The location of the chemical bond. Application of long covalent bond theory to the structure of silica.

Oxygen is the most abundant terrestrial element and is found in a variety of materials, but still wanting is a universal theory for the stability and structural organization it confers. Herein, a computational molecular orbital analysis elucidates the structure, stability, and cooperative bonding of α-quartz silica (SiO2). Despite geminal oxygen-oxygen distances of 2.61-2.64 Å, silica model complexes exhibit anomalously large O-O bond orders (Mulliken, Wiberg, Mayer) that increase with increasing cluster size-as the silicon-oxygen bond orders decrease. The average O-O bond order in bulk silica computes to 0.47 while that for Si-O computes to 0.64. Thereby, for each silicate tetrahedron, the six O-O bonds employ 52% (5.61 electrons) of the valence electrons, while the four Si-O bonds employ 48% (5.12 electrons), rendering the O-O bond the most abundant bond in the Earth's crust. The isodesmic deconstruction of silica clusters reveals cooperative O-O bonding with an O-O bond dissociation energy of 4.4 kcal/mol. These unorthodox, long covalent bonds are rationalized by an excess of O 2p-O 2p bonding versus anti-bonding interactions within the valence molecular orbitals of the SiO4 unit (48 vs. 24) and the Si6O6 ring (90 vs. 18). Within quartz silica, oxygen 2p orbitals contort and organize to avoid molecular orbital nodes, inducing the chirality of silica and resulting in Möbius aromatic Si6O6 rings, the most prevalent form of aromaticity on Earth. This long covalent bond theory (LCBT) relocates one-third of Earth's valence electrons and indicates that non-canonical O-O bonds play a subtle, but crucial role in the structure and stability of Earth's most abundant material.

Reductive Amination of Dialdehyde Cellulose: Access to Renewable Thermoplastics.

The reductive amination of dialdehyde cellulose (DAC) with 2-picoline borane was investigated for its applicability in the generation of bioderived thermoplastics. Five primary amines, both aliphatic and aromatic, were introduced to the cellulose backbone. The influences of the side chains on the course of the reaction were examined by various analytical techniques with microcrystalline cellulose as a model compound. The obtained insights were transferred to a 39%-oxidized softwood kraft pulp to study the thermal properties of thereby generated high-molecular-weight thermoplastics. The number-average molecular weights (Mn) of the diamine celluloses, ranging from 60 to 82 kD, were investigated by gel permeation chromatography. The diamine celluloses exhibited glass transition temperatures (Tg) from 71 to 112 °C and were stable at high temperatures. Diamine cellulose generated from aniline and DAC showed the highest conversion, the highest Tg (112 °C), and a narrow molecular weight distribution (D̵ of 1.30).

Adapting a Physical Earthquake-Aftershock Model to Simulate the Spread of COVID-19.

There exists a need for a simple, deterministic, scalable, and accurate model that captures the dominant physics of pandemic propagation. We propose such a model by adapting a physical earthquake/aftershock model to COVID-19. The aftershock model revealed the physical basis for the statistical Epidemic Type Aftershock Sequence (ETAS) model as a highly non-linear diffusion process, thus permitting a grafting of the underlying physical equations into a formulation for calculating infection pressure propagation in a pandemic-type model. Our model shows that the COVID-19 pandemic propagates through an analogous porous media with hydraulic properties approximating beach sand and water. Model results show good correlations with reported cumulative infections for all cases studied. In alphabetical order, these include Austria, Belgium, Brazil, France, Germany, Italy, New Zealand, Melbourne (AU), Spain, Sweden, Switzerland, the UK, and the USA. Importantly, the model is predominantly controlled by one parameter (α), which modulates the societal recovery from the spread of the virus. The obtained recovery times for the different pandemic waves vary considerably from country to country and are reflected in the temporal evolution of registered infections. These results provide an intuition-based approach to designing and implementing mitigation measures, with predictive capabilities for various mitigation scenarios.

High-speed, scanned laser structuring of multi-layered eco/bioresorbable materials for advanced electronic systems.

Physically transient forms of electronics enable unique classes of technologies, ranging from biomedical implants that disappear through processes of bioresorption after serving a clinical need to internet-of-things devices that harmlessly dissolve into the environment following a relevant period of use. Here, we develop a sustainable manufacturing pathway, based on ultrafast pulsed laser ablation, that can support high-volume, cost-effective manipulation of a diverse collection of organic and inorganic materials, each designed to degrade by hydrolysis or enzymatic activity, into patterned, multi-layered architectures with high resolution and accurate overlay registration. The technology can operate in patterning, thinning and/or cutting modes with (ultra)thin eco/bioresorbable materials of different types of semiconductors, dielectrics, and conductors on flexible substrates. Component-level demonstrations span passive and active devices, including diodes and field-effect transistors. Patterning these devices into interconnected layouts yields functional systems, as illustrated in examples that range from wireless implants as monitors of neural and cardiac activity, to thermal probes of microvascular flow, and multi-electrode arrays for biopotential sensing. These advances create important processing options for eco/bioresorbable materials and associated electronic systems, with immediate applicability across nearly all types of bioelectronic studies.

Aftershocks are fluid-driven and decay rates controlled by permeability dynamics.

One aspect of earthquake physics not adequately addressed is why some earthquakes generate thousands of aftershocks while other earthquakes generate few, if any, aftershocks. It also remains unknown why aftershock rates decay as ~1/time. Here, I show that these two are linked, with a dearth of aftershocks reflecting the absence of high-pressure fluid sources at depth, while rich and long-lasting aftershock sequences reflect tapping high-pressure fluid reservoirs that drive aftershock sequences. Using a physical model that captures the dominant aspects of permeability dynamics in the crust, I show that the model generates superior fits to observations than widely used empirical fits such as the Omori-Utsu Law, and find a functional relationship between aftershock decay rates and the tectonic ability to heal the co- and post-seismically generated fracture networks. These results have far-reaching implications, and can help interpret other observations such as seismic velocity recovery, attenuation, and migration.

Sustainable Synthesis and Polycondensation of Levoglucosenone-Cyrene-Based Bicyclic Diol Monomer: Access to Renewable Polyesters.

The already-reported, low-yielding, and non-sustainable Et3 N-mediated homocoupling of levoglucosenone (LGO) into the corresponding LGO-CyreneTM diketone has been revisited and greened-up. The use of methanol as both a renewable solvent and catalyst and K2 CO3 as a safe inorganic base improved the reaction significantly with regards to yield, purification, and green aspects. LGO-CyreneTM was then subjected to a one-pot, H2 O2 -mediated Baeyer-Villiger oxidation/rearrangement followed by an acidic hydrolysis to produce a new sterically hindered bicyclic monomer, 2H-HBO-HBO. This diol was further polymerized in bulk with diacyl chlorides to access new promising renewable polyesters that exhibit glass transition temperatures (Tg ) from -1 to 81 °C and a good thermostability with a temperature at which 50 % of the mass is lost (Td50 % ) of 349-406 °C.

Charge-transfer biexciton annihilation in a donor-acceptor co-crystal yields high-energy long-lived charge carriers.

Organic donor-acceptor (D-A) co-crystals have attracted much interest due to their important optical and electronic properties. Co-crystals having ⋯DADA⋯ π-stacked morphologies are especially interesting because photoexcitation produces a charge-transfer (CT) exciton, D˙+-A˙-, between adjacent D-A molecules. Although several studies have reported on the steady-state optical properties of this type of CT exciton, very few have measured the dynamics of its formation and decay in a single D-A co-crystal. We have co-crystallized a peri-xanthenoxanthene (PXX) donor with a N,N-bis(3-pentyl)-2,5,8,11-tetraphenylperylene-3,4:9,10-bis(dicarboximide) (Ph4PDI) acceptor to give an orthorhombic PXX-Ph4PDI ⋯DADA⋯ π-stacked co-crystal with a CT transition dipole moment that is perpendicular to the transition moments for S n ← S0 excitation of PXX and Ph4PDI. Using polarized, broadband, femtosecond pump-probe microscopy, we have determined that selective photoexcitation of Ph4PDI in the single co-crystal results in CT exciton formation within the 300 fs instrument response time. At early times (0.3 ≤ t ≤ 500 ps), the CT excitons decay with a t -1/2 dependence, which is attributed to CT biexciton annihilation within the one-dimensional ⋯DADA⋯ π-stacks producing high-energy, long-lived (>8 ns) electron-hole pairs in the crystal. These energetic charge carriers may prove useful in applications ranging from photovoltaics and opto-electronics to photocatalysis.

Ultrafast interligand electron transfer in cis-[Ru(4,4'-dicarboxylate-2,2'-bipyridine)2(NCS)2]4- and implications for electron injection limitations in dye sensitized solar cells.

Interligand electron transfer (ILET) of the lowest metal-to-ligand charge transfer (MLCT) state of N712 (cis-[Ru(dcb)2(NCS)2]4-, where dcb = 4,4'-dicarboxylate-2,2'-bipyridine) in a deuterated acetonitrile solution has been studied by means of femtosecond transient absorption anisotropy in the mid-IR. Time-independent B3LYP density functional calculations were performed to assign vibrational bands and determine their respective transition dipole moments. The transient absorption spectral band at 1327 cm-1, assigned to a symmetric carboxylate stretch, showed significant anisotropy. A rapid anisotropy increase (τ 1 ≈ 2 ps) was tentatively assigned to vibrational and solvent relaxation, considering the excess energy available after the excited singlet-triplet conversion. Thereafter, the anisotropy decayed to zero with a time constant τ 2 ≈ 240 ps, which was assigned to the rotational correlation time of the complex in deuterated acetonitrile. No other distinctive changes to the anisotropy were observed and the amplitude of the slow component at time zero agrees well with that predicted for a random mixture of MLCT localization on either of the two dcb ligands. The results therefore suggest that MLCT randomization over the two dcb ligands occurs on the sub-ps time scale. This is much faster than proposed by previous reports on the related N3 complex [Benkö et al., J. Phys. Chem. B, 2004, 108, 2862, and Waterland et al., J. Phys. Chem. A, 2001, 105, 4019], but in agreement with that found by Wallin and co-workers [J. Phys. Chem. A, 2005, 109, 4697] for the [Ru(bpy)3]2+ (bpy = 2,2'-bipyridine) complex. This suggests that electron injection from the excited dye into TiO2 in dye-sensitized solar cells is not limited by ILET.

Effects of Crystal Morphology on Singlet Exciton Fission in Diketopyrrolopyrrole Thin Films.

Singlet exciton fission (SF) is a promising strategy for increasing photovoltaic efficiency, but in order for SF to be useful in solar cells, it should take place in a chromophore that is air-stable, highly absorptive, solution processable, and inexpensive. Unlike many SF chromophores, diketopyrrolopyrrole (DPP) conforms to these criteria, and here we investigate SF in DPP for the first time. SF yields in thin films of DPP derivatives, which are widely used in organic electronics and photovoltaics, are shown to depend critically on crystal morphology. Time-resolved spectroscopy of three DPP derivatives with phenyl (3,6-diphenylpyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione, PhDPP), thienyl (3,6-di(thiophen-2-yl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione, TDPP), and phenylthienyl (3,6-di(5-phenylthiophen-2-yl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione, PhTDPP) aromatic substituents in 100-200 nm thin films reveals that efficient SF occurs only in TDPP and PhTDPP (τSF = 220 ± 20 ps), despite the fact that SF is most exoergic in PhDPP. This result correlates well with the greater degree of π-overlap and closer π-stacking in TDPP (3.50 Å) and PhTDPP (3.59 Å) relative to PhDPP (3.90 Å) and demonstrates that SF in DPP is highly sensitive to the electronic coupling between adjacent chromophores. The triplet yield in PhTDPP films is determined to be 210 ± 35% by the singlet depletion method and 165 ± 30% by the energy transfer method, showing that SF is nearly quantitative in these films and that DPP derivatives are a promising class of SF chromophores for enhancing photovoltaic performance.

Polymer-Metal-Organic Frameworks (polyMOFs) as Water Tolerant Materials for Selective Carbon Dioxide Separations.

Recently, polymer-metal-organic frameworks (polyMOFs) were reported as a new class of hybrid porous materials that combine advantages of both organic polymers and crystalline MOFs. Herein, we report a bridging coligand strategy to prepare new types of polyMOFs, demonstrating that polyMOFs are compatible with additional MOF architectures besides that of the earlier reported IRMOF-1 type polyMOF. Gas sorption studies revealed that these polyMOF materials exhibited relatively high CO2 sorption but very low N2 sorption, making them promising materials for CO2/N2 separations. Moreover, these polyMOFs demonstrated exceptional water stability attributed to the hydrophobicity of polymer ligands as well as the cross-linking of the polymer chains within the MOF.

Silicon Acetal Metathesis Polymerization.

A kinetic study revealed that the acid-catalyzed (p-TSA) equilibration of Me2Si(OMe)2 and Me2Si(OEt)2, forming Me2Si(OEt)OMe, is established in 300 min in benzene at room temperature. This silicon acetal metathesis reaction is exploited for the step-growth polymerization of bis-silicon acetals (MeOSiMe2OROSiMe2OMe) with metathetical loss of Me2Si(OMe)2. Thus, a convenient and generalized silicon acetal metathesis polymerization (SAMP) method is introduced as the acid-catalyzed copolymerization of a diol (HOROH) and Me2Si(OMe)2, driven by elimination of methanol and/or Me2Si(OMe)2. SAMP constitutes an effective and powerful strategy for manipulating the most common bond in the Earth's crust, the silicon-oxygen bond.

Raising the barrier for photoinduced DNA charge injection with a cyclohexyl artificial base pair.

The effects of an artificial cyclohexyl base pair on the quantum yields of fluorescence and dynamics of charge separation and charge recombination have been investigated for several synthetic DNA hairpins. The hairpins possess stilbenedicarboxamide, perylenediimide, or naphthalenediimide linkers and base-paired stems. In the absence of the artificial base pair hole injection into both adenine and guanine purine bases is exergonic and irreversible, except in the case of stilbene with adenine for which it is slightly endergonic and reversible. Insertion of the artificial base pair renders hole injection endergonic or isoergonic except in the case of the powerful naphthalene acceptor for which it remains exergonic. Both hole injection and charge recombination are slower for the naphthalene acceptor in the presence of the artificial base pair than in its absence. The effect of an artificial base pair on charge separation and charge recombination in hairpins possessing stilbene and naphthalene acceptor linkers and a stilbenediether donor capping group has also been investigated. In the case of the stilbene acceptor-stilbene donor capped hairpins photoinduced charge separation across six base pairs is efficient in the absence of the artificial base pair but does not occur in its presence. In the case of the naphthalene acceptor-stilbene donor capped hairpins the artificial base pair slows but does not stop charge separation and charge recombination, leading to the formation of long-lived charge separated states.

Sub-Picosecond Singlet Exciton Fission in Cyano-Substituted Diaryltetracenes.

Thin films of 5,11-dicyano-6,12-diphenyltetracene (TcCN) have been studied for their ability to undergo singlet exciton fission (SF). Functionalization of tetracene with cyano substituents yields a more stable chromophore with favorable energetics for exoergic SF (2E(T1)-E(S1)=-0.17 eV), where S1 and T1 are singlet and triplet excitons, respectively. As a result of tuning the triplet-state energy, SF is faster in TcCN relative to the corresponding endoergic process in tetracene. SF proceeds with two time constants in the film samples (τ=0.8±0.2 ps and τ=23±3 ps), which is attributed to structural disorder within the film giving rise to one population with a favorable interchromophore geometry, which undergoes rapid SF, and a second population in which the initially formed singlet exciton must diffuse to a site at which this favorable geometry exists. A triplet yield analysis using transient absorption spectra indicates the formation of 1.6±0.3 triplets per initial excited state.

polyMOFs: A Class of Interconvertible Polymer-Metal-Organic-Framework Hybrid Materials.

Preparation of porous materials from one-dimensional polymers is challenging because the packing of polymer chains results in a dense, non-porous arrangement. Herein, we demonstrate the remarkable adaptation of an amorphous, linear, non-porous, flexible organic polymer into a three-dimensional, highly porous, crystalline solid, as the organic component of a metal-organic framework (MOF). A polymer with aromatic dicarboxylic acids in the backbone functioned as a polymer ligand upon annealing with Zn(II), generating a polymer-metal-organic framework (polyMOF). These materials break the dogma that MOFs must be prepared from small, rigid ligands. Similarly, polyMOFs contradict conventional polymer chemistry by demonstrating that linear and amorphous polymers can be readily coaxed into a highly crystalline, porous, three-dimensional structure by coordination chemistry.

Singlet exciton fission in thin films of tert-butyl-substituted terrylenes.

Two terrylene chromophores, 2,5,10,13-tetra(tert-butyl)terrylene (1) and 2,5-di(tert-butyl)terrylene (2), were synthesized and studied to determine their singlet exciton fission (SF) efficiencies. Compound 1 crystallizes in one-dimensional stacks, whereas 2 packs in a slip-stacked, herringbone pattern of dimers motif. Strongly quenched fluorescence and rapid singlet exciton decay dynamics are observed in vapor-deposited thin films of 1 and 2. Phosphorescence measurements on thin films of 1 and 2 show that SF is only 70 meV endoergic for these chromophores. Femtosecond transient absorption experiments using low laser fluences on these films reveal rapid triplet exciton formation for both 1 (τ = 120 ± 10 ps) and 2 (τ = 320 ± 20 ps) that depends strongly on film crystallinity. The transient absorption data are consistent with formation of an excimer state prior to SF. Triplet exciton yield measurements indicate nearly quantitative SF in thin films of both chromophores in highly crystalline solvent-vapor-annealed films: 170 ± 20% for 1 and 200 ± 30% for 2. These results show that significantly different crystal morphologies of the same chromophore can both result in high-efficiency SF provided that the energetics are favorable.

The role of the deformational entropy in the miscibility of polymer blends investigated using a hybrid statistical mechanics and molecular dynamics model.

To fully understand the thermodynamic nature of polymer blends and accurately predict their miscibility on a microscopic level, a hybrid model employing both statistical mechanics and molecular dynamics techniques was developed to effectively predict the total free energy of mixing. The statistical mechanics principles were used to derive an expression for the deformational entropy of the chains in the polymeric blends that could be evaluated from molecular dynamics trajectories. Evaluation of the entropy loss due to the deformation of the polymer chains in the case of coiling as a result of the repulsive interactions between the blend components or in the case of swelling due to the attractive interactions between the polymeric segments predicted a negative value for the deformational entropy resulting in a decrease in the overall entropy change upon mixing. Molecular dynamics methods were then used to evaluate the enthalpy of mixing, entropy of mixing, the loss in entropy due to the deformation of the polymeric chains upon mixing and the total free energy change for a series of polar and non-polar, poly(glycolic acid), PGA, polymer blends.

Sterically expanded CGC catalysts: substituent effects on ethylene and α-olefin polymerization.

Several analogues of the sterically expanded constrained geometry catalyst Me2Si(η(1)-C29H36)(η(1)-N-tBu)ZrCl2·OEt2 (2) were synthesized to assess the effect on branching and molecular weight for ethylene homopolymerization. Catalysts based on tetramethyltetrahydrobenzofluorene (TetH), ethylTetH, t-butylTetH, and octamethyloctahydrodibenzofluorenyl (OctH) bearing a diphenylsilyl bridge were prepared and characterized: Me2Si(η(5)-C21H22)(η(1)-N-tBu)ZrCl2 (3); Me2Si(η(5)-C23H26)(η(1)-N-tBu)ZrCl2 (4); and Me2Si(η(5)-C25H30)(η(1)-N-tBu)ZrCl2 (5); Me2Si(η(5)-C21H22)(η(1)-N-tBu)ZrMe2 (6); and Ph2Si(η(5)-C29H36)(η(1)-N-tBu)ZrCl2 (7). Complexes 4, 5, 6, and 7 were characterized by X-ray crystallography and displayed η(5) hapticity to the carbon ring in each case, in contrast to 2. In comparison to 2, complexes 3, 4, 5, and 7 (in combination with methylaluminoxane = MAO) showed diminished branching, higher molecular weight, and higher polydispersity indices for obtained ethylene homopolymers. While 4/MAO produced the greatest molecular weight polymers, no branching was observed. Reactivity ratios were determined for the copolymerization of ethylene and 1-decene with 2/MAO. A value of r(ethylene) = 14.9 and an exceedingly high value of r(1-decene) = 0.49 were found--in line with previous reports of this catalyst's unusual affinity for α-olefins.

Sustainable Polymers: Opportunities for the Next Decade.

The field of sustainable polymers is growing and evolving at unprecedented rates. Researchers are increasingly concerned with the feedstock origins and the degradation behavior of, especially, large-scale commodity packaging plastics. A perspective is offered here for the design of sustainable polymers, specifically addressing opportunities for monomer development and polymer degradation. Key concepts include: water degradability instead of biodegradability; incorporation of novel main-chain functionality, such as acetals; utilization of lignin-based aromatics; and direct polymerization of biogenic C1 feedstocks.

Communication: uncovering molecule-TiO2 interactions with nonlinear spectroscopy.

Femtosecond transient grating experiments are used to investigate electronic structures and transport mechanisms in dye-sensitized nanocrystalline TiO(2) films. This study examines two molecular sensitizers spanning the weak (a phosphonated Ruthenium complex) and strong (catechol) molecule-TiO(2) coupling regimes. It is shown that strong molecule-TiO(2) interactions give rise to photoinduced vibrational coherences at the interface between species. We suggest that the amplitudes of these coherences reflect the molecule-TiO(2) coupling strength and signify the delocalization of excited state wavefunctions.

Polyalkylenehydroxybenzoates (PAHBs): biorenewable aromatic/aliphatic polyesters from lignin.

Polyalkylenehydroxybenzoates (PAHBs) are a new class of thermoplastic, biorenewable aromatic/aliphatic polyester having the general formula H-[O-(CH2)n-O-4-(3-X, 5-Y-C6H2)-CO]z-OH. These are synthesized from the lignin-derived aromatics 4-hydroxybenzoic acid (X = Y = H), vanillic acid (X = OMe; Y = H), and syringic acid (X = Y = OMe). Alkylation of these with several chloroalkanols affords hydroxy-acid monomers suitable for polyesterification under dynamic vacuum between 150 and 250 °C with 1 mol% Sb2O3 as catalyst. Thus, polyalkylene 4-hydroxybenzoates, polyalkylene vanillates, and polyalkylene syringates are prepared, characterized, and subjected to thermal property comparisons by differential scanning calorimetry and thermogravimetric analysis.