Electronic and Vibrational Dynamics of Heme Model Compounds-An Ultrafast Spectroscopic Study

The ultrafast electronic and vibrational dynamics of a model five-coordinate, high-spin heme Fe^IIOEP-2MeIm, following excitation in the Soret band, have been studied in a coordinated fashion using ultrafast transient absorption, time-resolved Stokes and anti-Stokes resonance Raman spectroscopies. Electronic excitation on the blue side of the Soret band region led to sub-100 fs S₂→S₁, followed by 800 fs nonradiative decay to a vibrationally hot, nonthermal ground state species S₀^*. S₀^* then evolved to the thermally equilibrated ground state species S₀ on ~10 ps timescale. The initial accepting modes during the S₁→S₀ internal conversion underwent rapid vibrational relaxation evidenced by 1-3 ps decay of transient anti-Stokes signals. Comparison to other studies in heme proteins where the initial excited electronic state was prepared by ligand photolysis led to insight into the state dependence of vibrational dynamics in hemes.

The ultrafast dynamics of a six-coordinate low-spin heme Fe^IIOEP-(Im)₂ were studied following photolysis of one of the imidazole ligands upon electronic excitation. No detectable vibrationally hot six-coordinate ground state heme is observed, indicating the intrinsic quantum yield of the photolysis to be close to unity. The intramolecular vibrational relaxation is the major contribution to the photoproduct cooling on the order of 0.6 to 4.5 ps time scale. The intermolecular vibrational cooling has a lifetime of approximately 10 ps. On average 75% of the photolysed ligand geminately recombines on the time scale of 10-20 ps strongly overlapping with the vibrational cooling dynamics. In addition to the two heme compounds, the effect of methyl groups on the vibrational relaxation processes of para-nitroaniline (PNA) was studied. Minimal impact upon electronic dynamics, significant impact upon vibrational dynamics was observed in 2-methyl, 2, 6-dimethyl and N, N-dimethyl PNA compounds. NO₂ group wagging overtone (~1495 cm^-1) acts as the coupling and primary accepting mode during internal conversion in all four compounds. N-methyl substitution increases vibrational cooling lifetimes, phenyl ring substitution populates two new low frequency ring vibrations not present in PNA or N, N-dimethyl PNA. Anti-Stokes intensity decay and peak-shift evolution yield different lifetimes. Off-diagonal anharmonic coupling to populated low frequency motions of the phenyl are likely to be important.