Modification ofThermally Activated Delayed FluorescenceEmitters Comprising Acridan–Pyrimidine and Spiro-Acridan–PyrimidineMoieties for Efficient Triplet Harvesting

In this study, we investigate the effect of substitution and conformational impact on the photophysical properties of novel 5-methylpyrimidine derivatives containing electron-donating groups with distinct rigidity. Research has revealed that all of the compounds showed pronounced thermally activated delayed fluorescence (TADF) features. The addition of the spiro-acridan moiety eliminated dual emission, simplifying the photophysical behavior of the compounds. Compounds containing spiro-acridan units exhibited a larger singlet–triplet energy gap, resulting in a reduced reverse intersystem crossing rate and an extended TADF lifetime in both toluene solutions and PMMA films. Additionally, the delayed fluorescence intensity was higher in these compounds, which was attributed to a slower nonradiative triplet quenching rate. Embedding TADF compounds into a rigid PMMA matrix significantly increases the quantum yield of delayed emission by minimizing nonradiative deactivation caused by intramolecular twisting. The considerable conformational disorder in the polymer-doped films leads to multiexponential fluorescence decay and noticeable shifts in both prompt and delayed fluorescence in time-resolved spectra. The attachment of electron-donating moieties at the fourth position in 5-methylpyrimidine reduces conformational disorder due to the restriction of the rotations caused by methyl attachment.