The photophysical and photochemical properties of a cationic porphyrin [bis(arginyl)porphyrin (BAP)] complexed to [poly(dG-dC)](2) and [poly(dA-dT)](2) have been investigated and compared to those of free BAP in aqueous solution. A drastic enhancement of the quantum yield of nonradiative deactivation of the lowest excited singlet state of BAP is observed upon intercalation between GC base pairs in [poly(dG-dC)](2) but not upon complexation with [poly(dA-dT)](2). Both picosecond time-resolved fluorescence and femtosecond transient absorption measurements give evidence for the occurrence of an ultrafast direct electron transfer (k greater than or equal to 1.25 x 10(13) s(-1)) from guanine to the lowest excited singlet state of BAP followed by an efficient back electron transfer (k = 8.3 x 10(12) s(-1)). A classical nonadiabatic Marcus model for this reverse electron transfer explains the experimental observations which allow one to estimate the electronic coupling energy (230 cm(-1)) involved for BAP-[poly(dG-dC)](2) complexes.