  Functionalization of carbon nanotubes with hydrosoluble porphyrins (TPPS) is
achieved by "$\pi$-stacking". The porphyrin/nanotube interaction is studied by
means of optical absorption, photoluminescence and photoluminescence excitation
spectroscopies. The main absorption line of the porphyrins adsorbed on
nanotubes exhibits a 120 meV red shift, which we ascribe to a flattening of the
molecule in order to optimize $\pi-\pi$ interactions. The porphyrin-nanotube
complex shows a strong quenching of the TPPS emission while the
photoluminescence intensity of the nanotubes is enhanced when the excitation
laser is in resonance with the porphyrin absorption band. This reveals an
efficient excitation transfer from the TPPS to the carbon nanotube.
