Electron transfer, charge stabilization and charge recombination in naphthalenediimide-tryptophan immobilized on silica particles

N,N′-Bis(2-phosphonoethyl)-1,4,5,8-naphthalenediimide (DPN) derivatives containing Ac-Trp (DPN-W) and Ac-Trp-Ala-Ala (DPN-AAW) were prepared on silica-gel solid supports by using a combined zirconium phosphonate and Fmoc-solid phase peptide synthesis strategy. A photophysical study of these particles revealed that the tryptophan (W) residue was subject to less polar surroundings on the DPN-AAW than in the DPN-W particle and a much more intense DPN fluorescence emission was observed on DPN-AAW than on DPN-W. The transient spectra of the particles showed the presence of DPN radical anion and Trp radical cation on the DPN-AAW but no signal was obtained for DPN-W. This finding is indicative of a very fast charge transfer between DPN and the tryptophan moiety that is followed by charge recombination on DPN-W and by charge stabilization on DPN-AAW.