Figure shows the dynamics of NiTMP in toluene excited in the B band at 400 nm for different optical-pump/optical-probe time delays Δt. The experimental details are given in S.I.3.The temporal resolution is set by the IRF estimated as ∼ 200 fs from the stimulated Raman signal. Comparing the OAS before and after measurements confirms that the photocycle is fully reversible S.I.3. At early times, the spectrum displays two broad bands E 1 and E 2 superimposed on the bleach signal B that matches the inverse of the GS OAS (gray line). The spectral lineshape exhibits a complex evolution over ∼ 10 ps until it reaches a profile that decays uniformly back to the zero baseline (Figure ). This transient spectrum corresponds to the difference between the LE-ES OAS and the GS OAS. The derivative-like lineshape closely resembles the profile constructed by taking the difference between the GS OAS shifted by ∼ 20 nm and the GS OAS itself. Consequently, the OAS of the LE-ES is very similar to the one of the GS, but presents a reduced HOMO-LUMO gap ascribed to a bond elongation expected for a MC state where the antibonding dx 2 -y 2 orbital is populated. According to Kasha's rule (Kasha1950), its multiplicity is expected to be triplet. DFT optimization of the LE-ES in the triplet multiplicity (noted T 1 ) reveals an NiN of 2.020 Å with a planarization of the ring (CNNC = 0°). The Kohn-Sham orbitals indicate the configuration (dπ) 6 (dz 2 ) (dx 2 -y 2 ) (S.I.2), confirming the MC character established experimentally. The absence of any stimulated emission signal suggests that ISC from singlet to triplet multiplicity completes within the pump pulse duration. The early spectral evolution reflects an intermediate stage in the deactivation from the FCS to the LE-ES T 1 via the HE-ES triplet manifold (noted T*). A more detailed view into the process is obtained by performing a global fit across the full photocycle to a sequential model involving three time constants (S.I.3). The corresponding components are shown in Figure . Figure displays representative kinetics across the probed spectral window, along with the traces built from the fit results of the global analysis, which returns three time constants of 0.5 ps, 5.4 ps, and 232 ps. The long time constant is unambiguously assigned to the lifetime of the LE-ES T 1 . In contrast, the interpretation of the first two components is not straightforward. Although previous work suggests that the intermediate stage involves CT (Shelby2019, Naumova2024), a precise assignment has not yet been proposed.