A Terrylene Bisimide based Universal Host for Aromatic Guests to Derive Contact Surface-Dependent Dispersion Energies.

π-π interactions are among the most important intermolecular interactions in supramolecular systems. Here we determine experimentally a universal parameter for their strength that is simply based on the size of the interacting contact surfaces. Toward this goal we designed a new cyclophane based on terrylene bisimide (TBI) π-walls connected by para-xylylene spacer units. With its extended π-surface this cyclophane proved to be an excellent and universal host for the complexation of π-conjugated guests, including small and large polycyclic aromatic hydrocarbons (PAHs) as well as dye molecules. The observed binding constants range up to 108 M-1 and show a linear dependence on the 2D area size of the guest molecules. This correlation can be used for the prediction of binding constants and for the design of new host-guest systems based on the herewith derived universal Gibbs interaction energy parameter of 0.31 kJ/molÅ2 in chloroform.

Energy landscape of perylenediimide chromophoric aggregates.

Understanding the self-assembly of conjugated organic materials at the molecular level is crucial in their potential applications as active components in electronic and optoelectronic devices. The type of aggregation significantly influences the intriguing electronic and optical characteristics differing from their constituent molecules. Perylenediimides (PDIs), electron-deficient molecules exhibiting remarkable n-type semiconducting properties, are among the most explored organic fluorescent materials due to their high fluorescence efficiency, photostability, and optoelectronic properties. PDI derivatives are reported to form well-tailored supramolecular architectures: cofacial with minor slip (H-aggregates), staggered with major slip (J-aggregates), magic angle stacking (M-aggregates), rotated (X-aggregates), rotated orthogonal ((+)-aggregates), etc. H*-aggregates are defined here as an ideal case of H-aggregate with an eclipsed configuration. Although numerous reports regarding the formation and optical properties of various PDI aggregates are known, the key driving force within the PDI units guiding the self-assembly to form distinct aggregate systems remains elusive. To unravel the molecular-level mechanisms behind the self-assembly of PDI units by probing the intermolecular interactions, symmetry-adapted perturbation theory-based energy decomposition, potential energy surface scans, and non-covalent interaction index analyses were employed on PDI dimer models. Quantum theory of atoms in molecules and frontier molecular orbital analyses were implemented on the dimer models to comprehend the effect of heteroatoms and orbital interactions in stabilising the X-aggregates over the other PDI aggregate systems. Competition between the attractive and repulsive non-covalent interactions dictates a stability order of X > H > J > M > (+) > H* for the PDI aggregate system, while in the parent perylene system, the stability order was found to be X > (+) > H > M > J > H*.

Global Optimization of Dinitrogen Clusters Bound to Monolayer and Bilayer Graphene: A Swarm Intelligence Approach.

Locating the global minimum of a potential energy surface is an arduous task. The complexity of the potential energy surface increases as the number of degrees of freedom of the system increases. The highly rugged nature of the potential energy surface makes the minimization of the total energy of the molecular clusters a difficult optimization problem. A solution to this conundrum is the use of metaheuristic techniques that efficiently track down the global minima through a trade-off between exploration and exploitation. Herein, we use the swarm intelligence technique, particle swarm optimization to locate the global minima geometries of N2 clusters of size 2-10, in free and adsorbed states. We have investigated the structures and energetics of bare N2 clusters, followed by N2 clusters adsorbed on graphene and intercalated between the layers in bilayer graphene. The noncovalent interactions between dinitrogen molecules are modeled using the Buckingham potential as well as the electrostatic point charge model, while those of the N2 molecules with the carbon atoms of graphene are modeled using the improved Lennard-Jones potential. The interactions of the carbon atoms belonging to different layers in a bilayer are modeled using the Lennard-Jones potential. The bare cluster geometries and intermolecular interaction energies obtained using particle swarm optimization are found to be the same as reported in the literature, validating the use of particle swarm optimization for studying molecular clusters. The N2 molecules are found to adsorb as a monolayer on top of the graphene sheet and intercalate themselves right in the middle of the two sheets of bilayer graphene. Our study establishes that particle swarm optimization is a feasible global optimization technique for performing the optimization of high-dimensional molecular clusters, both in pristine and in confined forms.

Elucidation of Noble Gas Cluster Configurations Bound on Graphdiyne: A Metaheuristic Approach.

Graphynes are a class of all-carbon two-dimensional membranes that have been intensely researched for various membrane-based technologies on account of their unique pore architectures. Herein, we report an investigation of the mechanism and energetics of adsorption of noble gases (He, Ne and Ar) on graphdiyne (GDY), the most popular form of graphynes. Two global optimization techniques, namely particle swarm optimization (PSO) and differential evolution are employed to predict the putative global minima configurations of rare gas clusters in the size range 1-30 when adsorbed on GDY. We use the 12-6 Lennard-Jones potential to represent the pairwise non-covalent interactions between various interacting atoms. Initially, the gas atoms adsorb as monolayers on GDY at the centers of the triangular pores until all the triangular pores are filled. This is followed by a second layer formation on top of the hexagonal pore centers or on top of the C-C bonds. The findings from the empirical approach are further validated by performing density functional theory calculations on the predicted adsorbed cluster configurations. We have also looked into the adsorption of noble gas clusters on bilayer GDY systems and have found that the intercalation of gas atoms within the bilayers is feasible. Our study suggests that the stochastic nature of the swarm intelligence technique, PSO can assist in an effective search of the potential energy surfaces for the global minima, eventually enabling large-scale simulations.

Modeling the Adsorption of Polycyclic Aromatic Hydrocarbons on Graphynes: An Improved Lennard-Jones Formulation.

Research on the development of theoretical methodologies for modeling noncovalent interactions governing the adsorption of polycyclic aromatic hydrocarbons (PAHs) on graphene and other two-dimensional materials is being intensely pursued in recent times. Highly accurate empirical potentials have emerged as a viable alternative to first-principles calculations for performing large-scale simulations. Herein, we report exploration of the potential energy surfaces for the adsorption of cata-condensed and peri-condensed PAHs on graphynes (GYs) using the improved Lennard-Jones (ILJ) potential. Initially, the ILJ potential is parametrized against benchmark electronic structure calculations performed on a selected set of PAH-GY complexes using dispersion-corrected density functional theory. The accuracy of the parametrization scheme is then assessed by a comparison of the adsorption features predicted from the ILJ potential with those computed using electronic structure calculations. The potential energy profiles as well as the single point energy calculations and geometry reoptimizations performed on the minimum-energy configurations predicted by the ILJ potential for a broader range of PAH-GY complexes provided a validation of the parametrization scheme. Finally, by an extrapolation of the PAH adsorption energies on various GYs, we estimated the interlayer cohesion energies for the van der Waals bilayer heterostructures of GYs with graphene to be in the range of 25-50 meV/atom.

An anisotropic dressed pairwise potential model for the adsorption of noble gases on boron nitride sheets.

Development of empirical potentials with accurate parameterization is indispensable while modeling large-scale systems. Herein, we report accurate parameterization of an anisotropic dressed pairwise potential model (PPM) for probing the adsorption of noble gases, He, Ne, Ar and Kr on boron nitride sheets. For the noble gas binding on B48N48H24, we carried out a least-squares fit analysis of the dispersion and dispersionless contributions of the interaction potential separately. The transferability of the parameters for a range of molecular model systems of boron nitride is further established. The dressed PPM is then used in conjunction with a global optimization technique, namely particle swarm optimization (PSO) to assess the possibility of performing large-scale simulations with the PPM-PSO methodology. The results obtained for the adsorption of 2-5 noble gases on BN sheets establish the proof-of-concept, encouraging the pursuit of large-scale simulations using the PPM-PSO approach.

In pursuit of accurate interlayer potentials for twisted bilayer graphynes.

Recent explorations of twist in bilayer graphene and the discovery of superconducting phases at certain magic angles have laid the groundwork for a new branch in materials science called twistronics. However, theoretical studies on twisted layered materials are impeded due to the computational expense associated with first-principles calculations. Empirical force field approaches that include anisotropic terms to describe interlayer interactions have come to the fore as excellent alternatives to deal with such a stumbling block. Taking a cue from these formulations, herein, we describe our pursuit of capturing the interlayer interactions in bilayer graphynes with atomistic empirical potentials. The choice of the potentials, namely the improved Lennard-Jones potential and Hod's interlayer potential, is motivated by the objective of bringing out the role of anisotropy explicitly. Empirical parameters for both the potentials are calibrated against dispersion-corrected DFT calculations that are performed to incorporate the stacking, sliding and twisting features of the bilayer configurations. Although the isotropic improved Lennard-Jones potential is able to describe the interlayer stacking of graphynes, it is inadequate to account for the interlayer twist properties. The anisotropic Hod's interlayer potential portrays the interlayer twisting energy profiles of the benchmark DFT calculations with a reasonable accuracy. Our potential formulations can bestow impetus to the research on the homo- and hetero-bilayer structures of graphynes and other two-dimensional materials.

A swarm intelligence modeling approach reveals noble gas cluster configurations confined within carbon nanotubes.

Confinement of atoms and molecules brings forth fascinating properties to chemical systems that are otherwise not known in the bulk. Carbon nanotubes (CNTs) and fullerenes are excellent hosts for probing the confinement effects. Herein, we explore the potential energy surfaces of large noble gas clusters, Ngn (Ng = He, Ne and Ar; n = 10, 20, 30, 40, and 50), in the confines of CNTs of various lengths. Our implementation involves integrating the continuum approximation for CNTs with the well-known swarm intelligence technique, particle swarm optimization (PSO), followed by a deterministic local optimization. Global search techniques such as PSO have been increasingly utilized in recent times to track down minimum energy configurations on highly rugged potential energy surfaces. Aside from the position vectors of the noble gas atoms, we have considered the radius of the CNTs as a design variable. Such an approach enabled us to predict the optimal CNT radii for the encapsulation of each of the clusters. Confined cluster geometries ranging from linear, zig-zag, and double-helical to spiral configurations are obtained on encapsulation, in sharp contrast to their bare cluster geometries. On increasing the CNT length, our approach yielded quasi-linear geometries, suggesting that the length of the CNTs plays a crucial role in determining the stable cluster configurations on confinement.

Quantum Transmission of He Isotopes through Crown Ether-Embedded Graphene Nanomeshes: An Eckart Potential Approach.

The remarkable performance of carbon membranes for the selective passage of various species has led to extensive research in designing smart membranes. The mechanical stability of graphene in conjunction with the excellent host-guest chemistry of crown ethers makes the recently synthesized family of crown ether-embedded graphene nanomeshes promising candidates for sieving applications. Inspired by the excellent control over pore architectures offered by such nanomeshes, we investigate the abilities of crown ether-embedded graphene nanomeshes for noble gas separation by the size-sieving mechanism and for He isotope separation by the quantum sieving mechanism. Unlike the previous studies that employ either a finite-difference approach or a wave packet approach, we employ an analytical Eckart potential approach to calculate the tunneling probabilities. Using tunneling-corrected transition-state theory, we examine the competing nature of the zero-point energy effects and tunneling effects in governing the total quantum transmission of the isotopes. Our analysis of the permeation barriers, diffusion rates, transmission probabilities, permeabilities, and selectivities suggests that crown ether-embedded graphene nanomeshes are a class of promising carbon membranes for He isotope separation.

Biogenic Cluster-Encased Gold Nanorods as a Targeted Three-in-One Theranostic Nanoenvelope for SERS-Guided Photochemotherapy against Metastatic Melanoma.

Effective treatment of malignant melanoma requires an appropriate combination of therapeutic intervention with long-term prognosis as it often survives by monotherapies. Herein, we report a novel melanoma-targeted theranostic nanoenvelope (MTTNe: ISQ@BSA-AuNC@AuNR@DAC@DR5) which has been constructed by assembling a bovine serum albumin (BSA) stabilized gold nanocluster on a gold nanorod (BSA-AuNC@AuNR), a three-in-one theranostic modality, i.e., photothermal therapy (PTT), photodynamic therapy (PDT), and chemotherapy, tethered with a surface-enhanced Raman scattering (SERS) detection technique. The resultant MTTNe was coloaded with the melanoma-specific FDA approved drug dacarbazine (DAC) and a newly synthesized near-infrared (NIR) absorbing squaraine molecule ISQ that served partly as a photosensitizer and multiplex Raman reporter. Finally, a nanoenvelope was anchored with anti-DR5 monoclonal antibodies as a targeting motif for highly expressed melanoma-specific death receptors in malignant cells. Significant phototherapies of MTTNe were initiated upon an 808 nm single laser trigger which showed a synergistic effect of photothermal hyperthermia as well as singlet oxygen (1O2) driven photodynamic effect in the presence of ISQ followed by on-demand thermoresponsive drug release in the intracellular milieu. Moreover, a multiplex SERS spectral pattern of ISQ (1345 cm-1) and DAC (1269 cm-1) has been utilized for monitoring precise drug release kinetics and target-specific recognition on melanoma cells by Raman imaging. Therapeutic performance of the nanoenvelope was evaluated by in vitro cytotoxicity studies in human melanoma cells (A375) and confirmed the apoptotic phenomenon by molecular-level monitoring of intracellular SERS fingerprints. Finally, to address the biocompatibility of MTTNe, in vivo subacute toxicity was conducted on BALB/c mice. Hence, the current studies mark a footstep of a facile strategy for the treatment of melanoma by synergistic multimodal photothermal/photodynamic/chemotherapy.

Selective Permeation through One-Atom-Thick Nanoporous Carbon Membranes: Theory Reveals Excellent Design Strategies!

Research on the permeation of various species through one-atom-thick nanoporous carbon membranes has gained an unprecedented importance in the past decade, thanks to the development of numerous theoretical design strategies for a plethora of applications ranging from gas separation, water desalination, isotope separation, and chiral separation, to DNA sequencing. Although some of the recent experiments have demonstrated successful performance of such carbon membranes in sieving, many of the suggested applications are yet to be realized in experiments. This review aims to draw the attention of the theoretical as well as the experimental researchers working on two-dimensional carbon materials toward the recent theoretical developments probing the permeation of various species such as atoms, ions, small molecules, and biopolymers like DNA through carbon frameworks like graphynes, graphdiyne, graphenylenes, and various forms of nanoporous graphene, including graphene crown ethers. The underlying guiding principles toward the design of carbon-based membranes for nanofiltration are established using estimates of the adsorption energies, barrier heights for permeation, rates of permeation, selectivities, permeances, etc. The crucial roles of tunneling, temperature effects, chemical functionalities, and dynamical aspects of the nanopores are also highlighted, paving the way to a comprehensive description of the theoretical design strategies for tailoring the applicability of novel nanoporous carbon membranes in sieving and related aspects.

Adsorption of Monocyclic Carbon Rings on Graphene: Energetics Revealed via Continuum Modeling.

Gas-phase spectroscopic detection of tiny carbon clusters is a recent success story in the area of carbon cluster research. However, experimental production and isolation of these clusters are extremely difficult because of their high reactivity. One possibility to isolate the generated clusters would be to deposit them on graphene and to desorb them for subsequent use. One of the pertinent questions toward realizing this would be the energetics of the adsorption process. Therefore, in this work, the energetics for the adsorption of the monocyclic carbon rings (C n with n = 10, 12, 14, 16, 18, 20, and 22) on a graphene sheet are investigated using the analytical approaches, developed earlier by Hill and co-workers. The adsorption process here is driven by the noncovalent interactions between the carbon rings and the graphene sheet. The analyses of the interaction energies as a function of both the vertical distance Z and the rotational angle ϕ are performed in order to determine the preferred orientations, equilibrium positions, and binding energies for the adsorption of various carbon rings on graphene. We find that the preferred orientation of the rings with respect to the graphene sheet is the parallel orientation. The results from continuum, discrete-continuum, and discrete models are in good agreement. Further, computations using density functional theory and quantum mechanics/molecular mechanics approaches are performed, and comparisons of the computed energetics with the data from the models are reported. Finally, we highlight the scope and the limitations of the analytical models.

Graphynes: indispensable nanoporous architectures in carbon flatland.

Theoretical design and experimental realization of novel nanoporous architectures in carbon membranes has been a success story in recent times. Research on graphynes, an interesting class of materials in carbon flatland, has contributed immensely to this success story. Graphyne frameworks possessing sp and sp2 hybridized carbon atoms offer a variety of uniformly distributed nanoporous architectures for applications ranging from water desalination, gas separation, and energy storage to catalysis. Theory has played a pivotal role in research on graphynes, starting from the prediction of various structural forms to the emergence of their remarkable applications. Herein, we attempt to provide an up-to-date account of research on graphynes, highlighting contributions from numerous theoretical investigations that have led to the current status of graphynes as indispensable materials in carbon flatland. Despite unsolved challenges in large-scale synthesis, the future appears bright for graphynes in present theoretical and experimental research scenarios.

Plexcitons: The Role of Oscillator Strengths and Spectral Widths in Determining Strong Coupling.

Strong coupling interactions between plasmon and exciton-based excitations have been proposed to be useful in the design of optoelectronic systems. However, the role of various optical parameters dictating the plasmon-exciton (plexciton) interactions is less understood. Herein, we propose an inequality for achieving strong coupling between plasmons and excitons through appropriate variation of their oscillator strengths and spectral widths. These aspects are found to be consistent with experiments on two sets of free-standing plexcitonic systems obtained by (i) linking fluorescein isothiocyanate on Ag nanoparticles of varying sizes through silane coupling and (ii) electrostatic binding of cyanine dyes on polystyrenesulfonate-coated Au nanorods of varying aspect ratios. Being covalently linked on Ag nanoparticles, fluorescein isothiocyanate remains in monomeric state, and its high oscillator strength and narrow spectral width enable us to approach the strong coupling limit. In contrast, in the presence of polystyrenesulfonate, monomeric forms of cyanine dyes exist in equilibrium with their aggregates: Coupling is not observed for monomers and H-aggregates whose optical parameters are unfavorable. The large aggregation number, narrow spectral width, and extremely high oscillator strength of J-aggregates of cyanines permit effective delocalization of excitons along the linear assembly of chromophores, which in turn leads to efficient coupling with the plasmons. Further, the results obtained from experiments and theoretical models are jointly employed to describe the plexcitonic states, estimate the coupling strengths, and rationalize the dispersion curves. The experimental results and the theoretical analysis presented here portray a way forward to the rational design of plexcitonic systems attaining the strong coupling limits.

Tunable Azacrown-Embedded Graphene Nanomeshes for Ion Sensing and Separation.

Remarkable selectivity with which crown ethers served as macrocyclic hosts for various guest species has led to numerous investigations on structure-specific interactions. Successful fabrication of graphene nanomeshes has opened up a plethora of avenues for sensing and separation applications. Embedding crown ether backbones in graphene frameworks can therefore be an interesting strategy for exploring the advantages offered by crown ether backbones, yet having the properties of graphene-based materials. Motivated by the recent success in fabrication of crown ether-based graphene nanopores, herein we investigate their performance toward ion sensing and separation using electronic structure methods. The effect of topology and electronic properties of the nanopore are probed by considering a series of oxygen-based and nitrogen-based graphene crown ethers (crown-n; n = 1-6). Our computations have revealed the excellent alkali ion binding properties of azacrown-based graphene nanomeshes over conventional oxygen crown-based graphene nanomeshes and normal crown ethers. Selectivity in ion transmission through the nanomeshes is demonstrated by employing graphene crown ethers [crown-n (n = 4-6)]. To the best of our knowledge, this article is the first report on azacrown-based graphene nanomeshes and their possible applications in ion sensing and separation, an aspect that we hope will be demonstrated in experiments soon.