DNA owes its remarkable photostability to its building blocks—the nucleosides—that efficiently dissipate the energy acquired upon ultraviolet light absorption. The mechanism occurring on a sub-picosecond time scale has been a matter of intense debate. Here we combine sub-30-fs transient absorption spectroscopy experiments with broad spectral coverage and state-of-the-art mixed quantum-classical dynamics with spectral signal simulations to resolve the early steps of the deactivation mechanisms of uridine (Urd) and 5-methyluridine (5mUrd) in aqueous solution. We track the wave packet motion from the Franck-Condon region to the conical intersections (CIs) with the ground state and observe spectral signatures of excited-state vibrational modes. 5mUrd exhibits an order of magnitude longer lifetime with respect to Urd due to the solvent reorganization needed to facilitate bulky methyl group motions leading to the CI. This activates potentially lesion-inducing dynamics such as ring opening. Involvement of the 1nπ* state is found to be negligible.

TT-Py exhibits anti-Kasha and multiple emissive behaviour due to the copresence of rigid TT and partially conformationally free Py.

Lewis acids have recently been recognized as catalysts enabling enantioselective photochemical transformations.M echanistic studies on these systems are however rare, either due to their absorption at wavelengths shorter than 260 nm, or due to the limitations of theoretical dynamic studies for larger complexes.I nt his work, we overcome these challenges and employs ub-30-fs transient absorption in the UV,incombination with ahighly accurate theoretical treatment on the XMS-CASPT2 level. We investigate 2-cyclohexenone and its complex to boron trifluoride and analyze the observed dynamics based on trajectory calculations including nonadiabatic coupling and intersystem crossing. This approach explains all ultrafast decaypathways observed in the complex. We show that the Lewis acid remains attached to the substrate in the triplet state,w hich in turn explains why chiral boronbased Lewis acids induce ah igh enantioselectivity in photocycloaddition reactions.

By combining UV transient absorption spectroscopy with sub-30-fs temporal resolution and CASPT2/MM calculations, we present a complete description of the primary photoinduced processes in solvated tryptophan. Our results shed new light on the role of the solvent in the relaxation dynamics of tryptophan. We unveil two consecutive coherent population transfer events involving the lowest two singlet excited states: a sub-50-fs nonadiabatic L a → L b transfer through a conical intersection and a subsequent 220 fs reverse L b → L a transfer due to solvent-assisted adiabatic stabilization of the L a state. Vibrational fingerprints in the transient absorption spectra provide compelling evidence of a vibronic coherence established between the two excited states from the earliest times after photoexcitation and lasting until the back-transfer to L a is complete. The demonstration of response to the environment as a driver of coherent population dynamics among the excited states of tryptophan closes the long debate on its solvent-assisted relaxation mechanisms and extends its application as a local probe of protein dynamics to the ultrafast time scales.

In this paper we demonstrate the first, to the best of our knowledge, all-fiber self-starting laser oscillator consisting entirely of polarization-maintaining large mode area (PLMA) fibers mode-locked with nonlinear optical loop mirror (NOLM). The system works in a Raman-free dissipative soliton regime and operates at a central wavelength of 1030 nm. It delivers stable ultrashort pulses of high energy of 12 nJ at a 7.56 MHz repetition rate which can be compressed down to the Fourier transform limit of ∼250 fs. Higher energies were limited by the formation of multiple pulses in the cavity.