Vibrational Relaxation in Metalloporphyrin CO Complexes

In order to better understand the relationships between molecular structure and vibrational dynamics, we have investigated the rates of vibrational relaxation (VR) of carbon monoxide bound to the active site of synthetic metalloporphyrin complexes. Here VR is used to denote Vibrational energy relaxation, the loss of energy from vibrationally excited CO to the rest of the system. VR rates were determined using picosecond mid-infrared (mid-IR) pump-probe experiments.1,2 In this work, we have investigated a large number of heme complexes and find a remarkable correlation between the vibrational lifetime and the carbonyl stretching frequency. By changing the metal atom, the trans axial ligand, and the porphyrin substituents, it becomes possible to tune the vibrational relaxation lifetime of CO over about a factor of 4. Never before has virtually continuous control of a vibrational lifetime been observed in a condensed phase polyatomic molecule. Loss of vibrational energy from CO inherently involves anharmonic coupling between the carbonyl vibrational fundamental and its surroundings.3 In broad terms, there are three ways this might occur, as illustrated schematically in Figure 1. VR might be a purely intramolecular process, involving COto-metalloporphyrin energy transfer via the covalent metalloporphyrin-CO bonds. There are both σand π-orbitals involved in M-CO bonding; the π-bonds are formed by backbonding, i.e., back-donation from the metal dπ and porphyrin pπ orbitals to the π* antibonding orbitals of CO.4 Alternatively, carbonyl VR might be a purely intermolecular process, involving CO-to-solvent energy transfer via nonbonded interactions. Finally, VR might involve a mixture of intermolecular and intramolecular processes. For example, some of the excess energy might be transferred to porphyrin vibrations and the remainder to low-frequency collective vibrations of the solvent (instantaneous normal modes or solvent phonons).5 In order to establish the mechanism of carbonyl VR, we systematically varied the structure of the heme using different metal atoms in an isoelectronic series, different porphyrins (which are the ligands cis to the CO), and different trans axial ligands. In addition, experiments were performed using different solvents and isotopic substitution with 13CO. We believe that the dominant mechanism of VR involves intramolecular anharmonic coupling from CO to the metalloporphyrin, via the metalloporphyrin-CO π-bonds, rather than the metal-CO σ-bonds. Structural and electronic factors that increase backbonding decrease both νCO and the VR lifetime by increasing the through π-bond coupling.6 The preparation of the metalloporphyrin complexes followed standard literature methods.7,8 The pump-probe experiment measures the vibrational lifetime of the CO stretching mode,1,9 which, for the compounds studied here, lies in the 1980-1880 cm-1 range. Pump-probe measurements were performed at the Stanford Free Electron Laser Center, as described in detail elsewhere.1 In Figure 2, we plot the VR rate versus carbonyl stretching frequency in CH2Cl2 solutions for M(porph)(CO)(L) for two different porphyrins (TPP and PHDME), three metals (M ) Fe, Ru, Os), and five nitrogenous ligands. We observe a very strong correlation between the VR rate and the carbonyl