Electronic rearrangement in molecular plasmons : An electron density and electrostatic potential-based study.

Plasmonic modes in single-molecule systems have been previously identified by scaling two-electron interactions while calculating excitation energies. Analysis of transition dipole moments for states of polyacenes based on configuration interaction was yet another method characterizing molecular plasmons. From our calculations using time-dependent density functional theory (TD-DFT) at B3LYP/cc-pVTZ basis, both the peaks were found to result from the same set of electronic transitions i.e. HOMO-n to LUMO and HOMO to LUMO+n, where n varies as the number of fused rings increases. In this work, the excited states of polyacenes, naphthalene through pentacene, have been analysed using electron densities and molecular electrostatic po tential (MESP) topography. The bright and dark plasmonic states involve the least electron rearrangement, as compared to other excited states. The MESP topography indicates that the variance in MESP values and the displacement in MESP minima positions, calculated with respect to the ground state, are lowest for plasmonic states. The excited state electronic density profiles and electrostatic potential topographies suggest least electron rearrangement for the plasmonic states. A high electron-rearrangement characterizes a single particle excitation. The molecular plasmon can be called an excited state most similar to the ground state in terms of one-electron properties. This is found to be true for silver (Ag6) and sodium (Na8) linear chains as well.