Jeffrey J. Rack

Two-Color Reversible Switching in a Photochromic Ruthenium Sulfoxide Complex†

Reversible, external triggering of molecules between two ground states is central to the operation of modern molecular machines. A desirable trigger is light, as it provides an ample amount of energy in a short burst of time. Although light is commonly employed for the formation of many different types of metastable states, in practice it is difficult to employ light to regenerate the initial ground state. The enthalpic and entropic factors that favor the photochemical generation of the metastable state from the ground state must necessarily be disfavored for the opposing photochemical reaction. Moreover, the photoproduct often reverts to the ground state thermally. Irradiation of photochromic compounds yields metastable states that exhibit distinct electronic structures compared to their ground states. Whilst a number of organic photochromic compounds feature reversible or two-color photoswitching between metastable and ground states, such reactivity is rare in transition metal complexes. Herein, we report our findings on a ruthenium sulfoxide complex that features two-color photochromism. The complex [Ru(bpy)2(pySO)](PF6)2 (bpy = 2,2’-bipyridine, pySO = 2-(isopropylsulfinylmethyl)pyridine) was prepared from reaction of cis-[Ru(bpy)2Cl2] with pySO. The structure and formulation of the complex was verified by oneand-two-dimensional H NMR, IR spectroscopy, and elemental analysis. The H NMR spectrum is consistent with the presence of two isomers, which are most likely diastereomers, as both the ruthenium and sulfur centers are chiral. Elemental analysis of the isomeric mixture supports this assignment. The lowest-energy transition in the electronic spectrum occurs at 370 nm (e = 7250 cm m ) and is assigned to a Ru dp!bpy p* charge-transfer transition based on its energy and intensity (Figure 1). This transition is similar to that observed for other chelating S-bonded sulfoxides. Furthermore, the structurally characterized complex [Ru(bpy)2(py)(dmso)] 2+ (dmso = dimethylsulfoxide), which has no chelation, features a maximum absorbance at 400 nm for the S-bonded isomer. Therefore, the chelate complex exhibits an additional 2000 cm 1 of stabilization relative to the non-chelate form. White light irradiation of [Ru(bpy)2(pySO)] 2+ in propylene carbonate solution results in a decrease in the absorbance at 370 nm concomitant with an increase at 472 nm (Figure 1, inset). In accord with other ruthenium sulfoxide complexes, the spectral changes indicate an intramolecular S!O isomerization, with an isosbestic point at 408 nm. However, the observed spectral changes are muted in comparison to these other complexes. We questioned whether or not this spectrum represented a photostationary state. Accordingly, irradiation of this solution with 355 nm light resulted in a more pronounced absorbance at 472 nm (quantum efficiency FS!O = 0.11(2)). This value is similar to that of the Obonded isomer of [Ru(bpy)2(py)(dmso)] 2+ (476 nm) and [Ru(bpy)2(pic)] + (483 nm; pic = 2-pyridinecarboxylate), which feature N and O donors in the Ru(bpy)2 coordination sphere. The S-bonded and extracted O-bonded spectra (e472nm = 6400 cm m ) are shown in Figure 1. The IR spectrum provides structural evidence for isomerization; the ground-state S-bonded isomer has a band at ñ(S=O) = 1090 cm 1 which is replaced by a band at ñ(S=O) = 1060 cm 1 upon irradiation. This new feature is ascribed to the O-bonded isomer. The presence of a photostationary state with white light irradiation indicates an excited-state photochemical pathway for O!S isomerization from longer wavelengths. Accordingly, 470 nm irradiation of a solution that contains predominately the O-isomer resulted in a decrease of the absorbance at 472 nm concomitant with an increase at 370 nm (FO!S = Figure 1. Absorption of S-bonded (c) and O-bonded (b) isomers. The spectrum of the O-bonded isomer was extrapolated from the photostationary state spectrum. Inset: Spectrum depicting the photostationary state obtained following white-light irradiation of the S-bonded isomer.

pH control of intramolecular energy transfer and oxygen quenching in Ru(II) complexes having coupled electronic excited states.

This work illustrates the control of excited state energy transfer processes via variation of pH in transition metal complexes. In these systems a Ru(II) complex having two carboxylated bipyridyl ligands is covalently linked to pyrene via one of two different pyrene derivitized bipyridyl ligands. The energy of the Ru to carboxy-bipyridine (3)MLCT state is pH dependent while the pyrene triplet energy remains unchanged with solution acidity. At pH 0 the (3)MLCT state is the lowest energy state, and as the pH is raised and the carboxy-bipyridyl ligands are successively deprotonated, the energy of the (3)MLCT state rises above that of the pyrene triplet, resulting in a significant increase in the lifetime of the observed emission. Detailed analysis of ultrafast and microsecond time-resolved excited state decays result in a description of excited state decay that involves initial equilibration of the (3)MLCT and pyrene triplet states followed by relaxation to the ground state. The lifetime of excited state decay is defined by the position of the equilibrium, going from 2 μs at pH 0 to >10 μs at higher pH as the equilibrium favors the pyrene triplet. In addition, quenching of the excited state by dissolved oxygen exhibits a pH dependence that parallels that of the excited state lifetime. The results illustrate the utility of exploiting excited state equilibria of this type in the development of highly effective luminescent oxygen sensors.

One Photon Yields Two Isomerizations: Large Atomic Displacements during Electronic Excited-State Dynamics in Ruthenium Sulfoxide Complexes

Photochromic compounds efficiently transduce photonic energy to potential energy for excited-state bond-breaking and bond-forming reactions. A critical feature of this reaction is the nature of the electronic excited-state potential energy surface and how this surface facilitates large nuclear displacements and rearrangements. We have prepared two photochromic ruthenium sulfoxide complexes that feature two isomerization reactions following absorption of a single photon. We show by femtosecond transient absorption spectroscopy that this reaction is complete within a few hundred picoseconds and suggest that isomerization occurs along a conical intersection seam formed by the ground-state and excited-state potential energy surfaces.

Pronounced photosensitivity of molecular [Ru(bpy) 2 (OSO)] + solutions based on two photoinduced linkage isomers

Photosensitive properties of [Ru(bpy)(2)(OSO)] PF(6) dissolved in propylene carbonate originating from photoinduced linkage isomerism have been studied by temperature and exposure dependent transmission and UV/Vis absorption spectroscopy. An exceeding photochromic photosensitivity of S = (63 +/- 10) x 10(3) cm l J(-1) mol(-1) is determined. It is attributed to a maximum population of 100% molecules in the photoinduced isomers, a unique absorption cross section per molecule and a very low light exposure of Q(0) = (0.25 +/- 0.03) Ws cm(-2) for isomerism. Relaxation studies of O-bonded to S-bonded isomers at different temperatures verify the existence of two distinct structures of linkage isomers determined by the activation energies of E(A,1) = (0.76 +/- 0.08) eV and E(A,2) = (1.00 +/- 0.14) eV.

Excited state dynamics and isomerization in ruthenium sulfoxide complexes.

Molecular photochromic compounds are those that interconvert between two isomeric forms with light. The two isomeric forms display distinct electronic and molecular structures and must not be in equilibrium with one another. These light-activated molecular switch compounds have found wide application in areas of study ranging from chemical biology to materials science, where conversion from one isomeric form to another by light prompts a response in the environment (e.g., protein or polymeric material). Certain ruthenium and osmium polypyridine sulfoxide complexes are photochromic. The mode of action is a phototriggered isomerization of the sulfoxide from S- to O-bonded. The change in ligation drastically alters both the spectroscopic and electrochemical properties of the metal complex. Our laboratory has pioneered the preparation and study of these complexes. In particular, we have applied femtosecond pump-probe spectroscopy to reveal excited state details of the isomerization mechanism. The data from numerous complexes allowed us to predict that the isomerization was nonadiabatic in nature, defined as occurring from a S-bonded triplet excited state (primarily metal-to-ligand charge transfer in character) to an O-bonded singlet ground state potential energy surface. This prediction was corroborated by high-level density functional theory calculations. An intriguing aspect of this reactivity is the coupling of nuclear motion to the electronic wave function and how this coupling affects motions productive for isomerization. In an effort to learn more about this coupling, we designed a project to examine phototriggered isomerization in bis-sulfoxide complexes. The goal of these studies was to determine whether certain complexes could be designed in which a single photon excitation event would prompt two sulfoxide isomerizations. We employed chelating sulfoxides in this study and found that both the nature of the chelate ring and the R group on the sulfoxide affect the photochemical reactivity. For example, this reactivity may be tuned such that two sulfoxide ligands isomerize sequentially following two successive excitations or that two sulfoxide ligands isomerize following a single excitation. This Account explains our understanding to date of this photochemistry.

Fluorescence probing of T box antiterminator RNA: Insights into riboswitch discernment of the tRNA discriminator base

The T box transcription antitermination riboswitch is one of the main regulatory mechanisms utilized by Gram-positive bacteria to regulate genes that are involved in amino acid metabolism. The details of the antitermination event, including the role that Mg(2+) plays, in this riboswitch have not been completely elucidated. In these studies, details of the antitermination event were investigated utilizing 2-aminopurine to monitor structural changes of a model antiterminator RNA when it was bound to model tRNA. Based on the results of these fluorescence studies, the model tRNA binds the model antiterminator RNA via an induced-fit. This binding is enhanced by the presence of Mg(2+), facilitating the complete base pairing of the model tRNA acceptor end with the complementary bases in the model antiterminator bulge.

Broadband femtosecond transient absorption spectroscopy for a CVD Mo S 2 monolayer

Carrier dynamics in monolayer

Complexes with Tunable Intramolecular Ferrocene to Ti(IV) Electronic Transitions: Models for Solid State Fe(II) to Ti(IV) Charge Transfer.

\mathrm{Mo}{\mathrm{S}}_{2}

Investigating the effects of solvent on the ultrafast dynamics of a photoreversible ruthenium sulfoxide complex.

have been investigated using broadband femtosecond transient absorption spectroscopy (FTAS). A tunable pump pulse was used while a broadband probe pulse revealed ground and excited state carrier dynamics. Interestingly, for pump wavelengths both resonant and nonresonant with the A and B excitons, we observe a broad ground state bleach around 2.9 eV, with decay components similar to A and B. Associating this bleach with the band nesting region between

Stimulating changes in the elastic modulus of polymer materials by molecular photochromism

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