Signatures of Conical Intersection Dynamics in the Time-Resolved Photoelectron Spectrum of Furan: Theoretical Modeling with an Ensemble Density Functional Theory Method

Non-adiabatic Couplings in Surface Hopping with Tight Binding Density Functional Theory: The Case of Molecular Motors

Nonadiabatic molecular dynamics (NAMD) has become an essential computational technique for studying the photophysical relaxation of molecular systems after light absorption. These phenomena require approximations that go beyond the Born-Oppenheimer approximation, and the accuracy of the results heavily depends on the electronic structure theory employed. Sophisticated electronic methods, however, make these techniques computationally expensive, even for medium size systems. Consequently, simulations are often performed on simplified models to interpret the experimental results. In this context, a variety of techniques have been developed to perform NAMD using approximate methods, particularly density functional tight binding (DFTB). Despite the use of these techniques on large systems, where ab initio methods are computationally prohibitive, a comprehensive validation has been lacking. In this work, we present a new implementation of trajectory surface hopping combined with DFTB, utilizing nonadiabatic coupling vectors. We selected the methaniminium cation and furan systems for validation, providing an exhaustive comparison with the higher-level electronic structure methods. As a case study, we simulated a system from the class of molecular motors, which has been extensively studied experimentally but remains challenging to simulate with ab initio methods due to its inherent complexity. Our approach effectively captures the key photophysical mechanism of dihedral rotation after the absorption of light. Additionally, we successfully reproduced the transition from the bright to dark states observed in the time-dependent fluorescence experiments, providing valuable insights into this critical part of the photophysical behavior in molecular motors.

Insights into Ultrafast Relaxation Dynamics of Electronically Excited Furfural and 5-Methylfurfural

The ultrafast relaxation dynamics of furfural and 5-methylfurfural following excitation in the ultraviolet range is investigated using the femtosecond time-resolved photoelectron spectroscopy method. Specifically, the pump wavelength-dependent decay dynamics of electronically excited furfural and 5-methylfurfural is discussed on the basis of a detailed analysis of our measured time-resolved photoelectron spectroscopy spectra. Irradiation at all pump wavelengths prepares both furfural and 5-methylfurfural molecules with different vibrational levels in the first optically bright S2 (1ππ*) state, the lifetime of which is measured to be at least hundreds of femtoseconds. Besides the prominent deactivation channels of ring-opening and ring-puckering pathways for the S2(1ππ*) state, we propose that there is a minor decay channel of internal conversion from the initially prepared S2(1ππ*) state to the S1(1nπ*) state. The wavepacket decays out of the Franck-Condon region on the S2(1ππ*) state potential energy surface and bifurcates into different parts somewhere. A small fraction of the wavepacket funnels down to the S1(1nπ*) state via internal conversion. The subsequently populated S1(1nπ*) state contains large vibrational excess energy and decays over a lifetime of 2.5-2.8 ps. One of the deactivation channels of the S1(1nπ*) state is intersystem crossing to the 3ππ* triplet state. In addition, methyl substitution effects on the excited-state dynamics of furfural are also discussed. This experimental study provides new insights into the excitation energy-dependent decay dynamics of photoexcited furfural and 5-methylfurfural.

Approximate Functionals for Multistate Density Functional Theory

Recently Lu and Gao [J. Phys. Chem. Lett. 2022, 13 (33), 7762-7769] published a new, rigorous density functional theory for excited states and proved that the projection of the kinetic and electron-repulsion operators into the subspace of the lowest electronic states are a universal functional of the matrix density D(r). This is the first attempt to find an approximation to the multistate universal functional F [ D ( r ) ] . It is shown that F (i) does not explicitly depend on the number of electronic states and (ii) is an analytic matrix functional. The Thomas-Fermi-Dirac-von Weizsäcker model and the correlation energy of the homogeneous electron gas are turned into matrix functionals guided by two principles: that each matrix functional should transform properly under basis set transformations and that the ground state functional should be recovered for a single electronic state. Lieb-Oxford-like bounds on the average kinetic and electron-repulsion energies in the subspace are given. When evaluated on the numerically exact matrix density of LiF, this simple approximation reproduces the matrix elements of the electron-repulsion operator in the basis of the exact eigenstates accurately for all bond lengths. In particular the off-diagonal elements of the effective Hamiltonian that come from the interactions of different electronic states can be calculated with the same or better accuracy than the diagonal elements. Unsurprisingly, the largest error comes from the kinetic energy functional. More exact conditions that constrain the functional form of F are needed to go beyond the local density approximation.

Formulation of transition dipole gradients for non-adiabatic dynamics with polaritonic states

A general formulation of the strong coupling between photons confined in a cavity and molecular electronic states is developed for the state-interaction state-average spin-restricted ensemble-referenced Kohn-Sham method. The light-matter interaction is included in the Jaynes-Cummings model, which requires the derivation and implementation of the analytical derivatives of the transition dipole moments between the molecular electronic states. The developed formalism is tested in the simulations of the nonadiabatic dynamics in the polaritonic states resulting from the strong coupling between the cavity photon mode and the ground and excited states of the penta-2,4-dieniminium cation, also known as PSB3. Comparison with the field-free simulations of the excited-state decay dynamics in PSB3 reveals that the light-matter coupling can considerably alter the decay dynamics by increasing the excited state lifetime and hindering photochemically induced torsion about the C=C double bonds of PSB3. The necessity of obtaining analytical transition dipole gradients for the accurate propagation of the dynamics is underlined.

Signatures of Conical Intersections in Extreme Ultraviolet Photoelectron Spectra of Furan Measured with 15 fs Time Resolution

Ultrafast internal conversion of furan upon deep UV excitation at 200 nm is studied by using extreme ultraviolet time-resolved photoelectron spectroscopy with a time resolution of 15 fs. Ballistic nuclear wavepacket motion from the 1B2(ππ*) state to the ground state is fully observed using 21.7 eV probe pulses. Through the performance of a comparison with the results of electronic structure calculations at the MS(3)-CASPT2(10,10)/cc-pVTZ level of theory, the photoelectron signals from the conical intersection regions are identified.

The Physical Chemistry and Chemical Physics (PCCP) Section of the International Journal of Molecular Sciences in Its Publications: The First 300 Thematic Articles in the First 3 Years

The Physical Chemistry and Chemical Physics Section (PCCP Section) is one of the youngest among the sections of the International Journal of Molecular Sciences (IJMS)-the year 2021 will only mark three years since its inception [...].