COMPUTATIONAL DESIGNING OF DONOR-LINKER-ACCEPTOR BASED THERMALLY ACTIVATED DELAYED FLUORESCENT MOLECULES

Abstract

Organic light emitting diodes (OLEDs) have gained wide attention during the last two decades due to their applications in various optoelectronic devices. They have been used in such devices to achieve low power consumption, quick response time and long operational lifetime. New types of OLEDs are designed and tested every year. According to the spin-statistics, when electric current is passed through a semiconducting material, holes and electrons are recombined forming an exciton. One singlet exciton is generated for every three triplet excitons. The theoretical quantum efficiency of fluorescent OLEDs is limited to 25% as only the singlet excitons can be utilized for light emission. To overcome this drawback, second-generation OLEDs based on phosphorescent molecules are emerged. Using this technique, a theoretical quantum efficiency of 100% was obtained by incorporating heavy metal complexes that are capable of harvesting both triplet and singlet excitons. This was achieved using heavy metals such as platinum, osmium, and iridium, mediated by spin-orbital coupling. Although they emit light, they are expensive because of the use of heavy metals. Thermally activated delayed fluorescence (TADF) molecules have been proposed in recent years as a potential alternative to heavy-metal complexes. In TADF, excitons are harvested via reverse intersystem crossing (RISC) from the lowest triplet excited state (T1) to the lowest singlet excited state (S1). The RISC requires no heavy metals, and the excitons can be harvested using small organic molecules. The rate of reverse intersystem crossing (𝑘𝑅𝐼𝑆𝐶) is inversely proportional to the energy gap between the lowest excited singlet and triplet states (ΔEST). The smaller the value of ΔEST, the higher is the rate of reverse intersystem crossing and more efficient is the triplet to singlet up-conversion. The OLEDs are generally made up of donor-acceptor (D-A), donor-linker-acceptor (D-L-A), donor-acceptor-donor (D-A-D) and acceptor-donor-acceptor (A-D-A) frameworks. The molecules that have D-L-A framework adopt angular geometry where acceptor and donor moieties are separated by the linker, resulting in spatial separation of frontier molecular orbitals in the molecule. Many experimental studies on the optoelectronic properties of such molecules have been reported. However, an in-depth theoretical understanding of numerous factors that influence the properties of TADF materials is still lacking. Therefore, computational investigations of organic molecules having the D-L-A framework with different donor, linker and acceptor groups for achieving a low ΔEST is crucial in the designing of different OLED materials and is carried out in the present thesis. The thesis consists of seven chapters. Chapter 1 discusses the introduction and history of different generations of OLEDs. A brief review of various organic TADF molecules having different types of frameworks including D-A, D-L-A and A-D-A is provided in this chapter. The second chapter of the thesis briefly reviews the computational methodologies used for the studies. It begins by exploring the Schrödinger equation, followed by a concise overview of quantum chemical techniques, including Hartree-Fock and Post-Hartree-Fock methods. In addition, the chapter examines density functional methods employed in the current research, outlining various types of functionals and basis sets. Various parameters such as ionization potential, electron affinity, electron extraction potential, hole extraction potential, reorganization energy due to hole and electron and the singlet-triplet energy gap (ΔEST) are outlined in this chapter. In chapter 3, the effect of aromatic linkers on the singlet-triplet energy gap (ΔEST) of molecules having carbazole donor, benzonitrile acceptor and aromatic linkers are investigated using the density functional B3LYP in conjunction with 6-311G(d) basis set. The energy gap between the highest occupied and lowest unoccupied molecular orbitals (ΔEHOMO-LUMO), ionization energy, electron affinity, and reorganizational energy due to the hole and electron of the molecules are determined. The analysis revealed that the HOMO and LUMO orbitals are majorly localized on the donor and acceptor moieties, respectively, fostering intramolecular charge transfer. The aromatic linkers spatially separate the HOMO and LUMO of the molecule resulting in a low energy gap between the excited singlet and triplet states suggesting more efficient reverse intersystem crossing, ultimately leading to thermally activated delayed fluorescence. Remarkably, the investigation identifies an inverse relationship between the singlet-triplet energy gap (ΔEST) and the aromaticity of the linker. The molecules benzene and furan as linkers stand out with the lowest and highest values of ΔEST, respectively. The structure, optoelectronic and charge transport properties of the donor-linker-acceptor framework having carbazole donor and benzonitrile acceptor with linkers such as biphenyl, naphthalene and bipyridine are discussed in chapter 4. The studies revealed that the molecule in which bipyridine is used as the linker has the ΔEST less than half of that for the molecules with biphenyl and naphthalene, highlighting the importance of bipyridine as linker. Further, the effect of electron-withdrawing and electron-donating substituents on the D-L-A framework with bipyridine is examined. A notable finding is the consistent reduction of ΔEST with an increase in the number of attached cyano groups at the acceptor site. The addition of methoxy groups at the donor site induces a pronounced redshift. The electron density distribution in the frontier molecular orbitals of the selected molecules showed that HOMO and LUMO are localized on the donor and acceptor moieties, respectively. The studies on the effect of electron-withdrawing substituents fluoro (-F), chloro (-Cl) and trifluoromethyl (-CF3) at the acceptor site were further examined and noted a decrease in ΔEST. However, the decrease was found to be negligible compared to that observed for the electron-withdrawing cyano groups. Among the fluoro, chloro and trifluoromethyl substituted derivatives, the decrease was found to follow the order -CF3 > -Cl > -F. Interestingly, the methoxy substitution of the donor in conjunction with the electron-withdrawing substituents of the acceptor leads to an inverted ΔEST, demonstrating the intricate interplay of substituents on TADF properties. In chapter 5 of the thesis, the TADF emitters with phenoxaborin acceptor, benzene linker and different types of donors are investigated using the M06 functional and 6-311G (d) basis set. The analysis unveils a distortion between the planes of acceptor-linker and linker-donor moieties. The frontier molecular orbital analysis revealed that HOMO and LUMO are localized on the donor and acceptor moieties, respectively. This indicates the intramolecular charge transfer in the molecules. The spatial separation of HOMO and LUMO due to the linker results in a reduced energy gap between the singlet and triplet states, enhancing reverse intersystem crossing facilitating thermally activated delayed fluorescence. Among the studied molecules, the molecule with 5,10-dihydrophenazine as the donor has the least ΔEST. The analysis of reorganization energy revealed that these molecules can function both as hole and as electron transporters, emphasizing their versatile applications in OLEDs. The effects of electron-donating substituents and their position on the TADF properties of the molecules of D-L-A framework with imidazopyridine acceptor, benzene linker and acridine donor moieties are studied and are discussed in chapter 6. The electron density distribution of the frontier molecular orbitals of the molecules indicated that the HOMO is localized majorly on the donor and the LUMO is confined to the acceptor suggesting the intramolecular charge transfer. Among ortho-, meta- and para-substituted molecules, the spatial overlap between HOMO and LUMO is found to be the least for para-substituted molecules leading to the lowest value of ΔEST for them. It is also observed that, by increasing the electron-donating capacity of the donor, the ΔEST can be further reduced. The disubstituted derivatives have lower spatial overlap and smaller ΔEST than those for the monosubstituted derivatives. The conclusion and future scope of the present studies are provided in chapter 7 of the thesis.