, where Δ𝑆 2 and Δ𝐸 are the uncertainties in 𝑆 2 and energy, respectively. This relation implies that it is not generally possible to specify the energy and total spin simultaneously and with arbitrary precision. Hence, the correct classification of the spin state of diradicals requires a detailed characterisation of the various interactions present in the spin Hamiltonian. In particular, one requires pulse ESR experiments which can measure the total spin quantum number directly. In this work, we present a continuous-wave, pulse ESR, and electron-nuclear double resonance (ENDOR) case study of the Chichibabin radicals shown in figure . These systems serve as model compounds for the wider class of open-shell PAHs. Given the above mentioned intricacies of classifying the spin states of diradicals, the focus of the study is on understanding the behaviour of liquid solutions of the FAAF molecule, which was previously investigated by Zeng et al. The AAF monoradical, which benefits from a well-defined electronic spin-1/2, will serve as an essential reference point for the ESR data obtained for FAAF. Additionally, the C substituted derivatives benefit from larger hyperfine couplings which facilitate the deconvolution of the cwESR spectra. The diradical FAAF was synthesized following the procedure reported by Zeng et al. (Figure ). Specifically, 10,10'-dibromo-9,9'-bianthracene (1) was treated sequentially with n-BuLi and fluorenone (2a) to yield diol (3a). Then, reaction of compound 3a with SnCl 2 afforded the diradical FAAF, which is stable enough to be purified by column chromatography and isolated under ambient conditions. A similar synthetic procedure was used for the preparation of monoradical AAF, from 10-bromo-9,9'-bianthracene (4). In this case, AAF was isolated in a 52% yield by reaction of alcohol 5a with SnCl 2 . For the preparation of isotopically labelled radicals 𝟏𝟑 C-FAAF and 𝟏𝟑 C-AAF, it was first necessary to prepare the labelled fluorenone 2b (see SI for details). Then, the corresponding isotopically labelled radicals were obtained following the same procedure as for the preparation of FAAF. 3a: X = C, 67% 3b: