Simulating the Electronic Circular Dichroism of Chlorophyll b in the Presence of a Gold Nanosphere

In this work, we apply a real-time multiscale approach to investigate plasmonic effects arising from a gold spherical nanoparticle (NP) on the electronic circular dichroism (ECD) spectrum of Chlorophyll b (Chlb). Chlb has been described within linear-response time-dependent density functional theory (TDDFT) to obtain excitation energies and transition dipole moments, which were then employed in a TD-CIS ansatz for the time-dependent wave function, subsequently propagated by solving the time-dependent Schrödinger equation. Three different distances (1, 3, and 5 nm) of the molecule with respect to the surface of the NP were considered. For each distance, three Chlb orientations (two perpendicular and one parallel) were taken into account, together with three pulse polarization directions. In addition, a simple descriptor of the plasmon-induced signal enhancement or quenching of the signal was introduced, based on the time-dependent coherences between the electronic ground state and selected excited states. In general, modifications in the ECD spectra of Chlb have been attributed to plasmonic effects. An interesting exception is the appearance of an intense peak at 2.36 eV when Chlb is oriented perpendicularly to the NP surface and the pulse is polarized perpendicular to the molecular plane. In this case, the magnetic transition dipole moment to the second state (providing the excitation at 2.36 eV) increases by a factor of four to five compared to bare Chlb or other orientations. The composition of the excitations was analyzed through the differential projected density of states.