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The interaction of three cationicpoly {9,9-bis[N,N-(trimethylammonium)hexyl]fluorene-co-1,4-phenylene}polymers with average chain lengths of ∼6, 12, and 100 repeatunits (PFP-NR36(I),12(Br),100(Br)) with both double andsingle stranded, short and long, DNA and DNA bases have been studiedby steady state and time-resolved fluorescence techniques. Fluorescenceof PFP-NR3 polymers is quenched with high efficiency by DNA (bothdouble and single stranded) and DNA bases. The resulting quenchingplots are sigmoidal and are not accurately described by using a Stern–Volmerquenching mechanism. Here, the quenching mechanism is well modeledin terms of an equilibrium in which a PFP-NR3/DNA aggregate complexis formed which brings polymer chains into close enough proximityto allow interchain excitation energy migration and quenching at aggregateor DNA base traps. Such an analysis gives equilibrium constants of8.4 × 106(±1.2 × 106) M–1for short-dsDNA and 8.6 × 106(±1.7× 106) M–1for short-ssDNA withPFP-NR36(I). [ABSTRACT FROM AUTHOR]

Zero-valent copper (Cu 0 ) is a promising co-catalyst in semiconductor-based photocatalysis as it is inexpensive and exhibits electronic properties similar to those of Ag and Au. However, its practical application in photocatalytic hydrogen production is limited by its susceptibility to oxidation, forming less active Cu species. Herein, we have carried out in situ encapsulation of Cu 0 nanoparticles with N-graphitic carbon layers (14.4% N) to stabilize Cu 0 nanoparticles (N/C-coated Cu) and improve the electronic communication with a TiO 2 photocatalyst. A facile solvothermal procedure is used to coat the Cu 0 nanoparticles at 200 °C, while graphitization is achieved by calcination at 550 °C under an inert atmosphere. The resultant N/C-coated Cu/TiO 2 composites outperform the uncoated Cu counterparts, exhibiting a 27-fold enhancement of the hydrogen evolution rate compared to TiO 2 and achieving a rate of 19.03 mmol g -1 h -1 under UV-vis irradiation. Likewise, the N/C-coated Cu co-catalyst exhibits a less negative onset potential of -0.05 V toward hydrogen evolution compared to uncoated Cu ( ca . -0.30 V). This superior activity is attributed to coating Cu 0 with N/C, which enhances the stability, electronic communication with TiO 2 , conductivity, and interfacial charge transfer processes. The reported synthetic approach is simple and scalable, yielding an efficient and affordable Cu 0 co-catalyst for TiO 2 .

The photochemistry and stability of fullerene films is found to be strongly dependent upon film nanomorphology. In particular, PC 61 BM blend films, dispersed with polystyrene, are found to be more susceptible to photobleaching in air than the more aggregated neat films. This enhanced photobleaching correlated with increased oxygen quenching of PC 61 BM triplet states and the appearance of a carbonyl FTIR absorption band indicative of fullerene oxidation, suggesting PC 61 BM photo-oxidation is primarily due to triplet-mediated singlet oxygen generation. PC 61 BM films were observed to undergo photo-oxidation in air for even modest (≤40 min) irradiation times, degrading electron mobility substantially, indicative of electron trap formation. This conclusion is supported by observation of red shifts in photo- and electro-luminescence with photo-oxidation, shown to be in agreement with time-dependent density functional theory calculations of defect generation. These results provide important implications on the environmental stability of PC 61 BM-based films and devices.

The effect of aggregation on the photophysical properties of three cationic poly{9,9-bis[N,N-(trimethylammonium)hexyl] fluorene-co-l,4-phenylene} polymers with average chain lengths of ∼6, 12, and 100 repeat units (PFP-NR3(6(I),12(Br),100(Br))) has been studied by steady-state and time-resolved fluorescence techniques. Conjugated polyelectrolytes are known to aggregate in solution and for these PFP-NR3 polymers this causes a decrease in the fluorescence quantum yield. The use of acetonitrile as a cosolvent leads to the breakup of aggregates of PFP-NR3 in water; for PFP-NR3(6(I)), this results in an ∼10-fold increase in fluorescence quantum yield, a ca. 2-fold increase in the molar extinction coefficient at 380 nm, and an increase in the emission lifetime, as compared with polymer behavior in water. Fluorescence anisotropy also decreases with increasing aggregation, and this is attributed to increased fluorescence depolarization by interchain energy transfer in aggregate PFP-NR3 clusters. Förster resonance energy transfer along the polymer chain is expected to be very fast, with a calculated FRET rate constant of 7.3 × 10(12) s(-1) and a Förster distance of 2.83 nm (cf. the polymer repeat unit separation of 0.840 nm) for PFP-NR3(100(Br)). The complex polymer excited-state decay kinetics in aggregated PFP-NR3 systems have been successfully modeled in terms of intrachain energy transfer via migration and trapping at interchain aggregate trap sites, with model parameters in good agreement with data from picosecond time-resolved studies and the calculated theoretical Förster energy-transfer rates.

The cationic conjugated polymer (CCP), poly{9,9-bis[N,N-(trimethylammonium)hexyl]fluorene-co-1,4-phenylene} iodide (PFP-NR3) induces compaction in DNA in an acetonitrile/water mixture, as seen by fluorescence microscopy. At high concentrations of PFP-NR3, some chain structures still appear to exist. However, these are larger than normal DNA coils and have "beads" of enhanced fluorescence along the chain. These structures have also been imaged on freshly cleaved mica surfaces using atomic force microscopy. Images obtained following deposition onto mica from mixtures of DNA and the polyelectrolyte PFP-NR3 in acetonitrile/water 25:75 show both the efficient compaction of DNA induced by the polymer and linking and bridging of DNA/PFP-NR3 aggregates. The strong interaction between DNA and PFP-NR3 results in the formation of DNA/PFP-NR3 networks across the mica surface in which several strands of DNA are linked with aggregated polymer/DNA structures at various points along these chains. The linking of DNA strands is confirmed by a large increase in the apparent length of the DNA, which increases from 775 (+/-82) nm (with no PFP-NR3 present) to 4050 (+/-800) nm in the presence of PFP-NR3. Larger aggregates, believed to be PFP-NR3/compacted DNA, which also link other DNA strands, can also be seen. The fluorescence of PFP-NR3 is quenched by DNA, and this is accompanied by a bathochromic wavelength shift of the CCP emission, indicating complexation. Although the quenching mechanism is not yet clear, it appears to be a consequence of DNA-CCP aggregate formation. These results have implications on the use of CCPs in DNA sensing and, because the particular polymer has a rigid backbone, on the effect of chain rigidity on compaction and on formation of extended and supramolecular structures, which may have implications in DNA nanotechnology.