INTRAMOLECULAR FLUORESCENCE QUENCHING AND EXCIPLEX FORMATION IN OMEGA-(1-PYRENYL)ALKYL PARASUBSTITUTED BENZOATES

Measurements of absorption and emission spectra, fluorescence quantum yields (Phi(f)) and lifetimes (tau(f)) of omega-(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. P2CF(3) also shows an exciplex emission in solvents of medium to high polarity. The solvent dependence of both Phi(f) and tau(f) increases as the electron-withdrawing ability of the para-sustituents increases. The relationship between fluorescence quenching by electron transfer sad para-substituents of PnX is discussed by means of the free energy for electron transfer, Delta G(ET), obtained from the oxidation and reduction potentials of pyrene and methyl para-substituted benzoates in acetonitrile, respectively. PnH, with positive Delta G(ET), does not show a solvent dependence of Phi(f) except for P1H, in which ca 40% of the fluorescence is quenched in acetonitrile. PnCl, with slightly negative Delta G(ET), shows more efficient quenching, but does not show exciplex emission. PnCN, PnCF(3) and PnNO(2) have Delta G(ET) values between -0.36 and -0.65 eV, and their fluorescence is fairly efficiently 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.