13 April 2017

Hot on the heels of their success last November, Durham University researchers Dr. Marc Etherington, Dr. Heather Higginbotham, and Prof. Andrew Monkman have published a second paper in Nature Communications based on results from the PHEBE project.

In the paper “Regio- and conformational isomerization critical to design of efficient thermally-activated delayed fluorescence emitters”, the researchers investigated the role of regio- and conformational isomerisation on the excited state properties of certain thermally activated delayed fluorescence (TADF) emitters.

The researchers studied two region-isomers of bis(10H-phenothiazin-10-yl)dibenzo[b,d]thiophene-S,S-dioxide, a donor–acceptor–donor TADF emitter:
  • 2,8-bis(10H-phenothiazin-10-yl)dibenzo[b,d]thiophene-S,S-dioxide exhibited only one quasi-equatorial conformer on both donor sites, with charge-transfer (CT) emission close to the local triplet state leading to efficient TADF via spin-vibronic coupling.
  • However, 3,7-bis(10H-phenothiazin-10-yl)dibenzo[b,d]thiophene-S,S-dioxide displayed both a quasi-equatorial CT state and a higher-energy quasi-axial CT state. No TADF was observed in the quasi-axial CT emission. The two CT states linked directly to the two folded conformers of phenothiazine. The presence of the low-lying local triplet state of the axial conformer implies that the quasi-axial CT is an effective loss pathway both photo-physically and in devices. Importantly, donors or acceptors with more than one conformer have negative repercussions for TADF in organic light-emitting diodes.

The research was undertaken together with other scientists from Durham University as well as Newcastle University.

About Durham University
The Durham University is a world top 100 university with a global reputation and performance in research and education. It is a member of the Russell Group of leading research-intensive UK universities. Research at Durham shapes local, national and international agendas, and directly informs the teaching of our students. It was ranked in the world top 25 for the employability of its students by blue-chip companies world-wide (QS World University Rankings 2014/15) and in the global top 50 for Arts and Humanities (THE World University Rankings 2013/14). In the 2015 Complete University Guide, Durham was the only UK university to receive a top ten ranking for all of its subjects and 19 of Durham’s 22 subjects were ranked in the top five. Furthermore, Durham was named as The Times and Sunday Times 'Sports University of the Year 2015' in recognition of outstanding performance in both the research and teaching of sport, and student and community participation in sport at all levels.

About PHEBE Project
Exploiting ground-breaking research in the field of thermally activated delayed fluorescence (TADF), the €4M PHEBE project aims to develop innovative, high-efficiency, blue emitters for white organic light emitting diodes (OLEDs). While OLEDs are already an efficient and resource-saving illumination system, the new emitters are expected to significantly enhance their cost performance and environmental-friendliness. The PHEBE project has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement number 641725. It started on 1st February 2015 and will run for 3 years.