title: The Role of Local Triplet Excited States and D-A Relative Orientation in Thermally Activated Delayed Fluorescence: Photophysics and Devices.
abstract: Here, a comprehensive photophysical investigation of a the emitter moleculeDPTZ-DBTO2, showing thermally activated delayed fluorescence (TADF), with near-orthogonal electron donor (D) and acceptor (A) units is reported. It is shown thatDPTZ-DBTO2has minimal singlet-triplet energy splitting due to its near-rigid molecular geometry. However, the electronic coupling between the local triplet (3LE) and the charge transfer states, singlet and triplet, (1CT,3CT), and the effect of dynamic rocking of the D-A units about the orthogonal geometry are crucial for efficient TADF to be achieved. In solvents with low polarity, the guest emissive singlet1CT state couples directly to the near-degenerate3LE, efficiently harvesting the triplet states by a spin orbit coupling charge transfer mechanism (SOCT). However, in solvents with higher polarity the emissive CT state inDPTZ-DBTO2shifts below (the static)3LE, leading to decreased TADF efficiencies. The relatively large energy difference between the1CT and3LE states and the extremely low efficiency of the1CT to3CT hyperfine coupling is responsible for the reduction in TADF efficiency. Both the electronic coupling between1CT and3LE, and the (dynamic) orientation of the D-A units are thus critical elements that dictate reverse intersystem crossing processes and thus high efficiency in TADF.
authors.affiliation: Physics Department Durham University South Road Durham DH1 3LE UK.
authors.forename: Fernando B
authors.initials: FB
authors.lastname: Dias
authors.affiliation: Physics Department Durham University South Road Durham DH1 3LE UK; IMDEA Nanociencia C/Faraday, 9 Campus Universitario de Cantoblanco 28049 Madrid Spain.
authors.forename: Jose
authors.initials: J
authors.lastname: Santos
authors.affiliation: Physics Department Durham University South Road Durham DH1 3LE UK.
authors.forename: David R
authors.initials: DR
authors.lastname: Graves
authors.affiliation: Physics Department Durham University South Road Durham DH1 3LE UK; Faculty of Chemistry Silesian University of Technology M. Strzody 944-100 Gliwice Poland.
authors.forename: Przemyslaw
authors.initials: P
authors.lastname: Data
authors.affiliation: Physics Department Durham University South Road Durham DH1 3LE UK.
authors.forename: Roberto S
authors.initials: RS
authors.lastname: Nobuyasu
authors.affiliation: Chemistry Department Durham University South Road Durham DH1 3LE UK.
authors.forename: Mark A
authors.initials: MA
authors.lastname: Fox
authors.affiliation: Chemistry Department Durham University South Road Durham DH1 3LE UK.
authors.forename: Andrei S
authors.initials: AS
authors.lastname: Batsanov
authors.affiliation: Centro de Quimica-Fisica Molecular Instituto Superior Tecnico 1049-001 Lisboa Portugal.
authors.forename: Tiago
authors.initials: T
authors.lastname: Palmeira
authors.affiliation: Centro de Quimica-Fisica Molecular Instituto Superior Tecnico 1049-001 Lisboa Portugal.
authors.forename: Mário N
authors.initials: MN
authors.lastname: Berberan-Santos
authors.affiliation: Chemistry Department Durham University South Road Durham DH1 3LE UK.
authors.forename: Martin R
authors.initials: MR
authors.lastname: Bryce
authors.affiliation: Physics Department Durham University South Road Durham DH1 3LE UK.
authors.forename: Andrew P
authors.initials: AP
authors.lastname: Monkman
keywords: OLEDs; TADF; charge transfer; electroluminescence; fluorescence