Simulation of electrical conductivity in a pi-conjugated polymeric conductor with infrared light

Irradn. with IR light is found to stimulate the elec. cond. of a film of an org. polymeric conductor [poly(3,4-ethylenedioxythiopene) with polystyrene sulfonate]. The change in cond. is found to be linear in the intensity of the irradn. (4-400 mW/cm2). Both frequency and time domain measurements reveal that the change in resistance induced by irradn., relaxes according to DR(t) ~ (1/t)0.6, with t as the time after excitation. As a possible mechanism for this relaxation, the authors model the diffusion of heat from the polymer film to the supporting glass substrate. By assuming that the change in resistance is linear with the raise in temp. caused by the IR irradn., one predicts a DR(t) ~ (1/t)0.5 dependence. The similarity between the model and exptl. behavior is taken as an indication that the relaxation is limited by heat transport from the polymer film and that the thermalization of the charge carriers occurs on a shorter time scale. Elec. characterization is complemented with optical measurements. These show IR-induced transient absorption of the polymer film with practically the same relaxation behavior as the change in resistance. This suggests that the optical transients are also due to thermal excitations. In the sub-ps time domain, measurements of the change in optical transmission (DT/T) induced by the IR pulse show a very short-lived component with a lifetime close to the instrumental resoln. (.apprx.500 fs). The rapid response is followed by a slow component that decays according to (DT/T)(t) ~ (1/t)0.65. This is interpreted in terms of cooling of the excited charge carriers limited by heat transport, indicating that the thermalization of the carriers occurs on the sub-ps time scal

pH Dependence of the enantioselective excited-state quencing of L,D-lanthanide(III) tris (pyridine-2,6-dicarboxylate) chelates by ferricytochrome c from horse heart and ferricytochrome c-550 from Paracoccus versutus

Macromolecular Biochemistr

Chiral recognition between dissymmetric Tb- and Eu(pyridine-2,6-dicarboxylate)(3)(3-) complexes and Fe(III) proteins in aqueous solution. Luminescence quenching by cytochrome c from horse heart and cytochrome c-550 from Thiobacillus versutus and its Lys14- Glu and Lys99- Glu mutants

Macromolecular Biochemistr

Structure, Photophysics and the Order-Disorder Transition to the Beta Phase in Poly(9,9-(di -n,n-octyl)fluorene)

X-ray diffraction, UV-vis absorption and photoluminescence (PL) spectroscopy have been used to study the well-known order-disorder transition (ODT) to the beta phase in poly(9,9-(di n,n-octyl)fluorene)) (PF8) thin film samples through combination of time-dependent and temperature-dependent measurements. The ODT is well described by a simple Avrami picture of one-dimensional nucleation and growth but crystallization, on cooling, proceeds only after molecular-level conformational relaxation to the so called beta phase. Rapid thermal quenching is employed for PF8 studies of pure alpha phase samples while extended low-temperature annealing is used for improved beta phase formation. Low temperature PL studies reveal sharp Franck-Condon type emission bands and, in the beta phase, two distinguishable vibronic sub-bands with energies of approximately 199 and 158 meV at 25 K. This improved molecular level structural order leads to a more complete analysis of the higher-order vibronic bands. A net Huang-Rhys coupling parameter of just under 0.7 is typically observed but the relative contributions by the two distinguishable vibronic sub-bands exhibit an anomalous temperature dependence. The PL studies also identify strongly correlated behavior between the relative beta phase 0-0 PL peak position and peak width. This relationship is modeled under the assumption that emission represents excitons in thermodynamic equilibrium from states at the bottom of a quasi-one-dimensional exciton band. The crystalline phase, as observed in annealed thin-film samples, has scattering peaks which are incompatible with a simple hexagonal packing of the PF8 chains.Comment: Submitted to PRB, 12 files; 1 tex, 1 bbl, 10 eps figure

Circular Polarization in absorption and emission of light by molecules and molecular assemblies

Chiral molecules and materials preferentially absorb and emit one of the circular polarizations of light. Yet the reverse statement is not necessarily true. Recent developments in circular polarization in topologically chiral molecules, supramolecular assemblies and materials with strong light-matter coupling are discussed.</p

Beta phase in chiral polyfluorene forms via a precursor

Chiral poly{9,9-bis[(3S)-3,7-dimethyloctyl]-2,7-fluorene) shows temperature-induced aggregation in 1-octanol. For polymer concentrations of > 0.01 mg/mL and cooling rates of = 10 °C/h, aggregates form, showing characteristics of the ß phase of polyfluorene. At higher cooling rates, lower concentration, or both, another type of aggregates forms (a), a and ß phases are distinguished by absorption, fluorescence, and circular dichroism (CD) spectroscopy. Temperature-dependent CD and dynamic light scattering show that the ß phase forms only via a precursor aggregate, whose formation is concentration-, temperature-, and time-dependent. The yield of the ß phase can be optimized by choosing processing conditions that favor the formation of the precursor

Macromolecular memory

Macromolecular materials, such as polymers, can be used to make electronic memory cells. This chapter focuses on data storage based on resistive switching in two terminal diodes incorporating macromolecules. Elementary physical chemistry of macromolecules relevant for the memory function is briefly discussed. Various approaches to realize macromolecular memory cell are discussed in relation to the chemical structure of the polymer and recent developments are reviewed

Enantioselective quenching of luminescence : molecular recognition of chiral lanthanide complexes by biomolecules in solution

Principles, experimental techniques, and applications of enantioselective quenching of lanthanide luminescence are addressed. Upon the addition of chiral, enantiomerically resolved quencher molecules to an aqueous, racemic solution of the tris (pyridine-2,6-dicarboxylate) chelate of Eu(III) or Tb(III), strong quenching of the lanthanide luminescence is observed, with a quenching rate that depends on the chirality of the lanthanide species. Quenching by c-type cytochromes and vitamin B12 derivatives is discussed in detail. For the latter quenchers, the energy transfer reaction held responsible for the quenching proceeds via formation of an encounter complex between donor and quencher in which the actual energy transfer takes place. A structural model for this transient pair has been constructed on the basis of data on complexation between B12 and ground-state lanthanide chelate obtained by measurements of lanthanide induced shifts and relaxation of the NMR signals of protons of B12 and from the induced circular dichroism spectra. The enantioselectivity in the quenching results mainly from selective binding of the two lanthanide enantiomers to B12. A smaller contribution to the selectivity comes from differences in the rate of the quantum mechanical energy transfer within the two diastereomeric encounter complexes. Finally, the use of the enantiodifferential quenching to study complexation of B12 with two B12 binding proteins (haptocorrin and a monoclonal antibody) is addressed

Insights from chiral polyfluorene on the unification of molecular exciton and cholesteric liquid crystal theories for chiroptical phenomena

The Maugin–Oseen–DeVries theory accounts for chiroptical properties of cholesteric films with long-range order. For molecular systems with short-range structural correlation lengths, molecular exciton theory is used. A consistent description for systems with intermediate correlation lengths is lacking. Films of chiral polyfluorene behave according to Maugin–Oseen–DeVries theory when the film thickness exceeds 300 nm. Properties of thin films are consistent with molecular exciton theory. We describe the crossover in the optical properties of the film in a phenomenological way using a dielectric tensor that contains terms from Maugin–Oseen–DeVries and molecular exciton theory. Guided by the experimental findings, we explore the possibility of a unification of the Maugin–Oseen–DeVries and molecular exciton theory involving a coupled oscillator model