Theoretical and computational investigations on linear and non-linear optical, electronic properties in organic conjugated molecules

Naskar, Sumit (2020) Theoretical and computational investigations on linear and non-linear optical, electronic properties in organic conjugated molecules. PhD thesis, Indian Institute of Science Education and Research Kolkata.

Text (PhD thesis of Sumit Naskar (14RS060)) 14RS060.pdf - Submitted Version Restricted to Repository staff only Download (16MB)

Abstract

Conjugated molecules attract immense research interest in recent times in the field of organic semiconductors, field effect transistors (FETs), light emitting diodes (LEDs), non-linear optoelectronics as well as molecular electronics. These optoelectronic properties in such conjugated molecules are predominately guided by the energy ordering of the low-lying singlet and triplet excited states with respect to ground states. In organic light emitting diode (OLED) applications, the generation of a photon due to the recombination of an electron and a hole ensures the formation of 25% of singlet and rest 75% triplet excitons following the spin statistics in noninteracting limit. Such comparatively low singlet yield will limit the effciency of OLEDs as emissive transition occurs only from the lowest transition dipole allowed excited singlet state(S₁) according to Kasha's law. Relative energetic location of lowest singlet(S₁) and lowest triplet(T1) excited state and their energy difference (ΔST )plays an important role in determining the optical properties of these conjugated molecules. Very small ΔST value helps non-emissive triplet excitons to repopulate themselves to singlet excitons through reverse intersystem crossing (RISC) to enhance the effciency of OLEDs. On the other hand, significantly large ΔST is desirable in the perspective of Singlet Fission (SF), a multiexciton generation process. In SF, one singlet exciton splits into two triplet excitons depending on energy conditions discussed in Chapter1 in details. The two triplets formed in the fission process can be considered as an overall singlet, thus SF process is spin allowed and can occur quickly. For intermolecular SF (xSF) coupled ¹(TT) state is located on both nearest neighbor chromophores. Occurrence of intermolecular SF process (xSF) depends on the suitable crystal packing of the molecular chromophores that can be circumvented through designing suitable intramolecular SF (iSF) molecule using molecular dimers. As multiple excitons are generated is SF, thus it can surpass the theoretical Shockley-Quieser limit of photoconversion efficiency (30%). Besides linear optical properties this thesis also deals with calculations of molecular conductance and non-linear optoelectronic properties of these molecules. Molecular charge transport calculations in single molecular device help us to predict the applicability of conjugated molecules as efficient diodes, rectifiers. In brief, we summarize the content of each chapter below. This thesis deals with eight chapters. The first chapter describes brie y some quantum mechanical correlated many body techniques to calculate the ground and low-lying excited states of conjugated molecules. It elaborately discusses the model Pariser-Parr-Pople Hamiltonian approach to compute the low-lying excitations of these systems. Exact diagonalization and an efficient Density Matrix Renormalization Group (DMRG) techniques are also discussed in this chapter to calculate the electronic structure. The second chapter of this thesis deals with ground and low-lying excited state calculations of cyclopenta-fused polycyclic aromatic hydrocarbons (CP-PAHs) using exact diagonalization technique within model Pariser-Parr-Pople Hamiltonian as well as time-dependent density functional theory (TDDFT) technique. The CP-PAHs include acenapthylene, isomers of pyracylene, cyclo-octa-pentalene and three isomers of dicyclo-pentacyclo-octenes (DCPCO). We used the inherent symmetries of these systems to calculate the energy ordering of the lowest singlet(S1) and lowest triplet excited(T1) states with respect to the ground state(S₀). The calculation shows that the lowest dipole allowed singlet absorption varies from 0.43 eV to 1.42 eV for most of these systems. Such optical absorption range is very promising in harvesting solar radiation ranging from visible to near infrared (NIR) region improving the effciency of photovoltaic device applications. The calculated optical gap for pyracylene, acenapthylene and two isomers of DCPCO are in very good agreement with experimental results reported in literature. The calculated S₁-T₁ energy gaps(ΔST ) in cyclo-octa-pentalene and in DCPCO isomers are very small ranging from 0.01 to 0.2 eV which is highly desirable in improving their electroluminescence efficiency in light emitting device applications. The low-lying excited states in oligoquinanes, a multiple fused five membered ring compounds, aryl fused oligoquinanes and their derivatives are investigated within the correlated PPP model Hamiltonian in the third chapter of this thesis. The numerical techniques in solving the low-lying excited states in singlet and triplet subspaces involve exact diagonalization for smaller molecules and Density Matrix Renormalization Group (DMRG) for comparatively larger molecules. The calculated low-lying singlet excited states are in very good agreement with the experimental values reported in literature. The lowest dipole allowed singlet excited state (S₁) calculated for most of the pentalene derivatives except 5,10 dihydroindolo[3,2-b]indole lies above the optically dark two-photon state resulting the molecules to be non-fluorescent. The energy ordering of S₁ and lowest triplet energy state (T₁) for pentalene and dicyclopenta[a,e]pentalene satisfies two energy criteria for intermolecular multiexcitonic SF process to occur. However, two thienocene-fused pentalene derivatives are found to satisfy only the first energy criteria of SF. The fourth chapter of this thesis considers S-indacene, indacenodithiophene derivatives and indeno uorene along with higher-order homologs to investigate the possibility of their applicability in optoelectronics. The low-lying singlet and triplet excited states are calculated within PPP model Hamiltonian for these molecules using exact diagonalization and DMRG techniques. Lowest singlet dipole allowed excited states calculated for these molecules are in excellent agreement with the experiment. S-indaceno[1,2-b:5,6-b'] dithiophene, diethno-s-indaceno dithiophene, indeno[1,2-b] uorene, and its higher-order homologs are found to be non- uorescent and show a promise in designing SF materials as they satisfy the two basic energetic criteria for SF. Calculations also show conjugated molecules with sulfur or nitrogen heteroatom substituted indeno uorene homologs are fluorescent and smallest ΔEST is found to be 0.3 eV for tertazanona cyclo tetratriaconta pentadecaene molecule. The fifth chapter deals with the low-lying excited state calculations of zethrene, its homologs, and conjugated molecules like bis-phenaleno-indeno uorene, diaryl pentaleno diacenaphthylene and pentalenodiphenalene. Density Matrix Renormalization Group (DMRG)technique has been implemented within model Pariser-Parr-Pople(PPP) Hamiltonian. DMRG calculation shows that the energy ordering of low-lying singlet and triplet excited states of most of these molecules with 4nπ electrons satisfy two essential energetic conditions related to SF. The lowest singlet and triplet excited states calculated in DMRG are in very good agreement with experimental results in the literature. Such low-lying excitations and intermediate diradical characters of these molecules calculated through projected spin unrestricted Hartree-Fock (PUHF) method establish the fact that these molecules are potential candidates for designing SF materials. In sixth chapter of this thesis, designing mechanism of intramolecular SF molecules are discussed. Indeno[1,2-b] uorene molecule itself satisfies two energetic conditions of SF. Dimer of indeno uorene molecules sandwiched through a covalent spacer is designed to act as intramolecular SF molecule. DMRG calculations suggest that all the dimers studied here satisfy the essential energy conditions for SF. Frontier orbital calculation through Density Functional Theory (DFT) depicts orbital localization of triplets on either side of the covalent spacers that supports the idea of entangled triplet-triplet state 1(TT). Here the process is intramolecular (iSF), which has many advantages over the intermolecular (xSF) process. DMRG and TDDFT calculations are in well agreement with experimental results found in the literature. Thus Indeno[1,2-b] uorene dimers can be designed for iSF applications. The seventh chapter of the thesis contains the quantum charge transport calculations at metal-molecule-metal junctions which leads to various electronic properties suitable in the field of miniaturization. Finite bias dependent conductivity is calculated through porphyrin, hexaphyrin and hexathia[26π]annulene molecular junction devices connected to metallic or semiconducting electrode using non-equilibrium Green's function (NEGF) technique along with Density Functional Theory (DFT) method. The (I - V) characteristic curves calculated for various donor-insulator-acceptor (D-ɑ-A) devices show ohmic, diode or rectifier like nature depending on the donor acceptor substitution effect in above molecules connected to electrode. The rectification ratio R (I⁺=I⁻ or I⁻=I⁺) calculated for such devices varies from 2 to 70 and maximum R is calculated for D-A substituted porphyrin molecular junction. I - V characteristics, rectification and negative differential resistance (NDR) effect found in such devices are well analyzed by projected density of states (PDOS) and molecular projected self consistent Hamiltonian (MPSH) eigenstate, local density of state calculations. Molecular conductivity calculations in D-ɑ-A devices using porphyrin, hexaphyrin and hexathia[26π]annulene show a promise in the field of molecular electronics and memory storage devices. The final chapter of the thesis we considered expanded porphyrin-based (Hexaphyrins) sensitizers which are promising due to their excellent light harvesting feature in dyesensitized solar cell (DSSC). The low-lying excitations of expanded porphyrins (EPs) as hexaphyrin and core modified hexaphyrin structures are calculated using Time- Dependent Density Functional Theory (TDDFT). Our calculation showed the EPs (both hexaphyrin and core modified hexaphyrin) have broad range of absorption band suitable for harvesting the visible and near infrared region of solar spectrum. All EPs studied here satisfy the energy condition of SF. The non-linear optical properties like first hyper polarizability β and second order hyper polarizability γ were calculated using coupled perturbed Hartree-Fock approach. From the second order NLO properties we carried out degenerate four wave mixing (DFWM) component (γ⁽²⁾(-ω; ω; ω;-ω)) and finally quadratic non linear refractive indices of these EPs are calculated. Calculation showed EPs are promising as organic dye for the opto-electronic applications and useful for high efficiency DSSC and also useful for potential NLO materials as their hyper polarizabilities showed higher order non linearities.

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