Purely Organic Emitters for Multiresonant Thermally Activated Delay Fluorescence: Design of Highly Efficient Sulfur and Selenium Derivatives

Multiresonance thermally activated delayed fluorescence (MR-TADF) emitters based on nitrogen- and/or oxygen-substituted organoboron molecules can exhibit high photoluminescence quantum yields, color purity, and thermal and chemical stability. Therefore, these emitters have recently attracted great interest for application in organic light-emitting diodes (OLEDs). The compositional diversity of MR-TADF materials is, however, limited to the use mainly of nitrogen and oxygen as electron-rich heteroatoms. Here, we expand the chemical range of these materials by considering the replacement of the O atoms with either S or Se atoms, with the objective of enhancing spin-orbit coupling via the heavy-atom effect. We theoretically evaluate the influence of these substitutions on the emissive properties. We investigate three series of MR molecules with structural motifs based on the following: (i) DOBNA (5,9-dioxa-13b-boranaphtho[3,2,1-de]anthracene); (ii) OAB-ABP (5,12-dioxa-8b-aza-16b,19b-diboraanthra[1,9-ab]benzo[j]perylene); and (iii) the variation of the positions of the chalcogen atoms within the OAB-ABP framework. The results of highly correlated quantum-chemical calculations show that the chemical nature and positions of the chalcogen atoms have a crucial impact on the photophysical properties. Several of the molecules incorporating sulfur or selenium are found to exhibit both high-energy emissive states and large reverse intersystem crossing rates, which makes them promising candidates as efficient deep-blue emitters.