5 research outputs found
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Cove-Edge Graphene Nanoribbon Semiconductors: from Molecules to Devices
This dissertation presents research conducted on the structure-property relationships of cove-edge graphene nanoribbon (GNR) semiconductors from the scale of molecular conformation to device performance. The ribbons described here are made derived from perylene-3,4,9,10-tetracarboxylic acid diimide (PDI) and adopt a helical conformation so we call them helical PDI (hPDI). They are n-type semiconductors with exceptional performance in field-effect transistors (FETs), organic photovoltaics (OPVs), narrowband photodetectors, and electron transporting materials in perovskite solar cells. In this work, reaction chemistry is used to design and synthesize new derivatives of hPDI to shine light on their molecular, bulk, and device properties.
The first chapter concerns the incorporation of hPDI into alternating donor- acceptor (D-A) macromolecules to create materials with internal charge transfer (CT). Computational and spectroscopic techniques, including femtosecond transient absorption spectroscopy (fsTA), are used to probe the CT character of these materials. A large dihedral angle between donor and acceptor portions limits orbital overlap, leading to lowest energy excited state with HOMO localized on the donor and LUMO localized on the acceptor. Notably, internal CT improves the OPV performance of these oligomers over their parent hPDI, while analogous macromolecules without internal CT exhibit reduced OPV performance.
Chapter 2 details a method for side chain engineering of hPDI by installing the side chain in the final step of the synthesis, rather than the first. The aromatic core of hPDI is built up with esters, rather than imides, appending the edges of the ribbons. The ester-appended ribbons are readily transformed into a late-stage intermediate for divergent installation of any desired side chains, including those that pose synthetic challenges when they are introduced into the parent PDI from the beginning. These side chains have a profound effect on the optical, thermal, and charge transport properties of hPDI in the solid state. This strategy of introducing imide side-chains into PDI-based materials in the final step can be generalized to other systems.
Chapter 3 demonstrates a method for controlling the conformation of cove-edge GNRs by changing the chemical substitution pattern at their edges. All-sp2 substituents that lock adjacent edge positions into a ring rigidify the aromatic core of these ribbons. When substituents at adjacent edge positions are no longer locked into a ring, the aromatic core becomes flexible. Modulating this flexibility dictates how these ribbons contort to accommodate their cove-edges, with rigid cores contorting into chiral helixes, and flexible cores contorting into a butterfly conformation. This may point the way forward for the use of GNRs in applications that rely on precise control of molecular conformatio
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Local electronic and chemical structure of oligo-acetylene derivatives formed through radical cyclizations at a surface.
Semiconducting π-conjugated polymers have attracted significant interest for applications in light-emitting diodes, field-effect transistors, photovoltaics, and nonlinear optoelectronic devices. Central to the success of these functional organic materials is the facile tunability of their electrical, optical, and magnetic properties along with easy processability and the outstanding mechanical properties associated with polymeric structures. In this work we characterize the chemical and electronic structure of individual chains of oligo-(E)-1,1'-bi(indenylidene), a polyacetylene derivative that we have obtained through cooperative C1-C5 thermal enediyne cyclizations on Au(111) surfaces followed by a step-growth polymerization of the (E)-1,1'-bi(indenylidene) diradical intermediates. We have determined the combined structural and electronic properties of this class of oligomers by characterizing the atomically precise chemical structure of individual monomer building blocks and oligomer chains (via noncontact atomic force microscopy (nc-AFM)), as well as by imaging their localized and extended molecular orbitals (via scanning tunneling microscopy and spectroscopy (STM/STS)). Our combined structural and electronic measurements reveal that the energy associated with extended π-conjugated states in these oligomers is significantly lower than the energy of the corresponding localized monomer orbitals, consistent with theoretical predictions
Local Electronic and Chemical Structure of Oligo-acetylene Derivatives Formed Through Radical Cyclizations at a Surface
Semiconducting
Ï€-conjugated polymers have attracted significant
interest for applications in light-emitting diodes, field-effect transistors,
photovoltaics, and nonlinear optoelectronic devices. Central to the
success of these functional organic materials is the facile tunability
of their electrical, optical, and magnetic properties along with easy
processability and the outstanding mechanical properties associated
with polymeric structures. In this work we characterize the chemical
and electronic structure of individual chains of oligo-(<i>E</i>)-1,1′-biÂ(indenylidene), a polyacetylene derivative that we
have obtained through cooperative C1–C5 thermal enediyne cyclizations
on Au(111) surfaces followed by a step-growth polymerization of the
(<i>E</i>)-1,1′-biÂ(indenylidene) diradical intermediates.
We have determined the combined structural and electronic properties
of this class of oligomers by characterizing the atomically precise
chemical structure of individual monomer building blocks and oligomer
chains (via noncontact atomic force microscopy (nc-AFM)), as well
as by imaging their localized and extended molecular orbitals (via
scanning tunneling microscopy and spectroscopy (STM/STS)). Our combined
structural and electronic measurements reveal that the energy associated
with extended π-conjugated states in these oligomers is significantly
lower than the energy of the corresponding localized monomer orbitals,
consistent with theoretical predictions
Macrocyclization in the Design of Organic n‑Type Electronic Materials
Here, we compare analogous cyclic
and acyclic π-conjugated
molecules as n-type electronic materials and find that the cyclic
molecules have numerous benefits in organic photovoltaics. This is
the first report of such a direct comparison. We designed two conjugated
cycles for this study. Each comprises four subunits: one combines
four electron-accepting, redox-active, diphenyl-perylenediimide subunits,
and the other alternates two electron-donating bithiophene units with
two diphenyl-perylenediimide units. We compare the macrocycles to
acyclic versions of these molecules and find that, relative to the
acyclic analogs, the conjugated macrocycles have bathochromically
shifted UV–vis absorbances and are more easily reduced. In
blended films, macrocycle-based devices show higher electron mobility
and good morphology. All of these factors contribute to the more than
doubling of the power conversion efficiency observed in organic photovoltaic
devices with these macrocycles as the n-type, electron transporting
material. This study highlights the importance of geometric design
in creating new molecular semiconductors. The ease with which we can
design and tune the electronic properties of these cyclic structures
charts a clear path to creating a new family of cyclic, conjugated
molecules as electron transporting materials in optoelectronic and
electronic devices