Photophysical properties of hexapyrrolidine C-60 adducts with T-h and D-3 symmetry: protonation of multiple basic sites

The effect of protonation on the absorption and emission properties of two isomeric hexapyrrolidine C-60 adducts with T-h and D-3 symmetry has been st udied. Results show that protonation does not modify substantially the inte resting emissive properties reported in previous work for the neutral forms . The absorption and fluorescence spectra, as well as the fluorescence deca y, are insensitive to protonation at any acid concentration for the D-3 iso mer, while small changes are seen for the T-h isomer. For the latter, as th e positive charge of the molecule increases the absorption spectrum shifts to lower wavelengths, the emission spectrum shifts to the red, and the fluo rescence Lifetime increases. The behavior of this isomer upon acid addition can be explained on the basis of a complex equilibrium between species hav ing different degrees of protonation. Results show that the equilibration r ates among the various species in the excited state are slower than the flu orescence decays, and consequently the measured decay is a weighted average decay of the individual species. The surprising difference in the behavior of the D-3 isomer was shown not to originate from significant differences in the basicity of the nitrogen atoms, but probably due to differences in t he polarizability of the two pi-systems. (C) 1999 Elsevier Science S.A. All rights reserved.