The optical excitation of close-by molecules can couple into collective states giving rise to phenomena such as ultrafast radiative decay and superradiance. Particularly intriguing are one-dimensional molecular chains that form inside nanotube templates, where the tubes align molecules into single- and multifile chains. The resulting collective excitations have strong fluorescence and shifted emission/absorption energies compared to the molecular monomer. We study the optical properties of α-sexithiophene chains inside boron nitride nanotubes by combining fluorescence with far- and near-field absorption spectroscopy. The inner nanotube diameter determines the number of encapsulated molecular chains. A single chain of α-sexithiophene molecules has an optical absorption and emission spectrum that is red-shifted by almost 300 meV compared to the monomer emission, which is much larger than expected from dipole–dipole coupling. For two or more parallel chains, the collective state splits into excitation and emission channels with a Stokes shift of 200 meV due to the chain–chain interaction. Our study emphasizes the formation of a delocalized collective state through Coulomb coupling of the molecular transition moments in one-dimensional molecular lattices. They show a remarkable tunability in the transition energy, which makes encapsulated molecules promising candidates for components in future optoelectronic devices and for analytic spectroscopy.
