Excited state behaviour of pentahelicene dinitriles

Abstract

The excited singlet and triplet states 2,13-dicyano[5]helicene ( 1) and two para-dicyno[5]helicenes containing one and two methyl groups ( 2 and 3, respectively) were studied in solvents of different polarity as a function of temperature. Fluorescence quenching by electron donors such as triethyl amine indicated photoinduced electron transfer. In the absence of additives triplet states were observed by flash photolysis. The triplet lifetime at room temperature was rather short (<1 μs) and the decay limited by intramolecular processes, e.g. charge transfer in the cases of 2 or 3. Luminescence of singlet molecular oxygen, O 2( 1 Δ g), was observed with moderate and low quantum yield for 1 and 3, respectively. For 1– 3, the triplet lifetime increases by six orders of magnitude on going to −196°C. Two subsequently formed triplet states were observed for 3 at lower temperatures. The effects of temperature and solvent polarity on the quantum yields of fluorescence and phosphorescence and the spectroscopic and kinetic triplet absorption properties were examined. The influence of substituents on the deactivation pathways of excited pentahelicenes are discussed.