Femtosecond Isomerization in a Photochromic Molecular Switch

Abstract

The concept of molecular information storage has long served as inspiration for chemists. [1][2][3] Light is often conceived as the trigger to switch between two (or more) molecular ground states. Light energy performs work on the molecule, thus storing photonic energy as potential energy. Maximizing the work performed on a molecule represents a new strategy in the design of molecular-based information systems. The development of bistable molecules with efficient switching mechanisms is tantamount to success in this field. Photochromic compounds are excellent candidates in this regard. These molecular devices convert light energy to potential energy for excited-state bond rupture and bond construction. Phototriggered molecular motions in stilbenes, azobenzenes, dithienylethenes, overcrowded alkenes, and spiro compounds demonstrate the versatility of organic-based structures that feature this reactivity.[4] An important aspect of this approach is that the switching reactions between states must be rapid to maximize the work performed on the molecule.Photochromic complexes based on ruthenium or osmium polypyridyl complexes have further appeal, as their electrochemical signatures provide an independent means of monitoring the color changes associated with these photoactive complexes. Our efforts in this field have focused on ruthenium and osmium sulfoxide complexes that feature intramolecular excited-state S!O and ground-state O!S isomerization reactions. The change in ligation shifts the redox potential E8' (M 3+/2+ ) between 0.3 V and 0.8 V for these two states, depending upon the compound. [5,6] The time constant for excited-state isomerization has been measured to be as rapid as 475 ps, with isomerization quantum yields as great as 0.80. [7,8] Recently, we incorporated the sulfoxide moiety within a chelate, while retaining the photochromic action associated with the sulfoxide. [9,10] We reasoned that limiting the degrees of freedom of the bound sulfoxide would provide an excited-state O!S isomerization pathway, thus maximizing work and limiting heat loss after excitation. Herein, we report a photochromic ruthenium disulfoxide complex that exhibits excited-state S!O and excited-state O!S isomerization on a femtosecond timescale by two different colors of light.The photochromic disulfoxide complex [Ru(bpy) 2 -(OSSO)] 2+ (bpy = 2,2'-bipyridine, OSSO = dimethylbis(methylsulfinylmethyl)silane) is prepared through oxidation of the dithioether parent with a peroxide oxidant, chloroperoxybenzoic acid (m-CPBA). The molecular structures of both ruthenium complexes were confirmed by single crystal X-ray diffractometry, and that of the disulfoxide complex is shown in Figure 1 . [11][12][13] Similar to [Ru(bpy) , the prototypical molecule in this class, the lowest energy transition is assigned as a Ru dp!bpy p* charge-transfer (CT) transition. Previous studies of [Ru(bpy) 3 ] 2+ have demonstrated femtosecond intersystem crossing following excitation to form a thermally

Isomerization in Photochromic Ruthenium Sulfoxide Complexes

“…[38,41] While this behavior has been observed in certain organic photochromes notably dithienylethenes, it remains rare in transition metal chemistry. We know of only a few examples of reversible, two color photochromic action in transition metal complexes.…”

“…[41] The ground state is a bis-S-bonded ground state, a meta- 2+ . Charge-Transfer excitation of the S,S isomer yields the S,O isomer and then the O,O-isomer in sequential photochemical reactions.…”

Effect of the Metal Ion on the Enantioselectivity and Linkage Isomerization of Thiosemicarbazone Helicates

“…[8] Other type of isomerism is the linkage isomerism,w hich is as tructural event that can be observed in complexes with ambidentate ligands. [9] Linkage isomerization can be stimulated by external factors sucha sr edox changes in the metal ion, [10] photoirradiation, [11] temperature changes, [12] or solvation. [13] These processes open up new pathways for the application of linkage isomers in optical devices or for information storagea tt he molecular level.…”

Holographic performance of silicon polymer films based on photoswitchable molecules

“…Careful adjustment of the optical beam paths ensured a high-contrast fringe pattern (modulation depth m = 2 √ I R × I S /(I R + I S ) ≈ 0.88 at a total intensity of I R + I S = (19.6 ± 0.2) mW/cm 2 , where I R and I S are the intensities of the reference and signal beams, respectively). In bright regions, the ultraviolet exposure triggers a Ru dπ → bpy π * metal-to-ligand-chargetransfer (MLCT) into an S-bonded excited state of the Ru-S-O bond [20][21][22][23]. This excited state may relax vibrationally to one of the two O-bonded (Ru-O-S bond) metastable states (MS 1 or MS 2 ) as schematically depicted in Fig.…”