Computational investigation on polycyclic aromatic hydrocarbons in the molecular and solid phases

In this Thesis we discuss the effects of specific chemical functional- ization and partial/complete atomic substitution on the electronic (ground-/excited-state) and charge-transport properties of small or- ganic compounds of interest for molecular electronics. In particular, we considered several Polycyclic Aromatic Hydro- carbons (PAHs) with different morphologies (small-compact, com- pact, angular and linear). For these molecules we study the effects of complete substitution of the peripheral H atoms with halogens (F and CL), the functionalization with Triisopropylsilylethynil (TIPS) group, and the partial substitution with chalcogen (S in particular) atoms on several physical properties. In the first part of this work we report a systematic comparative study on dibenzo[b,def]chrysene (angular) and dibenzo[def,mno]chrysene (compact) polyaromatic hydrocarbons and their bis-triisopropylsilylethynyl (TIPS)-functionalized and per- halogenated (F, Cl) counterparts. We used all-electrons density functional theory(DFT)and time- dependent DFT(TDDFT)to quantify the effects of morphology and chemical modifications ondifferent physical observables, namely electron affinity,ionization energy,quasi-particleenergy gap,optical absorption,excitonbindingenergy, and molecular reorganization energies for holesand electrons.For thispart of thework we used the hybrid exchange-correlation functional B3LYP in conjunction with a Gaussian localized basis-set.This adopted combination functional/basis-set has proven to yield good results for polyaromatic hydrocarbons and derivatives. In the second part of the work we used the same theoretical frame- work (DFT and TDDFT), to study the electronic, optical, and charge- transport properties of the hexathiapentacene (HTP) molecule. HTP is a derivative of pentacene (PNT) obtained by symmetric substi- tution of the six central H with S atoms. We discuss in a compar- ative way the key molecular properties of HTP and PNT. In par- ticular, electron affinities, ionization energies, quasi-particle energy- gaps, optical absorption spectra, exciton binding energies, and re- organization energies for holes and electrons are calculated for the molecules and compared with the corresponding results for PNT, as well as with the available experimental data. The DFT and TDDFT results are also validated by performing many-body perturbation theory calculations within the GW and Bethe-Salpeter equation for- malisms. In addition, for the crystal structures of PNT and HTP we perfomed DFT-based calculations using a pseudopotentials+plane- waves formalism and adopting the PBE exchange-correlation func- tional empirically corrected in order to take properly into account dispersive interactions.