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Singlet fission in covalently bound acene dimers in solution is driven by the interplay of excitonic and singlet correlated triplet

The chemical stability and the low-lying singlet and triplet excited states of BN-n-acenes (n = 1-7) were studied using single reference and multireference methodologies. From the calculations, descriptors such as the singlet-triplet splitting, the natural orbital (NO) occupations and aromaticity indexes are used to provide structural and energetic analysis. The boron and nitrogen atoms form an isoelectronic pair of two carbon atoms, which was used for the complete substitution of these units in the acene series. The structural analysis confirms the effects originated from the insertion of a uniform pattern of electronegativity difference within the molecular systems. The covalent bonds tend to be strongly polarized which does not happen in the case of a carbon-only framework. This effect leads to a charge transfer between neighbor atoms resulting in a more strengthened structure, keeping the aromaticity roughly constant along the chain. The singlet-triplet splitting also agrees with this stability trend, maintaining a consistent gap value for all molecules. The BN-n-acenes molecules possess a ground state with monoconfigurational character indicating their electronic stability. The low-lying singlet excited states have charge transfer character, which proceeds from nitrogen to boron.

3-Hydroxyflavon (3-HF) represents an interesting paradigmatic compound to study excited-state intramolecular proton transfer (ESIPT) and intermolecular (ESInterPT) processes to explain the experimentally observed dual fluorescence in solvents containing protic contamination (water) as opposed to single fluorescence in highly purified nonpolar solvents. In this work, adiabatic on-the-fly molecular dynamics simulations have been performed for isolated 3-HF in an aqueous solution using a polarizable continuum model and including explicit water molecules to represent adequately hydrogen bonding. For the calculation of the excited state, time-dependent density functional theory and the Becke-3-Lee-Yang-Parr (B3LYP) functional have been used. For the isolated 3-HF, ultrafast ESIPT from the enol group to the neighboring keto group has been observed. The calculated PT time of 48 fs agrees well with the experimental value of 39 fs. Addition of one water molecule quenches this ESIPT process but shows an intermolecular concerted or stepwise tautomerization process via the bridging water molecule. Adding a second or more water molecules inhibits this ESInterPT process to a large degree. Most of the trajectories do not show any PT, preserving the initial excited-state enol structure, which is the origin of the violet-blue fluorescence appearing in the solvents contaminated with protic components.

Fluorescence spectra and lifetimes were determined for 16 synthetic flavylium cation analogues of anthocyanin plant pigments in dry acetonitrile acidified with trifluoroacetic acid (TFA). Phosphorescence was also observed from the lowest excited triplet state for all of the flavylium cations at 77 K in a rigid TFA-acidified isopropanol glass. The fluorescence quantum yields and lifetimes depend in a systematic manner on the nature and position of the substituents on the flavylium chromophore and three specific substitution patterns associated with significant decreases in the fluorescence quantum yield were identified. A 4′-bromo or 4′-iodo substituent in the B-ring of the flavylium cation produced a small but normal heavy-atom effect, reducing the fluorescence quantum yield and the phosphorescence lifetime relative to analogues without the halogen atom. In contrast, three flavylium cations with a 3′-bromo substituent exhibited an “inverse” heavy atom effect, i.e., an increase in the fluorescence quantum yield rather than a decrease, which was rationalized on the basis of the nodal properties of the natural transition orbitals (NTOs) involved in the S0→S1 radiative transition.

The magnesium (Mg) particle surface can be used to regulate fluorination or oxidation reactions depending on the applied heating rate and Mg particle size. Magnesium particles are surrounded by a complex hydroxide shell composed of an inner layer of magnesium oxide (MgO) and outer layer of magnesium hydroxide (Mg(OH)2). As particles approach the nanoscale, the thick oxide shell (e.g., 22 nm) becomes an appreciable portion of the overall powder and can be exploited to regulate reactivity. In this study, the reactivity of 800 nm Mg particles (nMg) was compared to 44 µm Mg particles (µMg) when combined with Perfluoropolyethyer (PFPE), providing both fluorine and oxygen for Mg reactions. Experiments were performed at slow heating rates (10 °C/min) and separate experiments were performed at fast heating rates (6.0 × 105 °C/min). The slow heating rate studies used a differential scanning calorimeter (DSC) and thermogravimetric (TG) analyzer to examine reaction kinetics. The faster heating rate experiments used a hot wire to ignite a thermal run-away reaction. Powder X-ray diffraction (XRD) analysis of recovered residue at various temperatures corresponding to exothermic events in the DSC revealed reaction pathways for nMg + PFPE favoring oxidation reactions. For nMg powders, the outer Mg(OH)2 surface layer dehydrates at low temperatures (313 °C) creating highly reactive sites for surface oxidation reactions in the condensed phase leading to a higher conversion of Mg(OH)2 to MgO and greater consumption of Mg through oxidation reactions. For µMg, higher Mg(OH)2 dehydration temperatures (498 °C) stabilize µMg particles and the bulk of reactions occur at elevated temperatures and in the gas phase producing higher MgF2 concentrations. Under high heating rate conditions, MgF2 formation is favored over MgO formation for both particle sizes owing to the high reaction temperatures that promote gas phase reactions favoring MgF2 formation. Theoretical analysis using density functional theory (DFT) through cluster models for Mg(OH)2 and MgO surfaces further show that the Mg(OH)2 surface is more reactive with fluorine species than MgO, especially at elevated temperatures. The DFT results help explain the high heating rate reaction pathway that favors fluorination reactions independent of Mg particle size.

A preexisting chemisorbed defect is well-known to increase the reactivity of graphene which is normally chemically inert. Specifically, the presence of chemisorbed hydrogen atoms forming an sp3-hybridized C-H bond is known to increase the reactivity of neighboring carbon atoms toward additional hydrogenation with wide-ranging applications from materials science to astrochemistry. In this work, static DFT and DFT-based direct dynamics simulations are used to characterize the reactivity of a graphene sheet around an existing C-H bond defect. The spin density landscape shows how to guide subsequent H atom additions, always bonding most strongly to the carbon atom with greatest spin density. Molecular dynamics of an impinging H atom under thermal conditions with defect graphene was used to determine the statistics of probable reactions. The most frequent outcome is inelastic scattering (48%) and then Eley-Rideal (ER) abstraction of the chemisorbed H atom as vibrationally hot H2 (40%), while the least likely, but probably most interesting, result is formation of a novel C-H bond (12%). The C-H bonds always form in the β sublattice. The carbon atom in the para position shows to be most reactive toward the incoming H atom, followed by the ortho carbon, in agreement with the spin density computed in the static calculations. Globally, the graphene energy surface is repulsive, but the defects create local channels into this energy surface through which reactants can move locally through and react with the activated surface without a barrier.

Harnessing aluminum oxidation energy requires navigating the particle's passivation shell composed of alumina. The shell is a barrier to aluminum oxidation but can also exothermically react with halogenated species and therefore contribute to the overall energy generated during aluminum particle combustion. Fluorination reactions with alumina have been studied because fluorine is abundant in binder formulations that commonly surround aluminum particles in an energetic mixture. However, iodine has emerged as an alternative halogenated-based binder or oxidizer because iodine gas provides ancillary benefits such as chemical neutralization of biological agents or sterilization of contaminated environments. This study used density functional theory (DFT) calculations to evaluate potential reaction pathways for aluminum-iodine combustion. Relative to fluorinated fragments such as HF and F

Processing and characterization of films synthesized with the blade casting method were studied. The films include 80 nm average diameter aluminum (nAl) particles combined with a solvent–binder system composed of acetone and dimethylformamide (DMF) co-solvent and Poly(vinylidene fluoride) (PVDF) binder. The nAl powder was stress-altered by annealing to 300 °C then quenching at two different cooling rates to induce elevated dilatational strain and stress within the nAl particles. Moreover, the annealing and quenching process dehydrated and dehydroxylated the surface enough to cause stress-altered particles to create more viscous slurries that produced a more porous film microstructure than untreated (UN) nAl powder. Density functional theory calculations revealed defective (i.e., dehydroxylated and dehydrated) sites on the stress-altered nAl particle surface may be responsible for the differences in nAl + PVDF reactivity that showed a higher enthalpy for aluminum-fluorine interface reactions for stress-altered nAl particles. Further, faster quenching created delamination at the core–shell interface and promoted greater energy conversion via a weaker, unsupported oxide shell that became less of a barrier for diffusion reactions. Results from this study provide insight on optimizing nAl particle properties for energy conversion using a method that could be applicable to a range of filler materials and influence bulk film properties.

The explanation of the origin of the fluorescence properties of carbon dots (CDs) represents an important task because of the great interest in the promising capabilities of these nanomaterials. 5-...

Tuning of the electronic spectra of carbon dots by means of inserting heteroatoms into the π-conjugated polycyclic aromatic hydrocarbon (PAH) system is a popular tool to achieve a broad range of absorption and emission frequencies. Especially nitrogen atoms have been used successfully for that purpose. Despite the significant progress achieved with these procedures, the prediction of specific shifts in the UV-vis spectra and the understanding of the electronic transitions is still a challenging task. In this work, high-level quantum chemical methods based on multireference (MR) and single-reference (SR) methods have been used to predict the effect of different nitrogen doping patterns inserted into the prototypical PAH pyrene on its absorption spectrum. Furthermore, a simple classification scheme based on valence bond (VB) theory and the Clar sextet rule in combination with the harmonic oscillator measure of aromaticity (HOMA) index was applied to arrange the different doping structures into groups and rationalize their electronic properties. The results show a wide variety of mostly redshifts in the spectra as compared to the pristine pyrene case. The most interesting doping structures with the largest red shifts leading to absorption energies below one eV could be readily explained by the occurrence of diradical VB structures in combination with Clar sextets. Moreover, analysis of the electronic transitions computed with MR methods showed that several of the low-lying excited states possess double-excitation character, which cannot be realized by the popular SR methods and, thus, are simply absent in the calculated spectra.

The development of efficient organic-based photovoltaic devices is a vibrant area of research with the potential of providing a cheap source of sustainable energy to society. The attainable power conversion efficiencies could be strongly enhanced via the singlet fission (SF) mechanism, a quantum mechanical phenomenon that potentially doubles the number of electron–hole pairs in a photoexcitation process by splitting a high energy singlet into two triplets. Biradicaloid molecules are particularly appealing for SF applications due to the possibility of controlling the balance between open-shell and closed-shell resonance structures via chemical modifications, which open new opportunities to fine tune the singlet and triplet excitation energies, and thus maximize the SF efficiency. Recently, we have shown that doping acenes with boron (B) or nitrogen (N) atoms leads to a large modulation in its biradicaloid nature at the ground-state. Herein, this previous study is extended to the case of asymmetric substitutions by introducing a BN-pair in a tetracene molecule to form azaborine analogues of acenes. The consequences of the chemical doping on the excitonic properties of tetracene are investigated through high-level multireference calculations. From a pool of 60 proposed BN-tetracene chromophores, we identify 15 new promising candidates for SF as they satisfy the energy level matching conditions involving the low-lying singlet and triplet states of a monomer. Still, some of these compounds show good chemical stability as evidenced by their modest biradical character. These results are interpreted in terms of aromaticity changes, charge transfer effects and exciton properties. More generally, this study shows how the energetics of singlet fission materials can be dramatically altered by using fairly simple chemical substitutions and provides detailed insight into the underlying relationships between the molecular structure, the electronic structure, and the excited state energies.

In this work we discuss the calculation of the spin-density matrix from fundamental spin principles as implemented in the COLUMBUS Program System employing the graphical unitary group approach (GUGA). First, a general equation for the spin-density matrix is derived in terms of the one-and two-particle reduced density matrices, quantities that are spin-independent and readily available within the GUGA formalism. Next, the evaluation of this equation using the Shavitt loop values is discussed. Finally, the spatially resolved counterpart of the spin-density matrix, the spin distribution, is calculated for the phenalenyl radical and structures produced by heteroatoms with mono- and di-substitutions. The physical meaning of the spin-density along with its computational description using various methods is discussed putting special emphasis on negative contributions to the spin-density and their quantification via a spin-promotion index.

The microhydration effects on two polymer electrolyte membrane materials, Nafion and bis[(perfluoroalkyl)sulfonyl] imide (PFSI), were investigated by analyzing optimized geometries and normal modes of vibration. The calculations were performed on periodic structural models with systematic increases of the number of water molecules around the polar head groups. A strong effect of microhydration on the structure of the polar head groups was observed due to a large red shift of the SO-H (Nafion) and N-H (PFSI) stretching modes of about 600-1000 cm-1. In addition, frequency calculations showed how H2O stretching vibrations contribute to the overall high frequency vibrational spectra. Both asymmetric and symmetric stretching modes of the SO2 group (νas(SO2) and νs(SO2)) were red-shifted indicating the S-O bond weakening upon microhydration. PFSI models showed spectral changes upon microhydration comparable to those observed for Nafion. However, distinct spectral features related to the differences in the polar head groups were observed. Lower N-H stretching frequency compared to the SO-H mode explained a stronger acid character of the PFSI polar headgroup and its earlier proton transfer ability (less water molecules needed for the proton release).

Theoretical descriptions of excited state proton transfer (ESPT) have had various degrees of success. This work presents a theoretical description of the photodissociation of the 7-hydroxyflavylium cation (7-HF), the fundamental chromophoric moiety of anthocyanin natural plant pigments. ESPT of 7-HF is promoted by a significant shift of charge away from the OH group in the first singlet excited state, leading smoothly to the excited conjugate base and a protonated water cluster. Several factors contribute to the consistency of the results of the present study: (1) the theoretical approach (TD-DFT with the B3-LYP functional and def2-TZVP basis set utilizing Grimme's D3 dispersion correction); (2) the modeling of the solvent effect combining hydrogen bonding of the photoacid to a cluster of discrete water molecules in a water-like continuum solvent (COSMO); (3) the large S1 -S2 energy gap of flavylium cations; and (4) the electrostatics of the ESPT in which a proton is transferred from a cationic photoacid to water without Coulombic interaction between the proton and the conjugate base.

Pillar[5]arenes, a type of novel macrocycles containing di-substituted hydroquinone units linked by methylene bridges in para-positions, have attracted extensive attention in supramolecular chemistry as interesting candidates to be used in the preparation of host-guest complexes. Functionalization by means of rim substitution and sustaining an ordered substituent arrangement on both sides of the rim is important for the development of new pillararene-based materials. In order to achieve this, the rim inversion process of rotating the hydroquinone units through the pillar[5]arenes has to be controlled. In this context we have studied the effect of different types of hydroquinone substituents on the rotational energy profile using density functional theory combined with the hybrid M06-2X functional. The influence of polar ([sbnd]CH2F, [sbnd]CH2Cl, [sbnd]CH2OH, [sbnd]CH2SH, [sbnd]CH2NH2) and nonpolar alkyl ([sbnd]CH3, [sbnd]CH2CH3, [sbnd]CH2CH2CH3, [sbnd]CH(CH3)2 and [sbnd]CH2CH2CH2CH3) substituents on the on the energy barriers of the rotation mechanism, and different local minima was investigated. The stabilization of the intermediate structures by non-covalent van der Waals and interactions and also by hydrogen bonds constitute a major factor affecting barrier heights. In case of polar substituents, the largest barriers were found for [sbnd]CH2OH and [sbnd]CH3 substitutions and the lowest ones for [sbnd]CH2SH and [sbnd]CH2NH2. For the alkyl series, the barrier decreased significantly up to propyl due to increasing stabilizing dispersion interactions while it increased again for n-butyl since the chain did not fit in well the cavity to rotate through.

To harness greater power from aluminum (Al) combustion, one approach is to chemically transform the aluminum oxide (Al2O3) passivation layer surrounding the Al core particle by exploiting surface r...

Anthocyanins, which are responsible for most of the red, blue and purple colors of fruits and flowers, are very efficient at absorbing and dissipating light energy via excited state proton transfer or charge-transfer mediated internal conversion without appreciable excited triplet state formation. During the maturation of red wines, grape anthocyanins are slowly transformed into pyranoanthocyanins, which have a much more chemically stable pyranoflavylium cation chromophore. Development of straightforward synthetic routes to mono- and disubstituted derivatives of the pyranoflavylium cation chromophore has stimulated theoretical and experimental studies that highlight the interesting absorption and emission properties and redox properties of pyranoflavylium cations. Thus, p-methoxyphenyl substitution enhances the fluorescence quantum yield, while a p-dimethylaminophenyl substituent results in fast decay via a twisted intramolecular charge-transfer (TICT) state. Unlike anthocyanins and their synthetic analogs (flavylium cations), a variety of pyranoflavylium cations form readily detectable excited triplet states that sensitize singlet oxygen formation in solution and exhibit appreciable two-photon absorption cross sections for near-infrared light, suggesting a potential for applications in photodynamic therapy. These excited triplet states have microsecond lifetimes in solution and excited state reduction potentials of at least 1.3 V vs. SCE, features that are clearly desirable in a triplet photoredox catalyst.

The synthesis, structures and spectroscopic properties of M(MeOCH2CH2OMe2Si)3C BH3 (M-1-BH3) and M(MeOCH2CH2OMe2Si)3Si BH3 (M-2-BH3) (M = Li, Na, K) derived from reactions of BH3 with the alkali metal zwitterions [M(MeOCH2CH2OMe2Si)3C] (M-1) and [M(MeOCH2CH2OMe2Si)3Si] (M-2) (M = Li, Na, K), resp., are reported. X-ray analysis and DFT calculations reveal discrete zwitterionic structures with the octahedral alkali metal cations rigidly locked and charge separated from the BH3 units via pendant donors groups. Solution experiments with the hydride acceptors B(C6F5)3 and [Ph3C]2[B12C12] indicate that Na-1-BH3 can donate hydrides to form cations of formula [Na(MeOCH2CH2OMe2Si)3C BH2]+.

Density functional theory was used to compute proton affinity constants (pKa values) for the four step-wise hydrolysis reactions of the aluminum cation (Al3+ through [Al(OH)4]−). The hydrolysis spe...

There is increasing interest in using natural colorants like anthocyanins in cosmetics, food and pharmaceuticals as replacements for synthetic colorants. During the maturation of red wines, the anthocyanin pigments contained in grapes are transformed via reaction with copigments and metabolic products into pyranoanthocyanins, responsible in part for the final color of the wine. In order to understand structural effects on the absorption spectra of pyranoanthocyanins, the calculated excited state energies and spectroscopic states of a series of substitued pyranoflavylium cation analogs of pyranoanthocyanins have been compared to experimental spectroscopic data for these compounds. The vertical excitation energies, calculated by using the ADC(2) approach, gave excellent agreement with the experimental UV-Vis spectra and the nature of the lowest excited state correlates with the observed photophysical behavior in solution. The present results thus provide a basis for the design of new pyranoflavylium chromophores with the desired colors and photophysics, as well as for understanding the analogous properties of natural pyranoanthocyanin pigments in red wine.

The new phosphoranylideneketene, (cyclohexyl)3PCCO, was synthesized and structurally as well as spectroscopically characterized. Ph3PCCO and (cyclohexyl)3PCCO were found to be weak ambidentate Lewis bases capable of donating to strong Lewis acids via the ylidic carbon or the carbonyl oxygen.

The study of electronically excited states of stacked polycyclic aromatic hydrocarbons (PAHs) is of great interest due to promising applications of these compounds as luminescent carbon nanomaterials such as graphene quantum dots (GQDs) and carbon dots (CDs). In this study, the excited states and excitonic interactions are described in detail based on four CD model dimer systems of pyrene, coronene, circum-1-pyrene and circum-1-coronene. Two multi-reference methods, DFT/MRCI and SC-NEVPT2, and two single-reference methods, ADC(2) and CAM-B3LYP, have been used for excited state calculations. The DFT/MRCI method has been used as a benchmark method to evaluate the performance of the other ones. All methods produce useful lists of excited states. However, an overestimation of excitation energies and an inverted ordering of states, especially concerning the bright HOMO-LUMO excitation, are observed. In the pyrene-based systems, the first bright state appears among the first four states, whereas the number of dark states is significantly larger for the coronene-based systems. Fluorescence emission properties are addressed by means of geometry optimization in the S1 state. The inter sheet distances for the S1 state decrease in comparison to the corresponding ground-state values. These reductions are largest for the pyrene dimer and decrease significantly for the larger dimers. Several minima have been determined on the S1 energy surface for most of the dimers. The largest variability in emission energies is found for the pyrene dimer, whereas in the other cases a more regular behavior of the emission spectra is observed.

Hydrangea macrophylla is noted for the variation of the color of its sepals from red to blue when they are planted in acidic, aluminum-containing soil. The major anthocyanin present in the colored sepals is 3-O-glucosyl delphinidin and its color change from red to blue is attributed to chelation with Al(III). The pigment responsible for the blue color may also involve an additional organic copigment such as 5-O-caffeoylquinic acid, forming a 1:1:1 ternary anthocyanin–Al(III)–copigment complex. In the present work, we have employed the algebraic diagrammatic construction method to second order, ADC(2), to predict the ab initio excited state energies and oscillator strengths in the gas phase and in aqueous solution (via COSMO) of the various species that can potentially contribute to the red and blue colors of Hydrangea macrophylla sepals. These species include the anthocyanin, binary anthocyanin–copigment complexes without Al(III), binary anthocyanin–Al(III) complexes without the copigment and ternary anthocyanin–Al(III)–copigment complexes. The predicted and experimental spectra of the anthocyanin itself are in good agreement, providing evidence for the predictive ability of the calculations. The calculated spectrum of one of the binary anthocyanin–copigment complexes is shown to be close to the experimental spectrum of red Hydrangea sepals. In contrast the predicted spectra of the binary and ternary Al(III)-containing complexes exhibit maxima at substantially longer wavelengths than those of red sepals, much closer to the experimental spectrum of blue sepals. Indeed, when the predicted spectra are converted to color coordinates and plotted on a CIE chromaticity diagram, the species without Al(III) fall into more red-like regions around that of free anthocyanin and the red sepals, while those involving complexation with Al(III) fall into more blue-like regions near to the blue Hydrangea sepals. Thus, the direct calculation of the colors of the species potentially present in Hydrangea sepals provides strong evidence for the crucial role of complexation between the anthocyanin and aluminum(III) as the decisive structural feature responsible for the occurrence of the blue color.

Density functional theory calculations were used to reveal the mechanism for the fluorination reaction of active Lewis acid sites on alumina structures, which is important in understanding the pyrophoric processes involving Al particles. In this reaction, hydroxyl groups of active sites are replaced by fluorine anions. Alumina structures were represented by three aluminum aqua hydroxo clusters (labeled AlOOH), in which the Al atom had different coordination spheres, particularly four, five, or six. The F-bearing molecules HF, CH3F, and CF4 were taken as reactants for the fluorination reactions. The overall reaction was represented by four reaction steps as follows: (i) formation of the reaction complex, (ii) activation of the transition state (TS), (iii) deactivation of the TS with a formation of the product complex, and (iv) its decomplexation to individual products. The active reaction center of the TS structure is four-membered, in which two bonds break heterolytically and two form. The lowest reaction barriers were observed for the HF molecule, while the two other molecules had significantly higher reaction barriers. Similarly, the largest overall reaction energies (in absolute value) were found for HF, especially for the five- and six-coordinated Al centers. While the positive charge on the Al center remained almost constant throughout the reaction steps, large charge changes were observed for carbon bearing molecules with a formation of the carbenium cations in the TS step. Realizing the important role of HF in promoting exothermic reactions will enable new molecular design strategies for transforming energy release properties of aluminum powder fuels.

The ubiquitous presence of nitro-derivatives of polycyclic aromatic hydrocarbons like pyrene in the environment is a source of preoccupation given the fact that many of them have been shown to be toxic, mutagenic and/or carcinogenic. An understanding of their photophysics and photochemistry can provide insight into the potential for their sunlight-induced photodegradation in the environment. In the present work, ab initio quantum chemical methods (MP2/def2-TZVP and ADC(2)/def2-TZVP) were employed to calculate the geometries of the three mononitro pyrenes, three of the dinitropyrenes and a trinitropyrene in the ground state and in the lowest excited singlet and triplet states. Absorption spectra predicted from the vertical excitation energies and oscillator strengths of the first 10 excited singlet states (ADC(2)/def2-TZVP/COSMO acetonitrile) compare favorably with the experimental spectra in acetonitrile and adiabatic triplet energies with values derived from phosphorescence spectra. Except for 2-nitropyrene, which was predicted to be planar in the ground (S0) and lowest excited singlet (S1) and triplet (T1) states, the nitro groups of the other compounds were not in the plane of the ring in S0 or T1, but one of the nitro groups was predicted to become coplanar with the ring in the optimized geometry of S1. The theoretical results are discussed in the context of their overall consistency with the experimentally observed photophysical properties of these compounds.

The two hydroxyl groups of the 4′,7-dihydroxyfurano-3,2′-flavylium cation (1), a synthetic analog of the aurone pigments of plants, have been shown to have different relative acidities in the ground state (S0) and the lowest excited singlet state (S1). In the ground state, the 4′-OH group is slightly more acidic, while in the excited state, the molecule is strongly photoacidic and deprotonation occurs preferentially from the 7-OH group. In order to compare the relative acidities of these two OH groups via quantum chemical methodology, a common reference state was employed in which an explicit water molecule was hydrogen-bonded to each of the OH groups of 1. The relative acidities of the two OH groups were then inferred from the differential change in energy along the coordinate for proton transfer to the explicit water molecule via time-dependent density functional calculations (B3-LYP with Grimme’s D3 dispersion correction; TZVP basis set; and PCM to simulate an aqueous environment). The calculated acidity changes confirm the experimentally observed inversion in the relative acidities between S0 and S1. The enhanced photoacidity of S1 was also mirrored in the natural transition orbitals and the decrease in the negative change on the oxygen atoms of the OH groups. Employing a common reference state with an explicit water as the proton acceptor should thus serve as a convenient strategy for exploring the relative ground- and excited-state acidities of the OH groups of natural or synthetic dyes, especially when the values are not readily accessible through experiment.

Conjugated polymers and oligomers have great potentials in various fields, especially in materials and biological sciences because of their intriguing electronic and optoelectronic properties. In recent years, the through-space conjugation system has emerged as a new assembled pattern of multidimensional polymers. Here, a novel series of structurally condensed multicolumn/multilayer 3D polymers and oligomers have been designed and synthesized through one-pot Suzuki polycondensation (SPC). The intramolecularly stacked arrangement of polymers can be supported by either X-ray structural analysis or computational analysis. In all cases, polymers were obtained with modest to good yields, as determined by GPC and 1 H-NMR. MALDI-TOF analysis has proven the speculation of the step-growth process of this polymerization. The computational study of ab initio and DFT calculations based on trimer and pentamer models gives details of the structures and the electronic transition. Experimental results of optical and AIE research confirmed by calculation indicates that the present work would facilitate the research and applications in materials.

Functionalization of quantum carbon dots (QCDs) and graphene quantum dots (GQDs) is a popular way to tune their optical spectra increasing their potential applicability in material science and biorelated disciplines. Based on the experimental observation, functionalization by fluorine atoms induces substantial shifts in absorption and emission spectra and an intensity increase. Understanding of the effects due to fluorine functionalization at the atomic scale level is still challenging due to the complex structure of fluorinated QCDs. In this work, the effect of covalent edge-fluorination and fluorine anion doping on absorption and emission spectra of prototypical polycyclic aromatic hydrocarbons pyrene and circum-pyrene has been investigated. The ways to achieve efficient red-shifts in the UV spectra and obtaining reasonable intensities stood in the focus of the work. High-level quantum chemical methods based on density functional theory/multireference configuration interaction (DFT/MRCI) and single-reference second-order algebraic diagrammatic construction (ADC(2)) and density functional theory (DFT) using the CAM-B3LYP functional have been used for this purpose. The calculations show that doping with the fluoride anion can have significant effects on the electronic spectrum. However, the effect of the fluoride ion is strongly dependent on its position with respect to the QCD. The localization above the GQDs causes large red-shifts to both the absorption and emission of spectra of GQDs, while in-plane localization leads to only negligible shifts and a tendency to dissociation after electronic excitation. Thus, large red-shifts, observed in complexes with F

Excited state energy level diagrams of coumarin-3-carboxylic acid (HCCA) chromophore, Eu(CCA)Cl2(H2O)2 (1), Eu(CCA)2Cl(H2O)2 (2), Eu(CCA)3(H2O)3 (3), Tb(CCA)2Cl(H2O) (4) and Tb(CCA)2(NO3)(H2O) (5) in gas phase and polar solution have been calculated by means of DFT/TDDFT/ωB97XD methods. Based on these results, the ability of CCA to sensitize Eu(III) and Tb(III) luminescence has been examined. The competitive excited state processes in the complexes – fluorescence, intersystem crossing (ISC) and phosphorescence, were analyzed depending on the environment, number of the ligands, Ln(III) ion type (Eu and Tb) and counteranion (Cl− and NO3−). It has been found that the environment altered the S1 state energy, oscillator strength, fluorescence lifetime as well as the S1 character – polar solution stabilized the S1(ππ*) state, whereas non-polar solution (gas phase, solid state) stabilized the S1(nπ*) state. The S1(nπ*) state was decisive for the efficient energy transfer as it suppressed the S1 emission of CCA and favored ISC or direct transfer to the emitting levels of Eu(III). The HCCA triplet (T1) state minimum energy (~2.7, ~2.6ZPE eV) and (ππ*) character were retained in Eu/Tb-CCA complexes regardless of the environment. The energy gap between the higher energy T1 donor state and the acceptor levels 5D1 of Eu(III) (~0.5 eV) and 5D4 of Tb(III) (~0.1 eV) provided optimal resonance conditions for effective energy transfer for Eu(III), but less probability for Tb(III). The nonradiative energy (CCA → Eu(III)) transfer rates and quantum luminescence yield for 2 and 3 were calculated by a strategy combining DFT geometries, INDO/S excitation energies and calculated Judd-Ofelt parameters. The excitation channel T1 → 5D0 through an exchange mechanism was predicted as the most probable one to populate the main emissive Eu-centered state in complexes 2 and 3. The more efficient luminescence of 3 than that of 2 was discussed and explained.

Tuning of the electronic spectra of carbon dots by means of inserting heteroatoms into the π-conjugated polycyclic aromatic hydrocarbon (PAH) system is a popular tool to achieve a broad range of absorption and emission frequencies. Especially nitrogen atoms have been used successfully for that purpose. Despite the significant progress achieved with these procedures, the prediction of specific shifts in the UV-vis spectra and the understanding of the electronic transitions is still a challenging task. In this work, high-level quantum chemical methods based on multireference (MR) and single-reference (SR) methods have been used to predict the effect of different nitrogen doping patterns inserted into the prototypical PAH pyrene on its absorption spectrum. Furthermore, a simple classification scheme based on valence bond (VB) theory and the Clar sextet rule in combination with the harmonic oscillator measure of aromaticity (HOMA) index was applied to arrange the different doping structures into groups and rationalize their electronic properties. The results show a wide variety of mostly redshifts in the spectra as compared to the pristine pyrene case. The most interesting doping structures with the largest red shifts leading to absorption energies below one eV could be readily explained by the occurrence of diradical VB structures in combination with Clar sextets. Moreover, analysis of the electronic transitions computed with MR methods showed that several of the low-lying excited states possess double-excitation character, which cannot be realized by the popular SR methods and, thus, are simply absent in the calculated spectra.

Biphenylene is a well-characterized compound that includes the union of both aromatic and antiaromatic units: benzene and cyclobutadiene, respectively. In this work, we extend this structural theme to acene analogues with a cyclobutadiene linkage in a central position. At the single reference domain, perturbation theory and DFT calculations were applied to characterize the electronic structure of the extended biphenylene systems, as well as of some acenes of interest for comparison. Multireference calculations were also used to provide more accurate information about the energetic stability and insights into the radical character. Using descriptors such as the singlet–triplet splitting, the number of effectively unpaired electrons, fractional occupation number weighted density analysis and aromaticity indexes, the different compounds have been compared with respect to biradicaloid character and chemical stability through structural and energetic approaches. The results indicate that biphenylene derivatives should possess similar reactivities and stabilities as their half acenes fragments.

International audience; Surface hopping quantum mechanical/molecular dynamics simulations have been performed for the tetracyanoethylene–anthracene complex to investigate the evolution of charge-transfer (CT) states after excitation into a locally excited (LE) state of anthracene. The scaled opposite-spin (SOS) second-order algebraic diagrammatic construction (SOS-ADC(2)) has been used to achieve a balanced description of LE and CT states. The calculations have been performed for two media, the gas phase and water (described by molecular mechanics, MM). The two dynamics variants show strongly different behaviors. Even though in both cases the conversion from the LE state to lower-lying CT states occurs with 100 fs, in the gas phase, the system remains in the excited state for longer than 2 ps, while in water, it returns to the ground state within 0.5 ps. Moreover, while in the gas phase the original neutral equilibrium structure should be recovered, in water, the ion-pair (IPr) CT state is strongly stabilized, creating a new competing ground-state isomer. Thus, we predict that the ground state of the complex in water should be composed of two species, the original neutral state and an IPr state. The existence of an IPr ground state in strongly polar environments opens interesting possibilities for the design of efficient charge-separating organic donor–acceptor interfaces.

International audience; The core part of the program system COLUMBUS allows highly efficient calculations using variational multireference (MR) methods in the framework of configuration interaction with single and double excitations (MR-CISD) and averaged quadratic coupled-cluster calculations (MR-AQCC), based on uncontracted sets of configurations and the graphical unitary group approach (GUGA). The availability of analytic MR-CISD and MR-AQCC energy gradients and analytic nonadiabatic couplings for MR-CISD enables exciting applications including, e.g., investigations of π-conjugated biradicaloid compounds, calculations of multitudes of excited states, development of diabatization procedures, and furnishing the electronic structure information for on-the-fly surface nonadiabatic dynamics. With fully variational uncontracted spin-orbit MRCI, COLUMBUS provides a unique possibility of performing high-level calculations on compounds containing heavy atoms up to lanthanides and actinides. Crucial for carrying out all of these calculations effectively is the availability of an efficient parallel code for the CI step. Configuration spaces of several billion in size now can be treated quite routinely on standard parallel computer clusters. Emerging developments in COLUMBUS, including the all configuration mean energy multiconfiguration self-consistent field method and the graphically contracted function method, promise to allow practically unlimited configuration space dimensions. Spin density based on the GUGA approach, analytic spin-orbit energy gradients, possibilities for local electron correlation MR calculations, development of general interfaces for nonadiabatic dynamics, and MRCI linear vibronic coupling models conclude this overview.

The biradicaloid character of different types of polycyclic aromatic hydrocarbons is an important quality that guides the development of new materials with interesting magneto-optical properties. Diindenoacene-based systems represent such a class of promising compounds. In this work, the three types

The chemistry of strained unsaturated cyclic compounds has experienced remarkable growth in recent years via the development of metal–free click reactions. Among these reactions, the cycloaddition of cyclopropenes and their analogues to ortho-quinones has been established as a highly promising click reaction. The present work investigates the mechanism involved in the cycloaddition of strained dienes to ortho-quinones and structural factors that would influence this reaction. For this purpose, we use B97D density functional theory calculations throughout, and for relevant cases, we use spin component–scaled MP2 calculations and single–point domain-based local pair natural orbital coupled cluster (DLPNO-CCSD(T)) calculations. The outcomes are analyzed in detail using the distortion/interaction model, and suggestions for future experimental work are made.

The color of mature red wines is due in large part to the chemical transformation of grape anthocyanins into pyranoanthocyanins. Given the difficulties of isolation and purification of pyranoanthocyanins from wines, experimental investigations have focused on pyranoflavylium cations, synthetic analogs that contain the basic chromophoric moiety of pyranoanthocyanins. Quantum chemical methodologies have been extensively employed to predict the physical, spectroscopic and photophysical properties of anthocyanins and, more recently, pyranoflavylium cations. In the present work, we employ TD-DFT with the B3-LYP functional and the def2-TZVP basis set, combined with estimation of solvation free energies via COSMO, to estimate a priori the pKa values of the ground state and the lowest excited singlet and triplet states of a series of seven substituted hydroxypyranoflavylium cations. For the ground state and the first excited singlet state, the quantum chemical results compare favorably to experimental values for most of these compounds. Although there are currently no experimental data for the lowest excited triplet state, the acidity is predicted to be more similar to that of the ground state than to the excited singlet state, as is generally found experimentally for photoacids.

Although methyl transfer reactions are important in both chemical and biological systems, there is a need for thermodynamic parameters related to methyl affinity and O–CH3 bond dissociation enthalpies (BDEs) relevant to a full understanding of the mechanisms of methyl transfer reactions. As a prelude to the construction of a database of O–CH3 BDEs, the present work examines the reliability of a series of theoretical methods for the prediction of O–CH3 BDEs using a set of 25 compounds that included both aromatic and non-aromatic molecules. The BDEs calculated by density functional theory (DFT) with traditional exchange–correlation functions exhibited much larger errors than those obtained by either the M06-2X or G4 methods. For the non-aromatic compounds, M06-2X/def2-TZVP performed slightly better than G4, but G4 was more accurate for the aromatic molecules. As a result, we recommend G4 as the preferred method for the theoretical estimation of O–CH3 bond dissociation enthalpies, although M06-2X may be a good alternative for large complex molecules when the use of G4 is impractical.

Aromaticity is a multivariable concept in organic chemistry that plays a central role for understanding the structure, stability, and reactivity of polycyclic aromatic hydrocarbons (PAHs). Several types of PAHs are characterized as singlet biradicaloid species and their chemical stability is intimately linked to the degree of aromatic character. In this study, theoretically designed routes to tune the biradical character (and thereby its chemical stability) of nitrogen-substituted octacenes have been investigated on the basis of the high-level multireference averaged quadratic coupled-cluster MR-AQCC method necessary for the appropriate description of polyradicaloid systems. The influence of nitrogen centers on the aromaticity of octacene is probed through structural (HOMA) and electron localization (ELF) indices by comparing the N- against NH-doping cases. These analyses reveal that the aromaticity and biradical character of octacene is only slightly affected by replacing one pair of CH groups with N atoms, i.e., by N-doping. However, a significant aromatic stabilization can be obtained when NH-doping is applied at the inner octacene rings; this is also accompanied by an overall decrease of the open-shell character, as evidenced by the gradual quenching of the unpaired electrons and increase in the singlet-triplet splittings when the NH doping groups are moved toward the center of the octacene molecule. Our findings aid in the rational design of new PAH compounds with balanced biradicaloid character and chemical stability which is important, e.g., for practical applications in organic solar cells based on the singlet-fission mechanism.

Stimulated by its success in both bioconjugation and surface modification, we studied the strain-promoted oxidation-controlled cycloalkyne–1,2-quinone cycloaddition (SPOCQ) in three ways. First, the second-order rate constants and activation parameters (ΔH⧧) were determined of various cyclooctynes reacting with 4-tert-butyl-1,2-quinone in a SPOCQ reaction, yielding values for ΔH⧧ of 4.5, 7.3, and 12.1 kcal/mol, for bicyclo[6.1.0]non-4-yne (BCN), cyclooctyne (OCT), and dibenzoazacyclooctyne (DIBAC), respectively. Second, their reaction paths were investigated in detail by a range of quantum mechanical calculations. Single-configuration theoretical methods, like various DFT and a range of MP2-based methods, typically overestimate this barrier by 3–8 kcal/mol (after inclusion of zero-point energy, thermal, and solvation corrections), whereas MP2 itself underestimates the barrier significantly. Only dispersion-corrected DFT methods like B97D (yielding 4.9, 6.4, and 12.1 kcal/mol for these three reactions) and high-level CCSD(T) and multireference multiconfiguration AQCC ab initio approaches (both yielding 8.2 kcal/mol for BCN) give good approximations of experimental data. Finally, the multireference methods show that the radical character in the TS is rather small, thus rationalizing the use of single-reference methods like B97D and SCS-MP2 as intrinsically valid approaches.

In this work CASPT2 calculations of polyacenes (from naphthalene to heptacene) were performed to find a methodology suitable for calculations of the absorption spectra, in particular of the La (B2u state) and Lb (B3u state) bands, of more extended systems. The effect of the extension of the active space and of freezing σ orbitals was investigated. The MCSCF excitation energy of the B2u state is not sensitive to the size of the active space used. However, the CASPT2 results depend strongly on the amount of σ orbitals frozen reflecting the ionic character of the B2u state. On the other hand, the excitation energies of the B3u state are much more sensitive to the size of the active space used in the calculations reflecting its multiconfigurational character. We found a good agreement with experimental data for both bands by including 14 electrons in 14 π orbitals in the active space followed by the CASPT2(14,14) perturbation scheme in which both σ and π orbitals are included.

Density functional theory (DFT) calculations were performed to examine exothermic surface chemistry between alumina and four fluorinated, fragmented molecules representing species from decomposing fluoropolymers: F–, HF, CH3F, and CF4. The analysis has strong implications for the reactivity of aluminum (Al) particles passivated by an alumina shell. It was hypothesized that the alumina surface structure could be transformed due to hydrogen bonding effects from the environment that promote surface reactions with fluorinated species. In this study, the alumina surface was analyzed using model clusters as isolated systems embedded in a polar environment (i.e., acetone). The conductor-like screening model (COSMO) was used to mimic environmental effects on the alumina surface. Four defect models for specific active −OH sites were investigated including two terminal hydroxyl groups and two hydroxyl bridge groups. Reactions involving terminal bonds produce more energy than bridge bonds. Also, surface exothermic r...