We analyze how the photorelaxation dynamics of a molecule can be controlled by modifying its electromagnetic environment using a nanocavity mode. In particular, we consider the photorelaxation of the RNA nucleobase uracil, which is the natural mechanism to prevent photodamage. In our theoretical work, we identify the operative conditions in which strong coupling with the cavity mode can open an efficient photoprotective channel, resulting in a relaxation dynamics twice as fast as the natural one. We rely on a state-of-the-art chemically detailed molecular model and a non-Hermitian Hamiltonian propagation approach to perform full-quantum simulations of the system dissipative dynamics. By focusing on the photon decay, our analysis unveils the active role played by cavity-induced dissipative processes in modifying chemical reaction rates, in the context of molecular polaritonics. Remarkably, we find that the photorelaxation efficiency is maximized when an optimal trade-off between light–matter coupling strength and photon decay rate is satisfied. This result is in contrast with the common intuition that increasing the quality factor of nanocavities and plasmonic devices improves their performance. Finally, we use a detailed model of a metal nanoparticle to show that the speedup of the uracil relaxation could be observed via coupling with a nanosphere pseudomode, without requiring the implementation of complex nanophotonic structures.

The fast and slow components of the relaxation of photoexcited thiophene have been investigated by means of SHARC (surface hopping including arbitrary couplings) molecular dynamics based on multiconfiguration electronic structure calculations. Triplet states are included to ascertain their role in the relaxation process. After thiophene is excited to the S state, ultrafast dynamics (τ = 96 fs) initiates a ring opening due to cleavage of a carbon sulfur bond and simultaneous ring puckering. This time constant is in agreement with previous experimental and theoretical studies. The subsequent dynamics of the open-ring structures is characterized by the interplay of internal conversion and intersystem crossing. For the open-ring structures, the S, S, T and T states are nearly degenerate and the spin-orbit couplings are large. The underlying potential energy surface is flat and long-lived open-ring structures in the singlet as well as in the triplet states are formed. Both the participation of triplet states and the shape of the energy surface explain the experimentally observed slow ring closure in the ground state.

The rates and outcomes of virtually all photophysical and photochemical processes are determined by conical intersections. These are regions of degeneracy between electronic states on the nuclear landscape of molecules where electrons and nuclei evolve on comparable timescales and thus become strongly coupled, enabling radiationless relaxation channels upon optical excitation. Due to their ultrafast nature and vast complexity, monitoring conical intersections experimentally is an open challenge. We present a simulation study on the ultrafast photorelaxation of uracil, based on a quantum description of the nuclei. We demonstrate an additional window into conical intersections obtained by recording the transient wavepacket coherence during this passage with an X-ray free-electron laser pulse. Two major findings are reported. First, we find that the vibronic coherence at the conical intersection lives for several hundred femtoseconds and can be measured during this entire time. Second, the time-dependent energy-splitting landscape of the participating vibrational and electronic states is directly extracted from Wigner spectrograms of the signal. These offer a physical picture of the quantum conical intersection pathways through visualizing their transient vibronic coherence distributions. The path of a nuclear wavepacket in the vicinity of the conical intersection is directly mapped by the proposed experiment.

Oligothiophenes and polythiophenes are building blocks of organic-based energy conversion materials. Therefore the lifetime of the excited states plays a central role. As a first step to understand the factors influencing the performance, we investigated the deactivation processes from the first excited state S1 of thiophene and small oligothiophenes containing up to four rings using quantum chemical calculations. For thiophene a low-lying S1/S0 conical intersection seam is easily accessible and drives the fast internal conversion. In oligothiophenes barriers inhibit this passage while deactivation pathways via intersystem crossing channels open. The first one is responsible for the high triplet quantum yields and takes place shortly after the Franck-Condon region. The second one occurs in the vicinity of a local S1 minimum. The calculated spin-orbit coupling strength together with the singlet-triplet energy gaps can explain the decreasing triplet and increasing fluorescence quantum yields for growing chain length. From the triplets the ground state is reachable by inter-ring torsions and T1/S0 intersections. The present results allow a deeper understanding of the deactivation pathways of thiophene and small oligothiophenes and are of potential interest for the photophysics of longer oligothiophenes and polythiophenes used in optical devices.

Tyrosinase model systems pinpoint pathways to translating Nature's synthetic abilities for useful synthetic catalysts. Mostly, they use N-donor ligands which mimic the histidine residues coordinating the two copper centres. Copper complexes with bis(pyrazolyl)methanes with pyridinyl or imidazolyl moieties are already reported as excellent tyrosinase models. Substitution of the pyridinyl donor results in the new ligand HC(3-tBuPz) (4-CO MePy) which stabilises a room-temperature stable μ-η :η -peroxide dicopper(II) species upon oxygenation. It reveals highly efficient catalytic activity as it hydroxylates 8-hydroxyquinoline in high yields (TONs of up to 20) and much faster than all other model systems (max. conversion within 7.5 min). Stoichiometric reactions with para-substituted sodium phenolates show saturation kinetics which are nearly linear for electron-rich substrates. The resulting Hammett correlation proves the electrophilic aromatic substitution mechanism. Furthermore, density functional theory (DFT) calculations elucidate the influence of the substituent at the pyridinyl donor: the carboxymethyl group adjusts the basicity and nucleophilicity without additional steric demand. This substitution opens up new pathways in reactivity tuning.