Disentangling the absorption lineshape of methylene blue for nanocavity strong coupling

Cucurbit[7]uril molecules form non-covalent host - guest complexes with small molecular dyes. In addition, cucurbit[7]uril can also bind gold nanoparticles on gold surfaces with a 0.9 nm gap, creating plasmonic nanocavities for the dyes, with extreme confinement of the electromagnetic field. For methylene blue in such cavities, single molecule strong coupling was inferred from a complete disappearance of a characteristic shoulder in its spectrum, attributed to dimer removal. Yet, the shoulder's origin remains debated. Using atomistic simulations, we show that it arises from both dimerization and vibronic progression. While cucurbit[7]uril binding removes the dimer contribution, vibronic progression persists. As this conflicts with previous reports, we also measured the spectra. In line with our computations, the shoulder remains visible when cucurbit[7]uril binds methylene blue. These results clarify the spectral features and pave the way for atomistic models of single-molecule strong coupling in nanoparticle-on-mirror cavities.