5 research outputs found
Temporal fluctuations in excimer-like interactions between pi-conjugated chromophores
Inter- or intramolecular coupling processes between chromophores such as
excimer formation or H- and J-aggregation are crucial to describing the
photophysics of closely packed films of conjugated polymers. Such coupling is
highly distance dependent, and should be sensitive to both fluctuations in the
spacing between chromophores as well as the actual position on the chromophore
where the exciton localizes. Single-molecule spectroscopy reveals these
intrinsic fluctuations in well-defined bi-chromophoric model systems of
cofacial oligomers. Signatures of interchromophoric interactions in the excited
state - spectral red-shifting and broadening, and a slowing of
photoluminescence decay - correlate with each other but scatter strongly
between single molecules, implying an extraordinary distribution in coupling
strengths. Furthermore, these excimer-like spectral fingerprints vary with
time, revealing intrinsic dynamics in the coupling strength within one single
dimer molecule, which constitutes the starting point for describing a molecular
solid. Such spectral sensitivity to sub-Angstrom molecular dynamics could prove
complementary to conventional FRET-based molecular rulers
Chromophore Bending Controls Fluorescence Lifetime in Single Conjugated Polymer Chains
Single-molecule
spectroscopy of conjugated polymers offers unique
insight into the interplay between the spatial arrangement of monomer
unitstwisting and bendingand the characteristics of
the primary excitonic photoexcitation, provided that a single conjugated
segment can be isolated. β-phase polyfluorene constitutes an
ideal model to study variations in intermonomeric coupling, determined
by nanoscale molecular shape, on the fundamental optical transition.
If structural relaxation in the excited state is weak, exciton self-trapping
occurs stochastically along the conjugated segment. Bending of the
π-system is then revealed by a reduction in single-photon polarization
anisotropy, correlating directly with increased fluorescence lifetime.
Strong relaxation raises emission anisotropy because of deterministic
exciton localization, decelerating fluorescence decay due to a decrease
in exciton coherence length
Unraveling the Electronic Heterogeneity of Charge Traps in Conjugated Polymers by Single-Molecule Spectroscopy
Charge trapping is taken for granted
in modeling the characteristics
of organic semiconductor devices, but very few techniques actually
exist to spectroscopically pinpoint trap states. For example, trap
levels are often assumed to be discrete in energy. Using the well-known
keto defect in polyfluorene as a model, we demonstrate how single-molecule
spectroscopy can directly track the formation of charge and exciton
traps in conjugated polymers in real time, providing crucial information
on the energetic distribution of trap sites relative to the polymer
optical gap. Charge traps with universal spectral fingerprints scatter
by almost 1 eV in depth, implying that substantial heterogeneity must
be taken into account when modeling devices
Temporal Switching of Homo-FRET Pathways in Single-Chromophore Dimer Models of π‑Conjugated Polymers
A set of π-conjugated oligomer dimers templated
in molecular
scaffolds is presented as a model system for studying the interactions
between chromophores in conjugated polymers (CPs). Single-molecule
spectroscopy was used to reveal energy transfer dynamics between two
oligomers in either a parallel or oblique-angle geometry. In particular,
the conformation of single molecules embedded in a host matrix was
investigated via polarized excitation and emission fluorescence microscopy
in combination with fluorescence correlation spectroscopy. While the
intramolecular interchromophore conformation was found to have no
impact on the fluorescence quantum yield, lifetime, or photon statistics
(antibunching), the long-term nonequilibrium dynamics of energy transfer
within these bichromophoric systems was accessible by studying the
linear dichroism in emission at the single-molecule level, which revealed
reversible switching of the emission between the two oligomers. In
bulk polymer films, interchromophore coupling promotes the migration
of excitation energy to quenching sites. Realizing the presence and
dynamics of such interactions is crucial for understanding limitations
on the quantum efficiency of larger CP materials
Ultrafast Electron Emission from a Sharp Metal Nanotaper Driven by Adiabatic Nanofocusing of Surface Plasmons
We report photoelectron emission
from the apex of a sharp gold nanotaper illuminated via grating coupling
at a distance of 50 μm from the emission site with few-cycle
near-infrared laser pulses. We find a fifty-fold increase in electron
yield over that for direct apex illumination. Spatial localization
of the electron emission to a nanometer-sized region is demonstrated
by point-projection microscopic imaging of a silver nanowire. Our
results reveal negligible plasmon-induced electron emission from the
taper shaft and thus efficient nanofocusing of few-cycle plasmon wavepackets.
This novel, remotely driven emission scheme offers a particularly
compact source of ultrashort electron pulses of immediate interest
for miniaturized electron microscopy and diffraction schemes with
ultrahigh time resolution