DNA electron injection interlayers for polymer light-emitting diodes

Introduction of a DNA interlayer adjacent to an Al cathode in a polymer light-emitting diode leads to lower turn-on voltages, higher luminance efficiencies, and characteristics comparable to those observed using a Ba electrode. The DNA serves to improve electron injection and also functions as a hole-blocking layer. The temporal characteristics of the devices are consistent with an interfacial dipole layer adjacent to the electrode being responsible for the reduction of the electron injection barrier.     

Trifluoromethyl‐Substituted Conjugated Oligoelectrolytes

Conjugated oligoelectrolytes (COEs) are being introduced into a variety of optical and electronic technologies, yet the dependence of their properties as a function of molecular structure remains poorly understood. In response, we designed, synthesized, and examined a new tetracationic COE, namely, 1,4‐bis{9′,9′‐bis[6′′‐(N,N,N‐trimethylammonium)hexyl]‐2′‐fluorenyl}‐2,5‐bis(trifluoromethyl)benzene tetrabromide (FPF‐F6), which contains bulky electron‐withdrawing trifluoromethyl groups, and compared its properties with the unsubstituted counterpart 1,4‐bis{9′,9′‐bis[6′′‐(N,N,N‐trimethylammonium)hexyl]‐2′‐fluorenyl}benzene tetrabromide (FPF). The ground‐state geometry of FPF‐F6 is primarily twisted with little electronic communication between the aromatic units, as confirmed by single‐crystal X‐ray diffraction studies of the neutral precursor. However, absorption and photoluminescence spectroscopies reveal that the excited state of FPF‐F6 displays strong intramolecular charge‐transfer characteristics. Solution AFM in aqueous media shows that introduction of trifluoromethyl groups changes the size and aspect ratio of supramolecular aggregates that are brought together as a result of hydrophobic interactions. Furthermore, addition of ssDNA to FPF‐F6 leads to interoligoelectrolyte complexes wherein the backbone is more planar; the environment the chromophore experiences under these conditions is also considerably less polar. These findings provide considerable insight into the complex photophysics of electronically conjugated materials in aqueous media.     

Interpolyelectrolyte complexes of conjugated copolymers and DNA: platforms for multicolor biosensors

Interchain interactions modulate the frequency of emission from a cationic water-soluble conjugated polymer. The polymer, PFPB, is obtained by a Suzuki copolymerization of p-phenylenebisboronic acid with a 95:5 mixture of 2,7-dibromo-9,9-bis(6'-bromohexyl)fluorene and 4,7-dibromo-2,1,3-benzothiadiazole, followed by quarternization of the pendant groups by addition of NMe3. The structure of PFPB contains 5% of the 2,1,3-benzothiadiazole (BT) chromophore within a cationic poly(fluorene-co-phenylene) polymer chain. The emission of PFPB is blue under dilute conditions (<1 x 10-6 M in repeat units) and green at higher concentrations. Energy transfer to dye-labeled ss-DNA is more efficient, relative to the parent polymer poly(9,9-bis(6'-N,N,N,-trimethylammonium)hexyl)fluorene-co-alt-1,4-phenylene) dibromide (PFP), as a result of improved spectral overlap. By using a peptide nucleic acid (PNA-C*) labeled with a red-emitting chromophore one can obtain three different emission colors, depending on the nature of the substrate under interrogation. If no ss-DNA is present, the solution emits blue. With a ss-DNA that is noncomplementary to PNA-C*, green emission is observed. Red emission occurs upon addition of ss-DNA complementary to the PNA sequence.     

Synthesis of cationic water-soluble light-harvesting dendrimers

Four generations of phenylenefluorene (1F)- and phenylenebis(fluorene) (2F)-terminated polyamidoamine (PAMAM) dendrimers were synthesized by coupling activated esters with commercially available PAMAM precursors. Treatment of Boc-terminated pendant groups on the optically active units with 3 M HCl in dioxane yields cationic water-soluble dendrimers. Fluorescence resonance energy transfer (FRET) experiments with the cationic dendrimers as the donor and double stranded DNA containing a fluorescein label as the acceptor reveal cooperative optical behavior.     

Two-photon absorption in three-dimensional chromophores based on [2.2]-paracyclophane

A series of alpha,omega-bis donor substituted oligophenylenevinylene dimers held together by the [2.2]paracyclophane core were synthesized to probe how the number of repeat units and through-space delocalization influence two-photon absorption cross sections. Specifically, the paracyclophane molecules are tetra(4,7,12,15)-(4'-dihexylaminostyryl)[2.2]paracyclophane (3R(D)), tetra(4,7,12,15)-(4' '-(4'-dihexylaminostyryl)styryl)[2.2]paracyclophane (5R(D)), and tetra(4,7,12,15)-(4' "-(4' '-(4'-dihexylaminostyryl)styryl)styryl)[2.2]paracyclophane (7R(D)). The compounds bis(1,4)-(4'-dihexylaminostyryl)benzene (3R) and bis(1,4)-(4' '-(4'-dihexylaminostyryl)styryl)benzene (5R) were also synthesized to reveal the properties of the "monomeric" counterparts. The two-photon absorption cross sections were determined by the two-photon induced fluorescence method using both femtosecond and nanosecond pulsed lasers as excitation sources. While there is a red shift in the linear absorption spectra when going from the "monomer" chromophore to the paracyclophane "dimer" (i.e., 3R --> 3R(D), 5R --> 5R(D)), there is no shift in the two-photon absorption maxima. A theoretical treatment of these trends and the dependence of transition dipole moments on molecular structure rely on calculations that interfaced time-dependent density functional theory (TDDFT) techniques with the collective electronic oscillator (CEO) program. These theoretical and experimental results indicate that intermolecular interactions can strongly affect B(u) states but weakly perturb A(g) states, due to the small dipole-dipole coupling between A(g) states on the chromophores in the dimer.     

Aggregation-mediated optical properties of pH-responsive anionic conjugated polyelectrolytes

Conjugated polyelectrolyte copolymers containing 2,1,3-benzothiadiazole- (BT) and oligo(ethylene oxide)-substituted fluorene and phenylene units have been designed and synthesized. The phenylene pendent groups also have carboxylic acid functionalities, which allow probing the effect of pH on optical properties. The BT content in the backbone can be regulated at the synthesis stage. Dynamic light scattering studies show that polymers aggregate in water at low pH. Increased interchain contacts give rise to a lowering of the photoluminescence (PL) efficiency via self-quenching when the BT units are absent and increased levels of FRET from the phenylene-fluorene segments to BT. Furthermore, the PL efficiency of BT increases in the aggregated structures. Examination of solvent effects indicates that the increased BT efficiencies are likely due to decreased contact with water. The changes in PL efficiencies are reversible, showing that the aggregates are dynamic and not kinetically constrained.     

Solvent effects on the two-photon absorption of distyrylbenzene chromophores

A series of organic- and water-soluble distyrylbenzene-based two-photon absorption (TPA) fluorophores containing dialkylamino donor groups at the termini was designed, synthesized, and characterized. The central core was systematically substituted to modulate intramolecular charge transfer (ICT). These molecules allow an examination of solvent effects on the TPA cross section (delta) and on the TPA action cross section. In toluene, the delta values follow the order of ICT strength. The effect of solvent on delta is nonmonotonic: maximum delta was measured in an intermediate polarity solvent (THF) and was lowest in water. We failed to find a correlation between the observed solvent effect and previous theoretical predictions. Hydrogen bonding to the donor groups and aggregation of the optical units in water, which are not included in calculational analysis, may be responsible for the discrepancies between experimental results and theory.     

Triple tandem catalyst mixtures for the synthesis of polyethylenes with varying structures

Tandem catalysis in a single medium presents challenges and opportunities for creating novel synthetic protocols. Thus far, only two homogeneous catalysts have been used in tandem. Herein, we report that it is possible to coordinate the action of three well-defined homogeneous catalysts to produce a wide range of branched polyethylenes from a single monomer. Thus, ([(eta(5)-C(5)Me(4))SiMe(2)(eta(1)-NCMe(3))]TiMe)(MeB(C(6)F(5))(3)) (1), [(C(6)H(5))(2)PC(6)H(4)C(OB(C(6)F(5))(3))O-kappa(2)P,O]Ni(eta(3)-CH(2)C(6)H(5)) (2), and ((H(3)C)C[N(C(6)H(5))]C[O-B(C(6)F(5))(3)][N(C(6)H(5))]-kappa(2)N,N)Ni(eta(3)-CH(2)C(6)H(5)) (3) react with ethylene to produce branched polyethylene with structures that cannot be obtained using a single- or a two-component catalyst combination. The properties of the polyethylene depend on the ratio of the three catalysts. High-throughput screening techniques proved essential for optimizing reaction conditions and for probing how the catalyst composition influences the polymer properties.