Covalent functionalization of carbon nanohorns with porphyrins: Nanohybrid formation and photoinduced electron and energy transfer

The covalent attachment of carbon nanohorns (CNHs) to α-5-(2- aminophenyl)-α-15-(2-nitrophenyl)-10,20-bis(2,4,6-trimethylphenyl) -porphyrin (H₂P) via an amide bond is accomplished. The resulting CNH-H₂P nanohybrids form a stable inklike solution. High-resolution transmission electron microscopy (HRTEM) images demonstrate that the original dahlia-flowerlike superstructure of the CNHs is preserved in the CNH-H ₂P nanohybrids. Steady-state and time-resolved fluorescence studies show efficient quenching of the excited singlet state of H₂P, suggesting that both electron and energy transfer occur from the singlet excited state of H₂P to CNHs, depending on the polarity of the solvent. In the case of electron transfer, photoexcitation of H₂P results in the reduction of the nanohorns and the simultaneous oxidation of the porphyrin unit. The formation of a charge-separated state, CNH^--H₂P ⁺, has been corroborated with the help of an electron mediator, hexyl-viologen dication (HV²⁺), in polar solvents. Moreover, the charge-separated CNH^--H₂P⁺ states have been identified by transient absorption spectroscopy.