Reversible Photoproduction of Stable Charged Defects in Trans-Polyacetylene
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REVERSIBLE PHOTOPRODUCT'ION OF STABLE CHARGED DEFECTS IN TRANS-POLYACETYLENE
CAROLYN F. HOENER* Laboratory of Chemical Biodynamics, Lawrence Berkeley Laboratory, Berkeley, CA
ABSTRACT Photoproduction and photodepletion of stable charged defects in transpolyacetylene were observed, at low temperatures, by monitoring the IR signature of 1 1 these defects: absorptions at 1368, 1289, 640 cm- (1053, 600 cm- for (CD)x). These absorptions could be observed at long times (two to several hundred minutes) after photolysis, only when the sample was protected from photolysis by the probe beam, by blocking v > 2,300 cm- 1 . The location of these absorptions and the temperature dependence of their intensities were similar to those of previously observed photoinduced defects reported to have much shorter lifetimes (x - ms). The stable defects reported here can 1 be produced at much lower photolysis energies (v > 5,000 cm- ) than transient1 photoinduced defects (v > 9,000 cm- ). A mechanism for the photoproduction of stable defects, involving a two-photon process catalyzed by existing neutral defects, is proposed to explain this low onset. The slow decay of these defects observed when the sample is kept in the dark is This process was slow attributed to photoinduced depletion by blackbody irradiation. enough to allow the wavelength dependence of photodepletion (observed at 2,800 < v < 8,000 cm- 1 ) to be studied. The photodepletion was fastest near 4,500 cm-1, suggesting 1 that the defects possess an electronic absorption (with a peak near 4,500 cm- ) similar to that observed directly for the transient-photoinduced defects.
INTRODUCTION Linear polyacetylene has two structural isomers; cis- and trans-; both exhibit bond-alternation. In the trans- isomer the bond-alternation arises from a mixing of molecular orbitals, in the n system, which lowers the energy of the filled orbitals In the cis- isomer this effect is while raising the energy of the unoccupied orbitals. fortified by the fact that adjacent bonds are inequivalent, this provides an additional reason for one bond to be double and the next single resulting in a larger band-gap. Thus, end-effects or The bond positions in the trans- isomer are equivalent. other external forces can determine opposite directions of bond-alternation at either end of a length of chain, creating a "misfit region"[1] in the middle. These misfit regions, in which bond lengths do not alternate, extend over an odd number of carbons. Thus, neutral misfit regions contain unpaired electrons and can be detected with EPR. The unpaired electron in this non-bond-alternating defect is in a nonbonding xt orbital (of the defect moiety - divorced from the electronic structure of the surrounding polymer) located near the center of the trans-polyacetylene band-gap. This partially filled orbital provides the lowest energy position for both introduced Thus, when polyacetylene is chemically doped, similar nonelectrons and holes. bond-alternating defects are formed to accommodate the introduced charges. *Present add
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