Characterization of Excited States in a Multiple-Resonance-Type Thermally Activated Delayed Fluorescence Molecule Using Time-resolved Infrared Spectroscopy

We have investigated the correlation between the photophysical properties and the excited-state detailed characteristics in a multiple-resonance-type thermally activated delayed fluorescence (TADF) molecule, DABNA-1, using time-resolved infrared vibrational spectroscopy. In comparison of the distinctive vibrational spectra in the fingerprint region, 1000 - 1700 cm-1, to the simulated spectra by density functional theory calculations, we found the best calculation condition. On the basis of the calculations, we determined the excited-state geometries and molecular orbitals of the lowest excited singlet (S1) and triplet (T1) states as well as the ground state (S0). We revealed that the similarity of the potential surfaces between T1 and S0 suppresses the nonradiative decay and causes the high fluorescence quantum yield via TADF process

Fast, Efficient, Narrowband Room-Temperature Phosphorescence from Metal-Free 1,2-Diketones: Rational Design and Mechanism

We report metal-free organic 1,2-diketones that exhibit fast and highly efficient room-temperature phosphorescence (RTP) with high color purity under various conditions, including solutions. RTP quantum yields reached 38.2% in solution under Ar, 54% in a polymer matrix in air, and 50% in a crystalline solids in air. Moreover, the narrowband RTP consistently dominated the steady-state emission, regardless of the molecular environment. A detailed investigation of the emission mechanism using ultrafast spectroscopy, single-crystal X-ray structure analysis, and theoretical calculations revealed picoseconds intersystem crossing followed by RTP from a planar conformation. Thus, we attribute the high efficiency RTP across diverse molecular environments to an inherent ~5000-s–1 phosphorescence rate constant comparable to that of platinum porphyrin complexes. The planar conformation reflected a design principle for fast and narrowband RTP. This strategy complements the streamlined persistent RTP approach and enables the development of organic phosphors with emissions independent of environmental conditions, thereby offering alternatives to precious-metal based phosphors