As humanity moves into the future, the demand for new electronic devices increases. Flexible electronics that could be bendable, wearable, and/or biocompatible are more desired and, fortunately, closer to our grasp than ever. In order to produce these new devices, electronic materials not based on ridged, brittle crystals are needed. One candidate for these new electronics are organic electronic materials. Organic electronic materials have the potential to lead to devices that are flexible, simple to produce and that can take advantage of state-of-the-art processes like non-linear optics, spintronics and singlet fission. In order to access these exciting new devices, however, a better understanding of the type of conjugated organic molecules on which they will be based is needed. This dissertation explores the expansion of the indenofluorene project from a rotation student’s small spin off to an examination of a full class of materials. First, this document details the synthesis of donor-acceptor-donor triads using indenofluorene starting material, the dione, as the acceptor portion. What follows is an in-depth examination of the aromatic and antiaromatic properties of the class of materials we deemed diarenoantiaromatics. The computational techniques used are expanded along with the antiaromatic core of each molecule in order to evaluate the diradical character of the core expanded molecules being synthesized by my lab mates. Finally, the synthesis and characterization of a nine ring, linear dianthracenoindacene and the progress toward the thirteen ring dipentacenoindacene isomers are described. / 10000-01-01

The electronic state ordering is calculated in substituted meta-xylylene diradicals for reproduction and comparison of results previously published by other authors. The second hyperpolarizability, γ, is calculated in the same systems using different methods. The diradical character is calculated for comparison and discussion of the γ values. The obtained electronic state ordering agrees qualitatively with previous findings. The γ values obtained from the finite-field method, which are related to the diradical character, agree with theory. The γ values obtained from the sum-over-states method, which relate to the vertical state ordering, are unsatisfactory.

<p>The accurate description of ground and electronic excited states is an important and challenging topic in quantum chemistry. The pairing matrix fluctuation, as a counterpart of the density fluctuation, is applied to this topic. From the pairing matrix fluctuation, the exact electron correlation energy as well as two electron addition/removal energies can be extracted. Therefore, both ground state and excited states energies can be obtained and they are in principle exact with a complete knowledge of the pairing matrix fluctuation. In practice, considering the exact pairing matrix fluctuation is unknown, we adopt its simple approximation --- the particle-particle random phase approximation (pp-RPA) --- for ground and excited states calculations. The algorithms for accelerating the pp-RPA calculation, including spin separation, spin adaptation, as well as an iterative Davidson method, are developed. For ground states correlation descriptions, the results obtained from pp-RPA are usually comparable to and can be more accurate than those from traditional particle-hole random phase approximation (ph-RPA). For excited states, the pp-RPA is able to describe double, Rydberg, and charge transfer excitations, which are challenging for conventional time-dependent density functional theory (TDDFT). Although the pp-RPA intrinsically cannot describe those excitations excited from the orbitals below the highest occupied molecular orbital (HOMO), its performances on those single excitations that can be captured are comparable to TDDFT. The pp-RPA for excitation calculation is further applied to challenging diradical problems and is used to unveil the nature of the ground and electronic excited states of higher acenes. The pp-RPA and the corresponding Tamm-Dancoff approximation (pp-TDA) are also applied to conical intersections, an important concept in nonadiabatic dynamics. Their good description of the double-cone feature of conical intersections is in sharp contrast to the failure of TDDFT. All in all, the pairing matrix fluctuation opens up new channel of thinking for quantum chemistry, and the pp-RPA is a promising method in describing ground and electronic excited states.</p> / Dissertation

Les carburants dérivés du pétrole constituent la première source mondiale énergétique et leur approvisionnement constitue un défi actuel majeur impliquant des enjeux économiques et environnementaux cruciaux. Une des voies les plus efficaces pour peser simultanément sur ces deux enjeux passe par la diminution de la consommation en carburant. La simulation numérique constitue dès lors un outil précieux pour améliorer et optimiser les moteurs et les carburants. Les modèles chimiques détaillés sont nécessaires pour comprendre les phénomènes d’auto-inflammation et caractériser la nature et les quantités de polluants émis. Ces modèles mettent en jeu un nombre très important d’espèces et de réactions élémentaires, pour une espèce donnée et pour lesquelles la détermination des données thermodynamiques et cinétiques est un problème crucial. La chimie quantique constitue un outil précieux permettant d’une part de déterminer de façon précise les données thermocinétiques pour bon nombre de systèmes chimiques et d’autre part de mieux comprendre la réactivité de ces systèmes. Dans ce travail, les réactions unimoléculaires de décomposition d’hydrocarbures monocycliques et polycycliques (amorçages, réactions moléculaires, ß-scissions, formations d’éthers cycliques) ont été étudiées à l’aide des méthodes de la chimie quantique. Un mécanisme détaillé de pyrolyse d’un alcane polycyclique a été développé à partir des données thermodynamiques et cinétiques et des corrélations entre structure et réactivité déterminées pour les cyclanes à partir des calculs quantiques. Les simulations effectuées à partir de ce modèle sont en très bon accord avec les résultats expérimentaux de la littérature / Petroleum fuels are the world’s most important primary energy source and the need to maintain their supply is a major actual challenge involving both economical and environmental features. Decreasing fuels consumption is one of the more efficient ways to reconcile the goals of energy price and environmental protection. Numerical simulations become therefore a very important tool to optimize fuels and motors. Detailed chemical kinetic models are required to reproduce the reactivity of fuels and to characterize the amount of emitted pollutants. Such models imply a very large number of chemical species and elementary reactions, for a given species, and the determination of thermodynamic and kinetic data is a critical problem. Nowadays, quantum chemistry methods are able to calculate accurately thermodynamic data for a large number of chemical systems and to elucidate the reactivity of these systems. In this work we have used quantum chemistry to study the unimolecular reactions (initiation, molecular reactions, ß-scissions, cyclic ethers formations) involved in the decomposition of monocyclic and polycyclic hydrocarbons. From the results of quantum chemical calculations, a detailed chemical kinetic mechanism of the pyrolysis of a polycyclic alkane has been developed and validated against experimental data

Organic conjugated systems are attractive because of wide range of applications, which includes stimulated emission from -conjugated polymers, optical switches, organic solar cells and organic light emitting diodes, to name a few. They have the advantage of low cost, ease of processing and tunability of their linear and nonlinear optical properties by functionalization with donor or acceptor groups. In chapter 1, we provide an introduction to the π-conjugated systems and various interesting phenomena observed in these systems, This is followed by a brief description of the application of the above mentioned -conjugated systems for organic light emitting diodes, and organic photovoltaic cells. In the last section of this chapter, we give an introduction to magnetism due to π-electron systems. In chapter 2, we begin with energy band theory in one-dimension and its drawbacks. We introduce various model Hamiltonians which incorporate electron-electron inter-actions like Hubbard model, and Pariser-Parr-Pople (PPP) model. We present numerical techniques like valence-bond (VB) and constant MS techniques that are used to exactly solve the above model Hamiltonian. This is followed by an introduction to density matrix renormalization group method (DMRG) employed for the above model Hamiltonian for larger system in one-dimension and quasi-one-dimension. We give description of linear and nonlinear optical properties followed by the oriented gas model for ensemble of molecules. Various methods for computing polarizabilities and hyperpolarizibilities of molecules includes such as Finite-Field method, Sum-Over-State Method, and Correction Vector (CV) Methods are described in detail. In chapter 3, we look into fused azulene systems as a possible organic multiferroics. Azulene molecule with fused five and seven membered π-conjugated rings has a dipole moment, and the π-framework has geometric frustration. Hence in fused azulenes we can expect both ferroelectric and magnetic ground state. To explore this, we study low-lying correlated electronic states of fused azulenes using the long-range interacting PPP model and the finite DMRG method. The ground state is a singlet for oligomers up to 5 azulene units. For oligomers with more than 5 azulene units and up to 11 azulene units the oligomers have a triplet ground state. From the excitation gaps between the lowest MS = 0 state and the lowest states in MS=1, 2, and 3 sectors we predict that the ground-state spin of the fused azulene increases with the number of azulene units. In the thermodynamic limit, we expect the fused azulene to be a ferromagnet. Charge density calculations show that the ground state of the system has ferroelectric alignment of the dipoles of the monomeric units. Thus, a fused azulene system could be the first example of an organic molecule which is both ferromagnetic and ferroelectric, in the ground state. In chapter 4, we study the linear and nonlinear optical properties of diradical systems. We have studied linear and non-linear optical properties of π-conjugated diradicals because they are expected to exhibit large non-linear responses. The system studied are oligomers of dicyclopenta-fused acenes (DPA) and the s-indaceno[1,2,3-cd;5,6,7-c'd']diphenalene (IDPL) molecule. Spin-spin correlation functions within a correlated PPP model Hamiltonian, using exact diagonalization method, are used to characterize the diradical nature of DPA-2 and similar calculations on Anthracene have been performed to contrast this with a singlet character. The diradical character of DPA-2 is also manifest as low optical gap, low spin gap and large THG coefficients compared to Anthracene molecule. Larger DPA-k, k > 2, oligomers as well as the IDPL molecule have been studied within the DMRG technique. In the DPA-4, we nd a very small spin gap (0.04 eV), while in the oligomers with k > 4, we nd that the ground state is degenerate with the lowest triplet state. The energy of the second excited triplet state decreases with increasing size k and seems to saturate at ~0.36 eV in the thermodynamic limit. The lowest optical gap in DPA-4 is at 1.94 eV and has large transition dipoles, while for DPA-k, 4 < k ≥ 28, we have not been able to access states with large transition dipoles. The weak low-energy excitations seem to saturate at 0.5 eV and the two-photon gap also seems to be saturating at~ 0.3 eV in the thermodynamic limit. These polymers will not be IR uorescent by Kasha rule. The dominant component of the THG coefficient, γxxxx, is highest for DPA-4 which reduces almost by an order of magnitude in DPA-8; for k > 8 it increases up to the largest system with k=20 for which we have computed the coe cient. The variation of the charge gap of DPA oligomer with the increase in system size is small and in the polymer limit the charge gap is 4:24 eV. For IDPL molecule spin gap is 0.20 eV and next excited triplet state is at 1.48 eV. Two lowest singlet states in B space are nearly degenerate and have large transition dipole moments. Optical gaps to the above states are 2.20 eV and 2.22 eV. Two-photon gap in this system is 1.29 eV, hence this system is also non- uorescent. We calculated the dispersion of the major component of the THG coefficient, γxxxx, over a wide frequency range for this molecule, and we observe resonances corresponding to the 21Ag and 11Bu states. Extrapolated value of γxxxx at zero frequency is 15:58 x 106 a.u which is very large and the system does not have any donor or acceptor substituent groups. In chapter 5, we study absorption spectra and two photon absorption coefficient of expanded porphyrins (EPs). We nd that in the 4n+2 EPs there are two prominent low-lying one-photon excitations while in 4n systems there is only one such excitation. The two-photon gaps in both these types of systems are at energies close to the one-photon excitations. The spin gap in 4n+2 EPs are very small although the spin-1/2 Heisenberg calculations show that a pure spin system in this geometry will not have vanishing spin-gap. The charge density rearrangement in the one-photon excited state is most at the aza nitrogen site and at the meso carbon sites. In the two-photon states also the charge density rearrangement occurs mostly at the aza-ring sites. The bond order changes in these states is much more striking. In the one-photon state, the C-C bond length in the aza rings show a tendency to become uniform. Similar qualitative trend is also observed for the two-photon state. In chapter 6, we study linear and nonlinear optical properties of two push-pull polyenes stacked in head to head (HtH) and head to tail con gurations (HtT), at different stacking angles, exactly within the PPP model. Varying the stacking angle between polyenes, we nd that the optical gap varies slightly, but transition dipoles show large variation. The dominant component of first-order hyperpolarizability, βxxx for HtH and βyyy for HtT arrangement strongly depend on the distance between molecules. The βxxx for HtH configuration shows a maximum at a nonzero stacking angle, which varies with inter polyene distance. ZINDO study on two monomers, (4-hydroxy-40-nitro-azobenzene) connected by a conjugated bridge shows that βav is more than twice the monomer value and with a red-shift in the optical gap. In chapter 7, we have calculated the shifts in optical gaps and band edges as a function of the distance between two monomers within a correlated PPP model Hamiltonian for various stacking geometries. We have used as model monomers, both unsubstituted polyenes and push-pull substituted polyenes. We have carried out calculations with and without inter-chain hopping between sites on different molecules. We note that in the absence of inter-chain hopping, the energy level shifts are almost independent of the distances between the chains in all stacking geometries. It is also interesting to note that only electron-electron interactions yield a blue shift in the optical gaps for parallel stacking, but red shift in the gap for all other stacking geometries. We note that most of the shift in the gap is due to shifts in the excited state energy and the ground-state energy remains almost the same. With interchain transfer the shift in the optical gap increases with decrease in the interchain distance. We observe red-shifts in parallel stacking geometry when inter-chain electron hopping is turned on, at small interchain separations. In general interchain hopping increases significantly the red shift in the optical gaps for all geometries. Even for push-pull polyenes of | e| =2.0 eV, we observe the same trend in the shift in the optical gap for various stacking geometries. In this case the shift in optical gap is an order of magnitude higher when interchain hopping is turned on compared to that in the absence of interchain hopping. We find that the optical gap shifts are largest for the parallel stacking geometry, and it also shows stronger distance dependence. This is in close conformity with experimental observation of red-shift in absorption maxima when hydrostatic pressure is applied on the system. The shift in the HOMO (LUMO) level is small in the absence of t?, and the largest shift is in the case of parallel stacking compared to other stacking geometries. The distance dependence of the HOMO shifts is also rather weak. When t? is turned on, the level shifts become large by a factor of five or more. When we have push-pull groups electron-hole symmetry is broken and the shift is different for the HOMO and the LUMO level. Depending upon stacking geometry, the HOMO shifts vary from 0:1 ~ eV to 0.3 eV, which is larger than the shifts observed in unsubstituted polyenes. This large shift in the LUMO reduces the efficiency of exciton dissociation.