Macroporous Polyzwitterionic Gels As Versatile Intermediates for the Fixation and Release of Anions.

A new stable and functional polyzwitterion poly[1-(carboxymethyl)-4-methacrylamidopyridin-1-ium] was synthesized. The zwitterionic polymer shows its isoelectric point at a pH of 4.2, bidirectional pH responsiveness, and formation of dendritic fractal self-aggregated structures. Using this as a common intermediate, a simple, direct, and scalable single-step protocol was established to introduce various elementary anions like NO3-, HSO4-, H2PO4-, F-, Cl-, Br-, I-, CH3COO-, and HCOO- in their salt forms by reaction with the corresponding acids. FESEM studies on cross-linked polymeric hydrogels established the macroporous nature of these materials with their pore size in the range of 10-15 µm. Bidirectional swelling behavior was observed in these hydrogels from gel swelling kinetics and pH studies. Anion release studies in deionized water and buffer solutions showed ∼82 and ∼95% cumulative release for nitrate and phosphate anions, respectively, in 72 h. Our studies suggest that multifunctional polyzwitterionic gels are promising intermediates in the fixation and release of anions like nitrate and phosphate with potential applications in agriculture and healthcare.

Design and synthesis of N-substituted perylene diimide based low band gap polymers for organic solar cell applications.

In this study, we report on the synthesis and device studies of a series of new copolymers containing N-substituted perylene dimide and dioctylfluorene units as part of the main backbone. A facile synthetic approach avoiding non-selective bromination was used to synthesize the monomer M1 by the reaction of perylene-3,4,9,10-tetracarboxylic dianhydride with 2-amino-7-bromo-9,9-dioctylfluorene. The copolymers P1 and P2 were synthesized by Suzuki polycondensation of M1 with 2,2'-(9,9-dioctyl-9H-fluoren-2,7-diyl)bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolane) M2 and 9-(heptadecan-9-yl)-2,7-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-9H-carbazole M3, respectively. The copolymer P3 was synthesized by direct arylation polymerization of M1 with 4,7-bis(4-octylthiophen-2-yl)benzo[c]-1,2,5-thiadiazole M4. All the copolymers showed thermal stability greater than 380 °C as evidenced from thermogravimetric analysis. The copolymers exhibited a narrow optical band gap (1.80-2.08 eV) with their UV-visible absorption spectra extending up to the NIR region and they are found to be suitable for use in OSC applications. The molecular weights of the polymers P1-P3 were found to be in the range of 10.68 to 16.02 kg mol-1 as measured from GPC analysis. The surface morphology of the active layers based on P1/P2/P3:P3HT blend films was investigated by AFM and the rms values from height images range from 0.65 to 2.90 nm. The polymers were blended with P3HT to fabricate BHJ solar cells in three different weight ratios i.e. 1 : 1, 1.5 : 1 and 2 : 1 and the best power conversion efficiency was observed for the binary film of P3:P3HT blend device in a 1 : 1 weight ratio which reached up to 1.96% with a V oc of 0.55 V, J sc of 10.12 mA cm-2 and FF of 34.63% which is among the highest reported for BHJ solar cells with N-substituted PDI based acceptors.

Single molecule studies of a ladder type conjugated polymer: vibronic spectra, line widths, and energy transfer.

Confocal fluorescence microscopy and spectroscopy are employed to investigate single poly(ladder-type pentaphenylene) (LPPentP) molecules dispersed in thin poly(methyl methacrylate) (PMMA) films at 1.2 K. Emission spectra of single chains show single as well as multi-chromophore emission indicating variegated communication along the chains. The vibronic structure in the emission spectra resembles the one found for other ladder-type polymers. Purely electronic zero-phonon lines in emission are substantially broadened, most probably due to fast spectral diffusion. By surmounting the limitations of emission spectroscopy, nonemitting donor chromophores, which transfer their excitation energy in a radiationless manner to emitting chromophores, are accessed by excitation spectroscopy. Remarkably, by comparing the data of emitting and nonemitting chromophores a contribution to the zero-phonon excitation line width has to be considered which places a lower limit on the estimated energy transfer time of several picoseconds between adjacent chromophores. Finally, the data indicate qualitatively a restricted flexibility of LPPentP compared to poly[2-methoxy-5-(2'-ethyl-hexyloxy)-1,4-phenylene vinylene] (MEH-PPV).

Interplay of α,α- versus α,ß-conjugation in the excited states and charged defects of branched oligothiophenes as models for dendrimeric materials.

This article investigates the excited and charged states of three branched oligothiophenes with methyl-thienyl side groups as models to promote 3D arrangements. A comparison with the properties of the parent systems, linear all-α,α-oligothiophenes, is proposed. A wide variety of spectroscopic methods (i.e., absorption, emission, triplet-triplet transient absorption, and spectroelectrochemistry) in combination with DFT calculations have been used for this purpose. Whereas the absorption spectra are slightly blueshifted upon branching, both the emission spectra and triplet-triplet absorption spectra are moderately redshifted; this indicates a larger contribution of the ß-linked thienyl groups in the delocalization of the S1 and T1 states rather than into the S0 state. The delocalization through the α,ß-conjugated path was found to be crucial for the stabilization of the trication species in the larger branched systems, whereas the linear sexithiophene homologue can only be stabilized up to the dication species.

Copolymers comprising 2,7-carbazole and bis-benzothiadiazole units for bulk-heterojunction solar cells.

On the basis of theoretical considerations of the intramolecular charge transfer (ICT) effect, we have designed a series of donor (D)-acceptor (A) conjugated polymers based on bis-benzothiadiazole (BBT). A PPP-type copolymer of electron-rich 2,7-carbazole (CZ) and electron-deficient BBT units poly[N-(2-decyltetradecyl)-2,7-carbazole-co-7,7'-{4,4'-bis-(2,1,3-benzothiadiazole)}] (PCZ-BBT), a PPV-type copolymer poly[N-(2-decyltetradecyl)-2,7-carbazolevinylene-co-7,7'-{4,4'-bis-(2,1,3-benzothiadiazolevinylene)}] (PCZV-BBTV), and a tercopolymer based on carbazole, thiophene, and BBT poly[N-(2-decyltetradecyl)-2,7-(di-2-thienyl)carbazole-co-7,7'-{4,4'-bis-(2,1,3-benzothiadiazole)}] (PDTCZ-BBT) have been synthesized to understand the influence of BBT acceptor structure and linkage on the photovoltaic characteristics of the resulting materials. Both the HOMO and LUMO of the resulting polymers are found to be deeper-lying than those of benzothiadiazole-based polymers. The measured electrochemical band gaps (eV) are in the following order: PDTCZ-BBT (1.65 eV) < PCZV-BBTV (1.69 eV) < PCZ-BBT (1.75 eV). All the polymers provide a photovoltaic response when blended with a fullerene derivative as an electron acceptor. The best cell reaches a power conversion efficiency of 2.07 % estimated under standard solar light conditions (AM1.5G, 100 mW cm(-2)). We demonstrate for the first time that BBT-based polymers are promising materials for use in bulk-heterojunction solar cells.

Superexchange-mediated electronic energy transfer in a model dyad.

On the basis of time-dependent density functional theory (TD-DFT) calculations coupled to the polarizable continuum model (PCM) and single molecule spectroscopic studies, we provide a detailed investigation of excitation energy transfer within a model bi-chromophoric system where a perylene monoimide (PMI) donor is bridged to a terrylene diimide (TDI) acceptor through a ladder-type pentaphenylene (pPh) spacer. We find that the electronic excitation on the PMI donor significantly extends over the bridge giving rise to a partial charge transfer character and inducing a approximately 3-fold increase in the electronic interaction between the chromophores, which explains the failure of the Förster model in reproducing the observed energy migration rates when treating PMI as the donor. However, despite an increased charge transfer contribution in the effective donor state, the increase in solvent polarity is not accompanied by an enhancement in the electronic coupling between the subunits, which is rationalized from a detailed analysis of the excited-state wavefunctions.

A simple route toward the synthesis of bisbenzothiadiazole derivatives.

A simple and efficient route toward the synthesis of 4,4'-bis(2,1,3-benzothiadiazole) and 7,7'-dibromo-4,4'-bis(2,1,3-benzothiadiazole) has been developed. Oligomers were synthesized with bisbenzothiadiazole units either at the periphery or core, and each oligomer was characterized by X-ray crystallography. Both crystal structures display supramolecular interactions between the conjugated backbones, although the former, bearing two bisbenzothiadiazole units, has extended interactions within layers that engage all of the thiadiazole rings.

Singlet-singlet annihilation leading to a charge-transfer intermediate in chromophore-end-capped pentaphenylenes.

The excited-state properties of two peryleneimide chromophore end-capped pentaphenylene compounds were investigated in detail using femtosecond transient absorption and single-photon timing experiments. Singlet-singlet annihilation was found to promote one chromophore into a higher excited state and results in the formation of an ultra-short-living intermediate charge-transfer (CT) state in the S(n)-S(1) deactivation pathway. In low-polarity solvents, this CT state is found to be energetically higher than the first excited state and thus cannot be populated via one-photon excitation. The observed CT state decays with a time constant of about 1 ps to form the lowest singlet excited state. These results demonstrate the potential use of the singlet-singlet annihilation as a novel tool in studying reactions occurring in states that are energetically above the S(1).

CT-CT annihilation in rigid perylene end-capped pentaphenylenes.

The time-dependent spectral properties of a rigid, extended system consisting of three pentaphenylene units end-capped with perylene monoimide were investigated in detail by femtosecond transient absorption and single photon timing measurements. In polar solvents, the molecular system shows the occurrence of photoinduced charge transfer. Upon gradually increasing the excitation power, annihilation involving two states with charge-transfer character has been observed. Quantum-chemical calculations performed on the system consisting of two pentaphenylene units end-capped with perylene monoimide strongly support the experimental data.

Twin probes as a novel tool for the detection of single-nucleotide polymorphisms.

Single-nucleotide polymorphisms (SNPs) are the most common form of DNA sequence variation. There is a strong interest from both academy and industry to develop rapid, sensitive and cost effective methods for SNP detection. Here we report a novel structural concept for DNA detection based on fluorescence dequenching upon hybridization. The so-called "twin probe" consists of a central fluorene derivative as fluorophore to which two identical oligonucleotides are covalently attached. This probe architecture is applied in homogeneous hybridization assays with subsequent fluorescence spectroscopic analysis. The bioorganic hybrid structure is well suited for sequence specific DNA detection and even SNPs are identified with high efficiency. Additionally, the photophysical properties of the twin probe were investigated. The covalent attachment of two single stranded oligonucleotides leads to strong quenching of the central fluorescence dye induced by the nucleobases. The twin probe is characterized by supramolecular aggregate formation accompanied by red-shifted emission and broad fluorescence spectra.

Charge transfer enhanced annihilation leading to deterministic single photon emission in rigid perylene end-capped polyphenylenes.

Two new peryleneimide end-capped polyphenylenes are shown to be deterministic single photon sources in PMMA films due to efficient annihilation between charge transfer states.

Photophysical characterization of light-emitting poly(indenofluorene)s.

Time-resolved photoluminescence spectroscopy experiments of three poly(2,8-indenofluorene) derivatives bearing different pendant groups are presented. A comparison of the photophysical properties of dilute solutions and thin films provides information on the chemical purity of the materials. The photophysical properties of poly(2,8-indenofluorene)s are correlated with the morphological characteristics of their corresponding films. Wide-angle X-ray scattering experiments reveal the order in these materials at the molecular level. The spectroscopic results confirm the positive impact of a new synthetic approach on the spectral purity of the poly(indenofluorene)s. It is concluded that complete side-chain substitution of the bridgehead carbon atoms C-6 and C-12 in the indenofluorene unit, prior to indenofluorene ring formation, reduces the probability of keto formation. Due to the intrinsic chemical purity of the arylated derivative, identification of a long-delayed spectral feature, other than the known keto band, is possible in the case of thin films. Controlled doping experiments on the arylated derivative with trace amounts of an indenofluorene-monoketone provide quantitative information on the rates of two major photophysical processes, namely, singlet photoluminescence emission and singlet photoluminescence quenching. These results allow the determination of the minimum keto concentration that can affect the intrinsic photophysical properties of this polymer. The data suggest that photoluminescence quenching operates in the doped films according to the Stern-Volmer formalism.

Ladder-type pentaphenylenes and their polymers: efficient blue-light emitters and electron-accepting materials via a common intermediate.

A new route to ladder-type pentaphenylenes has been developed in which both good hole-accepting p-type and electron-accepting n-type materials can be prepared from a common intermediate. This key intermediate is a pentaphenylene diester 5 obtained in high yield by Suzuki coupling of 2 equiv of fluorene boronates with 2,5-dibromoterephthalate. Addition of aryllithium followed by ring closure with boron trifluoride produced a blue-emitting ladder-type pentaphenylene. Bromination followed by reductive polymerization with nickel(0) gave new high molecular mass polymers, which show efficient blue emission with a very small Stokes shift. These polymers bridge the gap in emission between polyfluorenes and fully ladder-type polyphenylenes. An alternative ring closure of the dibromopentaphenylene diester 14 with acid made a diketone that is a good electron-accepting material, as it displays a reversible two-electron reduction. The reduction onset potential of -0.875 V against Ag/Ag(+) corresponds to a lowest unoccupied molecular orbital (LUMO) energy level of 3.53 eV, comparable to the work function of magnesium, suggesting that this unit could be used to greatly increase the injection of electrons into polymers containing it in a light-emitting diode (LED) or solar cell. A red-emitting material was prepared by Suzuki coupling of the dibromopentaphenylene 10b with a perylene dye, thus offering the prospect of tuning the emission from pentaphenylene materials over the whole visible range by attachment of suitable dyes. Unoptimized single-layer organic LEDs that used 11b showed stable pure-blue emission with brightnesses of over 200 cd/m(2) at 7 V, with moderate efficiencies.

Selective conversion of diallylanilines and arylimines to quinolines.

A variety of diallylanilines are shown to undergo cobalt-carbonyl catalyzed rearrangement to quinolines. Diallylanilines are also used as allyl transfer reagents to convert benzaldimines into quinolines. Substitution in the 2- and 3-positions of the quinoline is featured in all transformations.