Quantifying Crystallographically Independent Optical Switching

Sep 9, 2013 - Sven O. Sylvester. † and Jacqueline M. Cole*. ,†,‡,§. †. Cavendish Laboratory, University of Cambridge, J. J. Thomson Avenue, C...
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Quantifying Crystallographically Independent Optical Switching Dynamics in Ru SO2 Photoisomers via Lock-and-Key Crystalline Environment Sven O. Sylvester† and Jacqueline M. Cole*,†,‡,§ †

Cavendish Laboratory, University of Cambridge, J. J. Thomson Avenue, Cambridge, CB3 0HE, United Kingdom Department of Chemistry, University of New Brunswick, P.O. Box 4400, Fredericton, E3B 5A3, Canada § Department of Physics, University of New Brunswick, P.O. Box 4400, Fredericton, E3B 5A3, Canada ‡

S Supporting Information *

ABSTRACT: The photophysical properties associated with solid-state Ru-based SO2 linkage photoisomerism are shown to differ with single-molecule recognition; this stands to afford superior optical resolution for data storage applications. Two compounds, [Ru(NH3)4SO2X]tosylate2 (X = isonicotinamide (1) and isonicotinic acid (2)), yield crystal structures, each with two chemically identical but crystallographically distinct Ru-based complexes (Ru01 and Ru02) and thus two different photoisomerizable SO2 environments. It was found that the SO2 photoconversion fraction in each crystallographically independent complex differed by over 20% (for 1: 51.0(12) % in Ru01 and 28.6(9) % in Ru02). These photophysical differences between neighboring molecules were attributed to the larger free volume around the groundstate SO2 in Ru01, allowing for higher photoconversion, i.e., a “lock-and-key” environment controls the photochemistry. Furthermore, the η2-side-S,O-bound (MS2) metastable photoisomer in Ru01 was 20 K more thermally stable in both 1 and 2; photoinduced intermolecular interactions are shown to dictate this thermal stability. SECTION: Spectroscopy, Photochemistry, and Excited States

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ideal to examine two different reaction cavities within the same single crystal to provide an internal standard for the analysis. Some work has been performed on dinitro compounds, e.g. the [NiL2(NO2)2] family, where L is the cis-ligand. However, in such cases, either the photoisomerizable NO2 ligands are symmetrically related and thus have identical reaction cavities, or only have one η1-N-bound geometry allowing only one crystallographically independent NO2 to isomerize.6,9,10 Another approach is to develop a photoisomerizable crystal with two crystallographically distinct metal centers (Z′ = 2), creating two different reaction cavities for the photoactive ligand within the same crystal, e.g., the Ru[(NH3)4SO2(isonicotinamide)]tosylate2 crystal. A photocrystallographic experiment on this compound was performed previously with a low-intensity halogen light,7 realizing an η2-side-S,O-bound (MS2) state that existed with different geometries but similar photoconversion levels in the two crystallographically independent Ru complexes. In one complex, there was a single MS2 geometry at 26.9(10)% photoconversion; in the other complex, the MS2 geometry was disordered over three sites with a total photoconversion of 22.4(10)%. Although a difference in the amount of disorder of the MS2 geometry was observed, an underlying η1-O-bound MS1 geometry was not crystallo-

rystalline materials with optically accessible metastable geometries have been examined as photonic switches, molecular transducers, and materials for holographic data storage.1−4 For such materials to be used in practical applications, the thermal stability of the photoinduced metastable geometry has to approach room temperature, and the occupational fraction of photoisomerization has to approach 100% conversion.5,6 It is known that the local chemical and crystallographic environment creates a unique reaction cavity that controls the photophysical property considerations upon photoisomerization, a specificity that is similar to the “lock-and-key” model observed in enzyme activity.7 The three major photophysical properties typically examined at the molecular level are (1) the geometry of the metastable states, (2) the amount of photoconversion of the isomerization, and (3) the thermal stability of the metastable states. As such, an in-depth understanding of these properties in the context of the reaction cavity is essential for tuning the photophysical property characteristics of the metastable geometries in order to create better devices. The reaction cavity has been investigated in a handful of chemical families containing linkage photoisomers by comparing the ligand cavity volume across chemicals with different trans-ligands and counterions.7,8 However, in these cases, it has not been possible to separate the effects of the reaction cavity from the chemical effects that also influence the photophysical properties of the metastable geometries. As such, it would be © 2013 American Chemical Society

Received: August 1, 2013 Accepted: September 9, 2013 Published: September 9, 2013 3221

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Figure 1. The geometry and photoconversion levels of the photoexcited states are displayed using the OLEX2 software.13 In all cases, except the MS1 isomer with the lowest occupancy in 1 Ru01, the free oxygen in the metastable state isomerizes into a position that is on the same side of the molecule as the NH2 of the isonicotinamide in 1 or the OH of the isonicotinic acid in 2 (i.e., the right-hand side of the molecule as shown in this Figure).

intensity LED broadband light source (λ = 425−650 nm) prior to the collection of another full X-ray data set at 100 K. Such a photocrystallographic experiment enables the differences between the “light” and “dark” X-ray data collections to be attributed to the photoinduced effects.4,12 For 1, the GS only model was introduced into the data, and a significant amount of photoinduced electron density was observed around the SO2 group in both Ru01 and Ru02 cations, demonstrating the partial photoisomerization of the SO2 group, which was subsequently modeled. In contrast to previous work,7 significant photoconversion levels were achieved in 1, affording an MS1 SO2 geometry, disordered over two sites about the Ru01 center, totalling 27.7(7) + 10.3(6) = 38.0(9) % MS1, and a single (nondisordered) MS2 geometry with 12.8(8) % photoconversion, leaving a residual 49.0(11) % GS geometry. In the Ru02 cation, there was no evidence of the MS1 geometry, although there was a significant amount of excess electron density around the site of the MS2 geometry; MS2 was disordered over two sites and photoconversion levels refined to 18.5(7) + 10.1(6) = 28.6(9) %, with a residual 71.4(3) % in the GS geometry. For 2, after photoisomerization the GS only model was introduced into the data and excess electron density surrounded three different photoexcited SO2 geometries. This was subsequently modeled and both Ru cations showed the same geometries: one MS1 geometry and the MS2 geometry disordered over two sites. The photoconversion levels refined to 9.2(7) % MS1, 26.5(7) + 5.3(9) = 31.8(9) % MS2 with a residual 59.0(6) % GS for Ru01 and 10.9(8) % MS1, 12.7(9) + 6.7(7) = 19.4(9) % MS2 with a residual 69.7(5) % GS for Ru02. Full crystallographic details and relevant bond lengths and angles are given in the Supporting Information. The differences in the level of photoconversion between 1 and 2 can be partially attributed to the chemical effects of having different trans-ligands with significantly different pKa values: isonicotinamide (pKa = 10.61) and isonicotinic acid

graphically resolved, and the differences in photoconversion and thermal stability between the different crystallographic centers were not examined.11 This communication analyzes the reaction cavity through the examination of two members of the [Ru(NH3)4SO2X]tosylate2 family of complexes, X = isonicotinamide (1) and isonicotinic acid (2), both of which have two crystallographically distinct Ru centers. The reaction cavity is analyzed using two methods, Voronoi−Dirichlet polyhedra (VDP) partitioning, which quantifies the free volume around the photoisomerizable SO2, and Hirshfeld surfaces, which identifies the intermolecular interactions around the photoisomeriziable SO2. Furthermore, each of the three aforementioned molecular photophysical properties are examined for each compound. It will be shown that the key metastable structural characteristics, i.e., the amount of photoisomerization and relative geometry of isomerization at 100 K, are primarily determined by the free volume around the ground-state (GS) SO2, as calculated via VDP. The free oxygen generated upon photoisomerization moves toward the position of the GS oxygen with the largest VDP volume, and the population of the metastable geometry is proportional to the total VDP volume of the GS SO2. The dynamic structural changes, which occur when the MS2 geometry decays into the GS geometry, are not able to be justified by simply looking at the free volumes; and it will be shown that the decay kinetics are primarily dominated by intermolecular interactions between the Ofree of the MS2 geometry and a neighboring benzene ring. Initially, the GSs of both 1 and 2 were determined using single-crystal X-ray diffraction and were found to be isomorphous, crystallizing in the P1̅ space group each with two crystallographically distinct metal complexes (labeled Ru01 and Ru02), formed owing to the dimerization of the amide/ carboxylic acid functional groups (Supporting Information (SI), Figure S2). The crystals were then irradiated in situ by a high 3222

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Figure 2. The VDP of the GS SO2 at each crystallographic center is imaged using TOPOS.16 The individual atom components are shown with the oxygens shown in red and the sulfur shown in yellow. The volume for the SO2 is larger in Ru01 in both 1 and 2, and the oxygen is on the same side of the molecule as the NH2 of the isonicotinamide in 1 or the OH of the isonicotinic acid in 2 has a larger volume than the other oxygen at all of its crystallographically unique sites.

(pKa = 4.96).8 A large associated change in the trans-influence in turn effects the bonding orientations and thermal stability of the metastable states in this family of complexes.8,14 However, the different photoconversion levels and photoexcitation geometries between the crystallographically distinct Ru complexes in 1 and 2 cannot be attributed to chemical effects; rather, they can only be explained by crystallographic effects. Examining these crystallographic effects, we first consider the static (metastable) photoexcited state structure of 1 at 100 K, where the geometry and photoconversion fraction of the photoexcited states varies greatly between the two different crystallographic centers. To analyze the crystallographic origins of these differences, the reaction cavity volume enveloping the GS SO2 was calculated using VDP to determine if we could predict the observed geometries.15 Similar to previous work,7 the VDP volumes were found to be 45.0(1) Å3 at Ru01 and 44.6(1) Å3 at Ru02 (Figure 2), significantly higher than other VDP volumes in other compounds of this family (others range from 38.3 Å3 to 41.5 Å3). The large VDPs observed in 1 are consistent with the observed trend of higher amounts of photoconversion in 1 compared to those other compounds with smaller VDPs. We thus empirically deduce that the larger free volume around the GS SO2 provides sufficient room for the generation of the higher levels of photoisomerization. The Z′ = 2 nature of 1 enables this deduction to be corroborated by the fact that Ru01 has a higher photoconversion fraction than Ru02. The SO2 VDP volume can be further partitioned into the individual atom components (Figure 2), with the volumes around the individual oxygen atoms shown. In Ru01, there is a significant difference (1.5 Å3) between the VDP volume of the different GS oxygens whereas in Ru02, a difference of only 0.2 Å3 is observed. At both crystallographic sites, the larger free volume around the oxygen occurs around the oxygen on the same side of the molecule as the NH2 of the isonicotinamide. Examining the geometric evolution of the photoisomerization (Figure 1), the free oxygen of the MS2 also forms on the same side of the molecule as the isonicotinamide NH2 which possesses a larger VDP volume. Furthermore, the less thermally stable MS1 geometry is only observed in the larger reaction

cavity (Ru01). The photoconversion level of the two states of the disordered MS1 isomer is higher where the free oxygen is on the same side of the molecule as the isonicotinamide NH2, consistent with the trends observed in the MS2 geometries. Investigating the reaction cavities of 2, the GS SO2 VDP volumes were calculated to be 45.7(1) Å3 for Ru01 and 44.6(1) Å3 for Ru02, similar to those of 1. To examine why the photoexcited states portray the same geometries but different photoconversion levels at the different crystallographically independent sites, the VDP volumes around individual oxygen atoms were also determined; they are similar to each other, and the trend follows that of Ru01 in 1. Similar to 1, the oxygen with the greater GS VDP volume is on the same side of the molecule as the OH of the isonicotinic acid, and the free oxygen in the metastable states (both MS1 and MS2) isomerizes into this volume. Furthermore, the greater photoconversion observed in Ru01 occurs where the GS oxygen VDP volume is 0.9 Å3 larger than in Ru02. By examining the VDP volumes around the GS SO2, it is clear that after photoexcitation the metastable free oxygen isomerizes directionally into the space where the GS oxygen has the larger free volume. Furthermore, the MS1 geometries are observed where the entire SO2 reaction cavity is the largest. Examining the trends across both 1 and 2, it is clear that there is not a linear relationship between the amount of photoconversion and VDP volume, possibly due to intermolecular interactions, stabilizing the photoexcited geometries, or a minor trans influence caused by the differing levels of pyridyl distortion in each crystallographically independent Ru cation (see SI, Figure S2). The thermal decay of the metastable geometries was subsequently examined across the different crystallographically unique sites to determine what effect, if any, the crystallographic environment has on the thermal stability. The temperature was raised from 100 to 190 K at 360 Kh−1. Interestingly, at both crystallographic sites in both 1 and 2, the multiple photoexcited geometries decayed into a single MS2 geometry, suggesting that all of the photoexcited geometries have decayed into the most thermally stable MS2 geometry. Multiple X-ray diffraction data collections were sequentially 3223

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Figure 3. A graph showing the decay of the MS2 geometry into the GS, the blue dots and lines of best fit showing 1, with 2 shown in red. Each point represents a full X-ray diffraction data collection, each conducted repeatedly, and the time is given as the relative midpoint of the scan.

Figure 4. Hirshfeld surfaces of the two crystallographically independent cations in 1 and 2 are generated using CrystalExplorer 2.1.17 Each isosurface encloses an MS2 photoisomerized cation, as defined by Hirshfeld partitioning. The red regions show a positive isoenergy (abnormally close atomic contact), in white regions there is neutral isoenergy, and blue regions display negative isoenergy. The top row of surfaces show the Ofree−H interaction (a) and the bottom row of shows the Sbound−O interactions (b).

collections were performed to determine the rate of its decay. A significant difference in the Ru01 MS2 decay rates between 1 and 2 was observed with a half-life of 14.8(8) h and 6.8(1) h, respectively (Figure 3). These results show that not only is the amount of photoexcitation in Ru01 greater than in Ru02, the onset of decay in the MS2 geometry is 20 K higher for Ru01. As the reaction cavity is constantly evolving during the dynamic process of the MS2 decay, the GS reaction cavity would not provide a useful concept by which to rationalize the thermal stability of the MS2 geometry. Intermolecular interactions around the MS2 geometry, however, provide a constant array of localized electrostatic sources around this photoisomerization environment, to a first approximation.

performed at 190 K, through which it was observed that the MS2 geometry in Ru02 decayed into the GS; however, there was no discernible change in the occupation of the MS2 geometry in Ru01 for both 1 and 2 (Figure 3). The decay of the MS2 geometry across Ru02 was successfully modeled via first-order kinetics, in common with decay kinetics for other compounds in this family, and the half-life was determined to be 10(1) h and 6.8(3) h for 1 and 2, respectively. The MS2 geometry of Ru01 in both 1 and 2 was completely stable over the entire duration of these successive diffraction experiments (>40 h), suggesting long-term stability at 190 K. Accordingly, the temperature was subsequently raised to 210 at 360 Kh−1, and a further set of multiple successive X-ray diffraction data 3224

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These intermolecular interactions were therefore examined in order to understand the crystallographic origins of the differences in thermal stability between the Ru01 and Ru02 MS2 states. These interactions were visualized by comparing the Hirshfeld surfaces of the cation with the MS2 SO2 orientation at each crystallographically independent site (Figure 4). Due to the large VDP volumes and correspondingly larger volumes enclosed by the Hirshfeld surface, there were not many intermolecular interactions shorter than the sum of the van der Waals radii, as evidenced by the primarily blue color of the isosurfaces. There are, however, two notably short interactions: first, the interaction between the Ofree-H on the neighboring tosylate anion (labeled ‘a’ in Figure 4); second, the Sbound−O interaction (labeled ‘b’ in Figure 4). The Ofree−H interaction appears to stabilize the MS2 geometry with the thermal stability increasing with the decreasing interaction length: 2.50(1) Å (1 Ru01), 2.55(1) Å (2 Ru01), 2.72(1) Å (1 Ru02) and 2.75(2) Å (2 Ru02), c.f. the sum of the O−H van der Waals radii is 2.72 Å. In contrast, the Sbound−O interaction is similar across both 1 and 2 in each of the crystallographic sites, with interaction lengths of 3.07(1) Å (1 Ru01), 3.05(1) Å (1 Ru02), 3.04(1) Å (2 Ru01), and 3.04(1) Å (2 Ru02), suggesting that this interaction does not contribute to the increased thermal stability observed in Ru01. In conclusion, this work demonstrates the large effect that, on one hand, the reaction cavity has on the geometry and photoconversion levels of the metastable states in photoisomerizable crystals, and on the other hand, the intermolecular interactions have on their thermal stability. By examining two different compounds, each with two crystallographically distinct, but chemically identical, metal complexes we can attribute the changes in photophysical properties to the reaction cavity shape and immediate solid-state environment. The reaction cavity volume has been shown to dictate the static geometry of the metastable states, and more photoisomerization was observed when there was a larger free volume around the SO2. Furthermore, a tendency for the photoexcited geometry to isomerize toward the oxygen (in the GS SO2) with the greatest free volume around it has been demonstrated. The reaction cavity therefore really serves as a “lock-and-key” system, ensuring a symbiosis of structure with photophysical properties at the level of single-molecule distinction. Regarding the structural dynamics, we have shown that the decay of the MS2 geometry is primarily dictated by intermolecular interactions; to this end, the onset of thermal decay of the MS2 geometry can vary by an order of 20 K from one molecule to the next, within the same compound, i.e. once again, singlemolecule resolution is observed. We have both demonstrated and quantified these findings via in situ photocrystallography. Developing a design protocol to control the reaction cavity is notoriously difficult. However, these results form a fundamental basis by which crystal engineering strategies can seek to tailor the free volume and intermolecular interactions surrounding the photoisomerizable SO2 ligand in order to tune the photophysical properties of photoisomers with single-molecule discrimination. If such tuning can be achieved, the singlemolecule optical recognition demonstrated in these materials will have very significant implications for their potential application in optical data storage.

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ASSOCIATED CONTENT

* Supporting Information S

Experimental procedures, refinement details, and selected crystallographic information. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS S.O.S. thanks the Cambridge Commonwealth Trust for a graduate student scholarship. J.M.C. thanks the Royal Society for a University Research Fellowship, and the University of New Brunswick, for the UNB Vice-Chancellor’s Research Chair.

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REFERENCES

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