Design and synthesis of purely organic chromophores for ambient triplet harvesting

Abstract

Harvesting the triplet excitons from organic chromophores is the key for achieving maximum efficiency in modern electroluminescent display devices such as organic light emitting diodes (OLEDs). In addition, due to the high sensitivity to air and slow-emitting characteristics of triplet excitons, organic triplet emitters are also very promising for various optical sensing and bio-imaging applications. Until recently, such triplet emitting phenomena (as phosphorescence) from purely organic molecules could only be observed under inert atmosphere and cryogenic conditions to prevent the non-radiative pathways that deactivate the excited triplets as heat. In addition, small organic chromophores generally lack strong spin-orbit coupling(SOC)which is the key for efficient triplet emission. In this regard, organometallic complexes have been utilized as phosphorescent emitting materials due to their high SOC strengthand therefore, high ambient phosphorescence efficiency. In fact, especially due to this ambient triplet stability they have been widely explored as OLED materials for past two decades and commercialized as modern display devices. However, their potential toxicity as well as high-cost and instability especially in the blue-region still remains a grand challenge for these materials to be mass produced. Hence, developing alternative “metal-free” organic triplet emitters are more interesting although it is a highly challenging task. There are two major pathways by which ambient triplet harvesting can be done from metal-free organic chromophores, namely, room temperature phosphorescence(RTP)and thermally activated delayed fluorescence(TADF). In this introductionChapter, we will be discussing the fundamental theory of phosphorescence and various mechanisms for delayed fluorescencein detail. In addition, we will also provide an overview on the design principles of such triplet emitting materials and their various state-of-the-art applications ranging from OLEDs, optical sensing and in vivo bio-imaging.