This work presents the analysis of in situ laser-induced fluorescence (LIF) and ex situ electron paramagnetic resonance (EPR) measurements performed in a sooting atmospheric methane co-flow diffusion flame stabilized on a Gülder-type burner. We implemented these techniques in order to obtain information on the species involved in the nucleation processes of soot particles in flames. The LIF experiments were performed using excitation-emission matrices (EEM) to study the spectroscopic properties of the compounds formed in the flame. The analysis of these images, carried out by means of EEM arrays of pure PAHs, allowed the identification of a type of intermediate species characterized by optical properties distinct from PAHs and soot. The EPR data were obtained using various methods (EPR imaging, continuous wave EPR and pulsed wave EPR) allowing the localization of persistent radicals along the flame height, their quantification and a partial analysis of their structure. A remarkable correspondence is found between the concentration profile of persistent radicals and that of the fluorescent intermediates in the visible range, determined from the EEM arrays. Finally, the analysis of hyperfine interactions by 1H hydrogen, 13C carbon HYSCORE spectroscopy indicates the presence of aromatic coronene-sized radical structures and highly branched PAH clusters in nascent soot.