Mesoscopic Quantum Emitters from Deterministic Aggregates of Conjugated Polymers

TL;DR

This paper investigates the properties of large aggregates of conjugated polymers, demonstrating they can act as single quantum emitters with enhanced brightness and revealing the nature of excitonic interactions within these mesoscale structures.

Contribution

It introduces a mesoscale approach to study deterministic polymer aggregates, distinguishing coherent and incoherent excitonic couplings and their spectroscopic signatures.

Findings

Aggregates with dozens of chains behave as single quantum emitters.

Coherent coupling increases excited-state lifetime and causes spectral red shift.

Incoherent FRET leads to blinking and exciton quenching in larger aggregates.

Abstract

An appealing definition of the term "molecule" arises from consideration of the nature of fluorescence, with discrete molecular entities emitting a stream of single photons. We address the question of how large a molecular object may become by growing deterministic aggregates from single conjugated polymer chains. Even particles containing dozens of individual chains still behave as single quantum emitters due to efficient excitation energy transfer, while the brightness is raised due to the increased absorption cross-section of the suprastructure. Excitation energy can delocalize between individual polymer chromophores in these aggregates by both coherent and incoherent coupling, which are differentiated by their distinct spectroscopic fingerprints. Coherent coupling is identified by a ten-fold increase in excited-state lifetime and a corresponding spectral red shift. Exciton quenching due to incoherent FRET becomes more significant as aggregate size increases, resulting in single-aggregate emission characterized by strong blinking. This mesoscale approach allows us to identify intermolecular interactions which do not exist in isolated chains and are inaccessible in bulk films where they are present but masked by disorder.