Potassium Ion Controlled Switching of Intra- to Intermolecular Electron Transfer in Crown Ether Appended Free-Base Porphyrin−Fullerene Donor−Acceptor Systems

Abstract

Photoinduced electron transfer in intramolecularly interacting free-base porphyrin bearing one or four 18-crown-6 ether units at different positions of the porphyrin macrocycle periphery and pristine fullerene was investigated in polar benzonitrile and nonpolar o-dichlorobenzene and toluene solvents. Owing to the presence of two modes of binding, stable dyads were obtained in which the binding constants, K, were found to range between 4.2 x 10(3) and 10.4 x 10(3) M(-1) from fluorescence quenching data depending upon the location and number of crown ether entities on the porphyrin macrocycle and the solvent. Computational studies using the B3LYP/3-21G() method were employed to arrive at the geometry and electronic structure of the intramolecular dyads. The energetics of the redox states of the dyads were established from cyclic voltammetric studies. Under the intramolecular conditions, both the steady-state and time-resolved emission studies revealed efficient quenching of the singlet excited free-base porphyrin in these dyads, and the measured rates of charge separation, k(CS), were found to be in the 10(8)-10(9) s(-1) range. Nanosecond transient absorption studies were performed to characterize the electron-transfer products and to evaluate the charge-recombination rates. Shifting of the electron-transfer pathway from the intra- to intermolecular route was achieved by complexing potassium ions to the crown ether cavity(ies) in benzonitrile. This cation complexation weakened the intramolecular interactions between fullerene and the crown ether appended free-base porphyrin supramolecules, and under these conditions, intermolecular type interactions were mainly observed. Reversible inter- to intramolecular electron transfer was also accomplished by extracting the potassium ions of the complex with the addition of 18-crown-6. The present study nicely demonstrates the application of supramolecular methodology to control the excited-state electron-transfer path in donor-acceptor dyads.

Self-assembled tetrapyrrole–fullerene and tetrapyrrole–carbon nanotube donor–acceptor hybrids for light induced electron transfer applications

“…Shifting of the electron-transfer pathway from the intra-to intermolecular route was achieved by complexing potassium ions to the crown ether moiety(ies) of porphyrins. 71,72 This cation complexation mainly weakened the intramolecular type interactions between fullerene and crown ether appended porphyrins and thus facilitated the intermolecular type interactions. Interestingly, reversible switching of inter-to intramolecular electron transfer path was also achieved by extracting the complexed potassium ions by external addition of 18-crown-6 to the solution.…”

“…Towards this, complexes of crown ether appended free-base or zinc porphyrins with C 60 , 55-56 were obtained. 71 The binding constants of these complexes were strongly dependent on the location and number of crown ether entities on the porphyrin ring, and the utilized solvent media. The stability of these complexes was increased by employing ammonium cation functionalized fullerene 48 and new complexes such as 57 were formed.…”

Supramolecular donor–acceptor hybrids of porphyrins/phthalocyanines with fullerenes/carbon nanotubes: electron transfer, sensing, switching, and catalytic applications

“…Control over the electron transfer path in photochemical reactions is important to further understand complex photochemical events occurring in natural photosynthesis 1 and also to build photonic devices. 2 In an effort to control the electron transfer events in donor-acceptor conjugates, 27 we designed a novel conjugate using fullerene functionalized with an alkyl ammonium cation (C 60 -NH 3 + ) and a porphyrin with a benzo-18-crown-6-ether entity (crown-P), as shown in Scheme 2. 28 Here, fullerene binds to the porphyrin primarily through the ammonium cation-crown ether by cation-dipole binding interactions, resulting in efficient intramolecular photoinduced electron transfer within the conjugate.…”

Helical Nanostructures of an Optically Active Metal‐Free Porphyrin with Four Optically Active Binaphthyl Moieties: Effect of Metal–Ligand Coordination on the Morphology

“…However, in the event that K + ions were added to the system, additional metal-ligand coordination bonding interactions between K + and crown ether moieties facilitate the linear stacking of porphyrin molecules in a "head-to-tail" manner. [25] This, in combination with the van der Waals interaction between the porphyrin rings and the chiral discrimination among the optically active binaphthyl units during the self-assembly process, dominates the formation of a helix with a right-and a left-handed helical arrangement of (R)-and (S)-1, respectively, which then further hierarchically self-assembles into highly ordered fibrous nanostructures with a helicity opposite to that of the original porphyrin stack. …”