Relaxation Dynamics and Structural Characterization of Organic Nanoparticles with Enhanced Emission

With a reprecipitation method, we prepared fluorescent organic nanoparticles of 1,4-di[(E)-2-phenyl-1-propenyl]benzene (PPB) that feature weak emission in solution but exhibit blue-shifted absorption and strong emission
as aggregates. Picosecond fluorescent transients of these PPB nanoparticles showed biexponential decay,
described with a consecutive kinetic model involving two emissive states. X-ray diffraction patterns of PPB
nanocrystals indicate long-range packing structures of two types, one the same as in a single crystal and the
other not yet determined. PPB molecules in a crystal show an arrangement of a herringbone type with three
benzene rings in a PPB unit being nearly planar and two methyl groups of the unit pointing along the same
direction, in contrast to the twisted structure of an isolated PPB molecule. Fluorescence transients of PPB on
a femtosecond scale indicate an efficient channel for isomerization that is activated for free PPB in solution
but inhibited in PPB forming nanoparticles, demonstrating the significance of molecular geometry and twisting
motions that affect the relaxation dynamics in the excited state. The versatile techniques combined in this
work provide strong evidence to improve our understanding of optical properties in organic nanoparticles
dependent on size.