Are Redox-Active Centers Bridged by Saturated Flexible Linkers Systematically Electrochemically Independent?

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.

On the Supra-LUMO Interaction: Case Study of a Sudden Change of Electronic Structure as a Functional Emergence.

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.

Environmental Control of Single-Molecule Junction Evolution and Conductance: A Case Study of Expanded Pyridinium Wiring.

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.

Triangulenium dyes: the comprehensive photo-absorption and emission story of a versatile family of chromophores.

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.

Electron Storage System Based on a Two-Way Inversion of Redox Potentials.

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, , and its -bridged cyclophane-like analogue, .

Designing expanded bipyridinium as redox and optical probes for DNA.

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.

Photoinduced Intercomponent Processes in Selectively Addressable Bichromophoric Dyads Made of Linearly Arranged Ru(II) Terpyridine and Expanded Pyridinium Components.

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.

Adsorption of Expanded Pyridinium Molecules at the Electrified Interface and Its Effect on the Electron-Transfer Process.

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.

Kinetics of Multielectron Transfers and Redox-Induced Structural Changes in N-Aryl-Expanded Pyridiniums: Establishing Their Unusual, Versatile Electrophoric Activity.

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.

Molecular dyads of ruthenium(II)- or osmium(II)-bis(terpyridine) chromophores and expanded pyridinium acceptors: equilibration between MLCT and charge-separated excited states.

The synthesis, characterization, redox behavior, and photophysical properties (both at room temperature in fluid solution and at 77 K in rigid matrix) of a series of four new molecular dyads (2-5) containing Ru(II)- or Os(II)-bis(terpyridine) subunits as chromophores and various expanded pyridinium subunits as electron acceptors are reported, along with the reference properties of a formerly reported dyad, 1. The molecular dyads 2-4 have been designed to have their (potentially emissive) triplet metal-to-ligand charge-transfer (MLCT) and charge-separated (CS) states close in energy, so that excited-state equilibration between these levels can take place. Such a situation is not shared by limit cases 1 and 5.

A new versatile class of hetero-tetra-metallic assemblies: highlighting single-molecule magnet behaviour.

Hetero-tetra-metallic species based on hexanuclear assemblies [((valen)M1)Ln(OH2)2(μM2(CN)8)]2(2-) (Ln = Gd(III), Tb(III); M1 = Cu(II), Ni(II) and M2 = Mo(IV), W(IV)) and co-crystallized mononuclear complexes [M3(tpy)2](2+) (M3 = Ni(II), Ru(II), Os(II)) were identified, fully characterized, and shown to constitute a new class of single-molecule magnets.

Modeling Dye-Sensitized Solar Cells: From Theory to Experiment.

Density functional theory (DFT) and time-dependent DFT are useful computational approaches frequently used in the dye-sensitized solar cell (DSSC) community in order to analyze experimental results and to clarify the elementary processes involved in the working principles of these devices. Indeed, despite these significant contributions, these methods can provide insights that go well beyond a purely descriptive aim, especially when suitable computational approaches and methodologies for interpreting and validating the computational outcomes are developed. In the present contribution, the possibility of using recently developed computational approaches to design and interpret the macroscopic behavior of DSSCs is exemplified by the study of the performances of three new TiO2-based DSSCs making use of organic dyes, all belonging to the expanded pyridinium family.

Tictoid expanded pyridiniums: assessing structural, electrochemical, electronic, and photophysical features.

In regard to semirigid donor-spacer-acceptor (D-S-A) dyads devised for photoinduced charge separation and built from an unsaturated spacer, there exists a strategy of design referred to as "geometrical decoupling" that consists in introducing an inner-S twist angle approaching 90° to minimize adverse D/A mutual electronic influence. The present work aims at gaining further insights into the actual impact of the use of bulky substituents (R) of the alkyl type on the electronic structure of spacers (S) of the oligo-p-phenylene type, which can be critical in the functioning of derived dyads. To this end, a series of 12 novel expanded pyridiniums (EPs), regarded as model S-A assemblies, was synthesized and its structural, electronic, and photophysical properties were investigated at both experimental and theoretical levels.

Photoinduced electron transfer in Os(terpyridine)-biphenylene-(bi)pyridinium assemblies.

A series of linearly arranged donor-spacer-acceptor (D-S-A) systems 1-3, has been prepared and characterized. These dyads combine an Os(II)bis(terpyridine) unit as the photoactivable electron donor (D), a biphenylene (2) or phenylene-xylylene (3) fragment as the spacer (S), and a N-aryl-2,6-diphenylpyridinium electrophore (with aryl = 4-pyridyl or 4-pyridylium in 1 or 2/3, respectively) as the acceptor (A). Their absorption spectra, redox behavior, and luminescence properties (both at 77 K in rigid matrix and at 298 K in fluid solution) have been studied.

Single-step versus stepwise two-electron reduction of polyarylpyridiniums: insights from the steric switching of redox potential compression.

Contrary to 4,4'-dipyridinium (i.e., archetypal methyl viologen), which is reduced by two single-electron transfers (stepwise reduction), the 4,1'-dipyridinium isomer (so-called "head-to-tail" isomer) undergoes two electron transfers at apparently the same potential (single-step reduction).

Theoretical procedure for optimizing dye-sensitized solar cells: from electronic structure to photovoltaic efficiency.

A step-by-step theoretical protocol based on density functional theory (DFT) and time-dependent DFT at both the molecular and periodic levels is proposed for the design of dye-sensitized solar cell (DSSC) devices including dyes and electrolyte additives. This computational tool is tested with a fused polycyclic pyridinium derivative as a novel dye prototype. First, the UV-vis spectrum of this dye alone is computed, and then the electronic structure of the system with the dye adsorbed on an oxide semiconductor surface is evaluated.

Designing multifunctional expanded pyridiniums: properties of branched and fused head-to-tail bipyridiniums.

The multifaceted potentialities of expanded pyridiniums (EPs), based on one pyridinium core bearing a 4-pyridyl or 4-pyridylium as the N-pyridinio group, are established at both experimental and theoretical levels. Two classes of head-to-tail (htt) EPs were designed, and their first representative elements were synthesized and fully characterized. The branched (B) family is made up of 2,6-diphenyl-4-aryl-1,4'-bipyridin-1-ium (or 1,1'-diium) species, denoted 1B and 2B for monocationic EPs (with aryl = phenyl and biphenyl, respectively) and 1B(Me) and 2B(Me) for related quaternarized dicationic species.

Expanded pyridiniums: bis-cyclization of branched pyridiniums into their fused polycyclic and positively charged derivatives--assessing the impact of pericondensation on structural, electrochemical, electronic, and photophysical features.

This study evaluates the impact of the extension of the π-conjugated system of pyridiniums on their various properties. The molecular scaffold of aryl-substituted expanded pyridiniums (referred to as branched species) can be photochemically bis-cyclized into the corresponding fused polycyclic derivatives (referred to as pericondensed species). The representative 1,2,4,6-tetraphenylpyridinium (1(H)) and 1,2,3,5,6-pentaphenyl-4-(p-tolyl)pyridinium (2(Me)) tetra- and hexa-branched pyridiniums are herein compared with their corresponding pericondensed derivatives, the fully fused 9-phenylbenzo[1,2]quinolizino[3,4,5,6-def]phenanthridinium (1(H)f) and the hitherto unknown hemifused 9-methyl-1,2,3-triphenylbenzo[h]phenanthro[9,10,1-def]isoquinolinium (2(Me)f).

Theoretical insights into branched and fused expanded pyridiniums by the means of density functional theory.

With the aim of getting insights into the peculiar electronic, structural, and photophysical properties of four expanded pyridinium systems of potential use as electron acceptors in supramolecular architectures, their electronic and geometrical structures, at both the ground and the excited states, were investigated by the means of density functional theory (DFT) and time-dependent DFT (TD-DFT). Solvent effects were included by the means of a polarizable continuum model (PCM) at both the ground and the excited states. In particular, the computed photophysical behaviors (absorption and emission) of the fused architectures were compared to those of the respective branched precursors in order to clarify the origin(s) of (i) the extension of their electronic absorption toward the visible region and (ii) the increase of their luminescence quantum yields and red-shifted emission wavelengths experimentally observed.

Photochemically driven intercalation of small molecules into DNA by in situ irradiation.

Intercalation of small molecules into DNA is photochemically achieved by in situ irradiation of a tetraaryl-pyridinium species. Such a "DNA intercalation on demand" process could highlight an alternative pathway to anticancer basic research, based on photo-activable DNA binders.

Intramolecular spin alignment within mono-oxidized and photoexcited anthracene-based pi radicals as prototypical photomagnetic molecular devices: relationships between electrochemical, photophysical, and photochemical control pathways.

AnOV is a pi-conjugated radical built from an anthracene (An) unit linked by a p-phenylene to an oxoverdazyl (OV) moiety. The mono-oxidized (cationic) form of AnOV was generated both electrochemically and photochemically (in the presence of an electron acceptor). The triplet nature (S=1) of the electronic ground state of AnOV(+) was demonstrated by combining spectroelectrochemistry, electron-spin resonance (ESR) experiments, and ab initio molecular orbital (MO) calculations.

Reaching optimal light-induced intramolecular spin alignment within photomagnetic molecular device prototypes.

Ground-state (GS) and excited-state (ES) properties of novel photomagnetic molecular devices (PMMDs) are investigated by means of density functional theory. These organic PMMDs undergo a ferromagnetic alignment of their intramolecular spins in the lowest ES. They are comprised of: 1) an anthracene unit (An) as both the photosensitizer (P) and a transient spin carrier (SC) in the triplet ES ((3)An*); 2) imino-nitroxyl (IN) or oxoverdazyl (OV) stable radical(s) as the dangling SC(s); and 3) bridge(s) (B) connecting peripheral SC(s) to the An core at positions 9 and 10.

Improving visible light sensitization of luminescent europium complexes.

The synthesis and characterization of the new ligands L(1), L(2) and L(4) are described with the series of four europium complexes of formula [EuL(n)(TTA)(3)] in which TTA refers to 2-thenoyltrifluoroacetonate and L(n) to tridentate ligands with nitrogen containing heterocyclic structure, such as a 2,6-bis(3-methyl-pyrazolyl)-4-(p-toluyl-ethynyl)-triazine for L(1), or terpyridines functionalized at the 4' position by a phenyl-vinylene for L(2), a p-dimethylamino-phenylene for L(3), or a p-aminophenyl-ethynylene for L(4). The spectroscopic properties of the ligands and of the complexes are studied by means of UV-Vis absorption spectroscopy, as well as steady-state and time-resolved luminescence spectroscopy. All complexes display europium centred luminescence upon ligand excitation.

Intramolecular spin alignment in photomagnetic molecular devices: a theoretical study.

Ground- and excited-state magnetic properties of recently characterized pi-conjugated photomagnetic organic molecules are analyzed by the means of density functional theory (DFT). The systems under investigation are made up of an anthracene (An) unit primarily acting as a photosensitizer (P), one or two iminonitroxyl (IN) or oxoverdazyl (OV) stable organic radical(s) as the dangling spin carrier(s) (SC), and intervening phenylene connector(s) (B). The magnetic behavior of these multicomponent systems, represented here by the Heisenberg-Dirac magnetic exchange coupling (J), as well as the EPR observables (g tensors and isotropic A values), are accurately modeled and rationalized by using our DFT approach.

Conformationally gated photoinduced processes within photosensitizer-acceptor dyads based on osmium(II) complexes with triarylpyridinio-functionalized terpyridyl ligands: insights from theoretical analysis.

A theoretical analysis, based on density functional theory, has been carried out to study photoinduced processes within a recently experimentally characterized (Lainé, P. P.; Bedioui, F.