Changing the shape of molecular ions: photoisomerization action spectroscopy in the gas phase

Abstract

A new approach for studying the photoisomerization of molecular ions in the gas phase is described. Packets of molecular ions are injected into a drift tube filled with helium buffer gas, where they are irradiated with tunable laser light. Photoisomerization changes the ions' cross section for collisions with helium atoms so that they arrive at the ion detector slightly earlier or later than the parent ions. By monitoring the photo-isomer peak as a function of laser wavelength one can record an action spectrum that is related to the ions' absorption spectrum modulated by the photoisomerization probability. The approach is demonstrated using the polymethine dye HITC (1,3,3,1',3',3'-hexamethylindotricarbocyanine). The data show that both trans and cis forms of HITC(+) exist in the gas phase with trans→cis photoisomerization predominating over the 550-710 nm range and cis→trans photoisomerization occurring over the 735-770 nm range. The gas-phase photoisomerization action spectrum is comparable to the absorption spectra of trans HITC and cis HTIC in the condensed phase, but with the absorption peaks shifted to shorter wavelength. The gas-phase photoisomerization action spectrum of the (HITC)2(2+) dication dimer is also reported. (HITC)2(2+) cations photoisomerize over the 550-770 nm range to form more compact structures.

An ion mobility mass spectrometer for investigating photoisomerization and photodissociation of molecular ions

“…Photoisomerization has also been accomplished using the output of a quasi-continuous Titanium:Sapphire laser with mechanical chopping of the light beam to record laser-on and laser-off ATDs and discern the laser-induced signal. 23 As shown in Figure 1, the drifting ions can either be excited at the beginning of the drift region (Mode 1), after a second ion gate halfway along the drift region (Mode 2), or coaxially in the central part of the drift region (Mode 3). The three excitation modes are described in more detail below.…”

“…[23][24][25] Normally, ions are injected into the drift region at 20 Hz with the laser fired at 10 Hz so that laser-on and laser-off ATDs are recorded. The photo-signal corresponds to the difference between laser-on and laser-off ATDs.…”

“…Previously, isomer specific spectra for molecular ions have been measured using IR-UV double resonance REPD, or by REPD with preselection of the ions using field asymmetric ion mobility spectrometry (FAIMS). 22 The new IMMS machine has been employed to investigate photoisomerization of molecules containing conjugated carbon chains including carbocyanine dyes HITC + and DTC + , 23,24 and the retinal protonated Schiff base. 25 The apparatus has also been used to structurally characterise charged fragments from retinal protonated Schiff base, helping to unravel the photodissociation mechanism.…”

Infrared Spectroscopy of Mobility-Selected H+-Gly-Pro-Gly-Gly (GPGG)

“…This region consists of two electrodes separated by 3 mm with a voltage drop that can be manipulated independently from the rest of the drift tube, allowing us to change the collision energy in this region in a controlled way. Bieske and coworkers measure visible absorption spectra of ions using a similar IMS-activation-IMS technique by relating photoinduced changes in collisional cross-section to visible light absorption [39][40][41][42][43].…”

Twisted Intramolecular Charge Transfer in Protonated Amino Pyridine

“…Such TICT reaction has been observed in solution and small clusters due to the large stabilization of the charge transfer state upon solvation. Bare isolated molecules such as the chromophore of the green fluorescent protein 12 and carbocyanine-like dye 13 can undergo cis-trans photo isomerization in the absence of solvent. As it will be shown here, we have found evidence for TICT reaction in protonated 4-aminopyridine.…”