Ultrafast vectorial and scalar dynamics of ionic clusters: Azobenzene solvated by oxygen

Abstract

The ultrafast dynamics of clusters of trans-azobenzene anion ͑A − ͒ solvated by oxygen molecules was investigated using femtosecond time-resolved photoelectron spectroscopy. The time scale for stripping off all oxygen molecules from A − was determined by monitoring in real time the transient of the A − rise, following an 800 nm excitation of A − ͑O 2 ͒ n , where n =1-4. A careful analysis of the time-dependent photoelectron spectra strongly suggests that for n Ͼ 1 a quasi-O 4 core is formed and that the dissociation occurs by a bond cleavage between A − and conglomerated ͑O 2 ͒ n rather than a stepwise evaporation of O 2 . With time and energy resolutions, we were able to capture the photoelectron signatures of transient species which instantaneously rise ͑Ͻ100 fs͒ then decay. The transient species are assigned as charge-transfer complexes: A ·O 2 − for A − O 2 and A ·O 4 − · ͑O 2 ͒ n−2 for A − ͑O 2 ͒ n , where n =2-4. Subsequent to an ultrafast electron recombination, A − rises with two distinct time scales: a subpicosecond component reflecting a direct bond rupture of the A − -͑O 2 ͒ n nuclear coordinate and a slower component ͑1.6-36 ps, increasing with n͒ attributed to an indirect channel exhibiting a quasistatistical behavior. The photodetachment transients exhibit a change in the transition dipole direction as a function of time delay. Rotational dephasing occurs on a time scale of 2 -3 ps, with a change in the sign of the transient anisotropy between A − O 2 and the larger clusters. This behavior is a key indicator of an evolving cluster structure and is successfully modeled by calculations based on the structures and inertial motion of the parent clusters.

Electron attachment to trans-azobenzene

“…Combining the calculated ground state anion relaxation energy of 0.22 eV with the predicted EA v of 0.83 eV from our scaling procedure, we find an EA a of 1.05 eV which is intermediate and entirely consistent with the threshold value of B1 eV determined by the PES experiment of Paik et al 26 and the EA a predicted by the B3LYP/6-31+G(d) calculations (1.19 eV, see Table 2).…”

“…In addition, experimental data for the gas-phase adiabatic electron affinity (EA a ) of azobenzene have been reported. There is however a large discrepancy between the values of 0.62 eV 24 and 0.57 eV 25 (obtained with electron-transfer equilibria techniques) and that (approximately 1 eV) recently obtained 26 by means of anion photoelectron spectroscopy. Similarly, the EA a s evaluated with different theoretical methods also range from 0.57 eV 27 to 1.05 eV.…”

Preparation of Labile Ni+(cyclam) Cations in the Gas Phase Using Electron-Transfer Reduction through Ion–Ion Recombination in an Ion Trap and Structural Characterization with Vibrational Spectroscopy

“…The production of the m / z = 258 ion is a necessary step in the application of ETR to the preparation of the key Ni(I)(cyclam) + species, but this method of preparation raises the important question of whether the organic scaffold survives the substantial exoergicity, Δ E , inherent to the bimolecular reaction: A rough estimate of Δ E based on the calculated ionization energy (IE) of Ni + (cyclam) of about 8.5 eV and the adiabatic electron affinities (AEA) of the anions (0.2 and 1.6 eV for fluoranthene and azobenene, respectively) 38 , 39 sets limiting values of Δ E in the range of 8.3 and 6.9 eV for the two anions (using Δ E = IE – AEA). Interestingly, we note that the more exothermic fluoranthene reaction yields more fragmentation peaks arising from eq 3 , as expected for the usual ETD processes.…”

“…A rough estimate of ΔE based on the calculated ionization energy (IE) of Ni + (cyclam) of about 8.5 eV and the adiabatic electron affinities (AEA) of the anions (0.2 and 1.6 eV for fluoranthene and azobenene, respectively) 38,39 sets limiting values of ΔE in the range of 8.3 and 6.9 eV for the two anions (using ΔE = IE − AEA). Interestingly, we note that the more exothermic fluoranthene reaction yields more fragmentation peaks arising from eq 3, as expected for the usual ETD processes.…”

“…The observation that azobenzene dramatically reduces the degradation byproducts (compare panels c and d of Figure ) indicates that lowering the exoergicity by 1.4 eV has a profound effect on the degree of fragmentation. Part of this enhancement likely also results from the strong geometry change in the azobenzene anion framework upon electron detachment, which has been quantified through negative ion photoelectron spectroscopy. , This is evident by the degree of vibrational excitation in the neutral upon vertical electron detachment, which in the case of azobenzene deposits considerable (∼1.6 eV) additional energy in the neutral azobenzene moiety, thus reducing the energy partitioned to the nascent Ni + (cyclam) ion. It would therefore be valuable to refine the ETR approach by identifying higher electron affinity electron donors as well as systems which feature large geometry changes upon charge transfer.…”