Investigation of the Mechanisms Giving Rise to TADF in Exciplex States

The donor-acceptor systems studied in this work have been demonstrated to show strong exciplex formation by their red-shifted PL emission when compared with the individual donor and acceptor molecules, and all give rise to delayed fluorescence. In all cases, the channels by which this delayed fluorescence is generated, along with the energy levels involved in reverse intersystem crossing (rISC), that is, which triplet levels the charge-transfer states couple to yield spin flip; have not previously been identified. Here the intermolecular charge transfer states formed in the donor-acceptor molecular pairs are studied. It is demonstrated that the local triplet excited states are the states that couple to the singlet charge-transfer excited state, defining the rISC process and hence thermally activated delayed fluorescence (TADF) mechanism. Moreover, in most systems there is a competition between delayed fluorescence mechanisms, triplet triplet annihilation and TADF, and this is analysed in detail. New design rules for exciplex materials showing dominant delayed fluorescence due to rISC for the device based on TADF are elucidated.