20 research outputs found
Advances in Polyynes to Model Carbyne
ConspectusThe formation and study of molecules
that model
the sp-hybridized
carbon allotrope, carbyne, is a challenging field of synthetic physical
organic chemistry. The target molecules, oligo- and polyynes, are
often the preferred candidates as models for carbyne because they
can be formed with monodisperse lengths as well as defined structures.
Despite a simple linear structure, the synthesis of polyynes is often
far from straightforward, due in large part to a highly conjugated
framework that can render both precursors and products highly reactive,
i.e., kinetically unstable. The vast majority of polyynes are formed
as symmetrical products from terminal alkynes as precursors via an
oxidative, acetylenic homocoupling reaction based on the Glaser, Eglinton–Galbraith,
and Hay reactions. These reactions are very efficient for the synthesis
of shorter polyynes (e.g., hexaynes and octaynes), but yields often
drop dramatically as a function of length for longer derivatives,
usually starting with the formation of decaynes. The most effective
approach to circumvent unstable precursors and products has been through
the incorporation of sterically demanding end groups that serve to
“protect” the polyyne skeleton. This approach was arguably
identified in the early 1950s by Bohlmann and co-workers with the
synthesis of tBu-end-capped polyynes. During the
next 50 years, a polyyne with 14 contiguous alkyne units remained
the longest isolated derivative until 2010, when the record was extended
to 22 alkyne units. The record length was broken again in 2020, when
a polyyne consisting of 24 alkynes was isolated and characterized.
Beyond polyynes, there have been several reports describing the potential
synthesis of carbyne, but conclusive characterization and proof of
structure have been tenuous. The sole example of synthetic carbyne
arises from synthesis within carbon nanotubes, when chains of thousands
of sp carbon atoms have been linked to form polydisperse samples of
carbyne. Thus, model compounds for carbyne, the polyynes, remain the
best means to examine and predict the experimental structure and properties
of this carbon allotrope.This Account will discuss the general
synthesis of polyynes using
homologous series of polyynes with up to 10 alkyne units as examples
(decaynes). The limited number of specific syntheses of series with
longer polyynes will then be presented and discussed in more detail
based on end groups. The monodisperse polyynes produced from these
synthetic efforts are then examined toward providing our best extrapolations
for the expected characteristics for carbyne based on 13C NMR spectroscopy, UV–vis spectroscopy, X-ray crystallography,
and Raman spectroscopy
Enhancing the Dispersibility of TiO<sub>2</sub> Nanorods and Gaining Control over Region-Selective Layer Formation
We
demonstrate that the dispersibility and reactivity of core–shell
TiO<sub>2</sub> nanorods (NRs) can be controlled significantly through
functionalization with a combination of ligands based on phosphonic
acid derivatives (PAs). Specifically, a glycol based PA allows dispersion
of the NRs in methanol (MeOH). On the other hand, incorporating an
alkyne terminated PA in the ligand shell of the NRs allows for a copper-catalyzed
alkyne–azide cycloaddition (CuAAC) reaction with an azide-patterned
aluminum oxide (AlO<sub><i>x</i></sub>) substrate and forms
a region-selectively deposited film of NRs. We clearly demonstrate
that the quality of the NR films correlates strongly with the stability
of the NR dispersions in the reaction medium. In particular, tuning
the concentration of alkyne PA in the ligand shell inhibits aggregation
of the NRs on the substrate, while reactivity for the CuAAC reaction
is maintained. The surface coverage with NRs fits the Langmuir model.
This study illustrates that surface functionalization of AlO<sub><i>x</i></sub> substrates can be effectively and conveniently controlled
through enhancing the dispersibility of the NRs using mixed ligand
shells
Addition Reactions of Olefins to Asphaltene Model Compounds
Addition reactions have been proposed
as a significant pathway
for coke formation in the liquid-phase cracking of heavy oils and
bitumens. In order to study the kinetics of olefin addition in the
liquid phase, two alkyl-bridged aromatic compounds, with molecular
weights of 899.70 g/mol and 1127.99 g/mol, were thermally cracked
with 1-hexadecene, 1-octadecene, or <i>trans</i>-stilbene,
in a batch microreactor at 375–430 °C for 15 to 45 min.
Reaction products were analyzed by gas chromatography, high-performance
liquid chromatography, matrix-assisted laser desorption/ionization
mass spectrometry (MALDI-MS), and proton nuclear magnetic resonance
(<sup>1</sup>H NMR) spectroscopy. Kinetic data indicate a first-order
reaction in model compound concentration, with energetics consistent
with a free-radical chain mechanism. Tandem MS/MS and <sup>1</sup>H NMR spectra of the products are consistent with olefin addition
through the alkyl bridge of the bridged aromatics. The results imply
that (i) the addition products are able to abstract hydrogen to give
detectable products faster than they decompose, and (ii) the addition
products can react even more readily than the parent compounds
Laser Desorption Mass Spectrometry of End Group-Protected Linear Polyynes: Evidence of Laser-Induced Cross-Linking
End group-protected
linear polyynes of composition Tr*–(CC)<sub><i>n</i></sub>–Tr* (with Tr* representing the super
trityl group and <i>n</i> = 2, 4, 6, 8, 10) and <i>t</i>Bu–(CC)<sub>6</sub>–<i>t</i>Bu (with <i>t</i>Bu being the tertiary butyl group) have
been studied by laser desorption ionization (LDI) time-of-flight (ToF)
mass spectrometry. <i>t</i>Bu-terminated polyyne molecules
show considerably higher stability during laser activation than Tr*-end-capped
polyyne molecules. A key feature is the abundant formation of oligomeric
species upon laser activation. Tandem mass spectrometry reveals strong
bonding within the oligomers which indicates cross-linking of the
former polyynes within the oligomers. The process is more abundantly
occurring and less energy demanding than the laser-induced coalescence
of C<sub>60</sub>. Cross-linking is more efficient with the smaller
end group (<i>t</i>Bu), and larger oligomers are formed
when the chain length of the polyyne increases, both a result of enhanced
interaction of the triple bonds in neighboring chains. The presence
of the matrix molecules in matrix-assisted (MA)ÂLDI hinders the polyyne
interaction, and oligomer formation is markedly reduced
Synthesis and Properties of Isomerically Pure Anthrabisbenzothiophenes
The synthesis of three heptacyclic heteroacenes is described, namely anthra[2,3-<i>b</i>:7,6-<i>b</i>′]bis[1]benzothiophenes (ABBTs). A stepwise sequence of aldol reactions provides regiochemical control, affording only the <i>syn</i>-isomer. The ABBTs are characterized by X-ray crystallography, UV–vis absorption, and emission spectroscopy, as well as cyclic voltammetry. Field effect transistors based on solution-cast thin films of ABBT derivatives exhibit charge-carrier mobilities of as high as 0.013 cm<sup>2</sup>/(V s)
Synthesis and Properties of Isomerically Pure Anthrabisbenzothiophenes
The synthesis of three heptacyclic heteroacenes is described, namely anthra[2,3-<i>b</i>:7,6-<i>b</i>′]bis[1]benzothiophenes (ABBTs). A stepwise sequence of aldol reactions provides regiochemical control, affording only the <i>syn</i>-isomer. The ABBTs are characterized by X-ray crystallography, UV–vis absorption, and emission spectroscopy, as well as cyclic voltammetry. Field effect transistors based on solution-cast thin films of ABBT derivatives exhibit charge-carrier mobilities of as high as 0.013 cm<sup>2</sup>/(V s)
Scalable, Chromatography-Free Synthesis of Alkyl-Tethered Pyrene-Based Materials. Application to First-Generation “Archipelago Model” Asphaltene Compounds
In
this paper, we report a highly efficient, scalable approach
to the total synthesis of conformationally unrestricted, electronically
isolated arrays of alkyl-tethered polycyclic aromatic chromophores.
This new class of modular molecules consists of polycyclic aromatic
“islands” comprising significant structural fragments
present in unrefined heavy petroleum, tethered together by short saturated
alkyl chains, as represented in the “archipelago model”
of asphaltene structure. The most highly branched archipelago compounds
reported here share an architecture with first-generation dendrimeric
constructs, making the convergent, chromatography-free synthesis described
herein particularly attractive for further extensions in scope and
applications to materials chemistry. The syntheses are efficient,
selective, and readily adaptable to a multigram scale, requiring only
inexpensive, “earth-abundant” transition-metal catalysts
for cross-coupling reactions and extraction and fractional crystallization
for purification. This approach avoids typical limitations in cost,
scale, and operational practicality. All of the archipelago compounds
and synthetic intermediates have been fully characterized spectroscopically
and analytically. The solid-state structure of one archipelago model
compound has been determined by X-ray crystallography
Synthesis of Polyyne Rotaxanes
Active-metal templating has been used to synthesize rotaxanes consisting of a phenanthroline-based macrocycle threaded around a C8, C12, or C20 polyyne chain. The crystal structure of the C12 rotaxane has been determined. In the rhenium(I) carbonyl complex of this rotaxane, with Re(CO)<sub>3</sub>Cl coordinated to the phenanthroline macrocycle, the proximity of the polyyne chain quenches the luminescence of the rhenium. These rotaxanes offer a new approach to controlling the environment and interactions of a polyyne chain
Synthesis of Polyyne Rotaxanes
Active-metal templating has been used to synthesize rotaxanes consisting of a phenanthroline-based macrocycle threaded around a C8, C12, or C20 polyyne chain. The crystal structure of the C12 rotaxane has been determined. In the rhenium(I) carbonyl complex of this rotaxane, with Re(CO)<sub>3</sub>Cl coordinated to the phenanthroline macrocycle, the proximity of the polyyne chain quenches the luminescence of the rhenium. These rotaxanes offer a new approach to controlling the environment and interactions of a polyyne chain
Polyyne Rotaxanes: Stabilization by Encapsulation
Active metal template Glaser coupling
has been used to synthesize
a series of rotaxanes consisting of a polyyne, with up to 24 contiguous <i>sp-</i>hybridized carbon atoms, threaded through a variety of
macrocycles. Cadiot–Chodkiewicz cross-coupling affords higher
yields of rotaxanes than homocoupling. This methodology has been used
to prepare [3]Ârotaxanes with two polyyne chains locked through the
same macrocycle. The crystal structure of one of these [3]Ârotaxanes
shows that there is extremely close contact between the central carbon
atoms of the threaded hexayne chains (C···C distance
3.29 Ă… vs 3.4 Ă… for the sum of van der Waals radii) and
that the bond-length-alternation is perturbed in the vicinity of this
contact. However, despite the close interaction between the hexayne
chains, the [3]Ârotaxane is remarkably stable under ambient conditions,
probably because the two polyynes adopt a crossed geometry. In the
solid state, the angle between the two polyyne chains is 74°,
and this crossed geometry appears to be dictated by the bulk of the
“supertrityl” end groups. Several rotaxanes have been
synthesized to explore gem-dibromoethene moieties as “masked”
polyynes. However, the reductive Fritsch–Buttenberg–Wiechell
rearrangement to form the desired polyyne rotaxanes has not yet been
achieved. X-ray crystallographic analysis on six [2]Ârotaxanes and
two [3]Ârotaxanes provides insight into the noncovalent interactions
in these systems. Differential scanning calorimetry (DSC) reveals
that the longer polyyne rotaxanes (C16, C18, and C24) decompose at
higher temperatures than the corresponding unthreaded polyyne axles.
The stability enhancement increases as the polyyne becomes longer,
reaching 60 °C in the C24 rotaxane