Pulse radiolysis studies on charge carriers in conjugated polymers

The charged states of the conjugated polymers poly(2-methoxy,5-(2'-ethylhex yloxy)-p- phenylenevinylene) (MEH-PPV) and poly(2,5-pyridinediyl) (PPY) hav e been studied by pulse radiolysis. Following pulse radiolysis of argon-sat urated solutions of MEH-PPV in chloroform, a new absorption is seen to grow in over a few hundred microseconds. This has a principal absorption at 1.4 3 eV and a weaker, low energy band (less than or equal to 0.80 eV), and is assigned to the positive one-electron charge state (positive polaron) of ME H-PPV. The slow absorption decay is unaffected by oxygen. Negative charge s tates (negative polarons), with absorptions around 1.4 eV, are produced upo n pulse radiolysis of MEH-PPV in argon-saturated solutions in tetrahydrofur an (THF) or benzonitrile. A small solvatochromic shift is observed. In cont rast to the behavior of the positive polaron, the MEH-PPV negative charge c arriers decay fairly rapidly, and are readily quenched by molecular oxygen. Previous results on chemically produced positive and negative charge state s of conjugated polymers and oligomers are discussed on the basis of these assignments, and comparison is made with theoretical calculations. Using be nzophenone as a charge scavenger, pulse radiolysis of formic acid is shown to generate one-electron reducing species. Various derivatives of PPY, incl uding a regioregular polymer (rPPY) and a hexyl substituted compound (HPPY) have been studied by pulse radiolysis in formic acid solution. With rPPY, new absorptions are observed at 2.59 and 1.40 eV, and are assigned to the o ne-electron reduced species. These are strongly quenched by molecular oxyge n. With HPPY, the lower energy transition is broadened to give a maximum be low 1.21 eV and a shoulder at 1.65 eV. The differences between rPPY and HPP Y are interpreted on the basis of differences in the rigidity of the polyme r. The relevance of these assignments to the identification of charged spec ies in photoinduced absorption measurements is indicated. (C) 2000 American Institute of Physics. [S0021-9606(00)51206-9].