Spectroscopy of discrete vertically oriented single-crystals of n-type tetraazaterrylene: understanding the role of defects in molecular semiconductor photovoltaics.

Recent synthetic work has realized a novel (n-type) small-molecule acceptor, 7,8,15,16-tetra-aza-terrylene (TAT), single-crystals of which can be grown oriented along the c-axis crystallographic direction, and over-coated with pentacene to form a highly ordered donor/acceptor interface for use in organic photovoltaic devices. However, characterization of single TAT crystals reveals highly variable emission spectra and excited state dynamics - properties which strongly influence photovoltaic performance. Through the use of single-crystal widefield imaging, photoluminescence spectroscopy, time correlated single photon counting, and resonant Raman studies, we conclude that this variability is a result of long-lived low-energy trap-emission from packing defects. Interestingly, we also discovered that TAT crystals whose width exceeds ∼200 nm begin acting as waveguides and optical microcavity resonators for their own photoluminescence. Several strategies are proposed for leveraging the size-dependant optical properties of TAT pillars to further enhance device performance using this active layer design.

Reduction and oxidation doping kinetics of an electropolymerized donor-acceptor low-bandgap conjugated copolymer.

The electrochemical synthesis of a new dithienylcyclopentadienone-derivative/3-methylthiopene copolymer was performed by cyclic voltammetry. The obtained material shows redox processes very close to those from the pristine DTCPD. A new redox process at -1.24 V, with a large anodic shift (0.51 V) related to the poly(3-methylthiophene) reduction, indicates the existence of a copolymer with a strong influence of the neighboring (n-doped) DTCPD comonomer. The new copolymer is electrochemically n-doped at more cathodic potentials than -750 mV and p-doped at more anodic potentials than 250 mV, with a bandgap of 1.0 eV. The cation's entrance in the film from the solution during n-doping and anion's entrance during p-doping for charge balance was checked by QCM. The reduction of the DTCPD part suffers a partial trapping of the negative charges that can be reoxidized only at high overpotentials (>1 V related to the reduction potentials). After polarization of the material at any potential inside the band gap, subsequent p- or n-doping reactions performed by potential steps start by nucleation-relaxation kinetic control, followed by anodic or cathodic, respectively, chronoamperometric maxima. At the maxima, both reactions were checked to occur under chemical kinetic control, allowing the determination of the reaction orders for p- and n-doping processes.

Intermolecular energy transfer involving an iridium complex studied by a combinatorial method.

A recently developed combinatorial method utilizing angular dependence of evaporation rate was used to create compositional spread thin film libraries of Tris(2-pyridin-2-yl-indolizino[3,4,5-ab] isoindole-C(1), N('))iridium(III) [Ir(pin)(3)] and 4,4(')-N,N(')-dicarbazol-biphenyl (CBP) composite, with the molar fraction of Ir(pin)(3) complex varying in the 0.0003Ir(pin)(3) energy transfer proceeds by the Forster mechanism with the Forster radius of 30 A. The CBPxIr(pin)(3) composite has the highest photoluminescence quantum efficiency approximately 0.95, for chi(Ir(pin)(3) )=0.03 and is characterized by a structured green emission (lambda(max)=538 nm) originating from the ligand-centered (pi-pi(*))(3) state of the Ir(pin)(3) complex. On the contrary, the PL spectra of Ir(pin)(3) bulk are characterized by a weak red emission (lambda(max)=673 nm) attributed to the lowest metal-to-ligand charge transfer state. A statistical analysis based on a binomial distribution indicates that the emission from the (pi-pi(*))(3) state is quenched in Ir(pin)(3) molecules that are in a direct contact with each other.

Mechanism of migration of the trimethylsilyl group during reactions of methoxy[(trimethylsilyl)ethoxy]carbene with N-phenylmaleimide and C(60).

A novel migration of the trimethylsilyl group during reaction of methoxy[(trimethylsilyl)ethoxy]carbene with N-phenylmaleimide (NPM) and with C(60), reported earlier, was examined by means of deuterium labeling of the carbene. For the NPM case it was found that the CD(2)CH(2)SiMe(3) group, initially bound to oxygen, became the CH(2)CD(2)SiMe(3) group bound to carbon in the end product. Not only had the trimethylsilylethyl group moved from oxygen to carbon, but the TMS group had also migrated 1,2 along the ethyl chain. For the C(60) case, complete scrambling of the CD(2) group was observed, strongly implying the involvement of a silacyclopropane carbocation responsible for product formation. The labeling study supports the mechanism that was tentatively advanced earlier for addition to NPM and one of the possibilities suggested for addition to C(60).

High-yield formation of giant bis(bicyclic) and crypt-tris(bicyclic) molecules under normal reaction conditions

We report the unexpected result of the reaction of 1, 3-bis[(9-anthrylmethoxy)methyl]benzene (1a) or 1,3, 5-tris[(9-anthrylmethoxy)methyl]benzene (1b) with tris(2-maleimidoethyl)amine (2) in homogeneous solution leading to giant bis(bicyclic) and crypt-tris(bicyclic) molecules. The anticipated, intractable solids are obtained in a condensed state reaction using an oscillating mill.

Highly sensitive biological and chemical sensors based on reversible fluorescence quenching in a conjugated polymer.

The fluorescence of a polyanionic conjugated polymer can be quenched by extremely low concentrations of cationic electron acceptors in aqueous solutions. We report a greater than million-fold amplification of the sensitivity to fluorescence quenching compared with corresponding "molecular excited states." Using a combination of steady-state and ultrafast spectroscopy, we have established that the dramatic quenching results from weak complex formation [polymer(-)/quencher(+)], followed by ultrafast electron transfer from excitations on the entire polymer chain to the quencher, with a time constant of 650 fs. Because of the weak complex formation, the quenching can be selectively reversed by using a quencher-recognition diad. We have constructed such a diad and demonstrate that the fluorescence is fully recovered on binding between the recognition site and a specific analyte protein. In both solutions and thin films, this reversible fluorescence quenching provides the basis for a new class of highly sensitive biological and chemical sensors.

Pharmacokinetics of a water-soluble fullerene in rats.

Fullerenes are the recently discovered third allotropic form of carbon. The biological activities of these compounds are being studied for various purposes. The bis(monosuccinimide) derivative of p p'-bis(2-amino-ethyl)-diphenyl-C60 (MSAD-C60) is a water-soluble fullerene derivative. MSAD-C60 has been shown to have antiviral activity against human immunodeficiency virus types 1 and 2 in vitro and to have virucidal and anti-human immunodeficiency virus protease activities. Moreover, MSAD-C60 has been shown to be well tolerated in mice after intraperitoneal administration. The purpose of the present study was to develop a high-performance liquid chromatographic analytical methodology for MSAD-C60 and to characterize the preclinical pharmacokinetics of the compound in rats. Following intravenous administration of the fullerene derivative at a dose of 15 mg/kg of body weight, the concentrations of MSAD-C60 in plasma declined either bi- or triexponentially. The mean terminal-phase half-life of MSAD-C60 was 6.8 +/- 1.1 h (mean +/- standard deviation). Binding studies indicated that the compound is greater than 99% bound to plasma proteins. The average total clearance of the compound was 0.19 +/- 0.06 liter/h/kg. Urine samples obtained 24 h after intravenous administration did not contain detectable levels of the compound, indicating the absence of a significant renal clearance mechanism. The steady-state volume of distribution of MSAD-C60 averaged 2.1 +/- 0.8 liters/kg, indicating that the compound distributes into tissues. At a dose of 15 mg/kg, MSAD-C60 appeared to be well tolerated. However, a dose of 25 mg/kg resulted in shortness of breath and violent movement of the rats, followed by death within 5 min of dosing. Further controlled toxicity studies are needed to fully evaluate the toxicity of the compound.

Enhanced optical limiting in derivatized fullerenes.

We have observed enhanced optical limiting behavior in solutions of a derivatized fullerene (phenyl-C(61)-butyric acid cholesteryl ester) from 532 to 700 nm. Transient absorption measurements determined the spectral and temporal regions of interest for optical limiting in C(60) and in C(60) derivatives that are due to a reverse saturable absorption mechanism and predicted enhanced limiting at longer wavelengths. Intensity-dependent transmission measurements made at several wavelengths confirmed these results. The increased solubility and the broadened ground-state absorption of the functionalized C(60) make it suitable for use as an optical limiter in the red and the near infrared.

Isolation of the Heterofullerene C59N as Its Dimer (C59N)2.

The heterofullerene ion C(59)N(+) is formed efficiently in the gas phase during fast atom bombardment mass spectroscopy of a cluster-opened N-MEM (N-methoxyethoxy methyl) ketolactam. This transformation is shown to occur also in solution in the presence of strong acid, affording biazafullerenyl (C(59)N)(2) in good yield. It is proposed that the azafullerene dimer is formed upon in situ reduction of the highly reactive azafulleronium ion. The isolation and characterization of biazafullerenyl opens a viable route for the preparation of other heterofullerenes in solution.

A 160-femtosecond optical image processor based on a conjugated polymer.

Degenerate ground-state conjugated polymers exhibit large third-order nonlinear optical susceptibilities, including substantial two-photon absorption. With the use of a machine architecture suited to these material properties, ultrafast optical processors are possible. A four-wave mixing optical correlator was built with an air-stable, processable, degenerate ground-state conjugated polymer, poly(1,6-heptadiester). The continuously updatable processor correlates two 5000-pixel images in less than 160 femtoseconds, achieving peak processing rates of 3 x 10(16) operations per second.

Synthesis and virucidal activity of a water-soluble, configurationally stable, derivatized C60 fullerene.

The bis(monosuccinimide) derivative of p,p'-bis(2-aminoethyl)diphenyl-C60 (compound 1), prepared by the fulleroid route, is active against human immunodeficiency virus type 1 (HIV-1) and HIV-2 (50% effective concentration [EC50] averaging approximately 6 microM) in acutely or chronically infected human lymphocytes and is active in vitro against 3'-azido-3'-deoxythymidine-resistant HIV-1 (EC50, approximately 3 microM). The virucidal properties of compound 1 were confirmed by virus inactivation assays. Compound 1 was noncytotoxic up to 100 microM in peripheral blood mononuclear cells and H9, Vero, and CEM cells. In cell-free assays, whereas the fullerene showed comparable activity against HIV-1 reverse transcriptase and DNA polymerase alpha (50% inhibitory concentration of approximately 5 microM), it demonstrated selective activity against HIV-1 protease.

Photoinduced electron transfer from a conducting polymer to buckminsterfullerene.

Evidence for photoinduced electron transfer from the excited state of a conducting polymer onto buckminsterfullerene, C(60), is reported. After photo-excitation of the conjugated polymer with light of energy greater than the pi-pi* gap, an electron transfer to the C(60) molecule is initiated. Photoinduced optical absorption studies demonstrate a different excitation spectrum for the composite as compared to the separate components, consistent with photo-excited charge transfer. A photoinduced electron spin resonance signal exhibits signatures of both the conducting polymer cation and the C(60) anion. Because the photoluminescence in the conducting polymer is quenched by interaction with C(60), the data imply that charge transfer from the excited state occurs on a picosecond time scale. The charge-separated state in composite films is metastable at low temperatures.