Institut des Sciences et Technologies Moléculaires d'Angers

An overview of some recent developments of the chemistry of molecular donor materials for organic photovoltaics (OPV) is presented. Although molecular materials have been used for the fabrication of OPV cells from the very beginning of the field, the design of molecular donors specifically designed for OPV is a relatively recent research area. In the past few years, molecular donors have been used in both vacuum-deposited and solution-processed OPV cells and both fields have witnessed impressive progress with power conversion efficiencies crossing the symbolic limit of 10 %. However, this progress has been achieved at the price of an increasing complexity of the chemistry of active materials and of the technology of device fabrication. This evolution probably inherent to the progress of research is difficult to reconcile with the necessity for OPV to demonstrate a decisive economic advantage over existing silicon technology. In this short review various classes of molecular donors are discussed with the aim of defining possible basic molecular structures that can combine structural simplicity, low molecular weight, synthetic accessibility, scalability and that can represent possible starting points for the development of simple and cost-effective OPV materials.

This tutorial review presents some recent developments in the design, synthesis and implementation of organic solution-processable molecular fluorophores for non-doped electroluminescent [corrected] devices. After a brief presentation of the basic principles of operation and main characteristics of electroluminescent devices, some examples of active emitters representative of the main classes of non-doped molecular electrofluorophores will be discussed. Emphasis is placed on the relationships between the molecular structure and the electronic properties of molecular emitters, in which high photoluminescence efficiency, synthetic accessibility and processability are combined by design with additional functions such as hole and/or electron injection and transport.

This Review discusses the structure-property relationships in chiral molecules, macromolecules (polymers), and supramolecules (crystals, liquid crystals, or thin films) containing main-group elements. Chirality is a major property in our world, having a prominent influence on processes in biology, chemistry, and physics. Its impact in optics due to its interaction with electromagnetic waves gave rise to a multitude of effects, such as the Cotton effect and circularly polarized luminescence, making possible applications such as 3D displays and polarized sunglasses. Herein, a particular emphasis will be given to the influence of chirality on the conducting and optical properties of molecules or materials containing frontier heteroelements, particularly boron, silicon, phosphorus, and sulfur. These synergic materials are expected to become game-changers in the field of materials science by bringing new properties into the realm of reality, such as chirality-induced spin-selectivity, circularly polarized luminescence, and electrical magnetochiral anisotropy. This Review should be of interest for chemists and also physicists working in the fields of molecular and supramolecular chemistry, and molecular materials in the broadest sense.

Intense current interest in supramolecular chemistry is devoted to the construction of molecular assemblies displaying controlled molecular motion associated to recognition. On this ground, molecular clips and tweezers have focused an increasing attention. This tutorial review points out the recent advances in the construction of always more sophisticated molecular clips and tweezers, illustrating their remarkably broad structural variety and focusing on their binding ability towards neutral guests. A particular attention is brought to recent findings in dynamic molecular tweezers whose recognition ability can be regulated by external stimuli. Porphyrin-based systems will not be covered here as this very active field has been recently reviewed.

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The search for hybrid organic-inorganic materials, which have the great advantage that they can be synthesized at moderate temperature (T < 200 °C), remains a great challenge in the field of ferroelectrics. Here, a room-temperature ferroelectric material with interesting characteristics, (MV)[BiI(3)Cl(2)] (MV(2+) = methylviologen), is reported. Its structure is based on polar inorganic chains resulting from a remarkable Cl/I segregation induced by methylviologen entities, which coincide with the fourfold polar axis of the tetragonal structure. Of great importance is that this room-temperature hybrid ferroelectric displays a clear electrical hysteresis loop with a large spontaneous polarization (>15 μC·cm(-2)).

Fixation of a 5-hexyl-2,2'-bithienyl unit on a conjugated BODIPY donor increases the conversion efficiency of the resulting molecular bulk heterojunction solar cells from 1.30 to 2.20%.

The controlled preparation of chiral structures is a contemporary challenge for supramolecular science because of the interesting properties that can arise from the resulting materials, and here we show that a synthetic nonamphiphilic C(3) compound containing π-functional tetrathiafulvalene units can form this kind of object. We describe the synthesis, characterization, and self-assembly properties in solution and in the solid state of the enantiopure materials. Circular dichroism (CD) measurements show optical activity resulting from the presence of twisted stacks of preferential helicity and also reveal the critical importance of fiber nucleation in their formation. Molecular mechanics (MM) and molecular dynamics (MD) simulations combined with CD theoretical calculations demonstrate that the (S) enantiomer provides the (M) helix, which is more stable than the (P) helix for this enantiomer. This relationship is for the first time established in this family of C(3) symmetric compounds. In addition, we show that introduction of the "wrong" enantiomer in a stack decreases the helical reversal barrier in a nonlinear manner, which very probably accounts for the absence of a "majority rules" effect. Mesoscopic chiral fibers, which show inverted helicity, i.e. (P) for the (S) enantiomer and (M) for the (R) one, have been obtained upon reprecipitation from dioxane and analyzed by optical and electronic microscopy. The fibers obtained with the racemic mixture present, as a remarkable feature, opposite homochiral domains within the same fiber, separated by points of helical reversal. Their formation can be explained through an "oscillating" crystallization mechanism. Although C(3) symmetric disk-shaped molecules containing a central benzene core substituted in the 1,3,5 positions with 3,3'-diamido-2,2'-bipyridine based wedges have shown peculiar self-assembly properties for amphiphilic derivatives, the present result shows the benefits of reducing the nonfunctional part of the molecule, in our case with short chiral isopentyl chains. The research reported herein represents an important step toward the preparation of functional mesostructures with controlled helical architectures.

A sustainable future requires highly efficient energy conversion and storage processes, where electrocatalysis plays a crucial role. The activity of an electrocatalyst is governed by the binding energy towards the reaction intermediates, while the scaling relationships prevent the improvement of a catalytic system over its volcano-plot limits. To overcome these limitations, unconventional methods that are not fully determined by the surface binding energy can be helpful. Here, we use organic chiral molecules, i.e., hetero-helicenes such as thiadiazole-[7]helicene and bis(thiadiazole)-[8]helicene, to boost the oxygen evolution reaction (OER) by up to ca. 130 % (at the potential of 1.65 V vs. RHE) at state-of-the-art 2D Ni- and NiFe-based catalysts via a spin-polarization mechanism. Our results show that chiral molecule-functionalization is able to increase the OER activity of catalysts beyond the volcano limits. A guideline for optimizing the catalytic activity via chiral molecular functionalization of hybrid 2D electrodes is given.

A novel stable bisazide molecule that can freeze the bulk heterojunction morphology at its optimized layout by specifically bonding to fullerenes is reported. The concept is demonstrated with various polymers: fullerene derivatives systems enable highly thermally stable polymer solar cells.

The structural and electronic properties of four isomers of didodecyl[1]-benzothieno[3,2-b][1]benzothiophene (C12-BTBT) have been investigated. Results show the strong impact of the molecular packing on charge carrier transport and electronic polarization properties. Field-induced time-resolved microwave conductivity measurements unravel an unprecedented high average interfacial mobility of 170 cm2 V−1 s−1 for the 2,7-isomer, holding great promise for the field of organic electronics. Since the dawn of organic electronics in the 1970's, academic and industrial research efforts have led to dramatic improvements of the solubility, stability, and electronic properties of organic semiconductors (OSCs).1, 2 The common benchmark to characterize the electrical performances of OSCs is their charge carrier mobility μ (cm2 V−1 s−1), defined as the drift velocity of the charge carrier (cm s−1) per unit of applied electric field (V cm−1). Reaching high mobilities in OSCs is highly desirable as it allows faster operation of transistors and energy savings by reduced calculation times.2, 3 However, OSCs performances (conventional values usually range from 1 to 10 cm2 V−1 s−1, with highest values obtained with single-crystal devices mostly exempt of structural defects) are still not comparable to that of state-of-the-art inorganic semiconductors (e.g., metal oxides with μ = 20–50 cm2 V−1 s−1 and polycrystalline silicon with μ > 100 cm2 V−1 s−1) thereby hampering important potential technological applications such as flexible organic light-emitting diode displays and wearable electronics.3, 4 Charge carrier mobilities on the order of 50–100 cm2 V−1 s−1at room temperature are conceivable for two reasons: First, a compound as simple as naphthalene exhibits mobilities ranging from 100 to 300 cm2 V−1 s−1 between 30 and 3 K;5 second, recent theoretical calculations prove that there is no reason to believe that achieving mobilities higher than 50 cm2 V−1 s−1 is impossible.6 The highest mobility reported to date, 43 cm2 V−1 s−1, has been achieved by Yuan et al. on aligned thin films of a polymorph of 2,7-dioctyl[1]benzothieno[3,2-b]benzothiophene.7 This value has, however, not been independently reproduced to date. Charge carrier mobility is a material property depending on multiple parameters among which we can cite molecular and crystalline structures, charge density, temperature, disorder, and defects.3, 8-10 Due to the relative ease of implementation, most of the reported mobility values are extracted from organic field-effect transistor (OFET) characteristics. Hall-effect measurements constitute an alternative robust method that is considerably more difficult to implement.8, 11 Both techniques are contact-based, i.e., implying the injection/collection of charge carriers at electrodes. It is worth mentioning here the recent work of Uemura et al.12 highlighting how to properly extract without any overestimation the mobility from high-mobility OSCs and the recommendation of Braga et al.13 Indeed, as a result of the possible dependence of the contact resistance with the gate voltage, devices can show a nonlinear drain current swing just above the threshold. It is thus of paramount importance to cross check high mobility values using gated four-point-probe and transmission line method measurements,8, 14 or another measurement method. The field-induced time-resolved microwave conductivity (FI-TRMC) technique has recently been introduced as a promising approach to avoid contact-related issues.9 The FI-TRMC technique can not only probe the intrinsic charge carrier mobility at the semiconductor/dielectric interface by microwave-based dielectric loss measurements but also investigate separately hole versus electron conduction by controlling the gate bias voltage. Holding great promises for the rapid screening of a large number of semiconductor/dielectric pairs, this technique has also recently proved its efficiency for quantitative probing of interfacial trap sites.10, 15-17 It must be emphasized that FI-TRMC is complementary to OFET and Hall-effect methods because it probes charge transport over shorter length- and time-scales, allowing thus to investigate the elementary steps of charge transport. FI-TRMC will certainly contribute to elucidate the charge transport mechanism exhibited by weakly van der Waals bonded systems. Indeed, the elaboration of an universal theoretical framework to describe charge transport in OSCs is a challenging task. Until now, extreme models based on a pure hopping regime (in which charges jump from one molecule to another one) or band-like regime (based on the scattering of the charges in delocalized electronic states by lattice phonons) are typically involved in theoretical studies aimed at assessing the influence of various parameters, such as molecular structure, crystalline packing, temperature, and energetic/positional disorder, on transport properties.3, 18 Moreover, as recently highlighted by Fratini et al., the strong dynamical disorder present in OSCs at room temperature imposes a transient localization of the charges which results in an intermediate regime of charge transport where the carriers exhibit both localized and extended characters.19 We report here on the structural and electronic properties of four isomers of didodecyl[1]benzothieno[3,2-b][1]benzothiophene (C12-BTBT-C12) varying by the isomerism of the alkyl side-chains. The choice of the BTBT core is motivated by the previously reported high mobility values, its high chemical stability, and the ease of derivatization.7, 20-23 FI-TRMC measurements performed on these four derivatives have been confronted to corresponding theoretical simulations in both the hopping and band regime. This work highlights that the molecular packing, driven by the molecular structure of the isomers, has not only a strong impact on the charge carrier mobility but also on the ionization potential (IP) due to changes in the magnitude of electronic delocalization and electronic polarization effects. The FI-TRMC measurements yield a strikingly high average interfacial mobility of 1.7 × 102 cm2 V−1 s−1 for 2,7-didodecyl[1]benzothieno[3,2-b][1]benzothiophene. Moreover, the performed simulations point to a band-like transport for this derivative. The four investigated isomers, whose molecular structures are depicted in Figure 1a, exhibit a symmetric conjugated backbone substituted by two dodecyl side chains. As expected, the four compounds exhibit rather comparable first oxidation potentials (Eox1) around 0.9 eV (vs Fc/Fc+) in solution with slightly higher values for 1 and 3 than for 2 and 4 (see Table 1, Section S2 and Figure S1 of the Supporting Information).20, 24 The thermal behavior of the different BTBTs was investigated by thermogravimetric analysis and differential scanning calorimetry (DSC). Evaporation of the materials occurs at temperatures ranging from 370 to 390 °C (Figure S2a, Supporting Information); DSC traces, as well as transition temperatures and their associated energies, are presented in Figure S2b and Table S1 in the Supporting Information. Almost all the isomers possess only one phase transition except 2 for which a smectic A phase (SmA) is observed prior to melting, as a result of its rod-like molecular shape.25 With its herringbone motif, the crystal of 2 melts at 117.4 °C. This value is higher than the melting point of the other materials, ranging from 86 to 103 °C. Interestingly, 4 crystallizes in a metastable polymorphic form, upon cooling, which converts into the stable single-crystal phase at 61 °C. Bulk structure determination of the different BTBT derivatives was realized by single-crystal X-ray diffraction; and the complete crystal data are available in Table S2 in the Supporting Information (the crystal structure of 2 has previously been solved by Takimiya and co-workers).20, 26 Compounds 1–4 exhibit a monoclinic unit cell containing half of the molecule in each asymmetric unit, i.e., Z′ = 0.5. 2 adopts a standard layer-by-layer herringbone packing motif mainly stabilized by CH···π interactions while cofacial interdigitated structures dominated by π···π interactions are observed for 1, 3, and 4 (Figure 1c,d). The shortest stacking distance between molecular planes, 3.48 Å, is obtained for 4; Table S3 (Supporting Information) highlights the different distances of stacking and slippage exhibited in the structures. 2 presents consequently favorable close contacts between the aromatic cores in two dimensions. Additional information relative to the different structures can be found in Section S4 in the Supporting Information. The crystal structure of the four isomers being firmly established, we turned our attention to the calculation of the electronic interactions between adjacent π-systems. Table 1 and Figure 2a collects the theoretical estimates of two important energetic parameters for charge transport, as calculated from density functional theory (DFT): (i) The transfer integral (J) is involved in both the hopping and band-like models and reflects the degree of electronic overlap/interactions between the interacting electronic levels; Figure 2a also shows the corresponding indexation of the molecules within the bulk single-crystal phase; (ii) the reorganization energy (λ) characterizes the degree of geometric relaxation accompanying the localization of one charge over a single molecule in a hopping picture. For compound 2, large transfer integrals are calculated for holes along several directions within a molecular layer (from 51 to 58 meV), thus pointing to a 2D charge transport. For all isomers, charge transport cannot be 3D since the transfer integrals between molecules belonging to adjacent layers are close to zero due to the insulating character of the long alkyl chains. In the case of 1, large transfer integrals (62 meV) are only calculated for dimer 1–2 (along the a axis) as a result of the short π-stacking distance, thus leading to a dominant 1D character for the charge transport. The transfer integrals in the other directions are quite small (<6 meV) because the columns of molecules within a layer are shifted parallel and hence do not favor a strong overlap between the highest occupied molecular orbital (HOMO) wave functions. Compounds 3 and 4 adopt a similar packing built from columns of slightly translated molecules (along the b direction). However, in contrast to 1, the molecules between adjacent columns within a layer adopt a herringbone-like arrangement, implying that large spatial overlaps of the HOMO wave functions are possible. This is particularly true for 3 for which significant transfer integrals (15 meV) are estimated within a column but also between adjacent columns within the layers (22 meV). For 4, very large transfer integrals are also calculated along the π-stacking direction (129 meV) while small values (7 meV) are obtained for hole transfer between adjacent columns, thus reflecting the high sensitivity of the transfer integral values to the relative position of the interacting units.27 These lower values are primarily attributed to the fact that the long axis of the molecules is almost lying parallel to the organic layer in 4 so that they are only slightly interacting through end-to-end contacts. Altogether, we expect better charge transport properties for 2 in the two regimes owing to its large transfer integrals and its pronounced 2D charge transport character which makes the transport less affected by the orientation of the crystals in the channel of the OFETs. The reorganization energies are very close for the four BTBT structures and range between 0.22 and 0.25 eV for holes, thus suggesting that differences in the charge transport properties among the BTBT derivatives are mainly governed by the amplitude of the transfer integrals in a hopping picture. It is worth mentioning that the calculations have been performed on structures solved at low temperature (except for 2) and can be affected to a small extent by the thermal expansion and the dynamics of the system.28 Second, we investigated to what extent the electronic properties are impacted by the solid-state packing starting with the ionization potentials. When it is calculated at the Hartree–Fock semi-empirical AM1 (Austin Model 1) level for a single molecule extracted from the crystals, without any further geometry optimization, the variation of IP among the four isomers is less than 50 meV, in good agreement with the small observed variation of Eox1. However, a radically new picture prevails when molecules interact in the crystals. In sharp contrast with compounds 1, 3, and 4, the isomer 2 exhibits a much lower IP in the solid state. The shift going from the gas phase to the solid state arises from the combination of intermolecular delocalization effects triggered by the electronic coupling between the molecules and electronic polarization effects driven by electrostatic and induction interactions (see the Experimental Section for details).29, 30 Our theoretical calculations, performed on molecular clusters of similar size, indicate that polarization effects are more pronounced for 2 and lead to a relative of the IP by eV with to the other Moreover, the energy associated with charge delocalization effects is also more pronounced for 2, i.e., they contribute to an energy of eV to the other isomers, Table S4 and Figure of the Supporting Information. Altogether, the IP values for both the molecules and the crystals are similar for 1, 3, and 4, while 2 shows a eV lower IP S4 and Figure Supporting These are in on films or with values of and eV for 1, 3, and 4 versus eV for 2 Table Supporting the of our these results the first report of an IP variation as large as eV among that an as large as eV in the ionization potential was observed by going from to as further by theoretical a result highlights the importance of intermolecular interactions in the of the ionization potential or electron in crystalline films and hence when assessing the ease of charge from electrodes. In order to the electronic behavior in the solid state of 2, also by its values of charge transport properties of the different isomers first in The technique has to be an screening method to the intrinsic charge carrier transport properties of is a method where the average of charge carriers to the in the bulk of thin is through microwave (the can information relative to the and FI-TRMC measurement techniques in the Experimental Figure (Supporting Information) and recent and As in Figure all films of 1–4 conductivity with a and upon of charge carriers and the yield of and of the mobilities of and charge Interestingly, 2 the highest with a value of = × cm2 V−1 the other 1, 3, and 4 of only and × cm2 V−1 s−1, This sharp over one order of magnitude in is in line with the behavior of isomer X-ray measurements on the different from to that these results not impacted by or the of a molecular the that charge transport. Indeed, presented in Figure 4 the of highly crystalline thin of the single-crystal bulk on of the molecules versus the favorable for probing the charge transport by The is slightly more in the films of 4 where two are mainly the single crystal phase and an phase that can charge transport. Additional information to the of the films can be found in Section in the Supporting Information. The higher value observed for 2 can thus be by the higher and of its charge transport as a result of its herringbone In 1, 3, and 4, more 1D charge transport are highly to the and thermal of the molecules and lead to lower mobility measurement in on polycrystalline thin films (see Section and Figure of the Supporting Information) only the good charge transport properties of 2, an average mobility of cm2 V−1 of 1, 3, and 4 impacted by their ionization hampering the of charges within the layer and leading to our to any in or contact Since not to the better charge transport properties of isomer 2, FI-TRMC measurements performed in using and as the and films of the different isomers as the layer The of the OSCs is by the FI-TRMC measurement that devices with and into the layer upon of a and microwave the charge carrier density (Figure for Interestingly, the interfacial hole mobility of 2 was estimated as 1.7 × 102 cm2 V−1 s−1 value for four at charge carrier and × 102 cm2 V−1 s−1 at (Figure Figure Supporting considerably higher than any previously reported values on the BTBT the of our this is the first report of an interfacial hole mobility 100 cm2 V−1 a molecular at room It is worth mentioning that FI-TRMC measurements to probe the charge transport properties of a material over the of the × cm2 = cm2 in our at short length- and (the of the charge carriers is estimated by = from several to as a of the mobility of the investigated and using a microwave of i.e., mainly than the of thus an intrinsic value of the charge a regime of charge transport, lower mobility values to of the value obtained at lower charge carrier is observed for charge carrier above Figure Figure Supporting a in mobility at higher charge carrier density has been observed in FI-TRMC Our studies lead to more mobility values, to typically around cm2 V−1 s−1 for and to cm2 V−1 s−1 for the most and The of a of mobility at short length- and on several OSCs and the influence of the charge carrier density for molecular semiconductors mobilities higher than 100 cm2 V−1 s−1 to the of charge transport mechanism in these It is worth here that a of mobility has been observed at room temperature on and devices upon of the charge carrier density, as a result of scattering A similar is most in our devices of 2 where charges are in the first molecular layer at the interface with the dielectric upon applications of higher than (see Section and Figure of the Supporting Information for more The of higher electric charge carrier certainly the carriers to by the interface where they have to be to scattering as previously reported in single-crystal by et to 2, the other isomers 1, 3, and 4 a much lower FI-TRMC hole mobility of and cm2 V−1 s−1, (Figure Supporting measurements of the films in the FI-TRMC measurements that 2 and 3 present their single-crystal phase (Figure The is more for 1 and 4 a between their single-crystal phase and an phase of 3 for 1 and only the phase observed in the films for 4 of only one of The of in the films of 1 and 4 can be attributed to the between and the and Since one of the most results of this is the unprecedented mobility of 2 highlighted by the FI-TRMC we on the theoretical of the bulk single-crystal of the different that a of the influence of charge carrier density and of the in 1 and 4 and its impact on charge transport is and the of this a into the behavior of 2 with to the other isomers, the hole mobility of all isomers has been at the theoretical level in two extreme versus band the Experimental Table 1 and Section of the Supporting Information. The highest mobilities in a hopping regime are obtained for 4 cm2 V−1 s−1) and in the order 2, 1, and 3 and cm2 V−1 s−1, for 2, 1, and 3, As by the amplitude of the transfer the mobility is the for 2 (see Figure of the Supporting In the band regime the a range of a of an from eV for isomer 3 to eV for isomer 2 (see Figure of the Supporting The of the highest band and conduction band along the direction are zero due to the of the alkyl We have the hole mobility of all isomers by the potential theory to the transport The values are obtained for 2 and cm2 V−1 s−1 along the b and a 1 and 4 exhibit hole mobilities and cm2 V−1 s−1 for 1 along the a and b directions and cm2 V−1 s−1 for 4 along the b while the values found for 3 and cm2 V−1 s−1 along both b and As expected, the hole mobilities calculated within the band regime are than obtained within the hopping the is for 1 and 2 to 3 and while the mobilities rather with the hopping values in 1, 3, and 4, the high μ value for 2 can only be reproduced by a band regime. This is in line with the that materials with a single dominant transport direction are more by thermal lattice which and localization of the charge hence the of a hopping picture. In we that thermal have a much more impact in the case of compound 2 a pronounced 2D transport. This is by calculations molecular dynamics and electronic structure methods that point to a of the transfer integrals at room temperature in 2 a standard of along the herringbone 1–4 in Figure to be to the corresponding values of and in the and Section and Figure of the Supporting results are also by the recent of et al. highlighting the of the of the aromatic along the long axis for BTBT that thermal lattice at room temperature and hence its on charge transport properties in As FI-TRMC mainly from an OFET measurement from the point of of the charge The that charges through molecules the molecules to only over a molecules a of the mobility of the investigated from a to 300 molecules in the case of a of to several the of this the charge delocalized over a distance of or higher for we be to mobilities these in is the mechanism hampering charge transport in In the structural and electronic properties of four isomers of didodecyl[1]benzothieno[3,2-b][1]benzothiophene have been investigated. Our theoretical and to the strong impact of the molecular packing on the ionization potential and transfer integrals the charge transport. FI-TRMC measurements to probe the intrinsic charge carrier transport of materials, among which was found to exhibit a strikingly high average interfacial mobility of 1.7 × 102 cm2 V−1 s−1, at room calculations that the transport in 2 within the band which we with the 2D character of the crystal and the thermal in electronic transfer a mobility great promise for the field of organic electronics and efforts be to further how to such high mobilities in the one this will a new range of associated to over large wearable the other it will better to solid-state and to the charge transport mechanism that prevails in and unprecedented and to this The from the from the from the of the from the from the for from a research of the of from a of the for the from a for and by the for the of from for and from The and the has from a of from the also the through the and the for in and are data for isomer 3 at the by the As a to our and this information by the materials are and be for but are not or from information than be to the The is not for the or of any information by the than be to the corresponding for the

Controlling the guest expulsion process from a receptor is of critical importance in various fields. Several coordination cages have been recently designed for this purpose, based on various types of stimuli to induce the guest release. Herein, we report the first example of a redox-triggered process from a coordination cage. The latter integrates a cavity, the panels of which are based on the extended tetrathiafulvalene unit (exTTF). The unique combination of electronic and conformational features of this framework (i.e. high π-donating properties and drastic conformational changes upon oxidation) allows the reversible disassembly/reassembly of the redox-active cavity upon chemical oxidation/reduction, respectively. This cage is able to bind the three-dimensional B12 F12 (2-) anion in a 1:2 host/guest stoichiometry. The reversible redox-triggered disassembly of the cage could also be demonstrated in the case of the host-guest complex, offering a new option for guest-delivering control.

Thiophene-based D-A-D π-conjugated systems containing triphenylamine end groups connected to a 1,1,4,4-tetracyanobuta-1,3-diene acceptor by oligothiophene chains of variable length have been synthesized. These compounds show interesting light-harvesting properties and low-lying HOMO levels. Preliminary results on bilayer heterojunction solar cells with C(60) as acceptor show power conversion efficiency higher than 1.0%.

Abstract An overview of various approaches for the realization of single‐material organic solar cells (SMOCs) is presented. Fullerene‐conjugated systems dyads, di‐block copolymers, and self‐organized donor‐acceptor molecules all represent different possible approaches towards SMOCs. Although each of them presents specific advantages and poses specific problems of design and synthesis, these different routes have witnessed significant progress in the past few years and SMOCs with efficiencies in the range of 1.50% have been realized. These performances are already higher than those of bi‐component bulk heterojunction solar cells some ten years ago, demonstrating that SMOCs can represent a credible approach towards efficient and simple organic solar cells. Possible directions for future research are discussed with the aim of stimulating further research on this exciting topic.

This review article gives an overview of past and current activities in the Linear Conjugated Systems Group of Angers and in the IPOC – Functional Polymers Group of the Institute of Polymer Chemistry of Stuttgart on the use of triphenylamine (TPA) as versatile building block for organic electronics. In the first part, the properties of TPA itself are introduced including geometrical and energy level considerations. Dimerization of TPA to tetraphenylbenzidine upon electrochemical oxidation is highlighted. The blocking of TPA para ‐positions and its implications in terms of electroactivity is further discussed. The second part shows that dimerization of TPA as pendant redox‐active moieties in polymers is a versatile strategy to crosslink polymer films. Coming from redox homopolymers the crosslinking strategy is extended towards conjugated redox polymers based on polythiophenes and block copolymers. Conductivity mechanisms and the influence of doping level on conductivity are probed with cyclic voltammetry coupled with in situ conductance and four‐point probe measurements. The last part is dedicated to the use of TPA as an electron‐donating block in the design of donor‐π‐acceptor chromophores and their use as active material in organic photovoltaics. An overview of some relevant TPA‐based push–pull molecules from the literature and our contribution to this field is presented emphasizing the progress of the photovoltaic performance of organic solar cells made over the last decade. © 2018 Society of Chemical Industry

A functionalized polymer film allowing for a complete and straightforward second-harmonic generation (SHG)-assisted high-contrast writing-reading-erasing-writing sequence is proposed. The whole process is supported by the reversible photoinduced dimerization of a coumarin chromophore and enables efficient optical data storage that can be detected only by SHG imaging.

Single material organic solar cells (SMOSCs) are based on ambivalent materials containing electron donor (D) and acceptor (A) units capable to ensure the basic functions of light absorption, exciton dissociation, and charge transport. Compared to bicomponent bulk heterojunctions, SMOSCs present several major advantages such as considerable simplification of cell fabrication and a strong stabilization of the morphology of the D/A interface, and thus of the cell lifetime. In addition to these technical issues, SMOSCs pose fundamental questions regarding the possible formation, and dissociation of excitons on the same molecular D-A architecture. SMOSCs are developed with various approaches, namely "double-cable" polymers, block copolymers, oligomers, and molecules that differ by the donor platform: polymer or molecule, the nature of A, the D-A connection, and the intra- and intermolecular interactions of D and A. Although for several years the maximum efficiency of SMOSCs has remained limited to 1.0-1.5%, impressive progress has been recently accomplished leading to SMOSCs with 4.0-5.0% efficiency. Here, recent advances in the synthesis of D-A materials for SMOSCs are presented in the broader context of the chemistry of organic photovoltaic materials in order to discuss possible directions for future research.

Syntheses, X-ray structural analyses, thermal behaviors, photochromism, and electrical properties of a series of methylviologen (MV(2+)) halobismuthate hybrids, namely, (MV)(3)[Bi(4)Cl(18)](H(2)O)(y) (1a, y approximately = 1.7), (MV)(4)[Bi(6)Cl(26)](H(2)O)(y) (2a, y approximately = 1.7), (MV)(4)[Bi(6)Cl(25.6)I(0.4)](H(2)O)(y) (3a, y approximately = 1.5), and (MV)(4)[Bi(6)Cl(24.6)I(1.4)](H(2)O)(y) (4a, y approximately = 1.3), are reported. Because of the thermal effect of a UV lamp or as a result of being heated up to 100 degrees C, all of the above compounds undergo a complete (1a, 2a, and 3a) or a partial (4a) dehydration together, in 2a and 3a, with an impressive structural reorganization involving a 90 degrees rotation of methylviologen dimers and, in 3a, a new Cl/I distribution, finally leading to (MV)(3)[Bi(4)Cl(18)] (1b), (MV)(4)[Bi(6)Cl(26)] (2b), (MV)(4)[Bi(6)Cl(25.6)I(0.4)] (3b), and (MV)(4)[Bi(6)Cl(24.6)I(1.4)](H(2)O)(x) (4a, x approximately = 0.65), respectively. In its turn, 4a (x approximately = 0.65) undergoes an abrupt structural change at 160 degrees C when water molecules are completely removed, leading to (MV)(4)[Bi(6)Cl(24.6)I(1.4)] (4b). Obviously, the two first dehydrated phases can be considered as the n = 2 (1b) and n = 3 (2b) members of the (MV)((2n+2)/2)[Bi(2n)Cl(8n+2)] family, and the ultimate member (n = infinity) with an infinite 1D double-chain inorganic framework, namely, (MV)[Bi(2)Cl(8)], has already been reported. According to the results of structural refinements, some positions of the Cl atoms in the [Bi(6)Cl(26)](8-) anionic cluster of 3a and 4a have been occupied by I atoms, finally leading to iodide-doped materials of the 2a type (percentage of doping: 3a, 1.5%; 4a, 5.4%). Upon UV irradiation, yellow crystals of 2a and 3a (which become 2b and 3b because of the thermal effect of the UV lamp) or yellow crystals of 2b, 3b, and 4a undergo a color change to black crystals (in the case of 2b), as observed in (MV)[Bi(2)Cl(8)], or light-brown crystals (in the cases of 3b and 4a). These photochromic properties are probably due to the photoinduced electron transfer from the anionic part to the methylviologen dications. In contrast, no color change is observed when yellow crystals of 1a or 1b and the iodide-doped (MV)[Bi(2)Cl(8-epsilon)I(epsilon)] material are irradiated. Because the relative positions of methylviologen to the host anionic frameworks are comparable in all structures (the N...Cl distances are about 3.4 A), these results indicate that such kinds of photochemical reactions depend on the dimension of the anionic networks, as well as the iodide doping. The single-crystal electrical conductivity measurements of 2b before and after irradiation were carried out between 150 and 393 K. The results prove that both of them are semiconductors with weak room temperature conductivity and that the band gap of the irradiated crystal (2b, 0.35 eV) is much smaller than that of the original hybrid 2a (1.0 eV).

Abstract Luminescent solar concentrators (LSCs) are optical systems that absorb, convert, and concentrate solar light by means of photoluminescence of an emitting material embedded in a transparent waveguide. LSCs combine large possibilities of variation of shape, flexibility, color, and transparency and can operate under direct or diffuse light. LSCs were actively investigated in the period 1975–1985 in view of photovoltaic (PV) conversion. After 20 years of sleep, research on LSCs has reemerged in the first years of the millennium driven by their potential application for PV conversion in built environment. Research on LSCs aims at the development of new active and passive components, namely emitting and light‐guiding materials, and at the reduction of the loss factors associated with the elemental processed involved in the operation in order to improve power conversion efficiency. After a brief historical account, the operating principles, characterization, components, technology, and applications are reviewed. Finally, the performance of LSCs are critically discussed in a global perspective with particular emphasis on the basic contradiction between light concentration and conversion efficiency leading to some suggestions for future development of the topic.

A simple strategy to avoid the formation of polyaryl layer during the functionalization of carbon surface by diazonium electroreduction is presented. The approach proposes to directly act on the polymerization mechanism by the use of a radical scavenger. The kinetic gap between the surface coupling and the multilayer formation is exploited to prevent the growth of the layer without interfering with the grafting. The well-known 4-nitrobenzenediazonium electrografting was used to demonstrate the possibility of reaching a monolayer surface coverage with an excess of DPPH (2,2-diphenyl-1-picrylhydrazyl). Experimental conditions were varied to validate the efficiency of the grafting limitation and the radical capture was confirmed by isolation of the aryl radical/DPPH coupling product.