Mono‐ and Bis(tetrathiafulvalene)‐1,3,5‐Triazines as Covalently Linked Donor–Acceptor Systems: Structural,Spectroscopic, and Theoretical Investigations

Reaction of 2,4,6‐trichloro‐1,3,5‐triazine with lithiated tetrathiafulvalene (TTF) in stoichiometric conditions, followed by treatment with sodium methanolate, provides mono‐ and bis(TTF)–triazines as new covalently linked (multi)donor–acceptor systems. Single‐crystal X‐ray analyses reveal planar structures for both compounds, with formation of peculiar segregated donor and acceptor stacks for the mono(TTF)–triazine compound, while mixed TTF–triazine stacks establish in the case of the bis(TTF) derivative. Cyclic voltammetry measurements show reversible oxidation of the TTF units, at rather low potential, with no splitting of the oxidation waves in the case of the dimeric TTF, whereas irreversible reduction of the triazine core is observed. Intramolecular charge transfer is experimentally evidenced through solution electronic absorption spectroscopy. Time‐dependent DFT calculations allow the assignment of the charge transfer band to singlet transitions from the HOMO of the donor(s) to the LUMO of the acceptor. Solution EPR measurements correlated with theoretical calculations were performed in order to characterize the oxidized species. In both cases the spectra show very stable radical species and contain a triplet of doublet pattern, in agreement with the coupling of the unpaired electron with the three TTF protons. The dication of the bis(TTF)–triazine is paramagnetic, but no spin–spin exchange interaction could be detected.     

A multistate switchable [3]rotacatenane

Rotacatenanes are exotic molecular compounds that can be visualized as a unique combination of a [2]catenane and a [2]rotaxane, thereby combining both the circumrotation of the ring component (rotary motion) and the shuttling of the dumbbell component (translational motion) in one structure. Herein, we describe a strategy for the synthesis of a new switchable [3]rotacatenane and the investigation of its switching properties, which rely on the formation of tetrathiafulvalene (TTF) radical π-dimer interactions-namely, the mixed-valence state (TTF(2) )(+.) and the radical-cation dimer state (TTF(+.) )(2) -under ambient conditions. A template-directed approach, based on donor-acceptor interactions, has been developed, resulting in an improved yield of the key precursor [2]catenane, prior to rotacatenation. The nature of the binding between the [2]catenane and selected π-electron-rich templates has been elucidated by using X-ray crystallography and UV/Vis spectroscopy as well as isothermal titration microcalorimetry. The multistate switching mechanism of the [3]rotacatenane has been demonstrated by cyclic voltammetry and EPR spectroscopy. Most notably, the radical-cation dimer state (TTF(+.) )(2) has been shown to enter into an equilibrium by forming the co-conformation in which the two 1,5-dioxynaphthalene (DNP) units co-occupy the cavity of tetracationic cyclophane, thus enforcing the separation of TTF radical-cation dimer (TTF(+.) )(2) . The population ratio of this equilibrium state was found to be 1:1. We believe that this research demonstrates the power of constructing complex molecular machines using template-directed protocols, enabling us to make the transition from simple molecular switches to their multistate variants for enhancing information storage in molecular electronic devices.     

Comparative Electrochemical and Photophysical Studies of Tetrathiafulvalene‐Annulated Porphyrins and Their ZnII Complexes: The Effect of Metalation and Structural Variation

A series of tetrathiafulvalene (TTF)‐annulated porphyrins, and their corresponding Zn^II complexes, have been synthesized. Detailed electrochemical, photophysical, and theoretical studies reveal the effects of intramolecular charge‐transfer transitions that originate from the TTF fragments to the macrocyclic core. The incremental synthetic addition of TTF moieties to the porphyrin core makes the species more susceptible to these charge‐transfer (CT) effects as evidenced by spectroscopic studies. On the other hand, regular positive shifts in the reduction signals are seen in the square‐wave voltammograms as the number of TTF subunits increases. Structural studies that involve the tetrakis‐substituted TTF–porphyrin (both free‐base and Zn^II complex) reveal only modest deviations from planarity. The effect of TTF substitution is thus ascribed to electronic overlap between annulated TTF subunits rather than steric effects. The directly linked thiafulvalene subunits function as both π acceptors as well as σ donors. Whereas σ donation accounts for the substituent‐dependent charge‐transfer transitions, it is the π‐acceptor nature of the appended tetrathiafulvalene groups that dominates the redox chemistry. Interactions between the subunits are also reflected in the square‐wave voltammograms. In the case of the free‐base derivatives that bear multiple TTF subunits, the neighboring TTF units, as well as the TTF ^{? +} generated through one‐electron oxidation, can interact with each other; this gives rise to multiple signals in the square‐wave voltammograms. On the other hand, after metalation, the electronic communication between the separate TTF moieties becomes restricted and they act as separate redox centers under conditions of oxidation. Thus only two signals, which correspond to TTF ^{. +} and TTF²⁺, are observed. The reduction potentials are also seen to shift towards more negative values after metalation, a finding that is considered to reflect an increased HOMO–LUMO gap. To probe the excited‐state dynamics and internal CT character, transient absorption spectral studies were performed. These analyses revealed that all the TTF–porphyrins of this study display relatively short excited‐state lifetimes, which range from 1 to 20 ps. This reflects a very fast decay to the ground state and is consistent with the proposed intramolecular charge‐transfer effects inferred from the ground‐state studies. Complementary DFT calculations provide a mechanistic rationale for the electron flow within the TTF–porphyrins and support the proposed intramolecular charge‐transfer interactions and π‐acceptor effects.     

Calix[4]arenes with 1,2- and 1,3-upper rim tetrathiafulvalene bridges

Herein we report the synthesis of several calix[4]arene derivatives with tetrathiafulvalene bridges at the upper rim. Calix[4]arene-tetrathiafulvalene (TTF) conjugates 4a–d, fixed in cone conformation and comprising two smaller 1,2-bridges, were prepared by cyclisation of tetrakis-chloromethylated calix[4]arene 1 with 2,3-dithiolates of TTFs. Larger calix[4]arene-TTF macrocycles 14 and 15, also in cone conformation, contain 1,3-bridges and were synthesised by cyclisation of 2,6- and 2,7-dithiolates of TTFs with bis-bromomethylated calix[4]arene 7. Redox properties of new calix[4]arene-TTF conjugates were characterised using cyclic voltammetry.