We report photoexcited-state crystal structures for two new members of the [Ru(SO_2)(NH_3)_4X]Y family: 1:X=H2O, Y=(+/-)-camphorsulfonate_2; 2:X=isonicotinamide, Y=tosylate_2. The excited states are metastable at 100 K, with a photoconversion fraction of 11.1(7)% achieved in 1, and 22.1(10)% and 26.9(10)% at the two distinct sites in 2.We further show using solid-state density-functional-theory calculations that the excited-state geometries achieved are strongly influenced by the local crystal environment. This result is relevant to attempts to rationally design related photoexcitation systems for optical data-storage applications.
Deep Dive into Effects of the reaction cavity on metastable optical excitation in ruthenium-sulfur dioxide complexes.
We report photoexcited-state crystal structures for two new members of the [Ru(SO_2)(NH_3)_4X]Y family: 1:X=H2O, Y=(+/-)-camphorsulfonate_2; 2:X=isonicotinamide, Y=tosylate_2. The excited states are metastable at 100 K, with a photoconversion fraction of 11.1(7)% achieved in 1, and 22.1(10)% and 26.9(10)% at the two distinct sites in 2.We further show using solid-state density-functional-theory calculations that the excited-state geometries achieved are strongly influenced by the local crystal environment. This result is relevant to attempts to rationally design related photoexcitation systems for optical data-storage applications.
Materials with optically accessible metastable states have recently attracted attention for their potential uses in optical data storage. 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 reread using suitable wavelengths of light. 1 Photorefractive properties arising from the existence of metastable states, rather than the Pockels effect, 2 give rise to unusual and potentially useful recording kinetics. 3 However, to date relatively few materials suitable for this purpose have been identified.
One potential source of suitable materials is the field of linkage isomerism complexes. In these systems, photoexcitation of a metal-ligand charge-transfer band causes a rearrangement of the molecular geometry so that the system relaxes to a different electronic ground state determined by the new, metastable nuclear potential. The most extensively studied of these materials is sodium nitroprusside ͑Na 2 ͓Fe͑CN͒ 5 ͑NO͔͒.2H 2 O͒, where the side-bound and oxygen-bound excited states of the NO ligand ͑in contrast to its nitrogen-bound ground state͒ have been identified by neutron 4 and x-ray 5 diffraction. This material has been the subject of density-functional-theory ͑DFT͒ calculations 6 and several reviews. 7 Furthermore, measurable phase gratings have been written in this material at low 8 and ambient temperatures. 9 Similar behavior is also known in other coordination complexes. For example, the general class of compounds ͓Ru͑SO 2 ͒͑NH 3 ͒ 4 X͔Y also exhibits a side-bound and an endbound metastable state ͑Fig. 1͒, studied initially in IR spectra 10 and later via x-ray diffraction and DFT calculations. [11][12][13] If these or related compounds are to be useful in optical data storage, then controlled, ideally complete, conversion to their photoisomers is important for ease of reading. However, only two recently reported systems have approached 100% conversion. 14,15 Unlike spin-crossover systems, for instance, there is little evidence of cooperativity between excited centers. Various factors are known to contribute to the low conversion achieved, including limited optical penetra-tion and the confines imposed by a relatively rigid crystal structure on the nuclear motion. 16 Comparison of gas phase with solid-state calculations has previously shown that in NO complexes the crystal surroundings have a small but important influence over the energy landscape of isomerization, in particular, influencing the barriers associated with rotation about the metal-ligand axis in the side-bound state. 17 In this paper we focus on the triatomic ligand SO 2 , in which the additional, free oxygen atom greatly increases the likelihood of steric interactions between the side-bound state and its crystal surroundings. The “reaction cavity” in which the ligand rotates is capable either of increasing or decreasing observed photoexcitation levels, as work on NO 2 complexes has demonstrated. 18 The dynamics of photoexcitation depend on the interplay between the energy costs of distorting the local and long-range structures. It is known that isomerization can cause sufficient strain on the lattice to crack a crystal. 19 We show that, on the other hand, the lattice in turn constrains the specific excited-state geometry seen in any particular case. I summarizes this and previous work. 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 the two subject complexes also represent in some sense entirely new data points from which to establish trends between photoexcitation behavior 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 the Diamond Light Source. A full ground-state data set was collected in the dark at 100 K. The crystal was then illuminated with a focused beam of light ͑from a tungsten lamp for 1, a xenon lamp for 2͒ for two hours, during which time the crystal was rotated about its mount ͑i.e., the axis͒. A data set for the “light” structure was subsequently collected, using the same parameters as the “dark” collection. A Fourier difference map was used to compare the light data with the dark structural model to reveal the extent of excitation and the geometry of the excited state. The excited-state structure was subsequently refined using a nonlinear least-squares procedure. 20 The crystallographic asymmetric unit of 1 contains a single ruthenium center, which in turn displays a single excitation geometry ͓Fig. 2͑a͔͒. The excited-state atoms were refined anisotropically and their occupancy allowed to vary; this gave a photoconversion fraction of 11.1͑7͒%. 21 By contrast, there are two crystallographically distinct Ru-SO 2 groups in structure 2, which we la
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