Hydrophilic Conjugated Polymers Prepared by Aqueous Horner–Wadsworth–Emmons Coupling

The synthesis of hydrophilic conjugated polymers typically relies on organometallic coupling methodologies. Here we present an approach to prepare polar poly­(arylene–vinylene)­s (PAVs) in water using the Horner–Wadsworth–Emmons (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 UV–vis 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 Addition–Fragmentation Chain Transfer Polymerization

Polystyrene-based polymers that feature pendant Ru­(II) polypyridine chromophores have been prepared by using reversible addition–fragmentation chain transfer (RAFT) polymerization combined with the azide–alkyne click reaction. RAFT polymerization was effected using 4-chloromethylstyrene to afford functional polymers with <i>M</i>_n values of 3300 and 8600 g/mol with low polydispersity. Reaction of the resulting poly­(4-chloromethylstyrene)­s with azide ion afforded the corresponding poly­(4-azidomethylstyrene)­s, which were further reacted in an azide–alkyne click reaction with (5-ethynyl-1,10-phenanthroline)-bis­(2,2-bipyridine)­ruthenium­(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. Stern–Volmer 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 poly­(3-hexylthiophene) (P3HT) scaffold and pendant Ru­(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 Ru­(II) was investigated by femtosecond transient absorption spectroscopy. Photoexcitation at 388 nm results in the excitation of both the polymer backbone and Ru­(II) complexes, with relative excitation probabilities of 60 and 40%, respectively. The dominant pathway (∼85%) for decay of the polymer excited state is direct electron transfer from the polymer scaffold to Ru­(II), forming a positive polaron and a reduced complex [Ru^II(L)₂(L^–)]⁺, denoted Ru­(I). The charge-separated state PT^+•-Ru­(I) is long-lived, persisting for 20–60 μs, 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 Ru­(II), forming Ru­(II)*. Ru­(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 π‑Conjugated Antenna Polymer Bound to TiO₂

This paper describes the photophysical and photoelectrochemical characterization of a light harvesting polychromophore array featuring a polyfluorene backbone with covalently attached Ru­(II) polypyridyl complexes (PF-Ru-A), adsorbed on the surface of mesostructured TiO₂ (PF-Ru-A//TiO₂). 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 Ru­(II) complexes bound to the metal oxide surface (proximal) results in electron injection into the conduction band of TiO₂, which is then followed by ultrafast hole transfer to the polymer to form oxidized polyfluorene (PF⁺). More interestingly, chromophores that are not directly bound to the TiO₂ 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 μs, 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 poly­(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 poly­(3,4-ethylenedioxythiophene):poly­(styrenesulfonate) (PEDOT:PSS) by over 1 eV, and an ultrathin interlayer of PSBMA reduced the electron injection barrier between indium tin oxide (ITO) and C₇₀ 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 Ru­(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 Ru­(II) complexes, producing Ru­(II)* excited states within 500 fs after photoexcitation. Femtosecond transient absorption data show the presence of electron transfer from PF* to Ru­(II) to form charge-separated (CS) products within 1–2 ps. The decay of the oxidized and reduced products, PF^+• and Ru­(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 substrate–source 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, PEDOT–PSS, ZnO, CuI, MoO₃, and MoS₂. 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