Unexpected expectation values for latent molecular properties

Latent molecular properties are not exhibited by the given molecular structure but are reproducibly exhibited by the same molecule in a different electronic state or if some molecular interactions have taken place. A consequence of the Holographic Electron Density Theorem, as applied to latent properties, provides a framework that allows an extension of the expectation value formalism, leading to “unexpected” expectation value expressions for latent properties. Connections of special cases of this approach to earlier density matrix extrapolation methods are pointed out.     

Optical and electro-optical properties of bimesogenic organosiloxane antiferroelectric liquid crystals with molecular tilt approaching 45 degrees

We have studied the optical and electro-optical properties of three different bimesogenic siloxane materials. The dimers are symmetrical and the two mesogenic groups are connected by a siloxane unit containing three Si atoms and methylene spacers. The mesogens contain F, Cl and Br substituents, respectively, laterally attached to the phenyl ring lying closest to the chiral centre of the mesogen. These materials each exhibited a broad temperature range antiferroelectric phase and large molecular tilt, which is almost temperature-independent. We have shown that, at low temperatures, the molecular tilt in the antiferroelectric phase is the same as the apparent molecular tilt in the field-induced ferroelectric state and is close to 45°. It was found that the antiferroelectric phase of these compounds, aligned in such way that the dimeric molecules were lying in a plane parallel to the confining substrates, exhibited very low in-plane birefringence. For this reason, samples of the compounds inserted between crossed polarizers in the antiferroelectric state exhibited an extremely dark state, with contrast ratios as high as 1000:1, which did not change noticeably on rotating the sample between the crossed polarizers. It was shown that the optically isotropic siloxane moiety did not contribute to the optical properties, viz. birefringence or tilt angle, but served to enhance the promotion of the antiferroelectric phase and the temperature independent properties. We also showed the existence of a linear electro-optic response in the pretransitional region of the antiferroelectric phase of these compounds over a moderate range of fields.     

A Multi‐Stimuli‐Responsive Oxazine Molecular Switch: A Strategy for the Design of Electrochromic Materials

A multi‐state and multi‐stimuli‐responsive oxazine molecular switch that combines an electro‐base property and sensitive base/acid‐responsive properties was designed and synthesized. The multi‐state structures of the molecular switch, with different colors, were predicted by comparing the optical properties with reference molecules and confirmed by using NMR spectroscopy. The color‐switching mechanism under stimulation with acids and bases was investigated by using DFT calculations. Three single states can be obtained and the switching is unidirectional under acid and base stimulation. The electrochromic phenomenon of the molecular switch, which combines its electro‐base and base‐sensitive properties, was demonstrated. An electrochromic device that exhibited good electrochromic properties with excellent reversibility (2000 cycles) and high coloration efficiency (804 cm² C^?1) was successfully constructed.     

Infrared study of the acidic and basic forms of betaxolol

Betaxolol and its respective hydrochloride salt were studied in solution by computational calculations and infrared spectroscopy. The solution molecular conformations were taken to be the same as those exhibited by the compounds in the solid state given by X-ray diffraction and calculated after full geometry optimization by ab initio Hartree-Fock methods using the 6-31G(d) basis set. Infrared spectra of carbon tetrachloride solutions provide valuable information on the structure of the compounds in non-polar solvents at different concentrations.     

Protonation states of the catalytic dyad of β-secretase (BACE1) in the presence of chemically diverse inhibitors: a molecular docking study

In this molecular docking study, the protonation states of the catalytic Asp dyad of the beta-secretase (BACE1) enzyme in the presence of eight chemically diverse inhibitors have been predicted. BACE1 catalyzes the rate-determining step in the generation of Alzheimer amyloid beta peptides and is widely considered as a promising therapeutic target. All the inhibitors were redocked into their corresponding X-ray structures using a combination of eight different protonation states of the Asp dyad for each inhibitor. Five inhibitors were primarily found to favor two different monoprotonated states, and the remaining three favor a dideprotonated state. In addition, five of them exhibited secondary preference for a diprotonated state. These results show that the knowledge of a single protonation state of the Asp dyad is not sufficient to search for the novel inhibitors of BACE1 and the most plausible state for each inhibitor must be determined prior to conducting in-silico screening.     

Ultrafast Excited State Relaxation Dynamics of New Fuchsine- a Triphenylmethane Derivative Dye

An all-inclusive investigation of the ultrafast excited state relaxation dynamics of a triphenylmethane derivative molecule, New Fuchsine (NF), using a combined approach of density functional theory (DFT), femtosecond transient absorption spectroscopy (fs-TAS), and photoluminescence spectroscopy is presented in this work. The DFT calculations confirmed the formation of twisted molecular structure in the excited state of NF in ethanol solution with bond rotation of ≈86°. TAS measurements of NF solution exhibited ultrafast ground state-recovery pathway via a conical intersection confirming an ultrafast structural reorientation. On the contrary, TAS measurements of NF thin-film exhibited a longer excited-state lifetime suggesting a hindered molecular twisted state formed as an intermediate step. Photophysical kinetic models are proposed to globally fit the fs-TAS data establishing the twisted intramolecular charge transfer (TICT) state mediated ground state recovery for NF in solution and thin film, respectively. Temperature-dependent photoluminescence study of NF film provided a clear insight into the effect of rotational motion of phenyl rings in NF molecules over the TICT mediated emission.     

Nanostructuration of phenylenevinylenediimide-bridged silsesquioxane: from electroluminescent molecular J-aggregates to photoresponsive polymeric H-aggregates

A new approach to control molecular aggregation of pi-conjugated chromophores in the solid state has been investigated. Our strategy was to use a modifiable bulky fragment which should induce a J-aggregation and offer the possibility to reach an H-aggregation upon its chemical modification by lateral slip of pi-conjugated molecules. The chosen fragment for that purpose was the hydrolyzable triethoxysilane function (Si(OEt)3). Our objective was to design and synthesize electroluminescent or solar cell hybrid organic-inorganic materials by the sol-gel process applied to a bifunctionalized silane. With this intention, the synthesis of the sol-gel processable phenylenevinylenediimide silsesquioxane 6 was accomplished and the study of spin-coated thin films of the pure silane precursor subjected or not to the sol-gel process has been carried out. Optical properties of 6 are consistent with the formation of J-aggregates in the solid state due to the steric hindrance introduced by the triethoxysilane units. Conversely, the spectroscopic behavior observed for the hybrid film 6F is attributed to an H-aggregation corresponding to a "card pack" orientation of the distyrylbenzeneimide chromophores in the compressed silicate network. Morevover, 6 and 6F also exhibited different electronic behaviors: light-emitting diodes exhibited high brightness with the native precursor 6 and almost no light output with the sol-gel processed silsesquioxane 6F. Photovoltaic cells showed the opposite behavior with low photocurrent generation in the precursor case and higher photocurrents with the sol-gel processed layers. These results provide a deeper understanding of the present self-assembly process that is strongly governed by the molecular packing of the oligosiloxane precursor.     

Energy Aspects of Thermal Molecular Switching: Molecular Thermal Hysteresis of Helicene Oligomers

Molecular switching is a phenomenon by which a molecule reversibly changes its structure and state in response to external stimuli or energy. Herein, molecular switching is discussed from thermodynamic and kinetic aspects in terms of energy supply with an emphasis on the thermal switching exhibited by helicene oligomers. It includes the inversion of relative thermodynamic stability induced by temperature changes and molecular thermal hysteresis in a closed system. The thermal phenomenon associated with the oligomers involves population/concentration changes between metastable states under nonequilibrium thermodynamic control.     

Tuning the Optical Properties of Sulfonylaniline Derivatives: Degeneracy Breaking of Benzene Orbitals and Linkage through Nodal Planes

The orbital degeneracy of benzene rings is resolved by an asymmetric push-pull system in 2,6-bis(methylsulfonyl)aniline (BMeSA), in which the highest occupied molecular orbital (HOMO) is located at the 4-position, while the lowest unoccupied molecular orbital (LUMO) is located at a different position and has a nodal plane through the carbon atoms at the 1- and 4-positions. Therefore, the π-extension of BMeSA at the 4-position reveals a strong overlap in the HOMO and a minimal overlap in the LUMO. Consequently, π-extended BMeSA derivatives exhibit longer absorbance and emission wavelengths in the order of the electron-donating abilities of their substituents at the 4-position, which is based on a decrease in an absolute HOMO-level-dependent HOMO-LUMO gap in accordance with the nodal arrangement. Positive fluorescent solvatochromism with polarity-dependent decrease in fluorescent intensity was also observed. The biaryls exhibited more planar geometries in the excited state than in the ground state. The charge transfer mechanism, which can be described as node-induced intramolecular charge transfer (NICT), differs from the planar intramolecular charge transfer (PICT) and twisted intramolecular charge transfer (TICT).     

Synthesis of phenylenevinylene and naphthylenevinylene homopolymers and block copolymers by ring‐opening metathesis polymerization

Homopolymers and copolymers containing phenylenevinylenes and naphthylenevinylenes can be synthesized by ring‐opening metathesis polymerization of strained monomers such as tetraoctyloxy‐substituted cyclophanedienes and naphthalenophanedienes initiated by the third‐generation Grubbs’ initiator. The resulting homopolymers exhibited low polydispersities. The block copolymers can also be synthesized by the sequential ring‐opening metathesis polymerization of two individual monomers. The structures of homopolymers and block copolymers were fully characterized by nuclear magnetic resonance spectroscopy. The molecular weight distribution of the block copolymers is relatively broad compared to their parent homopolymers possibly due to chain transfer reaction. The molar ratio of the two blocks can be tailored by the ratio of the monomers employed. The block copolymers exhibited a more efficient energy transfer in the solid state between the different blocks than those carried out in solution. The optical and electrochemical properties of the polymers were investigated and exhibited the potential uses in optoelectronics devices. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2018 , 56, 67–74     

Nonvolatile rewritable memory effects in graphene oxide functionalized by conjugated polymer containing fluorene and carbazole units

A new polymer, poly[{9,9-di(triphenylamine)fluorene}(9,9-dihexylfluorene)(4-aminophenylcarbazole)] (PFCz) was synthesized and used in a reaction with graphene oxide (GO) containing surface-bonded acyl chloride moieties to give a soluble GO-based polymer material GO-PFCz. A bistable electrical switching effect was observed in an electronic device in which the GO-PFCz film was sandwiched between indium-tin oxide (ITO) and Al electrodes. This device exhibited two accessible conductivity states, that is, a low-conductivity (OFF) state and a high-conductivity (ON) state, and can be switched to the ON state under a negative electrical sweep; it can also be reset to the initial OFF state by a reverse (positive) electrical sweep. The ON state is nonvolatile and can withstand a constant voltage stress of -1 V for 3 h and 10(8) read cycles at -1 V under ambient conditions. The nonvolatile nature of the ON state and the ability to write, read, and erase the electrical states, fulfill the functionality of a rewritable memory. The mechanism associated with the memory effects was elucidated from molecular simulation results and in-situ photoluminescence spectra of the GO-PFCz film under different electrical biases.     

Molecular Packing and Solid-State Photophysical Properties of 1,3,6,8-Tetraalkylpyrenes

The relationship between the photophysical properties and molecular orientation of 1,3,6,8-tetraalkylpyrenes in the solid state is described herein. The introduction of alkyl groups with different chain structures (in terms of length and branching) did not affect the photophysical properties in solution, but significantly shifted the emission wavelengths and fluorescence quantum yields in the solid state for some samples. Pyrenes bearing ethyl, isobutyl, or neopentyl groups at the 1-, 3-, 6-, and 8-positions showed similar emission profiles in both the solution and solid states. In contrast, pyrenes bearing other alkyl groups exhibited an excimer emission in the solid state, similar to that of the parent pyrene. On studying the photophysical properties in the solid state with respect to the obtained crystal structures, the observed solid-state photophysical properties were found to depend on the relative position of the pyrene chromophores. The solid-state photophysical properties can be controlled by the alkyl groups, which provide changing crystal packing. Among the pyrenes tested, 1,3,6,8-tetraethylpyrene showed the highest fluorescence quantum yield of 0.88 in the solid state.     

The effects of photochemical and mechanical damage on the excited state dynamics of charge-transfer molecular crystals composed of tetracyanobenzene and aromatic donor molecules

Charge-transfer molecular crystals are structurally well-defined systems whose electron transfer dynamics can be studied using time-resolved spectroscopy. In this paper, five 1:1 complexes, consisting of 1,2,4,5-tetracyanobenzene as the electron acceptor and durene, 9-methylanthracene, naphthalene, phenanthrene, and pyrene as electron donors, are studied using time-resolved fluorescence and transient absorption in the diffuse reflectance geometry. Two different sample morphologies were studied: single crystals and powders prepared by pulverizing the crystals and diluting them with barium sulfate microparticles. Fluorescence lifetime and transient absorption measurements performed on the crystals and the powders yielded different results. The crystals typically exhibited long-lived monoexponential fluorescence decays, while the powders had shorter multiexponential decays. Exposure of both types of samples to high laser fluence was also shown to induce faster excited state decay dynamics as observed using fluorescence and diffuse reflectance. In addition to the more rapid decays, these molecular crystals exhibited relatively high photobleaching quantum yields on the order of 10(-4). Previous work that interpreted picosecond decays in the transient absorption as evidence for rapid recombination and charge dissociation should be re-evaluated based on the susceptibility of this class of compounds to mechanical and photochemical damage.     

Analysis of Molecular Orientation in Organic Semiconducting Thin Films Using Static Dynamic Nuclear Polarization Enhanced Solid‐State NMR Spectroscopy

Molecular orientation in amorphous organic semiconducting thin‐film devices is an important issue affecting device performance. However, to date it has not been possible to analyze the “distribution” of the orientations. Although solid‐state NMR (ssNMR) spectroscopy can provide information on the “distribution” of molecular orientations, the technique is limited because of the small amount of sample in the device and the low sensitivity of ssNMR. Here, we report the first application of dynamic nuclear polarization enhanced ssNMR (DNP‐ssNMR) spectroscopy for the orientational analysis of amorphous phenyldi(pyren‐1‐yl)phosphine oxide (POPy₂). The ³¹P DNP‐ssNMR spectra exhibited a sufficient signal‐to‐noise ratio to quantify the distribution of molecular orientations in amorphous films: the P=O axis of the vacuum‐deposited and drop‐cast POPy₂ shows anisotropic and isotropic distribution, respectively. The different molecular orientations reflect the molecular origin of the different charge transport behaviors.     

Chiral diaminopyrrolic receptors for selective recognition of mannosides, part 2: a 3D view of the recognition modes by X-ray, NMR spectroscopy, and molecular modeling

The structural features of a representative set of five complexes of octyl α- and β-mannosides with some members of a new generation of chiral tripodal diaminopyrrolic receptors, namely, (R)-5 and (S)- and (R)-7, have been investigated in solution and in the solid state by a combined X-ray, NMR spectroscopy, and molecular modeling approach. In the solid state, the binding arms of the free receptors 7 delimit a cleft in which two solvent molecules are hydrogen bonded to the pyrrolic groups and to the benzenic scaffold. In a polar solvent (CD(3)CN), chemical shift and intermolecular NOE data, assisted by molecular modeling calculations, ascertained the binding modes of the interaction between the receptor and the glycoside for these complexes. Although a single binding mode was found to adequately describe the complex of the acyclic receptor 5 with the α-mannoside, for the complexes of the cyclic receptors 7 two different binding modes were required to simultaneously fit all the experimental data. In all cases, extensive binding through hydrogen bonding and CH-π interactions is responsible for the affinities measured in the same solvent. Furthermore, the binding modes closely account for the recognition preferences observed toward the anomeric glycosides and for the peculiar enantiodiscrimination properties exhibited by the chiral receptors.     

Mechanical force induced reversible fluorescence switching of two 3-aryl-2-cyano acrylamide derivatives

Two structural-simple 3-aryl-2-cyano acrylamide derivatives (CDPA-E and CDPA-P) were synthesized and characterized by NMR, MS, IR, and single crystal. It was found that both of them exhibited obvious piezofluorochromic properties. The as-prepared yellow-green compounds were both converted to orange-red after grinding with a spatula. All the ground samples can be recovered to its original color by heating at 80 °C over 5 min or fuming with solvents like DCM, ethanol, ethyl acetate etc. The changes in fluorescence color can be attributed to the phase transition between crystalline (order) state and amorphous (disorder) state according to the Powder XRD spectrum. At the molecular level, the extension of the conjugation length and the formation of excimers are confirmed to account for the color transition of CDPA-P and CDPA-E, respectively. In addition, CDPA-P exhibited AIE activity which was caused by the restricted rotation of the molecules in the crystal state.     

Anti-Stokes Luminescence in Multi-Resonance-Type Thermally-Activated Delayed Fluorescence Molecules

Photon-upconversion in organic molecular systems is one of the promising technologies for future energy harvesting systems because these systems can generate excitons that possess higher energy than excitation energy. The photon-upconversion caused by absorbing ambient heat as additional energy is particularly interesting because it could ideally provide a light-driving cooling system. However, only a few organic molecular systems have been reported. Here, we report the anti-Stokes photoluminescence (ASPL) derived from hot-band absorption in a series of multi-resonance-type thermally-activated delayed fluorescence (MR-TADF) molecules. The MR-TADF molecules exhibited an anti-Stokes shift of approximately 0.1 eV with a high PL quantum yield in the solution state. The anti-Stokes shift corresponded well to the 1–0 vibration transition from the ground state to the excited singlet state, and we further evaluated a correlation between the activation energy for the ASPL intensity and the TADF process. Our demonstration underlines that MR-TADF molecules have become a novel class of ASPL materials for various future applications, such as light-driving cooling systems.     

Characterization of folding intermediates of a domain-swapped protein by solid-state NMR spectroscopy

We have employed two-dimensional solid-state NMR to study structure and dynamics of insoluble folding states of the domain-swapped protein Crh. Starting from the protein precipitated at its pI, conformational changes due to a modest temperature increase were investigated at the level of individual residues and in real-time. As compared to the crystalline state, Crh pI-precipitates exhibited a higher degree of molecular mobility for several regions of the protein. A rigidly intact center was observed including a subset of residues of the hydrophobic core. Raising the temperature by 13 K to 282 K created a partially unfolded intermediate state that was converted into beta-sheet-rich aggregates that are mostly of spherical character according to electron microscopy. Residue-by-residue analysis indicated that two out of three alpha-helices in aggregated Crh underwent major structural rearrangements while the third helix was preserved. Residues in the hinge region exhibited major chemical-shift changes, indicating that the domain swap was not conserved in the aggregated form. Our study provides direct evidence that protein aggregates of a domain-swapped protein retain a significant fraction of native secondary structure and demonstrates that solid-state NMR can be used to directly monitor slow molecular folding events.     

Miscibility behavior and reequilibration of binary poly(amic acid) mixtures

Five poly (amic acid) solutions based on PMDA-PDA, PMDA-ODA, PMDA-6F, ODPA-ODA, and 6FDA-ODA were prepared in N-methylpyrrolidone at a polymer concentration of ca. 10 wt %. From these five poly (amic acid) solutions, six different binary blends were prepared: PMDA-PDA/PMDA-ODA, PMDA-PDA/PMDA-6F, PMDA-ODA/6FDA-ODA, PMDA-ODA/ODPA-ODA, PMDA-PDA/ODPA-ODA, and PMDA-PDA/6FDA-ODA. These blends were then characterized with respect to miscibility in the ternary state (polyamic acid-1/polyamic acid-2/N-methylpyrrolidone), the condensed state (ca. 70 wt % polymer), and the fully imidized state. All blends except for PMDA-PDA/PMDA-6F yielded homogeneous mixtures in the ternary solution of 10 wt % polymer concentration. The PMDA-PDA/PMDA-6F mixture eventually became homogeneous after 10 days of mixing at room temperature. Upon solvent evaporation (condensed state) and full cure (imidized state) two blends incorporating ODPA-ODA as one of the components exhibited apparent miscibility as evidenced by optical microscopy. The remaining blends exhibited large-scale phase separation upon solvent evaporation with no significant differences in the overall morphology between the condensed and imidized state. However, as in the case of the PMDA-PDA/PMDA-6F ternary system, the morphology in the condensed and imidized state was strongly dependent on the mixing time of the precursor poly(amic acid) components; the phase-separated domain size decreased with increasing mixing time, eventually leading to complete miscibility. These results are discussed with respect to the proposed “polymer-monomer” reequilibration reaction as well as its implications with respect to the preparation of polyimide molecular composites.     

Effect of Conjugation Mode on Intramolecular Charge Transfer in Fabricating Acid‐Responsive Fluorophores

Solid‐state acid‐responsive materials are promising for the tunability of their intrinsic properties. However, the relationship between molecular structure and emission shift as a response to acid stimuli has not been systematically studied. Herein, we report the effect of protonation and subsequent intramolecular hydrogen bonding on the photophysical properties of compounds (MPP‐s, MPP‐d, and MPP‐d‐CN) with different conjugation modes between the electron‐donating dimethoxyl phenyl and the electron‐withdrawing benzothiazole ring. The results established that the stronger the intramolecular charge transfer feature of the compound, the smaller is the emission shift after acid stimuli. Our studies also indicated that the conjugation mode significantly affected the solid‐state packing mode: MPP‐s and MPP‐d tended to form dimers, while MPP‐d‐CN exhibited the strongest aggregation‐induced emission enhancement (AIEE). The exploration of structure‐property relationship would provide experimental and theoretical guidance in designing acid‐responsive molecular switches and developing high‐performance AIEE‐active luminogens.