5 research outputs found
Revisiting Secondary Structures in NCA Polymerization: Influences on the Analysis of Protected Polylysines
Two series (degree of polymerization:
20–200) of polylysines
with Z and TFA protecting groups were synthesized, and their behavior
in a range of analytical methods was investigated. Gel permeation
chromatography of the smaller polypeptides reveals a bimodal distribution,
which is lost in larger polymers. With the help of GPC, NMR, circular
dichroism (CD), and MALDI-TOF, it was demonstrated that the bimodal
distribution is not due to terminated chains or other side reactions.
Our results indicate that the bimodality is caused by a change in
secondary structure of the growing peptide chain that occurs around
a degree of polymerization of about 15. This change in secondary structure
interferes strongly with the most used analysis method for polymersî—¸GPCî—¸by
producing a bimodal distribution as an artifact. After deprotection,
the polypeptides were found to exhibit exclusively random coil conformation,
and thus a monomodal GPC elugram was obtained. The effect can be explained
by a 1.6-fold increase in the hydrodynamic volume at the coil–helix
transition. This work demostrates that secondary structures need to
be carefully considered when performing standard analysis on polypeptidic
systems
Monitoring the On-Surface Synthesis of Graphene Nanoribbons by Mass Spectrometry
We present a mass spectrometric approach
to characterize and monitor
the intermediates of graphene nanoribbon (GNR) formation by chemical
vapor deposition (CVD) on top of Au(111) surfaces. Information regarding
the repeating units, lengths, and termini can be obtained directly
from the surface sample by a modified matrix-assisted laser desorption/ionization
(MALDI) method. The mass spectrometric results reveal ample oxidative
side reactions under CVD conditions that can be drastically diminished
by the introduction of protective H<sub>2</sub> gas at ambient pressure.
Simultaneously, the addition of hydrogen extends the lengths of the
oligophenylenes and thus the final GNRs. Moreover, the prematurely
formed cyclodehydrogenation products during the oligomer growth can
be assigned by the mass spectrometric technique. The obtained mechanistic
insights provide valuable information for optimizing and upscaling
the bottom-up fabrication of GNRs. Given the important role of GNRs
as semiconductors, the mass spectrometric analysis provides a readily
available tool to characterize and improve their structural perfection
Vapor-Phase Transport Deposition, Characterization, and Applications of Large Nanographenes
Recently, chemical synthesis of a
range of large nanographene molecules
with various shapes and sizes opened a new path to utilize them in
various applications and devices. However, due to their extended aromatic
cores and high molecular weight, film formation of large nanographene
molecules, bearing more than 90 sp<sup>2</sup> carbon atoms in aromatic
cores, is very challenging, which has prevented their applications
such as in thin-film transistors. Here, we developed an effective
approach to prepare films of such large nanographene molecules using
a vapor-phase transport (VPT) technique based on molecule sublimation.
The VPT of these molecules was made possible by combining the molecules
and the target substrate in a small confinement of vacuum-sealed glass
tube, so that a small amount of sublimation can be utilized to create
films. Surprisingly, such heavy and large molecules can be deposited
on any substrate by this method to create films of assembled large
nanographene molecules while maintaining their aromatic cores intact,
which was confirmed using mass spectrometry measurements. Moreover,
field-effect transistors based on these films are depleted and show
significantly improved current on/off ratio compared to previous large
nanographene-based transistors fabricated using liquid-phase-based
process. Our work shows that VPT deposition can be a viable technique
to prepare films based on large nanographene molecules and potentially
other high molecular weight compounds, which may find exciting applications
in electronics and optoelectronics
Chemical Vapor Deposition Synthesis and Terahertz Photoconductivity of Low-Band-Gap <i>N</i> = 9 Armchair Graphene Nanoribbons
Recent
advances in bottom-up synthesis of atomically defined graphene
nanoribbons (GNRs) with various microstructures and properties have
demonstrated their promise in electronic and optoelectronic devices.
Here we synthesized <i>N</i> = 9 armchair graphene nanoribbons
(9-AGNRs) with a low optical band gap of ∼1.0 eV and extended
absorption into the infrared range by an efficient chemical vapor
deposition process. Time-resolved terahertz spectroscopy was employed
to characterize the photoconductivity in 9-AGNRs and revealed their
high intrinsic charge-carrier mobility of approximately 350 cm<sup>2</sup>·V<sup>–1</sup>·s<sup>–1</sup>
Photoinduced C–C Reactions on Insulators toward Photolithography of Graphene Nanoarchitectures
On-surface chemistry for atomically
precise sp<sup>2</sup> macromolecules
requires top-down lithographic methods on insulating surfaces in order
to pattern the long-range complex architectures needed by the semiconductor
industry. Here, we fabricate sp<sup>2</sup>-carbon nanometer-thin
films on insulators and under ultrahigh vacuum (UHV) conditions from
photocoupled brominated precursors. We reveal that covalent coupling
is initiated by C–Br bond cleavage through photon energies
exceeding 4.4 eV, as monitored by laser desorption ionization (LDI)
mass spectrometry (MS) and X-ray photoelectron spectroscopy (XPS).
Density functional theory (DFT) gives insight into the mechanisms
of C–Br scission and C–C coupling processes. Further,
unreacted material can be sublimed and the coupled sp<sup>2</sup>-carbon
precursors can be graphitized by e-beam treatment at 500 °C,
demonstrating promising applications in photolithography of graphene
nanoarchitectures. Our results present UV-induced reactions on insulators
for the formation of all sp<sup>2</sup>-carbon architectures, thereby
converging top-down lithography and bottom-up on-surface chemistry
into technology