocrystals (Figure 1A). This is accompanied by the free motion and popping actuation of the micro/nano-sized crystalline fragments in random directions, with the travelling distance taking on several millimeters (Movie S1 and S2 in SI). TGA 6


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curves of 2NVP.TFA crystals before and after UV irradiation indicates that methanol molecules are not removed during the irradiation (Figure S7). Such light-driven burst behavior is known as the photosalient effect52 due to the stress generated along different axes for the anisotropic structure, which is rarely reported in molecular crystals.53

Figure 1. (A) The light-induced cracking and popping processes for the 2NVP.TFA cocrystal, accompanied by the fluorescence change during 0-8 minutes (B). Since optical signals of π-conjugated chromophores are highly sensitive to changes in their conformation and structure, in-situ fluorescent monitoring was performed on the selected 2D crystal. The slight fluorescence red-shift towards 495 nm of 2NVP.TFA occurs during the light-induced cracking and disintegration process, 7



accompanied by the gradually enhanced fluorescence intensity (Figure 1B). The red shift and enhancement of the photoemission suggest the appearance of new chromophore species. These observations show that despite the similar 2D morphology to the pristine NVP crystal, cocrystallization could achieve fast light-driven bursting and cracking during the UV photo-irradiation.

Figure 2. A) The flexible 1D NVP.TFA cocrystal with different bendable sharps; B) the light-induced fluorescence change during 0‒8 minutes, and C) the bending and motion processes for the cocrystal. 8


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For the 1D needle-like NVP.TFA, it is interesting that such macroscopic cocrystal is highly flexible, in contrast to the brittleness of the sheet-like 2NVP.TFA. Upon application of a pair of forces at both ends, the crystal can be elastically bent into a range of angles (0−180o) with different shapes (Figure 2A), and the crystal quickly recovers its originally straight shape without fracture after removal of the applied force. Such mechanical bending-relaxation process can be repeated for several reversible cycles (Movie S3 in SI), indicating its stress-dissipating and shape-restorative properties. To the best of our knowledge, the number of flexible molecular crystals are very limited to date, particularly for the two-component cocrystal systems.25 Under UV light (365 nm), the cocrystal sample presents bluish green emission (λem=498 nm, ØPLQY=6.60%, τ498nm=3.60 ns, Table S2 in SI). Compared with the pristine NVP, the red-shift of photoemission for both 2NVP.TFA and NVP.TFA can be attributed to that the protonation of chromophore containing pyridine ring could highly modify the electronic structures, and also reduce the band gap of NVP. Additionally, due to the different stacking fashions and packing modes in NVP.TFA and 2NVP.TFA cocrystals, the intermolecular interactions can be highly different between molecular crystals, which result in the different photoemission between NVP.TFA and 2NVP.TFA. Upon further UV light irradiation, the one-end fixed NVP.TFA cocrystal experiences gradually bending against the direction of the light resource during the time range of 0‒3 minutes (Figure S8 in SI). The fluorescence intensity also gradually increases with the emission wavelength towards 484 nm (Figure 2B). Similar light-induced florescence changes have also 9



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been reported in other photochromic emission systems, 54 which is beneficial to the remote monitoring of photo-mechanical conversion processes due to the high sensitivity and easy observation. Moreover, the bending and movement processes of the free 1D cocrystal can be in-situ observed under a fluorescence microscopy (Figure 2C and Movie S4 in SI), in which the NVP.TFA cocrystal presents rotation, translation and distortion at the 2D surface during photo-irradiation. This observation confirms that a light stimulus could effectively power the mechanical work in 1D flexible cocrystal at the macroscopic scale. The 1D crystal cannot recover back to the original shape after photoirradiation. Similar irreversible behaviors have also been observed in some previous studies.43,55 In this manner, it can be concluded that two cocrystals present completely different light-driven popping and bending behaviors, as two typical photoactuating modes for energy conversion. To detect whether the photo-responsive performances are available for other NVP-based cocrystals, another two 1:1 NVP-based cocrystals (Figure S9 and S10 in SI)

were

synthesized

as

control

samples.

The

co-formers

are

1,4-diiodotetrafluorobenzene (DITFB) and 4-bromotetrafluorobenzoic acid (BTFBA) (inset in Figure S9B and S10B in SI). The selection of these co-formers is due to their similar molecular sizes and shapes to TFA and the potential ability to transfer protons with NVP (i.e., BTFBA). However, neither a photo-induced mechanical response nor a fluorescence change was observed for these cocrystals (Figure S9C and S10C in SI), indicating the important role of molecular arrangement fashions in the light-driven motion and bending actuation processes in cocrystals. For cocrystals of 2NVP.TFA 10


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and NVP.TFA, the distances between lefinic C=C bonds in adjacent NVP molecules are 3.75 Å and 3.71 Å respectively (Figure S11 and Table S3), which can meet the need for reactive double bonds (distance less than 4.20 Å) in photoinduced cycloaddition reaction.56-61 However, the pairs of C=C bond are larger than 4.20 Å in NVP.DITFB and NVP.BTFBA, which are not suitable for [2+2] cycloaddition, and thus cannot exhibit the photomechanical effects as observed in experiment. Additionally, no proton transfer for the NVP.BTFBA cocrystal may also correspond to the lack of photo-responsive behavior (Figure S10 in SI). It was reported that acid condition favors the [2+2] cycloaddition for solutions of vinylpyridine-based compounds, due to formation of cation-π interactions.62 In this work, proton transfer from the carboxylic groups in TFA to N atoms in the pyridine unit of NVP could also play similar role in activation of cycloaddition in solid-state. Furthermore, the number of proton transfer is different for NVP.TFA and 2NVP.TFA, indicating changes in degree of proton transfer and hydrogen-bonding network could also result in the different reactive activation and energy distribution within the crystal upon photoirradiation. So the double bonds have photodimerized with different yields (based on NMR result below) and different behaviors of light driven motion and bending for 2NVP.TFA and NVP.TFA. To further uncover the origin of the photo-actuating performances of two cocrystals, we selected the 2NVP.TFA and NVP.TFA in single crystal form after UV irradiation for crystallographic study. The single crystal structures show that there are slight changes in their crystal structures (such as unit cells, interaction fashions between two 11



components and dihedral angles in NVP, Table S4 and S5 in SI). This is probably due to the fact that the number of reacted or rearranged molecules in large-sized crystals is much less than decomposed micro/nano-crystals. For example, some reacted molecules are only populated at the surface,13 which are not easy to be detected and reflected in single crystal XRD. NMR spectroscopy is also a powerful tool to detect the potential reaction and structural change during photo-induced reactions, and thus we have further performed 1H and

13

C NMR on the 2NVP.TFA and NVP.TFA

samples before and after UV irradiation. For the 2NVP.TFA, it was found that new peaks around 5.26 ppm (Figure 3A) and 45.1/43.3 ppm (Figure 3B) appear in the 1H and

13

C NMR spectra respectively, while other peaks nearly remain the same as the

initial cocrystal sample. The appearance of new peaks corresponds to the [2+2] cycloaddition product of NVP during the photo-irradiation process15. Similar changes in both 1H and

13

C NMR spectra are also observed in the NVP.TFA system (Figure

S12 in SI), confirming that the occurrence of [2+2] photo-induced cycloaddition reaction is the primary reason for their photo-actuating behaviors. Based on the integration of the signals from 1H spectra, the double bonds have photodimerized with a yield of 77.85% for 2NVP.TFA, and 19.02% for NVP.TFA, respectively. In the structures of both cocrystals, the parallel arrangement and suitable distance between vinyl units in NVP molecules facilitate the photodimerization, and the strain in photodimerization allows molecules easily away from the thermodynamic equilibrium state. Moreover, the synergetic and accumulative effect of individual molecules magnifies to macroscopic bending and motion as observed in the experiment. 12


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Additionally, the appearance of new photodimerization products also explains the irreversible recovery to the straight shape of 1D crystal after photoirradiation. Future work is still underway to detect how to achieve reversible photo-induced switching performances.

Figure 3. The 1H (A) and

13

C (B) NMR spectra of 2NVP.TFA before and after UV

irradiation. Some other solid-state measurements (such as IR, DSC, PXRD) of the cocrystals before and after UV irradiation were further performed as shown in Figure S13, 13



Figure S14, Figure S15 and Table S4. IR spectra (Figure S13) show that it is relative difficult to identify the change of functional groups, since the [2+2] cycloaddition reaction does not occur in all molecules in crystal under UV irradiation. The difference before and after UV irradiation can be reflected on the DSC result (Figure S14). The melting points of NVP.TFA and 2NVP.TFA were located at 173.82 °C and 173.17 °C before irradiation, respectively, which are different from the melting points of cocrystal NVP.TFA (195.48 °C) and 2NVP.TFA (191.34 °C) after irradiation for 30 min. In addition, the PXRD patterns of NVP.TFA and 2NVP.TFA (Figure S15) after UV irradiation provide some evidence for cycloaddition product, which show that the new diffraction peaks appear and some old peaks disappear. To further study the influence of stacking and packing fashions on the alternated photo-responsive mechanical properties between cocrystals with the same co-assembled units, crystal morphology prediction was performed on two crystal structures for linking the molecular stacking orientation and the macroscale crystal. The simulated morphologies of the pristine NVP (Figure S16 in SI) and two cocrystals (Figure 4 and Figure S17 in SI) are consistent with the experimental 2D sheet- and 1D needle-like characteristics. For the 2NVP.TFA system (Figure S17 in SI), the adjacent assembled motifs (NVP-TFA) in the (001) plane are mainly organized by van der Waals interactions, and will lead to easy cracking of the 2D sheet-like crystal at the (001) plane during photo-irradiation. However, for the NVP.TFA system, the self-assembly of NVP and TFA is nearly vertical to the long-axis [010] growth direction of the 1D crystal (Figure 4a), and both the 14


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hydrogen-bonding and electrostatic interactions in COO…HN units could act as a buffer layer (in the [010] direction) upon external stimuli (such as light and deforming stress), which could contribute to the high flexibility in the molecular crystals (Figure 4d). Therefore, to design and synthesize the next generation of flexible photomechanical materials, it is important to consider two important factors in molecular crystals: (1) the distance between the C=C bond is less than 4.20 Å towards [2+2] cycloaddition; (2) suitable 1D microstructure needs to be organized based on hydrogen-bonding interactions.

Figure 4. a) Formation of charge-assisted hydrogen-bonding layer (HBL) in NVP.TFA; b) structural conformation of the NVP and TFA; c) molecular stacking along the crystal [010] direction; (d) schematic profile for the bendable crystal under an external force or light stimulus. To quantitate the detailed intermolecular interactions in both cocrystals, molecular mechanics calculations were performed (Table S6 in SI). For the pristine NVP solid, non-covalent energy arises only from van der Waals interactions; while in the 15



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cocrystals, strong electrostatic and hydrogen-bonding interactions exist, which largely decrease

the

lattice

energy.

In

the

NVP.TFA system,

electrostatic

and

hydrogen-bonding interactions account for 66.3% of the absolute non-convent energy, which is much higher than that in 2NVP.TFA (16.7%). Additionally, H2O molecules also contribute to the hydrogen-bonding network in NVP.TFA, suggesting the occurrence of solvent molecules could also influence the macro-scaled mechanical properties.

These

results

indicate

that

the

formation

of

charge-mediated

hydrogen-bonding plays an important role in the flexibility of the molecular cocrystal. 3. Conclusion: In summary, we confirm that the rigid and flexible molecular crystals assembled with the same building blocks can be tuned based on the design of cocrystallization. As an alternative to familiar mechanisms (e.g., structural rearrangement, introduction of van der Waals interaction and π/σ-hole synthons),

63,64

this work reveals that the

charge-mediated hydrogen-bonding interactions as directional stress-buffer layers within cocrystals are important to achieve flexible molecular crystals. Moreover, the highly photosensitive units (protonated NVP) in 2D and 1D cocrystals result in different photochromic fluorescence (wavelength and intensity) and light-driven mechanical (bending and movement) responses. Several factors (such as distance between C=C bonds (

Tuning Light-driven Motion and Bending in ... - ACS Publications

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