Ultrafast vibrational and structural dynamics of dimeric cyclopentadienyliron dicarbonyl examined by infrared spectroscopy.

In this work, we carry out steady-state, femtosecond pump-probe and two-dimensional (2D) infrared spectroscopic studies on dimeric π-cyclopentadienyliron dicarbonyl [CpFe(CO)2]2 in the C≡O stretching vibration frequency region in CCl4 and CH2Cl2. The cis and trans isomers, in terms of the position of two terminal C≡O groups, are found to coexist in the two solvents. A weak asymmetric stretching peak of the cis-isomer is revealed under that of the IR-active trans-isomer by analyzing the 2D infrared cross peak, which is supported by ab initio computations. Furthermore, vibrational population relaxation is found to be both solute and solvent dependent (ranging from 21 ps to 32 ps)--the fastest dynamics is found for the trans-isomer in the polar solvent environment, which is believed to be associated with the availability and the number of efficient energy accepting channels for solvent molecules. The spectral diffusion dynamics of the C≡O stretching vibrations, occurring on an even faster time scale (1 ps to 3 ps), mainly exhibits solvent dependence--faster dynamics is found in the polar solvent, involving weak and rapidly fluctuating hydrogen bonding interactions between CH2 groups of the solvent and the terminal carbonyls of solutes.