On the Route to Metal-free Phosphors

Phosphorescence is a key phenomenon on which are based devices for solid-state lighting such as Organic Light Emitting Diodes (OLEDs) [1]. Despite an increasing demand for phosphorescent materials, purely organic (i.e., metal-free) compounds have been very rarely explored as emitters in phosphor applications [2]. Indeed, only a few organic molecules are known to exhibit efficient phosphorescence emission at room temperature, as their long-living triplet excited state are extremely sensitive to collisional quenching by O2 and other impurities. The main advantage of organic phosphorescent materials vs the inorganic analogues stems from the possibility to fabricate flexible devices and large-area displays by low-cost wet processing techniques (e.g., roll-to-roll printing), taking advantage of the fact that organic materials are typically soluble in a variety of common solvents. To defy the paradigm that organic materials are not suitable as phosphors, we describe here novel design principles to prepare purely organic phosphorescent molecular materials by incorporating calchogenic atoms into their structure [3]. The synthesis, X-Ray crystal structures, ground and excited state UV-V is absorption spectra and luminescence properties of two series of chalcogen-containing organic emitters equipped on both extremities with benzoxa-, benzothia-, benzoselena- and benzotellurazoles moieties are reported. The insertion of four different chalcogen atoms within the same molecular skeleton enables the investigation of the sole internal heavy-atom effect on the photophysical properties of the organic compounds. A progressive quenching of the fluorescence and concomitant onset of phosphorescence features with gradually shorter lifetimes are detected as the atomic number of the chalcogen heteroatom increases, with the Te derivatives exhibiting only phosphorescence emission. Notably, the phosphorescence spectra of the Se and the Te compounds can be recorded even at room temperature in dilute de-aerated solution, a very rare finding for organic emitters. These results open the route to the systematic exploitation of the internal heavy atom effect in the design of phosphorescent organic emitters, a rapidly expanding area of vast technological interest. This is a highly desirable goal in light of the di±culties still encountered in the preparation of cheap and stable triplet emitters based on transition metal complexes, particularly in the blue spectral region.