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

Styrene-Spaced Copolymers Including Anthraquinone and β-O-4 Lignin Model Units: Synthesis, Characterization and Reactivity Under Alkaline Pulping Conditions

A series of random copoly­(styrene)­s has been synthesized via radical polymerization of functionalized anthraquinone (AQ) and β-O-4 lignin model monomers. The copolymers were designed to have a different number of styrene spacer groups between the AQ and β-O-4 lignin side chains aiming at investigating the distance effects on AQ/β-O-4 electron transfer mechanisms. A detailed molecular characterization, including techniques such as size exclusion chromatography, MALDI-TOF mass spectrometry, and ¹H, ¹³C, ³¹P NMR and UV–vis spectroscopies, afforded quantitative information about the composition of the copolymers as well as the average distribution of the AQ and β-O-4 groups in the macromolecular structures. TGA and DSC thermal analysis have indicated that the copolymers were thermally stable under regular pulping conditions, revealing the inertness of the styrene polymer backbone in the investigation of electron transfer mechanisms. Alkaline pulping experiments showed that close contact between the redox active side chains in the copolymers was fundamental for an efficient degradation of the β-O-4 lignin model units, highlighting the importance of electron transfer reactions in the lignin degradation mechanisms catalyzed by AQ. In the absence of glucose, AQ units oxidized phenolic β-O-4 lignin model parts, mainly by electron transfer leading to vanillin as major product. By contrast, in presence of glucose, anthrahydroquinone units (formed by reduction of AQ) reduced the quinone-methide units (issued by dehydration of phenolic β-O-4 lignin model part) mainly by electron transfer leading to guaiacol as major product. Both processes were distance dependent

Spectroscopic Analysis of a Biomimetic Model of Tyr_Z Function in PSII

Using natural photosynthesis as a model, bio-inspired constructs for fuel generation from sunlight are being developed. Here we report the synthesis and time-resolved spectroscopic analysis of a molecular triad in which a porphyrin electron donor is covalently linked to both a cyanoporphyrin electron acceptor and a benzimidazole–phenol model for the Tyr_Z-D₁His190 pair of PSII. A dual-laser setup enabled us to record the ultrafast kinetics and long-living species in a single experiment. From this data, the photophysical relaxation pathways were elucidated for the triad and reference compounds. For the triad, quenching of the cyanoporphyrin singlet excited state lifetime was interpreted as photoinduced electron transfer from the porphyrin to the excited cyanoporphyrin. In contrast to a previous study of a related molecule, we were unable to observe subsequent formation of a long-lived charge separated state involving the benzimidazole–phenol moiety. The lack of detection of a long-lived charge separated state is attributed to a change in energetic landscape for charge separation/recombination due to small differences in structure and solvation of the new triad