Pulse radiolysis studies on charge carriers in conjugated polymers

Abstract

The charged states of the conjugated polymers poly(2-methoxy,5-(2′-ethylhexyloxy)-p- phenylenevinylene) (MEH-PPV) and poly(2,5-pyridinediyl) (PPY) have been studied by pulse radiolysis. Following pulse radiolysis of argon-saturated 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 1.43 eV and a weaker, low energy band (⩽0.80 eV), and is assigned to the positive one-electron charge state (positive polaron) of MEH-PPV. The slow absorption decay is unaffected by oxygen. Negative charge states (negative polarons), with absorptions around 1.4 eV, are produced upon pulse radiolysis of MEH-PPV in argon-saturated solutions in tetrahydrofuran (THF) or benzonitrile. A small solvatochromic shift is observed. In contrast to the behavior of the positive polaron, the MEH-PPV negative charge carriers decay fairly rapidly, and are readily quenched by molecular oxygen. Previous results on chemically produced positive and negative charge states of conjugated polymers and oligomers are discussed on the basis of these assignments, and comparison is made with theoretical calculations. Using benzophenone as a charge scavenger, pulse radiolysis of formic acid is shown to generate one-electron reducing species. Various derivatives of PPY, including 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 one-electron reduced species. These are strongly quenched by molecular oxygen. With HPPY, the lower energy transition is broadened to give a maximum below 1.21 eV and a shoulder at 1.65 eV. The differences between rPPY and HPPY are interpreted on the basis of differences in the rigidity of the polymer. The relevance of these assignments to the identification of charged species in photoinduced absorption measurements is indicated.

Transient absorption spectra of triplet states and charge carriers of conjugated polymers

“…The one-electron reduced species from MEH-PPV has also been prepared by pulse radiolysis in benzonitrile solution [26]. The spectrum is similar, but appears to show a small solvent shift.…”

“…We have shown that this can be used to unambiguously prepare the positive charge carriers of conjugated polymers [23,26]. We will take MEH-PPV as a typical case.…”

“…By appropriate choice of conditions, it is possible to speci®-cally produce either excited states or charged species. We have previously used this method to look at the lowest excited triplet states [22±25] and charged species [23,26] of some conjugated organic polymers in solution. In this paper we extend this study to include many of the most commonly used polymers in electroluminescent devices.…”

Triplet Energies ofπ-Conjugated Polymers

“…We recently reported on the use of pulse radiolysis to measure the triplet energy in poly[2-methoxy,5-(2 0 -(ethylhexoxy)-p-phenylenevinylene] (MEHPPV) [7]. We have also demonstrated that charged states, again generated by this method, have very different lifetimes and spectral signatures from the triplet states [11]. We now report on triplet state measurements on a range of the most widely used luminescent and conductive polymers in current display technologies; see Fig.…”

Room temperature and low-temperature absorption and emission spectra of some polypyridylruthenium(II) 3.2.1 complexes

“…This will favour Fo Èrster-type [14] triplet energy transfer from excited states of conjugated polymers. The triplet energies of a series of conjugated polymers have recently been determined [15,16]. Matching the energies of these with the lowest triplet states of polypyridylruthenium(II) complexes may provide a method for highly ef®cient , where tpy and py are 2,2 0 ,2 00 -terpyridine and pyridine, and L±L represents the bidentate ligands bipyridyl (bpy), 4,4 0 -dimethylbipyridyl (Me 2 bpy), 4-nitrobipyridyl (O 2 Nbpy), oxalate (ox) and acetylacetonate (acac) [17].…”