Optical and Infrared Spectroelectrochemical Studies of CN-Substituted Bipyridyl Complexes of Ruthenium(II)

Abstract

Ruthenium(II) polypyridyl complexes [Ru(CN-Me-bpy) x (bpy) 3−x ] 2+ (CN-Me-bpy = 4,4′-dicyano-5,5′-dimethyl-2,2′-bipyridine, bpy = 2,2′-bipyridine, and x = 1−3, abbreviated as 1 2+ , 2 2+ , and 3 2+ ) undergo four (1 2+ ) or five (2 2+ and 3 2+ ) successive one-electron reduction steps between −1.3 and −2.75 V versus ferrocenium/ferrocene (Fc + /Fc) in tetrahydrofuran. The CN-Me-bpy ligands are reduced first, with successive one-electron reductions in 2 2+ and 3 2+ being separated by 150−210 mV; reduction of the unsubstituted bpy ligand in 1 2+ and 2 2+ occurs only when all CN-Me-bpy ligands have been converted to their radical anions. Absorption spectra of the first three reduction products of each complex were measured across the UV, visible, near-IR (NIR), and mid-IR regions and interpreted with the help of density functional theory calculations. Reduction of the CN-Mebpy ligand shifts the ν(CN) IR band by ca. −45 cm −1 , enhances its intensity ∼35 times, and splits the symmetrical and antisymmetrical modes. Semireduced complexes containing two and three CN-derivatized ligands 2 + , 3 + , and 3 0 show distinct ν(C N) features due to the presence of both CN-Me-bpy and CN-Me-bpy •− , confirming that each reduction is localized on a single ligand. NIR spectra of 1 0 , 1 − , and 2 − exhibit a prominent band attributable to the CN-Me-bpy •− moiety between 6000 and 7500 cm −1 , whereas bpy •− -based absorption occurs between 4500 and 6000 cm −1 ; complexes 2 + , 3 + , and 3 0 also exhibit a band at ca. 3300 cm −1 due to a CN-Me-bpy •− → CN-Me-bpy interligand charge-transfer transition. In the UV−vis region, the decrease of π → π* intraligand bands of the neutral ligands and the emergence of the corresponding bands of the radical anions are most diagnostic. The first reduction product of 1 2+ is spectroscopically similar to the lowest triplet metal-to-ligand charge-transfer excited state, which shows pronounced NIR absorption, and its ν(CN) IR band is shifted by −38 cm −1 and 5−7-fold-enhanced relative to the ground state.

Good Vibrations Report on the DNA Quadruplex Binding of an Excited State Amplified Ruthenium Polypyridyl IR Probe

“…In our study, a significant photoenhancement of the nitrile stretching vibration is observed for the MCLT excited state of 1 formed upon 400 nm excitation (Figure ). A similar enhancement has been observed for aromatic nitriles and CN substituted bipyridyl complexes of ruthenium­(II). , However, this is the first time that such an effect is reported for an intercalating probe. This enhancement serves to amplify the environmental response of the nitrile vibration, which is found to display a linear shift to the DMSO/water environment and builds on observations of the solution response of nitriles in the ground state …”

“…18 cm –1 from the position of the bleach ground state absorption, indicating the presence of a weaker CN bond in the 3 MLCT* excited state relative to the ground state. The downshift of the ν­(CN) has been previously observed for ruthenium polypyridyl complexes and is attributed to the population of a ligand based π* antibonding orbital, which weakens the ν­(CN) bonds. , …”

“…The downshift of the ν(C�N) has been previously observed for ruthenium polypyridyl complexes and is attributed to the population of a ligand based π* antibonding orbital, which weakens the ν(C�N) bonds. 81,82 The C�N stretching vibration is known to be sensitive to the HB environment. 63,65,67,83 As the HB environment of 1 is expected to decrease upon DNA binding, the nitrile transient was recorded in different protic environments by increasing the water content in a DMSO/water mixture from 0% to 100% in 25%(v/v) increments (see Figure 3a).…”

The Contribution of Density Functional Theory to the Atomistic Knowledge of Electrochromic Processes

“…In recent years, however, the investigation has expanded to other promising organic and inorganic systems including those containing boron [ 41 , 42 , 43 ], quinodimethane derivatives [ 44 , 45 ], metal-phtalocyanine [ 46 ], metal oxides-based film [ 7 , 47 , 48 ], oligoaniline [ 49 ], antracenes [ 50 , 51 ], azacenes [ 52 ], polymethine and polyimides [ 53 , 54 ], and some organometallic complexes [ 55 , 56 ].…”

Photostable Ruthenium(II) Isocyanoborato Luminophores and Their Use in Energy Transfer and Photoredox Catalysis

“…Direct excitation of the substrates to their triplet excited states is spin-forbidden, and intersystem crossing from singlet to triplet excited states is frequently very inefficient in organic compounds; hence, metal-based triplet sensitizers are vital for many of these reactions . Iridium­(III) photosensitizers are often chosen due to their high triplet energies ( E T , up to 2.87 eV), long excited-state lifetimes, and their thermal and photostability. − In recent years, photosensitizers based on earth-abundant metals have gained increasing attention, − but many of them still require further development to become as widely applicable as their precious metal-based congeners, and until now, ruthenium­(II) and iridium­(III) sensitizers have remained the workhorses of synthetic photochemistry and applications in solar energy conversion. − Against this background, the development of ruthenium­(II)- and iridium­(III)-based photosensitizers continues to be important. − …”