Electronic structure and spin delocalization in Blatter radical derivatives: A computational study

Blatter radicals are well known for their exceptional thermodynamic stability. Here, a systematic density functional theory (DFT) investigation was carried out on the Blatter radical and its derivatives (1-7, 2-X, 3-X and 7-X) to explore how structural modification influences electronic structure and excited-state properties. Spin-density analysis confirms that the unpaired electron remains delocalized over the radical framework, consistent with the known radical stability. Time-dependent DFT calculations indicate that the dominant absorptions originate from the higher excited states. In contrast, the lowest doublet excited state (D₁), which is associated with the emission process, exhibits a weak oscillator strength, suggesting that radiative decay from the D₁ state is inherently limited. The calculated fluorescence radiative rate constants are on the order of 10⁶-10⁷ s^-1, accompanied by small structural differences between the ground (D₀) and D₁ states. Natural transition orbital (NTO) analyses indicate that the D₁ states of 2-X and 3-X derivatives are predominantly characterized by α-type transitions, while the 7-X derivatives exhibit mixed α/β transition character. Overall, the substituent effects do not fundamentally alter the open-shell electronic framework.