Exploring the Concept of Dimerization-Induced Intersystem Crossing: At the Origins of Spin-Orbit Coupling Selection Rules.

Singlet-triplet interconversions (intersystem crossing, ISC) in organic molecules are at the basis of many important processes in cutting-edge photonic applications (organic light-emitting devices, photodynamic therapy, etc.). Selection rules for these transitions are mainly governed by the spin-orbit coupling (SOC) phenomenon. Although the SOC relies on complex relativistic phenomena, theoreticians have, with time, developed increasingly sophisticated and efficient approaches to gain access to a satisfactory evaluation of its magnitude. However, recent works have highlighted the remarkable and somehow unexpected efficiency of dimers of small conjugated molecules in terms of ISC quantum yields, whose origin has not been completely investigated. In this work, we bring a coupled experimental and theoretical analysis of the origin of the unusually large ISC efficiency on a series of such dimers that differ by their nature (covalent or supramolecular). We show that considering the dynamical nature of the SOC, and especially its dependence on angular orientations between the dimer subunits sometimes overlooked in the literature, it is necessary to rationalize some counterintuitive experimental observations. This combined experimental and theoretical work paves the way for new molecular engineering rules for SOC control.

Theoretical and experimental investigation on the intersystem crossing kinetics in benzothioxanthene imide luminophores, and their dependence on substituent effects.

In spite of their remarkable luminescence properties, benzothioxanthene imide (BTXI, an imide containing rylene chromophores) derivatives have been largely overlooked compared to their perylene bisimide and naphthalene bisimide counterparts. Thus, their detailed photophysics are much less understood. In this paper, we show how relatively simple structural modifications of the backbone of BTXIs can lead to impressive variations in their inter-system crossing kinetics. Thus, through rational engineering of their structure, it is possible to obtain a triplet formation quantum yield that reaches unity, making BTXI a promising class of compounds for triplet-based applications (photodynamic therapy, electroluminescence, etc.).