Measurement of the S-0-T-1 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 conj ugated polymer poly(2-methoxy,5-(2'-ethylhexoxy)-p-phenylenevinylene) (MEH- PPV) in benzene. Using a range of triplet sensitisers we determine that che S-0-T-1 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 greate r than or equal to 100 mu s. The molar absorption coefficient at 1.5 eV is concentration dependent; at 200 mg/l, epsilon = 1.19 x 10(5) M-1 cm(-1). Up to 50 mg/l, the triplet decay rate showed no change, indicating self-quenc hing of triplets is not significant. Oxygen quenches the MEH-PPV triplet, l eading to singlet oxygen formation with a quantum yield of 2.5 +/- 1%. (C) 1999 Elsevier Science B.V. All rights reserved.