The ability of high oxidation potential zinc porphyrins acting as electron donors in photoinduced electron-transfer reactions is investigated. Donor−acceptor dyads were assembled via metal−ligand axial coordination of either pyridine or phenylimidazole functionalized fulleropyrrolidine with zinc porphyrin functionalized with different numbers of halogen substituents on the meso-aryl rings. Optical absorption studies on complex formation revealed relatively higher binding constants. Efficient quenching of fluorescence was observed for the newly assembled dyads, revealing their ability to undergo photoinduced events. Differential pulse voltammetry studies were performed to understand the structure−activity relationships with respect to the electron deficient nature of the porphyrins and to utilize these data to estimate free-energy change for charge-separation and charge-recombination processes. The absolute value of free-energy change for charge separation was found to be lower for halogenated porphyrins with higher oxidation potentials expecting to form high-energy radical ion pairs. Using femtosecond transient techniques, evidence for charge separation and kinetics of charge separation and recombination were obtained in toluene. The kinetic data obtained by analyzing the time profiles of the radical ions revealed occurrence of ultrafast charge separation and relatively slower charge recombination processes in the dyads. Notably, electron-transfer rates did not exactly follow the trends predicted based on Marcus theory of electron transfer. Donor− acceptor geometry and populating the triplet excited states of the sensitizers during charge recombination are considered to be possible reasons for this behavior.

Photoinduced electron transfer in self-assemblies of porphyrins ion-paired with ssDNA wrapped around single-wall carbon nanotubes (SWCNTs) has been reported. To accomplish the three-component hybrids, two kinds of diameter-sorted semiconducting SWCNT(n,m)s of different diameter ((n,m) = (6,5) and (7,6)) and free-base or zinc porphyrin bearing peripheral positive charges ((TMPyP(+))M (tetrakis(4-N-methylpyridyl)porphyrin); M = Zn and H(2)) serving as light-absorbing photoactive materials are utilized. The donor-acceptor hybrids are held by ion-pairing between the negatively charged phosphate groups of ssDNA on the surface of the SWCNT and the positively charged at the ring periphery porphyrin macrocycle. The newly assembled bionano donor-acceptor hybrids have been characterized by transmission electron microscopy (TEM) and spectroscopic methods. Photoinduced electron transfer from the excited singlet porphyrin to the SWCNTs directly and/or via ssDNA as an electron mediator has been established by performing systematic studies involving the steady-state and time-resolved emission as well as the transient absorption studies. Higher charge-separation efficiency has been successfully demonstrated by the selection of the appropriate semiconductive SWCNTs with the right band gap, in addition to the aid of ssDNA as the electron mediator.

The molecular self-assembly of surfactants on the surface of single-walled carbon nanotubes (SWCNT) is currently a common strategy for the tuning of nanotube properties and the stabilization of carbon nanotube dispersions. Here, we report direct measurements of the degree of interfacial ordering for sodium dodecyl sulfate (SDS) surfactants adsorbed on colloidal, single-chirality enriched, SWCNTs within a solid film and investigate the dependence of surface alkyl chain order on the surfactant concentration in the precursor solution. The degree of order for the SWCNT-bound SDS molecules, is probed by vibrational sum frequency generation (VSFG) spectroscopy. We find concrete evidence for the presence of highly ordered surface structures at sufficiently high SDS concentrations, attributed here to cylindrical-like micelle assemblies with the SWCNT at the core. As the SDS concentration decreases, the interfacial order is found to decrease as well, generating a more disordered or random adsorption of surfactants on the nanotube surfaces.

We discovered a rare phenomenon wherein a thieno-pyrrole fused BODIPY dye (SBDPiR690) generates singlet oxygen without heavy halogen atom substituents. SBDPiR690 generates both singlet oxygen and fluorescence. To our knowledge, this is the first example of such a finding. To establish a structure-photophysical property relationship, we prepared SBDPiR analogs with electron-withdrawing groups at the para-position of the phenyl groups. The electron-withdrawing groups increased the HOMO-LUMO energy gap and singlet oxygen generation. Among the analogs, SBDPiR688, a CF3 analog, had an excellent dual functionality of brightness (82290 m(-1)  cm(-1) ) and phototoxic power (99170 m(-1)  cm(-1) ) comparable to those of Pc 4, due to a high extinction coefficient (211 000 m(-1)  cm(-1) ) and balanced decay (Φflu =0.39 and ΦΔ =0.47). The dual functionality of the lead compound SBDPiR690 was successfully applied to preclinical optical imaging and for PDT to effectively control a subcutaneous tumor.

Photoinduced electron transfer in self-assembled via ion-pairing porphyrin-SWCNT nanohybrids has been reported. To accomplish this, two kinds of semiconducting SWCNTs of different diameters, and free-base or zinc porphyrin bearing peripheral positive or negative charges serving as light absorbing photo-active materials are utilized. The donor-acceptor hybrids are held by ion-pairing with the help of oppositely charged pyrene derivatives adhered to the side walls of SWCNTs. Higher charge-separation efficiency is established for thick SWCNT(7,6) with narrower band gap compared with the thin SWCNT(6,5) with wider band gap. Photoelectrochemical studies using FTO/SnO 2 electrodes modified with these donor-acceptor nanohybrids unanimously demonstrated the ability of these nanohybrids to harvest light energy into electricity. Importantly, the photocurrent generation followed the trend observed for charge-separation, that is, incident-photon-to-current conversion efficiency of a maximum of 8% is achieved for photocells with SWCNT(7,6), while such conversion efficiencies are smaller for the cells derived from SWCNT(6,5). These results indicate that higher light energy conversion efficiencies are possible to achieve by the selection of the appropriate SWCNTs with right band gap on combination of appropriate porphyrins.