Plasmons, coherent oscillations of conduction band electrons, have been well characterized in many different material systems including thin metal films, metal and metallodielectric nanoparticles, semiconductors, and graphene. Graphene's unique band structure provides a direct mechanism with which to tune the plasmon resonance energy with relatively small changes to its carrier . It has recently been shown that this remarkable electronic plasmon tunability persists as graphene is spatially confined to nanoribbons and nanoislands, down even to the molecular scale where Polycyclic Aromatic Hydrocarbon (PAH) molecules support molecular plasmons sensitive to the addition or removal of a single electron. We have previously demonstrated that molecular plasmons in PAHs exhibit behavior consistent with other plasmonic systems, notably the geometric and environmental tunability of the resonant wavelengths. This thesis reports on the further characterization of molecular plasmons and the use of their charge-sensitive response in electrochromic devices. Unlike larger graphene nanostructures, the PAH absorption spectra reveal rich features due to the coupling of the molecular plasmons with molecular vibrations. We investigate this surprising plasmon-phonon coupling with direct comparisons between experimental results and theoretical models accounting for the vibrational coupling. We proceed to demonstrate a series of electrochromic devices, color-changing glass, based on PAH plasmon resonances that can be reversibly switched between colorless and vivid colors dependent upon the chosen molecules. Finally, we explore the ultrafast dynamics of the molecular plasmon system and contrast this behavior with single-particle excitations. As the smallest examples of graphene and as readily available chemical species, PAHs provide an ideal platform for investigation of molecular plasmonics and are also well suited to large-scale applications since they are industrially available in large quantities and high purity.