Simulation of interlayer coupling for electroactive covalent organic framework design.

Porous, stacked two-dimensional covalent organic frameworks (2D COFs) bearing semiconducting linkers can support directional charge transfer across adjacent layers of the COF. To better inform the current and possible future design rules for enhancing electron and hole transport in such materials, an understanding of how linker selection and functionalization affects interlayer electronic couplings is essential. We report electronic structure simulation and analysis of electronic couplings across adjacent linker units and to encapsulated species in functionalized electroactive 2D COFs. The detailed dependence of these electronic couplings on interlayer interactions is examined through scans along key interlayer degrees of freedom and through configurational sampling from equilibrium molecular dynamics on semiempirical potential energy surfaces. Beyond affirming the sensitivity of the electronic coupling to interlayer distance and orientation, these studies offer guidance toward linker functionalization strategies for enhancing charge carrier transport in electroactive 2D COFs.

Sequential Deoxygenation of CO2 and NO2- via Redox-Control of a Pyridinediimine Ligand with a Hemilabile Phosphine.

The deoxygenation of environmental pollutants CO2 and NO2- to form value-added products is reported. CO2 reduction with subsequent CO release and NO2- conversion to NO are achieved via the starting complex Fe(PPhPDI)Cl2 (1). 1 contains the redox-active pyridinediimine (PDI) ligand with a hemilabile phosphine located in the secondary coordination sphere. 1 was reduced with SmI2 under a CO2 atmosphere to form the direduced monocarbonyl Fe(PPhPDI)(CO) (2). Subsequent CO release was achieved via oxidation of 2 using the NOx- source, NO2-. The resulting [Fe(PPhPDI)(NO)]+ (3) mononitrosyl iron complex (MNIC) is formed as the exclusive reduction product due to the hemilabile phosphine. 3 was investigated computationally to be characterized as {FeNO}7, an unusual intermediate-spin Fe(III) coupled to triplet NO- and a singly reduced PDI ligand.

Electronic transition dipole moments from time-independent excited-state density-functional tight-binding.

Computationally inspired design of organic electronic materials requires robust models of not only the ground and excited electronic states but also of transitions between these states. In this work, we introduce a strategy for obtaining electronic transition dipole moments for the lowest-lying singlet-singlet transition in organic chromophores from time-independent excited-state density-functional tight-binding (ΔDFTB) calculations. Through small-molecule benchmarks and applications to larger chromophores, we explore the accuracy, potential, and limitations of this semiempirical strategy. While more accurate methods are recommended for small systems, we find some evidence for the method's potential in high-throughput molecular screening applications and in the analysis of molecular dynamics simulations.

Building Blocks and COFs Formed in Concert-Three-Component Synthesis of Pyrene-Fused Azaacene Covalent Organic Framework in the Bulk and as Films.

A three-component synthesis methodology is described for the formation of covalent organic frameworks (COFs) containing extended aromatics. Notably, this approach enables synthesis of the building blocks and COF along parallel reaction landscapes, on a similar timeframe. The use of fragmental building block components, namely pyrene dione diboronic acid as aggregation-inducing COF precursor and the diamines o-phenylenediamine (Ph), 2,3-diaminonaphthalene (Naph), or (1R,2R)-(+)-1,2-diphenylethylenediamine (2Ph) as extending functionalization units in conjunction with 2,3,6,7,10,11-hexahydroxytriphenylene, resulted in the formation of the corresponding pyrene-fused azaacene, i.e., Aza-COF series with full conversion of the dione moiety, long-range order, and high surface area. In addition, the novel three-component synthesis was successfully applied to produce highly crystalline, oriented thin films of the Aza-COFs with nanostructured surfaces on various substrates. The Aza-COFs exhibit light absorption maxima in the blue spectral region, and each Aza-COF presents a distinct photoluminescence profile. Transient absorption measurements of Aza-Ph- and Aza-Naph-COFs suggest ultrafast relaxation dynamics of excited-states within these COFs.

Determinant Factors of Three-Dimensional Aromaticity in Antiaromatic Cyclophanes.

Three-dimensional aromaticity arising from the close stacking of two antiaromatic π-conjugated macrocycles has recently received considerable attention. Here, a cyclophane consisting of two antiaromatic Ni(II) norcorrole units tethered with two flexible alkyl chains was synthesized. The norcorrole cyclophane showed crystal polymorphism providing three different solid-state structures. Surprisingly, one of them adopted an aligned face-to-face stacking arrangement with negligible displacement along the slipping axis. Although the exchange repulsion between two π-clouds should be maximized in this orientation, the π-π distance is remarkably close (3.258 Å). Three-dimensional aromaticity in this conformation has been supported experimentally and theoretically as evidenced by small bond length alternations as well as the presence of a diatropic ring current. An analogous cyclophane with two aromatic Ni(II) porphyrin units was prepared for comparison. The porphyrin cyclophane exhibited a slipped-stacking conformation with a larger displacement (2.9 Å) and a larger interplanar distance (3.402 Å) without noticeable change of the aromaticity of each porphyrin unit. In solution, the norcorrole cyclophane forms a twist stacking arrangement with effective interplanar orbital overlap and exists in an equilibrium between stacked and nonstacked structures. Thermodynamic parameters of the stacking process were estimated, revealing an inherently large attractive interaction operating between two norcorrole units, which has been further supported by energy decomposition analysis.

Chemoselective Carbonyl Allylations with Alkoxyallylsiletanes.

Alkoxyallylsiletanes are capable of highly chemo- and diastereoselective carbonyl allylsilylations. Reactive substrates include salicylaldehydes and glyoxylic acids. Chemoselectivity in these reactions is thought to arise from a mechanism involving first exchange of the alkyoxy group on silicon with a substrate hydroxyl followed by activation of a nearby carbonyl by the Lewis acidic siletane and intramolecular allylation. In this way, substrates containing multiple reactive carbonyl groups (e.g., dialdehyde or triketone) can be selectively monoallylated, even overcoming inherent electrophilicity bias.

Hemilabile Proton Relays and Redox Activity Lead to {FeNO} x and Significant Rate Enhancements in NO2- Reduction.

Incorporation of the triad of redox activity, hemilability, and proton responsivity into a single ligand scaffold is reported. Due to this triad, the complexes Fe(PyrrPDI)(CO)2 (3) and Fe(MorPDI)(CO)2 (4) display 40-fold enhancements in the initial rate of NO2- reduction, with respect to Fe(MeOPDI)(CO)2 (7). Utilizing the proper sterics and p Ka of the pendant base(s) to introduce hemilability into our ligand scaffolds, we report unusual {FeNO} x mononitrosyl iron complexes (MNICs) as intermediates in the NO2- reduction reaction. The {FeNO} x species behave spectroscopically and computationally similar to {FeNO}7, an unusual intermediate-spin Fe(III) coupled to triplet NO- and a singly reduced PDI ligand. These {FeNO} x MNICs facilitate enhancements in the initial rate.

Theoretical rationalization for reduced charge recombination in bulky carbazole-based sensitizers in solar cells.

The search for greater efficiency in organic dye-sensitized solar cells (DSCs) and in their perovskite cousins is greatly aided by a more complete understanding of the spectral and morphological properties of the photoactive layer. This investigation resolves a discrepancy in the observed photoconversion efficiency (PCE) of two closely related DSCs based on carbazole-containing D-π-A organic sensitizers. Detailed theoretical characterization of the absorption spectra, dye adsorption on TiO2 , and electronic couplings for charge separation and recombination permit a systematic determination of the origin of the difference in PCE. Although the two dyes produce similar spectral features, ground- and excited-state density functional theory (DFT) simulations reveal that the dye with the bulkier donor group adsorbs more strongly to TiO2 , experiences limited π-π aggregation, and is more resistant to loss of excitation energy via charge recombination on the dye. The effects of conformational flexibility on absorption spectra and on the electronic coupling between the bright exciton and charge-transfer states are revealed to be substantial and are characterized through density-functional tight-binding (DFTB) molecular dynamics sampling. These simulations offer a mechanistic explanation for the superior open-circuit voltage and short-circuit current of the bulky-donor dye sensitizer and provide theoretical justification of an important design feature for the pursuit of greater photocurrent efficiency in DSCs. © 2017 Wiley Periodicals, Inc.

How Parallel Are Excited State Potential Energy Surfaces from Time-Independent and Time-Dependent DFT? A BODIPY Dye Case Study.

To support the development and characterization of chromophores with targeted photophysical properties, excited-state electronic structure calculations should rapidly and accurately predict how derivatization of a chromophore will affect its excitation and emission energies. This paper examines whether a time-independent excited-state density functional theory (DFT) approach meets this need through a case study of BODIPY chromophore photophysics. A restricted open-shell Kohn-Sham (ROKS) treatment of the S1 excited state of BODIPY dyes is contrasted with linear-response time-dependent density functional theory (TDDFT). Vertical excitation energies predicted by the two approaches are remarkably different due to overestimation by TDDFT and underestimation by ROKS relative to experiment. Overall, ROKS with a standard hybrid functional provides the more accurate description of the S1 excited state of BODIPY dyes, but excitation energies computed by the two methods are strongly correlated. The two approaches also make similar predictions of shifts in the excitation energy upon functionalization of the chromophore. TDDFT and ROKS models of the S1 potential energy surface are then examined in detail for a representative BODIPY dye through molecular dynamics sampling on both model surfaces. We identify the most significant differences in the sampled surfaces and analyze these differences along selected normal modes. Differences between ROKS and TDDFT descriptions of the S1 potential energy surface for this BODIPY derivative highlight the continuing need for validation of widely used approximations in excited state DFT through experimental benchmarking and comparison to ab initio reference data.

Hybridization of a flexible cyclooctatetraene core and rigid aceneimide wings for multiluminescent flapping π systems.

The hybridization of flexible and rigid π-conjugated frameworks is a potent concept for producing new functional materials. In this article, a series of multifluorescent flapping π systems that combine a flexible cyclooctatetraene (COT) core and rigid aceneimide wings with various π-conjugation lengths has been designed and synthesized, and their structure/properties relationships have been investigated. Whereas these molecules have a V-shaped bent conformation in the ground state, the bent structure changes to a planar conformation in the lowest excited singlet (S1 ) state irrespective of the lengths of the aceneimide wings. However, the fluorescence behavior in solution is distinct between the naphthaleneimide system and the anthraceneimide system. The former has a nonemissive S1 state owing to the significant contribution of the antiaromatic character of the planar COT frontier molecular orbitals, thereby resulting in complete fluorescence quenching in solution. In contrast, the latter anthraceneimide system shows an intense emission, which is ascribed to the planar but distorted S1 state that shows the allowed transition between the π-molecular orbitals delocalized over the COT core and the acene wings. The other characteristic of these π systems is the significantly redshifted fluorescence in the crystalline state relative to their monomer fluorescence. The relationship between the packing structures and the fluorescence properties was investigated by preparing a series of hybrid π systems with different sizes of substituents on the imide moieties, which revealed the effect of the twofold π-stacked structure of the V-shaped molecules on the large bathochromic shift in emission.

Two-dimensional tetrathiafulvalene covalent organic frameworks: towards latticed conductive organic salts.

The construction of a new class of covalent TTF lattice by integrating TTF units into two-dimensional covalent organic frameworks (2D COFs) is reported. We explored a general strategy based on the C2 +C2 topological diagram and applied to the synthesis of microporous and mesoporous TTF COFs. Structural resolutions revealed that both COFs consist of layered lattices with periodic TTF columns and tetragonal open nanochannels. The TTF columns offer predesigned pathways for high-rate hole transport, predominate the HOMO and LUMO levels of the COFs, and are redox active to form organic salts that exhibit enhanced electric conductivity by several orders of magnitude. On the other hand, the linkers between the TTF units play a vital role in determining the carrier mobility and conductivity through the perturbation of 2D sheet conformation and interlayer distance. These results open a way towards designing a new type of TTF materials with stable and predesignable lattice structures for functional exploration.

What can density functional theory tell us about artificial catalytic water splitting?

Water splitting by artificial catalysts is a critical process in the production of hydrogen gas as an alternative fuel. In this paper, we examine the essential role of theoretical calculations, with particular focus on density functional theory (DFT), in understanding the water-splitting reaction on these catalysts. First, we present an overview of DFT thermochemical calculations on water-splitting catalysts, addressing how these calculations are adapted to condensed phases and room temperature. We show how DFT-derived chemical descriptors of reactivity can be surprisingly good estimators for reactive trends in water-splitting catalysts. Using this concept, we recover trends for bulk catalysts using simple model complexes for at least the first-row transition-metal oxides. Then, using the CoPi cobalt oxide catalyst as a case study, we examine the usefulness of simulation for predicting the kinetics of water splitting. We demonstrate that the appropriate treatment of solvent effects is critical for computing accurate redox potentials with DFT, which, in turn, determine the rate-limiting steps and electrochemical overpotentials. Finally, we examine the ability of DFT to predict mechanism, using ruthenium complexes as a focal point for discussion. Our discussion is intended to provide an overview of the current strengths and weaknesses of the state-of-the-art DFT methodologies for condensed-phase molecular simulation involving transition metals and also to guide future experiments and computations toward the understanding and development of novel water-splitting catalysts.

Biological and Methodological Variability in Retinal Nerve Fiber Layer OCT: The Framingham Heart Study.

Objective: To explore participant-level biological attributes and scan-level methodological attributes associated with retinal nerve fiber layer (RNFL) thickness variability in a population-based sample of elderly United States adults. Design: Cross-sectional analysis using data from the Framingham Heart Study. Participants: One thousand three hundred forty-seven eyes from 825 participants with ≥1 OCT scan and axial length data were included. Methods: Three or more successive RNFL scans of each eye of each participant were obtained in a single session. Multivariable linear mixed models were employed to explore the associations between average RNFL thickness with participant-level biological attributes (age, gender, race, ethnicity, and axial length) and scan-level attributes (signal strength [SS]) as independent variables in the whole population as well as a subsample of adults with no self-reported history of glaucoma. Similar analyses were designed to assess methodological variability with average within-eye standard deviation (SD) for repeated scans as the dependent variable. Main Outcomes Measures: (1) Biological variability: average RNFL thickness, and (2) methodological variability: average within-participant SD across repeated scans. Results: Age (ß =  - 0.19 microns/year, [95% confidence interval {CI}: - 0.29, - 0.09]), female gender (ß = +1.48 microns vs. male, [95% CI: 0.09, 2.86]), axial length (ß =  - 1.24 microns/mm of greater length, [95% CI: - 1.80, - 0.67]), and SS (ß = +1.62 microns/1 unit greater SS, [95% CI: 1.16, 2.09]) were significantly associated with RNFL thickness, while race and ethnicity were not (P > 0.05). In analyses designed to assess methodological variability, higher RNFL thickness (ß = +0.02 per micron increase, [95% CI: 0.01, 0.03]), and lower SS (ß = +0.19 per 1 unit lower SS, [95% CI: 0.10, 0.27]) were significantly associated with greater RNFL variability. In adults with no self-reported history of glaucoma (n of eyes = 1165, n of participants = 712), female gender was not associated with RNFL, while African American race was associated with thicker RNFL (ß = +4.65 microns vs. Whites, [95% CI: 1.28, 8.03]). Conclusions: Retinal nerve fiber layer thickness is lower with older age, male gender, greater axial length, lower SS, and Whites (as compared with African Americans) without self-reported glaucoma. Measurement variability (SD) is higher with greater RNFL thickness and lower SS. Understanding these biological and methodological variations is important to aid in OCT interpretation. Financial Disclosures: Proprietary or commercial disclosure may be found in the Footnotes and Disclosures at the end of this article.

A multireference perturbation method using non-orthogonal Hartree-Fock determinants for ground and excited states.

In this article we propose the ΔSCF(2) framework, a multireference strategy based on second-order perturbation theory, for ground and excited electronic states. Unlike the complete active space family of methods, ΔSCF(2) employs a set of self-consistent Hartree-Fock determinants, also known as ΔSCF states. Each ΔSCF electronic state is modified by a first-order correction from Mo̸ller-Plesset perturbation theory and used to construct a Hamiltonian in a configuration interactions like framework. We present formulas for the resulting matrix elements between nonorthogonal states that scale as N(occ)(2)N(virt)(3). Unlike most active space methods, ΔSCF(2) treats the ground and excited state determinants even-handedly. We apply ΔSCF(2) to the H2, hydrogen fluoride, and H4 systems and show that the method provides accurate descriptions of ground- and excited-state potential energy surfaces with no single active space containing more than 10 ΔSCF states.

Excitation energies and Stokes shifts from a restricted open-shell Kohn-Sham approach.

Restricted open-shell Kohn-Sham (ROKS) theory provides a powerful computational tool for calculating singlet excited state energies and dynamics. However, the possibility of multiple solutions to the ROKS equations - with the associated difficulty of automatically selecting the physically meaningful solution - limits its usefulness for intensive applications such as long-time Born-Oppenheimer molecular dynamics. We present an implementation of ROKS for excited states which prescribes the physically correct solution from an overlap criterion and guarantees that this solution is stationary, allowing for straightforward evaluation of nuclear gradients. The method is used to benchmark ROKS for vertical excitation energies of small and large organic dyes and for the calculation of Stokes shifts. With common density functional approximations, ROKS vertical excitation energies, and Stokes shifts show similar accuracy to those from time-dependent density functional theory and Δ-self-consistent-field approaches. Advantages of the ROKS approach for excited state structure and molecular dynamics are discussed.

The diabatic picture of electron transfer, reaction barriers, and molecular dynamics.

Diabatic states have a long history in chemistry, beginning with early valence bond pictures of molecular bonding and extending through the construction of model potential energy surfaces to the modern proliferation of methods for computing these elusive states. In this review, we summarize the basic principles that define the diabatic basis and demonstrate how they can be applied in the specific context of constrained density functional theory. Using illustrative examples from electron transfer and chemical reactions, we show how the diabatic picture can be used to extract qualitative insight and quantitative predictions about energy landscapes. The review closes with a brief summary of the challenges and prospects for the further application of diabatic states in chemistry.

Assessment of the ΔSCF density functional theory approach for electronic excitations in organic dyes.

This paper assesses the accuracy of the ΔSCF method for computing low-lying HOMO→LUMO transitions in organic dye molecules. For a test set of vertical excitation energies of 16 chromophores, surprisingly similar accuracy is observed for time-dependent density functional theory and for ΔSCF density functional theory. In light of this performance, we reconsider the ad hoc ΔSCF prescription and demonstrate that it formally obtains the exact stationary density within the adiabatic approximation, partially justifying its use. The relative merits and future prospects of ΔSCF for simulating individual excited states are discussed.

Fluorescence quenching by photoinduced electron transfer in the Zn2+ sensor zinpyr-1: a computational investigation.

We report a detailed study of luminescence switching in the fluorescent zinc sensor Zinpyr-1 by density functional methods. A two-pronged approach employing both time-dependent density functional theory (TDDFT) and constrained density functional theory (CDFT) is used to characterize low-lying electronically excited states of the sensor. The calculations indicate that fluorescence activation in the sensor is governed by a photoinduced electron transfer mechanism in which the energy level ordering of the excited states is altered by binding Zn(2+). While the sensor is capable of binding two Zn(2+) cations, a single Zn(2+) ion appears to be sufficient to activate moderate fluorescence in aqueous solution at physiological pH. We show that it is reasonable to consider the tertiary amine as the effective electron donor in this system, although the pyridyl nitrogens each contribute some density to the xanthone ring. The calculations illustrate an important design principle: because protonation equilibria at receptor sites can play a determining role in the sensor's fluorescence response, receptor sites with a pK(a) near the pH of the sample are to be disfavored if a sensor governed by a simple PET fluorescence quenching model is desired.

Three-dimensional aromaticity in an antiaromatic cyclophane.

Understanding of interactions among molecules is essential to elucidate the binding of pharmaceuticals on receptors, the mechanism of protein folding and self-assembling of organic molecules. While interactions between two aromatic molecules have been examined extensively, little is known about the interactions between two antiaromatic molecules. Theoretical investigations have predicted that antiaromatic molecules should be stabilized when they stack with each other by attractive intermolecular interactions. Here, we report the synthesis of a cyclophane, in which two antiaromatic porphyrin moieties adopt a stacked face-to-face geometry with a distance shorter than the sum of the van der Waals radii of the atoms involved. The aromaticity in this cyclophane has been examined experimentally and theoretically. This cyclophane exhibits three-dimensional spatial current channels between the two subunits, which corroborates the existence of attractive interactions between two antiaromatic π-systems.