Photosalient Behavior of Photoreactive Crystals - American Chemical

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Photosalient Behavior of Photoreactive Crystals Raghavender Medishetty,† Subash Chandra Sahoo,‡ Caroline Evania Mulijanto,† Panče Naumov,*,‡ and Jagadese J. Vittal*,† †

Department of Chemistry, National University of Singapore, 3, Science Drive 3, Singapore 117543, Singapore New York University Abu Dhabi, P.O. Box 129188, Abu Dhabi, United Arab Emirates

‡

S Supporting Information *

ABSTRACT: Being documented with only about a dozen serendipitous observations, the photosalient effect, where crystals leap when exposed to light, is considered a very rare phenomenon. Here, with a set of structurally related materials that undergo [2 + 2] photocycloaddition we present evidence that this effect is more common than it has been realized in the past, and we seek to establish correlations with the kinematics and the crystal structure toward rational design of photosalient materials. To that end, nine photoreactive complexes AgL2X2 (L = 4-styrylpyridine, 2′-fluoro-4-styrylpyridine, and 3′-fluoro-4-styrylpyridine, X = BF4−, ClO4− and NO3−) were prepared. The [AgL2]+ cations in these structures pack by both head-to-head and head-to-tail alignment of the styrylpyridine ligands. Crystals of six out of the nine complexes were photosalient and popped, hopped, and/or leaped when exposed to UV light. It is concluded that the occurrence of the photosalient effect is determined not only by the nature of the ligand but also by the crystal packing which directs the magnitude, direction, and rate of volume expansion during the photoreaction.

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INTRODUCTION Mechanical effects of single crystals such as bending,1−7 jumping, leaping, hopping, splitting, and exploding8−11 are a subset of mechanochemical phenomena of organic materials that are gaining attention due to projected future applications as fast actuators for smart medical devices, artificial muscles, electromechanical devices, sensors, probes, and memory devices.8,12−18 Of these, photoinduced mechanical motions are mainly observed in single crystals that are capable of bending, twisting, and curling in effect to photoinduced transformations such as reversible cyclization,1,5,6,19,20 [4 + 4] dimerization,3,21,22 isomerization of azobenzene derivatives,4,23−25 tautomerization,17,26 and [2 + 2] photocycloaddition,10,27 along with other photoinduced phenomena.2,28 The ensuing mechanical effects are commonly attributed to strains that have accumulated during the photochemical reactions. The noncontact controllable response from these crystals, as opposed to molecular machines,29 are advantageous for remotely controllable actuation on a macroscopic scale.5,13 Recently, we have reported a serendipitous observation of the very rare phenomenon “photosalient effect” where single crystals of dinuclear Zn(II) complexes explode and shatter violently during a photochemical reaction similar to popping of corn.9 In these reactions, linear coordination polymers were produced by [2 + 2] photocycloaddition of a pair of olefin bonds. The resultant photosalient effect is an impressive visual demonstration of rapid conversion of UV light into mechanical energy mediated by the chemical energy of the photosalient medium. Deeper understanding of such phenomena requires insight into the underlying mechanisms, which was not possible © 2015 American Chemical Society

in the past due to the small number of available structurally related photosalient materials. It has been demonstrated recently that crystal motility can benefit from evolution of phase heterometry during photodimerization of synthetically accessible reactants.9,30 To decipher and quantify correlations with the structure, using crystal engineering principles here we prepared a series of nine structurally related linearly coordinated silver(I) complexes with ligands that contain olefin groups. These metal complexes are capable of photodimerization and exhibit a different mechanical response; only six out of the nine complexes are photomechanically active. The varying photoreactivity across this series of related structures provides unique opportunity to rationalize the mechanical response by considering the crystal packing within the broader concept of light-harvesting materials for conversion of light to mechanical energy.

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EXPERIMENTAL SECTION

Synthesis of [Ag(4spy)2]BF4, 1. Faint yellow blocks were obtained by slow evaporation of AgBF4 (5 mg, 0.025 mmol) and 4spy (9 mg, 0.05 mmol) in methanol/DMF solution. The crystals were filtered and dried at room temperature. Yield: 9 mg (63%). 1H NMR (D6-DMSO, 300 MHz, 298 K): δ 8.57 (d, 4H, pyridyl protons of 4spy), 7.25−7.68 (m, 18H, aromatic protons of 4spy). Elem. anal. (%) calcd for C26H22AgBF4N2 Received: January 5, 2015 Revised: February 9, 2015 Published: February 26, 2015 1821

DOI: 10.1021/acs.chemmater.5b00021 Chem. Mater. 2015, 27, 1821−1829

Article

Chemistry of Materials

(63%). 1H NMR (D6-DMSO, 300 MHz, 298 K): δ 8.58 (d, 4H, pyridyl proton of 2F-4spy), 7.25−7.83 (m, 16H, aromatic protons of 2F-4spy). Elem. anal. (%) calcd for C27H26AgF2N3O4 (600.37): C, 54.01; H, 4.03; N, 7.00. Found: C, 53.58; H, 4.25; N, 6.82.

(556.09): C, 56.05; H, 3.98; N, 5.03. Found: C, 56.01; H, 3.87; N, 5.01. Synthesis of [Ag(4spy)2]ClO4, 2. Pale yellow blocks were obtained by slow evaporation of AgClO4 (5.2 mg, 0.025 mmol) and 4spy (9 mg, 0.05 mmol) in methanol and DMSO solution. The crystals were filtered and dried at room temperature. Yield: 9.3 mg (65%). 1H NMR (D6-DMSO, 300 MHz, 298 K): δ 8.56 (d, 4H, pyridyl proton of 4spy), 7.24−7.68 (m, 18H, aromatic protons of 4spy). Elem. anal. (%) calcd for C26H22AgClN2O4 (568.03): C, 54.81; H, 3.89; N, 4.92. Found: C, 54.78; H, 3.97; N, 4.71. Synthesis of [Ag(2F-4spy)2]BF4, 3. Faint yellow rhombus shaped single crystals were obtained by slowly layering 2F-4spy (10 mg, 0.05 mmol) in 0.5 mL of methanol over AgBF4 (5 mg, 0.025 mmol) in 0.5 mL of methanol and dried at room temperature. Yield: 8.9 mg (59%). 1H NMR (300 MHz, 298 K, D6-DMSO): δ 8.58 (d, 4H, pyridyl proton of 2F-4spy), 7.25− 7.86 (m, 16H, aromatic protons of 2F-4spy). Elem. anal. (%) calcd for C26H20AgBF6N2 (592.07): C, 52.65; H, 3.40; N, 4.72. Found: C, 53.07; H, 3.38; N, 4.72. Synthesis of [Ag(2F-4spy)2]ClO4, 4. Pale yellow block shaped single crystals were obtained by slow evaporation of AgClO4 (5.2 mg, 0.025 mmol) and 4spy (9 mg, 0.05 mmol) in methanol/DMF solution. The crystals were filtered and dried at room temperature. Yield: 8.5 mg (56%). 1H NMR (D6-DMSO, 300 MHz, 298 K): δ 8.58 (d, 4H, pyridyl proton of 2F-4spy), 7.25−7.84 (m, 16H, aromatic protons of 2F-4spy). Elem. anal. (%) calcd for C26H20AgClF2N2O4 (605.77): C, 51.55; H, 3.33; N, 4.62. Found: C, 51.61; H, 3.32; N, 4.60. Synthesis of [Ag(BF4)(3F-4spy)2], 5. Faint yellow single crystals were obtained by slow evaporation of methanol solution of 3F-4spy (10 mg, 0.05 mmol) and AgBF4 (5 mg, 0.025 mmol). Yield: 7.2 mg (48%). 1H NMR (300 MHz, 298 K, D6-DMSO): δ 8.58 (d, 4H, pyridyl proton of 3F-4spy), 7.15−7.61 (m, 16H, aromatic protons of 3F-4spy). Elem. anal. (%) calcd for C26H20AgBF6N2 (592.07): C, 52.65; H, 3.40; N, 4.72. Found: C, 52.82; H, 3.46; N, 4.66. Synthesis of [Ag(ClO4)(3F-4spy)2], 6. These crystals were obtained by employing a similar synthetic procedure as in the case of 4 and using 3F-4spy instead of 4spy. Yield: 7.3 mg (48%). 1H NMR (D6-DMSO, 300 MHz, 298 K): δ 8.58 (d, 4H, pyridyl proton of 3F-4spy), 7.15−7.59 (m, 16H, aromatic protons of 3F-4spy). Elem. anal. (%) calcd for C26H20AgClF2N2O4 (605.77): C, 51.55; H, 3.33; N, 4.62. Found: C, 51.31; H, 3.28; N, 4.59. Synthesis of [Ag3(NO3)2(4spy)6](NO3)·H2O, 7. Light yellow blocks were obtained by a similar synthetic procedure as in the case of 1, by using AgNO3 instead of AgBF4. Yield: 9.0 mg (68%). 1H NMR (D6-DMSO, 300 MHz, 298 K): δ 8.56 (d, 4H, pyridyl proton of 4spy), 7.24−7.68 (m, 18H, aromatic protons of 4spy). Elem. anal. (%) calcd for C26H22AgN3O3.33 (537.67): C, 58.08; H, 4.12; N, 7.81. Found: C, 57.99; H, 4.29; N, 7.75. Synthesis of [Ag(NO3)(3F-4spy)2], 8. Pale yellow blocks were obtained from similar synthetic procedure as for 9 by using 3F-4spy instead of 2F-4spy. Yield: 7.8 mg (55%). 1H NMR (D6-DMSO, 300 MHz, 298 K): δ 8.58 (d, 4H, pyridyl proton of 3F-4spy), 7.15−7.59 (m, 16H, aromatic protons of 3F-4spy). Elem. anal. (%) calcd for C26H20AgF2N3O3 (567.05): C, 54.95; H, 3.55; N, 7.39. Found: C, 55.30; H, 3.49; N, 7.34. Synthesis of [Ag(NO3)(2F-4spy)2]·MeOH, 9. Colorless rod-like single crystals were obtained from a similar synthetic procedure as 2 by using AgNO3 instead of AgBF4. Yield: 9.4 mg

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RESULTS AND DISCUSSION Preparation and Characterization. Earlier comparative studies of crystals showing a thermosalient effect, a thermal counterpart of the photosalient effect, have indicated a number of commonalities among the crystal structures that are capable of self-actuation.15 Within the mechanistically simplified concept for crystal self-actuation, regardless of the inducting external stimulus, the salient effects are a consequence of a sudden release of latent strain due to structure transformation. Within that rudimentary model, the driving forces of the thermally and photochemically induced salient effects can be considered analogous. A purposeful “design” of a material that would be capable of exhibiting a photosalient effect based on the [2 + 2] photocycloaddition reaction thus requires that at least the general conditions that apply to thermosalient effects (or analogous prerequisites for a photoinduced process) be met. Specifically, for a crystal of a metal coordination structure to exhibit a photosalient effect, the olefin-bearing ligands should facilitate sudden albeit not necessarily large expansion of the unit cell by drastic conformational changes during the formation of a cyclobutane ring.9,30 This outcome is less probable if both sides of the photoreactive ligands are bonded to the metal; hence, terminal ligands having olefinic bonds are preferred over ditopic spacer ligands. The molecules should be photoreactive, which implies proximity of the parallel CC bond pairs from neighboring ligands. Although the ππ interactions between olefinic ligands are ideal for the purpose, we aimed at using additional supramolecular interactions to secure proper alignment of the CC bonds. Bis-coordinated silver(I) ions have linear geometry and appear an excellent choice because the coordinated ions tend to align with each other along the long molecular axis and their interaction could be reinforced by argentophilic (Ag···Ag) interactions. Since the resulting structure requires counterions for charge balance, this highly conserved linear geometry could be paired with a variety of counterions to create structurally related molecular scaffolds. With this strategy in mind, we prepared three sets of silver(I) complexes having tetrafluoroborate, perchlorate, and nitrate as counterions with each of three styrylpyridine ligands, namely 4styrylpyridine (4spy), 2′-fluoro-4-styrylpyridine (2F-4spy), and 3′-fluoro-4-styrylpyridine (3F-4spy). The chemical characterization revealed the following compositions: [Ag(4spy)2]BF4 (1), [Ag(4spy)2]ClO4 (2), [Ag(2F-4spy)2]BF4 (3), [Ag(2F4spy)2]ClO4 (4), [Ag(BF4)(3F-4spy)2] (5), [Ag(ClO4)(3F4spy)2] (6), [Ag3(NO3)2(4spy)6](NO3)·H2O (7), [Ag(NO3)(3F-4spy)2] (8), and [Ag(2F-4spy)2]NO3·MeOH (9). The crystal structures of all compounds but 2 were determined by single crystal X-ray crystallography (the quality of the single crystals of 2 were not satisfactory for analysis; however, the unit cell and the powder X-ray diffraction patterns indicate that 2 is isotypical with 1). The structures of the perchlorate salts are isotypical with the corresponding tetrafluoroborate salts (1 and 2, 3 and 4, 5 and 6); however the three nitrate salts (7−9) have distinct structures, as could be expected from the geometry of the ions. In continuation, the structures of 1−9 are briefly 1822


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rings. As shown in Figure 2, the neighboring 2F-4spy ligands are stacked in an HT fashion, but only one of the two 2F-4spy

described and correlated to their photoreactivity and photosalient properties. Crystal Structures and Predisposition for Photochemical Reactivity. [Ag(4spy)2]BF4 (1) and [Ag(4spy)2]ClO4 (2). There are four independent formula units in the asymmetric unit, and there are two types of packing patterns of [Ag(4spy)2] cations in the structure of 1 (Figure 1). The first

Figure 2. Stacking of one of the 2F-4spy ligand pairs in a head-to-tail manner in the crystal of 3. Color codes: C, golden; Ag, cyan; N, blue; F, green.

ligands is perfectly aligned in parallel with its partner, to which it is related by inversion center at a distance of 3.86 Å. The distance between the CC bonds in the second 2F-4spy ligand attached to Ag(I) is longer from its counterpart to be reactive. Thus, only 50% photodimerization is expected from the analysis of crystal packing. However, the rod-like Ag(I) complexes are aligned approximately along the [101] direction, and expansion of the unit cell is expected along the b axis. To our surprise, 3 underwent quantitative photodimerization within 1.5 h under UV light, as concluded from the appearance of the cyclobutane chemical shift in the 1H NMR spectrum at δ 4.63 ppm and the shift of the pyridyl protons peak at δ 8.58 to 8.35 ppm. A single-step photoreaction with complete conversion suggests that rotation of the second 2F-4spy ligand (atoms N2−C26) along the Ag1−N2 bond has occurred while the first 2F-4spy pair underwent [2 + 2] photodimerization. Interestingly, when subjected to UV radiation, single crystals of 3 started to jump violently to several centimeters within a few seconds. As a result, the crystals almost instantaneously turned into microcrystals or powder, indicating that a singlecrystal-to-single-crystal transformation is not possible in this case. 4 was also found to behave similar to 3 and underwent quantitative photodimerization in 40 min. The photoreaction was also accompanied by a photosalient effect. [Ag(BF4)(3F-4spy)2] (5) and [Ag(ClO4)(3F-4spy)2] (6). There are two formula units in the asymmetric unit of 5. The closest Ag···F distance is 2.78 Å, which indicates that BF4− interacts weakly with Ag(I) in bridging fashion to bring the two Ag(I) centers closer to each other, as can be concluded from the Ag··· Ag distances of 3.25 and 3.33 Å. Forming a centrosymmetric “dimer,” the two crystallographically different cations in the asymmetric unit are aligned in a head-to-head (HH) fashion. The CC bonds are parallel in the first dimer but are aligned in a crisscross manner in the second dimer with the centroid separation of 3.72 and 3.68 Å (Figure 3). All olefin bonds are expected to undergo quantitative photodimerization, resulting in formation of [Ag2(rctt-3F-ppcb)2] (rctt-3F-ppcb = regiocis,trans,trans-1,3-bis(4-pyridyl)-2,4-bis(3′-fluorophenyl)cyclobutane) after a pedal-like motion in one pair.35,36 As expected, 5 and 6 were photoreactive, and their reaction was accompanied by a photosalient effect whereby the compounds underwent quantitative cyclization under UV light as inferred from the 1H NMR spectra. [Ag3(NO3)2(4spy)6](NO3)·H2O (7). Compound 7 has three formula units along with a lattice water in the asymmetric unit. In all these cations, each Ag(I) is almost linearly bonded to 4spy ligands and aligned approximately along the c* axis (Figure 4a). The water molecule is hydrogen-bonded to two

Figure 1. (a) Two patterns of molecular packing observed in the structure of 1: slip-stacked misalignment (pink shade) and parallel head-to-tail alignment (blue shade). (b) Alignment of molecules in the blue-shaded part. (c) Alignment of molecules in the pink-shaded part. Color codes: C, golden yellow; B, yellow; Ag, cyan; N, blue.

one (Figure 1c) can be described as a slip-stacked misalignment where the pyridyl groups are in close proximity to the phenyl groups of the neighboring 4spy ligands (the distance between centroids is 3.79 Å) with which they interact through π−π interactions. In the second type of packing, the neighboring 4spy ligands are aligned in a head-to-tail (HT) manner where the olefin groups are separated by 3.78 and 3.87 Å (Figure 1b). From the alignment of the olefin pairs in 1 and 2, it is anticipated that only 25% of 4spy will undergo [2 + 2] cycloaddition under UV light.31−33 The photoreactivity of 1 under UV light was monitored at regular intervals after the sample was exposed to UV light using 1 H NMR spectroscopy. The formation of a photoproduct, evidenced by shift in the pyridyl protons peak at δ 8.55 to 8.31 ppm, was calculated by integrating the cyclobutane proton peak at δ 4.59 ppm. Surprisingly, the conversion to the photoproduct was 33% after 3 h, which is higher than the value expected from the crystal packing. This discrepancy can be attributed to motion of part of the misaligned 4spy during the photoreaction to a disposition that is compatible with the Schmidt’s criteria.34 The photoreactivity of 2 was similar to 1 with a maximum of 33.3% photoconversion after 2 h. [Ag(2F-4spy)2]BF4 (3) and [Ag(2F-4spy)2]ClO4 (4). In the isotypical structures 3 and 4, the two pyridyl groups bonded to Ag(I) are twisted by 35.6° between the coordinated pyridyl 1823


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Schmidt’s topochemical criteria. No such alignment was found for the third Ag(I) complex with the Ag3 atom. Hence, photoreactivity is expected only between 2/3 of the 4spy ligands. Interestingly, the time-dependent photoconversion of 7 shows a two-step dimerization reaction as determined by 1H NMR spectroscopy. In the first step, the sample reacts to 66% in 20 h, and then the reaction is at a stall to 40 h after the irradiation onset. However, the reaction proceeds further to completion in 45 h. It is evident that during the formation of a cyclobutane derivative from the 4spy ligands, the lonely Ag(I) coordination cation and its centrosymmetrically equivalent cation moved closer, resulting in a better alignment of the 4spy molecules and complete photoconversion. Such molecular movements preceding photodimerization are well documented.34 The coordinated cations are aligned along the [101] direction and the volume expansion is expected to occur approximately along the [101̅] and [010] directions. [Ag(NO3)(3F-4spy)2] (8). In 8, the silver cation forms a centrosymmetric “dimer” by bridging two nitrate anions at a Ag···Ag distance of 3.30 Å (Figure 5). Due to the presence of a

Figure 3. (a) Parallel head-to-head arrangement in 5 by anionsupported argentophilic interactions. (b) Parallel crisscross arrangement in 5 by anion-supported argentophilic interactions.

Figure 5. Relative orientations of the 3F-4spy ligands in the centrosymmetric dimer in 8 by anion-supported argentophilic interactions.

crystallographic inversion center between the Ag(I) atoms, the 3F-4spy ligands are aligned in a face-to-face, crisscross fashion, with the distance between the centroids of the pyridyl and phenyl rings and center-to-center distance of the double bond pairs of 3.48, 3.88, and 3.71 Å, respectively. A pedal-like motion is required for reactivity in this structure.35,36 The neighboring dimer molecules are slip-stacked and aligned approximately along the a* axis. Unit cell expansion due to the formation of a cyclobutane ring is expected in the bc plane, especially in the [011] and [011̅] directions. UV irradiation of 8 shows quantitative photoreaction of the olefin groups as determined by 1H NMR spectroscopy. Monitoring the reaction of 8 over time revealed that only 30 min was required for quantitative conversion. Crystals of 8 also showed photosalient behavior when exposed to UV light. [Ag(NO3)(2F-4spy)2]·MeOH (9). In the crystal structure of 9, the Ag(I) cations are paired up with other cations to which they are related by the crystallographic inversion center. Furthermore, two nitrate anions bridge the Ag(I) atoms through Ag− O bonds (Ag1−O2, 2.716 Å and Ag1−O3a, 2.686 Å, symmetry operation: 1 − x, 1 − y, 1 − z), as shown in Figure 6. As a result, the two metal complexes are aligned face-to-face, with a Ag···Ag distance of 3.43 Å, which is slightly longer than the sum of their van der Waals radii (3.24 Å). Within the “dimer,” the 2F-4spy ligands are stacked in HH fashion, with the centroid distances between the pyridyl and phenyl rings and the centerto-center distance of the olefin bonds shorter than 3.81 Å. Furthermore, it was also found that the dimers are further stacked infinitely and are parallel to each other. As a consequence, the intercenter distance between the CC bond pairs of the neighboring “dimers” is 3.76 Å (Figure 6b).

Figure 4. (a) Crystal packing of 7 showing the alignments of the Ag(I) cations. The water is omitted for clarity. (b) Orientations of one of the two cationic complexes showing head-to-head alignment. (c) Orientation of the cationic complexes relative to the Ag3 atom. Color codes: C, golden yellow; Ag, cyan; N, blue; O, pink.

nitrate anions. The oxygen atoms of the third nitrate anion are doubly bridging two Ag(I) atoms. Of these three Ag(I) complexes, two are packed in parallel with the neighboring Ag(I) complexes related by a crystallographic center of inversion. The 4spy ligands coordinating to Ag1 and Ag2 are well aligned parallel to each other in an HH fashion and are related by crystallographic inversion centers (−x, 1 − y, 1 − z and 1 − x, −y, 1 − z, respectively). The intercentroid distances between the pyridyl···pyridyl and phenyl···phenyl rings and the centerto-center distance between the olefin groups are in the range 3.67−3.98 Å (Figure 4b), and thus they comply with the 1824


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compounds retains single crystal integrity during the photochemical reaction, the packing of the cations indicates formation of a dinuclear complex or 1D coordination polymer during the reaction. All nine Ag(I) complexes were photoreactive, lending strong support to the design strategy for preparation of photoreactive solids by using monodentate ligands and cations that are capable of metallophilic interactions. Except 8 and 9, these compounds are expected to undergo only partial photoconversion in the absence of molecular motion during photoexcitation. Interestingly, quantitative formation of a cyclobutane ring was observed in crystals of 3−9. As revealed by the plots of photoconversion versus time, while 7 shows a two-step [2 + 2] photocycloaddition reaction, other compounds undergo a single-step transformation. It may be assumed that sliding of the misaligned cations occurs after 66% of the aligned CC bond pairs in 7 have reacted, as it has been observed in other cases.34 On the other hand, pedal motion of olefin bonds likely occurs in 5 and 6 during the photodimerization.35,36 The apparent rate constants, as derived from the kinetic plots (Figures S1−S9 in the SI) are in line with first order kinetics (Table 1; for 7 the rate constant is reported only for the first step). Although these apparent reaction orders are only an approximate estimate of the real solid-state kinetics, they provide useful means to qualitatively compare the reactivity of the related compounds. The values of the rate constants fall within the range of reported values.9 The rates of photodimerization are higher with fluorinated ligands, most notably with 3F-4spy. This could be due to the electron-withdrawing nature of the fluorine atom which lowers the LUMO of the C− F bonding orbitals and decreases the HOMO−LUMO gap which in turn speeds up the photodimerization.41 Furthermore, although the tetrafluoroborate and perchlorate salts are isotypical, the perchlorates undergo photodimerization almost twice as fast relative to the tetrafluoroborates, indicating that the perchlorate anions facilitate the photodimerization. Kinematic Analysis. General Considerations. Except for 1, 2 (isotypical with 1), and 9, all other compounds were found to be photosalient. The inactivity of 1 and 2 having a 4spy ligand as opposed to 9 with 2F-4spy as a ligand confirmed that the occurrence of the photosalient effect is not determined by the nature of the ligand; instead, it appears to be a consequence of the changes in the crystal packing during the photoreaction. The range of the in-plane component of the speed with which

Figure 6. (a) Head-to-head packing within the “dimer” unit in 9. (b) Packing of the “dimer” units in 9.

Since both distances fulfill the Schmidt’s topochemical criteria, it is hard to predict which pair would react under UV light. On exposure to UV light, 9 underwent quantitative photodimerization within 180 min; however the reaction was not accompanied by photoactuation of the crystals. Photochemical Reactions. In all structures, the Ag(I) cations with linear geometry are oriented parallel to each other and are thus coplanar. The anions are bridging Ag(I) cations in 5, 6, 8, and 9 and partially in 7. As a result, the ligands are aligned in HH disposition in these cations, and the expected photoproduct is a dinuclear cation.37,38 Interestingly, there is no Ag···Ag interaction in these structures. Although anions of similar shape were used, the role of BF4− and ClO4− was different in 1−4, which yielded partial HT arrangement of the olefin-bearing ligands. Formation of a 1D coordination polymer was expected from the dimerization in the coordination cations (Table 1). Thus, weakly coordinating anions are not able to dictate the alignment of the Ag(I) cations by bridging the Ag(I) atoms to form dinuclear species in all cases. This is not surprising, considering the similar conclusions reached previously.39,40 It is apparent that the substituents in the styrylpyridine ligands bonded to the Ag(I) cation affect and balance out the role of these anions. Since none of the Table 1. Structure−Photoreactivity Correlations no.

compound

CC alignmenta

photoproductb

rate constantc/min−1

photosalient activity

density changed

% vol. changee

1 2 3 4 5 6 7 8 9

[Ag(4spy)2]BF4 [Ag(4spy)2]ClO4 [Ag(2F-4spy)2]BF4 [Ag(2F-4spy)2]ClO4 [Ag(BF4)(3F-4spy)2] [Ag(ClO4)(3F-4spy)2] [Ag3(NO3)2(4spy)6](NO3)·H2O [Ag(NO3)(3F-4spy)2] [Ag(NO3)(2F-4spy)2]

HT HT HT HT HH HH HH HH HH

1D CP 1D CP 1D CP 1D CP dimer dimer dimer dimer dimer

0.0015 0.0155 0.0177 0.0383 0.1267 0.2482 0.0009 0.1103 0.0135

no no yes yes yes yes yes yes no

1.614/1.55(2) 1.614/1.53(2) 1.703/1.38(2) 1.706/1.45(2) 1.654/1.43(1) 1.673/1.43(3) 1.577/1.37(1) 1.669/1.54(4) 1.673/1.55(1)

3.97 5.20 18.97 15.01 13.54 14.52 13.13 7.73 7.35

a

Head-to-head (HH) and head-to-tail (HT) arrangement of the ligand. bThe product was predicted based on the crystal packing. cThe reaction rate constants were determined based on first-order kinetics. In case of 7, the rate constant is reported only for the first step. dThe density of the photoproduct was determined experimentally by the flotation method (g cm−3). eChange in volume of the crystalline solid calculated from the experimentally determined density change. 1825


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Chemistry of Materials Table 2. Kinematic Analysis of the Photosalient Effect no.

tetrafluoroborates 1 [Ag(4spy)2]BF4 3 [Ag(2F-4spy)2]BF4 5 [Ag(BF4)(3F-4spy)2] perchlorates 2 [Ag(4spy)2]ClO4 4 [Ag(2F-4spy)2]ClO4 6 [Ag(ClO4)(3F-4spy)2] nitrates 7 [Ag3(NO3)2(4spy)6] (NO3)·H2O 8 [Ag(NO3)(3F-4spy)2] 9 [Ag(NO3)(2F-4spy)2]· MeOH

vxy/(m s−1)e

Na

mave/kgb

Vave/m3c

tave/sd

no mechanical effect 50 35

(1.2 ± 0.4) × 10−8 (7.3 ± 0.1) × 10−9

(7.1 ± 0.2) × 10−12 (4.4 ± 0.1) × 10−12

5.2 ± 0.1) × 10−3 (9.7 ± 3.4) × 10−3

0.8 ± 0.1 0.9 ± 0.1

(3.7 ± 0.1) × 10−9 (2.9 ± 0.5) × 10−9

no mechanical effect 60 55

(1.5 ± 0.2) × 10−8 (8.7 ± 1.1) × 10−9

(8.5 ± 1.2) × 10−12 (5.2 ± 1.1) × 10−12

(1.2 ± 1.2) × 10−2 (5.1 ± 0.06) × 10−3

0.16 ± 0.07 0.13 ± 0.02

(1.9 ± 0.4) × 10−10 (7.0 ± 1.9) × 10−11

120

(1.3 ± 0.2) × 10−8

(7.9 ± 0.9) × 10−12

(3.9 ± 0.06) × 10−3

0.12 ± 0.01

(1.7 ± 0.2) × 10−10

55 (8.8 ± 1.0) × 10−9 no mechanical effect

(5.3 ± 0.6) × 10−12

(7.1 ± 1.2) × 10−3

formula

(7.44 ± 0.07) × 10−2

Ek,xy/Jf

(5.3 ± 0.5) × 10−11

a

Number of crystals sampled for statistics. Representative recordings of the effects are deposited as SI Movies S1−S19. bAverage mass of the crystals calculated using density from X-ray diffraction analysis. cAverage crystal volume. dTime of flight. eIn-plane velocity. fIn-plane component of the kinetic energy.

the debris of photomechanically responsive crystals fly off, 0.074−0.890 m s−1, is very close to the values observed earlier with thermosalient solids.15,42 Based on the xy component of the kinetic energy, the splintered crystals carry away approximately 0.053−3.67 nJ of the elastic energy accumulated during the prestressing of the crystal. This energy is the driving force of crystal motility. Comparison of the in-plane speed between the photosalientactive solids having different counter-anions (Table 2) indicates that on average crystals of the tetrafluoroborates travel faster than the perchlorates, which travel faster than the nitrates, as opposed to their rate constants (Table 1). This could be taken as an indication that the strength of the photosalient effect is tetrafluoroborates > perchlorates > nitrates. It should be noted, however, that this conclusion pertains only to the in-plane speed and thus only to the horizontal displacement of the crystal from its initial position (the kinematic analysis was performed on movies recorded in a two-dimensional space). In light of the correlation with the respective structures (vide inf ra), the tetrafluoroborates and the perchlorates are more directly comparable because they form three pairs of isomorphous structures. In each case, the weakly coordinating tetrafluoroborates exhibit a stronger photosalient effect. A peculiar observation made during the kinematic analysis was that under continuous irradiation, in many cases the mechanical effect was accompanied by a rapid loss of translucency of the crystals due to deterioration of crystallinity. In the high-speed recordings, one of these processes occurred after the other with a hiatus. In line with our earlier conclusions based on X-ray powder diffraction analysis of a prototypic photodimerization-related photosalient effect,9 this result indicates that the hopping of a photosalient crystal and its deterioration due to a photochemical reaction (or phase transition) are two distinct processes. In mechanistic terms, this result can be interpreted as evidence of a two-step mechanism of the photosalient effect.16 According to this mechanism, the mechanical effect, which occurs at a threshold value of the internal strain caused by the photochemical reaction, is preceded by a rapid phase transition where at sufficient yield the product lattice transforms into a new phase. In the current experiment, these two distinct processes become directly observable for the first time. We hypothesize that the reason

for the occurrence of the photosalient effect with some photochemical reactions (as opposed to other reactions where the crystal simply disintegrates without any apparent motion) is a rapid phase transition. This transition may be triggered by the photochemical reaction and facilitates conversion of the strain energy that accumulates during the reaction into kinetic energy of the crystal or the debris. A question then is posed as to why in some cases the mechanical effect precedes (e.g., see SI Movies 3a, 4b, and 5b), while in other cases it appears to succeed the phase transition (SI Movies 6a, 7d, and 8a). As mentioned above, the photosalient effect can occur as a result of a phase transition which occurs in response to accumulation of the product. In several cases, the spatial progression of this phase transition can be monitored as rapid advancement of the habit plane throughout the crystal (the border between the parent and product phases), similar to what was observed recently with a thermosalient phase transition in an organometallic compound.42 According to this scenario, the crystal is irradiated and reacts homogeneously until a threshold in the internal strain is reached. At that point, a phase transition is initiated, which advances to convert most of or the whole crystal, whereupon the crystal responds by splitting or reshaping. In the second case, when the loss of crystallinity occurs after the mechanical effect, the irradiated crystal first splits into several pieces. After a hiatus, these clear pieces turn opaque. This posthopping loss of translucency of continuously irradiated crystals can be interpreted as continuation of the photochemical reaction in the debris obtained after the crystal has splintered. Below, the photomechanical effects in each of the classes of compounds are described. It is noteworthy that such observations are very rarely reported in the literature. Tetrafluoroborates (1, 3, and 5; SI Movies S1−S4). Compound 1 did not show any mechanical effect. Colorless, air-stable crystals of 3 exhibited a very strong photosalient effect within ∼1 s of the radiation onset. The crystals remained transparent during the mechanical effect (Figure 8a and b) but turned opaque afterward. The mechanical effects consist of jumping, splitting, and displacement (Figure 7a and b). During exposure to UV light of 13 s, the crystals exhibited multiple effects separated by periods of latency (1−3 s). The crystal surface of 5 degraded substantially; the crystals expanded and 1826


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underwent several subsequent motions. The progress could be observed by the progression of the reaction front on the crystal surface. The mechanical effects included displacements, jumping, and splitting. Toward the end of irradiation (13 s), the crystals became voluminous. The effects of 3 and 5 were stronger than those of 4 and 6. Nitrates (7−9, SI Movies S10−S19). Crystals of 7 obtained from methanol (MeOH) are blocky, colorless, and clear, and they exhibit strong photosalient effects. The effects occurred immediately after the excitation onset as dramatic separation of debris off the surface of the crystals (Figure S29). Later on, the pieces that separated from the crystal exploded. Toward the end of the irradiation (∼10 s), the material became brown and sticky. Crystals obtained from ethanol (EtOH) had a plate habit with cracks on the surface and showed similar behavior as the crystals from MeOH. Crystals of 8 from MeOH solution were clear colorless prisms. Surface degradation occurred immediately after exposure to light and continued during 4−5 s of irradiation (Figure 8k and l). After prolonged irradiation, the crystals expanded and became voluminous. The overall photosalient effect was weak and occurred as small displacements or splitting (Figure S30). The crystals obtained from EtOH behaved in a similar manner. Compound 9 was photomechanically inert (Figure S31). Structure-Mechanical Activity Correlations. Based on the above observations and in conjunction with the recent results, it becomes apparent that the percentage of volume expansion, anisotropic expansion, and rate of expansion of solids during the photochemical reaction are responsible for the photosalient behavior.9 Crystals of 1 and 2 undergo only partial photodimerization. Although unit cell expansion can occur in one direction during the formation of a cyclobutane ring, the rate of expansion could be reduced by the overall “dilution effect” of the photoinert cations. It is not clear if 9 can maintain infinite parallel arrangement of cations after the suspected loss of MeOH during UV irradiation. Nevertheless, the low change in volume during photoreaction (Table 1) appears to correlate with the lack of photosalient activity. On the contrary, 3 and 4 have HT arrangement, and the volume expansion under UV light can be predicted to occur normal to the direction of the molecular axis. The difference in volume expansion apparently facilitates the strong photosalient activity in 3 and 4. The crystals containing tetrafluoroborate (3) show stronger motility, a higher percentage of volume change, and a higher rate of the dimerization process than the perchlorates. One possible explanation of this observation is that the tetrafluoroborate ion is relatively weakly coordinating and hence exerts weaker interactions on the cations along the direction of volume expansion relative to the perchlorate. Therefore, layers could easily be cleaved and fly apart to account for the observed behavior. On the basis of these observations, it appears that strongly binding anions make it harder for the crystals to fly apart. Interestingly, 5 and 6 are expected to form dinuclear complexes during the photoreaction. Their volume expansion and photosalient nature follow the same trend as those of 3 and 4. On the other hand, 7 forms a dimer in two steps, which accounts for the different behavior. The volume expansion in this process is comparable to that observed with 4−6. Shifts of the unreacted molecules are expected in the second step. Due to these reasons, this compound also shows a much weaker photosalient effect. Although in the case of 8 the change in volume is 7.73%, this material also shows a very weak

Figure 7. Optical images showing mechanical motion and splitting of single crystals of 3 (a and b) and 4 (c and d) under UV light.

lost their translucency before the mechanical effect occurred (Figure 8e and f). The motion occurred as small displacement, hopping, or splitting (see SI Figure S28a and b).

Figure 8. Deterioration of single crystals of 3−8 where the mechanical motion occurs before (a, c, e, g, i, and k) and after (b, d, f, h, j, and l) UV irradiation.

Perchlorates (2, 4, and 6; SI Movies S5−S9). When exposed to UV light, colorless irregular crystalline blocks of 2 became opaque, expanded, and finally turned into a brown solid. Within the aforementioned model, this result shows that although these crystals are reactive, they are devoid of phase transition; thus they are mechanically inactive. Transparent to yellowish blocks of 4 with an irregular shape exhibited a weak photosalient effect. The crystal surface degraded and turned opaque after 1−2 s of irradiation (Figure 8c and d). For crystals that were mechanically active, the effects occurred after substantial surface degradation and consisted of displacements, small jumps, and splitting (Figure 7c and d). 6 was transparent and colorless before irradiation but became less transparent within 1−2 s from the onset of irradiation. The mechanical effects occurred after surface degradation (SI Figure S28c and d). The compound exhibited a weaker photosalient effect compared to 4. Occasionally, the crystals of this compound 1827



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photosalient effect. It appears that the sole magnitude of volume expansion cannot account for the motility, and the anisotropy and rate of volume change should also be considered. The dissipation of the kinetic energy, the size and thickness of the crystals, and the intensity of the UV light are other important factors that affect the photoactuation behavior. Detailed assessment and quantification of these factors is now underway in our laboratory.

AUTHOR INFORMATION

Corresponding Authors

*Tel.: (+971) 2628-4572. Fax: (+971) 2628-8616. E-mail: [email protected]. *Tel.: (+65) 6516-2975. Fax: (+65)6779-1691. E-mail: [email protected]. Notes

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The authors declare no competing financial interest.

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CONCLUSIONS We prepared nine photoreactive bispyridyl Ag(I) complexes with linear geometry from three structurally closely related 4styrylpyridine ligands and three different anions with the intention to correlate the photosalient effect in some of these compounds with their structures. Of the nine photoreactive Ag(I) complexes synthesized, six (3−8) showed photosalient behavior under UV light. The two-coordinate [AgL2]+ cations with linear geometry tend to pack in parallel in the crystals. The weakly coordinated BF4− and ClO4− anions exert no control on the packing of the Ag(I) cations, and hence the monodentate ligands in 1−4 pack in an HT manner while they pack in an HH fashion in 5 and 6. In contrast, semicoordinating nitrate anions bridge the Ag(I) complexes and enforce HH packing in 6−9. All compounds undergo a [2 + 2] cycloaddition reaction under UV light from partial (in case of 1 and 2) to complete conversion of the olefin bonds to cyclobutane rings. The compounds with HT arrangement (1−4) will likely furnish 1D coordination polymers at the end of the photochemical reaction, while compounds 5−9 with HH alignment of monodentate ligands are expected to form simple dinuclear cations. The solid-state structures of the final photoproducts could not be determined by single crystal X-ray crystallography since the crystals shattered into pieces under UV light. Our attempts to grow single crystals by recrystallization of the photoproducts were unsuccessful. The parallel packing of the rod-like Ag(I) complexes and the formation of a cyclobutane ring in the orthogonal direction to the linear cations increases the volume during the popping of crystals. The percentage, anisotropy, and rate of volume change, in addition to the size and shape of the crystals, and the intensity of UV light are responsible for their motility. The rate and mechanical motion of these crystals is affected by the counteranion, and the photosalient effect decreases in the following order: tetrafluoroborates > perchlorates > nitrates. We also report two different patterns in the morphological changes that occur during the solid-state photocycloaddition reaction. Namely, some crystals lose translucency before they move, while others move first and then lose translucency. The light-induced motility of the crystals of a series of structurally related Ag(I) complexes that undergo a [2 + 2] photocycloaddition reaction under UV light provides a new and promising venue to explore the photosalient effect in metal−organic systems in light of the energy conversion of light to other forms of energy.

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Article

ACKNOWLEDGMENTS We thank the Ministry of Education, Singapore for financial support through NUS FRC Grant No. R-143-000-562-112 and New York University Abu Dhabi for the financial support of this work.

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ASSOCIATED CONTENT

S Supporting Information *

Experimental procedures, 1H NMR spectra, PXRD patterns, results of TG analysis, and other details are included. This material is available free of charge via the Internet at http:// pubs.acs.org 1828


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Photosalient Behavior of Photoreactive Crystals - American Chemical

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