Delocalization Pathway Research Articles

The curious case of S4N4 − A reinvestigation

This work employs quantum chemical methods to explore electronic properties and bonding of the S4N4 molecule. We utilized calculations of multidimensional off-nucleus magnetic shielding, anisotropy of the induced current density (ACID), natural bond orbitals (NBOs), and atoms in molecules (AIM). The unique S4N4 system comprises bent bonding, absence of NN, weakness in the centers of SS bonds, and polar SN bonds. Our findings uncover coexisting of both electron localization and delocalization in S4N4. The magnetic analysis discloses curved connections between the nitrogen atoms and the SS bonds, drawing possible electron delocalization pathways as two pyramid-like regions of magnetic activity. Our magnetic calculations were performed at DFT/GIAO-B3LYP using Jorge-TZP-DKH and 6-311++G(2d,2p) basis sets. No remarkable qualitative differences were noticed between the obtained magnetic features from these two basis sets. Capturing the magnetic details associated with structural features shows superiority of ZZ component of the magnetic shielding tensor (σzz(r)) over the isotropic magnetic shielding (σiso(r)).

Mechanisms of Ligand Hyperfine Coupling in Transition-Metal Complexes: σ and π Transmission Pathways.

Theoretical interpretation of hyperfine interactions was pioneered in the 1950s-1960s by the seminal works of McConnell, Karplus, and others for organic radicals and by Watson and Freeman for transition-metal (TM) complexes. In this work, we investigate a series of octahedral Ru(III) complexes with aromatic ligands to understand the mechanism of transmission of the spin density from the d-orbital of the metal to the s-orbitals of the ligand atoms. Spin densities and spin populations underlying ligand hyperfine couplings are analyzed in terms of π-conjugative or σ-hyperconjugative delocalization vs spin polarization based on symmetry considerations and restricted open-shell vs unrestricted wave function analysis. The transmission of spin density is shown to be most efficient in the case of symmetry-allowed π-conjugative delocalization, but when the π-conjugation is partially or fully symmetry-forbidden, it can be surpassed by σ-hyperconjugative delocalization. Despite a lower spin population of the ligand in σ-hyperconjugative transmission, the hyperfine couplings can be larger because of the direct involvement of the ligand s-orbitals in this delocalization pathway. We demonstrate a quantitative correlation between the hyperfine couplings of aromatic ligand atoms and the characteristics of the metal-ligand bond modulated by the trans substituent, a hyperfine trans effect.

New Perspectives on Delocalization Pathways in Aromatic Molecular Chameleons.

This study comprehensively analyzes the magnetically induced current density of polycyclic compounds labeled as "aromatic chameleons" since they can arrange their π-electrons to exhibit aromaticity in both the ground and the lowest triplet state. These compounds comprise benzenoid moieties fused to a central skeleton with 4n π-electrons and traditional magnetic descriptors are biased due to the superposition of local magnetic responses. In the S0 state, our analysis reveals that the molecular constituent fragments preserve their (anti)aromatic features in agreement with two types of resonant structures: one associated with aromatic benzenoids and the other with a central antiaromatic ring. Regarding the T1 state, a global and diatropic ring current is revealed. Our aromaticity study is complemented with advanced electronic and geometric descriptors to consider different aspects of aromaticity, particularly important in the evaluation of excited state aromaticity. Remarkably, these descriptors consistently align with the general features on the main delocalization pathways in polycyclic hydrocarbons consisting of fused 4n π-electron rings. Moreover, our study demonstrates an inverse correlation between the singlet-triplet energy difference and the antiaromatic character of the central ring in S0.

Linear Extension of Carbaporphyrin Chromophores: Synthesis, Protonation, and Metalation of Anthro[2,3-b]carbaporphyrins: Evidence for 30π-Electron Aromatic Circuits in a Palladium(II) Complex.

Acid-catalyzed condensation of a naphtho[2,3-f]indane dialdehyde with a tripyrrane, followed by an oxidation step, afforded an anthro[2,3-b]-21-carbaporphyrin. The presence of a fused anthracene unit induced minor bathochromic shifts and did not significantly affect the aromatic characteristics of the carbaporphyrin core. Protonation led to the formation of a monocation with similar diatropic properties, but the dication generated in the presence of a large excess of trifluoroacetic acid had a weakened Soret band absorption and a broad absorption at 754 nm. Nucleus-independent chemical shift (NICS) calculations indicate that the dication is only weakly aromatic and possesses a 32-atom 30π electron delocalization pathway. Alkylation with methyl iodide and potassium carbonate gave a 22-methyl derivative that reacted with palladium(II) acetate to afford an aromatic palladium(II) complex. Upon heating, the methyl group migrated from the nitrogen to the internal carbon atom and the resulting complex exhibited diminished aromatic character. A comparison with related carbaporphyrin complexes without ring fusion or with benzo- or naphtho-fused units demonstrated that the diatropic character decreased with increasing conjugation. NICS calculations and anisotropy of induced current density (AICD) plots confirmed this trend and showed that the remaining aromatic properties of the anthrocarbaporphyrin complex were due to a 30π electron circuit that extends around the entire anthracene unit.

Synthesis of Chrysoporphyrins and a Related Benzopyrene-Fused System.

Reaction of 6-nitrochrysene with ethyl isocyanoacetate in the presence of a non-nucleophilic base gave a c-annulated pyrrole ethyl ester that was used to prepare chrysene-fused tripyrranes and a chrysopyrrole dialdehyde. Chrysene-fused tripyrranes were reacted with a pyrrole dialdehyde, but poor yields of chrysoporphyrins were obtained. However, condensation of the chrysopyrrole dialdehyde with a series of tripyrranes afforded excellent yields of chrysoporphyrins and an acenaphtho-chrysoporphyrin. Iron(III) chloride mediated oxidative cyclization of a dihexylchrysoporphyrin afforded a benzopyrene-fused porphyrin that exhibited a strongly red-shifted electronic absorption spectrum. DFT calculations show that both chrysoporphyrins and the benzopyrene-fused porphyrin have tautomers that possess 34π electron delocalization pathways that pass through the porphyrin nucleus and the fused polycyclic aromatic hydrocarbon (PAH) units. Protonation gave dications that favor 36-atom 34π electron circuits. c-Annulated pyrrole dialdehydes were also condensed with a carbatripyrrin to generate PAH-fused carbaporphyrins that retained fully aromatic properties.

A study on the aromaticity and main conjugation pathways of triphyrin(2.1.1) and its heteroanalogues

The Topological Resonance Energy method was used to investigate the aromaticity of triphyrin(2.1.1) and its heteroanalogues. The calculations from indices of local aromaticity, such as Bond Resonance Energy (BRE) and Circuit Resonance Energy (CRE), enabled us to identify the origins of the aromaticity as well as the main conjugation pathways. The BRE and CRE results revealed that the macrocyclic delocalization pathways corresponding to the 14π-electron aromatic systems are not major contributors to global aromaticity. Instead, the five-membered rings show themselves to be the main contributors to the aromaticity of these molecules.

Synthesis, Structure, Spectral, and Anion Sensing Studies of an Aromatic meso-Fused Boron(III) Benzitriphyrin(2.1.1) Complex.

An aromatic tricoordinated organo B(III) complex of benzitriphyrin(2.1.1) was synthesized by treating the nonaromatic phlorin analogue of meso-fused benzitriphyrin(2.1.1) with BCl3 in triethylamine/toluene and refluxing for 2 h. The X-ray structure revealed that B(III) was in a trigonal-planar geometry and was coordinated to two nitrogen atoms and one carbon. Insertion of the small B(III) ion into the meso-fused phlorin analogue resulted in transforming the nonaromatic phlorin analogue to an aromatic B(III) benzitriphyrin(2.1.1) complex. Spectral and electrochemical studies supported the aromatic nature of the B(III) complex of benzitriphyrin(2.1.1). Theoretical studies supported the major contribution of the 18π delocalization pathway toward the aromatic nature of the B(III) meso-fused benzitriphyrin(2.1.1) in comparison to the 22π delocalization pathway. The B(III) benzitriphyrin(2.1.1) complex acts as a specific colorimetric and fluorogenic F-/CN- ion sensor.

Charge-transfer dynamics in van der Waals heterojunctions formed by thiophene-based semiconductor polymers and exfoliated franckeite investigated from resonantly core-excited electrons.

Organic/inorganic van der Waals heterojunctions formed by a combination of 2D materials with semiconductor polymer films enable the fabrication of new device architectures that are interesting for electronic and optoelectronic applications. Here, we investigated the charge-transfer dynamics at the interface between 2D layered franckeite (Fr) and two thiophene-based conjugated polymers (PFO-DBT and P3HT) from the resonantly core-excited electron. The unoccupied electronic states of PFO-DBT/Fr and P3HT/Fr heterojunctions were studied using near-edge X-ray absorption fine structure (NEXAFS) and resonant Auger (RAS) synchrotron-based spectroscopies. We found evidence of ultrafast (subfemtosecond charge-transfer times) interfacial electron delocalization pathways from specific electronic states. For the interface between the PFO-DBT polymer and exfoliated franckeite, the most efficient interfacial electron delocalization pathways were found through π*(S-N) and π*(S-C) electronic states corresponding to the benzothiadiazole and thiophene units. On the other hand, for the P3HT polymer, we found that electrons excited to π-π* and S1s-π*(C-C) electronic states of the P3HT polymer are the most affected by the presence of exfoliated franckeite and consequently are the main interfacial electron-transfer pathways in this heterojunction. Our results have important implications in understanding how ultrafast electron delocalization is taking place in organic/inorganic van der Waals heterojunctions, which is relevant information in designing new devices involving these systems.

Interfacial electronic coupling and band alignment of P3HT and exfoliated black phosphorous van der Waals heterojunctions

The band alignment and the interfacial electronic coupling of van der Waals heterojunctions formed by the Poly[3-hexylthiophene-2,5-diyl] (P3HT) semiconductor polymer and the mechanically exfoliated black phosphorous (BP) multilayers were investigated using different spectroscopies techniques. The energy band alignment was assessed using Ultraviolet (UPS) and X-ray (XPS) photoelectron spectroscopies. The result of this analysis shows that the P3HT-BP interface presents a type I band alignment with conduction band minimum and the valence band maximum located in the BP layer. A valence band offset of 0.45 eV and a conduction band offset of 1.25 eV parameters were found. Near-Edge X-ray Absorption Fine Structure (NEXAFS) and resonant Auger (RAS) synchrotron-based spectroscopies were used to investigate the interfacial electronic coupling between BP and P3HT. Interfacial charge transfer times obtained from core hole clock approach and using the S1s core–hole lifetime as a reference time were used as quantitative parameters to measure the degree of electronic coupling. We found that the π*(SC) electronic state is the faster electron delocalization pathway from the P3HT to the BP conduction band and therefore there is a strong electronic coupling between these states. This result was supported from density functional theoretical calculations.

Adsorption of greenhouse gases on the surface of covalent organic framework of porphyrin – An ab initio study

Porphyrin, a novel and recognized material for its unique properties and complex structure, is considered to be structured into 2-dimensional sheets of covalent organic frameworks. The stability and the functionality of the material is studied using density functional theory. The material is studied for its application as an adsorbing material for greenhouse gases (GHG) and the significant properties are noted. The gas molecules CO, CO2 and CH4 were interacted with the metal free porphyrin sheet (PS) and the efficiency to adsorb CO2 is found to be higher than for the other gas molecules under study. The adsorption energy of CO2 on PS is calculated to be −0.190 eV which is comparatively higher than the adsorption strength of graphene. Similar studies were conducted with porphyrin sheet with ferrous metal ion (PSF) as the adsorbing material. The adsorption strength of PSF for CO2 molecule is −0.914 eV which clearly demonstrates the tunability of the material. The HOMO-LUMO and the orbital localization plot along with the π-electron delocalization pathway confirm the adsorption of GHGs on PS and the efficiency of the material to attract gas molecules. The adsorption spectra confirm the adsorbing nature of both PS and PSF for the adsorption of gas molecules.

Aromatic and Antiaromatic Pathways in Triphyrin(2.1.1) Annelated with Benzo[b]heterocycles.

Understanding of the aromatic properties and magnetically induced current densities of highly conjugated chromophores is important when designing molecules with strongly delocalized electronic structure. Linear extension of the triphyrin(2.1.1) skeleton with an annelated benzo[b]heterocycle fragment modifies the aromatic character by extending the electron delocalization pathway. Two-electron reduction leads to an antiaromatic triphyrin(2.1.1) ring and an aromatic benzo[b]heterocycle subunit. Current-density calculations provide detailed information about the observed pathways and their strengths.

Design, Synthesis, and Characterization of a Water–Stable Li+ ion-Conducting Sulfonate-Based Neutral Metal-Organic Framework

The desire for ion permeable electrode separators in rechargeable batteries has led to an increase in interest in lithium ion-conducting metal-organic frameworks (MOFs). A novel, water-stable, 2D sheet-like neutral Cu (I) −sulfonate MOF, could be the first example of significant lithium ion-conductivity in a neutral, practically solvent-free, not post-synthetically modified MOF. The naphthalenediimide (NDI) ligands of this MOF are capable of simultaneously binding guest lithium ions to its carbonyls and uncoordinated sulfonate oxygen atoms, while charge diffuse perchlorate anions coordinate through anion−π interaction. When made into pristine MOF pellets, the intrinsic electrical conductivity was rather poor (4.65x10−10 S/m) at room temperature. This was due to deficient electron delocalization pathways and charge carrier density. Yet, upon the insertion of LiClO4, the ionic conductivity grew by a factor of 106 to 2.3x10−4 S/m and the activation energy for the charge carrier transport to 0.167 eV. To show the impact of the Li+ binding, the MOF was treated with Bu4NClO4, and the conductivity was essentially unchanged from the original value. The lack of conductivity can be attributed to the size exclusion and facile removal of large uncoordinated Bu4N+ cations. Thus, this presents a new strategy to develop ion-conducting sulfonate MOFs for potential battery applications

Azulichlorins and Benzocarbachlorins Derived Therefrom.

Acid-catalyzed condensation of azulidipyrrane aldehydes with a dihydrodipyrrin carbaldehyde afforded the first examples of azulichlorins. These macrocyclic products were isolated in a monoprotonated form, and the free bases proved to be somewhat unstable. The monocations were strongly diatropic, and proton NMR spectroscopy showed the internal C-H at ca. -2 ppm. Addition of TFA gave the related dications, but these exhibited significantly reduced aromatic ring currents. Reaction of an azulichlorin with tert-butyl hydroperoxide and KOH in dichloromethane/methanol gave a benzocarbachlorin and two related aldehydes. The UV-vis spectrum for the benzocarbachlorin showed a split Soret band at 414 and 430 nm, together with a strong chlorin-like absorption at 684 nm. The proton NMR spectrum indicated that the carbachlorin is strongly aromatic and the internal C-H was observed at -4.64 ppm. Addition of TFA afforded a C-protonated dication with a significantly increased diatropic ring current. The proton NMR spectrum, NICS calculations, and AICD plots indicated that the system favors a 22π electron delocalization pathway that runs through the fused benzo unit. Addition of TFA to the benzocarbachlorin aldehydes primarily led to the formation of monocations, and the generation of C-protonated dications was no longer favored due to the presence of electron-withdrawing formyl moieties.

Adj-Dicarbaporphyrinoid Systems: Synthesis, Spectroscopic Characterization, and Reactivity of 23-Carbabenziporphyrins.

A new family of adj-dicarbaporphyrinoids has been prepared using the "2 + 2" MacDonald methodology. Dibutylboron triflate catalyzed condensation of 3-iodo-4-methoxybenzaldehyde with an indene enamine afforded an iodofulvene aldehyde, and a related dimethoxyfulvene was similarly prepared in two steps from 2,4-dimethoxybenzaldehyde. Following protection as the corresponding dimethyl acetals, the iodofulvenes were metalated with Bu3MgLi at -100 °C and reacted with dimethylformamide to give the required fulvene dialdehyde intermediates. Acid-catalyzed condensation with three different dipyrrylmethanes afforded a series of benzo-23-carbabenziporphyrins in 52-70% yields. The proton nuclear magnetic resonance spectra for these adj-dicarbaporphyrinoids indicate that these macrocycles are slightly diatropic. Monoprotonation afforded cationic species with slightly larger aromatic ring currents, and under strongly acidic conditions, C-protonated dications were generated with substantial diatropic properties. The aromatic character of these structures was supported by nucleus-independent chemical shifts and anisotropy of induced current density calculations. The computational results indicate that the dications favor 23-atom 22π electron delocalization pathways. The benzo-23-carbabenziporphyrins were selectively oxidized with silver(I) acetate in dichloromethane-methanol to give stable nonaromatic structures with two additional methoxy substituents connected to sp3 hybridized bridging carbons. The intriguing reactivity and unique spectroscopic properties of benzo-23-carbabenziporphyrins make these novel structures promising candidates for further investigations.

Li+ Ion-Conducting Sulfonate-Based Neutral Metal–Organic Framework

Lithium ion-conducting metal–organic frameworks (MOFs) are rapidly gaining interest because of their potential application as ion-permeable, robust electrode separators in rechargeable batteries, arguably the most ubiquitous portable clean energy storage devices developed to date. A novel, water-stable 2D sheet-like neutral Cu(I)–sulfonate MOF featuring π-acidic naphthalenediimide (NDI) ligands that can simultaneously bind guest lithium ions with its carbonyl and uncoordinated sulfonate oxygen atoms and charge diffuse perchlorate anions through anion−π interaction has been constructed. While the pristine MOF pellets displayed poor intrinsic electrical conductivity (4.65 × 10–10 S/m) at room temperature due to inadequate charge carrier density and electron delocalization pathway, upon infiltration of LiClO4, its ionic conductivity surged almost million times to 2.3 × 10–4 S/m, and the activation energy for charge carrier transport dropped to a mere 0.167 eV. In contrast, the conductivity of Bu4NClO4-treate...

Tropylium and Porphyrinoid Character in Carbaporphyrinoid Systems. Relative Stability and Aromatic Characteristics of Azuliporphyrin and Tropiporphyrin Tautomers, Protonated Species, and Related Structures

DFT studies have been carried out on 46 azuliporphyrin (AzP) and tropiporphyrin (TrP) tautomers, cations, and dications. The structures were minimized using DFT-B3LYP/6-311++G(d,p), and the relative stabilities of the tautomers for each series were computed with M06-2X and B3LYP-D3 functionals. Nucleus independent chemical shifts, both NICS(0) and NICS(1) zz, were calculated for both the center of the macrocycles and the individual pyrrolic and carbocyclic subunits. In addition, anisotropy of induced current density (AICD) plots were generated for all of the species under investigation. The results provided insights into the favored conjugation pathways and aromatic properties of these important carbaporphyrinoid systems. The π delocalization pathways for AzP were complex, and contrary to previous speculations, there was no evidence for tropylium characteristics in the favored free base structure. The global diatropicity of AzP was found to increase with increasing solvent polarity, in agreement with experimental observations. TrP was shown to be planar and to possess a large global diamagnetic ring current. However, NICS calculations and AICD plots indicated that the cycloheptatrienyl components of the two most favored TrP tautomers have paratropic characteristics. These results are not fully consistent with the proton NMR spectra for TrPs. A dihydroTrP and a Diels-Alder adduct were investigated, and these showed exceptionally large shielding effects with NICS(1) zz values of -36.53 and -36.92 ppm, respectively, in agreement with experimental results. A silver(III) complex of tropiporphyrin was also examined, and this showed no paratropic character for the seven-membered ring, although in this case, the system was found to be severely twisted. Overall, the results provide important information about the electronic properties of AzP and TrP tautomers, as well as for related protonated species, and complement the extensive experimental studies that have been conducted on these systems.

Aromaticity switching via azulene transformations in azulene-bridged A,D-dithiahexaphyrin.

The incorporation of an azulene bridge into an aromatic hexaphyrin framework allows manipulating π-electron delocalization pathways. The palladium(ii) complex undergoes hydroxyl-triggered azulene contraction or isomerization to an oxynaphthalene unit, transforming the hexaphyrin framework into meso-linked carbaporphyrins. This converts the 26π-electron pathway into the 18π one.

Ultrafast charge transfer dynamics pathways in two-dimensional MoS2-graphene heterostructures: a core-hole clock approach.

Two-dimensional van der Waals heterostructures are attractive candidates for optoelectronic nanodevice applications. The charge transport process in these systems has been extensively investigated, however the effect of coupling between specific electronic states on the charge transfer process is not completely established yet. Here, interfacial charge transfer (CT) in the MoS2/graphene/SiO2 heterostructure is investigated from static and dynamic points of view. Static CT in the MoS2-graphene interface was elucidated by an intensity quenching, broadening and a blueshift of the photoluminescence peaks. Atomic and electronic state-specific CT dynamics on a femtosecond timescale are characterized using a core-hole clock approach and using the S1s core-hole lifetime as an internal clock. We demonstrate that the femtosecond electron transfer pathway in the MoS2/SiO2 heterostructure is mainly due to the electronic coupling between S3p-Mo4d states forming the Mo-S covalent bond in the MoS2 layer. For the MoS2/graphene/SiO2 heterostructure, we identify, with the support of density functional calculations, new pathways that arise due to the high density of empty electronic states of the graphene conduction band. The latter makes the transfer process time in the MoS2/graphene/SiO2/Si twice as fast as in the MoS2/SiO2/Si sample. Our results show that ultrafast electron delocalization pathways in van der Waals heterostructures are dependent on the electronic properties of each involved 2D material, creating opportunities to modulate their transport properties.

(Invited) Predicting the Degree of Aromaticity of Novel Porphyrinoid Compounds

Electronic and spectroscopic properties of porphyrinoid macrocycles are, to a large extent, determined by the number of π-electrons participating in the conjugation network. Full control of the number of π-electrons implies full control of the aromatic or antiaromatic character of the molecule. To achieve this, knowledge about the electron delocalization pathways within a molecule is essential. Experimentally, these pathways are not easily accessible. Therefore theoretical studies are necessary to complement experiments that aim at systematically synthesizing porphyrinoids with distinct properties. A computational approach to achieve this is the gauge including magnetically induced current density method (GIMIC). GIMIC is a independent program that is used for the calculation of magnetically induced current densities using London orbitals.[1, 2] Numerical integration of the current flow around molecular rings and along selected chemical bonds can be used for determining current pathways and the degree of aromaticity of various molecules according to the magnetic criterion.[3,4] In my talk I am going to present the GIMIC method and briefly discuss the idea behind it. I will illustrate what type of information is accessible through current density studies. Some very recent results on carbaporphyrinoids will be highlighted showing the potential and the challenges of the method.[5,6]

New insights into aromatic pathways of carbachlorins and carbaporphyrins based on calculations of magnetically induced current densities.

Magnetically induced current densities have been calculated and analyzed for a number of synthesized carbachlorins and carbaporphyrins using density functional theory and the gauge including magnetically induced current (GIMIC) method. Aromatic properties have been determined by using accurate numerical integration of the current flow yielding reliable current strengths and pathways that are related to the degree of aromaticity and the aromatic character of the studied molecules. All investigated compounds are found to be aromatic. However, the obtained aromatic pathways differ from those previously deduced from spectroscopic data and magnetic shielding calculations. For all studied compounds, the ring current divides into an outer and an inner branch at each pyrrolic subring, showing that all π-electrons of the pyrrolic rings take part in the delocalization pathway. The calculations do not support the common notion that the main share of the current takes the inner route at the pyrrolic rings without an inner hydrogen and follows an 18π aromatic pathway. The aromatic pathways of the investigated carbaporphyrins and carbachlorins are very similar, since the current strength via the Cβ[double bond, length as m-dash]Cβ' bond of the cyclopentadienyl ring of the carbaporphyrins is almost as weak as the current density passing the corresponding saturated Cβ-Cβ' bond of the carbachlorins.