Transient absorption spectroscopy of the iron(II) [Fe(phen)3]2+ complex: Study of the non-radiative relaxation of an isolated iron(II) complex

We report on the light induced spin crossover dynamics of [Fe(phen)3] (BF4)2 (phen = 1,10-phenantroline) complex dissolved in acetonitrile. This complex was excited by picosecond and femtosecond laser pulses whose central frequency was centered on the 1A1 → 1MLCT optical transition. Our analysis that well reproduces the recorded transient absorptions suggests this complex relaxes following inter-system crossing cascade: 1MLCT(S = 0) → 3MLCT(S = 1) → 5T2(S = 2) → 1A1(S = 0). The 3MLCT and 5T2 states were identified, thanks to the triplet–triplet 3MLCT → 3MLCT∗ and 5T2 → 5MLCT optical transitions. Our experiments well agree with the relaxation mechanism recently proposed and clearly indicate the impact of the solvent on the measured relaxation constant time.