Polyaromatic Systems Research Articles

Unraveling the Photophysical Characteristics, Aromaticity, and Stability of π-Extended Acene-Quinodimethyl Thioamides†.

Poly-aromatic systems that contain quinodimethyl (QDM) units are appealing for several photonic and spintronic applications owing to the unique electronic structure, aromaticity, and spin state(s) of the QDM ring. Herein, we report the synthesis and characterization of novel QDM-based chromophores 1-3, which exhibit unique photo-excited behavior and aromaticity. Extending the aromatic core with a biphenyl/phenanthryl- and a pyrrolo-fragment led to reducing the optoelectronic bandgap and modulating the photophysics QDM 1-3. Yet, QDM 2 and 3 suffer from "aromaticity imbalance" and become relatively unstable compared to the parent compound QDM 1. Further assessment of local aromaticity using computational tools revealed that the pseudo-quinoidal ring B is the main driving force allowing to easily populate the excited triplet state of these chromophores. The present study provides complementary guidelines for designing novel non-classical poly-aromatic systems.

(Invited) Stereoselective Synthesis of Molecular Nanographenes

The precise control of stereogenic elements in chemical structures has always been a higly demanding synthetic task.1 In this regard, most of the currently available asymmetric methods require the presence of a heteroatom in the starting substrates that serves as an anchoring point for the successful transfer of chiral information. This eventually emerges, typically, in the form of a new stereogenic centre while the heteroatom remains in the product. This general procedure could account for the lack of stereoselective protocols in all-carbon atom compounds.2,3 Actually, optically active nanographenes have so far been obtained only by either top-down or bottom-up racemic syntheses, followed by time consuming chiral chromatographic separations.4-6 In this presentation, we will introduce a general overview on the efforts carried out in our group in the search for stereoselectivity to introduce and extend stereogenic elements into polyaromatic sp2 carbon-based molecular nanographenes. Acknowledgements The author acknowledge support from the MICIN of Spain (project: PID2020-114653RB-I00) and the European Research Council (ERC) Synergy Grant “TOMATTO” (Project 951224). References H. Kuang, C. Xu, Z. Tang, “Emerging Chiral Materials”, Adv. Mater. 2020, 32, e2005110.M. Rickhaus, M. Mayor, M. Juricek, “Strain-induced helical chirality in polyaromatic systems”, Chem. Soc. Rev. 2016, 45, 1542-1556.J. M. Fernández-García, P. Izquierdo-Garcia, M. Buendia, S. Filippone, N. Martín, “Synthetic chiral molecular nanographenes: the key figure of the racemization barrier”, Chem. Commun. 2022, 58, 2634-2645.P. J. Evans, J. Ouyang, L. Favereau, J. Crassous, I. Fernández, J. Perles-Hernáez, N. Martín "Synthesis of a Helical Bilayer Nanographene" Angew. Chem. Int. Ed. 2018, 57, 6774 –6779.P. Izquierdo-García, J. M. Fernández-García, I. Fernández, J. Perles, N. Martín, “Helically Arranged Chiral Molecular Nanographenes", J. Am. Chem. Soc., 2021, 143, 11864−11870.M. Vázquez-Nakagawa, L. Rodríguez-Pérez, N. Martín, M. A. Herránz, “Supramolecular Assembly of Edge Functionalized Top-Down Chiral Graphene Quantum Dots”, Angew. Chem. Int. Ed. 2022, 61, e202211365.

Novel strategy to produce polyaromatic compounds at low temperature for the production of secondary chars

The production of polyaromatic compounds during biomass thermochemical processing at temperatures over 700 °C from the secondary reactions pyrolysis vapors is a well study phenomena. These compounds are sometimes used to produce carbonaceous materials (such as carbon black, carbon nanotubes). In the case of gasification these polyaromatic compounds form tars and are a major source of problems for the operation of these reactors. In this paper we explore the use of acid impregnation, particularly using phosphoric acid, to reduce the temperature at which small polyaromatic rings are formed and released from the biomass. The acid treatment of biomass was found to increase the yield of biochar (from 17 wt% to 40 wt%) and to be an excellent source of polyaromatic compounds (close to 15 wt% on biomass dry basis). In this manuscript we explored the use of the small polyaromatic ring systems produced from the carbonization of acid-pretreated biomass as precursors for the synthesis of secondary carbons. A two-step pyrolysis process was conducted on acid-treated biomass, revealing that the volatiles produced in the first stage were primarily derived from the dehydration of anhydrosugars. In contrast, the volatiles produced in the second stage were found to be primarily composed of aromatic compounds similar to those produced in gasification processes, with low to no oxygen content. In this research, we have introduced an innovative strategy to maximize the benefits of acid-treated biomass pyrolysis by utilizing the low oxygen aromatic hydrocarbons compounds produced as a carbon source for the production of carbon nanostructures (CNS) on biochar-supported nickel catalysts. This methodology allows to produce polyaromatic compounds at very low temperature from lignocellulosic materials. This not only optimizes the utilization of biomass resources but also enhances the yield of high-value products and aligns with the principles of a circular economy.

Effect of H3PO4 and NaOH additives on the Co-carbonization of cellulose and N-containing compounds to produce N-doped chars

N-doped carbons have significant applications in electronics, materials, and as tools for environmental services. However, few studies focus on N-containing compounds’ carbonization strategies to increase char N retention. For this purpose, H3PO4 and NaOH were used as additives during the co-carbonization of cellulose and four N-containing compounds (lysine, melamine, chitosan, and dicyandiamide (DCD)). Thermogravimetric and Py-GC/MS studies showed significant interactions between the N-containing compounds and cellulose with acid and basic additives. In all cases, an increase in char yield was observed. Acid-base treated samples were carbonized in a spoon reactor at 500 °C, and the resulting chars were thoroughly characterized. The maximum char yields were obtained in the presence of H3PO4 due to the impact of this additive on cellulose dehydration. N’s content in the char product varied from 2 to 12 wt. % depending on the N source. N conversion efficiency increases in the presence of H3PO4. In the case of chitosan, we obtained N conversion efficiencies as high as 77 % in acidic conditions. The deconvoluted XPS N 1s spectra showed that each acid-treated char contained a dominant pyrrolic N form, likely due to the acid’s effect on forming furanic compounds. NaOH-treated chars were richer in pyridinic structures. The first step in N-integration to biochar is the Maillard reaction. Carbonyls and amines react to form glycosylamine and the Amadori product. These intermediates form a Schiff’s base of hydroxymethyl furfural in acidic conditions. In basic conditions, reductones are produced. These are important mediators of melanoidins. The melanoidins are subjected to aromatization and polycondensation reactions to form N-containing polyaromatic ring systems at high temperatures.

Multi-spectroscopic characterization of bitumen and its polarity-based fractions

• Insights on H/C-ratio, aromatic systems and average molecular sizes are obtained. • NMR reveals varying chain lengths of aliphatic substituents throughout the fractions. • Characteristic NMR shifts in the resins suggest presence of some esters or amides. • 31 P NMR after derivatization, shows minor amounts of alcohols and carboxylic acids. Bitumen contains a complex mixture of molecules, ranging from simple linear alkanes to complex polyaromatic systems. Used in road construction, those structures are permanently influenced by atmospheric processes, which result in oxidation, degradation, loss of volatiles and structural rearrangements. In order to understand those molecular changes, a profound description of the unaltered system is needed. In this study, structural and chemical peculiarities of bitumen and its polarity-based fractions were analysed using nuclear magnetic resonance (NMR) techniques in combination with infrared/fluorescence spectroscopy and elemental analysis. The bitumen sample was separated into saturates, aromatics, resins and asphaltenes. Elemental analysis provided insights into variations of heteroatom content and hydrogen-carbon ratio among the fractions. Fluorescence spectroscopy showed an increase in the aromatic condensation degree with rising polarity. The combination of heteronuclear single quantum coherence (HSQC) and elemental analysis facilitated the assessment of length and branching extent of aliphatic side chains, throughout the fractions. Another 2D NMR technique, heteronuclear multiple bond coherence (HMBC) found evidence for minor contributions of carboxylic esters or amides within the resins. Hydroxyl and amino groups were analysed by 31 P NMR after derivatization, revealing minor amounts of alcohols, amides and carboxylic acids in the most polar fractions. Additionally, diffusion ordered spectroscopy was carried out to obtain estimations on average molecular sizes and weights. Applying in-depth NMR analysis to bitumen and combining it with complementary analytical techniques, this study provides a unique basis for future ageing studies and demonstrates how multi-spectroscopy can give new insights into bitumen structure and chemistry.

IR, UV-visible, NMR spectra and aromaticity of the covalent organic tetraoxa[8]circulene frameworks

In this paper, we explore the aromaticity, electronic, vibrational and NMR spectra of the novel tetraoxa[8]circulene-based covalent organic frameworks which were first synthesized in 2022 [Angew. Chem. Int. Ed. 61 (2022) e202116527] being predicted by quantum chemical modeling in 2014 [Phys. Chem. Chem. Phys. 16 (2014) 6555–6559]. Both studied 2x2 and 3x3 tetraoxa[8]circulene (TOC) oligomers represent polyaromatic systems consisting of 108π (2x2) and 168π (3x3) electrons in the outer perimeter with a global diatropic (“aromatic”) current; they includes also two types of internal rings: the cyclooctatetraene cores with 8π electrons and larger C16(CH4)O4 cavities with 28π electrons. Both internal rings provide local paratropic (“antiaromatic”) ring currents.The elongated π-conjugated system of the 2x2 and 3x3 TOC oligomers is responsible for the unique nature of photoinduced charge transfer and vibrational spectra, which obey strict selection rules within the D4h symmetry point group. The nature of photoinduced charge transfer was interpreted using visualization methods including charge density difference and transition density matrix. It was found that strong optical absorption around 500 nm is for the doubly degenerated S3(4) (1Eu) states, in which one pair of oppositely located anthracene fragments has an alternating distribution of holes and electrons, while in the other pair of anthracene fragments, the holes and electrons are mainly localized on the central benzene fragments and the ethylene bridges of adjacent furan rings, respectively. For both 2x2 and 3x3 TOC oligomers, the optical absorption strength for S1 and S2 states is vanished. The density functional calculations of 2x2 and 3x3 TOC models have also allowed us to make the assignment of all the observed lines in the IR and Raman spectra of experimental pTOC samples. We believe that the TOC oligomers represent a new-generation of macrocyclic host molecules in heterocirculene and supramolecular chemistry.

Benzodithiophene-Fused Cyclopentannulated Aromatics via a Palladium-Catalyzed Cyclopentannulation and Scholl Cyclodehydrogenation Strategy.

We report the synthesis of a new class of cyclopenta-fused polyaromatic hydrocarbon (CP-PAH) incorporating fused benzodithiophene subunits. These CP-PAHs were prepared utilizing a two-step process involving a palladium catalyzed cyclopentannulation followed by a Scholl cyclodehydrogenation. This work broadens the scope of annulation chemistry by employing 1,2-bis(5-hexylthiophen-3-yl)ethyne and dibromoaryl derivatives based on anthracene, pyrene, and perylene to give 4,4',4'',4'''-(cyclopenta[hi]aceanthrylene-1,2,6,7-tetrayl)tetrakis(2-hexylthiophene), 4,4',4'',4'''-(dicyclopenta[cd,jk]pyrene-1,2,6,7-tetrayl)tetrakis(2-hexylthiophene), and 1,2,7,8-tetrakis(5-hexylthiophen-3-yl)-1,2,7,8-tetrahydrodicyclopenta[cd,lm]perylene. Scholl cyclodehydrogenation of the pendant thiophene units provided access to the π-extended polyaromatic systems 2,5,11,14-tetrahexylrubiceno[5,4-b:6,7-b':12,11-b'':13,14-b''']tetrathiophene, 2,5,11,14-tetrahexyldithieno-[4,5:6,7]indeno[1,2,3-cd]dithieno[4,5:6,7]indeno-[1,2,3-jk]pyrenes, and 2,9,12,19-tetrahexyldithieno[4,5:6,7]indaceno[1,2,3-cd]dithieno[4,5:6,7]indaceno[1,2,3-lm]perylene that possess helicene-like fragments. The anthracene-based CP-PAH was contorted owing to [5]helicene-like arrangements, while the pyrene- and perylene-based systems were essentially planar. The fully conjugated small molecules give low optical gaps (1.7-2.1 eV) with broad light absorption. The HOMO and LUMO energies of the CP-PAHs were found to be in the range of -5.48 to -5.05 eV and -3.48 to -3.14 eV, respectively. Finally, the anthracene-based CP-PAH was found to be a p-type semiconductor when tested in an organic field effect transistor.

A Cyclic Iminoborane-NHC Adduct: Synthesis, Reactivity, and Bonding Analysis.

Lewis-base coordinated iminoborane adducts, in contrast to their isoelectronic analogue imines, remain largely unexplored given the lack of efficient synthetic strategies for generating robust compounds. Herein, we report the preparation of a cyclic amino iminoborane carbene complex 2 obtained in quantitative yield by adding NHC to the 1,8-(trimethylsilyl)aminonaphthalene complex of boron 1 to induce the elimination of trimethylsilyl chloride (TMSCl). The iminoborane-NHC adduct 2 shows unprecedented thermal stability both in the solid and solution phases, due to the rigid, pre-established geometry of the 1,8-diaminonaphthalene scaffold. Theoretical calculations reveal an exceptionally strong iminoborane-NHC bond as a consequence of the enhanced boron-center acidity in combination with the lower steric and electronic shielding. We show that the chemical bond can be understood as donor-acceptor interaction, leading to a different kind of electronic situation of the B═N π-bond. The high conjugation between the pz-lone pair of the tricoordinated sp2 hybridized N atom and the B═N π-system results in a particularly long B═N double bond distance. Taking advantage of the pendant lone pair of the dicoordinated sp2 hybridized N atom, the iminoborane-NHC adduct gives access to NHC-stabilized borenium cation 3 through the reaction with trimethylsilyl triflate (Me3SiOTf) or to the gallium adduct 4 by reacting with GaCl3. Incorporating an iminoborane functional group into a π-conjugated system brings a new bonding situation for broadening the scope of BN-containing polyaromatic systems.

Supramolecular adducts between macrocyclic Gd(iii) complexes and polyaromatic systems: a route to enhance the relaxivity through the formation of hydrophobic interactions.

The set-up of reversible binding interactions between the hydrophobic region of macrocyclic GBCAs (Gadolinium Based Contrast Agents) and SO3−/OH containing pyrene derivatives provides new insights for pursuing relaxivity enhancements of this class of MRI contrast agents. The strong binding affinity allows attaining relaxation enhancements up to 50% at pyrene/GBCA ratios of 3 : 1. High resolution NMR spectra of the Yb-HPDO3A/pyrene system fully support the formation of a supramolecular adduct based on the set-up of hydrophobic interactions. The relaxation enhancement may be accounted for in terms of the increase of the molecular reorientation time (τR) and the number of second sphere water molecules. This effect is maintained in blood serum and in vivo, as shown by the enhancement of contrast in T1w-MR images obtained by simultaneous injection of GBCA and pyrene derivatives in mice.

Stable Luminescent Radicals and Radical-Based LEDs with Doublet Emission

Radical-based light-emitting diodes are an innovative type of organic light-emitting diodes (OLEDs), which adopt luminescent radicals as emitters, aiming at improving the external quantum efficienc...

Solvent-Controlled Divergent Syntheses of Polycyclic N-Fused Heteroaromatics

Due to a growing interest in aza-fused polyaromatic systems among various sciences, enormous attention has been continuously paid to design and synthesize novel chemotypes of N-fused hetero­cycles. During the course of continued efforts in this line, it was found that divergent access to new polycyclic N-fused heteroaromatics was enabled by choice of reaction solvent. Described herein are solvent-controlled selective approaches to three novel N-fused azacycles, benzo-[d]imidazole-pyrrolo[1,2-a]pyrazine hybrids, under mild conditions. The plausible reaction mechanism for each class of compound is suggested as well.

Polyaromatic Systems Combining Increasing Optical Gaps and Amplified Nonlinear Optical Properties. A Comprehensive Theoretical Study on B3N3 Doped Nanographenes

Materials combining amplified nonlinear optical responses and wide optical gaps that ensure high laser-induced damage thresholds and broadband transparency are highly required for photonic applicat...

Valence Bond Account of Triangular Polyaromatic Hydrocarbons with Spin: Combining Ab Initio and Phenomenological Approaches

We present computational analyses, methodological advances, and heuristic conclusions applied on a series of polyaromatic systems condensed in the shape of regular triangles, carrying spin, because...

Predicting the Outcome of Photocyclisation Reactions: A Joint Experimental and Computational Investigation.

Photochemical oxidative cyclodehydrogenation reactions are a versatile class of aromatic ring-forming reactions. They are tolerant to functional group substitution and heteroatom inclusion, so can be used to form a diverse range of extended polyaromatic systems by fusing existing ring substituents. However, despite their undoubted synthetic utility, there are no existing models-computational or heuristic-that predict the outcome of photocyclisation reactions across all possible classes of reactants. This can be traced back to the fact that "negative" results are rarely published in the synthetic literature and the lack of a general conceptual framework for understanding how photoexcitation affects reactivity. In this work, we address both of these issues. We present experimental data for a series of aromatically substituted pyrroles and indoles, and show that quantifying induced atomic forces upon photoexcitation provides a powerful predictive model for determining whether a given reactant will photoplanarise and hence proceed to photocyclised product under appropriate reaction conditions. The propensity of a molecule to photoplanarise is related to localised changes in charge distribution around the putative forming ring upon photoexcitation. This is promoted by asymmetry in molecular structures and/or charge distributions, inclusion of heteroatoms and ethylene bridging and well-separated or isolated photocyclisation sites.

Stereoelectronic Influence of a “Spectator” Propargylic Substituent Can Override Aromaticity Effects in Radical Peri-Cyclizations en Route to Expanded Polyaromatics

Computational analysis quantifies key trends in " peri"-radical cyclizations, a recently developed type of ring-forming reaction for the expansion of polyaromatic systems at the zigzag edge. Comparison of vinyl radical attack on the peri-position versus a topologically similar six-membered ring formation at the armchair edge reveals that the barriers for the peri-ring closure are slightly higher, even though the peri-attack is more exergonic. On the other hand, the intramolecular competition between the formation of a five-membered ring by ortho-attack at the armchair edge and formation of a six-membered ring by peri-attack at the zigzag edge clearly favors six-membered ring formation. The key novel finding is the unprecedented sensitivity of peri-cyclization to the presence and spatial orientation of a "spectator" propargylic -OMe substituent. Remarkably, formation of cis-products proceeds, in general, through a significantly (∼2-4 kcal/mol) lower barrier than formation of the trans-products, even when the cis-products are less stable. The origin of this unexpected effect is clearly stereoelectronic. These findings identify such remote substitution as a conceptually new tool for the control of rate and selectivity of radical reactions. The correlations of activation barriers for vinyl radical attack with aromaticity of the target show the expected relationship in phenanthrenes and pyrenes but not in anthracenes. In the latter case, the attack at the less aromatic ring corresponds to a higher barrier because a steric penalty on the stereoelectronically favorable cis-TS negates the accelerating influence of the properly aligned C-O and C-Sn bonds.

Diphosphahexaarenes as Highly Fluorescent and Stable Materials.

Oligoarenes are regarded as subunits of π-extended carbon nanoforms, such as graphene and nanotubes, with exceptional technological importance. Fused arenes can thus provide fundamental insight into the nature of the electronic properties of fused polyaromatic rings and pave the way for the design of extended graphene-like materials. However, large π-extended arenes often show low stability. Herein we report the straightforward preparation of linearly fused diphosphahexaarenes containing two six-membered phosphorus heterocycles. They are highly stable towards air, water, and light over months in both solution and the solid state. Single-crystal X-ray crystallography confirmed the molecular structure of all derivatives. Investigations of their optoelectronic properties revealed that the diphosphahexaarenes exhibit ambipolar redox behavior and high fluorescence quantum yields. Embedding six-membered phosphorus rings into large acenes thus opens up new opportunities for the investigation of polyaromatic systems.

Diphosphahexaarenes as Highly Fluorescent and Stable Materials

AbstractOligoarenes are regarded as subunits of π‐extended carbon nanoforms, such as graphene and nanotubes, with exceptional technological importance. Fused arenes can thus provide fundamental insight into the nature of the electronic properties of fused polyaromatic rings and pave the way for the design of extended graphene‐like materials. However, large π‐extended arenes often show low stability. Herein we report the straightforward preparation of linearly fused diphosphahexaarenes containing two six‐membered phosphorus heterocycles. They are highly stable towards air, water, and light over months in both solution and the solid state. Single‐crystal X‐ray crystallography confirmed the molecular structure of all derivatives. Investigations of their optoelectronic properties revealed that the diphosphahexaarenes exhibit ambipolar redox behavior and high fluorescence quantum yields. Embedding six‐membered phosphorus rings into large acenes thus opens up new opportunities for the investigation of polyaromatic systems.

Lysoquinone-TH1, a New Polyphenolic Tridecaketide Produced by Expressing the Lysolipin Minimal PKS II in Streptomyces albus.

The structural repertoire of bioactive naphthacene quinones is expanded by engineering Streptomyces albus to express the lysolipin minimal polyketide synthase II (PKS II) genes from Streptomyces tendae Tü 4042 (llpD-F) with the corresponding cyclase genes llpCI-CIII. Fermentation of the recombinant strain revealed the two new polyaromatic tridecaketides lysoquinone-TH1 (7, identified) and TH2 (8, postulated structure) as engineered congeners of the dodecaketide lysolipin (1). The chemical structure of 7, a benzo[a]naphthacene-8,13-dione, was elucidated by NMR and HR-MS and confirmed by feeding experiments with [1,2-13C2]-labeled acetate. Lysoquinone-TH1 (7) is a pentangular polyphenol and one example of such rare extended polyaromatic systems of the benz[a]napthacene quinone type produced by the expression of a minimal PKS II in combination with cyclases in an artificial system. While the natural product lysolipin (1) has antimicrobial activity in nM-range, lysoquinone-TH1 (7) showed only minor potency as inhibitor of Gram-positive microorganisms. The bioactivity profiling of lysoquinone-TH1 (7) revealed inhibitory activity towards phosphodiesterase 4 (PDE4), an important target for the treatment in human health like asthma or chronic obstructive pulmonary disease (COPD). These results underline the availability of pentangular polyphenolic structural skeletons from biosynthetic engineering in the search of new chemical entities in drug discovery.

A Guide for the Design of Functional Polyaromatic Organophosphorus Materials.

The impact of integrating six-membered phosphorus heterocycles into a poly(hetero)aromatic materials is investigated. Mechanistic studies reveal the key synthetic requirements to embed the latter phosphorus heterocycles in polyaromatic molecules. DFT calculations indicate that introducing six-membered phosphorus rings into π-extended molecules induces a particular electron distribution over the π-extended system. Electrochemical investigations confirm that inserting six-membered phosphacycles into polyaromatics triggers ambipolar redox behavior. Steady-state spectroscopy reveals that fusing pyrroles with phosphorus-containing polyaromatic molecules induces fluorescence quantum yields as high as 0.8, whereas transient absorption spectroscopy demonstrates that fusing thiophenes promote the formation of non-emissive triplet-excited states. As a whole, the optoelectronic properties of fused phosphorus-containing materials give rise to promising performances in photoelectrochemical cells. Moreover, X-ray analyses confirm that the 3D arrangement in the solid state of polyaromatic systems containing six-membered phosphorus rings can be tailored through post-functionalization of the phosphorus center. Altogether, this investigation sets the bedrock for the design of a new generation of highly functional polyaromatic organophosphorus materials, keeping control over their electrochemical properties, fluorescence features, photo-induced excited states, and 3D molecular arrangement.

Exploring energetics of dirhodium paddlewheel complexes with π-ligands of different topologies

The first comprehensive theoretical exploration of the energetics of the interaction between π-ligands with isolated multiple carbon-carbon bonds (acetylene and ethylene), aromatic (benzene), planar polyaromatic (naphthalene, acenaphthalene, pyrene), curved polyaromatic (corannulene) and closed-cage C60-fullerene systems and Lewis acidic [Rh2(O2CCF3)4] complex has been carried out. Importantly, two energy decomposition schemes (EDA-NOCV and NBO-NEDA) that are based on very different approaches and algorithms were utilized in order to shed light on the energy and nature of bonding. The total bonding energy was sequentially represented as a combination of attractive covalent (ΔEorb and ECT in EDA and NBO approaches, respectively) and ionic (ΔEelstat and EEL), as well as repulsive (ΔEPauli and ECO) terms. This approach provides a balanced and complete picture of bonding between two (or more) interacting molecular fragments. All computational methods applied unambiguously indicate that the actual stability of adducts formed by unsaturated hydrocarbons with the selected Lewis acidic dirhodium complex is indeed a multifaceted phenomenon. Important contributions come from a different interplay between attractive (ΔEorbvs. ΔEelstat or ECTvs. EEL) and repulsive (ΔEPauli or ECO) components of the bonding and from variations in aromatic behavior of the selected π-systems.