title: Ultrafast Coherent Dynamics of Electronic Excitations in Single Conjugated Polymers and Supramolecular Aggregates
Creators: name: Wilma, Kevin
Publisher: name: University of Bayreuth
Year: 2019
Subjects: subject: Ultrafast Spectroscopy, Coherent Control, Organic Matter, Two-Photon Absorption, Single Molecules, Supramolecular, Aggregation, Conjugated Polymers, Non-linear Optics, Coherence, Dephasing, Ultrafast Dynamics
subject: 500
subject: 530

description: The spectroscopic characterisation of fundamental photophysical processes is highly
important for the final physical picture of organic nanostructures and the further
transfer of this knowledge to technical-relevant applications. The first few femtoseconds
after a photo-excitation has taken place, determine the complete evolution to a large
extend of the generated quantum mechanical properties and therefore the microscopic
characteristics. This work provides a joint experimentally and theoretically approach,
which is capable of extracting the ultrafast dynamics and quantum mechanical properties
of single and supramolecular nano structures. By using an ultrafast spectroscopy setup
and quantum dynamics identification (QDI) procedure the samples were measured and
the important parameters identified.
The first chapter of this work was about the photoluminescence (PL) detected
measurement of ultrafast dynamics of single methyl-substituted ladder type poly(paraphenylenes)
(MeLPPP). MeLPPP is a suitable system for the analysis of two-photon (2P)
processes, as the very rigid backbone is responsible for the strict quantum mechanical
symmetry selection rules and its extraordinary 2P cross section. By using amplitudeshaping
techniques, the ultrafast decay of the generated coherence between the electronic
ground singlet state S0 and the second excited singlet state S2 was measured. The
obtained data was subsequently analysed with the QDI-procedure and the underlying
processes identified. All data was reliably reproduced by introducing a 3-level model,
where a pure 2P transition takes place between S0 and S2 and the emission originates
from the first excited singlet state S1. The obtained histograms of the optimised
dephasing and relaxation rates as well as E2, the energy of S2 reflect the inhomogeneity
of the complex environments of each molecule. However, simply measuring the coherence
decay is not enough to fully characterise MeLPPP with respect to the S0-S2 2P Rabicoupling,
and hence more complex experiments need to be carried out.
These additional measurements were done by phase-only shaping techniques, where
the PL intensity of single MeLPPP molecules was detected in dependency of the
variation of the linear chirp. Most of the traces analysed, exhibit a symmetric PL
intensity under variation of the linear chirp. This symmetric response was nicely
reproduced by the QDI-procedure with the previous 3-level model. Surprisingly a
considerable fraction of the data exhibits asymmetric PL dependencies on the linear chirp. The QDI procedure was not able to find a minimum model which reproduces
the asymmetric data within the 3-level model with physically reasonable parameters.
Therefore the model was extended to a 4-level model, including a highly excited state
(with a PL quantum yield nearly 0), denoted with S3. S3 exhibits an energy mismatch
of around one one-photon energy of the laser spectrum with respect to S2. Hence, for
resonant conditions this level opens up the path for an additional 1P transition S2 !
S3, after excitation was generated in S2 by a 2P absorption. Including this additional
transition into as loss-channel for the generated population, the QDI-procedure found
a minimum model reproducing the asymmetric data with high quality. The model
describes a 4-level system with a (2+1)-photon transition, which is highly sensitive to
the interplay between the electronic parameters of S2 and S3 and the instantaneous
excitation energy. Both of these chapters proof that the described joint approach is
capable of a reliable and fast extraction and characterisation of quantum mechanical
and ultrafast parameters for single quantum systems. Furthermore this represents
the first time proof of genuinely active coherent control of multi-photon processes via
phase-shaping of single organic molecules in complex environment.
The second chapter is about similar ultrafast measurements on an ensemble of
carbonyl-bridged triarylamine derivative with s-chiral side chains (S-CBT). S-CBT
exhibits different aggregation behaviours, dependent on the used solvent. While SCBT
emerges fully dissolved in tetrahydrofuran (THF), n-dodecane (DOD) induces
H-type like aggregation of the single building blocks to supra molecular nanofibres. The
measurement of the coherence decay S-CBT in THF revealed a exclusively 2P allowed
electronic state S2, which was not known before. Hence, the induced transition in these
experiments does not take place between the electronic ground state S0 and the first
excited state S1, but between S0 and S2. The results on the linear chirp variation and the
two-color-pulse sequence both exhibited asymmetric PL intensities, as a function of the
linear chirp parameter and the delay between the double pulse sequence, respectively, in
analogy to MeLPPP. Two possible processes were considered, which could give rise to
these asymmetric shapes: (i) the simultaneous stimulated emission (SE) of two-photons
and (ii) the intersystem crossing (ISC) rate between the accessed state S2 and an
iso-energetic excited triplet state T2, which is dependent on the spectral phase of the
laser pulse. Although the first option can not be excluded, further measurements on
the existence of the triplet manifold, its energetic landscape and calculations of the
shape of the generated wave packet in S2 strongly corroborate the second option.
Changing the solvent from THF to DOD opens up the path for supramolecular
nanofibres with clear H-type interaction characteristics between the S-CBT building
blocks. In analogy to S-CBT in THF similar experiment for DOD were carried out. In contrast to the H-type characteristic decrease of the PL quantum yields of S-CBT in
DOD and the suppression of the 0-0 transition from the lowest excited singlet exciton
band, the results of the ultrafast measurements for S-CBT in DOD resemble the data
obtained for S-CBT in THF. In other words, the aggregation only seems to have
minor influence on the accessed second excited singlet state S2, if at all. This agrees
with theoretical predictions, which state that in case of a non-resonant two-photon
transition, the limiting lifetime of the virtual state impedes efficient interaction of the
dipole moments. Also for S-CBT in DOD the asymmetric linear chirp and two-color
pulse sequence measurements could be explained by either stimulated emission of two
photons or the wave packet shape dependent intersystem crossing rate between the
excited singlet and triplet states. Latter option is further corroborated by the results on
S-CBT in THF, but also by the fact that the electronic structure of the triplet manifold
does not change considerably for the structural change from the isolated to the single
nanofibre.
In conclusion, it has been shown, that an experimentally and theoretically joint
approach is capable of measuring the ultrafast dynamics and energetic landscape of
single organic molecules in a complex environment at room temperature. In doing so
non-resonant 2P absorption processes were exploited to measure the ultrafast coherence
decay envelope and achieve quantum mechanical coherent control over a (2+1)-photon
transition. In the second part ultrafast measurement on a compound were presented,
which exhibits an aggregation behaviour, strongly dependent on the used solvent.
For the isolated monomer and its H-aggregated form it was shown, that next to the
electronic states found in the linear absorption, this system exhibits an exclusively 2P
allowed singlet state, which does not change its electronic structure upon aggregation.
Furthermore, it is highly likely that phase-only modulation approaches are capable
of coherently controlling this singlet-triplet transition by the generation of different
shapes of wave packets in the excited state.