Bridging Unit Research Articles

Dopant-Free Pyrene-Based Hole Transporting Material Enables Efficient and Stable Perovskite Solar Cells.

Dopant-free hole transporting materials (HTMs) is significant to the stability of perovskite solar cells (PSCs). Here, we developed a novel star-shape arylamine HTM, termed Py-DB, with a pyrene core and carbon-carbon double bonds as the bridge units. Compared to the reference HTM (termed Py-C), the extension of the planar conjugation backbone endows Py-DB with typical intermolecular π-π stacking interactions and excellent solubility, resulting in improved hole mobility and film morphology. In addition, the lower HOMO energy level of the Py-DB HTM provides efficient hole extraction with reduced energy loss at the perovskite/HTM interface. Consequently, an impressive power conversion efficiency (PCE) of 24.33 % was achieved for dopant-free Py-DB-based PSCs, which is the highest PCE for dopant-free small molecular HTMs in n-i-p configured PSCs. The dopant-free Py-DB-based device also exhibits improved long-term stability, retaining over 90 % of its initial efficiency after 1000 h exposure to 25 % humidity at 60 °C. These findings provide valuable insights and approaches for the further development of dopant-free HTMs for efficient and reliable PSCs.

Dopant‐Free Pyrene‐Based Hole Transporting Material Enables Efficient and Stable Perovskite Solar Cells

AbstractDopant‐free hole transporting materials (HTMs) is significant to the stability of perovskite solar cells (PSCs). Here, we developed a novel star‐shape arylamine HTM, termed Py‐DB, with a pyrene core and carbon‐carbon double bonds as the bridge units. Compared to the reference HTM (termed Py‐C), the extension of the planar conjugation backbone endows Py‐DB with typical intermolecular π–π stacking interactions and excellent solubility, resulting in improved hole mobility and film morphology. In addition, the lower HOMO energy level of the Py‐DB HTM provides efficient hole extraction with reduced energy loss at the perovskite/HTM interface. Consequently, an impressive power conversion efficiency (PCE) of 24.33 % was achieved for dopant‐free Py‐DB‐based PSCs, which is the highest PCE for dopant‐free small molecular HTMs in n‐i‐p configured PSCs. The dopant‐free Py‐DB‐based device also exhibits improved long‐term stability, retaining over 90 % of its initial efficiency after 1000 h exposure to 25 % humidity at 60 °C. These findings provide valuable insights and approaches for the further development of dopant‐free HTMs for efficient and reliable PSCs.

Metal-Ligand Exchange Coupling Alters the Open-Shell Ligand Electronic Structure in a Bis(semiquinone) Complex.

The electronic structure of the bis(dioxolene) bridging ligand -SQ2Th2- is responsive to metal-ligand magnetic exchange coupling. Comparison of the crystal structure of (NiSQ)2Th2 to that of (ZnSQ)2Th2 indicates an open-shell biradical ground state for the dinuclear Ni(II) complex compared to the closed-shell quinoidal character found in the dinuclear Zn(II) complex. Consistent with a comparison of bond lengths obtained by X-ray diffraction, the analysis of the variable-temperature magnetic susceptibility data for crystalline (NiSQ)2Th2 yields reduced SQ-SQ radical-radical magnetic exchange coupling (JSQ-SQ = -203 cm-1) compared to that of (ZnSQ)2Th2 (JSQ-SQ = -321 cm-1). The reduced SQ-SQ exchange coupling in (NiSQ)2Th2 derives from an attenuation of the SQ spin densities, which in turn is derived from the Ni-SQ antiferromagnetic exchange interactions. This reduction in SQ--SQ exchange that we observe for (NiSQ)2Th2 correlates with an effective lengthening of the bridge unit by ∼2.1 Å relative to that of (ZnSQ)2Th2. This magnitude of the effective increase in the bridge distance is consistent with the (NiSQ)2Th2 JSQ-SQ value lying between those of (ZnSQ)2Th2 and (ZnSQ)2Th3. The ability to modulate spin populations on an organic radical via pairwise Ni-SQ magnetic exchange interactions is a general way to affect electronic coupling in the Th-Th bridge. Our results suggest that metal-radical exchange coupling represents a powerful mechanism for tuning organic molecular electronic structure, with important implications for molecular electronics and molecular electron transport.

1]Ferrocenophane Bridged by a 9-Silafluorenylidene Moiety

Sila[1]ferrocenophane bearing a 9-silafluorenylidene moiety (1) as a bridging unit was synthesized and isolated as a stable crystalline compound. Sila[1]ferrocenophane 1, which was newly obtained in this study, was characterized by spectroscopic analyses, a single-crystal X-ray diffraction (SC-XRD) analysis, and electrochemical measurements. Due to the characteristic 9-silafluorenyl moiety, 1 exhibited large electron affinity and a slightly higher oxidation potential relative to that of ferrocene. In addition, 1 was found to undergo ring-opening polymerization (ROP) triggered by thermolysis at a lower temperature relative to that of Ph2Sifc (1′, fc = 1,1′-ferrocenylidene). It also underwent ROP through reduction by KC8 to give the corresponding polymeric compound. The DFT calculations suggested that one-electron reduction of 1 would promote ring-opening polymerization, as shown in the experimental results.

Computational evaluation of zirconocene catalysts for ε-caprolactone cationic ring-opening polymerization

This quantum chemical study presents the ligand effect and a structure–property relationship in the cationic ring-opening polymerization (CROP) of ε-caprolactone using zirconocene catalysts. We first examined the effects of catalyst structure on the initiation and chain propagation steps of the CROP process. A total of 54 catalyst structures were investigated to understand the influence of the ligand structure on the stability of the catalyst–monomer complex and polymerization activity. The properties of the catalysts were analyzed in terms of ancillary ligands, ligand substituents, and bridging units. Calculations showed that the polymerization follows a proposed cationic mechanism, with ring opening occurring via alkyl-bond cleavage. A correlation between complex stability and activation energy was also observed, with ligand substituents dominating in both steps. While the ancillary ligands directly affect the HOMO energy level, the bridges are mainly responsible for the catalyst geometries, resulting in reduced complex stability and higher activation energy for the propagation step. This study contributes to a better understanding of the structural characteristics of zirconocene catalysts, which offers guidance for improving CROP activities in lactone polymerization.

Amino-containing donor-acceptor biaryls with lactone bridging units as three-state halochromic pi-expanded coumarins

This report describes the synthesis and study of two new amino-containing donor-acceptor lactone-bridged biaryls as pi-expanded, or benzannulated, coumarins based on the 6H-benzo[c]chromen-6-one (benzo[c]coumarin) structural unit. These fluorescent and planar compounds exhibit strong intramolecular charge transfer (ICT) through the conjugated biaryl unit via a diethylamino donor on one arene and either a nitro or cyano acceptor unit on the opposing arene. The ICT of these systems can be reversibly altered via two different stimuli: 1) addition of a strong base (hydroxide) results in nucleophilic cleavage of the lactone bridge which dramatically weakens ICT by disrupting conjugation between the individual arenes and 2) addition of a strong acid protonates the diethylamino donor unit, disrupting its ability to donate, thereby causing ICT to be attenuated. The presence of strong acid or base results in dramatic changes in optical absorption and emission properties of these systems which can be readily observed both spectroscopically and visually. Ultimately compounds containing the benzo[c]coumarin structural unit with amine donors could act as three-state halochromic molecular switches and dyes.

Bisazathienoisoindigo-Based Conjugated Polymers: Synthesis, Characterization, and the Influence of Inner Bridging Units on the Quinoidal Character and Charge Transport Property

Bisazathienoisoindigo-Based Conjugated Polymers: Synthesis, Characterization, and the Influence of Inner Bridging Units on the Quinoidal Character and Charge Transport Property

High-performance transmissive-to-black electrochromism derived from diphenylamine-based polyimides with tetraphenylethylene as bridging units

Transmissive-to-black switching electrochromic (EC) polymers have attracted considerable attention toward the development of low-cost, low-energy-demand passive displays.

PPX/PXP-type ligands (X = O and S) and their transition metal complexes: synthesis, properties and applications.

Short-bite diphosphines of the form R2P-X-PR2 (PXP; X = O, S; R = aryl, alkyl), incorporating an oxygen or sulphur atom as bridging unit X, are widely underexplored compared to their N- and C-containing PNP- and PCP-type counterparts. However, these PXP ligands undergo an interesting phosphorotropic equilibrium with the PPX (R2P(X)-PR2) tautomer, which opens up a very versatile coordination chemistry. This article covers the impact of the ligand backbone in short-bite ligands on their coordination chemistry, reactivity and applications. Especially in PXP-type complexes, metallophillic interactions can be induced in the case of coinage metals, which lead to fascinating photo-optical properties. Furthermore, PPX/PXP-type complexes are believed to exhibit a promising behavior in catalysis, due to the potential hemilability of the ligand and the therewith involved availability of free active sites for substrate binding.

Taming 2,2'-biimidazole ligands in trivalent chromium complexes.

Complete or partial replacement of well-known five-membered chelating 2,2'-bipyridine (bipy) or 1,10-phenanthroline (phen) ligands with analogous didentate 2,2'-biimidazole (H2biim) provides novel perspectives for exploiting the latter pH-tuneable bridging unit for connecting inert trivalent chromium with cationic partners. The most simple homoleptic complex [Cr(H2biim)3]3+ and its stepwise deprotonated analogues are only poorly soluble in most solvents and their characterization is limited to some solid-state structures, in which the pseudo-octahedral [CrN6] units are found to be intermolecularly connected via peripheral N-H⋯X hydrogen bonds. Moreover, the associated high-energy stretching N-H vibrations drastically quench the targeted near infrared (NIR) CrIII-based phosphorescence, which makes these homoleptic building blocks incompatible with the design of molecular-based luminescent assemblies. Restricting the number of bound 2,2'-biimidazole ligands to a single unit in the challenging heteroleptic [Cr(phen)2(Hxbiim)](1+x)+ (x = 2-0) complexes overcomes the latter limitations and allows (i) the synthesis and characterization of these [CrN6] chromophores in the solid state and in solution, (ii) the stepwise and controlled deprotonation of the bound 2,2'-biimidazole ligand and (iii) the implementation of Cr-centered phosphorescence with energies, lifetimes and quantum yields adapted for using the latter chromophores as sensitizers in promising 'complex-as-ligand' strategies.

Adducts of Lewis acidic stibanes with phosphane chalcogenides.

Starting from ClSbRF2 (RF = C2F5, 3,5-(CF3)2C6H3) and H(E)P(tBu)2 (E = O, S), we prepared the oxy- and sulphanediyl-bridged adducts RF2Sb(Cl)-E-(H)P(tBu)2, which are stable against the elimination of HCl. The different electron-withdrawing substituents and chalcogen bridging units influence the size of the Sb-E-P angle. ClSb(C2F5)2 and nBu3SnH react to give HSb(C2F5)2, which seems to interact weakly with H(O)P(tBu)2 in solution as observed via NMR. All products were characterised by NMR spectroscopy and all the stable ones were additionally characterised by X-ray diffraction and elemental analyses.

Supramolecular self-sorting predicted by a simple harmonic oscillator model.

Macrocycles that encapsulate two guests can self-sort those into homo- and heterodimers. We report here a family of self-sorting homobimetallic Pt(II) terpyridyl acetylide dimers secured together with a pair of Cucurbit[8]uril macrocycles (CB[8]). The rigid bridging unit between both Pt centers introduces varying "hinge" angles, resulting in disparities in Pt-Pt distances in the heterodimers, and leads to recognition motif mismatch. We found that the self-sorting process can be quantified using a simple model, in which each complex behaves as a simple harmonic oscillator, whose heteroassembly tends to minimize geometry distorsions through C(aryl)-CC-Pt axis deformation.

Control of electronic and exchange coupling by bridge substituents in donor acceptor triads with triptycene bridges.

A series of triads, consisting of a triarylamine electron donor and a perylene diimide electron acceptor which were attached to two different wings of a triptycene bridging unit, was investigated concerning the dynamics of photoinduced charge separation and charge recombination processes with a particular focus on the involved spin-chemical aspects. Attaching electron-donating or electron-withdrawing substituents to the third wing of the triptycene bridge allowed tuning the electron transfer processes. These processes were investigated via fs-transient absorption spectroscopy and ns-transient absorption spectroscopy in an external magnetic field. The resulting magnetic field-dependent decay dynamics were analysed and modelled using the stochastic Liouville equation which yielded rate constants for the charge recombination and the exchange energy. In combination with a diabatic rate theory and Anderson's perturbative treatment of the exchange energy, these data gave a complete set of rate constants for charge separation and charge recombination from which the diverse electronic couplings between the involved states were derived. These couplings depend linearly on the inverse energy of virtual triptycene bridge states which allows tuning the electron transfer dynamics by modifying the triptycene bridge.

Fine-tuning the color hue of the solution-processable electrochromic copolymers based on the diketopyrrolopyrrole, benzodithiophene and dithienosilole units

Electrochromic materials (ECMs) have attracted increasing attentions on account of their extensive prospects and numerous demands. Here, the classical donor-acceptor (D-A) alternative copolymerization strategy was used to synthesize three kinds of novel conjugated polymers PBPS-1, PBPS-2, and PBPS-3. The diketopyrrolopyrrole (M1) unit with a long alkyl side-chain was employed as the core acceptor while the dithienosilole (M2) used as the donor units. More importantly, the benzodithiophene (M3) units which have the high skeleton coplanarity was selected as the bridge units to enhance the charge transport. Variations of the feed ratios of monomers was used to regulate the energy level and spectral absorption range of the solid films. Three polymers exhibited the abundant color hues in the redox process, which including grape purple/brownness/aircraft ash color (PBPS-1), dark grey/red soap/dusty blue color (PBPS-2) and marine blue/grayish/celadon color (PBPS-3). Electrochromic performances demonstrated that PBPS-2 possessed the fast discoloration speed as low as 0.26 s and the outstanding coloration efficiency (550.1 cm2·C−1), which portended a valuable material for smart ECMs applications. Furthermore, the effect of different feed ratios for three polymer in electrochemical, optical and kinetic properties were investigated in detail, which showed the satisfactory optical contrasts (>70% in the NIR region) and good kinetic stability (>97% retention in 1000 s). In general, this exploration supplied a handy design strategy for fine tuning of polymer color and conducive to strengthen the understanding of EC behaviors of the ternary polymers, which deserve further researches to explore more promising ECMs.

Quantum modelling of multi-directional fused-ring electron acceptors for organic photovoltaics

Multi-directional fused-ring electron acceptors (FREAs) feature a fused-ring backbone that extends in multiple directions through bridging and acceptor units. This unique structural design imparts several advantageous properties, like increased light absorption, precise energy level adjustments, improved electron mobility, reduced energy loss, and enhanced efficiency in organic photovoltaic (OPV) devices. In this study, we designed seven novel multi-directional FREAs and conducted a comprehensive computational analysis using Density Functional Theory (DFT) and Time-Dependent DFT (TD-DFT). The designed molecules feature a benzotrithienopyrrole central core connected to a π-bridge acceptor, either benzothiadiazole or benzotriazole, and three varying terminal acceptors, including malononitrile, 2-(5,6-difluoro-3-oxo-2,3-dihydro-1H-inden-1-ylidene)malononitrile, and 2-(3-methyl-4-oxothiazolidin-2-ylidene)malononitrile. Our investigation encompassed the evaluation of geometry, frontier molecular orbitals (FMOs), optical properties, density of states (DOS), charge mobilities, dipole moment, global reactivity descriptors, molecular electrostatic potential (MEP), transition density matrix (TDM), and OPV device-related parameters. The results revealed significant potential of the designed molecules as FREAs for OPVs, thus highlighting their suitability for further experimental exploration.

Theoretical study of the effect of D-A type polymer donors on the performance of Y6-based organic solar cells

Non-fullerene organic solar cells (OSCs) based on Y6 molecule have developed rapidly in recent years, and the power conversion efficiency (PCE) varies greatly when the OSC device constructed by Y6 and donor molecules with small structural difference. In this work, the influence of six donor-acceptor (D-A) polymer donors on Y6-based OSCs were focused. The ground-state properties (planarity, frontier molecular orbital energy levels, driving force), excited-state properties of donors (primary indicators, Coulomb attraction) and donor/acceptor interfaces (Frenkel excitation (FE) and charge transfer (CT) states, electron coupling and charge separation rates) were discussed. The results show that symmetrical alkyl chains in the acceptor unit is favorable for maintaining planarity of molecule, while fluorination induces twisting in the thiophene bridge and acceptor unit. And the investigated high-performance molecules have desirable driving force, smaller Coulomb attraction and significant degree of holes and electrons delocalization. Also, the highest-performance system has multiple charge transfer pathways, which can improve charge separation efficiency. At the same time, high-performance systems often have better planarity and more favorable substituent positions. The high-performance donor/Y6 systems have more advantage in charge separation than the low-performance systems. This work provides a fundamental understanding of the impact of D-A type donor materials on the cell performance and theoretical guidance for further optimization of polymer donor materials in Y6-based OSCs.

Benzodithiophene‐Thienothiadiazole Copolymers as Dopant‐Free Hole‐Transporting Layers Enabling Efficient and Stable Perovskite Solar Cells

The exploration of dopant‐free hole‐transporting materials (HTMs) with excellent optoelectronic properties and defect passivation ability is of great significance for simultaneously improving the efficiency and stability of perovskite solar cells (PSCs). Herein, two donor–acceptor type conjugated polymers PTTDZ‐F and PTTDZ‐Cl with alternating benzodithiophene (BDT) and thienothiadiazole (TTD) units are successfully developed with desirable hole mobilities and conductivities. The formation of non‐covalent interaction (S···N) between thiophene bridge and TTD units significantly improves the co‐planarity of the molecular backbone and promotes the intramolecular charge transfer from BDT to TTD. The hydrophobicity and energy levels of the polymers are finely regulated via introducing different halogen atoms on the BDT moiety. Moreover, the functional groups in polymers efficiently passivate the surface charged traps in perovskite. As a result, champion power conversion efficiencies of 21.50% and 22.20% along with negligible hysteresis have been achieved for devices based on dopant‐free PTTDZ‐F and PTTDZ‐Cl, respectively. The unencapsulated devices also demonstrate excellent long‐term operational stability. Over 93% of their initial efficiencies can be retained after 320 h of maximum output power point tracking under 1 sun illumination. Herein, it paves a new avenue for developing highly stable and efficient HTMs for PSCs.

Intramolecular Electron Transfer in Multi-Redox Systems Based on Cyclic [3]Spirobifluorenylene Compound.

Cyclic [3]spirobifluorenylene with bulky alkyl groups at the ends (1) was designed and synthesized to investigate the electron transfer phenomena in a π-conjugated system including orthogonal π-conjugated chains. The three bifluorenyl units in 1 are conjugated to each other via spiro-conjugation, resulting in the splitting of the HOMO levels to a small extent. Therefore, the SOMO-HOMO gap of the radical cation species is small, which is considered to allow the facile intramolecular electron transfer. In the TD-DFT calculations for the radical cation species of 1, the geometry-featured interchain electronic transitions were visualized by NTO calculations. The radical cation species of 1 generated by chemical oxidation with SbCl5 exhibited a broadened and lower-energy shifted NIR absorption band compared to those of its partial structures. Considering the results of the TD-DFT calculations, the NIR band of the radical cation of 1 was attributed to the intramolecular electron transfer processes among the bifluorenyl units in the macrocycle. ESR experiments also indicated the delocalization of a spin of 1·+ in the whole molecule via hole hopping in the ESR time scale at room temperature. This work demonstrates the usefulness of spiro-conjugation as a bridging unit in molecular wires to facilitate smooth electron transfer.

Supramolecular Nucleic Acid-Based Organosilica Nanoparticles Responsive to Physical and Biological Inputs.

Organosilica nanoparticles that contain responsive organic building blocks as constitutive components of the silica network offer promising opportunities for the development of innovative drug formulations, biomolecule delivery, and diagnostic tools. However, the synthetic challenges required to introduce dynamic and multifunctional building blocks have hindered the realization of biomimicking nanoparticles. In this study, capitalizing on our previous research on responsive nucleic acid-based organosilica nanoparticles, we combine the supramolecular programmability of nucleic acid (NA) interactions with sol-gel chemistry. This approach allows us to create dynamic supramolecular bridging units of nucleic acids in a silica-based scaffold. Two peptide nucleic acid-based monoalkoxysilane derivatives, which self-assemble into a supramolecular bis-alkoxysilane through direct base pairing, were chosen as the noncovalent units inserted into the silica network. In addition, a bridging functional NA aptamer leads to the specific recognition of ATP molecules. In a one-step bottom-up approach, the resulting supramolecular building blocks can be used to prepare responsive organosilica nanoparticles. The supramolecular Watson-Crick-Franklin interactions of the organosilica nanoparticles result in a programmable response to external physical (i.e., temperature) and biological (i.e., DNA and ATP) inputs and thus pave the way for the rational design of multifunctional silica materials with application from drug delivery to theranostics.

Excitation-Wavelength-Dependent Charge Injection and Hole Localization in Diblock Oligomers Anchored to TiO2.

A series of diblock oligomers containing oligothiophene (Tn, n = 4, 5) and 4,7-di(thiophen-2-yl)benzo[c][1,2,5]thiadizole (TBT) segments, functionalized with carboxylic acid anchoring groups, were prepared and anchored to mesoporous TiO2 films to study wavelength-dependent interfacial electron transfer mechanisms. Thin films of the surface-anchored diblock oligomers contained two absorption bands centered at 400 and 500 nm, corresponding to the Tn and TBT blocks, respectively. Pulsed-laser excitation of the oligomer-sensitized films yielded local excited-states that promoted electron injection into TiO2. The injection pathway was dependent on the excitation wavelength, as electron injection occurred from the oligomer block that was locally excited. Recombination between the injected electron and the oxidized oligomer was sensitive to the bridging unit that separates the oligomer conjugated segments (-C≡C- vs trans-Pt(PBu3)2-). When the bridge facilitated strong coupling between the two blocks (-C≡C- bridge), the excitation wavelength had no effect on the recombination pathway, as the hole was delocalized over the entire oligomer. However, in the weak coupling case (Pt(PBu3)2- bridge), selective excitation resulted in wavelength-dependent hole localization that persisted to the μs time scale, providing control over the recombination pathway by varying the excitation wavelength. Dye-sensitized solar cells (DSSCs) were fabricated by using the diblock oligomers as sensitizers. The photocurrent action spectra were measured, and the absorbed photon-to-current efficiency (APCE) provided further insight into the electron-transfer mechanisms that are operative under continuous illumination.