Styrylpyrimidine chromophores with bulky electron-donating substituents: experimental and theoretical investigation

Styrylpyrimidines with bulky 9,9-dimethylacridan, phenoxazine and phenothiazine electron-donating fragments were designed. Thermally activated delayed fluorescence (TADF) properties were expected for these structures. These chromophores exhibit peculiar emission properties. For 9,9-dimethylacridan and phenoxazine derivatives, a single emission highly sensitive to the polarity is observed in solution whereas for phenothiazine derivative a dual emission is observed in solution and is attributed to the coexistence of quasi-axial (Qax) and quasi-equatorial (Qeq) conformers. This study intends to understand through theoretical and experimental works, why the studied chromophores do not exhibit TADF properties, contrary to what was expected. The absence of phosphorescence both at room temperature and 77 K tends to indicate the impossibility to harvest triplet states in these systems. Wave-function based calculations show that for both conformers of the three chromophores the S1-T1 splitting is significantly larger than 0.2 eV. The second triplet state T2 of Qeq conformers is found very close in energy to the singlet S1 state, but S1 and T2 states possess similar charge transfer characters. This prevents efficient spin-orbit coupling between the states, which is consistent with the absence of TADF. The photophysical properties of styrylpyrimidines with bulky electron-donating substituents are studied. The absence of thermally activated delayed fluorescence is rationalized thanks to experimental and theoretical investigations.