Controlling the rate of processes of electronically excited states of organic molecules is key to obtain efficient and stable organic-electronic devices. The insights gained from the last century of research within the fields of energy and electron transfer has been conveyed into organic electronic devices recently. However, a key parameter which still not completely resolved in organic electronics is the conversion of electronic spin states. For instance, in organic light-emitting diodes (OLED) singlet states are of preference, whereas triplet states cause a nuisance since the emission of light from those is quantum mechanically forbidden.
Förster type energy transfer offers another possibility to transfer energy, and the process is in theory not dependent on spin changes. However recently, this picture of a spin-independent energy transfer mechanism has been challenged. In this project, we show that it is indeed possible to transfer excited state energy between states of different multiplicity. Therefore, researchers from Gothenborg University designed a novel organic dye, which was synthesised and could be structurally characterised at an atomic resolution at BioMAX. Using spectroscopical methods, Alexei Cravcenco from the Börjesson group could show a 36 times higher energy transfer efficiency compared to the classical emission form isolated electronic triplet states.
Uwe Mueller, a structural biologist and beamline scientist at BioMAX, was supporting this very interdisciplinary research and enabled the structure determination at MAX IV. – It is fascinating to see that BioMAX, which has been optimised for structural biology application, has the capability to contribute besides its main application to challenging material science research. The collaboration with the GU scientist was very dynamic and inspiring, says Uwe.
The ability to convert between molecular spin states is of utmost importance in materials chemistry. Förster-type energy transfer is based on dipole-dipole interactions and can therefore theoretically be used to convert between molecular spin states. Here, a molecular dyad that is capable of transferring energy from an excited triplet state to an excited singlet state is presented. The rate of conversion between these states was shown to be 36 times faster than the rate of emission from the isolated triplet state. This dyad provides the first solid proof that Förster-type triplet-to-singlet energy transfer is possible, revealing a method to increase the rate of light extraction from excited triplet states.
Multiplicity conversion based on intramolecular triplet-to-singlet energy transfer
A. Cravcenco, M. Hertzog, C. Ye, M. N. Iqbal, U. Mueller, L. Eriksson and K. Börjesson
Science Advances 20 Sep 2019: Vol. 5, no. 9, eaaw5978