Supramolecular carbon nanotube-fullerene donor-acceptor hybrids for photoinduced electron transfer

Photoinduced electron transfer in a self-assembled single-wall carbon nanotube (SWNT)-fullerene-(C₆₀) hybrid with SWNT acting as an electron donor and fullerene as an electron acceptor has been successfully demonstrated. Toward this, first, SWNTs were noncovalently functionalized using alkyl ammonium functionalized pyrene (Pyr-NH₃⁺) to form SWNT/Pyr-NH₃⁺ hybrids. The alkyl ammonium entity of SWNT/Pyr-NH₃⁺ hybrids was further utilized to complex with benzo-18-crown-6 functionalized fullerene, crown-C₆₀, via ammonium-crown ether interactions to yield SWNT/Pyr-NH₃ ⁺/crown-C₆₀ nanohybrids. The nanohybrids were isolated and characterized by TEM, UV-visible-near IR, and electrochemical methods. Free-energy calculations suggested possibility of electron transfer from the carbon nanotube to the singlet excited fullerene in the SWNT/Pyr-NH ₃⁺/crown-C₆₀ nanohybrids. Accordingly, steady-state and time-resolved fluorescence studies revealed efficient quenching of the singlet excited-state of C₆₀ in the nanohybrids. Further studies involving nanosecond transient absorption studies confirmed electron transfer to be the quenching mechanism, in which the electron-transfer product, fullerene anion radical, was possible to spectrally characterize. The rates of charge separation, k_cs, and charge recombination, k_CR, were found to be 3.46 × 10⁹ and 1.04 × 10⁷ s^-1, respectively. The calculated lifetime of the radical ion-pair was found to be over 100 ns, suggesting charge stabilization in the novel supramolecular nanohybrids. The present nanohybrids were further utilized to reduce hexyl-viologen dication (HV²⁺) and a sacrificial electron donor, 1-benzyl-1,4-dihydronicotinamide, in an electron-pooling experiment, offering additional proof for the occurrence of photoinduced charge-separation and potential utilization of these materials in light-energy harvesting applications.