Photodissociation Dynamics of the Iodine−Arene Charge-Transfer Complex

Abstract

The photodissociation reaction of the molecular iodine:arene charge-transfer (CT) complex into an iodine atom and an iodine atom−arene fragment has been investigated using femtosecond pump−probe, resonance Raman, and molecular dynamics simulations. In the condensed phase the reaction 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 about 1 ps. Since little CT resonance enhancement is found in Raman studies on the I2-stretch vibration, it is concluded that rapid curve crossing 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 ∼0.12 in pure mesitylene and ∼0.34 in mixed mesitylene/cyclohexane solutions. Semiempirical configuration-interaction calculations show that, except for the axial CT complex, the transition dipole is aligned almost parallel to the normal of the arene plane. The oscillator strength of the CT transition is found to be maximal in the oblique conformation with the I2 molecule positioned at an angle of about 30° with respect to the arene normal. This iodine angular dependence of the oscillator strength leads to photoselection of bent I2:ar complexes in pump−probe experiments. Molecular dynamics simulations confirm earlier findings that the I2:benzene complex is a fragile entity and that it persists only for a few hundred femtoseconds. These simulations also provide the proper time scale for the decay of the polarization anisotropy. The fact that the photoproduct experiences a substantial torque in the dissociation process explains the absence of a cage effect in this reaction.