Through-bond Interactions Research Articles

Electron Transfer in Oligothiophene-Bridged Bisporphyrins

Oligothiophene-bridged zinc-tin bisporphyrinates were synthesized. Their absorption spectra have been analyzed in terms of exciton interaction and porphyrin-bridge coupling by through-bond interaction and the steady-state fluorescence spectra in terms of differential Stokes shifts for the electron-donating zinc and the electron-accepting tin porphyrinates. Strong quenching of the fluorescence intensity and acceleration of the fluorescence decay as compared to porphyrinate monomers (ZnTPP, SnTPP) were observed. Both phenomena were traced back to light-induced electron transfer by the occurrence of ion pair absorption bands in picosecond transient absorption spectra. Similar absorption spectra of both chromophores caused always simultaneous excitation and, consequently, two concurrent photoreactions. Combined evaluation of the time-dependent absorption and fluorescence data allowed the estimation of rates for the electron transfer reactions. The found dependence on the separation distance was much smaller than for donor-acceptor systems with saturated spacers. A damping factor of 0.05 was calculated for the charge separation proceeding from the excited state of the zinc porphyrin. The polarity of the solvent had a profound influence on the transfer rates. The charge recombination was 300 times faster in polar tetrahydrofuran than in nonpolar toluene.

Fluorescence Dynamics of Directly Meso−Meso Linked Porphyrin Rings Probed by Single Molecule Spectroscopy

Porphyrin rings CZ4, CZ6, and CZ8 that respectively comprise four, six, and eight porphyrins, immobilized in a thin PMMA film, have been investigated using single molecule fluorescence spectroscopy with a focus on the influences of the overall structural rigidity as the ring size of porphyrin array increases. Neighboring porphyrin moieties were linked directly to enhance through-bond electronic interactions and, as a consequence, efficient excitation energy migration processes like the natural LH2 complex. Unlike the ensemble study, the single molecule study using confocal microscopy could eliminate the averaging effect, and consequently provide detailed information on individual molecular behaviors. Indeed, in solution, as a dihedral angle between neighboring porphyrins decreases in the order of CZ6 > CZ8 > CZ4, red-shifted Q-absorption bands and faster excitation energy hopping rates were observed. However, at the single molecule level, we found that they show longer survival times, less frequent on-off behaviors, narrower fluorescence lifetime distributions, and high relative single molecular brightness in the order of CZ8 > CZ6 > CZ4, by recording fluorescence intensity trajectories. Especially, CZ4 reveals high photostability with its rigid structure, and about 3 porphyrin units among the 4 chromophores-constituted molecule behave as a collective coherent domain. Thus, our results single out CZ4 as a potential and promising candidate for application in artificial light harvesting solid-state devices.

Strong through-space two-halide magnetic exchange of −234 K in (2,5-dimethylpyrazine)copper(ii) bromide

An unprecedented antiferromagnetic exchange of -234 K in Cu(2,5-dimethylpyrazine)Br(2) is shown experimentally and theoretically to propagate via through-space interactions between Br ions rather than through-bond interactions.

Development of a versatile synthesis method for trinuclear Co(iii), Rh(iii), and Ir(iii) dithiolene complexes, and their crystal structures and multi-step redox properties

Triangular trinuclear Co(iii), Rh(iii), and Ir(iii) dithiolene complexes, [[M(3)(eta(5)-C(5)Me(5))(3)(S(6)C(6))] (M = Co, Rh, Ir), which have not been obtained by the trimerization of mononuclear complexes, were synthesized via a new synthetic route, that is, the reaction of benzenehexathiol and [(eta(5)-C(5)Me(5))Co(CO)I(2)], [Rh(eta(5)-C(5)Me(5))Cl(2)](2) and [Ir(eta(5)-C(5)Me(5))Cl(2)](2), respectively, under basic conditions. Single crystal X-ray diffraction measurements were carried out and their crystal structures were determined. Electrochemical measurements revealed that the trinuclear complexes exhibit three-step one-electron redox reactions and form mixed valence complexes. Studies on the stabilities of their mixed valence states and the X-ray crystal structure analyses revealed that the through-bond internuclear electronic interaction and the aromaticity of central benzene ring decrease in the order of Co > Rh > Ir.

3,3′- and 4,4′-Biphenylene-Bridged Subporphyrin Dimers

3,3'- and 4,4'-biphenylene-bridged subporphyrin dimers were synthesized by means of a Ni-catalyzed homocoupling reaction. Distinctly red-shifted absorption bands and enhanced fluorescence were observed only for the latter, indicating effective through-bond interactions through a 4,4'-biphenylene bridge, which is not possible for porphyrins.

Nanoparticle-Mediated Intervalence Transfer

Nanoparticle-mediated intervalence transfer was reported with ferrocene moieties that were attached onto the ruthenium nanoparticle surface by ruthenium-carbene pi bonds. The resulting particles exhibited two pairs of voltammetric waves with a potential spacing of about 200 mV and a rather intense absorption peak in the near-infrared range (approximately 1930 nm) at mixed valence. Both features suggested Class II characteristics of the intraparticle intervalence transfer that mainly arose from through-bond interactions between the metal centers. Quantum calculations based on density functional theory showed that the nanoparticle core electrons served as conducting band states for the effective charge delocalization between particle-bound ferrocene moieties.

Synthesis and Excited‐state Photodynamics of a Chlorin–Bacteriochlorin Dyad—Through‐space Versus Through‐bond Energy Transfer in Tetrapyrrole Arrays

Understanding energy transfer among hydroporphyrins is of fundamental interest and essential for a wide variety of photochemical applications. Toward this goal, a synthetic free base ethynylphenylchlorin has been coupled with a synthetic free base bromobacteriochlorin to give a phenylethyne-linked chlorin-bacteriochlorin dyad (FbC-pe-FbB). The chlorin and bacteriochlorin are each stable toward adventitious oxidation because of the presence of a geminal dimethyl group in each reduced pyrrole ring. A combination of static and transient optical spectroscopic studies indicate that excitation into the Qy band of the chlorin constituent (675 nm) of FbC-pe-FbB in toluene results in rapid energy transfer to the bacteriochlorin constituent with a rate of approximately (5 ps)(-1) and efficiency of >99%. The excited bacteriochlorin resulting from the energy-transfer process in FbC-pe-FbB has essentially the same fluorescence characteristics as an isolated monomeric reference compound, namely a narrow (12 nm fwhm) fluorescence emission band at 760 nm and a long-lived (5.4 ns) Qy excited state that exhibits a significant fluorescence quantum yield (Phif=0.19). Förster calculations are consistent with energy transfer in FbC-pe-FbB occurring predominantly by a through-space mechanism. The energy-transfer characteristics of FbC-pe-FbB are compared with those previously obtained for analogous phenylethyne-linked dyads consisting of two porphyrins or two oxochlorins. The comparisons among the sets of dyads are facilitated by density functional theory calculations that elucidate the molecular-orbital characteristics of the energy donor and acceptor constituents. The electron-density distributions in the frontier molecular orbitals provide insights into the through-bond electronic interactions that can also contribute to the energy-transfer process in the different types of dyads.

Synthesis and through-bond spin interaction of stable 1,3-phenylene linked poly(phenothiazine cation radical)

A 1,3-phenylene linked poly(phenothiazine cation radical) was synthesized and its through-bond spin interaction was investigated. Monomer and dimer models were also synthesized to investigate the behaviour of spins in poly(phenothiazine cation radical). X-Ray crystallographic analysis of the monomer model showed that it has a near-planar structure, which is favourable for spin interaction through π-conjugation. MO calculation of the dimer model showed its exchange integral J (+31.4 K) is larger by about two times than that of diaminoxyl (+14.5 K) which is a dimer model of a polyradical studied previously. The spin state of poly(phenothiazine cation radical) was S = 2/2–3/2 in the ground state, indicating that, taking into account the average degree of polymerization (5–6) and the spin concentration (0.77 spins per repeating unit), 2–3 spins out of 4–5 spins per molecule were ferromagnetically aligned.

Links between through-bond interactions and assembly structure in simple piperidones

3,5-Disubstituted piperidones provide an opportunity to explore donor–acceptor through-bond interactions in the context of molecular and supramolecular structure. The crystal structure of cis-3,5-dibenzyl-1-phenylpiperidin-4-one 3 is disordered and the lattice accommodates a ∼3 : 1 ratio of the N-Ph equatorial (3-eq) and N-Ph axial (3-ax) epimers, based on refined values of occupancy factors. The fortuitous result allows a side-by-side comparison of the two configurations with respect to their donor–acceptor through-bond interactions. The energy difference between 3-ax and 3-eq (ΔEax–eq) has been evaluated in the gas phase using extensive first principles calculations, and for many levels of theory this difference parallels the experimentally-observed configurational ratio in the solid state (where the epimers share nearly identical packing environments). The calculations further show a difference in the through-bond stabilization for 3-ax and 3-eq, with larger contributions for 3-ax. Natural bond order (NBO) analysis quantifies the delocalization of the donor nitrogen lone pair into the adjacent carbon–carbon bonds and carbonyl acceptor for the 3-axepimer. The work concludes that molecular-level structural perturbations that arise from or otherwise influence through-bond donor–acceptor interactions have consequences on solid-state and supramolecular assembly structure.

Boron atom as an electron density relay

The electronic structure of tetrapyrrolidinodiborane [(C 4H 8N) 2B] 2 has been investigated by HeI and HeII UV photoelectron spectroscopy (UPS) and DFT/OVGF calculations. Our results indicate that the title molecule has very low first ionization energy and can be readily oxidized in solution. We estimated the magnitudes of alkyl substituent effects on nitrogen and boron atoms and the efficiency of boron atom to act as an electron density relay facilitating through-bond interactions between the nitrogen lone pairs.

Impact of Bridging Units on the Dynamics of Photoinduced Charge Generation and Charge Recombination in Donor–Acceptor Dyads

We estimate, at a full quantum-chemical level, the various molecular parameters governing the rate of photoinduced charge generation and charge recombination in model organic structures containing a donor and an acceptor unit in view of the possible use of such systems in organic solar cells. The rate of through-space excitation dissociation, as predicted in the framework of the Marcus-Levich-Jortner theory, is found to be low in comparison to intramolecular decay processes when the donor and acceptor molecules are lying in a head-to-tail arrangement and high when the donor and acceptor molecules are superimposed in a cofacial arrangement. The charge separation rates for side-by-side donor-acceptor dyads are significantly increased by promoting through-bond interactions in covalently linked donor and acceptor units. This has motivated a detailed quantitative analysis of the influence of the nature, size, and conformation of the bridging moiety on the calculated transfer rates.

Photoelectron spectroscopy and substituent effects in halonaphthalenes

The electronic structure of two isomeric dibromonaphthalenes (C 10H 6Br 2) has been investigated by HeI/HeII photoelectron spectroscopy. The spectra were assigned by Green's functions calculations and comparison with the spectra of related dibromobenzenes (C 6H 4Br 2). The analysis of π-orbital and halogen lone pair ionization energies, enabled us to determine the magnitude of bromine-bromine intramolecular interactions and distinguish between through-bond and through-space type interactions. We also discuss the halogen–halogen interactions in other polynuclear aromatics.

Computational Study about Through-Bond and Through-Space Interactions in [2.2]Cyclophanes

An analysis of the electron density, obtained by B3PW91/6-31+G(d,p), B3LYP/6-31+G(d,p), and MP2/6-31+G(d,p) for [2,2]cyclophanes isomers, [2.2]paracyclophane, anti-[2.2]metacyclophane, syn-[2.2]metacyclophane, and [2.2]metaparacyclophane, was made through natural bond orbitals (NBO), natural steric analysis (NSA), and atoms in molecules (AIM) methods and through analysis of frontier molecular orbitals (MOs). NBO indicates that all compounds present through-bond interactions, but only the conformers of [2.2]metacyclophane present significant through-space interactions. The last interactions are observed in AIM analysis and by the plots of MOs. AIM indicates that these through-space interactions are closed-shell ones, and they stabilize the conformers. In contrast, all isomers present through-bond and through-space repulsive interactions. In addition, the atomic properties, computed over the atomic basins, showed that the position of the bridges and the relative displacement of the rings can affect the atomic charges, the first atomic moments, and the atomic volumes.

Synthesis and through-bond spin interaction of stable 1,3-phenylene-linked polyradical carrying aminoxyls in the π-conjugated main chain

A stable polyradical carrying aminoxyls in the π-conjugated main chain was synthesized and its through-bond spin interaction was investigated. The polyradical is constructed with 1,3-phenylene as a spin coupler and 9,9-di-n-propyl-9,10-dihydroacridin-10-yloxy as a stable spin source. The synthetic method of the spin source moiety was modified by using conventional organic reactions from a previously reported method. The polymeric structure was synthesized by Suzuki–Miyaura coupling. The degree of polymerization and the spin concentration of the polyradical were ca. 11 and 0.71 spins per repeating unit, respectively, indicating that there were ca. 8 spins per molecule. The average S value of the polyradical was 3/2, therefore, 3 spins out of 8 spins were aligned in the polyradical. This showed that the through-bond spin interaction occurs more effectively than in our previous polyradical carrying aminoxyls in the side chain.

Intramolecular energy transfer in a tetra-coumarin perylene system: influence of solvent and bridging unit on electronic properties

The synthesis and characterisation of a novel coumarin donor-perylene bisimide acceptor light-harvesting system is reported, in which an energy-transfer efficiency of >99% is achieved. Comparison of the excited-state properties of the donor-acceptor system with model compounds revealed that although the photophysical properties of the perylene bisimide acceptor unit are affected considerably by the nature of the substituent at the imide positions and the solvent employed, through-bond interaction between the donor and acceptor units is negligible. Energy transfer in the present system can be described as occurring via a through-space energy-transfer mechanism. Careful consideration of the redox properties of the donor relative to the acceptor units allows for avoidance of potentially deleterious excited-state electron-transfer processes.

Weak distance dependence of through-bond interactions in tetrahydro-4H-thiopyran-4-ylidene end-capped oligo(cyclohexylidenes); a computational survey

Calculations on members of the oligo(cyclohexylidene) series [(n), n = 1-5)] and related tetrahydro-4H-thiopyran end-capped analogues [(n), n = 1-4)] show a strong through-bond coupling between their pi bonds and sulfur lone pairs (Lp(pi)S). This coupling is mediated by an interaction between the H(ax)-C-C-H(ax) structural sub-units and the pi bonds connecting the cyclohexyl moieties. A comparison of the length dependency of the through-bond coupling via an oligo(cyclohexylidene) and an alkane bridge [divinyl alkanes (n)] shows that oligo(cyclohexylidenes) are more efficient in mediating through-bond couplings over large distances. Oligo(cyclohexylidene) bridges exhibit molecular wire characteristics.

Chromophores in phenylenevinylene-based conjugated polymers: Role of conformational kinks and chemical defects

The influence of chemical defects and conformational kinks on the nature of the lowest electronic excitations in phenylenevinylene-based polymers is assessed at the semiempirical quantum-chemical level. The amount of excited-state localization and the amplitude of through-space (Coulomb-like) versus through-bond (charge-transfer-like) interactions have been quantified by comparing the results provided by excitonic and supermolecular models. While excitation delocalization among conjugated segments delineated by the defects occurs in the acceptor configuration, self-confinement on individual chromophores follows from geometric relaxation in the excited-state donor configuration. The extent of excited-state localization is found to be sensitive to both the nature of the defect and the length of the conjugated chains. Implications for resonant energy transfer along conjugated polymer chains are discussed.

Selective NMR Measurements of Homonuclear Scalar Couplings in Isotopically Enriched Solids

Scalar (J) couplings in solid-state NMR spectroscopy are sensitive to covalent through-bond interactions that make them informative structural probes for a wide range of complex materials. Until now, however, they have been generally unsuitable for use in isotopically enriched solids, such as proteins or many inorganic solids, because of the complications presented by multiple coupled but nonisolated spins. Such difficulties are overcome by incorporating a z-filter that results in a robust method for measuring pure J-coupling modulations between selected pairs of nuclei in an isotopically enriched spin system. The reliability of the new experimental approach is established by using numerical simulations and tested on fully (13)C-labeled polycrystalline L-alanine. It is furthermore shown to be applicable to partially enriched systems, when used in combination with a selective double-quantum (DQ) filter, as demonstrated for the measurement of (2)J((29)Si-O-(29)Si) couplings in a 50% (29)Si-enriched surfactant-templated layered silicate lacking long-range 3D crystallinity. J-coupling constants are obtained with sufficient accuracy to distinguish between different (29)Si-O-(29)Si pairs, shedding insight on the local structure of the silicate framework. The new experiment is appropriate for fully or partially enriched liquid or solid samples.

Lone-Pair Orbital Interactions in Polythiaadamantanes

The electronic structures of a series of polythiaadamantanes from thiaadamantane through 2,4,6,8,9,10-hexathiaadamantane (HTA) have been analyzed using density functional theory calculations in conjunction with Hückel and natural bond orbital analysis. The effects of multiple sulfur p-type lone-pair orbital interactions on ionization potentials, hole mobilities, and electronic coupling have been determined. An overall increase in the average energy of the lone-pair orbitals as the number of sulfur atoms increases is predicted, with the exact positioning of the HOMO depending on specific lone-pair interactions. Separation of through-bond (TB) and through-space (TS) interactions between intramolecular sulfur atoms has been performed using localized molecular orbitals and model systems based on interacting hydrogen sulfide molecules. TB interations were found to reduce orbital splitting, while TS interactions were found to increase orbital splitting. TS interactions were more or less constant from one polythiaadamantane to the next, and the contributions of TB effects to individual orbital energies vary depending on the relative orientation of sulfur atoms as determined by the sigma molecular framework. Electronic coupling between intermolecular sulfur lone-pair orbitals was determined by investigating unique dimer pairs observed in the crystal structure of HTA. Electronic coupling is not as strong as expected given the short intermolecular S-S distances observed in the crystal structure. In general, B3LYP/6-31G(d) and B3LYP/6-311+G(d,p) give very similar orbital energies and splittings.

Impact of Transition Metal Substituents on Polysilane Properties: Iron versus Ruthenium

The previously unknown ruthenio disilanes Rp–Si2Me4–C6H4X (Rp = η5-C5H5Ru(CO)2; X = H, Br, –CHO, CH=C(CN)2) were synthesized from ClSi2Me4C6H4X (X = H, Br) and Rp− using conventional chemical methods. Trends in the UV/Vis absorption spectra indicate strong electronic coupling within the Rp–Si–Si–Caryl fragment and, therefore, closely resemble the ones observed for the corresponding iron complexes. The four compounds however, were shown to be less sensitive towards UV irradiation. The crystal structure of Rp–Si2Me4–C6H4CH=C(CN)2 was determined by X-ray diffraction and exhibits an all-trans-array of the Ru–Si–Si–Caryl moiety, what is a basic requirement for optimal through-bond interaction.