“The Importance of Vibronic Coupling for Efficient Reverse Intersystem Crossing in TADF Molecules”

The communication between low lying singlet and triplet excited states is of great importance in the field of organic electronics and plays a fundamental role in determining key molecular and material properties such as the lifetime and mobility of charge separated states. Consequently, a thorough understanding of the basic principles governing the interplay between these manifolds of spin states is of great importance, with direct implications for the performance of devices such as Organic Photovoltaics and Organic Light-Emitting Diodes (OLEDs).

Herein I will present some of our recent results on reserve intersystem crossing (rISC), which plays a central part in the thermally activated delay fluorescence (TADF) mechanism of 3rd generation OLEDs. Despite intense research interest, present understanding of the TADF mechanism does not extend far beyond a thermal equilibrium between the lowest singlet and triplet states. This has encouraged research to focused almost exclusively on the energy gap between these two (T1 and S1, Figure b) states. Using quantum dynamics and a model Hamiltonian we have studied a high performing complex (Figure a) and revealed the crucial role of non-Born-Oppenheimer effects in determining krISC. Indeed, this can increase it by over 4 orders of magnitude! Crucially this insight, which explains recent experimental observations, shows the dynamical mechanism of rISC. It also demonstrates the importance of not just tuning the ES1–T1 gap but playing close attention to the gaps between the multiple excited states.