Theoretical and computational study in understanding optoelectronic properties of conjugated polycyclic hydrocarbons within correlated model Hamiltonian

Karmakar, Naomi (2023) Theoretical and computational study in understanding optoelectronic properties of conjugated polycyclic hydrocarbons within correlated model Hamiltonian. PhD thesis, Indian Institute of Science Education and Research Kolkata.

Text (PhD thesis of Naomi Karmakar (16RS039)) 16RS039.pdf - Submitted Version Restricted to Repository staff only Download (3MB)

Abstract

In recent times research interest has focused on the electronic structure of the π conjugated molecules attributable to their applications such as photovoltaic devices, Light Emitting Diodes (LEDs), Field Effect Transistor (FET) and solar cells. Moreover, over the last few decades polycyclic aromatic hydrocarbons (PAHs) have been extensively studied due to its wide applications in the field of designing singlet fission (SF) materials for improving the efciency of organic solar cell (OSCs). The relative excited state energy ordering of the lowest transition dipole allowed singlet state (S₁) and lowest triplet state (T₁) is a deciding factor for such applications. A very small ΔST (S₁ - T₁) gap aids in the enhancement of the effciency of the OLED devices. On the other hand there are certain energy conditions necessary for singlet fission mechanism that enhances the effciency of solar cell. The thesis contains eight chapters in total. We summarize briefy the contents of each chapter below. The first chapter is devoted to explaining the quantum mechanical many body techniques used in calculating the ground state and the low-lying excited states of the conjugated molecules. It describes the Pariser-Parr-Pople (PPP) model Hamiltonian approach used in all our calculations. It gives a detailed explanation of the methods used to calculate the electronic structure properties of the molecules namely the Atomic Orbital Valence Bond (AOVB) and the Density Matrix Renormalization Group (DMRG) technique. It also explicitly discusses about the Singlet Fission (SF) process which holds the potential to increase the effciency of photovoltaic devices by surpassing the Shockley-Queisser limit of solar cells. It is a multiple exciton generation process where a singlet exciton gives rise to two triplet excitons residing in two neighbouring molecules. As a result the process of intermolecular SF (xSF) requires close crystal packing and proximity which is difficult to achieve as there are defects and dislocations in reality. The problem can be overcome by designing molecular dimer as chromophores where the triplets generated resides in the same molecule. The process is known as intramolecular SF (iSF). A thorough understanding of the above procedures and the energy level criteria required for SF to occur are extensively discussed in this chapter. On the other hand, to enhance the efficiency of the OLED devices the relative energy difference ΔST in these molecules is desirable to be very small so that the singlet states can be repopulated by the nonemissive triplet excitons through Reverse Intersystem Crossing (RISC). This chapter also gives an elaborate description of the above process. In the second chapter, the low-lying excited states of the isomers of stilbene and phenanthrene are investigated under the in uence of donor-acceptor substitution as well as rotation around central vinyl bond. Both exact diagonalization (ED) technique within the model Pariser-Parr-Pople (PPP) Hamiltonian and EOM-CCSD have been implemented for our calculations. Suitable donor-acceptor substitutions show considerable effect on low-lying excitations over geometrical change due to rotation in these systems. Calculated excited state energies based on model Hamiltonian match well with experiments over EOM-CCSD. Study shows some of these substituted chromophores experiences a considerable drop in ΔST value and may find potential applications in OLED devices. In the third chapter, the low-lying excited states of α,ω-Diphenylpolyenes namely 1,6- diphenyl-1,3,5-hexatriene (DPH), 1,8-diphenyl-1,3,5,7-octatetraene (DPO), 1,10-diphenyl- 1,3,5,7-decapentaene (DPD) molecules and their derivatives are calculated within Pariser- Parr-Pople (PPP) model Hamiltonian implementing DMRG technique. The calculations reveal that all three molecules are promising candidates for SF mechanism. Excited state energies in singlet subspace have been calculated for 1,6-diphenyl-1,3,5-hexatriene (DPH) comprising 18 carbon atoms within exact diagonalization method which are in excellent agreement with results obtained in DMRG technique. Moreover, calculations on α,ω-diphenyl-μ, ν-dicyano oligoenes (DPDCs) show with increasing chain length, these derivatives support endoergic to isoergic SF. Until now, a handful of molecules are designed to exhibit intramolecular singlet fission (iSF). In the fourth chapter, calculations on low-lying excited states of covalently linked para-phenylene bridged and meta-phenylene bridged tetracene and pentacene dimers and their electron donor substituted conformers in the singlet and triplet manifold are carried out in understanding their photophysical properties. These excited states have been studied using the Density Matrix Renormalization Group (DMRG) technique within the Pariser-Parr-Pople model Hamiltonian on these acene dimers. DMRG calculations on four oligoacene dimers exhibit that relative energy ordering of the singlet and triplet excited states satisfies the required energy criteria for inter SF mechanism and these dimers can be designed to undergo the iSF mechanism. In the fifth chapter, we employed Density Matrix Renormalization Group (DMRG) technique within correlated Pariser-Parr-Pople (PPP) π electron model Hamiltonian to study and compare these low-lying excited states of polycyclic aromatic hydrocarbons (PAH) isomers containing 22, 30 and 38 carbon atoms such as isomers of pentaphene, heptaphene and nonaphene. We found that the first energetic criterion for singlet fission (SF) is satisfied for all these molecules except picene. Conjugated microporous polymers based on thiophene (ThPOPs) have high porosities and very high specific surface area. Thus, they act as excellent material for gas adsorbtion and gas storage. In the sixth chapter, we have considered two such molecules namely tetrathienylethylene (TTE) and tetrathienylbenzene (TTB) which are thiophene containing star shaped molecules. The low-lying excited states of TTE and TTB have been studied employing the Density Matrix Renormalization group (DMRG) technique within the Pariser-Parr-Pople (PPP) correlated π electron model Hamiltonian. In this work, DMRG method has been implemented successfully in calculating the electronic structure in these star shaped quasi one-dimensional TTE and TTB molecules. Relative energy ordering of ground state (S₀) and low lying excited states mainly lowest singlet (S₁) and the lowest triplet excited state (T₁) exhibit that TTE and TTB can act as singlet fission (SF) material and as an Organic Light Emitting Diode (OLED) respectively. The study has been further extended to calculate the low-lying excited states in donor-acceptor (D-A) substituted TTE and TTB for comparison with the pristine molecule. In the seventh chapter, we performed a comparative study on the low-lying excited states of acenedithiophenes namely Benzodithiophene (BDT), Naphthodithiophene (NDT) and Anthradithiophene (ADT) with that of anthracene, tetracene and pentacene molecules respectively. We study the changes in optical properties upon replacement of the phenyl units in anthracene, tetracene and pentacene molecules at the two ends with thiophene units. The low-lying excited states of anthracene, BDT and NDT have been investigated using the AOVB technique. In case of ADT molecule, DMRG calculation have been performed. The calculations for tetracene and pentacene are discussed in Chapter 4 of this thesis. The study shows ADT to satisfy the best SF criteria over others. In the eighth chapter we discuss the future work that can be done based on the results obtained in all the previous chapters.

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