Thermally Activated Delayed Fluorescence (TADF) is a process in Organic LEDs to improve the internal quantum efficiency from 25% up to 100%. By studying this process and their interactions with the environment, we can build devices with better and brighter screens. NAI-DMAC is a known TADF molecule and studying it in the glass-forming solvent methyl tetrahydrofuran (mTHF) allows us to measure the effects of molecular dynamics on fluorescence and the singlet-triplet energy splitting (ΔE_ST). An orthogonal orientation of the donor and acceptor moieties results in the smallest ΔE_ST but very little fluorescence. However, as the molecule leaves orthogonality, the ΔE_ST increases but results in more fluorescence. In solutions, the solvent allows free rotation between the donor and acceptor to access these necessary conformations given enough thermal energy. In mTHF solution, decreasing the temperature permits comparison of the fluorescence spectrum from the singlet state at 300 K to the phosphorescence spectrum from the triplet state at 80 K. From this comparison, a value of ΔE_STof 0.25 eV was obtained. Decreasing the temperature also reduces the variation of molecular conformations in the TADF process, leading to the photoluminescence (PL) intensity decreasing by a factor of 10 between 300 and 80 K. However, no such reduction of PL intensity occurs in the solid-state because NAI-DMAC molecules aggregate, allowing energy transfer among molecules of various conformations. The energy transfer process permits the TADF process to occur even at low temperature but obscures the true ΔE_STvalue so the traditional approach of comparing the fluorescence and phosphorescence will result in a ΔE_STvalue of 0.03 eV, which is considerably smaller than the solution study.

Gary Chen

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