Photodetachment band of the fluorenyl anion: a theoretical rationalization.

In order to rationalize the experimental photodetachment spectra of the fluorenyl anion, nuclear dynamics studies are performed using adiabatic and non-adiabatic quantum chemistry approaches. The adiabatic dynamics calculations are performed on the basis of FC_Class and Poisson simulations. The outcomes from these simulations explain most of the experimental observations. Later, time-independent non-adiabatic nuclear dynamics calculations using vibronic coupling theory (VCT) are performed to elucidate the origin of a few vibrational peaks that do not appear in the results from adiabatic dynamics calculations. In addition, time-dependent non-adiabatic nuclear dynamics calculations using the same VCT approach are performed to rationalize the lifetime of the excited state of the fluorenyl anion. It is found that the photodetachment band obtained from the non-adiabatic simulations is in better agreement with the experimental findings.

Dihydropyrene/Cyclophanediene Photoswitching Mechanism Revisited with Spin-Flip Time-Dependent Density Functional Theory: Nature of the Photoisomerization Funnel at Stake!

The complex photoisomerization mechanism of the dihydropyrene (DHP) photochromic system is revisited using spin-flip time-dependent density functional theory (SF-TD-DFT). The photoinduced ring-opening reaction of DHP into its cyclophanediene isomer involves multiple coupled electronic states of different character. A balanced treatment of both static and dynamic electron correlations is required to determine both the photophysical and photochemical paths in this system. The present results provide a refinement of the mechanistic picture provided in a previous complete active space self-consistent field plus second-order perturbation theory (CASPT2//CASSCF) study based on geometry optimizations at the CASSCF level. In particular, the nature of the conical intersection playing the central role of the photochemical funnel is different. While at the CASSCF level, the crossing with the ground state involves a covalent doubly excited state leading to a three-electron/three-center bond conical intersection, SF-TD-DFT predicts a crossing between the ground state and a zwitterionic state. These results are supported by multi-state CASPT2 calculations. This study illustrates the importance of optimizing conical intersections at a sufficiently correlated level of theory to describe a photochemical path involving crossings between covalent and ionic states.

Synthesis and Electron Accepting Properties of Two Di(benz[f]indenone)-Fused Tetraazaanthracene Isomers.

We designed and synthesized a novel di(benz[f]indenone)-fused tetraazaanthracene derivative and isolated its two isomers, 1a and 1s, having anti and syn configurations, respectively. Their structure and that of the condensation reaction intermediates, anti-2a and syn-2s, were fully characterized using one- and two-dimensional nuclear magnetic resonance spectroscopy and single-crystal X-ray diffraction. The optical and electronic properties of 1a and 1s were investigated using ultraviolet-visible absorption and fluorescence spectroscopies, cyclic voltammetry, and time-dependent density functional theory calculations. The presence of the carbonyl and ethynyltris(isopropyl)silane groups endows the di(benzoindenone)-fused azaacene derivatives with a strong electron accepting character. With an electron affinity of approximately -3.7 eV, the two isomers represent attractive electron-deficient molecular systems for the generation of n-channel semiconducting materials. Organic field effect transistors of 1a and 1s showed electron transport, and organic solar cells gave a proof of concept of the potential of the two compounds as electron acceptor materials when they are paired with an electron donor polymer.

Understanding of the Photodetachment Spectrum of Anionic Mixed Carbon-Boron Cluster C3B5- Following Adiabatic and Nonadiabatic Quantum Chemistry Approaches.

The presence of nonadiabaticity in the photodetachment bands of the anionic mixed carbon-boron cluster C3B5- has been realized through ab initio electronic structure calculations and detailed analyses of quantum dynamics study on top of those electronic structures. In the course of our study, we traverse extensive first principles electronic structure calculations to compute potential energy curves and to trace the energetic locations for the conical intersections in the multidimensional surfaces. All the ab initio calculations are performed on the four low-lying electronic states of the C3B5 cluster, while quantum nuclear dynamics are pursued on those electronic states by applying both time-dependent and time-independent quantum chemistry frameworks. In particular, we rely on the diabatic electronic representation to construct the molecular Hamiltonian. Altogether, the simulated theoretical spectra offer exceptional agreement with the experimental attainments.

Absorption band structure of the photochromic dimethyldihydropyrene/metacyclophanediene couple. Insight from vibronic coupling theory.

A detailed insight behind the structure of absorption bands of the photochromic couple dimethyldihydropyrene (DHP)/metacyclophanediene (CPD) is studied employing vibronic coupling theory. Two separate model molecular Hamiltonians, including a maximum of four electronic states and 18 vibrational modes for DHP and five electronic states and 20 vibrational modes for CPD, are constructed in a diabatic electronic representation. The parameters of the Hamiltonians are estimated from the electronic energies obtained from extensive density functional theory (DFT) and time-dependent DFT calculations. Based on these Hamiltonians' parameters, a detailed analysis of potential energy curves is performed in conjunction with positional and energetic locations of several stationary points in multi-dimensional potential energy surfaces. Based on the results of electronic structure calculations, quantum nuclear dynamics studies on the electronic excited states of DHP and CPD are performed to understand the impact of non-adiabatic effects on the formation of vibronic structures of absorption bands of these photo-isomers.

Elucidation of vibronic structure and dynamics of first eight excited electronic states of pentafluorobenzene.

Vibronic coupling in the first eight electronic excited states of Pentafluorobenzene (PFBz) is investigated in this article. In particular, the vibronic coupling between the optically bright ππ* and optically dark πσ* states of PFBz is considered. A model 8 × 8 diabatic Hamiltonian is constructed in terms of normal coordinate of vibrational modes using the standard vibronic coupling theory and symmetry selection rule. The Hamiltonian parameters are estimated with the aid of extensive ab initio quantum chemistry calculations. The topography of the first eight electronic excited states of PFBz is examined at length, and multiple multi-state conical intersections are established. The nuclear dynamics calculations on the coupled electronic surfaces are carried out from first principles by the wave packet propagation method. Theoretical results are found to be in good accord with the available experimental optical absorption spectrum of PFBz.

An unbiased confirmation of the participating isomers of C2B5- in the formation of its photo-detachment spectra: a theoretical study.

The primary goal of the present article is to provide an unbiased structural confirmation of C2B5-, relying on its available experimental photo-detachment spectra. The study is performed from scratch by optimizing the lowest energy isomers of C2B5- and later, suitable molecular vibronic Hamiltonians are constructed by analyzing the normal modes of these optimized isomers. The Hamiltonians' parameters are evaluated from the fits of the calculated ab initio single point energies using a state of the art multireference configuration (MRCI) level of theory employing a correlation consistent polarized triple zeta (cc-pVTZ) basis set. The state-averaged variant of the MRCI level of theory is also applied to deal with the highly interactive electronic states of both of the isomers. A detailed analysis of the potential energy curves along the totally symmetric vibrational modes is performed to understand the energy modulation between the different electronic states and also to find the energetic locations of the conical intersections. The introduction of the non-symmetric vibrational modes in the Hamiltonians help to understand the impact of non-adiabaticity during energy modulation in the coupled surfaces. Later, both adiabatic and non-adiabatic nuclear dynamics are performed on the electronic states of both of the isomers using the constructed reduced and full-dimensional Hamiltonians. The results of the adiabatic dynamics are used to assign the positions of the simulated photo-detachment bands, while the non-adiabatic dynamics improve the shape of those bands. Finally, we compare our theoretical findings with the available experimental photo-detachment spectra of C2B5- to provide an unbiased structural confirmation of the participating isomers of C2B5- in its photo-detachment spectra.

Electronic Excited States and UV-Vis Absorption Spectra of the Dihydropyrene/Cyclophanediene Photochromic Couple: a Theoretical Investigation.

Dihydropyrene (DHP)/cyclophanediene (CPD) is a fascinating photoswitchable organic system displaying negative photochromism. Upon irradiation in the visible region, the colored DHP can be converted to its open-ring CPD colorless isomer, which can be converted back to DHP by UV light. DHP and CPD thus possess very different absorption spectra whose absorption bands have never been assigned in detail so far. In this work, we characterize the vertical electronic transitions of the first six and seven excited states of DHP and CPD, respectively, aiming for a realistic comparison with experiment. We used state-of-the-art electronic structure methods [e.g., complete active space second-order perturbation theory (CASPT2), n-electron valence-state perturbation theory (NEVPT2), extended multiconfigurational quasi-degenerate perturbation theory (XMCQDPT2), and third-order algebraic diagrammatic construction ADC(3)] capable of describing differential electron correlation. Vertical transition energies were also computed with time-dependent density functional theory (TD-DFT) and compared to these accurate methods. After the reliability of TD-DFT was validated for the main optical transitions, this efficient method was used to simulate the absorption spectra of DHP and CPD in the framework of the Franck-Condon Herzberg-Teller approximation and also using the nuclear ensemble approach. Overall, for both methods, the simulated absorption spectra reproduce nicely the main spectral features of the DHP and CPD isomers, that is, the main four absorption bands of increasing intensity of DHP and the absorption rise below 300 nm for CPD.

Rationalization of photo-detachment spectra of the indenyl anion (C9H7-) from the perspective of vibronic coupling theory.

The nuclear dynamics of the low-lying first four electronic states of the prototypical indenyl radical is investigated based on first principles calculations to rationalize the experimental vibronic structure of the radical. The study is performed following both time-dependent and time-independent quantum-chemistry approaches using a model diabatic Hamiltonian. The construction of model Hamiltonians is based on the fits of the adiabatic energies calculated from the electronic structure method. The analyses of the static and dynamics results of the present study corroborate the experimental findings regarding the shape of the spectrum, vibrational progressions and the lifetime of the excited state. Finally, the present theoretical investigations suggest that the electronic non-adiabatic effect is extremely important for a detailed study of the vibronic structure and the electronic relaxation mechanism of the low-lying electronic states of the indenyl radical.

Theoretical Rationalization of the Dual Photophysical Behavior of C60.

Interest in fullerenes has been renewed recently in astrophysics as a consequence of their detection in circumstellar environments. In particular, C60+ was detected in the diffuse interstellar medium and its presence has been related to some diffuse interstellar bands (DIBs) whose origin was previously unknown. A single recent laboratory experiment ( J. Phys. Chem. A 2017, 121, 7356-7361) shows that upon laser excitation at 785 nm, C60+ in neon matrixes exhibits a radiative decay at 965 nm, while UV photoexcitation does not lead to any significant luminescence. To rationalize this original dual photophysical behavior, we have performed time-dependent density functional theory (TD-DFT) calculations on C60+ to investigate the potential energy surfaces of the relevant electronic states, completed by the simulations of vibrationally resolved absorption and emission spectra. The proposed photophysical pathways shed light on the experimental measurements: The near-IR laser excitation populates the 11th doublet excited state (D11) that decays to the lowest first bright excited state D5, from which photoluminescence is predicted. Indeed, D5 is largely separated from the lower electronic states (D0-D4). Thus, D5 behaves effectively as the first excited state, while the D0-D4 set of states act as the electronic ground state. In addition, there are no low-lying conical intersections between D5 and lower excited states energetically accessible upon near-IR excitation that can provide efficient nonradiative decay channels for this state, leaving radiative decay as the most likely deactivation pathway. However, a sloped conical intersection between D5 and D4 was located around 2.9 eV above D0. While it is too high in energy to be accessible upon near-IR excitation, it provides a funnel for efficient nonradiative decay down to the ground state (D0) accessible upon UV light excitation. Thus, the photophysics of C60+ is controlled by the ability to access this funnel: Upon near-IR excitation, the system fluoresces because the funnel for nonradiative decay cannot be reached, while UV irradiation provides a different route by opening up a radiationless decay channel via this funnel, accounting for the absence of fluorescence.

An Organic Receptor Isolated in an Unusual Intermediate Conformation: Computation, Crystallography, and Hirshfeld Surface Analysis.

1,1″-1,4-Phenylene-bis(methylene)bis-4,4'-bipyridinium cation [C28H24N4]2+ (c), an organic receptor that generally crystallizes in its anti conformation, has recently been shown to be isolated in its syn conformation in an ion-paired compound [C28H24N4][Zn(dmit)2]·2DMF (dmit2- = 1,3-dithiole-2-thione-4,5-dithiolate; DMF = dimethylformamide). In this article, we demonstrated that the same receptor [C28H24N4]2+ (c) can also be stabilized in an unusual intermediate conformation (neither syn nor anti) with PF6- anion in compound [C28H24N4](PF6)2·(1,4-dioxane) (1·(1,4-dioxane)). The energetically favored anti conformation has been described in its nitrate salt [C28H24N4](NO3)2·2H2O (2·2H2O). Compounds 1·(1,4-dioxane) and 2·2H2O, crystallizing in triclinic and monoclinic systems with space groups P1̅ and P21/n, respectively, were additionally characterized by Hirshfeld surface analysis. The density functional theory calculations are performed to understand the internal mechanism of the stability of various conformers of cationic receptor c, compound 1, and compound 2. In conjunction with the electronic stability of the conformers, the natural bond orbital analysis and conformational equilibrium constants at different temperatures are also calculated to find out the sources of the different stability of the various conformers of experimentally synthesized compounds.

Theoretical study of photodetachment spectroscopy of hydrogenated boron cluster anion H2B7- and its deuterated isotopomer.

Photodetachment spectroscopy of H2B7- and its deuterated isotopomer probing the energetically low-lying electronic states of the respective neutral cluster is theoretically investigated in this paper. The theoretical methodology is based on detailed quantum chemistry calculations of electronic state energies, construction of a vibronic coupling model in the diabatic electronic basis, and nuclear dynamics calculations from first principles using time-dependent and time-independent quantum mechanical methods. The theoretical model consists of five coupled electronic states and fifteen vibrational modes. Several reduced dimensional calculations are performed to identify the relevant vibrational modes contributing to the vibronic structure of electronic bands and the impact of non-adiabatic coupling on them. The low-energy part of the spectrum of both H2B7 and its deuterated analogue is assigned by examining the vibronic wavefunctions and the results are compared with the experimental findings. The nonadiabatic decay dynamics of the electronic excited states of the neutral clusters is examined at length.

Insights into extended coupled polymethines through the investigation of dual UV-to-NIR acidochromic switches based on heptamethine-oxonol dyes.

A series of heptamethine-oxonol dyes featuring different heterocyclic end groups were designed with the aim to explore structure-property relationships in π-extended coupled polymethines. These dyes can be stabilised under three different protonation states, affording dicationic derivatives with an aromatic core, cationic heptamethines, and zwitterionic bis-cyanine forms. The variation of the end groups directly impacts the absorption and emission properties and mostly controls reaching either a colourless neutral dispirocyclic species or near-infrared zwitterions. The acidochromic switching between the three states involves profound electronic rearrangements leading to notable shifts of their optical properties that were investigated using a parallel experiment-theory approach, providing a comprehensive description of these unique systems.

Assessing the Performances of CASPT2 and NEVPT2 for Vertical Excitation Energies.

Methods able to simultaneously account for both static and dynamic electron correlations have often been employed, not only to model photochemical events but also to provide reference values for vertical transition energies, hence allowing benchmarking of lower-order models. In this category, both the complete-active-space second-order perturbation theory (CASPT2) and the N-electron valence state second-order perturbation theory (NEVPT2) are certainly popular, the latter presenting the advantage of not requiring the application of the empirical ionization-potential-electron-affinity (IPEA) and level shifts. However, the actual accuracy of these multiconfigurational approaches is not settled yet. In this context, to assess the performances of these approaches, the present work relies on highly accurate (±0.03 eV) aug-cc-pVTZ vertical transition energies for 284 excited states of diverse character (174 singlet, 110 triplet, 206 valence, 78 Rydberg, 78 n → π*, 119 π → π*, and 9 double excitations) determined in 35 small- to medium-sized organic molecules containing from three to six non-hydrogen atoms. The CASPT2 calculations are performed with and without IPEA shift and compared to the partially contracted (PC) and strongly contracted (SC) variants of NEVPT2. We find that both CASPT2 with IPEA shift and PC-NEVPT2 provide fairly reliable vertical transition energy estimates, with slight overestimations and mean absolute errors of 0.11 and 0.13 eV, respectively. These values are found to be rather uniform for the various subgroups of transitions. The present work completes our previous benchmarks focused on single-reference wave function methods ( J. Chem. Theory Comput. 2018, 14, 4360; J. Chem. Theory Comput. 2020, 16, 1711), hence allowing for a fair comparison between various families of electronic structure methods. In particular, we show that ADC(2), CCSD, and CASPT2 deliver similar accuracies for excited states with a dominant single-excitation character.

Benchmarking TD-DFT and Wave Function Methods for Oscillator Strengths and Excited-State Dipole Moments.

Using a set of oscillator strengths and excited-state dipole moments of near full configuration interaction quality determined for small compounds, we benchmark the performances of several single-reference wave function methods [CC2, CCSD, CC3, CCSDT, ADC(2), and ADC(3/2)] and time-dependent density-functional theory (TD-DFT) with various functionals (B3LYP, PBE0, M06-2X, CAM-B3LYP, and ωB97X-D). We consider the impact of various gauges (length, velocity, and mixed) and formalisms: equation of motion versus linear response, relaxed versus unrelaxed orbitals, and so forth. Beyond the expected accuracy improvements and a neat decrease of formalism sensitivity when using higher-order wave function methods, the present contribution shows that, for both ADC(2) and CC2, the choice of gauge impacts more significantly the magnitude of the oscillator strengths than the choice of formalism and that CCSD yields a notable improvement on this transition property as compared to CC2. For the excited-state dipole moments, switching on orbital relaxation appreciably improves the accuracy of both ADC(2) and CC2 but has a rather small effect at the CCSD level. Going from ground to excited states, the typical errors on dipole moments for a given method tend to roughly triple. Interestingly, the ADC(3/2) oscillator strengths and dipoles are significantly more accurate than their ADC(2) counterparts, whereas the two models do deliver rather similar absolute errors for transition energies. Concerning TD-DFT, one finds: (i) a rather negligible impact of the gauge on oscillator strengths for all tested functionals (except for M06-2X); (ii) deviations of ca. 0.10 D on ground-state dipoles for all functionals; (iii) strong differences between excited-state dipoles obtained with, on the one hand, B3LYP and PBE0 and, on the other hand, M06-2X, CAM-B3LYP, and ωB97X-D, the latter group being markedly more accurate with the selected basis set; and (iv) the better overall performance of CAM-B3LYP for the two considered excited-state properties. Finally, for all investigated properties, both the accuracy and consistency obtained with the second-order wave function approaches, ADC(2) and CC2, do not clearly outperform those of TD-DFT, hinting that assessing the accuracy of the latter (or selecting a specific functional) on the basis of the results of the former is not systematically a well-settled strategy.

Designing UiO-66-Based Superprotonic Conductor with the Highest Metal-Organic Framework Based Proton Conductivity.

Metal-organic framework (MOF) based proton conductors have received immense importance recently. The present study endeavors to design two post synthetically modified UiO-66-based MOFs and examines the effects of their structural differences on their proton conductivity. UiO-66-NH2 is modified by reaction with sultones to prepare two homologous compounds, that is, PSM 1 and PSM 2, with SO3H functionalization in comparable extent (Zr:S = 2:1) in both. However, the pendant alkyl chain holding the -SO3H group is of different length. PSM 2 has longer alkyl chain attachment than PSM 1. This difference in the length of side arms results in a huge difference in proton conductivity of the two compounds. PSM 1 is observed to have the highest MOF-based proton conductivity (1.64 × 10-1 S cm-1) at 80 °C, which is comparable to commercially available Nafion, while PSM 2 shows significantly lower conductivity (4.6 × 10-3 S cm-1). Again, the activation energy for proton conduction is one of the lowest among all MOF-based proton conductors in the case of PSM 1, while PSM 2 requires larger activation energy (almost 3 times). This profound effect of variation of the chain length of the side arm by one carbon atom in the case of PSM 1 and PSM 2 was rather surprising and never documented before. This effect of the length of the side arm can be very useful to understand the proton conduction mechanism of MOF-based compounds and also to design better proton conductors. Besides, PSM 1 showed proton conductivity as high as 1.64 × 10-1 S cm-1 at 80 °C, which is the highest reported value to date among all MOF-based systems. The lability of the -SO3H proton of the post synthetically modified UiO-66 MOFs has theoretically been determined by molecular electrostatic potential analysis and theoretical p Ka calculation of models of functional sites along with relevant NBO analyses.

Functional Molecular System of Bis(pyrazolyl)pyridine Derivatives: Photophysics, Spectroscopy, Computation, and Ion Sensing.

A new series of conjugated donor-π-acceptor type of 2,6-bis(pyrazolyl)pyridine derivatives (compounds IK-(3-9)) have been synthesized via Horner-Wadsworth-Emmons (HWE) reaction, starting from a common phosphonate precursor and diverse donor aromatic aldehydes and characterized by routine spectral analysis including elemental analysis. Compound IK-2, one of the starting precursors, and molecule IK-3, the first member of the donor-π-acceptor series, are additionally characterized by single-crystal X-ray structure determination. Compounds IK-2 and IK-3 are crystallized in P1̅ (triclinic) and P21/c (monoclinic) space groups, respectively. The absorption maxima in the electronic spectra of the title compounds shift mainly due to intramolecular charge transfer (ICT) between different donor (dibutyl and cyclic pyrrolidine) groups and the acceptor moiety [2,6-bis(pyrazolyl) pyridine]. Solution-state emission spectral studies of all these compounds show large solvent sensitive behavior with significant amounts of Stokes shifts. The large solvent dependence of the emission indicates that the excited state is stabilized in more polar solvents due to the ICT. All chromophores exhibit solid-state fluorescence behavior except compound IK-7. The role of the position and nature of the donor functionalities in the conjugated backbone of overall donor moiety of compounds IK-(3-9), on the electronic absorption properties of the title chromophores has been demonstrated, which has further been corroborated by density functional theory (DFT) and time-dependent DFT (TDDFT) computational studies. The emission spectral results of compounds IK-3, IK-5, and IK-7 have also been supported by the DFT and TDDFT calculations. A fluorescence lifetime study on this series also shows that the excited states are stabilized in more polar solvents. Finally, one of the chromophores (chromophore IK-4) in the title series has been shown to act as a selective molecular sensor (turn-off switch) for the Cu(II) ion.

Synthesis, characterization and unravelling the molecular interaction of new bioactive 4-hydroxycoumarin derivative with biopolymer: Insights from spectroscopic and theoretical aspect.

In the progress of small molecule as drug candidates, 4-hydroxycoumarin based compounds bearing a crucial place as potent antibiotic agents with appreciable safety in drug invention. Being synthetically and easily obtainable, 4-hydroxycoumarin related compounds with planar structure have been promoted predominantly as DNA targeting agent. Nevertheless, here we elucidate the synthesis, characterization and theoretical study of bio-active small molecule 4-hydroxy-3,4'-bichromenyl-2,2'-dione (4HBD). Then we have illuminated the binding interactions of 4HBD with calf thymus DNA (ctDNA), which is particularly designed for biological application. Extensive investigations of the binding of 4HBD with ctDNA are provided by utilizing multi-spectroscopic and molecular docking approaches, including UV-vis absorbance, steady-state, time-resolved fluorescence spectroscopy and circular dichroism study. The calculated binding and quenching constant value from quantitative data analysis of absorption and emission spectroscopy shows that 4HBD binds to the ctDNA groove. Further confirmation of the same is found by comparative displacement and iodide quenching studies. Negative enthalpy, negative free energy and positive entropy change imply a hydrophobic force monitors the association of 4HBD with the biomacromolecule. Interestingly the small molecule (4HBD) shows potential anti-bacterial activity against the model pathogenic gram-negative (Escherichia coli and Pseudomonas aeruginosa) and gram-positive (Bacillus subtilis and Staphylococcus aureus) bacteria. The noncytotoxic nature of the 4HBD is demonstrated in vitro with the help of MTT assay by normal kidney epithelial (NKE), breast cancer cells (MCF-7) and human prostate cancer cell (PC3) lines. Hemolytic assay exhibits insignificant hemolysis of human erythrocyte cells at the minimum inhibitory concentration (MIC) of these tested bacteria. In this regard the present invention of 4-hydroxycoumarin based antimicrobial and noncytotoxic 4HBD molecule holds future promise in the development of new antibiotics.