  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.
