Jerry Perlstein

Efficient photogeneration of charge carriers in a layered photoreceptor using a conjugated polymer

The field‐dependent quantum efficiency φ(E) for photogeneration of charge carriers in a bilayer photoreceptor consisting of a poly(p‐phenylene benzobisthiazole) charge generation layer and a hole transporting layer has been measured by the xerographic photodischarge technique. φ(E) varied from 0.10 at low fields (<105 V/cm) to 0.32 at 106 V/cm. The conjugated polymer‐based photoreceptors have a photosensitivity of 13 ergs/cm2 and negligible dark decay (∼0.2 V/s). Photocarrier generation in the bilayer devices is suggested to be due to field‐assisted dissociation of exciplexes formed between the conjugated polymer and the hole transporting molecule.

Supramolecular aggregates of photoreactive aromatics. Structure, photophysics and photochemistry of stilbene and azobenzene phospholipids

Abstract Stilbene and azobenzene-derivatized phospholipids have been synthesized as building blocks to study H-aggregate formation, structure and properties in both bilayer vesicles and LB films. The observed H-aggregates in bilayers display similar spectral characteristics to that in LB films and a strong induced circular dichroism spectrum. Studies of interconversion between H-aggregates and monomers or dimers have established relatively small, integral numbers for aggregate sizes. Based on these results as well as molecular cooling simulation, chiral cyclic pinwheel structures are proposed for these H-aggregates. Single crystal structure of a trans -stilbene derivative ( 4 ) has been obtained, and its remarkable similarity in structure to compressed monolayer obtained from molecular simulation and almost identical spectra to H-aggregate of stilbene support the proposed herringbone lattice for these aggregates. The very large arrays of aggregates may consist of a “mosaic” of small aggregates. The presence of aggregates as either stable or metastable entities has special consequences for microscopic as well as macroscopic properties of the medium in which they are generated. The aggregates of azobenzene in DPPC vesicles promote reagent release much more efficiently than the correspondent monomer and dimer. Trans—cis photoisomerization of azobenzene aggregates opens a convenient way to establish photoregulatable membrane and related materials.

Photoinduced absorption of conducting polymer-C60 composites

Experimental studies of photoinduced absorption, PA, have been carried out by several groups on composites of C60 with conducting polymers such as poly[2-methoxy, 5-(ethyl hexyloxy) -p- phenylene vinylene], MEH-PPV, to understand the quenching of photoluminescence and increase of photoconductivity caused by the addition of small amounts of C60. To account for the observed PA it is first necessary to know where the C60 molecules sit relative to the MEH-PPV chains. We found with the use of a Monte Carlo cooling algorithm that the most probable position of the C60 is in a channel formed by the alkoxy side chains, its center approximately 10 angstrom from that of the MEH-PPV backbone. We identify the PA peak seen in MEH-PPV as the transition between the lower and upper levels of a polaron coupled to one of opposite sign on a neighboring chain. Calculations are carried out for the energy levels of neutral C60 and C60- using a tight binding Hamiltonian with the parameters obtained by Harigaya. To determine the effect on the levels of the Coulomb interaction between C60- and a positively charged polaron, P+, on MEH-PPV we use coupled equations. Finally, we suggest that the peak of the PA in the composite is due to transitions between the two polaron levels of a free P+ on MEH-PPV. Additional PA features in the composite can be identified as due to C60- and neutral C60.