Measurement of the S0–T1 energy gap in poly(2-methoxy,5-(2′-ethyl-hexoxy)–p-phenylenevinylene) by triplet–triplet energy transfer

We have elucidated the lowest triplet state in the soluble luminescent conjugated polymer poly(2-methoxy,5-(2′-ethyl-hexoxy)–p-phenylenevinylene) (MEH–PPV) in benzene. Using a range of triplet sensitisers we determine that the S0–T1 energy separation is 1.27±0.07 eV. Triplet–triplet absorption is observed at 1.5 eV. In solution, the triplet state has a lifetime ⩾100 μs. The molar absorption coefficient at 1.5 eV is concentration dependent; at 200 mg/l, ε=1.19×105 M−1 cm−1. Up to 50 mg/l, the triplet decay rate showed no change, indicating self-quenching of triplets is not significant. Oxygen quenches the MEH–PPV triplet, leading to singlet oxygen formation with a quantum yield of 2.5±1%.