Tuning the vibrational relaxation of CO bound to heme and metalloporphyrin complexes

Picosecond mid-infrared pump-probe experiments were used to investigate vibrational relaxation (VR, which here denotes loss of excess vibrational energy) of CO bound to synthetic heme and porphyrin complexes with different metal atoms (M = Fe, Ru, Os) and different proximal ligands (imidazoles and pyridines). Isotope effects of ¹³CO vs ¹²CO and solvent effects were also studied. A remarkable correlation between the carbonyl vibrational lifetime and the carbonyl vibrational frequency ν_CO is observed. The vibrational lifetime decreases as ν_CO decreases. The lifetime-frequency correlation is consistent with a linear relation between carbonyl vibrational relaxation rate and ν_CO. Hemes and porphyrins show similar lifetime-frequency correlations, but the absolute value of the VR rate in meso-tetraphenylporphyrin complexes is slightly faster than in protoporphyrin IX dimethyl ester heme complexes. The predominant VR process is shown to be intramolecular transfer from CO to heme vibrations, rather than intermolecular transfer from CO to solvent vibrations. The intramolecular process occurs by anharmonic coupling via π-bonding between CO and the metalloporphyrin or heme. In metalloporphyrin and heme complexes, changes in back-bonding to CO simultaneously affect both CO frequency and the strength of anharmonic coupling, accounting for the observed lifetime-frequency correlation. Increasing back-bonding lowers the CO frequency and increases the anharmonic coupling, shortening the vibrational lifetime. Similar lifetime-frequency correlations are observed in wild-type and mutant heme proteins. It is possible to continuously tune the vibrational relaxation rate of CO over a range spanning about a factor of 4, by systematic modification of the chemical structure of the heme or porphyrin complex to which it is bound.