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Diamondoids are hydrogen-saturated molecular motifs cut out of diamond, forming a class of materials with tunable optoelectronic properties. In this work, we extend previous work on neutral, closed-shell diamondoids by computing with hybrid density functional theory and time-dependent correlation functions vibrationally broadened absorption spectra of cations and radicals derived from the simplest diamondoid, adamantane, namely, the neutral 1- and 2-adamantyl radicals (C10H15), the 1- and 2-adamantyl cations (C10H15+), and the adamantane radical cation (C10H16+). For selected cases, we also report vibrationally broadened emission, photoelectron, and resonance Raman spectra. Furthermore, the effect of the damping factor on the vibrational fine-structure is studied. The following trends are found: (1) Low-energy absorptions of the adamantyl radicals and cations, and of the adamantane cation, are all strongly red-shifted with respect to adamantane; (2) also, emission spectra are strongly red-shifted, whereas photoelectron spectra are less affected for the cases studied; (3) vibrational fine-structures are reduced compared to those of adamantane; (4) the spectroscopic signals of 1- and 2-adamantyl species are significantly different from each other; and (5) reducing the damping factor has only a limited effect on the vibrational fine-structure in most cases. This suggests that removing hydrogen atoms and/or electrons from adamantane leads to new optoelectronic properties, which should be detectable by vibronic spectroscopy.
Nowadays, reactions on surfaces are attaining great scientific interest because of their diverse applications. Some well known examples are production of ammonia on metal surfaces for fertilizers and reduction of poisonous gases from automobiles using catalytic converters. More recently, also photoinduced reactions at surfaces, useful, \textit{e.g.}, for photocatalysis, were studied in detail. Often, very short laser pulses are used for this purpose. Some of these reactions are occurring on femtosecond (1 fs=$10^{-15}$ s) time scales since the motion of atoms (which leads to bond breaking and new bond formation) belongs to this time range. This thesis investigates the femtosecond laser induced associative photodesorption of hydrogen, H$_2$, and deuterium, D$_2$, from a ruthenium metal surface. Many interesting features of this reaction were explored by experimentalists: (i) a huge isotope effect in the desorption probability of H$_2$ and D$_2$, (ii) the desorption yield increases non-linearly with the applied visible (vis) laser fluence, and (iii) unequal energy partitioning to different degrees of freedom. These peculiarities are due to the fact that an ultrashort vis pulse creates hot electrons in the metal. These hot electrons then transfer energy to adsorbate vibrations which leads to desorption. In fact, adsorbate vibrations are strongly coupled to metal electrons, \textit{i.e.}, through non-adiabatic couplings. This means that, surfaces introduce additional channels for energy exchange which makes the control of surface reactions more difficult than the control of reactions in the gas phase. In fact, the quantum yield of surface photochemical reactions is often notoriously small. One of the goals of the present thesis is to suggest, on the basis of theoretical simulations, strategies to control/enhance the photodesorption yield of H$_2$ and D$_2$ from Ru(0001). For this purpose, we suggest a \textit{hybrid scheme} to control the reaction, where the adsorbate vibrations are initially excited by an infrared (IR) pulse, prior to the vis pulse. Both \textit{adiabatic} and \textit{non-adiabatic} representations for photoinduced desorption problems are employed here. The \textit{adiabatic} representation is realized within the classical picture using Molecular Dynamics (MD) with electronic frictions. In a quantum mechanical description, \textit{non-adiabatic} representations are employed within open-system density matrix theory. The time evolution of the desorption process is studied using a two-mode reduced dimensionality model with one vibrational coordinate and one translational coordinate of the adsorbate. The ground and excited electronic state potentials, and dipole function for the IR excitation are taken from first principles. The IR driven vibrational excitation of adsorbate modes with moderate efficiency is achieved by (modified) $\pi$-pulses or/and optimal control theory. The fluence dependence of the desorption reaction is computed by including the electronic temperature of the metal calculated from the two-temperature model. Here, our theoretical results show a good agreement with experimental and previous theoretical findings. We then employed the IR+vis strategy in both models. Here, we found that vibrational excitation indeed promotes the desorption of hydrogen and deuterium. To summarize, we conclude that photocontrol of this surface reaction can be achieved by our IR+vis scheme.
Vibrationally resolved absorption and emission (fluorescence) spectra of perylene and its N-derivatives in gas phase and in solution (acetonitrile) were simulated using a time-dependent approach based on correlation functions determined by density functional theory. By systematically varying the number and position of N atoms, it is shown that the presence of nitrogen heteroatoms has a negligible effect on the molecular structure and geometric distortions upon electronic transitions, while spectral properties change: in particular the number of N atoms is important while their position is less decisive. Thus, the N-substitution can be used to fine-tune the optical properties of perylene-based molecules.
The time-dependent approach to electronic spectroscopy, as popularized by Heller and coworkers in the 1980's, is applied here in conjunction with linear-response, time-dependent density functional theory to study vibronic absorption, emission and resonance Raman spectra of several diamondoids. Two-state models, the harmonic and the Condon approximations, are used for the calculations, making them easily applicable to larger molecules. The method is applied to nine pristine lower and higher diamondoids: adamantane, diamantane, triamantane, and three isomers each of tetramantane and pentamantane. We also consider a hybrid species “Dia = Dia” – a shorthand notation for a recently synthesized molecule comprising two diamantane units connected by a C[double bond, length as m-dash]C double bond. We resolve and interpret trends in optical and vibrational properties of these molecules as a function of their size, shape, and symmetry, as well as effects of “blending” with sp2-hybridized C-atoms. Time-dependent correlation functions facilitate the computations and shed light on the vibrational dynamics following electronic transitions.
Time-dependent correlation function based methods to study optical spectroscopy involving electronic transitions can be traced back to the work of Heller and coworkers. This intuitive methodology can be expected to be computationally efficient and is applied in the current work to study the vibronic absorption, emission, and resonance Raman spectra of selected organic molecules. Besides, the "non-standard" application of this approach to photoionization processes is also explored. The application section consists of four chapters as described below.
In Chapter 4, the molar absorptivities and vibronic absorption/emission spectra of perylene and several of its N-substituted derivatives are investigated. By systematically varying the number and position of N atoms, it is shown that the presence of nitrogen heteroatoms has a negligible effect on the molecular structure and geometric distortions upon electronic transitions, while spectral properties are more sensitive: In particular the number of N atoms is important while their position is less decisive. Thus, N-substitution can be used to fine-tune the optical properties of perylene-based molecules.
In Chapter 5, the same methods are applied to study the vibronic absorption/emission and resonance Raman spectra of a newly synthesized donor-acceptor type molecule. The simulated absorption/emission spectra agree fairly well with experimental data, with discrepancies being attributed to solvent effects. Possible modes which may dominate the fine-structure in the vibronic spectra are proposed by analyzing the correlation function with the aid of Raman and resonance Raman spectra.
In the next two chapters, besides the above types of spectra, the methods are extended to study photoelectron spectra of several small diamondoid-related systems (molecules, radicals, and cations). Comparison of the photoelectron spectra with available experimental data suggests that the correlation function based approach can describe ionization processes reasonably well. Some of these systems, cationic species in particular, exhibit somewhat peculiar optical behavior, which presents them as possible candidates for functional devices.
Correlation function based methods in a more general sense can be very versatile. In fact, besides the above radiative processes, formulas for non-radiative processes such as internal conversion have been derived in literature. Further implementation of the available methods is among our next goals.
Optical properties of modified diamondoids have been studied theoretically using vibrationally resolved electronic absorption, emission and resonance Raman spectra. A time-dependent correlation function approach has been used for electronic two-state models, comprising a ground state (g) and a bright, excited state (e), the latter determined from linear-response, time-dependent density functional theory (TD-DFT). The harmonic and Condon approximations were adopted. In most cases origin shifts, frequency alteration and Duschinsky rotation in excited states were considered. For other cases where no excited state geometry optimization and normal mode analysis were possible or desired, a short-time approximation was used. The optical properties and spectra have been computed for (i) a set of recently synthesized sp(2)/sp(3) hybrid species with CQC double-bond connected saturated diamondoid subunits, (ii) functionalized (mostly by thiol or thione groups) diamondoids and (iii) urotropine and other C-substituted diamondoids. The ultimate goal is to tailor optical and electronic features of diamondoids by electronic blending, functionalization and substitution, based on a molecular-level understanding of the ongoing photophysics.
Optical properties of modified diamondoids have been studied theoretically using vibrationally resolved electronic absorption, emission and resonance Raman spectra. A time-dependent correlation function approach has been used for electronic two-state models, comprising a ground state (g) and a bright, excited state (e), the latter determined from linear-response, time-dependent density functional theory (TD-DFT). The harmonic and Condon approximations were adopted. In most cases origin shifts, frequency alteration and Duschinsky rotation in excited states were considered. For other cases where no excited state geometry optimization and normal mode analysis were possible or desired, a short-time approximation was used. The optical properties and spectra have been computed for (i) a set of recently synthesized sp2/sp3 hybrid species with C[double bond, length as m-dash]C double-bond connected saturated diamondoid subunits, (ii) functionalized (mostly by thiol or thione groups) diamondoids and (iii) urotropine and other C-substituted diamondoids. The ultimate goal is to tailor optical and electronic features of diamondoids by electronic blending, functionalization and substitution, based on a molecular-level understanding of the ongoing photophysics.
Optical properties of modified diamondoids have been studied theoretically using vibrationally resolved electronic absorption, emission and resonance Raman spectra. A time-dependent correlation function approach has been used for electronic two-state models, comprising a ground state (g) and a bright, excited state (e), the latter determined from linear-response, time-dependent density functional theory (TD-DFT). The harmonic and Condon approximations were adopted. In most cases origin shifts, frequency alteration and Duschinsky rotation in excited states were considered. For other cases where no excited state geometry optimization and normal mode analysis were possible or desired, a short-time approximation was used. The optical properties and spectra have been computed for (i) a set of recently synthesized sp2/sp3 hybrid species with C[double bond, length as m-dash]C double-bond connected saturated diamondoid subunits, (ii) functionalized (mostly by thiol or thione groups) diamondoids and (iii) urotropine and other C-substituted diamondoids. The ultimate goal is to tailor optical and electronic features of diamondoids by electronic blending, functionalization and substitution, based on a molecular-level understanding of the ongoing photophysics.
Vibrationally resolved lowest-energy bands of the photoelectron spectra (PES) of adamantane, diamantane, and urotropine were simulated by a time-dependent correlation function approach within the harmonic approximation. Geometries and normal modes for neutral and cationic molecules were obtained from B3LYP hybrid density functional theory (DFT). It is shown that the simulated spectra reproduce the experimentally observed vibrational finestructure (or its absence) quite well. Origins of the finestructure are discussed and related to recurrences of autocorrelation functions and dominant vibrations. Remaining quantitative and qualitative errors of the DFT-derived PES spectra refer to (i) an overall redshift by ∼0.5 eV and (ii) the absence of satellites in the high-energy region of the spectra. The former error is shown to be due to the neglect of many-body corrections to ordinary Kohn-Sham methods, while the latter has been argued to be due to electron-nuclear couplings beyond the Born-Oppenheimer approximation [Gali et al., Nat. Commun. 7, 11327 (2016)].
Visible-Light-Mediated Photodynamic Water Disinfection @ Bimetallic-Doped Hybrid Clay Nanocomposites
(2019)
This study reports a new class of photocatalytic hybrid clay nanocomposites prepared from low-cost sources (kaolinite clay and Carica papaya seeds) doped with Zn and Cu salts via a solvothermal process. X-ray diffraction analysis suggests that Cu-doping and Cu/Zn-doping introduce new phases into the crystalline structure of Kaolinite clay, which is linked to the reduced band gap of kaolinite from typically between 4.9 and 8.2 eV to 2.69 eV for Cu-doped and 1.5 eV for Cu/Zn hybrid clay nanocomposites (Nisar, J.; Arhammar, C.; Jamstorp, E.; Ahuja, R. Phys. Rev. B 2011, 84, 075120). In the presence of solar light irradiation, Cu- and Cu/Zn-doped nanocomposites facilitate the electron hole pair separation. This promotes the generation of singlet oxygen which in turn improves the water disinfection efficiencies of these novel nanocomposite materials. The nanocomposite materials were further characterized using high-resolution scanning electron microscopy, fluorimetry, therrnogravimetric analysis, and Raman spectroscopy. The breakthrough times of the nanocomposites for a fixed bed mode of disinfection of water contaminated with 2.32 x 10(7) cfu/mL E. coli ATCC 25922 under solar light irradiation are 25 h for Zn-doped, 30 h for Cu-doped, and 35 h for Cu/Zn-doped nanocomposites. In the presence of multidrug and multimetal resistant strains of E. coli, the breakthrough time decreases significantly. Zn-only doped nanocomposites are not photocatalytically active. In the absence of light, the nanocomposites are still effective in decontaminating water, although less efficient than under solar light irradiation. Electrostatic interaction, metal toxicity, and release of singlet oxygen (only in the Cu-doped and Cu/Zn-doped nanocomposites) are the three disinfection mechanisms by which these nanocomposites disinfect water. A regrowth study indicates the absence of any living E. coli cells in treated water even after 4 days. These data and the long hydraulic times (under gravity) exhibited by these nanocomposites during photodisinfection of water indicate an unusually high potential of these nanocomposites as efficient, affordable, and sustainable point-of-use systems for the disinfection of water in developing countries.
The anisotropic effects of the phenyl, alpha- and beta-naphthyl moieties in four series of 1,3-oxazino[4,3- a]isoquinolines on the H-1 chemical shifts of the isoquinoline protons were calculated by employing the Nucleus Independent Chemical Shift (NICS) concept and Visualized as anisotropic cones by a through-space NMR shielding grid. The signs and extents of these spatial effects on the H-1 chemical shifts of the isoquinoline protons were compared with the experimental H-1 NMR spectra. The differences between the experimental delta (H-1)/ppm values and the calculated anisotropic effects of the aromatic moieties are discussed in terms of the steric compression that occurs in the Compounds studied.
The anisotropic effect of the olefinic C=C double bond has been calculated by employing the NICS (nucleus independent chemical shift) concept and visualized as an anisotropic cone by a through space NMR shielding grid. Sign and size of this spatial effect on 1H chemical shifts of protons in norbornene, exo- and endo-2-methylnorbornenes, and in three highly congested tetracyclic norbornene analogs have been compared with the experimental 1H NMR spectra as far as published. 1H NMR spectra have also been calculated at the HF/6-31G* level of theory to get a full, comparable set of proton chemical shifts. Differences between ;(1H)/ppm and the calculated anisotropic effect of the C=C double bond are discussed in terms of the steric compression that occurs in the compounds studied.
Prototypes for homoaromaticity in cations, neutral molecules, and anions are theoretically studied at the MP2 level of theory. For the global minimum structures on the potential energy surface both 1H/13C chemical shifts and spatial magnetic properties as through space NMR shieldings (TSNMRS) were calculated by the GIAO perturbation method. The TSNMRS are visualized as iso-chemical-shielding surfaces (ICSS) of different sign and size. Coincident experimental and computed 1H/13C chemical shifts afforded the possibility to decide from the TSNMRSs at hand on both the existence and the size of homoaromaticity in the molecules studied.
Through space NMR shieldings of aromatic (benzene, mono-substituted and annelated benzenes, ferrocene, [14]- and [18]-annulenes, phenylenes and tetra- to heptahelicene) and anti-aromatic molecules (cyclobutadiene and pentalene) were assessed by ab initio molecular-orbital calculations. Employing the nucleus-independent chemical shifts (NICS) concept, these through space NMR shieldings were visualized as iso-chemical-shielding surfaces (ICSSs) and can be applied quantitatively to determine the stereochemistry of proximal nuclei. In addition, the distances in Å at ICSS values of ±0.1 ppm in-plane and perpendicular-to-center of the aromatic ring system were employed as a simple means to compare and estimate qualitatively the aromaticity of the systems at hand.
Dynamic and direct visualization of interfacial evolution is helpful in gaining fundamental knowledge of all-solid-state-lithium battery working/degradation mechanisms and clarifying future research directions for constructing next-generation batteries. Herein, in situ and in operando synchrotron X-ray tomography and energy dispersive diffraction were simultaneously employed to record the morphological and compositional evolution of the interface of InLi-anode|sulfide-solid-electrolyte during battery cycling. Compelling morphological evidence of interfacial degradation during all-solid-state-lithium battery operation has been directly visualized by tomographic measurement. The accompanying energy dispersive diffraction results agree well with the observed morphological deterioration and the recorded electrochemical performance. It is concluded from the current investigation that a fundamental understanding of the phenomena occurring at the solid-solid electrode|electrolyte interface during all-solid-state-lithium battery cycling is critical for future progress in cell performance improvement and may determine its final commercial viability.
Watching the Vibration and Cooling of Ultrathin Gold Nanotriangles by Ultrafast X-ray Diffraction
(2016)
We study the vibrations of ultrathin gold nanotriangles upon optical excitation of the electron gas by ultrafast X-ray diffraction. We quantitatively measure the strain evolution in these highly asymmetric nano-objects, providing a direct estimation of the amplitude and phase of the excited vibrational motion. The maximal strain value is well reproduced by calculations addressing pump absorption by the nanotriangles and their resulting thermal expansion. The amplitude and phase of the out-of-plane vibration mode with 3.6 ps period dominating the observed oscillations are related to two distinct excitation mechanisms. Electronic and phonon pressures impose stresses with different time dependences. The nanosecond relaxation of the expansion yields a direct temperature sensing of the nano-object. The presence of a thin organic molecular layer at the nanotriangle/substrate interfaces drastically reduces the thermal conductance to the substrate.