Kirk S. Schanze

Conjugated Polyelectrolytes: Synthesis, Photophysics, and Applications

Organic optoelectronic polymers have evolved to the point where fine structural control of the conjugated main chain, coupled with solubilizing and property-modifying pendant substituents, provides an entirely new class of materials. Conjugated polyelectrolytes (CPEs) provide a unique set of properties, including water solubility and processability, main-chain-controlled exciton and charge transport, variable band gap light absorption and fluorescence, ionic interactions, and aggregation phenomena. These characteristics allow these materials to be considered for use in applications ranging from light-emitting diodes and electrochromic color-changing displays, to photovoltaic devices and photodetectors, along with chemical and biological sensors. This Review describes the evolution of CPE structures from simple polymers to complex materials, describes numerous photophysical aspects, including amplified quenching in macromolecules and aggregates, and illustrates how the physical and electronic properties lead to useful applications in devices.

Conjugated Polyelectrolyte-Based Real-Time Fluorescence Assay for Alkaline Phosphatase with Pyrophosphate as Substrate

The fluorescence of the anionic, carboxylate-substituted poly(phenylene ethynylene) polymer PPECO2 is quenched very efficiently via the addition of 1 equiv of Cu(2+). Addition of pyrophosphate (PPi) into the weakly fluorescent solution of PPECO2 and Cu(2+) induces recovery of the polymers fluorescence; the recovery occurs because PPi complexes with Cu(2+), effectively sequestering the ion so it cannot bind to the carboxylate groups of the polymer. A calibration curve was developed that relates the extent of fluorescence recovery to [PPi], making the PPECO2-Cu(2+) system a sensitive and selective turn-on sensor for PPi. Using the PPECO2-Cu(2+) system as the signal transducer, a real-time fluorescence turn-off assay for the enzyme alkaline phosphatase (ALP) using PPi as the substrate is developed. The assay operates with [PPi] in the micromolar range, and it offers a straightforward and rapid detection of ALP activity with the enzyme present in the nanomolar concentration range, operating either in an end point or real-time format. Kinetic and product inhibition parameters are derived by converting time-dependent fluorescence intensity into PPi (substrate) concentration, thus allowing calculation of the initial reaction rates (v(o)). Weak, nonspecific fluorescence responses are observed concomitant to addition of other proteins to the assay solution; however, the signal response to ALP is demonstrated to arise from the ALP catalyzed hydrolysis of PPi to phosphate (Pi).

Mechanistic understanding of surface plasmon assisted catalysis on a single particle: cyclic redox of 4-aminothiophenol

Surface plasmon assisted catalysis (SPAC) reactions of 4-aminothiophenol (4ATP) to and back from 4,4′-dimercaptoazobenzene (DMAB) have been investigated by single particle surface enhanced Raman spectroscopy, using a self-designed gas flow cell to control the reductive/oxidative environment over the reactions. Conversion of 4ATP into DMAB is induced by energy transfer (plasmonic heating) from surface plasmon resonance to 4ATP, where O2 (as an electron acceptor) is essential and H2O (as a base) can accelerate the reaction. In contrast, hot electron (from surface plasmon decay) induction drives the reverse reaction of DMAB to 4ATP, where H2O (or H2) acts as the hydrogen source. More interestingly, the cyclic redox between 4ATP and DMAB by SPAC approach has been demonstrated. This SPAC methodology presents a unique platform for studying chemical reactions that are not possible under standard synthetic conditions.

Platinum-acetylide polymer based solar cells: involvement of the triplet state for energy conversion.

Relatively efficient photovoltaic devices were fabricated using blends of a phosphorescent platinum-acetylide polymer and a fullerene (PCBM); involvement of the triplet excited state of the platinum-acetylide polymer in photoinduced charge transfer is believed to contribute to the device efficiency.

Light-Induced Biocidal Action of Conjugated Polyelectrolytes Supported on Colloids

A series of water soluble, cationic conjugated polyelectrolytes (CPEs) with backbones based on a poly(phenylene ethynylene) repeat unit structure and tetraakylammonium side groups exhibit a profound light-induced biocidal effect. The present study examines the biocidal activity of the CPEs, correlating this activity with the photophysical properties of the polymers. The photophysical properties of the CPEs are studied in solution, and the results demonstrate that direct excitation produces a triplet excited-state in moderate yield, and the triplet is shown to be effective at sensitizing the production of singlet oxygen. Using the polymers in a format where they are physisorbed or covalently grafted to the surface of colloidal silica particles (5 and 30 microm diameter), we demonstrate that they exhibit light-activated biocidal activity, effectively killing Cobetia marina and Pseudomonas aeruginosa. The light-induced biocidal activity is also correlated with a requirement for oxygen suggesting that interfacial generation of singlet oxygen is the crucial step in the light-induced biocidal activity.

Low-Band-Gap Platinum Acetylide Polymers as Active Materials for Organic Solar Cells

We report on two pairs of platinum acetylide based polymers and model oligomers utilizing a 2,1,3-benzothiadiazole (BTD) acceptor moiety flanked on either side by either 2,5-thienyl donor units (Pt2BTD-Th and p-PtBTD-Th) or (3,4-ethylenedioxy)-2,5-thienyl donors (Pt2BTD-EDOT and p-PtBTD-EDOT). Both oligomer/polymer pairs absorb strongly throughout the visible region; however, because the (ethylenedioxy)thiophene moiety is a stronger donor than thiophene, the latter oligomer/polymer pair has a correspondingly lower band gap and, therefore, harvests light more efficiently at longer wavelengths. p-PtBTD-Th exhibits a relatively narrow molecular weight distribution with a number-average molecular weight (Mn) of 22 kDa, while p-PtBTD-EDOT exhibits a comparable Mn of 33 kDa but has a high polydispersity index likely due to aggregation. We provide a complete report of the photophysical and electrochemical characterization of the two oligomer/polymer pairs. The photophysical studies reveal that the materials undergo relatively efficient intersystem crossing. In a discussion of the energetics of photoinduced electron transfer from the platinum polymers to [6,6]-phenyl C61 butyric acid methyl ester (PCBM), it is noted that while the singlet state is quenched efficiently, the triplet state is not quenched, indicating that charge generation in the photovoltaic materials must ensue from the singlet manifold. Finally, organic photovoltaic devices based on blends of p-PtBDT-Th or p-PtBDT-EDOT with PCBM were characterized under monochromatic and simulated solar (AM1.5) illumination. Optimized devices exhibit an open-circuit voltage (Voc) of approximately 0.5 V, a short-circuit current density (Isc) of approximately 7.2 mA cm(-2), and a fill factor of approximately 35%, which yields overall power conversion efficiencies of 1.1-1.4%.

Photophysics of metal-organic π-conjugated polymers

Abstract A series of polyaryleneethynylene π-conjugated polymers that contain fac -(5,5′-diethynyl-2,2′-bipyridine)Re I (CO) 3 Cl as part of the π-conjugated polymer backbone have been synthesized by Pd-mediated coupling chemistry. Three metal-organic polymers P10, P25 and P50 , have been prepared which contain, respectively, 10, 25 and 50 mol.% of the Re(I) repeat units in the polymer chain. These polymers have been characterized by 1 H- and 13 C-NMR and FTIR spectroscopy and gel permeation chromatography. The analysis indicate that the pholymers contain the intact Re(I) chromophore and M n values of 10–15 kDa are typical (GPC, relative to polystyrene). The metal-organic polymers feature two spectrally-distinct absorption bands, one due to the π, π ∗ absorption of the polymer backbone and another at lower energy which is attributed to the d π( Re )→π poly ∗ metal to ligand charge transfer (MLCT) absorption. The luminescence properties of the polymers have been examined in fluid solution at 298 K and in a 2-methyltetrahedrofuran (MTHF) solvent glass at 77 K. These spectroscopic studies reveal that: (1) fluorescence from the 1 π, π ∗ exciton state is observed at 2.80 eV in all of the polymers at 298 and 77 K, but the intensity and lifetime of the fluorescence is quenched as the mole fraction of Re in the polymers increases; (2) phosphorescence from the 3 π, π ∗ state of the conjugated polymer backbone is observed at 1.93 eV in P10–P50 at 77 K; (3) luminescence from the d π→π ∗ MLCT state is observed at 1.8 eV in the metal-organic polymers at 77 K.

Near-infrared electroluminescence from conjugated polymer/lanthanide porphyrin blends

Near-infrared-emitting polymer light-emitting diodes (PLEDs) have been fabricated using blends of conjugated polymers and lanthanide tetraphenylporphyrin complexes. Host polymers include MEH–PPV and a bis-alkoxy-substituted poly(p-phenylene) (PPP–OR11), and the lanthanide complexes include Yb(TPP)acac and Er(TPP)acac (where TPP=5,10,15,20-tetraphenylporphyrin and acac=acetylacetonate). Electroluminescence (EL) is observed at 977 nm from devices fabricated using MEH–PPV or PPP–OR11 blended with Yb(TPP)acac, and EL is observed at 1560 nm from a device fabricated using a blend of MEH–PPV and Er(TPP)acac. Visible EL from the host polymers is strongly suppressed in all of the devices, however, in the device fabricated using the PPP–OR11 polymer blue emission from the host is completely quenched. Very efficient quenching of the EL from the host in the PPP–OR11 device is believed to occur due to efficient Forster energy transfer, which is facilitated by the excellent spectral overlap between the PPP–OR11 fluores...

It Takes More Than an Imine: The Role of the Central Atom on the Electron-Accepting Ability of Benzotriazole and Benzothiadiazole Oligomers

We report on the comparison of the electronic and photophysical properties of a series of related donor-acceptor-donor oligomers incorporating the previously known 2H-benzo[d][1,2,3]triazole (BTz) moiety as the acceptor and the recently reported BTzTD acceptor, a hybrid of BTz and 2,1,3-benzothiadiazole (BTD). Although often implied in the polymer literature that BTz has good acceptor character, we show that this moiety is best described as a weak acceptor. We present electrochemical, computational, and photophysical evidence supporting our assertion that BTzTD is a strong electron acceptor while maintaining the alkylation ability of the BTz moiety. Our results show that the identity of the central atom (N or S) in the benzo-fused heterocyclic ring plays an important role in both the electron-accepting and the electron-donating ability of acceptor moieties with sulfur imparting a greater electron-accepting ability and nitrogen affording greater electron-donating character. We report on the X-ray crystal structure of a BTzTD trimer, which exhibits greater local aromatic character in the region of the triazole ring and contains an electron-deficient sulfur that imparts strong electron-accepting ability. Additionally, we examine the transient absorption spectra of BTzTD and BTz oligomers and report that the BTz core promotes efficient intersystem crossing to the triplet state, while the presence of the thiadiazole moiety in BTzTD leads to a negligible triplet yield. Additionally, while BTz does not function as a good acceptor, oligomers containing this moiety do function as excellent sensitizers for the generation of singlet oxygen.

Fluorescent ratiometric sensing of pyrophosphate via induced aggregation of a conjugated polyelectrolyte.

Direct detection of pyrophosphate (PPi) in aqueous solution is demonstrated using a cationic poly(phenylene-ethynylene) with polyamine side chains. Pyrophosphate-induced polymer aggregation causes a significant spectroscopic change, which in turn allows quantification of dissolved PPi using ratiometric signals.