Photodissociation Dynamics of the Iodine-Arene Charge-Transfer Complex

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.

Femtosecond photochemistry in solution

In dit proefschrift wordt een aantal modelreacties bestudeerd om deze problemen nader te onderzoeken. Beide genoemde experimentele technieken worden gebruikt om de moleculaire bewegingen in vier unimoleculaire fotochemische reacties in oplossing in kaart te brengen. Deze reacties vinden alle plaats zonder noemenswaardige potentiaalbarrière, wat betekent dat de eerste stap van de reactie plaatsvindt onmiddellijk na absorptie van een foton. ... Zie: Samenvatting

Photodissociation dynamics of dimethylnitrosamine studied by resonance Raman spectroscopy

The initial molecular dynamics in the dissociative S1 (n, π*) state of dimethylnitrosamine (DMN) is investigated using resonance Raman spectroscopy. We find that photochemical N-N bond cleavage in DMN proceeds via a bent conformation around the amine N atom, which supports the outcome of ab initio and classical trajectory calculations. Additional information is obtained about the other motions that accompany the photodissociation: a stretch of the N-N bond and a change of the NNO angle.

Femtochemistry of the iodine-arene charge-transfer complex

Femtosecond spectroscopy is used to investigate the ultrafast photochemistry of the I2:arene charge-transfer complex that occurs upon excitation into the charge-transfer state. We find that the photo-excited complex branches into two different channels within 25 fs. The dominant channel involves breaking up of the I2:arene complex in its constituent molecules, in the other channel the I:arene complex and an iodine atom are formed. From the polarization anisotropy and the absence of fast geminate recombination of the I:arene and atomic iodine fragments we conclude that in the transition state the I2:arene complex is severely distorted. The molecular I2 formed in a separate channel of the reaction recombines with arene to I2:ar in about 10 ps.

Femtosecond fragmentation of tetraphenylhydrazine in solution

The photo-induced dissociation of tetraphenylhydrazine (TPH) in alkane solutions is studied by femtosecond pump-probe spectroscopy and by fluorescence lifetime measurements. The absorption of the photoproduct, a pair of diphenylaminyl radicals, is found to rise with a time constant of 80±25 fs. The excited-state lifetime of TPH in cyclohexane is measured to be 16 ps and found to be only slightly dependent on solvent viscosity. All our measurements and those of others can be explained by assuming the upper-state potential in TPH to be a double well. Fragmentation of TPH proceeds via two channels. First, the excited wavepacket branches ultrafast into a dissociative continuum and into a bound state. Subsequently, this bound state tunnels on a picosecond time scale into the dissociative channel as well.