Metal-free photochemical silylations and transfer hydrogenations of benzenoid hydrocarbons and graphene

The Covalent Functionalization of Surface-Supported Graphene: An Update

In the last decade, the use of graphene supported on solid surfaces has broadened its scope and applications, and graphene has acquire a promising role as a major component of high-performance electronic devices. In this context, the chemical modification of graphene has become essential. In particular, covalent modification offers key benefits, including controllability, stability, and the facility to be integrated into manufacturing operations. In this Review, we critically comment on the latest advances in the covalent modification of supported graphene on substrates. We analyze the different chemical modifications with special attention to radical reactions. In this context, we review the latest achievements in reactivity control, tailoring electronic properties, and introducing active functionalities. Finally, we extended our analysis to other emerging 2D materials supported on surfaces, such as transition metal dichalcogenides, transition metal oxides, and elemental analogs of graphene.

Visible-Light Assisted Covalent Surface Functionalization of Reduced Graphene Oxide Nanosheets with Arylazo Sulfones

We present an environmentally benign methodology for the covalent functionalization (arylation) of reduced graphene oxide (rGO) nanosheets with arylazo sulfones. A variety of tagged aryl units were conveniently accommodated at the rGO surface via visible-light irradiation of suspensions of carbon nanostructured materials in aqueous media. Mild reaction conditions, absence of photosensitizers, functional group tolerance and high atomic fractions (XPS analysis) represent some of the salient features characterizing the present methodology. Control experiments for the mechanistic elucidation (Raman analysis) and chemical nanomanipulation of the tagged rGO surfaces are also reported.

Electron-Beam-Induced Fluorination Cycle for Long-Term Preservation of Graphene under Ambient Conditions

The aging in air inevitably results in the accumulation of airborne hydrocarbon contaminations on a graphene surface, which causes considerable difficulties in the subsequent application of graphene. Herein, we report an electron-beam-activated fluorination/defluorination cycle for achieving a long-term preservation of CVD graphene. After experiencing such cycle, the accumulation of airborne hydrocarbon on the graphene surfaces is strongly reduced, and the initial chemical status of graphene can be restored, which is confirmed by employing atomic force microscopy and X-ray photoelectron microscopy. Our reported approach provides an efficient method for the cleaning and long-term preservation of graphene, and it is particularly useful for graphene microscopy characterizations.

Porphyrinoids, a unique platform for exploring excited-state aromaticity

Recently, Baird (anti)aromaticity has been referred to as a description of excited-state (anti)aromaticity. With the term of Baird's rule, recent studies have intensively verified that the Hückel aromatic [4n + 2]π (or antiaromatic [4n]π) molecules in the ground state are reversed to give Baird aromatic [4n]π (or Baird antiaromatic [4n + 2]π) molecules in the excited states. Since the Hückel (anti)aromaticity has great influence on the molecular properties and reaction mechanisms, the Baird (anti)aromaticity has been expected to act as a dominant factor in governing excited-state properties and processes, which has attracted intensive scientific investigations for the verification of the concept of reversed aromaticity in the excited states. In this scientific endeavor, porphyrinoids have recently played leading roles in the demonstration of the aromaticity reversal in the excited states and its conceptual development. The distinct structural and electronic nature of porphyhrinoids depending on their (anti)aromaticity allow the direct observation of excited-state aromaticity reversal, Baird's rule. The explicit experimental demonstration with porphyrinoids has contributed greatly to its conceptual development and application in novel functional organic materials. Based on the significant role of porphyrinoids in the field of excited-state aromaticity, this review provides an overview of the experimental verification of the reversal concept of excited-state aromaticity by porphyrinoids and the recent progress on its conceptual application in novel functional molecules.

Antiaromaticity-Promoted Radical Stability in α-Methyl Heterocyclics

Aromaticity is a fundamental and important concept in chemistry, and usually, the enhancement of aromaticity brings additional thermodynamic stability to a compound. Moreover, since radicals can act as intermediates in chemical reactions, they have attracted considerable attention from both experimental and theoretical chemists for a long time. However, it remains unclear whether there is a relationship between the thermodynamic stability of cyclic planar radicals and their aromaticity. In this work, using various aromaticity indices including anisotropy of the induced current density analysis and nucleus-independent chemical shifts against the radical stabilization energy, we systematically investigated the relationship between aromaticity and the thermodynamic stability of α-methyl heterocyclics. Density functional theory calculations suggest that the stronger the antiaromaticity of the original form heterocyclics, the higher the thermodynamic stability of the corresponding radicals, which is in sharp contrast to the general knowledge that aromaticity brings compounds' thermodynamic stabilities. The principal interacting spin orbital analysis shows that the stronger the π-bond formed between the heterocyclics and the α-methyl carbon, the more spin density the radicals tend to be distributed on the heterocyclics. Thus, the strong π-bonding is one of the factors for improving the thermodynamic stability of radicals.

Barrier-Lowering Effects of Baird Antiaromaticity in Photoinduced Proton-Coupled Electron Transfer (PCET) Reactions

Many popular organic chromophores that catalyze photoinduced proton-coupled electron transfer (PCET) reactions are aromatic in the ground state but become excited-state antiaromatic in the lowest ππ* state. We show that excited-state antiaromaticity makes electron transfer easier. Two representative photoinduced electron transfer processes are investigated: (1) the photolysis of phenol and (2) solar water splitting of a pyridine-water complex. In the selected reactions, the directions of electron transfer are opposite, but the net result is proton transfer following the direction of electron transfer. Nucleus-independent chemical shifts (NICS), ionization energies, electron affinities, and PCET energy profiles of selected [4n] and [4n + 2] π-systems are presented, and important mechanistic implications are discussed.

Photocycloadditions of Arenes Derived from Lignin

Intramolecular photocycloaddition reactions of 3,4-alkoxybenzonitriles derived from vanillin with alkenes have been investigated. In contrast to previous reports on photochemical reactions with these compounds, mainly [2 + 3] cycloaddition has been observed. A competing [2 + 2] photocycloaddition plays a minor role. Most probably, these additions occur at the singlet state S₁. In the case of a triplet reaction, a different regioselectivity of the [2 + 2] cycloaddition would be observed. Linear and angular [2 + 3] cycloadducts are formed as major products. The first isomer is transformed in the second one by a photochemical vinyl-cyclopropane rearrangement, which increases the selectivity of the reaction. The influence of the substitution pattern on the reactivity and the selectivity has also been investigated.

Benzene Excimer and Excited Multimers: Electronic Character, Interaction Nature, and Aromaticity

In this Letter we analyze the forces involved in the formation of the benzene excimer and its electron structure, and (anti)aromatic character. We extend our study to excited states in molecular aggregates, the triplet excimer and the benzene-tricyanobenzene exciplex. Electronic wave functions are decomposed in terms of localized excitations and ion-pair configurations through diabatization, and we show that excimer (anti)aromaticity can be described as the linear combination of ground, excited, and ionic molecular states. Our analysis concludes that the benzene excimer must be characterized as antiaromatic, with weaker antiaromaticity than the molecular excited singlet. Moreover, we define a model electronic Hamiltonian for the excimer state and we use it as a building block for the extrapolation of electronic Hamiltonians in molecular aggregates. Benzene multimers present a nonuniform (anti)aromatic character, with the center of the column being antiaromatic and the edges behaving as aromatic. The implications of this work go beyond the study of the excimer, providing a general framework for the calculation and characterization of excited states in aggregates.

Singlet/Triplet State Anti/Aromaticity of CyclopentadienylCation: Sensitivity to Substituent Effect

It is well known that singlet state aromaticity is quite insensitive to substituent effects, in the case of monosubstitution. In this work, we use density functional theory (DFT) calculations to examine the sensitivity of triplet state aromaticity to substituent effects. For this purpose, we chose the singlet state antiaromatic cyclopentadienyl cation, antiaromaticity of which reverses to triplet state aromaticity, conforming to Baird’s rule. The extent of (anti)aromaticity was evaluated by using structural (HOMA), magnetic (NICS), energetic (ISE), and electronic (EDDBp) criteria. We find that the extent of triplet state aromaticity of monosubstituted cyclopentadienyl cations is weaker than the singlet state aromaticity of benzene and is, thus, slightly more sensitive to substituent effects. As an addition to the existing literature data, we also discuss substituent effects on singlet state antiaromaticity of cyclopentadienyl cation.

Acenes and phenacenes in their lowest-lying triplet states. Does kinked remain more stable than straight?

The larger stability of phenacenes compared to their acene isomers in their ground states is attributed to the larger aromaticity of the former. To our knowledge the relative stability of acenes and phenacenes in their lowest-lying triplet states (T1) has not been discussed yet. Using unrestricted density functional theory calculations, our results show that for the smallest members of the series, acenes in their T1 states are more stable than the corresponding phenacenes. However, when the number of the rings (n) involved increases, the energy difference is reduced and for n > 12, phenacenes become more stable than acenes in their T1 states. To rationalize this trend, we analyze the aromaticity of acenes and phenacenes using a set of aromaticity descriptors. We find that in the T1 states of both acenes and phenacenes, the outer rings form aromatic Clar π-sextets. In acenes, delocalization of spin density in the central rings leads to the preferred formation of the largest antiaromatic diradical. Resonant structures in the form of antiaromatic diradical Baird π-octadectets and π-tetradectets are the major contributors, while the smaller ones, such as π-doublets and π-sextets, contribute the least. In phenacenes, structures with diradical antiaromatic Baird π-sextets in some of the central rings contribute the most. These results are relevant to understand the (anti)aromaticity of larger polycyclic aromatic hydrocarbons in their triplet states.

Prediction of Spin Density, Baird-Antiaromaticity, and Singlet-Triplet Energy Gap in Triplet-State Polybenzenoid Systems from Simple Structural Motifs

Triplet-state aromaticity has been recently proposed as a strategy for designing functional organic electronic compounds, many of which are polycyclic aromatic systems. However, in many cases, the aromatic nature of the triplet state cannot be easily predicted. Moreover, it is often unclear how specific structural manipulations affect the electronic properties of the excited-state compounds. Herein, the relationship between the structure of polybenzenoid hydrocarbons (PBHs) and their spin-density distribution and aromatic character in the first triplet excited state is studied. Although a direct link is not immediately visible, classifying the PBHs according to their annulation sequence reveals regularities. Based on these, a set of guidelines is defined to qualitatively predict the location of spin and paratropicity and the singlet-triplet energy gap in larger PBHs, using only their smaller tri- and tetracyclic components, and subsequently tested on larger systems.

Exfoliation and Noncovalent Functionalization of Graphene Surface with Poly-N-Vinyl-2-Pyrrolidone by In Situ Polymerization

Heteroatom functionalization on a graphene surface can endow the physical and structural properties of graphene. Here, a one-step in situ polymerization method was used for the noncovalent functionalization of a graphene surface with poly-N-vinyl-2-pyrrolidone (PNVP) and the exfoliation of graphite into graphene sheets. The obtained graphene/poly-N-vinyl pyrrolidone (GPNVP) composite was thoroughly characterized. The surface morphology of GPNVP was observed using field emission scanning electron microscopy and high-resolution transmission electron microscopy. Raman spectroscopy and X-ray diffraction studies were carried out to check for the exfoliation of graphite into graphene sheets. Thermogravimetric analysis was performed to calculate the amount of PNVP on the graphene surface in the GPNVP composite. The successful formation of the GPNVP composite and functionalization of the graphene surface was confirmed by various studies. The cyclic voltammetry measurement at different scan rates (5–500 mV/s) and electrochemical impedance spectroscopy study of the GPNVP composite were performed in the typical three-electrode system. The GPNVP composite has excellent rate capability with the capacitive property. This study demonstrates the one-pot preparation of exfoliation and functionalization of a graphene surface with the heterocyclic polymer PNVP; the resulting GPNVP composite will be an ideal candidate for various electrochemical applications.

Triplet state (anti)aromaticity of some monoheterocyclic analogues of benzene, naphthalene and anthracene

By employing DFT calculations, we show the influence of heteroatom substitution on the triplet state (anti)aromaticity of benzene, naphthalene and anthracene.

Fabrication of BP2T functionalized graphene via non-covalent π–π stacking interactions for enhanced ammonia detection

Graphene has stimulated great enthusiasm in a variety of fields, while its chemically inert surface still remains challenging for functionalization towards various applications. Herein, we report an approach to fabricate non-covalently functionalized graphene by employing π–π stacking interactions, which has potentialities for enhanced ammonia detection. 5,5′-Di(4-biphenylyl)-2,2′-bithiophene (BP2T) molecules are used in our work for the non-covalent functionalization through strong π–π interactions of aromatic structures with graphene, and systematic investigations by employing various spectroscopic and microscopic characterization methods confirm the successful non-covalent attachment of the BP2T on the top of graphene. From our gas sensing experiments, the BP2T functionalized graphene is promising for ammonia sensing with a 3-fold higher sensitivity comparing to that of the pristine graphene, which is mainly attributed to the enhanced binding energy between the ammonia and BP2T molecules derived by employing the Langmuir isotherm model. This work provides essential evidence of the π–π stacking interactions between graphene and aromatic molecules, and the reported approach also has the potential to be widely employed in a variety of graphene functionalizations for chemical detection.

Non-covalent functionalization of graphene has been achieved by employing π–π stacking interactions, and it is promising for ammonia detection with greatly enhanced sensitivity.

Click Chemistry Enabling Covalent and Non-Covalent Modifications of Graphene with (Poly)saccharides

Graphene is a material with outstanding properties and numerous potential applications in a wide range of research and technology areas, spanning from electronics, energy materials, sensors, and actuators to life-science and many more. However, the insolubility and poor dispersibility of graphene are two major problems hampering its use in certain applications. Tethering mono-, di-, or even poly-saccharides on graphene through click-chemistry is gaining more and more attention as a key modification approach leading to new graphene-based materials (GBM) with improved hydrophilicity and substantial dispersibility in polar solvents, e.g., water. The attachment of (poly)saccharides on graphene further renders the final GBMs biocompatible and could open new routes to novel biomedical and environmental applications. In this review, recent modifications of graphene and other carbon rich materials (CRMs) through click chemistry are reviewed.

Photoinduced Changes in Aromaticity Facilitate Electrocyclization of Dithienylbenzene Switches

The concepts of excited-state aromaticity and antiaromaticity have in recent years with increasing frequency been invoked to rationalize the photochemistry of cyclic conjugated organic compounds, with the long-term goal of using these concepts to improve the reactivities of such compounds toward different photochemical transformations. In this regard, it is of particular interest to assess how the presence of a benzene motif affects photochemical reactivity, as benzene is well-known to completely change its aromatic character in its lowest excited states. Here, we investigate how a benzene motif influences the photoinduced electrocyclization of dithienylethenes, a major class of molecular switches. Specifically, we report on the synthesis of a dithienylbenzene switch where the typical nonaromatic, ethene-like motif bridging the two thienyl units is replaced by a benzene motif, and show that this compound undergoes electrocyclization upon irradiation with UV-light. Furthermore, through a detailed quantum chemical analysis, we demonstrate that the electrocyclization is driven jointly and synergistically by the loss of aromaticity in this motif from the formation of a reactive, antiaromatic excited state during the initial photoexcitation, and by the subsequent relief of this antiaromaticity as the reaction progresses from the Franck–Condon region. Overall, we conclude that photoinduced changes in aromaticity facilitate the electrocyclization of dithienylbenzene switches.

Impact of Excited-State Antiaromaticity Relief in a Fundamental Benzene Photoreaction Leading to Substituted Bicyclo[3.1.0]hexenes

Benzene exhibits a rich photochemistry which can provide access to complex molecular scaffolds that are difficult to access with reactions in the electronic ground state. While benzene is aromatic in its ground state, it is antiaromatic in its lowest ππ* excited states. Herein, we clarify to what extent relief of excited-state antiaromaticity (ESAA) triggers a fundamental benzene photoreaction: the photoinitiated nucleophilic addition of solvent to benzene in acidic media leading to substituted bicyclo[3.1.0]hex-2-enes. The reaction scope was probed experimentally, and it was found that silyl-substituted benzenes provide the most rapid access to bicyclo[3.1.0]hexene derivatives, formed as single isomers with three stereogenic centers in yields up to 75% in one step. Two major mechanism hypotheses, both involving ESAA relief, were explored through quantum chemical calculations and experiments. The first mechanism involves protonation of excited-state benzene and subsequent rearrangement to bicyclo[3.1.0]hexenium cation, trapped by a nucleophile, while the second involves photorearrangement of benzene to benzvalene followed by protonation and nucleophilic addition. Our studies reveal that the second mechanism is operative. We also clarify that similar ESAA relief leads to puckering of S₁-state silabenzene and pyridinium ion, where the photorearrangement of the latter is of established synthetic utility. Finally, we identified causes for the limitations of the reaction, information that should be valuable in explorations of similar photoreactions. Taken together, we reveal how the ESAA in benzene and 6π-electron heterocycles trigger photochemical distortions that provide access to complex three-dimensional molecular scaffolds from simple reactants.

Substituent Effect on Triplet State Aromaticity of Benzene

Density functional theory calculations have been performed to explore the substituent effect on benzene's structure and aromaticity upon excitation to the first triplet excited state (T₁). Discussion is based on spin density analysis, HOMA (harmonic oscillator model of aromaticity), NICS (nucleus-independent chemical shift), ACID (anisotropy of the induced current density), and monohydrogenation free energies and shows that a large span of aromatic properties, from highly antiaromatic to strongly aromatic, could be achieved by varying the substituent. This opens up a possibility of controlling benzene's physicochemical behavior in its excited state, while molecular motion, predicted for several derivatives, could be of interest for the development of photomechanical materials.

Photochemical reaction on graphene surfaces controlled by substrate-surface modification with polar self-assembled monolayers

The unique thinness of two-dimensional materials enables control over chemical phenomena at their surfaces by means of various gating techniques. For example, gating methods based on field-effect-transistor configurations have been achieved. Here, we report a molecular gating approach that employs a local electric field generated by a polar self-assembled monolayer formed on a supporting substrate. By performing Raman scattering spectroscopy analyses with a proper data correction procedure, we found that molecular gating is effective for controlling solid phase photochemical reactions of graphene with benzoyl peroxide. Molecular gating offers a simple method to control chemical reactions on the surfaces of two-dimensional materials because it requires neither the fabrication of a transistor structure nor the application of an external voltage.

All-metal Baird aromaticity

We have proven that Baird's rule can also be applied to a series of all-metal species with both σ- and π-aromaticity.

Through-space aromatic character in excimers

Aromaticity, though widely used to delineate diverse photochemical phenomena, remains to be examined in excimers, a fundamental and extensively studied entity in the excited states. Herein, the first theoretical evidence for the excited state through-space aromatic character in triplet state (T1) excimers of benzene, naphthalene and anthracene is reported using multiple aromaticity descriptors based on magnetic, electronic and geometric criteria. The calculated chemical shifts and induced current densities manifest the presence of transannular π-electronic currents in the excimers. The results open up enormous research potential from exploring the possibility of through-space aromatic character in singlet excimers to its possible implications in photoexcited state processes of aromatic supramolecular systems.

Two-electron transfer stabilized by excited-state aromatization

The scientific significance of excited-state aromaticity concerns with the elucidation of processes and properties in the excited states. Here, we focus on TMTQ, an oligomer composed of a central 1,6-methano[10]annulene and 5-dicyanomethyl-thiophene peripheries (acceptor-donor-acceptor system), and investigate a two-electron transfer process dominantly stabilized by an aromatization in the low-energy lying excited state. Our spectroscopic measurements quantitatively observe the shift of two π-electrons between donor and acceptors. It is revealed that this two-electron transfer process accompanies the excited-state aromatization, producing a Baird aromatic 8π core annulene in TMTQ. Biradical character on each terminal dicyanomethylene group of TMTQ allows a pseudo triplet-like configuration on the 8π core annulene with multiexcitonic nature, which stabilizes the energetically unfavorable two-charge separated state by the formation of Baird aromatic core annulene. This finding provides a comprehensive understanding of the role of excited-state aromaticity and insight to designing functional photoactive materials.

Excited state aromaticity gives rise to unique photophysical properties which may aid the design of functional photoactive materials. Here, the authors spectroscopically characterize an acceptor-donor-acceptor system featuring a two-electron transfer process stabilized by aromatization in the lower energy excited state.

Triplet-State Structures, Energies, and Antiaromaticity of BN Analogues of Benzene and Their Benzo-Fused Derivatives

It is well known that benzene is aromatic in the ground state (the Hückel's rule) and antiaromatic in the first triplet (T₁) excited state (the Baird's rule). Whereas its BN analogues, the three isomeric dihydro-azaborines, have been shown to have various degrees of aromaticity in their ground state, almost no data are available for their T₁ states. Thus, the purpose of this work is to theoretically [B3LYP/6-311+G(d,p)] predict structures, energies, and antiaromaticity of T₁ dihydro-azaborines and some benzo-fused derivatives. Conclusions are based on spin density analysis, isogyric and hydrogenation reactions, HOMA, NICS, and ACID calculations. The results suggest that singlet-triplet energy gaps, antiaromaticity, and related excited-state properties of benzene, naphthalene, and anthracene could be tuned and controlled by the BN substitution pattern. While all studied compounds remain (nearly) planar upon excitation, the spin density distribution in T₁ 1,4-dihydro-azaborine induces a conformational change by which the two co-planar C-H bonds in the ground state become perpendicular to each other in the excited state. This predicted change in geometry could be of interest for the design of new photomechanical materials. Excitation of B-CN/N-NH₂ 1,4-azaborine would have a few effects: intramolecular charge transfer, aromaticity reversal, rotation, and stereoelectronic Umpolung of the amino group.

Carbon quantum dots derived by direct carbonization of carbonaceous microcrystals in mesophase pitch

Aggregation of the central aromatic ring system of asphaltene molecules due to π-π interaction can lead to the formation of carbon quantum dots (CQDs). However, to date, such a roadmap has not been demonstrated. Here, we present a simple approach to the synthesis of CQDs by direct carbonization of dispersed carbonaceous microcrystals in mesophase pitch. The size of the as-prepared CQDs is modulated by adjusting the nucleation temperature for mesophase formation. Due to the oxygen-free character, the CQDs exhibit excitation-independent fluorescent behavior with a quantum yield up to 87%. The CQDs were successfully applied to fluorescent detection of Fe³⁺ ions with good specificity and sensitivity. Our results not only provide a scalable production of CQDs at low cost, but also give valuable clues to understand the solidification of asphaltene at nanoscale.

Nanoresolution patterning of hydrogenated graphene by electron beam induced C-H dissociation

Direct writing of semi-conductive or insulative nanopatterns on graphene surfaces is one of the major challenges in the application of graphene in flexible and transparent electronic devices. Here, we demonstrate that nanoresolution patterning on hydrogenated graphene can be approached by using electron beam induced C-H dissociation when the electron accelerating voltage is beyond a critical voltage of 3 kV. The resolution of the patterning reaches 18 nm and remains constant as the accelerating voltage is beyond 15 kV. The origin of the nanoresolution pattering as well as the dependence of the resolution on voltage in this technique is well explained by studying the cross-section of the C-H bond under electron impact. This work constitutes a new approach to fabricate graphene-based electronic nanodevices, with the reduced hydrogenated graphene channel utilized as conductive or semi-conductive counterpart in the structure.

Molecular Photoswitching Aided by Excited-State Aromaticity

Central to the development of optoelectronic devices is the availability of efficient synthetic molecular photoswitches, the design of which is an arena where the evolving concept of excited-state aromaticity (ESA) is yet to make a big impact. The aim of this minireview is to illustrate the potential of this concept to become a key tool for the future design of photoswitches. The paper starts with a discussion of challenges facing the use of photoswitches for applications and continues with an account of how the ESA concept has progressed since its inception. Then, following some brief remarks on computational modeling of photoswitches and ESA, the paper describes two different approaches to improve the quantum yields and response times of switches driven by E/Z photoisomerization or photoinduced H-atom/proton transfer reactions through simple ESA considerations. It is our hope that these approaches, verified by quantum chemical calculations and molecular dynamics simulations, will help stimulate the application of the ESA concept as a general tool for designing more efficient photoswitches and other functional molecules used in optoelectronic devices.

Reaction Electronic Flux Perspective on the Mechanism of the Zimmerman Di-π-methane Rearrangement

The reaction electronic flux (REF) offers a powerful tool in the analysis of reaction mechanisms. Noteworthy, the relationship between aromaticity and REF can eventually reveal subtle electronic events associated with reactivity in aromatic systems. In this work, this relationship was studied for the triplet Zimmerman di-π-methane rearrangement. The aromaticity loss and gain taking place during the reaction is well acquainted by the REF, thus shedding light on the electronic nature of reactions involving dibenzobarrelenes.

Triplet state homoaromaticity: concept, computational validation and experimental relevance

Conjugation through space can give rise to aromaticity in the lowest excited triplet state, with impact for photochemistry.

Cyclic conjugation that occurs through-space and leads to aromatic properties is called homoaromaticity. Here we formulate the homoaromaticity concept for the triplet excited state (T₁) based on Baird's 4n rule and validate it through extensive quantum-chemical calculations on a range of different species (neutral, cationic and anionic). By comparison to well-known ground state homoaromatic molecules we reveal that five of the investigated compounds show strong T₁ homoaromaticity, four show weak homoaromaticity and two are non-aromatic. Two of the compounds have previously been identified as excited state intermediates in photochemical reactions and our calculations indicate that they are also homoaromatic in the first singlet excited state. Homoaromaticity should therefore have broad implications in photochemistry. We further demonstrate this by computational design of a photomechanical “lever” that is powered by relief of homoantiaromatic destabilization in the first singlet excited state.

Cyclopropyl Group: An Excited-State Aromaticity Indicator?

The cyclopropyl (cPr) group, which is a well-known probe for detecting radical character at atoms to which it is connected, is tested as an indicator for aromaticity in the first ππ* triplet and singlet excited states (T₁ and S₁ ). Baird's rule says that the π-electron counts for aromaticity and antiaromaticity in the T₁ and S₁ states are opposite to Hückel's rule in the ground state (S₀ ). Our hypothesis is that the cPr group, as a result of Baird's rule, will remain closed when attached to an excited-state aromatic ring, enabling it to be used as an indicator to distinguish excited-state aromatic rings from excited-state antiaromatic and nonaromatic rings. Quantum chemical calculations and photoreactivity experiments support our hypothesis; calculated aromaticity indices reveal that openings of cPr substituents on [4n]annulenes ruin the excited-state aromaticity in energetically unfavorable processes. Yet, polycyclic compounds influenced by excited-state aromaticity (e.g., biphenylene), as well as 4nπ-electron heterocycles with two or more heteroatoms represent limitations.

Energetics of Baird aromaticity supported by inversion of photoexcited chiral [4n]annulene derivatives

For the concept of aromaticity, energetic quantification is crucial. However, this has been elusive for excited-state (Baird) aromaticity. Here we report our serendipitous discovery of two nonplanar thiophene-fused chiral [4n]annulenes Th4 COT Saddle and Th6 CDH Screw , which by computational analysis turned out to be a pair of molecules suitable for energetic quantification of Baird aromaticity. Their enantiomers were separable chromatographically but racemized thermally, enabling investigation of the ring inversion kinetics. In contrast to Th6 CDH Screw , which inverts through a nonplanar transition state, the inversion of Th4 COT Saddle , progressing through a planar transition state, was remarkably accelerated upon photoexcitation. As predicted by Baird's theory, the planar conformation of Th4 COT Saddle is stabilized in the photoexcited state, thereby enabling lower activation enthalpy than that in the ground state. The lowering of the activation enthalpy, i.e., the energetic impact of excited-state aromaticity, was quantified experimentally to be as high as 21-22 kcal mol-1.Baird's rule applies to cyclic π-conjugated molecules in their excited state, yet a quantification of the involved energetics is elusive. Here, the authors show the ring inversion kinetics of two nonplanar and chiral [4n]annulenes to support Baird's rule from an energetic point of view.

Modulating the electronic and magnetic properties of graphene

Graphene, an sp²hybridized single sheet of carbon atoms organized in a honeycomb lattice, is a zero band gap semiconductor or semimetal.