The vibrational energy relaxation of iodine-aromatic hydrocarbon charge-transfer complexes

Ultrafast transient absorption measurements on CT complexes between iodine and (m) xylene (MXY), iodine and mesitylene (MST) and iodine and hexamethylbenzene (HMB) have been carried out.; The ultrafast dynamics in these systems are quite similar. Upon excitation with 400 nm laser pulses, dissociation takes place in less than 250 fs along both the I-I and I-arene coordinates. Three geminate recombination processes along the I-I channel are observed. The fastest one occurs in less than 2 ps and produces vibrationally excited iodine on the ground state surface. The hot iodine transients subsequently undergo vibrational relaxation along the ground state potential and give rise to a large cooling signal observed in our experimental measurements. The second geminate recombination of two iodine fragments occurs within the solvent cage and have time constants of 19.4 ps, 14.8 ps and 11.9 ps in MXY, MST and HMB respectively. The third process is related to the diffusion controlled recombination from escaped iodine atoms and can be regarded as a non-decaying component in our analysis.; Vibrational relaxation dynamics are modeled based on the assumption that vibrational absorption spectrum is associated with its population distribution function. The population distribution function at each vibrational level can be calculated by solving a master equation. The excess energy decay function was bi-exponential and can be represented in terms of the solvent friction power spectrum G(ω). In condensed media solvent frictions originate from the perturbations on the hot vibrations due to solute-solvent interactions. Such perturbations occur on time scales that corresponding to the Fourier transform of the far-infrared spectrum of neat solvent (150–200 cm −1). The iodine cooling rate follows the order HMB > MST > MXY. The relaxation dynamics are strongly dependent on the attractions between the I₂-arene charge-transfer (CT) complexes. Fluctuating dipoles between solutes and solvents do not have significant effects on the cooling processes. The attractions either shift the G(ω) along the frequency channel or modify the magnitude of G(ω). In all cases the G(ω) values are the highest in HMB and lowest in MXY at the same frequency. The model can reproduce the experimental data rather well. Comparisons among different systems were made to demonstrate the dependence of transfer rate on the solute-solvent interaction strength.