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 a recent collaboration with our colleagues at the Leibniz Institute of Polymer Research Dresden (IPF), we have developed polymers that show thermally activated delayed fluorescence (TADF) properties with high efficiency. This work has now been published in Advanced Functional Materials under the title: Conjugation-Induced Thermally Activated Delayed Fluorescence (TADF): From Conventional Non-TADF Units to TADF-Active Polymers. Interestingly, the monomer building block does not show TADF but rather only phosphorescence. Hence, the TADF property is induced as a consequence of increased conjugation during polymer formation. Ultimately, the singlet-triplet splitting is reduced in the polymer to allow for TADF. The emitter shows sky-blue emission with roughly 70% PLQY. This report includes the synthesis of the monomer and polymer materials, quantum chemical calculations and a detailed photo-physical characterization.
Ramon Springer joined the group of Prof. Jang Hyuk Kwon (Department of Information Display, Kyung Hee University, South Korea) to carry out a Master thesis topic within the international Masters course Organic and Molecular Electronics (OME) at the TU Dresden. His thesis task was to develop a white-light emitting, multiple OLED stack based on blue and yellow units to be used in AMOLED displays. Here, aside from the optimization of device efficiency, the color quality and angular stability were parameters to be optimized. His work led to a recent publication in Optics Express entitled “Cool white light-emitting three stack OLED structures for AMOLED display applications“. Congratulations to a very successful research stay abroad.
Our work on highly efficient biluminescent organic emitters at room temperature is featured in Advances in Engineering. Find the appropriate direct link here. The work describes an organic molecule, namely (BzP)PB that shows highly efficient fluorescence and phosphorescence at room temperature. Here, the intermixing between singlet and triplet manifold only determines the relative shares of fluorescence and phosphorescence, turning this emitter into a dual state emitter, where intercombination from one spin manifold to another does not represent an internal loss channel.
In my last post, I highlighted our most recent publication in Scientific Reports, which discusses novel strategies to achieve room temperature phosphorescence of organic semiconductors by means of sample engineering and exciton management (see: ‘Room temperature triplet state spectroscopy of organic semiconductors‘). In today’s post, I’d like to give some very convincing evidence, how well these approaches work out in real time and space.
In the video below, you see a couple of glass slides that are covered with a thin film composed of the polymer PMMA [Poly(methyl 2-methylpropenoate)], into which 2 wt% of the well known organic material NPB [N,N′-di(naphtha-1-yl)-N,N′-diphenyl-benzidine] is embedded. The sample is optically excited with a 365 nm LED, giving rise to blue fluorescence of NPB. Whenever the LED is turned off, the sample shows a persistent emission of green/yellow color, which is the phosphorescence of NPB. Conditions: room temperature, nitrogen atmosphere.
In a paper recently published in Journal of the American Chemical Society (Selective Turn-On Ammonia Sensing Enabled by High-Temperature Fluorescence in Metal–Organic Frameworks with Open Metal Sites), we show that highly fluorescent metal organic frameworks (MOFs) can be used to selectively sense ammonia at high temperatures. The work is lead by Mircea Dincă (Dincă group), professor at the MIT Chemistry Department. This has been a fun and interesting collaboration, which I’d like to pursue in the future.
A new paper was just published in Applied Physics Letters entitled ‘High efficiency, dual emission from an organic semiconductor‘. The work describes the observation of highly efficient luminescence of both singlet and triplet states of a purely organic semiconductor, namely N,N’-bis(4-benzoyl-phenyl)-N,N’-diphenyl-benzidine, at room temperature. This unusual observation is the result of a very effective suppression of non-radiative modes within the triplet manifold of the molecule, enabling high efficiency phosphorescence. Together with efficient fluorescence, this molecule transforms in a dual state emitter, a phenomenon we term ‘biluminescence’. As neither singlet nor triplet states are a loss channel in the emitter molecule, the mixing between the two states through intersystem crossing (ISC) and back (reverse ISC) only dictates the relative intensities of fluorescence and phosphorescence. An biluminescence emitter may find future applications as ultra broadband emitters, exciton probes, various types of sensors, and spin independent energy transfer intermediates.