New paper: Interplay of Fluorescence and Phosphorescence in Organic Biluminescent Emitters

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).

ERC Starting Grant awarded – Project BILUM

Exciting news: The ERC has just announced the 291 young researchers, who were awarded a 2015 ERC Starting Grant, where one funding slot was reserved for my proposal. The project is called BILUM and runs under the title ‘Novel applications based on organic biluminescence’. The project will start with April 2016 and will last for 5 years; the host institution is the Technische Universität Dresden.

The project is all about the investigation and utilization of a recently discovered phenomena called biluminescence, which is unique to organic molecules. In these molecules, two differently excited states can be formed giving rise to completely different emission properties. The key here is that both states can emit photons efficiently, typically with different color, turning these molecules into dual-state emitters – hence: biluminophores.

BILUM Scheme

BILUM has many objectives that span from fundamental research to the quest to discover novel applications. Especially, we are interested in:

    • the structure-property relationships that lead to efficient biluminescence,
    • the exciton dynamics of these dual state emitter systems,
    • the photo-stability of such compounds,
    • the compatibility of biluminophores with known processing techniques (solution and vacuum processing),
    • the quantum chemical and synthetic pathways to novel biluminophores with improved performance,
    • excitonic and photonic concepts that make use of the unique spectral and temporal characteristics of biluminescence,
    • the ratio between fluorescence and phosphorescence and ways to alter it,
    • and many more…

The project abstract reads:

Organic semiconducting molecules often make for very good luminescent materials. Fundamental excitations are localized on single molecules, which is in stark contrast to inorganic semiconductors, such that exchange interactions lead to energetically distinct singlet and triplet states. The singlet-excited state is the origin of conventional fluorescence. However, once an excitation is in the molecular triplet state, emission of photons is very unlikely, because spin conservation needs to be broken. Here, non-radiative recombination outcompetes the radiative. Recent research efforts led to the discovery of highly efficient biluminescence. Here, in addition to the fluorescence from the singlet state, the phosphorescence (triplet state emission) is unlocked by suppression of non-radiative channels at room temperature. The dynamics of both states is vastly different with nanosecond fluorescence and millisecond phosphorescence. If both channels are highly luminescent, then there is no room for loss channels. Within BILUM, the virtually unexplored phenomenon of biluminescence will be the central point: On the basic science side, efforts will be focused on the detailed understanding of structure-property relationships that are key for efficient dual state emission. At the same time, with a curiosity driven engineering approach, known bilumophores will be carefully tested in different scenarios to set the ground for future applications. Biluminescence has the potential to access non-radiative triplet states that are in many cases system limiting, to serve as ultra-broadband emitters, to introduce persistent (ultra long-lived) emission, to store photonic energy, and to allow optical sensing with internal reference emission – all on the molecular level. New bilumophores will be identified through systematic screening that will employ quantum chemical calculations and developed through organic synthesis.

For the interested reader, here are some links to our recent research publications on the matter:

– C. Salas Redondo and S. Reineke, ‘Simultaneous fluorescence and phosphorescence from organic molecules‘, SPIE newsroom (2015). DOI

– S. Reineke and M. A. Baldo, Sci. Rep. 4; DOI: 10.1038/srep03797 (2014). DOI

– S. Reineke, N. Seidler, S. R. Yost, F. Prins, W. A. Tisdale, and M. A. Baldo, Appl. Phys. Lett. 103, 093302 (2013). DOI

Below, find a short video that demonstrates the biluminescence of one example molecule at room temperature. You see a solid sample (thin film) at room temperature that is repetitively excited with a UV flashlight. Whenever the excitation source is turned off, the persistent phosphorescence remains as afterglow.