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 characterizing 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 that are capable of forming an intramolecular excimer, by an electron/charge-accepting quencher, nitromethane, is investigated in four different solventsnonpolar (cyclohexane (CH)), polar-aprotic (acetonitrile (ACN)), polar-protic (ethanol (EtOH)), and chlorinated (dichloromethane (DCM))under ambient conditions. For a given probe in a solvent, fluorescence from the monomer and the intramolecular excimer are quenched with similar efficiencies; the efficiency of quenching is higher for the probe with the 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 the longer alkyl chain compound. It is suggested that, while the donor electronic excited-state energetics is more favorable for the long alkyl chain compound, the approach of the quencher to the intramolecular excimer appears to be hindered by the presence of a longer alkyl chain.