Oligothiophene/graphene supramolecular ensembles managing light induced processes: Preparation, characterization, and femtosecond transient absorption studies leading to charge‐separation

Advances  in  organic  synthetic  chemistry  combined  with  the  exceptional  electronic  properties  of  carbon  allotropes,  particularly graphene, is the basis to design and fabricate novel electron donor-­‐acceptor ensembles with desired properties for technological applications. Thiophene-­‐based materials, mainly thiophene-­‐containing polymers, are known for  their  notable  electronic  properties.  In  this  frame  moving  from  polymer  to  oligomer  forms,  new  fundamental  information would help to the better understanding of their electrochemical and photophysical properties. Furthermore, a successful  combination  of  their  electronic  properties  with  those  of  graphene  is  a  challenging  goal.  In  this  work  two  oligothiophene compounds consists of three and nine thiophene-­‐rings, abbreviated as 3T and 9T, respectively, were synthesized and noncovalently associated with liquid phase exfoliated few-­‐layered graphene sheets (abbreviated as eG), forming donor-­‐acceptor 3T/eG and 9T/eG nanoensembes. Markedly, intra-­‐ensemble electronic interactions between the two  components  in  the  ground  and  excited  states  were  evaluated  with  the  aid  of  UV-­‐Vis  and  photoluminescence  spectroscopy. Furthermore, redox assays revealed an one-­‐electron oxidation of 3T accompanied by one-­‐electron reduction due  to  eG  in  3T/eG,  while  two  reversible  one-­‐electron  oxidations  of  9T  accompanied  by  one-­‐electron  reduction  of  eG  9T/eG. The electrochemical band gap for 3T/eG and 9T/eG ensembles were calculated and verified that the negative free-­‐energy change for the charge-­‐separated state of 3T/eG and 9T/eG via the singlet excited state of 3T and 9T respectively, were  thermodynamically  favorable.  Finally,  results  of  transient  pump-­‐probe  spectroscopic  studies  at  the  femtosecond  time scale were supportive of charge transfer type interactions in the 3T/eG and 9T/eG ensembles. The estimated rates for intra-­‐ensemble charge separation were found to be 9.52 x 109 s-­‐1 and 2.2 x 1011 s-­‐1, respectively, for 3T/eG and 9T/eG in THF, revealing moderate to ultrafast photoinduced events in the oligothiophene/graphene supramolecular ensemble

The Polar Cosolvent Effect on Caffeine Solvation in Supercritical CO2-Ethanol Mixtures: A Molecular Modeling Approach

The effect of the addition of a small amount of ethanol cosolvent in supercritical CO2 on the solvation structure and dynamics of caffeine in a mixed supercritical solvent has been investigated using a systematic multiscale molecular modeling approach. An effective interaction potential model has been employed for caffeine, using the intramolecular geometry and charge distribution from quantum chemical calculations performed in the present treatment and adopting well-established Lennard-Jones parameters from the literature. The solvation structure and related dynamics have been further investigated by means of classical molecular dynamics simulations. The results obtained have revealed an enhancement of the local mole fraction of ethanol around caffeine due to the formation of hydrogen bonds between caffeine and its nearest ethanol molecules. This effect becomes less pronounced as the pressure of the system increases due to the denser packing of CO2 molecules in the first solvation shell of caffeine. The reorientational dynamics of caffeine is controlled by the intermittent hydrogen-bond dynamics, and its translational diffusion has been found to be significantly lower in comparison with the values obtained for ethanol and CO2. The pressure effects on the self-diffusion have also been found to be more pronounced in the cases of CO2 and EtOH in comparison with caffeine. The findings of the present study confirm a previous hypothesis in the literature, according to which polar solutes approach the polar domains formed by the alcohol aggregates and become more easily dissolved in the mixed CO2-ethanol solvent than in pure supercritical CO2. © 2021 American Chemical Society

The solvent effect on a styryl-bodipy derivative functioning as an AND molecular logic gate

We study via DFT and TDDFT calculations the photophysical processes of a styryl-bodipy derivative, (1), of its monometallic complexes 1-M2+ (M = Ca, Zn, and Hg), and its trimetallic complex (2) unprotonated, protonated and complexed with water molecules in water solvent and in acidic conditions. The main targets of this study are to gain information regarding published reports on fluorophore species mentioning that fluorescent switching results from trace water, to study how 1 behaves in water solvent which is a common used solvent for molecular logic gates (MLG), and how it behaves in acidic conditions. We conclude that in water solvent, as in acetonitrile solvent (which was found before both theoretically and experimentally) there will be a quenching of emission spectra in 1 and 1-M2+ and a retaining of emission in 2. However, contrary to acetonitrile solvent, in water, a weak peak will be observed for 1 and 1-M2+, due to a small ratio of reversible protonation, showing that in acetonitrile 1 acts as a better MLG candidate than in water solvent. On the other hand, in acidic conditions all five species will emit and as a result, 1 will not be an AND MLG, showing that the selection of the solvent conditions is crucial for a species to act as an MLG candidate. Finally, we conclude that the retaining of emission is accomplished by the simultaneous tetrahedral geometry of all three aniline N atoms. © 2020 Wiley Periodicals, Inc

Theoretical calculations on electronic transitions for H3, including Rydberg and transition state spectra

MRD-CI calculations have been carried out on the ground and excited electronic states of H3 for D3h, D∞h, C∞v, and C2v, geometries. Dipole transition moments between the various electronic states have been also obtained at the different geometries calculated. The present work provides accurate theoretical information relevant to the transition state spectroscopy of H + H2 along a collinear path and also along a perpendicular path. In addition, the present work is the first all-electron configuration interaction treatment of the Rydberg states of H3, and the results are in excellent agreement with the observed spectra

Potential energy surface for large-amplitude motion and vibrational spacings for FH2 +

Multireference configuration interaction calculations have been carried out on the ground electronic state of the fluoronium ion FH2 +. Both local (expansion about equilibrium geometry) and global (expansion about linear equidistant geometry) surface fits are obtained. The equilibrium geometry occurs at Re = 1.812 35 bohr and 〈HFH = 112.30 deg. The proton affinity is 116.5 kcal/mol and the inversion barrier height is 19.25 kcal/mol. The surface is suitable for the study of large-amplitude motion, and we obtain vibrational energies up to 2 eV, which is well above the barrier height. For higher vibrational levels, we note the effect of the potential energy barrier on the vibrational spacing. The minimum in vibrational spacing for the bending progression is found to be in excellent agreement with the calculated barrier height

Theoretical investigation involving electronic and vibrational calculations of the 2 2A1(3p)→1 2B2(3p) and the 3 2A1(4s)→1 2B2(3p) transitions in FH2 and FD2

Theoretical calculations have been carried out on the X 2A 1, 1 2B2(3p), 2 2A1(3p), and 3 2A1(4s) electronic states of FH2. Equilibrium geometries and rotational constants as well as the first few vibrational levels of the excited states have been calculated, in order to obtain theoretical information on the 2 2A1(3p)→1 2B2(3p) and the 3 2A1(4s)→1 2B2(3p) transitions in FH2, which might be relevant to the observed spectra at about 7500 and 8000 Å. The results show that the equilibrium geometry of the first excited state of FH2, 1 2B2(3p), is quite different from those of the other excited states. The estimated transition energies (△E0) in FH2 are 1.68 and 1.97 eV for the transitions 2 2A 1(3p)→1 2B2(3p) and 3 2A 1(4s)→1 2B2(3p), respectively, while in FD2 the corresponding quantities are 1.65 and 1.95 eV, respectively. A search for a minimum on the ground state surface of FH2, which has been carried out near two saddle point geometries, has not found one. Thus the present calculations do not find a metastable ground state species

The Rydberg states of FH2

The ground and first few excited electronic states of FH2 have been calculated by the MRD-CI method, in an effort to make predictions on the Rydberg spectrum of this molecule. The results show that the excited Rydberg states, up to the 4p levels (in the united atom notation), are bound and have minima at geometries similar to that of the cation FH2 + except the first excited state, which is also found to be bound but with minimum energy at a considerably longer bond length. The most intense bound-bound transitions are predicted to occur from the 3d, 4p, and 4s states to the first excited state 1 2B2(3p)

The role of electric field, peripheral chains, and magnetic effects on significant1H upfield shifts of the encapsulated molecules in chalcogen-bonded capsules

The chalcogen-bonded homo-cavitand and hetero-cavitandAY+AY′capsules (Y, Y′ = Se, Te), as well as their encapsulated complexes with one or two guest molecules have been studied theoreticallyviadensity functional theory (DFT), while the1H NMR spectra of the homo-cavitand encapsulated complexes (inASe+ASe) have been measured experimentally. There is excellent agreement between theoretical and experimental spectra. In all cases, we found significant1H upfield shifts which are more intense in theASe+ASecage compared to theATe+ATeandASe+ATecages. The non-uniform electron distribution which gives rise to an inherent electric field and a non-zero electric dipole moment of the encapsulated complexes, the induced electric field effects, the magnetic anisotropy which is enhanced due to the polarizability of chalcogen atoms, and the peripheral chains, which are responsible for the solubility of the cages, increase the upfield shifts of1H of the encapsulated molecules; the peripheral chains lead to an increase of the upfield shifts by up to 1.8 ppm for H of the rim and up to 1.2 ppm for the terminal H in the interior of the cage. Hence, substantial1H upfield chemical shifts of the guests in these capsules are consequences of (i) the enhanced aromaticity of the walls of the capsules due to the polarizability of chalcogen atoms, (ii) the induced and inherent electric field effects, and (iii) the peripheral chains. © the Owner Societies 2021

Semi-empirical dielectric descriptions of the Bethe surface of the valence bands of condensed water

The Bethe surface of a material is an essential element in the study of inelastic scattering at low impact energies where the optical approximation fails. In this work we examine various semi-empirical models for the dielectric response function of condensed water towards an improved description of the energy-loss function over the whole energy–momentum plane (i.e. Bethe surface). The experimental “optical” data (i.e. at zero momentum transfer) for the valence bands of liquid and solid water are analytically represented by a sum-rule constrained linear combination of Drude-type functions. The dependence on momentum transfer is introduced through various widespread “extension” schemes which are compared against the available Compton scattering data. It is shown that the widely used Lindhard function along with its “single-pole” (or “δ-oscillator”) approximation used in the Penn and Ashley models, as well as the Ritchie and Howie extended-Drude scheme with a simple quadratic dispersion, predict a sharp Bethe ridge which compares poorly with the experimental profile. In contrast, the Mermin dielectric function provides a more realistic account of the observed broadening with momentum transfer. An improved fully-extended-Drude model is presented which incorporates the momentum broadening and line-shift of the Bethe ridge and distinguishes between the different dispersion of the discrete and continuum spectra of water.This work has been partially supported by the Spanish Ministerio de Educación y Ciencia (Projects FIS2006-13309-C02-01 and FIS2006-13309-C02-02)

Electron inelastic mean free paths in biological matter based on dielectric theory and local-field corrections

The inelastic mean free path (IMFP) of electrons with energies up to a few keV is calculated from the dielectric electron-gas theory for densities corresponding to those of biological matter. The effect of the many-body local-field correction on the Lindhard dielectric response function is examined using some of the available analytical approximations to its static limit. We have tested the performance of several Hubbard-type local-field corrections along with the formula proposed by Corradini and co-workers [M. Corradini, R. Del Sole, G. Onida, M. Palumno, Phys. Rev. B 57 (1998) 14569] which is extensively used in connection with the exchange-correlation kernel of time-dependent density functional theory. It is shown that the Lindhard dielectric function provides reasonable estimates of electron IMFPs below about 50 eV, where the majority of semi-empirical dielectric calculations based on the extended-optical-data methodology fail. The use of LFC results in a sizeable reduction of the IMFP which, at low energies, may reach ∼20%.DE and IK acknowledge financial support by the European Union FP7 ANTICARB (HEALTH-F2-2008-201587) research programme and by an internal grand from the Research Committee of the University of Ioannina (Grant No. 80037). IA and RGM acknowledge financial support by the Spanish Ministerio de Educación y Ciencia (Projects FIS2006-13309-C02-01 and FIS2006-13309-C02-02)