Sub-picosecond injection of electrons from excited [Ru(2,2′-bipy-4,4′-dicarboxy)₂(SCN)₂] into TiO₂ using transient mid-infrared spectroscopy

We have used femtosecond pump-probe spectroscopy to time resolve the injection of electrons into nanocrystalline TiO2 film electodes under ambient conditions following photoexcitation of the adsorbed dye, [Ru(4,4’-dicarboxy-2,2’-bipyridine)2(NCS)2] (N3). Pumping at one of the metal-to-ligand charge transfer adsorption peaks and probing the absorption of electrons injected into the TiO2 conduction band at 1.52 µm and in the range of 4.1 to 7.0 µm, we have directly observed the arrival of the injected electrons. Our measurements indicate an instrument-limited ~50-fs upper limit on the electron injection time under ambient conditions in air. We have compared the infrared transient absorption for noninjecting (blank) systems consisting of N3 in ethanol and N3 adsorbed to films of nanocrystalline Al2O3 and ZrO2, and found no indication of electron injection at probe wavelengths in the mid-IR (4.1 to 7.0 µm). At 1.52 µm interferences exist in the observed transient adsorption signal for the blanks

Striking the Right Balance of Intermolecular Coupling for High-Efficiency Singlet Fission

Singlet fission is a process that splits collective excitations, or excitons, into two with unity efficiency. This exciton splitting process, unique to molecular photophysics, has the potential to considerably improve the efficiency of optoelectronic devices through more efficient light harvesting. While the first step of singlet fission has been characterized in great detail, subsequent steps critical to achieving overall highly-efficient singlet-to-triplet conversion are only just beginning to become well understood. One of the most elementary suggestions, which has yet to be tested, is that an appropriately balanced coupling is necessary to ensure overall highly efficient singlet fission; that is, the coupling needs to be strong enough so that the first step is fast and efficient, yet weak enough to ensure the independent behavior of the resultant triplets. In this work, we show how high overall singlet-to-triplet conversion efficiencies can be achieved in singlet fission by ensuring that the triplets comprising the triplet pair behave as independently as possible. We show that side chain sterics govern local packing in amorphous pentacene derivative nanoparticles, and that this in turn controls both the rate at which triplet pairs form and the rate at which they decay. We show how compact side chains and stronger couplings promote a triplet pair that effectively couples to the ground state, whereas bulkier side chains promote a triplet pair that appears more like two independent and long-lived triplet excitations. Our results show that the triplet pair is not emissive, that its decay is best viewed as internal conversion rather than triplet–triplet annihilation, and perhaps most critically that, in contrast to a number of recent suggestions, the triplets comprising the initially formed triplet pair cannot be considered independently. This work represents a significant step toward better understanding intermediates in singlet fission, and how molecular packing and couplings govern overall triplet yields

Guillain-Barré syndrome: a century of progress

In 1916, Guillain, Barré and Strohl reported on two cases of acute flaccid paralysis with high cerebrospinal fluid protein levels and normal cell counts — novel findings that identified the disease we now know as Guillain–Barré syndrome (GBS). 100 years on, we have made great progress with the clinical and pathological characterization of GBS. Early clinicopathological and animal studies indicated that GBS was an immune-mediated demyelinating disorder, and that severe GBS could result in secondary axonal injury; the current treatments of plasma exchange and intravenous immunoglobulin, which were developed in the 1980s, are based on this premise. Subsequent work has, however, shown that primary axonal injury can be the underlying disease. The association of Campylobacter jejuni strains has led to confirmation that anti-ganglioside antibodies are pathogenic and that axonal GBS involves an antibody and complement-mediated disruption of nodes of Ranvier, neuromuscular junctions and other neuronal and glial membranes. Now, ongoing clinical trials of the complement inhibitor eculizumab are the first targeted immunotherapy in GBS

Relationship Between Risk Factors and Mortality in Type 1 Diabetic Patients in Europe: The EURODIAB Prospective Complications Study (PCS)

OBJECTIVE—The purpose of this study was to examine risk factors for mortality in patients with type 1 diabetes

Vibrational echo correlation spectroscopy as a new probe of complex dynamics and mixtures

Vibrational echo correlation spectroscopy (VECS) is a powerful new method that can be applied to diverse problems such as hydrogen bond dynamics and the detn. of the compn. of complex mixts. First, the nature of a hydrogen bond network following hydrogen bond breaking is addressed for MeOD oligomers in CCl4. The correlation spectra demonstrate MeOD oligomers retain a detailed memory of the hydrogen bonded network that existed prior to hydrogen bond breaking through the observation of spectral diffusion in the photoproduct band formed following vibrational relaxation. Second, VECS is used to demonstrate the assignment of the chem. species in a complex mixt. In the example, while the linear absorption spectrum does not provide enough information to identify which peaks belong to the same mol., the off-diagonal coherence transfer peaks in the 2-dimensional correlation spectrum make the assignment straightforward. [on SciFinder(R)

Ultrafast heterodyne detected infrared multidimensional vibrational stimulated echo studies of hydrogen bond dynamics

Multidimensional vibrational stimulated echo correlation spectra with full phase information are presented for the broad hydroxyl stretch band of methanol-OD oligomers in CCl4 using ultrashort (1 ps) shows that there is frequency correlation between the initially excited hydroxyl stretch and the frequency shifted hydroxyl stretch formed by hydrogen bond breaking. [on SciFinder(R)

Using ultrafast infrared multidimensional correlation spectroscopy to aid in vibrational spectral peak assignments

Ultrafast IR heterodyne detected vibrational stimulated echoes with full phase information are used to obtain the vibrational correlation spectrum from a mixt. of metal-carbonyl compds. The linear absorption spectrum displays 4 peaks in the carbonyl stretching region. In the absence of knowledge of the mols. that make up the mixt., the absorption spectrum could arise from 4 mols. that each produces a single peak to 1 mol. with 4 peaks. But the correlation spectrum displays 4 peaks on the diagonal and off-diagonal peaks that make it straightforward to det. which peaks belong to a particular mol. [on SciFinder(R)