PHOTODISSOCIATION DYNAMICS OF THE IODINE-ARENE CHARGE-TRANSFER COMPLEX

The photodissociation reaction of the molecular iodine:arene charge-tr ansfer (CT) complex into an iodine atom and an iodine atom-arene fragm ent has been investigated using femtosecond pump-probe, resonance Rama n, and molecular dynamics simulations. In the condensed phase the reac tion proceeds on a time scale of less than 25 fs, in sharp contrast to the gas phase where the excited state lifetime of the complex is abou t 1 ps. Since little CT resonance enhancement is found in Raman studie s on the I-2-stretch vibration, it is concluded that rapid curve cross ing occurs from the CT state to a dissociative surface. Of particular interest is the finding that the polarization anisotropy of the iodine atom:arene (I:ar) photoproduct decays on a time scale of 350 fs both in pure arene solvents as well as in mixed arene/cyclohexane solutions . This latter finding rules out that secondary I:ar complex formation is the main cause of this ultrafast depolarization effect. The initial polarization anisotropy is found to be similar to 0.12 in pure mesity lene and similar to 0.34 in mixed mesitylene/cyclohexane solutions. Se miempirical configuration-interaction calculations show that, except f or the axial CT complex, the transition dipole is aligned almost paral lel to the normal of the arene plane. The oscillator strength of the C T transition is found to be maximal in the oblique conformation with t he I-2 molecule positioned at an angle of about 30 degrees with respec t to the arene normal. This iodine angular dependence of the oscillato r strength leads to photoselection of bent I-2:ar complexes in pump-pr obe experiments. Molecular dynamics simulations confirm earlier findin gs that the I-2:benzene complex is a fragile entity and that it persis ts only for a few hundred femtoseconds. These simulations also provide the proper time scale for the decay of the polarization anisotropy. T he fact that the photoproduct experiences a substantial torque in the dissociation process explains the absence of a cage effect in this rea ction.