title: Theoretical rationalization of the singlet-triplet gap in OLEDs materials: impact of charge-transfer character.
abstract: New materials for OLED applications with low singlet-triplet energy splitting have been recently synthesized in order to allow for the conversion of triplet into singlet excitons (emitting light) via a Thermally Activated Delayed Fluorescence (TADF) process, which involves excited-states with a non-negligible amount of Charge-Transfer (CT). The accurate modeling of these states with Time-Dependent Density Functional Theory (TD-DFT), the most used method so far because of the favorable trade-off between accuracy and computational cost, is however particularly challenging. We carefully address this issue here by considering materials with small (high) singlet-triplet gap acting as emitter (host) in OLEDs and by comparing the accuracy of TD-DFT and the corresponding Tamm-Dancoff Approximation (TDA), which is found to greatly reduce error bars with respect to experiments thanks to better estimates for the lowest singlet-triplet transition. Finally, we quantitatively correlate the singlet-triplet splitting values with the extent of CT, using for it a simple metric extracted from calculations with double-hybrid functionals, that might be applied in further molecular engineering studies.
authors.affiliation: Departamento de Química Fsica, Universidad de Alicante , 03080 Alicante, Alicante, Spain.
authors.forename: M
authors.initials: M
authors.lastname: Moral
authors.affiliation: Department of Industrial Chemistry "Toso Montanari", University of Bologna , 40136 Bologna, Bologna, Italy.|Laboratoire de Chimie des Polymères Organiques, UMR 5629, University of Bordeaux , 33607 Pessac, France.
authors.forename: L
authors.initials: L
authors.lastname: Muccioli
authors.affiliation: Samsung Advanced Institute of Technology, Yongin, Gyeonggi-do 443-803, South Korea.
authors.forename: W-J
authors.initials: WJ
authors.lastname: Son
authors.affiliation: Laboratory for Chemistry of Novel Materials, University of Mons , 7000 Mons, Belgium.
authors.forename: Y
authors.initials: Y
authors.lastname: Olivier
authors.affiliation: Departamento de Química Fsica, Universidad de Alicante , 03080 Alicante, Alicante, Spain.
authors.forename: J C
authors.initials: JC
authors.lastname: Sancho-García