Molecular interactions: a study of charge transfer effects

Polymer photovoltaic cells currently achieve power conversion efficiencies (PCE) above 10% on lab scale. To compete with the efficiencies above 20% of inorganic solar cells, understanding and elimination of all the loss channels is necessary. This thesis investigates charge generation and recombination processes in polymer-fullerene solar cells with the aim of elucidating the nature of such losses. Chapter 1 provides a general introduction to the subject of the thesis and an overview of the most relevant literature. In Chapter 2 the effect of the thermodynamic driving force on the efficiency of photoinduced charge transfer between semiconducting polymers and fullerene is investigated. A polyfluorene copolymer is mixed with a variety of fullerene mono- and bisadducts having different LUMO energy levels. The difference in LUMO energy of the acceptors results in different energies of the charge transfer (CT) states in the blends. By spectroscopic characterization of bulk heterojunction thin films and photovoltaic devices it is found that the CT state needs to be at least 0.1 eV lower in energy than the singlet excited state of the fullerene to be formed efficiently. The central finding of Chapters 3 to 5 is that the nanoscale morphology of the polymer-fullerene heterojunction strongly influences the CT state dissociation and recombination. Blends composed of a small optical bandgap semiconducting polymer (PCPDTBT) mixed with fullerene are studied with various techniques. The nanoscale phase separation of these blends is controlled by adding high-boiling point co-solvents to the solutions used to process the thin films. Without co-solvent the blends are very finely mixed and the addition of co-solvents increases the phase separation. In blends that are too finely mixed we observe recombination of the CT state to the triplet state of the polymer (Chapter 3). We monitor the formation of triplets and charges in the thin films by means of photo-induced absorption (PIA).The recombination to the triplet state is a loss mechanism, in competition with the dissociation of the CT state into free charges. This process is reduced in favor of free charge formation when the phase separation is increased, correlating with the increased device performance. Specifically, increased fill factor and short circuit current are observed in the devices with optimized morphology. These results show that photophysical processes are controlled by the nanoscale morphology. In Chapter 4 we perform charge extraction experiments on solar cells having different nanoscale phase separation and we support the results with PIA experiments on corresponding thin films. In the charge extraction and PIA experiments we study the same set of long lived charges. We confirm that CT recombination (to the triplet and to the ground state) is largely responsible for the poor device performances of not-optimized blends. Finally, in Chapter 5 we show a correlation between population of triplet excited states and decreased photo-stability of the blends. We suggest that triplet states can lead to the formation of singlet oxygen, which in turn can initiate destructive chemical reactions in the polymer chains. In Chapters 6 and 7 the influence of excess photon energy (i.e. the difference between the energy of the absorbed photon and that of the CT state) in the CT state dissociation is investigated by means of charge extraction and spectral response measurements in different polymer:fullerene blends. We find that excess energy is not necessary for CT dissociation in blends with coarse phase separation. A measurable influence of the excess photon energy on CT dissociation is observed only in PCPDTBT:PCBM finely dispersed blends, which were shown in the previous chapters to be strongly affected by CT state recombination. Chapter 8 investigates what is limiting the photo-current of solar cells close to the open-circuit voltage. It is found that under these conditions space-charge can build up, so that the photocurrent becomes space-charge limited. This condition is reached only in blends where CT dissociation is efficient at low electric fields. Impedance spectroscopy is the technique used here. The experimental results are supported by an analytical model and by drift diffusion simulations. Summarizing, in this thesis it is shown that the CT state is a critical intermediate state in the charge generation process in polymer solar cells. The energy of the CT state needs to be low enough to convert neutral excitations into charges. The blend morphology is a crucial factor determining the branching ratio between dissociation into free carriers and recombination to the ground state or to the triplet state of the polymer.

In the chemistry of metallic complexes, two important concepts have been used to rationalize the recognition and selectivity of a host by a guest: preorganization and complementarity. Both of these concepts stem from geometrical features. Less explored in the literature has been the interactional complementarity, where mainly the electronic factors in the intermolecular forces are involved. Because the charge transfer between a species rich in electrons (ligand) and another deficient in them (cation) is one of the main intermolecular factors that control the binding energies in metallic complexes, for such systems, we propose a simple model based on density functional theory. We define an interactional energy in which the geometrical energy changes are subtracted from the binding energies and just the electronic factors are taken into account. The model is tested for the complexation between bidentate and cyclic ligands and Ca, Pb, and Hg metal dications. The charge-transfer energy described by our model fits nicely with the interactional energy. Thus, when the geometrical changes do not contribute in a significant way to the complexation energy, the interactional energy is dominated by charge-transfer effects.

The dependence of exciplex emission spectra on the solvent polarity was studied for exciplexes with the Gibbs free energy of excited-state electron transfer, ΔG*et, exceeding –0.1 eV (for pyrene, fluoranthene, 1,12-benzoperylene, and 9-cyanoanthracene with methoxybenzenes or dimethylnaphthalene). These exciplexes showed stronger changes in the spectral shift of exciplex emission and the extent of charge transfer with increasing solvent polarity than the exciplexes having more negative ΔG*etvalues. The parameters (difference in energy of charge transfer (CT) and locally excited (LE) states in a vacuum, (H022– H011), and the matrix element for electronic coupling of CT and LE states H12and mrelated to the dipole moment of the CT state and the size of the exciplex) determining the extent of charge transfer, the spectral shift, and other properties of exciplexes were evaluated. The parameters H12and mfor the exciplexes examined fall in the interval 0.1–0.5 and 1.0–1.7 eV, respectively, and the difference (H022– H011) is proportional to ΔG*et.

An article in this issue of IUCrJ (Milas ˇinovic ´et al., 2023) addresses a long-asked question concerning the dominant component of strong -hole interactions. A deep understanding of intra-and intermolecular non-covalent interactions is crucial for progress in many areas of supramolecular chemistry, especially in the field of molecular recognition and self-assembly, which is the basis of e.g. drug design and crystal engineering. By pondering the nature of non-covalent interactions, quantum theory leads us to the conclusion that the interactions within and between such chemical entities as molecules, ions etc. are Coulombic, and include electrostatic, polarization, dispersion and charge transfer contributions. Theoretical calculations and crystallographic studies (Politzer et al., 2021; Stevens, 1979; Batsanov, 2000) show a non-uniform electron density distribution on an atom in a molecule. Obviously, the region with a higher electron density is located at the covalent bond, while a region of lower electronic density is at the side opposite to the covalent bond. The difference in electron density between the regions depends on atomic polarizability and is most notable for covalently bonded atoms of Groups 14-17 (Grabowski, 2017) . If an atom participates in a multiple-bond formation, e.g. double or triple, the electron-depleted areas are perpendicular to the main molecule. Depending on the positive region location, it is usually defined as a -, -, lonepair n-, or radical RÁ-hole. These electron density holes are responsible for the formation of attractive interactions with any electron-rich site (a lone pair or a -system of a molecule, an anion etc.). Depending on the atoms on which the hole is observed, the most studied kinds of interactions are called hydrogen bonding, and tetrel, pnictogen, chalcogen and halogen interactions. Among all the above-mentioned hole interactions, the nÁÁÁ-hole one has been attracting particular attention during the past decade for several reasons: (1) significant polarizability of the -electrons, especially those in conjugated systems, makes estimation of the correct whole system energy hard from the calculational point of view; (2) the important role of the nÁÁÁ-hole interactions in biochemistry (

Many years ago, the gas-phase infrared fundamental intensities of Cl2CS were determined within experimental error from the experimental intensities and frequencies of F2CO, Cl2CO, and F2CS. An additive characteristic substituent shift relationship between atomic polar tensors of these molecules formed the basis for these calculations. Here, QCISD/cc-pVTZ-level Quantum Theory of Atoms In Molecules (QTAIM) individual charge, charge transfer, and polarization contributions to these atomic polar tensor elements are shown to obey the same basic relationship for the extended X2CY (Y = O, S; X = H, F, Cl, Br) family of molecules. QTAIM charge and polarization contributions, as well as the total equilibrium dipole moments of the X2CY molecules, also follow this characteristic substituent shift model. The root-mean-sqaure error for the 231 estimates of these parameters is 0.14 e or only about 1% of the total 10 e range of the Atomic Polar Tensor (APT) contributions determined from the wave functions. The substituent effect APT contribution estimates were used to calculate the infrared intensities of the X2CY molecules. Although one serious discrepancy was observed for one of the CH stretching vibrations of H2CS, accurate values were within 45 km·mol-1 or about 7% of the 656 km·mol-1 intensity range predicted by the QCISD/cc-pVTZ wave functions. Hirshfeld charge, charge transfer, and polarization contributions are also found to follow this model, although their charge parameters do not follow electronegativity expectations.

ABSTRACTWe present the ionization of decaborane (B10H14) and formation of hydrogen- and boron-contents-controlled B10-yHx+ through the charge transfer from ambient gas ion to decaborane molecules in an external quadrupole static attraction ion trap. The charge transfer energy is estimated from the experimentally observed products. PBE0/6-311+G(d)//B3LYP/6-31G(d) level of DFT calculations are conducted to investigate the mechanism of charge transfer from ambient gas ion. The calculation of the difference of ionization energies and mismatch of orbital energies between decaborane and ambient gas reveals the mechanism of ionization.

The absorption spectrum of dissolved organic matter (DOM) is a topic of interest to environmental scientists and engineers as it can be used to assess both the concentration and physicochemical properties of DOM. In this study, the UV-vis spectra for DOM model compounds were calculated using time-dependent density functional theory. Summing these individual spectra, it was possible to re-create the observed exponential shape of the DOM absorption spectra. Additionally, by predicting the effects of sodium borohydride reduction on the model compounds and then calculating the UV-vis absorbance spectra of the reduced compounds, it was also possible to correctly predict the effects of borohydride reduction on DOM absorbance spectra with a relatively larger decrease in absorbance at longer wavelengths. The contribution of charge-transfer (CT) interactions to DOM absorption was also evaluated, and the calculations showed that intra-molecular CT interactions could take place, while inter-molecular CT interactions were proposed to be less likely to contribute.

Interaction energies of halide-water dimers, X-(H2O), and trimers, X-(H2O)2, with X = F, Cl, Br, and I, are investigated using various many-body models and exchange correlation functionals selected across the hierarchy of density functional theory (DFT) approximations. Analysis of the results obtained with the many-body models demonstrates the need to capture important close-range interactions in the regime of large intermolecular orbital overlap, such as charge transfer and charge penetration. Failure to reproduce these effects can lead to large deviations relative to reference data calculated at the coupled cluster level of theory. Decompositions of interaction energies carried out with the absolutely localized molecular orbital energy decomposition analysis (ALMO-EDA) method demonstrate that permanent and inductive electrostatic energies are accurately reproduced by all classes of XC functionals (from generalized gradient corrected (GGA) to hybrid and range-separated hybrid functionals), while significant variance is found for charge transfer energies predicted by different XC functionals. Since GGA and hybrid XC functionals predict the most and least attractive charge transfer energies, respectively, the large variance is likely due to the delocalization error. In this scenario, the hybrid XC functionals are then expected to provide the most accurate charge transfer energies. The sum of Pauli repulsion and dispersion energies is the most varied among the XC functionals, but it is found that a correspondence between the interaction energy and the ALMO-EDA total frozen energy may be used to determine accurate estimates for these contributions.

Calculated variation trends across the actinide dioxide series: electronic structure and charge transfer transitions

Spectroscopic characterization of charge transfer complexes of 2,3-diaminopyridine with chloranilic acid and dihydroxy-p-benzoquinone in polar solvent

The electrostatic energy of aromatic charge-transfer (CT) and free radical (FR) crystals is obtained by using the many-electron site representation and the modified Hubbard model previously developed to correlate the unusual magnetic, electric and optical properties of CT and FR crystals. Both π-electron overlap between adjacent ion radicals in a stack and configuration interaction with CT states are included, thus improving on the Madelung approximation of disjoint charge distributions. π-electron overlap leads to a Coulomb exchange energy E ex which stabilizes both ionic CT and FR crystals, while configuration interaction scales the ground state charge densities, and thus the Madelung energy, in CT crystals. E ex is obtained for 1 : 1 CT crystals and a numerical estimate of 10–15 kcal/mole is found for both TMPD-TCNQ and for TMPD-chloranil.

Molecular interactions between uracil and nitrous acid (U–NA) [C4N2O2H4NO2H] have been studied using B3LYP, B3PW91, and MP2 methods with different basis sets. The optimized geometries, harmonic vibrational frequencies, charge transfer, topological properties of electron density, nucleus‐independent chemical shift (NICS), and nuclear magnetic resonance one‐ and two‐bonds spin–spin coupling constants were calculated for U–NA complexes. In interaction between U and NA, eight cyclic complexes were obtained with two intermolecular hydrogen bonds N(C)HU…N(O) and OHNA…OU. In these complexes, uracil (U) simultaneously acts as proton acceptor and proton donor. The most stable complexes labeled, UNA1 and UNA2, are formed via NH bond of U with highest acidity and CO group of U with lowest proton affinity. There is a relationship between hydrogen bond distances and the corresponding frequency shifts. The solvent effect on complexes stability was examined using B3LYP method with the aug‐cc‐pVDZ basis set by applying the polarizable continuum model (PCM). The binding energies in the gas phase have also been compared with solvation energies computed using the PCM. Natural bond orbital analysis shows that in all complexes, the charge transfer takes place from U to NA. The results predict that the Lone Pair (LP)(O)U → σ*(OH) and LP(N(O)NA → σ*(N(C)H)U donor–acceptor interactions are most important interactions in these complexes. Atom in molecule analysis confirms that hydrogen bond contacts are electrostatic in nature and covalent nature of proton donor groups decreases upon complexation. The relationship between spin–spin coupling constant (1hJH…Y and 2hJH…Y) with interaction energy and electronic density at corresponding hydrogen bond critical points and H‐bonds distances are investigated. NICS used for indicating of aromaticity of U ring upon complexation. © 2013 Wiley Periodicals, Inc.

A model is presented for the contribution of ground-state charge transfer between a metal and adsorbate to surface-enhanced Raman scattering (SERS). It is shown that this contribution can be understood using the vibronic theory for calculating Raman intensities. The enhancement is due to vibronic coupling of the molecular ground state to the metal states, the coupling mechanism being a modulation of the ground-state charge-transfer energy by the molecular vibrations. An analysis of the coupling operator gives the selection rules for this process, which turn out to be dependent on the overall symmetry of the adsorbate-metal system, even if the charge transfer is small enough for the symmetry of the adsorbate to remain the same as that of the free molecule. It is shown that the model can yield predictions on the properties of SERS, e.g., specificity to adsorption geometry, appearance of forbidden bands, dependence on the applied potential, and dependence on the excitation wavelength. The predictions are in good agreement with experimental results. It is also deduced from this model that in many cases atomic-scale roughness is a prerequisite for the observation of SERS. A result on the magnitude of the enhancement can only be given in a crude approximation. Although in most cases an additional electromagnetic enhancement seems to be necessary to give an observable signal, this charge-transfer mechanism should be important in many SERS systems.

Lab-energy distributions of HD+ ions formed in the D+(H2,H)HD+ reaction were measured with high resolution for collisions with relative energy W in the range 1 eV⩽W⩽5 eV. In addition to observing the transition from long- to short-lived collision complexes and the relative growth of repulsive interactions, we also observed the possible participation of charge transfer in the entrance channel of the reaction. The charge transfer contribution was suggested by independent study of the H2+(D,H)HD+ reaction, which follows predominantly direct interactions over the range 0.1 eV⩽W⩽10 eV. HD+ ions formed in the H2+(D,H)HD+ reaction have recoil energies in the same range as HD+ ions which appear as a distinct shoulder in the D+(H2,H)HD+ product distributions. For W⩾8 eV, the H2+(D,H)HD+ reaction yields a small fraction of H atoms which may be in an electronically excited state.

TiO2 is a promising material for high‐power battery and supercapacitor applications. However, in general TiO2 suffers from an initial irreversible capacity that limits its use in different applications. A combination of a microbead morphology, Nb‐doping, and the use of an ionic liquid electrolyte is shown to significantly decrease the irreversible capacity loss. A change in the electrochemical response in the first cycles indicates formation of a solid–electrolyte interphase (SEI) or a modification of the structure of the surface layer of the TiO2/Nb microbeads, which apparently stabilises the performance. The change in the response is manifested in an increased charge transfer resistance and the presence of two charge transfer contributions. During prolonged cycling the TiO2/Nb electrode shows an excellent stability over 5000 cycles. Ex situ analysis after cycling shows that the overall microbead morphology is intact and that there are no changes in the crystal structure. However, a decrease in the intensity of the XRD pattern can point to a decrease in size of the nanocrystals building up the microbeads or the formation of amorphous phases.