The structure of the lowest excited singlet (S1) state of 4,4′-bipyridine: a picosecond time-resolved Raman analysis

Electrochemical behavior of N-methyl-N'-carboxydecyl-4,4'-bipyridinium probed by surface-enhanced Raman spectroscopy

Interfacial structure determines the activity and selectivity of a sensor and plays important roles in interfacial electrochemistry, electroanalysis, biosensing, etc. In situ electrochemical Raman spectroscopy appears to be a powerful tool to probe the electrochemical interface and surface process by providing the molecular fingerprint information. Herein, the electrochemical behaviors of N-methyl-N'-carboxydecyl-4,4'-bipyridinium (derivatives of methyl viologens, MV(2+)) with different alkyl chain lengths (n=2 and 10) on roughened Au electrodes were systematically investigated by the electrochemical surface-enhanced Raman spectroscopy (SERS). Three systems with different interfacial structures were constructed. One is to anchor the MV(2+) molecules via esterification with the 2-mercaptoethanol molecule pre-assembled on the Au surface. The second system is similar to the first one but without esterification. The third system is the direct adsorption of MV(2+) molecules on the bare roughened Au surfaces. The three systems gave different spectral response upon the change of the electrode potential. A drastically increased relative Raman intensity of 19a/8a modes of the MV(2+) molecules was observed at negative potentials. The phenomenon is attributed to the formation of the reduced form of MV(2+) molecules, which produces resonant Raman effect to enhanced the signal of 19a mode. The third system showed the highest electrochemical reduction activity towards the reduction of MV(2+) molecules, followed by the first and second systems. The result indicates that the interfacial structure can sensitively influence electrochemical activity of the electrode.

Probing the dynamics of solvation and structure of the OH- ion in aqueous solution from picosecond transient absorption measurements

The reaction of intracomplex proton transfer (44BPY^-....HO-H) → 44BPYH^. + OH^- that follows the photoreduction of 4,4’-bipyridine (44BPY) into its anion radical 44BPY^- in the presence of 1,4-diazabicyclo[2.2.2]octane (DABCO) is investigated in acetonitrile-water mixtures by using picosecond transient absorption. The dependence of the appearance kinetics of the 44BPYH^. radical on the water content reveals a highly diffusional proton transfer process that is controlled by the dynamics of solvation of the released hydroxide ion. The results are interpreted on the basis of a two-step mechanism where an intermediate solvation complex (44BPYH^.)OH^-(H₂O)₃ is formed first before evolving toward a final four-water hydration structure OH^-(H₂O)₄.

Photoreduction of 4,4‘-Bipyridine by Amines in Acetonitrile−Water Mixtures: Influence of H-Bonding on the Ion-Pair Structure and Dynamics

The photoreduction of 4,4'-bipyridine (44BPY) by diazabicyclo[2.2.2]octane and triethylamine (TEA) is investigated by using picosecond transient absorption and time-resolved resonance Raman spectroscopy in various acetonitrile-water mixtures. The results are interpreted on the basis of a preferential solvation effect resulting from the presence of a specific interaction between 44BPY and water by hydrogen bonding. Below 10% water, the free 44BPY species is dominant and leads upon photoreduction to a contact ion pair that undergoes efficient intrapair proton transfer if TEA is the amine donor. Above 10% water, most of the 44BPY population is H-bonded and leads upon photoreduction to a hydrated ion pair in which the intrapair proton transfer is inhibited. Instead, the 44BPY(-*) species is protonated by water through the hydrogen bond with a rate constant that increases by more than 3 orders of magnitude on going from 10% to 100% water. The dependence of this rate constant on the solvent mixture composition suggests that the reaction of intracomplex proton transfer is controlled by the hydration of the residual OH(-) species by three molecules of water, leading to a trihydrated HO(-)(H(2)O)(3) species.