Collective states of {\alpha}-sexithiophene chains inside boron nitride nanotubes

TL;DR

This study investigates how {eta}-sexithiophene chains inside boron nitride nanotubes form collective excitations with tunable optical properties, revealing significant energy shifts and delocalized states due to Coulomb coupling.

Contribution

It demonstrates the formation of delocalized collective states in encapsulated {eta}-sexithiophene chains and their tunable optical properties, a novel insight into molecular-nanotube interactions.

Findings

Single chain emission red-shifted by 300 meV compared to monomer.

Collective states split into excitation and emission channels with 200 meV Stokes shift.

Optical properties are highly tunable based on nanotube diameter.

Abstract

Nanotubes align molecules into one dimensional chains creating collective states through the coupling of the molecular transition dipole moments. These collective excitations have strong fluorescence, narrow bandwidth, and shifted emission/absorption energies. We study the optical properties of {\alpha}-sexithiophene chains in 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 {\alpha}-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. The collective state splits into excitation and emission channels with a Stokes shift of 200 meV for chains with two or more files. Our study emphasises the formation of a delocalized collective state through Coulomb coupling of the transition moments that shows a remarkable tuneability in transition energy.