The Effect of Regioisomerism on the TADF Properties of Organic Dyes

The properties of thermally activated delayed fluorescence (TADF) chromophores are highly dependent on the molecular design. The choice of the donor (D) and acceptor (A) group are as important as the choice of the π-linker and the substitution positions. Herein, the influence of regioisomerism through the substitution pattern of 9,9’-spirobi[fluorene] (SBF) on the TADF characteristics is investigated. In our studies, phenothiazine and carbonitrile have been chosen as D and A groups, and their substitution varied among the 2, 2’, 7 and 7’ positions of the spiro core resulting in two pairs of regioisomers. Depending on these changes, the physical properties differ from one to another with changes in emission wavelength maximum, photoluminescence quantum yield (PLQY), fluorescence lifetime and thermal stability. Three of the investigated molecules exhibit TADF properties in solid phase as thin films and crystals.