Research Projects

Organic semiconductors have emerged as promising alternatives to inorganic materials due to their mechanical flexibility, low-cost fabrication, and compatibility with lightweight substrates. However, a key bottleneck is the development of efficient, cost-effective emitter materials. TADF enables efficient harvesting of triplet excitons via ISC/RISC offering a metal-free route to harvest triplets by converting them into singlets via reverse intersystem crossing (RISC), enabled by a small singlet-triplet energy gap (ΔEST). However, fundamental challenges remain in understanding and controlling the excited-state dynamics, particularly the processes governing intersystem crossing and RISC. A common strategy to enhance the rate of RISC is to minimize the energy gap between the singlet (1CT) and triplet (3CT) excited states. Recent theoretical works suggested that the vibronic coupling (non-adiabatic) through a localized triplet state, 3LE states mediates the spin-flip. This is formally known as spin-vibronic coupling (SVC), which influences not only emission properties and excitation relaxation pathways but also governs how triplet-to-singlet conversion occurs in TADF and how energy is transferred to fluorescent dopants in HF systems. While strong vibronic coupling can accelerate desirable processes such as RISC, it can also facilitate non-radiative losses, especially in red emitters where the energy gap law dictates vibrational relaxation. Hence, optimum and strategic tuning of exciton-vibronic interactions are required to achieve high IQE in TADF emitters. Over the past decade, research in TADF has rapidly evolved and significant progress has been made in understanding mechanisms, designing efficient materials and achieving device-level performance. Yet, critical questions remain still unanswered due to lack of conclusive direct experimental evidences: which vibrational modes mediates ISC and RISC processes in different classes of TADF materials? how do structural rigidity or flexibility modulate vibronic coupling strength? can design strategies be used to decouple excitons from loss-inducing vibrations? Advances in ultrafast spectroscopic techniques including broadband femtosecond transient absorption spectroscopy and time-resolved impulsive stimulated Raman spectroscop, enable real-time observation of vibrational wavepacket dynamics and their role in triplet harvesting. These methods can resolve coherent nuclear motions and reveal vibrational modes that control ISC and RISC rate and efficiencies. In this project, we aim to address these questions by exploring vibrational wavepacket dynamics using ultrafast coherent spectroscopy tools, which will be supported by computational study. By mapping the exciton-vibrational landscape, it aims to provide key insights for the rational design of next-generation OLED with maximized efficiency and minimized energy loss.