Carbon Nanohorn–Porphyrin Dimer Hybrid Material for Enhancing Light-Energy Conversion

The covalent grafting through a rigid ester bond of a dimeric porphyrin [(H<sub>2</sub>P)<sub>2</sub>] and carbon nanohorns (CNHs) was accomplished. The newly formed CNH–(H<sub>2</sub>P)<sub>2</sub> hybrid was found to be soluble or dispersible in several organic solvents. Application of diverse spectroscopic techniques verified the successful formation of the CNH–(H<sub>2</sub>P)<sub>2</sub> hybrid material. In addition, thermogravimetric analysis revealed the amount of (H<sub>2</sub>P)<sub>2</sub> loading onto CNHs, and TEM studies showed the characteristic secondary spherical superstructure morphology of the hybrid material. Efficient fluorescence quenching of (H<sub>2</sub>P)<sub>2</sub> in the CNH–(H<sub>2</sub>P)<sub>2</sub> hybrid suggests that photoinduced events occur from the photoexcited (H<sub>2</sub>P)<sub>2</sub> to CNHs. Nanosecond transient absorption spectroscopy revealed the formation of transient species such as (H<sub>2</sub>P)<sub>2</sub><sup>•+</sup> and CNH<sup>•–</sup> by photoinduced charge separation in CNH–(H<sub>2</sub>P)<sub>2</sub>. Additional proof for the photoinduced charge-separated state CNH<sup>•–</sup>–(H<sub>2</sub>P)<sub>2</sub><sup>•+</sup> was obtained, from which the electron mediates to added hexyl viologen dication (HV<sup>2+</sup>). Finally, the CNH–(H<sub>2</sub>P)<sub>2</sub> was adsorbed on nanostructured SnO<sub>2</sub> electrode, to construct a photoactive electrode, which reveals photocurrent and photovoltage responses with an incident photon-to-current conversion efficiency value as large as 9.6%, without the application of any bias voltage.