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.