Topological and Conformational Effects on Electron Transfer Dynamics in Porphyrin-[60]Fullerene Interlocked Systems

The effect of molecular topology and conformation on the dynamics of photoinduced electron transfer (ET) processes has been studied in interlocked electron donor–acceptor systems, specifically rotaxanes with zinc­(II)-tetraphenylporphyrin (ZnP) as the electron donor and [60]­fullerene (C₆₀) as the electron acceptor. Formation or cleavage of coordinative bonds was used to induce major topological and conformational changes in the interlocked architecture. In the first approach, the tweezer-like structure created by the two ZnP stopper groups on the thread was used as a recognition site for complexation of 1,4-diazabicyclo[2.2.2]­octane (DABCO), which creates a bridge between the two ZnP moieties of the rotaxane, generating a catenane structure. The photoinduced processes in the DABCO-complexed (ZnP)₂-[2]­catenate-C₆₀ system were compared with those of the (ZnP)₂-rotaxane-C₆₀ precursor and the previously reported ZnP-[2]­catenate-C₆₀. Steady-state emission and transient absorption studies showed that a similar multistep ET pathway emerged for rotaxanes and catenanes upon photoexcitation at various wavelengths, ultimately resulting in a long-lived ZnP^•+/C₆₀^•– charge-separated radical pair (CSRP) state. However, the decay kinetics of the CSRP states clearly reflect the topological differences between the rotaxane, the catenate, and DABCO-complexed-catenate architectures. The lifetime of the long-distance ZnP^•+–[Cu­(I)­phen₂]⁺–C₆₀^•– CSRP state is more than four times longer in <b>3</b> (1.03 μs) than in <b>1</b> (0.24 μs) and approaches that in catenate <b>2</b> (1.1 μs). The results clearly showed that creation of a catenane from a rotaxane topology inhibits the charge recombination process. In a second approach, when the Cu­(I) ion used as the template to assemble the (ZnP)₂–[Cu­(I)­phen₂]⁺–C₆₀ rotaxane was removed, it was evident that a major structural change had occurred. since charge separation between the chromophores was no longer observed upon photoexcitation in nonpolar as well as in polar solvents. Only ZnP and C₆₀ triplet excited states were observed upon laser excitation of the Cu-free rotaxane. These results highlight the critical importance of the central Cu­(I) ion for long-range ET processes in these nanoscale interlocked electron donor–acceptor systems

Stabilizing Ion and Radical Ion Pair States in a Paramagnetic Endohedral Metallofullerene/π‑Extended Tetrathiafulvalene Conjugate

Electron donor–acceptor conjugates of paramagnetic endohedral metallofullerenes and π-extended tetrathiafulvalene (exTTF) were synthesized, characterized, and probed with respect to intramolecular electron transfer involving paramagnetic fullerenes. UV–vis–NIR absorption spectroscopy complemented by electrochemical measurements attested to weak electronic interactions between the electron donor, exTTF, and the electron acceptor, La@C₈₂, in the ground state. In the excited state, photoexcitation powers a fast intramolecular electron transfer to yield an ion and radical ion pair state consisting of one-electron-reduced La@C₈₂ and of one-electron-oxidized exTTF

A Paradigmatic Change: Linking Fullerenes to Electron Acceptors

The potential of Lu₃N@C₈₀ and its analogues as electron acceptors in the areas of photovoltaics and artificial photosynthesis is tremendous. To this date, their electron-donating properties have never been explored, despite the facile oxidations that they reveal when compared to those of C₆₀. Herein, we report on the synthesis and physicochemical studies of a covalently linked Lu₃N@C₈₀–perylenebisimide (PDI) conjugate, in which PDI acts as the light harvester and the electron acceptor. Most important is the unambiguous evidencein terms of spectroscopy and kineticsthat corroborates a photoinduced electron transfer evolving from the ground state of Lu₃N@C₈₀ to the singlet excited state of PDI. In stark contrast, the photoreactivity of a C₆₀–PDI conjugate is exclusively governed by a cascade of energy-transfer processes. Also, the electron-donating property of the Lu₃N@C₈₀ moiety was confirmed through constructing and testing a bilayer heterojunction solar cell device with a PDI and Lu₃N@C₈₀ derivative as electron acceptor and electron donor, respectively. In particular, a positive photovoltage of 0.46 V and a negative short circuit current density of 0.38 mA are observed with PDI/Ca as anode and ITO/Lu₃N@C₈₀ as cathode. Although the devices were not optimized, the sign of the <i>V</i>_OC and the flow direction of <i>J</i>_SC clearly underline the unique oxidative role of Lu₃N@C₈₀ within electron donor–acceptor conjugates toward the construction of novel optoelectronic devices

A Paradigmatic Change: Linking Fullerenes to Electron Acceptors

The potential of Lu₃N@C₈₀ and its analogues as electron acceptors in the areas of photovoltaics and artificial photosynthesis is tremendous. To this date, their electron-donating properties have never been explored, despite the facile oxidations that they reveal when compared to those of C₆₀. Herein, we report on the synthesis and physicochemical studies of a covalently linked Lu₃N@C₈₀–perylenebisimide (PDI) conjugate, in which PDI acts as the light harvester and the electron acceptor. Most important is the unambiguous evidencein terms of spectroscopy and kineticsthat corroborates a photoinduced electron transfer evolving from the ground state of Lu₃N@C₈₀ to the singlet excited state of PDI. In stark contrast, the photoreactivity of a C₆₀–PDI conjugate is exclusively governed by a cascade of energy-transfer processes. Also, the electron-donating property of the Lu₃N@C₈₀ moiety was confirmed through constructing and testing a bilayer heterojunction solar cell device with a PDI and Lu₃N@C₈₀ derivative as electron acceptor and electron donor, respectively. In particular, a positive photovoltage of 0.46 V and a negative short circuit current density of 0.38 mA are observed with PDI/Ca as anode and ITO/Lu₃N@C₈₀ as cathode. Although the devices were not optimized, the sign of the <i>V</i>_OC and the flow direction of <i>J</i>_SC clearly underline the unique oxidative role of Lu₃N@C₈₀ within electron donor–acceptor conjugates toward the construction of novel optoelectronic devices