Regio- and conformational isomerization critical to design of efficient thermally-activated delayed fluorescence emitters

Etherington, Marc, Franchello, Flavio, Gibson, Jamie, Northey, Thomas, Santos, Jose, Ward, Jonathan S., Higginbotham, Heather F., Data, Przemyslaw, Kurowska, Aleksandra, dos Santos, Paloma L., Graves, David R., Batsanov, Andrei S., Dias, Fernando B., Bryce, Martin R., Penfold, Thomas J. and Monkman, Andrew P. (2017) Regio- and conformational isomerization critical to design of efficient thermally-activated delayed fluorescence emitters. Nature Communications, 8 (1). p. 14987. ISSN 2041-1723

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Abstract

Regio- and conformational isomerization are fundamental in chemistry, with profound effects upon physical properties, however their role in excited state properties is less developed. Here two regioisomers of bis(10H-phenothiazin-10-yl)dibenzo[b,d]thiophene-S,S-dioxide, a donor–acceptor–donor (D–A–D) thermally-activated delayed fluorescence (TADF) emitter, are studied. 2,8-bis(10H-phenothiazin-10-yl)dibenzo[b,d]thiophene-S,S-dioxide exhibits 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 displays both a quasi-equatorial CT state and a higher-energy quasi-axial CT state. No TADF is observed in the quasi-axial CT emission. These two CT states link directly to the two folded conformers of phenothiazine. The presence of the low-lying local triplet state of the axial conformer also means that this quasi-axial CT is an effective loss pathway both photophysically and in devices. Importantly, donors or acceptors with more than one conformer have negative repercussions for TADF in organic light-emitting diodes.

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