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 pi-pi interacting long alkyl chain substituted porphyrins (tetrakis(4-dodecyloxyphenyl)porphyrins; abbreviated as MP(alkyl)(4)) (M = Zn and H-2), to which imidazole functionalized fullerene[60] (C(60)Im) is coordinated, have been investigated in organic solvents. The intermolecular alkyl-pi and pi-pi interactions between the MP (alkyl)(4) and SWCNTs, in addition, coordination between C(60)Im 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) ((MP)-M-1*(alkyl)(4)) with the rate constants of charge separation (k(CS)) in the range of (1-9) x 10(9) s(-1). A nanosecond transient absorption technique confirmed the electron transfer products, MP center dot+(alkyl)(4)/SWCNT center dot- and/or MP center dot-(alkyl)(4)/SWCNT center dot+ for the two-layer nanohybrids. Upon further coordination of C(60)Im to ZnP, acceleration of charge separation via (ZnP)-Zn-1* in C(60)Im -> ZnP(alkyl)(4)/SWCNT is observed to form C(60)(center dot-)Im -> ZnP center dot+(alkyl)(4)/SWCNT and C(60)(center dot-)Im -> ZnP(alkyl)(4)/SWCNT center dot+ 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) x 10(6) s(-1), corresponding to the lifetimes of the radical ion-pairs of 100-170 ns. The electro! n transfer in the nanohybrids has further been utilized for light-to-electricity conversion by the construction of proof-of-concept photoelectrochemical solar cells.