Simulating One-Photon Absorption and Resonance Raman Scattering Spectra Using Analytical Excited State Energy Gradients within Time-Dependent Density Functional Theory

Capturing Coupled Structural and Electronic Motions During Excited-State Intramolecular Proton Transfer via Computational Multiedge Resonant Inelastic X-ray Scattering

Proton transfer processes form the foundation of many chemical processes. In excited-state intramolecular proton transfer (ESIPT) processes, ultrafast proton transfer is impulsively initiated through light. Here, we explore time-dependent coupled atomic and electronic motions during and following ESIPT through computational time-resolved resonant inelastic X-ray scattering (RIXS). Excited-state ab initio molecular dynamics simulations combined with time-dependent density functional theory calculations were performed for a model ESIPT system, 10-hydroxybenzo[h]quinoline, to obtain transient RIXS signatures. The RIXS spectra at both the nitrogen and oxygen K-edges were computed to resolve the electronic and atomic structural dynamics from both the proton donor and acceptor perspective. The results demonstrate that RIXS provides unprecedented details of the local electronic structure, the coupling between different core and valence excited electronic states, and the reorganization of the electronic structure coupled to the proton transfer process. We also develop a spectroscopic ruler correlating spectral shifts of a RIXS peak to the proton transfer distance during ESIPT. This work highlights the exciting potential of time-resolved RIXS experiments at newly commissioned soft X-ray free electron laser facilities for measuring coupled electronic and structural changes during ultrafast chemical processes.

Resonance Raman spectra and excited state properties of methyl viologen and its radical cation from time-dependent density functional theory

Time-dependent density functional theory (TDDFT) was applied to gain insights into the electronic and vibrational spectroscopic properties of an important electron transport mediator, methyl viologen (MV2+ ). An organic dication, MV2+ has numerous applications in electrochemistry that include energy conversion and storage, environmental remediation, and chemical sensing and electrosynthesis. MV2+ is easily reduced by a single electron transfer to form a radical cation species (MV•+ ), which has an intense UV-visible absorption near 600 nm. The redox properties of the MV2+ /MV•+ couple and light-sensitivity of MV•+ have made the system appealing for photo-electrochemical energy conversion (e.g., solar hydrogen generation from water) and the study of photo-induced charge transfer processes through electronic absorption and resonance Raman spectroscopic measurements. The reported work applies leading TDDFT approaches to investigate the electronic and vibrational spectroscopic properties of MV2+ and MV•+ . Using a conventional hybrid exchange functional (B3-LYP) and a long-range corrected hybrid exchange functional (ωB97X-D3), including with a conductor-like polarizable continuum model to account for solvation, the electronic absorption and resonance Raman spectra predicted are in good agreement with experiment. Also analyzed are the charge transfer character and natural transition orbitals derived from the TDDFT vertical excitations calculated. The findings and models developed further the understanding of the electronic properties of viologens and related organic redox mediators important in renewable energy applications and serve as a reference for guiding the interpretation of electronic absorption and Raman spectra of the ions.

Computational infrared and Raman spectra by hybrid QM/MM techniques: a study on molecular and catalytic material systems

Vibrational spectroscopy is one of the most well-established and important techniques for characterizing chemical systems. To aid the interpretation of experimental infrared and Raman spectra, we report on recent theoretical developments in the ChemShell computational chemistry environment for modelling vibrational signatures. The hybrid quantum mechanical and molecular mechanical approach is employed, using density functional theory for the electronic structure calculations and classical forcefields for the environment. Computational vibrational intensities at chemical active sites are reported using electrostatic and fully polarizable embedding environments to achieve more realistic vibrational signatures for materials and molecular systems, including solvated molecules, proteins, zeolites and metal oxide surfaces, providing useful insight into the effect of the chemical environment on the signatures obtained from experiment. This work has been enabled by the efficient task-farming parallelism implemented in ChemShell for high-performance computing platforms.  This article is part of a discussion meeting issue 'Supercomputing simulations of advanced materials'.

Efficient simulation of resonance Raman spectra with tight-binding approximations to Density Functional Theory

Resonance Raman spectroscopy has long been established as one of the most sensitive techniques for detection, structure characterization and probing the excited-state dynamics of biochemical systems. However, the analysis of resonance Raman spectra is much facilitated when measurements are accompanied by Density Functional Theory (DFT) calculations which are expensive for large biomolecules. In this work, resonance Raman spectra are therefore computed with the Density Functional Tight-Binding (DFTB) method in the time-dependent excited-state gradient approximation. To test the accuracy of the tight-binding approximations, this method is first applied to typical resonance Raman benchmark molecules like β-carotene and compared to results obtained with pure and range-separated exchange-correlation (xc) functionals. We then demonstrate the efficiency of the approach by considering a computationally challenging heme variation. Overall, we find that the vibrational frequencies and excited-state properties (energies and gradients) which are needed to simulate the spectra are reasonably accurate and suitable for interpretation of experiments. We can therefore recommend DFTB as a fast computational method to interpret resonance Raman spectra.

Evaluation of molecular photophysical and photochemical properties using linear response time-dependent density functional theory with classical embedding: Successes and challenges

Time-dependent density functional theory (TDDFT) based approaches have been developed in recent years to model the excited-state properties and transition processes of the molecules in the gas-phase and in a condensed medium, such as in a solution and protein microenvironment or near semiconductor and metal surfaces. In the latter case, usually, classical embedding models have been adopted to account for the molecular environmental effects, leading to the multi-scale approaches of TDDFT/polarizable continuum model (PCM) and TDDFT/molecular mechanics (MM), where a molecular system of interest is designated as the quantum mechanical region and treated with TDDFT, while the environment is usually described using either a PCM or (non-polarizable or polarizable) MM force fields. In this Perspective, we briefly review these TDDFT-related multi-scale models with a specific emphasis on the implementation of analytical energy derivatives, such as the energy gradient and Hessian, the nonadiabatic coupling, the spin-orbit coupling, and the transition dipole moment as well as their nuclear derivatives for various radiative and radiativeless transition processes among electronic states. Three variations of the TDDFT method, the Tamm-Dancoff approximation to TDDFT, spin-flip DFT, and spin-adiabatic TDDFT, are discussed. Moreover, using a model system (pyridine-Ag20 complex), we emphasize that caution is needed to properly account for system-environment interactions within the TDDFT/MM models. Specifically, one should appropriately damp the electrostatic embedding potential from MM atoms and carefully tune the van der Waals interaction potential between the system and the environment. We also highlight the lack of proper treatment of charge transfer between the quantum mechanics and MM regions as well as the need for accelerated TDDFT modelings and interpretability, which calls for new method developments.

Efficient and Automated Quantum Chemical Calculation of Rovibrational Nonresonant Raman Spectra

An outline of a newly developed program for the simulation of rovibrational nonresonant Raman spectra is presented. This program is an extension of our recently developed code for rovibrational infrared spectra [J. Chem Phys. 152 (2020) 244104] and relies on vibrational wavefunctions from variational configuration interaction theory to allow for an almost fully automated calculation of such spectra in pure ab initio fashion. Due to efficient contraction schemes this program requires modest computational resources and it can be controlled by only a few lines of input. As the required polarizability surfaces are also computed in an automated fashion, this implementation enables the routine application to small molecules. For demonstrating its capabilities, benchmark calculations for water H216O are compared to reference data and spectra for the beryllium dihydride dimer, Be2H4 (D2h), are predicted. The inversion symmetry of the D2h systems lead to complementary infrared and Raman spectra, which are needed both for a comprehensive investigation of this system.

Femtosecond X-ray Spectroscopy Directly Quantifies Transient Excited-State Mixed Valency

Quantifying charge delocalization associated with short-lived photoexcited states of molecular complexes in solution remains experimentally challenging, requiring local element specific femtosecond experimental probes of time-evolving electron transfer. In this study, we quantify the evolving valence hole charge distribution in the photoexcited charge transfer state of a prototypical mixed valence bimetallic iron-ruthenium complex, [(CN)₅Fe^IICNRu^III(NH₃)₅]^-, in water by combining femtosecond X-ray spectroscopy measurements with time-dependent density functional theory calculations of the excited-state dynamics. We estimate the valence hole charge that accumulated at the Fe atom to be 0.6 ± 0.2, resulting from excited-state metal-to-metal charge transfer, on an ∼60 fs time scale. Our combined experimental and computational approach provides a spectroscopic ruler for quantifying excited-state valency in solvated complexes.

Spectral Signatures of Ultrafast Excited-State Intramolecular Proton Transfer from Computational Multi-edge Transient X-ray Absorption Spectroscopy

Excited-state intramolecular proton transfer (ESIPT) is a fundamental chemical process with several applications. Ultrafast ESIPT involves coupled electronic and atomic motions and has been primarily studied using femtosecond optical spectroscopy. X-ray spectroscopy is particularly useful because it is element-specific and enables direct, individual probes of the proton-donating and -accepting atoms. Herein, we report a computational study to resolve the ESIPT in 10-hydroxybenzo[h]quinoline (HBQ), an intramolecularly hydrogen bonded compound. We use linear-response time-dependent density functional theory (LR-TDDFT) combined with ab initio molecular dynamics (AIMD) and time-resolved X-ray absorption spectroscopy (XAS) computations to track the ultrafast excited-state dynamics. Our results reveal clear X-ray spectral signatures of coupled electronic and atomic motions during and following ESIPT at the oxygen and nitrogen K-edge, paving the way for future experiments at X-ray free electron lasers.

Delocalized exciton formation in C60 linear molecular aggregates

Organic semiconducting materials containing C60 molecules are efficient acceptors for planar perovskite solar cells. In this work, we theoretically investigate the optical and excitonic properties of C60 linear molecular aggregates (composed of 1 to 7 C60 molecules) via the real-time-propagation rt-TDDFT technique. In the case of a single C60 molecule, the photoabsorption peaks are dominated by localized molecular excitons. We furthermore demonstrate that, in the case of linear molecular aggregates, the photoabsorption peaks are contributed by localized molecular excitons, charge transfer excitons, and Wannier-like delocalized excitons. This result is different to the accepted theory that only localized molecular excitons or charge transfer excitons can be produced in organic semiconducting materials. This work provides additional insights into the exciton formation in C60 molecular aggregates and may help in the rational design of efficient solar cells.

Software for the frontiers of quantum chemistry: An overview of developments in the Q-Chem 5 package

This article summarizes technical advances contained in the fifth major release of the Q-Chem quantum chemistry program package, covering developments since 2015. A comprehensive library of exchange-correlation functionals, along with a suite of correlated many-body methods, continues to be a hallmark of the Q-Chem software. The many-body methods include novel variants of both coupled-cluster and configuration-interaction approaches along with methods based on the algebraic diagrammatic construction and variational reduced density-matrix methods. Methods highlighted in Q-Chem 5 include a suite of tools for modeling core-level spectroscopy, methods for describing metastable resonances, methods for computing vibronic spectra, the nuclear-electronic orbital method, and several different energy decomposition analysis techniques. High-performance capabilities including multithreaded parallelism and support for calculations on graphics processing units are described. Q-Chem boasts a community of well over 100 active academic developers, and the continuing evolution of the software is supported by an "open teamware" model and an increasingly modular design.

Highly efficient implementation of the analytical gradients of pseudospectral time-dependent density functional theory

The accuracy and efficiency of time-dependent density functional theory (TDDFT) excited state gradient calculations using the pseudospectral method are presented. TDDFT excited state geometry optimizations of the G2 test set molecules, the organic fluorophores with large Stokes shifts, and the Pt-complexes show that the pseudospectral method gives average errors of 0.01-0.1 kcal/mol for the TDDFT excited state energy, 0.02-0.06 pm for the bond length, and 0.02-0.12° for the bond angle when compared to the results from conventional TDDFT. TDDFT gradient calculations of fullerenes (C_n, n up to 540) with the B3LYP functional and 6-31G** basis set show that the pseudospectral method provides 8- to 14-fold speedups in the total wall clock time over the conventional methods. The pseudospectral TDDFT gradient calculations with a diffuse basis set give higher speedups than the calculations for the same basis set without diffuse functions included.

An Ab Initio Multiple Cloning Method for Non-Adiabatic Excited-State Molecular Dynamics in NWChem

The recently developed ab initio multiple cloning (AIMC) approach based on the multiconfigurational Ehrenfest (MCE) method provides a powerful and accurate way of describing the excited-state dynamics of molecular systems. The AIMC method is a controlled approximation to nonadiabatic dynamics with a particular strength in the proper description of decoherence effects because of the branching of vibrational wavepackets at a level crossing. Here, we report a new implementation of the AIMC algorithm in the open source NWChem computational chemistry program. The framework combines linear-response time-dependent density functional theory with Ehrenfest mean-field theory to determine the equations of motion for classical trajectories. The multidimensional wave function is decomposed into a superposition of Gaussian coherent states guided by Ehrenfest trajectories (i.e., MCE approach), which can clone with fully quantum mechanical amplitudes and phases. By using an efficient time-derivative based nonadiabatic coupling approach within the AIMC method, all observables are calculated on-the-fly in the nonadiabatic molecular dynamics process. As a representative example, we apply our implementation to study the ultrafast photoinduced electronic and vibrational energy transfer in a pyridine molecule. The effects of the cloning procedure on electronic and vibrational coherence, relaxation and unidirectional energy transfer are discussed. This new AIMC implementation provides a high-level nonadiabatic molecular dynamics framework for simulating photoexcited dynamics in complex molecular systems and experimentally relevant ultrafast spectroscopic probes, such as nonlinear coherent optical and X-ray signals.

Direct observation of coherent femtosecond solvent reorganization coupled to intramolecular electron transfer

It is well known that the solvent plays a critical role in ultrafast electron-transfer reactions. However, solvent reorganization occurs on multiple length scales, and selectively measuring short-range solute-solvent interactions at the atomic level with femtosecond time resolution remains a challenge. Here we report femtosecond X-ray scattering and emission measurements following photoinduced charge-transfer excitation in a mixed-valence bimetallic (FeⁱⁱRuⁱⁱⁱ) complex in water, and their interpretation using non-equilibrium molecular dynamics simulations. Combined experimental and computational analysis reveals that the charge-transfer excited state has a lifetime of 62 fs and that coherent translational motions of the first solvation shell are coupled to the back electron transfer. Our molecular dynamics simulations identify that the observed coherent translational motions arise from hydrogen bonding changes between the solute and nearby water molecules upon photoexcitation, and have an amplitude of tenths of ångströms, 120-200 cm^-1 frequency and ~100 fs relaxation time. This study provides an atomistic view of coherent solvent reorganization mediating ultrafast intramolecular electron transfer.

Ab Initio Approach to Femtosecond Stimulated Raman Spectroscopy: Investigating Vibrational Modes Probed in Excited-State Relaxation of Quaterthiophenes

Femtosecond stimulated Raman spectroscopy (FSRS) is an ultrafast pump-probe technique designed to elucidate excited-state molecular dynamics by means of vibrational spectroscopy. We present a first-principles protocol for the simulation of FSRS that integrates ab initio molecular dynamics with computational resonance Raman spectroscopy. Theoretical calculations can monitor the time-dependent evolution of specific vibrational modes and thus provide insight into the nature of the motion responsible for the experimental FSRS signal, and we apply this technique to study quaterthiophene derivatives. The S₁ state of two different quaterthiophene derivatives relaxes via in-phase and out-of-phase stretching modes whose frequencies are coupled to the dihedral backbone angle, such that the spectral evolution reflects the excited-state relaxation toward a planar conformation. The simulated spectra aid in confirming the experimental assignment of the vibrational modes that are probed in the existing FSRS experiments on quaterthiophenes.

NWChem: Past, present, and future

Specialized computational chemistry packages have permanently reshaped the landscape of chemical and materials science by providing tools to support and guide experimental efforts and for the prediction of atomistic and electronic properties. In this regard, electronic structure packages have played a special role by using first-principle-driven methodologies to model complex chemical and materials processes. Over the past few decades, the rapid development of computing technologies and the tremendous increase in computational power have offered a unique chance to study complex transformations using sophisticated and predictive many-body techniques that describe correlated behavior of electrons in molecular and condensed phase systems at different levels of theory. In enabling these simulations, novel parallel algorithms have been able to take advantage of computational resources to address the polynomial scaling of electronic structure methods. In this paper, we briefly review the NWChem computational chemistry suite, including its history, design principles, parallel tools, current capabilities, outreach, and outlook.

Localized surface plasmon resonances of a metal nanoring

Using the linear combination of atomic orbitals real-time-propagation rt-TDDFT technique and transition contribution maps, we study the optical and plasmonic features of a metal nanoring made up of sodium atoms.

Role of Explicit Solvation in the Simulation of Resonance Raman Spectra within Short-Time Dynamics Approximation

In short-time dynamics approximation, relative resonance Raman (RR) intensity of a vibrational mode primarily depends on the magnitude of square of the excited-state gradient along the corresponding normal coordinate, ground-state normal mode eigenvector, and harmonic vibrational wavenumbers. In this study, through simulation of RR spectra of guanosine-5'-monophosphate (GMP) in two ππ* singlet excited states, we analyze how the explicitly hydrogen-bonded local solvation structure of the chromophore dictates intensities of the RR active modes in an unprecedented manner. We show that the accuracy of the structural model of solvated chromophore plays a decisive role in determining an optimal theoretical method for prediction of the Franck-Condon region of the ππ* excited states. 9-Methylguanine (9-meG) in complex with six water molecules (9-meG·6H₂O) is found out to be the most accurate one for describing GMP in two different bright electronic states. We find that explicit hydrogen-bonded water molecules strongly influence computed RR intensities of GMP by modulating both the ground-state normal mode vectors and the excited-state energy gradients. We find that simultaneous inclusion of six explicit waters to describe the solute-solvent interaction near all hydration sites is essential for reliable prediction of the features of RR spectra in L_b and B_b electronic states of GMP.

Ab Initio Investigation of the Resonance Raman Spectrum of the Hydrated Electron

According to the conventional picture, the aqueous or "hydrated" electron, e^-(aq), occupies an excluded volume (cavity) in the structure of liquid water. However, simulations with certain one-electron models predict a more delocalized spin density for the unpaired electron, with no distinct cavity structure. It has been suggested that only the latter (non-cavity) structure can explain the hydrated electron's resonance Raman spectrum, although this suggestion is based on calculations using empirical frequency maps developed for neat liquid water, not for e^-(aq). All-electron ab initio calculations presented here demonstrate that both cavity and non-cavity models of e^-(aq) afford significant red-shifts in the O-H stretching region. This effect is nonspecific and arises due to electron penetration into frontier orbitals of the water molecules. Only the conventional cavity model, however, reproduces the splitting of the H-O-D bend (in isotopically mixed water) that is observed experimentally and arises due to the asymmetric environments of the hydroxyl moieties in the electron's first solvation shell. We conclude that the cavity model of e^-(aq) is more consistent with the measured resonance Raman spectrum than is the delocalized, non-cavity model, despite previous suggestions to the contrary. Furthermore, calculations with hybrid density functionals and with Hartree-Fock theory predict that non-cavity liquid geometries afford only unbound (continuum) states for an extra electron, whereas in reality this energy level should lie more than 3 eV below vacuum level. As such, the non-cavity model of e^-(aq) appears to be inconsistent with available vibrational spectroscopy, photoelectron spectroscopy, and quantum chemistry.

Structure of the aqueous electron

A cavity or excluded-volume structure best explains the experimental properties of the aqueous or “hydrated” electron.

Excited-state absorption in tetrapyridyl porphyrins: comparing real-time and quadratic-response time-dependent density functional theory

Three meso-substituted tetrapyridyl porphyrins (free base, Ni(ii), and Cu(ii)) were investigated for their optical limiting (OL) capabilities using real-time (RT-), linear-response (LR-), and quadratic-response (QR-) time-dependent density functional theory (TDDFT) methods. These species are experimentally known to display a prominent reverse saturable absorption feature between the Q and B bands of the ground-state absorption (GSA), which has been attributed to increased excited-state absorption (ESA) relative to GSA. A recently developed RT-TDDFT based method for calculating ESA from a LR-TDDFT density was utilized with eight exchange-correlation functionals (BLYP, PBE, B3LYP, CAM-B3LYP, PBE0, M06, BHLYP, and BHandH) and contrasted with calculations of ESA using QR-TDDFT with five exchange-correlation functionals (BLYP, B3LYP, CAM-B3LYP, BHLYP, and BHandH). This allowed for comparison between functionals with varying amounts of exact exchange as well as between the ability of RT-TDDFT and QR-TDDFT to reproduce OL behavior in porphyrin systems. The absorption peak positions and intensities for GSA and ESA are significantly impacted by the choice of DFT functional, with the most critical factor identified as the amount of exact exchange in the functional form. Calculating ESA with QR-TDDFT is found to be significantly more sensitive to the amount of exact exchange than GSA and ESA with RT-TDDFT, as well as GSA with LR-TDDFT. An analogous behavior is also demonstrated for the polycyclic aromatic hydrocarbon coronene. This is problematic when using the same approximate functional for calculation of both GSA and ESA, as the LR- and QR-TDDFT excitation energies will not have similar errors. Overall, the RT-TDDFT method with hybrid functionals reproduces the OL features for the porphyrin systems studied here and is a viable computational approach for efficient screening of molecular complexes for OL properties.

Elucidation of charge-transfer SERS selection rules by considering the excited state properties and the role of electrode potential

The goal of this study is to shed light on the charge-transfer (CT) mechanism of surface-enhanced Raman scattering (SERS) by considering the properties of CT excited states. The calculations have been done by means of an excited-state gradient approximation for a pyridine molecule interacting with a silver cluster, and provided a satisfactory improvement in comparison to previous work. The effect of electrode potential on the SERS-CT spectra has been modelled theoretically by applying an external electric field for selected CT transitions and the enhancement of the ν_6a and ν_9a modes and a decline in the intensity of the ν_8a mode under a negative electric field (which is directed toward the cluster) have been observed. These results match well with the experimental studies and also explain the effect of electrode potentials on the patterns of spectra, as experimental evidence of the CT mechanism. Finally, this study demonstrated that the excited state vector gradient can be used as a distinguishing factor to explain the SERS selection rules.

Sulfate radical induced degradation of β2-adrenoceptor agonists salbutamol and terbutaline: Phenoxyl radical dependent mechanisms

The present study investigated the reactivity and oxidation mechanisms of salbutamol (SAL) and terbutaline (TBL), two typical β2-adrenoceptor agonists, towards sulfate radical (SO₄^-) by using photo-activated persulfate (PS). The reaction pathways and mechanisms were proposed based on products identification using high resolution HPLC-ESI-MS, laser flash photolysis (LFP) and molecular orbital calculations. The results indicated that SO₄^- was the dominant reactive species in the UV/PS process. The second-order rate constants of sulfate radical reaction with SAL and TBL were measured as (3.7 ± 0.3) × 10⁹ and (4.2 ± 0.3) × 10⁹ M^-1 s^-1 by LFP, respectively. For both SAL and TBL, phenoxyl radicals were found to play key roles in the orientation of the primary pathways. For SAL, a benzophenone derivative was generated by oxidation of the phenoxyl radical. However, in the case of TBL, the transformation of the phenoxyl radical into benzoquinone was impossible. Instead, the addition of OSO₃H on the aromatic ring was the major pathway. The same reactivity pattern was observed in the case of TBL structural analogs resorcinol and 3,5-dihydroxybenzyl alcohol. Our results revealed that basic conditions inhibited the decomposition of SAL and TBL, while, increasing PS dose enhanced the degradation. The present work could help for a better understanding of the difference in oxidation reactivity of substituted phenols widely present in natural waters.

Sub-50 fs excited state dynamics of 6-chloroguanine upon deep ultraviolet excitation

The photophysical properties of natural nucleobases and their respective nucleotides are ascribed to the sub-picosecond lifetime of their first singlet states in the UV-B region (260-350 nm). Electronic transitions of the ππ* type, which are stronger than those in the UV-B region, lie at the red edge of the UV-C range (100-260 nm) in all isolated nucleobases. The lowest energetic excited states in the UV-B region of nucleobases have been investigated using a plethora of experimental and theoretical methods in gas and solution phases. The sub-picosecond lifetime of these molecules is not a general attribute of all nucleobases but specific to the five primary nucleobases and a few xanthine and methylated derivatives. To determine the overall UV photostability, we aim to understand the effect of more energetic photons lying in the UV-C region on nucleobases. To determine the UV-C initiated photophysics of a nucleobase system, we chose a halogen substituted purine, 6-chloroguanine (6-ClG), that we had investigated previously using resonance Raman spectroscopy. We have performed quantitative measurements of the resonance Raman cross-section across the Bb absorption band (210-230 nm) and constructed the Raman excitation profiles. We modeled the excitation profiles using Lee and Heller's time-dependent theory of resonance Raman intensities to extract the initial excited state dynamics of 6-ClG within 30-50 fs after photoexcitation. We found that imidazole and pyrimidine rings of 6-ClG undergo expansion and contraction, respectively, following photoexcitation to the Bb state. The amount of distortions of the excited state structure from that of the ground state structure is reflected by the total internal reorganization energy that is determined at 112 cm(-1). The contribution of the inertial component of the solvent response towards the total reorganization energy was obtained at 1220 cm(-1). In addition, our simulation also yields an instantaneous response of the first solvation shell within an ultrafast timescale of less than 30 fs following photoexcitation.

Theoretical Assessment of Excited State Gradients and Resonance Raman Intensities for the Azobenzene Molecule

The ground state geometries and vibrational frequencies as well as the excitation energies and excited state gradients of the S₁(nπ*) and S₂(ππ*) states of trans- and cis-azobenzene are investigated by several DFT methods, namely B3LYP, PBE, M06-2X, CAM-B3LYP, and ωB97X. Excited state properties and in particular gradients are also assessed using the wave function based methods EOM-CCSD and RASPT2/RASSCF. Comparison with experimental data shows that the B3LYP functional gives the most accurate results for the ground state geometry and vibrational frequencies. The analysis of the vertical excitation energies reveals that the RASPT2 approach provides the most accurate excitation energies with deviations of the order of 0.1 eV. Among the TDDFT methods, the CAM-B3LYP functional shows the best performance on the excitation energies. By assessing the excited state gradients with respect to the reference RASPT2 data, the most accurate gradients are obtained with B3LYP, whereas other functionals as well as the EOM-CCSD and RASSCF calculations give less consistent results. Overall, despite the tendency of B3LYP to underestimate the excitation energies, this functional provides the most balanced description of both ground and excited state properties for both isomers of azobenzene in the Franck-Condon region.

Rational Design of Push-Pull Fluorene Dyes: Synthesis and Structure-Photophysics Relationship

Our work surveyed experimental and theoretical investigations to construct highly emissive D-π-A (D=donor, A=acceptor) fluorenes. The synthetic routes were optimised to be concise and gram-scalable. The molecular design was first rationalised by varying the electron-withdrawing group from an aldehyde, ketotriazole or succinyl to methylenemalonitrile or benzothiadiazole. The electron-donating group was next varied from aliphatic or aromatic amines to saturated cyclic amines ranging from aziridine to azepane. Spectroscopic studies correlated with TD-DFT calculations provided the optimised structures. The selected push-pull dyes exhibited visible absorptions, significant brightness, important solvatofluorochromism, mega-Stokes shifts (>250 nm) and dramatic shifts in emission to the near-infrared. The current library includes the comprehensive characterization of 16 prospective dyes for fluorescence applications. Among them, several fluorene derivatives bearing different conjugation anchors were tested for coupling and demonstrated to preserve the photophysical responses once further bound.

Infrared and Raman Spectroscopy from Ab Initio Molecular Dynamics and Static Normal Mode Analysis: The C-H Region of DMSO as a Case Study

Carbon-hydrogen (C-H) vibration modes serve as key probes in the chemical identification of hydrocarbons and in vibrational sum-frequency generation spectroscopy of hydrocarbons at the liquid/gas interface. Their assignments pose a challenge from a theoretical viewpoint. In this work, we present a detailed study of the C-H stretching region of dimethyl sulfoxide using a new ab initio molecular dynamics (AIMD) module that we have implemented in NWChem. Through a combination of AIMD simulations and static normal mode analysis, we interpret experimental infrared and Raman spectra and explore the role of anharmonic effects in this system. Comprehensive anharmonic normal mode analysis of the C-H stretching region casts doubt upon previous experimental assignments of the shoulder on the symmetric C-H stretching peak. In addition, our AIMD simulations also show significant broadening of the in-phase symmetric C-H stretching resonance, which suggests that the experimentally observed shoulder is due to thermal broadening of the symmetric stretching resonance.

Resonant Raman spectra of molecules with diradical character: multiconfigurational wavefunction investigation of neutral viologens

In search for a relationship between the diradical character and resonance Raman signatures of neutral viologens by multiconfigurational methods.