The Role of ‘Local Excited States’ in Reverse Intersystem Crossing Process to Achieve Efficient TADF Molecular Emitters

Mondal, Madalasa (2024) The Role of ‘Local Excited States’ in Reverse Intersystem Crossing Process to Achieve Efficient TADF Molecular Emitters. PhD thesis, Indian Institute of Science Education and Research Kolkata.

Text (PhD thesis of Madalasa Mondal (18RS042)) 18RS042.pdf - Submitted Version Restricted to Repository staff only Download (21MB)

Abstract

In the last few years, Thermally Activated Delayed Fluorescence (TADF) has become the most efficient approach for triplet exciton harvesting. They represent a significant advancement in optoelectronics, particularly in enhancing the performance of OLEDs and other light-emitting devices. The TADF mechanism provides ~100% internal quantum efficiency (IQE) by the up-conversion of dark triplet excitons into singlet excitons through an effective reverse intersystem crossing (RISC) process. However, many aspects of the TADF mechanism are still unclear and need to be illuminated. Though the basic key parameter to achieve TADF property is small ΔEST. Still, other essential factors also play an important role in the management of kRISC, which requires detailed understanding. My thesis work is mainly focused on improving the kRISC to achieve maximum TADF efficiency using a novel molecular design strategy. In-depth analysis of the photophysical properties to understand the TADF mechanism involving intermediate triplet excited states of different electronic characters than the lowest excited CT states. Observation of significant mixing coefficient between the CT and locally lying triplet states, and the active participation of hybrid electronic states facilitates the RISC process. Using this approach, we reported that modulation of local triplet states is demonstrated by incorporating secondary donor substituents to the core donor unit in conventional D-A and unconventional acceptor-free emitters, which played an essential role in achieving efficient RISC. Moreover, the effect of the dielectric environment on the electronic energy levels and, hence the kRISC, controlling the activation energy barrier for RISC is also explored. Fine-tune between the CT and LE states with the variation of different polymer host matrices. The emitter and host combination minimized the RISC barrier and maximized the TADF. In addition, the radiative rate was also optimized by manipulating oscillator strength with suitable ΔEST for a fast spin-flip process.

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