Synthesis and characterization of dinuclear ruthenium complexes covalently linked to Ru(II) tris-bipyridine: an approach to mimics of the donor side of photosystem II

To mimic the electron-donor side of photosystem II (PSII), three trinuclear ruthenium complexes (2, 2a, 2b) were synthesized. In these complexes, a mixed-valent dinuclear Ru2(II,III) moiety with one phenoxy and two acetato bridges is covalently linked to a Ru(II) tris-bipyridine photosensitizer. The properties and photoinduced electron/energy transfer of these complexes were studied. The results show that the Ru2(II,III) moieties in the complexes readily undergo reversible one-electron reduction and one-electron oxidation to give the Ru2(II,III) and Ru2(III,III) states, respectively. This could allow for photooxidation of the sensitizer part with an external acceptor and subsequent electron transfer from the dinuclear ruthenium moiety to regenerate the sensitizer. However, all trinuclear ruthenium complexes have a very short excited-state lifetime, in the range of a few nanoseconds to less than 100 ps. Studies by femtosecond time-resolved techniques suggest that a mixture of intramolecular energy and electron transfer between the dinuclear ruthenium moiety and the excited [Ru(bpy)3]2+ photosensitizer is responsible for the short lifetimes. This problem is overcome by anchoring the complexes with ester- or carboxyl-substituted bipyridine ligands (2a, 2b) to nanocrystalline TiO2, and the desired electron transfer from the excited state of the [Ru(bpy)3]2+ moiety to the conduction band of TiO2 followed by intramolecular electron transfer from the dinuclear Ru2(II,III) moiety to photogenerated Ru(III) was observed. The resulting long-lived Ru2(III,III) state decays on the millisecond timescale.     

Carboxymethylated nanofibrillated cellulose: rheological studies

The rheological properties of carboxymethylated nanofibrillated cellulose (NFC), investigated with controlled shear rate- and oscillatory measurements, are reported for the first time. It was shown that the rheological properties of the studied system are similar to those reported for other NFC systems. The carboxymethylated NFC systems showed among other things high elasticity and a shear thinning behaviour when subjected to increasing shear rates. Further, the shear viscosity and storage modulus of the system displayed power-law relations with respect to the dry content of the NFC suspension. The exponential values, 2 and 2.4 respectively, were found to be in good agreement with both theoretical predictions and published experimental work. Furthermore, it was found that the pulp consistency at which NFC is produced affects the properties of the system. The rheological studies imply that there exists a critical pulp concentration below which the efficiency of the delamination process diminishes; the same adverse effect is also observed when the critical concentration is significantly exceeded due to a lower energy input during delamination.     

Hot Branching Dynamics in a Light‐Harvesting Iron Carbene Complex Revealed by Ultrafast X‐ray Emission Spectroscopy

Iron N‐heterocyclic carbene (NHC) complexes have received a great deal of attention recently because of their growing potential as light sensitizers or photocatalysts. We present a sub‐ps X‐ray spectroscopy study of an Fe^IINHC complex that identifies and quantifies the states involved in the deactivation cascade after light absorption. Excited molecules relax back to the ground state along two pathways: After population of a hot ³MLCT state, from the initially excited ¹MLCT state, 30 % of the molecules undergo ultrafast (150 fs) relaxation to the ³MC state, in competition with vibrational relaxation and cooling to the relaxed ³MLCT state. The relaxed ³MLCT state then decays much more slowly (7.6 ps) to the ³MC state. The ³MC state is rapidly (2.2 ps) deactivated to the ground state. The ⁵MC state is not involved in the deactivation pathway. The ultrafast partial deactivation of the ³MLCT state constitutes a loss channel from the point of view of photochemical efficiency and highlights the necessity to screen transition‐metal complexes for similar ultrafast decays to optimize photochemical performance.     

UV‐Dissipation Mechanisms in the Eumelanin Building Block DHICA

The UV‐dissipative mechanisms of the eumelanin building block 5,6‐dihydroxyindole‐2‐carboxylic acid (DHICA) and the 4,7‐dideutero derivative (DHICA‐d₂) in buffered H₂O or D₂O have been characterized by using ultrafast time‐resolved fluorescence spectroscopy. Excitation of the carboxylate anion form, the dominating state at neutral pH, leads to dual fluorescence. The band peaking at λ=378 nm is caused by emission from the excited initial geometry. The second band around λ=450 nm is owed to a complex formed between the mono‐anion and specific buffer components. In the absence of complex formation, the mono‐anion solely decays non‐radiatively or by emission with a lifetime of about 2.1 ns. Excitation of the neutral carboxylic acid state, which dominates at acidic pH, leads to a weak emission around λ=427 nm with a short lifetime of 240 ps. This emission originates from the zwitterionic state, formed upon excitation of the neutral state by sub‐ps excited‐state intramolecular proton transfer (ESIPT) between the carboxylic acid group and the indole nitrogen. Future studies will unravel whether this also occurs in larger building blocks and ESIPT is a built‐in photoprotective mechanism in epidermal eumelanin.     

A discussion of the problem of determining multiple lifetimes from picosecond absorption recovery data as encountered in two carbocyanine dyes

Absorption recovery kinetics data on the picosecond time-scale of a high signalto-noise ratio can be obtained by using continuously operating synchronously modelocked dye lasers. We show that such data make it possible to determine multiple exponential decays with small differences in rate constants. We also show that an absolute prerequisite to accomplish this is that the measurements are performed at a number of different wavelengths, and that the decays are monitored in a time interval corresponding to several lifetimes. These considerations are demonstrated by measurements on two carbocyanine dyes, DQOCI and pinacyanol, respectively.     

High spin collective excitations in 101Ru

The energy level structure of ¹⁰¹Ru was studied using the reactions ¹⁰⁰Mo(α, 3n)¹⁰¹Ru and ¹⁰⁰Mo(³He, 2n)¹⁰¹Ru. Excitation functions, γγ coincidences and γ-ray angular distributions were measured. Three ΔI = 2 cascades proceeding to a $\text{5}\text{2}{}^{\mathrm{+}}$, $\text{7}\text{2}{}^{\mathrm{+}}$ and $\text{11}\text{2}{}^{\mathrm{?}}$ state were observed. A decay scheme is presented showing energies up to 5849 keV and spins up to $\text{35}\text{2}$. The bands are discussed within the framework of the Nilsson model with Coriolis coupling in which the recoil effect has been properly introduced.