Role of the Bridge in Photoinduced Electron Transfer in Porphyrin–Fullerene Dyads

The role of π‐conjugated molecular bridges in through‐space and through‐bond electron transfer is studied by comparing two porphyrin–fullerene donor–acceptor (D–A) dyads. One dyad, ZnP–Ph–C₆₀ (ZnP=zinc porphyrin), incorporates a phenyl bridge between D and A and behaves very similarly to analogous dyads studied previously. The second dyad, ZnP–EDOTV–C₆₀, introduces an additional 3,4‐ethylenedioxythienylvinylene (EDOTV) unit into the conjugated bridge, which increases the distance between D and A, but, at the same time, provides increased electronic communication between them. Two essential outcomes that result from the introduction of the EDOTV unit in the bridge are as follows: 1) faster charge recombination, which indicates enhanced electronic coupling between the charge‐separated and ground electronic states; and 2) the disappearance of the intramolecular exciplex, which mediates photoinduced charge separation in the ZnP–Ph–C₆₀ dyad. The latter can be interpreted as a gradual decrease in electronic coupling between locally excited singlet states of D and A when introducing the EDOTV unit into the D–A bridge.     

Impact of Molecular Size and Shape on the Supramolecular Co-Assembly of Chiral Tricarboxamides: A Comparative Study

A comparative investigation of the chiral amplification features of a series of three families of C₃-symmetric tricarboxamides, 1,3,5-triphenylbenzenetricarboxamides (TPBAs), benzenetricarboxamides (BTAs) and oligo(phenylene ethynylene) tricarboxamides (OPE-TAs), is here reported. As previously observed for BTAs and OPE-TAs, a similar dichroic response is obtained for TPBAs decorated with one, two or three chiral side chains bearing stereogenic centers, thus confirming the efficient transfer of point chirality to the supramolecular helical aggregates. Unlike BTAs and OPE-TAs, the chiral amplification ability of TPBAs in majority rules experiments shows a negligible dependence on the number of chiral centers per monomeric unit, and stands the largest among the series of tricarboxamides. Detailed experimental and theoretical studies demonstrate that the rotation angle between the TPBA units in the helical stack is intermediate to that observed for BTAs and OPE-TAs. This feature strongly conditions the steric interactions between vicinal molecules in the stack and the final chiral amplification outcome. Furthermore, theoretical calculations show that achiral side chains favor the interdigitation of the helical aggregates and thereby the formation of bundle superstructures.     

Dihydrogen Catalysis of the Reversible Formation and Cleavage of CH and NH Bonds of Aminopyridinate Ligands Bound to (η5‐C5Me5)IrIII

This study focuses on a series of cationic complexes of iridium that contain aminopyridinate (Ap) ligands bound to an (η⁵‐C₅Me₅)Ir^III fragment. The new complexes have the chemical composition [Ir(Ap)(η⁵‐C₅Me₅)]⁺, exist in the form of two isomers ( 1⁺ and 2⁺ ) and were isolated as salts of the BAr_F^? anion (BAr_F=B[3,5‐(CF₃)₂C₆H₃]₄). Four Ap ligands that differ in the nature of their bulky aryl substituents at the amido nitrogen atom and pyridinic ring were employed. In the presence of H₂, the electrophilicity of the Ir^III centre of these complexes allows for a reversible prototropic rearrangement that changes the nature and coordination mode of the aminopyridinate ligand between the well‐known κ²‐N,N′‐bidentate binding in 1⁺ and the unprecedented κ‐N,η³‐pseudo‐allyl‐coordination mode in isomers 2⁺ through activation of a benzylic C?H bond and formal proton transfer to the amido nitrogen atom. Experimental and computational studies evidence that the overall rearrangement, which entails reversible formation and cleavage of H?H, C?H and N?H bonds, is catalysed by dihydrogen under homogeneous conditions.     

Shedding Light on the Origin of Solid-State Luminescence Enhancement in Butterfly Molecules

Different molecular strategies have been carefully evaluated to produce solid-state luminescence enhancement (SLE) in compounds that show dark states in solution. A set of α-phenylstyrylarene derivatives with a butterfly shape have been designed and synthesised, for the first time, with the aim of improving the solid-state fluorescence emission of their parent styrylarene compounds. Although these butterfly molecules are not fluorescent in solution, one of them (1,2,4,5-tetra(α-phenylstyryl)benzene) exhibits a fluorescence quantum yield as high as 68 % in a drop-cast sample and 31 % in its crystalline form. In contrast, 1,3,5-tris(α-phenylstyryl)benzene and 4,6-bis(α-phenylstyryl)pyrimidine do not show SLE. A range of fluorescence spectroscopy experiments and DFT calculations were carried out to unravel the origin of different photophysical behaviour of these compounds in the solid state. The results indicate that a rational strategy to control the SLE effect in luminogens depends on a delicate balance between molecular properties and inter-/intramolecular interactions in the solid state.     

Palladium catalysed alkyne hydrogenation and oligomerisation: a parahydrogen based NMR investigation

The role phosphine ligands play in the palladium(ii)-bis-phosphine-hydride cation catalysed hydrogenation of diphenylacetylene is explored through a PHIP (parahydrogen induced polarization) NMR study. The precursors Pd(LL')(OTf)(2) () [LL' = dcpe (PCy(2)CH(2)CH(2)PCy(2)), dppe, dppm, dppp, cppe (PCy(2)CH(2)CH(2)PPh(2))] are used. Alkyl palladium intermediates of the type [Pd(LL')(CHPhCH(2)Ph)](OTf) ( and ) are detected and demonstrated to play an active role in hydrogenation catalysis. Magnetization transfer experiments reveal chemical exchange from the alpha-H of the alkyl ligand of (LL' = dcpe) and linkage isomer ' (LL' = cppe) into trans-stilbene on the NMR timescale. Activation parameters (DeltaH( not equal) and DeltaS( not equal)) for this transformation have been determined. These experiments, coupled with GC/MS data, indicate that the catalytic activity is greater in methanol, where it follows the order: dcpe > cppe > dppp > dppe > dppm, than in CD(2)Cl(2). All five of the phosphine systems described are less active than those based on bcope [where bcope is (C(8)H(14))PCH(2)-CH(2)P(C(8)H(14))] and (t)bucope [where (t)bucope is (C(8)H(14))PC(6)H(4)CH(2)P((t)Bu)(2)]. cis, cis-1,2,3,4-Tetraphenyl-buta-1,3-diene is detected as a minor reaction product with Pd(LL')(PhCH-CHPh-CPh[double bond, length as m-dash]CHPh)(+) () also being shown to play a role in the alkyne dimerisation step.     

Ionic Liquid Crystalline Calixarene with Photo-Switchable Proton Conduction

We have developed a new strategy for the preparation of a light-responsive ionic liquid crystal (LC) that shows photo-switchable proton conduction. The ionic LC consists of a bowl-shaped calix[4]arene core ionically functionalized with azobenzene moieties. The non-covalent architectures were obtained by the formation of ionic salts between the carboxylic acid group of an azo-derivative and the terminal amine groups of a calixarene core. The presence of ionic salts results in a hierarchical self-assembly process that extends to the formation of a nanostructured lamellar LC arrangement (smectic A phase). In this LC phase, the ionic LC calixarene is able to display proton conductive properties, since the ionic nanosegregated areas (formed by the ionic pairs) generate the continuous channels that favor proton transport. The optical and photo-responsive properties were studied by UV-Vis spectroscopy, demonstrating that the azobenzene moieties of the ionic LC undergo reversible (E)-to-(Z) isomerization by irradiation with UV light. Interestingly, this (E)-to-(Z) photoisomerization results in a decrease of the proton conductivity values since the bent-shaped (Z)-isomer disrupts the lamellar LC phase. This isomerization process is totally reversible and leads to an ionic LC material with unique photo-switchable proton conductive properties.     

Raman spectroscopy shows interchain through space charge delocalization in a mixed valence oligothiophene cation and in its pi-dimeric biradicaloid dication

The vibrational Raman spectra of a decathiophene are provided in three relevant oxidations states: for the radical cation, its class III mixed valence system and its "frozen, -170 degrees C" class II MV analogue; for the dication, its singlet biradical pi-dimer and its "hot, +70 degrees C" magnetically active triplet excited state. Everything is compatible with interpentathiophene charge delocalization occurring at distances of 4-5 A similar to those found in the crystals of pi-stacked oligothiophenes. This stresses the interest of this spectroscopic tool for the analyses of electronic processes in crystals or in thin films of conjugated organic molecules.     

Relationship between Electron Affinity and Half‐Wave Reduction Potential: A Theoretical Study on Cyclic Electron‐Acceptor Compounds

A high‐level ab initio protocol to compute accurate electron affinities and half‐wave reduction potentials is presented and applied for a series of electron‐acceptor compounds with potential interest in organic electronics and redox flow batteries. The comprehensive comparison between the theoretical and experimental electron affinities not only proves the reliability of the theoretical G3(MP2) approach employed but also calls into question certain experimental measurements, which need to be revised. By using the thermodynamic cycle for the one‐electron attachment reaction A+e^?→A^?, theoretical estimates for the first half‐wave reduction potential have been computed along the series of electron‐acceptor systems investigated, with maximum deviations from experiment of only 0.2 V. The precise inspection of the terms contributing to the half‐wave reduction potential shows that the difference in the free energy of solvation between the neutral and the anionic species (ΔΔG_solv) plays a crucial role in accurately estimating the electron‐acceptor properties in solution, and thus it cannot be considered constant even in a family of related compounds. This term, which can be used to explain the occasional lack of correlation between electron affinities and reduction potentials, is rationalized by the (de)localization of the additional electron involved in the reduction process along the π‐conjugated chemical structure.