Intramolecular through-space charge transfer between benzofuran and ynone groups on a naphthalene spacer.

We have developed a new scaffold that exhibits an efficient intramolecular through-space charge transfer (CT). In this design, electron-rich benzofuran and electron-deficient ynone groups are placed strategically in proximity via a naphthalene spacer. Charge transfer is supported by distinct CT bands in the visible region (>500 nm) in their UV-vis absorption and emission spectra.

Synthesis of New Flexible Coumarin Dimers for Sodium and Potassium Differentiation.

Using fluorescence to detect biologically relevant metals has been studied extensively due to its rapid and low detection limit ability. Sodium and potassium differentiation is significant in diagnosis of many medical conditions. For this, we designed coumarin dimers as flexible fluorescent probes using ethylene glycol units for differentiation of sodium and potassium. To our best knowledge, use of these easy-to-synthesize coumarin dimers linked through ethylene glycol units are first in the literature. In fluorescence titration experiments, diethylene glycol linked coumarin-3-carboxylate dimer is responsive for sodium ions but not for potassium ions. The driving force for the complexation of metal cation and fluorescence probes is thought to be size-matching. To further explain the phenomenon, we synthesized coumarin dimer using 1,8- octanediol as the linker, and methyl ester of coumarin-3-carboxylic acid to investigate the effect of structural changes on the fluorescence intensity. These two compounds could not differentiate the sodium and potassium. Flexible coumarin dimers as fluorophores are shown to be useful for sensing sodium cation in the presence of potassium cation.

Structure and photophysics of indigoids for singlet fission: Cibalackrot.

We report an investigation of structure and photophysics of thin layers of cibalackrot, a sturdy dye derived from indigo by double annulation at the central double bond. Evaporated layers contain up to three phases, two crystalline and one amorphous. Relative amounts of all three have been determined by a combination of X-ray diffraction and FT-IR reflectance spectroscopy. Initially, excited singlet state rapidly produces a high yield of a transient intermediate whose spectral properties are compatible with charge-transfer nature. This intermediate more slowly converts to a significant yield of triplet, which, however, does not exceed 100% and may well be produced by intersystem crossing rather than singlet fission. The yields were determined by transient absorption spectroscopy and corrected for effects of partial sample alignment by a simple generally applicable procedure. Formation of excimers was also observed. In order to obtain guidance for improving molecular packing by a minor structural modification, calculations by a simplified frontier orbital method were used to find all local maxima of singlet fission rate as a function of geometry of a molecular pair. The method was tested at 48 maxima by comparison with the ab initio Frenkel-Davydov exciton model.

Fabrication of functionalized citrus pectin/silk fibroin scaffolds for skin tissue engineering.

In this study, novel porous three-dimensional (3D) scaffolds from silk fibroin (SF) and functionalized (amidated and oxidized) citrus pectin (PEC) were developed for skin tissue engineering applications. Crosslinking was achieved by Schiff's reaction in borax presence as crosslinking coordinating agent and CaCl2 addition. After freeze-drying and methanol treatment, plasma treatment (10 W, 3 min) was applied to remove surface skin layer formed on scaffolds. 3D matrices had high porosity (83%) and interconnectivity with pore size about 120 µm that providing suitable microenvironment for cells. Modifications on PEC chain and crosslinking of scaffolds were verified by fourier-transform infrared spectroscopy (FTIR) analysis and spectrophotometric assay. Scaffolds showed low weight loss (21.3% in 40 days) and high water uptake ability in phosphate-buffered saline (800% in 24 h). Mechanical properties of 3D matrices satisfied the stability of scaffolds under compressive stress and supported adhesion, proliferation and penetration of fibroblast cells. Our results suggested that modified PEC-SF scaffolds would be proposed for use in tissue engineered skin dermal substitutes. © 2018 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 106B: 2625-2635, 2018.

Excitation Localization/Delocalization Isomerism in a Strongly Coupled Covalent Dimer of 1,3-Diphenylisobenzofuran.

Two isomers of both the lowest excited singlet (S1) and triplet (T1) states of the directly para, para'-connected covalent dimer of the singlet-fission chromophore 1,3-diphenylisobenzofuran have been observed. In one isomer, excitation is delocalized over both halves of the dimer, and in the other, it is localized on one or the other half. For a covalent dimer in solution, such "excitation isomerism" is extremely rare. The vibrationally relaxed isomers do not interconvert, and their photophysical properties, including singlet fission, differ significantly.

Covalent dimers of 1,3-diphenylisobenzofuran for singlet fission: synthesis and electrochemistry.

The synthesis of covalent dimers in which two 1,3-diphenylisobenzofuran units are connected through one phenyl substituent on each is reported. In three of the dimers, the subunits are linked directly, and in three others, they are linked via an alkane chain. A seventh new compound in which two 1,3-diphenylisobenzofuran units share a phenyl substituent is also described. These materials are needed for investigations of the singlet fission process, which promises to increase the efficiency of solar cells. The electrochemical oxidation and reduction of the monomer, two previously known dimers, and the seven new compounds have been examined, and reversible redox potentials have been compared with results obtained from density functional theory. Although the overall agreement is satisfactory, some discrepancies are noted and discussed.

Toward designed singlet fission: solution photophysics of two indirectly coupled covalent dimers of 1,3-diphenylisobenzofuran.

In order to identify optimal conditions for singlet fission, we are examining the photophysics of 1,3-diphenylisobenzofuran (1) dimers covalently coupled in various ways. In the two dimers studied presently, the coupling is weak. The subunits are linked via the para position of one of the phenyl substituents, in one case (2) through a CH2 linker and in the other (3) directly, but with methyl substituents in ortho positions forcing a nearly perpendicular twist between the two joint phenyl rings. The measurements are accompanied with density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations. Although in neat solid state, 1 undergoes singlet fission with a rate constant higher than 10(11) s(-1); in nonpolar solutions of 2 and 3, the triplet formation rate constant is less than 10(6) s(-1) and fluorescence is the only significant event following electronic excitation. In polar solvents, fluorescence is weaker because the initial excited singlet state S1 equilibrates by sub-nanosecond charge transfer with a nonemissive dipolar species in which a radical cation of 1 is attached to a radical anion of 1. Most of this charge transfer species decays to S0, and some is converted into triplet T1 with a rate constant near 10(8) s(-1). Experimental uncertainties prevent an accurate determination of the number of T1 excitations that result when a single S1 excitation changes into triplet excitation. It would be one if the charge-transfer species undergoes ordinary intersystem crossing and two if it undergoes the second step of two-step singlet fission. The triplet yield maximizes below room temperature to a value of roughly 9% for 3 and 4% for 2. Above ∼360 K, some of the S1 molecules of 3 are converted into an isomeric charge-transfer species with a shorter lifetime, possibly with a twisted intramolecular charge transfer (TICT) structure. This is not observed in 2.

Evidence for an Intermediate in the Methylation of CB11 H12 - with Methyl Triflate: Comparison of Electrophilic Substitution in Cage Boranes and in Arenes.

The trideuteriomethylation of BH vertices in CB11 H12 - and its derivatives with CD3 OTf (OTf=triflate, trifluoromethanesulfonate) yields a mixture of BCD3 and BCHD2 substitution products, thus demonstrating the intermediacy of a species with a long enough lifetime for hydrogen scrambling between the boron vertex and the methyl substituent. No such scrambling is observed if CD3 OTf is used to methylate toluene. According to density functional theory calculations, the intermediate in BH vertex methylation is a three-center bonded σ adduct of a methyl cation to the BH bond and the proton scrambling occurs via a transition structure containing a distorted square-pyramidal methane attached axially to a "naked" boron vertex. The subsequent proton or deuteron loss is presently not understood in detail. A general comparison of electrophilic substitution on closo-boranes and arenes is provided and similarities as well as differences are discussed. A recalculation of the optimized geometry of the CB11 Me12 . radical produced a second Jahn-Teller distorted minimum and resulted in a somewhat improved agreement between calculated and measured proton hyperfine coupling constants.

The 16 CB11(CH3)n(CD3)(12-n)• radicals with 5-fold substitution symmetry: spin density distribution in CB11Me12(•).

The syntheses of all 16 CB(11)(CH(3))(n)(CD(3))(12-n)(•) radicals with 5-fold substitution symmetry are described. The variation in the width of their broad and featureless electron paramagnetic resonance signals as a function of the deuteriation pattern is used to deduce the relative values of the average hyperfine coupling constants a(H) of the hydrogen atoms in the ipso (1), ortho (2-6), meta (7-11), and para (12) methyl groups, a(H)(i):a(H)(o):a(H)(m):a(H)(p) = (0.18 ± 0.09):(0.71 ± 0.02):(1.00 ± 0.03):(0.52 ± 0.05), and these can be compared with ratios expected from a B3LYP/EPRII calculation, 0.04:0.55:1:0.51.

Search for a small chromophore with efficient singlet fission: biradicaloid heterocycles.

Of the five small biradicaloid heterocycles whose S(1), S(2), T(1), and T(2) adiabatic excitation energies were examined by the CASPT2/ANO-L-VTZP method, two have been found to meet the state energy criterion for efficient singlet fission and are recommended to the attention of synthetic chemists and photophysicists.

Mechanism of photolytic decomposition of N-halamine antimicrobial siloxane coatings.

Generally, antimicrobial N-halamine siloxane coatings can be rehalogenated repetitively upon loss of their biocidal efficacies, a marked advantage over coatings containing other antimicrobial materials. However, the N-halamine materials tend to slowly decompose upon exposure to ultraviolet irradiation as in direct sunlight. In this work the mechanism of photolytic decomposition for the N-halamine siloxanes has been studied using spectroscopic and theoretical methods. It was found that the N-chlorinated coatings slowly decomposed upon UVA irradiation, whereas the unhalogenated coatings did not. Model compound evidence in this work suggests that upon UVA irradiation, the N-Cl bond dissociates homolytically, followed by a Cl radical migration to the alkyl side chain connected to the siloxane tethering group. An alpha and/or beta scission then occurs causing partial loss of the biocidal moiety from the surface of the coated material, thus precluding complete rechlorination. NMR, FTIR, GCMS, and computations at the DFT (U)B3LYP/6-311++G(2d,p) level of theory have been employed in reaching this conclusion.

Singlet exciton fission for solar cell applications: energy aspects of interchromophore coupling.

Singlet exciton fission, a process that converts one singlet exciton to a pair of triplet excitons, has the potential to enhance the efficiency of both bulk heterojunction and dye-sensitized solar cells and is understood in crystals but not well understood in molecules. Previous studies have identified promising building blocks for singlet fission in molecular systems, but little work has investigated how these individual chromophores should be combined to maximize triplet yield. We consider the effects of chemically connecting two chromophores to create a coupled chromophore pair and compute how various structural choices alter the thermodynamic and kinetic parameters likely to control singlet fission yield. We use density functional theory to compute the electron transfer matrix element and the thermodynamics of fission for several promising chromophore pairs and find a trade-off between the desire to maximize this element and the desire to keep the singlet fission process exoergic. We identify promising molecular systems for singlet fission and suggest future experiments.

Toward designed singlet fission: electronic states and photophysics of 1,3-diphenylisobenzofuran.

Single crystal molecular structure and solution photophysical properties are reported for 1,3-diphenylisobenzofuran (1), of interest as a model compound in studies of singlet fission. For the ground state of 1 and of its radical cation (1(+*)) and anion (1(-*)), we report the UV-visible absorption spectra, and for neutral 1, also the magnetic circular dichroism (MCD) and the decomposition of the absorption spectrum into purely polarized components, deduced from fluorescence polarization. These results were used to identify a series of singlet excited states. For the first excited singlet and triplet states of 1, the transient visible absorption spectra, S(1) --> S(x) and sensitized T(1) --> T(x), and single exponential lifetimes, tau(F) = approximately 5.3 ns and tau(T) = approximately 200 micros, are reported. The spectra and lifetimes of S(1) --> S(0) fluorescence and sensitized T(1) --> T(x) absorption of 1 were obtained in a series of solvents, as was the fluorescence quantum yield, Phi(F) = 0.95-0.99. No phosphorescence has been detected. The first triplet excitation energy of solid 1 (11,400 cm(-1)) was obtained by electron energy loss spectroscopy, in agreement with previously reported solution values. The fluorescence excitation spectrum suggests an onset of a nonradiative channel at approximately 37,000 cm(-1). Excitation energies and relative transition intensities are in agreement with those of ab initio (CC2) calculations after an empirical 3000 cm(-1) adjustment of the initial state energy to correct differentially for a better quality description of the initial relative to the terminal state of an absorption transition. The interpretation of the MCD spectrum used the semiempirical PPP method, whose results for the S(0) --> S(x) spectrum require no empirical adjustment and are otherwise nearly identical with the CC2 results in all respects including the detailed nature of the electronic excitation. The ground state geometry of 1 was also calculated by the MP2, B3LYP, and CAS methods. The calculations provided a prediction of changes of molecular geometry upon excitation or ionization and permitted an interpretation of the spectra in terms of molecular orbitals involved. Computations suggest that 1 can exist as two nearly isoenergetic conformers of C(2) or C(s) symmetry. Linear dichroism measurements in stretched polyethylene provide evidence for their existence and show that they orient to different degrees, permitting a separation of their spectra in the region of the purely polarized first absorption band. Their excitation energies are nearly identical, but the Franck-Condon envelopes of their first transition differ to a surprising degree.

Electrospun polyacrylonitrile nanofibrous biomaterials.

An N-halamine precursor, 3-(5'-methyl-5'-hydantoinyl)acetanilide (I), was synthesized in our laboratory and loaded onto electrospun polyacrylonitrile fiber to prepare nanosized biocidal materials, which could be rendered antimicrobial by exposure to household bleach. Differential scanning calorimetry was used to study the thermal properties of the nanofibers with and without the N-halamine precursor and its chlorinated derivative loaded. Scanning electron microscopy demonstrated that the ultrafine fibers formed with diameters from 250 to 600 nm. Chlorinated nanofibrous mats composed of the fibers were challenged with Staphylococcus aureus (ATCC 6538) and Escherichia coli O157:H7 (ATCC 43895); they showed promising inactivation efficacies against the two bacterial species within 5 minutes of contact. Potential uses of the antimicrobial fibers include filters for industrial water and air disinfection and protective clothing.

Carbocyclic 4'-Epiformycin.

Formycin is a naturally occurring biologically responsive C-nucleoside. In pursuing the design and syntheses of novel C-nucleosides, convenient access to carbocyclic C-nucleosides based on the formycin framework was a goal. One such target was carbocyclic 4'-epiformycin (4). This compound is reported via a procedure based on an asymmetric aldol/ring closure methathesis strategy. To provide a preliminary glimpse into the biological characterization of 4 an antiviral assay was conducted. Target 4 was found to be inactive and to lack cytotoxicity to the host cells.

Why does Kevlar decompose, while Nomex does not, when treated with aqueous chlorine solutions?

Kevlar and Nomex are high-performance polymers which have wide varieties of applications in daily life. Recently, they have been proposed to be biocidal materials when reacted with household bleach (sodium hypochlorite solution) because they contain amide moieties which can be chlorinated to generate biocidal N-halamine functional groups. Although Nomex can be chlorinated without any significant decomposition, Kevlar decomposes under the same chlorination conditions. In this study, two mimics for each of the polymers were synthesized to simulate the carboxylate and diaminophenylene components of the materials. It was found that the p-diaminophenylene component of the Kevlar mimic is oxidized to a quinone-type structure upon treatment with hypochlorous acid, which then decomposes. However, such a mechanism for the Nomex mimic is not possible. In this paper, based upon these observations, a plausible answer will be provided to the title question.

Mechanism of 5,5-Dimethylhydantoin Chlorination: Monochlorination through a Dichloro Intermediate.

The hydantoin moiety is an important pharmacore, and when halogenated, hydantoin derivatives act as excellent biocides. However, there have been no computational studies concerning the chlorination mechanism for the hydantoin moiety reported. Herein we describe a computational mechanistic study of the chlorination of 5,5-dimethylhydantoin (H) at the B3LYP/6-311+G(2d,p) level. Under a 1:1 molar ratio of hydantoin and a chlorinating agent (HOCl), conproportionation is calculated to be favorable to give the N1 monochloro derivative as the major predicted product, which is in agreement with experiment. Initial direct chlorination at the N1 position is prevented by a high kinetic barrier. The first step involves the deprotonation of the hydantoin moiety (at the N3 position) which is followed by a SN2 step transferring a chloronium ion (Cl(+)) from HOCl to the ionized hydantoin anion. A mechanism is proposed where the N3 nitrogen is chlorinated first followed by the N1 position to form the dichloro derivative. When CPCM solvation free energies (ΔG(solv)) were added to the gas-phase free energies (ΔG(gas)) along the SN2 reaction path, a sudden decrease in free energy was observed due to the incipient formation of the hydroxide ion. Explicit consideration of solvation within a box of 512 water molecules led to a much more gradual free energy change along the reaction path but a very similar free energy of activation.

Mechanism of formation of biocidal imidazolidin-4-one derivatives: an Ab initio density-functional theory study.

N-halamine chemistry has been a research topic of considerable importance in these laboratories for over 2 decades because N-halamine compounds are very useful in preparing biocidal materials. To understand the utility of these compounds, the stabilities and mechanism of halogenation of cyclic N-halamine compounds should be resolved. The important precursor biocidal compound, 2,2,5,5-tetramethylimidazolidin-4-one (TMIO) was considered as a model in this theoretical study. The thermodynamic and kinetic products of monohalogenation were investigated along with tautomerization of TMIO and succinimide theoretically at the level of B3LYP/6-311+G(2d,p). Solvation effects (water and chloroform) were included using the CPCM solvation model with UAKS cavities. Several mechanisms have been proposed for the chlorine migration from the 3-position (kinetic product) to the 1-position (thermodynamic product) of the TMIO ring. The results are in agreement with experimental NMR data.

The Stabilities of N-Cl Bonds in Biocidal Materials.

N-halamine chemistry has been a research topic of considerable importance in these laboratories for over two decades. N-halamine compounds are useful in preparing biocidal materials. There are three N-Cl moieties available in cyclic N-halamine compounds: imide, amide, and amine. The stabilities toward the release of free halogen have been experimentally shown to decrease in the order amine > amide > imide. In this work, this generalization has been tested theoretically at the level of B3LYP/6-31+G(d) and using the conductor-like polarizable continuum aqueous solvation model with UAKS cavities. Excellent accord was observed between theory and experiment. It was also found that the imide and amide N-halamine stabilities on hydantoin rings could be reversed with substitution patterns at the 5 position.