Photoinduced charge separation in three-layer supramolecular nanohybrids: fullerene–porphyrin–SWCNT

Abstract

Photoinduced charge separation processes of three-layer supramolecular hybrids, fullerene–porphyrin–SWCNT, which are constructed from semiconducting (7,6)- and (6,5)-enriched SWCNTs and self-assembled via π–π interacting long alkyl chain substituted porphyrins (tetrakis(4-dodecyloxyphenyl)porphyrins; abbreviated as MP(alkyl)₄) (M = Zn and H₂), to which imidazole functionalized fullerene[60] (C₆₀Im) is coordinated, have been investigated in organic solvents. The intermolecular alkyl–π and π–π interactions between the MP(alkyl)₄ and SWCNTs, in addition, coordination between C₆₀Im and Zn ion in the porphyrin cavity are visualized using DFT calculations at the B3LYP/3-21G(*) level, predicting donor–acceptor interactions between them in the ground and excited states. The donor–acceptor nanohybrids thus formed are characterized by TEM imaging, steady-state absorption and fluorescence spectra. The time-resolved fluorescence studies of MP(alkyl)₄ in two-layered nanohybrids (MP(alkyl)₄/SWCNT) revealed efficient quenching of the singlet excited states of MP(alkyl)₄ (¹MP*(alkyl)₄) with the rate constants of charge separation (k_CS) in the range of (1–9) × 10⁹ s⁻¹. A nanosecond transient absorption technique confirmed the electron transfer products, MP˙⁺(alkyl)₄/SWCNT˙⁻ and/or MP˙⁻(alkyl)₄/SWCNT˙⁺ for the two-layer nanohybrids. Upon further coordination of C₆₀Im to ZnP, acceleration of charge separation via¹ZnP* in C₆₀Im→ZnP(alkyl)₄/SWCNT is observed to form C₆₀˙⁻Im→ZnP˙⁺(alkyl)₄/SWCNT and C₆₀˙⁻Im→ZnP(alkyl)₄/SWCNT˙⁺ charge separated states as supported by the transient absorption spectra. These characteristic absorptions decay with rate constants due to charge recombination (k_CR) in the range of (6–10) × 10⁶ s⁻¹, corresponding to the lifetimes of the radical ion-pairs of 100–170 ns. The electron transfer in the nanohybrids has further been utilized for light-to-electricity conversion by the construction of proof-of-concept photoelectrochemical solar cells.