Enhanced triplet formation by twisted intramolecular charge-transfer excited states in conjugated oligomers and polymers

The triplet yield and intersystem crossing rate of a set of conjugated oligomers and polymers that, in polar solvents, form a charge-transfer state with a twisted conformation has been investigated. It was observed that in these dibenzothiophene-fluorene oligomers a greater than 10-fold increase on the triplet yield is achieved by simply changing the medium polarity to favor the formation of the twisted charge-transfer state, while the fluorescence lifetime is only slightly increased. The increase in the intersystem crossing rate is attributed to the improved mixing between the singlet and triplet states in the twisted excited state. In analogous polymers, the intersystem crossing, rate does not show the same increase, most likely because of the greater energetic and conformational disorder increasing the intersystem crossing rate at all times, regardless of the formation of the twisted charge-transfer state or not.