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 I 2 -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 I 2 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 I 2 :ar complexes in pump-probe experiments. Molecular dynamics simulations confirm earlier findings that the I 2 :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.

Femtosecond real-time probing of reactions. XXI. Direct observation of transition-state dynamics and structure in charge-transfer reactions

“…With ps resolution, Langhoff et al 40͑a͒ and Hilinski et al 40͑b͒ explored the dynamics of these complexes and showed the ultrafast nature of the reaction, though not resolved. Two fs studies of D•I 2 reactions in the liquid phase have been reported recently by the groups of Wiersma 41,42 and Sension. 43 Here, we will discuss these results and compare the initial dynamics ͑Ͻ1 ps͒ reported in solutions with our findings.…”

“…With the advent of fs laser spectroscopy, the photochemistry of D•I 2 complexes has attracted new interest. Fs laser studies [41][42][43][44] have been reported recently and shown that the formation of D•I complexes is indeed extremely fast ͑25-500 fs͒ upon CT excitation of I 2 /D solutions. We will review and compare results of the liquid-phase fs studies with our gas-phase and cluster results in Sec.…”

“…42 The main conclusion of their calculations is that the transition dipole component parallel to the benzene plane is always much smaller than the perpendicular component. With the exception of the perfectly axial conformation, the direction of the transition dipole moment was found to be pointing ͑within ϳ10°͒ from the center of the benzene ring to the I 2 c.m.…”

“…Thus, their calculations suggest that the 11 component is much more important than the 10 component, making the transition dipole always perpendicular to the benzene plane. Since the experimental studies by Lenderink et al 42 were conducted in solutions, the excitation of tilted configurations is done by photoselections through the •⑀ interaction.…”

Solvation Ultrafast Dynamics of Reactions. 14. Molecular Dynamics and ab Initio Studies of Charge-Transfer Reactions of Iodine in Benzene Clusters

“…A wealth of studies has been devoted to studying this system in the solution phase; a review of the work can be found in recent publications from Wiersma's group 2 and from this laboratory 3 .…”