Femtosecond Excited-State Dynamics of an Iron(II) Polypyridyl Solar Cell Sensitizer Model

Time-resolved electronic absorption spectroscopy on a 100 fs time scale has been used to study excited-state dynamics in an Fe^II polypyridyl complex. [Fe(tren(py)₃)]²⁺, where tren(py)₃ is tris(2-pyridylmethyliminoethyl)amine, forms a ¹MLCT excited state upon irradiation at 400 nm and is known from previous studies to undergo relaxation to a low-lying ligand-field state having S = 2. Static absorption measurements on the low-spin parent complex and a high-spin analogue have been used to identify spectroscopic signatures for the S = 0 and S = 2 ligand-field states, respectively. Comparison of these data with femtosecond and nanosecond differential absorption spectra establishes that the net ΔS = 2 intersystem crossing is essentially complete in well under 1 ps. Spectroelectochemistry on [Fe(tren(py)₃)]²⁺ has also been used to find an absorption feature characteristic of the initially formed ¹MLCT state at λ 600 nm. Analysis of single-wavelength kinetics data in this spectral region reveals that the charge-transfer → ligand-field manifold conversion, observed here for the first time, occurs with a nearly instrument-response limited time constant of less than 100 fs. Additional dynamics occurring with a time constant of 8 ± 3 ps are tentatively assigned as vibrational cooling in the high-spin ligand-field state. The ultrafast intersystem crossing is interpreted as calling into question the utility of spin selection rules for understanding and predicting excited-state relaxation dynamics in transition metal complexes, whereas the sub-100 fs MLCT → LF conversion is discussed in terms of its implications for the dynamics of electron injection in Fe^II-sensitized TiO₂-based solar cells.