Perimeter Model Research Articles

Zero trust implementation in the emerging technologies era: a survey

This paper presents a comprehensive analysis of the shift from the traditional perimeter model of security to the Zero Trust (ZT) framework, emphasizing the key points in the transition and the practical application of ZT. It outlines the differences between ZT policies and legacy security policies, along with the significant events that have impacted the evolution of ZT. Additionally, the paper explores the potential impacts of emerging technologies, such as Artificial Intelligence and quantum computing, on the policy and implementation of ZT. The study thoroughly examines how Artificial Intelligence can enhance ZT by utilizing Machine Learning algorithms to analyze patterns, detect anomalies, and predict threats, thereby improving real-time decision-making processes. Furthermore, the paper demonstrates how a chaos theory-based approach, in conjunction with other technologies such as eXtended Detection and Response, can effectively mitigate cyberattacks. As quantum computing presents new challenges to ZT and cybersecurity as a whole, the paper delves into the intricacies of ZT migration, automation, and orchestration, addressing the complexities associated with these aspects. Finally, the paper provides a best practice approach for the seamless implementation of ZT in organizations, laying out the proposed guidelines to facilitate organizations in their transition towards a more secure ZT model. The study aims to support organizations in successfully implementing ZT and enhancing their cybersecurity measures.

Fine-tuning of radiative properties by "mild" substituents: searching for a perfectly soft chromophore.

Controlling spectral properties to achieve desired characteristics is an attractive goal in application-oriented research, e.g., in the design of fluorescence sensors. "Soft" chromophores, molecules with strong spectral responses to internal or external perturbations are good candidates for such studies. In this work, absorption, fluorescence, and magnetic circular dichroism (MCD) spectra were obtained for a series of porphyrins, substituted at the meso-positions with n-hexyl groups. As the number of substituents increases from 1 to 4, significant changes are observed. The intensity of the S0-S1 transition (Qx) in the 0-0 region strongly decreases in mono-substituted porphyrin, but upon additional substitutions it increases to values larger than in the parent, unsubstituted molecule. Such behavior can be explained, using the perimeter model, by changes in the energy splittings between the two highest (HOMO) and two lowest (LUMO) frontier molecular orbitals. Single substitution makes porphyrin a nearly perfect soft chromophore, but upon introduction of a larger number of n-hexyl groups it is transformed into a hard one. DFT simulations incorrectly predict a continuous transition from a soft to hard chromophore, because the calculated ordering of two HOMO orbitals is opposite to that obtained by experiment. On the other hand, for those porphyrins that can be classified as hard chromophores, the calculations nicely reproduce contributions of Franck-Condon and Herzberg-Teller terms to absorption and fluorescence spectra.

Multicomponent Plasma Expansion Model for Arc Discharges on Large-Area Solar Arrays

For decades it has been known that spacecraft charging-induced electrostatic arc discharge on solar arrays can cause cell degradation and damaging secondary arcs. While discharging on large-area solar arrays is the concern, there is no space data on primary arc current magnitudes on large arrays. Laboratory testing is normally performed on coupons with just a few solar cells. External capacitors and circuitry based on theoretical models are used to simulate the arc current profile from the charge stored on the rest of the array. Previously, the current profiles for large arrays were generated assuming that the arc plasma expanded at a constant velocity and instantaneously discharged cells at the plasma perimeter. Arc-current profiles using this perimeter model failed to agree with laboratory tests on large arrays. An improved model needs to account for the range of expansion velocities in a multicomponent plasma, and that the cell neutralization at the perimeter is not instantaneous but is limited by the arc-plasma thermal electron current density. A new, multifluid model of arc-plasma expansion addresses both concerns. The model assumes as input primary arc ion generation rates. Ion expansion velocities are calculated self-consistently from electric potentials in the expanding plasma.

Trust No One? A Framework for Assisting Healthcare Organisations in Transitioning to a Zero-Trust Network Architecture

Traditional networks are designed to be hard on the outside and soft on the inside. It is this soft inside which has made the traditional perimeter model laughable to attackers, who can easily breach a network and run away with the data without even having to deal with the hardened perimeter. The zero-trust security model, created by John Kindervag in 2010, addresses the security flaws of the traditional perimeter model and asserts that all network traffic on the inside should not be trusted by default. Other core principles of zero trust include verification and continuous monitoring of all communication, as well as encryption of all data in transit and data at rest, since the goal of zero trust is to focus on protecting data. Although the zero-trust model was created in 2010, with some of the associated security practices existing even before that, many healthcare organisations are still choosing to focus primarily on securing the perimeter instead of focusing on the vulnerabilities within them. The current COVID-19 pandemic which healthcare providers are struggling with further highlights the need for improvements to security within the network perimeter, as many healthcare providers and vaccine developers are still using vulnerable, outdated legacy systems which could become compromised and indirectly have a detrimental effect on patient care. Legacy systems which are technologically limited, as well as medical devices which cannot be controlled or managed by network administrators, create boundaries to transitioning to a zero-trust architecture. It is challenges like this that have been explored during the research phase of this project in order to gain a better understanding of how a health organisation can adopt zero-trust practices despite the limitations of their current architecture. From the information gathered during this research, a framework was developed to allow a health organisation to transition to a more secure architecture based on the concept of zero-trust. Aspects of the proposed framework were tested in Cisco Modelling Labs (CML), and the results were evaluated to ensure the validity of some of the recommendations laid out in the framework. The main objective of this research was to prove that if a host within the local area network (LAN) were to be compromised, the damage would be limited to that host and would not spread throughout the rest of the network. This was successful after the qualitative research performed in CML. One of the other takeaways from testing the framework in CML was that medical devices could be secured by placing firewalls directly in front of them. This placement of firewalls may seem like an unorthodox approach and was shown to increase latency, but the blocking of all unnecessary traffic on the rest of the network will result in a performance boost and should balance it out in a real-world application.

Magnetic Circular Dichroism of meso-Phenyl-Substituted Pd-Octaethylporphyrins.

Absorption and magnetic circular dichroism (MCD) spectra have been measured and theoretically simulated for a series of palladium octaethylporphyrins substituted at the meso positions with phenyl groups (n = 0–4). Analysis of the spectra included the perimeter model and time-dependent density functional theory (TDDFT) calculations. With the increasing number of phenyl substituents, the molecule is transformed from a positive hard (ΔHOMO > ΔLUMO) to a soft (ΔHOMO ≈ ΔLUMO) chromophore. This is manifested by a drastic decrease of the absorption intensity in the 0–0 region of the Q-band and by the strongly altered ratio of MCD intensities in the Q and Soret regions. Such behavior can be readily predicted using perimeter model, by analyzing frontier orbital shifts caused by various perturbations: alkyl and aryl substitution, insertion of a metal, and deviations from planarity. TDDFT calculations confirm the trends predicted by the perimeter model, but they fail in cases of less symmetrical derivatives to properly reproduce the MCD spectra in the Soret region. Our results confirm the power of the perimeter model in predicting absorption and MCD spectra of large organic molecules, porphyrins in particular. We also postulate, contrary to previous works, that the isolated porphyrin dianion is not a soft chromophore, but rather a strongly positive-hard one.

Antiaromatic or Nonaromatic? 21 H,61 H-2,6(2,5)-Dipyrrola-1,5(2,6)-dipyridinacyclooctaphane-3,7-diene: a Porphycene Derivative with 4 N π Electrons.

A porphycene-derived compound with a 20 π-electron skeleton has been obtained by replacing two pyrrolene units of porphycene by pyridine rings. NMR, electronic absorption and MCD spectra, and the lack of fluorescence are typical for 4 N cyclic π electron systems. The electronic structure and the differences with respect to porphycene can be rationalized by treating these compounds as perturbed, doubly positively charged [22]annulene and [20]annulene perimeters, respectively. Even though the spectroscopic and photophysical criteria proposed for antiaromatic systems are fulfilled, the molecule is very stable. We argue that the compound should be characterized as nonaromatic rather than antiaromatic. The perimeter model is recommended as a powerful tool for predicting the electronic structure and spectra and as a useful addition to other methods that probe the aromaticity.

Magnetic circular dichroism of an unaromatic planar [8]annulene

AbstractAn overview of magnetic circular dichroism (MCD) spectroscopy of π‐electron systems derived from a 4N‐electron perimeter is provided, with emphasis on the hypothetical parent cycloocta‐1,3,5,7‐tetraene of D8h symmetry (1) and its D4h symmetry derivatives. UV‐visible absorption and MCD spectra of 2, a D4h symmetric cycloocta‐1,3,5,7‐tetraene planarized by the effect of 4 bicyclo[2.1.1]hexeno units fused to its 8‐membered ring, are reported and interpreted. The perimeter model is applied to obtain an understanding of the nature of electronic states in 1 and 2 and to predict general trends in the spectra. The electronic excitation patterns are found to be different in the antiaromatic D8h and unaromatic D4h species, and their states cannot be unequivocally correlated. The results of time‐dependent density functional theory and extended multistate complete active space second‐order perturbation theory (XMS‐CASPT2) computations agree with the algebraic perimeter model analysis and reproduce the spectra of 2 well, including three of the four observed MCD signs of A and B terms.

Building situational awareness for network threats in fog/edge computing: Emerging paradigms beyond the security perimeter model

The growing interest in fog and edge computing is gradually but inexorably outlining new architectural and usage models, distinguished by geographical dispersion and device heterogeneity. Unfortunately, the evolution of cyber-security paradigms has not gone with the same pace, leading to a substantial difficulty in protecting the new forms of distributed and heterogeneous systems against cyber-threats.In this paper, we focus on situational awareness for network threats. We briefly review the main limitations of current cyber-security paradigms with respect to emerging fog/edge architectures, and we discuss how current challenges and emerging trends are pushing from vertical security frameworks to horizontal and distributed architectures. In this respect, we outline the main elements and relevant technologies for a multi-layer framework that create the necessary knowledge and awareness in relation to network threats over large and heterogeneous computing and networking environments.

Expanded, Contracted, and Isomeric Porphyrins: Theoretical Aspects

The use of cyclic polyene perimeter-model approaches, such as Gouterman's four-orbital model and Michl's perimeter model, to analyze trends in the electronic structures and optical properties of expanded, contracted, and isomeric porphyrins is described with an emphasis on the use of magnetic circular dichroism (MCD) spectroscopy to validate the results of TD-DFT calculations. Trends in the electronic structures and optical properties of isomeric porphyrins are examined by comparing the properties of porphycenes, corrphycenes, hemiporphycenes, isoporphycenes, N-confused and neoconfused porphyrins, and norroles, whereas those of ring-contracted porphyrins are examined by comparing the properties of subporphyrins, triphyrins, and vacataporphyrins. The ring-expanded compounds that are examined include cyclo[n]pyrroles, [22]pentaphyrins(1.1.1.1.1), sapphyrins, smaragdyrins, isosmaragdyrins, orangarins, ozaphyrins, [26]hexaphyrins(1.1.1.1.1.1), rubyrins, rosarins, amethyrins, isoamethyrins, bronzaphyrins, and doubly N-confused hexaphyrins.

Synthesis, Characterization, and Electronic Structures of Porphyrins Fused with Polycyclic Aromatic Ring Systems

A series of porphyrins fused with acenaphthylene, phenanthroline, and benzofluoranthene polycyclic aromatic rings were prepared by means of a 3+1 porphyrin synthesis approach and subsequent retro-Diels-Alder reaction of bicyclo[2.2.2]octadiene-fused precursors. Analysis of the magnetic circular dichroism spectra and the results of time-dependent DFT calculations are used to identify the reasons for the trends observed in the wavelengths and relative intensities of the Q bands of the products. Michl's perimeter model is used as a conceptual framework to explain the changes in the relative energies of the frontier π-molecular orbitals.

Spectroscopy and Tautomerization Studies of Porphycenes

Tautomerization in porphycenes, constitutional isomers of porphyrins, is strongly entangled with spectral and photophysical parameters. The intramolecular double hydrogen transfer occurring in the ground and electronically excited states leads to uncommon spectroscopic characteristics, such as depolarized emission, viscosity-dependent radiationless depopulation, and vibrational-mode-specific tunneling splittings. This review starts with documentation of the electronic spectra of porphycenes: Absorption and magnetic circular dichroism are discussed, together with their analysis based on the perimeter model. Next, photophysical characteristics are presented, setting the stage for the final part, which discusses the developments in research on tautomerism. Porphycenes have been studied in different experimental regimes: molecules in condensed phases, isolated in supersonic jets and helium nanodroplets, and, recently also on the level of single molecules investigated by optical and scanning probe microscopies. Because of the rich and detailed information obtained from these diverse investigations, porphycenes emerge as very good models for studying the complex, multidimensional phenomena involved in the process of intramolecular double hydrogen transfer.

Synthesis, characterization, and electrochemistry of the manganese(I) complexes of meso-substituted [14]tribenzotriphyrins(2.1.1).

Metalation of 6,13,20,21-tetraaryl-22H-[14]tribenzotriphyrins(2.1.1) (TriP, 1 a-d) with [Mn(CO)5 Br] provided Mn(I) tricarbonyl complexes of [14]tribenzotriphyrins(2.1.1) 2 a-d in 85-93 % yield. The complexes were characterized by mass spectrometry and UV/Vis absorption, IR, and NMR spectroscopy. Single-crystal X-ray analyses revealed that 2 b and 2 c adopt bowl-shaped conformations. The redox properties of [(TriP)Mn(I) (CO)3 ] (2 a-d) were studied by cyclic voltammetry. Each compound undergoes two reversible one-electron reductions to form a porphyrin π anion radical and a dianion in CH2 Cl2 . Two oxidation waves were observed, the first of which corresponds to a metal-centered electron-transfer process. The redox potentials of 2 a-d are consistent with the optical spectroscopic data and the relatively narrow HOMO-LUMO gaps that were predicted in DFT calculations. The optical spectra can be assigned by using Michl's perimeter model. TDDFT calculations predict the presence of several metal-to-ligand charge-transfer bands in the L-band region between 500 and 700 nm.

Trends in the TD-DFT calculations of porphyrin and phthalocyanine analogs

In 2005, Kobayashi and coworkers reported trends in the TD-DFT spectra of 17 Zn ( II ) porphyrinoids [J. Am. Chem. Soc. 2005; 127: 17697] that were analyzed using Michl's perimeter model as part of a study of the anomalous magnetic circular dichroism (MCD) spectroscopy of zinc tetraphenyltetraacenaphthoporphyrin. In recent years, it has become increasingly clear that TD-DFT calculations with the commonly used hybrid B3LYP exchange-correlation functional of the Gaussian software package are problematic in the B -band region of porphyrinoid spectra, since the degree of configurational interaction between the B and higher energy ππ* state appears to be significantly overestimated. The CAM-B3LYP functional is now often preferred for analyzing the optical properties of porphyrinoids, since it includes a long-range correction of the exchange potential, which incorporates an increasing fraction of Hartree–Fock (HF) exchange as the interelectronic separation increases, making it better suited for studying compounds where there is significant charge transfer in the electronic excited states. The trends in the TD-DFT calculations are reexamined with a wider range porphyrinoid compounds including several with pyrazino moieties and are found to provide a closer agreement with the experimental in the B -band region for complexes such as zinc tetraphenylporphyrin and phthalocyanine.

Optical and magneto-optical properties of metal phthalocyanine and metal porphyrin thin films

The optical constants together with the magneto-optical Voigt constants of several phthalocyanine (Pc) and methoxy functionalized tetraphenylporphyrin (TMPP) thin films prepared on silicon substrates are presented. The materials investigated are MePc with Me = Fe, Co, Ni, Cu, Zn and MeTMPP with Me = Cu, Ni. We also compared our results to the metal-free H2Pc, H2TPP and H2TMPP. The experimental results will be supported by electronic structure calculations based on density functional theory (DFT) and interpreted using the perimeter model initially proposed by Platt. The model allows for qualitative understanding of the forbidden character of transitions in planar, aromatic molecules, and is able to qualify differences between Pc and TMPP type materials.

Structure, Electronic States, and Anion-Binding Properties of Cyclo[4]naphthobipyrroles

Three octaalkyl-substituted cyclo[4]naphthobipyrroles, studied in solution in the form of their sulfates, reveal absorption and magnetic circular dichroism (MCD) spectra very similar to those of the parent cyclo[8]pyrrole. A unique feature of these systems is a strong absorption in the near IR region. The analysis of MCD patterns based on a perimeter model reveals a hard-chromophore character of cyclo[4]naphthobipyrroles, i.e., ΔHOMO ≪ ΔLUMO. Comparison of Raman spectra obtained for crystalline samples and solutions, combined with the analysis of absorption and MCD spectra based on quantum chemical calculations reveals that cyclo[4]naphthobipyrroles exist in solutions as undissociated sulfates of the doubly protonated forms.

Acenaphthylene‐Fused Cyclo[8]pyrroles with Intense Near‐IR‐Region Absorption Bands

An acenaphthylene-fused cyclo[8]pyrrole was synthesized by using an oxidative coupling reaction of the corresponding 2,2'-bipyrrole. Two conformational isomers 1 a and 1 b were isolated, and their molecular structures were elucidated by X-ray crystallographic studies. The less-polar and lower-symmetry 1 b isomer can be converted into the 1 a isomer through a thermal ring flip. Application of the perimeter model developed by Michl to magnetic circular dichroism spectroscopic data and theoretical calculations demonstrate that there is a marked redshift of the near-IR absorption maxima relative to cyclo[8]isoindole because there is a significant stabilization of the LUMO due to the differing effects of a fused ring expansion with acenaphthylene and benzene moieties on the frontier π molecular orbitals.

Synthesis, characterization and solid-state emission properties of arylsilyl-substituted pyrene derivatives

Arylsilyl substituted pyrene derivatives were synthesized via Suzuki-Miyaura and Hagihara-Sonogashira cross-coupling reactions with boronic acid-terminated and ethynyl-terminated tetraphenylsilane at the 1-, 3-, 6- and 8-positions. The spectroscopic properties of the dyes were investigated in various solvents and in films to assess the suitability of these compounds for optoelectronic applications. A triphenylsilylphenyl substituted pyrene was found to exhibit intense emission in the solid state with an absolute fluorescence quantum yield (ΦF) of 0.65. In contrast, the emission properties of a triphenylsilylphenyl(ethynyl) substituted pyrene were found to be somewhat weaker in the solid state (ΦF = 0.16) and solvent polarity dependent in solution. Magnetic circular dichroism (MCD) spectroscopy, cyclic voltammetry, and theoretical calculations have been used to analyze the structure-property relationships with Michl's perimeter model used as a conceptual framework for rationalizing the key trends.

Magnetic circular dichroism spectroscopy and electronic structures of C3 symmetry buckybowls

TD-DFT and ZINDO/s calculations of C3v and C3 symmetry buckybowls are reported. Magnetic circular dichroism (MCD) spectroscopy has been used to validate the theoretical descriptions of the electronic structures and the optical spectroscopy, with Michl’s perimeter model used as a conceptual framework to analyze the results. The MCD spectroscopy has been found to be broadly similar to that reported previously for triphenylene. Two weak Gaussian-shaped Faraday B0 terms at lower energy can be readily assigned to forbidden L1 and L2 transitions; while an intense derivative-shaped Faraday A1 term at higher energy is assigned to the allowed B transition.

Synthesis, Characterization, MCD Spectroscopy, and TD-DFT Calculations of Copper-Metalated Nonperipherally Substituted Octaoctyl Derivatives of Tetrabenzotriazaporphyrin, cis- and trans-Tetrabenzodiazaporphyrin, Tetrabenzomonoazaporphyrin, and Tetrabenzoporphyrin

Synthesis of the title compounds has been achieved through refinement of a recently reported synthetic protocol whereby varying equivalents of MeMgBr are reacted with 1,4-dioctylphthalonitrile to produce mixtures favoring specific hybrid structures. The initially formed magnesium-metalated compounds are obtained as pure materials and include, for the first time, both isomers (cis and trans) of tetrabenzodiazaporphyrin. The compounds were demetalated to the metal-free analogues, which were then converted into the copper-metalated derivatives. The X-ray structure of the copper tetrabenzotriazaporphyrin derivative is reported. The metal-free and copper-metalated macrocycles exhibit columnar mesophase behavior, and it is found that the mesophase stability is unexpectedly reduced in the diazaporphyrin derivatives compared to the rest of the series. The results of time-dependent density functional theory calculations for the copper complexes are compared to the observed optical properties. Michl's perimeter model was used as a conceptual framework for analyzing the magnetic circular dichroism spectral data, which predicted and accounted for trends in the observed experimental spectra.

Synthesis, Optical Properties, and Electronic Structures of Fully Core‐Modified Porphyrin Dications and Isophlorins

The synthesis, structures, optical properties, and electronic structures of the tetraphenyltetrathiaporphyrin dication (S(4)TPP(2+), 6) and tetrakis(pentafluorophenyl)tetrathiaisophlorin (S(4)F(20)TPP, 7) are reported. S(4)TPP(2+) (6) and S(4)F(20)TPP (7) were synthesized by acid-catalyzed condensation of the corresponding hydroxylmethylthiophene, followed by oxidation. The electronic structures of S(4)TPP(2+) (6) and S(4)F(20)TPP (7) were analyzed by using UV/Vis-absorption spectroscopy and by magnetic circular dichroism (MCD) spectroscopy and the bands were assigned by using time-dependent density functional theory (TD-DFT) and ZINDO/s calculations. A red-shift of the Q bands of S(4)TPP(2+) (6) is observed relative to the spectra of tetraphenylporphyrins because a destabilization of the HOMO leads to a narrower HOMO-LUMO band-gap. Michl's perimeter model was used to assign the absorption bands and MCD spectra of S(4)F(20)TPP (7). Current-density maps and nucleus-independent chemical-shift (NICS) calculations of S(4)TPP(2+) (6) and of a model compound predict marked modification to the diamagnetic ring current, whilst nonaromatic character is predicted for S(4)F(20)TPP (7).