Solvent-Induced Shift of the Lowest Singlet π → π* Charge-Transfer Excited State of p-Nitroaniline in Water: An Application of the TDDFT/EFP1 Method

Ab initio electronic absorption spectra of para-nitroaniline in different solvents: Intramolecular charge transfer effects

Intramolecular charge transfer (ICT) effects of para-nitroaniline (pNA) in eight solvents (cyclohexane, toluene, acetic acid, dichloroethane, acetone, acetonitrile, dimethylsulfoxide, and water) are investigated extensively. The second-order algebraic diagrammatic construction, ADC(2), ab initio wave function is employed with the COSMO implicit and discrete multiscale solvation methods. We found a decreasing amine group torsion angle with increased solvent polarity and a linear correlation between the polarity and ADC(2) transition energies. The first absorption band involves π → π* transitions with ICT from the amine and the benzene ring to the nitro group, increased by 4%-11% for different solvation models of water compared to the vacuum. A second band of pNA is characterized for the first time. This band is primarily a local excitation on the nitro group, including some ICT from the amine group to the benzene ring that decreases with the solvent polarity. For cyclohexane, the COSMO implicit solvent model shows the best agreement with the experiment, while the explicit model has the best agreement for water.

Solvatochromism in Mixtures of Hydrogen Bond Acceptor (HBA) Solvents with Water

The UV/Vis absorption energies νmax of Reichardt's dye B30 with respect to ET(30) and 4-nitroaniline (NA) are investigated as a function of the solvent composition Nav,z. in co-solvent/water mixtures. Nav,z. is the average molar concentration of the solvent mixture at a given solvent fraction z. The z can be the mole, the volume or the mass fraction. The co-solvents considered were acetonitrile, acetone, tetrahydrofuran, pyridine, piperidine and 2-(diethylamino)-ethanol. Acetone and acetonitrile can be expected to slightly enhance the water structure at low co-solvent concentrations. This interpretation is supported by the analysis of the refractive index as a function of the solvent composition. In general, it can be stated that the structural complexity of the binary solvent mixtures is mainly responsible for the evolution of the absorption energies ET(30) or νmax(NA) as a function of the mixture composition. In particular, the endothermic solvation of NA in co-solvent/water mixtures and its effect on the νmax(NA) is highlighted.

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.

Theoretical Study of Raman Intensities of p-Nitroaniline in Different Solvent Conditions by Using a Reference Interaction Site Model Self-Consistent Field Explicitly Including Constrained Spatial Electron Density Distribution

Raman spectroscopy is one of the most powerful tools to understand and characterize the states and structures of systems in several environments. To obtain highly accurate changes in Raman intensities of systems in solution, theoretical treatment, which can deal with not only the states and structures of systems but also the environment around molecules, proves to be significant. Hence, in this study, we developed the calculation of changes in Raman intensities of systems in different solvent conditions by using the reference interaction site model self-consistent field study explicitly including constrained spatial electron density distribution; this model is designed based on elements from both quantum mechanics and statistical mechanics. We showed that our calculation method could reproduce the changes in Raman intensities of p-nitroaniline (pNA) under different solvent conditions, including supercritical water, which has been observed in previous experimental studies. Based on the analysis of the calculation results, we observed that the ratio of the Raman intensity change of pNA in different solvent conditions is strongly correlated with the charge-transfer character of pNA.

Spatial Decay and Limits of Quantum Solute-Solvent Interactions

Molecular excitations in the liquid-phase environment are renormalized by the surrounding solvent molecules. Herein, we employ the GW approximation to investigate the solvation effects on the ionization energy of phenol in various solvent environments. The electronic effects differ by up to 0.4 eV among the five investigated solvents. This difference depends on both the macroscopic solvent polarizability and the spatial decay of the solvation effects. The latter is probed by separating the electronic subspace and the GW correlation self-energy into fragments. The fragment correlation energy decays with increasing intermolecular distance and vanishes at ∼9 Å, and this pattern is independent of the type of solvent environment. The 9 Å cutoff defines an effective interacting volume within which the ionization energy shift per solvent molecule is proportional to the macroscopic solvent polarizability. Finally, we propose a simple model for computing the ionization energies of molecules in an arbitrary solvent environment.

Assessing the quality of QM/MM approaches to describe vacuo-to-water solvatochromic shifts

The performance of different quantum mechanics/molecular mechanics embedding models to compute vacuo-to-water solvatochromic shifts is investigated. In particular, both nonpolarizable and polarizable approaches are analyzed and computed results are compared to reference experimental data. We show that none of the approaches outperform the others and that errors strongly depend on the nature of the molecular transition to be described. Thus, we prove that the best choice of embedding model highly depends on the molecular system and that the use of a specific approach as a black box can lead to significant errors and, sometimes, totally wrong predictions.

All-fluorescence white organic light-emitting diodes with record-beating power efficiencies over 130 lm W‒1 and small roll-offs

Improving power efficiency (PE) and reducing roll-off are of significant importance for the commercialization of white organic light-emitting diodes (WOLEDs) in consideration of energy conservation. Herein, record-beating PE of 130.7 lm W⁻¹ and outstanding external quantum efficiency (EQE) of 31.1% are achieved in all-fluorescence two-color WOLEDs based on a simple sandwich configuration of emitting layer consisting of sky-blue and orange delayed fluorescence materials. By introducing a red fluorescence dopant, all-fluorescence three-color WOLEDs with high color rendering index are constructed based on an interlayer sensitization configuration, furnishing ultrahigh PE of 110.7 lm W⁻¹ and EQE of 30.8%. More importantly, both two-color and three-color WOLEDs maintain excellent PEs at operating luminance with smaller roll-offs than the reported state-of-the-art WOLEDs, and further device optimization realizes outstanding comprehensive performances of low driving voltages, large luminance, high PEs and long operational lifetimes. The underlying mechanisms of the impressive device performances are elucidated by host-tuning effect and electron-trapping effect, providing useful guidance for the development of energy-conserving all-fluorescence WOLEDs.

High power efficiency and low roll-off values are essential to the commercialization of white organic light-emitting diodes. Here, the authors construct all-fluorescence devices with an orange emitting layer sandwiched between two sky-blue emitting layers, achieving figure-of-merit of 130.7 lm/W.

Analytical second derivatives of the free energy in solution by the reference interaction site model self-consistent field explicitly including constrained spatial electron density distribution

The application of analytical derivative methods to solution systems is important because several chemical reactions occur in solution. The reference interaction site model (RISM) is one of the solvation theories used to study solution systems and has shown good performance, especially in the polar solvent systems. Although the analytical first derivative based on the RISM coupled with quantum methods (RISM-SCF) has already been derived, the analytical second derivative has not been proposed yet. Therefore, in this study, the analytical second derivative was derived using RISM-SCF explicitly including constrained spatial electron density distribution (RISM-SCF-cSED). The performance of this method was validated with the Hessian calculations of formaldehyde and para-nitroaniline in solution, and the results demonstrated that the method accurately calculated frequency values at a small computational cost.

Synthesis, spectroscopic characterization, structural studies, thermal analysis and molecular docking of N-(2-methyl-5-nitrophenyl)-4-(pyridin-2-yl)pyrimidin-2-amine, a precursor for drug design against chronic myeloid leukemia

The synthesis, crystal structure and spectroscopic and electronic properties of N-(2-methyl-5-nitrophenyl)-4-(pyridin-2-yl)pyrimidin-2-amine (NPPA), C₁₆H₁₃N₅O₂, a potential template for drug design against chronic myelogenous leukemia (CML), is reported. The design and construction of the target molecule were carried out starting from the guanidinium nitrate salt (previously synthesized) and the corresponding enaminone. X-ray diffraction analysis and a study of the Hirshfeld surfaces revealed important interactions between the nitro-group O atoms and the H atoms of the pyridine and pyrimidine rings. A crystalline ordering in layers, by the stacking of rings through interactions of the π-π type, was observed and confirmed by a study of the shape-index surfaces and dispersion energy calculations. Quantitative electrostatic potential studies revealed the most positive value of the molecule on regions close to the N-H groups (34.8 kcal mol^-1); nevertheless, steric impediments and the planarity of the molecule do not allow the formation of hydrogen bonds from this group. This interaction is however activated when the molecule takes on a new extended conformation in the active pocket of the enzyme kinase (PDB ID 2hyy), interacting with protein residues that are fundamental in the inhibition process of CML. The most negative values of the molecule are seen in regions close to the nitro group (-35.4 and -34.0 kcal mol^-1). A molecular docking study revealed an energy affinity of ΔG = -10.3 kcal mol^-1 for NPPA which, despite not having a more negative value than the control molecule (Imatinib; ΔG = -12.8 kcal mol^-1), shows great potential to be used as a template for new drugs against CML.

A Machine-Learning Protocol for Ultraviolet Protein-Backbone Absorption Spectroscopy under Environmental Fluctuations

Ultraviolet (UV) absorption spectra are commonly used for characterizing the global structure of proteins. However, the theoretical interpretation of UV spectra is hindered by the large number of required expensive ab initio calculations of excited states spanning a huge conformation space. We present a machine-learning (ML) protocol for far-UV (FUV) spectra of proteins, which can predict FUV spectra of proteins with comparable accuracy to density functional theory (DFT) calculations but with 3-4 orders of magnitude reduced computational cost. It further shows excellent predictive power and transferability that can be used to probe structural mutations and protein folding pathways.

Combining multilevel Hartree–Fock and multilevel coupled cluster approaches with molecular mechanics: a study of electronic excitations in solutions

We present the coupling of different quantum-embedding approaches with a third molecular-mechanics layer, which can be either polarizable or non-polarizable.

Recent developments in the general atomic and molecular electronic structure system

A discussion of many of the recently implemented features of GAMESS (General Atomic and Molecular Electronic Structure System) and LibCChem (the C++ CPU/GPU library associated with GAMESS) is presented. These features include fragmentation methods such as the fragment molecular orbital, effective fragment potential and effective fragment molecular orbital methods, hybrid MPI/OpenMP approaches to Hartree-Fock, and resolution of the identity second order perturbation theory. Many new coupled cluster theory methods have been implemented in GAMESS, as have multiple levels of density functional/tight binding theory. The role of accelerators, especially graphical processing units, is discussed in the context of the new features of LibCChem, as it is the associated problem of power consumption as the power of computers increases dramatically. The process by which a complex program suite such as GAMESS is maintained and developed is considered. Future developments are briefly summarized.

Electronic transitions for a fully polarizable QM/MM approach based on fluctuating charges and fluctuating dipoles: Linear and corrected linear response regimes

The fully polarizable Quantum Mechanics/Molecular Mechanics (QM/MM) approach based on fluctuating charges and fluctuating dipoles, named QM/FQFμ [T. Giovannini et al., J. Chem. Theory Comput. 15, 2233 (2019)], is extended to the calculation of vertical excitation energies of solvated molecular systems. Excitation energies are defined within two different solvation regimes, i.e., linear response (LR), where the response of the MM portion is adjusted to the QM transition density, and corrected-Linear Response (cLR) in which the MM response is adjusted to the relaxed QM density, thus being able to account for charge equilibration in the excited state. The model, which is specified in terms of three physical parameters (electronegativity, chemical hardness, and polarizability) is applied to vacuo-to-water solvatochromic shifts of aqueous solutions of para-nitroaniline, pyridine, and pyrimidine. The results show a good agreement with their experimental counterparts, thus highlighting the potentialities of this approach.

Hydrogen bond strengthening between o-nitroaniline and formaldehyde in electronic excited states: A theoretical study

To study the hydrogen bonds upon photoexcited, the time dependent density function method (TD DFT) was performed to investigate the excited state hydrogen bond properties of between o-nitroaniline (ONA) and formaldehyde (CH₂O). The optimized structures of the complex and the monomers both in the ground state and the electronically excited states are calculated using DFT and TD DFT method respectively. Quantum chemical calculations of the electronic and vibrational absorption spectra are also carried out by TD DFT method at the different level. The complex ONA⋯CH₂O forms the intramolecular hydrogen bond and intermolecular hydrogen bonds. Since the strength of hydrogen bonds can be measured by studying the vibrational absorption spectra of the characteristic groups on the hydrogen bonding acceptor and donor, it evidently confirms that the hydrogen bonds is strengthened in the S₁/S₂/T₁ excited states upon photoexcitation. As a result, the hydrogen bonds cause that the CH stretch frequency of the proton donor CH₂O has a blue shift, and the electron excitations leads to a frequency red shift of NO and NH stretch modes in the o-nitroaniline(ONA) and a small frequency blue shift of CH stretch mode in the formaldehyde(CH₂O) in the S₁ and S₂ excited states. The excited states S₁, S₂ and T₁ are locally excited states where only the ONA moiety is excited, but the CH₂O moiety remains in its ground state.

Electronic Polarization Effect of the Water Environment in Charge-Separated Donor-Acceptor Systems: An Effective Fragment Potential Model Study

The electronic polarization (POL) of the surrounding environment plays a crucial role in the energetics of charge-separated systems. Here, the mechanism of POL in charge-separated systems is studied using a combined quantum mechanical and effective fragment potential (QM/EFP) method. In particular, the POL effect caused by charge separation (CS) is investigated at the atomic level by decomposition into the POL at each polarizability point. The relevance of the electric field generated by the CS is analyzed in detail. The model systems investigated are Na⁺-Cl^- and guanine-thymine solvated in water. The dominant part of the POL arises from solvent molecules close to the donor (D) and acceptor (A) units. At short D-A distances, the electric field shows both positive and negative interferences. The former case enhances the POL energy. At longer distances, the interference is weakened, and the local electric field determines the POL energy.

Assessing Molecular Dynamics Simulations with Solvatochromism Modeling

For the modeling of solvatochromism with an explicit representation of the solvent molecules, the quality of preceding molecular dynamics simulations is crucial. Therefore, the possibility to apply force fields which are derived with as little empiricism as possible seems desirable. Such an approach is tested here by exploiting the sensitive solvatochromism of p-nitroaniline, and the use of reliable excitation energies based on approximate second-order coupled cluster results within a polarizable embedding scheme. The quality of the various MD settings for four different solvents, water, methanol, ethanol, and dichloromethane, is assessed. In general, good agreement with the experiment is observed when polarizable force fields and special treatment of hydrogen bonding are applied.

Probing Lewis Acid-Base Interactions with Born-Oppenheimer Molecular Dynamics: The Electronic Absorption Spectrum of p-Nitroaniline in Supercritical CO2

The structure and dynamics of p-nitroaniline (PNA) in supercritical CO2 (scCO2) at T = 315 K and ρ = 0.81 g cm(-3) are investigated by carrying out Born-Oppenheimer molecular dynamics, and the electronic absorption spectrum in scCO2 is determined by time dependent density functional theory. The structure of the PNA-scCO2 solution illustrates the role played by Lewis acid-base (LA-LB) interactions. In comparison with isolated PNA, the ν(N-O) symmetric and asymmetric stretching modes of PNA in scCO2 are red-shifted by -17 and -29 cm(-1), respectively. The maximum of the charge transfer (CT) absorption band of PNA in scSCO2 is at 3.9 eV, and the predicted red-shift of the π → π* electronic transition relative to the isolated gas-phase PNA molecule reproduces the experimental value of -0.35 eV. An analysis of the relationship between geometry distortions and excitation energies of PNA in scCO2 shows that the π → π* CT transition is very sensitive to changes of the N-O bond distance, strongly indicating a correlation between vibrational and electronic solvatochromism driven by LA-LB interactions. Despite the importance of LA-LB interactions to explain the solvation of PNA in scCO2, the red-shift of the CT band is mainly determined by electrostatic interactions.

Nonlinear response time-dependent density functional theory combined with the effective fragment potential method

This work presents an extension of the linear response TDDFT/EFP method to the nonlinear-response regime together with the implementation of nonlinear-response TDDFT/EFP in the quantum-chemistry computer package GAMESS. Included in the new method is the ability to calculate the two-photon absorption cross section and to incorporate solvent effects via the EFP method. The nonlinear-response TDDFT/EFP method is able to make correct qualitative predictions for both gas phase values and aqueous solvent shifts of several important nonlinear properties.

One lithium atom binding with P-nitroaniline: lithium salts or lithium electrides?

Recently, both lithium (Li) salts and Li electrides formed by one Li atom interacting with ligand complexes, have been widely investigated. An interesting question emerges: is the configuration of one Li atom interacting with ligand complexes a Li salt or electride? In the present work, four configurations n-Li-PNA (n = 1-4) were obtained by binding one Li atom with the p-nitroaniline (PNA) at different positions to explore this question. The results show that 1-Li-PNA and 2-Li-PNA are typical Li salts, and 4-Li-PNA is a typical Li electride. Significantly, 3-Li-PNA possesses both characteristics of Li salt and electride. At the same time, 3-Li-PNA has the largest first hyperpolarizability (2.9 × 10(6) au) by ROMP2 method compared with the other three configurations. Furthermore, the first hyperpolarizability of 3-Li-PNA is about 2600 times larger than that of PNA. Further, the vertical ionization potential (VIP) and interaction energy (E int) indicate that 3-Li-PNA is less stable than 1-Li-PNA and 2-Li-PNA (Li salts), but is more stable than 4-Li-PNA (Li electrides).

Accurate first principles model potentials for intermolecular interactions

The general effective fragment potential (EFP) method provides model potentials for any molecule that is derived from first principles, with no empirically fitted parameters. The EFP method has been interfaced with most currently used ab initio single-reference and multireference quantum mechanics (QM) methods, ranging from Hartree-Fock and coupled cluster theory to multireference perturbation theory. The most recent innovations in the EFP model have been to make the computationally expensive charge transfer term much more efficient and to interface the general EFP dispersion and exchange repulsion interactions with QM methods. Following a summary of the method and its implementation in generally available computer programs, these most recent new developments are discussed.

Solvent induced shifts in the UV spectrum of amides

Solvent effects on the electronic spectra of formamide and trans-N-methylacetamide are studied using four different levels of theory: singly excited configuration interaction (CIS), equations of motion coupled-cluster theory with singles and doubles (EOM-CCSD), completely renormalized coupled-cluster theory with singles and doubles with perturbative triple excitations (CR-EOM-CCSD(T)), and time-dependent density functional theory (TDDFT), employing small clusters of water molecules. The simulated electronic spectrum is obtained via molecular dynamics simulations with 100 waters modeled with the effective fragment potential method and exhibits a blue-shift and red-shift, respectively, for the n → π* and πnb → π* vertical excitation energies, in good agreement with the experimental electronic spectra of amides.