A Quantum Chemical Look at Energy and Charge Transfer in N-Heteropolycycles

Abstract

The transport of charge and energy are two essential processes in optoelectronic devices. In this thesis, using quantum chemical methods, molecular properties, as well as charge and energy transfer performance are studied in novel N-heteropolycycles. N-heteropolycycles are formally N-doped heterocyclic nanographene segments. The position and number of the nitrogen substitution, as well as further modification, can fine-tune their molecular properties such as energy levels, diradical characters, and charge and energy transfer rates. For the investigation of energy transfer, particular interest lies in singlet fission (SF), which has the potential to dramatically increase solar cell efficiency by converting one singlet exciton to two free triplet excitons or a correlated triplet pair. In chapter 3, quantum chemical methods based on DFT and constrained DFT are applied to rationalize how SF is affected by systematic chemical modifications introduced into phenazinothiadiazoles (PTD). The results indicate that unlike unsubstituted tetracene, PTDs fulfill the energetic requirement of SF (E(S_1)≥2×E(T_1)), and the effective coupling can be up to 75.8 meV. Hence, PTDs are promising candidates for SF. In chapter 4, a single-reference DFT-based protocol is proposed to simulate the absorption spectra of excited states involved in SF. The resulting spectra show good agreement with the experiment. This could be helpful for the identification of various species in SF and the understanding of SF dynamics. On the other hand, N-heteroacenes are known as electron-poor counterparts of the acenes, and they are electron transport (n-type) materials. Since the charge transport moiety in bulk films of azaacenes is thought to be the radical anion, in chapter 5, the energetics, electronic structures, and spectroscopic properties of negatively charged N-heteroacenes are investigated. It is found that the anions of the azapentacenes and their derivatives are stable with respect to electron loss and disproportionation into the dianion and the neutral compound. This motivates a further look into their electron transport properties. The results of electron transfer integrals and charge mobilities are demonstrated in chapter 6. Excellent performance of electron transport has been proved for halogenated 6,13-Diethynyl-5,7,12,14-tetraazapentacenes, especially for the bromine and iodine derivatives.