High performing D-π-A-π-D Benzothiadiazole based hybrid local and charge transfer emitters in solution-processed OLEDs
Transforming triplet excitons to singlet excitons is a topic of great interest to material scientists in order to surpass the exciton utilization efficiency (EUE) limit of 25 % in electro-fluorescent devices. Towards this the donor-acceptor strategy of the molecular design has been proven to be an effective approach to obtain high electroluminescence (EL) efficiency. In this context, we have reported benzothiadiazole (BTD) based donor-acceptor π-conjugated fluorescent molecules (1-2) that exquisite external quantum efficiency (EQE) as high as 7.0% for 1 and 8.1% for 2 in solution-processed doped green OLED devices. The observed high photoluminescence quantum yields of 81% and 85% for 1 and 2, respectively corroborates to the high EUE values of 43% and 48% exceeding its traditional limit (25%), hence suggesting the utilization of triplet excitons. The reported emitters combine the two parameters i.e high photoluminescence efficiency and high EUE which are of key importance to harvest maximum EL in OLED devices. Based on the photophysical (solvatochromic experiment) and quantum chemical calculations, the impacts of D-π-A-π-D molecular design on the regulation of the locally excited and charge transfer components are unveiled, that explained the observed high EQE (>5%) and EUE (>25%) values for the BTD based emitters. The emitter with hybrid local and charge transfer state in combination with the ‘hot exciton’ mechanism is an important strategy to produce highly efficient fluorescent emitter materials. Apart from the impressive EL properties of the emitters, the studied fluorophore 1 as a chemosensor shows the selective sensing of metal cation (Fe2+) and anion (I-) along with the staining agent of Hibiscus rosa-sinensis pollen grains.