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.

Thermochromism and solvatochromism of solutions of poly(3‐alkylthienylenes) are reported. The experimental results indicate the presence of two coexisting phases: polymer in solution and polymer in microcrystalline aggregates. From the concentration independence of the thermochromism, it is concluded that the transition is fundamentally driven by a single chain mechanism, and that the aggregation (microcrystallization) of the poly(3‐alkylthienylene) macromolecules can occur only after the single chain conformational change has occurred.

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The existence of transiently open states in DNA and synthetic polynucleotide double helices as been demonstrated by hydrogen exchange measurements; base pairs reversibly separate and reclose, exposing nucleotide protons to exchange with solvent protons. Recently it has been possible to define the equilibrium kinetic, and activation parameters of the major open state tdat determines base pair hydrogen exchange. However, there is no direct information at the moment about the conformation of the open form. Here we consider the possibility that the low energy and slow opening and closing rates observed reflect a deformation involving several adjacent base pairs. Assuming a mobile open unit capable of diffusing along the double helix, we find that available data are consistent with structures of 10 or so adjacent open pairs. It is further suggested that these structures correspond to thermally induced so on excitations of the double helix, which retain coherence by sharing the energy of a twist deformation among several base tairs. Solitons

Light-emissive polymers are outstanding laser materials because they are intrinsically ª4-levelº systems, they have luminescence efficiencies higher than 60 % even in undiluted films, they emit at colors that span the visible spectrum, and they can be processed into optical quality films by spin casting. The important materials issues are reviewed and the prospects for making polymer diode lasers are discussed.

at pH 9.0, 64 %. Similarly, 0.1 M UMP increases the size of a typical polymer film at pH 7.0 by 28 %, but at pH 11.0 by 140 %. Solely pH-dependent macroscopic changes have been described earlier with natural polymers such as chitosan [6] and gelatin derivatives, [7] and they are due to repulsions of charged loci within and between the polymer chains.[8] The significantly increased macroscopic dimension change by effector uptake at higher pH observed here can be understood by the presence of a less tight polymer network in the more deprotonated state, which, due to its higher flexibility, can better accommodate the added guest compounds. The observed cooperativity creates new possibilities for designing macroscopic supramolecular systems, which can be fine-tuned to respond only to a given combination of different effector compounds at distinct concentrations. Such systems can be of interest for actuators including artificial muscles, vessels, or implants, for self-regulating valves in process control, for sensors, and other related applications. Possible applications for drug release or uptake can take advantage of the necessary presence of a certain combination of effectors. The new self-controlled logic gates could directly communicate with the outside world like a natural organ. Obviously, supramolecular chemistry within suitably functionalized polymers extends significantly over that possible with traditional host± guest complexes in solution, as evidenced by the observed strong cooperativity based on relatively simple binding units. It should be emphasized that the sensitivity of such systems can be greatly enhanced by changes in volume and capacity of the polymer particles, and that manifold known recognition sites can be incorporated into such intelligent materials.

We report the spectral response and slow decay of the steady-state photoconductivity in poly (pphenylenevinylene) (PPV) films. The spectral response of the photoconductivity is in good agreement with that calculated from the absorption data with the assumption of rapid recombination at the surface of the film; the results indicate direct photogeneration of free charge carriers via an interband transition. The photoconductivity is, therefore, consistent with a description of the electronic structure of PPV in terms of a semiconductor band model (rather than an exciton model). The very slow stretchedexponential relaxation of the photoinduced conductivity is reminiscent of the persistent photoconductivity observed in inorganic semiconductors. By assuming that the photocurrent is carried predominantly by mobile polarons near the surface, one can construct a model for the persistent photoconductivity in which the recombination of long-lived bipolarons is inhibited in the bulk where bipolarons have a lower free energy than polarons. The persistent photoconductivity, therefore, is caused by the slow dispersive diffusion of photogenerated bipolarons to the surface where they dissociate into polarons and where both polaron transport and recombination occur.