Watching the Interplay between Photoinduced Ultrafast Charge Dynamics and Nuclear Vibrations

Effects of Nuclear Motion on the Photoinduced Interfacial Charge Transfer Dynamics at a NiO/P1 Photocathode

The performance of dye-sensitized photoelectrochemical cells is presently limited by the photocathode component. Here, we investigate the impact of nuclear dynamics on the photoinduced charge separation of the benchmark NiO/P1 system (P1 = 4-(bis-4-(5-(2,2-dicyano-vinyl)-thiophene-2-yl)-phenyl-amino)-benzoic acid). Transient absorption (TA) studies in aqueous environments with different viscosities show that photoinduced hole injection either proceeds ultrafast (<100 fs) or in a sub-ps time window. We assign the fastest component to a surface species strongly coupled to the NiO. Interestingly, the slower injection component and charge recombination are slowed down considerably in more viscous media. Quantum-classical dynamics simulations of a system with the dye standing perpendicular to the surface yield an injection lifetime remarkably close to the slow component from kinetic modeling of the TA results. Simulations including nuclear thermal motion yield a 2-fold increase in hole transfer rate compared to simulations on fixed nuclei, highlighting the role of nuclear motion and providing new design principles for dye-sensitized photocathodes.

Second-Order Mass-Weighting Scheme for Atom-Centered Density Matrix Propagation Molecular Dynamics

The atom-centered density matrix propagation (ADMP) method is an extended Lagrangian approach to ab initio molecular dynamics, which includes the density matrix in an orthonormalized atom-centered Gaussian basis as additional, fictitious, electronic degrees of freedom, classically propagated along with the nuclear ones. A high adiabaticity between the nuclear and electronic subsystems is mandatory in order to keep the trajectory close to the Born-Oppenheimer (BO) surface. In this regard, the fictitious electronic mass μ, being a symmetric, nondiagonal matrix in its most general form, represents a free parameter, exploitable to optimize the propagation of the electronic density. Although mass-weighting schemes in ADMP exist, a systematic procedure to define an optimal value of the fictitious masses is not available yet. In this work, in order to rationally evaluate the electronic mass, fictitious electronic normal modes are defined through the diagonalization of the Hessian of the electronic density matrix. If the same frequency is imposed on all such modes (compatible with the chosen integration time step), then the corresponding μ matrix can be calculated and then employed for the following propagation. Analysis of several ADMP test simulations reveals that such Hessian-based mass-weighting approach is able to ensure, together with a 0.1/0.2 fs time steps, a high separation between the (real) nuclear and the (fictitious) electronic frequencies, which determines a high adiabaticity. This high, unprecedented, accuracy in the propagation leads, in turn, to low errors in the estimated nuclear vibrational frequencies, making the ADMP method totally comparable to a fully converged BO molecular dynamics simulation but more computationally efficient. This work, therefore, contributes to a further development of the ADMP ab initio molecular dynamics method, aimed at improving its accuracy through a more rational evaluation of the fictitious electronic mass parameter.

Photophysics of a nucleic acid-protein crosslinking model strongly depends on solvation dynamics: an experimental and theoretical study

We present a combined experimental and theoretical study of the photophysics of 5-benzyluracil (5BU) in methanol, which is a model system for interactions between nucleic acids and proteins. A molecular dynamics study of 5BU in solution through efficient DFT-based hybrid ab initio potentials revealed a remarkable conformational flexibility - allowing the population of two main conformers - as well as specific solute-solvent interactions, which both appear as relevant factors for the observed 5BU optical absorption properties. The simulated absorption spectrum, calculated on such an ensemble, enabled a molecular interpretation of the experimental UV-Vis lowest energy band, which is also involved in the induced photo-reactivity upon irradiation. In particular, the first two excited states (mainly involving the uracil moiety) both contribute to the 5BU lowest energy absorption. Moreover, as a key finding, the nature and brightness of such electronic transitions are strongly influenced by 5BU conformation and the microsolvation of its heteroatoms.