Fluorescence Quenching of Dipyrenylalkanes by an Electron/Charge Acceptor.

Compounds possessing two fluorophoric moieties may exhibit dual fluorescence; one characterizing the monomeric fluorophore unit and the other the intramolecular aggregate. Fluorescence quenching of two dipyrenylalkanes, 1,3-bis-(1-pyrenyl)propane [1Py(3)1Py] and 1,10-bis-(1-pyrenyl)decane [1Py(10)1Py] having different alkyl chains separating the two termini pyrenyl groups capable of forming intramolecular excimer, by an electron/charge accepting quencher, nitromethane, is investigated in four different solvents, a non-polar (cyclohexane, CH), a polar-aprotic (acetonitrile, ACN), a polar-protic (ethanol, EtOH), and a chlorinated (dichloromethane, DCM), under ambient conditions. The fluorescence from the monomer and the intramolecular excimer are quenched with similar efficiencies for both the probes in a given solvent; the efficiency of quenching is higher for the compound with longer alkyl chain separating the two fluorophores. Quenching efficiency is significantly higher in chlorinated solvent DCM. The bimolecular quenching rate constants for intramolecular excimer, however, are either comparable or lower for longer alkyl chain compound. It is suggested that while the donor electronic excited-state energetics is more favorable for long alkyl chain compound, the approach of the quencher to the intramolecular excimer appears to be hindered by the presence of longer alkyl chain.