Towards systematic design of optical data storage materials

Materials with metastable excited states hold great potential for optical data storage applications. Their functionality arises directly from their ability to act as binary switches: if the ground state is taken to signify a ‘0’ and the metastable state a ‘1’, data can be written and re-read using suitable wavelengths of light. However, only a handful of such materials have been reported and little is known about the correlation between their structure and optical properties. As part of an ongoing investigation of these materials, in recent experiments at beamline I19 we have determined the metastable excited-state structures of two new ruthenium-sulfur dioxide complexes. Our results demonstrate the importance of crystal packing effects in determining the observed geometry of the excited state. The insight thus provided will help to direct crystal engineering efforts to produce materials suitable for data storage applications. Further, these results represent an important technical development on beamline I19, which heralds excited-state crystallography as one of its scientific priorities.

In an era characterised by ever-increasing amounts of data being generated and stored electronically, the search for more efficient and compact methods of storing this data is of great importance. Techniques such as holographic data storage offer great potential, but their successful deployment will rely crucially on the identification of suitable materials. To this end, we have investigated a series of linkage isomerism complexes: systems in which photoexcitation of a metal-ligand charge transfer band causes the molecular geometry to rearrange. Due to the significant technical challenges of determining photoinduced crystal structures [1], only a few such complexes have had their excited state structures reported, and little is understood about the correlations between their structures and the properties which would make them useful in applications. For instance, a wide range of quantum yields has been reported, with 100% excitation being very rare [2], yet high yields will be essential in data storage applications for ease of reading.

Figure 1: Ground and metastable state geometries in ruthenium-sulfur dioxide complexes.

Our research program has focused on the general class of compounds [Ru(SO2)(NH3)4X]Y, in which the SO2 ligand, which exhibits regular S-bound geometry in the ground state, can be excited to side-bound (η2) and O-bound geometries (Fig. 1). In the course of two recent experiments at beamline I19, we have solved the photoexcited structures of two new members of this family: the aqua camphorsulfonate (X=H2O, Y=(±)-(C10H15SO3)2) and the isonicotinamide tosylate (X=NC5H4CONH2, Y=(CH3C6H4SO3)2). Each of these compounds contains a newly introduced moiety X or Y and one common to previously studied complexes [3]. Of course, changes in the structure of individual ions, however small or systematic, do not map predictably onto changes in the crystal structure formed by packing such ions optimally. Thus these two complexes also represent in some sense entirely new data points from which to draw trends between photoexcitation behaviour and the chemical and structural characteristics that give rise to it.

Single-crystal X-ray diffraction data on these compounds were collected on beamline I19 at Diamond Light Source. A full ground-state structure was collected in the dark at 100 K; the crystal was then illuminated with a focused beam of light (from a tungsten lamp for the aqua camphorsulfonate, a xenon lamp for the isonicotinamide tosylate) for two hours. The same collection strategy was then repeated to obtain “light” data. A Fourier difference map was used to compare the “light” data with the “dark” structural model to reveal the extent of excitation and geometry of the excited state. The excited-state structure was refined to afford the quantum yield and geometric parameters. These were comparable with previous results on related systems: in particular, in the metastable state the S-O bond bound to Ru is longer than the free S-O bond, as expected for a bond weakened by coordination.

Figure 2: Ground (dark) and metastable (light) geometries observed in (a) the aqua camphorsulfonate, and (b) and (c) the two independent sites in the isonicotinamide tosylate. For clarity only the ligated N atom of the isonicotinamide is shown. The four possible orientations are labelled A to D. Ruthenium atoms are shown in orange, nitrogen in blue, and hydrogen in grey. In (a) and (c), sulfur atoms are shown in yellow and oxygen in red; in (b), where some oxygen and sulfur atoms overlap, the colours reflect the three observed excitation geometries.

In the gas phase there are four degenerate geometries for the excited state, but in solid-state experiments not all of these are observed. The isonicotinamide tosylate is particularly interesting in this regard, as it contains two crystallographically independent ruthenium centres which yield different excited-state geometries. In the first, only a single geometry was observed, while in the second, the excited-state population was spread over three out of the four possible geometries (Fig. 2). As these centres are chemically identical and, as part of the same crystal, were exposed to identical experimental conditions, the differences between the observed geometries can be unambiguously attributed to local crystal packing effects.

Our results were supported by solid-state DFT calculations, which demonstrated good correlation between the energetic favourability of the possible metastable state geometries and their observed quantum yields. On the other hand, the Dirichlet volume of the “reaction cavity” in which the SO2 ligand rotates was not a good predictor of the yields obtained. This confirms the importance of the shape of the reaction cavity in determining the properties of the metastable state.

Our calculations further suggest that suitable restraints on the position of the free O atom favour the O-bound metastable state, which has only been experimentally achieved at much lower temperature (12 K) [4]. These results strongly imply that the shape of the reaction cavity is the aspect of the crystal structure that will require careful optimisation to produce materials suitable for data storage applications. It is of course notoriously difficult to predict the effects on the structure of a molecular crystal of changes in that of its components, but we believe that embedding the photoactive centres within a rigid framework may be a useful tactic and are further investigating the possibilities this might afford.

In conclusion, photoinduced isomerism has been experimentally observed in two new complexes in the rutheniumsulfur dioxide family: the aqua camphorsulfonate and the isonicotinamide tosylate. While the general characteristics of these excited states are similar to previously reported members of this family, the geometric detail varies substantially, even within the one compound, as a function of the local environment. In particular, the two Ru sites in the isonicotinamide tosylate structure provide a sort of “internal standard” for each other: as excitation at these sites necessarily occurs under identical experimental conditions and to chemically identical species, geometric differences can be directly attributed to local crystal-packing effects. We have shown using DFT calculations that, although the crystal lattice is not sufficiently rigid to prevent excitation from occurring at all, it has a strong influence on the relative occupancies observed for the four possible geometries, degenerate in the gas phase, of the excited state. Our results afford further understanding of factors impacting both the quantum yield and the geometric manifestation of optical conversion. This leads towards the ultimate goal of being able to tailor linkage isomerism compounds to meet the technical needs of the data storage industry.

References:
[1] P. Coppens, D. V. Fomitchev, M. D. Carducci, and K. Culp, J. Chem. Soc., Dalton Trans., 865 (1998); J. M. Cole, Chem. Soc. Rev. 33, 501 (2004).
[2] M. R. Warren, S. K. Brayshaw, A. L. Johnson, et al., Angew. Chem. Int. Ed. 48, 5711 (2009).
[3] A. Y. Kovalevsky, K. A. Bagley, and P. Coppens, J. Am. Chem. Soc. 124, 9241 (2002); A. Y. Kovalevsky, K. A. Bagley, J. M. Cole, and P. Coppens, Inorg. Chem. 42, 140 (2003).
[4] K. F. Bowes, J. M. Cole, S. L. G. Husheer, et al., Chem. Comm. 2448 (2006).

Principal Publications and Authors
A. E. Phillips, J. M. Cole, T. d’Almeida, K. S. Low, Phys. Rev. B, (submitted), 2010.

Funding Acknowledgement
Trinity College, Cambridge, the Royal Society , the Leverhulme Trust, and the University of New Brunswick. Engineering and Physical Sciences Research Council (ESPRC) Grant (EP/P504120/1).