While the concept of storing electrons in chemical bonds is not new, innovative approaches to achieve it continue to emerge. This applies to the structronic (STR) electrochemical paradigm. Typically, when strategically positioned in space, electrophoric components within STR functional assemblies—featuring potent electrophilic centers (i.e., atomic orbitals with a significant contribution to the lowest unoccupied molecular orbital (LUMO) of their redox‐active components)—can engage in direct electronic interactions through σ‐type orbital overlap, creating a set of so‐called supra‐molecular orbitals (supra‐MOs). The core idea is to fill the potentially bonding supra‐LUMO with electrons, thereby storing them in the form of a weakened (elongated) reservoir bond. This approach bypasses electrostatic limitations by leveraging the quantum behavior of electrons within covalent bonds. STR assemblies are bistable systems that can be electrochemically cycled, allowing for the charging and discharging of stored electrons. This concept article aims to define the conditions necessary for generating these supra‐MOs, explore their electrochemical characteristics (such as hysteretic electrochemical behaviors and inversion of redox potentials), and highlight potential applications in advanced energy storage and other fields within the dynamic scope of electrochemistry.

The extent to which electrophores covalently bridged by a saturated linker are electrochemically independent was investigated considering the charge/spin duality of the electron and functionality of the electrophore as a spin carrier upon reduction. By combining computational modeling with electrochemical experiments, we investigated the mechanism by which tethered electrophores react together within 4,4'-oligo[n]methylene-bipyridinium assemblies (with n=2 to 5). We show that native dicationic electrophores (redox state Z=+2) are folded prior to electron injection into the system, allowing the emergence of supra-molecular orbitals (supra-MOs) likely to support the process of the reductive σ bond formation giving cyclomers. Indeed, for Z=+2, London Dispersion (LD) forces contribute to flatten the potential energy surface such that all-trans and folded conformers are approximately isoenergetic. Then, upon one-electron injection, for radical cations (Z=+1), LD forces significantly stabilize the folded conformers, except for the ethylene derivative deprived of supra-MOs. For radical cations equipped with supra-MOs, the unpaired electron is delocalized over both heterocycles through space. Cyclomer completion (Z=0) upon the second electron transfer occurs according to the inversion of redox potentials. This mechanism explains why intramolecular reactivity is favored and why pyridinium electrophores are not independent.

The context of molecular structronics (from "molecular structure" and "electronics") is that of molecular-level electrochemical storage of energy of sustainable origin (wind, solar). Due to its discontinuous availability, storage of this energy is a key issue. The targeted type of storage relies on implementing "electron reservoirs" within the structronic molecules by electrochemically forming dedicated chemical bonds according to non-catalytic processes. Reservoir bonds are therefore integral parts of the molecular backbone of structronic assemblies. When filled, electron reservoirs manifest themselves in the form of elongated covalent bonds that are to be cleaved for electron releasing (discharging) on demand. The scope of this short review is limited to pyridinium electrophores as particularly suited building blocks for the development of structronics.

The synergistic functioning of redox-active components that emerges from prototypical 2,2 '-di(N-methylpyrid-4-ylium)-1,1 '-biphenyl is described. Interestingly, even if a trans conformation of the native assembly is expected, due to electrostatic repulsion between cationic pyridinium units, we demonstrate that cis conformation is equally energy-stabilized on account of a peculiar LUMO (SupLUMO) that develops through space, encompassing the two pyridiniums in a single, made-in-one-piece, electronic entity (superelectrophoric behavior). This SupLUMO emergence, with the cis species as superelectrophore embodiment, originates in a sudden change of electronic structure. This finding is substantiated by insights from solid state (single-crystal X-ray diffraction) and solution (NOE NMR and UV-vis-NIR spectroelectrochemistry) studies, combined with electronic structure computations. Electrochemistry shows that electron transfers are so strongly correlated that two-electron reduction manifests itself as a single-step process with a large potential inversion consistent with inner creation of a carbon-carbon bond (digital simulation). Besides, absence of reductive formation of dimers is a further indication of a preferential intramolecular reactivity determined by the SupLUMO interaction (cis isomer pre-organization). The redox-gated covalent bond, serving as electron reservoir, was studied via atropisomerism of the reduction product (VT NMR study). The overall picture derived from this in-depth study of 2,2 '-di(N-methylpyrid-4-ylium)-1,1 '-biphenyl proves that trans and cis species are worth considered as intrinsically sharply different, that is, as doubly-electrophoric and singly-superelectrophoric switchable assemblies, beyond conformational isomerism. Most importantly, the through-space-mediated SupLUMO may come in complement of other weak interactions encountered in Supramolecular Chemistry as a tool for the design of electroactive architectures.

Abstract Environmental control of single‐molecule junction evolution and conductance was demonstrated for expanded pyridinium molecules by scanning tunneling microscopy break junction method and interpreted by quantum transport calculations including solvent molecules explicitly. Fully extended and highly conducting molecular junctions prevail in water environment as opposed to short and less conducting junctions formed in non‐solvating mesitylene. A theoretical approach correctly models single‐molecule conductance values considering the experimental junction length. Most pronounced difference in the molecular junction formation and conductance was identified for a molecule with the highest stabilization energy on the gold substrate confirming the importance of molecule–electrode interactions. Presented concept of tuning conductance through molecule–electrode interactions in the solvent‐driven junctions can be used in the development of new molecular electronic devices.

Molecular-level multielectron handling toward electrical storage is a worthwhile approach to solar energy harvesting. Here, a strategy which uses chemical bonds as electron reservoirs is introduced to demonstrate the new concept of "structronics" (a neologism derived from "structure" and "electronics"). Through this concept, we establish, synthesize, and thoroughly study two multicomponent "super-electrophores": 1,8-dipyridyliumnaphthalene, 2, and its N,N-bridged cyclophane-like analogue, 3. Within both of them, a covalent bond can be formed and subsequently broken electrochemically. These superelectrophores are based on two electrophoric (pyridinium) units that are, on purpose, spatially arranged by a naphthalene scaffold. A key characteristic of 2 and 3 is that they possess a LUMO that develops through space as the result of the interaction between the closely positioned electrophoric units. In the context of electron storage, this "super-LUMO" serves as an empty reservoir, which can be filled by a two-electron reduction, giving rise to an elongated C-C bond or "super-HOMO". Because of its weakened nature, this bond can undergo an electrochemically driven cleavage at a significantly more anodic-yet accessible-potential, thereby restoring the availability of the electron pair (reservoir emptying). In the representative case study of 2, an inversion of potential in both of the two-electron processes of bond formation and bond-cleavage is demonstrated. Overall, the structronic function is characterized by an electrochemical hysteresis and a chemical reversibility. This structronic superelectrophore can be viewed as the three-dimensional counterpart of benchmark methyl viologen (MV).

Molecular-level multielectron handling toward electrical storage is a worthwhile approach to solar energy harvesting. Here, a strategy which uses chemical bonds as electron reservoirs is introduced to demonstrate the new concept of “structronics” (a neologism derived from “structure” and “electronics”). Through this concept, we establish, synthesize, and thoroughly study two multicomponent “super-electrophores”: 1,8-dipyridyliumnaphthalene, 2, and its N,N-bridged cyclophane-like analogue, 3. Within both of them, a covalent bond can be formed and subsequently broken electrochemically. These superelectrophores are based on two electrophoric (pyridinium) units that are, on purpose, spatially arranged by a naphthalene scaffold. A key characteristic of 2 and 3 is that they possess a LUMO that develops through space as the result of the interaction between the closely positioned electrophoric units. In the context of electron storage, this “super-LUMO” serves as an empty reservoir, which can be filled by a two-electron reduction, giving rise to an elongated C–C bond or “super-HOMO”. Because of its weakened nature, this bond can undergo an electrochemically driven cleavage at a significantly more anodicyet accessiblepotential, thereby restoring the availability of the electron pair (reservoir emptying). In the representative case study of 2, an inversion of potential in both of the two-electron processes of bond formation and bond-cleavage is demonstrated. Overall, the structronic function is characterized by an electrochemical hysteresis and a chemical reversibility. This structronic superelectrophore can be viewed as the three-dimensional counterpart of benchmark methyl viologen (MV).

The triangulenium dyes constitute a family of versatile chromophores whose impressive photo-absorption and emission properties are currently highlighted in numerous novel experimental applications. In this investigation, we provide a comprehensive TDDFT characterization of their spectroscopic properties elucidating the origin of their large and complex absorption and emission vibronic spectra spread over the (whole) visible region. More precisely, by benchmarking the performance of 10 commonly-used exchange-correlation density functionals belonging to different classes of approximation, we develop and validate a computational protocol allowing the accurate modeling of both the position and optical line-shape of their vibrationally-resolved absorption and emission band structures. We find that semilocal approximations provide the best estimate of the structure of the vibronic spectra, however they spuriously and strongly underestimate their position. We finally show that global-hybrid density functionals mixing between 20 and 30% of exact-like exchange are an excellent compromise to get a satisfactory estimate of both of these properties.

We report on the light-switch behaviour of two head-to-tail expanded bipyridinium species as a function of their interaction with calf thymus DNA and polynucleotides. In particular, both DNA and polynucleotides containing exclusively adenine or guanine moieties quench the luminescence of the fused expanded bipyridinium species. This behaviour has been rationalized demonstrating that a reductive photoinduced electron transfer process takes place involving both adenine or guanine moieties. The charge separated state so produced recombines in the tens of picoseconds. These results could help in designing new organic substrates for application in DNA probing technology and lab on chip-based sensing systems.

Three new linearly arranged bichromophoric systems 1-3 have been prepared, and their photophysical properties have been studied, taking also advantage of femtosecond pump-probe transient absorption spectroscopy. The three compounds contain the same chromophores, that is a Ru(II)-terpy-like species and a fused expanded bipyridinium (FEBP) unit, separated by three different, variously methylated biphenylene-type bridges. The chromophores have been selected to be selectively addressable, and excitation involving the Ru-based or the FEBP-based dyes results in different excited-state decays. Upon Ru-based excitation at 570 nm, oxidative photoinduced electron transfer (OPET) takes place in 1-3 from the 3MLCT state; however, the charge-separated species does not accumulate, indicating that the charge recombination rate constant exceeds the OPET rate constant. Upon excitation of the organic dye at 400 nm, the FEBP-based 1π-π* level is prepared, which undergoes a series of intercomponent decay events, including (i) electron-exchange energy transfer leading to the MLCT manifold (SS-EnT), which successively decays according to 570 nm excitation, and (ii) reductive photoinduced electron transfer (RPET), leading to the preparation of the charge-separated (CS) state. Reductive PET, involving the FEBP-based singlet state, is much faster than oxidative PET, involving the MLCT triplet state, essentially because of driving force reasons. The rate constant of CR is intermediate between the rate constants of OPET and RPET, and this makes 1-3 capable to selectively read the 400 nm excitation as an active input to prepare the CS state, whereas excitation at wavelengths longer than 480 nm is inefficient to accumulate the CS state. Moreover, intriguing differences between the rate constants of the various processes in 1-3 have been analyzed and interpreted according to the superexchange theory for electron transfer. This allowed us to uncover the role of the electron-transfer and hole-transfer superexchange pathways in promoting the various intercomponent photoinduced decay processes occurring in 1-3.

Twelve new polycyclic aromatic chromophores whose structures recall 9,10-substituted anthracene have been prepared and their absorption spectra, and luminescence properties (both in ethanol and dichloromethane solution at room temperature and in MeOH/EtOH (4:1 v/v) rigid matrix at 77 K) have been analyzed as well as pump-probe transient absorption spectroscopy and computational studies have been performed. The compounds have variously decorated pyrimidyl groups as substituents of the anthracene framework. The compounds are conveniently grouped in two series: 1-4 and 11, in which the pyrimidine nitrogen atoms are on the external side with respect to the anthracene framework (Ext-type compounds), and 5-10, where the pyrimidine nitrogen atoms are pointing towards the anthracene platform (Int-type compounds). Compound 12 contains both a pyrimidine substituent with "inner" nitrogen atoms ("Int") and a pyrimidine with "outer" nitrogen atoms ("Ext"). All the new species are quite efficient luminophores (spectral range of emission maxima: 400-450 nm at 77 K; 415-520 nm at room temperature; emission lifetimes: 0.25-8.7 ns range; emission quantum yields ranging between 0.08 to 0.99, with a single exception), with their photophysical properties depending on the connection scheme of the pyrimidyl groups to the anthryl platform (i.e., Int-type vs Ext-type compounds). Luminescence originating from locally excited pi-pi* anthracene-based singlet states, delocalized states largely involving the pyrimidyl moieties, and charge transfer states has been identified. For the int-type series, the initially-prepared excited state can deactivate via interconversion to a saddle-shaped conformation, opening the way to fast nonradiative decays.

Photosensitizers used in homogeneous photo catalytic systems for artificial photosynthesis, such as hydrogen production, are typically based on expensive transition metal complexes such as d(6) ruthenium(II) or iridium(III). In this work, we demonstrate efficient H-2 production in acidic water by using an organic dye derived from the triazatriangulenium (TATA(+)) family as a visible-light-absorbing photosensitizer (PS). By associating the hydrosoluble tris(ethoxyethanol)triazatriangulenium with an efficient H-2-evolving cobalt catalyst and ascorbic acid as sacrificial electron donor (SD), remarkable photocatalytic performances were reached in aqueous solution at pH 4.5, under visible-light irradiation, with up to 8950 catalytic cycles versus catalyst. The performances of this dye largely exceed those of the benchmark Ru tris-bipyridine in the same experimental conditions when low concentrations of catalyst are used. This higher efficiency has been clearly ascribed to the remarkable robustness of the reduced form of the organic dye, TATA(center dot). Indeed, the combination of the planar structure of TATA+ together with the presence of the three electron-donating nitrogen atoms promotes the stabilization of TATA(center dot) by delocalization of the radical, thereby preventing its degradation in the course of photocatalysis. By contrast, the reduced form of the Ru photosensitizer, [Ru-II(bpy)(2)(bpy(center dot-))](+) ("Ru-"), is much less stable. Nanosecond transient absorption experiments confirm the formation of TATA(center dot) in the course of the photocatalytic process in accordance with the mechanism initiated by the reductive quenching of the singlet excited state of TATA(+) by ascorbate. The second electron transfer from TATA(center dot) to the catalyst has also been evidenced by this technique with the detection of the signature of the reduced Co (I) form of the catalyst. The present study establishes that certain organic dyes are to be considered as relevant alternatives to expensive metal-based PSs insofar as they can exhibit a high stability under prolonged irradiation, even in acidic water, thereby providing valuable insights for the development of robust molecular systems only based on earth-abundant elements for solar fuel generation.

Two expanded pyridinium-based compounds 1 and 2 were studied by cyclic voltammetry in two different environments. The nonaqueous solution suppresses the adsorption of the compounds on the electrode surface. Adsorption process in aqueous environment was confirmed by typical shape of the curse as well as by the linear dependence of peak current on c scan rate The shift of standard redox potential in aqueous solution compared to nonaqueous environment toward more negative potential indicates the adsorption of reactant on the electrode surface Larger shift observed for flat conjugated molecule I confirms its stronger adsorption than for the second molecule.

Adsorption properties of a series of redox-active expanded pyridinium molecules were studied at an electrified interface by cyclic and alternating current voltammetry methods. It was shown that the adsorbed state can sufficiently block N-pyramidalization of the pyridinium redox center of 2',6'-diphenyl-[4,1':4',4''-terpyridin]-1'-ium tetrafluoroborate (2), leading to a change of the mechanism from a single two-electron-transfer process to stepwise transfer of two electrons. Chemically locked molecules 1, 9-(pyridin-4-yl)benzo[ c]benzo[1,2]quinolizino[3,4,5,6- ija][1,6]naphthyridin-15-ium tetrafluoroborate (ring fusion), and 3, 3,5-dimethyl-2',6'-diphenyl-[4,1':4',4''-terpyridin]-1'-ium tetrafluoroborate (steric hindrance) do not enable N-pyramidalization of the redox center upon electron transfer (ET) and serve as references. It was shown that 1 follows Langmuir-type adsorption around a potential of zero charge and that 1-3 form a close-packed film with some repulsive interactions between individual molecules at potentials where ET takes place. It has been suggested that all three molecules lie flat on the electrode surface, with the lowest free energy of adsorption found for 2. Maximum surface concentration Γ* equal to (1.4 ± 0.1) × 10-10 mol·cm-2 was found for 1, (1.5 ± 0.1) × 10-10 mol·cm-2 for 2, and (1.6 ± 0.1) × 10-10 mol·cm-2 for 3. These findings will help to clarify the role of molecular contacts with conducting substrate in the single-molecule electron-transport measurements of 1-3 during the metal-molecule-metal junction formation process.

A series of four expanded pyridinium molecules were used to investigate the correlation between a single molecule conductance (electron transport) and redox (electron transfer) properties at the electrode|electrolyte interface. Quantum chemical calculations of the transmission functions using DFT and non-equilibrium Green's function approach confirmed LUMO‒mediated electron transport in the break junction experiment. Single molecule conductance data can be rationalized within the framework of the non‒resonant tunneling mechanism. More interestingly, a linear correlation was found between the conductance values and the apparent electron transfer rate constants for three molecules of this series.

The single molecule conductance of expanded pyridinium derivative terminated by pyridyl\nnitrogen as an anchoring group at both ends of the molecule in two different environments\nwas measured by scanning tunneling microscopy break junction technique (STM-BJ). The\nvalues obtained in commonly used 1,3,5-trimethylbenzene (TMB) significantly differ from\nthe values acquired in a mixture of TMB and ethanol. Markedly lower junction length and\nhigher conductance indicate strong influence of adsorption of the molecule to the substrate\nsurface in pure TMB compared to the TMB and ethanol mixture.

The derivates of pyridinium belong to Promising group of molecules applicable in molecular electronic. for this purpose, four expanded branched pyridinium compounds were synthetized and the influence of the molecular structure on their electrochemical properties and single-molecule conductances has been studied. The highest values of conductance PM.. compounds 9-(pyridin-4-yl)benzo[c]benzo[1,2]quinolizino[3,4,5,6-ija][1,6]naphthyridin-15-ium tetralluomborate and 3,5-dimethyl-2',6'-diphenyl-[4,1',4',4'-terpyridin]-1'-ium tetratluoroborate. By the electrochemical reduction both of these compounds received two electrons in two separated one-electron steps. On the other hand, the compounds 2',6'-dipheny1-[4,1',4',4 "-terpyridin]-1'-ium tetrafluoroborate and 2,6-dipheny1-4-(4-(pyridin-4-yl)phenyl)-[1,4'-bypyridin]-1-ium tetralluoroborate are reduced by different mechanism and their conductance was lower.

A combined electrochemical and theoretical study of a series of pyridinium-based electrophores, consisting of reference N-alkyl-2,4,6-triarylpyridiniums (1-3) and N-aryl-expanded pyridiniums (EPs), i.e. N-aryl-2,4,6-triarylpyridiniums (4-10), is presented with the aim of elucidating multifaceted mechanisms underpinning the complex electrophoric activity of fluxional EP systems. Series 1-10 constitutes a library of model electrophores showing an incremental variation of their composition, charge, and steric hindrance. By kinetic mapping of the first two heterogeneous electron transfers (ETs) of 1-10 and computational mapping, at the density functional theory level, of their electronic and geometrical features in various redox states, it is established that, depending on whether EPs are made of one (4, 5) or two "head-to-tail"-connected pyridinium rings (6-10), the nature of the redox-triggered distortions (when allowed) is different, namely, N-pyramidalization due to hybridization change in the former case versus saddle-shaped distortion originating from conflicting intramolecular interactions in the latter case (8-10). When skeletal relaxations are sterically hampered, zwitterionic states and electron delocalization with quinoidal features are promoted as alternative relaxation modes. It follows that "potential compression" is changed to "potential expansion" (i.e., a further separation of redox potentials) in single-pyridinium EPs (4, 5), whereas "potential inversion" (i.e., single-step two-electron transfer; 8-10) is changed to stepwise ETs of the Weitz type for two-pyridinium EPs (6, 7). Overall, kinetic rate constants not only consistently indicate the most prominent mechanistic aspects of the reduction pathways of EPs, but they are also instrumental in establishing EPs as a unique class of electrophores.

This work is focused on the comparison of two techniques of single-molecule conductance measurements: Scanning Tunneling Microscopy Break Junction technique and Mechanically Controlled Break Junction technique. The structure of studied compound 9-( pyridin-4-yl)benzo[c]benzo[1,2]quinolizino[3,4,5,6-ija][1,6]naphthyridin-15-ium allows the formation of the molecular bridge between two gold electrodes, which are connected to a source of the constant voltage in both of the used methods. The differences, advantages and disadvantages of both of the techniques will be discussed. Both techniques provided two values of conductance of studied compound depending on the experimantal conditions.

Photoinduced electron transfer continues to be a key process for the design of artificial systems capable to perform an efficient solar energy conversion. In particular, linearly-arranged donor bridge acceptor dyads have greatly contributed to shine light on the various factors that must be taken into account when designing systems for obtaining long-lived charge separation, a useful property on the route to artificial photosynthesis. Here we summarized the results we recently obtained on the photoinduced electron transfer processes occurring in Os(II)-bis(terpyridine)-(bi)pyridinium dyads. In particular, we will focus on the role of the bridge in forward and backward electron transfer processes, and on the possibility of obtaining efficient photoinduced charge separation even when the driving force for the electron transfer process approaches zero. This latter point can be of considerable interest when several electron transfer steps are considered to ultimately yield long-range charge-separated state, with minimal energy losses from the initial, light-prepared localized excited state. (C) 2014 Elsevier B.V. All rights reserved.