Density‐dependent photochemical branching ratio in supercritical CO₂: Photodissociation and isomerization of diiodomethane

Abstract

The photodissociation of diiodomethane following excitation at 305 nm in supercritical CO2 was investigated by femtosecond pump‐probe absorption spectroscopy at pressures between 10 and 100 MPa. As in liquid solution, transient absorption signals in the wavelength range from 350 to 450 nm generally show an instantaneous peak, followed by a fast initial decay (200–300 fs) and a subsequent rise on a 10‐ps timescale. The initial fast decay time is found to be linearly dependent on viscosity, suggesting that dissociative motion on the repulsive surface is damped by solvent friction. Both amplitude and formation rate of the rising component, which is assigned to formation of iso‐diiodomethane within the solvent cage, increase with increasing pressure. Spectral narrowing of the transient absorption band indicates vibrational cooling of hot isomer by energy transfer to CO2 in about 20–40 ps. Immediately after excitation, this band shows absorption anisotropy for about 3 ps. The anisotropy decay rate increases from 3·1011 s−1 to 2·1012 s−1 as the pressure is lowered from 80 to 10 MPa. It is tentatively assigned to rotational relaxation of “hot” CH2I radicals generated in ultrafast photodissociation of the parent molecule. The observed density dependence of formation rate and relative yield of iso‐diiodomethane are described in terms of a simple kinetic model.

Formation and relaxation dynamics of iso-CH2Cl–I in cryogenic matrices

“…16,17 We have measured similar time scales at λ probe = 435 nm following the photolysis of dilute CH 2 ClI in room-temperature cyclohexane, where the signal rises with τ 1 = 4.4 ± 0.4 ps and decays with τ 2 = 68 ± 4 ps. The isomer CH 2 I-I formed by photolysis of CH 2 I 2 in super-critical CO 2 loses vibrational energy to the solvent in 20-40 ps, 55 again pointing to vibrational relaxation time scales that are relatively insensitive to the details of the condensed-phase surroundings.…”

Hirata Award: A. Brik / Liebig Lectureship: R. Klajn / Chaim Weizmann Prize for Exact Sciences: I. Marek / New Members of the Nationale Akademie der Wissenschaften Leopoldina: L. S. Cederbaum, U. Diebold, C. Hertweck, F. Meyer, and R. Weissleder / Otto Hahn Prize: J. Troe / Blaise Pascal Medal in Chemistry: H. W. Roesky

“…Troe’s research interests include reaction kinetics, spectroscopy, photochemistry, and atmospheric chemistry. He has reported in the Israel Journal of Chemistry on the photodissociation of diiodomethane 9…”

Ultrafast intramolecular electronic energy transfer in rigidly linked aminopyrenyl–aminobenzanthronyl dyads—a femtosecond study

“…

Ultrafast electronic excitation transfer (EET) followed by structural and vibrational relaxation (VER) of the acceptor have been characterised using transient absorption and transient lens techniques.

ExperimentalDetails of the pump-probe set-up for transient absorption measurements have been described elsewhere. 32 In brief, we used a 1 kHz Ti:sapphire regenerative amplifier (Clark-MXR CPA2001) as the femtosecond light source delivering pulses of 0.9 mJ energy and 150 fs duration at 773 nm which were used to pump two commercial optical parametric amplifiers, a Jobin Yvon two stage NOPA (1/3 of pump intensity) and a Light Conversion TOPAS (2/3 of pump intensity). After Scheme 1 Building blocks and compounds investigated in this study.

…”