Title:	Spirocarbons in TADF systerms
DNr:	NAISS 2024/22-1583
Project Type:	NAISS Small Compute
Principal Investigator:	Karl Börjesson <karl.borjesson@gu.se>
Affiliation:	Göteborgs universitet
Duration:	2024-12-02 – 2026-01-01
Classification:	10402
Keywords:

Abstract

Lighting is one of the crucial technologies for civilization. Historically it allowed us to work and study without sunlight being available. Today it is also used as a means to design our indoor and outdoor surroundings, communicate information in a broad sense, and for entertainment. The two latter examples most often use active displays, in which each pixel is a point light source. Two display technologies dominate, the liquid crystal display (LCD) and the organic light emitting diode (OLED) display. To build an OLED, molecules should be constructed to effectively emit a photon after being excited by a charge recombination process. This is immensely difficult due to spin statistics, which results in triplet states being generated. Triplet states are always lower in energy than the corresponding singlet state, but the triplet states of organic dyes are so long-lived that the emission from thermally populated singlet state can be the main channel of deactivation. Thus, forming a photophysical scheme that do not rely on having a heavy (and scarce) atom in the structure. This photophysical process is called thermally activated delayed fluorescence (TADF), or historically e-type delayed fluorescence from the work by Parker and Hatchard using the dye eosin (a fluorescein derivative).1 The breakthrough for TADF in OLEDs came from Adachi in 2011.2 In this seminal work, the authors propose molecular design criteria for making organic dyes having a small energy gap between the excited singlet and triplet states, and still have a high rate of fluorescence. The idea is in principle to separate the HOMO and LUMO orbitals onto separate parts of the molecule as this overlap is proportional to the singlet/triplet energy gap. The idea presented by Adachi was to separate the HOMO and LUMO levels by implementing a charge transfer transition. This results in very low HOMO-LUMO overlaps, resulting in small singlet-triplet energy gaps, but the envelope of the emitted light is typically quite broad. The colour purity of the light thus becomes low. In 2016 a new dye family, the multi-resonance TADF (MR-TADF) emitters was introduces by Hatakeyama and co-workers.3 As the HOMO and LUMO orbitals alternate in the molecular framework, their overlap is small without the need of a charge transfer transition. This class of molecule are the so far most suited one’s ever developed for OLED applications. We are on the path to further optimize MR-TADF dyes by incorporating a strong exciton coupling scheme into these. To do so, we would need to see how substitutions of known emitters affect the energy separation between the S1 and T1 states, which is best calculated using deltaSCF and DFT methods.