Photodissociation UV–Vis Spectra of Cold Protonated Azobenzene and 4-(Dimethylamino)azobenzene and Their Benzenediazonium Cation Fragment

Abstract

Gas phase photodissociation electronic spectra of protonated azobenzene (ABH(+)) and 4-(dimethylamino)azobenzene (dmaABH(+)) were measured in a cryogenically cooled ion trap at temperatures of a few tens of Kelvin. Experimental results were complemented with electronic structure calculations in the ground state at the MP2/cc-pVDZ level of theory, and in the low lying excited states using the RI-CC2 method. Calculated energies revealed that only the trans isomers of the azonium molecular ions (protonation site on the azo group) will likely exist in the trap at the temperatures achieved in the experiment. The first transition of trans-ABH(+) is π* ← π, and the absorption band in the spectrum appears strongly red-shifted from that of the neutral molecule. The calculations showed that upon excitation the quasi-planar ground state (S0) transforms into a chairlike excited state (S1) by twisting the CNNC dihedral angle about 96°. A 41 cm(-1) active vibrational progression found in the ABH(+) spectrum may be associated with the twisting of the azo bond. Conversely, the electronic spectrum of dmaABH(+) exhibits a steep and unstructured S1 ← S0 absorption corresponding to a less distorted S1 state. The next two quasi-degenerate bands in the ABH(+) spectrum evidence sharper onsets and a charge transfer character. Using a second fragmentation laser and an additional He cooling pulse in the trap, it was possible to measure the UV spectrum of cold benzenediazonium fragments.

Photoisomerization of Protonated Azobenzenes in the Gas Phase

“…No photoisomerization response was discernible for the S 2 ← S 0 or S 3 ← S 0 transitions of ABH + or NH 2 ABH + , similar to the situation for the higher ππ* transitions of neutral trans -azobenzene . Overall, the PISA spectra are consistent with previously reported PD spectra of ABH + and NMe 2 ABH + , although the bands are broader due to the higher temperature in the drift tube (close to 300 K) compared to the cryogenic ion trap used for the PD measurements (40 K) . Perhaps most significantly we have shown that protonation of azobenzenes leads to a significant red-shift in their absorption maxima in the gas phase while retaining the trans to cis isomerization response required for their application as photoswitch molecules.…”

“…The equilibrium geometry of trans -ABH + was found to be planar at the ωB97X-D/6-311+G­(2df,p) level of theory, unlike the previously published twisted structure . However, as discussed in the Supporting Information, the energy difference between the planar and nonplanar structures and the barrier separating them are small (≤1 kJ/mol), such that the molecule should be quasi-planar at 300 K. Therefore, the planar DFT geometry was used for calculating vertical excitation energies.…”

“…Another approach to modifying the absorption profile involves coordinating the azo group to a strongly electron withdrawing BF 2 group, which has the effect of red-shifting the absorption into the near-infrared while maintaining the isomerization quantum yield above 70%. , The third approach to red-shifting the transition, first detailed by Woolley and co-workers, , is protonation of the azo group to give an azonium protomer. The azonium ions exhibit a red-shift which can exceed 100 nm that is caused by delocalization of the positive charge over the π-system and stabilization of the LUMO . For azobenzenes with hydrogen bond acceptors such as methoxy groups incorporated at the ortho -position, the azo group can become sufficiently basic to be protonated at physiological pHs, leading to absorption maxima in the 560–620 nm range.…”

“…Although the dynamics of neutral azobenzenes have been thoroughly studied both in solution and in the gas phase, less is known about the protonated species. Féraud et al recently measured the S 1 ← S 0 photodissociation (PD) spectra of protonated azobenzene (ABH + ) and protonated 4-(dimethyl­amino)­azobenzene cations (NMe 2 ABH + ) in a cryogenic ion trap, showing that the ππ* transition was significantly red-shifted compared to the unprotonated molecules. Although the authors could not directly observe photoisomerization for either of these protonated azobenzenes in the gas phase, they postulated a similar excited state isomerization mechanism to that established for azobenzene based on the presence of a pronounced 41 cm –1 vibronic progression, assigned to a vibrational mode involving torsion around the NN bond.…”

Gas-Phase Ion Spectroscopy of Congo Red Dianions and Their Complexes with Betaine

“…The spectroscopic properties of dyes often depend on their immediate environment, e.g., solvent molecules or counterions, a fact exploited extensively to report on microenvironments in biological systems from absorption or emission band maxima . In recent years significant work has been done in elucidating the intrinsic properties of ionic dyes, some acting as photoswitches like the azo dyes, from spectroscopy experiments on isolated ions in vacuo . − For example, protonated azobenzene and other photoswitches were studied by Bieske and co-workers − using photoisomerization action spectroscopy (PISA). These experiments take advantage of different ion mobilities of the two isomers when they travel through a drift tube filled with N 2 as their collision cross sections differ.…”