Optimising conformational effects on thermally activated delayed fluorescence

The design and engineering of delayed fluorescent dyes that are able to convert non-emissive triplet excitons into fluorescent singlet excitons is a challenging task, due to the contrasting nature of the various requirements that a material should satisfy to successfully exploit thermally activated delayed fluorescence (TADF) in devices. Indeed, the typical donor-acceptor designs present opposite dependencies of the various photophysical processes that contribute to the overall efficiency on conformations. To tackle this problem, in this work, we introduce a rigorous mathematical protocol borrowed from the field of multiobjective optimisation that leads to the systematic identification of the conformations showing the best compromise among all the properties relevant to TADF applications. We perform a computational analysis of two typical TADF blue emitters, studying the dependence of quantities relevant for TADF on internal degrees of freedom, and find the conformations that optimise all parameters simultaneously, proposing structural modifications to constrain the dyes in their optimal conformations. We show that, in contrast with what is commonly done in the literature, analysing molecular properties only might not be enough: the interplay among parameters that determine the rates of the fundamental processes relevant for TADF is so delicate that one cannot prescind from the computations of the actual rates. Notably, while this work focuses on the best compromise for TADF, the mathematical framework is entirely general and can be extended to other applications where multiple objectives must be satisfied at the same time.