Time-dependent quantum/continuum modeling of plasmon-enhanced electronic circular dichroism

In this work, we present a multiscale real-time approach to study the plasmonic effects of a metal nanoparticle (NP) on the electronic circular-dichroism (ECD) spectrum of a chiral molecule interacting with it. The method is based on the time-evolution of the molecule's time-dependent wavefunction, expanded in the eigenstates of a perturbed Hamiltonian. A quantum description of the molecular system is coupled to a classical representation of the NP via a continuum model. The method is applied to methyloxirane and peridinin at various distances (1, 3, and 5 nm) with respect to a gold NP surface. While no remarkable effect is observed for methyloxirane at any studied distance, an enhancement appears when the peridinin lies at 1 nm and the pulse is linearly polarized perpendicularly to the molecular axis, with the ECD signal centered at 4.1 eV increased by a factor of around 20. These results are rationalized looking at the gap between the plasmonic peak of the NP at around 2.5 eV and the molecular excitations: the smaller the gap between molecular and plasmonic excitations, the larger the plasmonic enhancement of the ECD signal. Moreover, ECD peaks are selectively enhanced due to the favorable coupling between the pulse polarization and the combined effect of electric and magnetic dipole moments. This approach allows one to go through the electronic structure and dynamics of chiral molecules for obtaining a realistic description of plasmon-mediated ECD spectra, e.g., paving the way to applications to molecules of biological relevance interacting with nanostructures of experimental interest.