Roger C. Helgeson

DUAL-COLOR POLYMER LIGHT-EMITTING PIXELS PROCESSED BY HYBRID INKJET PRINTING

A hybrid inkjet printing (HIJP) technology, which combines a pin-hole free polymer buffer layer and an inkjet printed polymer layer, allows the patterning of high quality polymer light-emitting devices. In this letter, we present a successful demonstration of controllable patterning of dual-color polymer light-emitting pixels using this HIJP technique. In this demonstration, the polymer buffer layer is a wide band gap, blue emitting semiconducting polymer prepared by the spin-casting technique. The inkjet printed layer is a red-orange semiconducting polymer which was printed onto the buffer layer. When a proper solvent was selected, the printed polymer diffused into the buffer layer and efficient energy transfer took place generating a red-orange photoluminescence and electroluminescence from the inkjet printed sites. Based on this principle, blue and orange-red dual-color polymer light-emitting pixels were fabricated on the same substrate. The use of this concept represents an entirely new technology for...

Chiral recognition in complexation of guests by designed host molecules

ABSTRACT Host molecules have been designed and synthesized to selectively complex and lipophilize guest molecules. Examples of the use of the following binding interactions are given: hydrogen bonding, ion pairing, cation to n-electrons, carbonyl to n-electrons and pi-pi bonding. Multiheteromacrocycles have been prepared whose association constants with tert -butylammonium salts in chloroform range from 6 M -1 . Host molecules with built-in counterions have been prepared that selectively complex and lipophilize metal and alkylammonium cations. Locations of complementary binding sites and non-complementary steric barriers provide for selective binding by host molecules of candidate guest molecules. Locations of appropriate chiral barriers and multiple complexing sites in guest compounds have led to the complete optical resolution of host compounds by optically active amino acids, and of amino acid esters by optically active host compounds. Ratios of association constants for diastereomeric complexes in excess of ten have been obtained. A molecular basis for designing an amino acid resolving machine has been developed.

Building highly sensitive dye assemblies for biosensing from molecular building blocks

Fluorescence superquenching is investigated for polyelectrolytes consisting of cyanine dye pendant polylysines ranging in number of polymer repeat units (NPRU) from 1 to 900, both in solution and after adsorption onto silica nanoparticles. As NPRU increases, the absorption and fluorescence evolve from monomer spectra to red-shifted features indicative of molecular J aggregates. In solution, the superquenching sensitivity toward an anionic electron acceptor increases by more than a millionfold over the NPRU range from 1 to 900. The dramatic increase is attributed to enhanced equilibrium constants for binding the quenchers, and the amplified quenching of a delocalized exciton of ≈100 polymer repeat units. The self-assembly of monomer onto silica and clay nanoparticles leads to formation of J aggregates, and surface-activated superquenching enhanced 10,000× over the monomer in solution, indicating the formation of “self-assembled polymers” on the nanoparticle surface. Utilization of these self-assembled polymers as high-sensitivity biosensors is demonstrated.

Computational Design of Catalytic Dyads and Oxyanion Holes for Ester Hydrolysis

Nucleophilic catalysis is a general strategy for accelerating ester and amide hydrolysis. In natural active sites, nucleophilic elements such as catalytic dyads and triads are usually paired with oxyanion holes for substrate activation, but it is difficult to parse out the independent contributions of these elements or to understand how they emerged in the course of evolution. Here we explore the minimal requirements for esterase activity by computationally designing artificial catalysts using catalytic dyads and oxyanion holes. We found much higher success rates using designed oxyanion holes formed by backbone NH groups rather than by side chains or bridging water molecules and obtained four active designs in different scaffolds by combining this motif with a Cys-His dyad. Following active site optimization, the most active of the variants exhibited a catalytic efficiency (k(cat)/K(M)) of 400 M(-1) s(-1) for the cleavage of a p-nitrophenyl ester. Kinetic experiments indicate that the active site cysteines are rapidly acylated as programmed by design, but the subsequent slow hydrolysis of the acyl-enzyme intermediate limits overall catalytic efficiency. Moreover, the Cys-His dyads are not properly formed in crystal structures of the designed enzymes. These results highlight the challenges that computational design must overcome to achieve high levels of activity.

Ultrafast competition between energy and charge transfer in a functionalized electron donor/fullerene derivative

Abstract The fact that fullerenes are good electron acceptors has generated interest in covalently linked complexes between electron donors and fullerenes; photoinduced charge transfer in these dyads has great potential for use in photovoltaic devices. In this Letter, we investigate the excited-state properties of a perylene–fulleropyrrolidine dyad using steady-state and femtosecond time-resolved spectroscopies. Following photoexcitation, charge separation and energy transfer occur in nearly equal proportion; both processes take place on a sub-picosecond timescale. This suggests that competition between energy and charge transfer is common in these molecular systems, so that the best molecules for device applications are not necessarily those with the fastest electron transfer rates.

One-, two-, and three-photon pumped lasing in a novel liquid dye salt system

We report the observation of one-, two-, and three-photon pumped lasing in the same medium, a novel liquid dye salt system when excited by pulsed 0.532-, 1.06-, and /spl sim/1.49-/spl mu/m coherent radiation pulses, respectively. Since the gain medium is a liquid and not a solution, it contains a significantly higher effective dye concentration and, therefore, is highly suitable for multiphoton pumped lasing and optical power limiting applications. The lasing spectra, temporal waveforms, near- and far-field intensity distributions, and output/input efficiency were measured under the conditions of one-, two-, and three-photon pump configurations.

Controlled unidirectional energy transfer in luminescent self-assembled conjugated polymer superlattices

Abstract We report the synthesis and characterization of multi-layered organic superlattices made by polyelectrolyte self-assembly. Self-assembled films were formed from a water-soluble form of poly(phenylene vinylene) with high-photoluminescence quantum efficiency (QE). We observed a self-quenching of the luminescence with increasing film thickness. This quenching can be reversed by inserting spacer layers between each active conjugated layer. A red shift of the luminescence was also observed as additional poly(phenylene vinylene) layers were added. We attribute the red shift and increasing QE to changing polymer conformation, together with efficient unidirectional energy transfer. We rule out quantum confinement as the origin of the red shift.

Building on Cram’s Legacy: Stimulated Gating in Hemicarcerands

Conspectus Donald Cram’s pioneering Nobel Prize-winning work on host–guest molecules led eventually to his creation of the field of container molecules. Cram defined two types of container molecules: carcerands and hemicarcerands. Host–guest complexes of carcerands, called carceplexes, are formed during their synthesis; once a carceplex is formed, the trapped guest cannot exit without breaking covalent bonds. Cram defined a quantity called constrictive binding, arising from the mechanical force that prevents guest escape. The constrictive binding in carceplexes is high. In contrast, hemicarcerands have low constrictive binding and are able to release the incarcerated guests at elevated temperatures without breaking covalent bonds. We have designed molecules that can switch from carcerand to hemicarcerand through a change in structure that we call gating. The original discovery of gating in container molecules involved our computational studies of a Cram hemicarceplex that was observed to release a guest upon heating. We found that the side portals of this hemicarceplex have multiple thermally accessible conformations. An eight-membered ring that is part of a portal changes from a “chair” to a “boat” structure, leading to the enlargement of the side portal and the release of the guest. This type of gating is analogous to phenomena often observed with peptide loops in enzymes. We refer to this phenomenon as thermally controlled gating. We have also designed and synthesized redox and photochemically controlled gated hemicarceplexes. Gates are built onto host molecules so that the opening or closing of such gates is stimulated by reducing or oxidizing conditions, or by ultraviolet irradiation. In both cases, the appropriate stimuli can produce a carceplex (closed gates) or hemicarceplex (open gates). A hemicarceplex with closed gates behaves like a carceplex, due to its very high constrictive binding energy. When the gates are opened, constrictive binding is dramatically lowered, and guest entrance and exit become facile. This stimulated switching between open and closed states controls access of the guest to the binding site. The experimental and computational investigations of gated hemicarcerands and several potential applications of gated hemicarceplexes are described in this Account.

Synthesis, X-ray Structure, and Properties of a Tetrabenzannelated 1,2,4,5-Cyclophane†

Parylene is the most frequently used material in the protective encapsulation of modern electronic components and medical implants.[1] This high-performance polymer is produced by the pyrolytic decomposition of [2.2]paracyclophane.[1c] Another high-performance organic material with even stronger C C bonds would be produced if another highly strained, all-aromatic cyclophane could be pyrolyzed, thus resulting in a TMsuperparylene∫. However, contrary to the mode of pyrolytic decomposition of [2.2]paracyclophane, where scission of the Csp3 Csp3 bond is the important first step leading to a p-xylylene monomer, in the case of a molecule such as 1 (Scheme 2), cleavage of a biaryl bond would produce a very reactive diradical monomer. Angle and bond strain in organic molecules and their effect on properties also continue to be an active field of research.[2] Over the last five decades, a substantial number of chemists have prepared many fascinating, strained saturated and unsaturated molecules.[2,3] The most notable of the strained unsaturated molecules are those of the fullerene C60 and the cyclophane families.[5] In the former, the hexagons are essentially cyclohexatrienes[6] and in the latter, the hexagons, while considerably distorted, still retain their benzenoid character. Since the first synthesis of [2.2]paracyclophane diene by Dewhirst and Cram,[7] a variety of [2.2]paracyclophanes with unsaturated or benzannelated bridges have been synthesized.[8] The influence of the bridges on the transannular benzene interactions and the geometry of the strained cyclophanes has been widely investigated.[9] To date, only a few unsaturated bridged and benzannelated cyclophanes are known,[9] but no benzannelated [2n]cyclophane with more than two bridges (n> 2) has been reported.[10] One would expect that, as the number of o-phenylene bridges increased, the total strain would also increase. To prepare a superparylene and to test the effect of benzo bridges in place of the alkyl bridges of cyclophanes one needs a rapid, reasonably high-yield synthetic entry. A priori, based on existing cyclophane synthetic methodology, the preparation of a symmetrical tetrabenzannelated [2n]cyclophane tetraene would appear to be rather difficult and lengthy. However, careful consideration of the molecular symmetry of the target revealed that the synthesis could be easily achieved. Herein, we describe the synthesis, X-ray structure, and some of the properties of the symmetrically benzannelated [24]cyclophane tetraene 1. In future publications we will report on the results of its pyrolytic decomposition. In Scheme 1 we depict the retrosynthetic analysis with a rather unusual disconnection leading to two dibenzocyclooctadiene-diynes and four methine units. As shown, the latter can originate from a meso-ionic precursor.

Ionic strength and solvent control over the physical structure, electronic properties and superquenching of conjugated polyelectrolytes

In this paper, we investigate the photophysical properties of the conjugated poly electrolyte poly(2-methoxy-5-propyloxy sulfonate phenylene vinylene) (MPS-PPV), dissolved in both water and DMSO as a function of the solution ionic strength. Dynamic light scattering indicates that MPS-PPV chains exist in a highly agglomerated conformation in both solvents, and that the size of the agglomerates depends on both the ionic strength and the charge of the counter-ion. Even though the degree of agglomeration is similar in the two solvents, we find that the fluorescence quantum yield of MPS-PPV in DMSO is nearly 100-times greater than that in water. Moreover, intensity-dependent femtosecond pump-probe experiments show that there is a significant degree of exciton-exciton annihilation in water but not in DMSO, suggesting that the MPS-PPV chromophores interact to form interchain electronic species that quench the emission in water. Given that the emission quenching properties depend sensitively on the chain conformation and degree of chromophore contact, we also explore the superquenching may be either enhanced or diminished in either of the solvents via addition of simple salts, and we present a molecular picture to rationalize how the conformational properties of conjugated polyelectrolytes can be tuned to enhance their emissive behavior for sensing applications.