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We have used pulsed radiolysis to elucidate the neutral tnplet and charged excited states in soluble conjugated polymers, including poly(2-methoxy-5-(2'-ethyl-hexoxy)-p-phenylenevinylene), (MEH-PP\T), poly(2,5-pyridinediyl) (PPY) and poly(4-hexyl-2,5-pyndinediyl) (HIPPY) in various solvents. Using a range of tnplet sensitisers we have determined the S0 -T1 energy separation in these polymers to be 1.27 0.07 e\T, 2.4 0.1 eV and 2.5 0.1 eV respectively. Our experimental results confirm that photomduced absorption features observed m the range 1 to 2 eV in these polymers, are due to triplet-triplet absorption. Tnplet state lifetimes and molar absorption coefficients in solution have also been determined. This work not only demonstrates the versatility of the pulse radiolysis technique but also gives, for the first time, important and unambiguous assignment and detail on the various excited state species in conjugated polymers.

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Pulse radiolysis was used to determine the triplet state energy (2.3 eV) of Poly(2,5-pyridine diyl) (PPY) ®lm which was found to be coincident with the emission energy. Detailed time-resolved photoluminescence (PL) and pump-probe measurements have been applied to study the photoexcited state relaxation dynamics. In ®lms, a very large spectral red-shift ($0.35 eV) for the PL occurred within the ®rst 100 ps whereas no spectral red-shift was observed for the PPY in solution. This result shows clear evidence for the evolution of short-lived singlet emission (S 1 3 S 0 ) at $450 nm to long-lived triplet emission (T 1 3 S 0 ) at $520 nm for the PPY thin ®lm. Streak camera measurement indicates the long live component has a decay time constant of several ns. The picosecond photo-induced triplet state absorption (T 1 3 T 2 transition) peaks at $600 nm as measured by pump-probe which is consistent with both the radiolysis and cw photoinduced absorption measurements. A triplet lifetime of $6 ns is measured which is again consistent with the streak camera measurement. These results lead us to believe that the long live component of the emission from PPY ®lm is in fact phosphorescence. Furthermore, there is evidence that oxygen plays a very important role in the fast triplet radiative lifetime in PPY ®lms. # 2001 Elsevier Science B.V. All rights reserved.Keywords: Poly(2,5-pyridine diyl); Triplet state energy; PPY ®lms; Time-resolved photoluminescence Excited states and their relaxation processes in conjugated polymers have been of signi®cant experimental and theoretical interest in the past decade. With the recent demonstration of polymer light emitting diodes (PLED) [1] and near commercialisation of these devices, a better understanding of the excited state properties of conjugated polymers became even more important. Luminescence in PLED is normally via recombination of singlet excitons. Spin statistics dictate that there is a 3:1 probability of triplet exciton compared to singlet exciton formation from electrical injected charge carrier recombination in PLED, thus a polymer in which ef®cient radiative triplet state recombination can occur will greatly enhance the PLED performance. Indeed a recent realisation of radiative triplet state recombination (i.e. phosphorescence) in organic light emitting device with a quantum ef®cient as high as 7% [2] compared to 3±4% for the singlet exciton emission PLED demonstrated the important role of the triplet state. Therefore, understanding the triplet state manifold and singlet±triplet intersystem crossing for the light emitting polymeric systems are important.The existence of triplet states in conjugated polymers has been well established [3±6]. The photo-induced triplet state absorption (T 1 3 T 2 transition) measurements were the most frequent experimental technique to investigate these triplet states, however, direct triplet exciton emission (i.e. T 1 3 S 0 ) with microsecond lifetime has been observed in poly(3-hexylthiophene) [7] and ladder-type poly(paraphenylene) [8]....

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