Bidirectional Electron Transfer Capability in Phthalocyanine–Sc₃N@<i>I</i>_<i>h</i>–C₈₀ Complexes

To activate oxidative and/or reductive electron transfer reactions, <i>N</i>-pyridyl-substituted Sc₃N@<i>I</i>_<i>h</i>–C₈₀ (<b>4</b>) and C₆₀ (<b>3</b>) fulleropyrrolidines have been prepared and axially coordinated to electron-rich (<b>1</b>) or electron-deficient (<b>2</b>) Zn­(II)­phthalocyanines (Zn­(II)­Pcs) through zinc-pyridyl, metal–ligand coordination affording a full-fledged family of electron donor–acceptor ensembles. An arsenal of photophysical assays as they were carried out with, for example, <b>1</b>/<b>4</b> and <b>2</b>/<b>4</b> show unambiguously that a Zn­(II)­Pc-to-Sc₃N@<i>I</i>_<i>h</i>–C₈₀ photoinduced electron transfer takes place in the former ensemble, whereas a Sc₃N@<i>I</i>_<i>h</i>–C₈₀-to-Zn­(II)­Pc electron transfer occurs in the latter ensemble. To the best of our knowledge, this is the first time that a fullerene-based molecular building block shows an electron transfer dichotomy, namely acting both as electron-acceptor or electron-donor, and its outcome is simply governed by the electronic nature of its counterpart. In light of the latter, the present work, which involves the use of Sc₃N@<i>I</i>_<i>h</i>–C₈₀, one of the most abundant and easy-to-purify endohedral metallofullerenes, is, on one hand, a paradigmatic change and, on the other hand, an important milestone <i>en-route</i> toward the construction of easy-to-prepare molecular materials featuring switchable electron transfer reactivity

Mediating Reductive Charge Shift Reactions in Electron Transport Chains

We report the synthesis of a full-fledged family of covalent electron donor–acceptor₁–acceptor₂ conjugates and their charge-transfer characterization by means of advanced photophysical assays. By virtue of variable excited state energies and electron donor strengths, either Zn­(II)­Porphyrins or Zn­(II)­Phthalocyanines were linked to different electron-transport chains featuring pairs of electron accepting fullerenes, that is, C₆₀ and C₇₀. In this way, a fine-tuned redox gradient is established to power a unidirectional, long-range charge transport from the excited-state electron donor via a transient C₆₀^•– toward C₇₀^•–. This strategy helps minimize energy losses in the reductive, short-range charge shift from C₆₀ to C₇₀. At the forefront of our investigations are excited-state dynamics deduced from femtosecond transient absorption spectroscopic measurements and subsequent computational deconvolution of the transient absorption spectra. These provide evidence for cascades of short-range charge-transfer processes, including reductive charge shift reactions between the two electron-accepting fullerenes, and for kinetics that are influenced by the nature and length of the respective spacer. Of key importance is the postulate of a mediating state in the charge-shift reaction at weak electronic couplings. Our results point to an intimate relationship between triplet–triplet energy transfer and charge transfer

Long-Range Orientational Self-Assembly, Spatially Controlled Deprotonation, and Off-Centered Metalation of an Expanded Porphyrin

Expanded porphyrins are large-cavity macrocycles with enormous potential in coordination chemistry, anion sensing, photodynamic therapy, and optoelectronics. In the last two decades, the surface science community has assessed the physicochemical properties of tetrapyrrolic-like macrocycles. However, to date, the sublimation, self-assembly and atomistic insights of expanded porphyrins on surfaces have remained elusive. Here, we show the self-assembly on Au(111) of an expanded aza-porphyrin, namely, an “expanded hemiporphyrazine”, through a unique growth mechanism based on long-range orientational self-assembly. Furthermore, a spatially controlled “writing” protocol on such self-assembled architecture is presented based on the STM tip-induced deprotonation of the inner protons of individual macrocycles. Finally, the capability of these surface-confined macrocycles to host lanthanide elements is assessed, introducing a novel off-centered coordination motif. The presented findings represent a milestone in the fields of porphyrinoid chemistry and surface science, revealing a great potential for novel surface patterning, opening new avenues for molecular level information storage, and boosting the emerging field of surface-confined coordination chemistry involving f-block elements