Fluorescence Investigations on Intramolecular Energy Transfer in Alkyl-Fluorenole and Alkyl-Anthracene

Fluorescence investigations of alkyl fluorenole- and alkyl anthracene-solutions in n-hexane or cyclohexane resp. were performed for β- and UV-excitation. The observed influence of scavengers for electrons or positive ions on the fluorescence intensity for β-excitation reveals that charge transfer is the predominant process for the intermolecular energy transfer from solvent to solute. The fluorescence intensity of alkyl fluorenoles and alkyl anthracenes increases with increasing number n of CH₂-groups in the alkyl chain, reaching a plateau for n ~ 6. This gives evidence for intramolecular energy transfer within the CH₂-chain with a limited range. Using a simple model in alkyl fluorenoles for the transfer probability from C_n to C_n-1 in the chain a value of 0.65 could be deduced. The model derived for n-alkyl fluorenoles fits to the values of branched alkyl fluorenoles as well. The results obtained from the fluorescence measurements are compared with those on C—H-bond breakage in the aliphatic chain, derived from investigations of hydrogen production and radical formation. Possible mechanisms for intramolecular energy transfer are discussed.