Origin and TDDFT Benchmarking of the Plasmon Resonance in Acenes

The origin of plasmon resonance in acenes is described by analyzing the excitation spectrum of naphthalene in terms of configuration interaction. The strong longitudinal beta-peak in the UV region of the spectrum results from a constructive interaction of the transition dipole moments of two degenerate configurations V-1 and V-2. V-1 corresponds to the excitation of an electron from the HOMO to the LUMO+1. V-2 corresponds to the excitation of an electron from the HOMO-1 to the LUMO. The weak longitudinal a-peak in the visible results from a destructive interaction of the dipole moments of the same two configurations. Previous time-dependent density functional theory (TDDFT) calculations showed a similar behavior for silver and gold nanoparticles but often with more than two interacting configurations. The plasmon occurs at the frequency where all configurations interact constructively. The beta-peak of acenes can therefore be identified as the plasmon peak. The natural transition orbitals involved in the alpha- and beta-peaks of naphthalene have identical shapes, which reflects the fact that the transitions involved in these two peaks are similar, but they may have opposite phases. An analysis of the transition density of the beta-peak of naphthalene reveals that the electron density moves from one side of the molecule to the other upon excitation, as expected for a dipolar plasmon. The plasmonic character of the beta-peak is compared to the single-particle transition character of the transverse p-band. Several exchange-correlation functionals have been benchmarked. Hybrid functionals give the best description of the beta-peak and the alpha-peak. The couplings between the two interacting configurations at all levels of theory are similar to experimental values. On the other hand, long-range corrected functionals give the most accurate energies for the transverse p-band.