Photoinduced Charge Separation in Ion-Paired Porphyrin-Single-Wall Carbon Nanotube Donor-Acceptor Hybrids

Photoinduced electron transfer in self-assembled (via ion-pairing) porphyrin-single-wall carbon nanotube (SWNT) donor-acceptor hybrids has been demonstrated. Toward this, first, SWNTs were solubilized by pi-pi stacking of pyrene (Pyr) functionalized with either a cationic (alkyl anirnonium; Pyr-NH3+) or anionic (butyric acid; Pyr-COO-) terminal groups. The SWNT/Pyr-NH3+ and SWNT/Pyr-COO- hybrids thus obtained were subsequently used to generate donor-acceptor hybrids by ion-pairing with free-base or zinc(II) porphyrins bearing either peripheral anionic (sulfonatophenyl) or cationic (N-methylpyridyl) groups. The SWNT/PyrNH(3)(+)/(TPPS)M and SWNT/Pyr-COO-/(TMPyP)M (where TPPS = tetrakis(4-sulfonatophenyl)porphyrin, TMPyP = tetrakis(N-methylpyridyl)porphyrin, and M = 2H or Zn(II)) thus obtained were spectrally and electrochemically characterized to arrive at the structure and redox potentials. Both steady-state and time-resolved emission studies revealed efficient quenching of the singlet excited state of porphyrins in the nanohybrids, suggesting either electron or energy transfer from the excited porphyrins to SWNTs. Nanosecond transient absorption spectra revealed the formation of one-electron oxidation of the porphyrins with a simultaneous one-electron reduction of SWNT. The charge separation process is further confirmed with the aid of an electron mediator, methyl or hexyl viologen dication (MV2+ or HV2+) and an electron-hole shifter in the studied solvents, water, or dimethylformamide (DMF). As a consequence of the photoinduced processes, accumulations of MV center dot+ and HV center dot+ as a result of the electron-pooling were observed with 50-100% yields in the presence of a sacrificial electron donor, thus demonstrating the importance of the photosensitizer-functionalized SWNT nanohybrids in the photogeneration of redox products.