30 research outputs found
All-Conjugated, All-Crystalline DonorâAcceptor Block Copolymers P3HTâ<i>b</i>âPNDIT2 via Direct Arylation Polycondensation
The synthesis and
characterization of all-conjugated, all-crystalline
donorâacceptor block copolymers (BCPs) containing polyÂ(3-hexylÂthiophene)
(P3HT) and polyÂ{[<i>N</i>,<i>N</i>â˛-bisÂ(2-octylÂdodecyl)Ânaphthalene-1,4,5,8-bisÂ(dicarboximide)-2,6-diyl]-<i>alt</i>-5,5â˛-(2,2â˛-bithiophene)} (PNDIT2) is presented.
Direct arylation polycondensation (DAP) of dibromoÂnaphthaleneÂdiimide
and bithiophene is carried out in the presence of P3HT end-cappers
to allow the in situ formation of BCPs P3HT-<i>b</i>-PNDIT2.
As-prepared, well-defined H-P3HT-Br with hydrogen and bromine chain
termini shows nonoptimal reactivity under the DAP conditions used.
Therefore, H-P3HT-Br is converted into either H-P3HT-Th (thiophene)
or H-P3HT-Mes (mesitylene), giving Îą,Ď-hetero-CâH
functionalized P3HT with modulated CâH reactivity. The influence
of the different CâH chain termini of P3HT on the ability to
act as end-capper and the resulting block structures is investigated
in detail using wavelength-dependent size exclusion chromatography
(SEC) and NMR spectroscopy. Different CâH reactivities of Îą,Ď-hetero-CâH
functionalized P3HT cause different contents of multiblocks, which
in turn lead to varied degrees of crystallinity. These results show
that careful tuning of CâH reactivity is a promising way to
obtain well-defined, all-conjugated block copolymers via DAP
Rational Use of Aromatic Solvents for Direct Arylation Polycondensation: CâH Reactivity versus Solvent Quality
The solvent for direct arylation
polycondensation (DAP) is of crucial
importance. For conjugated polymers exhibiting reduced solubility,
the choice of solvent decides on the maximum molecular weight that
can be achieved, hence, good aromatic solvents are generally desirable.
However, unintentional activation of CâH bonds present in aromatic
solvents under DAP conditions leads to in situ solvent termination
which competes with step growth. Here we evaluate relative CâH
reactivity and solvent quality of seven aromatic solvents for the
DAP of defect-free naphthalene diimide (NDI)-based copolymers of different
solubility. CâH reactivity is strongly reduced with increasing
degree of substitution for both chlorine and methyl substituents.
Mesitylene is largely CâH unreactive and, thus, albeit being
a moderate solvent, enables very high molecular weights at elevated
temperature for NDI copolymers with limited solubility
Expanding the Scope of Electron-Deficient CâH Building Blocks: Direct Arylation of Pyromellitic Acid Diimide
Direct
CâH activation of pyromellitic diimide (PMDI) is
reported for the first time. The method avoids cumbersome pathways
involving bromination usually required for further cross-coupling.
Good to excellent yields of mono- and di-substituted PMDI derivatives
can be obtained under optimized reaction conditions. The reaction
scope was also explored, and the materials were characterized with
respect to their thermal, optical, and electronic properties
Conformer Ring Flip Enhances Mechanochromic Performance of <i>ansa</i>-DonorâAcceptorâDonor Mechanochromic Torsional Springs
Mechanochromophores
based on conformational changes of
donorâacceptorâdonor
(DAD) springs allow sensing of forces acting on polymer chains by
monotonic changes of absorbance or photoluminescence (PL) wavelength.
Here, we identify a series of thiophene (D)-flanked quinoxalines (A)
as molecular torsional springs for force sensing in bulk polymers
at room temperature. The mode of DAD linkage to the polymer matrix
and linker rigidity are key parameters that influence the efficacy
of force transduction to the DAD spring and thus mechanochromic response,
as probed by in situ PL spectroscopy of bulk films during stressâstrain
experiments. The largest shift of the PL maximum, and thus the highest
sensitivity, is obtained from an ansa-DAD spring
exhibiting bridged D units and a stiff A linker. Using detailed spectroscopy
and density functional theory calculations, we reveal conformer redistribution
in the form of a thiophene ring flip as the major part of the overall
mechanochromic response. At forces as low as 27 pN at early stages
of deformation, the ring flip precedes mechanically induced planarization
of the ansa-DAD spring, the latter process producing
a PL shift of 21 nm nNâ1. Within the stressâstrain
diagram, the thiophene ring flip and DAD planarization are thus two
separated processes that also cause irreversible and reversible mechanochromic
responses, respectively, upon sample failure. As the thiophene ring
flip requires much smaller forces than planarization of the DAD spring,
such micromechanical motion gives access to sensing of tiny forces
and expands both sensitivity and the force range of conformational
mechanochromophores
Defect-free Naphthalene Diimide Bithiophene Copolymers with Controlled Molar Mass and High Performance via Direct Arylation Polycondensation
A highly efficient, simple, and environmentally
friendly protocol
for the synthesis of an alternating naphthalene diimide bithiophene
copolymer (PNDIT2) via direct arylation polycondensation (DAP) is
presented. High molecular weight (MW) PNDIT2 can be obtained in quantitative
yield using aromatic solvents. Most critical is the suppression of
two major termination reactions of NDIBr end groups: nucleophilic
substitution and solvent end-capping by aromatic solvents via CâH
activation. In situ solvent end-capping can be used to control MW
by varying monomer concentration, whereby end-capping is efficient
and MW is low for low concentration and vice versa. Reducing CâH
reactivity of the solvent at optimized conditions further increases
MW. Chain perfection of PNDIT2 is demonstrated in detail by NMR spectroscopy,
which reveals PNDIT2 chains to be fully linear and alternating. This
is further confirmed by investigating the optical and thermal properties
as a function of MW, which saturate at <i>M</i><sub>n</sub> â 20 kDa, in agreement with controls made by Stille coupling.
Field-effect transistor (FET) electron mobilities Ο<sub>sat</sub> up to 3 cm<sup>2</sup>/(V¡s) are measured using off-center
spin-coating, with FET devices made from DAP PNDIT2 exhibiting better
reproducibility compared to Stille controls
Direct S<sub>0</sub>âT Excitation of a Conjugated Polymer Repeat Unit: Unusual Spin-Forbidden Transitions Probed by Time-Resolved Electron Paramagnetic Resonance Spectroscopy
A detailed
understanding of the electronic structure of semiconducting
polymers and their building blocks is essential to develop efficient
materials for organic electronics. (Time-resolved) electron paramagnetic
resonance (EPR) is particularly suited to address these questions,
allowing one to directly detect paramagnetic states and to reveal
their spin-multiplicity, besides its clearly superior resolution compared
to optical methods. We present here evidence for a direct S<sub>0</sub>âT optical excitation of distinct triplet states in the repeat
unit of a conjugated polymer used in organic photovoltaics. These
states differ in their electronic structure from those populated via
intersystem crossing from excited singlet states. This is an additional
and so far unconsidered route to triplet states with potentially high
impact on efficiency of organic electronic devices
Spectroscopic Signature of Two Distinct HâAggregate Species in Poly(3-hexylthiophene)
In
an endeavor to correlate the optoelectronic properties of Ď-conjugated
polymers with their structural properties, we investigated the aggregation
of P3HT in THF solution within a temperature range from 300 to 5 K.
By detailed steady-state, site-selective, and time-resolved fluorescence
spectroscopy combined with FranckâCondon analyses, we show
that below a certain transition temperature (265 K) aggregates are
formed that prevail in different polymorphs. At 5 K, we can spectroscopically
identify two H-type aggregates with planar polymer backbones yet different
degree of order regarding their side chains. Upon heating, the H-character
of the aggregates becomes gradually eroded, until just below the transition
temperature the prevailing âaggregateâ structure is
that of still phase-separated, yet disordered main and side chains.
These conclusions are derived by analyzing the vibrational structure
of the spectra and from comparing the solution spectra with those
obtained from thin films that were cooled slowly from the melting
temperature to room temperature and that had been analyzed previously
by various X-ray techniques. In addition, site selectively recorded
fluorescence spectra show that there isî¸dependent on temperatureî¸energy
transfer from higher energy to lower energy aggregates. This suggests
that they must form clusters with dimensions of the exciton diffusion
length, i.e., several nanometers in diameter
Compatibilization of All-Conjugated Polymer Blends for Organic Photovoltaics
Compatibilization
of an immiscible binary blend comprising a conjugated
electron donor and a conjugated electron acceptor polymer with suitable
electronic properties upon addition of a block copolymer (BCP) composed
of the same building blocks is demonstrated. Efficient compatibilization
during melt-annealing is feasible when the two polymers are immiscible
in the melt, i.e. above the melting point of âź250 °C of
the semicrystalline donor polymer P3HT. To generate immiscibility
at these high temperatures, the acceptor polymer PCDTBT is equipped
with fluorinated side chains leading to an increased FloryâHuggins
interaction parameter. Compatibilization in bulk and thin films is
demonstrated, showing that the photovoltaic performance of pristine
microphase separated and nanostructured BCPs can also be obtained
for compatibilized blend films containing low contents of 10â20
wt % BCP. Thermodynamically stable domain sizes range between several
tens of microns for pure blends and âź10 nm for pure block copolymers.
In addition to controlling domain size, the amount of block copolymer
added dictates the ratio of edge-on and face-on P3HT crystals, with
compatibilized films showing an increasing amount of face-on P3HT
crystals with increasing amount of compatibilizer. This study demonstrates
the prerequisites and benefits of compatibilizing all-conjugated semicrystalline
polymer blends for organic photovoltaics
TBT Entirely Dominates the Electronic Structure of the Conjugated Copolymer PCDTBT: Insights from Time-Resolved Electron Paramagnetic Resonance Spectroscopy
Insight into the electronic structure
of conjugated polymers used
for organic electronics applications is of outstanding importance.
Time-resolved electron paramagnetic resonance spectroscopy of light-induced
triplet excitons provides access to the electronic structure with
molecular resolution. Systematically investigating building blocks
of increasing length and comparing the results with the polymer deepens
our understanding of the structureâfunction relationship in
organic semiconductors. Applying this approach to the copolymer polyÂ[<i>N</i>-9â˛-heptadecanyl-2,7-carbazole-<i>alt</i>-5,5-(4â˛,7â˛-di-2-thienyl-2â˛,1â˛,3â˛-benzoÂthiadiazole)]
(PCDTBT) known for its efficiency and device stability reveals the
electronic structure of the polymer as well as each of the smaller
building blocks to be dominated entirely by the TBT moiety. Hence,
the usual description of PCDTBT as a carbazole derivative is somewhat
misleading. Furthermore, delocalization extends along the backbone,
over at least two repeat units, and is consistent for singlet and
triplet excitons, quite in contrast to other pushâpull systems
previously investigated. DFT calculations of the spin density distribution
agree well with the experimental results and show the BP86 functional
to be superior to B3LYP in the given context. The polymer and all
its building blocks show a remarkable homogeneity that by ruling out
aggregation phenomena is ascribed to a rather rigid and planar backbone
geometry
In Situ Synthesis of Ternary Block Copolymer/Homopolymer Blends for Organic Photovoltaics
A detailed
investigation of in situ-synthesized all-conjugated block copolymer
(BCP) compatibilized ternary blends containing polyÂ(3-hexylthiophene)
(P3HT) and polyÂ{[<i>N</i>,<i>N</i>â˛-bisÂ(2-octyldodecyl)Ânaphthalene-1,4,5,8-bisÂ(dibcarboximide)-2,6-diyl]-<i>alt</i>-5,5â˛-(2,2â˛-bithiophene)} (PNDIT2) as donor
and acceptor polymers, respectively, is presented. Both polymers are
incompatible and show strong segregation in blends, which renders
compatibilization with their corresponding BCPs promising to enable
nanometer-phase-separated structures suitable for excitonic devices.
Here, we synthesize a ternary block copolymer/homopolymer blend system
and investigate the phase behavior as a function of block copolymer
molecular weight and different annealing conditions. The device performance
decreases on increasing annealing temperatures. To understand this
effect, morphological investigations including atomic force microscopy,
high-resolution transmission electron microscopy (HR-TEM), and grazing
incidence wide- and small-angle X-ray scattering (GIWAXS/GISAXS) are
carried out. On comparing domain sizes of pristine and compatibilized
blends obtained from GISAXS, a weak compatibilization effect appears
to take place for the in situ-synthesized ternary systems. The effect
of thermal annealing is most prevalent for all samples, which, for
the highest annealing temperature above the melting point of PNDIT2
(310 °C), ultimately leads to a change from the face-on to edge-on
orientation of PNDIT2, as seen in GIWAXS. This effect dominates and
decreases all photovoltaic parameters, irrespective of whether a pristine
or compatibilized blend is used