title: Regio- and conformational isomerization critical to design of efficient thermally-activated delayed fluorescence emitters.
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.
authors.affiliation: Department of Physics, Durham University, South Road, Durham DH1 3LE, UK.
authors.forename: Marc K
authors.identifier: 0000-0003-2101-5757
authors.initials: MK
authors.lastname: Etherington
authors.affiliation: Department of Physics, Durham University, South Road, Durham DH1 3LE, UK.
authors.forename: Flavio
authors.initials: F
authors.lastname: Franchello
authors.affiliation: School of Chemistry, Newcastle University, Newcastle upon Tyne NE1 7RU, UK.
authors.forename: Jamie
authors.initials: J
authors.lastname: Gibson
authors.affiliation: School of Chemistry, Newcastle University, Newcastle upon Tyne NE1 7RU, UK.
authors.forename: Thomas
authors.initials: T
authors.lastname: Northey
authors.affiliation: Department of Chemistry, Durham University, South Road, Durham DH1 3LE, UK.
authors.forename: Jose
authors.identifier: 0000-0002-9702-2315
authors.initials: J
authors.lastname: Santos
authors.affiliation: Department of Chemistry, Durham University, South Road, Durham DH1 3LE, UK.
authors.forename: Jonathan S
authors.initials: JS
authors.lastname: Ward
authors.affiliation: Department of Physics, Durham University, South Road, Durham DH1 3LE, UK.
authors.forename: Heather F
authors.initials: HF
authors.lastname: Higginbotham
authors.affiliation: Department of Physics, Durham University, South Road, Durham DH1 3LE, UK.|Faculty of Chemistry, Silesian University of Technology, Marcina Strzody 9, 44-100 Gliwice, Poland.
authors.forename: Przemyslaw
authors.initials: P
authors.lastname: Data
authors.affiliation: Faculty of Chemistry, Silesian University of Technology, Marcina Strzody 9, 44-100 Gliwice, Poland.
authors.forename: Aleksandra
authors.initials: A
authors.lastname: Kurowska
authors.affiliation: Department of Physics, Durham University, South Road, Durham DH1 3LE, UK.
authors.forename: Paloma Lays
authors.initials: PL
authors.lastname: Dos Santos
authors.affiliation: Department of Physics, Durham University, South Road, Durham DH1 3LE, UK.
authors.forename: David R
authors.initials: DR
authors.lastname: Graves
authors.affiliation: Department of Chemistry, Durham University, South Road, Durham DH1 3LE, UK.
authors.forename: Andrei S
authors.initials: AS
authors.lastname: Batsanov
authors.affiliation: Department of Physics, Durham University, South Road, Durham DH1 3LE, UK.
authors.forename: Fernando B
authors.identifier: 0000-0001-9841-863X
authors.initials: FB
authors.lastname: Dias
authors.affiliation: Department of Chemistry, Durham University, South Road, Durham DH1 3LE, UK.
authors.forename: Martin R
authors.initials: MR
authors.lastname: Bryce
authors.affiliation: School of Chemistry, Newcastle University, Newcastle upon Tyne NE1 7RU, UK.
authors.forename: Thomas J
authors.initials: TJ
authors.lastname: Penfold
authors.affiliation: Department of Physics, Durham University, South Road, Durham DH1 3LE, UK.
authors.forename: Andrew P
authors.initials: AP
authors.lastname: Monkman