jp8b03963_si_001.pdf (1.6 MB)
The Nature of Excimer Formation in Crystalline Pyrene Nanoparticles
journal contribution
posted on 2018-08-30, 22:04 authored by Ryan D. Pensack, Rachel J. Ashmore, Angela L. Paoletta, Gregory D. ScholesPyrene
is an exemplary conjugated organic chromophore with a strong
propensity for self-association through an excited-state process known
as excimer formation. Pyrene “excimers” and molecular
“excimers” more generally are strongly avoided in some
applications, such as in light harvesting, yet have found widespread
use in others, such as in sensing and structure determination. Despite
this disparate view and despite their widespread use, a fundamental
understanding of the structure and dynamics of these collective excitations
remains outstanding. In this work, we shed key insights into the nature
of excimer formation in crystalline pyrene. We developed a flash precipitation
procedure incorporating a polymer additive that enabled us to prepare
aqueous suspensions of crystalline pyrene nanoparticles. We provide
evidence that the molecular-level packing in the nanoparticles is
equivalent to the equilibrium packing of the single crystal and show
that excimer formation is the primary excited-state decay pathway.
We find that excimer formation in the crystalline pyrene nanoparticles
occurs in two stages on a picosecond time scale and suggest that intermolecular
structural dynamics are largely responsible for the observed two-stage
dynamics. We discuss an exciton theory description of molecular “excimers”
and provide insights into their mechanism of formation, which we argue
is best viewed simply as the relaxation of a singlet exciton into
an excimer geometry.