Excited state behaviour of pentahelicene dinitriles

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, res pectively) were studied in solvents of different polarity as a function of temperature. Fluorescence quenching by electron donors such as triethyl ami ne indicated photoinduced electron transfer. In the absence of additives tr iplet states were observed by flash photolysis. The triplet lifetime at roo m temperature was rather short (<1 mu s) and the decay limited by intramole cular processes, e.g. charge transfer in the cases of 2 or 3. Luminescence of singlet molecular oxygen, O-2((1)Delta(g)), was observed with moderate a nd low quantum yield for 1 and 3, respectively. For 1-3, the triplet lifeti me increases by six orders of magnitude on going to -196 degrees C. Two sub sequently formed triplet states were observed for 3 at lower temperatures. The effects of temperature and solvent polarity on the quantum yields of fl uorescence and phosphorescence and the spectroscopic and kinetic triplet ab sorption properties were examined. The influence of substituents on the dea ctivation pathways of excited pentahelicenes are discussed. (C) 1999 Elsevi er Science S.A. All rights reserved.