Reciprocity of CO⋯π interactions with the dominant anion-π on fullerene (C60)- amine-based organocatalysts: a mechanistic elucidation for addition vs. decarboxylation reaction.

Reiterating the counterintuitive anion-π interactions that J. López-Andarias and coworkers [J. Am. Chem. Soc., 2017, 139, 13296-13299] have experimentally discussed in their pioneering work, the current investigation explores the role of such interactions in the fullerene-amine conjugate-based organocatalysis reaction via density functional theory (DFT) protocols where the underlying catalytic reaction paths have been ascribed to unique transition state geometries. The reaction between MAHT (malonic acid half thioester) and nitrostyrene was reported to follow the addition and decarboxylation pathways. Our findings exclusively help to visualize and quantify anion-π interactions operating in the planar enolate intermediates. We substantiate that the synergistic effects of anion-π and CO⋯π surface interactions play a central role in distinguishing the planar and bent tautomers with delocalized and localized charges, respectively, on the π-acidic surfaces of fullerene C60 catalysts. Overall, the theoretical pieces of evidence suggest a selective acceleration of the addition pathway, leading to a higher yield of the addition product, as observed in the experiments [J. Am. Chem. Soc., 2017, 139, 13296-13299].

Through-Bond-Driven Through-Space Interactions in a Fullerene C60 Noncovalent Dyad: An Unusual Strong Binding between Spherical and Planar π Electron Clouds and Culmination of Dyadic Fractals.

Hydrogen-bond-induced π-depletion as a criterion for π-stacking, a configurationally unique noncovalent strategy enabled an unconventional strong binding between the spherical N-fulleropyrrolidine (NFP) and the planar distributions of π electron clouds of three substituted pybates to form noncovalent fulleropyrrolidino-4-(pyrenyl) butanoate dyads of large computed interaction energies, varying between 37.49 and 44.93 kcal/mol. The geometrical distortion/bending of the alkyl tail of pybate in the noncovalent dyad was experimentally corroborated via UV-vis absorption spectroscopy, which translated into spectral broadening along with pronounced shifts in the n-π* transitions of the oxy-substituted pyrene in different solvents, ensuring through-bond interactions. Facile electron transfer through H-bond influenced the dynamic dispersive forces to be active, revealing the supremacy of through-bond over through-space interactions. The analyses of intermolecular forces using an absolutely localized molecular orbital-based energy decomposition analysis (ALMO-EDA) scheme revealed intricate insights into the intermolecular interactions and characteristic charge transfer; the dominance of forward electron transfer (pybate to NFP) over the reverse in offering stabilization was noted. Charge transfer was investigated further from natural bond orbital (NBO) and absolutely localized molecular orbital-based charge-transfer analysis (ALMO-CTA) methods, establishing the supremacy of donor-to-acceptor electron transfer over the reverse (acceptor-to-donor) one. The characteristic self-assembly of the noncovalent dyad in suitable solvents led to the formation of fractal networks via reaction-limited cluster aggregation with a fractal dimension of 2.37. Adoption of constrained molecular dynamics simulations indicated probable wrapping of pybates around NFP, leading to fractal-like assembly.

Synergistic effect of hydrophobic and hydrogen bonding interaction-driven viologen-pyranine charge-transfer aggregates: adenosine monophosphate recognition.

Understanding the role of non-covalent interactions that dictate and fine-tune the direction of self-assembly of functional molecules is crucial for developing stimuli responsive materials. Herein, we systematically designed and synthesized viologen derivatives with hydrophobic dodecyl chains and alkyl carboxylic acid functionalities. The complementary electronic and electrostatic counterpart of viologens was chosen as pyranine. Viologens comprising of a hydrophobic dodecyl chain on one terminal and hydrogen bonding alkyl carboxylic acid on the other (V1 and V2) underwent aggregation to a varying extent upon interaction with pyranine. The length of the alkyl carboxylic acid had a greater impact on the nature and morphology of the aggregates. Control molecules (V3 and V4) in which 4,4'-bipyridine was symmetrically quaternized with alkyl carboxylic acids did not aggregate upon interaction with pyranine. The delicate balance existing between the hydrophobicity of the dodecyl chains and the intermolecular hydrogen bonding interaction between the alkyl carboxylic acid groups in V1 or V2 of the corresponding charge transfer (CT) complexes was instrumental in driving the aggregation. The CT aggregates of [V1-Pyr] and [V2-Pyr] exhibited excellent stability in water which disaggregated at physiological pH. We emphasize on the importance of synergy between hydrophobic and hydrogen bonding interactions in reinforcing each other to drive the supramolecular aggregation of the CT complexes. Such pH dependent CT aggregates are of importance as scaffolds in pH controlled drug release. In the present study, the CT aggregates were evaluated for adenosine nucleotide recognition in water; [V1-Pyr] and [V2-Pyr] exhibited selective response towards adenosine monophosphate via deprotonation induced dissolution of aggregates in water leading to emission enhancement.

Natural rubber-SiO2 nanohybrids: interface structures and dynamics.

Homogeneous dispersion of silica nanoparticles (SiO2 NPs) in natural rubber (NR) is a key challenge for engineering high-performance nanocomposites and elucidation of their structure on a molecular basis. Towards this, the present work devised a novel route for obtaining 3D self-assembled SiO2 NP-NR nanocomposites under aqueous conditions and in the presence of Mg2+, by establishing a molecular bridge that clamped the negatively charged NR and SiO2 colloidal particles with a favoured NR-SiO2 NP hetero-aggregation. The characteristic NR-SiO2 NP hetero-aggregates displayed a decreased heat capacity with increase in the SiO2 mass-fraction, implying a restricted NR chain mobility. Such changes in the interfacial layers were tapped by 29Si NMR, DFT calculations and molecular dynamics simulations towards a mechanistic understanding of the structure and dynamics of the NR/SiO2 NP hybrid. Simple models were used to illustrate basic ideas; specific electrostatic interactions such as ion-dipole and H-bonding interactions proved to be the driving forces for the organized assembly leading to the NR-SiO2 hetero-aggregate over the NR-NR or SiO2 NP-SiO2 NP homo-aggregate. Molecular dynamics simulation of the aqueous canonical ensemble of the hybrid showed the stable molecular conformation to reveal a SiO2 NP spherical core encapsulated by a hydrophobically interconnected NR polymer layer as the outer shell, as a unique structural model. Specifically, the lipid end of the NR was involved electrostatically while the lysine end (the protein part of NR) H-bonded to the core silica cluster thereby restricting random aggregation. The calculated negative free energy changes for the hetero-aggregate composites via their vibrational and rotational spectra proved the spontaneity of composite formation.

Oxidation State-Dependent Electronic Properties of Sulfur-Containing Thermally Activated Delayed Fluorescence Molecules.

Comparative studies of a series of sulfur-containing thermally activated delayed fluorescence (TADF) molecules and their oxidized compounds are carried out by means of electronic structure calculations. Aiming at investigating the effects of oxidation of bridged sulfur on the modulation of electronic structures of sulfur-containing TADF molecules, their geometrical structures, singlet (S1) and triple (T1) energies and their gap (ΔEST), the transition dipole moment, the spin-orbit coupling (SOC) between S1 and T1 states, the ionization potentials, and electron affinities are analyzed in detail to determine the structure-property relationships in these investigated TADF molecules and their corresponding oxidized counterparts. The electronic structure calculations show that the oxidation of bridged sulfur into the corresponding sulfoxide and sulfone significantly changes the electronic properties of TADF molecules. Interestingly, a substantial reduction in the singlet-triplet energy difference is possible with an increase in the oxidation state of the sulfur atom in the core. Moreover, the sulfone-containing molecules exhibit both S1 and T1 states having a large charge transfer (CT) excitation characteristic, which helps reduce the singlet-triplet energy gap and facilitates the reverse intersystem crossing (RISC) from the triplet state to the singlet state. SOC values increase with an increase in the oxidation state of the sulfur atom. Particularly, a sulfoxide-containing core moiety exhibits higher SOC values when compared with the sulfone-containing acceptor core.

Carbon Nanosheets by Morphology-Retained Carbonization of Two-Dimensional Assembled Anisotropic Carbon Nanorings.

Two-dimensional (2D) carbon nanomaterials possessing promising physical and chemical properties find applications in high-performance energy storage devices and catalysts. However, large-scale fabrication of 2D carbon nanostructures is based on a few specific carbon templates or precursors and poses a formidable challenge. Now a new bottom-up method for carbon nanosheet fabrication using a newly designed anisotropic carbon nanoring molecule, CPPhen, is presented. CPPhen was self-assembled at a dynamic air-water interface with a vortex motion to afford molecular nanosheets, which were then carbonized under inert gas flow. Their nanosheet morphologies were retained after carbonization, which has never been seen for low-molecular weight compounds. Furthermore, adding pyridine as a nitrogen dopant in the self-assembly step successfully afforded nitrogen-doped carbon nanosheets containing mainly pyridinic nitrogen species.

A New Class of Nitroanilinic Dimer, the PNA O-Dimer: Electronic Structure and Emission Characteristics of O-Dimeric Aggregates.

p-Nitroaniline (PNA) has been reported as a "J" aggregate species. In retrospect, this communication confirms a radically different "oblique" orientation of the PNA units in all three solid, liquid, and gas phases of the dimer, the O-dimer. The nonvanishing transition dipole moments (TDM) associated with the allowed electronic excitations of the O-dimer, computed using electron-hole pair density distribution (EDD and HDD) analyses ascertained the two monomers to be inclined at slippage (θ) and polarization (α) angles of 18.5° and 55.4°, respectively. A detailed structure-property relationship of the PNA O-dimeric aggregate was carried out using UV-vis absorption and matrix scan emission spectroscopy, supported by electronic structure calculations at DFT-M062X/6-31G+(d,p) level using integral equation formalism polarizable continuum model (IEFPCM). The computed potential energy surface (PES) implied the global minimum of the PNA O-dimer stabilized by 4.8 kcal.mol(-1), owing to bifurcated intermolecular hydrogen bonding. In the excited PNA O-dimeric aggregate, an exchange of excitation energy between the monomeric units resulted in two distinct electronic states separated by an interaction energy of -1644 cm(-1). The TD-DFT computed excited state equilibrium structures of the PNA O-dimer corroborated the experimentally observed pronounced Stoke's shift to internal conversion following vibrational relaxation of the allowed electronic excited states. On the basis of the detailed structural analysis of PNA O-dimer, the observed energy shifts in optical absorption spectroscopy were evident within the framework of exciton coupling model.

Structure and dynamics of H2O vis-á-vis phenylalanine recognition at a DPPC lipid membrane via interfacial H-bond types: insights from polarized FT-IRRAS and ADMP simulations.

Preferential and enantioselective interactions of L-/D-Phenylalanine (L-Phe and D-Phe) and butoxycarbonyl-protected L-/D-Phenylalanine (LPA and DPA) as guest with 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (L-DPPC) as host were tapped by using real time Fourier transform infrared reflection absorption spectroscopy (FT-IRRAS). Polarization-modulated FT-IRRAS of DPPC monolayers above the phenylalanine modified subphases depicted fine structure/conformation differences under considerations of controlled 2D surface pressure. Selective molecular recognition of D-enantiomer over L-enantiomer driven by the DPPC head group via H-bonding and electrostatic interactions was evident spectroscopically. Accordingly, binding constants (K) of 145, 346, 28, and 56 M(-1) for LPA, DPA, L-Phe, and D-Phe, respectively, were estimated. The real time FT-IRRAS water bands were strictly conformation sensitive. The effect of micro-solvation on the structure and stability of the 1:1 diastereomeric L-lipid⋯, LPA/DPA and L-lipid⋯, (L/D)-Phe adducts was investigated with the aid of Atom-centered Density Matrix Propagation (ADMP), a first principle quantum mechanical molecular dynamics approach. The phosphodiester fragment was the primary site of hydration where specific solvent interactions were simulated through single- and triple- "water-phosphate" interactions, as water cluster's "tetrahedral dice" to a "trimeric motif" transformation as a partial de-clusterization was evident. Under all the hydration patterns considered in both static and dynamic descriptions of density functional theory, L-lipid/D-amino acid enantiomer adducts continued to be stable structures while in dynamic systems, water rearranged without getting "squeezed-out" in the process of recognition. In spite of the challenging computational realm of this multiscale problem, the ADMP simulated molecular interactions complying with polarized vibrational spectroscopy unraveled a novel route to chiral recognition and interfacial water structure.

Tailoring recognition clefts from non-specific recognition matrices in mixed molecular arrays.

Multi-component organic interfaces with molecular-level mixing were prepared by integrating benzoic acid appended thiophene amphiphile [4-(6-(thiophene-3-carbonyloxy)hexyloxy)benzoic acid] (T6BA) and (±)-α-lipoic acid onto the Au surface. On a flat surface with infinite radii of curvature, T6BA and (±)-α-lipoic acid, endowed with chemically distinct end-groups, provided sufficient length mismatch to gain conformational entropy leading to stripe-like patterns when the immiscible ligands co-adsorbed. Good quality multi-component organic interfaces and molecular islands could be fabricated via composition variation of the participating ligands. Host-guest chemistry between benzoic acids and ß-cyclodextrin was used to confirm the molecular-level mixing. T6BA and (±)-α-lipoic acid, each being a non-specific recognition matrix for dopamine, could thus be organized into mixed molecular arrays having well defined cavities for guest inclusion. This mixed molecular array behaved as a 'recognition matrix' for dopamine (DA, 15 nm) in the presence of ascorbic acid (AA). The surface patterns described here on a flat surface should in principle be applicable to other geometrical structures like spheres and cylinders. Further, charge transfer through the T6BA self-assembled monolayers depended on the anion type present in the supporting electrolyte, monitored through cyclic voltammetry.

Phosphonium Ionic Liquid-Activated Sulfur Vulcanization: A Way Forward to Reduce Zinc Oxide Levels in Industrial Rubber Formulations.

Extensive use of zinc oxide and accelerators such as diphenyl guanidine (DPG) in the vulcanization of rubber composites entail potential environmental risks. These are pervasive contaminants of roadway runoff originating from tire wear particles (TWPs). Herein, the effect of phosphonium ionic liquids (PILs) in styrene-butadiene rubber compounds was demonstrated with reduced ZnO loading and no DPG to minimize the environmental footprint of the vulcanization process. The structure and chemistry of PILs were found to be the influencing parameters impelling the cross-linking kinetics, enabling shorter induction times. The generation of active Zn2+ sites by PILs was examined through FTIR spectroscopy, calorimetry, and molecular dynamics simulations. From a tire application perspective, the PILs not only enhanced the cure kinetics but also improved the dynamic-mechanical behavior of the rubber composites. Consequently, the harm caused by TWPs to the atmosphere, fuel intake, and CO2 emissions was minimal, thereby confirming the potential use of PILs in the tire industry.

Molecular Insights into Antioxidant Efficiency of Melanin: A Sustainable Antioxidant for Natural Rubber Formulations.

N-(1,3-Dimethyl butyl)-N'-phenyl-p-phenylenediamine (6-PPD) is a worldwide antioxidant commonly added to delay the thermo-oxidative degradation of tire rubbers. Unfortunately, 6PPD and its transformation product 6PPD-quinone are toxic to aquatic organisms (e.g., coho salmon). Herein, we explore the free radical scavenging activity and protective mechanism of melanin (MLN) on natural rubber's (NR's) oxidative resistance using molecular dynamics (MD) and quantum mechanical (QM) calculations. The relationship between the molecular structure and the chemical nature of the antioxidant molecules via transition state calculations is explored to unravel the reaction mechanisms of antioxidants interacting with peroxy radicals (ROO·) of NR with the estimation of reaction barriers. Following this, the radical scavenging activity of antioxidants was quantified via a hydrogen atom transfer mechanism and bond dissociation energy calculations. Parallel MD simulations were considered to study the interfacial interactions of antioxidant molecules with polymer chains and fillers with a quantifiable structure-property correlation. Given these results, the nanocomposite (NR-MLN-SiO2) with natural antioxidant melanin manifested outstanding antioxidant properties by preferentially bagging the ROO· radicals, thus improving NR's thermal-oxidative aging relative to 6-PPD. The MD results revealed that the intermolecular interactions at the NR/antioxidant interface benefited the antioxidant MLN to bind tightly to the NR in NR-MLN-SiO2 composite, thus exhibiting improved dispersion, O2 barrier properties, and thermo-oxidative stability, which could extend the service life of NR products (e.g., tires). In addition, as a sustainable antioxidant, MLN could replace toxic antioxidants like 6-PPD. More importantly, the QM/MD simulations provided a fundamental understanding of the mechanistic pathways of antioxidant molecules in NR composites, which are conducive to designing high-performance and sustainable green elastomers.

L-tryptophan-induced electron transport across supported lipid bilayers: an alkyl-chain tilt-angle, and bilayer-symmetry dependence.

Molecular orientation-dependent electron transport across supported 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) lipid bilayers (SLBs) on semiconducting indium tin oxide (ITO) is reported with an aim towards potential nanobiotechnological applications. A bifunctional strategy is adopted to form symmetric and asymmetric bilayers of DPPC that interact with L-tryptophan, and are analyzed by surface manometry and atomic force microscopy. Polarization-dependent real-time Fourier transform infrared reflection absorption spectroscopy (FT-IRRAS) analysis of these SLBs reveals electrostatic, hydrogen-bonding, and cation-π interactions between the polar head groups of the lipid and the indole side chains. Consequently, a molecular tilt arises from the effective interface dipole, facilitating electron transport across the ITO-anchored SLBs in the presence of an internal Fe(CN)(6)(4-/3-) redox probe. The incorporation of tryptophan enhances the voltammetric features of the SLBs. The estimated electron-transfer rate constants for symmetric and asymmetric bilayers (k(s) = 2.0×10(-2) and 2.8×10(-2) s(-1)) across the two-dimensional (2D) ordered DPPC/tryptophan SLBs are higher compared to pure DPPC SLBs (k(s) = 3.2×10(-3) and 3.9×10(-3) s(-1)). In addition, they are molecular tilt-dependent, as it is the case with the standard apparent rate constants k(app)(0), estimated from electrochemical impedance spectroscopy and bipotentiostatic experiments with a Pt ultramicroelectrode. Lower magnitudes of k(s) and k(app)(0) imply that electrochemical reactions across the ITO-SLB electrodes are kinetically limited and consequently governed by electron tunneling across the SLBs. Standard theoretical rate constants (k(th)(0)) accrued upon electron tunneling comply with the potential-independent electron-tunneling coefficient ß = 0.15 Å(-1). Insulator-semiconductor transitions moving from a liquid-expanded to a condensed 2D-phase state of the SLBs are noted, adding a new dimension to their transport behavior. These results highlight the role of tryptophan in expediting electron transfer across lipid bilayer membranes in a cellular environment and can provide potential clues towards patterned lipid nanocomposites and devices.

Interfacial Janus gold nanoclusters as excellent phase- and orientation-specific dopamine sensors.

This investigation, following our recent report on the one-pot hemi-micellar interfacial synthesis of Janus gold nanoclusters and the inter-cluster electron coupling establishing insulator-metal transition in the oriented Janus monolayers [Langmuir, 2010, 26(17), 14047], was to fabricate modified electrodes for sensing dopamine, the neurotransmitter. With a detection limit in the sub-nanomolar range, the apparent electron transfer rate constants for dopamine detection signified an intricate Janus cluster 2D phase dependency. Surface pressure as a thermodynamic variable controlled the electronic communication between the clusters as a result of varied inter-cluster distance and size, ultimately reflecting on the sensitivity and detection limit for dopamine sensing. The non-covalent nature of the ligands on the core metal clusters facilitated the overall electro-catalytic oxidation of dopamine. The notable feature of this precise work was that it established a more effective phase- and orientation-specific Janus cluster sensing than those reported through patterned gold nanowire based sensors.

Solvent-polarity-tunable dimeric association of a fullerene (C60)-N,N-dimethylaminoazobenzene dyad: modulated electronic coupling of the azo chromophore with a substituted 3D fullerene.

The tunable self-assembly of a fullerene (C(60))-N,N-dimethylaminoazobenzene (DPNME) molecular system as a function of solvent polarity in THF/water binary solvent is reported. Gradual increase of the volume fraction of the nonsolvent water in a 1×10(-5) M THF solution of DPNME at a mixed dielectric constant ε(mix)≈42 resulted in initial redshifting of the (1)(π→π(*)) absorption band, which signified the 1D head-to-tail or J-type arrangement of the DPNME molecular system. Further increase in the solvent polarity to ε(mix)≈66 evidenced formation of an antiparallel head-to-tail or H-type molecular arrangement in conjunction with the J-aggregates, thereby establishing a solvent-polarity-dependent dynamic equilibrium between the monomer ↔ J-aggregate ↔ H-aggregate. The controlled aggregation was governed by the synergetic effect of intermolecular donor-acceptor interaction between the electron-deficient fullerene ring and the electron-rich N,N-dimethylamino-substituted aromatic ring; typically, van der Waals and π-π interactions between the molecules constituting a pair of dimers were envisaged. An agreement between the semiempirically calculated drastically reduced oscillator strength of the DPNME H-dimer in the antiparallel configuration (0.69 vs. 1.29 in the monomeric DPNME) and the experimental electronic absorption spectra beyond ε(mix)=66 further strengthened this assignment to the hitherto forbidden antiparallel H-dimer. Complementing the above, the periodicity of molecular self-assembly dictated a monoclinic unit cell in the single-crystal XRD packing pattern with a C2/c space group; the molecules packed laterally with mutual interdigitation with the donor (E)-N,N-dimethyl-4-(p-tolyldiazenyl)aniline (AZNME) parts in an antiparallel fashion (contrary to the usual expectation for H-aggregates) with strong inter- and intrapair van der Waals and π-π interactions between the constituent fullerene moieties. Unlike those of porphyrin/phthalocyanine bowl-like donor-initiated architectures, a rare class of DPNME dyadic supramolecular self-assemblies was realized with π-extended 2D fullerene networks, in which the linear geometry of the AZNME donor and the conformational rigidity of the fullerene acceptor played crucial roles.

Tunable one-, two-, and three-dimensional self-assemblies from an acceptor-donor fullerene-N,N-dimethylaminoazobenzene dyad: interfacial geometry and temporal evolution.

Controllable fabrication of spontaneously ordered and varied geometry fullerene C(60) based molecular architecture was achieved upon hierarchical self-assembly of the fullerene-N,N-dimethylaminoazobenzene acceptor-donor hybrid (DPNME). Simple preparation techniques, such as Langmuir-Blodgett (LB), solution-cast, and immersion at the liquid-air and solid-air interfaces, were used without templates as a function of DPNME concentration, media pH, time, and supporting substrate characteristics. The resulting structures depending upon the preparation methods were investigated with field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), atomic force microscopy (AFM), and molecular modeling, which revealed a delicate role of intermolecular donor-acceptor, π-π, and van der Waals interactions between the electron deficient fullerene core and the N,N-dimethylaminoazobenzene electron donor under neutral conditions. Upon protonation, the electrostatics associated with the charged DPNME moiety and the dominant intermolecular fullerene-fullerene interactions guided the self-assembly process. Increased time scales led the molecular subunits to grow by maximizing the most favored orientations and yielded one-dimensional (1D) and two-dimensional (2D) structures in neutral and acidic conditions, respectively, which upon solvent evaporation formed the final multipods or stacked squares upon oriented attachment. For the protonated DPNME, 2D lamellar sheets formed from the bilayers gained cohesive energy, forming ultimately rectangular sheets. Interestingly, the Si(100) supported multilayer DPNME Langmuir films as a function of surface pressure and pH yielded a uniform and directional structure pattern in comparison with the geometry obtained from drop casting methods. This controllable structure architecture of the fullerene-azobenzene hybrid opens up a new alley in fullerene C(60) based self-assembly.

Intramolecular electronic communication in a dimethylaminoazobenzene-fullerene C60 dyad: an experimental and TD-DFT study.

An electronically push-pull type dimethylaminoazobenzene-fullerene C(60) hybrid was designed and synthesized by tailoring N,N-dimethylaniline as an electron donating auxochrome that intensified charge density on the beta-azonitrogen, and on N-methylfulleropyrrolidine (NMFP) as an electron acceptor at the 4 and 4' positions of the azobenzene moiety, respectively. The absorption and charge transfer behavior of the hybrid donor-bridge-acceptor dyad were studied experimentally and by performing TD-DFT calculations. The TD-DFT predicted charge transfer interactions of the dyad ranging from 747 to 601 nm were experimentally observed in the UV-vis spectra at 721 nm in toluene and dichloromethane. A 149 mV anodic shift in the first reduction potential of the N=N group of the dyad in comparison with the model aminoazobenzene derivative further supported the phenomenon. Analysis of the charge transfer band through the orbital picture revealed charge displacement from the n((N=N)) (nonbonding) and pi ((N=N)) type orbitals centered on the donor part to the purely fullerene centered LUMOs and LUMO+n orbitals, delocalized over the entire molecule. The imposed electronic perturbations on the aminoazobenzene moiety upon coupling it with C(60) were analyzed by comparing the TD-DFT predicted and experimentally observed electronic transition energies of the dyad with the model compounds, NMFP and (E)-N,N-dimethyl-4-(p-tolyldiazenyl)aniline (AZNME). The n((N=N)) --> pi*((N=N)) and pi((N=N)) --> pi*((N=N)) transitions of the dyad were bathochromically shifted with a significant charge transfer character. The shifting of pi((N=N)) --> pi*((N=N)) excitation energy closer to the n --> pi*((N=N)) in comparison with the model aminoazobenzene emphasized the predominant existence of charge separated quinonoid-like ground state electronic structure. Increasing solvent polarity introduced hyperchromic effect in the pi((N=N)) --> pi*((N=N)) electronic transition at the expense of transitions involved with benzenic states, and the extent of intensity borrowing was quantified adopting the Gaussian deconvolution method. On a comparative scale, the predicted excitation energies were in reasonable agreement with the observed values, demonstrating the efficiency of TD-DFT in predicting the localized and the charge transfer nature of transitions involved with large electronically asymmetric molecules with HOMO and LUMO centered on different parts of the molecular framework.

Tunable electronic properties of a proton-responsive N,N-dimethylaminoazobenzene fullerene (C60) dyad.

A basic N,N-dimethylaminoazobenzene-fullerene (C(60)) dyad molecular skeleton is modelled and synthesized. In spite of the myriad use of azobenzene as a photo- and electrochromic moiety, the idea presented herein is to adopt a conceptually different path by using it as a bridge in a donor-bridge-acceptor single-molecular skeleton, connecting the electron acceptor N-methylfulleropyrrolidine with an electron donor N,N-dimethylaniline. Addition of trifluoroacetic acid (TFA) results in a drastic colour change of the dyad from yellow to pink in dichloromethane (DCM). The structure of the protonated species are established from electronic spectroscopy and time-dependent density functional theory (TD-DFT) calculations. UV/Vis spectroscopic investigations reveal the disappearance of the 409 nm (1)(π→π*) transition with appearance of new features at 520 and 540 nm, attributed to protonated ß and α nitrogens, respectively, along with a finite weight of the C(60) pyrrolidinic nitrogen. Calculations reveal intermixing of n((N=N))→π*((N=N)) and charge transfer (CT) transitions in the neutral dyad, whereas, the n((N=N))→π*((N=N)) transition in the protonated dyad is buried under the dominant (1)(π →π*) feature and is red-shifted upon Gaussian deconvolution. The experimental binding constants involved in the protonation of N,N-dimethylanilineazobenzene and the dyad imply an almost equal probability of existence of both α- and ß-protonated forms. Larger binding constants for the protonated dyads imply more stable dyad complexes than for the donor counterparts. One of the most significant findings upon protonation resulted in frontier molecular orbital (FMO) switching with the dyad LUMO located on the donor part, evidenced from electrochemical investigations. The appearance of a new peak, prior to the first reduction potential of N-methylfulleropyrrolidine, clearly indicates location of the first incoming electron on the donor-centred LUMO of the dyad, corroborated by unrestricted DFT calculations performed on the monoanions of the protonated dyad. The protonation of the basic azo nitrogens thus enables a rational control over the energetics and location of the FMOs, indispensable for electron transport across molecular junctions in realizing futuristic current switching devices.

One pot hemimicellar synthesis of amphiphilic Janus gold nanoclusters for novel electronic attributes.

A one-pot hemimicellar synthesis of oriented, amphiphilic, and fluorescent Janus gold clusters, establishing the Janus character in terms of ligand asymmetry and distribution, has been demonstrated. The method was based on the efficient Langmuir strategy, where the in situ two-dimensional (2D) reduction of Au(3+) in the sprayed micellar electrostatic complex, TOA(+)-AuCl(4)(-), was accomplished by subphase tryptophan that acted as the hydrophilic protecting ligand on one hemisphere of the spherical gold cluster. In contrast to the reported micelle-assisted Janus cluster formation, here the cluster growth occurred inside the surface pressure driven hemimicelles, which rapidly formed 2D cluster arrays without any interfacial reorientation. The Janus structure was validated using angle dependent polarized Fourier Transform Infrared Reflection-Absorption Spectroscopy (FT-IRRAS), where orientation dependent vibrational changes in the adsorbed ligand functionalities were detected. Electrochemical impedance measurements of the transferred Janus layers onto hydrophobized ITO revealed the heterogeneous electron transfer rate constant k(ET) to show a clear orientational odd-even parity effect with the odd layers showing much higher rates. Isobaric area relaxation investigations further evidenced toward a hemispherical instantaneous nucleation with edge growth mechanism of the nanoclusters formed at the tryptophan subphase. Surface pressure as a thermodynamic variable effectively controlled the interparticle separation; intercluster electron coupling exhibited insulator-metal transition in the Janus cluster monolayers through scanning electrochemical microscopy investigations.

Design Principles of Interfacial Dynamic Bonds in Self-Healing Materials: What are the Parameters?

Polymers and polymer nanocomposites (PNCs) are extensively used in daily life. However, the growing requirement of advanced PNCs laid persistent environmental issues due to deformation-induced damage that once formed, does not vanish at future stages. Therefore, self-healing materials with significantly enhanced long life and safety have been designed to epitomize the forefront of recent advances in materials chemistry and engineering. Self-healing PNC (SH-PNCs) materials are a class of smart composites in which nanoparticles induce interfacial reconstruction via multiple covalent and non-covalent interactions culminating in improved mechanical strength and self-healing capability. However, since the filler nanoparticles are independent of the reversible supramolecular network, the filler incorporation destroys the self-healing ability but could enhance the mechanical strength. Hence, the molecular parameters controlling the alliance of robust mechanical strength with virtuous self-healing ability is a crucial challenge. Herein, we review the latest developments that have been made in self-healing materials and puts advancing insights into the fabrication of SH-PNCs in which the combination of covalent bonds and non-covalent interactions provides an optimal balance between their mechanical performance and self-healing capability. We highlight the importance of specific entropic, enthalpic changes, polymer chain conformations and flexibility that enable the reconstruction of damaged surface and physical reshuffling of dynamic bonds at the interface of cut surfaces.

Role of Interface Structure and Chain Dynamics on the Diverging Glass Transition Behavior of SSBR-SiO2-PIL Elastomers.

Intermolecular interactions between the constituents of a polymer nanocomposite at the polymer-particle interface strongly affect the segmental mobility of polymer chains, correlated with their glass transition behavior, and are responsible for the improved dynamical viscoelastic properties. In this work, we emphasized on the evolution of characteristic interfaces and their dynamics in silica (SiO2 NP)-reinforced, solution-polymerized, styrene butadiene rubber (SSBR) composites, whose relative prevalence varied with the phosphonium ionic liquid (PIL) volume fraction, used as an interfacial modifier. The molecular origins of such interfaces were examined through systematic dielectric spectroscopy, molecular dynamics (MD) simulations, and dynamic-mechanical analyses. The PIL facilitated H-bonding, cation-π, surface-phenyl, and van der Waals interfacial interactions between SSBR and SiO2 NP, thereby regulating the polymer chain dynamics, orientation, and mean-square displacement. Specifically, the mass density profiles from MD simulations revealed the dynamic gradient of polymer chains in the interfacial region as a function of radial distance from the center of mass of the SiO2 NP surface. The results showed a structuring effect to result in well-resolved density peaks at specific radial distances with the tangential orientation of styrene monomers in the vicinity of the SiO2 NP surface. These domino effects highlighted strong interfacial interactions to have an indispensable effect on the viscoelastic performance and thermal motion of SSBR molecular chains, leading to a higher glass transition temperature (T g) by ∼15 K, validating the experimental data. More importantly, our results gave new insights into the fundamental understanding of the fact that the strength of intermolecular interactions induced by PIL at the polymer-particle interface is the key to control the α-relaxation dynamics and T g optimization, desired for specific applications.