Picosecond excited-state lifetimes of protonated indazole and benzimidazole: The role of the N–N bond

Abstract

Certain chemical groups give rise to characteristic excited-state deactivation mechanisms. Here, we target the role of a protonated N–N chemical group in the excited-state deactivation of protonated indazole by comparison to its isomer that lacks this group, protonated benzimidazole. Gas-phase protonated indazole and protonated benzimidazole ions are investigated at room temperature using picosecond laser pump–probe photodissociation experiments in a linear ion-trap. Excited state lifetimes are measured across a range of pump energies (4.0–5.4 eV). The 1ππ* lifetimes of protonated indazole range from 390 ± 70 ps using 4.0 eV pump energy to ≤18 ps using 4.6 eV pump energy. The 1ππ* lifetimes of protonated benzimidazole are systematically longer, ranging from 3700 ± 1100 ps at 4.6 eV pump energy to 400 ± 200 ps at 5.4 eV. Based on these experimental results and accompanying quantum chemical calculations and potential energy surfaces, the shorter lifetimes of protonated indazole are attributed to πσ* state mediated elongation of the protonated N–N bond.

The combination of laser photodissociation, action spectroscopy, and mass spectrometry to identify and separate isomers

“…In these cases, pure samples of structural isomers were available so-provided no isomerisation occurs upon delivery of ions-the spectra from these isomers can be compared. Such studies have targeted substituted pyridines, [79][80][81][82] azaindoles, 83,84 diazabenzenes, 85,86 indazole and benzimidazole, 87 and diazanaphthalenes, 82 as well as anionic systems: deprotonated azaindoles, 84 nitrophenolate ions, [88][89][90] molecular-iodide clusters, 91 and protonated quinoline and isoquinoline. 54,92 The case of the structural isomers protonated quinoline and isoquinoline is rather straightforward.…”

“…Other groups have deployed femtosecond or picosecond pump-probe laser configurations combined with photodissociation mass-spectrometry to interrogate excited-state lifetimes of ions in tandem time-of-flight mass spectrometers, [133][134][135] triple-quadrupole mass spectrometers, 136 ion storage rings, 137,138 custom built ion traps at room temperature, [139][140][141] custom built ion traps that are cryogenically cooled, 81,85,132,142 and commercial ion traps. 87,[143][144][145][146] There are also setups using electronic delays for pump-probe photodissociation on the nanosecond to millisecond timescale. 117,132,[146][147][148][149][150][151] The excited 63 This journal is © The Royal Society of Chemistry 2022 state dynamics of few gas-phase ions has been investigated experimentally or computationally, especially relative to solution-phase species.…”

Ultrafast Transient Vibrational Action Spectroscopy of Cryogenically Cooled Ions

“…While time-resolved photofragmentation signals have been measured on gaseous molecular ions to track reaction kinetics 16,17 and excited-state lifetimes, 18 frequency-resolved action-based ultrafast spectroscopies of isolated molecular ions has yet to be demonstrated. Extension of ultrafast nonlinear spectroscopies to molecular ions is highly desirable, as well-established mass spectrometric techniques can be used to precisely generate, manipulate, and composition-select the desired chemical species of interest.…”

Two-Dimensional Infrared Spectroscopy of Isolated Molecular Ions

“…The merger of mass spectrometric ion processing techniques with nonlinear ultrafast spectroscopies is particularly enticing. − Soft ionization techniques such as electrospray ionization enable the extraction of complex and diverse chemical, biological, and nanomaterial systems into the gas phase from nM−μM solutions. Extracted ions can be further processed to controllably produce the desired chemical species, for example, through H–D exchange, generation of reactive intermediates, or clustering with solvent molecules. , Buffer gas cooling in cryogenic ion traps offers additional advantages by quenching molecular systems into their lowest-energy configurations.…”