Characterisation of the triplet state of a fluorene-terthiophene alternating copolymer

The triplet state behaviour of the alternating copolymer poly[2,7-(9,9-bis(2′-ethylhexyl)fluorene)-alt-2,5-terthiophene] (PF3T) was studied in benzene solution. This shows a transient absorption at 730 nm, which decays with a lifetime of 10 μs. Using the singlet depletion method, a quantum yield ΦT = 0.335 was determined. This efficient intersystem crossing results from extensive spin-orbit coupling by the sulfur atoms. In the presence of air, the triplet state sensitises singlet oxygen formation in nearly quantitative yield. A lowest triplet state energy of 1.84 eV was determined from time-resolved photoacoustic calorimetry. This is intermediate between polyfluorenes and terthiophene, in agreement with theoretical predictions.