A theoretical study of carbon-based functionalized materials
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This dissertation will examine the electronic properties of several materials used in state-of-the art technology and ongoing scientific investigation by means of high-level ab initio quantum chemical calculations. In this way, the application of such calculations to graphene reactivity and defects, organic photovoltaic processes, and excited states and biradical character of polycyclic aromatic hydrocarbons (PAH) is presented. Graphene reactivity and defect characterization is currently the source of intense scrutiny. The reactivity of pristine graphene and single and double carbon vacancies toward a hydrogen radical was assessed using complete active space self-consistent field theory (CASSCF) and multireference configuration interaction singles and doubles (MR-CISD) calculations using a pyrene model. While all carbon centers formed stable, novel C-H bonds, the single carbon defect was found to be most stabilizing thanks to the dangling bond at the six-membered ring. Bonding at this carbon center shows interaction with several closely-spaced electronic excited states. A rigid scan was also used to show the dynamic landscape of the potential energy surfaces of these structures. Double carbon vacancy pyrene and circumpyrene were used to investigate the structure and electronic states of an intrinsic silicon impurity in graphene by density functional theory (DFT) methods. The ground state was found to be low spin, non-planar and D2 symmetric with a nearby low spin, planar state with D2h symmetry. Characterization with natural bond orbital (NBO) analyses showed that the hybridization in the non-planar structure is sp3 and sp2d in the planar structure. Charge transfer from the silicon dopant to the surrounding carbon atoms is demonstrated via analysis with natural charges and molecular electrostatic potential (MEP) plots. Experiment shows drastically enhanced power conversion efficiency (PCE) in a bulk heterojunction (BHJ) device consisting of poly(3-hexylthiophene-2, 5-diyl) (P3HT), terthiophene (T3), and fullerene (C60). This BHJ scheme was investigated using DFT and several environmental models. The state-specific (SS) environment was found to best reproduce the experimental findings showing that initial photoexcitation of a local, bright exciton at P3HT was able to decay through several charge transfer (CT) bands of P3HT→C60 and T3→C60 character explaining the increased PCE. The spectra calculated for the isolated system and linear response (LR) environment were very similar to each other and showed the initial photoexcitation at P3HT decayed instead into bands of local C60 excitonic states quenching the CT states. Crucial benchmark calculations of the structure and electronic excited states missing from the current literature were performed for oligomers of poly(p-phenylenevinylene) (PPV) dimers using high-level scaled opposite-spin (SOS) second-order Møller-Plesset (SOS-MP2) and second-order algebraic diagrammatic construction (SOS-ADC(2)) calculations. Comparisons were also made to DFT methods. The dimer structures were studied in two conformations: sandwich-stacked and displaced; the latter exhibited greater stacking interactions. Good agreement was found between the cc-pVQZ and aug-cc-pVQZ basis sets and the cc-pVTZ basis. Comparisons of the interaction energies were made to the complete basis set limit. Basis set superposition error was also accounted for. The excited state spectra presented a varied and dynamic picture that was analyzed using transition density matrices and natural transition orbitals (NTOs). The assessment of the biradical character of several trans-diindenoacenes, cis-diindenoacenes, and trans-indenoindenodi(acenothiophenes) was accomplished utilizing the multireference averaged quadratic coupled-cluster (MR-AQCC) method. These systems have been previously identified as highly reactive in experiments, often subject to unplanned side-reactions. The biradical character was characterized using the singlet-triplet splitting energy (∆ES-T), effective number of unpaired electrons (NU), and unpaired electron density. For all structures, low spin ground states were found. In addition, increasing the number of core benzene rings decreased the ∆ES-T, increased the NU, and increased the unpaired electron density. Good agreement to experimental values for the ∆ES-T is also demonstrated.