Collective States of α-Sexithiophene Chains Inside Boron Nitride Nanotubes

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.