Theoretical study of the structural and electronic properties of oligothiophene and pentacene systems

Resumen

Oligoacenes and oligothiophenes are two of the most promising families of organic semiconductors due to their application in thin-film organic electronic devices, such as organic light-emitting diodes (OLEDs), organic solar cells and organic field-effect transistors (OFETs). This Thesis tackles the study of the structural, vibrational, electronic, optical and redox properties of different families of pi-conjugated materials (oligothiophenes, oligothienoacenes and pentacenes) that play a relevant role in the field of organic electronics by using quantum-chemical calculations. Our aim is to establish precise structure-property relationships which are crucial to the design of novel and enhanced pi-conjugated materials. This Thesis is divided in seven chapters. In the first chapter, the relevance and applications of oligoacenes and oligothiophenes are put into context. The valuable information that quantum-chemical calculations can provide in the characterization of these pi-conjugated compounds is also highlighted. The main goal of the Thesis and the systems studied are indicated at the end of this chapter. In the second chapter, a brief description of quantum chemistry methodologies mainly used in this Thesis is given. The chapter continues with the equations and steps needed to simulate the absorption and fluorescence spectra at room and low temperature. Finally, a revision of the modern quantum chemistry methods capable of taking into account pi-pi interactions and predicting reliable charge-transfer excitations in molecular systems is done. In the third chapter, a theoretical characterization of the structural, vibrational and electronic properties of two families of carbonyl-functionalized quaterthiophenes and diferrocenyl quaterthiophenes is presented. The main goal is to analyze the effect of the substitution pattern on the properties studied. The fourth chapter presents a theoretical analysis of the structural, vibrational, electronic and optical properties of a series of fully-fused thiophene systems. The chapter is divided into three sections: i) Oligothioenoacenes versus Oligothiophenes, where the impact of backbone rigidity on the optical properties of thiophene-based compounds is studied by analyzing in detail the geometrical, electronic, optical and vibronic features of a family of oligothienoacenes in comparison with non-fused oligothiophenes; ii) Neutral and Oxidized Triisopropylsilyl End-Capped Oligothienoacenes, which focuses on the changes induced by oxidation on the structural and optical properties of a whole family of oligothienoacenes with triisopropylsilyl terminal groups, and iii) Octathio[8]circulene: A Peculiar Oligothienoacene, where the structural and optical properties of this molecule are compared with those obtained for its oligothienoacene counterpart (heptathienoacene). In the fifth chapter, a combined theoretical and experimental characterization of the structural, vibrational and optical properties of a family of functionalized pentacenes is presented. Emphasis is made on the effect of the functional group in the structural and optical properties. In the sixth chapter, a thorough quantum-chemical characterization of the terthiophene-tetracyanoquinodimethane (3T-TCNQ) molecular complex is performed. Our aim is to evaluate the performance of different density functionals to account for the pi-pi interactions and to predict charge-transfer excitations in the 3T-TCNQ complex. Theoretical calculations have revealed that the wB97X-D functional is able to accurately treat pi-pi intermolecular interactions and charge-transfer excitations in a balanced way, for the 3T-TCNQ complex, thanks to the combination of dispersion and long-range corrected terms. Hence, the wB97X-D functional might be reliably used to further investigate pi-pi donor-acceptor complexes. Finally, the last chapter lists the main conclusions attained in this Thesis.