3 research outputs found
Controlling Electronic Transitions in Fullerene van der Waals Aggregates via Supramolecular Assembly
Morphologies
crucially determine the optoelectronic properties of organic semiconductors.
Therefore, hierarchical and supramolecular approaches have been developed
for targeted design of supramolecular ensembles of organic semiconducting
molecules and performance improvement of, <i>e.g</i>., organic
solar cells (OSCs), organic light emitting diodes (OLEDs), and organic
field-effect transistors (OFETs). We demonstrate how the photonic
properties of fullerenes change with the formation of van der Waals
aggregates. We identified supramolecular structures with broadly tunable
absorption in the visible spectral range and demonstrated how to form
aggregates with targeted visible (vis) absorption. To control supramolecular
structure formation, we functionalized the C60-backbone with polar
(bis-polyethylene glycol malonate-MPEG) tails, thus yielding an amphiphilic
fullerene derivative that self-assembles at interfaces. Aggregates
of systematically tuned size were obtained from concentrating MPEGC60
in stearic acid matrices, while different supramolecular geometries
were provoked via different thin film preparation methods, namely
spin-casting and Langmuir–Blodgett (LB) deposition from an
air–water interface. We demonstrated that differences in molecular
orientation in LB films (<i>C</i><sub>2<i>v</i></sub> type point group aggregates) and spin-casting (stochastic
aggregates) lead to huge changes in electronic absorption spectra
due to symmetry and orientation reasons. These differences in the
supramolecular structures, causing the different photonic properties
of spin-cast and LB films, could be identified by means of quantum
chemical calculations. Employing supramolecular assembly, we propounded
that molecular symmetry in fullerene aggregates is extremely important
in controlling vis absorption to harvest photons efficiently, when
mixed with a donor molecule, thus improving active layer design and
performance of OSCs
Absorption and Fluorescence Features of an Amphiphilic <i>meso</i>-Pyrimidinylcorrole: Experimental Study and Quantum Chemical Calculations
Corroles
are emerging as an important class of macrocycles with
numerous applications because of their peculiar photophysical and
metal chelating properties. <i>meso</i>-Pyrimidinylcorroles
are easily deprotonated in certain solvents, which changes their absorption
and emission spectra as well as their accessible supramolecular structures.
To enable control over the formation of supramolecular structures,
the dominant corrole species, i.e., the deprotonated form or one of
the two NH-tautomers, needs to be identified. Therefore, we focus
in the present article on the determination of the UV–vis spectroscopic
properties of the free-base NH-tautomers and the deprotonated form
of a new amphiphilic <i>meso</i>-pyrimidinylcorrole that
can assemble to supramolecular structures at heterointerfaces as utilized
in the Langmuir–Blodgett and liquid–liquid interface
precipitation techniques. After quantification of the polarities of
the free-base NH-tautomers and the deprotonated form by means of quantum
chemically derived electrostatic potential distributions at the corroles’
van der Waals surfaces, the preferential stabilization of (some of)
the considered species in solvents of different polarity is identified
by means of absorption spectroscopy. For the solutions with complex
mixtures of species, we applied fluorescence excitation spectroscopy
to estimate the relative weights of the individual corrole species.
This technique might also be applied to identify dominating species
in molecularly thin films directly on the subphase’ surface
of Langmuir–Blodgett troughs. Supported by quantum chemical
calculations we were able to differentiate between the spectral signatures
of the individual NH-tautomers by means of fluorescence excitation
spectroscopy
On the Control of Chromophore Orientation, Supramolecular Structure, and Thermodynamic Stability of an Amphiphilic Pyridyl-Thiazol upon Lateral Compression and Spacer Length Variation
The supramolecular
structure essentially determines the properties of organic thin films.
Therefore, it is of utmost importance to understand the influence
of molecular structure modifications on supramolecular structure formation.
In this article, we demonstrate how to tune molecular orientations
of amphiphilic 4-hydroxy thiazole derivatives by means of the Langmuir–Blodgett
(LB) technique and how this depends on the length of an alkylic spacer
between the thiazole chromophore and the polar anchor group. Therefore,
we characterize their corresponding supramolecular structures, thermodynamic,
absorption, and fluorescence properties. Particularly, the polarization-dependence
of the fluorescence is analyzed to deduce molecular orientations and
their possible changes after annealing, i.e., to characterize the
thermodynamic stability of the individual solid state phases. Because
the investigated thiazoles are amphiphilic, the different solid state
phases can be formed and be controlled by means of the Langmuir–Blodgett
(LB) technique. This technique also allows to deduce atomistic supramolecular
structure motives of the individual solid phases and to characterize
their thermodynamic stabilities. Utilizing the LB technique, we demonstrate
that subtle molecular changes, like the variation in spacer length,
can yield entirely different solid state phases with distinct supramolecular
structures and properties