[Ru(bpy)(3)](2+) as a reference in transient absorption spectroscopy: differential absorption coefficients for formation of the long-lived (3)MLCT excited state.

Transient absorption spectroscopy and other time-resolved methods are commonly used to study chemical reactions and biological processes induced by absorption of light. In order to scale the signal amplitude or to compare results obtained under different conditions, it is advisable to use a reference system, a standard of convenient and well-defined properties. Finding Tris(bipyridine)ruthenium(ii), [Ru(bpy)(3)](2+), a suitable candidate for a transient-absorption spectroscopy reference due to its favourable photochemical properties, we have determined accurate relative values of differential molar absorption coefficients (Δε) for light-induced formation of the metal-to-ligand charge transfer (MLCT) excited triplet state at several relevant wavelengths (wavelengths of commercially available lasers) in the UV and visible regions. We have also attempted to determine the absolute value of Δε close to the wavelength of maximum bleaching (∼450 nm) and we propose to narrow down the interval of conceivable values for Δε(450) from the broad range of published values (-0.88 × 10(4) M(-1)cm(-1) to -1.36 × 10(4) M(-1)cm(-1)) to -1.1 × 10(4) M(-1)cm(-1)± 15%. Having ourselves successfully applied [Ru(bpy)(3)](2+) as a standard in a recent time-resolved study of enzymatic DNA repair, we would like to encourage other scientists to use this convenient tool as a reference in their future spectroscopic studies on time scales from picoseconds to hundreds of nanoseconds.