In our recent publication entitled ‘Interplay of Fluorescence and Phosphorescence in Organic Biluminescent Emitters‘ published in the Journal of Physical Chemistry C, we discuss how the population of triplet excitons in emitters which sport efficient phosphorescence at room temperature influence the overall luminescence properties. An important emphasis here is on the exciton dynamics of the fast fluorescence (nanoseconds) and the slow phosphorescence (milliseconds), which span over six orders of magnitude in excited state lifetimes, depending on the respective sample composition. All of these results are obtained at room temperature.
We acknowledge the funding from the German excellence cluster cfaed (TU Dresden) and from European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (grant agreement no. 679213).
In the world of organic electronics, many devices and applications make use of the excitonic properties of organic semiconductors, namely light-emitting diodes, solar cells, photo-detectors, lasers, sensors, luminescent solar concentrators, optical up- and down-converters, and more. Excitons in organic molecules are highly localized states, giving rise to singlet and triplet excitons that differ in almost every aspect. Triplets are ‘dark’ states, who’s transition to the ground state is quantum mechanically forbidden, therefore, they are long-lived states, in crystals they can diffuse like crazy, and are typically significantly lower in energy than the molecule’s singlet state, as a consequence of exchange interactions. Still, if incorporated in the right way, one can specifically make use of triplets, e.g. through singlet exciton fission, thermally activated delayed fluorescence, or spin-orbit coupling.
Without any doubt, it is essential to know the triplet state energy of a given organic molecule to be able to design excitonic devices. Given the vast of organic molecules known and possible to design, the task of experimentally determining the triplet state is time consuming and can get very frustrating. This is because the ‘dark’ triplet generally only unmasks it’s properties at cryogenic (< 77 K, often at ~ 10 K) temperatures, where non-radiative modes are frozen out. This task would be simplified to great extend, if spectroscopy could be carried out at room temperature.
In our new paper published in Scientific Reports today, ‘Room temperature triplet state spectroscopy of organic semiconductors‘, we report on our recent efforts to simplify the determination of triplet states through sample engineering that allows to carry out these experiments at room temperature. Key trick to unlock room temperature phosphorescence from random organic materials is the engineering of a rigid matrix-enviroment that suppresses many non-radiative modes, very similar to the cooling of the sample. We test this on a variety of materials well known in the field of organic electronics. This scheme is very effective and powerful. Beyond simple spectroscopy of triplet states, it also allowed the observation of biluminescence, i.e. efficient, simultaneous fluorescence and phosphorescence of organic molecules.