XH/π (X = C, Si) Interactions in Graphene and Silicene: Weak in Strength, Strong in Tuning Band Structures

Identification of Cadmium Compounds in a Solution Using Graphene-Based Sensor Array

Rapid detection of heavy metals in solution is necessary to ensure human health and environmental protection. Some heavy-metal compounds are present in solution as compounds instead of as ions owing to their low ionization. Therefore, the development of sensor devices for the detection of heavy-metal compounds is important. In this study, as a proof of concept, we propose a sensor device using graphene and a chelating agent, which were used to develop an identification technique for three types of cadmium compounds. Pristine-graphene and two types of chelator-modified graphene-based sensors were successfully used to detect cadmium compounds at concentrations ranging from 50 to 1000 μM. The detection time was less than 5 min. The three type of graphene-based sensors responded differently to each cadmium compound, which indicates that they detected cadmium as a cadmium compound instead of as cadmium ions. Furthermore, we successfully identified cadmium compounds by operating these three types of sensors as a sensor array on the same substrate. The results indicate that sensors that focus on heavy-metal compounds instead of heavy-metal ions can be used for the detection of heavy metals in solution.

Robust pure spin current induced by the photogalvanic effect in half-silicane with spatial inversion symmetry

The spin-dependent photogalvanic (PG) effect in low-dimensional spin semiconductors has attracted great interest recently. Here, we have studied the spin semiconducting feature and spin-dependent photocurrent in a two-dimensional (2D) silicene-based device with spatial inversion symmetrical half-hydrogenation, in which half of the silicene is hydrogenated on the upper surface and half is hydrogenated on the lower surface. Because of the unique spin semiconductor properties and symmetry of the system, pure spin current can be robustly produced in both the zigzag and armchair directions for linearly and elliptically polarized light. The behavior of the spin-dependent photoresponse in the spin PG effect is highly anisotropic and can be tuned by the polarization/phase angles or photon energy (E_ph). Moreover, the produced pure spin current in such a half-silicane device with spatial inversion symmetry via the PG effect is several orders of magnitude larger than that obtained in metal/semiconductor/metal systems. These findings suggest a promising approach for generating pure spin current by the PG effect and provide a new possibility for the application of 2D half-silicane in spintronics.

Azobenzene-Based Solar Thermal Fuels: A Review

The energy storage mechanism of azobenzene is based on the transformation of molecular cis and trans isomerization, while NBD/QC, DHA/VHF, and fulvalene dimetal complexes realize the energy storage function by changing the molecular structure. Acting as "molecular batteries," they can exhibit excellent charging and discharging behavior by converting between trans and cis isomers or changing molecular structure upon absorption of ultraviolet light. Key properties determining the performance of STFs are stored energy, energy density, half-life, and solar energy conversion efficiency. This review is aiming to provide a comprehensive and authoritative overview on the recent advancements of azobenzene molecular photoswitch system in STFs fields, including derivatives and carbon nano-templates, which is emphasized for its attractive performance. Although the energy storage performance of Azo-STFs has already reached the level of commercial lithium batteries, the cycling capability and controllable release of energy still need to be further explored. For this, some potential solutions to the cycle performance are proposed, and the methods of azobenzene controllable energy release are summarized. Moreover, energy stored by STFs can be released in the form of mechanical energy, which in turn can also promote the release of thermal energy from STFs, implying that there could be a relationship between mechanical and thermal energy in Azo-STFs, providing a potential direction for further research on Azo-STFs.

Two-Dimensional Sb/InS van der Waals Heterostructure for Electronic and Optical Related Applications

Stable configurations with excellent optical adsorption are crucial for the photovoltaics or photocatalysis. Two-dimensional materials with intrinsic electric-field have been proposed suitable for electric and optical device. Here, we have...

New Insight on Photoisomerization Kinetics of Photo-Switchable Thin Films Based on Azobenzene/Graphene Hybrid Additives in Polyethylene Oxide

In this work, we reported a new insight on the kinetics of photoisomerization and time evolution of hybrid thin films considering the azo-dye methyl red (MR) incorporated with graphene accommodated in polyethylene oxide (PEO). The kinetics of photoisomerization and time-evolution of hybrid thin films were investigated using UV-Vis s and FTIR spectroscopies, as well as appropriate models developed with new analytical methods. The existence of azo-dye MR in the complex is crucial for the resource action of the trans↔cis cycles through UV-illumination ↔ Visible-illumination relaxations. The results of the UV–Vis and the FTIR investigations prove the cyclical trans ↔ cis-states. Consequently, PEO-(MR-Graphene) hybrid composite thin films can be introduced as possible applicants for photochromic molecular switches, light-gated transistors, and molecular solar thermal energy storage media.

The liquid-exfoliation of graphene assisted with hyperbranched polyethylene-g-polyhedral oligomeric silsesquioxane copolymer and its thermal property in polydimethylsiloxane nanocomposite

Thermal interface materials with high thermal conductivity are essential to transfer the redundant heat and improve the reliability of integrated circuits. Here we reported high thermal conductivity in polydimethylsiloxane (PDMS) nanocomposite incorporated with few-layer graphene, which was exfoliated in chloroform with assistance of hyperbranched polyethylene-g-polyhedral oligomeric silsesquioxane copolymer (HBPE@POSS) as the stabilizer. In order to improve the compatibility and enhance the thermal property, the HBPE@POSS copolymer was synthesized via the unique chain walking polymerization mechanism, which subsequently was applied to exfoliate natural graphite into few-layer graphene in low-boiling-point solvents. The majority of resultant nanosheets with low defects was verified with lateral dimension of ~400 nm and the thickness of ~1.6 nm, which is attributed to the presence of CH-π noncovalent interaction between graphene and HBPE@POSS copolymer. The graphene nanoplates (GNPs)/polydimethylsiloxane (PDMS) nanocomposites were prepared by solution casting, in which graphene nanofillers were dispersed uniformly in the matrix due to good compatibility between PDMS and oligomeric silsesquioxane segments adsorbed on the nanosheets. The thermal conductivity of 4.0 wt% GNPs/PDMS nanocomposite reaches 0.93 W m^-1  K^-1, which is 400% higher than that of pure PDMS. The PDMS nanocomposite incorporated with few-layer graphene exhibits a promising prospect in thermal interface for thermal management of electronic devices, and sheds a light on the interfacial improvement mechanism of thermal conductivity for polymer composite.

A High Energy Density Azobenzene/Graphene Oxide Hybrid with Weak Nonbonding Interactions for Solar Thermal Storage

Incorporating photochromic chromophores into polymer composites provides the possibility of a reversible photoswitch of the intrinsic properties of these materials. In this paper we report a route to attach azobenzene (AZO) moiety covalently to graphene oxide (GO) to create chromophore/graphene oxide (AZO-GO) hybrid, in which GO is both part of the chromophore and the template. Due to the high grafting density of AZO moiety and the low mass of the novel structure, the hybrid is a potential solar thermal storage material with high energy density of about 240 Wh·kg-1. It is found that C-H···π interaction between the cis-AZO chromophores and the aromatic rings of the substrate induces collective electronic modifications of GO at critical percentage of cis-isomers and reduce the thermal barrier of π-π* transition of the chromophores directly, which results in two sections of first-order reactions during the photoisomerization of trans- to cis-hybrid and also thermally stabilizes the cis-hybrid. Our findings demonstrate that high-performance AZO-GO hybrid can be manipulated by optimizing intermolecular nonbonding interactions.

Steric "attraction": not by dispersion alone

Non-covalent interactions between neutral, sterically hindered organic molecules generally involve a strong stabilizing contribution from dispersion forces that in many systems turns the 'steric repulsion' into a 'steric attraction'. In addition to London dispersion, such systems benefit from electrostatic stabilization, which arises from a short-range effect of charge penetration and gets bigger with increasing steric bulk. In the present work, we quantify this contribution for a diverse set of molecular cores, ranging from unsubstituted benzene and cyclohexane to their derivatives carrying tert-butyl, phenyl, cyclohexyl and adamantyl substituents. While the importance of electrostatic interactions in the dimers of sp2-rich (e.g., π-conjugated) cores is well appreciated, less polarizable assemblies of sp3-rich systems with multiple short-range CH···HC contacts between the bulky cyclohexyl and adamantyl moieties are also significantly influenced by electrostatics. Charge penetration is drastically larger in absolute terms for the sp2-rich cores, but still has a non-negligible effect on the sp3-rich dimers, investigated herein, both in terms of their energetics and equilibrium interaction distances. These results emphasize the importance of this electrostatic effect, which has so far been less recognized in aliphatic systems compared to London dispersion, and are therefore likely to have implications for the development of force fields and methods for crystal structure prediction.

Hot-Chemistry Structural Phase Transformation in Single-Crystal Chalcogenides for Long-Life Lithium Ion Batteries

Tuned chalcogenide single crystals rooted in sulfur-doped graphene were prepared by high-temperature solution chemistry. We present a facile route to synthesize a rod-on-sheet-like nanohybrid as an active anode material and demonstrate its superior performance in lithium ion batteries (LIBs). This nanohybrid contains a nanoassembly of one-dimensional (1D) single-crystalline, orthorhombic SnS onto two-dimensional (2D) sulfur-doped graphene. The 1D nanoscaled SnS with the rodlike single-crystalline structure possesses improved transport properties compared to its 2D hexagonal platelike SnS₂. Furthermore, we blend this hybrid chalcogenide with biodegradable polymer composite using water as a solvent. Upon drying, the electrodes were subjected to heating in vacuum at 150 °C to induce polymer condensation via formation of carboxylate groups to produce a mechanically robust anode. The LIB using the as-developed anode material can deliver a high volumetric capacity of ∼2350 mA h cm^-3 and exhibit superior cycle stability over 1500 cycles as well as a high capacity retention of 85% at a 1 C rate. The excellent battery performance combined with the simplistic, scalable, and green chemistry approach renders this anode material as a very promising candidate for LIB applications.

Novel electronic and optical properties of ultrathin silicene/arsenene heterostructures and electric field effects

The variable band-gap of the Si/As heterostructure (left) and optical absorption spectra for AA-stacking under a vertical electric field (right).

Band gap opening of bilayer graphene by graphene oxide support doping

In contrast to the metallic monolayer graphene by graphene oxides (GOs) doping, the sizable band gap of bilayer graphene is opened by GOs.

Modulation of silicene properties by AsSb with van der Waals interaction

Our present work provides a new promising material AsSb monlayer as the substrate for silicene with a negligible mismatch, sizable band gap and high carrier mobility.

Two-dimensional stanane: strain-tunable electronic structure, high carrier mobility, and pronounced light absorption

By means of state-of-the-art density functional theory (DFT) computations, we systematically studied the structural, electronic, and optical properties of a novel two dimensional material, namely stanane (SnH). According to our computational results, stanane is semiconducting with a direct band gap of 1.00 eV, which can be flexibly tuned by applying an external strain. Remarkably, stanane has much higher electron and hole mobilities than those of a MoS2 monolayer at room temperature. Moreover, stanane has rather strong optical absorption in the visible as well as infrared regions of the solar spectrum. These results provide many useful insights for the wide application of stanane in electronics and optoelectronics.

First principles study of silicene symmetrically and asymmetrically functionalized with halogen atoms

Using first principles calculations, we predicted that a direct-band-gap between 0.98 and 2.13 eV can be obtained in silicene by symmetrically and asymmetrically (Janus) functionalisation with halogen atoms and applying elastic tensile strain.

First-Principles Study of Phosphorene and Graphene Heterostructure as Anode Materials for Rechargeable Li Batteries

There is a great desire to develop the high-efficient anodes materials for Li batteries, which require not only large capacity but also high stability and mobility. In this work, the phosphorene/graphene heterostructure (P/G) was carefully explored based on first-principles calculations. The binding energy of Li on the pristine phosphorene is relatively weak (within 1.9 eV), whereas the phosphorene/graphene heterostructure (P/G) can greatly improve the binding energy (2.6 eV) without affecting the high mobility of Li within the layers. The electronic structures show that the large Li adsorption energy and fast diffusion ability of the P/G origin from the interfacial synergy effect. Interestingly, the P/G also displays ultrahigh stiffness (Cac = 350 N/m, Czz = 464 N/m), which can effectively avoid the distortion of the pristine phosphorene after the insertion of lithium. Thus, P/G can greatly enhance the cycle life of the battery. Owing to the high capacity, good conductivity, excellent Li mobility, and ultrahigh stiffness, P/G is a very promising anode material in Li-ion batteries (LIBs).

Evidence of covalent synergy in silicon-sulfur-graphene yielding highly efficient and long-life lithium-ion batteries

Silicon has the potential to revolutionize the energy storage capacities of lithium-ion batteries to meet the ever increasing power demands of next generation technologies. To avoid the operational stability problems of silicon-based anodes, we propose synergistic physicochemical alteration of electrode structures during their design. This capitalizes on covalent interaction of Si nanoparticles with sulfur-doped graphene and with cyclized polyacrylonitrile to provide a robust nanoarchitecture. This hierarchical structure stabilized the solid electrolyte interphase leading to superior reversible capacity of over 1,000 mAh g⁻¹ for 2,275 cycles at 2 A g⁻¹. Furthermore, the nanoarchitectured design lowered the contact of the electrolyte to the electrode leading to not only high coulombic efficiency of 99.9% but also maintaining high stability even with high electrode loading associated with 3.4 mAh cm⁻². The excellent performance combined with the simplistic, scalable and non-hazardous approach render the process as a very promising candidate for Li-ion battery technology.

Silicon anodes are promising for lithium-ion battery development, but suffer from problems such as undesired volume expansion and solid-electrolyte interface formation. Here, the authors report a hierarchical silicon-sulfur-graphene composite anode which mitigates the problems leading to high performance.

Strong orbital deformation due to CH-π interaction in the benzene-methane complex

The orbital distribution and composition of the benzene-methane complex have been investigated systemically using ab initio calculations for the first time. Surprisingly, we find strong deformation in the HOMO-4 and LUMO+2 induced by CH-π interaction, extending the general view that nonbonding interaction does not cause orbital change of molecules.

Small molecules make big differences: molecular doping effects on electronic and optical properties of phosphorene

Systematical computations on the density functional theory were performed to investigate the adsorption of three typical organic molecules, tetracyanoquinodimethane (TCNQ), tetracyanoethylene (TCNE) and tetrathiafulvalene (TTF), on the surface of phosphorene monolayers and thicker layers. There exist considerable charge transfer and strong non-covalent interaction between these molecules and phosphorene. In particular, the band gap of phosphorene decreases dramatically due to the molecular modification and can be further tuned by applying an external electric field. Meanwhile, surface molecular modification has proven to be an effective way to enhance the light harvesting of phosphorene in different directions. Our results predict a flexible method toward modulating the electronic and optical properties of phosphorene and shed light on its experimental applications.

Removal of NO with silicene: a DFT investigation

NO can be adsorbed onto silicene and then reduced into N₂via a dimer mechanism.

Carbon nanoscroll from C4H/C4F-type graphene superlattice: MD and MM simulation insights

Carbon nanoscrolls from a C₄H/C₄F-type graphene superlattice.

Graphane versus graphene: a computational investigation of the interaction of nucleobases, aminoacids, heterocycles, small molecules (CO2, H2O, NH3, CH4, H2), metal ions and onium ions

We compared the binding affinity of graphane and graphene with various molecules and ions.

Tunable band gap and doping type in silicene by surface adsorption: towards tunneling transistors

By using first-principles calculations, we predict that a sizable band gap can be opened at the Dirac point of silicene without degrading silicene's electronic properties with n-type doping by Cu, Ag, and Au adsorption, p-type doping by Ir adsorption, and neutral doping by Pt adsorption. A silicene p-i-n tunneling field effect transistor (TFET) model is designed by the adsorption of different transition metal atoms on different regions of silicene. Quantum transport simulations demonstrate that silicene TFETs have an on-off ratio of 10(3), a small sub-threshold swing of 77 mV dec(-1), and a large on-state current of over 1 mA μm(-1) under a supply voltage of about 1.7 V.

Studying reaction intermediates formed at graphenic surfaces

We report in-situ production and detection of intermediates at graphenic surfaces, especially during alcohol oxidation. Alcohol oxidation to acid occurs on graphene oxide-coated paper surface, driven by an electrical potential, in a paper spray mass spectrometry experiment. As paper spray ionization is a fast process and the time scale matches with the reaction time scale, we were able to detect the intermediate, acetal. This is the first observation of acetal formed in surface oxidation. The process is not limited to alcohols and the reaction has been extended to aldehydes, amines, phosphenes, sugars, etc., where reaction products were detected instantaneously. By combining surface reactions with ambient ionization and mass spectrometry, we show that new insights into chemical reactions become feasible. We suggest that several other chemical transformations may be studied this way. This work opens up a new pathway for different industrially and energetically important reactions using different metal catalysts and modified substrate.

Structures and chemical properties of silicene: unlike graphene

The discovery of graphene and its remarkable and exotic properties have aroused interest in other elements and molecules that form 2D atomic layers, such as metal chalcogenides, transition metal oxides, boron nitride, silicon, and germanium. Silicene and germanene, the Si and Ge counterparts of graphene, have interesting fundamental physical properties with potential applications in technology. For example, researchers expect that silicene will be relatively easy to incorporate within existing silicon-based electronics. In this Account, we summarize the challenges and progress in the field of silicene research. Theoretical calculations have predicted that silicene possesses graphene-like properties such as massless Dirac fermions that carry charge and the quantum spin Hall effect. Researchers are actively exploring the physical and chemical properties of silicene and tailoring it for wide variety of applications. The symmetric buckling in each of the six-membered rings of silicene differentiates it from graphene and imparts a variety of interesting properties with potential technological applications. The pseudo-Jahn-Teller (PJT) distortion breaks the symmetry and leads to the buckling in silicenes. In graphene, the two sublattice structures are equivalent, which does not allow for the opening of the band gap by an external electric field. However, in silicene where the neighboring Si atoms are displaced alternatively perpendicular to the plane, the intrinsic buckling permits a band gap opening in silicene in the presence of external electric field. Silicene's stronger spin orbit coupling than graphene has far reaching applications in spintronic devices. Because silicon prefers sp(3) hybridization over sp(2), hydrogenation is much easier in silicene. The hydrogenation of silicene to form silicane opens the band gap and increases the puckering angle. Lithiation can suppress the pseudo-Jahn-Teller distortion in silicene and hence can flatten silicene's structure while opening the band gap. So far, chemists have not successfully synthesized and characterized a free-standing silicene. But recently chemists have successfully produced silicene sheets and nanoribbons over various substrates such as silver, diboride thin films, and iridium. The supporting substrate critically controls the electronic properties of silicene, and the match of the appropriate support and its use is critical in applications of silicene.

Characterization of the Mechanical Properties of Monolayer Molybdenum Disulfide Nanosheets Using First Principles

Recently, synthesized inorganic two-dimensional monolayer nanostructures are very promising to be applied in electronic devices. This article explores the mechanical properties of a monolayer molybdenum disulfide (MoS2) including Young's bulk and shear moduli and Poisson's ratio by applying density functional theory (DFT) calculation based on the generalized gradient approximation (GGA). The results demonstrate that the elastic properties of MoS2 nanosheets are less than those of graphene and hexagonal boron-nitride (h-BN) nanosheets. However, their Poisson's ratio is found to be higher than that of graphene and h-BN nanosheet. It is also observed that due to the special structure of MoS2, the thickness of nanosheet changes when the axial strain is applied.

Effect of solvent polarizability on the assembly and ordering of nanoscale polyhedral oligomeric silsesquioxane films

Understanding the factors that affect molecular self-assembly is crucial to building designed nanoscale structures. We have deposited nanoscale films of polyhedral oligomeric silsesquioxane (POSS) onto polished silicon substrates from a range of organic solvents. We studied these films using synchrotron X-ray reflectivity and found that dip-coating from benzene, toluene, or chloroform results in near-substrate ordering only, but when acetone, hexane, or THF is used, self-assembled layers are formed throughout the entire deposited film. We conclude that solvent polarizability is the factor that determines the alignment of the POSS molecules. We have successfully tested this prediction using additional solvents selected on the basis of their calculated polarizabilities.

Tunable band gaps in silicene–MoS2heterobilayers

A sizable and tunable bandgap is realized in silicene–MoS₂heterobilayers.

Silicene oxides: formation, structures and electronic properties

Understanding the oxidation of silicon has been critical to the success of all types of silicon materials, which are the cornerstones of modern silicon technologies. For the recent experimentally obtained two-dimensional silicene, oxidation should also be addressed to enable the development of silicene-based devices. Here we focus on silicene oxides (SOs) that result from the partial or full oxidation of silicene in the framework of density functional theory. It is found that the formation of SOs greatly depends on oxidation conditions, which concern the oxidizing agents of oxygen and hydroxyl. The honeycomb lattice of silicene may be preserved, distorted or destroyed after oxidation. The charge state of Si in partially oxidized silicene ranges from +1 to +3, while that in fully oxidized silicene is +4. Metals, semimetals, semiconductors and insulators can all be found among the SOs, which show a wide spectrum of electronic structures. Our work indicates that the oxidation of silicene should be exquisitely controlled to obtain specific SOs with desired electronic properties.

Self-assembly of C4H-type hydrogenated graphene

We demonstrate by molecular dynamic (MD) simulations that patterned partially hydrogenated graphene (C4H) can self-assemble at room temperature. The main driving force of the self-assembly of C4H is due to the one-sided distribution of hydrogen and the corresponding asymmetric orientation of sp(3) bonding, there exists strong electrostatic repulsion between the relatively close H atoms. The simulations show that C4H can self-assemble into various carbon nanoscroll (CNS) structures, this is mainly controlled by its geometry (size and aspect ratio). And the carbon nanotube (CNT) is a good candidate to activate and guide C4H to form CNS, whose core size can be controlled. Meanwhile, a novel CNT/C4H core/shell composite nanostructure is also formed. The theoretical results shed important light on a feasible approach to fabricate high-quality CNS and other novel nanostructures including core/shell structures, which hold great potential applications in optics, optoelectronic devices, hydrogen storage, sensors, and energy storage in supercapacitors or batteries.

Influence of solvent on ion aggregation and transport in PY15TFSI ionic liquid-aprotic solvent mixtures

Molecular dynamics (MD) simulations using a many-body polarizable APPLE&P force field have been performed on mixtures of the N-methyl-N-pentylpyrrolidinium bis(trifluoromethanesulfonyl)imide (PY15TFSI) ionic liquid (IL) with three molecular solvents: propylene carbonate (PC), dimethyl carbonate (DMC), and acetonitrile (AN). The MD simulations predict density, viscosity, and ionic conductivity values that agree well with the experimental results. In the solvent-rich regime, the ionic conductivity of the PY15TFSI-AN mixtures was found to be significantly higher than the conductivity of the corresponding -PC and -DMC mixtures, despite the similar viscosity values obtained from both the MD simulations and experiments for the -DMC and -AN mixtures. The significantly lower conductivity of the PY15TFSI-DMC mixtures, as compared to those for PY15TFSI-AN, in the solvent-rich regime was attributed to the more extensive ion aggregation observed for the -DMC mixtures. The PY15TFSI-DMC mixtures present an interesting case where the addition of the organic solvent to the IL results in an increase in the cation-anion correlations, in contrast to what is found for the mixtures with PC and AN, where ion motion became increasingly uncorrelated with addition of solvent. A combination of pfg-NMR and conductivity measurements confirmed the MD simulation predictions. Further insight into the molecular interactions and properties was also obtained using the MD simulations by examining the solvent distribution in the IL-solvent mixtures and the mixture excess properties.

Local control of cis-peptidyl-prolyl bonds mediated by CH···π interactions: the Xaa-Pro-Tyr motif

Compared to generic peptide bonds, the peptidyl-prolyl bond shows a strong propensity for the cis conformer. The presence of a sequence-contiguous aromatic (Aro) residue can further stabilize the cis conformer, as observed for the Aro-Pro motif. The cis propensity of the reverse sequence motif, Pro-Aro, is not so well understood, especially the effect of N-capping the Pro-Aro motif with different amino acid residues. From a comparative nuclear magnetic resonance study of two peptide series with the general sequences Ac-Xaa-Pro-Tyr-NH2 and Ac-Xaa-Pro-Ala-NH2, we present a relative thermodynamic scale that reflects how the nature of the Xaa side chain influences the cis propensity of the Xaa-Pro-Tyr motif, with Gly, Pro, and Ala at position Xaa giving the greatest enhancement of the cis-peptidyl-prolyl population. We also show that CH···π interaction between Xaa and Tyr is responsible for the enhanced cis population. However, the mere presence of the CH···π interaction does not guarantee that the peptidyl-prolyl bond will have a higher cis content in Xaa-Pro-Tyr than in Xaa-Pro-Ala. Xaa-dependent intramolecular interactions present in Xaa-trans-Pro-Tyr can nullify favorable CH···π interactions in Xaa-cis-Pro-Tyr. The relative cis-peptidyl-prolyl stabilizing propensities of Xaa (Xaa-Pro-Tyr) in proteins and in our peptide series show strong linear correlation except when Xaa is aromatic. We also explore the Xaa-Pro-Gly-Tyr sequence motif and show that mediated by a Pro-Tyr CH···π interaction, the cis-peptidyl-prolyl bond in the motif is stabilized when Xaa is Pro.