Vibrational Coherence Spectroscopy Identifies Ultrafast Branching in an Iron(II) Sensitizer.

The introduction of N-heterocyclic carbene ligands has greatly increased the lifetimes of metal-to-ligand charge transfer states (MLCT) in iron(II) complexes, making them promising candidates for photocatalytic applications. However, the spectrally elusive triplet metal-centered state (3MC) has been suggested to play a decisive role in the relaxation of the MLCT manifold to the ground state, shortening their lifetimes and consequently limiting the application potential. In this work, time-resolved vibrational spectroscopy and quantum chemical calculations are applied to shed light on the 3MCs' involvement in the deactivation of the MLCT manifold of an iron(II) sensitizer. Two distinct symmetric Fe-L breathing vibrations at frequencies below 150 cm-1 are assigned to the 3MC and 3MLCT states by quantum chemical calculations. On the basis of this assignment, an ultrafast branching directly after excitation forms not only the long-lived 3MLCT but also the 3MC as an additional loss channel.

Effect of point mutations on the ultrafast photo-isomerization of Anabaena sensory rhodopsin.

Anabaena sensory rhodopsin (ASR) is a particular microbial retinal protein for which light-adaptation leads to the ability to bind both the all-trans, 15-anti (AT) and the 13-cis, 15-syn (13C) isomers of the protonated Schiff base of retinal (PSBR). In the context of obtaining insight into the mechanisms by which retinal proteins catalyse the PSBR photo-isomerization reaction, ASR is a model system allowing to study, within the same protein, the protein-PSBR interactions for two different PSBR conformers at the same time. A detailed analysis of the vibrational spectra of AT and 13C, and their photo-products in wild-type ASR obtained through femtosecond (pump-) four-wave-mixing is reported for the first time, and compared to bacterio- and channelrhodopsin. As part of an extensive study of ASR mutants with blue-shifted absorption spectra, we present here a detailed computational analysis of the origin of the mutation-induced blue-shift of the absorption spectra, and identify electrostatic interactions as dominating steric effects that would entail a red-shift. The excited state lifetimes and isomerization reaction times (IRT) for the three mutants V112N, W76F, and L83Q are studied experimentally by femtosecond broadband transient absorption spectroscopy. Interestingly, in all three mutants, isomerization is accelerated for AT with respect to wild-type ASR, and this the more, the shorter the wavelength of maximum absorption. On the contrary, the 13C photo-reaction is slightly slowed down, leading to an inversion of the ESLs of AT and 13C, with respect to wt-ASR, in the blue-most absorbing mutant L83Q. Possible mechanisms for these mutation effects, and their steric and electrostatic origins are discussed.

Two-step kinetic model of the self-assembly mechanism for diphenylalanine micro/nanotube formation.

Peptide nanostructures compose a new class of materials that have gained attention due to their interesting properties. Among them, nanotubes of diphenylalanine (FF) and its analogues have been one of the most studied structures in the last few years. Their importance originates from the need to better understand the formation of ß-amyloid fibrils which are associated with Alzheimer's disease. In this work, the FF self-assembly process was probed using time-resolved Raman microscopy. The changes in the Raman spectra are followed over time after injecting water into a FF-film until micro/nanotubes (MNTs) are formed. Specific features of the Raman spectra clearly suggest that FF-molecules after water injection form an intermediate species before forming FF-MNTs. The broad Raman bands observed for the intermediate species suggest the presence of very heterogeneous structures based on FF. The FF-MNTs appear almost instantaneously (detected via the rise of the typical Raman bands of FF-MNTs at 761, 1249 and 1426 cm-1) after the intermediate structures are formed. This delayed formation of FF-MNTs supports a nucleation process. The formation via nucleation of FF-MNTs is further corroborated by a simulation of the Raman spectra based on a 2-step kinetic model and the respective vibrational Raman modes are identified using Density Functional Theory vibrational calculations. Our results indicate that the driving force for the FF-MNT patterning process is the electric dipole re-orientation originating from the FF dipeptide unit connectivity over time.

Parametrically amplified ultrashort pulses from a shaped photonic crystal fiber supercontinuum.

By seeding a noncollinear optical parametric amplifier with a photonic crystal fiber supercontinuum, temporally well-defined amplified output pulses have been generated with durations down to 13 fs. The phase of the supercontinuum seed has been characterized by the ZAP-SPIDER technique and can be tailored with a femtosecond pulse shaper. Thus, a very flexible source for arbitrarily shaped, amplified ultrashort laser pulses has been realized.