Effect of aggregation on thermally activated delayed fluorescence and ultralong organic phosphorescence: QM/MM study

Abstract

Aggregation-induced thermally activated delayed fluorescence (TADF) phenomena have attracted extensive attention recently. In this paper, several theoretical models including monomer, dimer, and complex are used for the explanation of the luminescent properties of (R)-5-(9H-carbazol-9-yl)-2-(1,2,3,4-tetrahydronaphthalen-1-yl)isoindoline-1,3-dione ((R)-ImNCz), which was recently reported [Chemical Engineering Journal 418 129167 (2021)]. The polarizable continuum model (PCM) and the combined quantum mechanics and molecular mechanics (QM/MM) method are adopted in simulation of the property of the molecule in the gas phase, solvated in acetonitrile and in aggregation states. It is found that large spin–orbit coupling (SOC) constants and a smaller energy gap between the first singlet excited state and the first triplet excited state (ΔE st) in prism-like single crystals (SCp-form) are responsible for the TADF of (R)-lmNCz, while no TADF is found in block-like single crystals (SCb-form) with a larger ΔE st. The multiple ultralong phosphorescence (UOP) peaks in the spectrum are of complex origins, and they are related not only to ImNCz but also to a minor amount of impurities (ImNBd) in the crystal prepared in the laboratory. The dimer has similar phosphorescence emission wavelengths to the (R)-lmNCz-SCp monomers. The complex composed of (R)-lmNCz and (R)-lmNBd contributes to the phosphorescent emission peak at about 600 nm, and the phosphorescent emission peak at about 650 nm is generated by (R)-lmNBd. This indicates that the impurity could also contribute to emission in molecular crystals. The present calculations clarify the relationship between the molecular aggregation and the light-emitting properties of the TADF emitters and will therefore be helpful for the design of potentially more useful TADF emitters.