J. Kawakami et M. Iwamura, INTRAMOLECULAR FLUORESCENCE QUENCHING AND EXCIPLEX FORMATION IN OMEGA-(1-PYRENYL)ALKYL PARASUBSTITUTED BENZOATES, Journal of physical organic chemistry, 7(1), 1994, pp. 31-42
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 interacti
on in the ground state is not observed in any of these compounds. PnCN
(n = 2-4) shows an intramolecular exciplex emission in solvents of lo
w to high polarity. The broad, structureless emission at longer wavele
ngth observed in ethyl acetate solution of P1CN is ascribed to an 'exc
iplex-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-susti
tuents increases. The relationship between fluorescence quenching by e
lectron 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-substitu
ted 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, w
ith 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 fa
irly efficiently quenched. The fluorescence of PnCN is concluded to be
strongly quenched by intersystem crossing from the singlet exciplex t
o the locally excited pyrene, and by electron transfer from the pyrene
part to the benzoate part. That the formation of a singlet exciplex i
s necessary for intersystem crossing in bichromophoric compounds conta
ining pyrene is thus clearly illustrated.