Designing High-Spin Organic Radicals for Spintronics: A Computational Exploration of Substituent and Structural Effects in Blatter Tri-Radicals

High-spin Blatter-type tri-radicals represent a unique class of polyradicals with promising applications in materials science, electronics, and spintronics, attributed to their stability, antiferromagnetic behaviour, and π-spin delocalisation. However, broader application of these compounds has been hindered by limited understanding of how structural variations, including geometry and substituent effects, influence their electronic properties and stability, particularly the doublet-quartet (D–Q) energy gap. This thesis addresses these gaps by examining how geometry and substituent modifications affect the electronic structure, ground-state multiplicity, and excited-state topology of Blatter-type tri-radicals and additional triazine biradicals using DFT benchmarks against high-level multireference calculations. The findings demonstrate that substituents play a pivotal role: F, as an electron-withdrawing substituent, shortens bond lengths, while Cl and Br induce rotational distortions due to steric hindrance, especially in inner substituent positions. Electron-donating and electron-withdrawing groups also exhibit unique behaviours, with NH2 groups introducing deviations and hydrogen bonding, while COOH and CHO groups maintain planarity yet contribute out-of-plane distortions. Analysis of D–Q gaps reveals that inner substitutions generally reduce gaps due to steric effects, while outer substitutions tend to favour quartet ground states, irrespective of the substituent tested. Further analysis of fused dithiazole-triazine derivatives reveals that, while ring heteroatoms and functional groups influence singlet-triplet energy gaps, all these systems consistently exhibit closed-shell singlet ground states. This thesis provides deeper insights into the electronic properties of Blatter-type tri-radicals and biradical triazines, establishing key design principles for creating stable, high-performance organic polyradicals and advancing the potential of highspin systems in magnetic and spintronic devices.