14 research outputs found
Vibrational Dynamics in Dendridic Oligoarylamines by Raman Spectroscopy and Incoherent Inelastic Neutron Scattering
Vibrational dynamics in triarylamine
dendrimers was studied in
a complementary way by Raman and infrared (IR) spectroscopies and
incoherent inelastic neutron scattering (IINS). Three molecules were
investigated, namely, unsubstituted triarylamine dendrimer of the
first generation and two dendrimers of the first and second generation,
substituted in the crown with butyl groups. To facilitate the assignment
of the observed IR and Raman modes as well as the IINS peaks, vibrational
models, based on the general valence force field method (GVFF), were
calculated for all three compounds studied. A perfect consistency
between the calculated and experimental results was found. Moreover,
an important complementarity of the vibrational spectroscopies and
IINS was established for the investigated dendrimers. The IINS peaks
originating mainly from the CâH motions were not restricted
by particular selection rules and only dependent on the IINS cross
section. To the contrary, Raman and IR bands were imposed by the selection
rules and the local geometry of the dendrimers yielding mainly CâC
and CâN deformation modes with those of CâH nature of
much lower intensity. Raman spectroscopy was also applied to the studies
of the oxidation of dendrimers to their cationic forms. A strong Raman
resonance effect was observed, since the spectra of the studied compounds,
registered at different levels of their oxidation, strongly depended
on the position of the excitation line with respect to their electronic
spectrum. In particular, the blue (458 nm) excitation line turned
out to be insensitive toward the cationic forms yielding very limited
spectral information. To the contrary, the use of the red (647 nm)
and infrared (1064 nm) excitation lines allowed for an unambiguous
monitoring of the spectral changes in dendrimers oxidized to nominally
monocationic and tricationic states. The analysis of oxidation-induced
spectral changes in the tricationic state indicated that the charge
storage configuration predominantly involved one spinless dication
of the quinoid bond sequence and one radical cation. However, small
numbers of dications were also found in a nominally monocationic state,
where only radical cations should have been present. This finding
was indicative of some inhomogeneity of the oxidation
Ferromagnetic Spin Coupling through the 3,4â˛-Biphenyl Moiety in Arylamine Oligomersî¸Experimental and Computational Study
This
report describes the study of a dimer <b>d</b><sup><b>2+</b></sup> and a linear trimer <b>t</b><sup><b>3+</b></sup> of amminium radical cations coupled by 3,4â˛-biphenyl
spin coupling units. The synthesis of the parent diamine and triamine
and their optical and electrochemical properties obtained by UVâvisible
and cyclic voltammetry are presented. The chemical doping of the parent
diamine <b>d</b> and triamine <b>t</b> was performed quantitatively
to obtain samples containing the corresponding dimer <b>d</b><sup><b>2+</b></sup> and trimer <b>t</b><sup><b>3+</b></sup> in almost pure high-spin states as evidenced by pulsed EPR
nutation spectroscopy. The <i>J</i> coupling constants of
the corresponding <i>S</i> = 1 and <i>S</i> =
3/2 spin states were measured (<i>J</i>/<i>k</i> = 135 K) and compared quantitatively to DFT calculations
Direct Evidence of Chlorine-Induced Preferential Crystalline Orientation in Methylammonium Lead Iodide Perovskites Grown on TiO<sub>2</sub>
Mixed
halide hybrid perovskites CH<sub>3</sub>NH<sub>3</sub>PbI<sub>3â<i>x</i></sub>Cl<sub><i>x</i></sub> (MAPICl)
show higher charge carrier diffusion lengths, improved solar cell
characteristics, and enhanced stability as compared to their single
halide counterparts CH<sub>3</sub>NH<sub>3</sub>PbI<sub>3</sub> (MAPI).
This is surprising in view of the fact that the actually observed
Cl content is very low (<i>x</i> < 0.1) as the incorporation
of large amounts of Cl in the MAPI lattice is crystallographically
not possible. Understanding the role of chlorine in these systems
is therefore of the utmost importance and has been the subject of
several experimental and theoretical studies. The main conclusions
are that Cl addition leads to larger grain sizes and preferential
crystalline orientation; however, so far a more quantitative description
of these aspects is lacking. Synchrotron X-ray diffraction microscopy
is a very promising technique in this respect, as it allows for the
direct imaging of crystalline domains oriented in a selected direction
with a resolution of some hundreds of nanometers. When imaging perovskite
thin films deposited on an FTO-covered glass substrate containing
a dense polycrystalline TiO<sub>2</sub> layer at the perovskite (220)
reflection, diffraction micrographs show an increase in surface coverage
of oriented grains from 2.5% (MAPI) to 5.5% (MAPICl; <i>x</i> = 0.07). This is a clear evidence of the increased preferential
orientation in the mixed perovskite. At the same time the integrated
intensity increases by a factor of 4, which can be related to a larger
perovskite grain size. When a single crystalline TiO<sub>2</sub>(001)
substrate is used for the perovskite film deposition, both effects
are dramatically enhanced in the case of MAPICl, leading to a surface
coverage of 80%, a 55-fold increase in integrated diffracted intensity
with respect to MAPI, and a mean crystallite size of 2.5 Îźm.
In contrast, MAPI does not show any differences when grown on TiO<sub>2</sub>(001), which demonstrates that chlorine locates preferentially
at the TiO<sub>2</sub>âperovskite interface and induces the
growth of larger crystallites showing preferential orientation along
the (110) direction
Tuning of Ferromagnetic Spin Interactions in Polymeric Aromatic Amines via Modification of Their ĎâConjugated System
Polyarylamine containing <i>metaâparaâpara</i>-aniline units in the main chain and <i>metaâpara</i>-aniline units in the pendant chains was synthesized. The polymer
can be oxidized to radical cations in chemical or electrochemical
ways. The presence of <i>meta</i>-phenylenes in the polymer
chemical structure allows for the ferromagnetic coupling of electronic
spins, which leads to the formation of high spin states. Detailed
pulsed-EPR study indicates that the <i>S</i> = 2 spin state
was reached for the best oxidation level. Quantitative magnetization
measurements reveal that the doped polymer contains mainly <i>S</i> = 2 spin states and a fraction of <i>S</i> =
3/2 spin states. The efficiency of the oxidation was determined to
be 74%. To the best of our knowledge, this polymer is the first example
of a linear doped polyarylamine combining such high spin states with
high doping efficiency
Tuning of Ferromagnetic Spin Interactions in Polymeric Aromatic Amines via Modification of Their ĎâConjugated System
Polyarylamine containing <i>metaâparaâpara</i>-aniline units in the main chain and <i>metaâpara</i>-aniline units in the pendant chains was synthesized. The polymer
can be oxidized to radical cations in chemical or electrochemical
ways. The presence of <i>meta</i>-phenylenes in the polymer
chemical structure allows for the ferromagnetic coupling of electronic
spins, which leads to the formation of high spin states. Detailed
pulsed-EPR study indicates that the <i>S</i> = 2 spin state
was reached for the best oxidation level. Quantitative magnetization
measurements reveal that the doped polymer contains mainly <i>S</i> = 2 spin states and a fraction of <i>S</i> =
3/2 spin states. The efficiency of the oxidation was determined to
be 74%. To the best of our knowledge, this polymer is the first example
of a linear doped polyarylamine combining such high spin states with
high doping efficiency
Tuning of Ferromagnetic Spin Interactions in Polymeric Aromatic Amines via Modification of Their ĎâConjugated System
Polyarylamine containing <i>metaâparaâpara</i>-aniline units in the main chain and <i>metaâpara</i>-aniline units in the pendant chains was synthesized. The polymer
can be oxidized to radical cations in chemical or electrochemical
ways. The presence of <i>meta</i>-phenylenes in the polymer
chemical structure allows for the ferromagnetic coupling of electronic
spins, which leads to the formation of high spin states. Detailed
pulsed-EPR study indicates that the <i>S</i> = 2 spin state
was reached for the best oxidation level. Quantitative magnetization
measurements reveal that the doped polymer contains mainly <i>S</i> = 2 spin states and a fraction of <i>S</i> =
3/2 spin states. The efficiency of the oxidation was determined to
be 74%. To the best of our knowledge, this polymer is the first example
of a linear doped polyarylamine combining such high spin states with
high doping efficiency
Tuning of Ferromagnetic Spin Interactions in Polymeric Aromatic Amines via Modification of Their ĎâConjugated System
Polyarylamine containing <i>metaâparaâpara</i>-aniline units in the main chain and <i>metaâpara</i>-aniline units in the pendant chains was synthesized. The polymer
can be oxidized to radical cations in chemical or electrochemical
ways. The presence of <i>meta</i>-phenylenes in the polymer
chemical structure allows for the ferromagnetic coupling of electronic
spins, which leads to the formation of high spin states. Detailed
pulsed-EPR study indicates that the <i>S</i> = 2 spin state
was reached for the best oxidation level. Quantitative magnetization
measurements reveal that the doped polymer contains mainly <i>S</i> = 2 spin states and a fraction of <i>S</i> =
3/2 spin states. The efficiency of the oxidation was determined to
be 74%. To the best of our knowledge, this polymer is the first example
of a linear doped polyarylamine combining such high spin states with
high doping efficiency
Activation Energy of Organic Cation Rotation in CH<sub>3</sub>NH<sub>3</sub>PbI<sub>3</sub> and CD<sub>3</sub>NH<sub>3</sub>PbI<sub>3</sub>: Quasi-Elastic Neutron Scattering Measurements and First-Principles Analysis Including Nuclear Quantum Effects
The
motion of CH<sub>3</sub>NH<sub>3</sub><sup>+</sup> cations in the low-temperature phase of
the promising photovoltaic material methylammonium lead triiodide
(CH<sub>3</sub>NH<sub>3</sub>PbI<sub>3</sub>) is investigated experimentally
as well as theoretically, with a particular focus on the activation
energy. Inelastic and quasi-elastic neutron scattering measurements
reveal an activation energy of âź48 meV. Through a combination
of experiments and first-principles calculations, we attribute this
activation energy to the relative rotation of CH<sub>3</sub> against
an NH<sub>3</sub> group that stays bound to the inorganic cage. The
inclusion of nuclear quantum effects through path integral molecular
dynamics gives an activation energy of âź42 meV, in good agreement
with the neutron scattering experiments. For deuterated samples (CD<sub>3</sub>NH<sub>3</sub>PbI<sub>3</sub>), both theory and experiment
observe a higher activation energy for the rotation of CD<sub>3</sub> against NH<sub>3</sub>, which results from the smaller nuclear quantum
effects in CD<sub>3</sub>. The rotation of the NH<sub>3</sub> group,
which is bound to the inorganic cage via strong hydrogen bonding,
is unlikely to occur at low temperatures due to its high energy barrier
of âź120 meV
Tuning of Ferromagnetic Spin Interactions in Polymeric Aromatic Amines via Modification of Their ĎâConjugated System
Polyarylamine containing <i>metaâparaâpara</i>-aniline units in the main chain and <i>metaâpara</i>-aniline units in the pendant chains was synthesized. The polymer
can be oxidized to radical cations in chemical or electrochemical
ways. The presence of <i>meta</i>-phenylenes in the polymer
chemical structure allows for the ferromagnetic coupling of electronic
spins, which leads to the formation of high spin states. Detailed
pulsed-EPR study indicates that the <i>S</i> = 2 spin state
was reached for the best oxidation level. Quantitative magnetization
measurements reveal that the doped polymer contains mainly <i>S</i> = 2 spin states and a fraction of <i>S</i> =
3/2 spin states. The efficiency of the oxidation was determined to
be 74%. To the best of our knowledge, this polymer is the first example
of a linear doped polyarylamine combining such high spin states with
high doping efficiency
Self-Assembly Properties of Semiconducting DonorâAcceptorâDonor Bithienyl Derivatives of Tetrazine and Thiadiazoleî¸Effect of the Electron Accepting Central Ring
Scanning
tunneling microscopy was used to study the effect of the
electron-accepting unit and the alkyl substituentâs position
on the type and extent of 2D supramolecular organization of penta-ring
donorâacceptorâdonor (DAD) semiconductors, consisting
of either tetrazine or thiadiazole central acceptor ring symmetrically
attached to two bithienyl groups. Microscopic observations of monomolecular
layers on HOPG of four alkyl derivatives of the studied adsorbates
indicate significant differences in their 2D organizations. Ordered
monolayers of thiadiazole derivatives are relatively loose and, independent
of the position of alkyl substituents, characterized by large intermolecular
separation of acceptor units in the adjacent molecules located in
the face-to-face configuration. The 2D supramolecular architecture
in both derivatives of thiadiazole is very sensitive to the alkyl
substituentâs position. Significantly different behavior is
observed for derivatives of tetrazine (which is a stronger electron
acceptor). Stronger intermolecular DA interactions in these adsorbates
generate an intermolecular shift in the monolayer, which is a dominant
factor determining the 2D structural organization. As a consequence
of this molecular arrangement, tetrazine groups (A segments) face
thiophene rings (D segments) of the neighboring molecules. Monolayers
of tetrazine derivatives are therefore much more densely packed and
characterized by similar Ď-stacking of molecules independently
of the position of alkyl substituents. Moreover, a comparative study
of 3D supramolecular organization, deduced from the X-ray diffraction
patterns, is also presented clearly confirming the polymorphism of
the studied adsorbates