7 research outputs found
Hydrophilic Conjugated Polymers Prepared by Aqueous HornerâWadsworthâEmmons Coupling
The synthesis of hydrophilic conjugated
polymers typically relies on organometallic coupling methodologies.
Here we present an approach to prepare polar polyÂ(aryleneâvinylene)Âs
(PAVs) in water using the HornerâWadsworthâEmmons (HWE)
reaction. The additional preparation of discrete arylene vinylene
(AVs) afforded insight into HWE kinetics and regioselectivity. Nine
novel PAVs and AVs were synthesized, characterized by UVâvis
absorption and ultraviolet photoelectron spectroscopy, and studied
for their utility in sensing and photovoltaic applications
Light Harvesting Arrays of Polypyridine Ruthenium(II) Chromophores Prepared by Reversible AdditionâFragmentation Chain Transfer Polymerization
Polystyrene-based polymers that feature pendant RuÂ(II)
polypyridine
chromophores have been prepared by using reversible additionâfragmentation
chain transfer (RAFT) polymerization combined with the azideâalkyne
click reaction. RAFT polymerization was effected using 4-chloromethylstyrene
to afford functional polymers with <i>M</i><sub>n</sub> values
of 3300 and 8600 g/mol with low polydispersity. Reaction of the resulting
polyÂ(4-chloromethylstyrene)Âs with azide ion afforded the corresponding
polyÂ(4-azidomethylstyrene)Âs, which were further reacted in an azideâalkyne
click reaction with (5-ethynyl-1,10-phenanthroline)-bisÂ(2,2-bipyridine)ÂrutheniumÂ(II)
to afford the chromophore loaded polymers. The reactions were followed
by using nuclear magnetic resonance and infrared spectroscopy, and
the results suggest that the click reactions lead to nearly quantitative
functionalization of the azidomethyl functional polymers. The photophysical
and electrochemical properties of the Ru functional polymers were
characterized in solution. Emission quantum yield and lifetime studies
reveal that the metal-to-ligand charge transfer excited state is quenched
in the polymers relative to a model Ru complex chromophore. This finding
indicates that the thiol end-group on the polymers that arises from
the thiocarbonylthio RAFT chain transfer agent is able to quench the
MLCT state, presumably by a charge transfer mechanism. SternâVolmer
quenching studies show that the polymers are quenched with very high
efficiency by negatively charged ions compared to model systems, revealing
amplified quenching takes place
Ultrafast Formation of a Long-Lived Charge-Separated State in a Ru-Loaded Poly(3-hexylthiophene) Light-Harvesting Polymer
A light-harvesting macromolecular
assembly (PT-Ru) consisting of
a polyÂ(3-hexylthiophene) (P3HT) scaffold and pendant RuÂ(II) polypyridyl
complexes that exhibits rapid and efficient formation of a long-lived
charge-separated state is described here. Photoinduced electron transfer
from the polymer backbone to RuÂ(II) was investigated by femtosecond
transient absorption spectroscopy. Photoexcitation at 388 nm results
in the excitation of both the polymer backbone and RuÂ(II) complexes,
with relative excitation probabilities of 60 and 40%, respectively.
The dominant pathway (âź85%) for decay of the polymer excited
state is direct electron transfer from the polymer scaffold to RuÂ(II),
forming a positive polaron and a reduced complex [Ru<sup>II</sup>(L)<sub>2</sub>(L<sup>â</sup>)]<sup>+</sup>, denoted RuÂ(I). The charge-separated
state PT<sup>+â˘</sup>-RuÂ(I) is long-lived, persisting for 20â60
Îźs, and is attributed to the high mobility of holes on the polymer
backbone, which facilitates spatial separation of the electron and
hole, delaying recombination. The remaining 15% of the polymer excited
states undergo an alternate deactivation mechanism, possibly energy
transfer to RuÂ(II), forming RuÂ(II)*. RuÂ(II)* formed by either direct
excitation or energy transfer undergoes back energy transfer to the
scaffold, forming the low-lying polymer triplet state on the nanosecond
time scale
Light Harvesting and Charge Separation in a ĎâConjugated Antenna Polymer Bound to TiO<sub>2</sub>
This
paper describes the photophysical and photoelectrochemical
characterization of a light harvesting polychromophore array featuring
a polyfluorene backbone with covalently attached RuÂ(II) polypyridyl
complexes (PF-Ru-A), adsorbed on the surface of mesostructured TiO<sub>2</sub> (PF-Ru-A//TiO<sub>2</sub>). The surface adsorbed polymer
is characterized by transmission electron microscopy (TEM), scanning
electron microscopy (SEM), and attenuated total reflectance-Fourier
transform infrared (ATR-FTIR) spectroscopy, providing evidence for
the morphology of the surface adsorbed polymer and the mode of binding.
Photoexcitation of the RuÂ(II) complexes bound to the metal oxide surface
(proximal) results in electron injection into the conduction band
of TiO<sub>2</sub>, which is then followed by ultrafast hole transfer
to the polymer to form oxidized polyfluorene (PF<sup>+</sup>). More
interestingly, chromophores that are not directly bound to the TiO<sub>2</sub> interface (distal) that are excited participate in site-to-site
energy transfer processes that transport the excited state to surface
bound chromophores where charge injection occurs, underscoring the
antenna-like nature of the polymer assembly. The charge separated
state is long-lived and persists for >100 Îźs, a consequence
of the increased separation between the hole and injected electron
Poly(sulfobetaine methacrylate)s as Electrode Modifiers for Inverted Organic Electronics
We
demonstrate the use of polyÂ(sulfobetaine methacrylate) (PSBMA),
and its pyrene-containing copolymer, as solution-processable work
function reducers for inverted organic electronic devices. A notable
feature of PSBMA is its orthogonal solubility relative to solvents
typically employed in the processing of organic semiconductors. A
strong permanent dipole moment on the sulfobetaine moiety was calculated
by density functional theory. PSBMA interlayers reduced the work function
of metals, graphene, and polyÂ(3,4-ethylenedioxythiophene):polyÂ(styrenesulfonate)
(PEDOT:PSS) by over 1 eV, and an ultrathin interlayer of PSBMA reduced
the electron injection barrier between indium tin oxide (ITO) and
C<sub>70</sub> by 0.67 eV. As a result, the performance of organic
photovoltaic devices with PSBMA interlayers is significantly improved,
and enhanced electron injection is demonstrated in electron-only devices
with ITO, PEDOT:PSS, and graphene electrodes. This work makes available
a new class of dipole-rich, counterion-free, pH insensitive polymer
interlayers with demonstrated effectiveness in inverted devices
Competition between Ultrafast Energy Flow and Electron Transfer in a Ru(II)-Loaded Polyfluorene Light-Harvesting Polymer
This Letter describes the synthesis and photophysical
characterization
of a RuÂ(II) assembly consisting of metal polypyridyl complexes linked
together by a polyfluorene scaffold. Unlike many scaffolds incorporating
saturated linkages, the conjugated polymer in this system acts as
a functional light-harvesting component. Conformational disorder breaks
the conjugation in the polymer backbone, resulting in a chain composed
of many chromophore units, whose relative energies depend on the segment
lengths. Photoexcitation of the polyfluorene by a femtosecond laser
pulse results in the excitation of polyfluorene, which then undergoes
direct energy transfer to the pendant RuÂ(II) complexes, producing
RuÂ(II)* excited states within 500 fs after photoexcitation. Femtosecond
transient absorption data show the presence of electron transfer from
PF* to RuÂ(II) to form charge-separated (CS) products within 1â2
ps. The decay of the oxidized and reduced products, PF<sup>+â˘</sup> and RuÂ(I), through back electron transfer are followed using picosecond
transient absorption methods
Intrinsic and Extrinsic Parameters for Controlling the Growth of Organic Single-Crystalline Nanopillars in Photovoltaics
The
most efficient architecture for achieving high donor/acceptor
interfacial area in organic photovoltaics (OPVs) would employ arrays
of vertically interdigitated p- and n- type semiconductor nanopillars
(NPs). Such morphology could have an advantage in bulk heterojunction
systems; however, precise control of the dimension morphology in a
crystalline, interpenetrating architecture has not yet been realized.
Here we present a simple, yet facile, crystallization technique for
the growth of vertically oriented NPs utilizing a modified thermal
evaporation technique that hinges on a fast deposition rate, short
substrateâsource distance, and ballistic mass transport. A
broad range of organic semiconductor materials is beneficial from
the technique to generate NP geometries. Moreover, this technique
can also be generalized to various substrates, namely, graphene, PEDOTâPSS,
ZnO, CuI, MoO<sub>3</sub>, and MoS<sub>2</sub>. The advantage of the
NP architecture over the conventional thin film counterpart is demonstrated
with an increase of power conversion efficiency of 32% in photovoltaics. This technique will advance the
knowledge of organic semiconductor crystallization and create opportunities
for the fabrication and processing of NPs for applications that include
solar cells, charge storage devices, sensors, and vertical transistors