Tailoring the properties of carbon nanotubes by chemical functionalization is one of the key challenges towards the realization of carbon nanotube-based opto-electronic devices. This technique aims at combining the great transport properties of nanotubes with the versatility of the optical properties of organic molecules. We present new results about "pi-stacking" functionalization of single wall carbon nanotubes with porphyrin molecules. The optical properties of these complexes are investigated by means of photoluminescence excitation and ultrafast pump/probe experiments. The nanotubes/porphyrin complexes show a total quenching of the porphyrin fluorescence, while the luminescence of the nanotubes is preserved and even enhanced when the excitation is tuned in resonance with the Soret band of the porphyrins. This brings evidence for an efficient energy transfer within the nanotube/porphyrin complex, which corresponds to the implementation of a key functionality: an organic graft acting as an antenna and the nanotube acting as a collector. The dynamics of this transfer turn out to be extremely fast, in qualitative agreement with the high transfer ratio observed in permanent regime.