Absorption spectra of p-nitroaniline derivatives: charge transfer effects and the role of substituents.

CONTEXT: Push-pull compounds are model systems and have numerous applications. By changing their substituents, properties are modified and new molecules for different applications can be designed. The work investigates the gas-phase electronic absorption spectra of 15 derivatives of push-pull para-nitroaniline (pNA). This molecule has applications in pharmaceuticals, azo dyes, corrosion inhibitors, and optoelectronics. Both electron-donor and electron-withdrawing groups were investigated. Employing machine learning-derived Hammett's constants σm, σm0, σR, and σI, correlations between substituents and electronic properties were obtained. Overall, the σm0 constants presented the best correlation with HOMO and LUMO energies, whereas the σR constants best agreed with the transition energy of the first band and HOMO-LUMO energy gap. Electron-donors, which have lower σR values, redshift the absorption spectrum and reduce the HOMO-LUMO energy gap. Conversely, electron-withdrawing groups (higher σR's) blueshift the spectrum and increase the energy gap. The second band maximum energies, studied here for the first time, showed no correlation with σ but tended to increase with σ. A comprehensive charge transfer (CT) analysis of the main transition of all systems was also carried out. We found that donors (lower σ's) slightly enhance the CT character of the unsubstituted pNA, whereas acceptors (higher σ's) decrease it, leading to increased local excitations within the aromatic ring. The overall CT variation is not large, except for pNA-SO2H, which considerably decreases the total CT value. We found that the strong electron donors pNA-OH, pNA-OCH3, and pNA-NH2, which have the smallest HOMO-LUMO energy gaps and lowest σ's, have potential for optoelectronic applications. The results show that none of the studied molecules is fluorescent in the gas phase. However, pNA-NH2 and pNA-COOH in cyclohexane and water reveal fluorescence upon solvation. METHODS: We investigated theoretically employing the second-order algebraic diagrammatic construction (ADC(2)) ab initio wave function and time-dependent density functional theory (TDDFT) the gas-phase electronic absorption spectra of 15 derivatives of p-nitroaniline (pNA). The investigated substituents include both electron-donor (C6H5, CCH, CH3, NH2, OCH3, and OH,) and electron-withdrawing (Br, CCl3, CF3, Cl, CN, COOH, F, NO2, and SO2H) substituents.

The effect of the electronic structure method and basis set on the accuracy of the electric multipoles computed with the distributed multipole analysis (DMA).

CONTEXT: An accurate description of the molecular charge density is crucial for investigating intra- and inter-molecular properties. Among the different ways of describing and analyzing it, the widely used distributed multipole analysis (DMA) is an accurate method for decomposing the molecular charge density into atom-centered electric multipoles (monopole, dipole, quadrupole, and so on) that have a direct chemical interpretation. In this work, DMA was employed to decompose the molecular charge density of six chemically distinct molecules, namely, (2R)-2-amino-3-[(S)-prop-2-enylsulfinyl] propanoic acid (AAP), 4-amine-2-nitro-1,3,5 triazole (ANTA), (RS)-Propan-2-yl methylphosphonofluoridate (SARIN), chloromethane (CLMET), and 2-aminoacetic acid (GLY) into monopole, dipole, and quadrupole values. A hypothetical variation of ANTA built by exchanging all the nitrogen atoms with phosphorus that we named 4-phosphine-2-phosphite-1,3,5-phosphorine (ANTAP) was also studied. These molecules have different chemical structures bearing distinct carbon skeletons, electronegative atoms, and electron-withdrawing/donating groups. We found that although DFT multipole values can depend considerably on the exchange-correlation functional for specific atomic sites, the associated root-mean-square errors (RMSEs) compared to benchmark MP4 mainly were about [Formula: see text] The most significant variations were for monopoles and dipoles of sites highly polarized by adjacent atoms, and to a lesser degree, for the quadrupoles. The double hybrid B2PLYP and the hybrid meta M06-2X functionals, as expected in the framework of Jacob's ladder, overall give the most accurate results among the DFT methods. The MP2 DMA multipole values have an RMSE in relation to the MP4 benchmark mainly in the range [Formula: see text], thus representing a lower computational cost to obtain results with similar good accuracy without the ambiguity of choosing a DFT functional. The deviations of the HF multipoles from the benchmark in most cases were less than 20%, in agreement with the well-known fact that non-correlated charge densities have a slight dependence on the electronic correlation. We also confirmed that DMA values have a small dependence on the size of the basis set: deviations did not exceed 5% in most cases. However, the dependence of the DMA values on the size of the basis set increases with the rank of the electric multipole. To compute accurate values of DMA multipoles of an atom bonded to very electronegative atoms, especially dipoles and quadrupoles, a large basis set including diffuse functions is necessary. Despite that, for a given polarized basis set, the choice of the basis set to compute accurate DMA multipole values is not critical. METHODS: The molecular charge densities were computed using the electronic structure methods Hartree-Fock (HF), MP2, MP4, DFT/PBE, DFT/B3LYP, DFT/B3PW91, DFT/M06-2X, and DFT/B2PLYP implemented in the Gaussian 09 package. MP4 was the benchmark method. The DMA multipoles were obtained with the GDMA program of Stone. The 6-311G + + (d,p) basis set was used for the production calculations, and the augmented correlation-consistent Dunning's hierarchy of basis sets was employed to evaluate the dependence of the DMA multipoles on the basis set size.

CO adsorption on MgO thin-films: formation and interaction of surface charged defects.

Two-dimensional (2D) materials formed by thin-films of metal oxides that grow on metal supports are commonly used in heterogeneous catalysis and multilayer electronic devices. Despite extensive research on these systems, the effects of charged defects at supported oxides on surface processes are still not clear. In this work, we perform spin-polarized density-functional theory (DFT) calculations to investigate formation and interaction of charged magnesium and oxygen vacancies, and Al dopants on MgO(001)/Ag(001) surface. The results show a sizable interface compressive effect that decreases the metal work function as electrons are added on the MgO surface with a magnesium vacancy. This surface displays a larger formation energy in a water environment (O-rich condition) even with additional Al-doping. Under these conditions, we found that a polar molecule such as CO is more strongly adsorbed on the low-coordination oxygen sites due to a larger contribution of the channeled electronic transport with the silver interface regardless of the surface charge. Therefore, these findings elucidate how surface intrinsic vacancies can influence or contribute to charge transfer, which allows one to explore more specific reactions at different surface topologies for more efficient catalysts for CO2 conversion.

A comprehensive analysis of charge transfer effects on donor-pyrene (bridge)-acceptor systems using different substituents.

The alternant polycyclic aromatic hydrocarbon pyrene has photophysical properties that can be tuned with different donor and acceptor substituents. Recently, a D (donor)-Pyrene (bridge)-A (acceptor) system, DPA, with the electron donor N,N-dimethylaniline (DMA), and the electron acceptor trifluoromethylphenyl (TFM), was investigated by means of time-resolved spectroscopic measurements (J. Phys. Chem. Lett. 2021, 12, 2226-2231). DPA shows great promise for potential applications in organic electronic devices. In this work, we used the ab initio second-order algebraic diagrammatic construction method ADC(2) to investigate the excited-state properties of a series of analogous DPA systems, including the originally synthesized DPAs. The additionally investigated substituents were amino, fluorine, and methoxy as donors and nitrile and nitro groups as acceptors. The focus of this work was on characterizing the lowest excited singlet states regarding charge transfer (CT) and local excitation (LE) characters. For the DMA-pyrene-TFM system, the ADC(2) calculations show two initial electronic states relevant for interpreting the photodynamics. The bright S1 state is locally excited within the pyrene moiety, and an S2 state is localized ~0.5 eV above S1 and characterized as a donor to pyrene CT state. HOMO and LUMO energies were employed to assess the efficiency of the DPA compounds for organic photovoltaics (OPVs). HOMO-LUMO and optical gaps were used to estimate power conversion and light-harvesting efficiencies for practical applications in organic solar cells. Considering the systems using smaller D/A substituents, compounds with the strong acceptor NO2 substituent group show enhanced CT and promising properties for use in OPVs. Some of the other compounds with small substituents are also found to be competitive in this regard.

A Hammett's analysis of the substituent effect in functionalized diketopyrrolopyrrole (DPP) systems: Optoelectronic properties and intramolecular charge transfer effects.

Diketopyrrolopyrrole (DPP) systems have promising applications in different organic electronic devices. In this work, we investigated the effect of 20 different substituent groups on the optoelectronic properties of DPP-based derivatives as the donor ( D )-material in an organic photovoltaic (OPV) device. For this purpose, we employed Hammett's theory (HT), which quantifies the electron-donating or -withdrawing properties of a given substituent group. Machine learning (ML)-based σ m , σ p , σ m 0 , σ p 0 , σ p + , σ p - , σ I , and σ R Hammett's constants previously determined were used. Mono- (DPP-X1 ) and di-functionalized (DPP-X2 ) DPPs, where X is a substituent group, were investigated using density functional theory (DFT), time-dependent DFT (TDDFT), and ab initio methods. Several properties were computed using CAM-B3LYP and the second-order algebraic diagrammatic construction, ADC(2), an ab initio wave function method, including the adiabatic ionization potential ( I P A ), the electron affinity ( E A A ), the HOMO-LUMO gaps ( E g ), and the maximum absorption wavelengths ( λ max ), the first excited state transition 1 S0 → 1 S1 energies ( ∆ E ) (the optical gap), and exciton binding energies. From the optoelectronic properties and employing typical acceptor systems, the power conversion efficiency ( PCE ), open-circuit voltage ( V OC ), and fill factor ( FF ) were predicted for a DPP-based OPV device. These photovoltaic properties were also correlated with the machine learning (ML)-based Hammett's constants. Overall, good correlations between all properties and the different types of σ constants were obtained, except for the σ I constants, which are related to inductive effects. This scenario suggests that resonance is the main factor controlling electron donation and withdrawal effects. We found that substituent groups with large σ values can produce higher photovoltaic efficiencies. It was also found that electron-withdrawing groups (EWGs) reduced E g and ∆ E considerably compared to the unsubstituted DPP-H. Moreover, for every decrease (increase) in the values of a given optoelectronic property of DPP-X1 systems, a more significant decrease (increase) in the same values was observed for the DPP-X2 , thus showing that the addition of the second substituent results in a more extensive influence on all electronic properties. For the exciton binding energies, an unsupervised machine learning algorithm identified groups of substituents characterized by average values (centroids) of Hammett's constants that can drive the search for new DDP-derived materials. Our work presents a promising approach by applying HT on molecular engineering DPP-based molecules and other conjugated molecules for applications on organic optoelectronic devices.

Machine Learning Determination of New Hammett's Constants for meta- and para-Substituted Benzoic Acid Derivatives Employing Quantum Chemical Atomic Charge Methods.

Hammett's constants σ quantify the electron donor or electron acceptor power of a chemical group bonded to an aromatic ring. Their experimental values have been successfully used in many applications, but some are inconsistent or not measured. Therefore, developing an accurate and consistent set of Hammett's values is paramount. In this work,we employed different types of machine learning (ML) algorithms combined with quantum chemical calculations of atomic charges to predict theoretically new Hammett's constants σm, σp, σm0, σp0, σp+, σp-, σR, and σI for 90 chemical donor or acceptor groups. New σ values (219), including previously unknown 92, are proposed. The substituent groups were bonded to benzene and meta- and para-substituted benzoic acid derivatives. Among the charge methods (Mulliken, Löwdin, Hirshfeld, and ChelpG), Hirshfeld showed the best agreement for most kinds of σ values. For each type of Hammett constant, linear expressions depending on carbon charges were obtained. The ML approach overall showed very close predictions to the original experimental values, with meta- and para-substituted benzoic acid derivative values showing the most accurate values. A new consistent set of Hammett's constants is presented, as well as simple equations for predicting new values for groups not included in the original set of 90.

Molecular modeling of Mannich phenols as reactivators of human acetylcholinesterase inhibited by A-series nerve agents.

The A-series is the most recent generation of chemical warfare nerve agents (CWA) which act directly on the inhibition of the human acetylcholinesterase (HssAChE) enzyme. These compounds lack accurate experimental data on their physicochemical properties, and there is no evidence that traditional antidotes effectively reactivate HssAChE inhibited by them. In the search for potential antidotes, we employed virtual screening, molecular docking, and molecular dynamics (MD) simulations for the theoretical assessment of the performance of a library of Mannich phenols as potential reactivators of HssAChE inhibited by the Novichok agents A-230, A-232, and A-234, in comparison with the commercial oximes pralidoxime (2-PAM), asoxime (HI-6), trimedoxime (TMB-4), and obidoxime. Following the near-attack conformation (NAC) approach, our results suggest that the compounds assessed would face difficulties in triggering the proposed nucleophilic in-line displacement mechanism. Despite this, it was observed that certain Mannich phenols presented similar or superior results to those obtained by reference oximes against A-232 and A-234 model, suggesting that these compounds can adopt more favourable conformations. Additional binding energy calculations confirmed the stability of the model/ligands complexes and the reactivating potential observed in the molecular docking and MD studies. Our findings indicate that the Mannich phenols could be alternative antidotes and that their efficacy should be evaluated experimentally against the A-series CWA.

Which molecular properties determine the impact sensitivity of an explosive? A machine learning quantitative investigation of nitroaromatic explosives.

We decomposed density functional theory charge densities of 53 nitroaromatic molecules into atom-centered electric multipoles using the distributed multipole analysis that provides a detailed picture of the molecular electronic structure. Three electric multipoles, (the charge of the nitro groups), (the total dipole, i.e., polarization, of the nitro groups), (the total electron delocalization of the C ring atoms), and the number of explosophore groups (#NO2) were selected as features for a comprehensive machine learning (ML) investigation. The target property was the impact sensitivity h50 (cm) values quantified by drop-weight measurements, with a large h50 (e.g., 150 cm) indicating that an explosive is insensitive and vice versa. After a preliminary screening of 42 ML algorithms, four were selected based on the lowest root mean square errors: Extra Trees, Random Forests, Gradient Boosting, and AdaBoost. Compared to experimental data, the predicted h50 values of molecules having very different sensitivities for the four algorithms have differences in the range 19-28%. The most important properties for predicting h50 are the electron delocalization in the ring atoms and the polarization of the nitro groups with averaged weights of 39% and 35%, followed by the charge (16%) and number (10%) of nitro groups. A significant result is how the contribution of these properties to h50 depends on their actual sensitivities: for the most sensitive explosives (h50 up to ∼50 cm), the four properties contribute to reducing h50, and for intermediate ones (∼50 cm ≲ h50 ≲ 100 cm) #NO2 and contribute to increasing it and the other two properties to reducing it. For highly insensitive explosives (h50 ≳ 200 cm), all four properties essentially contribute to increasing it. These results furnish a consistent molecular basis of the sensitivities of known explosives that also can be used for developing safer new ones.

Non-Kasha fluorescence of pyrene emerges from a dynamic equilibrium between excited states.

Pyrene fluorescence after a high-energy electronic excitation exhibits a prominent band shoulder not present after excitation at low energies. The standard assignment of this shoulder as a non-Kasha emission from the second-excited state (S2) has been recently questioned. To elucidate this issue, we simulated the fluorescence of pyrene using two different theoretical approaches based on vertical convolution and nonadiabatic dynamics with nuclear ensembles. To conduct the necessary nonadiabatic dynamics simulations with high-lying electronic states and deal with fluorescence timescales of about 100 ns of this large molecule, we developed new computational protocols. The results from both approaches confirm that the band shoulder is, in fact, due to S2 emission. We show that the non-Kasha behavior is a dynamic-equilibrium effect not caused by a metastable S2 minimum. However, it requires considerable vibrational energy, which can only be achieved in collisionless regimes after transitions into highly excited states. This strict condition explains why the S2 emission was not observed in some experiments.

Nonradiative relaxation mechanisms of the elusive silole molecule.

Silole derivatives have been extensively employed for developing organic optoelectronics, but few studies focused on the photophysical properties of the silole molecule. In this work, we investigate these properties by computing the absorption spectra and performing nonadiabatic molecular dynamics of silole employing the algebraic diagrammatic construction [ADC(2)] and extended multi-state XMS-CASPT2 ab initio electronic structure methods. For vertical excitations and excited state optimizations, the equation of motion coupled-cluster singles and doubles (EOM-CCSD) was also used. The nuclear ensemble and the fewest-switches surface hopping molecular dynamics methods coupled with the first two high-level electronic structure methods were applied to probe the relaxation mechanisms of silole. We could reproduce the experimental first absorption maximum value and found an ultrafast relaxation process occurring exclusively through ring-puckering distortions without breaking ring bonds or hydrogen elimination. Minimum energy conical intersection optimizations were carried out and potential energy curves, including triplet states, were calculated to further elucidate the relaxation process of silole.

Simulation of the electron ionization mass spectra of the Novichok nerve agent.

Novichok is one of the most feared and controversial nerve agents, which existence was confirmed only after the Salisbury attack in 2018. A new attack on August 2020, in Russia, was confirmed. After the 2018 attack, the agent was included in the list of the most dangerous chemicals of the Chemical Weapons Convention (CWC). However, information related to its electron ionization mass spectrometry (EI/MS), essential for unambiguous identification, is scarce. Therefore, investigations about Novichok EI/MS are urgent. In this work, we employed Born-Oppenheimer molecular dynamics through the Quantum Chemistry Electron Ionization Mass Spectrometry (QCEIMS) method to simulate and rationalize the EI/MS spectra and fragmentation pathways of 32 Novichok molecules recently incorporated into the CWC. The comparison of additional simulations with the measured EI spectrum of another Novichok analog is very favorable. A general scheme of the fragmentation pathways derived from simulation results was presented. The present results will be useful for elucidation and prediction of the EI spectra and fragmentation pathways of the dangerous Novichok nerve agent.

Quantifying bond strengths via a Coulombic force model: application to the impact sensitivity of nitrobenzene, nitrogen-rich nitroazole, and non-aromatic nitramine molecules.

The quantification of bond strengths is a useful and general concept in chemistry. In this work, a Coulombic force model based on atomic electric charges computed using the accurate distributed multipole analysis (DMA) partition of the molecular charge density was employed to quantify the weakest N-NO2 and C-NO2 bond strengths of 19 nitrobenzene, 11 nitroazole, and 10 nitramine molecules. These bonds are known as trigger linkages because they are usually related to the initiation of an explosive. The three families of explosives combine different types of molecular properties and structures ranging from essentially aromatic molecules (nitrobenzenes) to others with moderate aromaticity (nitroazoles) and non-aromatic molecules with cyclic and acyclic skeletons (nitramines). We used the results to investigate the impact sensitivity of the corresponding explosives employing the trigger linkage concept. For this purpose, the computed Coulombic bond strength of the trigger linkages was used to build four sensitivity models that lead to an overall good agreement between the predicted values and available experimental sensitivity values even when the model included the three chemical families simultaneously. We discussed the role of the trigger linkages for determining the sensitivity of the explosives and rationalized eventual discrepancies in the models by examining alternative decomposition mechanisms and features of the molecular structures.

The generality of the GUGA MRCI approach in COLUMBUS for treating complex quantum chemistry.

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.

Molecular dynamics simulation of the electron ionization mass spectrum of tabun.

Tabun (ethyl N,N-dimethylphosphoramidocyanidate), or GA, is a chemical warfare nerve agent produced during the World War II. The synthesis of its analogs is rather simple; thus, it is a significant threat. Furthermore, experiments with tabun and other nerve agents are greatly limited by the involved life risks and the severe restrictions imposed by the Chemical Weapons Convention. For these reasons, accurate theoretical assignment of fragmentation pathways can be especially important. In this work, we employ the Quantum Chemistry Electron Ionization Mass Spectra method, which combines molecular dynamics, quantum chemistry methods, and stochastic approaches, to accurately investigate the electron ionization/mass spectrometry (EI/MS) fragmentation spectrum and pathways of the tabun molecule. We found that different rearrangement reactions occur including a McLafferty involving the nitrile group. An essential and characteristic pathway for identification of tabun and analogs, a two-step fragmentation producing the m/z 70 ion, was confirmed. The present results will be also useful to predict EI/MS spectrum and fragmentation pathways of other members of the tabun family, namely, the O-alkyl/cycloalkyl N,N-dialkyl (methyl, ethyl, isopropyl, or propyl) phosphoramidocyanidates.

Correlation between molecular charge densities and sensitivity of nitrogen-rich heterocyclic nitroazole derivative explosives.

Nitroazole derivatives are nitrogen-rich heterocyclic ring molecules with potential application as energetic materials. Thirty-three of them-nitroimidazoles, nitrotriazoles, and nitropyrazoles-were investigated. Computed density functional theory molecular charge densities were partitioned employing the accurate distributed multipole analysis (DMA) method. Based on the magnitude of the DMA atom-centered electric multipoles (monopole, dipole, and quadrupole values), mathematical models were developed to compute the impact sensitivity of the explosives composed of these molecules. Charge localization and delocalization of the ring nitrogen atoms as well as charges of the atoms of the nitro group affect the sensitivity of explosives composed of nitroazole derivatives. The sensitivity is strongly dependent on the ring position of the nitrogen atoms and the bonding site of the substituent groups. The N/C ratio and the repulsion of the non-bonding electron pairs of the vicinal nitrogen atoms of the ring also play an important role in the stability of nitroazoles. The influence of the withdrawing group (NO2) and the electron injector groups (NH2 and CH3) including their bonding position on the ring could be quantified.

Pronounced changes in atomistic mechanisms for the Cl- + CH3I SN2 reaction with increasing collision energy.

In a previous direct dynamics simulation of the Cl- + CH3I → ClCH3 + I- SN2 reaction, predominantly indirect and direct reaction was found at collision energies Erel of 0.20 and 0.39 eV, respectively. For the work presented here, these simulations were extended by studying the reaction dynamics from Erel of 0.15 to 0.40 eV in 0.05 eV intervals. A transition from a predominantly indirect to direct reaction is found for Erel of 0.27-0.28 eV, a finding consistent with experiment. The simulation results corroborate the understanding that in experiments indirect reaction is characterized by small product translational energies and isotropic scattering, while direct reaction has higher translational energies and anisotropic scattering. The traditional statistical theoretical model for the Cl- + CH3I SN2 reaction assumes the Cl--CH3I pre-reaction complex (A) is formed, followed by barrier crossing, and then formation of the ClCH3-I- post-reaction complex (B). This mechanism is seen in the dynamics, but the complete atomistic dynamics are much more complex. Atomistic SN2 mechanisms contain A and B, but other dynamical events consisting of barrier recrossing (br) and the roundabout (Ra), in which the CH3-moiety rotates around the heavy I-atom, are also observed. The two most important mechanisms are only formation of A and Ra + A. The simulation results are compared with simulations and experiments for Cl- + CH3Cl, Cl- + CH3Br, F- + CH3I, and OH- + CH3I.

Dynamics of benzene excimer formation from the parallel-displaced dimer.

Excimers play a key role in a variety of excited-state processes, such as exciton trapping, fluorescence quenching, and singlet-fission. The dynamics of benzene excimer formation in the first 2 ps after S1 excitation from the parallel-displaced geometry of the benzene dimer is reported here. It was simulated via nonadiabatic surface-hopping dynamics using the second-order algebraic diagrammatic construction (ADC(2)). After excitation, the benzene rings take ∼0.5-1.0 ps to approach each other in a parallel-stacked structure of the S1 minimum and stay in the excimer region for ∼0.1-0.4 ps before leaving due to excess vibrational energy. The S1-S2 gap widens considerably while the rings visit the excimer region in the potential energy surface. Our work provides detailed insight into correlations between nuclear and electronic structure in the excimer and shows that decreased ring distance goes along with enhanced charge transfer and that fast exciton transfer happens between the rings, leading to the equal probability of finding the exciton in each ring after around 1.0 ps.

Electronic structure theory gives insights into the higher efficiency of the PTB electron-donor polymers for organic photovoltaics in comparison with prototypical P3HT.

The electron donor poly-thienothiophene-benzodithiophene (PTB) polymer series displays remarkable properties that lead to more efficient bulk heterojunction (BHJ) organic solar cells. In this work, the ground and four excited states (bright S 1 and dark S 2-S 4) of three different members of the PTBn (n = 1, 6, 7) series were studied and compared with the prototypical poly(3-hexylthiophene) (P3HT) donor polymer. Time-dependent density functional theory was employed to investigate oligomers of similar sizes (∼50 Å). Charge alternation electron accumulation and depletion regions of the four transitions are concentrated on the inner units, thereby favoring interaction with the electron acceptor in a BHJ. The bright S 1 transition energies of PTBn are about 0.2 eV lower as compared to P3HT, thereby allowing a better match of their levels with the typical C60-type acceptor moiety in a BHJ. Side chains play a minor role in the electronic spectrum (less than ∼0.1 eV). The most efficient PTB7 transfers more electronic charge from its electron-rich benzodithiophene subunit to its electron-deficient thieno[3,4-b] thiophene subunit as compared to PTB1 and PTB6. We show that the dipolar effect, a partial concentration of negative and positive charges on the different parts of the donor polymer that favors charge separation, is more pronounced in PTBn polymers and typically an order of magnitude larger as compared to P3HT. These effects are conspicuous for the most efficient polymer of the series, PTB7, with its fluorine substituent shown to play a crucial role.

Elucidating the mass spectrum of the retronecine alkaloid using DFT calculations.

Pyrrolizidine alkaloids are natural molecules playing important roles in different biochemical processes in nature and in humans. In this work, the electron ionization mass spectrum of retronecine, an alkaloid molecule found in plants, was investigated computationally. Its mass spectrum can be characterized by three main fragment ions having the following m/z ratios: 111, 94, and 80. In order to rationalize the mass spectrum, minima and transition state geometries were computed using density functional theory. It was showed that the dissociation process includes an aromatization of the originally five-membered ring of retronecine converted into a six-membered ring compound. A fragmentation pathway mechanism involving dissociation activation barriers that are easily overcome by the initial ionization energy was found. From the computed quantum chemical geometric, atomic charges, and energetic parameters, the abundance of each ion in the mass spectrum of retronecine was discussed.

Discrete and continuum modeling of solvent effects in a twisted intramolecular charge transfer system: The 4-N,N-dimethylaminobenzonitrile (DMABN) molecule.

The 4-N,N-dimethylaminobenzonitrile (DMABN) molecule is a prototypical system displaying twisted intramolecular (TICT) charge transfer effects. The ground and the first four electronic excited states (S1-S4) in gas phase and upon solvation were studied. Charge transfer values as function of the torsion angle between the donor group (dimethylamine) and the acceptor moiety (benzonitrile) were explicitly computed. Potential energy curves were also obtained. The algebraic diagrammatic construction method at the second-order [ADC(2)] ab initio wave function was employed. Three solvents of increased polarities (benzene, DMSO and water) were investigated using discrete (average solvent electrostatic configuration - ASEC) and continuum (conductor-like screening model - COSMO) models. The results for the S3 and S4 excited states and the S1-S4 charge transfer curves were not previously available in the literature. Electronic gas phase and solvent vertical spectra are in good agreement with previous theoretical and experimental results. In the twisted (90°) geometry the optical oscillator strengths have negligible values even for the S2 bright state. Potential energy curves show two distinct pairs of curves intersecting at decreasing angles or not crossing in the more polar solvents. Charge transfer and electric dipole values allowed the rationalization of these results. The former effects are mostly independent of the solvent model and polarity. Although COSMO and ASEC solvent models mostly lead to similar results, there is an important difference: some crossings of the excitation energy curves appear only in the ASEC solvation model, which has important implications to the photochemistry of DMABN.