In this section, we are reporting on the ultrafast photodynamics following the photoexcitation of the molecular complex [Pyr2Bpy]Cp*RhCl2 in acetonitrile (MeCN, ε = 37.5), the solvent typically used for the photocatalytic CO2 reduction, propionitrile (ProCN, ε = 27.7) and butyronitrile (BuCN ε = 20.7). The UV-vis absorption spectra recorded for [Pyr2Bpy]Cp*RhCl2 in MeCN, ProCN, BuCN are depicted in Figure . In the three solvents, the absorption spectrum of the complex exhibits a first absorption band with a maximum peaking at 420 nm and a set of narrow peak typical of the pyrene unit around 330 nm. The lowest optical transition at 420 nm is 70 nm red-shifted in comparison with the free Pyr2Bpy ligand in THF. The absorption spectrum of [Pyr2Bpy]Cp*RhCl2 in MeCN is similar to that one of protonated [Pyr2BpyH] . Considering that the proton H + is equivalent to a reduced form of the Rhodium cation, the similitude in the absorption is suggesting a charge transfer from the Bpy to the metal center. This is also demonstrated by DFT calculation of the HOMO/LUMO of [Pyr2Bpy]Cp*RhCl2 showing that the lowest transition correspond to a charge transfer from the pyrene moieties towards the bipyridine-rhodium moiety. We can observe in ProCN and BuCN a slight decrease of the relative intensity of the lowest absorption band and an increase of the absorption at 350-375 nm for BuCN. The polarity of solvent is known to assists the charge separation, therefore this effect support the assignment of the lowest optical transition to a charge transfer between the photoactive pyrene chromophore and the bipyridine-rhodium centre forming a TICT state. Contrary to the S1 state of the Pyr2Bpy molecular ligand, the lowest excited state of [Pyr2Bpy]Cp*RhCl2 is weakly fluorescent ( < 1 %) with an emission maximum at 620 nm when excited at 420 nm (Fig. ) and it decays within less than 200 ps (Fig. ). We focused our study on the characterization of the excited state by carrying out femtosecond transient absorption measurement using a pump in resonance with the lowest S0 → Sn optical transition (pump = 400 nm). The transient absorption data of [Pyr2Bpy]Cp*RhCl2 in MeCN are reported in Figure for selected pump-probe delays between -200 fs and 1.2 ps and in Figure for delays up to 1.5 ns. The transient spectra show the ultrafast continuous interconversion between a first excited state absorption band exhibiting a maximum at 620 nm (ESA1) and a second absorption band peaking at 520 nm (ESA2). ESA1 is formed in less than 150 fs and it decays within 300-500 fs. In addition, the transient spectra exhibit a ground state bleach (GSB) contribution between 360 -420 nm. In Figure we selected three spectra representative of the ultrafast sub-ps photodynamic of the system. Qualitatively, the first step is identified at t = 0.14 ps, by the absorption ESA1, the negative GSB contribution, and in addition a second minimum at 500 nm that corresponds to the stimulated emission contribution (SE) from the generated excited state. SE is continuously red-shifted and decreasing (step 2, from ca 0.1 ps to 0.3 ps). Between 0.3 and 1 ps (step 3) we observe the conversion of ESA1 into ESA2 with an isobestic point at 580 nm. The amplitude of GSB (420 nm) is constant during this phase, showing the absence of internal conversion to the ground state. Based on these transient spectra, the ultrafast sub-ps photodynamic of [Pyr2Bpy]Cp*RhCl2 corresponds to the population of the Sn excited state by absorption of the photon at 400 nm, followed by an ultrafast evolution of the Franck-Condon geometry in response to the instantaneous change of the electronic density, and then, the subsequent internal conversion to the S1 state. This behaviour is typical of the ultrafast conversion between the close lying S2 and S1 excited state of pyrene and pyrene derivatives and we are noticing that the absorption band at 650 nm is lasting to the nanosecond time scale. The transient absorption after 1.5 ns is significantly more important in BuCN than in MeCN, notably due to the more pronounce component at 650 nm. The dynamics of conversion S2→ S1 is also significantly slower in BuCN, as it is clearly seen in the traces of Figure . From the fit of the decay, the growing time of the band at 530 nm is 360 fs in MeCN and 0.9 ps in BuCN. In MeCN, the band at 530 decays with two times constants 1 = 14 ps and 2 = 37 ps (see SI), while in BuCN, the process is slower with 1 = 33 ps and 2 = 225 ps (see SI). We also performed transient measurements in the presence of TEAO. In MeCN:TEOA, the process is not modified compared to the process in pure MeCN, excepted a slight increase of the decay time at 530 nm, but without consequence of the final spectra at 1.5 ns. For BuCN, we observe an increase of intensity and lifetime of the absorption band at 530 nm (Figure and). The spectrum at 1.5 ns exhibits more absorption contribution at 530 nm, and more bleach contribution in BuCN:TEOA than in BuCN. Interestingly, while in ProCN the transient data are similar to that measured in MeCN, while in ProCN:TEAO (see SI section 3-2) we can notice a transient absorption at 1.5 ns that is very close to that observed in BuCN:TEAO (see SI section 3-2). The sub-ns photodynamics measured for the molecular complex [Pyr2Bpy]Cp*RhCl2 can be described based on the photophysic of the excited-states of the aryl-substituted pyrene molecular derivatives. For the [Pyr2Bpy]Cp*RhCl2 compounds. the transient spectrum detected immediately after the pump excitation (t = 100-200 fs in Figure and Figure ) is similar to that one of the molecular ligands (Fig 4) showing the formation of a MICT state delocalized over the Py2Bpy ligand part of the compounds. It is then converted within a few hundreds of femtoseconds into a combination of the MICT and TICT state of the ligand. In MeCN, the TICT is favoured by the polarity of the solvent while, as the polarity of the solvent is reduced in BuCN, the mixing between the MICT and TICT is more pronounced and we observed the presence of the two maxima (530 nm and 650 nm) suggesting that the two configurations of the excited ligand are in equilibrium. This excited state decays time in the 10 ps-100 ps range corresponds to the formation of the ligand to metal (LMCT) charge transfer state from the excited ligand to the Rh 3+ centre. This assignment is supported by the DFT calculation . Our time-resolved data revealed the presence of two absorption band at 520 nm and 600 nm after 1.5 ns suggesting the co-existence of two different conformers that we tentatively propose as the asymmetric and the symmetric charge transfer, involving respectively one or two pyrene moieties (Scheme 3). The existence of two conformer and two reaction channels is supported by the difference in the kinetic at 530 nm and 650 nm. However, additional kinetics analysis and timeresolved measurements (structural or vibrational, for examples) are necessary to precise the nature and the yields of formation of these different LMCT species.