INTRAMOLECULAR FLUORESCENCE QUENCHING AND EXCIPLEX FORMATION IN ω-(1-PYRENYL)ALKYL PARA-SUBSTITUTED BENZOATES

Article abstract of DOI:10.1002/poc.610070107

Measurements of absorption and emission spectra, fluorescence quantum yields (Φ_f) and lifetimes (τ_f) of ω-(1-pyrenyl)alkyl para-substituted benzoates PnX (X = H, Cl, CF3, CN, NO2, with n = 1-4) were carried out in solvents of various polarity.Intramolecular interaction in the ground state is not observed in any of these compounds.PnCN (n = 2-4) shows an intramolecular exciplex emission in solvents of low to high polarity.The broad, structureless emission at longer wavelength observed in ethyl acetate solution of P1CN is ascribed to an 'exciplex-type' emission which does not require actual overlapping of the two chromophores.P2CF3 also shows an exciplex emission in solvents of medium to high polarity.The solvent dependence of both Φ_f and τ_f increases as the electron-withdrawing ability of the para-subtituents increases.The relationship between fluorescence quenching by electron transfer and para-substituents of PnX is discussed by means of the tree energy for electron transfer, ΔG_ET, obtained from the oxidation and reduction potentials of pyrene and methyl para-substituted benzoates in acetonitrile, respectively.PnH, with positive ΔG_ET, does not show a solvent dependence of Φ_f, except for P1H, in which ca 40percent of the fluorescence is quenched in acetonitrile.PnCl, with slightly negative ΔG_ET, shows more efficient quenching, but does not show exciplex emission.PnCN, PnCF3 and PnNO2 have ΔG_ET values between -0.36 and -0.65 eV, and their fluorescence is fairly effciently quenched.The fluorescence of PnCN is concluded to be strongly quenched by intersystem crossing from the singlet exciplex to the locally excited pyrene, and by electron transfer from the pyrene part to the benzoate part.That the formation of a singlet exciplex is necessary for intersystem crossing in bichromophoric compounds containing pyrene is thus clearly illustrated.