Carbon Nanotubes with Covalently Linked Porphyrin Antennae: Photoinduced Electron Transfer

Functionalization of carbon nanotubes through surface modification has attracted significant interest recently. Covalent and noncovalent functionalization strategies involving reactions of organic or polymeric molecules onto carbon nanotubes have primarily focused on dispersion or dissolution properties. Incorporation of light absorbing antenna chromophores through a covalent linkage with the extended π electrons of a carbon nanotube would constitute an ideal supramolecular nanoassembly for generating singlet excited energy and its conversion to chemical energy. Porphyrins are one such class of molecules used in assemblies of donor-acceptor materials in molecular electronics and photovoltaic devices. Several fullerene-based molecular systems with covalently linked porphyrins and metalloporphyrins have been synthesized; their interesting photoinduced electron-transfer processes have been studied. Noncovalent interaction of metalloporphyrins and freebase porphyrins with single-wall carbon nanotubes (SWNTs) has been used for dispersion and also for separation of semiconducting and metallic tubes. Recently, a noncovalently interacting donoracceptor system consisting of an anionically functionalized porphyrin and a cationically functionalized pyrene stacked on SWNTs has been shown to exhibit electron-transfer properties. A donor-acceptor system with a covalent linkage between the light-harvesting antenna and the acceptor reaction center could enhance the efficiency of photoinduced electron transfer and energy transfer. Supramolecular structures consisting of covalently grafted porphyrins to carbon nanotubes have not been synthesized so far as an efficient donor-acceptor system. In this communication, we report the synthesis of meso-substituted porphyrin-grafted carbon nanotubes ((por)n-g-CNTs), including multiwalled nanotubes ((por)n-g- MWNTs) and SWNT ((por)n-g-SWNTs), and the study of their photoinduced electron-transfer properties