Electronic band structure in the solid and its relation to the energy gap of the monomer is all about studying how intermolecular interactions change electronic structure. In experimental studies this results in broader absorption bands and by extension a lowering of the LUMO and raising of HOMO energy to the conduction and valence band edges respectively. This electronic change involves splitting of the molecular energy levels into bands of non-degenerate energies and can be calculated either quantum mechanically (QM) or by classical force field models through the change in ionization potential (IP) and electron affinity (EA), called the apparent polarization energy, and its relation to HOMO and LUMO through Koopman’s and Janak’s theorem. The study of the formation of a ‘band’ like structure is important in regimes and systems where conventional quantum mechanical (QM) methods become infeasible. Specifically, when systems are non-periodic and plane wave approximations fail, such as in amorphous structures, or in regimes between where the plane wave bulk approximation and the gas phase single molecule QM methods where the scaling of conventional gas phase atomic orbital methods becomes exorbitantly costly and the plane wave approximation fails for open systems. Therefore, the objective of this work is to highlight the changing optoelectronic properties of molecular solids within this regime using both density functional theory and molecular mechanics. The scalability of DFT limits it to multimer systems, leaving the larger nanoscale materials to be studied using molecular mechanics. Here we have utilized a variety of dispersion sensitive functionals in order to characterize the intermolecular interactions and splitting energies in small multimers of some of the smallest oligoacene species, benzene and anthracene. Benzene and anthracene nanoclusters from 0.8 to 5.0 nm in radius have had their changes in electronic band energy calculated due to polarization using the AMOEBA force field and bulk values have also been extrapolated. AMOEBA’s explicit polarization terms allow for direct handling of the polarization energy, control of nanocluster size and shape in a regime that QM methods cannot probe efficiently, and the ability to specify the position of charge carriers in order to examine specific electronic surface behavior. Using differing DFT methods the change in the HOMO and LUMO energy from the single molecule state to multimers of the size of 10 and 12 units for anthracene and benzene respectively. The HOMO band of benzene was raised by ~0.3 eV and LUMO lowered by 0.35 eV. In anthracene the HOMO was lowered by ~0.1 eV and the LUMO by ~0.15 eV. These values remain within 0.1 eV across all dispersion functionals. Using Ren’s parameterization procedure and MP2 for the AMOEBA force field he apparent polarization was calculated. The extrapolated values for the change in the HOMO and LUMO of benzene from single molecule to bulk were 1.42 eV and 0.49 eV respectively. For anthracene the crystalline bulk changes the HOMO and LUMO by 1.34 eV and 1.16 eV respectively. The regression for bulk extrapolation also predicts that benzene clusters of 12 units will be 0.77 eV for HOMO and -0.41 eV for LUMO. Similarly for an anthracene cluster made up of 10 molecular units the apparent polarization is predicted through linear regression to be 0.58 eV for HOMO and 0.53 eV for LUMO.

This Thesis describes the systematic theoretical study aimed at prediction of the essential properties for the functional organic molecules that belong to diarylethene (DA) family of compounds. Diarylethenes present the distinct ability to change color under the influence of light, known as photochromism. This change is due to ultrafast chemical transition from open to closed ring isomers (photocyclization). It can be used for optical data storage, photoswitching, and other photonic applications. In this work we apply Density Functional Theory methods to predict 6 of the related properties: (i) molecular geometry; (ii) resonant wavelength; (iii) thermal stability; (iv) fatigue resistance; (v) quantum yield and (vi) nanoscale organization of the material. In order to study sensitivity at diode laser wavelengths, we optimized geometry and calculated vertical absorption spectra for a benchmark set of 28 diarylethenes. Bond length alternation (BLA) parameters and maximum absorption wavelengths (λmax) are compared to the data presently available from X-ray diffraction and spectroscopy experiments. We conclude that TD-M05/6-31G*/PCM//M05-2X/6-31G*/PCM level of theory gives the best agreement for both the parameters. For our predictions the root mean square deviation (RMSD) are below 0.014 Å for the BLAs and 25 nm for λmax. The polarization functions in the basis set and solvent effects are both important for this agreement. Next we consider thermal stability. Our results suggest that UB3LYP and UM05-2X functionals predict the activation barrier for the cycloreversion reaction within 3-4 kcal/mol from experimental value for a set of 7 photochromic compounds. We also study thermal fatigue, defined as the rate of undesirable photochemical side reactions. In order to predict the kinetics of photochemical fatigue, we investigate the mechanism of by-product formation. It has been established experimentally that the by-product is formed from the closed isomer; however the mechanism was not known. We found that the thermal by-product pathway involves the bicyclohexane (BCH) ring formation as a stable intermediate, while the photochemical by-product formation pathway may involve the methylcyclopentene diradical (MCPD) intermediate. At UM05-2X/6-31G* level, the calculated barrier between the closed form and the BCH intermediate is 51.2 kcal/mol and the barrier between the BCH intermediate and the by-product 16.2 kcal/mol. Next we investigate two theoretical approaches to the prediction of quantum yield (QY) for a set of 14 diarylethene derivatives at the validated M05-2X/6-31G* theory level. These include population of ground-state conformers and location of the pericycylic minimum on the potential energy surface 2-A state. Finally, we investigate the possibility of nanoscale organization of the photochromic material based on DNA template, as an alternative to the amorphous polymer matrix. Here we demonstrate that Molecular Dynamic methods are capable to describe the intercalation of π-conjugated systems between DNA base pairs and accurately reproduced the available photophysical properties of these nanocomposites. In summary, our results are in good agreement with the experimental data for the benchmark set of molecules we conclude that Density Functional Theory methods could be successfully used as an important component of material design strategy in prediction of accurate molecular geometry, absorption spectra, thermal stability of isomers, fatigue resistance, quantum yield of photocyclization and photophysical properties of nanocomposites.

This dissertation describes the development and testing of modified embedded atom method (MEAM) interatomic potentials for Al, Si, Mg, Cu, Fe, and their alloys, with primary concentration on Mg-Al system. We performed the density functional theory (DFT) based ab initio calculations to determine the structural and elastic properties of element pairs that are impractical to obtain from experimental measurements. Specifically, we estimated the cohesive energy, equilibrium atomic volume, bulk modulus, and elastic moduli of every element pair in the NaCl reference structure. Based on the results of DFT calculations, MEAM parameters for each element pair were constructed. We extensively tested the new MEAM potential for Mg-Al alloy system. The new Mg-Al MEAM potential was compared with DFT calculations, previously published semi-empirical interatomic potentials, and experiments. Applicability of the new MEAM potential to atomistic modeling was demonstrated by calculating stress-strain responses from molecular dynamics (MD) simulations of Mg and Al systems in a variety of configurations. The effects of alloying, porosity, and strain rate conditions on the stress-strain response were quantified. The underlying mechanisms for tension-compression asymmetry observed in the macroscale experiments of Mg alloys were investigated at the nanoscale. This work presents a contribution to the task of bridging quantum-mechanical and classical atomistic scale simulations. Information from ab initio electronic structure calculations was used to construct parameters of semi-empirical MEAM potentials for large-scale atomistic simulations of alloys. The results of the new MEAM models compare extremely well to those from other published interatomic potentials. The applicability of the new MEAM potential to investigate nanoscale mechanisms of the deformation and fracture for Al, Mg and Mg-Al alloys was demonstrated. It has been shown that the MEAM provides a single universal formalism for classical atomistic simulations of a wide range of elements and their alloys.

No description available.

No description available.

No description available.

No description available.

No description available.

No description available.

No description available.