Intramolecular Exchange Research Articles

Electrode Design to Overcome Substrate Transparency Limitations for Highly Efficient 1 cm2 Mesoscopic Perovskite Solar Cells

Summary Fluorine-doped tin oxide glass substrate is typically used for state-of-the-art perovskite solar cells (PSCs). However, indium-doped tin oxide (ITO) is better due to higher transparency for a given conductivity, although it has lower tolerance to high-temperature processes required for the compact and mesoporous TiO2 layers. Here we overcome this challenge by developing and utilizing a new electrode design. We successfully demonstrate high-efficiency mesoscopic PSCs on annealed ITO substrates showing improved photocurrent without sacrificing fill factor. After further optimizations of cell geometry and substrate conductivity guided by simulation, a certified 19.63% efficiency is achieved on 1 cm2 for ITO-based mesoscopic PSC, which is the highest among PSCs prepared by gas quenching. This work is useful for providing design principles and methods for optimizing cell geometry, metal electrode design, and substrate conductivity requirements for large-area PSCs.

Flatbands and Emergent Ferromagnetic Ordering in Fe_{3}Sn_{2} Kagome Lattices.

A flatband representing a highly degenerate and dispersionless manifold state of electrons may offer unique opportunities for the emergence of exotic quantum phases. To date, definitive experimental demonstrations of flatbands remain to be accomplished in realistic materials. Here, we present the first experimental observation of a striking flatband near the Fermi level in the layered Fe_{3}Sn_{2} crystal consisting of two Fe kagome lattices separated by a Sn spacing layer. The band flatness is attributed to the local destructive interferences of Bloch wave functions within the kagome lattices, as confirmed through theoretical calculations and modelings. We also establish high-temperature ferromagnetic ordering in the system and interpret the observed collective phenomenon as a consequence of the synergetic effect of electron correlation and the peculiar lattice geometry. Specifically, local spin moments formed by intramolecular exchange interaction are ferromagnetically coupled through a unique network of the hexagonal units in the kagome lattice. Our findings have important implications to exploit emergent flat-band physics in special lattice geometries.

A Combined Theoretical and Experimental Study of CH3NH3PbI3 Containing AVAI Films Prepared via an Intramolecular Exchange Process

Methylammonium lead iodide (CH3NH3PbI3) has emerged as a superior light-absorbing material for perovskite solar cells, mainly because of its excellent optical and electronic properties. However, insufficient long-term stability of CH3NH3PbI3 has severely hindered its practical applications. The direct addition of 5-ammonium valeric acid iodide (HOOC(CH2)4NH3I or AVAI) to perovskite precursors, such as CH3NH3I and PbI2, was found to improve the stability of CH3NH3PbI3, but can limit the incorporation of AVA+ into the CH3NH3PbI3 structure because of strong competitive interactions between AVA+ and CH3NH3+ with PbI2 through the same electrostatic interactions. Here, we present an alternative approach to prepare high-quality CH3NH3PbI3 containing AVAI films via an intramolecular exchange process. The films were prepared from precursor complexes, that is, organic lead iodide complexes, by the intramolecular exchange of dimethyl sulfoxide intercalated in the complexes with CH3NH3+. Scanning electron microscopy,...

Effect of Magnetic Coupling on Water Proton Relaxivity in a Series of Transition Metal GdIII Complexes.

A fundamental challenge in the design of bioresponsive (or bioactivated) GdIII-based magnetic resonance (MR) imaging probes is the considerable background signal present in the "preactivated" state that arises from outer-sphere relaxation processes. When sufficient concentrations of a bioresponsive agent are present (i.e., a detectable signal in the image), the inner- and outer-sphere contributions to r1 may be misinterpreted to conclude that the agent has been activated, when it has not. Of the several parameters that determine the observed MR signal of an agent, only the electron relaxation time ( T1e) impacts both the inner- and outer-sphere relaxation. Therefore, strategies to minimize this background signal must be developed to create a near zero-background (or truly "off" state) of the agent. Here, we demonstrate that intramolecular magnetic exchange coupling when GdIII is coupled to a paramagnetic transition metal provides a means to overcome the contribution of second- and outer-sphere contributions to the observed relaxivity. We have prepared a series of complexes with the general formula LMLn(μ-O2CCH3)(O2CCH3)2 (M = Co, Cu, Zn). Solid-state magnetic susceptibility measurements reveal significant magnetic coupling between GdIII and the transition metal ion. Nuclear magnetic relaxation dispersion (NMRD) analysis confirms that the observed differences in relaxivity are associated with the modulation of T1e at GdIII. These results clearly demonstrate that magnetic exchange coupling between GdIII and a transition metal ion can provide a significant decrease in T1e (and therefore the relaxivity of GdIII). This design strategy is being exploited to prepare new generations of preclinical bioresponsive MR imaging probes with near zero-background.

UV and IR Photochemistries of Malonaldehyde Trapped in Cryogenic Matrices.

UV and IR photochemistries of malonaldehyde, the simplest molecule exhibiting an intramolecular proton exchange, have been studied in four cryogenic matrices at 4.3 K, N2, Ne, Ar, and Xe. Samples have been irradiated using a UV and IR OPO type tunable laser, and with a broad band UV mercury lamp. UV and IR spectra have been recorded and compared with theoretical calculations carried out at the SAC-CI/6-31++G(d,p) (UV transitions) and B3LYP/6-311++G(2d,2p) (IR spectra) levels of theory. After deposition, the intramolecularly H-bonded form is found exclusively, while several open forms are formed upon UV irradiation. These open forms show ability to interconvert upon UV irradiation too. Some of them are also able to isomerize upon selective IR irradiations. The whole set of results allowed us to identify seven isomers among the eight postulated. The photodynamics of the electronic relaxation of malonaldehyde have also been investigated. By following the decay or rise of suited specific vibrational bands in the IR spectra, and by comparing the results with an earlier study of the homologous acetylacetone, we deduced that the electronic relaxation of malonaldehyde proceeds through singlet states, most probably through a 3-fold conical intersection, as postulated from theoretical calculations. In contrast with acetylacetone, malonaldehyde does not show fragmentation after UV excitation.

Syntheses, spectroscopy, and crystal structures of 3-(4-bromo-phen-yl)-1,5-di-phenyl-formazan and the 3-(4-bromo-phen-yl)-1,5-di-phenyl-verdazyl radical and the crystal structure of the by-product 5-anilino-3-(4-bromo-phen-yl)-1-phenyl-1H-1,2,4-triazole.

The title compounds, C19H15BrN4, C20H16BrN4 and C20H15BrN4, are nitro-gen-rich organic compounds that are related by their synthesis. The verdazyl radical, in which stacking leads to anti-ferromagnetic inter-actions, was reported previously [Iwase et al. (2013 ▸). Phys. Rev. B, 88, 184431]. For this compound, improved structural data and spectroscopic data are presented. The other two compounds have been crystallized for the first time and form stacks of dimers, roughly along the a-axis direction of the crystal. The formazan mol-ecule shows signs of rapid intra-molecular H-atom exchange typical for this class of compounds and spectroscopic data are provided in addition to the crystal structure. The triazole compound appears to be a side-product of the verdazyl synthesis.

Evaporation-Induced Self-Assembly of Semi-Crystalline PbI2(DMSO) Complex Films as a Facile Route to Reproducible and Efficient Planar p-i-n Perovskite Solar Cells

ABSTRACTHigh quality active layers for hybrid organic-inorganic perovskite solar cells are essential for achieving maximum device performance. However, perovskite active layers in solar cells are frequently prepared with unoptimized processes that lead to layers of inferior quality. This is often the case when research focuses on other aspects of the solar cell device, such as device design and architecture, carrier transport layers, electrodes, interlayers, etc. In this study, a single-step spin-coating method was used to prepare semi-crystalline PbI2(DMSO) complex films via evaporation-induced self-assembly. These optimized intermediate films were then used to form homogeneous methylammonium lead iodide (MAPbI3) perovskite films of optimum thickness (ca. 400 nm) with uniform surface coverage, good crystallinity, high purity, and grain sizes up to one micron, by employing a sequential deposition process involving intramolecular exchange between the PbI2(DMSO) complex film and a methylammonium iodide (MAI) layer deposited on top of it. We found that for certain ranges of MAI concentration, the formation of optimal-quality perovskite active layers was independent of MAI concentration, so long as MAI deposition occurred at specific corresponding spin speeds. Planar p-i-n perovskite solar cells comprising the optimized active layers were fabricated, and they exhibited negligible hysteresis and a maximum power conversion efficiency (PCE) of 16.72%, without any additional compositional and interfacial engineering. The latter can be used in the future to further enhance the PCE. These findings demonstrate the importance of an optimized perovskite active layer for reproducibly fabricating high-efficiency planar p-i-n photovoltaic devices. Additionally, the simplicity of the PbI2(DMSO) complex film preparation and the versatility of the MAI deposition with this fabrication method further enhances the potential of this material for large-scale processing.

Coupling of charge, lattice, orbital, and spin degrees of freedom in charge density waves in 1T−TaS2

Two-dimensional layered transition-metal-dichalcogenide (TMDC) materials often exhibit exotic quantum phases due to the delicate coupling and competitions of charge, lattice, orbital, and spin degrees of freedom. Surprisingly, we here present, based on first-principles density-functional theory calculations, the incorporation of all such degrees of freedom in a charge density wave (CDW) of monolayer (ML) TMDC $1T\text{\ensuremath{-}}{\mathrm{TaS}}_{2}$. We reveal that this CDW accompanying the lattice distortion to the ``David-star'' (DS) superstructure constituted of one cental, six nearest-neighbor, and six next-nearest-neighbor Ta atoms is driven by the formation of quasimolecular orbitals due to a strong hybridization of Ta ${t}_{2\mathrm{g}}$ orbitals. The resulting weakly overlapped nonbonding orbitals between the DS clusters form a narrow half-filled band at the middle of the CDW gap, leading to the Stoner-type magnetic instability caused by an intramolecular exchange interaction. It is thus demonstrated that the Stoner parameter $I$ corresponding to the effective on-site Coulomb interaction $U$ opens a Mott gap. Our finding of the intricate charge-lattice-orbital-spin coupling in ML $1T\text{\ensuremath{-}}{\mathrm{TaS}}_{2}$ provides a framework for the exploration of various CDW phases observed in few-layer or bulk $1T\text{\ensuremath{-}}{\mathrm{TaS}}_{2}$.

Hydrogen's isotopic exchange reaction in the C‐methyl sides in the medicinal agent xymedon: NMR spectroscopy and ab initio calculations

AbstractThe kinetics of intramolecular and intermolecular exchange processes in xymedon (1‐(2‐hydroxyethyl)‐4,6‐dimethyl‐1,2‐dihydropyrimidin‐2‐one, a regeneratory, wound‐healing drug) and its analogue were investigated in the solution. Hydrogen's mobility was detected in the C‐methyl sides of these compounds. This mobility was monitored via NMR in the hydrogen/deuterium exchange reaction in water. Two models were proposed as explanations for this hydrogen‐deuterium exchange. According to the main model, the key intermediates of these reactions are low‐energy tautomers of xymedon in which the N3 is protonated following which one proton leaves either 6‐Me or 4‐Me and thus its hybridization is changed. This hydrogen‐to‐deuterium exchange reaction is much faster under acidic conditions although it also occurs in alkaline conditions.Methylation via MeOTs or MeI leads to products with a quaternized ring N3 atom in which a hydrogen‐to‐deuterium exchange reaction also takes place, although the rates of the 6‐Me and 4‐Me hydrogens exchange are reversed.According to density functional theory calculations, the presence of methyl groups at the C4/C6 positions and of the C═O fragment is crucial to remarkably lower the energies of these “rare” tautomers. The exact position of the C═O in heterocycle is also very important in the tautomers' relative stability.

Exploring the intramolecular catalysis of the proton exchange process to modulate the relaxivity of Gd(iii)-complexes of HP-DO3A-like ligands.

The Gd(iii)-complexes of three novel HP-DO3A-like ligands have been investigated to assess the relationship between relaxometry and intramolecular catalysis of the proton exchange. The structures of these ligands differ from the parent HP-DO3A because the methyl group of the hydroxy-propyl arm has been replaced by -Ph-OH, -Ph-NH2 and -Ph-COOH, respectively. The phenol, amine and carboxylate functionalities display an intramolecular H-bonding with the coordinated hydroxyl moiety that affects either the pK values of the involved functionalities and the rate of the proton exchange process.

How does chiral self-sorting take place in the formation of homochiral Pd6L8 capsules consisting of cyclotriveratrylene-based chiral tritopic ligands?

The chiral self-sorting process during the self-assembly of homochiral Pd6L8 capsules from cyclotriveratrylene (CTV)-based chiral tritopic ligands (L) and (Py*: 3-chloropyridine) was investigated by an NMR-based approach (QASAP: quantitative analysis of the self-assembly process). From the beginning to the formation of the immature capsules (ICs), enantiomeric ligands are distributed in the intermediates in a non-self-sorting manner, which leads to the isomers of heterochiral ICs over 99% yield. The mismatch of the chirality in the heterochiral ICs prevents intramolecular ligand exchanges in ICs to form the heterochiral capsules. The correction of the chirality in the heterochiral ICs (chiral self-sorting) takes place very slowly to finally lead to the homochiral capsules. The reason why the chiral self-sorting took place in the late stage of the self-assembly (after the formation of the heterochiral ICs) would be due to the relatively high flexibility of the CTV-based ligand.

Magnetic and transport properties of conjugated and cumulated molecules: the π-system enlightens part of the story

We have investigated the intramolecular magnetic exchange coupling constants (J) for a series of nitronyl nitroxide diradicals connected by a range of linear conjugated and cumulene couplers focusing on the unusual π-interaction properties within the couplers. Distance between radical centers, spin density within the couplers, as well as the dihedral angles between the radical centers and the plane of the coupler influence the strength of magnetic coupling. We also establish that with the increase in the length of the coupler, the strength of magnetic interaction in conjugated and cumulated systems varies in a different way. Transport calculations show that with the increase in chain length, diradicals based on cumulene containing an even number of carbon atoms act as better conductors than cumulenes with an odd number of carbon atoms. It is also observed that with the increase in the length of the conjugated coupler based diradicals, transmission does not vary in a sequential way.

Realizing Efficient Lead-Free Formamidinium Tin Triiodide Perovskite Solar Cells via a Sequential Deposition Route.

Recently, the evolved intermediate phase based on iodoplumbate anions that mediates perovskite crystallization has been embodied as the Lewis acid-base adduct formed by metal halides (serve as Lewis acid) and polar aprotic solvents (serve as Lewis base). Based on this principle, it is proposed to constitute efficient Lewis acid-base adduct in the SnI2 deposition step to modulate its volume expansion and fast reaction with methylammonium iodide (MAI)/formamidinium iodide (FAI) (FAI is studied hereafter). Herein, trimethylamine (TMA) is employed as the additional Lewis base in the tin halide solution to form SnY2 -TMA complexes (Y = I- , F- ) in the first-step deposition, followed by intercalating with FAI to convert into FASnI. It is shown that TMA can facilitate homogeneous film formation of a SnI2 (+SnF2 ) layer by effectively forming intermediate SnY2 -TMA complexes. Meanwhile, its relatively larger size and weaker affinity with SnI2 than FA+ ions will facilitate the intramolecular exchange with FA+ ions, thereby enabling the formation of dense and compact FASnI3 film with large crystalline domain (>1 µm). As a result, high power conversion efficiencies of 4.34% and 7.09% with decent stability are successfully accomplished in both conventional and inverted perovskite solar cells, respectively.

Effect of core substituents on the intramolecular exchange interaction in N,N′‐dioxy‐2,6‐diazaadamantane biradical: DFT studies

AbstractDensity‐functional theory calculations of a series of organic biradicals on the basis of the N,N′‐dioxy‐2,6‐diazaadamantane core with different substituents at carbon atoms adjacent to the nitroxyl groups have been performed by the UB3LYP/6‐311++G(2d,2p) method. Using the breaking symmetry approach, the values of the exchange interaction parameter, J, between the radical centers are calculated. It is shown that the intramolecular exchange interaction for the most part is ferromagnetic in nature, but the J parameter gradually decreases, changing its sign to antiferromagnetic interaction for the last substituent in the following sequence: CF3(CH3)COH > CH2F(H)COH > CH2OH > H > CBr3 > CH2F > CCl3 > CF3 > CH2Br > CH2Cl > CH3 > C2H5 > C3H7 > i‐C4H9 > F > Br > OCH3 > Cl > CH2C6H5. The calculations at the UHSEH1PBE/6‐311++G(2d,2p) level with the most of substituents show nearly the same variation sequence for the J parameter. It is concluded that spin polarization effects in the diazaadamantane cage and a direct through‐space antiferromagnetic exchange interaction between the nitroxyl groups are the main mechanisms contributing to the exchange interaction parameter value in the studied series of compounds. The exchange coupling constant, J, depends on the electronic effects and geometry of the substituents, as well as on their specific interactions with the nitroxyl radical groups.

Quantitative Analysis of Self-Assembly Process of a Pd2 L4 Cage Consisting of Rigid Ditopic Ligands.

The self-assembly process of a Pd2 L4 cage complex consisting of rigid ditopic ligands, in which two 3-pyridyl groups are connected to a benzene ring through acetylene bonds and PdII ions was revealed by a recently developed quantitative analysis of self-assembly process (QASAP), with which the self-assembly process of coordination assemblies can be investigated by monitoring the evolution with time of the average composition of all the intermediates. QASAP revealed that the rate-determining steps of the cage formation are the intramolecular ligand exchanges in the final stage of the self-assembly: [Pd2 L4 Py*2 ]4+ →[Pd2 L4 Py*1 ]4+ +Py* and [Pd2 L4 Py*1 ]4+ →[Pd2 L4 ]4+ +Py* (Py*: 3-chloropyridine, which was used as a leaving ligand on the metal source). The energy barriers for the two reactions were determined to be 22.3 and 21.9 kcal mol-1 , respectively. DFT calculations of the transition-state (TS) structures for the two steps indicated that the distortion of the trigonal-bipyramidal PdII center at the TS geometries increases the activation free energy of the two steps.

Synthesis and Magnetic Properties of Trioxytriphenylamine Dimers in their Di(radical cationic) States.

Three structural isomers of trioxytriphenylamine (TOT) dimers, 4,4'''-bis(2,2':6',2":6",6-trioxytriphenylamine) (4), 3,3'''-bis(2,2':6',2":6",6-trioxytriphenylamine) (5), and 3,4'''-bis(2,2':6',2":6",6-trioxytriphenylamine) (6), have been prepared and their electronic and magnetic properties in their di(radical cationic) states have been investigated. These di(radical cationic) species can be handled under ambient conditions because of their high stability under aerated conditions even in solution. The X-ray crystal structure analysis demonstrated that the TOT moieties of all the di(radical cation)s have planar structures similar to that of the parent TOT radical cation 3+ . The UV/Vis spectra of the di(radical cation)s show characteristic absorptions depending on the connecting pattern. Thus, in the long-wavelength region (600-900 nm), 42+ exhibits strong and broad characteristic absorptions, whereas compounds 52+ and 62+ exhibit weak absorptions. Notably, in the 450-600 nm region, 52+ displays very similar absorptions (with twice the intensity) to 3+ , whereas small differences were observed for 62+ . Finally, we investigated in detail the magnetic properties of the corresponding di(radical cation)s by electron spin resonance spectroscopy and magnetic susceptibility measurements, which indicated intramolecular exchange interactions with a singlet ground state and a large singlet-triplet (S-T) gap for 42+ , a singlet ground state and a small S-T gap for 52+ , and a triplet ground state for 62+ .

Time-Resolved Resonance Raman Spectroscopy and Application to Studies on Ultrafast Protein Dynamics

Abstract Protein dynamics play a fundamental role in allosteric regulation, which is vital to the function of many proteins. In many proteins, rather than a direct interaction, mutual modulation of properties such as ligand affinity at spatially separated sites is achieved through a conformational change. Conformational changes of proteins are thermally activated processes that involve intramolecular and intermolecular energy exchanges. In this account, I review the work of my team on the development and applications of ultrafast time-resolved resonance Raman spectroscopy to observe functionally important protein dynamics. We gained insights into conformational dynamics upon external stimulus and energy flow with a spatial resolution of a single amino acid residue using time-resolved visible and ultraviolet resonance Raman spectroscopy. The results have contributed to a deeper understanding of the structural nature of protein motion and the relationship of dynamics to function. I discuss the protein dynamics and allosteric mechanism in terms of the nature of the high packing density of protein structures. In addition, I present a view of the future of molecular science on proteins.

Synthesis of Intrinsically Blue-Colored bis-Nitronyl Nitroxide Peptidomimetic Templates and Their Conformational Preferences as Revealed by a Combined Spectroscopic Analysis.

The intrinsically blue-colored Ullman imidazolinyl nitronyl nitroxide (NN) mono-radicals have found various applications, in particular as spin probes and organic magnetic materials. Here, we present the solution-phase synthesis, extensive characterization, and conformational analysis of the first peptidomimetics with two pendant, chiral nitronyl nitroxide free radical units. Two (R)-Aic(NN) residues, where Aic(NN) is 2-amino-5-nitronylnitroxide-indan-2-carboxylic acid, have been inserted at positions i and i+3 of the pentapeptide Boc-(R)-Aic(NN)-(Ala)2-(R)-Aic(NN)-Ala-OMe and the hexapeptide Boc-[Ala-(R)-Aic(NN)-Ala]2-OMe as well. The two compounds were obtained in good yields and high purities. Thanks to a combination of several spectroscopic techniques (IR absorption, NMR, VCD, and EPR) we gained clear evidence that both compounds adopt a right-handed 310-helical conformation with both nitronyl nitroxide pendants positioned on the same side of the helix. This peptidomimetic/free radical system is a potentially excellent template for the preparation of a set of appropriate analogs with exciting applications in the area of host-guest organic chemistry, or to spectroscopically evaluate in-depth the intramolecular exchange interactions in this type of probe.

Enhancing the crystallinity of HC(NH2)2PbI3 film by incorporating methylammonium halide intermediate for efficient and stable perovskite solar cells

For organic-inorganic hybrid perovskite solar cells (PerSCs), the crystallinity and the grain size of the perovskite films are crucial for effective exciton dissociation and charge transportation. Herein, we introduce methylammonium halide (MABr and MACl) intermediates to control the perovskite formation processes via intramolecular exchange. X-ray diffraction (XRD), scanning electron microscopy (SEM), and X-ray Photoelectron Spectroscopy (XPS) are used to monitor the transition from intermediate to perovskite. It is particularly noteworthy that as Cl- and MA+ are removed from the film as MACl vapor, the intermediate derived from Cl-Hybrid eventually turns into FAPbI3 perovskite after thermal annealing. The morphology, crystallinity, fluorescence lifetime and the stability of perovskite films derived from different methylammonium halide intermediates are also investigated, and PerSCs based on different perovskite films are fabricated. The PerSCs derived from MACl intermediate exhibit the best photovoltaic performance with a PCE of 19.16%, and the PCE reaches 15.87% for MABr intermediated PerSCs, much higher than that (13.72%) of the control device without methylammonium halide intermediates. Furthermore, MACl intermediated PerSCs demonstrate the best stability due to the highest crystallinity. Our findings indicate that methylammonium halide intermediating is a feasible and effective approach to acquire high crystallinity of perovskite film for high performance and stable PerSCs.

Mixed Phenyl and Thiophene Oligomers for Bridging Nitronyl Nitroxides.

The synthesis of four nitronyl nitroxide (NN) biradicals is described which are conjugatively linked through p-ter-phenyl (PPP), ter-thiophene (TTT) and alternating phenylene (P) and thiophene (T) units as PTP and TPT. We first utilized Suzuki and Stille coupling reactions through protection and deprotection protocols to synthesize these (NN) biradicals. Single crystals were efficiently grown for radical precursors of 3, 5, 6, PPP-NNSi, PTP-NNSi, and final biradicals of TTT-NN, TPT-NN, and PPP-NN, whose structures and molecular packing were examined by X-ray diffraction studies. As a result, much smaller torsions between the NN and thiophene units (∼10°) in TTT-NN and TPT-NN than for NN and phenyl units (∼29°) in PPP-NN were observed due to smaller hindrance for a five vs a six membered ring. All four biradicals TTT-NN, TPT-NN, PTP-NN, and PPP-NN were investigated by EPR and optical spectroscopy combined with DFT calculations. The magnetic susceptibility was studied by SQUID measurements for TTT-NN and TPT-NN. The intramolecular exchange interactions for TPT-NN and TTT-NN were found in good agreement with the ones calculated by broken symmetry DFT calculations.