High spin ! low spin ultrafast excitation and relaxation of an isolated iron ( II ) complex

Among spin crossover (SCO) compounds, iron(II) complexes are of particular interest. For iron(II) SCO crystals, it has been shown that optical excitations along the weak d-d singlet (1A1,S=0) Low Spin (LS) to triplet (T, S=1) or quintet (5T2, S=2) High Spin (HS) to quintet ( E) optical transition of iron(II) ion can drive back and forth iron(II) from LS to HS states [1]. On the other hand, McGarvey et al. were the first to discover that in a solution of iron(II) complexes, the HS state could be populated at the expense of the LS state by pulsed laser excitation [2]. In this case, the doorway to the HS state is usually achieved via the singlet metal-to-ligand-charge-transfer (MLCT) that exhibits strong absorption bands in the visible spectrum. The latter process has been shown to be very efficient and, in less than 300 fs, ~100% of the complexes excited in the MLCT state relax toward the 5T2 state [3-4]. The associated experiments resolved a long-standing issue about the population mechanism of quintet states in iron(II)-based complexes, which were identified as a simple 1MLCT!3MLCT!5T2 cascade starting from the LS state [3]. However nobody has, to the best of our knowledge, demonstrated the reverse phenomenon: i.e. the HS to LS photo-excitation of isolated iron(II) complexes in a solution. Therefore for isolated molecules in solution, one can question if the absence of cooperative phenomena does not forbid such a photo-switching. Hereafter, using picosecond and femtosecond time resolved experiments, we demonstrate that it is indeed possible to study both the LS!HS and HS ! LS excitation and relaxation processes in a solution of iron(II) complexes. Both processes are shown to occur in less than 300 fs. The relaxation pathways of the photo-excited complexes are presented and discussed.

Among spin crossover (SCO) compounds, iron(II) complexes are of particular interest.For iron(II) SCO crystals, it has been shown that optical excitations along the weak d-d singlet ( 1 A 1 ,S=0) Low Spin (LS) to triplet ( 1,3 T, S=1) or quintet ( 5 T 2 , S=2) High Spin (HS) to quintet ( 5 E) optical transition of iron(II) ion can drive back and forth iron(II) from LS to HS states [1].On the other hand, McGarvey et al. were the first to discover that in a solution of iron(II) complexes, the HS state could be populated at the expense of the LS state by pulsed laser excitation [2].In this case, the doorway to the HS state is usually achieved via the singlet metal-to-ligand-charge-transfer ( 1 MLCT) that exhibits strong absorption bands in the visible spectrum.The latter process has been shown to be very efficient and, in less than 300 fs, ~100% of the complexes excited in the 1 MLCT state relax toward the 5 T 2 state [3][4].The associated experiments resolved a long-standing issue about the population mechanism of quintet states in iron(II)-based complexes, which were identified as a simple 1 MLCTĺ 3 MLCTĺ 5 T 2 cascade starting from the LS state [3].However nobody has, to the best of our knowledge, demonstrated the reverse phenomenon: i.e. the HS to LS photo-excitation of isolated iron(II) complexes in a solution.Therefore for isolated molecules in solution, one can question if the absence of cooperative phenomena does not forbid such a photo-switching.Hereafter, using picosecond and femtosecond time resolved experiments, we demonstrate that it is indeed possible to study both the LSĺHS and HS ĺ LS excitation and relaxation processes in a solution of iron(II) complexes.Both processes are shown to occur in less than 300 fs.The relaxation pathways of the photo-excited complexes are presented and discussed.
We studied, at room temperature, two very similar [Fe(phen) 3 ] 2+ and [Fe(2 CH 3 -phen) 3 ] 2+ (phen=1,10-phenantroline) complexes dissolved in acetonitrile.Under these conditions, the [Fe(phen) 3 ] 2+ complexes are in the LS state whereas for the [Fe(2 CH 3 -phen) 3 ] 2+ solution, ~80% (resp.~20%) of the complexes are in the HS (resp.LS) state [5].We have previously shown that the excitation of [Fe(phen) 3 ] 2+ solution by ~100 fs pulses centered at 500 nm makes it possible to promote complexes from the LS to the HS state in less than 300 fs [8].For these complexes, the photo-induced HS state that relaxes in 1.6±0.4ns absorbs at O ~330 nm.This absorption was attributed to the quintet ( 5 T 2 ) metal-to-ligand-charge-transfer ( 5 MLCT).Thus, one may consider to perform transient LSĺHS or HSĺLS photo-excitation by exciting the [Fe(2 CH 3 -phen) 3 ] 2+ at O ~500 nm or O ~330 nm respectively.Firstly, we excited the [Fe(2 CH 3 -phen) 3 ] 2+ complexes with picosecond pulses centered at O =500 nm and recorded the transient optical density of the sample versus the pump delay.The associated absorption spectrogram recorded by a streak camera is shown in Figure 1a.In good agreement with [Fe(phen) 3 ] 2+ , upon excitation we noticed a bleaching of the singlet metal-to-ligand-charge-transfer band that is centered at O ~500 nm.This bleaching relaxes in ~3.6±0.2 ns (Figure 1c).This clearly indicates promotion of [Fe(2 CH 3 -phen) 3 ] 2+ complexes from LS to HS state upon such a pulsed excitation.We performed the same experiment exciting the solution at O =355 nm. Figure 1d shows the recorded spectrogram.As shown in Figure 1e, the narrower width of the weak absorption band recorded at O =500 nm upon excitation as well as the slight bleaching at the edges of the 1 A 1 ĺ 1 MLCT band invalidates a pure HS to LS photo-switching.However, as shown in Figure 1e, one could reproduce this transient spectrogram by subtracting the weighted stationary absorption spectra of [Fe(phen) 3 ] 2+ and [Fe(2 CH 3 -phen) 3 ] 2+ complexes.This indicates that the transient recorded spectra can be accounted by a slight increase of the population of LS state.In fact upon excitation two opposite processes are taking place.On the one hand, we previously showed the photo-induced HS of [Fe(2 CH 3 -phen) 3 ] 2+ do absorbs at O =330 nm [6].Hence, one can populate the LS state along the excitation of the quintet ( 5 T 2 ) to metal-to-ligand-charge-transfer ( 5 MLCT) band that subsequently relax toward the LS state.On the other hand, the pump excitation could also results in a promotion of the [Fe(2 CH 3 -phen) 3 ] 2+ complexes from the LS to HS state.Indeed, we have shown that the kinetic recorded upon an excitation of the [Fe(phen) 3 ] 2+ complexes by a ~100 fs laser centered at O=320 nm is almost similar to the one recorded upon an excitation of the same solution by a ~100 fs laser centered at O =500 nm.Hence, one cannot either discard LS ĺ HS nor HS ĺ LS photo-switching of the complexes.We therefore consider, the transient absorption (resp.bleaching) recorded at O ~500 nm (resp.O ~450 nm and O ~550 nm) indicates the evolution of the average number of complexes promoted from the HS to LS state (resp.LS to HS state).To confirm this hypothesis, we recorded the evolutions of the optical density of the [Fe(2 CH 3phen) 3 ] 2+ excited by a ~100 fs pulse centered at O=330 nm and probed with ~100 fs pulses centered at O=450, 500 and 550 nm (Figure 2).As shown in Figure 2, the kinetics of the recorded signals could be nicely fitted.Interestingly for both probe pulses centered at O=450 and 550 nm, we could fit the data using the same time constants: 250±40 fs, 7.0±0.5 ps and 3.6±0.2ns.The latter are very close to the ones we recorded for the LS to HS state photo-switching of [Fe(phen) 3 ] 2+ complexes [6].The first relaxation steps were identified as a simple 1 MLCTĺ 3 MLCTĺ 5 T 2 ĺ 1 A 1 cascade from the initially excited state [3].In our case, upon excitation of the 1 A 1 ground state, the 1 MLCTĺ 3 MLCT transition takes place is less than 100 fs.The 3 MLCTĺ 5 T 2 transition occurs in ~250 fs living the complex in a highly vibrationaly excited state that relaxes toward the valley of 5 T 2 well in ~7 ps and subsequently to the ground state in ~3.6 ns.The latter process cannot account for the population of the LS state recorded in Figure 1d.Moreover, with these previous time constants, we could not fit the data recorded for O=500 nm.While the increase of the absorption does also occur during the pump-probe pulses temporal overlap, the decrease of the signal was fitted by a tri-exponential decay law with time constants of 200±30 fs, 5.1±0.3ps and ~3.6±0.5 ns respectively.This phenomenon can be accounted if one considers that part of the excited complexes relax as follow: 5 MLCTĺ 3 T 2 ĺ 1 A 1 ĺ 5 T 2 .Here again the 5 MLCTĺ 3 T 2 transition takes place is less than 100 fs.The 3 T 2 ĺ 1 A 1 transition occurs in ~200 fs living the complex in a highly vibrationaly excited state that relaxes toward the valley of 1 A 1 well in ~5 ps and subsequently to the 5 T 2 in ~3.6 ns.
In conclusion, we have demonstrated that it is indeed possible to study both the LSĺHS and HS ĺ LS excitation and relaxation processes in a solution of iron(II) complexes.The kinetics and the relaxation pathways of the photo-excited complexes have presented and discussed.

Fig. 1 :
Fig. 1: Spectro-temporal evolution of the differential absorption spectrum of the [Fe(2CH 3 -phen) 3 ] 2+ complex after its excitation at (a) O=500 nm and (d) O=355 nm.The black dots in (b) and (e) are the differential absorption spectrum recorded ~1 ns after an excitation.The grey lines are the absorption of [Fe(phen) 3 ] 2+ in the LS state and the absorption obtained subtracting the weighted stationary absorption spectra of [Fe(phen) 3 ] 2+ and [Fe(2 CH 3 -phen) 3 ] 2+ respectively.Fig. (c) and (f) present the experimental data and the mono exponential fits of the relaxation kinetic of the differential absorption spectrum at the selected wavelength.

Fig. 2 :
Fig.2: Kinetic traces recorded at different probe wavelengths (450, 500 and 550 nm).The solid lines are the fits of the kinetic.