Femtosecond excited-state dynamics of an iron(II) polypyridyl solar cell sensitizer model

Time-resolved electronic absorption spectroscopy on a similar to 100 fs time scale has been used to study excited-state dynamics in an Fen polypyridyl complex. [Fe(tren(py)(3))](2+), where tren(py)(3) is tris(2-pyridylmethyliminoethyl)amine, forms a (MLCT)-M-1 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 Delta S = 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 (MLCT)-M-1 state at lambda greater than or similar to 600 nn, 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 an 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 Fen-sensitized TiO2-based solar cells.