A mid infrared study of dynamic processes in iodine–pyridine charge transfer complexes Original Research Article

The mid IR absorption spectra of the iodine–pyridine charge-transfer complex diluted in an `inertʹ solvent have been measured as a function of the donor concentration in the ternary mixture. The enhanced internal modes of pyridine as well as the new vibrational modes of the complex were assigned on the basis of a recent ab-initio study. The results obtained confirm the observations and assignment reported in earlier literature measurement for the iodine–pyridine system very diluted in ternary mixture. The spectral characteristics (shifts, band centre frequencies and widths) of the profiles associated with the internal modes of the complex have been characterised using Lorentzian bandshape. The interpretation of the spectral changes upon the variation of the donor concentration in the mixture has been performed on the profiles associated with both the ν1 and ν3 internal modes (A1 symmetry) using a model of the dipolar interaction between the transition dipole moment of the internal mode with the dipoles of the surrounding solvent molecule. In this theoretical framework, the more relevant result concerns the mechanism governing the bandshapes. This is found to involve vibrational relaxation by pure dephasing, with an associated correlation time close to the rotational correlation time of the solvent molecules. These conclusions support fully the ones obtained from the study of the acceptor (νI–I) and intermolecular (νD–I) vibrations using far infrared spectroscopy that we have reported before. Finally, the merits and limits of the two approaches is critically assessed.