Synthesis, characterization, redox properties, and photodynamics of donor-acceptor nanohybrids composed of size-controlled cup-shaped nanocarbons and porphyrins

Cup-shaped nanocarbons (CNC) generated by the electron-transfer reduction of cup-stacked carbon nanotubes have been functionalized with porphyrins (H ₂P) as light-capturing chromophores. The resulting donor-acceptor nanohybrid has been characterized by thermogravimetric analysis (TGA), Raman and IR spectroscopy, transmission electron microscopy, elemental analysis, and UV/Vis spectroscopy. The weight of the porphyrins attached to the cup-shaped nanocarbons was determined as 20% by TGA and elemental analysis. The UV/Vis absorption spectrum of CNC-(H₂P)_n in DMF agrees well with that obtained by the superposition of reference porphyrin (ref-H₂P) and cupshaped nanocarbons. The photoexcitation of the CNC-(H2P)n nanohybrid results in formation of the charge-separated (CS) state to attain the longest CS lifetime (0.64 ± 0.01 ms) ever reported for donor-acceptor nanohybrids, which may arise from efficient electron migration following the charge séparation. The formation of a radical ion pair was detected directly by electron spin resonance (ESR) measurements under photoirradiation of CNC(H ₂P)n with a high-pressure mercury lamp in frozen DMF at 153 K. The observed ESR signal at g = 2.0044 agrees with that of TCf-H ₂P"1" produced by one-electron oxidation with [Ru(bpy) ₃]³⁺ in deaerated CHCl₃, indicating the formation of H₂P"1". The electron-acceptor ability of the reference CNC compound (ref-CNC) was also examined by the electron-transfer reduction of ref-CNC by a series of semiquinone radical anions.